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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).
Substation, Its Function And Types

Substation, Its Function And Types

An electrical sub-station is an assemblage of electrical components including busbars, switchgear, power transformers, auxiliaries etc.

These components are connected in a definite sequence such that a circuit can be switched off during normal operation by manual command and also automatically during abnormal conditions such as short-circuit. Basically an electrical substation consists of No. of incoming circuits and outgoing circuits connected to a common Bus-bar systems. A substation receives electrical power from generating station via incoming transmission lines and delivers elect. power via the outgoing transmission lines.

Sub-station are integral parts of a power system and form important links between the generating station, transmission systems, distribution systems and the load points.

MAIN TASKS

…Associated with major sub-stations in the transmission and distribution system include the following:

  1. Protection of transmission system.
  2. Controlling the Exchange of Energy.
  3. Ensure steady State & Transient stability.
  4. Load shedding and prevention of loss of synchronism. Maintaining the system frequency within targeted limits.
  5. Voltage Control; reducing the reactive power flow by compensation of reactive power, tap-changing.
  6. Securing the supply by proving adequate line capacity.
  7. Data transmission via power line carrier for the purpose of network monitoring; control and protection.
  8. Fault analysis and pin-pointing the cause and subsequent improvement in that area of field.
  9. Determining the energy transfer through transmission lines.
  10. Reliable supply by feeding the network at various points.
  11. Establishment of economic load distribution and several associated functions.

TYPES OF SUBSTATION

The substations can be classified in several ways including the following :

  1. Classification based on voltage levels, e.g. : A.C. Substation : EHV, HV, MV, LV; HVDC Substation.
  2. Classification based on Outdoor or Indoor : Outdor substation is under open skv. Indoor substation is inside a building.
  3. Classification based on configuration, e.g. :
    • Conventional air insulated outdoor substation or
    • SF6 Gas Insulated Substation (GIS)
    • Composite substations having combination of the above two
  4. Classification based on application
    • Step Up Substation : Associated with generating station as the generating voltage is low.
    • Primary Grid Substation : Created at suitable load centre along Primary transmission lines.
    • Secondary Substation : Along Secondary Transmission Line.
    • Distribution Substation : Created where the transmission line voltage is Step Down to supply voltage.
    • Bulk supply and industrial substation : Similar to distribution sub-station but created separately for each consumer.
    • Mining Substation : Needs special design consideration because of extra precaution for safety needed in the operation of electric supply.
    • Mobile Substation : Temporary requirement.
      NOTE :
    • Primary Substations receive power from EHV lines at 400KV, 220KV, 132KV and transform the voltage to 66KV, 33KV or 22KV (22KV is uncommon) to suit the local requirements in respect of both load and distance of ultimate consumers. These are also referred to ‘EHV’ Substations.
    • Secondary Substations receive power at 66/33KV which is stepped down usually to 11KV.
    • Distribution Substations receive power at 11KV, 6.6 KV and step down to a volt suitable for LV distribution purposes, normally at 415 volts

SUBSTATION PARTS AND EQUIPMENTS

Each sub-station has the following parts and equipment.

  1. Outdoor Switchyard
    • Incoming Lines
    • Outgoing Lines
    • Bus bar
    • Transformers
    • Bus post insulator & string insulators
    • Substation Equipment such as Circuit-beakers, Isolators, Earthing Switches, Surge Arresters, CTs, VTs, Neutral Grounding equipment.
    • Station Earthing system comprising ground mat, risers, auxiliary mat, earthing strips, earthing spikes & earth electrodes.
    • Overhead earthwire shielding against lightening strokes.
    • Galvanised steel structures for towers, gantries, equipment supports.
    • PLCC equipment including line trap, tuning unit, coupling capacitor, etc.
    • Power cables
    • Control cables for protection and control
    • Roads, Railway track, cable trenches
    • Station illumination system
  2. Main Office Building
    • Administrative building
    • Conference room etc.
  3. 6/10/11/20/35 KV Switchgear, LV
    • Indoor Switchgear
  4. Switchgear and Control Panel Building
    • Low voltage a.c. Switchgear
    • Control Panels, Protection Panels
  5. Battery Room and D.C. Distribution System
    • D.C. Battery system and charging equipment
    • D.C. distribution system
  6. Mechanical, Electrical and Other Auxiliaries
    • Fire fighting system
    • D.G. Set
    • Oil purification system

An important function performed by a substation is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be “planned” or “unplanned”. A transmission line or other component may need to be deenergized for maintenance or for new construction; for example, adding or removing a transmission line or a transformer. To maintain reliability of supply, no company ever brings down its whole system for maintenance. All work to be performed, from routine testing to adding entirely new substations, must be done while keeping the whole system running.

Perhaps more importantly, a fault may develop in a transmission line or any other component. Some examples of this: a line is hit by lightning and develops an arc, or a tower is blown down by a high wind. The function of the substation is to isolate the faulted portion of the system in the shortest possible time.

There are two main reasons: a fault tends to cause equipment damage; and it tends to destabilize the whole system. For example, a transmission line left in a faulted condition will eventually burn down, and similarly, a transformer left in a faulted condition will eventually blow up. While these are happening, the power drain makes the system more unstable. Disconnecting the faulted component, quickly, tends to minimize both of these problems.

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Shielding Of Power Cables

Shielding Of Power Cables

Shielding of an electric power cable is accomplished by surrounding the assembly or insulation with a grounded, conducting medium.

This confines the dielectric field to the inside of this shield.

Two distinct types of shields are used:

- Metallic

- Nonmetallic

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The purposes of the insulation shield are to:

  • Obtain symmetrical radial stress distribution withh the insulation.
  • Eliminate tangential and longitudinal stresses on the surface of the insulation.
  • Exclude from the dielectric field those materials such as braids, tapes, and fillers that are not intended as insulation.
  • Protect the cables from induced or direct aver-voltages. Shields do this by making the surge impedance uniform along the length of the cable and by helping to attenuate surge potentials.

