Demonstrate the matching compatibility between UPS and generator set

Demonstrate the matching compatibility between UPS and generator set

With the rapid development of the Internet, the demand for high-power UPS and generators in the data center is growing rapidly, and some new problems have arisen. In this paper, the power factor of the UPS input and the influence of the input filter on the generator are analyzed theoretically and the actual case is explained to clarify the cause of the problem, and then find a solution.

1 matching problem between generator set and UPS

Manufacturers and users of UPS power supply systems have long noticed the matching problem between the generator set and the UPS, especially the current harmonics generated by the rectifier are not good for the power supply system such as the voltage regulator of the generator set and the synchronous circuit of the UPS. The impact is very obvious. Therefore, the technician designed the input filter and applied it to the UPS to successfully control the current harmonics in the UPS application. These filters play a key role in the compatibility of the UPS with the generator set.

Virtually all input filters use capacitors and inductors to absorb destructive current harmonics at the UPS input. The design of the input filter takes into account the percentage of the maximum possible harmonic distortion inherent to the UPS circuit and at full load. Another benefit of most filters is to increase the input power factor of the on-load UPS. However, another consequence of the application of the input filter is that the overall efficiency of the UPS is reduced. Most filters consume about 1% of UPS power. The design of the input filter has always sought a balance between favorable and unfavorable factors.

In order to maximize the efficiency of the UPS system, recent UPS engineers have made improvements in the power consumption of the input filter. The increase in filter efficiency depends to a large extent on the application of IGBT (Insulated Gate-Level Transistor) technology to UPS designs. The high efficiency of the IGBT inverter has led to a redesign of the UPS. The input filter can absorb certain current harmonics while absorbing a small fraction of the active power. In summary, the ratio of inductive factors to capacitive factors in the filter is reduced, and the UPS is reduced in size and efficiency. The problem of compatibility between UPS and generators has arisen again.

2 power factor problem

Often, people pay attention to the working state of the UPS at full load or near full load. Most engineers understand the UPS operating characteristics at full load, especially the characteristics of the input filter, but few people are interested in the condition of the filter at no load or near no load. After all, the UPS and its electrical system have little effect on the current harmonics under light load conditions. However, the operating parameters of the UPS at no load, especially the input power factor, are important for the compatibility of the UPS with the generator.

The newly designed input filter has a good effect in reducing current harmonics and increasing the power factor at full load. However, under no-load or very small load conditions, a very low power factor of capacitive lead is derived, especially those that meet the 5% maximum current distortion. In general, the input filter of most UPS systems can cause significant power factor reduction when the load is less than 25%. Despite this, the input power factor is rarely less than 30%, and some new systems have even reached a no-load power factor of less than 2%, close to the ideal capacitive load.

This situation does not affect the UPS output and critical loads, and the mains transformers and transmission and distribution systems are not affected. But the generators are different. Experienced generator engineers know that the generator will not work properly when it has a large capacitive load. When it is connected to a lower power factor load, typically less than 15% to 20% capacitive, The generator may be shut down due to a system imbalance. The occurrence of such a shutdown after the mains power failure, the emergency generator system to drive the UPS system load will cause a catastrophic accident. Downtime poses a hazard to critical loads due to two reasons: (1) the generator needs to be manually restarted and must be before the end of the UPS battery discharge; (2) the generator may cause an "overvoltage" of the system before the shutdown, it Communication equipment, fire alarm systems, monitoring networks, and even UPS modules may be damaged.

To make matters worse, after an accident, it is difficult to distinguish the responsibilities, identify the problem and correct it. The manufacturer said that the UPS system was in good condition and pointed out that similar problems occurred in the same equipment elsewhere. The generator manufacturer said it was a load problem and could not adjust the generator to solve the problem. At the same time, the user engineer explains the specifications and wants to be compatible with each other. To understand why an accident occurs and how to avoid it (or how to find a solution in a critical application), you first need to understand the working relationship between the generator and the load.