2012 International Conference on Computing, Electronics and Electrical Technologies [ICCEET]

Generalized UPQC system with an improved Control Method under Distorted and

Unbalanced Load Conditions. A.Jeraldine Viji\Dr.M.Sudhakaran2

Asst.Professor,Research scholar!, MEC,JNTU,Associate pro-f,PEC

Abstract- Power quality has become an important factor in power systems, for consumer and household appliances with proliferation of various electric and electronic equipment and computer systems. The main causes of a poor power quality are harmonic currents, poor power factor, supply-voltage variations, etc. A technique of achieving both active current distortion compensation, power factor correction and also mitigating the supply-voltage variation at the load side, is compensated by unique device of UPQC presented in this paper and this paper presents a modified synchronous-reference frame (SRF)-based control method to Shunt active filter and instantaneous PQ (IPQ) theory based control technique for series active filter to compensate power-quality (PQ) problems through a three-phase four-wire unified PQ conditioner (UPQC) under unbalanced and distorted load conditions. The proposed UPQC system can improve the power quality at the point of common coupling on power distribution systems under unbalanced and distorted load conditions. The simulation results based on Matlab/Simulink are discussed in detail in this paper.

Index Terms-Active power filter (APF), harmonics, modified phase locked loop (MPLL), power quality (PQ), synchronous reference frame (SRF), unified power-quality (PQ) conditioner (UPQC).

I INTRODUCTION

The modem power distribution system is becoming highly vulnerable to the different power quality problems [1-2]. The extensive use of non-linear loads is further contributing to increased current and voltage harmonics issues. Furthermore, the penetration level of small/large-scale renewable energy systems based on wind energy, solar energy, fuel cell, etc., installed at distribution as well as transmission levels is increasing significantly.

A. Jeraldine viji is a Research Scholar in JNTUK, Andra Pradesh. and currently working Asst.Professor in Mailam Engineering college,. E-mail: jeraldrovan@gmail..com. Dr.M.Sudhakaran is working as a prof in Pondicherry Engg college,E-mail: sudhakaran@pec.edu

978-1-4673-0210-4112/$31.00 2012 IEEE 193

Unified power quality control was widely studied by many researchers as an eventual method to improve power quality of electrical distribution system [1-3]. The function of unified power quality conditioner is to compensate supply voltage flicker/imbalance, reactive power, negativesequence current, and harmonics. In other words, the UPQC has the capability of improving power quality at the point of installation on power distribution systems or industrial power systems. Therefore, the UPQC is expected to be one of the most powerful solutions to large capacity loads sensitive to supply voltage flicker/imbalance [2]. The UPQC consisting of the combination of a series active power filter (APF) and shunt APF can also compensate the voltage interruption if it has some energy storage or battery in the dc link [3]. The shunt APF is usually connected across the loads to compensate for all current-related problems such as the reactive power compensation, power factor improvement, current harmonic, compensation, and load unbalance compensation (3,4)whereas the series APF is connected in a series with the line through series transformers. It acts as controlled voltage source and can compensate all voltage related problems, such as voltage harmonics, voltage sag, voltage swell, flicker, etc.

The proposed control technique has been evaluated and tested under non-ideal mains voltage and unbalanced load conditions using Matlab/Simulink software. The proposed method is also validated through experimental study. The following diagram shows the generalized UPQC system

Voltage

i:>., iC. il.

rv = r.J',, + rL".; .. J '

pce

Fig-I Generalized Diagram ofUPQC system

2012 International Conference on Computing, Electronics and Electrical Technologies [ICCEET]

II. UPQC CONTROL ALGORITHM

DVR APF

ContmJCf Centrdla.: $\I'dlddtUion H.aJTnonu:c:'iIr1ion . .. Rei'ertnc\'ollage gmtr.lioo. GalesigM generation Gate,ignjgenaanon ReJCti\'i:powercalru3tion'

Fig-2 control Diagram ofUPQC system

The UPQC consists of two voltage source inverters Connected back to back with each of them sharing a common dc link. Fig-2 shows the control diagram of Upqc system. One inverter work as a variable voltage source is called series APF, and the other as a variable current source in called shunt APF.The main aim of the series APF is harmonic isolation between load and Supply; it has the capability of voltage flicker/ imbalance compensation as well as voltage regulation and harmonic compensation at the utility-consumer PCC. The shunt APF is used to absorb current harmonics, compensate for reactive power and negative-sequence current, and regulate the dc link voltage between both APFs. The proposed UPQC control algorithm block diagram in Matlab/Simulink simulation software is shown in Fig. 2.

A. Reference Voltage Signal Generation for Series APF

.. ---1r-.---+i .. ----jf-H--H

===--------------------- ---------------------------- ----------------------------

Fig-3 control diagram of Series Active Filter

The series APF control algorithm calculates the reference value to be injected by the series APF transformers, comparing the positive-sequence component with the load side line voltages. In

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equation (1), supply voltages VSabc are transformed to d-q-O coordinates.