Conductor Shielding

In cables rated over 2,000 volts, a conductor shield is required by indusby standards. The purpose of the semiconducting, also called screening, material over the conductor is to provide a smooth cylinder rather than the relatively rough surface of a stranded conductor in order to reduce the stress concentration at the interface with the insulation. Conductor shielding has been used for cables with both laminar and extruded insulations. The materials used are either semiconducting materials or ones that have a high dielectric constant and are known as stress control materials. Both serve the same function of stress reduction.

Conductor shields for paper insulated cables are either carbon black tapes or metallized paper tapes. The conductor shieldmg materials were originally made of semiconducting tapes that were helically wrapped over the conductor. Present standards still permit such a tape over the conductor. This is done, especially on large conductors, in order to hold the strands together firmly during the application of the extruded semiconducting material that is now required for medium voltage cables. Experience with cables that only had a semiconductingtape was not satisfactory, so the industry changed their requirements to call for an extruded layer over the conductor.

In extruded cables, this layer is now extruded directly over the conductor and is bonded to the insulation layer that is applied over this stress relief layer. It is extremely important that there be no voids or extraneous material between those two layers.

Presentday extruded layers are not only clean (free from undesirable impurities) but are very smooth and round. This has greatly reduced the formation of water tress that could originate from irregular surfaces. By extruding the two layers at the same time, the conductor shield and the insulation are cured at the same time. This provides the inseparable bond that minimizes the chances of the formation of a void at the critical interface. For compatibility reasons, the extruded shielding layer is usually made from the same or a similar polymer as the insulation. Special carbon black is used to make the layer over the conductor semiconducting to provide the necessary conductivity. Industry standards require that the conductor semiconducting material have a maximum resistivity of 1,000 meter-ohms. Those standards also require that this material pass a long-time stability test for resistivity at the emergency operating temperature level to insure that the layer remains conductive and hence provides a long cable life.

A water-impervious material can be incorporated as part of the conductor shield to prevent radial moisture transmission. This layer consists of a thin layer of aluminum or lead sandwiched between semiconducting material. A similar laminate may be used for an insulation shield for the same reason.

There is no definitive standard that describes the class of extrudable shielding materials known as “super smooth, super clean”. It is not usually practical to use a manufacturer’strade name or product number to describeany material. The term “super smooth, super clean” is the only way at this writing to describe a class of material that provides a higher quality cable thanan earlier version. This is only an academic issue since the older type of materials are no longer used for medium voltage cable construction by known suppliers. The point is that these newer materials have tremendously improved cable performance in laboratory evaluations.

Insulation Shielding For Medium-Voltage Cables

The insulation shield for a medium voltage cable is made up of two components:

  • Semiconducting or stress relief layer
  • Metallic layer of tape or tap , drain wires, concentric neutral wires, or a metal tube.

They must function as a unit for a cable to achieve a long service life

Stress Relief Layer

The polymer layer used with exbuded cables has replaced the tapes shields that were used many years ago. This extruded layer is called the extruded insulation shield or screen. Its properties and compatibility requirements are similar to the conductor shield previously described except that standards require that the volume resistivity of thisexternal layer be limited to 500 meter-ohms.

The nonmetallic layer is directly Over the insulation and the voltage stress at that interface is lower than at the conductor shield interface.. This outer layer is not required to be bonded for cables rated up to 35 kV. At voltages above that, it is strongly recommendedt that this layer be bonded to the insulation .
Since most users want this layer to be easily removable, the Association of Edison Illuminating Companies (AEIC) has established strip tension limits. Presently these limits are that a 1/2 inch wide strip cut parallel to the conductor peel offwith a minimum of 6 pounds and a minimum of 24 pounds of force that is at a 90º angle to the insulation surface.

Metallic Shield

The metallic portion of the insulation shield or screen is necessary to provide a low resistance path for charging current to flow to ground. It is important to realize that the extruded shield materials will not survive a sustained current flow of more than a few milliamperes. These materials are capable of handing the small amounts of charging current, but cannot tolerate unbalanced or fault currents.

The metallic component of the insulation shield system must be able to accommodate these higher currents. On the other hand, an excessive amount of metal in the shield of a single-conductor cable is costly in two ways. First, additional metal over the amount that is actually required increases the initial cost of the cable. Secondly, the greater the metal component of the insulation shield, the higher the shield losses that result h m the flow of current in the central conductor.

A sufficient amount of metal must be provided in the cable design to ensure that the cable will activate the back-up protection in the event of any cable fault over the life of that cable. There is also the concern for shield losses.

It therefore becomes essential that:

  • The type of circuitinterruptingequipmentto be analyzed.What is the design and operational setting of the hse, recloser, or circuit breaker?
  • What fault current will the cable encounter over its life?
  • What shield losses can be tolerated? How many times is the shield to be grounded? Will there be shield breaks to prevent circulating currents?
Concentric Neutral Cables

When concentric neutral cables are specified, the concentric neutrals must be manufactured in accordance with ICEA standards. These wires must meet ASTM B3 for uncoated wires or B33 for coated wires. These wires are applied directly over the nonmetallic insulation shield with a lay of not less than six or more than ten times the diameter over the concentric wires.

Shielding Of Low Voltage Cables

Shielding of low voltage cables is generally required where inductive interference can be a problem. In numerous communication, instrumentation, and control cable applications, small electrical signals may be transmitted on the cable conductor and amplified at the receiving end. Unwanted signals (noise) due to inductive interference can beaslargeasthedesiredsignal. This can result in false signals or audible noise that can effect voice communications.

Across the entire frequency spectrum, it is necessary to separate disturbances into electric field ef€ects and magnetic field effects.

