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Magnifying Transmitter - Nikola TeslaThe Colorado Springs lab possessed the largest Tesla Coil ever built, known as the “Magnifying Transmitter”. This was not identical to the classic Tesla Coil. According to accounts, Tesla managed to transmit tens of thousands of watts of power without wires using the magnifier. Tesla posted a large fence around the coil with a sign, “Keep Out – Great Danger”. Tesla’s Magnifying Transmitter, at fifty-two feet in diameter, generated millions of volts of electricity and produced lightning bolts one-hundred-thirty feet long (forty-one metres). It was a three-coil magnifying system requiring alternative forms of analysis than lumped-constant coupled resonant coils presently described to most. The Magnifying Transmitter resonated at a natural quarter wavelength frequency.

Tesla also worked with the magnifying transmitter in a continuous-wave mode and in a damped-wave resonant mode. The Magnifying Transmitter produced thunder which was heard as far away as Cripple Creek. He became the first man to create electrical effects on the scale of lightning.

People near the lab would observe sparks emitting from the ground to their feet and through their shoes. Some people observed electrical sparks from the fire hydrants (Tesla for a time grounded out to the plumbing of the city). The area around the laboratory would glow with a blue corona (similar to St. Elmo’s Fire). One of Tesla’s experiments with the Magnifying Transmitter destroyed Colorado Springs Electric Company’s generator by back feeding the city’s power generators, and blacked out the city.

The city had a backup generator and company officials denied Tesla further access to their feed if he did not repair the city’s primary generator at his own expense. The generator was working again in a few days.

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UPS design criteria and selection

UPS design criteria and selection

An UPS system is an alternate or backup source of power with the electric utility company being the primary source. The UPS provides protection of load against line frequency variations, elimination of power line noise and voltage transients, voltage regulation, and uninterruptible power for critical loads during failures of normal utility source. An UPS can be considered a source of standby power or emergency power depending on the nature of the critical loads. The amount of power that the UPS must supply also depends on these specific needs.

These needs can include emergency lighting for evacuation, emergency perimeter lighting for security, orderly shut down of manufacturing or computer operations, continued operation of life support or critical medical equipment, safe operation of equipment during sags and brownouts, and a combination of the preceding needs.

The UPS selection process involves several steps as discussed briefly here.

Determine need

Prior to selecting the UPS it is necessary to determine the need. The types of loads may determine whether local, state, or federal laws mandate the incorporation of an UPS. An UPS may be needed for a variety of purposes such as lighting, startup power, transportation, mechanical utility systems, heating, refrigeration, production, fire protection, space conditioning, data processing, communication, life support, or signal circuits.

Some facilities need an UPS for more than one purpose. It is important to determine the acceptable delay between loss of primary power and availability of UPS power, the length of time that emergency or backup power is required, and the criticality of the load that the UPS must bear. All of these factors play into the sizing of the UPS and the selection of the type of the UPS.

Determine safety

It must be determined if the safety of the selected UPS is acceptable. The UPS may have safety issues such as hydrogen accumulation from batteries, or noise pollution from solid-state equipment or rotating equipment. These issues may be addressed through proper precautions or may require a selection of a different UPS.

Determine availability

The availability of the selected UPS must be acceptable. The criticality of the loads will determine the necessary availability of the UPS. The availability of an UPS may be improved by using different configurations to provide redundancy. It should be noted that the C4ISR facilities require a reliability level of 99.9999 percent.

Determine maintainability

The selected UPS must be maintainable. Maintenance of the unit is important in assuring the unit’s availability. If the unit is not properly cared for, the unit will be more likely to fail. Therefore, it is necessary that the maintenance be performed as required. If the skills and resources required for the maintenance of the unit are not available, it may be necessary to select a unit requiring less maintenance.

Determine if affordable

The selected UPS must be affordable. While this is the most limiting factor in the selection process, cost cannot be identified without knowing the other parameters. The pricing of the unit consists of the equipment cost as well as the operating and maintenance costs. Disposal costs of the unit should also be considered for when the unit reaches the end of its life.

Re-evaluate steps

If these criteria are not met, another UPS system must be selected and these steps re-evaluated.

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PowerLogic System lets you optimise the cost, quality and reliability of an electrical installation. It combines communicating devices with power monitoring software operating under Windows. PowerLogic System provides information on the entire electrical installation.

It offers a wide range of possibilities and can carry out a number of tasks including:
- alarm processing
- automatic tasks (e.g. automatic reports)
- precision instrumentation
- power quality and disturbance measurements
- data transfer
- etc.

PowerLogic System can be used for all electrical distribution systems. It creates a network of communicating devices connected to one or more supervision stations.

PowerLogic System is made up of three main parts:

  • communicating devices
  • communication interfaces
  • SMS software.

The products listed below are part of the PowerLogic System:

  • Circuit Monitor
  • Power Meter
  • low-voltage circuit breakers
  • Digipact DC150 interfaces
  • Sepam protection relays
  • Vigilohm System
  • and all third-party devices using the Modbus protocol (specific configuration is required).

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The Power Factor Correction

The Power Factor Correction

The power factor of a load, which may be a single power-consuming item, or a number of items (for example an entire installation), is given by the ratio of P/S i.e. kW divided by kVA at any given moment.

The value of a power factor will range from 0 to 1. If currents and voltages are perfectly sinusoidal signals, power factor equals cos ϕ.

A power factor close to unity means that the reactive energy is small compared with the active energy, while a low value of power factor indicates the opposite condition.

Power vector diagram
  • Active power P (in kW)
    • Single phase (1 phase and neutral): P = V x I x cos ϕ
    • Single phase (phase to phase): P = U x I x cos ϕ
    • Three phase (3 wires or 3 wires + neutral): P = √3 x U x I x cos ϕ
  • Reactive power Q (in kvar)
    • Single phase (1 phase and neutral): P = V x I x sin ϕ
    • Single phase (phase to phase): Q = UI sin ϕ
    • Three phase (3 wires or 3 wires + neutral): P = √3 x U x I x sin ϕ
  • Apparent power S (in kVA)
    • Single phase (1 phase and neutral): S = VI
    • Single phase (phase to phase): S = UI
    • Three phase (3 wires or 3 wires + neutral): P = √3 x U x I

where:

V = Voltage between phase and neutral
U = Voltage between phases

  • For balanced and near-balanced loads on 4-wire systems

The power factor is the ratio of kW to kVA. The closer the power factor approaches its maximum possible value of 1, the greater the benefit to consumer and supplier.
PF = P (kW) / S (kVA)
P = Active power
S = Apparent power

Current and voltage vectors, and derivation of the power diagram

The power vector diagram is a useful artifice, derived directly from the true rotating vector diagram of currents and voltage, as follows:

The power-system voltages are taken as the reference quantities, and one phase only is considered on the assumption of balanced 3-phase loading. The reference phase voltage (V) is co-incident with the horizontal axis, and the current (I) of that phase will, for practically all power-system loads, lag the voltage by an angle ϕ. The component of I which is in phase with V is the wattful component of I and is equal to I cos ϕ, while VI cos ϕ equals the active power (in kV) in the circuit, if V is expressed in kV.

The component of I which lags 90 degrees behind V is the wattless component of I and is equal to I sin ϕ, while VI sin ϕ equals the reactive power (in kvar) in the circuit, if V is expressed in kV.

If the vector I is multiplied by V, expressed in kV, then VI equals the apparent power (in kVA) for the circuit. The above kW, kvar and kVA values per phase, when multiplied by 3, can therefore conveniently represent the relationships of kVA, kW, kvar and power factor for a total 3-phase load, as shown in Figure K3 .

SOURCE: Schneider Electric

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Reduction In The Cost Of Electricity

Reduction In The Cost Of Electricity

Good management in the consumption of reactive energy brings with it the following economic advantages.

These notes are based on an actual tariff structure of a kind commonly applied in Europe, designed to encourage consumers to minimize their consumption of reactive energy.

The installation of power-factor correcting capacitors on installations permits the consumer to reduce his electricity bill by maintaining the level of reactive-power consumption below a value contractually agreed with the power supply authority.

In this particular tariff, reactive energy is billed according to the tan ϕ criterion.

As previously noted:

forumula

At the supply service position, the power supply distributor delivers reactive energy free, until:

  • The point at which it reaches 40% of the active energy (tan ϕ = 0.4) for a maximum period of 16 hours each day (from 06-00 h to 22-00 h) during the mostheavily loaded period (often in winter)
  • Without limitation during light-load periods in winter, and in spring and summer.

During the periods of limitation, reactive energy consumption exceeding 40% of the active energy (i.e. tan ϕ > 0.4) is billed monthly at the current rates. Thus, the quantity of reactive energy billed in these periods will be:

kvarh (to be billed) = kWh (tan ϕ – 0.4) where kWh is the active energy consumed during the periods of limitation, and kWh tan ϕ is the total reactive energy during a period of limitation, and 0.4 kWh is the amount of reactive energy delivered free during a period of limitation.

Tan ϕ = 0.4 corresponds to a power factor of 0.93 so that, if steps are taken to ensure that during the limitation periods the power factor never falls below 0.93, the consumer will have nothing to pay for the reactive power consumed.

Against the financial advantages of reduced billing, the consumer must balance the cost of purchasing, installing and maintaining the power-factor-improvement capacitors and controlling switchgear, automatic control equipment (where stepped levels of compensation are required) together with the additional kWh consumed by the dielectric.

Losses of the capacitors, etc. It may be found that it is more economic to provide partial compensation only, and that paying for some of the reactive energy consumed is less expensive than providing 100% compensation.

The question of power-factor correction is a matter of optimization, except in very simple cases.

