Study on Control Strategy of High Power Multiple Connection Rectifier

High power DC speed control power supply which can improve the power factor is a kind of multiple connection rectifier using sequential control, but it will bring the grid large harmonic which cannot be ignored in shallow control. To solve this problem, wave and theory of the input side current,harmonic, total factor of harmonic distortion, fundamental wave factor and power factor are contrastively analyzed in this paper. Moreover, the best control strategy of reducing harmonic is also proposed through the calculating formulas. For the control of high power multiple connection rectifier, the best control strategy is to use sequential control in depth control and synchronous control (parallel control) in shallow control which was proved by experiment. In this method, harmonic is greatly reduced but the power factor is the same.


Introduction
In the field of high power DC speed control power supply, in order to improve the grid power factor and reduce the motor current ripple, multiple connection rectifiers are always used to supply electricity for DC motor.Such as 12-phase, 18-phase and 24-phase power rectifier [1- 5] .Many other literatures searched varietals topology structure for improving the power factor.But those methods just increased the pulse number of rectifier, never discussed or analyzed the control method [6][7][8] .They just mentioned sequential control and synchronous control (parallel control), neither did they show quantitative analysis or measurement of harmonic.They also didn't discuss the control method of rectifier [9] .They could not give the best control strategy because there was no precise quantitative analysis for the current harmonic in the rectifier side which used multiple connection and sequential control method.The theoretical analysis and practical tests show that the power factor can be improved when we adopt depth control for multiple connection sequential control rectifier, but large harmonic will harm the grid in shallow control.This paper will introduce the best control strategy by quantitively analyzed the power factor and harmonic in different control level of multiple connection rectifier.At last, the research will be proved by experiment.

Harmonic current quantities analysis of multiple connection rectifier in different control methods
Generally, to obtain a 12-pulse rectifier output voltage, we often use two three-phase full-controlled rectifier circuits in series circuit, also called reconnection rectifier, whose main circuit wiring is shown in Figure .1.The transformer ratio of rectifier transformer is 1:1: 3 , the line current of input side is , and its values is equal to the sum of and , which is winding of output side converted to the input side .In order to facilitate the analysis, it is assumed that the rectifier transformer leakage reactance is 0, inductance of flat wave reactor is infinity.Assuming 1bridge firing angle is α The common control method is：α 1 fixed ，α 2 continuously changing or fixed.Corresponding to the input side of the current is shown in Figure 2. Then: In the formulas: is rectifier output DC current, ω is the power Angular frequency here 100π.

d I
Applying the principle of superposition to get the total current of network side: Formula (2) shows the input side current of the rectifier tran current content is the key to harmonic ana rmonic content of the current I H is: ( sformer by sequential control, the 5, 7, 17, 19 ... harmonics still exist, the amplitude of all harmonics are related to Δα.
Harmonic lysis.The ha Valid values of fundamental current: 3 Fig. 3. Different Δα the net side current waveform ide of th rectifier transformer, since α 1 is fixed in the control process A I is the sum of two winding current in output s e , while α 2 adjusted continuously, the current waveform of A I also changes the same time.The waveforms of A I as Δα take different angles current are shown in Figure .3. The waveforms in Figure 3 shows that, although shapes of A I are different when Δα range from 0 to 30 °, but each current has the same effect time within a half cycle time.
When Δα =0 °,   When Δα =30°,   appears once each, continue 60°, it is obvious: om 0 vali lues of When Δα ranges fr to 30°, d va When Δα ranges from 150° to 180°, the variation regularity of in a half cycle is the same to when Δα Through above analysis, we can lid values of input side is a constant current when Δα in inter obtain that the va regional of 0°～30°and 150°～180°.When 30°<Δα<90, By the above analysis, we can know e valid values of current are equal within two intervals, th Put the formula (4 when different Δα, ), ( 7) into (3), it an be drawn that  4), ( 5), ( 6), (7), the value of ξ can be determ cosφ

Power fa connection s
The power factor of dual connection sequence control ξ cosφ, ξ is fundamental wave fa ined corresponding to different Δα, fundamental wave factor ξ change in the range of 0～0.989., the φ in can be represented by following formula: In general, the firing angle of one bridge is fixed, then, variation of t α 1 fixed, α 2 c he other in the sequence control, assuming that hanges, then, The power factor is: The power factor of each segment can be obtained as follow, When 0°≤Δα≤ In the above fo mula, α is the firing angle of ronous control, T the power factor of synchronous control, which is equivalent to the sequential con l: the power factor of synchronous control, which is equivalent to the sequential con l: According to ( 12), ( 13), ( 14) and ( 15

The optimal control strategy
equipment in a reversible operation, we need to minimum trigger often take as follow: α min =β min =30°.
Δα in this case is often about 30 °, it can be seen from the preceding analysis of the power factor and harmonic content that the harmonic of the sequence control is maximum in this case and the value of it is 0.468, assuredly, the harm to grid is large in this situation .Meanwhile, if sequence control becomes synchronization control, that is to say Δα=0°, then the harmonic content is 0.24, harmonic content is reduced by 97%.while the power of sequence control and synchronization control is the same when Δα≤ 30°, there is no doubt that synchronous control is the best control strategy at this time.Δα can take 150° when motor is running at low speed, then the harmonic content is only 0.125.

Simulation and experiment
In order to verify the theoretical analysis, usi sis.The simulation circuit is shown in Figure 6.System parameters are as follows: supply voltage 380V, load 10Ω, 0.1mH.The simulation results shown in Figure 7, is percentage of each small figure at is the grid current waveform, the harmonic content of 1-50 bar graph below is the grid current, the unit content of the fundamental current.
By analyzing simulation results it can be obtained that, grid current harmonic content is the smaller than in situations, when α 1 =α 2 =30 °, while α 1 = 30 °, α 2 = 60 °, the grid current harmonic content is largest.

Conclusions
Through the analysis of connecting multiple sequential control rectifier input harmonic current, power factor and the simulation results we can obtain that, when Δα ranges from 0 ° to 180 °, the harmonic content of the input current is a three continuous function, as Δα= 30 °, when compared with synchronization control, the harmonic content has nearly doubled.When Δα ≤ 30 °, the power factor of two control modes are equal.Once adopt synchronous control, the power factor basically unchanged, while the harmonic has been greatly reduced, so for the high-power direct current power supply with multiple connection speed circuit, the best control strategy is sequential control when in deep control and synchronous control in shallow control.This control strategy under the premise of not reducing the power factor, greatly reduces the grid current harmonic content, reducing the harmonic pollution to the grid, but also increases the life of the motor, and also improves the speed performance.

I
by calculating the valid values of the input side line current A I .

14 )
Make two control modes rectified output the same voltage.Compare synchronous and sequential control of the dual connection rectifier po r synch dS , then,  wer factor.Make U dT , U dS respectively output voltage of Synchronous control and sequence control, then, U dT =U do cos α.
), we can  T (15) draw curves of the two power fact α 1 = 30 ° which is shown in Figure. 5. or control mode as α 1 = 0