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2012 IEEE 7th International Power Electronics and Motion Control Conference - ECCE Asia June 2-5, 2012, Harbin, China

Study on Three-Leg-Based Three-Phase Four-Wire

Shunt Active Power Filter

Dong Mingxuan, Huang Zhihao, Wu Jian, Xu Dianguo

Department of Electrical Engineering Harbin Institute of Technology

Harbin, China E-mail: [email protected]

Abstract-Power quality problems such as three-phase unbalanced and aberrance, harmonics and so on, have become serious increasingly and the development of APF has been one of the ways to solve these problems. The detection algorithm based­on FBD theory for command current in three-phase and four­wire system, can extract the zero-sequence current and harmonic current from the load current. This algorithm is adopted in the paper to design the main circuit of three-leg with split phase capacitance and hysteresis control is used to regulate the output current. A prototype of APF based-on DSP, which provide experimental environment for high-power, is developed to test the performance of the APF and results have proved that the compensation effect of the device to zero-sequence current, harmonics and reactive power is reliable.

Keywords-three-phase andfour-wire; active power filter (APF);

Fryze-Buchholz-Dpenbrock (FBD); hysteresis control; three-leg

I. INTRODUCTION

With the development of industrialization process, requirement of consumers for the power quality is becoming higher and higher. Power problems like the lack of reactive compensation capacity of grid and aggravating harmonics pollution increase the loss and influence the power supply quality[11. To solve the problems, static var generator (SYG) is proposed with advantages of rapid response, small volume, wide tuning range, high quality of current waveform in grid[21. The instantaneous reactive power theory was first proposed by H. Kin in 1980s. The definition of reactive power in the p-q-O coordinate system in the theory makes the full compensation of zero-sequence component in the three phase system possible[31. As for the unbalance three phase system, the proposing of the instantaneous reactive power theory promotes the development and application of electrical energy management largely.

Active power filter (APF) of three-phase three-wire has been widely applied[41. While it is only suitable for the balance three phase system but not unbalance three phase system[51. APF of three-phase four-wire can compensate the zero­sequence component effectively which means that it is very suitable to compensate the three-phase four-wire system[61. Command current detection algorithm of three-phase four-wire system based on improved Fryze-Buchholz-Dpenbrock (FBD) algorithm is proposed to eliminate the influence of zero­sequence component. A three-leg APF of three-phase four-wire

This work has been supported by "the Fundamental Research Funds/or the Central Universities" (Grant No. HIT NSRlF2010102)

552

HuaKe

Heilongjiang Electric Power Company Limited Heilongjiang, 150090

[email protected]

system is studied and verified the compensate effect by experiments of prototype.

IT. MAIN CIRCUIT STRUCTURE OF THREE-LEG THREE-

PHASE FOUR-WIRE APF

Main circuit structure of three-phase four-wire APF based on the three-leg circuit has two kinds that one is based on the split-phase capacitor and the other one is based on three-leg topology. There are merits and faults of each. 8 IGBTs and 4 current limited reactors are required in the 4-leg main circuit syructure. And because of the existence of forth leg among three phases, decoupling of the phase control of system can be realized. But the 4-leg structure is expensive. At the same time, with only 6 TGBTs and 3 reactors, 3-leg structure with split­phase capacity is much cheaper. But the voltage of split-phase capacity vibrates largely, and the balance control of direct current (DC) bus voltage is complex. Considering the costs and that the capacity of prototype blows 1 OOkY A, 3-leg structure with split-phase capacity is selected.

Main circuit of three-phase four-wire APF with 3 legs is shown in Fig. l. C1 and C2 are split-phase capacities; they have the same voltage value, withstand voltage and capacity. Zero line of the system is connected to the neutral point of DC bus to provide the circulation path of compensate current of zero line. And the main circuit of APF is connected to the grid and load through series current limited reactors.

