Generator Paralleling Issues and Troubleshooting Techniques

Contents

• Introduction
• Basic Principles of Generator Paralleling
• Synchronisation Mechanisms and Devices
• Conditions for Paralleling and Why They Matter
• Common Failure Modes
• Detailed Troubleshooting Process
• Frequency Control Issues
• Voltage and Excitation Faults
• Phase Sequence Problems
• Load Sharing and Kilowatt Balancing
• Governor and AVR Diagnosis
• Protective Devices, Trips and Nuisance Shutdowns
• Measuring and Logging Key Parameters
• Escalation, Best Practice and When to Call for Help
• Operational Scenarios: Case Studies
• Review Questions
• Glossary
• ASCII Diagrams

Introduction

Generator paralleling is a crucial operation aboard nearly all commercial vessels. Successful synchronisation and load-sharing between generators ensures redundancy, continuity, and safe power supply to vessel systems. When issues occur during paralleling, risks include blackouts, generator or switchboard damage, and even loss of propulsion. This article provides a chief engineer’s perspective on real-world generator paralleling challenges, offering guidance on troubleshooting, checks, escalation, and best practice for all engineering ranks.

The emphasis is on mechanisms and sequence. Each issue is detailed with observable symptoms, fault mechanisms, checks, and practical steps for diagnosis and mitigation. The article assumes basic knowledge of switchboard layouts and generator controls, but it is relevant to everyone from cadets to seasoned chief engineers.

Basic Principles of Generator Paralleling

Paralleling generators means connecting two or more AC generators to a common busbar such that they share load. This allows for redundancy, the ability to take rotating machines off line for maintenance, or to match power output to fluctuating demand. For safe paralleling, both electrical and sequence conditions must be met—including synchronised voltage, frequency, and phase sequence.

Marine switchboards may use manual or automatic systems to control synchronisation and paralleling. Manual systems require the operator to manipulate generator speed and excitation to match incoming and running generators before closing the breaker. Automatic systems rely on synchronisers, load-sharing modules, and protection relays.

Failure to follow correct procedure or to address system faults can lead to currents many times the generator’s rated value passing across the bus. This can inflict severe damage. Thus, familiarity with operational mechanisms and swift, methodical troubleshooting are essential skills in the engine room.

Synchronisation Mechanisms and Devices

The synchronising process matches three key parameters: voltage, frequency, and phase sequence. Manual synchronising typically involves a synchroscope as well as voltmeters and frequency meters. Modern ships mostly use automatic synchronisers and dead-bus relays, interfaced with the switchboard and generator control systems.

A manual synchronising station has:

• Synchroscope: Indicates the phase difference and relative speed between incomer and busbar.
• Volt/frequency meters: Allow operator to adjust excitation and engine governor.

Automatic synchronisers will sense all parameters, signalling governor and AVR for fine adjustments and auto-closing the breaker under ideal conditions. Both systems must include safety interlocks and rejection mechanisms to prevent out-of-phase closure.

The devices involved can fail—by drifting out of calibration, developing electrical faults, or suffering from human error in operation. Periodic testing and calibration of synchronising gear is a vital routine for any engineer. Any signs of drifting readings, sluggish meters, or sticky breakers must be attended to before a synchronisation attempt.

Conditions for Paralleling and Why They Matter

Safe paralleling requires that the following conditions are met:

| Parameter        | Acceptable Deviation |
|------------------|---------------------|
| Voltage          | +/- 5%              |
| Frequency        | +/- 0.2 Hz          |
| Phase Sequence   | Exact match         |
| Phase Angle      | < 10 degrees        |

Voltage mismatch can cause large circulating currents, which produce overheating. Frequency mismatch will force hunting and immediately result in load-sharing instability, or it may lead to one set acting as a motor. Phase sequence error is catastrophic, causing severe current surges—which can physically damage windings and cause arc flash.

In practice, prior to breaker closure, an engineer must observe and confirm all parameters—visually if using a manual system, or checking auto-synchroniser status lights and instrument readings with a ‘trust but verify’ mentality.

Best practice is to repeat the key checks twice, and to continuously monitor phase angle drift and frequency difference during closing. Interlocks must never be bypassed — doing so has led to court-marshals and loss of tickets in serious causalities.

Common Failure Modes

In generator paralleling, the most common failure modes encountered by engineers include:

Failure of synchronising gear: Synchroscope or synchroniser not receiving correct signals due to broken connections, faulty PTs (potential transformers), or stuck relays. This is often seen as readings that are static, inconsistent, or fail to respond to governor or AVR adjustments. The consequence can be an out-of-phase closure if left undiscovered.

