How to improve power quality: comparison and selection of UPS rectification methods

At present, the main problems of power quality are as follows: harmonic distortion, power outage, over (under) voltage, voltage sag, transient, surge, etc. The reasons for these problems come from infrastructure sharing, such as a fault in the power grid affects other users in the grid, and from the power-consuming equipment itself. At present, since the equipment generally uses switching power supply devices, the load current waveform is seriously distorted and presents a non-sinusoidal waveform. In addition, there is a certain impedance in the power supply line, and the current waveform distorts the voltage waveform, which will seriously pollute the upper-level power grid. In view of the above, power quality issues have become a difficult problem faced by power supply workers.

1 Harmonics generated by UPS

1.1 Concept and hazards of harmonics

In an ideal power system, the voltage and current waveforms are both smooth sine waves. In reality, when the electrical equipment is a nonlinear load, such as a switching power supply, electronic ballast, variable speed drive, UPS, etc., the current waveform will be non-sinusoidal. Voltage or current with an integer multiple of the fundamental power frequency is called harmonics. By performing Fourier series expansion on the waveform, it can be seen that any periodic waveform can be decomposed into a sine wave of the fundamental frequency and multiple sine waves of the harmonic frequency. For symmetrical waveforms, all even harmonics are zero.

The hazards caused by harmonics can be divided into hazards caused by harmonic current and hazards caused by harmonic voltage. The hazards caused by harmonic current include the superposition of 3N harmonic currents on the neutral line, which causes the distribution cable to be used at a reduced capacity, increased transformer losses, and harmonics causing circuit breakers to trip in error, etc. The hazards caused by harmonic voltage mainly include voltage distortion affecting the normal operation of electronic equipment and zero-crossing noise, etc.

1.2 UPS harmonic generation mechanism

The original waveforms related to power electronic devices are almost all non-sinusoidal, and the waveforms all contain harmonic components. UPS is one of the representatives. The rectification method of UPS is an important reason for the generation of harmonics. Under ideal conditions, the AC power supply is considered to be a three-phase symmetrical industrial frequency sinusoidal voltage, and the harmonics of the power supply itself are ignored. At the same time, the influence of the commutation overlap angle of the pulse bridge rectifier circuit is not considered. The influence of the DC loop current ripple is ignored and it is assumed that the trigger pulse of the circuit is symmetrical and the conduction angle α is equal. Under the above ideal conditions, when the device is in a stable working state, the switching function method is used to express the analyzed waveform in the form of a series of products and sums of known waveforms and switching functions. The known waveforms and switching functions are expanded into the form of series, and the product sums are sorted and simplified. Finally, the analyzed waveform is expressed in the form of series to facilitate the discussion of the order and content of the harmonics. 1.3 Characteristics of harmonics generated by UPS rectification

The harmonics generated by UPS rectification can be divided into rectified DC voltage harmonics and current harmonics on the power supply side. For multi-pulse rectification circuits, the following conclusions can be drawn:

For DC voltage:

(1) The more total pulses in the circuit, the more harmonics are offset in the DC voltage waveform, and the better the DC voltage waveform is;

(2) The harmonic frequency appearing in the DC side voltage of the system is an integer multiple of the number of voltage pulses, and the harmonic order n=pm. For example, in a 6-pulse circuit, the voltage harmonic frequency is 6m times the input frequency;

(3) The lower the harmonic order, the greater the harmonic amplitude.

For AC current:

(1) For an ideal p-pulse rectifier, there are only harmonic currents of the following orders on the AC side: n = Pm ± 1 times, where m = 1, 2, 3, .... The amplitude of each harmonic current component is l/n of the fundamental current. For example, for a 6-pulse rectifier, there are only odd harmonics such as 5, 7, 11, 13, 17, 19, etc. in the line current, and the amplitudes of the 5th and 7th harmonics are 1/5 and 1/7 of the fundamental current respectively; for a 12-pulse device, there are only odd harmonics such as 11, 13, 23, 25, etc. in the line current, and their amplitudes are also significantly reduced;

(2) Increasing the pulse number p of the rectifier has a decisive influence on reducing the harmonic components on both sides of the AC and DC. Therefore, constructing a system with as large a pulse number as possible is the fundamental measure for the rectifier device to reduce the harmonic current and harmonic voltage on both sides of the AC and DC.

This paper mainly analyzes and studies the power supply side current harmonics generated by UPS rectification.

2 Relationship between UPS rectification and filtering methods and harmonic suppression

2.1 Classification of high-power UPS in information center

The National Meteorological Information Center has a total of 12 high-power UPS, which almost cover all forms of high-power UPS currently on the market. According to the different rectification and filtering methods, they can be divided into the following types, as shown in Table 1.