Technical/economic optimization

A high power factor allows the optimization of the components of an installation. Overating of certain equipment can be avoided, but to achieve the best results, the correction should be effected as close to the individual items of inductive plant as possible.

Reduction of cable size

Figure 1 shows the required increase in the size of cables as the power factor is reduced from unity to 0.4.

Fig. 1 : Multiplying factor for cable size as a function of cos φ

Fig. 1 : Multiplying factor for cable size as a function of cos φ

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The Nature Of Reactive Energy

The Nature Of Reactive Energy

All inductive machines i.e. electromagnetic and devices that operate on AC systems convert electrical energy from the powersystem generators into mechanical work and heat. This energy is measured by kWh meters, and is referred to as active or wattful energy. In order to perform this conversion, magnetic fields have to be established in the machines, and these fields are associated with another form of energy to be supplied from the power system, known as reactive or wattless energy.

The reason for this is that inductive plant cyclically absorbs energy from the system (during the build-up of the magnetic fields) and re-injects that energy into the system (during the collapse of the magnetic fields) twice in every power-frequency cycle.

The effect on generator rotors is to (tend to) slow them during one part of the cycle and to accelerate them during another part of the cycle. The pulsating torque is stricly true only for single-phase alternators. In three-phase alternators the effect is mutually cancelled in the three phases, since, at any instant, the reactive energy supplied on one (or two) phase(s) is equal to the reactive energy being returned on the other two (or one) phase(s) of a balanced system. The nett result is zero average load on the generators, i.e. the reactive current is “wattless”.

An exactly similar phenomenon occurs with shunt capacitive elements in a power system, such as cable capacitance or banks of power capacitors, etc. In this case, energy is stored electrostatically. The cyclic charging and discharging of capacitive plant reacts on the generators of the system in the same manner as that described above for inductive plant, but the current flow to and from capacitive plant is in exact phase opposition to that of the inductive plant. This feature is the basis on which powerfactor improvement schemes depend.

It should be noted that while this “wattless” current (more accurately, the wattless component of a load current) does not draw power from the system, it does cause power losses in transmission and distribution systems by heating the conductors.

In practical power systems, wattless components of load currents are invariably inductive, while the impedances of transmission and distribution systems are predominantly inductively reactive. The combination of inductive current passing through an inductive reactance produces the worst possible conditions of voltage drop (i.e. in direct phase opposition to the system voltage).

Active and reactive power

Fig. 1 : An electric motor requires active power P and reactive power Q from the power system

For these reasons, viz:

  • Transmission power losses and
  • Voltage drop

The power-supply authorities reduce the amount of wattless (inductive) current as much as possible. Wattless (capacitive) currents have the reverse effect on voltage levels and produce voltage-rises in power systems.

The power (kW) associated with “active” energy is usually represented by the letter P. The reactive power (kvar) is represented by Q. Inductively-reactive power is conventionally positive (+ Q) while capacitively-reactive power is shown as a negative quantity (- Q). S represents kVA of “apparent” power.

Figure 1 shows that the kVA of apparent power is the vector sum of the kW of active power plus the kvar of reactive power.

Alternating current systems supply two forms of energy:

  • Active energy measured in kilowatt hours (kWh) which is converted into mechanical work, heat, light, etc
  • Reactive energy, which again takes two forms:
    • “Reactive” energy required by inductive circuits (transformers, motors, etc.),
Plant and appliances requiring reactive energy

All AC plant and appliances that include electromagnetic devices, or depend on magnetically-coupled windings, require some degree of reactive current to create magnetic flux. The most common items in this class are transformers and reactors, motors and discharge lamps (i.e. the ballasts of).

The proportion of reactive power (kvar) with respect to active power (kW) when an item of plant is fully loaded varies according to the item concerned being:

  • 65-75% for asynchronous motors
  • 5-10% for transformers

SOURCE: Schneider Electric

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Storage Systems

Storage Systems | ABB Battery

Energy storage technologies are of great interest to electric utilities, energy service companies, and automobile manufacturers (for electric vehicle application). The ability to store large amounts of energy would allow electric utilities to have greater flexibility in their operation because with this option the supply and demand do not have to be matched instantaneously. The availability of the proper battery at the right price will make the electric vehicle a reality, a goal that has eluded the automotive industry thus far. Four types of storage technologies (listed below) are discussed in this section, but most emphasis is placed on storage batteries because it is now closest to being commercially viable. The other storage technology widely used by the electric power industry, pumped-storage power plants, is not discussed as this has been in commercial operation for more than 60 years in various countries around the world.

  • Flywheel storage
  • Compressed air energy storage
  • Superconducting magnetic energy storage
  • Battery storage

Flywheel Storage

Flywheels store their energy in their rotating mass, which rotates at very high speeds (approaching 75,000 rotations per minute), and are made of composite materials instead of steel because of the composite’s ability to withstand the rotating forces exerted on the flywheel. In order to store enegy the flywheel is placed in a sealed container which is then placed in a vacuum to reduce air resistance. Magnets embedded in the flywheel pass near pickup coils. The magnet induces a current in the coil changing the rotational energy into electrical energy.

Flywheels are still in research and development, and commercial products are several years away.

Compressed Air Energy Storage

As the name implies, the compressed air energy storage (CAES) plant uses electricity to compress air which is stored in underground reservoirs. When electricity is needed, this compressed air is withdrawn, heated with gas or oil, and run through an expansion turbine to drive a generator. The compressed air can be stored in several types of underground structures, including caverns in salt or rock formations, aquifers, and depleted natural gas fields. Typically the compressed air in a CAES plant uses about one third of the premium fuel needed to produce the same amount of electricity as in a conventional plant. A 290-MW CAES plant has been in operation in Germany since the early 1980s with 90% availability and 99% starting reliability. In the U.S., the Alabama Electric Cooperative runs a CAES plant that stores compressed air in a 19-million cubic foot cavern mined from a salt dome. This 110-MW plant has a storage capacity of 26 h. The fixed-price turnkey cost for this first-of-a-kind plant is about $400/kW in constant 1988 dollars.

The turbomachinery of the CAES plant is like a combustion turbine, but the compressor and the expander operate independently. In a combustion turbine, the air that is used to drive the turbine is compressed just prior to combustion and expansion and, as a result, the compressor and the expander must operate at the same time and must have the same air mass flow rate. In the case of a CAES plant, the compressor and the expander can be sized independently to provide the utility-selected “optimal” MW charge and discharge rate which determines the ratio of hours of compression required for each hour of turbine-generator operation. The MW ratings and time ratio are influenced by the utility’s load curve, and the price of off-peak power.

For example, the CAES plant in Germany requires 4 h of compression per hour of generation. On the other hand, the Alabama plant requires 1.7 h of compression for each hour of generation. At 110-MW net output, the power ratio is 0.818 kW output for each kilowatt input. The heat rate (LHV) is 4122 BTU/kWh with natural gas fuel and 4089 BTU/kWh with fuel oil. Due to the storage option, a partial-load operation of the CAES plant is also very flexible. For example, the heat rate of the expander increases only by 5%, and the airflow decreases nearly linearly when the plant output is turned down to 45% of full load. However, CAES plants have not reached commercial viability beyond some prototypes.

Superconducting Magnetic Energy Storage

A third type of advanced energy storage technology is superconducting magnetic energy storage (SMES), which may someday allow electric utilities to store electricity with unparalled efficiency (90% or more). A simple description of SMES operation follows.
The electricity storage medium is a doughnut-shaped electromagnetic coil of superconducting wire. This coil could be about 1000 m in diameter, installed in a trench, and kept at superconducting temper- ature by a refrigeration system. Off-peak electricity, converted to direct current (DC), would be fed into this coil and stored for retrieval at any moment. The coil would be kept at a low-temperature supercon- ducting state using liquid helium.

The time between charging and discharging could be as little as 20 ms with a round-trip AC–AC efficiency of over 90%.

Developing a commercial-scale SMES plant presents both economic and technical challenges. Due to the high cost of liquiud helium, only plants with 1000-MW, 5-h capacity are economically attractive. Even then the plant capital cost can exceed several thousand dollars per kilowatt. As ceramic superconductors, which become superconducting at higher temperatures (maintained by less expensive liquid nitrogen), become more widely available, it may be possible to develop smaller scale SMES plants at a lower price.

Battery Storage

Even though battery storage is the oldest and most familiar energy storage device, significant advances have been made in this technology in recent years to deserve more attention. There has been renewed interest in this technology due to its potential application in non-polluting electric vehicles. Battery systems are quiet and non-polluting, and can be installed near load centers and existing suburban substations. These have round-trip AC–AC efficiencies in the range of 85%, and can respond to load changes within 20 ms. Several U.S., European, and Japanese utilities have demonstrated the application of lead–acid batteries for load-following applications. Some of them have been as large as 10 MW with 4 h of storage.

The other player in battery development is the automotive industry for electric vehicle application. In 1991, General Motors, Ford, Chrysler, Electric Power Research Institute (EPRI), several utilities, and the U.S. Department of Energy (DOE) formed the U.S. Advanced Battery Consortium (USABC) to develop better batteries for electric vehicle (EV) applications. A brief introduction to some of the available battery technologies as well some that are under study is presented in the following (Source:http://www.eren. doe.gov/consumerinfo/refbriefs/fa1/html).

Battery Types

Chemical batteries are individual cells filled with a conducting medium-electrolyte that, when connected together, form a battery. Multiple batteries connected together form a battery bank. At present, there are two main types of batteries: primary batteries (non-rechargeable) and secondary batteries (rechargeable). Secondary batteries are further divided into two categories based on the operating temperature of the electrolyte. Ambient operating temperature batteries have either aqueous (flooded) or nonaqueous elec- trolytes. High operating temperature batteries (molten electrodes) have either solid or molten electrolytes. Batteries in EVs are the secondary-rechargeable-type and are in either of the two sub-categories. A battery for an EV must meet certain performance goals.

