The variable amplitude 12-phase two-superimposed rectifier is the most widely used in UPS. When the harmonic injection technology is adopted, its application will be wider. For this reason, we will briefly introduce the influence of the input transformer on the harmonic current. Provide time reference.
There are two ways to connect the main circuit of the variable amplitude 12-phase input rectifier transformer: one is a single-unit 12-phase rectifier circuit composed of a transformer and two three-phase bridge rectifiers; the other is two completely independent rectifier circuits. The transformer and two three-phase bridge rectifiers form a dual-unit 12-phase rectifier circuit. The two connection methods are shown in Figure 1 and Figure 2. These two connection methods have different effects on the symmetry of the output voltage or current waveform of the 12-phase two-superimposed rectifier and on the distortion rate THDi of the mains input current waveform, which must be paid attention to.
- The influence of two connection methods on harmonic current
Under ideal conditions, during the operation of the 12-phase rectifier circuit, if the variable amplitude two superimposed addition is selected, the 5th and 7th harmonics of the mains input current will not be generated. However, because the output voltage and impedance of the two connecting transformers of the single-unit 12-phase rectifier circuit and the secondary winding of the bridge rectifier are not easy to be consistent, there is a serious problem of uneven load distribution during operation, which requires This deviation is corrected by the excitation adjustment of the thyristor phase control or the saturable reactor, so that the phase difference of the conduction of the thyristors of the two three-phase bridge rectifiers cannot be strictly maintained at 30°, and finally the mains input current still exists 5 times and 7th harmonic.
For the dual-unit 12-phase rectifier circuit, since the output voltage and impedance of the two secondary windings of the transformer connected to the three-phase rectifier bridge are easy to be consistent, the symmetry of the 12-phase rectifier will not be destroyed.
- The problem of uneven distribution of load current between secondary windings
1) Single unit 12-phase rectifier circuit
The transformer primary of the single-unit 12-phase rectifier circuit has only one winding. The reason for the uneven load distribution between the two secondary windings is that the turns ratio WY/WΔ between the two windings of the Y connection and the delta connection deviates, so that the no-load DC voltage Ud10 of the two three-phase bridge rectifiers and Ud20s are not equal, so the load distribution cannot be equal either.
The turns ratio WY/WΔ= between the two secondary windings of the transformer can be approximated as (deviation 1.04%), (deviation 1.02%), (deviation 0.27%). However, due to the rationality of the transformer structure and manufacturing reasons, it is actually not easy to obtain WY/WΔ.
When WY/WΔ=, the difference between the no-load DC voltages of the two secondary windings WY/WΔΔUd0=1.04%, because the high voltage group has a reverse bias effect on the low voltage group rectifier, so the load of the two groups of three-phase rectifier bridges The current distribution is very different. Because the reactance X1 of the primary winding of the transformer is shared by each rectifier and has no adjustment effect on the uneven load distribution, the load distribution depends entirely on the secondary winding reactance X2=XY+XΔ and the DC side connection The reactance of the busbar XM. According to the relevant data, the calculation results show that: when XΔ=XY=5%, ldY=0.2938Idn, ldΔ=0.7072ldn; when XΔ=X*Y=10%, ldY=0.3964ldn, ldΔ= 0.6036ldn. It can be seen that the smaller the value of the secondary reactance of the transformer, the greater the difference in load distribution.
Here, X1, X2, XY, XΔ, XM are the magnitudes of the reactance, and “” is added to represent the per unit value. The reference value is UN/IN, where UN and IN are rated phase voltage and rated phase current respectively. ldY and ldΔ are the rectified output currents of the secondary Y winding and the delta winding in Figure 1. ldn=ldY+ldΔ is the rectified output current of the transformer secondary.
2) Dual-unit 12-phase rectifier circuit
The dual-unit 12-phase rectifier circuit does not have the problem of uneven load current distribution. This is because the number of turns of the primary winding of the transformer is much larger than that of the secondary winding, so it is relatively easy to make the two primary windings WY/WΔ=. And because the number of turns per volt of the windings of the two transformers can be selected differently, the secondary voltages of the two transformers U2Y=U2Δ, ΔUdo=0, and the primary reactance X*1 of the transformer is not shared, so the current distribution has Adjustment effect, so that the purpose of equalizing the load current of the two transformers can be achieved.
