Can UPS be used with a diesel generator?

When the UPS is used with the generator, you need to configure a small number of spare batteries for the UPS in case of a switchover. The power conversion mode between the generator and the mains is a manual or automatic switching device (ATS). When the mains supply fails, the ATS will automatically switch to the generator, and the generator will automatically start to provide power after a certain time delay (which can be set according to the customer’s needs). In practical application, according to the availability requirements of different customers for the power supply system, UPS and generators are configured in different ways. Generally speaking, they are commonly divided into single machine mode and parallel machine mode, etc. They have their own characteristics. Today, Gosun will introduce the solution when UPS and generators are used together:


Single-machine operation configuration

Select a single UPS based on the actual load power. The schematic diagram of the power supply system is as follows:


  • Cost-effective. Although the availability of the power supply system is general, the system composition is simple and economical.
  • Generally speaking, it is unnecessary to consider the later capacity expansion of the system, and it is unnecessary to use the UPS of parallel machine models, so the equipment utilization rate is high.
  • The system has a single point of failure.

The main components of the system are:

  1. UPS host: The UPS host provides high-quality power supply to the load regardless of whether the mains power supply is input. There is no output interruption when the mains power supply runs and the battery runs. The built-in static bypass system improves the availability of the UPS and works with the maintenance bypass switch to ensure uninterrupted load power supply during UPS shutdown maintenance.
  2. Backup battery pack + battery DC Circuit breaker: Backup batteries provide power for loads when the mains supply is interrupted. The stored energy is supplied to loads through UPS inverters. Battery circuit breakers protect battery packs from overcurrent and circuit breakers to prevent battery damage and UPS faults.
  3. Diesel generator: In the high-power UPS system, the battery configuration required by the long backup time is often a solution with large footprint and high investment, so the scheme of diesel generator plus UPS is often considered. Even in the case of dual mains, backup diesel can be used as the ultimate backup and is more cost-effective than large capacity backup battery packs. The conversion between the generator and the mains power supply can be manually or automatically switched equipment (ATS), and the capacity ratio relationship between the generator and the UPS is often due to the harmonic feedback of the UPS, load current mutation and other interference, the capacity of the generator is required to be 2 to 3 times the maximum load of the UPS, at the same time, it should also consider other load factors carried by the generator to determine its capacity.
  1. Others: include the relevant input and output power distribution system.

Redundant parallel configuration

When a single system cannot meet the requirements, two or more UPS with the same power are often used to run in parallel to improve the reliability of the parallel system. The schematic diagram of the power supply system is as follows.

UPS works with diesel generators

Connect the mains power supply R, S, T, and N to the UPS according to the phase sequence, and then connect the ground cable by hand to check whether the cable is loose, and then lock it. After confirming the phase sequence of R, S and T, a multimeter was used to measure the voltage of R-N, S-N and T-N within the range of 220V±25%, and the frequency was 50Hz±1. Turn off the machine’s mains power input switch, and then turn off the machine’s automatic bypass switch. If there is a long alarm sound, it is the mains power input phase error, and it should be corrected immediately (any replacement of the two phases of the input three-phase, no long alarm means that the phase is accurate).

In high-power UPS systems, the customer’s need for a long backup time often means a large number of backup battery packs. This configuration scheme has certain limitations. A large number of backup battery packs require large storage space, so load-bearing should be considered in this case. In addition, the backup battery pack has a certain replacement cycle, which also requires a large cost input. In order to solve these problems, it is often considered that the diesel generator is used in the UPS configuration, and the diesel generator is used as the ultimate backup means.

It is necessary to consider the matching problem when connecting the diesel generator with the UPS. From the external characteristics of the diesel generator, the factors affecting the frequency and output voltage stability are mainly the high harmonic components of the load current. The instantaneous start of the load. The input power factor of the traditional double transform UPS is only 0.8, and the high harmonic of the input current is more than 30%. When the diesel generator is used for power supply, it will inevitably seriously affect the stability of the frequency and output voltage of the oil generator. Therefore, the power capacity of the generator must be up to 2.5-3 times the capacity of the UPS, in order to ensure the normal operation of the system.

UPS selection

UPS input rectifier mode. The special problem encountered when the UPS is powered by the diesel generator set is that the AC power sent by the diesel generator set not only has a large voltage distortion (waveform distortion), but also increases its internal resistance as the output power of the diesel generator set decreases. This effect is not perceptible when the diesel generator set has a resistive load. However, if the diesel generator set with rectifier filter type load and various thyristor phase shift control equipment, often will encounter great trouble. The reason is that the above nonlinear load will reflect a large number of high-order harmonics to the diesel generator set, especially the fifth and 11th harmonics, which are more harmful to the operation of the diesel generator set. Table 1 shows the influence of different nonlinear loads on the output voltage distortion of the diesel generator set (note: The capacity of the diesel generator used in this experiment is 200kVA).

Table: Influence of different rectifiers on generator operation

Allowable voltage distortion of generator set

Percentage of allowable load
12 pulse rectifier UPS6 pulse rectifier UPSSingle phase rectifier UPS

As can be seen from the above table, different loads have a great influence on the output voltage distortion of diesel generator sets. For example, for a machine room that requires a high quality supply with a voltage distortion of less than 5%, a 12-pulse rectifier UPS will only be allowed to use 78% of the volume of the diesel generator set. If the customer is using a 6-pulse rectifier load, the load should not exceed 42% of the rated output power of the generator set. Obviously, this will inevitably lead to the reduction of the utilization rate of the generator set equipment. If the allowable voltage distortion standard is reduced to 10% of the general civil electricity standard, it can be allowed to take 100% of the 12-pulse rectifier UPS equipment. At this point, a UPS unit with a 6-pulse rectifier can only deliver 69 percent of the rated power of the generator. The UPS has the following functions to better match with the generator set.

1 Bypass phase lock frequency range and inverter synchronization rate field adjustable function. The UPS with this function can be adjusted based on the type of the power supply device to improve system reliability.

2 High-speed broadband rectifier control circuit. The UPS rectifier requires a high-speed broadband control circuit to adjust the working state of the rectifier to adapt to the frequency drift of the input power supply. A good rectifier control circuit can make the rectifier run normally when the frequency drift of the input power exceeds ±5Hz.