Fault Level=fault MVA/P.U reactance
If P.U reactance increases,then fault level decreases.
P.U reactance=Actual reactance/Base reactance
Shunt and series capacitors improve the voltage profile of the transmission line. at the point of compensation. Hence the line reactance does decrease; but for the same ATC of lines and fault level rating of the associated station, the fault current can reduce, especially at higher frequencies typical of faults. However rating and location of capacitors are critical.
With Series Capacitors introduced in the Line Z + R + j( XL -XC ) and the Transfer Reactance between S.E. and R.E. decreases increasing the Power Transfer Capability and Reducing the Power Angle,thus increasing Steady State Stability and to a somewhat increase in the difference between the initial and Critical Clearing Angle under Faults, if XC < XL, in the case of Transient Stability, If the Two Reactanes are Equal, under Fault,the Fault Level will be very high due to the Very Small Resistance of the Line and Fault Resistance being practically nil for Symmetrical Faults.If there are Parallel Lines and only one Lines is Compensated and the Fault occurs on that Line there is a Possibility of increased Transient Stability since some Power can be Transferred through the other Health Line under Fault.
TRANSIENT RECOVERY VOLTAGE
The installation of a series capacitor in a transmission line can have a significant effect on the amplitude of the transient voltage which appears when a line circuit breaker opens to clear a fault [5]. The voltage , which affects the breaking performance of the line circuit breaker, is referred to as the Transient Recovery Voltage (TRV). Increased TRV is caused by the trapped voltage over the series capacitors when the line breaker is opened. Avoiding this is enabled by means of the FPD, allowing very fast by-passing of the series capacitors in conjunction with line breaker opening in the transmission lines carrying the series capacitors. This means the impact of trapped charges in the series capacitors is eliminated, and does not add to the TRV stressing of the circuit breakers.
Extensive PSCAD simulations were performed for different fault types, locations, point-of-wave inceptions and by-pass delays. It is concluded that as long as the series capacitor can be fully by-passed 2 ms prior to the first pole opening of the terminal breaker, TRV can be managed to an acceptable level. Tele-by-passing via line protection was decided to be the most reliable mechanism to achieve the
required margin.
Refer Link:
http://www05.abb.com/global/scot/scot221.nsf/veritydisplay/c43e38616760760548257a28006ee3ce/$file/ieee%20epec%202012_london,%20on,%20canada.pdf
However use of Solid State Fault Current Limiters can give both the benefits of Greater Power Transfer and Limitation of Fault Currents,wherein an Inductance is Immediately introduced and Capacitor disconnected immediately after the Fault Occurs
Solid-state fault current limiter
The SSFCL mainly consists of the capacitor bank C1, reactor L1 and GTO (gate-turn-off) or fast-closing switch SW1. It operates as follows: normally, the capacitor C1 and the reactor L1 combine to give a ‘zero’impedance. Thus, the ‘zero’ impedance is presented within the circuit. When a fault occurs, SW1 bypasses the capacitor C1 at a high speed within 3ms [21], and the reactor L1 is immediately inserted into the network working as the fault current limiter.
The low impedance Z1 limits the inrush current through SW1. The over voltage protection device ZnO (zinc-oxide arrester), and the bypass switch BPS, which backs up the switch SW1 are also connected in parallel to the capacitors C1. The low impedance Z2 restrains the inrush current when BPS closes .
However, the SSFCL is still not widely used in practice due to its high cost, low reliability, and complicated auxiliary system..
Refer Link:
http://www.tyndall.ac.uk/sites/default/files/wp25.pdf
P.S.
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