In the grid-connected operation of high-voltage diesel generator sets, the rationality of reactive power distribution is directly related to unit stability, power grid safety and equipment service life. As an enterprise focusing on power equipment operation and maintenance and technical services, we combine on-site practical experience to comprehensively analyze the core issues, common faults and solutions of reactive power distribution for grid-connected high-voltage (10.5kV/6.3kV) diesel generator sets, providing practical reference for industry partners.
I. Core Principles: Key Premises for Reactive Power Distribution
Compared with low-voltage units, the core logic of reactive power distribution for grid-connected high-voltage diesel generator sets is the same, but the requirements for parameter matching and insulation protection are more stringent. Its core principles can be summarized into three points: consistent AVR Droop, matched excitation reference, and in-place circulating current suppression. Once these three principles are violated, problems such as reactive power imbalance, excessive circulating current, voltage oscillation, and even AVR device or unit overheating and tripping are likely to occur, seriously affecting the stability of the grid-connected system.
In terms of principle, reactive power Q is determined by excitation current and terminal voltage, and realizes decoupled control with active power (controlled by the governor). When a single unit is in operation, an increase in excitation current will increase the terminal voltage, which in turn increases the reactive power and decreases the power factor; when multiple units are grid-connected, the system voltage is unique, and each unit needs to distribute reactive power according to the Q–V droop characteristic (droop). The core formula is (where is the no-load voltage setting, is the droop coefficient, and is the reactive power of the unit itself).
The three key conditions to ensure stable grid connection are: all units must be set with positive droop (, conventional range 2%–5%), and direct parallel operation with no droop or negative droop is prohibited; the droop coefficients of each unit must be consistent (the same slope for units of the same capacity, and matching in inverse proportion to capacity for units of different capacities); the no-load voltage must be calibrated consistently to avoid inherent circulating current.
II. Unique Difficulties and Risk Tips for High-voltage Grid Connection
In addition to the common problems of low-voltage units, the reactive power distribution of grid-connected high-voltage diesel generator sets (10.5kV/6.3kV) has the following unique difficulties that need to be focused on:
1. Strict Requirements for Insulation and Voltage Withstand
The insulation level of high-voltage excitation systems, AVR devices, PT (Potential Transformers), CT (Current Transformers) and connecting cables must match the high-voltage environment; otherwise, problems such as creepage, insulation breakdown and equipment misoperation are likely to occur. It is particularly important to note that the harm of reactive power circulating current on the high-voltage side is much greater than that on the low-voltage side. Excessive circulating current will increase the stator current and cause insulation overheating, which in turn leads to serious faults such as inter-turn short circuit and winding burnout.
2. PT/CT Accuracy and Wiring Cannot Be Ignored
Errors in the transformation ratio, polarity and phase sequence of PT and CT will lead to AVR sampling distortion, which in turn causes excitation regulation disorder, and ultimately results in serious imbalance of reactive power distribution and voltage oscillation. At the same time, the secondary circuit of CT on the high-voltage side is strictly prohibited from opening, otherwise it will generate thousands of volts of overvoltage, directly damaging the AVR and control circuit equipment.
3. AVR Droop Mismatch is a Common Hidden Danger
AVR droop coefficient mismatch is the most common cause of uneven reactive power distribution in high-voltage grid connection: if the difference of droop coefficients between units of the same capacity exceeds 0.5%, the reactive power distribution error will exceed 10%; if units of different capacities do not set the droop coefficient in inverse proportion to the capacity, the large unit will be underloaded and the small unit will be overloaded with reactive power. Due to the larger excitation current of high-voltage units, the circulating current and equipment heating problems caused by droop mismatch will be more prominent.
4. Excitation System Differences and Grid Connection Risks with Municipal Power
If brushless excitation and brushed excitation, phase compound excitation and controllable excitation are mixed in grid-connected units, it will lead to inconsistent external characteristics of the units, causing reactive power distribution drift and voltage instability; differences in the impedance of the excitation windings of high-voltage units will also cause uneven excitation current, which in turn leads to reactive power imbalance. In addition, when grid-connected with municipal power (large power grid, non-droop characteristic), the diesel generator set must be set with a positive droop of 3%–5%, otherwise it will be "pulled out of balance" by the power grid, resulting in problems such as reactive power backfeeding, AVR saturation and unit tripping; insufficient synchronization accuracy of voltage, frequency and phase before grid connection will also cause excitation system disturbance, leading to reactive power distribution imbalance.
III. Common Fault Phenomena and Rapid Troubleshooting Directions
In on-site operation, the following fault phenomena can be used to quickly locate reactive power distribution problems and improve troubleshooting efficiency:
- Phenomenon 1: One unit has large reactive power and low power factor (e.g., 0.7), while the other unit has small reactive power and high power factor (e.g., 0.95) — Core cause: Inconsistent AVR droop slope and unequal no-load voltage settings.
