STSTM32L152 32-bit MCU development and evaluation solution

Central topics:

• Comparison of Schemes for Suppressing Input Harmonic Current in High-Power UPS

Solution:

• Adopt 6-pulse UPS + active harmonic filter
• Adopt 6-pulse UPS + 5th harmonic filter
• Pseudo 12-pulse scheme using phase-shifting transformer + 6-pulse rectifier
• Adopt 12 pulse UPS + 11th harmonic filter

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 positive integers), - sequence (3k+2 times, k is 0 and positive integers), and 0 sequence (3k times, k is a positive integer) according to the current phase sequence. + sequence current increases the loss, - sequence current causes the motor to reverse and heat up, and 0 sequence current causes the neutral current to increase abnormally.

At present, there are 4 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. Although this configuration has very good input indicators, the technology is still immature and there are problems such as miscompensation and overcompensation, which may cause the main input switch to trip or be damaged. The technical defects of THM active harmonic filter are:

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

b) Low reliability: For UPS with 6 pulses + active filter, since IGBT tubes are used as the power drive tubes of its rectifier and converter in its active filter, its failure rate is relatively high. 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 400KVA UPS parallel, under 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 fee 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, if the input harmonic current content is to be fully compensated to <5%, at least the compensation current needs to be calculated: 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% for a 6-pulse 200KVA UPS.

Option 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 too 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;

Option 3: Use a pseudo 12-pulse solution of phase-shifting transformer + 6-pulse rectifier , which is composed of two 6-pulse rectifier UPS:

a) a standard 6-pulse rectifier
b) a phase-shifting 30-degree transformer + 6-pulse rectifier

A pseudo 12-pulse rectifier UPS. On the surface, the full-load input current harmonic is 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 are cut corners, and a whole set of equipment is missing.
2) If the rectifier of a UPS fails, it will be converted into a 6-pulse UPS, and the harmonic content will increase sharply.
3) And the control of the DC bus line is an open-loop control system. The input current sharing cannot be very good. The harmonic current under light load will still be very large.
4) The expansion of the system will be extremely difficult
5) The installed phase-shifting transformer is not an original product, and the matching with the original system will definitely not be very good.
6) The floor space will be relatively large
7) The performance is 12-15%, which is not as good as the 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 harm to the power grid caused by harmonic current content can be basically eliminated after adding the 11th harmonic filter. The price is much cheaper than that of active filter.

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.