Improvement of the Voltage Compensation Performance of the Series Active Power Filter Using a Simple PI-Control Method

Juha Turunen and Heikki Tuusa Tampere University of Technology

P.O.BOX 692, FI-33101 Tampere, Finland Tampere, Finland

Tel.: +358 3 3115 3684 Fax: +358 3 3115 2088

E-Mail: juha.turunen@tut.fi, heikki.tuusa@tut.fi URL: http://www.tut.fi

Keywords «Active Filter», «Harmonics», «Power Quality»

Abstract The quality of the power is impaired by several reasons. This degradation of the quality of the power is seen e.g. as supply interruptions, transient overvoltages, voltage dips, flicker, harmonics and voltage unbalance. The quality of the power may be improved by using power conditioning equipment, which may be either passive or active. This paper discusses one of the most modern power conditioning devices, i.e. the unified power quality conditioner (UPQC), which consists of the parallel active power filter (PAPF) and the series active power filter (SAPF). The UPQC is a power conditioning device able to compensate all kinds of power quality faults. In this paper the UPQC is used to filter both current and voltage harmonics and to compensate the voltage dip. The paper deals with a way to improve the voltage compensation performance of the SAPF at fundamental frequency in steady-state operation. This improvement is achieved by improving the conventional SAPF control system. The control system of the SAPF is improved by using closed-loop control instead of open-loop control. Finally, the results of the experimental tests verifying the functioning of the proposed control system are presented.

Introduction The quality of the power in the supply network is impaired due to several reasons and therefore this degradation is seen as various phenomena. The end user of the electricity may suffer e.g. from supply interruptions, transient overvoltages, voltage dips, flicker, harmonics and voltage unbalance. These voltage failures and their maximum allowable values in the distribution network are defined by the European standard EN 50160 [1]. The quality of the power may be improved by several means [2]. One solution is to use power conditioning equipment, which may be either passive or active. One of the most modern power conditioning devices is a unified power quality conditioner (UPQC), which is shown in Fig. 1 [3],[4],[5]. It consists of a parallel active power filter (PAPF) and a series active power filter (SAPF), which are connected back-to-back using a common dc-link. The SAPF is connected in series with the supply using a coupling transformer. The PAPF is connected in parallel with the load. Both active power filters consist of a pulse width modulated voltage source converter (PWM-VSC) and an LC(L) filter. Since the UPQC is made of two active power filters, it also combines their characteristics and is thus capable of compensating all kinds of current and voltage failures, such as current and voltage harmonics, reactive power, flicker, short voltage interruptions, voltage dips etc. depending on the control systems of the PAPF and the SAPF.

SAPF

SAPFcontrol system

+-

udcus,ABC PAPFcontrol system

ip,ABC

ilo,ABC

SW SW

ib

Rb

ulo

up

Lb

Cp ip

Lp

uaudc

iloisLoad

Supply

us

Cdc

Lc

ia

Cc

u2

PAPF

ulo,ABC ulo,ABC

Fig. 1: Unified power quality conditioner. In this paper, the UPQC is used to filter the harmonics of the supply current and the load voltage and to compensate the supply voltage dip. The paper presents a way to improve the voltage compensation performance of the SAPF at fundamental frequency in steady-state operation. Next, the operating principles of the proposed UPQC and its control system are discussed and after this the experimental test results verifying the functioning of the presented improvement are presented.

Operating principle of the UPQC The operating principle of the UPQC is twofold. The purpose of the SAPF is to improve the quality of the load voltage by injecting the compensation voltage u2, whose amplitude is equal and phase opposite to the failures of the supply voltage us. On the other hand, the purpose of the PAPF is to improve the quality of the supply current by injecting the compensation current ib, whose amplitude is equal and phase opposite to the failures of the load current ilo. Both SAPF and PAPF have their own control systems. The control systems are based on the space vector calculation in the rotating reference frame, where the angular speed of the reference frame corresponds to the fundamental frequency, i.e in the synchronous reference frame.

