When it comes to synchronous generator operation, terms like lagging, leading, and compensating mode are often used by electrical engineers. These concepts are crucial in optimizing generator performance and maintaining grid stability, especially in large-scale power systems.
Whether you’re working with a diesel generator for industrial use, utility-scale systems, or exploring advanced grid compensation techniques, understanding these three modes is essential.
1. What is Lagging Power Factor Operation?
In lagging operation (also known as overexcited mode), the generator supplies both active (real) power and reactive (inductive) power to the grid. The generator’s excitation system is in an overexcited state, and the power factor is positive.
This is the default or normal mode of generator operation. It means the stator current lags behind the terminal voltage — a standard condition for supplying both real and reactive power to inductive loads in the grid.
✅ Common in industrial diesel generators and backup power systems.
2. What is Leading Power Factor Operation?
Leading operation occurs when the excitation current of the generator is reduced, causing the internal generated voltage to drop. The power factor angle becomes leading, and the generator still supplies real power but begins to absorb reactive power from the grid.
This is useful in scenarios where the grid voltage is too high due to excess reactive power. By shifting the generator to a leading mode, it helps reduce system voltage, maintaining operational stability.
However, prolonged operation in this mode can reduce generator stability, increase the power angle, and potentially lead to step-out (loss of synchronism). Therefore, leading mode is functional but requires careful control.
⚠️ Use with caution in high-voltage systems or parallel generator setups.
3. What is Compensating (Synchronous Condenser) Operation?
In compensating operation, the generator does not supply active power, but works only to regulate voltage by supplying or absorbing reactive power. This is also known as running the generator in synchronous condenser mode.
In overexcited compensating mode: the generator supplies inductive (lagging) reactive power.
In underexcited mode: it supplies capacitive (leading) reactive power.
This mode is typically used when a generator acts as a grid voltage regulator, especially in large power systems where reactive power support is needed without real power generation.
🔌 Ideal for voltage control and reactive power balancing.
4. Key Differences Between the Three Modes
| Mode | Active Power (P) | Reactive Power (Q) | Power Factor | Application |
| Lagging (Overexcited) | Supplied | Supplied | Positive | Standard operation |
| Leading (Underexcited) | Supplied | Absorbed | Leading | Grid voltage reduction |
| Compensating (No load) | Not supplied | Supplied/Absorbed | Leading or Lagging | Grid stabilization |
5. Generator Operation and Life Analogy
Interestingly, one technical author humorously compared generator modes to life stages:
1st Quadrant (Lagging): You have a job (active power) and love (reactive power).
2nd Quadrant (Compensating): Only love, no job — emotionally fulfilling, financially unstable.
3rd Quadrant (Idle): No job, no love.
4th Quadrant (Leading): You have a job, no love — stable but emotionally lacking.
Just as a generator can sustain leading mode longer than compensating mode, a person can survive longer with a job but no relationship than the other way around.
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