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Electric Power Systems Research 79 (2009) 1553–1560

Contents lists available at ScienceDirect

Electric Power Systems Research

journa l homepage: www.e lsev ier .com/ locate /epsr

ower quality improvement with an extended custom power park

. Emin Meral ∗, Ahmet Teke, K. Cagatay Bayindir, Mehmet Tumayukurova University, Department of Electrical and Electronics Engineering, Balcali, 01330, Adana, Turkey

r t i c l e i n f o

rticle history:eceived 10 September 2008eceived in revised form 3 June 2009ccepted 4 June 2009vailable online 4 July 2009

a b s t r a c t

This paper describes the operation principles of an extended custom power park (CPP). The proposed parkis more effective when it is compared to the conventional power parks regarding the yield of improvingboth current and voltage quality of linear and nonlinear loads using dynamic voltage restorer (DVR),active power filter (APF), static transfer switch (STS) and diesel generator (DG). Moreover, a supervisorypower quality control centre is presented to coordinate these custom power (CP) devices by providing

eywords:ustom powerustom power parkower quality control centreTS

pre-specified quality of power. A fast sag/swell detection unit is also presented to improve the systemresponse. The ability of the extended CPP for power quality improvements is further analyzed usingPSCAD/EMTDC through a set of simulation tests.

© 2009 Elsevier B.V. All rights reserved.

PFVR

. Introduction

The control of most of the industrial loads is mainly basedn semiconductor devices and microprocessors, which cause suchoads to be more sensitive against power system disturbances suchs voltage sag, voltage swell, current harmonics, interruption andhase shift. Thus, the prevention of negative effects of the PQ dis-urbances has gained more interest for the last twenty years [1,2].

CP is a power electronic based solution against PQ disturbancesr electromagnetic disturbances. CP devices, namely DVR, APF andTS, are applied in the distribution system of an electric utility withhe purpose of protecting an entire plant, feeder, a block of cus-omers or loads [3]. CP devices include an acceptable combinationf the following features; no (or rare) power interruptions, magni-ude and the duration of voltage reductions within specified limits,

agnitude and the duration of over voltages within specified limitsnd low harmonic currents [4].

The STS is used to transfer the load from the preferred sourceo an alterative healthy source. The DVR is capable of generating orbsorbing independently the controllable real and reactive powert its ac output voltage in series with the distribution feeder in syn-

hronism with the voltages of the distribution system. The APF isne of the CP devices and it is generally shunt connected to theystem via a reactance. It can mitigate the harmonic currents gen-rated by nonlinear loads by controlling the compensation current

∗ Corresponding author. Tel.: +90 322 3386868.E-mail addresses: emeral@Cu.Edu.Tr (M.E. Meral), ahmetteke@Cu.Edu.Tr

A. Teke), cbayindir@Cu.Edu.Tr (K.C. Bayindir), mtumay@Cu.Edu.Tr (M. Tumay).

378-7796/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.epsr.2009.06.001

[3,5]. The concept of CPP has been introduced in order to meet cus-tomer’s needs. CPP concept means the integration of multiple CPdevices within the Industrial/Commercial Park that offers the cus-tomers a high quality power at the distribution system voltage level[6]. In the literature, there are various studies about a high qualitypower park concept apart from CPP (unlike CPP). One of the mostimportant studies is the power quality park (PQP) [7]. The classi-fication of customers is the distinguishing feature of PQP and CPP.PQP does not classify their customers while CPP classifies the cus-tomers, so that each customer can be offered different tariff ratesfor required power quality needs.

In this paper, an extended CPP is proposed and variousPSCAD/EMTDC simulation studies are performed to validate theperformance of the park. The designed park and case studies dif-fer from the conventional power park studies in [8–10] from thefollowing ways:

• Power Quality Control Centre (PQCC) provides a coordination ofextended CPP including CP devices and loads, thus resulting in areliable distribution system and a required qualified power.