3) Compatibility issues between the two rectifiers
The 12-phase rectifier circuit is composed of two 6-phase rectifier bridges with a phase difference of 30°, powered by two independent secondary windings. This will cause the voltage waveform of each point of the mains grid to be distorted during the operation, and interfere with the normal operation of other electrical equipment on the mains grid. When the disturbance of the power grid exceeds a certain limit, the specified performance of the rectifier will also be degraded, and the operation will be interrupted or even damaged. This is the compatibility of the rectifier with the mains grid where it is located. According to the provisions of the national standard GB 10236-88, the meaning of compatibility: first, the interference of the rectifier to the mains grid should be within the allowable range of the grid; second, after the rectifier is connected to the mains grid, the voltage fluctuation, frequency, waveform, etc. The disturbance of the parameters (including the disturbance caused by its own connection) should be lower than the anti-jamming limit of the rectifier.
According to the provisions of the national standard GB10236-88, the limit value of the commutation gap allowed for a rectifier with a class B immunity level is: the maximum depth is 40% of the original; the maximum width is 30°; the maximum area is 1 of the product of the maximum depth and the maximum width /10, that is, 40×30×0.1=120. If the commutation gap is too large, it will cause trigger failure, false trigger or unstable rectification. The Ministry of Industry and Information Technology promulgated the industry standard of “Uninterruptible Power Supply for Communication – UPS” in 2000, and made clear regulations on the electrical parameters input by the UPS, as shown in Figure 3.
- 12-phase two superimposed rectifier for UPS
Through the above analysis, it can be seen that the 12-phase two-superimposed rectifier suitable for UPS applications should be a dual-unit variable-amplitude 12-phase two-superimposed rectifier circuit as shown in Figure 2. Because this rectifier circuit has strong harmonic elimination ability, high input power factor of mains, easy design and manufacture, and stable and reliable operation. However, this rectifier circuit also has two disadvantages: one is that the volume and mass are large, and the cost is high; the other is that the amplitude of the 11th and 13th harmonics in the mains input current is too large.
The first shortcoming can be solved by removing the △/△ transformer in Figure 2 and using the inductance instead, as shown in Figure 4. As for the second disadvantage, it can be solved by installing an 11th harmonic passive filter at the input end of the rectifier circuit, or by using a harmonic injection method. The method of removing the △/△ transformer and installing the 11th harmonic passive filter, or adopting the harmonic injection method, can be used in the actual UPS circuit. The comparison of the mains input current waveform before and after the installation of the 11th harmonic passive filter on the variable amplitude 12-phase two-superimposed rectifier is shown in Figure 5. It can be seen that the waveform of the mains input current has been significantly improved after installing the 11th harmonic passive filter.
The 12-phase two-superimposed rectifier for UPS is to connect two sets of three-phase bridge rectifiers in accordance with the same polarity and connect them together to supply power to the load. Since the line voltages between the two secondary windings of the input transformer differ by 30° phase angle, the instantaneous values of the output voltages of the two groups of bridge rectifiers are also different by 30°, so an AC circulating current occurs between the two groups of voltages. In order to limit this circulating current, it is necessary to add a balancing reactor between the two groups of bridge rectifiers. Even so, the instantaneous value voltages between the two groups of rectifiers are still different. The group with high instantaneous line voltage works in the rectified output state, while the low group is blocked by reverse bias.
After 30°, the working states of the two groups are interchanged, and the two groups are turned on for 30° in turn. In the three-phase bridge rectifier, each thyristor is only turned on for 30° at a time, and it is turned on twice in one cycle, with a total conduction of 60°, but the normal on-time of one thyristor of the three-phase bridge rectifier is 120°, so that the two sets of secondary windings of the input transformer work alternately. In order to make them work in parallel rather than alternately, balanced reactors are also required. The instantaneous value voltage output by the two groups of rectifiers after parallel connection is the average value of Ud1 and Ud2, namely:
In fact, the balanced reactor also has a positive effect on the improvement of the input power factor of the 12-phase two-layer rectifier. The larger the balance reactance, the higher the mains input power factor of the rectifier.