Question

Draw the electrical diagram of a VSC-STATCOM and explain how it operates. (Keep your answer within 100 words maximum)

Answer 1

Working Principle of STATCOM:

To understand the working principle of STATCOM, we will first have a look at the reactive power transfer equation. Let us consider two sources V1 and V2 are connected through an impedance Z = Ra + jX as shown in figure below.

In the above reactive power flow equation, angle δ is the angle between V1 and V2. Thus if we maintain angle δ = 0 then Reactive power flow will become

Q = (V2/X)[V1-V2]

and active power flow will become

P = V1V2Sinδ / X =0

To summarize, we can say that if the angle between V1 and V2 is zero, the flow of active power becomes zero and the flow of reactive power depends on (V1 – V2). Thus for flow of reactive power there are two possibilities.

1)    If the magnitude of V1 is more than V2, then reactive power will flow from source V1 to V2.

2)    If the magnitude of V2 is more than V1, reactive power will flow from source V2 to V1.

This principle is used in STATCOM for reactive power control. Now we will discuss about the design of STATCOM for better correlation of working principle and design.

A simplified diagram along with equivalent electrical circuit of STATCOM is shown in figure below.

Now we will understand how the STATCOM works though we have discussed the basic operating principle of STATCOM. As can be seen from the figure above, source V1 represents the output voltage of the STATCOM. In case of reactive power demand increases in the power system, STATCOM increases its output voltage V1 while maintain the phase difference between V1 and V2 to zero (it shall be noted here that there will always exists small phase angle between V1 and V2 to cater for the leakage impedance drop in the interconnecting Transformer ). As V1 > V2, reactive power will flow from STATCOM to the power system. Thus STATCOM, supplies reactive power and acts as reactive power generator.

Again, if the voltage of power system increase due to load throw off, STATCOM will reduce its output voltage V1 and therefore will absorb reactive power to stabilize the voltage to normal value.

The above mode of operation of STATCOM is called Voltage Regulation Mode.

But as we know every equipment has got their own limitations, so STATCOM must also have some limitation of supplying or absorbing reactive power. Yes, there exists a limitation and this limitation is imposed by the current carrying capacity of force commutated devices like IGBT, GTO etc. Therefore, if the operation of STATCOM reaches their limitation, it does not further increases or decreases its output voltage V1 rather it supplies or absorbs fixed reactive power equal to its limiting value at a fixed voltage and current and acts like constant current source. This mode of operation of STATCOM is called VAR Control Mode.