These goals include quick discharge and recharge capability, long cycle life (the number of discharges before becoming unserviceable), low cost, recycla- bility, high specific energy (amount of usable energy, measured in watt-hours per pound [lb] or kilogram [kg]), high energy density (amount of energy stored per unit volume), specific power (determines the potential for acceleration), and the ability to work in extreme heat or cold. No battery currently available meets all these criteria.

Lead–Acid Batteries

Lead–acid starting batteries (shallow-cycle lead–acid secondary batteries) are the most common battery used in vehicles today. This battery is an ambient temperature, aqueous electrolyte battery. A cousin to this battery is the deep-cycle lead–acid battery, now widely used in golf carts and forklifts. The first electric cars built also used this technology. Although the lead–acid battery is relatively inexpensive, it is very heavy, with a limited usable energy by weight (specific energy). The battery’s low specific energy and poor energy density make for a very large and heavy battery pack, which cannot power a vehicle as far as an equivalent gas-powered vehicle. Lead–acid batteries should not be discharged by more than 80% of their rated capacity or depth of discharge (DOD). Exceeding the 80% DOD shortens the life of the battery. Lead–acid batteries are inexpensive, readily available, and are highly recyclable, using the elaborate recycling system already in place. Research continues to try to improve these batteries.

A lead–acid nonaqueous (gelled lead acid) battery uses an electrolyte paste instead of a liquid. These batteries do not have to be mounted in an upright position. There is no electrolyte to spill in an accident. Nonaqueous lead–acid batteries typically do not have as high a life cycle and are more expensive than flooded deep-cycle lead–acid batteries.

Nickel Iron and Nickel Cadmium Batteries

Nickel iron (Edison cells) and nickel cadmium (nicad) pocket and sintered plate batteries have been in use for many years. Both of these batteries have a specific energy of around 25 Wh/lb (55 Wh/kg), which is higher than advanced lead–acid batteries. These batteries also have a long cycle life. Both of these batteries are recyclable. Nickel iron batteries are non-toxic, while nicads are toxic. They can also be discharged to 100% DOD without damage. The biggest drawback to these batteries is their cost. Depend- ing on the size of battery bank in the vehicle, it may cost between $20,000 and $60,000 for the batteries. The batteries should last at least 100,000 mi (160,900 km) in normal service.

Nickel Metal Hydride Batteries

Nickel metal hydride batteries are offered as the best of the next generation of batteries. They have a high specific energy: around 40.8 Wh/lb (90 Wh/kg). According to a U.S. DOE report, the batteries are benign to the environment and are recyclable. They also are reported to have a very long cycle life. Nickel metal hydride batteries have a high self-discharge rate: they lose their charge when stored for long periods of time. They are already commercially available as “AA” and “C” cell batteries, for small consumer appliances and toys. Manufacturing of larger batteries for EV applications is only available to EV manufacturers. Honda is using these batteries in the EV Plus, which is available for lease in California.

Sodium Sulfur Batteries

This battery is a high-temperature battery, with the electrolyte operating at temperatures of 572°F (300°C). The sodium component of this battery explodes on contact with water, which raises certain safety concerns. The materials of the battery must be capable of withstanding the high internal temper- atures they create, as well as freezing and thawing cycles. This battery has a very high specific energy: 50 Wh/lb (110 Wh/kg). The Ford Motor Company uses sodium sulfur batteries in their Ecostar, a converted delivery minivan that is currently sold in Europe. Sodium sulfur batteries are only available to EV manufacturers.

Lithium Iron and Lithium Polymer Batteries

The USABC considers lithium iron batteries to be the long-term battery solution for EVs. The batteries have a very high specific energy: 68 Wh/lb (150 Wh/kg). They have a molten-salt electrolyte and share many features of a sealed bipolar battery. Lithium iron batteries are also reported to have a very long cycle life. These are widely used in laptop computers. These batteries will allow a vehicle to travel distances and accelerate at a rate comparable to conventional gasoline-powered vehicles. Lithium polymer batteries eliminate liquid electrolytes. They are thin and flexible, and can be molded into a variety of shapes and sizes.

Neither type will be ready for EV commercial applications until early in the 21st century.

Zinc and Aluminum Air Batteries

Zinc air batteries are currently being tested in postal trucks in Germany. These batteries use either aluminum or zinc as a sacrificial anode. As the battery produces electricity, the anode dissolves into the electrolyte. When the anode is completely dissolved, a new anode is placed in the vehicle. The aluminum or zinc and the electrolyte are removed and sent to a recycling facility. These batteries have a specific energy of over 97 Wh/lb (200 Wh/kg). The German postal vans currently carry 80 kWh of energy in their battery, giving them about the same range as 13 gallons (49.2 liters) of gasoline. In their tests, the vans have achieved a range of 615 mi (990 km) at 25 miles per hour (40 km/h).
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SOURCE: Rahman, Saifur “Electric Power Generation: Non-Conventional Methods”
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Acti 9 - The Fifth Generation Of Modular Systems

Acti 9 - The Fifth Generation Of Modular Systems

Acti 9 represents the fifth generation of Schneider’s low voltage modular systems. His older brother Multi 9 has finally evolved to much better and smarter system. Multi 9 was the famous and most known product of Schneider’s ex brand Merlin Gerin (now is incorporated into Schneider global brand), and now new Acti 9 is ready to inherit it.

Before Acti 9 – iC60 and Multi 9 – C60 modular systems, there was also Multi 9 – C32, F32 and F70 at the beginning of development.

Acti 9 covers all applications, especially in polluted environments and networks, for absolute safety and improved continuity of service.

Acti 9 exclusivities

For absolute safety and improved continuity of service.

  • VISI-SAFE – Guaranteed safe intervention on site
  • VISI-TRIP – Fast location of the faulty outgoer to minimize dowtime
  • The super immunization “Si” on RCD – Improved continuity of service, especially in polluted environments and networks
  • Front face class 2 - Continuous safety for operators and non-qualified personnel

Acti 9 exclusivities

VISI-SAFE concept is combining:

  • Contact position indication with the green strip
  • Impulse voltage withstand: Uimp 6 kV
  • Insulation voltage: Ui 500 V
  • Pollution degree: level 3 (conductive pollution, dust,etc.)

Easy to choose

  • Compliance with both IEC/EN 60898 & IEC/EN 60947-2 - Suitable for commercial and industrial applications
  • RCDs fully coordinated up to the MCB’s breaking capacity – Peace of mind, easy to select
    .

Easy to install

  • Quick and ergonomic wiring, safe connections
    - IP20 insulated flap terminals
    - Distribloc system
  • Twice the standard terminal tightening torque
    .

Easy to operate

  • Great readability:
    - large circuit labelling area
    - specific colour code system.
  • Upgradeability with Multiclip system
  • Load rebalancing and addition of new outgoers.
  • Device removable with comb busbar in place
  • Double locking.

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Galaxy 7000 UPS 250-500kVA by APC

Galaxy 7000 UPS 250-500kVA by APC

Three-phase Galaxy 7000, 250 to 500 kVA (up to 4 MVA in parallel configurations), includes cutting-edge technologies for high-power applications.

Calling on over 40 years of experience in the Critical Power and Cooling Services division of Schneider Electric, the leader in complete, high-quality electrical power solutions, Galaxy 7000 offers optimised performance for data centers, infrastructure and industrial processes. The compact size, back-to-back or back-to-wall installation enables more room to be utilised for other equipments in the technical room than traditional UPS systems. The UPS can be upgraded to ensure available and secure power infrastructure for current as well as future demands.

Applications

Galaxy 7000 increases the productivity of these applications by providing continuity of service through secure supply solutions that are flexible, adaptable and upgradeable. Galaxy 7000 provides high-quality energy, compatible with all loads, with a very high level of availability. The variety of architectures meets the specific needs of each installation and allows easy upgrading. The communication capabilities and proactive services provided by Schneider Electric, the most complete and available worldwide, make for highly effective maintenance.

  • Data Centers
    The strategic and economic importance of data centers made it necessary to set up the ANSI/TIA site typology (TIER I to IV). It presents the necessary functions of the major components, including supply via UPSs, to ensure consistency and obtain a high level of overall availability.
    .
    Galaxy 7000, through its design and many possibilities for parallel connection, as well as its compatibility with STS (static transfer switch) systems, meets TIER IV requirements for fault-tolerant sites offering the highest level of availability (99.995%).
    .
    Combined with STS units, Galaxy 7000 can supply energy via two or three different channels for dual or triple-attach applications and also offers supervision, network administration and remote-control solutions. Galaxy 7000 is ideal for the large (over 500 square metre) data centers of banks, insurance companies, internet and colocation services, telecoms, etc. where 24/365 operation is mandatory and preventive maintenance and upgrades must not require system shutdown.
  • Fig. 2. Applications such as data centers, infrastructure and industrial processes.

    Fig. 2. Applications such as data centers, infrastructure and industrial processes.

