Ac to Dc Rectifier Series

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I. Theoretical derivation
1.6 pulse rectifier principle:
6 pulse refers to the full bridge rectifier composed of 6 thyristors (thyristors), because there are 6 switching pulses to control 6 thyristors respectively, so it is called 6 pulse rectifier.






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When ignoring the three-phase bridge thyristor commutation process and current pulsation, assuming that the AC side reactance is zero, the DC inductance is infinite, and the delay triggering Angle a is zero, the Fourier series expansion of the AC side current is:


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(1, 1)







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It can be concluded from the formula (1-1) that:
Does the current contain 6K? 1 (k is a positive integer) subharmonics, i.e. 5, 7, 11, 13... The effective value of each harmonic is inversely proportional to the harmonic number, and the ratio of the effective value of the fundamental wave is the reciprocal of the harmonic number.

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Figure 1.1 Input voltage and current waveform of 6-pulse A-phase simulated by computer






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2. Principle of 12-pulse rectifier:
12 pulse refers to the original 6 pulse rectifier on the basis of the input end, increase the phase shift transformer after adding a group of 6 pulse rectifier, so that the DC bus current is completed by 12 thyristor rectifier, so it is also called 12 pulse rectifier.
As shown in the following figure, the two three-phase rectifier circuits I and II constitute a 12-phase rectifier circuit through different connections of the transformer.

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Schematic diagram of 12 pulse rectifier (composed of 2 6 pulses in parallel)






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(1-2)



The Fourier series expansion of the network side current of Bridge 1 is















(1-3)



Bridge II network side line voltage is 30 ahead of bridge I? , because the side line current of the network is 30?















(1-4)



The resultant side current of the network








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Visible, two rectification Bridges produce 5, 7, 17, 19,... Subharmonics cancel each other, and only 12k is injected into the grid? 1 (k is a positive integer) harmonic, that is, 11, 13, 23, 25 and other harmonics, and its effective value is inversely proportional to the harmonic number, and the ratio of the fundamental wave effective value is the reciprocal of the harmonic number.
FIG. 1.2 Input voltage and current waveform of A phase of 12-pulse UPS simulated by computer






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2. Analysis of measured data.
The above calculation is the ideal state, ignoring many factors, such as commutation process, DC side current pulsation, trigger delay Angle, AC side reactance and so on. Therefore, there are some differences between the measured value and the calculated value. Theoretical harmonic calculation table:






Harmonic number
5th
7th
11th
13th
17th
19th
23th


6 Pulse harmonic content
20%
14%
9%
8%
6%
5%
4%


12 Pulse harmonic content
0%
0%
9%
8%
0%
0%
4%






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A model of high-power UPS harmonic measurement data table:






Harmonic frequency
5th
7th
11th
13th
17th
19th
23th


6 Pulse harmonic content
32%
3%
8%
3%
4%
2%
2%


12 Pulse harmonic content
1%
1%
9%
4%
1%
1%
2%




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From the above two tables, it can be seen that the harmonic content of the 6-pulse rectifier is up to 5 harmonics, and the intensity of the 12-pulse rectifier is up to 11 harmonics, which is consistent with the theoretical calculation results. The measured value of the 5th harmonics of the 6-pulse rectifier is larger than the calculated value, and the measured value of the 11th harmonics of the 12-pulse rectifier is the same as the calculated value



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Third, harmonic analysis and improvement countermeasures

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Harmonics may cause distribution cable, transformer heating, reduce the quality of talk, air switch misoperation, 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), 0 sequence (3k times, k is a positive integer).

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+ sequence current makes the loss worse, -sequence current makes the motor reverse and heat up, and 0 sequence current makes the center line current increase abnormally.



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From the measured value can be seen, the 6-pulse rectifier 5 harmonic maximum, can be installed 5 filters to suppress harmonics; The 12-pulse rectifier has the largest 11 harmonics, and can add 11 filters to suppress harmonics. The principle diagram of the filter is as follows:


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Figure: A common LC filter schematic



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The harmonic comparison table of a certain type of high-power UPS after installing the filter is as follows:






Harmonic frequency
3th
5th
7th
11th
13th
17th
19th
Total


IEC6000-3-4 Indicator requirements
21.6%
10.7%
7.2%
3.1%
2%
1.2%
1.1%
25%


6 Pulse harmonic content

32%
3%
8%
3%
4%
2%
> 30%


6-pulse rectifier + 5-pass filter

2%
1%
7%
2%
3%
2%
9%


12 Pulse harmonic content

1%
1%
9%
4%
1%
1%
10%


12-pulse rectifier + 11-pass filter

1%
1%
3%
2%
1%
1%
4.5%




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As can be seen from the above table, the addition of filters has a very obvious effect on harmonic suppression.



