Capacitors are generally not connected directly to Diesel Generators (DG) because of the following reasons:
1. Voltage Fluctuations and Instability:
- Diesel generators produce varying levels of voltage and frequency, especially during load changes or when starting or stopping. Capacitors are designed to work with stable voltage conditions, and connecting them to an unstable source like a DG could lead to over- or under-compensation of reactive power.
- These voltage fluctuations can cause the capacitors to absorb or supply too much reactive power, which may lead to a phenomenon called "overcompensation," potentially causing voltage instability or surges that can damage sensitive equipment.
2. Leading Power Factor Risks:
- Capacitors improve power factor by supplying reactive power. In some cases, if the capacitors supply too much reactive power to the generator, it may cause a "leading" power factor, which is when the current leads the voltage.
- Diesel generators are generally designed to operate with a "lagging" or near-unity power factor. A leading power factor can cause the generator to become unstable and may even trip the generator's protective relays. This can also cause overheating in the generator's winding, leading to damage.
3. Automatic Voltage Regulator (AVR) Interference:
- Diesel generators are often equipped with Automatic Voltage Regulators (AVRs) to maintain voltage output within a certain range. Capacitors can interfere with the AVR’s ability to regulate the voltage, causing further instability.
- If a capacitor bank is used with a generator, it can confuse the AVR's readings, leading to incorrect adjustments and voltage regulation issues.
4. High Harmonics Generation:
- Generators, especially under partial or fluctuating loads, can generate harmonics, which are unwanted frequencies that distort the electrical waveform. Capacitors can amplify these harmonics when they are present, potentially causing resonance in the electrical system.
- Resonance can lead to increased currents through the capacitors, causing them to overheat and fail prematurely.
5. Size and Design of DG Systems:
- Diesel generators are often sized to handle both active and reactive power loads of a facility. Hence, the need for capacitors to correct power factor is less compared to a standard grid power system where capacitors are used to reduce the reactive power demand from the utility.
- In situations where power factor correction is needed in systems with diesel generators, it’s typically managed with proper generator sizing or by using Variable Frequency Drives (VFDs) that already include power factor correction.
Alternative Approach:
- In some cases, capacitors can be connected to the load side rather than directly with the DG. For example, Automatic Power Factor Correction (APFC) panels can be used downstream from the generator to improve power factor without directly interfering with the generator's operation.
- In larger systems where power factor correction is critical, reactors or filters might be used in combination with capacitors to control harmonic resonance.
For these reasons, it is not generally recommended to connect capacitors directly with diesel generators. Instead, careful planning and analysis are required to manage reactive power and power factor in such systems.