Electric Fields

Electric field effects are those which are a function of the capacitive coupling or mutual capacitance between the circuits. Shielding can be effected by a continuous metal shield to isolate the disturbed circuit fiom the disturbing circuit. Even semiconducting extrusions or tapes supplemented by a grounded dmin wirecan serve some shielding function for electric field effects.

Magnetic Fields

Magnetic field effects are the result of a magnetic field coupling between circuits. This is a bit more complex thanfor electrical effects.

At relatively low frequencies, the energy emitted from the source is treated as radiation. This increases with the square of the frequency. This electromagnetic radiation can cause dislxrbancesat considerable distance and will penetrate any “openings” in the shielding. This can occur with braid shields or tapes that are not overlapped. The type of metal used in the shield also can effect the amount of disturbance. Any metallic shield material, as opposed to magnetic metals, will provide some shield due to the eddy currents that are set up in the metallic shield by the impinging field. These eddy currents tend to neutralize the disturbing field. Non-metallic, semiconducting shielding is not effective for magnetic effects. In general, the most effective shielding is a complete steel conduit, but thisis not always practical.

The effectiveness of a shield is called the “shielding factor” and is given as:

SF = Induced voltage in shield circuit / Inducted voltage in unshielded circuit

Test circuits to measwe the effectiveness of various shielding designs against electrical field effects and magnetic field effects have been reported by Gooding and Slade.

AUTHORS: Lawrence J. Kelly and Carl C. Landinger

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Figure 1: Three generations of transformer substations

Figure 1: Three generations of transformer substations

Early transformers were located at the top of pylons and could achieve powers of up to 1000kVA. Column-type transformer substations provided the interface between overhead and underground networks. These were equipped essentially with air-insulated MV switchgear, a liquid- insulated transformer and a low voltage distribution switchboard.

These were fabricated from bricks, and thanks to the chimney effect provided by the column the substations had good airflow, and consequently there were no problems with overheating of the equipment.

However the next generation of substations for underground networks also built from brick, had a reduced height and no chimney effect, and for the first time the equipment designers had to confront overheating problems.

This second generation of transformer substations was also the subject of the first internal fault tests, intended to provide operating personnel and the general public with greater levels of safety. The next step was the introduction of factory assembled prefabricated transformer substations.

Figure 2: 630kVA qualified by EDF, validated against internal faults in the air and easily integrated into buildings.

Figure 2: 630kVA qualified by EDF, validated against internal faults in the air and easily integrated into buildings.

This third generation of transformer substations were subject to the first international regulations. These substations were characterised by the use of more compact, environmentally insensitive equipment, factory assembled and standardised which means that Utilities can be supplied with series produced products, the production and performance of which are guaranteed by the tests carried out by the manufacturers in accredited laboratories.

For the third generation substations the first Utility specifications demanded small surface areas, leading to a standardised layout of the substations (Figure 1).

This enclosure can be metallic, GRC (Glass Reinforced Concrete), GRP (Glass Reinforced Plastic), SFRC (Steel Fibres Reinforced Concrete). The world wide trend is for reinforced concrete enclosures for the following reasons :

  • Improved mechanical strength
  • Reduced effect of solar radiation
  • Reduced condensation
  • Improved fire behaviour
  • Weathering
  • Improved aesthetic.

Finally, a fourth generation of transformer substations (Figure 2) has recently appeared, where the detailed technical specification has been replaced by functional specification.

AUTHORS: Thierry CORMENIER, ALSTOM-France; Robert DIDES.

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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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Transformer Ratings

Transformer Ratings

Transformer size or capacity is most often expressed in kVA. “We require 30 kVA of power for this system” is one example, or “The facility has a 480 VAC feed rated for 112.5 kVA”.

However, reliance upon only kVA rating can result insafety and performance problems when sizing transformers to feed modern electronic equipment.

Use of off-the-shelf, general purpose transformers for electronics loads can lead to power quality and siting problems:

  • Single phase electronic loads can cause excessive transformer heating.
  • Electronic loads draw non-linear currents, resulting in low voltage and output voltage distortion.
  • Oversizing for impedance and thermal performance can result in a transformer with a significantly larger footprint.

It is vital for the systems designer to understand all of the factors that affect transformer effectiveness and performance.
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Thermal Performance

Historically, transformers have been developed to supply 60 Hz, linear loads such as lights, motors, and heaters. Electronic loads were a small part of the total connected load. A system designer could be assured that if transformer voltage and current ratings were not exceeded, the transformer would not overheat, and would perform as expected. A standard transformer is designed and specified with three main parameters: kVA Rating, Impedance, and Temperature Rise.
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KVA Rating

The transformer voltage and current specification. KVA is simply the load voltage times the load current. A single phase transformer rated for 120 VAC and 20 Amperes would be rated for 120 x 20 = 2400 VA, or 2.4 KVA (thousand VA).
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Impedance

Transformer Impedance and Voltage Regulation are closely related: a measure of the transformer voltage drop when supplying full load current. A transformer with a nominal output voltage of 120 VAC and a Voltage Regulation of 5% has an output voltage of 120 VAC at no-load and (120 VAC – 5%) at full load – the transformer output voltage will be 114 VAC at full load. Impedance is related to the transformer thermal performance because any voltage drop in the transformer is converted to heat in the windings.
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Temperature Rise

Steel selection, winding capacity, impedance, leakage current, overall steel and winding design contribute to total transformer heat loss. The transformer heat loss causes the transformer temperature to rise. Manufacturers design the transformer cooling, and select materials, to accommodate this temperature rise.

Transformer Heat Loss

Transformer Heat Loss

Use of less expensive material with a lower temperature rating will require the manufacturer to design the transformer for higher airflow and cooling, often resulting in a larger transformer. Use of higher quality materials with a higher temperature rating permits a more compact transformer design.

