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.

Common Problem: Excessive Circulating Current

When generators are connected to an isolated bus, excessive circulating currents can develop due to improper settings or faulty components in the reactive droop compensation circuit. To troubleshoot this, follow these steps:

Step 1: Initial Checks

1. Verify system connections – Ensure that the current transformer (CT) and potential transformer (PT) are correctly wired and phased.
2. Bring generators online – Start the turbine/generator, turn on the AVR, and bring generator voltages to normal levels.
3. Measure field voltage – Record field voltages for both generators before synchronization.
4. Synchronize generators – Match voltage, frequency, and phase angle before connecting the generators.

Step 2: Checking Field Voltage and Reactive Power (Vars)

1. If generator vars and line current remain stable after breaker closure, the system is functioning correctly.
2. If generator vars drastically increase, accompanied by a sudden rise in generator line current, there may be a problem in the AVR reactive droop compensation circuit.
3. If generator vars tend to drift between machines over time, check and record the drift rate in field voltage.

Diagnosing the Problem

If excessive circulating current persists, follow these troubleshooting steps:

Step 3: Checking Current Transformers (CTs)

1. Open generator breakers and short CT1 on Generator 1.

2. Match voltage, frequency, and phase angle with Generator 2.

3. Close breakers and monitor generator vars:

• If generator vars increase, open breakers, shut down generators, and reverse CT2.
• If vars remain stable, the issue is not with Generator 2.

4. Re-check CT installation and wiring:

• Ensure CTs are installed on the correct generator line.
• Verify that the CT provides 3-5A signal at full load.
• Measure 3-5V drop across the burden resistor in the AVR paralleling circuit.

Step 4: Addressing AVR and Droop Adjustment Issues

• Check if CT circuit is miswired – The circuit may be incorrectly connected within the AVR.
• Verify droop settings – Incorrect droop adjustment on the voltage regulator can lead to insufficient compensation.
• Inspect governor operation – If high circulating current persists despite corrections, the governor may be malfunctioning.

Final Verification

1. Remove any shorted CT connections and resynchronize the generators.
2. Apply rated load at a 0.8 power factor to observe system behavior.
3. Confirm that voltage droop is approximately 5% under rated power factor load.

Conclusion

By methodically checking CT phasing, AVR settings, and generator synchronization, excessive circulating current issues can be resolved. This ensures efficient generator operation, reduces stress on electrical components, and improves overall system reliability.

If problems persist despite following these steps, a deeper inspection of AVR circuits, PT connections, and governor functions may be necessary.

Articles about Automatic Voltage Regulator:

1. Automatic Voltage Regulators. What is a generator AVR or Automatic Voltage Regulator?
2. Automatic Voltage Regulator and Parallel Operation of generators. Voltage droop
3. Automatic Voltage Regulator. Real power, Reactive power, Apparent power. KW, KVAR, KVA
4. Function of Voltage Regulator and Parallel Generator Operation
5. The Current Transformer Unit - Parallel Switch. Paralleling Different Size Generators
6. Checking and Troubleshooting a Reactive Compensation Circuit for the AVR in an Isolated AC Bus
7. ACB Trouble. The generator does not connect to the main busbars. Troubleshooting