  • Infrastructure and buildings
    Service continuity is also required for infrastructure (airports, ports, tunnels) as well as the operation and technical monitoring (via SCADA and BMS systems) of shopping centres, hospitals, office buildings, etc. Galaxy 7000 is perfectly suited to the needs of these communicating, frequently upgraded applications thanks to its power ratings, extension possibilities and communication capabilities.
    .
  • Industrial processes
    Operation in industrial environments requires equipment capable of maintaining processes, without failure, under difficult conditions, including dust, humidity, vibrations, major variations in temperature, etc. Due to its high electrical and mechanical level of performance, Galaxy 7000 meets these specific needs.
    .
    Technical files are available for all the applications specified by design offices, including Ni-Cad batteries for the chemical and petrochemical industries, high IP values, heavy-duty reinforced cabinets, dust filters, marine configurations, rated voltages up to 440 V, etc. The MGE UPS SYSTEMS design office, in conjunction with a specialised industrial organisation, can also handle uncommon conditions, e.g. outdoor installations, anti-vibration bases for marine applications, special paints with the corresponding labels, etc.
    .
  • Colour
    Light grey RAL 9023

Strong Points

Galaxy 7000 design combines the best technology (double conversion) with the most recent innovations to supply high-quality power, available 24/365, to high-power applications, whatever the situation in the distribution system.

  • Double conversion technology (VFI as per IEC 62040-3/EN 62040-3).
    This is the only technology that insulates the load from the upstream network and completely regenerates the output voltage, thus providing high-quality, stable power.
  • IGBT-based, PFC sinusoidal-current input rectifier. The rectifier draws sinusoidal current, without any reactive power, thus avoiding disturbances upstream by reducing harmonic reinjection.
    • Very low current total harmonic distortion THDI, less than 5%.
    • Input power factor (PF) greater than 0.99 from 50% load upwards.
    .
    These performance levels, combined with the three-phase input not requiring neutral, offer substantial savings in terms of cables and equipment.

    Fig. 3. Three-phase, PFC sinusoidal-current rectifier, with DualPack IGBTs.

    Fig. 3. Three-phase, PFC sinusoidal-current rectifier, with DualPack IGBTs.

  • Battery charger separated from the AC input. The chopper is supplied via the rectifier output and is thus protected against fluctuations on the AC input. Battery recharge is adjusted as a function of the temperature.
  • A check on the phase sequence is run to protect the power system from the effects of incorrect connections.
  • Wide input-voltage (250 to 470 V) and frequency (45 to 65 Hz) range. This is made possible by double-conversion technology and the PFC rectifier which is compatible with all sources and with disturbed distribution systems (voltages as low as 250 V for 30% load).

    Fig. 4. Wide input-voltage range.

    Fig. 4. Wide input-voltage range.

  • Soft start. This system provides total compatibility with gensets through gradual start of the rectifier, in addition to a PF of 0.99. It makes it possible:
    • when AC power is absent, to progressively transfer the load from the battery to the genset
    • when the normal AC source returns to tolerances, to delay transfer from the battery to the rectifier, thus avoiding excessive variations on the AC source.
    • in a parallel system, to set up sequential start-up of the inverters.

    Fig. 5. Soft start walk-in ramp with time delay.

    Fig. 5. Soft start walk-in ramp with time delay.

  • Cold start on battery power if the AC source is absent or disturbed, even in parallel systems.
  • High-quality output voltage (380, 400, 415 or440 V) for all types of loads with:
    • THDU < 3%
    • output PF = 09 for all types of load.
    The range is suitable for the most recent non-linear and computer loads, called capacitive loads, with a leading PF close to 0.9 and a high crest factor.
  • Output voltage adjustable to ± 3% of the rated value (in 0.5 V steps) to take into account voltage drops in long cables.
  • Excellent response to load step changes, < 2% for 100 to 0% or 0 to 100% and return to the ± 1% tolerances within 100 ms.
  • High current-limiting capacity for inverter short-circuits (2.5 In, 150 ms) to facilitate discrimination with downstream protective devices and accept high crest factors, 2.4:1 up to 3:1, depending on the voltage and power levels.
  • Intelligent thermal-overload capacity curve for better performance. The curve is adjusted as a function of the ambient temperature, 1.5 In / 30 seconds, 1.25 In / 10 minutes.

Advanced management to extend battery life

Galaxy 7000 proposes backup times from 5 minutes to 2 hours with a rapid charger (recharge < 6 hours for 10-minute backup time). Backup time remains available due to digital battery management and the protection systems.

  • The standard “DigiBat™” system monitors the battery to forward information and maximise performance. Based on a large number of parameters (percent load, temperature, battery type and age), DigiBat controls the battery charge voltage and continuously calculates:
    • the real available backup time
    • the remaining battery life (1).
    .
    DigiBat also offers:
    • automatic entry of battery parameters
    • test on battery status to preventively detect operating faults
    • automatic battery discharge test at adjustable time intervals
    • protection against deep discharge taking into account the discharge rate, with a circuit breaker to isolate the battery. The breaker opens automatically after double the specified backup time plus two hours (sleep mode to provide vital functions)
    • limiting of the battery charge current (0.05 C10 to 0.1 C10)
    • gradual alarm signalling the end of backup time
    • numerous automatic tests.Fig. 10. Digibat.
  • The B1000 battery monitoring or “Cellwatch” option monitors all battery strings 24/365 and displays a failure prediction for each block.

User-friendly interface for more dependable operation

Galaxy 7000 has a control and display interface offering intuitive functions. Based on graphs and pictograms, the interface can be set up in 19 languages including Chinese, Korean, Thai, Indonesian, Turkish and Greek. It includes a graphical display for time-stamped events and useful operating statistics. For operating personnel, that is an essential aspect in facilitating decisions. Simple and user-friendly, the interface enhances safety and comfort.

  • Graphical display with HD, B&W touch screen (SVGA).
  • Animated mimic panel.
  • Menu keys and direct access to display functions.
  • Buzzer.
  • Remote supervision that can run under many BMS systems and network
    supervisors via the communication cards.
  • Time-stamping of last 2500 events.
  • On-line help to assist with the displayed messages.
  • Multi-mode for parallel units (modular UPSs in parallel, parallel UPSs with SSC, frequency converters in parallel), i.e. it is possible to read the measurements of any unit on any unit, or to read system data.Fig. 11. HMI with display, LEDs, keys and mimic panel

Advanced communication

Galaxy 7000 offers the entire MGE UPS SYSTEMS range of communication systems designed to meet three essential needs.

  • Inform on UPS operation and its environment, warn users, wherever they may be, concerning any potential and existing problems.
  • Protect server data by automatic, clean shutdown of operating systems
  • Actively supervise an entire set of UPSs.
    These functions are carried out by hot-swappable communication cards running
    under different protocols, depending on the environment in which Galaxy 7000 is
    installed.
  • Standard relay card with programmable dry contacts (4 logic inputs and 6 logic
    outputs).
  • INMC (Industrial Network Management Card) communication card with two
    ports:
    • JBus/ModBus RS485/RS232 protocol for communication with a BMS
    • Ethernet 10/100 Mbps protocol using the Https standard (secure connection) for
    supervision via the web.
    Each UPS then has its own IP address making it possible for the user to:
    • supervise and control the UPS via a simple browser (HTTP)
    • interface with an SNMP administration system (HP Openview, etc.)
    • communicate with shutdown modules installed on the protected servers
    • set up external temperature and humidity monitoring (sensor environment)
    • receive e-mail alarms.
  • NMTC (Network Management Teleservice Card) communication card with three
    ports:
    • JBus/ModBus RS232/RS485
    • Ethernet 10/100 Mbps using the Https protocol.
    The functions are identical to the INMC card, with in addition:
    • a modem connection may be used to connect the UPS to the MGE Teleservice
    centre for remote monitoring.
  • Life Cycle Monitoring software for optimised maintenance.
  • Compatible with CPSOL software from Schneider Electric for complete
    installation design.

Installation

Galaxy 7000 uses the latest technical and mechanical advances in terms of electrical components and power electronics. The greatly reduced number of components offers:

  • an overall solution that is very compact, but high accessible for maintenance
  • integration of many functions in a single cabinet. The batteries, installed in a
    separate cabinet, can be hot-swapped (with the UPS supplying the load).

The UPS can be installed in both technical and computer rooms.

  • The UPS can operate correctly back to the wall or back to back, but it is preferable to leave some space (> 600 mm) for easier maintenance.
  • Leave one meter of free space in front of the UPS for door opening.
  • At least 500 mm of clearance above the UPS is required.

Equipment and diagrams

Galaxy 7000 UPS units offer the following equipment and functions.

Standard configuration

  • IGBT-based, PFC three-phase rectifier
  • Phase-sequence check on input
  • Chopper for battery charging, insulated from AC input, charge adjusted for ambient temperature
  • Static switch (except parallel UPS units)
  • Manual bypass (except parallel UPS units)
  • Three-phase IGBT inverter with freefrequency PWM chopping
  • Redundant ventilation for power components
  • HMI with graphical interface, 19 languages, menu, function and ON/OFF keys
  • Mimic panel with status LEDs
  • Time-stamping of last 2500 events
  • Battery protected against deep discharges by a circuit breaker
  • Cold start on battery power
  • Soft start with walk-in ramp and sequential start in parallel configurations
  • EMC, level B
  • Parallel connection of modular or parallel UPS units
  • DigiBat digital battery monitoring and calculation of true backup time
  • Programmable relay card, dry contacts, 4 logic inputs, 6 logic outputs

Options

  • Connection through the top
  • Isolation/voltage matching transformer
  • Synchronisation module
  • B1000 or Cellwatch battery-monitoring system for block by block management
  • Lightning arrestor (built into the UPS cabinet)
  • Backfeed protection
  • Synchronisation module
  • Multi-standard communication cards:
    .
    - Jbus/Modbus + Ethernet 10/100
    - Jbus/Modbus + Ethernet 10/100 + Modem
    - 2 ports with dry contacts and/or remote shutdown
  • Battery circuit breaker unit
  • Supervision and shutdown software:
  • Enterprise Power Manager V.2

Electrical characteristics

Electrical characteristics

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Related articles

EES Kvalitet električne energije - viši harmonici (2 deo)

EES Kvalitet električne energije - viši harmonici (2 deo)

Nastavak prvog dela članka:
EES Kvalitet električne energije – viši harmonici (1)

Postavljanje filtera

U slučajevima kada navedena rešenja nisu dovoljna ili nemoguće ih je izvesti primenjuje se rešenje ugradnjom filtera. Postoje tri vrste filtera:

• Pasivni
• Aktivni
• Hibridni

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Pasivni filteri

Pasivni filteri se najčešće postavljaju paraleno potrošaču i sastoje se od kondenzatora sa pridodatom prigušnicom slika 5.4. Rezonantna frekvencija filtera se proračunava uvek da bude nešto ispod frekvencije najnižeg dominantnog harmonika. Time se obezbeđuje da filter pravilno radi i u slučaju oscilacija parametara kondenzatora zbog temperature i sl., a i da se izbegne da se antirezonantna učestanost približi učestanosti harmonika. Primena serijskih filtera se ređe primenjuje, a cilj im je da predstavljaju visoku impedansu za harmonike struje i na taj način blokiraju njihovo širenje u mrežu.

