Synchronizing Generators Problems | Common Issues & Field Solutions

 In every industrial power plant, synchronizing generators is one of the most delicate and technically demanding operations. It’s not simply about connecting two electrical sources — it’s a precise process of aligning frequency, voltage, and phase angle so that the generator can safely connect to a live bus without causing current surges, torque shocks, or instability.

Whether you’re working in a cement plant, steel mill, water treatment station, or backup power facility, synchronization problems are among the most common and critical issues maintenance and commissioning engineers face daily.

The reality is: even small mismatches in timing or settings can trigger reverse power trips, unequal load sharing, or even complete blackouts. Below are the real questions and problems engineers encounter in the field, based on actual site experiences — along with detailed technical explanations and effective solutions that help keep power systems stable and efficient.

Why does synchronization fail even when voltage and frequency look correct?

This is one of the most frustrating problems for field engineers. You’ve adjusted the generator’s voltage and frequency to match the bus readings perfectly, but the synchronizing relay still refuses to close the breaker.
The cause usually lies in the phase angle difference between the generator and the grid. Even when the voltmeter and frequency meter show nearly identical values, the waveform of the generator may still be “out of step” by several degrees.

Most synchronization systems have a phase-angle window, typically ±10°, within which the closing command is allowed. If the phase lag exceeds that limit, the sync-check relay blocks the closing coil.
To fix this, operators must slow down the generator’s speed until the synchroscope pointer rotates very slowly — ideally one revolution every 10–15 seconds — and close the breaker when the pointer is exactly at the 12 o’clock position. In automatic systems, ensure the synchronizer’s closing time compensation is correctly set for the breaker’s operating delay.

What causes reverse power trips immediately after synchronization?

Reverse power protection trips are common and often misunderstood. The generator, instead of delivering power, begins absorbing it — acting like a motor driven by the bus. This happens when the prime mover fails to supply sufficient torque to maintain generation.
In diesel generators, it’s often due to slow fuel rack response or governor tuning lag. In turbines, steam pressure or valve control delays may cause similar effects.

Another possibility is an incorrect relay setting: if the reverse power relay is too sensitive, it may trip during minor transients after closing.
The correct approach is to verify that the engine ramp-up after synchronization is fast enough, adjust the governor gain parameters, and set the relay pickup around 3–5% of rated generator power. Also, check the polarity of current transformers — a reversed CT connection can falsely indicate reverse power.

Read about: PLC Communication Error with HMI | Causes & Resolution

Why do generators share load unequally after synchronization?

Unequal load sharing is a classic field challenge, especially when multiple generators operate in parallel. After synchronization, one generator carries most of the load while others contribute very little.
This typically occurs due to mismatched governor droop settings or incorrect AVR sharing calibration. Droop determines how much frequency or voltage changes as load increases — if one generator’s droop setting is lower, it will try to pick up more load.

To solve this, engineers must ensure all governors have identical droop characteristics (e.g., 4–5% for frequency control). In electronic or digital systems, use the control panel or software interface to adjust the droop curve and fine-tune proportional load sharing. For voltage balance, verify that the reactive power sharing circuit or reactive droop compensation is calibrated correctly.

When both active and reactive loads are balanced, generators operate more efficiently, avoiding overheating and unnecessary fuel consumption.

Why does the automatic synchronizer fail to match parameters?

Automatic synchronizers are designed to handle voltage and frequency matching automatically, but they rely entirely on accurate input signals.
If there’s a drift in the analog signal from voltage or frequency transducers, or a wiring issue, the synchronizer may endlessly “hunt” without ever reaching the closing conditions.

Field engineers often discover that one phase’s PT or CT input has a loose connection or incorrect scaling. Another common issue is firmware mismatch between the synchronizer module and the PLC system controlling it.
To resolve this, verify all analog signal inputs using a calibrated multimeter, check signal scaling in the control logic, and recalibrate sensors periodically. Always ensure that automatic synchronizers are tested manually after firmware updates or logic changes.

What happens when the phase sequence is wrong during synchronization?

This is one of the most dangerous mistakes that can occur.
If the generator’s phase sequence (R-Y-B) doesn’t match the bus sequence, closing the breaker will cause massive circulating currents between the two sources.
The mechanical torque shock can damage the alternator shaft and coupling, and electrical stress may trip protection devices or damage equipment.

This error usually happens after maintenance or during installation when cables are mistakenly swapped.
To avoid it, always use a phase sequence indicator before synchronization, especially if the wiring has been modified. Once confirmed, label all terminals clearly and document the rotation direction to prevent future mistakes.