Transformer Insulation Systems

Transformer Insulation Systems

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“K” Factor Transformer Rating

In the 1980′s, power quality engineers began encountering a new phenomenon: non-linear loads, such as computers and peripherals, began to exceed linear loads on some distribution panels. This resulted in large harmonic currents being drawn, causing excessive transformer heating due to eddy-current losses, skin effect, and core flux density increases.

Standard transformers, not designed for nonlinear harmonic currents were overheating and failing even though RMS currents were well within transformer ratings.

In response to this problem, IEEE C57.110-1986 developed a method of quantifying harmonic currents. A “k” factor was the result, calculated from the individual harmonic components and the effective heating such a harmonic would cause in a transformer. Transformer manufacturers began designing transformers that could supply harmonic currents, rated with a “k” factor. Typical “K” factor applications include:

  • K-4: Electric discharge lighting, UPS with input filtering, Programmable logic controllers and solid state controls
  • K-13: Telecommunications equipment, UPS systems, multi-wire receptacle circuits in schools, health-care, and production areas
  • K-20: Main-frame computer loads, solid state motor drives, critical care areas of hospitals

“K” factor is a good way to assure that transformers will not overheat and fail. However, “K” factor is primarily concerned with thermal issues. Selection of a “K” factor transformer may result in power quality improvement, but this depends upon manufacturer and design.
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Transformer Impedance

Transformer impedance is the best measure of the transformer’s ability to supply an electronic load with optimum power quality. Many power problems do not come from the utility but are internally generated from the current requirements of other loads.

While a “K” factor transformer can feed these loads and not overheat, a low impedance transformer will provide the best quality power. As an example, consider a 5% impedance transformer. When an electronic load with a 200% inrush current is turned on, a voltage sag of 10% will result. A low impedance transformer (1%) would provide only a 2% voltage sag – a substantial improvement. Transformer impedance may be specified as a percentage, or alternately, in Ohms (Ω) from Phase- Phase or Phase-Neutral.
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High Frequency Transformer Impedance

Most transformer impedance discussions involve the 60 Hz transformer impedance. This is the power frequency, and is the main concern for voltage drops, fault calculations, and power delivery. However, nonlinear loads draw current at higher harmonics. Voltage drops occur at both 60 Hz and higher frequencies. It is common to model transformer impedance as a resistor, often expressed in ohms. In fact, a transformer behaves more like a series resistor and inductor.

The voltage drop of the resistive portion is independent of frequency, the voltage drop of the inductor is frequency dependent.

Standard Transformer impedances rise rapidly with frequency. However, devices designed specifically for use with nonlinear loads use special winding and steel lamination designs to minimize impedance at both 60 Hz and higher frequencies. As a result, the output voltage of such designs is far better quality than for standard transformers.
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Recommendations for Transformer Sizing

System design engineers who must specify and apply transformers have several options when selecting transformers.
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Do It Yourself Approach

With this approach, a larger than required standard transformer is specified in order to supply harmonic currents and minimize voltage drop. Transformer oversizing was considered prudent design in the days before transformer manufacturers understood harmonic loads, and remains an attractive option from a pure cost standpoint. However, such a practice today has several problems:

  • A larger footprint and volume than low impedance devices specifically designed for non-linear loads
  • Poor high frequency impedance
  • Future loads may lead to thermal and power quality problems
Standard Isolation Transformer

Standard Isolation Transformer

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“K”-factor Rated Transformers

Selecting and using “K”-factor rated transformers is a prudent way to ensure that transformer overheating will not occur. Unfortunately, lack of standardization makes the “K” factor rating a measure only of thermal performance, not impedance or power quality.

Percent Impedance

Percent Impedance

Some manufacturers achieve a good “K” factor using design techniques that lower impedance and enhance power quality, others simply derate components and temperature ratings. Only experience with a particular transformer manufacturer can determine if a “K” factor transformer addresses both thermal and power quality concerns.
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Transformers Designed for Non-Linear Loads

Transformers designed specifically for non-linear loads incorporate substantial design improvements that address both thermal and power quality concerns. Such devices are low impedance, compact, and have better high frequency performance than standard or “K” factor designs. As a result, this type of transformer is the optimum design solution.

This type of transformer may be more expensive than standard transformers, due to higher amounts of iron and copper, higher quality materials, and more expensive winding and stacking techniques. However, the benefits of such a design in power quality and smaller size justify the extra cost, and make the low impedance transformer the most cost effective design overall.

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How Wind Turbines Work

How Wind Turbines Work

Wind is a form of solar energy. Winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth’s surface, and rotation of the earth. Wind flow patterns are modified by the earth’s terrain, bodies of water, and vegetation. Humans use this wind flow, or motion energy, for many purposes: sailing, flying a kite, and even generating electricity.

The terms wind energy or wind powmegawatts.er describe the process by which the wind is used to generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. This mechanical power can be used for specific tasks (such as grinding grain or pumping water) or a generator can convert this mechanical power into electricity.

So how do wind turbines make electricity? Simply stated, a wind turbine works the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity. Take a look inside a wind turbine to see the various parts. View the wind turbine animation to see how a wind turbine works.

Wind turbines operate on a simple principle. The energy in the wind turns two or three propeller-like blades around a rotor. The rotor is connected to the main shaft, which spins a generator to create electricity. Wind turbines are mounted on a tower to capture the most energy.

At 100 feet (30 meters) or more above ground, they can take advantage of faster and less turbulent wind.

Wind turbines can be used to produce electricity for a single home or building, or they can be connected to an electricity grid (shown here) for more widespread electricity distribution.

This aerial view of a wind power plant shows how a group of wind turbines can make electricity for the utility grid. The electricity is sent through transmission and distribution lines to homes, businesses, schools, and so on.