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In particular, it should be pointed out that: The configuration of 6 pulse +5 harmonic filter can reach the harmonic requirements of about 9%, but because the capacitance of the 5 harmonic (250Hz) filter is larger, when the UPS load is light (<15% rated load), the rectifier input current will be ahead of the input voltage, if the generator excitation winding is self-excited, it is easy to produce armature positive feedback effect, The output voltage of the generator will rise abnormally, resulting in the generator entering the protection state and stopping. Therefore, the 6 pulse +5 harmonic filter scheme is not recommended for use when the UPS load is light. When the actual load is light, the 5th harmonic filter can be removed from the rectifier.

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The single 12-pulse rectifier can also reach about 10% of the current harmonic index, but there is no large capacitor LC circuit, to avoid the positive feedback effect with the generator excitation.
The current harmonic index of less than 4.5% can be achieved by using 12 pulse rectifier +11 harmonic device. The single harmonics and total harmonics content meet the index requirements of IEC61000-3-4.

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Schedule 1: Input current harmonic content data under different load conditions

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Input the total harmonic content
No-load
25% load
50% load
>75% load


6 Pulses
86%
65%
50%
> 30%


6 pulse +5 pass filter
> 50%
30%
15-20%
10-12%


12 pulses
20%
15%
12%
9.5%


12 pulse + 11-pass filter
12 ~ 15%
8 ~12%
5 ~ 8%
4.5%




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Schedule 2: Table of the total harmonic content of the input current of a 400KVA 12-pulse + 11-pass filter UPS of a certain model







Load percentage



Battery harmonic component THDI%



Total harmonic current absolute value




100%
4.5%
33A


75%
5%
28A


50%
8%
29A


40%
10%
29A


30%
12%
27A






It is the absolute value of harmonic current that affects the grid, not the percentage of harmonic current. As can be seen from the table above, the absolute value of harmonic current is the largest at full load; In the case of half load and light load, the absolute value of harmonic current does not exceed 100% of the absolute value of full load harmonic current. 12 pulse +11 harmonic filter has the minimum effect of the input current total harmonics, but also can avoid active filter "error compensation", low system efficiency shortcomings, because 12 pulse +11 harmonic filter the lowest pollution of the power grid, suitable for use in occasions with high reliability requirements.



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Fourth, performance comparison:

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Project
6-pulse rectifier + 5-pass filter
12 pulse rectifier +11 degree filter
Technical advantages of 12-pulse rectifier + 11-pass filter


1, the input current harmonic content radiated to the power grid
く10%
く4.5%
Due to the introduction of rectifier filter load into the mains power grid will cause the grid to be "polluted", resulting in a large number of high order harmonic current flowing through the whole power supply system. For this reason, it will cause the current flowing through the center line to overcurrent and the motor load abnormal heating. In order to solve the above problems, it is recommended to configure 12 pulse rectifier and 11 harmonic filter, so as to ensure that the power supply quality of the user's grid reaches the "green power supply" standard.


2. Energy consumption
Larger
Smaller
The input current radiated to the power grid has a small harmonic content
High order harmonics will cause heating loss on the input power transformer and cable, resulting in energy waste.


3. Generator down
easy
Will not
The positive feedback effect of the field windings is avoided


4. Cause switch misoperation
easy
Not easy



5. The UPS front end needs generator capacity
Large, according to 1.6~2 times the capacity of the generator configuration
Smaller, according to 1.5 to 1.8 times the capacity of the generator configuration
6 Pulse rectifier UPS needs more than 1.8 times the oil engine capacity
12 Pulse rectifiers require only 1.5 times the oil engine capacity


6. UPS input switch, cable capacity configuration
Add
No increase
Due to the large peak current of the 6 pulse rectifier input, transformers, circuit breakers, cables, etc., need to be derated, resulting in increased investment costs.


7. Rectifier output ripple voltage
Larger
50% lower
The ripple of the DC voltage output of the 12-pulse rectifier is 50% of that of the 6-pulse, which greatly reduces the ripple effect of the battery when charging, and can effectively extend the life of the battery.




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As can be seen from the table above, 12-pulse rectifiers are superior to 6-pulse rectifiers in a number of performance indicators. 12 pulse rectifier technology was born since the 1970s, after continuous improvement and improvement, has gradually become the preferred technology of high-power rectifier.

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Finally, as mentioned above, in the case of sufficient investment, try to choose the configuration of 12-pulse rectifier and 11-time filter. This configuration to meet the Ministry of information industry power supply industry standard input current harmonic component Class I requirements (YD/T1095-2000) and the International Electrotechnical Commission IEC61000-3-4 index requirements.