Essential Conditions For Parallel Operation Of Diesel Generator Sets: Common Fault Causes And Solutions

Essential Conditions for Parallel Operation of Diesel Generator Sets: Common Fault Causes and Solutions

 

1. Four Mandatory Conditions for Parallel Operation of Diesel Generator Sets (None Can Be Omitted)

1. Voltage Equality
The line voltage difference between two or more sets must be ≤±0.5%. Large voltage differences will immediately generate large circulating currents, causing switchgear burning and winding damage.

 

2. Frequency Equality
For rated 50Hz operation, the frequency difference before grid synchronization must be ≤±0.2Hz. Large frequency differences during parallel operation can cause severe shocks and tripping.

 

3. Phase Alignment
Phases must be strictly synchronized. The greater the phase angle deviation, the larger the inrush current, which can directly damage the generator in severe cases.

 

4. Phase Sequence Consistency
Phase sequences must correspond (ABC to ABC). Parallel operation with incorrect phase sequence is absolutely prohibited, as it can instantly short-circuit switchgear and burn the generator.

 

Additional Supporting Conditions:

- Identical capacity/model sets are preferred; for different capacities, the load must be proportionally shared.
- Parallel operation controllers, synchroscopes, reverse power protection, and overcurrent/undervoltage/overfrequency protection must be in place.
- Sets must share a busbar section, with consistent grounding methods and wiring groups.

 

 

2. Common faults and causes in parallel operation and solutions

 

1. Grid connection failure and inability to synchronize

The reason

- Unstable speed controller, frequency fluctuates;

- Unstable voltage regulation with large voltage fluctuations;

- Incorrect synchronous wiring or phase sequence;

- Incorrect setting of parallel module parameters.

Solution

- Adjust the unit speed control plate to stabilize frequency; Adjust AVR voltage stabilization;

- Check wiring and phase sequence;

- Re-calibrate the synchronization window, frequency difference, and pressure differential parameters of the parallel controller.

 

2. After paralleling, large circulation current and unit heat may occur

The reason

- Inconsistent voltage and voltage regulation characteristics;

- The voltage regulation slope of the two AVRs does not match;

- Large differences in internal resistance and cable cross-section/length of the unit.

Solution

- Uniformly calibrate AVRs of two units and level the no-load voltage;

- Matches the voltage regulation drop slope to achieve reactive power equalization;

- Standardize bus cable specifications and lengths to reduce line voltage drop differences.

 

3. Uneven load distribution after paralleling (one fully loaded, one light)

The reason

- Inconsistent speed controller sensitivity;

- The active and reactive power allocation parameters of the parallel controller are not properly adjusted;

- The throttle actuator response varies in speed.

Solution

- Fine-tuning the speed control plate gain and sag coefficient;

- Setting active and reactive power equalization parameters in the parallel screen;

- Calibrate throttle actuator stroke and response speed.

 

4. Reverse power protection trips after paralleling

The reason

- One unit is towed by another and turns into an electric motor state;

- Forced grid connection due to frequency difference or misalignment;

- The set value for reverse power protection is set too low.

Solution

- Close the circuit and connect to the grid only after strict synchronization; forced paralleling is prohibited;

- Appropriately and reasonably set the delay and action value of reverse power protection;

- Check if the speed controller is out of control and is running around.

 

5. Voltage fluctuations and flickering lights after paralleling

The reason

- Poor AVR voltage regulation performance and unstable excitation;

- Imbalance in the distribution of unpaid power, competing for non-effective work;

- Sudden increase in busbar load.

Solution

- Repair or replace AVR excitation boards;

- Adjust reactive power sag parameters to balance reactive power output;

- Heavy loads are delivered in batches to avoid sudden unloading.

 

6. Multiple units running in parallel under light load, emitting black smoke and carbon accumulation

The reason

- Load too low, forcing multiple units to run in parallel with load;

- The parallel logic does not have the "automatic shutdown or machine reduction" function.

Solution

- Parallel operation logic: When load is low, it automatically disconnects excess units, leaving only 1~2 units under load;

- Ensures single-unit load ≥ run at over 30%, reducing carbon buildup and wear.

 

3. Safety operation mnemonic for parallel operation

Four Identities: Same voltage, same frequency, same phase, same phase sequence
Three prohibitions: No forced paralleling out of sync or unclear, no unclear phase sequence, and no protection failure or parallel operation
Two points to note: first stabilize frequency and voltage, then check the synchronization meter; Load distribution is evenly distributed and then operated long-term