Reactive power and voltage balance

Bus voltage magnitude in a power system is governed by the reactive power balance: Q supplied at or near a bus must equal Q consumed by loads and line reactances. A deficit lowers the voltage, a surplus raises it.

Why Q controls V

For a line of reactance $X$ with $X \gg R$:

$$\Delta V = V_1 - V_2 \approx \frac{Q_{12}X}{V_1}$$

So the voltage drop depends mainly on the reactive power carried. Q flows from the higher-voltage bus to the lower. Since $I^2X$ losses are large, Q cannot be sent over long distances; it must be balanced locally.

Sources and sinks of Q

• Sources: over-excited generators, shunt capacitors, line charging (lightly loaded lines), synchronous condensers.
• Sinks: induction motors, transformers and line reactances at heavy load, shunt reactors, under-excited generators.

How balance is maintained

1. AVR on generators: compares terminal voltage with the set value and changes field current; more excitation → more Q output → voltage rises.
2. Shunt capacitors switched in at heavy load; shunt reactors switched in at light load.
3. Synchronous condensers and SVC/STATCOM: continuous, fast Q adjustment.
4. On-load tap changers on transformers adjust the voltage ratio.
5. Series capacitors reduce net line reactance and hence the reactive drop.

Voltage is normally held within about ±5% of nominal; if Q support is not enough, voltage may fall progressively (voltage collapse).