Guidelines for Operating Diesel Generator Sets in Parallel

Diesel generator sets, as a significant source of emergency power, are widely used in these power-critical places. Although the use of single generator sets is widely accepted, many organizations and operators still find themselves unfamiliar with or face technical challenges when it comes to running multiple diesel generator sets in parallel. This article provides a detailed exploration of the operations and commissioning methods for parallel operation of diesel generator sets, aimed at helping operators better understand and implement this technical solution.

 

Diesel generator sets, as emergency power sources, are typically installed in units and locations that require backup power. While they are not used frequently, they must be ready for standby to handle unexpected power interruptions. However, as electricity consumption grows, many organizations are gradually expanding from using a single generator set to multiple sets with different models and power ratings.

In such cases, issues related to load distribution, unit coordination, and power supply stability become pressing concerns. A single diesel generator set may struggle to maintain stable operation under large load changes, leading to inefficiencies where its load capacity is not fully utilized, often resulting in problems such as "oversized engine for a small load" or "undersized engine for a large load." This leads to frequency fluctuations, voltage instability, and ultimately affects the quality of the power supply. Additionally, the efficiency of a single generator set is lower, leading to greater fuel consumption and an inability to fully utilize the generator's potential, which causes unnecessary energy waste.

To address these issues, the most effective solution is to operate multiple diesel generator sets in parallel. Parallel operation allows for the flexible operation of the required number of sets based on the actual electricity demand, preventing overload on individual generators and ensuring more efficient use. It also allows each generator to operate near its full load, thereby improving overall efficiency and reducing fuel consumption. Furthermore, through parallel operation, multiple sets can share the load, stabilize the grid, reduce voltage and frequency fluctuations, and improve the quality of the power supply.

 

When operating diesel generator sets in parallel, there are several technical requirements and considerations that must be addressed to ensure smooth and efficient operation. The following are key technical points for parallel operation.

 

One of the basic requirements when paralleling generator sets is that their phase sequence must be consistent. If the phase sequence is mismatched, the generators will not synchronize properly and may even cause damage or operational failure. A simple method to check the phase sequence is by using a small asynchronous motor. Connect the motor to the two power sources and observe whether its direction of rotation is consistent. If the rotation direction matches, the phase sequence is correct. If not, adjust the phase sequence of one of the sources to ensure synchronization.

 

The voltage of the generator sets operating in parallel must be close to the grid voltage, and the voltage difference should be minimized. Typically, the generator's voltage should be adjusted to around 400V to ensure that the voltage difference between the two generators is as small as possible, thereby avoiding power supply instability caused by voltage fluctuations.

 

In addition to voltage matching, the phases of the parallel generator sets must also be synchronized. If the phase difference is too large, it can cause severe current fluctuations, increasing system load and potentially leading to a failure in synchronization. Therefore, it is essential to ensure that the generator's phase is aligned with the grid's phase. During synchronization, synchronization meters or light indicators can be used to help determine the phase alignment. Once synchronization is achieved, operators can quickly close the parallel switch and complete the grid connection.

 

To ensure stable parallel operation, the speed regulation performance of the diesel generator sets is critical. The speed controllers on the diesel generators need to be full-speed controllers, with quick and sensitive response times that adjust the speed rapidly in response to load changes. The adjustment range of the speed controller should generally be between 2% to 3% to avoid excessive speed changes that could destabilize the system. Additionally, the speed characteristics of the different generator sets should be as similar as possible to ensure coordinated operation once paralleled.

 

Matching the frequency of the generator sets with the grid frequency is equally important. The frequency of the parallel generator sets must be close to the grid frequency, typically within a range of ±3% to ±5%. If the frequency difference is too large, it may lead to synchronization failure or affect the stability of the power supply. Before paralleling, operators must adjust the frequency to ensure that the generator's frequency matches the grid frequency.

 

The key to successful parallel operation is precise commissioning and synchronization, ensuring that the sets operate smoothly and efficiently. Below are the detailed operational steps for parallel operation.

 

Before paralleling, disconnect the main circuit breaker and balancing line switch to ensure the sets operate independently. Start the diesel generator sets and check oil pressure, coolant temperature, and other operational parameters to ensure that the equipment is functioning normally. Once the generator set is operating stably, gradually increase the speed to the synchronization speed of 1500 rpm. At this point, the frequency meter should read 50Hz to ensure that the generator has reached the synchronization speed.

 

Adjust the generator's voltage by tuning the field resistance to bring the voltage closer to the grid voltage. Typically, the generator's voltage should be adjusted to around 400V. If the voltage difference is significant, operators can adjust the resistance value to raise or lower the voltage to make the two generators' voltages as close as possible.

 

Synchronization is one of the key steps in parallel operation. Operators can choose either manual or automatic synchronization methods. During the process, synchronization meters or light indicators can be used to determine if the two generators are synchronized. When synchronization is achieved, quickly close the parallel switch to complete the grid connection.

 

Once the generator set is started, do not immediately apply excitation. First, adjust the field resistance to the generator's no-load voltage position. As the generator approaches the rated speed, the frequency difference between the system and the generator should be kept within ±3% to ±5%. At this point, close the parallel switch and quickly engage the excitation system to synchronize the generator with the grid and complete the parallel operation.

 

There are several key precautions to be aware of during parallel operation to ensure stable operation of the system.

 

To ensure smooth parallel operation, it is recommended to start the larger units first, leaving the smaller units as standby machines. The larger units should be loaded with a portion of the load before paralleling to avoid excessive load fluctuations that may lead to synchronization failure.

 

During parallel operation, operators must monitor both active and reactive power outputs to ensure they remain within reasonable ranges. In the case of over-excitation or under-excitation, operators can adjust the throttle or excitation resistance to control the output power. If adjustment is difficult, the speed controller of the fuel injection pump may need to be checked, especially for older sets, which may require readjusting the injection pump's speed spring.

 

During parallel operation, operators must pay attention to how load changes affect the sets. If the load varies significantly, it may lead to desynchronization or overload. To avoid this, generator sets should be equipped with damping windings to reduce the impact of load fluctuations on system stability. Additionally, operators should monitor the flywheel inertia effect to ensure that the natural oscillation frequency of the set aligns with the forced impact frequency of the prime mover.

 

During the parallel operation of diesel generator sets, operators may encounter technical problems that, if not handled properly, could lead to equipment failure or power supply interruptions.

 

If the generator's speed cannot stabilize at 1500 rpm, the speed regulator may be malfunctioning. This is especially true for older sets, where the speed spring may have weakened due to long-term use, affecting the speed regulation accuracy. In such cases, the speed controller of the fuel injection pump needs to be readjusted to ensure stable operation at the rated speed.

 

If generators fail to synchronize during the parallel process, it is often because the generator's flywheel inertia is insufficient, causing the generator to fail to synchronize with the grid. In this case, damping windings can be added, or the electrical system's synchronization parameters should be checked to ensure stable operation of the two sets.

 

The parallel operation of diesel generator sets is an essential technique for improving power supply stability and operational efficiency. Through proper operation and commissioning, it is possible to fully utilize the potential of each generator, reduce energy waste, and significantly enhance the overall stability and reliability of the power system. Successful parallel operation requires operators to have a solid understanding of the generator sets' operating principles, operational steps, and commissioning methods. With precise operation, continuous monitoring, and prompt troubleshooting, the parallel system can maintain stable and reliable operation under varying load conditions.