The cooperation between high-power apcups power supply and generator

Compatibility of generator set and UPS

Manufacturers and users of apcups power supply system have long noticed the cooperation between generator set and apcups power supply, especially the current harmonic generated by rectifier has a very obvious adverse impact on power supply system (such as voltage regulator of generator set, synchronous circuit of apcups power supply). Because the input (rectifier) of UPS is a nonlinear load, and the generator can only provide limited current, the combination of the two will produce many problems. Therefore, the input filter technology is used in the design of UPS system, which plays a key role in ensuring the compatibility between ups and generator set. In this paper, the influence of power factor and input filter of apcups power supply on the generator is analyzed theoretically and illustrated with a practical case, the causes of the problems are clarified, and the solutions are found out.

1. Current harmonic "pollution" of oil unit

When the apcups power supply enters into the power supply by the oil unit, the special problem encountered is: because the power supply sent by the oil unit itself not only has large voltage distortion (large waveform distortion), but also has limited rated output power of the oil unit, the problem of large internal resistance will become more prominent. When the oil unit is charged with resistive load, this effect is not obvious. However, if the oil unit is used to bring rectifier and filter type load (such as computer and communication equipment) and various thyristor phase-shift control equipment, all kinds of incompatibilities often occur. The reason is that the nonlinear load will reflect a large number of higher harmonics to the oil unit, especially the 5th and 11th harmonics, which are the most serious hazards to the operation of the oil unit. Under the specified voltage distortion, the influence of various non-linear loads on the allowable load percentage of the generator is shown in table 4-3.

Note: the capacity of the oil unit used in this experiment is 200KVA.

It can be seen from table 4-3 that different types of user loads have a great influence on the voltage distortion of the output power supply of the oil unit. For the machine room which is required to meet the high-quality power supply standard with voltage distortion less than 5%, if the user uses 12 pulse rectifier load, 78% of the rated capacity of the oil unit is allowed to be used; if the user uses 6 pulse rectifier load, the load shall not exceed 14% of the rated output power of the generator unit. Obviously, overheating will cause the utilization rate of generator set equipment to be greatly reduced. Of course, if the allowable standard of voltage distortion is reduced to the general civil standard (10%), the 12 pulse rectifier filter type electrical equipment with 100% can be allowed. At this time, even with 6 pulse rectifier load, about 35% of the rated capacity of the generator set can be used.

2. Power factor

Generally, people pay attention to the working state of apcups when the power supply is full or close to full load. Although it can describe the working characteristics of apcups under full load, especially the characteristics of the input filter, it is rarely able to describe the status of the filter under no-load or near no-load, after all, the current harmonic effect of ups and its electrical system under light load is very small. However, the working parameters of UPS, especially the input power factor, are very important for the compatibility of apcups and generator.

The newly designed input filter has achieved good results in reducing current harmonics and improving power factor at full load. However, a very low power factor with capacitance leading has been derived at no load or very small load, especially for those filters that meet the maximum current distortion of 5%. In general, when the load is less than 25%, the input filter of most UPS systems will significantly reduce the power factor. However, the no-load power factor of some new systems is even lower than 25, which is close to the ideal capacitive load. In this case, the city power transformer and the transmission and distribution system are not affected, but the oil unit is different, the generator with large capacity load will not work normally. When low power factor loads (such as capacitive loads with power factor lower than 15% - 20%) are connected, the generator may be shut down due to system maladjustment. This kind of shutdown phenomenon (the emergency generator system drives the UPS system load) will cause catastrophic accidents after the power outage.

3. Resonance

Capacitance self excitation may be aggravated or covered by other electrical states. For example, for LRC Series circuit with AC power supply, take appropriate values for L and C, and the voltage u can oscillate to a very high value. When the ohmic value of the inductive reactance of the generator and the ohmic value of the capacitive reactance of the input filter are close to each other, and the resistance value of the system is small, there will be oscillation, and the voltage may exceed the fixed value of the power system.

The newly designed apcups power supply system is essentially 100% capacitive input impedance. A 5ookva UPS may have 150KVA capacitance and a power factor close to zero. Parallel inductor, series choke and input isolation transformer are the common parts of apcups, which are all inductive. In fact, together with the capacitor of the filter, they make the overall performance of apcups capacitive, which may have some oscillations in the ups.

4. In practical application, when the type selection of oil unit and apcups power supply is unreasonable, the following phenomena often occur.

① Generator set output voltage oscillation. The oscillation range of the output voltage of the generator set is as high as - 10% ~ 20% of the rated voltage. When the AVR is adjusted to the best, the oscillation is still greater than 2%.

② Current oscillations. When the load of APCUPS is stable, the output current of generator oscillates within ± 20% ～± 50%, and the current oscillates