• The extra functionality is added by integrating APF to the parkand thus an extended CPP is performed.

• A fast fault detection method is presented both for STS and DVR.• The coordination and interaction between the CP devices are pre-

sented comprehensively.

The paper is organized as follows: after this introductory section,general operations of the CP devices in the CPP are described inSection 2. The innovative contributions of the study, the proposedCPP and power quality control centre are presented in Section 3. The

1554 M.E. Meral et al. / Electric Power Systems Research 79 (2009) 1553–1560

iagram

car

2

aadb

ittmtdsfdip

tstPl

Fig. 1. The single line d

ase studies and discussions showing power quality improvementsre presented in Section 4. The main contributions and significantesults of the study are summarized in Section 5.

. The extended custom power park concept

The extended CPP offers a high quality power (grades of A, AAnd AAA) to customers and meets the needs of sensitive loads withn Industrial/Commercial business park. Fig. 1 shows the single lineiagram of the proposed CPP including STS, DVR, APF, DG, the circuitreakers and loads.

STS protects sensitive loads against voltage sags, swells andnterruptions. STS ensures a continuous high quality power supplyo sensitive loads by transferring, within a time scale of 4–8 ms,he load from a faulted bus to a healthy one [11]. STS with a

ake-before-break transfer strategy [12] is used to satisfy the unin-errupted transfer of the power to the critical loads in this study. Theetection and transfer logic must function properly for all the pos-ible operating conditions. In this study, the control method usedor voltage compensation in [13] is developed for voltage sag/swelletection. By using this approach the detection time can be further

mproved with the respect to conventional methods using a lowass filter [14,15].

APF mitigates current harmonic disturbances and compensate

he reactive power of nonlinear loads. The shunt connected voltageource inverter topology is used in the power circuit. The compensa-ion signal is calculated using the concept of Instantaneous Reactiveower Theory (IRPT) [16], which is based on both load voltage andoad current samples.

of the extended CPP.

DVR is connected in series to the distribution circuit by means ofa set of single-phase injection transformers and has capable of gen-erating or absorbing the real and reactive power at its ac terminals.To maximize the dynamic performance, a direct feed-forward-typecontrol [2] is applied to the control unit of DVR. With this control,a fast response time (approximately 1 ms) can be achieved to com-pensate the voltage disturbances. The voltage reference is obtainedfrom the pre-fault line voltage and the compensation signal is cal-culated using the PQR theory [17].

The coordination of CP devices in the CPP is clearly described inthe following sections. A detailed circuit diagram of the CPP systemand the circuit parameters are given in Appendix A.

2.1. The profiles of CPP loads and grades of powers

The loads in the park are divided into three categories. LoadsL-A1, L-AA and L-AAA are balanced and harmonic-free, while LoadL-A2 is a harmonic polluting load. L-AA and L-AAA are the sensitiveloads and they require almost an uninterrupted electrical power. L-AAA is the most critical load and cannot tolerate any disturbances.CPP has two incoming feeders designed for an improved groundingand insulation. Thus, all loads benefit from a high quality powersupply. L-A1 (and also L-A2), L-AA and L-AAA receive the powersQP-A, QP-AA and QP-AAA, respectively, as shown in Fig. 2.

The grades of the powers are explained below.

2.1.1. Qualified Power-A (QP-A)QP-A is a harmonic free and sag/swell free power. This is the

least qualified power at the park. This grade power requires the use

M.E. Meral et al. / Electric Power System

oohb

2

bfcb

2

fb

pai

3

bFcsQf

R(�) = (1)

TO

O

C

D

T

Fig. 2. The grades of the powers at the CPP.

f STS and APF. STS reduces the duration of the voltage sag/swellr the interruption to 4–10 ms by rapidly transferring the loads to aealthy feeder. APF reduces the harmonic distortion at the CPP loadus created by nonlinear loads.