Primenjuju  se:

  • U postrojenjima koja sadrže nelinearna opterećenja čije snage idu preko  200kVA,
  • U postrojenjima koja traže popravku faktora snage,
  • U postrojenjima gde se izobličenje napona mora smanjiti na dopuštene vrednosti,da bi se izbegao uticaj na osetljive prijemnike.
Slika 5.4. Pasivni filter

Slika 5.4. Pasivni filter

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Aktivni filteri

Aktivni filteri su u stvari energetski elektronski pretvarači, koji su tako programirani da vrše kompenzaciju viših harmonika.

Aktivni filteri kompenzuju više harmonike,proizvedene od strane nelinearnih prijemnika,tako što proizvode iste takve harmonike samo suprotnih faza. Sa takvim filterom obezbeđuje se “čista” sinusoidna struja mreže, a često i faktor snage 1. Složenije konfiguracije omogućuju potpuno otklanjanje svih poremećaja, koji utiču na kvalitet električne energije.

Primenjuju  se postrojenjima koja sadrže nelinearna opterećenja čije snage su manje od  200kVA i u postrojenjima kod kojih bi usled velikog izobličenja struje došlo do preopterećenja.

Slika 5.5. Aktivni filter

Slika 5.5. Aktivni filter

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Slika 5.5. Aktivni filter

Slika 5.6. Talasni oblici struje opterećenja pre i posle filtriranja

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Popravka faktora snage

Najjednostavniji način da se ostvari kontrola viših harmonika i pritom popravi faktor snage je ugrađivanje kondenzatorskih baterija za kompenzaciju reaktivne energije.
Njihova rezonantna učestanost je često blizu učestanosti karakterističnih harmonika,pa dolazi do neželjenih neagativnih pojava.Dodavanjem redne impedanse u kolo kondenzatora negativne pojave se mogu otkloniti,šematski je to pokazano na slici 5.9. Ugradnjom kondenzatorskih baterija ostvaruje se značajna kontrola petog harmonika.

Slika 5.9. Kondenzatorska baterija sa pridodatim rednim impedansama

Slika 5.9. Kondenzatorska baterija sa pridodatim rednim impedansama

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Standardi i preporuke

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Standardi i preporuke u Francuskoj

U Francuskoj su prema preporuci Regulations Concerning the Installation of Power Convertors Taking into Account the Characteristics of the Supply Network, definisane sledeće granične vrednosti viših harmonika harmonijskog izobličenja, kada je samo jedan potrošač vezan za tačku priključenja na mrežu, i iznose:

  • za parne harmonike 0,6% osnovnog harmonika napona,
  • za neparne harmonike 1% osnovnog harmonika napona,
  • ukupno harmonisko izobličenje napona u tački priključenja 1,6%.

Ove granice su izabrane tako da se osigura nivo od oko 5% THD napona u tački priključenja, da ne bude premašen kada su svi potrošači priključeni.

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Nemački standard  DIN 57160 (VDE 0160/11.81)

U Nemačkoj standard DIN 57160 (VDE 0160/11.81) određuje dozvoljene nazivne vrednosti uređaja koji generišu više harmonike, koje nisu veće od 1% vrednosti snage kratkog spoja. Pojedinačni nivoi harmonika, do 15-tog harmonika iznose 5% osnovnog harmonika napona,dozvoljeni nivo harmonika opada do 1% osnovnog harmonika napona za 100-ti harmonik, prema definisanoj krivoj liniji.

Ukupno harmonisko izobličenje napona u tački priključenja ne sme da pređe 10%.

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Standardi i preporuke u Švedskoj

Švedska, probleme viših harmonika opisuje u posebnoj preporuci i definiše zahteve kojima se ograničava priključenje potrošača u zavisnosti od mesta priključenja,vrste potrošača i ukupnog harmonijskog izobličenja.
U tabeli 1. su prikazane vrednosti ukupnog dozvoljenog harmonijskog izobličenja napona u zavisnosti od nazivnog napona napojne mreže.

Tabela 1. Ukupno dozvoljeno harmonijsko izobličenje u Švedskoj

Tabela 1. Ukupno dozvoljeno harmonijsko izobličenje u Švedskoj

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Standardi i preporuke u Australiji

Istorijski gledano, prvi kompletan standard koji razmatra probleme viših harmonika u distributivnim i prenosnim mrežama je izdat u Australiji (Standard AS 2279-1991)

Standard se sastoji iz četiri dela:

  • prvi deo razmatra i definiše dozvoljene vrednosti viših harmonika prouzrokovanih priključenjem aparata za domaćinstvo i sličnih uređaja,
  • drugi deo razmatra i definiše dozvoljene vrednosti viših harmonika prouzrokovanih priključenjem industrijskih postrojenja,
  • treći i četvrti deo se odnose na dozvoljene fluktuacije napona prouzrokovane priključenjem aparata za domaćinstvo i sličnih uređaja i industrijskih postrojenja.

Prvi deo standarda se primenjuje na aparate za domaćinstvo i slične uređaje naznačene snage manje od 4,8 kVA priključene na distributivnu niskonaponsku mrežu nazivnog napona 240 V monofaznog sistema i 240/415 V trofaznog sistema. U ovom delu standarda se određuju:

  • dozvoljene vrednosti viših harmonika struje koje mogu u distributivnu mrežu unositi pomenuti uređaji,
  • referentna impedansa mreže,
  • praktične metode merenja viših harmonika.

Za monofazne uređaje naznačenog napona 240 V, kao i za trofazne uređaje naznačenog napona 415 V, primenjuju se granične vrednosti harmonika struje Ih, i napona Vh prema tabelama 2 i 3.

Tabela 2. Granične vrednosti harmonika struje prema Australijskom standardu AS 2279-1991

Tabela 2. Granične vrednosti harmonika struje prema Australijskom standardu AS 2279-1991

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Tabela 3. Granične vrednosti harmonika napona prema Australijskom standardu AS 2279-1991

Tabela 3. Granične vrednosti harmonika napona prema Australijskom standardu AS 2279-1991

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U drugom delu standarda date su maksimalno dozvoljene vrednosti viših harmonika koje u distributivnu mrežu unosi industrijska oprema napajana sa srednjeg i visokog napona,naznačene snage veće od 4,8 kVA. Kako je pomenuta oprema raznovrsna, izvršena je njena podela u tri velike grupe:

  • Prvu grupu čine uređaji koji se mogu priključiti bez posebne dozvole na distributivnu mrežu, a čija naznačena snaga je manja od 0,3% snage trofaznog kratkog spoja u tački priključenja.
    • Pri tome je nazivna snaga uređaja manja od 75 kVA, za priključenje na sekundarnu mrežu, odnosno 500 kVA za primarnu distributivnu mrežu (napon mreže od 415 V do 33 kV). Pri tome snaga kratkog spoja mora biti najmanje 5 MVA za sekundarnu mrežu (415/240 V), odnosno 50 MVA za primarnu distributivnu mrežu (6,6; 11 i 24 kV).
    • U slučaju priključenja više manjih pretvarača suma njihovih snaga ne sme da pređe granicu od 75 kVA;
    • Pri monofaznom priključenju naznačena snaga pretvarača nije veća od 5 kVA pri naponu 240 V, odnosno 7,5 kVA pri naponu 415 V.
  • Druga grupa potrošača obuhvata uređaje koji se mogu priključiti na distributivnu mrežu ako postojeće ukupno harmonijsko izobličenje u tački priključenja nije veće od 75% vrednosti harmonika napona pre priključenja.
  • U treću grupu spadaju uređaji velikih snaga i za njih su potrebna posebna merenja i studije.

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Standardi i preporuke međunarodne elektrotehničke komisije (IEC)

Problemom viših harmonika se bavi više radnih grupa Međunarodne elektrotehničke komisije (IEC). Prvi standard se pojavio 1982. godine, (IEC 555) koji se sastoji iz tri dela: IEC 555-1 Definicije, IEC 555-2 Harmonici, IEC 555-3 Fluktuacije napona. Ovaj standard je preveden i primenjuje se u Srbiji i Crnoj Gori od 1989-godine, (JUS N.A6.101, JUS N.A6.102, JUS N.A6.103).

Dalji rad IEC komiteta TC 77 rezultovao je nizom standarda kojima se ograničavaju struje i naponi viših harmonika na vrednosti za koje se smatra da električna mreža može da ih toleriše. Tu se posebno izdvajaju standardi iz grupe IEC 61000, u kojima je obrađena problematika viših harmonika (osnovne definicije,merenja, proračuni i dozvoljene granične vrednosti).

