Safe and stable parallel operation of
diesel generator sets and the grid requires meeting a series of stringent conditions, which can be categorized into three main categories: synchronization (the most crucial requirement), protection and control system requirements, and unit-specific requirements.
The following is a detailed description of these conditions:
Synchronization (synchronization)
This is a critical requirement for successful grid connection. When closing the grid-connecting switch (circuit breaker), the AC characteristics of the diesel generator set output must be highly consistent with those at the connection point (synchronization point) to avoid significant surge current and torque that could damage equipment or trigger protective measures. These conditions include:
1. Identical or similar voltage amplitudes.
As required, the three-phase voltages (line voltage or phase voltage) output by the generator must be very close to the voltage amplitudes (dimensions) of the corresponding phases of the grid.
Tolerance: The voltage difference is typically controlled within a range of ±0.5% to ±5% of the rated voltage (the specific standard depends on system requirements; the more accurate, the better). For example, for a 380V system, the voltage difference should be between ±1.9V and ±19V. Regulation Method: Precisely control the output voltage by adjusting the generator set's automatic voltage regulator.
2. Similar or Close Frequency:
The AC power frequency output by the generator must be exactly the same as, or very close to, the grid frequency.
Tolerance: The frequency difference is typically required to be within the tolerance range of ±0.05 Hz to ±0.2 Hz (again, the more accurate the better). For example, the frequency of a 50 Hz grid should be maintained between 49.95 Hz and 50.05 Hz (±0.1 Hz).
Regulation Method: Accurately control the output frequency by adjusting the generator set's speed regulator (which controls engine speed).
3. Phase Alignment or Minimal Phase Difference:
The AC power output by the generator must be perfectly aligned with the grid phase (zero phase difference) at the moment of grid connection.
Tolerance: This is the most critical and difficult condition to control. Phase difference typically needs to be managed within a controllable range of ±10 degrees, with high-precision systems requiring ±5 degrees or even less. Adjustment Method: By fine-tuning the generator speed (i.e., frequency), the circuit breaker is closed at the optimal time when the phase difference approaches zero, causing the generator voltage phase to rotate slowly relative to the grid voltage phase (slip). This is typically accomplished using an automatic synchronization device.
4. Identical Sequence:
The generator output voltage phase sequence must completely match the grid voltage phase sequence (usually positive sequence: A-B-C). This is a fundamental requirement that must be ensured during system design and wiring. This should typically be verified using a phase sequence meter after initial installation or maintenance. If the phase sequence is incorrect, grid connection is absolutely prohibited!
5. Identical Waveform:
The generator output voltage waveform (sine wave) should be as consistent as possible with the grid waveform, and the total distortion (THD) should meet relevant standards (typically low requirements, such as less than 5%). Modern generators generally meet this requirement.
II. Status Management of Protection and Control Systems
1. Synchronization Device:
A reliable manual synchronization device or (preferably) an automatic synchronization device is required. An automatic synchronization device continuously monitors the voltage, frequency, and phase difference mentioned above and automatically issues a shutdown command at the optimal time, significantly improving the success rate and safety of grid connection.
2. Protection System:
Reverse power protection: This is one of the most important protections during grid-connected operation. When the generator not only does not output power but also draws power from the grid (becoming a motor), the protection must trip quickly to prevent damage to the diesel engine due to "reverse drag" and unnecessary fuel consumption.
Overcurrent/short-circuit protection: Protects the generator and system from short-circuit fault currents.
Overvoltage/undervoltage protection: Protects equipment from abnormal voltages.
Overfrequency/underfrequency protection: Protects equipment from abnormal frequencies (During grid-connected operation, these protection settings must be coordinated with grid-side protection to avoid false tripping).
Differential protection (optional, more important for large-capacity units): Provides fast and sensitive protection against internal faults in the generator windings.
Ground fault protection: Detects and handles ground faults.
Asynchronous parallel protection: Forces shutdown or tripping when a failure of the automatic synchronization device or manual error causes severe differential period errors.
3. Control and Monitoring System:
Speed Control System: This system stably and accurately adjusts the engine speed (thus controlling the frequency) while also providing necessary speed regulation features (such as droop control) to ensure optimal distribution of active power between the grid and other paralleled units.
Automatic Voltage Regulation System: This system stably and accurately adjusts the generator excitation (controlling the voltage) while providing the necessary voltage regulation features to ensure optimal reactive power distribution with the grid.
Power Management System (optional, important for complex systems): For systems with multiple units in parallel or requiring precise load distribution/shedding, a more advanced power management system is required for coordinated control.
Comprehensive Monitoring Instruments: This system provides real-time display of key parameters such as voltage, current, frequency, power (active, reactive, and visual), power factor, and speed, as well as the status of synchronization devices (voltage difference, frequency difference, and phase difference).
4. Reliable Grid-Connecting Switch (Circuit Breaker):
A circuit breaker with appropriate capacity, reliable performance, and sufficient switching capacity should be used as the grid-connecting switch. The circuit breaker must accurately and quickly execute switching commands.
III. Generator and System Conditions
1. Generator unit operating conditions are good.
The diesel generator set has completed startup and warm-up, and parameters such as engine water temperature, oil pressure, and oil temperature are within normal operating ranges.
The generator excitation system has established voltage normally.
There are no generator alarms or fault signals.
2. Relationship between unit capacity and grid capacity:
After integration into the larger grid, the grid capacity far exceeds that of a single diesel generator set, and the grid voltage and frequency can be considered stable (similar to an infinite grid). The diesel generator set primarily needs to meet synchronization requirements, and its power output has little impact on the grid frequency and voltage. Adjustments are primarily dependent on its own speed governor and AVR.
For small or island grids: If the diesel generator set accounts for a high capacity proportion, more sophisticated power management strategies, such as multi-unit droop control, are required to jointly maintain grid frequency and voltage stability. In this case, synchronization requirements may be more stringent, and adjustments may be more complex.
3. Correct system wiring and switching status:
The connecting lines between the generator and the grid are intact and correctly wired (especially the phase sequence!). Relevant circuit breakers and disconnectors are in the correct open/closed positions (e.g., generator output circuit breaker open, synchro switch engaged, grid-side circuit breaker closed, etc.).
4. Compliance with Standards and Regulations:
The design, installation, and commissioning of the entire grid-connected system (generator, protection, control, wiring, etc.) must comply with relevant national, industry, or enterprise electrical regulations and standards (e.g., IEC, IEEE, GB, etc.).
Key Points:
Synchronization is crucial: Voltage, frequency, phase, and phase sequence must be identical.
Protection is the bottom line: Key protections such as reverse power protection must be reliably engaged and correctly set.
Control Assurance: The speed governor, speed regulator, AVR, and other devices must function properly simultaneously.
Genome Condition: The generator meets standards and operates smoothly.
System Matching: Consider the relationship between unit capacity and the grid and implement appropriate control strategies.
Operational Specifications: Operators must receive training and strictly follow operating procedures.
Important Note: Paralleling a diesel generator set with the grid is a highly specialized operation, involving significant risks such as high voltage, high current, and asynchronous paralleling. Qualified and professionally trained electrical personnel must fully understand and meet all parallel connection requirements and strictly follow the operating procedures. For first-time grid connection or after major system changes, it is recommended to perform the operation under the guidance of experienced personnel.
