In case if any system has to operate close to the stability limits in steady state, That is with minimum margins then the fast control of reactive power is essential.since some last years, thyristor controlled static var compensator is used for the fast reactive power control.
Basic static var compensator schemes :-
A reactor and a thyristor valve are incorporated in each and every single phase branch.in this system Power is changed by controlling the current through the reactor via thyristor valve. On any state interval is controlled by the delaying triggering of the thyristor valve relative to the natural zero current crossing through it. Any thyristor controlled reactor TCR is used in the combination with a fixed capacitor that is FC when reactive power generation or alternatively, absorption and generation is needed .This is often optimum solution for sub transmission and distribution.
TCR or FCs are characterized by
- Continuous control
- No transients
- Compact design
- Thyristor switched capacitor, TSC
Shunt capacitor bank is divided into appropriate number of branches and Each branch is individually switched on or off via thyristor valve then The Switching takes place and voltage across thyristor valve Which is zero, making this virtually transient-free. Disconnection can be effected by the suppressing firing pulses to the thyristors that will be blocked when the current reaches zero.
TSCs are characterized by:-
- Stepped control
- No transients
- No harmonics
- Low losses
- Redundancy and flexibility
Thyristor controlled reactor and Thyristor switched capacitor TCR or TSC :
The combined TCR and TSC is optimum solution in many of the cases. With a TCR / TSC compensator, continuously variable the reactive power is obtained across entire control range and full control of both an inductive and the capacitive parts of the compensator. the main Principal benefit of this is the optimum performance during major disturbances in power system like line faults and load rejections.
TCR / TSC combinations are characterized by these :
- No transients
- Low losses
- Flexible control and operation.
- Benefits of static var compensator to the power transmission:
- Stabilized voltages in weak systems
- Reduced transmission losses
- Transmission capacity Increased , to reduce, defer or eliminate need for new lines
- Higher transient stability limit
- Damping can be Increased of the minor disturbances
- voltage control and the stability Greater
- Power oscillation damping benefits of the static var compensator to power distribution.
- Stabilization of the voltage at receiving end of long lines.
- Increased productivity as the stabilized voltage which means better utilized capacity
- Reduced reactive power consumption can give lower losses and improved tariffs
- Asymmetrical load Balanced can reduce system losses and it enables lower stresses in the rotating machinery
- Enables better use of equipment (particularly transformers and cables)
- It Reduced voltage fluctuations and the light flicker
- Decreased harmonic distortion .
Q1: Explain the benefits in Railways of static var compensator ?
Ans: The increase in the traffic on existing tracks combined with the new rail projects of high speed mean rail traction is fast becoming an important load on electrical supply grids .In turn This is focusing a lots of attention on the efficiency of the centenary as well as the power quality of the surrounding grids. Trains taking power from centenary need to be sure supply voltages are stable and it do not sag.
Q2: What are the types of static var compensator ?
Ans: Three basic types of static var compensator
1. Variable impedance type
2. Current source type
3. Voltage source type
Q3: What are the static var compensator Control Characteristics?
Ans:-The high voltage is measured at the high voltage side of the transformer feeding the static var compensator and the current is the reactive current, considered to be positive when static var compensator is inductive (absorbing reactive power).
• In the control range, the static var compensator voltage is not maintained constant. The slope of the control characteristics is positive that is from 3 to 5% and helps in
(a) stable parallel operation of more than one static var compensator connected at the same or neighboring buses and
(b) prevent static var compensator hitting the limits too frequently.
Q4: What are the uses of static var compensator ?
Ans: static var compensator are used for
1. Increasing power transfer in long lines
2. Stability improvement (both steady state and transient) with fast acting voltage regulation
3. Damping of low frequency oscillations (corresponding to electromechanical modes)
4. Damping of sub synchronous frequency oscillations (due to torsional modes)
5. Control of dynamic over voltages.
Q5: What is the full form of SR ?
Ans: Saturated Reactor.
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