Standard Electrical Operation Procedures for Generator Sets: No-load Operation, Loading, Unloading and Shutdown

This paper systematically specifies the complete electrical operation specifications of generator sets, covering pre-start inspection, no-load startup, staged loading, graded unloading, normal shutdown and emergency shutdown. The operation logic complies with electrical safety principles, which can effectively prevent reverse electromotive force impact, three-phase unbalance damage, carbon deposition and other common faults, and guarantee the service life and power supply stability of generator equipment.

1 Pre-start Electrical Inspection (Mandatory Steps)

1.Disconnect the main output circuit breaker (QF) of the generator set to guarantee no-load startup; startup with load is strictly forbidden.

2.Check the control cabinet: ensure all meters, voltage/frequency regulators and protection modules (overcurrent, undervoltage, overspeed, high water temperature and low oil pressure protection) show no fault alarms.

3.Verify reliable grounding protection; the grounding connections of the unit shell and neutral point shall be firm.

4.For parallel operation systems: open the bus tie switch and reset the synchronizing device. For mains automatic transfer scenarios: place the ATS in the open position to realize physical isolation between mains and generator circuits.

5.Inspect battery voltage and excitation circuit wiring without looseness.

2 No-load Startup Procedures

1. Switch the control cabinet to manual mode and press the start button.

2. The starter drives the engine to rotate, then release the button. Single startup duration shall not exceed 5 seconds; the interval between repeated startup attempts shall be no less than 15 seconds. Stop operation and troubleshoot if startup fails three times consecutively.

3. After the unit rises to rated speed and enters no-load state:

①Observe instrument readings: voltage at 400V±5%, frequency at 50Hz±0.5Hz, and balanced three-phase voltage.

②Standard no-load warm-up time: 5–10 minutes at normal temperature, 10–15 minutes in low-temperature environments.

③Monitor stable oil pressure and water temperature without overspeed, undervoltage or electric leakage alarms.

4. Closing the output breaker to carry loads during no-load warm-up is prohibited. This stage is used to fully establish excitation, warm up the engine, and check abnormal noise and electrical faults.

 3 Loading Specifications (Loading Only After Qualified No-load Parameters)

3.1 Loading Steps for Single Independent Generator Set

1. Close the main output circuit breaker QF only when no-load parameters are stable with no fault alarms.

2. Implement gradual staged loading; full-load input at one time is forbidden:

Stage 1: Run under 30% rated load for 3 minutes.

Stage 2: Increase to 60% rated load and operate for 5 minutes.

Stage 3: Gradually raise the load up to 100% rated power.

3. Monitor the whole loading process: maintain balanced three-phase current, voltage drop less than 10%, and no sharp frequency fluctuation.

4. Avoid sudden input of high-power inductive loads such as water pumps and air compressors; put them into service in batches.

5. For parallel units: complete synchronization first, then distribute loads by stages to strictly control reverse power.

3.2 Key Monitoring Points During Load Operation

 The economic operating load range is 70%–80% of rated power.

 Long-term operation under less than 30% load is forbidden, which will cause cylinder carbon deposition, exhaust blockage and higher fuel consumption.

 Keep the three-phase current difference below 10% of rated current to prevent stator winding burnout caused by three-phase unbalance.

4 Unloading Specifications (Unload Fully Before Shutdown; Shutdown with Load Is Prohibited)

1. Cut off loads in separate circuits and reduce load step by step; power off all loads at once is not allowed:

 Step 1: Disconnect high-power motors and inductive loads.

 Step 2: Reduce to 50% load and run for 2 minutes.

 Step 3: Reduce to 30% load and run for 3 minutes.

2. Remove all loads and open all load switches completely.

3. Disconnect the main output circuit breaker QF to restore the unit to pure no-load state.

Core Electrical Principle

Instant shutdown with load will generate reverse induced electromotive force, breaking excitation modules and voltage regulators and causing permanent damage to electrical control components.

5 Standard Normal No-load Shutdown Procedures

1. After complete unloading and opening the output breaker, perform no-load cooling operation:

o Cool down for 5–10 minutes if the unit previously ran at full load; cool down for 3 minutes after short-term light-load operation.

o Cooling can slowly lower the temperature of cylinders and exhaust manifolds to avoid cracking caused by rapid cooling under high temperature.

2. Confirm stable no-load voltage, frequency and no fault alarms.

3. Press the shutdown button on the control cabinet to cut off fuel supply and stop the engine.

4. Disconnect the main battery switch after shutdown to cut off power of the control circuit.

5. Conduct on-site inspection: keep the output switch open, and fill in operation records including running time, load conditions and abnormal phenomena.

6 Specifications for Special Shutdown Conditions

6.1 Emergency Shutdown (Only for Fault Conditions, Not for Daily Operation)

Trigger conditions: overspeed, sharp drop of oil pressure, ultra-high water temperature, short circuit, fire, electric leakage and severe abnormal noise. Operation: Press the red emergency stop button on the control cabinet, and disconnect the output breaker immediately. Post-fault requirement: Do not restart the unit directly. Carry out comprehensive inspection on electrical and power faults, and eliminate hidden dangers before restarting.

6.2 Start-stop Logic of ATS Automatic Transfer Generator Sets

1. Mains power failure → ATS disconnects mains circuit → The unit automatically starts and warms up under no-load condition.

2. After parameters meet standards, ATS automatically closes the generator load circuit and carries loads by stages.

3. Mains power recovery → ATS cuts off generator loads first → The unit runs no-load for cooling delay → Automatic shutdown.

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