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Types of Wind Turbines

Modern wind turbines fall into two basic groups: the horizontal-axis variety, as shown in the photo, and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor.

Horizontal-axis wind turbines typically either have two or three blades. These three-bladed wind turbines are operated “upwind,” with the blades facing into the wind.
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Sizes of Wind Turbines

Utility-scale turbines range in size from 100 kilowatts to as large as several megawatts. Larger turbines are grouped together into wind farms, which provide bulk power to the electrical grid.

Single small turbines, below 100 kilowatts, are used for homes, telecommunications dishes, or water pumping. Small turbines are sometimes used in connection with diesel generators, batteries, and photovoltaic systems.

These systems are called hybrid wind systems and are typically used in remote, off-grid locations, where a connection to the utility grid is not available.

Many wind farms have sprung up in the Midwest in recent years, generating power for utilities. Farmers benefit by receiving land lease payments from wind energy project developers.

Many wind farms have sprung up in the Midwest in recent years, generating power for utilities. Farmers benefit by receiving land lease payments from wind energy project developers.

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GE Wind Energy's 3.6 megawatt wind turbine is one of the largest prototypes ever erected. Larger wind turbines are more efficient and cost effective.

GE Wind Energy's 3.6 megawatt wind turbine is one of the largest prototypes ever erected. Larger wind turbines are more efficient and cost effective.

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Inside the Wind Turbine

Inside the Wind Turbine

Inside the Wind Turbine

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Anemometer:
Measures the wind speed and transmits wind speed data to the controller.
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Blades:
Most turbines have either two or three blades. Wind blowing over the blades causes the blades to “lift” and rotate.
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Brake:
A disc brake, which can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies.
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Controller:
The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they might be damaged by the high winds.
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Gear box:
Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring “direct-drive” generators that operate at lower rotational speeds and don’t need gear boxes.
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Generator:
Usually an off-the-shelf induction generator that produces 60-cycle AC electricity.
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High-speed shaft:
Drives the generator.
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Low-speed shaft:
The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.
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Nacelle:
The nacelle sits atop the tower and contains the gear box, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on.
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Pitch:
Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity.
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Rotor:
The blades and the hub together are called the rotor.
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Tower:
Towers are made from tubular steel (shown here), concrete, or steel lattice. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity.
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Wind direction:
This is an “upwind” turbine, so-called because it operates facing into the wind. Other turbines are designed to run “downwind,” facing away from the wind.
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Wind vane:
Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.
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Yaw drive:
Upwind turbines face into the wind; the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don’t require a yaw drive, the wind blows the rotor downwind.
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Yaw motor:
Powers the yaw drive.
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SOURCE: U.S. Department Of Energy | How Wind Turbines Work

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ePlusMenuCAD 9 - New Polished Version

ePlusMenuCAD 9 - Advanced Electrical Design Tool

ePlusMenuCAD 9 is finally released! Since version 3, ePlusMenuCAD has been changed and improved a lot, and now, version 9 is fine polished and most complete version so far.

There are many improvements and some new things that will be very usefull to designers.

Many electrical designers use AutoCAD platform in their daily work. ePlusMenuCAD is an integrated tool within AutoCAD which contains almost every aspect of electrical design.

Few Words About ePlusMenuCAD 9

ePlusMenuCAD is a software tool for professonal electrical design in AutoCAD environment. If you do electrical design using AutoCAD, then you certainly know how much time you lose on inserting varius blocks of luminaires, sockets, panels, generating technical specifications, drawing single line diagrams, etc.

If you use your own blocks in AutoCAD which are placed somewhere on your HDD and insert them when needed in the drawing, and after that manually copy each time  – then ePlusMenuCAD is for you. All the symbols are placed in one place, available from the drop-down menu , 26 toolbars, and also from intuitive shorcuts from command line. No more boring inserting and copyng blocks! ePlusMenuCAD offers efficency and high speed in generating technical specifications for Bill of Quantities, as well as automation in inserting electrical symbols into drawing.

In ePlusMenuCAD there are two modules integrated: Mosaic Design and X-functions. Mosaic Design is advanced tool for creating single line diagrams and application diagrams. Large database of (universal) symbols covers almost any kind of scheme. Insertion of symbols and feeders, and generation of BOM is completely automated and very easy for use in drawing. Second module X-functions, has more than 50 extra usefull functions (commands) that saves a lot of time durin daily work in AutoCAD. Working with layers, blocks, polylines  etc. is much much easier .

ePlusMenuCAD can be translated in to two languages English and Serbian/Croatian.

Example

Example of using ePlusMenuCAD in project Hotel Splendid in Budva (Montenegro), where it was used for designing lighting, power distribution,  technology, installations of sockets and single line diagrams.
ePlusMEnuCAD - Hotel Splendid - energetski razvodePlusMenuCAD - Hotel Splendid - tehnička specifikacijaePlusMenuCAD - Hotel Splendid - osvetljenjeePlusMenuCAD - Hotel Splendid - tehnologijaePlusMenuCAD - Hotel Splendid - utičnice

AutoCAD support

AutoCAD versions 2006, 2007, 2008, 2009 and 2010 are fully supported, and ePlusMenuCAD can be installed and used simultany on this versions. That means that ones ePlusMenuCAD is installed, you can use it in all (supported) installed AutoCAD versions.

Conception
Drop-down menu (click to enlarge)

ePlusMenuCAD drop-down menu (click to enlarge)

More than 1200 electrical symbols are placed in its categories (outlets, luminaires, types of installation, DEA, Cable verticals, labels of cables, transformers, cable feeds, TKS, EIB KONNEX ..). Every category has its layer. Layers carry the prefix “EnJS_” and “EnTS_” so that can be easily sorted in Layer Manager in the AutoCAD.

