ECE4053 Chap.5 Voltage and Reactive Power Control
Voltage and Reactive Power Control
This chapter treats voltage as a local network variable linked strongly to reactive-power balance. A local reactive deficit tends to lower voltage, while an excess tends to raise it.
The direction rule is checked within the full AC network because line reactance, taps, neighbouring voltages and device limits determine sensitivity.
Generator excitation, capacitors, reactors, static compensators and tap changers provide different directions, speeds and durations of control. Primary local regulation sits inside wider secondary coordination and slower scheduling.
What this chapter covers
- 01
Active-power and reactive-power balance
- 02
Local Q-V sensitivity
- 03
Generator excitation and capability
- 04
Capacitor-bank injection
- 05
Reactor absorption
- 06
Static reactive compensators
- 07
Transformer tap control
- 08
Primary and coordinated voltage regulation
Worked example · free
Screen a local reactive deficit
- +1Identify the 18 MVAr added demand as a local reactive deficit.
- +1Switch the capacitor to supply 12 MVAr, leaving 6 MVAr.
- +1Request 6 MVAr from the generator, within its 10 MVAr spare capability.
- +1Check 12 + 6 = 18 MVAr and 4 MVAr generator reserve remains.
- +2State that AC power flow must verify the restored voltage and neighbouring buses.
Key terms
- Reactive Deficit
- A local condition in which reactive demand exceeds available supply, tending to depress voltage.
- Reactive Compensation
- Injection or absorption of reactive power to influence voltage, power factor or network loading.
- Excitation Control
- Adjustment of synchronous-machine field excitation to regulate terminal voltage and reactive output within capability.
- Shunt Capacitor
- A device that injects reactive power and supports voltage, with output dependent on voltage for fixed susceptance.
- Shunt Reactor
- A device that absorbs reactive power and helps control high voltage, especially in lightly loaded networks.
- Tap Changer
- A transformer mechanism that changes effective turns ratio to alter voltage levels and network reactive flows.
Voltage and Reactive Power Control FAQ
Why is voltage control described as local?
Reactive transfer produces voltage drops across network reactance, so distant Q support can be inefficient and may worsen loading. Nearby generators, shunts or compensators usually have stronger sensitivity at the affected bus, although the full network remains coupled.
What should be done for low voltage?
Check local reactive demand, generator and compensator limits, tap positions and transfer levels. Add or release reactive injection where feasible, then rerun AC power flow to verify the complete profile rather than promising an exact voltage from a local balance.
When is a reactor preferable to a capacitor?
A reactor absorbs reactive power and is useful for high voltage or excess-Q conditions, such as lightly loaded lines. A capacitor injects Q and would tend to worsen an already high-voltage condition if all other factors were unchanged.
Why can a PV bus become a PQ bus?
A generator holds its voltage target only while reactive output remains inside capability limits. At a binding Q limit, reactive output is fixed and voltage magnitude becomes an unknown, so the bus is represented as PQ for the next solution.
Exam move
Begin voltage problems with direction. Decide whether the bus is low or high and whether local reactive power appears deficient or excessive. State the injection convention so generator, capacitor, load and reactor signs are not mixed. Then identify network factors that can change sensitivity: line reactance, neighbouring voltage, tap ratio and device limits. Build a device table by action, speed, duration and boundary.
Generator excitation can supply or absorb Q within a capability curve. Capacitors inject in discrete or controlled steps and their output falls with voltage for fixed susceptance. Reactors absorb Q. Static compensators respond quickly within current limits. Tap changers redistribute voltage and reactive demand but do not create a new reactive source. Practise recommendations that include both benefit and second effect.
A capacitor can support low voltage but consume switching flexibility and provide less output as voltage falls. A reactor can control light-load high voltage but deepen a low-voltage event. A tap change may improve one side while shifting stress elsewhere. Generator support uses reserve that may be valuable after a contingency. Connect voltage control back to power flow.
When a PV generator reaches a reactive limit, convert it to PQ and resolve. After any device action, inspect the full voltage profile, reactive outputs, branch loading and remaining reserve. A single corrected bus is not enough if a neighbouring violation appears. Distinguish control layers.
Primary control is the immediate local automatic loop, secondary control coordinates regional setpoints and reserves, and slower scheduling positions taps and shunts for expected conditions. Name the measured variable and actuator whenever using these labels.
The captured folder includes detailed Week 9 source but not later teaching files, so keep revision within grounded local Q-V principles and device control rather than inventing uncaptured stability formulas. Close every answer with the AC network check needed to validate the recommendation, and state which reactive reserve remains afterward.
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