Control system of the PAPF The control system of the PAPF is presented in Fig. 2. The control system of the PAPF is conventional, although its performance is improved by using the control delay compensation (CDC) and cross-coupling compensation (CCC) methods. The purpose and functioning of these methods are well presented in the reference and therefore they are not explained here [6]. The operating principle of the control system of the PAPF is next briefly presented. The load current ilo is first measured and its space vector in the synchronous reference frame calculated using (1) and (2). In these equations, the phases are denoted as “A”, “B” and “C”, the space vector components in the stationary reference frame as “α” and “β” (ilo = ilo,α+jilo,β) and the space vector components in the synchronous reference frame as “d” and “q” (ik

lo = ilo,d+jilo,q, where k refers to the synchronous reference frame).

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

⎥⎦

⎤⎢⎣

⎡−−−

=⎥⎥⎦

⎤

⎢⎢⎣

⎡

Clo,

Blo,

Alo,

βlo,

αlo,

2323021211

32

iii

ii

(1)

ilo,ABC

dq

ABC

ip,d

+ -HPF CDC

CDC

ip,q

PID

PID

+ + ++ +

Pc

Pc

-

-+700

Pe2

up,ref,dip,ref,d

ip,ref,q

PLL

SW

SVM

udc

ip,ABC ABC

dq

f

ip,d

ip,q

up,ref,q

f

f

dq

ABC

fulo,ABC

ulo,d

ulo,q

ulo,d

ulo,q

Fig. 2: Control system of the PAPF.

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎥⎦

⎤⎢⎣

⎡−

=⎥⎥⎦

⎤

⎢⎢⎣

⎡

βlo,

αlo,

qlo,

dlo,

cossinsincos

ii

ii

ϕϕϕϕ

(2)

The angle ϕ of the synchronous reference frame is generated by a phase-locked loop (PLL). Next, the d-component of the load current space vector is high-pass filtered (HPF) and the delays of the control system compensated (CDC). The output signal of the dc-link voltage controller is added to the d-component of the output current reference in order to maintain the constant dc-link voltage. The achieved PAPF current reference components ip,ref,d and ip,ref,q are compared to the space vector components of the measured PAPF output current ip,d and ip,q and the error signals are fed into the PID-controllers. Next, the cross-coupling terms Pc⋅ip,ref,q and -Pc⋅ip,ref,d are summed to the outputs of the PID-controllers. Finally, the achieved space vector components are subtracted from the load voltage space vector components ulo,d and ulo,q. The resulting PAPF output voltage reference vector uk

p,ref is fed to the space vector modulator (SVM). The dc-link controller, which is included in the control system of the PAPF, is based on the proportional error squared (Pe2) controller. The error between the measured dc-link voltage and its reference value is fed into the Pe2-controller, whose output signal is added to the d-component of the PAPF output current reference.

Control system of the SAPF The control system of the SAPF is next discussed. The operation of the conventional SAPF control system is based on open-loop control. The conventional control system is shown in Fig. 3. This is also the basis of the proposed control system and it can be seen as the lowest calculation branch in Fig. 4, which presents the proposed control system.

CDCABC

dqus,ABC

u lo,ref

f

ua,ref

PLLSW

SVM

k

TRus,ref

k k

Fig. 3: Conventional control system of the SAPF.

CDCABC

dqus,ABC

u lo,ref

f

ua,ref

PLLSW

SVM

k

ulo,ABCdq

ABCLPF

+

-

-

+

LPF

PIu2,diff

zero sig.

Logic

k

++

TRus,ref

k

u2,PIk

k

Fig. 4: Proposed control system of the SAPF.

The operating principle of the conventional control system is the following: the supply voltage us is first measured and its space vector calculated using (1) and (2) (where ilo is replaced by us). The angle ϕ of the synchronous reference frame is generated by a phase-locked loop (PLL). The space vector of the supply voltage us is first compared to its reference value uk

lo,ref = 325+j0. The result of this is the SAPF output voltage reference uk

s,ref. After the control delay compensation (CDC) the voltage reference vector is multiplied by the transformation ratio of the coupling transformer (TR) and the resulting vector uk

