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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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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:
- Protection of transmission system.
- Controlling the Exchange of Energy.
- Ensure steady State & Transient stability.
- Load shedding and prevention of loss of synchronism. Maintaining the system frequency within targeted limits.
- Voltage Control; reducing the reactive power flow by compensation of reactive power, tap-changing.
- Securing the supply by proving adequate line capacity.
- Data transmission via power line carrier for the purpose of network monitoring; control and protection.
- Fault analysis and pin-pointing the cause and subsequent improvement in that area of field.
- Determining the energy transfer through transmission lines.
- Reliable supply by feeding the network at various points.
- Establishment of economic load distribution and several associated functions.
TYPES OF SUBSTATION
The substations can be classified in several ways including the following :
- Classification based on voltage levels, e.g. : A.C. Substation : EHV, HV, MV, LV; HVDC Substation.
- Classification based on Outdoor or Indoor : Outdor substation is under open skv. Indoor substation is inside a building.
- 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
- 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.
- 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
- Main Office Building
- Administrative building
- Conference room etc.
- 6/10/11/20/35 KV Switchgear, LV
- Indoor Switchgear
- Switchgear and Control Panel Building
- Low voltage a.c. Switchgear
- Control Panels, Protection Panels
- Battery Room and D.C. Distribution System
- D.C. Battery system and charging equipment
- D.C. distribution system
- 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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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).
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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Connecting wind turbines to the power grid
Precautions to be taken when connecting wind turbines to the power grid: The procedure for connecting wind turbines to an electric distribution network normally consists of 2 steps:
1. First, the HV/LV transformer is energized from the high voltage side,
2. Then, in the right wind conditions and further to wind turbine adjustment tests (initial pole test, pole test sequence, etc.), the turbine is connected to the power grid as follows:
- The rotation of the wind turbine’s blades triggers the aerogenerator (motorgenerator set), which acts as a generator,
- The transformer’s LV winding is energized by the wind turbine’s stator (connected by a star or delta connection) and hence provides electrical energy to the HV network.
However, during this 2-step process, the HV/LV transformer must not, in any event whatsoever, be supplied with high and low voltage currents at the same time. In such an event, there would be a risk of energizing the LV voltage side in opposite phase to the HV side.
The result would be an extremely strong current, the intensity of which would be greater than the brief, 3-phase short-circuit current stipulated in the contract (usually 2 seconds).
General diagram of a wind turbine installation
As the electrodynamic stress on the windings is proportional to the square of the current intensity (F = K.I2), the transformer can not, in general, withstand the extremely intense stress caused by a current greater than the contractual short-circuit current. This type of stress would automatically lead to significant, unacceptable and irreversible mechanical deformation of the LV and HV windings, and the LV connections: hence it would, in due course, totally destroy the transformer.
On-site transformer failures have occurred, as a result of energizing the LV and HV sides at the same time and failing to comply with the phase sequence of the LV network.
The LV winding was subjected to a current much stronger than the contractual 3-phase short-circuit current and, as a result, the transformer was completely destroyed by huge electrodynamic stress.
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Measures to apply in all circumstances…
Power HV/LV Transformer
Therefore, when connecting a wind turbine transformer to a power grid, it is absolutely essential not to energize the LV and HV sides of the transformer at the same time, which may cause the LV winding to be in opposite phase.
Hence, it is extremely important not to interfere with the various tripping sequences, and to comply with the adjustment specifications for the transformer in question.
If the transformer is energized from both sides and, in addition, the phase sequence of the LV network is not respected, the result will be total transformer failure.
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SOURCE: France Transfo
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