1 1 1

71 - - - r' 1 ,= ; n(t) n(cut - ) n(cut+ ) Vs!l a a v. 4 CQOJt) CilOJt - ) CilOJtt ;) a

(1)

The voltage in d axes (VSd) given in (2) consists of A verage and oscillating components of source voltages (VSd and VSd ) . The average voltage VSd is calculated by using second order LPF (low pass filter).

(2)

The load side reference voltages V l'!< 12bc are calculated as given in equation (3). The switching signals are assessed by comparing reference voltagesV Lbc and the load voltages (VLabc ) via sinusoidal PWM controller.

(3)

The three-phase load reference voltages are compared with load line voltages and errors are then processed by sinusoidal PWM controller to generate the required switching signals for series APF switches.

B. Reference Current Signal Generation for Shunt APF

Fig-4 control diagram of Shunt Active Filter

The above figure shows the control diagram of shunt active filter. The shunt active filter compensates the current harmonics and reactive power generated by the nonlinear load. The instantaneous active power (p-q) theory is used to

2012 International Conference on Computing, Electronics and Electrical Technologies [ICCEET]

control of shunt APF in real time. In this theory, the instantaneous three-phase currents and voltages are transformed to a-p-O coordinates as shown in equation (4) and (5).

[t l. v, T 1 l.J [ ' 1 t]= - ' 1 ft '\ a "'/, V

"1 1

' a

1 l .:l !!] l

(4)

(5)

The source side instantaneous real and imaginary power components are calculated by using source currents and phase-neutral voltages. The instantaneous real and imaginary powers include both oscillating and average components as shown in (7). Average components of p and q consist of positive sequence components (p andq)of source current. The oscillating components (p and q) of p and q include harmonic and negative sequence components of source currents [4]. In order to reduce neutral current, p 0 is calculated by using average and oscillating components of imaginary power and oscillating component of the real power; as given in (8) if both harmonic and reactive power compensation is required. i*sa i *s, i*so are the reference currents of shunt APF in a-p-O coordinates. These currents are transformed to three-phase system as shown in (9).

(6)

p=p+ P (7)

(8)

1 1 @ 1. "Il ro] (9) Oi"" ' jC 1. ..p

The reference currents are calculated in order to compensate neutral, harmonic and reactive currents

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in the load. These reference source current signals are then compared with sensed three-phase source currents, and the errors are processed by hysteresis band PWM controller to generate the required switching signals for the shunt APF Switches [6].

III. SIMULA TOIN RESULTS

Fig

Fig-5 Simulink diagram of Series Active Filter

In this study, a new control algorithm for the UPQC is evaluated by using simulation results given in Matlab/Simulink software. The simulated

UPQC system parameters are given in Table 1. In simulation studies, the results are specified before and after UPQC system are operated. Before harmonic compensation, the THD of the supply current is 26.23% and after is 0.5%.Similally voltage harmonic is 24.6% before compensation and after is 0.3%.

"

Fig-6 Overall simulink diagram of proposed UPQC system

Parameters Value Source

Voltage VSabc 380 Vnn, Frequency f 50 Hz

Load 3-Phase ac Line Inductance LLabc 3 mH Shunt APF Ac Line Inductance LCabo 4 mH Filter Resistor RCabc 60 Filter Capacitor CCabo 10 "F Switching Frequency fp",m 10 kHz Series APF

2012 International Conference on Computing, Electronics and Electrical Technologies [ICCEET]

ac Line Inductance LTabc 2 mH Filter Resistor RTabc 60 Filter Capacitor CTabc 20 "F Switching Frequency fpwm 15 kHz

The proposed UPQC control algorithm has ability to compensate both harmonics and reactive power of the load and neutral current is also eliminated. The proposed control technique has been evaluated and tested under dynamical and steady-state load conditions.

Fig-7. Three phase source current before compensation

Fig-7. Three phase source voltage before compensation

Fig-8 Supply voltage after adding upqc system

Fig-9 .Supply current after adding upqc system

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200 ..

Fig-10 Three phase compensating current

100 .

,;! j 0 ,r r r' r'1l r'f

-3lO ...

-8lO 0 0.01 0.112 0.111 O.IM 0.00 O.IE 0.07 0.111 0.111 0.1 Tme(5)

Fig-II. Three phase load current

_(6IlIz)'6!.1IIFO.51\

i ,

j 0 >

100 200 300 400 500 600 700 8110 900 18110 fr!

2012 International Conference on Computing, Electronics and Electrical Technologies [ICCEET]

voltages and source voltage, the shunt APF provides three-phase balanced and rated currents for the loads.

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