Why does voltage fluctuate during synchronization?

Voltage instability, known as AVR hunting, can disrupt synchronization. The AVR (Automatic Voltage Regulator) is responsible for maintaining output voltage, but poor tuning or faulty feedback causes it to overcorrect continuously.
As the generator’s excitation keeps oscillating, the bus and generator voltages never stabilize long enough for synchronization.

Common causes include loose feedback wiring, poor grounding, or unstable PID tuning within the AVR. In plants with multiple generators, mutual interference between AVRs can also lead to oscillation.
The solution is to tighten all connections, retune AVR parameters, and use shielded cables to minimize electrical noise. Installing line filters or ferrite cores on feedback circuits can also help suppress oscillations.

Why do synchronization lamps or synchro scope behave erratically?

Manual synchronization using lamps or a synchro scope is still common in small and medium power systems. However, engineers often report irregular or confusing indications.
If the lamps flash inconsistently or the synchro scope rotates erratically, it usually means there’s a frequency or voltage instability or a fault in the PT circuit.

A defective lamp, incorrect polarity, or bad wiring can also lead to misleading readings. In such cases, always cross-check using digital meters and verify that the PT ratio and polarity are correct.
For better reliability, modern control panels integrate electronic synchronizing meters that directly show phase angle difference, eliminating manual error.

Why does the breaker fail to close even when parameters look correct?

Sometimes, synchronization looks perfect, but the breaker simply doesn’t close.
This issue often comes down to mechanical lag in the breaker mechanism or coil delay. When the closing command is sent, the breaker might take 100–200 milliseconds to physically close, which is enough for the phase angle to drift out of the safe window.

Older breakers are particularly prone to sluggish operation due to worn-out linkages or inadequate lubrication.
The fix is to perform breaker timing tests, clean and lubricate mechanical parts, and verify that the closing coil voltage is stable. Some systems use “advance closing” logic — triggering the breaker slightly earlier to compensate for the mechanical delay.

Why does synchronization cause a sudden bus voltage dip?

A noticeable voltage drop across the bus immediately after synchronization is a sign that the incoming generator’s voltage was slightly lower than the bus, or that it picked up reactive load too abruptly.
This happens when the AVR response is too slow or load sharing circuits are not tuned properly.

To prevent this, ensure the incoming generator voltage is set slightly above the bus voltage (by around 1%) before closing. Once synchronized, fine-tune the reactive power sharing so both generators maintain similar excitation levels.
In some cases, it’s also necessary to inspect the bus voltage regulator or capacitor bank response time.

Why does synchronization sometimes fail intermittently?

Intermittent synchronization failures often stem from communication problems between the synchronizer, PLC, and monitoring systems.
If voltage or frequency signals flicker due to electrical noise, or if Modbus communication drops momentarily, the synchronizer logic interprets it as unsafe and aborts the operation.

Checking grounding, shielding, and cable routing is essential — especially in industrial sites with heavy drives or VFDs. Using twisted-shielded pairs and proper earthing can drastically improve signal stability.
Additionally, engineers should verify Modbus/Profibus address mapping and baud rate configurations to prevent data loss between control units.

Why does the system fail to synchronize automatically but works manually?

This is a subtle but common problem. In many installations, automatic synchronization logic fails repeatedly, yet manual synchronization works fine.
This typically means that the control logic or interlocks in the PLC program are not aligned with real-world timing or sequence.
For example, the sync-check relay might require a signal from the synchronizer and an auxiliary signal from the generator breaker that arrives slightly late.

To resolve this, engineers must review the PLC sequence step-by-step, verify interlocks, and adjust logic timers to accommodate real breaker operation time. Logging synchronization attempts in the PLC history helps pinpoint the exact step where failure occurs.

Why is load sharing unstable after several minutes of synchronization?

Sometimes synchronization succeeds initially, but after a few minutes, the generators’ load sharing drifts and becomes unstable.
This typically indicates droop line mismatch or governor temperature drift. As the engine warms up, its mechanical characteristics change, slightly altering the droop response.

Engineers can fix this by performing extended load tests and adjusting droop coefficients after the system reaches stable operating temperature.
If the issue persists, inspect the load sharing lines for grounding noise or poor connections that could affect signal quality between governors.

Conclusion

Generator synchronization is both an art and a science. Successful synchronization requires precise alignment of voltage, frequency, and phase angle — but in real industrial environments, countless small factors can throw this balance off.
From poorly tuned governors and AVRs to communication noise and timing mismatches, every stage of synchronization demands attention to detail.