This is a serious issue that needs immediate attention to prevent further damage and ensure safety. Arcing between bus bars when a generator drops offline indicates a significant voltage difference exists where there shouldn't be, and this difference is enough to ionize the air and create a conductive path. Here's a breakdown of the potential causes and how to investigate them

Likely Causes:
Out-of-Phase Synchronization (Most Likely)

This is the prime suspect. When a generator is taken offline (or faults), it MUST be properly synchronized with the remaining grid. If the generator is not perfectly synchronized before disconnecting, a significant phase angle difference can exist. This means the voltage waveforms are not aligned. When a generator is taken offline with a phase angle difference, the voltage difference between phases on the dead generator and the live grid will appear across the busbars. This voltage can be high enough to cause arcing, especially under high load conditions. Even a small phase difference can create a substantial voltage transient.



Incorrect Droop Settings / Poor Load Sharing

If the generators aren't sharing the load properly, the remaining generators may be heavily loaded. When one drops off, the sudden surge in current demanded by the load can create a voltage dip and phase shift on the remaining generators, potentially contributing to the arcing on the disconnected generator.

Circulating Currents

If the generator voltage regulators are not correctly configured, circulating currents can flow between the generators. When a generator drops off, the sudden change in these currents can induce voltage transients in the disconnected generator's output wiring.

Neutral Issues/Grounding Problems

A floating neutral or ground fault can lead to voltage imbalances and circulating currents. If the neutral point of one generator isn't properly connected, or if there's a ground fault in the system, the potential between the disconnected generator's phases and the combiner bus can become significant.

Transformer Issues (Less Likely, but Possible)

If the generators are connected to the combiner through transformers, a fault within a transformer (e.g., winding fault) can create voltage imbalances and transients.

Harmonics

High harmonic content in the load current can exacerbate voltage imbalances and create transient voltage spikes when a generator is switched. However, while pumps and motors can generate harmonics, it's less likely to be the primary cause of such dramatic arcing.

Component Failure in the Generator's Protective Gear

A malfunctioning circuit breaker, contactor, or other protective device in the failing generator might not be opening cleanly, leading to delayed or incomplete isolation and allowing voltage differences to develop.

Investigation and Troubleshooting Steps


IMMEDIATE ACTION



STOP Operations

Until the problem is identified and resolved, shut down the system. Continued arcing can lead to catastrophic equipment failure and potential fire.

SAFETY FIRST

Ensure all personnel are aware of the hazard. Only qualified electrical personnel should perform the following checks. Use proper PPE.

Visually Inspect

Carefully inspect the bus bars, connections, and wiring in the generator output closet for signs of damage, overheating, or loose connections.2.

Synchronization System Checks



Verify Synchronization Settings

Confirm that the automatic synchronizers on each generator are properly configured and functioning correctly. Check the voltage, frequency, and phase angle tolerances.

Manual Synchronization

If the system allows, attempt to manually synchronize the generators. Observe the synchroscope or synchronizing lights carefully to ensure proper phase alignment before closing the breaker. If manual synchronization also causes issues, it strongly points to a fundamental synchronization problem.

Synchronization Relays

Examine the operation and settings of any synchronization relays. A faulty relay might be allowing out-of-phase generators to be connected.3.

Generator and AVR (Automatic Voltage Regulator) Checks



Voltage Regulation

Verify that all generators are maintaining stable and consistent voltage levels under varying loads. Test the AVRs for proper functionality. A malfunctioning AVR can cause voltage fluctuations and instability.

Droop Settings

Confirm that the droop settings on each generator's governor control are correctly set to ensure proper load sharing. Incorrect droop settings can lead to one generator carrying too much load, making it more susceptible to voltage dips and phase shifts when another generator drops off.

Field Current

Monitor the field current of each generator. Unstable or fluctuating field current can indicate a problem with the exciter or AVR.4.

Load Sharing and Power Quality Analysis



Load Distribution

Use a power analyzer to monitor the real and reactive power being supplied by each generator under normal operating conditions. Ensure that the generators are sharing the load proportionally.

Harmonic Analysis

Use a power quality analyzer to measure the harmonic content in the system's voltage and current. High harmonic levels can exacerbate voltage imbalances.

Transient Recording

Connect a transient voltage recorder to the bus bars to capture the voltage and current waveforms during a generator disconnection event. This will provide valuable information about the nature and magnitude of the transient voltages.5.

Grounding and Neutral Checks



Neutral Grounding Resistor (NGR)

If the system uses an NGR, verify its integrity and proper connection.

Ground Fault Detection

Check the operation of the ground fault protection system.

Neutral Voltage

Measure the voltage between the neutral point of each generator and ground. Excessive neutral voltage can indicate a problem with the grounding system.6.

Protection System Review



Relay Settings

Examine the settings of all protective relays (overcurrent, undervoltage, reverse power, etc.) to ensure they are properly coordinated and are not contributing to the problem.

Breaker Operation

Verify that the circuit breakers protecting the generators are opening cleanly and quickly when a fault occurs. A slow-opening breaker can allow a voltage difference to develop.7.

Professional Consultation



Engage a qualified electrical engineer

This issue is complex and potentially dangerous. Consult with an experienced electrical engineer specializing in power systems to review your findings, perform further analysis, and recommend a solution.

Generator Manufacturer

Consult the generator manufacturer for specific troubleshooting advice and recommendations.

Specific Things to Look For Regarding Synchronization



Synchroscope

If you have a synchroscope, carefully observe the rotation speed and direction before attempting to connect a generator. The speed should be very slow, and the pointer should be rotating in the "fast" direction. Ideally, you want to close the breaker just before the pointer reaches the 12 o'clock position.

Synchronizing Lights

If you're using synchronizing lights, the lights should be dimming and brightening in unison. The breaker should be closed when the lights are at their dimmest.

Automatic Synchronizer Tuning

The automatic synchronizer has settings that control how quickly and accurately it attempts to match the voltage, frequency, and phase angle of the incoming generator to the grid. These settings may need to be adjusted based on the characteristics of your system.

Why Reverse Power is Unlikely to be the Primary Cause

While reverse power can be a symptom of a problem, it's less likely to be the root cause of the arcing. Reverse power occurs when a generator starts drawing power from the grid, rather than supplying it. This typically happens due to:

Prime Mover Failure

If the engine driving the generator fails, the generator can start acting like a motor and draw power from the grid.

Incorrect AVR Settings

An AVR that is set too low can cause the generator to underexcited and draw reactive power from the grid.Reverse power is detected by a reverse power relay, which trips the generator's breaker to protect it. The arcing is more likely happening before the reverse power relay has a chance to act, due to the phase angle difference. However, the fact that you might also be seeing reverse power indicates that the synchronization is definitely failing, because even a small reverse power signal on the running generators might exacerbate voltage instabilities and imbalances if one of the generators were to drop off-line.

In summary, the most likely cause of the arcing is a synchronization issue that results in a significant voltage difference between the disconnected generator's bus bars and the live combiner bus. Thoroughly investigate the synchronization system, voltage regulation, load sharing, and grounding to identify and resolve the problem.* Flag for review