Voltage Control and Reactive Power Management

Why it matters

Voltage control and reactive power management are crucial for maintaining the stability and efficiency of power systems. Proper management ensures that electrical equipment operates within safe voltage limits, reducing losses and improving the reliability of power supply.

Key ideas

• Voltage Control: The process of maintaining the voltage levels within prescribed limits across the power system. This is essential to ensure the proper functioning of electrical devices and to minimize power losses.
• Reactive Power: Reactive power is the component of electrical power that does not perform any work but is necessary for maintaining the voltage levels required for active power to do useful work. It is measured in VAR (Volt-Ampere Reactive).
• Sources of Reactive Power: Capacitors, inductors, synchronous condensers, and FACTS devices (Flexible AC Transmission Systems) are commonly used to manage reactive power.
• Methods of Voltage Control:
  • Tap Changing Transformers: Adjust the voltage level by changing the transformer tap settings.
  • Shunt Capacitors and Reactors: Used to inject or absorb reactive power, respectively.
  • Synchronous Condensers: Synchronous machines that can generate or absorb reactive power.
  • FACTS Devices: Advanced electronic devices that provide dynamic voltage control.

Formulas

For sinusoidal single-phase quantities, Q = Vrms Irms sinφ; balanced three-phase total Q = √3 VL IL sinφ. Positive Q denotes reactive absorption for lagging current under the load convention. For a short line, per-phase phasors satisfy Vs = Vr + IZ. Treating this as addition of voltage magnitudes is generally invalid. For fixed active load P corrected from lagging power factor cosφ1 to cosφ2, required capacitive compensation magnitude is Qc = P(tanφ1 − tanφ2).

Worked example

A balanced three-phase load takes 100 kW at power factor 0.8 lagging. Improve it to 0.95 lagging without changing active power. Initial Q = 100 tan(arccos0.8) = 75 kvar. Final Q = 100 tan(arccos0.95) = 32.87 kvar. Required capacitor-bank rating at operating voltage is 75 − 32.87 = 42.13 kvar. At 400 V line-to-line, current falls from 100000/(√3 × 400 × 0.8) = 180.4 A to 151.9 A. This ideal calculation neglects capacitor losses and harmonics; a real installation requires appropriate ratings and resonance assessment.

Common mistakes

• Confusing reactive power with active power.
• Incorrectly calculating phase angles, leading to errors in reactive power calculations.
• Neglecting the effects of line impedance on voltage drop.

For GATE EE

• Questions often involve calculating reactive power, voltage drops, and the use of devices for voltage control.
• Practice problems on the application of tap changing transformers and FACTS devices.

Quick check

1. What is the unit of reactive power?
2. Name two devices used for voltage control.
3. What is the role of a synchronous condenser?

Answers: 1. VAR, 2. Tap changing transformers, shunt capacitors, 3. To generate or absorb reactive power.