Rectifier device Rectification method Isolation transformer Input filter method UPS typical

Thyristor (SVR) 6 pulse None 120kVA Brand A UPS 120kVA Brand B UPS 250kVA Brand A UPS

Active filter + passive filter 300kVA A brand UPS

12 pulse 30° phase shift transformer 11th harmonic filter 300kVA C brand UPS

Primary: triangle Secondary: star + triangle 11th harmonic filter 13th harmonic filter 625kVA D brand UPS

IGBTPWM without 120kVA E brand UPS 2.2 Harmonic analysis of 6-pulse UPS without input isolation transformer and input filter

The 6-pulse thyristor rectifier UPS represented by the 250kVAA brand UPS has no input isolation transformer and input filter. The harmonic spectrum analysis diagram and current waveform of the UPS are obtained by the FLUKEF434 power quality analyzer, as shown in Figure 2. As can be seen from the figure, the input current waveform is seriously distorted, and the total harmonic distortion of the A-phase input current reaches 45.3%, of which the 5th harmonic accounts for a large proportion, about 41.5% of the A-phase, and the 7th harmonic is second, about 13.9% of the A-phase, which is consistent with the characteristics of the harmonics generated by the UPS6-pulse rectification mentioned above. The ratio of the input power factor PF (0.79) to the fundamental power (displacement) power factor COSφ (0.86) is large, that is, the current distortion factor ζ (0.92) deviates greatly from 1, indicating that the input current distortion is serious. In addition, when the current waveform is non-sinusoidal, the distortion power D, that is, the reactive power generated by the harmonic current, cannot be ignored.

2.3 Harmonic Analysis of 12-Pulse UPS with Phase-Shifting Isolation Transformer and Input Filter

For a transformer with a triangle winding, all 3N harmonics are in phase. Therefore, the 3N harmonic current circulates in the winding and will not spread to the power grid. The input isolation transformer has a certain inhibitory effect on current harmonics. The two 30OkVAC brand UPS in the Information Center have a phase-shifting transformer at the input end based on the original 6-pulse rectification, and then a set of 6-pulse rectifiers are added, so that the DC bus current is rectified by 12 thyristors, which greatly reduces the input harmonic current of the UPS. In addition, the C brand UPS is installed with an input filter that absorbs 11th harmonics, that is, the resonance point of the LC series resonant circuit is adjusted to 55OHz, so that most of the 11th harmonics generated by the rectifier circuit flow into the LC series resonant circuit, thereby suppressing the harmonic circuit flowing into the power grid within the allowable value. In order to improve the accuracy of the reference of power quality test data, all parallel redundant UPS in the center are switched to a single UPS for power supply. In this way, the load rate of the single UPS is increased, and the load rates of all the UPSs with several rectification and filtering modes involved in the comparison are within 45%~65%, so that the compared data is less affected by the load rate and is more referenceable and convincing. Through power quality analysis, it can be obtained (see Figure 3): the total harmonic distortion of the 30OkVAC UPS input is small, the highest phase is only 5.2%, the harmonics are mainly 7th, and the harmonic content is small, the input power factor is not high (only 0.89), and the fundamental power factor COSφ (0.89) is equal to the total input power factor, indicating that the current distortion is very small.

2.4 Harmonic Analysis of 12-Pulse UPS with Input Isolation Transformer and Input Filter

D Jing brand UPS adopts 12-pulse thyristor full-bridge rectifier circuit. Both rectifiers are isolated from the input end. The primary side is connected in triangle, the secondary rectifier A is connected in triangle, and the rectifier B is connected in star. The UPS is equipped with an intelligent input filter, which can decide whether to put some filter capacitors into use according to the load. For UPS using 12-pulse rectification, the harmonics of the input current are mainly 11th and 13th. Therefore, the UPS filter is designed to absorb the 11th and 13th harmonics. It can be seen from the power quality analysis diagram (see Figure 4) that the filtering effect for the 11th and 13th harmonics is very good, and the 11th and 13th harmonic contents are only 0.3% and 0.4%, respectively, and the total current harmonic distortion is also within 5.2%. The current presents a good sinusoidal waveform and the input power factor is as high as 0.94. The intelligent input filter used by brand D UPS can effectively purify the pulses, surge voltages, peak voltages, high-frequency electromagnetic interference, etc. from the mains power grid that may cause harm to the UPS. It can also reduce the harmonic pollution of the current harmonics formed by the rectifier to the mains power grid and improve the input power factor. 2.5 Harmonic Analysis of Pulse Width Modulation Rectification UPS without Input Isolation Transformer and Input Filter