U standardu IEC 61000-3-6 , definišu se osnovni zahtevi koje treba da ispune nelinearni potrošači da bi se priključili na distributivnu mrežu. Granične vrednosti viših harmonika su tako određene da se održi zadovoljavajući kvalitet napona i to kako u tački priključenja nelinearnog potrošača na distributivnu mrežu tako i prema ostalim potrošačima.

Tabela 4. Granične vrednosti harmonika napona na nivou elektromagnetne kompatibilnosti prema standardu IEC 61000-2-2

Tabela 4. Granične vrednosti harmonika napona na nivou elektromagnetne kompatibilnosti prema standardu IEC 61000-2-2

Na osnovu ove procedure može se proceniti da li će amplitude unetih viših harmonika u distributivnu mrežu zadovoljiti planirani nivo, odnosno nivo elektromagnetne kompatibilnosti. Ukoliko potrošači zadovolje navedene granične vrednosti sadržaja viših harmonika, dozvoljava se priključenje, a u protivnom se priključenje uslovljava odgovarajućim metodom za eliminaciju viših harmonika.

Takođe, ovim standardom se definiše da za pojavu viših harmonika u havarijskim radnim režimima odgovornost snosi isporučilac električne energije a ne potrošač. Za priključenje malih potrošača koji ne unose značajna harmonijska izobličenja struje i napona ne mora se tražiti posebna dozvola. Priključenja ovih potrošača na niskom naponu su regulisana standardima IEC 61000-3-2 i IEC 61000-3-4.

Zaključak

Otvaranjem tržišta, električna energija postaje roba kao i svaki drugi proizvod te mora zadovoljavati zadate kriterijume kvaliteta. U prenosnoj mreži probleme predstavljaju elektrolučne peći,vetroelektrane, i železnica, dok su u distributivnoj mreži najveći problem uređaji koji se zasnivaju na energetskoj elektronici.

Ograničavanje viših harmonika u distributivnim mrežama pomoću odgovarajućih standarda i preporuka neophodno je iz više razloga:

  • da se ograniči nivo izobličenja talasnih oblika struje i napona na vrednosti koje sistem i njegovi elementi mogu da tolerišu i time omoguće kvalitetnu isporuku električne energije potrošačima,
  • da ne utiču na dalje širenje upotrebe energetskih pretvarača i drugih uređaja koji unose nelinearnost u distributivnu mrežu,
  • da se ograniči ometanje drugih uređaja i sistema od strane distributivne mreže (telefonske mreže i sl.)

Iz pregleda nacionalnih standarda i preporuka se vidi da većina njih određuje granične vrednosti ukupnog harmonijskog izobličenja THD, koje se razlikuju na pojedinim naponskim nivoima. U većini zemalja se ovaj faktor usvaja da bude za niski napon THD <5%, za srednji napon THD je između 3 i 5%, a za visoki 1-1,5%.

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AUTOR STRUČNOG TEKSTA:

Dragan Simović
Dragan Simovic

Seminarski rad iz predmeta: Eksploatacija EES, tema: Kvalitet električne energije – viši harmonici
Visoka Škola Tehničkih Strukovnih Studija Čačak
Specijalističke Strukovne Studije Elektrotehnike i Računarstva | Modul Studijskog Programa: Elektroenergetika

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EES Kvalitet električne energije - viši harmonici (prvi deo)

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Prisustvo velikog broja nelinearnih potrošača u distributivnim mrežama dovodi do niza negativnih efekata koji se odražavaju kako na samu mrežu tako i na ostale priključene potrošače.

Zajednički interes potrošača i proizvođača električne energije je poslednjih godina doveo u žižu interesovanja probleme vezane za kvalitet električne energije, odnosno sadržaj harmonika u distributivnoj mreži i druge aspekte kvaliteta električne energije (neprekidnost napajanja, prisustvo kratkotrajnih fluktuacija i distorzija,…).

Danas je u svetu pred proizvođače i projektante uređaja energetske elektronike postavljen čitav niz standarda i preporuka iz oblasti kvaliteta električne energije. Tradicionalno se smatralo da je kvalitet električne energije u stvari pouzdanost,odnosno nepostojanje trajnih prekida u snabdevanju električnom energijom, dok moderno shvatanje kvaliteta električne energije podrazumeva i sigurno (neprekidno) napajanje i fizički kvalitet napona. Problemi neprekidnosti napajanja se uglavnom rešavaju u toku postupka planiranja i izgradnje mreže, dok je problem fizičkog kvaliteta napona usko vezan za eksploataciju. Dominantan uticaj na fizički kvalitet napona imaju nelinearni potrošači (uređaji energetske elektronike, zasićene električne mašine,elektrolučne peći, itd…),tranzijentne pojave usled komutacija u sistemu (rad prekidača),rad elektroenergetskog sistema na granicama mogućnosti, itd…

Narušavanje kvaliteta električne energije podrazumeva narušavanje osnovnih parametara napona u ustaljenim ili prelaznim režimima i deformaciju talasnih oblika. Osnovni parametri napona su njegova efektivna vrednost, frekvencija i simetrija faznih napona.Dalje će biti razmatrani standardi vezani za sadržaj viših harmonika kako napojnog napona tako i struje koju potrošač uzima, dok drugi aspekti kvaliteta električne energije se neće razmatrati.

Viši harmonici

Napon viših harmonika je sinusni napon, čija je frekvencija celobrojni umnožak frekvencije osnovnog harmonika.Viši harmonici su nepoželjni u mrežama, jer se zbrajaju na osnovni talas i izobličuju ga, što uzrokuje problem u napajanju osetljivih potrošača, npr. medicinske opreme,koja zahteva čisti sinusni napon.

Slika 2.Talasni oblici napona prvog,petog i sedmog harmonika

Slika 2.Talasni oblici napona prvog,petog i sedmog harmonika

Dopuštene vrednosti viših harmonika (h od 2 do 40) tablično se prikazuju, i to:

  • pojedinačno, njihovim amplitudama (Uh), svedenim na amplitudu osnovnog harmonika  (U1),
  • zajednički, pomoću ukupnog sadržaja viših harmonika: THD (eng. Total Harmonic Distortion –    ukupno harmonijsko izobličenje), koje se izračunava kao:
    .
    formula 1

Tokom svakog desetminutnog intervala vrednost THD-a mora biti < 8% vrednosti prvog harmonika, dok vrednosti pojedinih harmonika mogu imati vrednosti najčešće u pojasu od 0,5% (npr. od 6., do 24. harmonika) do 6% (npr. za “poznati” 5. harmonik) od vrednosti prvog harmonika.Više harmonike u mrežnom naponu najčešće proizvode viši harmonici struja nelinearnih opterećenja potrošača, koji su priključeni na različitim nivoima distributivne mreže. Ti viši harmonici struje opterećenja stvaraju na impedansama unutar distributivne mreže odgovarajuće više harmonike napojnog napona. S druge strane, sve veća primena pretvarača frekvencije i sličnih upravljačkih uređaja utiče na povećanje vrednosti međuharmonika, čije se dopuštene vrednosti u okviru norme EN 50160 još razmatraju.

U pojedinim situacijama i međuharmonici malih intenziteta izazivaju treperenje (flikere) ili smetnje u sistemu mrežnog tonfrekventnog upravljanja.

Merenje ukupnog harmonijskog izobličenja napona

Za izračunavanje  THD U   koriste se izmerene (RMS) vrednosti svakog od prvih 40 harmonika (Un) i vrednost nazivnog napona (osnovni harmonik), koja prema normi EN 50160 iznosi npr.: U1 = 220 V, a prema jednačini:

formula 2

Pomnoženo sa 100%, THD U % ne sme biti veće od 8% vrednosti nazivnog napona. Ta jednačina u skladu je sa normom EN 61000-4-7.

Izračunavanje ukupnoga harmonijskog izobličenja napona i struje

Za izračunavanje THD U i THD I,  primenjuju se sledeće jednačine:

formula 3

Pri čemu su:
Urms – efektivna vrednost (RMS – Root Mean Square) ukupnog napona
U1 – efektivna (RMS) vrednost napona osnovnog harmonika
Irms – efektivna vrednost ukupnog signala struje
I1 – RMS vrednost struje osnovnog harmonika (nazivna vrednost signala na 50 Hz).

Izvori viših harmonika

Izvori viših harmonica su:

  • Prekidačke napojne jedinice
  • Elektronske prigušnice za fluo cevi
  • Regulisani elektromotorni pogoni
  • Besprekidna napajanja
  • Energetski ispravljači i pretvarači
  • Transformatori sa nelinearnim magnećenjem
  • Elektrolučne peći
  • Indukcione peći
  • Aparati za elektrolučno zavarivanje
Slika 3.1 Šema trofaznog ispravljača | Slika 3.2. Talasni oblik struje trofaznog ispravljača

Slika 3.1 Šema trofaznog ispravljača | Slika 3.2. Talasni oblik struje trofaznog ispravljača

.

Slika 3.5. Talasni oblici struje:

Slika 3.5. Talasni oblici struje: a) magnećenja transformatora,b) hladnjaka zamrzivača,c) klima uređaja,d) jednofaznog pretvarača sa sklopnim načinom rada,e) fluorescentne cevi sa elektromagnetnom prigušnicom,d) fluorescentne cevi sa elektronskom prigušnicom

Problemi zbog viših harmonika

Problemi koji u elektroenergetskom sistemu nastaju zbog prisustva viših harmonika su brojni i ovde će biti navedeni samo neki, kao što su:

Manja iskoristivost snage. Mrežni kablovi su dimenzionisani i osigurani na osnovu struje koju mogu sigurno isporučiti. Pošto mali faktor snage povećava prividnu struju iz izvora, iznos korisne snage koju može povući kolo je smanjen zbog toplotnih ograničenja.
Enormno smanjenje raspoložive snage izazvano je ili faznim pomakom ili distorzijom.