.1.2. Qualified Power-AA (QP-AA)QP-AA is harmonic free and sag/swell free caused by the distri-

ution faults and long interruption free. The grade of QP-AA is overrom the grade of QP-A and it receives the benefit of a DG whichan come up to about 5–10 s in the case of two feeder loss causedy the transmission line faults.

.1.3. Qualified Power-AAA (QP-AAA)QP-AAA is a harmonic free, sag/swell free and long interruption

ree power. Grade QP-AAA is over grade QP-AA and it receives theenefit of DVR.

Consequently, the loads of the CPP receive the superior qualityower compared to the regular power of ordinary loads. In addition,more sensitive load gets more power quality in the CPP as shown

n Fig. 2.

. Power quality control centre

When different types of devices are used to solve multiple distur-ances simultaneously, a coordination of these devices is needed.

or the flexibility of the system, some control functions may beentralized [7]. On–Off states of the proposed CPP equipments arehown in Table 1 and these devices are controlled by the Poweruality Control Centre. The distribution system voltage is assumed

aultless if the voltage is within ±10% of the nominal value. CP

able 1n–Off states of CPP devices and loads.

n–Off states of park equipments

onditions

istribution line faults1. Less than 10% sag/swell at preferred and alternate feeder (nominal operation)2. Less than 10% sag/swell at preferred feeder between 10% and 50% sag/swell at alterna3. Less than 10% sag/swell at preferred feeder more than 50% sag/swell at alternate feed4. Between 10% and 50% sag/swell preferred feeder less than 10% sag/swell at alternate5. More than 50% sag/swell at preferred feeder less than 10% sag/swell at alternate feed

ransmission line faults6. More than 50% sag or interruption at preferred and alternate feeder during start-up d7. More than 50% sag or interruption at preferred and alternate feeder after start-up del8. Between 10% and 50% sag at pref. feeder preferred and alternate feeder

s Research 79 (2009) 1553–1560 1555

devices are operated when the system voltage exceeds these limitsas given in Table 1. DVR is designed to compensate maximum 50%sag as in similar studies [4,11,15,17]. The voltage sags higher than50% are considered as an interruption, as given in Table 1.

The voltage waveforms of the both feeders and the harmoniccurrent-source load are monitored by the PQCC and power qualityevents are captured and managed for a periodic assessment of theservice being provided.

The DG shown in Fig. 1 normally stays off and is not connectedto the CPP load bus. When both of the feeders are lost (more than51% sag or interruption), the generator is started-up immediatelyand connected to the CPP load bus. It should take 5–10 s (condition6 in Table 1) for the generator to come on line and pick up the loadsof both L-AA and L-AAA [4]. L-AA and L-AAA experience power lossonly for 5–10 s during this event. However, L-A1 and L-A2 do notreceive power until one of the feeders is back in service (condition7th in Table 1).

When the 6th or 8th condition occurs, DVR protects L-AAAagainst voltage disturbances. This is the distinguishing feature ofL-AAA from L-AA. During this condition, L-A1, L-A2 and L-AA aresubject to these disturbances. During 4th and 5th conditions, CPPvoltage remains at desired values by transferring the entire loadsto an alternate feeder. However, for the conditions 1st, 2nd and 3rd,there is no need to transfer the loads because the CPP load bus volt-age remains within desired values (90%–110% of nominal voltage)[18].

The APF can filter the current harmonics produced by the har-monic polluting load. It is “On” during the load L-A2 is “On” state.As explained above, for achieving all the conditions appropriately,the coordination of STS (STS a and STS p), DVR, DG, APF and thecircuit breakers are needed. The flowchart of the proposed coordi-nation scheme according to above conditions is shown in Fig. 3. Acommon fault detection method is used for the coordination of allthe devices.

The most important part of the PQCC is the sag/swell (fault)detection unit. In the proposed fault detection method shown inFig. 4, the line-to-line supply voltages named as Vab, Vbc, Vca arefirstly transformed into Stationary Reference Frame (STRF) usingClarke transformation [19] and then transformed into SynchronousRotating Frame (SRF) using Park transformation [20]. dq voltages inthe SRF and their relationship with ˛ˇ voltages in STRF are shownin Fig. 5.