Governor or AVR malfunctions: Governor fails to adjust speed finely, leading to persistent frequency mismatch. AVR (automatic voltage regulator) issue results in persistent voltage mismatch. Symptoms are visible via meters or in the inability to fine-tune the incomer.

Breaker malfunctions: Breaker fails to close, chatters, or closes prematurely. An unresponsive breaker is sometimes due to failed electrical interlocks or lack of auxiliary supply. A chattering breaker poses a fire risk and may ultimately lock out the system.

Phase sequence errors: Seen if maintenance was recently carried out on busbars or incoming cable terminations, or if generators have been rewired. If missed, this error will trip busbar protection and isolate incoming generator with high-current alarms.

Detailed Troubleshooting Process

Troubleshooting must begin with observation and instrument verification. If synchronising fails or causes trips, observe which readings do not match expected conditions. Step one is always to verify actual busbar voltage, frequency, and phase sequence using a calibrated hand-held meter if standard instrumentation is suspected.

Where synchroniser devices are auto, check for correct lamp indication (usually colour-coded). For manual, check that the synchroscope rotates at correct speed and direction: clock-wise meaning incomer is fast, anti-clockwise slow, and should be adjusted to near stationary at 12 o’clock before closing.

Should instrument readings be conflicting, pay particular attention to auxiliary fuses and circuit breaker feeds to instrumentation and control panels. A blown fuse hides a hundred sins. Often, corroded terminals behind switchboard access panels are culprits for inconsistent readings.

If load-sharing fails after successful paralleling, immediate checks should be made on governor droop settings and that load-sharing wires or cabling are intact and secure. Never forget to check the simple things—plug connectors, loose terminals, and local stop buttons inadvertently pressed.

Frequency Control Issues

Frequency control is the domain of the prime mover governor. Before paralleling, adjust the incoming generator so its frequency is fractionally higher than the bus. This allows the incomer to pick up some load after closing. If the incoming frequency is not stable or matching, the breaker should not be closed.

Issues here include oversensitive or unresponsive electronic governors, mechanical linkage problems on older engines, or control system drift due to temperature or vibration. Symptoms include a synchroscope needle that never slows down, or two generators constantly hunting after paralleling.

To rectify, conduct a manual governor bump test if safe: manually adjust the speed setting and observe response. For electronic systems, ensure control voltage is within range and check for error codes on the governor panel. If unresolved, switch to hand control (if available) and test governor stability before further attempts.

Best practice mandates logging all frequency readings before and after synchronisation, noting engine room temperature, oil pressure, and possible induced factor. This builds a trend, helpful for both troubleshooting and maintenance forecasting.

Voltage and Excitation Faults

AVR (automatic voltage regulator) governs generator output voltage via the excitation circuit. Common causes for voltage mismatch are defective AVR cards, loose sense wiring, brush gear faults (for older machines), or poorly adjusted voltage setpoints. Sudden loss of excitation will force the generator to act as a motor or trip offline.

Symptoms of voltage or excitation faults include inability to balance voltmeters, abnormal sparking (on machines with brush gear), or fluctuating voltages during adjustment. Always check for correct supply voltage to AVR, disconnected or oxidised sense wires, or blown fuses in AVR supply.

In case of sudden voltage collapse, isolate the machine immediately and begin condition checks on excitation circuit. For brushless exciter types, check diode wheel continuity and stator resistance with a megger tool, only when stationary and after correct isolation procedures.

Routine excitation health checks—especially insulation and brush contact (for non-brushless)—must be completed every watch, with findings logged and anomalies reported to the chief before escalation.

Phase Sequence Problems

Proper phase sequence is fundamental to paralleling. A phase sequence error occurs if A-B-C is not identical between incomer and bus. Commonly, this happens after maintenance or when a generator is rewired or a main breaker is replaced and connections reversed by accident.

Symptoms are clear: synchronising lamps do not extinguish in sequence, synchroscope moves erratically, or breaker will refuse to close despite all other readings within spec. In rare cases, a mistaken sequence causes instant high-current trip and severe electrical stress.

To check sequence, use a phase sequence indicator on both incomer and bus, or visually confirm cable terminations from wiring diagrams. If discrepancy exists, swapping any two incomer phases (after isolating the generator, with permit to work, and confirming with dead-testing methods) corrects this. Never attempt live phase reversal; this is fatal and has led to serious injury.

Post-correction, visually mark terminations for future reference and enter full details in logbooks. All personnel should be briefed on the alteration to avoid confusion during subsequent operations.