a,ref is fed into the space vector modulator (SVM). In the case of the conventional control system, the control method is basically an open-loop control, where the P-controller is used with gain P=1. However, the voltage compensation performance of the SAPF is poor at the fundamental frequency if the open-loop control system is used, because in practice it always produces a steady-state error [7]. This can be avoided by using the closed-loop control method with the PI-controller. However, the PI-controller works properly only when its reference signal is a dc-component. In the control system of the SAPF the fundamental frequency component can be seen as a dc-component because of the space vector calculation in the synchronous reference frame. Therefore, the steady-state error of the load voltage ulo at the fundamental frequency can be reduced to zero by using the closed-loop control system with the PI-controller. Now the operating principle of the proposed control system of the SAPF shown in Fig. 4 is discussed. Besides the aforementioned open-loop calculation branch, the control system also includes another calculation branch, which provides a feedback path in the control system. In this closed-loop calculation branch the PI-controller is used. The operating principle of this calculation branch is the following: first, the space vector of the compensation voltage u2 is found by calculating the difference of the space vectors of the load and supply voltages ulo and us. This vector is low-pass filtered (LPF) to obtain the fundamental component of the load voltage, which is seen as a dc-component in the control system. The reference vector, which is also a dc-component in the given reference frame is achieved by low-pass filtering the voltage reference vector uk

s,ref. After this, the difference of these vectors uk2,diff is fed into the PI-controller and its output added to

the voltage reference vector uks,ref. The PI-controller is used only in steady-state. If the dynamic change in

the supply voltage is observed by the control block “Logic”, it disables the PI-controller (zero sig.). The “zero signal” resets the output signal uk

2,PI and also the internal variables of the PI-controller and keeps them at zero for 10 ms. The idea behind this feedback-loop is that the actual coupling transformer voltage u2 at the fundamental frequency is compared to its reference value and the difference between these signals is fed into the PI-controller. Now the purpose of the PI-controller is to force the error between these signals to zero in steady-state operation, which means that the fundamental frequency component of the compensation voltage u2 follows exactly its reference value. It must be noted that this concerns only the fundamental

frequency component, since it can be seen as a dc-component in this reference frame and may therefore be controlled using the PI-controller. In the case where the system is in dynamic change it is advantageous to disable the PI-controller, since its integrating part makes the response of the control system slower. The PI-control may be enabled again after the dynamic change to ensure a good steady-state performance. Because of the dynamic changes the proposed control system includes the “Logic” block whose operation was presented above.

Experimental tests

In this section the experimental tests verifying the functioning of the proposed control system of the SAPF are presented. First, the prototype of the UPQC as well as the test setup and the execution of the tests are presented. After this, the test results of these tests are shown and analysed.

UPQC prototype and the test set-up The operation of the proposed control system was tested experimentally using the UPQC prototype. The main circuit of the test setup was similar to that presented in Fig. 1. In the prototype, Motorola MPC563CZP66 microcontrollers were used for calculation and Semikron SKM40GD123D IGBT modules were used as switching devices with the PAPF and the SAPF. The supply voltage was 230/400 V. The most important main circuit parameters are presented in Table I. The experimental tests were performed in two cases. In the first case the steady-state harmonics filtering performance of the UPQC was tested using the conventional control system and the proposed control system with the SAPF. The harmonics were produced by two sources: the supply and the load. The supply voltage, which was produced by ELGAR SW 5250A power supply, was distorted. The supply voltage harmonics are presented in Table II a). This table also shows the maximum permissible values of the supply voltage harmonics according to EN 50160 [1]. The load was a 2.3 kVA three-phase diode rectifier with the RL-load on its dc-side. The load current harmonics are presented in Table II b). In the second case it was tested how well the UPQC compensates a voltage dip in cases where the conventional or proposed control system is used with the SAPF. In this test the same load was used as in the previous test. However, in this test the three-phase voltage dip, whose duration was 200 ms, was produced to the supply voltage by the power supply device. During this voltage dip the supply voltage was decreased by 23 %. In this test, sinusoidal supply voltage was used, i.e. it did not contain any harmonics as was the case in the steady-state test.

Table I: Main circuit parameters.

Component Value DC-link capacitance Cdc 1.65 mF DC-link voltage 700 V Modulation frequency (PAPF+SAPF) 10 kHz PAPF LC-filter capacitance Cp 5.0 μF PAPF LC-filter inductance Lb 1.2 mH PAPF LC-filter inductance Lp 5.2 mH SAPF LC-filter capacitance Cc 1.0 μF SAPF LC-filter inductance Lc 5.3 mH Transformer transformation ratio N1/N2 2:1

Table II: a) Phase-A supply voltage harmonics. b) Phase-A load current harmonics.