- Phenomenon 2: Periodic voltage oscillation and back-and-forth reactive power drift after grid connection — Core cause: Droop coefficient close to zero (no droop), negative droop, or unstable excitation system.
- Phenomenon 3: Frequent tripping of high-voltage switches, excessive stator temperature, and AVR overheating alarm — Core cause: Excessive reactive power circulating current, reactive power overload of a single unit, or PT/CT failure.
- Phenomenon 4: After grid connection with municipal power, the reactive power of the diesel generator set is negative (absorbing reactive power) and the power factor is leading — Core cause: The voltage setting of the diesel generator set is lower than the grid voltage, the droop is too small, or the excitation is insufficient.
IV. On-site Practical Solutions
Aiming at the problem of reactive power distribution for grid-connected high-voltage diesel generator sets, combined with on-site practical experience, we can start from three dimensions: pre-grid connection calibration, post-grid connection fine-tuning, and high-voltage-specific governance to ensure reasonable reactive power distribution and stable system operation.
1. Pre-grid Connection: Conduct Parameter Consistency Calibration
Parameter calibration before grid connection is the basis for avoiding reactive power distribution problems. Three key points need to be focused on: first, AVR droop setting. The droop coefficient of units with the same capacity is controlled at 2%–5% (conventional 4%), and all units are completely consistent; for units with different capacities, the droop coefficient is set in inverse proportion to the capacity (). For example, a 1000kVA unit is set to 4%, and a 500kVA unit is set to 8%. Second, no-load voltage calibration. The secondary voltage of PT on the high-voltage side is unified (e.g., 100V), and the deviation of AVR no-load voltage is controlled within ±0.5%. Third, PT/CT inspection. Check whether the transformation ratio, polarity and phase sequence are correct, ensure reliable grounding of the secondary circuit, and strictly prohibit CT secondary circuit opening.
2. Post-grid Connection: Precisely Fine-tune Reactive Power Distribution
After grid connection, the principle of "stabilizing active power first, then adjusting reactive power" should be followed to gradually optimize reactive power distribution: first, observe the reactive power meter, power factor meter and voltage meter data of each unit; if a unit has high reactive power (low power factor), the excitation of the unit can be reduced (lower AVR given value); if the reactive power is low (high power factor), the excitation of the unit can be increased. The ultimate goal is to realize reactive power distribution in proportion to capacity, with the distribution error controlled within ±10% (in line with GB/T 2820 standard), voltage deviation ≤±5%, and power factor maintained at 0.8–0.9 lagging. If conditions permit, the AVR automatic load distribution function (equalizing line/circulating current compensation) can be turned on. For high-voltage units, DC equalizing lines (of the same model) or reactive power droop control are preferred to improve adjustment accuracy.
3. High-voltage-specific Governance: Strengthen Protection and Insulation
According to the characteristics of high-voltage units, additional measures for circulating current suppression and insulation enhancement are required: install a high-voltage side circulating current monitoring and protection device, which will realize delayed alarm or tripping when the circulating current exceeds the standard (exceeding 5% of rated current) to avoid equipment damage; high-voltage excitation circuits, AVR devices and connecting cables adopt insulation grade F or above, and withstand voltage tests are carried out regularly to timely check insulation hidden dangers; high-voltage diesel generator sets at the same site should try to adopt the same excitation mode and AVR model to avoid inconsistent external characteristics caused by mixing.
V. Standard Limits and Enterprise Suggestions
According to the national standard GB/T 2820, the reactive power distribution of grid-connected high-voltage diesel generator sets must meet the following limits: reactive power distribution error, ≤±10% for units of the same capacity, ≤±10% for large units and ≤±20% for small units of different capacities; voltage regulation rate (droop) is controlled at 2%–5% (positive droop), and direct parallel operation with no droop or negative droop is prohibited; circulating current ≤5% of rated current, which should be strictly controlled for high-voltage units.
Combined with years of industry experience, we suggest that enterprises strictly follow the principles of "pre-grid connection calibration, post-grid connection monitoring and regular maintenance" when high-voltage diesel generator sets are in grid-connected operation: focus on calibrating droop coefficient, no-load voltage and PT/CT parameters before grid connection; real-time monitor reactive power distribution, circulating current and equipment temperature after grid connection; regularly detect and maintain the excitation system and insulation performance to avoid reactive power distribution-related faults from the source and ensure the stable operation of the unit and power grid.
If you encounter specific problems in the reactive power distribution of grid-connected high-voltage diesel generator sets, you can contact our technical team, and we will provide one-on-one on-site guidance and solutions.
Post time: Apr-28-2026