(3) is obtained in positive SRF by using Clarke and Park transfor-mations

(cos(�) sin(�)

)

− sin(�) cos(�)

C =(

1 −1/2 −1/20

√3/2 −

√3/2

)(2)

STS p STS a DVR GEN L-A1 and L-A2 L-AA L-AAA APF

On Off Off Off On On On Onte feeder On Off Off Off On On On Oner On Off Off Off On On On Onfeeder Off On Off Off On On On Oner Off On Off Off On On On On

elay On Off Off Off Off Off Off Offay On Off Off On Off On On Off

On Off On Off On On On On

1556 M.E. Meral et al. / Electric Power Systems Research 79 (2009) 1553–1560

Fig. 3. Flowchart for the coordination scheme of CPP.

Fig. 4. The block diagram of proposed fault detection method.

M.E. Meral et al. / Electric Power Systems Research 79 (2009) 1553–1560 1557

(

w

tdairsa((

oFtcacaa

Fig. 5. Voltages in STRF and SRF.

Vd(p)

Vq(p)

)= 2

3∗ R(−�) ∗ C ∗

⎛⎜⎝

Vab

Vbc

Vca

⎞⎟⎠ (3)

here � = wt.R(�) rotates at the phase angle wt. The subscript (p) represents

hat this is the value in the positive sequence SRF. The subscriptsand q represent d-axis and q-axis values in SRF, respectively. Forpositive sequence SRF, the positive sequence component rotates

n the counter clockwise and the negative sequence componentotates in the clockwise direction in the STRF, so, the positiveequence becomes a DC component and the negative sequence has100 Hz (for 50 Hz network frequency) component as expressed in

4)

Vd(p)

Vq(p)

)=

(Vdp

Vqp

)+ R(−2wt)

(Vdn

Vqn

)(4)

The subscripts p and n show that related parameter is the valuef original positive or negative sequence components, respectively.or balanced faults, there is no need to extract the original posi-ive and negative sequence SRF. Using only the positive sequence qomponent is sufficient for fault detection since q component has

DC value for balanced faults. An unbalanced voltage sag/swell

auses negative sequence components to appear in the feeder volt-ge. Fig. 6 shows q component for a 40% three phase balanced fault,nd for a 40% single phase unbalanced fault.

Fig. 6. q and d components for a balanced fault and an unbalanced fault.

Fig. 7. The filtered Vp and the Vqp signals for fault detection.

Conventionally, (5) is used for fault detection [12]

VP =√

V2d(p) + V2

q(p) (5)

But, the term Vp has 100 Hz ripples. For effective fault detec-tion, the original positive sequence components that have only DCvalue should be separated. A low pass filter (50 Hz) is used [12] toseparate the DC component and ripples in conventional method.Nevertheless, the “original positive sequence component” cannotbe obtained. Furthermore, the filter also causes in a certain amountof delay in an error signal.

In order to overcome this problem, a differential controller isused in the proposed fault detection method. Eq. (6) is obtained bydifferentiating (4).

(V̇d(p)

V̇q(p)

)= −2wR

(�

2

)R(−2wt)

(Vdn

Vqn

)(6)

Since the value of positive sequence is constant, the derivationof it becomes zero. (6) is rotated by 90◦ and divided by −2w, asfollows:

− 12w

R(

�

2

)(V̇d(p)

V̇q(p)

)= R(�)R(−2wt)

(Vdn

Vqn

)(7)

Since the sum of a vector and the value of that vector shiftedby 180◦ is zero, the sum of (4) and (7) leave an only positivesequence component. Thus all the negative sequence componentsare removed and the result is a DC component

(Vd(p)

Vq(p)

)− 1

2wR(

�

2

)(V̇d(p)

V̇q(p)

)=

(Vdp

Vqp

)(8)

Finally, for obtaining original q component; following equationis used:

Vqp = Vq(p) + 12w

V̇d(p) (9)

Fig. 7 shows the filtered (with 50 Hz filter) Vp signal which iscalculated by (5), and Vqp signal which is calculated by (9) in caseof a single phase unbalanced fault occurred at 160 ms. As shownfrom Fig. 7, there is a certain delay because of the filtering on Vp.