Load Sharing and Kilowatt Balancing

Once paralleling is achieved, equal load sharing is mandatory. Failure results in overloading one set, risking generator overheat and eventual blackout. Older ships use droop-based mechanical governors and voltage balancing through manual AVR adjustment. Modern vessels use electronic load-sharing modules that sense kilowatt (kW) and kilovolt-ampere reactive (kVAR) flows.

Imbalanced sharing shows as one generator consistently with higher ammeter or kW reading. On switchboards fitted with load-sharing indicators, observe needle movement after paralleling is initiated. A drifting pointer or one that pegs out signals failed cross-wiring or defective load-sharing module.

For mechanical droop, check droop characteristic settings and adjust governor as per maker’s instructions. For electronic systems, check continuity of load-sharing wires between control panels and modules. Disconnected or broken sense wires render load-sharing dead, with the system defaulting to settings inherent in governor or AVR.

Correct kilowatt balancing typically requires fine adjustment after initial paralleling—gradual movement of speed control or voltage pot, observing effect on load distribution and busbar stability. Log all adjustments, and revert to previous conditions if instability increases, until fault is corrected or escalation requested.

Governor and AVR Diagnosis

Governor and AVR units are both the heart of generator paralleling control. Diagnosing their faults is a methodical process. For governors, check response to speed setting changes, monitor hunting or surging, and inspect all physical linkages for looseness and excess wear. For electronic types, consult control panel diagnostics and manufacturer troubleshooting guides.

For AVRs, check all fuse protection on excitation supplies, inspect external wiring (especially sense and reference signals), then test field excitation current with a clamp meter (following all lockout/tagout protocols). Any drop in field current, or lack of continuity through the exciter winding, points to either an open-circuit or a failed semiconductor component.

Multiple AVRs or governors in the circuit—common on synchronised sets—demand coordinated tuning. Never adjust both simultaneously without understanding interaction. Unskilled fiddling can produce wild instability. Best practice is to reset to manufacturer settings, verify all hardware connections, then tweak one parameter at a time whilst monitoring results.

Logbooks should include all serial numbers, adjustment settings, and incidents of governor/AVR replacement or tuning. Often, a six-month trend in these logs points to underlying vibration, temperature, or moisture ingress issues.

Protective Devices, Trips and Nuisance Shutdowns

Switchboard protection relays—overcurrent, reverse power, differential, and earth fault—protect both generators and busbars from dangerous faults. Incorrect paralleling procedures or faulty sensors can cause these protections to trip erroneously, leading to a blackout or temporary loss of power sources.

Nuisance shutdowns usually manifest as a generator tripping immediately after synchronisation, or breaker failing to close due to an interlock alarm. First, check that relay settings correspond to the loading—overly sensitive overcurrent or reverse power settings lead to chronic tripping and must be checked against the protection coordination diagram.

If a trip occurs, immediately record trip indicators and sequence lights before resetting anything. Many incidents are lost for diagnosis because crew reset too quickly. For differential trips, check CT (current transformer) wiring integrity, especially for loose or shorted terminations after recent board works.

Where protective devices are found to be faulty, e.g. sticky relays or old thermal trip mechanisms, isolation for repair is essential. Always notify the bridge and chief engineer, and where possible, arrange for power redundancy before any protection rack is bypassed by exception permit.

Measuring and Logging Key Parameters

Configurable digital meters have largely replaced analogue gauges for real-time monitoring. Nonetheless, the principle is unchanged: the engineer must trust none and verify all readings—especially if issues persist. Carry a calibrated hand-held multimeter and clamp meter for independent spot checks of voltage, frequency, current, and phase order. Confirm that busbar meters agree with local generator panel meters before making control adjustments.

Log every adjustment, incident and resolution. Date, time, generator number, readings before and after—minimum expected for troubleshooting. For recurring issues, create event logs and look for patterns tied to watchkeepers, ambient temperature, humidity, or after periods of vibration (heavy weather, close-quarter manoeuvring).

As a general rule, spot-check insulation resistance, voltages, and circuit breaker trip times every six months, or following any incident. Forward logs to superintendent if patterns of instability, hunting, nuisance trips, or excessive adjustments are found. The paper trail is vital for both technical diagnosis and insurance liability investigations.

Escalation, Best Practice and When to Call for Help

Responsible escalation is the mark of a mature engineer. When repeated improper paralleling, unstable load sharing, persistent trips, burnt insulation smell, arcing, or instrument blackouts occur, you must escalate. Any sign of overheating, smell of burning insulation, or visible arcing demands instant shutdown and chief/off-site notification.

