Precautions for Grid Connection of Diesel Generator Sets with Municipal Power

(2) Verify Equipment Parameters to Ensure Matching Consistency

The core premise for grid connection of generator sets with municipal power is that the electrical parameters of the two are strictly matched. This is like the meshing of two rotating gears, which must achieve "consistent size, the same speed, and aligned teeth". Specifically, the following 4 key conditions must be met, none of which can be missing:

• Consistent Phase Sequence: The A, B, and C three-phase outputs of the generator set must strictly correspond to the A, B, and C three phases of the municipal power, which is the primary premise of grid connection. If the phase sequence is wrong, it will lead to serious accidents such as short circuits and equipment explosions. The phase sequence can be verified by a phase sequence meter to ensure correct wiring.
• Equal Voltage: The effective value of the output voltage of the generator set must be consistent with the municipal power voltage, usually allowing a small deviation of ±0.5%. If the voltage is too high, it will transmit reactive power to the grid; if the voltage is too low, the generator set cannot bear the preset load. The excitation current can be adjusted through the Automatic Voltage Regulator (AVR) to achieve voltage matching.
• Same Frequency: The standard frequency of municipal power in China is 50Hz. The output frequency of the generator set must be completely synchronized with the municipal power frequency, with the deviation controlled within ±0.05Hz. Frequency mismatch will cause power oscillation between the unit and the grid, damaging equipment. Frequency synchronization can be achieved by adjusting the speed of the diesel engine.
• Synchronous Phase: The phase angle of the output voltage of the generator set must be completely consistent with that of the municipal power voltage, and the instantaneous voltage difference is close to zero. This is the most demanding condition. Excessive phase deviation will generate huge inrush current, burn the generator winding, and break the rotating shaft. It can be monitored and adjusted through a synchroscope or automatic synchronization device.

(3) Check Equipment Status and Eliminate Potential Hidden Dangers

Before grid connection, a comprehensive inspection of the diesel generator set and supporting equipment must be carried out to ensure no faults or hidden dangers: first, check the generator set itself, start the unit for no-load operation, confirm that the water temperature, oil temperature, and oil pressure are normal, without abnormal noise or violent vibration, and the output power is not less than 85% of the rated power; second, check grid-connected related equipment, including synchronization control devices, grid-connected circuit breakers, protection relays, etc., to ensure firm wiring, sensitive instruments, and intact and effective devices such as reverse power protection, overcurrent protection, and short-circuit protection; third, check the computer room environment, the temperature of the computer room is controlled at 5℃-35℃, the relative humidity is less than 75%, and the ventilation is good to avoid poor heat dissipation of the unit. At the same time, reserve sufficient diesel oil, set more than two oil storage tanks to facilitate the precipitation of new oil, and ensure that the cooling water is supplied by gravity to avoid being affected by the interruption of municipal power.

II. During Grid Connection: Standardize Operation Procedures and Avoid Operational Risks

Grid connection operations must follow the principle of "first detection, then adjustment, and finally closing", and be operated by professional operators throughout the process. Non-professional personnel are strictly prohibited from participating. The specific procedures and precautions are as follows:

(1) Choose the Appropriate Grid Connection Method

There are two common grid connection methods, which should be selected according to the actual scenario: first, automatic grid connection. Through a synchronization control device (such as high-end controllers like Deep Sea and ComAp), it real-time monitors the parameters of the power grid and the generator set, automatically adjusts the speed and voltage, captures the synchronization point, and then automatically issues a closing command. It is suitable for large-scale units or scenarios with high requirements for power supply continuity, with efficient and accurate operation; second, manual grid connection. Observe the parameters through a synchroscope and synchronization indicator lights, manually adjust the speed of the diesel engine and the excitation current, and manually close the switch after the parameters are matched. It is suitable for small-scale units, requiring operators to have rich experience and be able to accurately judge the synchronization point.

It should be noted that manual grid connection can use the dark lamp method or the light rotation method to judge the synchronization state: the dark lamp method judges the voltage difference through the on-off of the bulb, and the light rotation method judges the frequency difference and phase sequence through the rotation speed and direction of the light. If the lights turn on and off at the same time, it indicates a phase sequence error, and the wiring must be adjusted before trying again.

(2) Strictly Implement Closing Operation Specifications

When the synchronization device shows that the parameters are completely matched (the voltage difference, frequency difference, and phase difference are all within the allowable range), the closing operation can be carried out: the grid-connected circuit breaker must be operated slowly during closing to avoid inrush current caused by too fast operation; immediately observe the unit operation status after closing, including parameters such as current, voltage, frequency, and power, and confirm that there is no abnormality. If there are problems such as excessive inrush current and abnormal noise of the unit, the switch must be opened immediately, and the grid connection can be tried again after troubleshooting.

Special reminder: Forcible closing when parameters are not matched is strictly prohibited. This asynchronous grid connection is the most dangerous operation, which will generate huge mechanical torque and inrush current, possibly leading to damage to the generator winding, explosion of the circuit breaker, and even affecting the municipal power grid, expanding the fault range.