Troškovi distribucije. Ako postoji mnoštvo opterećenja sa malim faktorom snage, postavljaju se zahtevi za dodatnim proizvodnim i distributivnim kapacitetima. Troškovi, rastu proporcionalno sa inverznom vrednošću faktora snage. Gubici u disipativnim elementima (žice i namotaji transformatora) proporcionalni su kvadratu prividne struje pa troškovi za obezbeđenje ove disipirane snage su takođe u inverznoj vezi sa faktorom snage. Brojila električne energije registrovaće samo aktivnu snagu pa korisnici ne plaćaju reaktivnu snagu.

Distorzija napona. Impedanse realnih izvora su konačne. Kablovi su sve tanji prema krajnjim potrošačima električne energije. Mali preseci provodnika u uređajima i velika strujna distorzija utiču na oblik napona i on postaje nesinusoidan.Distorzija napona izaziva probleme u radu napojnih jedinica i drugih obližnjih uređaja spojenih na isti izvor.

Trofazni sistemi. Nesimetrično opterećenje izaziva neželjene struje u neutralnom provodniku. Ali, čak i kod potpuno simetrčnog opterećenja koje generiše više harmonike, harmonijski sadržaj će se pojaviti u neutralnom provodniku ( to su tzv. harmonici trećeg reda, 3-ći, 6-ti, 9-ti itd.).
Prethodno nabrojani negativni efekti koje izaziva distorzija mrežne struje i viši harmonici, doveli su do potrebe za postavljanjem ograničenja na strujne harmonike koje u mreži izazivaju priključeni uređaji.

Metode za neutralisanje viših harmonika

Da bi se harmonijski problem smanjio ili eliminisao postoji nekoliko osnovnim rešenja:

  • smanjenje intenziteta harmonijskih struja
  • postavljanje filtera
  • popravka faktora snage

Metode smanjenja intenziteta harmonijskih struja

Metode smanjenja intenziteta harmonijskih struja obično podrazumevaju menjanje načina rada pogona, koji generišu harmonike. Takav pristup je teško praktično izvesti, jer to može da utiče na kompletan proizvodni proces, odnosno moguće je jedino u fazi projektovanja.
Neka od rešenja koja se koriste pri ograničavanju viših harmonika u fazi projektovanja su:

  • Izmeštanje nelinearnih prijemnika što dalje od osetljive opreme.Slika 5. Izmeštanje nelinearnih prijemnika
    Slika 5. Izmeštanje nelinearnih prijemnika
  • Grupisanje nelinearnih prijemnika,koji se priključuju na odvojene sabirnice.
    Slika 5.1. Priključenje više nelinearnih prijemnika
    Slika 5.1. Priključenje više nelinearnih prijemnika
    .
  • Instaliranje više transformatora,jedni napajaju nelinearne prijemnike,dok drugi napajaju linearne prijemnike.
    Slika 5.2. Posebni transformatori za posebne vrste prijemnika
    Slika 5.2. Posebni transformatori za posebne vrste prijemnika
    .
  • Odgovarajućim sprezanjem transformatora mogu se ograničiti viši harmonici. Sprega namotaja u trougao dovodi do blokiranja daljeg toka svih harmonika, koji su umnozak od 3. Unošenjem faznog pomeraja od 30 stepeni, sprezanjem sekundara transformatora u zvezdu i u trougao, dobija se efekat 12-pulsnog ispravljača, odnosno eliminišu se 5-ti i 7-mi harmonik.
        * Slika 5.3. Različito sprezanje namotaja utiče na eliminisanje pojedinih harmonica
    Slika 5.3. Različito sprezanje namotaja utiče na eliminisanje pojedinih harmonika

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Uskoro u nastavku stručnog teksta: EES Kvalitet električne energije – viši harmonici (2):

  • Postavljanje filtera (pasivni, aktivni i hibridni)
  • Popravka faktora snage
  • Standardi i preporuke (u Francuskoj, Švedskoj, Australiji)
  • Standardi i preporuke međunarodne elektrotehničke komisije (IEC)

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AUTOR STRUČNOG TEKSTA:

Dragan Simović
Dragan Simovic

Seminarski rad iz predmeta: Eksploatacija EES, tema: Kvalitet električne energije – viši harmonici
Visoka Škola Tehničkih Strukovnih Studija Čačak
Specijalističke Strukovne Studije Elektrotehnike i Računarstva | Modul Studijskog Programa: Elektroenergetika

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Grounded or Ungrounded Systems

Designing a quality grounding system is not only for the safety of employees but also provides the protection required for buildings and equipment.

Ungrounded systems may provide greater continuity of operations in the event of a ground fault. However, the second fault will most likely be more catastrophic than a grounded system fault. Whenever ungrounded systems are used in a facility, the maintenance personnel should receive training in how to detect and troubleshoot the first ground on an ungrounded system.

Electrical systems can be operated grounded or ungrounded, depending on the condition of the systems use. Electrical systems are grounded to protect circuits, equipment, and conductor enclosures from dangerous voltages and personnel from electrical shock. See NEC Sections 110-9, 110-10, 230-65, 250-1, and 250-2 that list the requirements to provide this protection.
“Grounded” means that the connection to ground between the service panel and earth has been made. Ungrounded electrical systems are used where the designer does not want the overcurrent protection device to clear in the event of a ground fault.

Ground detectors can be installed per NEC Section 250-5(b) FPN to sound an alarm or send a message to alert personnel that a ground fault has occurred on one of the phase conductors. Ground detectors will detect the presence of leakage current or developing fault current conditions while the system is still energized and operating. By warning of the need to take corrective action before a problem occurs, safe conditions can usually be maintained until an orderly shutdown is implemented.

Grounded Systems

Grounded systems are equipped with a grounded conductor that is required per NEC Section 250- 23(b) to be run to each service disconnecting means. The grounded conductor can be used as a current-carrying conductor to accommodate all neutral related loads. It can also be used as an equipment grounding conductor to clear ground faults per NEC Section 250-61(a).
A network of equipment grounding conductors is routed from the service equipment enclosure to all metal enclosures throughout the electrical system. The equipment grounding conductor carries fault currents from the point of the fault to the grounded bus in the service equipment where it is transferred to the grounded conductor. The grounded conductor carries the fault current back to the source and returns over the faulted phase and trips open the overcurrent protection device.

Note: A system is considered grounded if the supplying source such as a transformer, generator, etc., is grounded, in addition to the grounding means on the supply side of the service equipment disconnecting device per NEC Sections 250-23(a) or 250-26 for seperately derived systems.
The neutral of any grounded system serves two main purposes: (1) it permits the utilization of line- to-neutral voltage and thus will serve as a current-carrying conductor to carry any unbalanced current, and (2) it plays a vital role in providing a low-impedance path for the flow of fault currents to facilitate the operation of the overcurrent devices in the circuit. (See picture below).

Consideration should be given to the sizing of the neutral conductor for certain loads due to the presence of harmonic currents (See NEC Sections 210-4 and 310-10).

A grounded system is equipped with a grounded (neutral) conductor routed between the supply transformer and the service equipment.

A grounded system is equipped with a grounded (neutral) conductor routed between the supply transformer and the service equipment.

Ungrounded Systems

Ungrounded systems operate without a grounded conductor. In other words, none of the circuit conductors of the electrical system are intentionally grounded to an earth ground such as a metal water pipe, building steel, etc. The same network of equipment grounding conductors is provided for ungrounded systems as for solidly grounded electrical systems. However, equipment grounding conductors (EGCs) are used only to locate phase-to-ground faults and sound some type of alarm.

Therefore, a single sustained line-to-ground fault does not result in an automatic trip of the overcurrent protection device. This is a major benefit if electrical system continuity is required or if it would result in the shutdown of a continuous process. However, if an accidental ground fault occurs and is allowed to flow for a substantial time, overvoltages can develop in the associated phase conductors. Such an overvoltage situation can lead to conductor insulation damage, and while a ground fault remains on one phase of an ungrounded system, personnel contacting one of the other phases and ground are subjected to 1.732 times the voltage they would experience on a solidly neutral grounded system. (See picture below).

Note: All ungrounded systems should be equipped with ground detectors and proper maintenance applied to avoid, as far as practical, the overcurrent of a sustained ground fault on ungrounded systems. If appropriate maintenance is not provided for ungrounded systems, a grounded system should be installed to ensure that ground faults will be cleared and the safety of circuits, equipment, and that personnel safety is ensured.

An ungrounded system does not have a grounded (neutral) conductor routed between the supply transformer and the service equipment because the supply transformer is not earth grounded.

An ungrounded system does not have a grounded (neutral) conductor routed between the supply transformer and the service equipment because the supply transformer is not earth grounded.

High impedance grounding

Electrical systems containing three-phase, three-wire loads, as compared to grounded neutral circuit conductor loads, can be equipped with a high-impedance grounded system. High-impedance grounded systems shall not be used unless they are provided with ground fault indicators or alarms, or both, and qualified personnel are available to quickly locate and eliminate such ground faults.

Ground faults must be promptly removed or the service reliability will be reduced. See NEC Section 250-27 for requirements pertaining to installing a high-impedance grounding system. (See picture below).

A high-impedance grounding system has a high-impedance unit, installed between the grounded (neutral) conductor and the grounding electrode conductor, which is used to regulate fault current.

A high-impedance grounding system has a high-impedance unit, installed between the grounded (neutral) conductor and the grounding electrode conductor, which is used to regulate fault current.