However, the Vqp is an ideal signal to obtain error signal.

The obtained original positive sequence Vqp signal is comparedwith a DC reference and passed through a noise filter with a highcut-off frequency (greater than 1 kHz). Thus, the response time ofthe sag/swell detection is decreased compared to the conventionalmethod [12]. An absolute value block is used for the swell detection(because, the value of Vqp is greater than 1 in the case of swell) andthe hysteresis relay is used to generate the transfer signal.

1558 M.E. Meral et al. / Electric Power Systems Research 79 (2009) 1553–1560

4

boosTi

•

•

•

4

tmtqec

4.2. Load bus transferring with STS

As stated in Table 1, conditions 4th, 5th or 6th should be satisfiedto transfer the loads to the alternate feeder.

Fig. 8. CPP load bus currents in case of APF is offline and online.

. Power quality improvements

The extended CPP is tested under the different types of distur-ances such as current harmonics, voltage sags/swells/interruptionccurred on a preferred feeder and voltage sags/swells occurredn both of feeders. The circuit scheme of proposed CPP and theimulation parameters are given in more details in Appendix A.he following case studies are presented to test the power qualitymprovements with the proposed extended CPP.

Simulation results for APF which improves the quality of bus cur-rents with mitigating the current harmonics drawn by nonlinearload.Simulation results for STS which improves the quality of bus volt-ages with transferring bus to a healthy feeder.Simulation results for DVR which improves the quality of mostcritical load voltages with compensating voltage sag.

.1. Harmonic mitigation with shunt APF

L-A2 draws harmonic currents that give rise to a distortion ofhe bus currents and the bus voltages due to line resistance. This

ay affect other loads that are connected to the same bus. In ordero overcome this problem and in order to provide a good poweruality, active power filter is connected to the PCC. Fig. 8 shows theffect of harmonic current components drawn by the L-A2 to the busurrent. Fig. 8 also shows the bus current when the APF is on line.

Fig. 9. Voltage waveforms of preferred feeder and CPP load bus.

Fig. 10. Load bus currents during transition from preferred feeder to alternatefeeder.

The value of current THD should be smaller than the limits statedin IEEE Standard 519-1992 [21]. A considerable reduction of THD isobtained at the CPP load bus currents and L-A2 line currents asfollows:

• THD of L-A2 currents (%): 25.50 in case of APF-offline; 4.02 in caseof APF-online.

• THD of CPP load bus currents (%): 9.30 in case of APF-offline; 1.52in case of APF-online.

THD values are kept below the current distortion limits statedin [21] using APF.

Fig. 11. Waveforms of CPP load bus voltages, L-AA voltages and L-AAA voltages.

M.E. Meral et al. / Electric Power Systems Research 79 (2009) 1553–1560 1559

Table 2Parameters of the simulated CPP.

Symbol in Fig. 12 Description Value/profile

S p and S a Preferred and alternate AC sources L–L 380 VSag/swell generator Disturbance generator –Z pref, Z alt Preferred and alternate feeder impedances NegligibleVT Voltages measurement –CT Currents measurement –BRK As circuit breakers, normally open or normally close –Z a1 Load L-A1 impedance/per phase 145 �T R For start-up delay –GEN Diesel generator L–L 380 VZ aa Load L-AA impedance/per phase 145 �VSI apf Voltage source inverter of APF Six pulse bridge inverterL apf Smoothing inductor 25 mHL r Choke inductor 8 mHNonlinear load Harmonic current source load Thyristor bridge rectifierR a2 Resistor as DC load 90 �TR inj Injection transformer Single phase, 1:1, 1 kVA

orDVR

r phas

Avtnanps

C filter and L filter Filter capacitor and inductVSI dvr Voltage source inverter ofDC s DC source of DVRZ aaa Load L-AAA impedance/pe