Pulse rectifier is a rectifier that works in pulse width modulation (PWM) mode. Compared with phase-controlled rectifier, it has the advantages of high power factor, low harmonic content, AC side current close to sine, and fast rectifier dynamic response. The 12OkVAE brand UPS of the National Meteorological Information Center is a UPS that uses IGBT PWM rectification mode. From the power quality analysis diagram (see Figure 5), it can be concluded that when the load rate is 62.2%, the total harmonic distortion of its input current is within 10.9%, mainly the 5th harmonic and the 7th harmonic, the current waveform is close to the sine wave, but not smooth, and the power factor is very high (0.94), which meets the characteristics of PWM rectification. However, after analysis and comparison, it can be seen that the total harmonic distortion generated by PWM rectification is greater than that of 12-pulse phase-controlled rectification, which is slightly different from the traditional view that PWM rectification has the characteristic of "lowest harmonic content".

2.6 Harmonic analysis of 6-pulse UPS with active + passive input filter

300kVAA brand UPS adopts 6-pulse rectification, no input isolation transformer, and is equipped with a hybrid filter (THM) of active filter and passive filter as a measure to suppress harmonics. The basic principle of active filter is: by detecting the current i1 of the compensation object, the command signal ic* of the compensation current is calculated through the compensation current detection circuit. This signal is used as the compensation current reference value to obtain the actual compensation current value ic through the compensation current generation circuit. The compensation current and the harmonics and reactive current to be compensated in the load current are offset (ic=ih), and finally the desired power supply current (is=if) is obtained. 300kVAA brand UPS adopts a hybrid filter (THM). When the harmonic content is large, the active filter is automatically put into use. When the harmonic content is small, LC passive filtering is used. Figures 6 and 7 are schematic diagrams of the principle of the hybrid filter and the THM schematic diagram of the 300kVAA brand UPS, respectively. Figures 8 and 9 are power quality analysis diagrams detected at the rear end and front end of the hybrid filter THM, respectively. Comparing the two diagrams, it can be seen that the effect of measuring at the rear end of the hybrid filter is equivalent to directly measuring the harmonics generated by the 6-pulse SCR rectifier. Observing its waveform, it is found that it is extremely similar to the input waveform of the 250kVAA brand UPS. In addition, its total input harmonic current distortion is very large, with the highest phase reaching 47.6%, especially the 5th harmonic (43.6%), followed by the 7th harmonic (17.2%), and the input power factor is very low, only 0.77. Comparing the detection data at the front end of the hybrid filter, it can be seen that the input current waveform presents a smooth and intact sine wave, the input total harmonic distortion is within 4.4%, the 3rd harmonic content is slightly larger (3.6%), and the rest of the harmonic content is very small. The process of harmonic suppression by active filters can be understood as follows: the active filter generates the following waveform current, that is, the waveform current required by the rectifier minus the grid waveform current. In short, the harmonic current is provided by the active filter, not the grid. However, since the active filter itself cannot generate any electrical energy, it must consume grid power to compensate for harmonics. Therefore, active filtering has the disadvantage of low efficiency when used in conjunction with UPS. 3 Comparison and summary

Rectification and filtering methods Input total harmonic distortion Input power factor Advantages Disadvantages

6-pulse SCR rectification is very low and the harmonic distortion is serious

12-pulse SCR rectifier + 30° phase-shifting transformer + 11th harmonic filter is smaller and has better high harmonic suppression effect. The harmonic suppression effect of phase-shifting transformer is not as good as that of input isolation transformer.

12-pulse SCR rectifier + input isolation transformer + 11th harmonic filter + 13th harmonic filter is very small and high (1) input total harmonic distortion is very low (2) high power factor (3) high reliability and mature technology (4) relatively low cost and large size

IGBT PWM rectifier has a small and low current (1) It has a certain suppression effect on harmonics (2) The power factor is high (only when the load rate is high) It is currently impossible to produce high-power IGBT rectifiers

6-pulse SCR rectifier + hybrid filter has good minimum harmonic suppression effect and low input total harmonic distortion (1) There is a miscompensation problem (2) Low reliability (3) Low system efficiency (4) High cost

For example, in a site environment like the National Meteorological Information Center, where a large computer room is the main load, the 12-pulse thyristor (SCR) rectifier, combined with 11th and 13th harmonic filters or IGBTPWM rectifier, and 6-pulse SCR rectifier with a hybrid filter can all have a good harmonic suppression effect. However, if the load efficiency is low, the IGBTPWM rectifier can be excluded because its power factor is also low when the load rate is low. The computer room of the information center has a large load and energy consumption must be considered. Therefore, considering many factors, the 12-pulse thyristor (SCR) rectifier, combined with 11th and 13th harmonic filters, is currently more suitable and has certain advantages in harmonic suppression and power factor improvement.