Also, there are a lot of various types of luminaires from metal-halid throug incadescent sorted by category and with predict shortcuts from the command line. Lamps that are designed to be supplied rom Diesel Agregate DEA, have cross symbol, and as such are also located in generated technical specification. Almost every area in which there are elements is covered with IEC symbols.

Drop-down menu is well organized, all symbols and functions are divided into categories, the most important are shown below:

Electrical distribution of power
• Predefined types of power supply lines (network, aggregate, UPS, diesel supply…)
• Power transformers – dry type and distribution oil transformers (with and without conservator) typical powers 630, 1000, 1600, 2000, 2500 and 3200kVA
• Distribution boards and panels, panels supplied from diesel agregate, and all with labels
• Cable or busbar vertical runs with their labels of incoming or outgoing connections
• Predefined labels for the cables in the colors (to distinguish cables of differnt type and supply…)

Installation of power sockets
• Power sockets 2P and 3P in the IEC variations and variations GOST standards (Russian standard)
• TV, antenna, computer plug and terminal space in the floor, fan-coil connection…
• Thermostats, rails for the main and additional equipotential deuce …
• Cable feeds for direct consumers, in wall and ceiling (2P, 3P), luminaires

Power and distribution transformers 10-20/0, 4kV
• Dry type transformer, powers: 630kVA – 3200kVA
• Oil type transformers, powers: 630kVA – 2500kVA with and without conservator

Installation of earthing
• Vertical runs of FeZn earthing bar (predefined in various colors)
• Tables for power sockets, cable feeds, lamps, and elements of Earthing with predefined default values

Legends
• Legends for the power sockets, luminaires, electric distribution and cables (2p and 3p)
• Stamp basis (the ability to post the logo of your company)
• Unique symbol of current round ECM
• The automatic marking ECM and (increasing, decreasing or all of the same series)

Installation of lighting
• Fluoroscent lamps built-in and built-on, powers from 1x18W to 4x36W with DEA symbols
• Fluroscent tubes, powers from 1x18W to 2x36W
• Fluo-compact lamps built-in and built-on, powers from 1x9W to 2x36W with DEA symbols
• Incadescent lamps, built-in and built-on, powers from 40W to 100W with DEA symbols
• Incadescent-reflect lamps, built-in and built-on, with DEA symbols
• Halogen lamps built-in and built-on, powers from 20W to 1000W with DEA symbols
• Metal-halid lamps built-in and built-on, powers from 70W do 2000W with DEA symbols
• Reflectors
• Crystal chandeliers for the salons and kitchen
• Decorative lamps for billiard tables, halls, theaters…
• Lamps for outdoor lighting (pillars, underwater lamps …)
• Anti-panic lamp
• Sensors and feeds the optical cable …
• Installation switches, 2p, 3p, alternate, serial …
• Dimers, tasters…

Telecommunications and signal systems
• predefined types of installations (fire, access control, anti-burglary, structural wiring…)
• Anti-burglary
• Anti-fire
• Access control
• Video surveillance – CCTV
• TV and Radio
• Phone and intercom
• Clock
• Gas
• Audio-Video Systems
• Speakers
• Power supply
• Wireless transfer of information

EIB KONNEX
• Instabus elements (system, input / output, lighting, heating / cooling, display, infrared …)

What can be designed with ePlusMenuCAD?
Electric systems up to 1000V
Designing transformer substations 20/10/6/0,4 kV
Installations of power sockets (+ IEC symbols)
Installations of power distribution (+ cable labels)
Single line diagrams and application schemes of switchgears 0,4 kV
Reserv power supply (Diesel agregate, UPS system)
External cable distribution 20/10/6/0, 4 kV
Installation of interior lighting (general, technology and decorative)
Installation of external lighting (lighting roads, promenades, courts …)
Installation of lighting open trade centers and parking space
Installation of decorative lighting for public facilities and open sports facilities
Lightning protection
Earthing system
Technological installations
Telecommunications and signaling systems
Telephone system and installations
Intercom system and installations
Systems and installations for reception and distribution R / TV
Speaker systems
Anti-fire system and installations
Anti-burglary system and installations
Access control system
Hotel management system
Clock system
Conference system
Gas detection system
Wireless information transfer system

Highlights:

Mosaic DesignMOSAIC DESIGN: ePlusMenuCAD is fully capable to draw single line diagrams and application schemes using built-in modul Mosaic Design. Main feature is the fact that all pages of scheme are in one DWG drawing, and that user can create complete distributive or motor feeders in a minute, just by picking on one of the many predefined feeders.User can also plot one or  one hunderd and one scheme just with one click. All elements and feeders are intuitive sorted in iNteLLi Elements, with options of zoom preview of each element or feeder. Mosaic Design runs when you open one of it’s templates from default folder (new drawing). There are several offered templates that are copied during installation of ePlusMenuCAD in default AutoCADs template folder. Now, all you have to do is to choose one  template and Mosac Design module will be automatically loaded, and you can use any command from the menu or a toolbar. You can also simply change existing template and save it as your own template .

Scale FactorSCALE FACTOR: All symbols (blocks) in ePlusMenuCAD are defined by ScaleFactor. This is the scale of symbols with respect to the drawing. Default value is 1, but it can be changed at any time to any positive value. Symbols of electric current mark ECM and tables of power sockets and luminaires have scale factor SFecm, and symbol of junction box has its scale factor SFjb. In this way, you can intelligently control the scale of symbols in the drawing. Scale can therefore  be changed very easy. If you don’t want to think about the Scale Factor, then set the Master SF to some value that applies to all drawings.