(3) Smoothly Complete Load Transfer

After successful closing, the entire load cannot be transferred to the generator set immediately. The load must be increased gradually: first, run with a light load for a period of time, observe whether the unit parameters are stable, and then gradually increase the load to the preset proportion to avoid frequency oscillation and unit flameout caused by sudden load addition. At the same time, the load distribution should be adjusted through the power management system to ensure that the generator set and municipal power share the load reasonably. If multiple generator sets are connected to the grid, it is necessary to ensure that the active power and reactive power are evenly distributed, avoiding the situation where one unit is overloaded and another is underloaded, and preventing reactive power circulation from burning equipment.

III. After Grid Connection: Strengthen Safety Monitoring and Ensure Stable Operation

Successful grid connection is not the end. Subsequent real-time monitoring and safety protection are the keys to ensuring long-term stable operation. The following points should be focused on:

(1) Real-Time Monitoring of Operation Parameters

Arrange special personnel on duty to real-time monitor the operation parameters of the generator set and municipal power, including voltage, frequency, current, power, phase, water temperature, oil temperature, etc., and keep operation records. If abnormal parameters are found (such as frequency fluctuation exceeding the allowable range, sudden rise and fall of voltage, reverse power, etc.), measures must be taken immediately. If necessary, cut off the grid-connected switch, disconnect the generator set from the grid, and reconnect to the grid after troubleshooting. Among them, reverse power protection is the top priority. When the power grid transmits power back to the generator set (the generator set becomes an electric motor), the reverse power relay must act immediately to cut off the circuit and prevent the diesel engine from being dragged and damaged.

(2) Do a Good Job in Electrical Isolation and Grounding Protection

When the generator set is connected to the municipal power grid, a reliable electrical isolation device must be set up, and an interlocking device must be installed to prevent incorrect grid connection. At the same time, the metal shell, bracket, etc. of the generator set should adopt a common grounding method, and the grounding body should be correctly and firmly connected, with an insulation resistance value greater than 0.5MΩ, to avoid electric shock accidents or equipment damage caused by poor grounding. In addition, it is strictly prohibited to randomly parallel the generator set power supply with the municipal power, and unauthorized grid connection is not allowed without setting an interlocking device.

(3) Standardize the Disconnection Operation

When it is necessary to shut down or the municipal power returns to normal, the load of the generator set must be smoothly transferred back to the municipal power first. After the unit load is close to zero (or a very small reverse power), the opening command is issued to disconnect the generator set from the grid. After disconnection, the generator set should run at no load for a period of time and then shut down after cooling to avoid damage to the equipment caused by high-temperature shutdown. If the municipal power returns to normal, it is necessary to ensure that the unit automatically switches to municipal power supply, and the unit automatically exits work and shuts down with a delay, complying with the requirements of GB51348-2019 standard.

IV. Daily Operation and Maintenance: Regular Inspection and Maintenance to Reduce Fault Probability

Daily operation and maintenance is the foundation for ensuring grid connection safety. A complete maintenance system must be established to regularly inspect, maintain, and test the equipment. The specific points are as follows:

• Regular Trial Operation: Conduct a no-load trial operation of the generator set once a month, each time for no less than 15 minutes, to check the unit start-up performance, instrument sensitivity, and equipment operation status; conduct a load test once a quarter to simulate the grid connection scenario and verify the effectiveness of the synchronization device and protection device.
• Equipment Maintenance: Regularly replace diesel oil, engine oil, coolant, and air filters, and check for leaks in fuel pipelines and cooling systems; regularly calibrate equipment such as synchroscopes, voltage regulators, and protection relays to ensure accurate parameters; in cold areas, heating should be provided for the computer room to ensure that the unit can start smoothly in an emergency.
• Fault Troubleshooting: Establish a fault account, timely record and troubleshoot problems occurring during grid connection (such as grid connection failure, uneven power distribution, excessive circulation, etc.), analyze the cause of the fault, and avoid the recurrence of similar problems. For example, if there is uneven power distribution, it may be due to stuck governor or wrong wiring of current transformer, which needs to be repaired and adjusted in time.
• Personnel Training: Operators must receive professional training, be familiar with grid connection operation procedures, equipment performance, and safety precautions, and can only take up the job after passing the assessment. Unauthorized operation is strictly prohibited; organize emergency drills regularly to improve the operators' ability to handle sudden faults.

V. Summary: Respect Specifications and Achieve Safe Grid Connection

The grid connection of diesel generator sets with municipal power is a precise operation that "a miss is as good as a mile". It not only needs to meet strict technical conditions but also follow standardized operation procedures, and attach great importance to daily operation and maintenance and safety protection. From the qualification handling and parameter verification before grid connection, to the standardized operation and load transfer during grid connection, and then to the real-time monitoring and disconnection processing after grid connection, every link cannot be careless.

It should be particularly emphasized that non-professional personnel are strictly prohibited from operating grid-connected equipment. Unauthorized grid connection and irregular operation will not only damage the equipment but also cause power grid failures and personal safety accidents. For ordinary backup power users, if long-term grid connection is not required, the standard "municipal power-generation" automatic transfer switch (ATS) is a safer, more economical, and compliant choice.

Only by respecting technical specifications, strictly implementing operation procedures, and doing a good job in daily operation and maintenance can we give full play to the advantages of grid connection of diesel generator sets with municipal power, achieve safe, stable, and efficient power supply, and escort production and life.