Ill. WORKING PRINCIPLES OF THREE-PHASE FOUR-WIRE

APF WITH THREE LEGS

Block diagram of APF is shown in the Fig. 2. SYG with three legs consists of command current detection module, DC bus voltage control module and average voltage of split-phase capacities in DC bus control module. The workflow of three­phase four-wire APF with three legs is as followed. After put into work, three-phase load current and three-phase compensate current of APF are detected and then, through a sampling circuit, transported to digital signal processor (DSP) in which command signal of compensate current is obtained based on the improved FBD algorithm. Then drive pulse is produced by the hysteretic control of command signal. After the power amplification, the drive pulse will be put on the gate of TGBT to complete the compensate work. Tn this process,

978-1-4577-2088-8/11/$26.00 ©20121EEE

DSP has real-time communication with host computer, receIVIng instruction of parameter modification from host computer and sending parameter needed by host computer. In addition, voltages of DC bus and split-phase capacities are required to control the voltage of DC bus and the result will playa role in the calculation of command current[51.

A. Improved FBD Algorithm

FBD algorithm was proposed by German scholar Fryze in 1930s and then improved by others like F. Buchholz and becomes FBD algorithm in contemporary finally. It has good real-time property and simple calculation and is not only suitable for three-phase system but also for the multi-phase. While, for the three-phase four-wire system, the study of the algorithm is limited to the theoretical research I long time. Compared with traditional ip-iq algorithm, the FBD harmonic detection has a different expression in the energy flow and is simpler in calculation in the extraction process of command current. The fundamental thought of calculation of command current is as followed. Initially, loads in each phase of APF should be transformed to the equivalent conductance on which all energy of system is considered to be consumed without other loss. And then, load current can be decomposed into parts including harmonics component and reactive power component. Based on the positive sequence component of unbalanced and non-sinusoidal voltage, reference voltage signal which is in phase with positive sequence component of three phase fundamental wave can be extracted and restored by phase­locked loop (PLL). At last, after the calculation using the reference voltage and three phase load current with zero­sequence separation, active power component of three phase fundamental wave can be achieved.

Fig 1. Mian circuit structure of APF

However, as for the three-phase four-wire system with unbalanced three phase, the traditional FBD algorithm cannot be applied directly with the lack of zero-sequence detection. So, before the calculation of command current, zero-sequence component should be separated from the load current.

[U"(ll] sin OJt

Ub (t) = sin(OJt-~Jr) (I) 3

uc(t) sin(OJt + ~Jr)

3

I,Dad

===~>

<===~1J Fig 2. Block diagram of APF

ia(t) = L~~J:k sin(kOJt + rp;) + l:k sin(kOJt + rp;)]

ih(t) = L~~J:k sin(kOJt + rp; -~Jr)+l':k sin(kOJt

2 + rn- - -Jr)]

't'k 3

ic (t) = L~~l/~ sin(kOJt + rp; + ~ Jr) + I~ sin(kOJt

2 + rp; +-Jr)]

3

G (t) = (u, i) = Ua (tVa (t) + Ub (t)ib (t) + UC (tK (t) f! (u,U) Ua(t)2 +Ub(t)2 +Uc (t)2

(2)

3 [Ua (t)ia (t) + Uh (t)i" (t) + Uc (t)ic (t)](3) --cos20Jt 2

sin OJt

ip(t) = Op(t) sin(OJt-~Jr) 3

sin(OJt+~Jr) 3

[ ~aJ (t)] = lbJ (t)

id (t)

B. Average Voltage Control of Split-Phase Capacitors

(4)

(5)

Circuit structure with split-phase capacitor is proposed. In the unbalanced three phase system, zero- sequence component of three phase flows back to the neutral line through capacitors Cl or C2 which may cause the voltage concussion of split­phase capacitors. And if the amplitude of concussion is extensive, the stability of DC bus voltage would be lost and the compensation of SVG would be influenced. So the average voltage control of split-phase capacitors is in need. Voltage changes of split-phase capacitors are shown in the Table 1. As shown in the Table 1, whenick > 0, the voltage of Cl will hike

and the voltage of C2 will descend. However, when ick < 0, the

condition is in opposite. And this is the principle of voltage concussIOn.

Compensation command current detection algorithm of APF system which combines the average voltage control with FBD algorithm is shown in Fig. 3.The DC bus voltage control of three-phase three-wire system is achieved as followed. The real-time sampled values of DC bus voltage is compared with the given value, then, the result will be stacked on the linear conductance branch circuit which stands for the active power in the FBD method after passing through the proportional­integral (PI) control. And the average voltage control of three­phase four-wire system is adding an outer loop control based on the DC bus voltage PI control. The voltage of split-phase control is compared with the half of the DC bus voltage reference. After the calculation of PI control, the result is added on the output of command current and the average voltage control is completed. . !