InfoIt ProInfoIt PRO: Is a function to be used for generating Bill of Quantaties as well as for getting a lot of information about the symbols in the drawing. What can you do with InfoIt? You can take out a detailed technical specification from DWG drawing, calculate installed single-phase and three-phase el. powers of sockets and cable feeds from their tables, export report to MS Word, take out a list of all non-ePlusMenuCAD blocks, take out all luminaires by tags. InfoIt PRO is integrated part of ePlusMenuCAD. InfoIt Database is a unique datsbase of blocks that are within ePlusMenuCAD, and it offers the possibility to add your own symbol definitions – your own blocks . It is also possible to edit descriptions of all blocks in the InfoIt Database.

iNaLL Professional 6iNaLL Professional 6: A unique tool for every-day work in AutoCAD. It can make changes in the content of text objects TEXT, MTEXT, ATTRIBUTE, BLOCK, DIM. Inall PRO can store any text that you select into internal memory, so you can use it later in some other drawing. It has the support of the Serbian latin letters ČĆĐŠŽ, as well as all Russian letters, which can be used with any font. You can also import content of any text file into iNaLL PROs memory and use it for pasting in text  objects in drawing.

Download link: Here

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Power transformer

Power transformer

In a real transformer, some power is dissipated in the form of heat. A portion of these power losses occur in the conductor windings due to electrical resistance and are referred to as copper losses. However, so-called iron losses from the transformer core are also important. The latter result from the rapid change of direction of the magnetic field, which means that the microscopic iron particles must continually realign themselves technically, their magnetic moment—in the direction of the field (or flux). Just as with the flow of charge, this realignment encounters friction on the microscopic level and therefore dissipates energy, which becomes tangible as heating of the material.
Taking account of both iron and copper losses, the efficiency (or ratio of electrical power out to electrical power in) of real transformers can be in the high 90% range. Still, even a small percentage of losses in a large transformer corresponds to a sig- nificant amount of heat that must be dealt with. In the case of small transformers inside typical household adaptors for low-voltage d.c. appliances, we know that they are warm to the touch. Yet they transfer such small quantities of power that the heat is easily dissipated into the ambient air . By contrast, suppose a 10MVA transformer at a distribution substation operates at an efficiency of 99%: A 1% loss here corresponds to a staggering 100 kW.
In general, smaller transformers like those on distribution poles are passively cooled by simply radiating heat away to their surroundings, sometimes assisted by radiator vanes that maximize the available surface area for removing the heat.

Large transformers like those at substations or power plants require the heat to be removed from the core and windings by active cooling, generally through circulat- ing oil that simultaneously functions as an electrical insulator.

The capacity limit of a transformer is dictated by the rate of heat dissipation. Thus, as is true for power lines, the ability to load a transformer depends in part on ambient conditions including temperature, wind, and rain. For example, if a transformer appears to be reaching its thermal limit on a hot day, one way to salvage the situation is to hose down its exterior with cold water—a procedure that is not “by the book,” but has been reported to work in emergencies. When transformers are operated near their capacity limit, the key variable to monitor is the internal or oil temperature. This task is complicated by the problem that the temperature may not be uniform throughout the inside of the transformer, and damage can be done by just a local hot spot. Under extreme heat, the oil can break down, sustain an electric arc, or even burn, and a transformer may explode.
A cooling and insulating fluid for transformers has to meet criteria similar to those for other high-voltage equipment, such as circuit breakers and capacitors: it must conduct heat but not electricity; it must not be chemically reactive; and it must not be easily ionized, which would allow arcs to form. Mineral oil meets these criteria fairly well, since the long, nonpolar molecules do not readily break apart under an electric field.

Another class of compounds that performs very well and has been in widespread use for transformers and other equipment is polychlorinated biphenyls, commonly known as PCBs. Because PCBs and the dioxins that contaminate them were found to be carcinogenic and ecologically toxic and persistent, they are no longer manufactured in the United States; the installation of new PCB-containing utility equipment has been banned since 1977.11 However, much of the extant hardware predates this phase-out and is therefore subject to careful maintenance and disposal procedures (somewhat analogous to asbestos in buildings).

Introduced in the 1960s, sulfur hexafluoride (SF6) is another very effective arc-extinguishing fluid for high-voltage equipment. SF6 has the advantage of being reasonably nontoxic as well as chemically inert, and it has a superior ability to with- stand electric fields without ionizing. While the size of transformers and capacitors is constrained by other factors, circuit breakers can be made much smaller with SF6 than traditional oil-filled breakers. However, it turns out that SF6 absorbs thermal infrared radiation and thus acts as a greenhouse gas when it escapes into the atmos- phere; it is included among regulated substances in the Kyoto Protocol on global climate change. SF6 in the atmosphere also appears to form another compound by the name of trifluoromethyl sulfur pentafluoride (SF5CF3), an even more potent greenhouse gas whose atmospheric concentration is rapidly increasing.

COOLING EQUIPMENT
Transformer fan

Transformer fan

Heat from core losses and copper losses must be dissipated to the environment. In dry type transformers, cooling is accomplished simply by circulating air around and through the coil and core assembly, either by natural convection or by forced air flow from fans. This cooling method is usually limited to low-voltage indoor transformers (5 kV and below) having a three-phase rating below 1500 KVA. At higher voltages, oil is required to insulate the windings, which prevents the use of air for cooling the core and coils directly. At higher KVA ratings, the losses are just too high for direct air cooling to be effective. In outdoor environments, direct air cooling would introduce unacceptable amounts of dirt and moisture into the windings.
Transformers come in various cooling classes, as defined by the industry standards. In recent years, there have been attempts to align the designa- tions that apply to transformers manufactured in North America with the IEC cooling-class designations. Table below gives the IEC designations and the earlier designations that are used in this book. All of the IEC designations use four letters. In some respects, the IEC designations are more descriptive than the North American designations because IEC makes a distinction between forced-oil/air cooled (OFAF) and directed-flow-air cooled (ODAF). Some people find using the four-letter designations somewhat awkward, and this book uses the earlier designations throughout.
In small oil-filled distribution transformers, the surface of the tank is sufficient for transferring heat from the oil to the air. Ribs are added to the tanks of some distribution transformers to increase the surface area of the tank and to improve heat transfer. Large distribution transformers and small power transformers generally require radiator banks to provide cooling. Regardless of whether the tank surface, ribs, or radiators are used, transformers that trans-fer heat from oil to air through natural convection are all cooling class OA transformers.

