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.