The voltage waveform of the preferred feeder is shown in Fig. 9.ccording to EN 50160 standards [18], the admissible maximumoltage variation should be within 10% of the nominal value. Whenhe voltage on the bus is greater than 90% of nominal, there is no

eed to perform the source transfer. The feeder transfer is occurredt 360 ms because the preferred feeder voltage drops to 65% of theominal voltage. STS instantaneously transfers the loads from thereferred feeder to the alternate feeder in a few ms when voltageag occurs on the preferred feeder. Fig. 10 shows the transition of

Fig. 12. The circuit schem

18 �F and 5 mH1-phase H-bridge inverter150 V

e 48 �

load bus currents from the preferred feeder to an alternate feeder.When a fault is detected, the load is transferred to the alternatefeeders and the preferred feeder currents are interrupted. CPP loadbus voltage is almost kept constant.

4.3. Voltage compensation with DVR

A transmission line fault causes a 25% voltage reduction onboth alternate and the preferred feeder voltages at 240 ms. The

e of simulated CPP.

1 ystem

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Transactions on Industry Applications (2003) 844–853.[20] U.A. Miranda, M. Aredes, L.G.B. Rolim, A dq synchronous reference frame current

560 M.E. Meral et al. / Electric Power S

VR starts to operate according to the condition 8th, as stated inable 1.

The CPP load bus and L-AAA voltage waveforms for this conditionre shown in Fig. 11. A voltage sag to 80% of the nominal valuends at 350 ms. During this fault, the (all) loads except L-AAA areubject to a voltage sag and L-AAA voltage is almost kept constant.ccording to IEEE Standard 519-1992 [21], the voltage THD of a lowoltage system should be smaller than 5%. DVR keeps the sensitiveoad voltage magnitude between 0.9–1.0 per unit (pu), and THDower than 3.7%.

. Conclusions

An extended CPP for the improvement of power quality is pre-ented in this paper. A high quality power and an improved powerervice are achieved with the extended CPP to satisfy the needs ofustomers in a power park. The loads of the CPP receive a supe-ior quality power compared to the regular power of ordinaryoads. In addition, more sensitive load gets more power quality andmproved power service. The main contributions of this study arehe coordination of CP devices with a supervisory PQCC and provid-ng the rapid control of the switching devices with a fast sag/swelletection method. An extra functionality to CPP is also providedy introducing a Shunt APF to the park. It ensures the elimina-ion of current harmonics drawn by nonlinear loads at load bus.lso, the DVR keeps the voltage of more sensitive load constant,

he STS transfers all off the loads from a preferred feeder to a alter-ate feeder, and the DG protects the critical loads against the faultsccurred in the transmission line.

Consequently, the simulation results point out that the extendedPP with the new added functionalities has the ability to improveoth voltage and current quality. The extended CPP providesn overall solution to most common power quality disturbancesncountered in power systems.

cknowledgements

The authors would like to acknowledge Electrical, Electronicsnd Informatics Research Group of the TUBITAK (Project No: EEEAG-06E188) for full financial support.

ppendix A.

.1. The scheme and parameters of simulated CPP

The ratings of devices and loads are as follows: low voltage STS,

VR: 1.5 kVA, shunt APF: 1 kVA, generator: 9.5 kVA, L-A1: 1 kVA,-AA: 1 kVA, L-A2: 3 kVA and L-AAA: 3 kVA. The sample time of sim-lation is 25 �s. PSCAD/EMTDC program is used to test the validityf proposed extended CPP. Table 2 gives the parameters of the CPPhown in Fig. 12.

[

s Research 79 (2009) 1553–1560

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