The Compatibility Between UPS Systems and Generator Sets

In modern power systems, the compatibility between uninterruptible power supply systems (UPS) and generators is receiving increasing attention. One major concern is the current harmonics generated by rectifiers, which can significantly impact voltage regulators and the synchronization circuits of UPS systems. To address this issue, UPS system engineers have designed and implemented input filters that successfully control current harmonics in UPS applications, thereby improving compatibility with generators.

Role and Design of Input Filters

Input filters play a crucial role in UPS systems. They use a combination of capacitors and inductors to absorb the most damaging current harmonics at the UPS input. The design of these filters takes into account not only the inherent harmonic distortion of the UPS circuit but also the maximum possible harmonic percentage under full load. Additionally, these filters enhance the input power factor of the UPS during load operation, improving the overall efficiency of the power system.

1. Impact of Current Harmonics

Current harmonics can adversely affect not only the performance of the UPS but also the voltage stability of the generator. Harmonic currents produced by rectifiers can cause voltage fluctuations, which may disrupt the normal operation of the generator.

2. Design Considerations

When designing input filters, it is essential to consider the frequency, amplitude, and impact of current harmonics on the system. By selecting appropriate capacitor and inductor parameters, engineers can maintain effective harmonic suppression while minimizing the impact on system efficiency. However, the use of input filters can also introduce some downsides, typically consuming about 1% of the UPS power and thus reducing overall efficiency. Recent design improvements have focused on reducing filter power consumption. By applying insulated gate bipolar transistor (IGBT) technology in UPS designs, filter efficiency has significantly improved, allowing them to absorb current harmonics while consuming less active power, thereby reducing the size and enhancing the efficiency of the UPS.

Power Factor Issues at No Load

While input filters perform well under full load, they can experience low power factor issues when the system is at no load or light load. Specifically, filters designed to meet a maximum current distortion of 5% may lead to noticeably lower UPS input power factors when the load is below 25%. In some cases, the no-load power factor of new systems can drop below 2%, approaching an ideal capacitive load.
Although this situation has little effect on the UPS output and critical loads, it poses potential risks for the generator. Experienced generator engineers understand that connecting a generator to a large capacitive load can lead to system instability and generator shutdown. This risk is particularly pronounced when the UPS system connects to the generator’s output load first after a power outage, potentially causing serious accidents and impacting the operation of critical equipment.

Working Principle of Generators

The operation of a generator relies on a voltage regulator to control output voltage. The regulator detects the three-phase output voltage, compares it to a set voltage value, and adjusts the generator’s rotating magnetic field to maintain stable voltage output.

1. Voltage Regulation Mechanism

The electromotive force (EMF) within the generator is produced by the rotor excitation coil. The voltage regulator automatically adjusts the strength of the rotor magnetic field based on the difference between the output voltage and the target voltage, ensuring stable voltage output.

2. Load Characteristics Impact on Generators

When the load is purely inductive, the current lags behind the voltage, allowing the generator to operate normally. However, with purely capacitive loads, the current leads the voltage, potentially causing output voltage to rise excessively due to internal resistance voltage drop opposing the output voltage. In such cases, the voltage regulator may struggle to manage the excessive voltage, leading to overvoltage conditions or generator shutdown.

Common Solutions

To address the compatibility issues between UPS systems and generators, power factor correction becomes necessary. Here are several possible solutions.

1. Install Automatic Transfer Switches

Prioritize connecting motor loads to ensure they are connected to the generator before the UPS. This requires transfer switches to effectively manage the sequence of load connections to avoid instability under high capacitive loads.

2. Add Reactive Reactance

Install parallel-wound reactors on the generator output to compensate for capacitive loads. This method is simple and cost-effective but may continuously absorb current, affecting the load’s power factor.

3. Install Inductive Reactors in Each UPS

This method can provide more precise compensation for UPS capacitance but increases installation complexity and costs. Properly configured contactors can control the inductors during low load conditions.

4. Install Contactors Before Filtering Capacitors

Disconnecting capacitors during low load can help reduce power factor issues. This approach requires precise factory tuning to ensure system stability.

5. Regular Maintenance and Tuning

It is advisable to regularly maintain and tune both the UPS and generators to ensure compatibility and stability under different load conditions.

Conclusion

When selecting the best solution, it's essential to consider the actual site conditions and equipment performance to ensure efficient compatibility between UPS systems and generators. Through thoughtful design and improvements, it is possible to effectively resolve compatibility issues, maintain stable operation of the power system, and protect critical loads, ultimately enhancing the reliability and safety of the entire system.