Interference elimination and protection of UPS power system (harmonic current)

　　For high-power UPS, if the UPS rectifier is a three-phase full-controlled bridge 6-pulse rectifier, the harmonics generated by the rectifier account for nearly 25-33% of all harmonics, which is very harmful to the power grid. Harmonics can cause heating of distribution cables and transformers , reduce call quality, malfunction of air switches, generator surge and other adverse consequences; harmonics are divided into + sequence (3k+1 times, k is 0 and a positive integer), - sequence (3k+2 times, k is 0 and a positive integer), and 0 sequence (3k times, k is a positive integer) according to the current phase sequence. The + sequence current increases the loss, the - sequence current causes the motor to reverse and heat up, and the 0 sequence current causes the neutral line current to increase abnormally.

　　Currently, there are four solutions for large UPS input harmonic current suppression:

　　Solution 1: Use 6-pulse UPS + active harmonic filter, input current harmonics <5% (rated load), input power factor 0.95. This configuration, although the input indicators are very good, but the technology is still immature, there are problems such as miscompensation and overcompensation, resulting in false tripping or damage of the main input switch; THM active harmonic filter technical defects are:

　　a) There is a problem of "false compensation": Since its compensation response time is more than 40ms, there is a hidden danger of "false compensation". When performing the cut-off/on-off operation on the input power supply or cutting off/on-off operation of a large load on the input upstream side of the UPS, "false compensation" is easy to occur. In the mild case, it causes the input harmonic current of the UPS to "mutate". In the severe case, it will cause the input switch of the UPS to "falsely trip".

　　b) Low reliability: For UPS with 6 pulses + active filter, the failure rate is high because IGBT tubes are used as power drive tubes for rectifiers and converters in its active filter. On the contrary, for UPS with 12 pulses + passive filter, inductors and capacitors with high reliability are used in its filter.

　　c) Reduce system efficiency and increase operating costs: The system efficiency of the active filter is about 93%. For a 400KVA UPS parallel machine, under the conditions of full load and 33% input harmonic current compensation, if the electricity fee is paid at 0.8 yuan per KW*hr, the operating cost to be paid in one year is: 400KVA*0.07/3=9.3KVA; the annual power consumption is 65407KW.Hr, and the additional electricity cost is: 65407X0.8 yuan=52,000 yuan.

　　d) The price of adding active filter is extremely expensive: the nominal input current of 200KVA UPS with active filter is: 303A;

　　Harmonic current estimation: 0.33* 303A = 100A. To fully compensate the input harmonic current content to <5%, the minimum compensation current to be calculated is: 100A.

　　Actual configuration: a set of 100A active filter. According to the current quotation of 1500-2000 yuan per ampere, the total cost increases by 150,000 to 200,000 yuan, which is almost 60%-80% increase in cost for a 6-pulse 200KVA UPS.

　　Solution 2: Use 6-pulse UPS + 5th harmonic filter. If the UPS rectifier is a three-phase full-controlled bridge 6-pulse rectifier, the harmonics generated by the rectifier account for nearly 25-33% of all harmonics. After adding the 5th harmonic filter, it is reduced to less than 10%, and the input power factor is 0.9, which can partially reduce the harm of harmonic current to the power grid. In this configuration, the input current harmonics are still large, and the generator capacity ratio is required to be more than 1:2, and there is a hidden danger of abnormal increase in generator output;

　　Solution 3: A pseudo 12-pulse solution using a phase-shifting transformer + 6-pulse rectifier, which consists of two 6-pulse rectifier UPS:

　　a) A standard 6-pulse rectifier

　　b) One phase-shifted 30-degree transformer + 6-pulse rectifier

　　The pseudo 12-pulse rectifier UPS is formed. On the surface, the full-load input current harmonics are 10%. This configuration has a serious single-point failure. When a UPS fails, the system input harmonic current increases sharply, seriously endangering the safety of the power supply system. Main disadvantages:

　　1) The original components were cut corners and a whole set of equipment was missing.

　　2) If the rectifier of a UPS fails, it will be transformed into a 6-pulse UPS and the harmonic content will increase dramatically.

　　3) The control of the DC bus line is an open-loop control system. The input current sharing cannot be very good. The harmonic current will still be very large when the load is light. 4) The expansion of the system will be extremely difficult

　　5) The installed phase-shifting transformer is not an original product and will not match the original system very well.

　　6) The floor space will be relatively large

　　7) The performance is 12-15%, which is not as good as a 12-pulse UPS.

　　Solution 4: Use 12-pulse UPS + 11th harmonic filter. If the UPS rectifier is a three-phase full-controlled bridge 12-pulse rectifier, the 11th harmonic filter can be added to reduce the harmonic current to less than 4.5%, which can basically completely eliminate the harm of harmonic current content to the power grid. The price is much cheaper than that of active filters.

　　Using 12-pulse UPS + 11th harmonic filter, the input current harmonic is 4.5% (rated load), and the input power factor is 0.95. This configuration is the most mature and reliable solution in the UPS industry, and the generator capacity requirement is 1:1.4;

Cost-effectiveness of four solutions

　　Based on the above analysis, in practice, the harmonic elimination solution of 12-pulse rectifier + 11th harmonic filter will be recommended, which has the best performance, highest reliability, and has been proven to be stable and reliable through long-term operation practice, as well as the best cost-effectiveness.