Test results The test results of the steady-state performance test are presented in Figs. 5 and 6 and in Tables III a) and III b). In the test results it can be seen that there is no notable difference in the harmonic filtering performance of the UPQC between the cases where the conventional or proposed control system of the SAPF is used. This is natural, since the aim of the proposed control system compared to the conventional one is to improve the compensation performance only at fundamental frequency. However, in Table III b) it can be seen that the fundamental component of the load voltage is closer to its reference value 230 V in the case where the proposed control system is used. Actually, in the case where the conventional control system is used the fundamental frequency component of the load voltage ulo is smaller than that of the supply voltage us. This is because the supply current is generates a voltage drop in the coupling transformer and since there is no feedback from the load voltage ulo, this voltage drop cannot be compensated.

a) b) c) d) Fig. 5: Phase-A quantities using the conventional control system. a) Supply voltage us,A. b) Load voltage ulo,A. c) Load current ilo,A. d) Supply current is,A.

a) b) c) d)

Fig. 6: Phase-A quantities using the proposed control system. a) Supply voltage us,A. b) Load voltage ulo,A. c) Load current ilo,A. d) Supply current is,A.

b)

Harmonic no. Ilo,A (A) 1 3.37 5 0.76 7 0.39

11 0.31 13 0.18 17 0.18 19 0.11 23 0.10 25 0.08

THD2.5kHz (%) 28.6

a)

Harmonic no. Test values EN 50160

n Us,A,n/Us,A,1 (%)

Us,A,n/Us,A,1(%)

5 12.0 6.0 7 10.0 5.0

11 3.5 3.5 13 3.0 3.0 17 0 2.0 19 0 1.5 23 0 1.5 25 0 1.5

Table III: a) Harmonic content of the phase-A load current ilo,A and the supply current is,A. b) Harmonic content of the phase-a supply voltage us,A and the load voltage ulo,A.

On the other hand, in the case where the proposed control system is used, it can be seen that the fundamental frequency component of the load voltage is very close to 230 V because of the closed-loop control. Next, the voltage compensation performance of the UPQC was tested in the case of a voltage dip by using the conventional and proposed control systems with the SAPF. The results of this test are presented in Figs. 7 and 8. As is presented in Figs 7 a) and 7 b), at the time instant 80 ms there is a voltage dip of 23 % in the supply voltage us. The duration of the voltage dip is 200 ms.

a) b) Fig. 7: RMS-values of the Phase-A supply voltage us,A (--) and the load voltage ulo,A (-). a) Conventional control system is used with the SAPF. b) Proposed control system is used with the SAPF.

a) b)

Fig. 8: Phase-A quantities using the proposed control system. a) Load current ilo,A. b) Supply current is,A.

b)

Control system Conven-

tional Propo-

sed Harmonic

no. Us,A (V) Ulo,A (V)

Ulo,A (V)

1 223.2 210.4 228.1 5 27.5 4.5 3.9 7 23.7 6.9 7.8

11 8.0 1.0 0.9 13 6.9 2.5 4.6 17 0.2 1.6 2.8 19 0.1 0.8 1.8 23 0.1 1.0 1.4 25 0.1 0.8 0.9

THD2.5kHz 16.9 4.5 5.0 THD20kHz 17.0 4.7 5.2

a)

Control system Conven-

tional Propo-

sed Harmonic

no. Ilo,A (A) Is,A (A) Is,A (A)

1 3.37 3.77 4.42 5 0.76 0.15 0.07 7 0.39 0.14 0.21

11 0.31 0.02 0.03 13 0.18 0.08 0.18 17 0.18 0.08 0.13 19 0.11 0.04 0.09 23 0.10 0.02 0.04 25 0.08 0.01 0.02