Source: DOE HANDBOOK – ELECTRICAL SAFETY

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Energy management control center
Energy management control center

Energy management is the process of monitoring, coordinating, and controlling the generation, transmission, and distribution of electrical energy. The physical plant to be managed includes generating plants that produce energy fed through transformers to the high-voltage transmission network, interconnecting generating plants, and load centers. Transmission lines terminate at substations that perform switching, voltage transformation, measurement, and control. Substations at load centers transform to subtransmission and distribution levels. These lower-voltage circuits typically operate radially, i.e., no normally closed paths between substations through subtransmission or distribution circuits. (Underground cable networks in large cities are an exception.)

Since transmission systems provide negligible energy storage, supply and demand must be balanced by either generation or load. Production is controlled by turbine governors at generating plants, and automatic generation control is performed by control center computers remote from generating plants. Load management, sometimes called demand-side management, extends remote supervision and control to subtransmission and distribution circuits, including control of residential, commercial, and industrial loads.

Events such as lightning strikes, short circuits, equipment failure, or accidents may cause a system fault. Protective relays actuate rapid, local control through operation of circuit breakers before operators can respond. The goal is to maximize safety, minimize damage, and continue to supply load with the least inconvenience to customers. Data acquisition provides operators and computer control systems with status and measurement information needed to supervise overall operations. Security control analyzes the consequences of faults to establish operating conditions that are both robust and economical.

Energy management is performed at control centers (see picture below), typically called system control centers, by computer systems called energy management systems (EMS). Data acquisition and remote control is performed by computer systems called supervisory control and data acquisition (SCADA) systems. These latter systems may be installed at a variety of sites including system control centers. An EMS typically includes a SCADA ‘‘front-end’’ through which it communicates with generating plants, substations, and other remote devices.

Picture below illustrates the applications layer of modern EMS as well as the underlying layers on which it is built: the operating system, a database manager, and a utilities=services layer.

Layers of a modern EMS

Layers of a modern EMS (Energy management systems)

SCADA – Supervisory control and data acquisition

A SCADA system consists of a master station that communicates with remote terminal units (RTUs) for the purpose of allowing operators to observe and control physical plants.

Generating plants and transmission substations certainly justify RTUs, and their installation is becoming more common in distribution substations as costs decrease. RTUs transmit device status and measurements to, and receive control commands and setpoint data from, the master station.

Communication is generally via dedicated circuits operating in the range of 600 to 4800 bits=s with the RTU responding to periodic requests initiated from the master station (polling) every 2 to 10 s, depending on the criticality of the data.
The traditional functions of SCADA systems are summarized:

  • Data acquisition: Provides telemetered measurements and status information to operator.
  • Supervisory control: Allows operator to remotely control devices, e.g., open and close circuit breakers. A ‘‘select before operate’’ procedure is used for greater safety.
  • Tagging: Identifies a device as subject to specific operating restrictions and prevents unauthorized operation.
  • Alarms: Inform operator of unplanned events and undesirable operating conditions. Alarms are sorted by criticality, area of responsibility, and chronology. Acknowledgment may be required.
  • Logging: Logs all operator entry, all alarms, and selected information.
  • Load shed: Provides both automatic and operator-initiated tripping of load in response to system emergencies.
  • Trending: Plots measurements on selected time scales.

Since the master station is critical to power system operations, its functions are generally distributed among several computer systems depending on specific design. A dual computer system configured in primary and standby modes is most common. SCADA functions are listed below without stating which computer has specific responsibility.

  • Manage communication circuit configuration
  • Downline load RTU files
  • Maintain scan tables and perform polling
  • Check and correct message errors
  • Convert to engineering units
  • Detect status and measurement changes
  • Monitor abnormal and out-of-limit conditions
  • Log and time-tag sequence of events
  • Detect and annunciate alarms
  • Respond to operator requests to:
    • Display information
    • Enter data
    • Execute control action
    • Acknowledge alarms
  • Transmit control action to RTUs
  • Inhibit unauthorized actions
  • Maintain historical files
  • Log events and prepare reports
  • Perform load shedding

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Power quality

Power quality

The term power quality seeks to quantify the condition of the electrical supply. It not only relates largely to voltage, but also deals with current and it is largely the corrupting effect of current disturbances upon voltage. Power quality can be quantified by a very broad range of parameters, some of which have been recognized and studied for as long as electrical power has been utilized. However, the advent of the term itself is more modern and it has created a useful vehicle for discussing and quantifying all factors that can describe supply quality. Power quality is yet another means of analysing and expressing electromagnetic compatibility (EMC), but in terms of the frequency spectrum, power quality charac- terizes mainly low-frequency phenomena. Perhaps because of this and because of the manner in which it affects electrical equipment, power quality has largely been dealt with by engineers with electrical power experience rather than those with an EMC expertise. In reality, resolving power problems can benefit from all available expertise, particularly since power quality disorders and higher frequency emissions can produce similar effects.

In 1989, the European Community defined the supply of electricity as a product, and it is therefore closely related to the provisions and protection of the EMC Directive (89/336/EEC), but in drawing a comparison between electricity and other manufactured product it is essential to recall a significant difference.

Electricity is probably unique in being a product which is manufactured, delivered and used at the same time. An electricity manufacturer cannot institute a batch testing process for example and pull substandard products out of the supply chain. By the time electricity is tested it will have been delivered and used by the customer whether it was of good quality or not.

Key parameters

The parameters that are commonly used to characterize supplies are listed in Table 1 together with the typical tolerance limits which define acceptable norms. Within Europe these power quality limits are defined by the EN 61000 series of standards in order to be compatible with the susceptibility limits set for equipment.

Table 1: Summary of power quality levels defined by EN 50160
.Power frequency (50Hz).Interconnected systems
.±1% (95% of week)
.+4% (absolute level)
.−6% (absolute level)
.Supply voltage variations on 230V nominal.±10% (95% of week based on
.10 min samples, rms)
.Rapid voltage changes.±5% Frequent
.±10% Infrequent
.Flicker.Pk=1.0 (95% of week)
.Supply voltage dips.Majority
.Few 10s
.Duration <1s
.Depth <60%
.Some locations
.Few 1000 per year of <15% depth
.Short interruptions.20–500 per year
.Duration 1s of 100% depth
.Long interruptions.10–50 per year
.Duration >180s of 100% depth
.Temporary power frequency overvoltage.<1.5kV
.Transient overvoltages.Majority
.<6kV
.Exceptionally
.>6kV
.Supply voltage unbalance.Majority
.<2%(95% of the week)
.Exceptionally
.>2%, <3%(95% of the week)
.Harmonic voltage distortion.THD <8%(95% of the week)
.Interharmonic voltage distortion.Under consideration
.Mains signalling.95 to 148.5kHz at up to 1.4Vrms (not in MV)

The more a supply deviates from these limits, the more likely it is that malfunction could be experienced in terminating equipment. However, individual items of equipment will have particular sensitivity to certain power quality parameters while having a wider tolerance to others. Table 2 provides examples of equipment and the power quality parameters to which they are particularly sensitive. Table 2 shows a preponderance of examples with a vulnerability to voltage dips. Of all the power quality parameters, this is probably the most troublesome to the manufacturing industry; and in the early 1970s, as the industry moved towards a reliance on electronic rather than electromagnetic controls, it was commonly observed how much more vulnerable the industrial processes were to supply disturbances.

Supply distortion (characterized by harmonics) is another power quality parameter that has received enormous attention, with many articles, textbooks and papers written on the subject. However, the modern practices that will be discussed later have reduced the degree to which this currently presents a problem. Other parameters tend to be much less problematic in reality, although that is not to say that perceptions sometimes suggest otherwise. Voltage surge and tran- sient overvoltage in particular are often blamed for a wide range of problems.

Table 2: Examples of sensitivity to particular power quality parameters
Equipment typeVulnerable power quality parameter Effect if exceededRang
.Induction motor .Voltage unbalance.Excessive rotor heating.<3%
.Power factor correction .capacitors.Spectral frequency .content.This is usually .defined .in terms of harmonic .distortion.Capacitor failure due to .excessive current flow or .voltage.Most sensitive if .resonance occurs.In resonant .conditions
.PLCs .Voltage dips.Disruption to the programmed .functionality.V tr
.Computing systems .Voltage dips.Disruption to the programmed .functionality.V
.Variable speed drives, .motor starters and .attracted .armature control .relays .Voltage dips.Disruption to the control system .causing shutdown. V
.Power transformers .Spectral frequency content of .load current.This is usually .defined in terms of harmonic .distortion.Increased losses leading to excessive temperature rise.At full load
.Devices employing .phase .control, such as .light .dimmers and .generator .automatic .voltage regulator .(AVRs) .Alteration in waveform zero .crossing due to waveform .distortion, causing multiple .crossing or phase asymmetry.Instability.Will depend .upon the r
.Motor driven .speed-.sensitive plant .Induction and synchronous .motor shaft speed are .proportional to supply .frequency. Some driven loads .are .sensitive to even small .speed variations.The motors themselves are .tolerant of small speed .variations.At high supply .frequencies (>10%) shaft .stresses may be excessive due .to high running speeds.Limits .depend on .the .sensitivity

However, very often this is a scapegoat when the actual cause cannot be identified. Even when correlation with switching voltage transients is correctly observed, the coupling introduced by poor wiring installations or bad earth bonding practices can be the real problem. Unlike the other power quality parameters, voltage transients have a high frequency content and will couple readily through stray capacitance and mutual inductance into neighbouring circuits. Coupling into closed conductor loops that interface with sensitive circuits such as screens and drain wires can easily lead to spurious events.

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