Checking and Troubleshooting a Reactive Compensation Circuit for the Automatic Voltage Regulator in an Isolated AC Bus

One of the common issues encountered in generator systems operating on an isolated AC bus is excessive circulating current between generators. This problem can lead to system inefficiencies, potential damage to components, and unexpected shutdowns. To address this, it is essential to ensure proper setup and functionality of the Automatic Voltage Regulator (AVR) reactive droop compensation circuit.

This guide provides a step-by-step approach to diagnosing and resolving issues related to reactive power compensation in a generator system.

The Current Transformer Unit - Parallel Switch. Paralleling Different Size Generators

A switch is installed on the secondary side of the current transformer to short-circuit both the CT and the burden resistor, preventing any signal from reaching the generator. When set to the “Unit” position, the switch allows the generator to operate independently of the parallel generating system, eliminating the effects of the droop circuit.

When a generator operates independently in a parallel droop system without the short-circuit switch set to the "Unit" position, an unwanted droop occurs in the generator's output voltage. The same effect happens in a crosscurrent system, though to a lesser extent, because other burden resistors in series function as a voltage divider, proportionally reducing the voltage.

Function of Voltage Regulator and Parallel Generator Operation

The voltage regulator's primary function is to maintain a stable and precise generator voltage under no-load conditions and as loads fluctuate. When generators operate in parallel, a parallel compensation circuit is required to help voltage regulators manage reactive power distribution among the generators.

Reactive power imbalances between generators can occur when the voltage regulator adjusts the generator’s excitation due to load variations, prime mover speed changes, thermal drift, and other factors. 

Automatic Voltage Regulator. Real power, Reactive power, Apparent power. KW, KVAR, KVA

In a DC analogy with batteries in parallel, only voltage needs to be controlled for proper load sharing. Similarly, in a mechanical AC analogy, only torque requires control. However, when paralleling AC generators, both variables — torque and excitation — must be properly managed.

• Torque control regulates the division of real power (kilowatts, kW).
• Excitation control manages the division of reactive power (kilovolt-amperes reactive, kVAR).

Both must be precisely controlled to ensure stable and efficient generator operation.

Real power represents the actual work performed by the electrical energy generated. It is supplied by the prime mover in the form of torque, which the generator converts into electrical energy. This energy is then delivered to a load, where it is transformed into useful forms such as heat, light, or mechanical motion (e.g., in a motor).

Reactive power, on the other hand, is the power required by inductive or capacitive loads to store energy during each half-cycle. In the example shown in Drawing 10, the load is purely resistive, such as a heating element. Since no reactive power is needed at any point on the sine wave, the current remains directly proportional to the voltage at all times. Real power can be calculated using Ohm’s Law in this scenario.

Automatic Voltage Regulator and Parallel Operation of generators. Voltage droop

Generator sets are commonly operated in parallel to enhance fuel efficiency and increase the reliability of the power supply. When generators are connected in parallel, fuel economy is optimized by activating only the necessary generators to meet the load demand at any given moment. Operating each generator close to its full capacity ensures efficient fuel consumption.

The reliability of the power system is also improved since backup generators are available when others are in use. Additionally, protective systems can be implemented to detect faults and isolate the affected part while maintaining power to the rest of the system. If one generator fails or develops a problem, it can be shut down, with the remaining generators taking over the load.