1. Passive Shunt Compensation
Presented by,
Sudarshan B S
Asst. Professor
Dept. of EEE
RVCE, Bangalore

2. Introduction Objectives of passive shunt compensation
to provide reactive power compensation for linear AC loads for improving the voltage profile (even for zero voltage regulation or power factor correction) at the AC mains in single-phase and three-phase circuits
to do so using lossless passive elements such as capacitors and inductors.
Nowadays passive shunt compensators are also used in distributed, stand-alone, and renewable power generating systems.
Consider the two-machine system shown here.

3. Passive Compensators
Shunt capacitors and/or reactors could be located at any of the buses in the system.
For example, shunt capacitors may be connected to prevent low voltage during peak load conditions.
During light load conditions, shunt reactors may be connected to cancel part of the capacitive reactive power of the line to prevent unduly high voltages.
The presence of capacitors and reactors also influences the dynamics of system following a disturbance.
The compensators are not switched (ON/OFF) during disturbances, hence their effects follows from the fact that they modify the network parameters, in particular, its surge impedance, its electrical length, and the driving point impedances at the system buses.

4. Passive Compensators – TRANSIENT PERIOD
The relationship between reactive power and voltage can be represented graphically. Such a representation is called the reactive load line.
The thevenin equivalent of the two-machine system as viewed from the mid- system bus is shown here along with the system load line.
To analyse the transient period, let us consider the case where a line outage occurs. Because of this disturbance, the load line transition occurs.

5. Passive Compensators – TRANSIENT PERIOD
If no reactor or capacitor was connected at bus m, then the load line transition occurs as shown in figure here.
The open circuit voltage Vm decreases from E1 to E2, with Xs1 > Xs2 where the pre-fault slope is Xs1 and the post-fault slope is Xs2.
Transition from
E1 to E2
with Xs1 > Xs2

6. Passive Compensators – TRANSIENT PERIOD – Presence of Shunt Capacitor
With the presence of shunt capacitor Xcy, the voltage change across the capacitor Vc1 → Vc2 is less than the voltage change E1 → E2 without capacitor. This is shown in figure (next slide).

7. Passive Compensators – TRANSIENT PERIOD – Presence of Shunt Capacitor
If the capacitor rating were doubled (Xcy/2), a voltage rise would take place instead of decrease. i.e. the voltage would rise from Vc1’ to Vc2’.

8. Passive Compensators – Effect of Shunt Capacitors
In case of fixed capacitors, their reactive contribution reduces with square of voltage during voltage depressions.
Therefore, it becomes highly uneconomical to depend completely on fixed capacitors for providing voltage support to improve transient stability.
In case load rejection occurs and a large bank of capacitors are used, there is a high possibility of overvoltages.
Therefore, the size of capacitor bank is also limited. Feasibility of switching them very often under such condition poses a big limitation.

9. Passive Compensators – TRANSIENT PERIOD – Presence of Shunt Reactor
If a shunt reactor with reactance Xly was connected at bus 1, then a line outage would result in voltage drop VL1 → VL2 being greater than E1 → E2 without the inductor.

10. Passive Compensators – Effect of Shunt Reactor
Fixed reactors tend to reduce the steady state voltage, particularly during peak load periods. They therefore reduce the steady-state power transfer capability of the line.
If during peak power periods we disconnect the reactor using conventional control & switchgear, then it will be difficult to rapidly reconnect them to suppress the overvoltages in case of sudden load rejections. Hence, their disconnection should be avoided.
However, one could use high-speed mechanical switches, which close in 2-3 cycles. In this case, the number of allowable operations per day will be very low (to keep the wear and tear on moving parts and contacts low).

11. Passive Compensators – FIRST SWING PERIOD
Shunt capacitors and reactors have a limited effect on the total change in voltage during first swing (unless they are switched on and off at required moments).
In order to be truly effective in reducing the large voltage dip during the first-swing period, shunt capacitors, which are not fully energized, would have to be switched on line during or immediately following the fault.
If sufficient capacitance is switched on at precisely the time the fault is removed, the dashed voltage curve in figure could result.
The minimum voltage experienced during first-swing is higher. This tends to increase the post-fault power and decelerates the generators which accelerated during the fault. This reduces the load angle excursions.

12. Passive Compensators – FIRST SWING PERIOD
Even with all these potential advantages, one major limitation has led to lower use of shunt capacitors and reactors.
Modern breakers and relays that trip during line outage operate within 3-7 cycles. On the other hand, conventional switching devices for capacitors operate in 6-30 cycles.
Such a slower switching would never be fast enough to impact the first swing voltage dip, as the capacitors/reactors need to be switched precisely at the time of occurrence of fault (3-7 cycles) for maximum impact.
Also, the capacitor bank should switch off immediately after the first swing to prevent sustained high voltages during subsequent oscillations.

13. Passive Compensators – OSCILLATORY PERIOD
As in the case of first swing period, fixed shunt compensation has only a limited influence on voltage, power, and machine angle swings during oscillatory period.
To be effective in damping the oscillations in voltage, power, and machine angles, shunt capacitors and reactors would have to be switched ON and OFF repeatedly, at precise times. Only then the transfer reactance between synchronous machines would effectively increase and decrease.

14. SUMMARY
Shunt capacitors and reactors do influence the post-disturbance voltage variations, and, to a limited extent, the stability of the generators in the system.
Fixed shunt capacitors and reactors merely bias the average value of the voltage up or down during the post-disturbance transition period.
If they are to correct for momentary overvoltages or voltage dips, they must be switched ON or OFF rapidly, repeatedly. This is not generally practical with conventional switches.