Radiators used on OA transformers generally have round cooling tubes or flat fins with large cross section areas in order to allow oil to flow by natural convection with minimal resistance. Hot oil from the core and coils rises to the top of the tank above the inlet to the radiator. Cool oil from the radiator sinks to the bottom of the radiator through the outlet and into the bottom of the core and coils. This process is called thermo-siphoning and the oil velocity is relatively slow throughout the transformer and radiators. For this reason, OA transformers have relatively large temperature gradients between the bot- tom oil and the top oil, and relatively large temperature gradients between the winding temperatures and the top oil temperature. Likewise, the air circulates through the radiator through natural convection, or is aided by the wind.

Designations and descriptions of the cooling classes used in power transformers
Previous designationIEC designationDescription
.OA
.ONAN
Oil-air cooled (self-cooled)
.FA
.ONAF
Forced-air cooled
.OA/FA/FA
.ONAN/ONAF/ONAF
Oil-air cooled (self-cooled), followed by two stages of forced-air cooling (fans)
.OA/FA/FOA.ONAN/ONAF/OFAFOil-air cooled (self-cooled), followed by one stage of forced-air cooling (fans), followed by 1 stage of forced oil (oil pumps)
.OA/FOA.ONAF/ODAF
Oil-air cooled (self-cooled), followed by one stage of directed oil flow pumps (with fans)
. OA/FOA/FOA.ONAF/ODAF/ODAFOil-air cooled (self-cooled), followed by two stages of directed oil flow pumps (with fans)
.FOA
.OFAF
Forced oil/air cooled (with fans) rating only—no self-cooled rating
.FOW
.OFWF
Forced oil / water cooled rating only (oil / water heat exchanger with oil and wa- ter pumps)—no self-cooled rating
.FOA .ODAF
Forced oil / air cooled rating    only    with    di- rected oil flow pumps and fans—no self-cooled rating
.FOW .ODWF
Forced oil / water cooled rating only (oil / water heat exchanger with directed oil flow pumps and water pumps)— no self-cooled rating

As the transformer losses increase, the number and size of the radiators that are required to cool the oil must increase. Eventually, a point is reached where wind and natural convection are not adequate to remove the heat and air must be forced through the radiators by motor-driven fans. Transformers that have forced air cooling are cooling class FA transformers. FA transform- ers require auxiliary power to run the fan motors, however, and one of the advantages of OA transformers is that they require no auxiliary power for cooling equipment. Since additional cooling is not usually needed until the transformer is heavily loaded, the fans on most FA transformers are turned off until temperatures exceed some threshold value, so under light load the transformer is cooled by natural convection only. These transformers are cool- ing class OA/FA transformers.

Some transformers are cooled by natural convection below temperature T1, turn on one stage of fans at a higher temperature T2 and turn on a second stage of fans at an even higher temperature T3. These transformers are cooling class OA/FA/FA transformers. The direction of air flow in forced-air units is either horizontally outward or vertically upward. The vertical flow pattern has the advantage of being in the same direction as the natural air convection, so the two air flows will reinforce each other.

Although the cooling capacity is greatly increased by the use of forced air, increasing the loading to take advantage of the increased capacity will increase the temperature gradients within the transformer. A point is reached where the internal temperature gradients limit the ability to increase load any further. The solution is to increase the oil velocity by pumping oil as well as forcing air through the radiators. The usual pump placement is at the bottom of the radiators, forcing oil from the radiator outlets into the bottom of he transformer tank in the same direction as natural circulation but at a much higher velocity. Such transformers are cooling class FOA transformers. By directing the flow of oil within the transformer windings, greater cooling effi- ciency can be achieved. In recognition of this fact, the calculation of hot-spot temperatures is modified slightly for directed-flow cooling class transformers.

As in forced-air designs, forced-oil cooling can be combined with OA cooling (OA/FOA) or in two stages (OA/FOA/FOA). A transformer having a stage of fans and a stage of oil pumps that are switched on at different temperatures would be a cooling class OA/FA/FOA transformer.
The radiator design on FOA transformers can differ substantially with the radiator design on FA transformers. Since the oil is pumped under consid- erable pressure, the resistance to oil flow is of secondary importance so the radiator tubes can be designed to maximize surface area at the expense of cross section area. FOA radiators are sometimes called coolers instead, and tend to resemble automotive radiators with very narrow spaces between the cooling tubes and flat fins in the spaces between the cooling tubes to provide additional surface area. The comparison of the two types is illustrated in picture left (OA/FA type) and right (FOA type).

OA/FA radiator construction

OA/FA radiator construction. The large radiator tubes minimize restric- tion of oil flow under natural convection. The fan is shown mounted at the bottom with air flow directed upward.

FOA cooler construction

FOA cooler construction. The oil is forced through narrow tubes from top to bottom by means of oil pumps. The cooling fans direct air horizontally outward.

Cooling equipment requires maintenance in order to run efficiently and provide for a long transformer life. There is the obvious need to main- tain the fans, pumps, and electrical supply equipment. The oil coolers them- selves must be kept clean as well, especially FOA-type coolers. Many transformers have overheated under moderate loads because the cooling fins were clogged with insect and bird nests, dust, pollen, and other debris. For generator step-up transformers, where the load is nearly at nameplate rating continuously, steam-cleaning the coolers once every year is a good mainte- nance practice.

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