Do not operate under unsafe conditions or bypass interlocks/protection devices unless authorised with a formal permit and full, logged risk assessment. Consult manufacturer manuals, class requirements and safety officers as required. Photographic and written evidence of any abnormality aids future investigations.

In any doubt, advise the bridge and superintendent before forcing breakers or removing protection. Temporary solutions must be recorded, notified, and risk managed. Ultimately, the cost of downtime or inconvenience is less than a major switchboard failure or injury.

Bear in mind, full incident reports should follow every significant issue, detailing readings, actions, escalation chain, and outcome. Peer review of incident logs enhances safety culture and prevents recurrence.

Operational Scenarios: Case Studies

Case Study 1: Hunting after Paralleling
Two generators set to parallel using automatic synchroniser. After breaker closed, both sets hunted and kW readings fluctuated. Investigation traced fault to a failed load-sharing cable between panels, verified by resistance measurement. Temporary jumper was fitted pending permanent repair. Lesson: Always check wiring continuity first.

Case Study 2: Blackout after Out-of-Phase Closure
Manual paralleling attempted in rough weather. Governor adjustment sluggish, synchroscope misread. Breaker closed at 30-degree phase difference, causing instantaneous blackout and damage to incomer machine. Root cause: Synchroscope stuck from vibration-induced wiring fracture. Lesson: Always confirm instrument operation before closing, especially in heavy weather.

Case Study 3: Nuisance Reverse Power Trip
Following major overhaul, generator tripped on reverse power after paralleling. Investigation showed phase sequence error from misconnected incomer leads. Proper phase sequence testing before connecting would have prevented trip. Lesson: Always test phase rotation after switchboard or cable maintenance.

Case Study 4: Load Sharing Instability in New System
Post-replacement of a new electronic governor, load sharing between two new and one old generator was unstable. Tuning process revealed incompatible droop settings between governor types. Adjustment of droop parameters and a common reference point resolved issue. Lesson: Always match droop/isochronous settings when mixing governor types.

Review Questions

• What are the three main electrical conditions that must be met before paralleling generators?
• Why is phase sequence matching vital before closing the incomer breaker?
• Describe the consequences of closing a generator breaker when voltage and frequency are not synchronised.
• How does a synchroscope operate, and what does a ‘stationary at 12 o’clock’ indication mean?
• What are the failure modes associated with AVRs during paralleling operations?
• Explain how frequency mismatch affects load sharing after synchronisation.
• How would you verify phase sequence using shipboard equipment?
• What is the significance of matching generator droop settings in load sharing?
• Describe a systematic approach to troubleshooting a generator that will not parallel.
• What are the typical symptoms of a defective load-sharing module?
• Why must all instrumentation be independently checked by hand-held meters?
• How should nuisance trips on generator protection relays be investigated?
• Describe key steps and checks after a reverse power trip event during paralleling.
• What risks are associated with bypassing generator interlocks or protection relays?
• List best practice steps for escalating a repeated synchronising failure to management or class.
• How does a failed current transformer manifest in switchboard protection performance?
• Which measurements must be logged after each failed synchronisation attempt?
• How does mechanical governor ‘droop’ affect paralleling performance with electronic governors?
• What steps must be taken to correct a phase sequence error after maintenance?
• Why is incident logging and post-event peer review important for generator paralleling safety?

Glossary

• AVR (Automatic Voltage Regulator): Unit for controlling output voltage by regulating field excitation.
• Governor: Device controlling engine speed, thereby regulating generator frequency.
• Synchroniser: Device ensuring electrical parameters are matched before paralleling.
• Droop: Change in frequency as load increases; used to share loads between generators.
• Breaker: Switch connecting or disconnecting a generator to/from busbar.
• Dead Bus: A busbar without voltage, awaiting connection to a live generator.
• Reverse Power: Power flowing from bus to generator, indicating loss of driving torque.
• Phase Sequence: Order of voltage waveform peaks (A-B-C) in a generator set.
• CT (Current Transformer): Instrument transformer for measuring current or for protection circuits.
• Busbar: Main conductor connecting several power circuits together in the switchboard.

ASCII Diagrams

Basic Manual Paralleling (Synchroscope and Meters):

Generator 1      Generator 2 (Incomer)
   |                    | 
   +--------+   +-------+  
            |   |        
        [Synchroscope]
            |   |      
           [Switchboard Busbar]


Typical Paralleling Sequence (Automatic):

[Generator 2 Control]--[Synchroniser]--[Breaker]--[Busbar]
                             |         /
                   (signals to Engine Governor & AVR)