THD2.5kHz 28.6 7.1 8.2 THD20kHz 28.6 7.2 8.3

The results shown in Fig. 7 a) are achieved using the conventional control system. In this case no change in the load voltage ulo can be seen despite the supply voltage dip because of the UPQC. However, as can be seen, the steady state performance at the fundamental frequency is poor. The load voltage ulo is continuously approximately 9 % lower than its reference value 230 V. A small ripple component, which can be seen in the load voltage ulo,A presented in Figs. 7 a) and b) is a result of harmonics in the load voltage. Although there are no harmonics in the supply voltage, the harmonics are generated in the load voltage because of the coupling transformer for two reasons. First, because of its impedance, there is a voltage drop across the coupling transformer at the harmonic frequencies due to the load current harmonics. Second, if the transformer is saturated, it generates harmonics whose amplitude depends on the degree of its saturation. Since the control system of the SAPF is based on open-loop control at the harmonic frequencies, i.e. the harmonics are compensated on the basis of the supply voltage measurement, these load voltage harmonics cannot be filtered. Fig. 7 b) presents a test case where the proposed control system is used with the SAPF. In this case it can be seen that at the beginning and end of the supply voltage dip the load voltage ulo decreases approximately 9% for 10 ms. This is because the “Logic”-block shown in Fig. 4 disables the PI -control for 10 ms because of a dynamic change. After 10 ms the PI-control is again enabled, and the steady-state value of the load voltage ulo improves close to its reference value 230 V. Fig. 8 b) presents the supply current is during the voltage dip in the case where the proposed control system is used with the SAPF. It can be seen that its value is increased during the dip. This is because the PAPF draws more current during the voltage dip to maintain the dc-link voltage. The behavior of the supply current is similar in the case where the conventional control system is used with the SAPF.

Conclusion Deterioration of the power quality in the supply network is seen as various phenomena, such as supply interruptions, transient overvoltages, voltage dips, flicker, harmonics and voltage unbalance. The quality of the power may be improved by using power conditioning equipment, which can be either passive or active. In this paper, a unified power quality conditioner (UPQC), which is one of the most modern power conditioning devices, was discussed. The UPQC is a multifunctional power conditioning device, which can be used to compensate all kinds of power quality faults depending on the control system used with it. In this paper the UPQC was used to filter both current and voltage harmonics and to compensate the voltage dip. The UPQC consists of two active power filters, which are the parallel active power filter (PAPF) and the series active power filter (SAPF). The paper presented a way to improve the voltage compensation performance of the SAPF at fundamental frequency in steady-state operation. This improvement was achieved by using the improved control system with the SAPF. This control system was based on closed-loop control and therefore enabled the improvement of the voltage compensation performance at the fundamental frequency. At the beginning of the paper, the UPQC was presented and the operation principles of the PAPF and SAPF discussed. Next, the control systems of the PAPF and SAPF were presented. The proposed control system of the SAPF and its improvements compared to the conventional control system were presented. The improvement of the proposed control system was to use the closed-loop control method to enhance the steady-state compensation performance at fundamental frequency.

Finally, experimental tests verifying the functioning of the proposed control system were presented. Two test cases were presented. In the first the steady-state harmonic filtering performance of the UPQC was tested. In the second the performance of the UPQC was tested in a case of voltage dip. Based on these two experiments it was concluded that the steady-state compensation performance of the SAPF could be improved by using the proposed control system.

References [1] Voltage Characteristics of Electricity Supplied by Public Distribution Systems. European Standard EN 50160.

February 2000. [2] Kennedy, B. W. Power Quality Primer. McGraw-Hill, 361 p. 2000. [3] Cao, R., Zhao, J., Shi, W., Jiang, P., Tang, G. Series Power Quality Compensator for Voltage Sags, Swells,

Harmonics and Unbalance. Proceedings of the Conference on Transmission and Distribution, Vol. 1, pp. 543-547. November 2001.

[4] Chen, Y., Zha, X., Liu, H., Sun, J., Tang. H. Unified Power Quality Conditioner (UPQC): the Theory, Modeling and Application. Proceedings of the International Conference on Power System Technology, Vol. 3, pp. 1329-1333. December 2000.

[5] Fujita, H., Akagi, H. The Unified Power Quality Conditioner: The Integration of Series Active Filters and Shunt Active filters. IEEE Transactions on Power Electronics, Vol. 13, pp. 315-322. March 1996.

[6] Turunen, J., Salo, M., Tuusa, H. Improvement of the Compensation Performance of UPQC. Proceedings of the Nordic Workshop on Power and Industrial Electronics, June 2006.

[7] Dorf, R. H., Bishop, R. C., Modern Control Systems. Addison Wesley Longman, 855 p. 1998.