IV. RESULT OF THREE-LEG APF IN THE THREE-PHASE

FOUR WIRE SYSTEM

Firstly, a simulation based on Matlab on three-leg APF in three-phase four wire system was done in the paper to prove the validity of the control strategy. The result is shown in Fig. 4.

sin OJ t

Ua PLL

From Fig. 4, it can be seen that the zero current and the harmonics have been compensated completely, the validity of the APF is proved.

Based on the proposed APF prototype, the programming task of command current detection algorithm is accomplished on DSP. And the operation condition of APF in the three-phase system with unbalanced load is also tested.

TABLE 1. CHANG RULE OF VOLTAGE OF SPLIT-PHASE CAPACITORS

i > 0 dick < 0 ck 'dt Udc1 t

di Udcl t i <0 ~<O

ck 'dt

. 0 dick 0 I'k < ,--> , dt Udc -Udcl t

. dick Udc -Udc1 t Ick > 0, -- > 0

dt

sin(OJt + 120") I i cc*

Fig 3. Command current algorithm based on the improved FBD method

~~ .. ~.f:j ... = ~ ~ ~ ~ ~ ~ ~ ~ ~

~~>-"~~--'~~-~'~'-~'~'-~'~'-"~Q-,,6u-~.c~~ •• '-~~ tis tis (b) zero current after compensation

(a) three-phase current after compensation Fig 4. Waves after compensation

The unbalanced load consists of three parts like three-phase thyristor rectifier circuit with resistance-inductance load and single phase diode rectifier circuit with resistance-inductance load. Single phase load is connected between the A phase and neutral line. The line voltage of grid is 380V. Four split-phase capacitors in DC bus all have the capacity of 10000jlF. Each two of them are connected in series and then these two branches are connected in parallel. The capacity of reactors at the exit side of APF is l.2mH. The detection device is PW3196 power quality analyzer produced by HlOKI Company.

Current of load and voltage of grid of three-phase system before the APF is applied are shown in the Fig. 5. And current of load and voltage of grid of three-phase system when the APF is applied are shown in the Fig. 6. As shown in these two figures, before the APF is applied, the unbalanced phenomenon is very obvious and the reactive power component is very large. While, after the APF is applied, the unbalanced phenomenon is nearly disappeared and the three-phase current is balanced and almost sinusoidal. Waveforms shown in the Fig. 7 are current of neutral line, voltage of split-phase capacitors and voltage of DC bus from top to the bottom successively. As shown in the figure, voltage of both DC bus and split-phase capacitors occurs concussion of which the peak value is 7V. The compensation of the current in neutral line is very obvious, and the current after the compensation is almost zero.

- 74.96 ' � �

(.'C?a.'C?a.'C?a'C?�

v?0iVZ�iVZ�iVZ�� 103.11 . . .

?'SC2z=:.;:;z=:.;:;�,. .;:;?0 ...... ! ...... ! 100.00 A/diu

........... O:..B� ................................................................... .... , .. . � n n n

v v v

100.00 A/diu

v

Fig 5. three-phase voltage and current before compensation

- 5.00

__ .� __ ���� __ �,o���.� __ ��� ____ �� ................................ ....................... ...... ......... ..... .............. ..............................

100.00 A/diu

Fig 6. three-phase voltage and current after compensation

555

(a) before the APF is applied

...

... I , .. • • • Is

(b) before the APF is applied

Fig 7. voltage of DC bus and phase-split capacitor

CONCLUSION

A SVG for the unbalanced three-phase phenomenon in the three-phase four-wire system is designed. And the three-leg structure is applied in the main circuit. The flow path of neutral line current is provided by the split-phase capacitors in the DC bus. Based on the improved FBD algorithm, command current detection algorithm of APF is designed. Dynamic hysteresis­band control is applied to achieve the simultaneous compensation of harmonics and reactive power. In this way, power factor at grid side can up to 0.998. Prototype of APF for the three-phase four-wire system is designed on which control algorithm is tested. The correctness of control strategy and favorable compensation performance of APF for the harmonics and reactive power is testified by the result.

REFERENCES

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