A Control Strategy for Unified Power Quality Conditioner Based On

8/7/2019 A Control Strategy for Unified Power Quality Conditioner Based On

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A CONTROL STRATEGY FOR UNIFIED POWER QUALITY

CONDITIONER BASED ON

INSTANTANEOUS ACTIVE AND REACTIVE POWERS

Document BySANTOSH BHARADWAJ REDDY

Email: [email protected]

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Abstract : - One of the serious problems in electrical systems is the increasing number of

electronic- components of devices that are used by industry as well as residences. These

devices, which need high-quality energy to work properly, at the same time, are the most

responsible ones for injections of harmonics in the distribution system. Therefore, devices that

soften this drawback have been developed. One of them is the unified power quality

conditioner (UPQC). This paper presents a control strategy for a Unified Power Quality

Conditioner. This control strategy is used in three-phase three-wire systems. The UPQC device

combines a shunt-active filter together with a series-active filter in a back -to- back

configuration, to simultaneously compensate the supply voltage and the load current. Some of

the other control strategy for shunt-active filter that guarantees sinusoidal balanced and

minimized source currents even if under unbalanced and / or distorted system voltages, also

known as “Sinusoidal Fryze Currents”. Then, this control strategy was extended to develop a

dual control strategy for series-active filter. Now, this paper deals about the integration

principles of shunt current compensation and series voltages compensation, both based on

instantaneous active and non-active powers, directly calculated from a-b-c phase voltages and

line currents. Literature-simulated results are presented to validate the proposed

Index Terms: Active Filters, Active Power Line Conditioners, Instantaneous Active and

Reactive Power, Sinusoidal Fryze Currents, Sinusoidal Fryze Voltages.

I. Introduction

ONE of the serious problems in electrical systems is the increasing number of

electronic- components of devices that are used by industry as well as residences. These

devices, which need high-quality energy to work properly, at the same time, are the most

responsible ones for injections of harmonics in the distribution system. Therefore, devices that

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soften this drawback have been developed. One of them is the unified power quality

conditioner (UPQC), as shown in Fig.1. It consists of a shunt- active filter together with a

series-active filter. This combination allows a simultaneous compensation of the load currents

and the supply voltages, so that compensated current drawn from the network and the

compensated supply voltage delivered to the load are sinusoidal, balanced and minimized. The

series and shunt-active filters are connected in a back-to-back configuration, in which the shunt

converter is responsible for regulating the common DC-link voltage.

Fig1. General Configuration of UPQC

In the 30’s of the last century, Fryze [1] proposed a set of active and non-active

(reactive) power definitions in the time domain. From these concepts, Tenti et al [2] developed

a control strategy for shunt-active filters that guarantees compensated currents in the network

that are sinusoidal even if the system voltage at the point of common coupling (PCC) already

contains harmonics. However, this control strategy does not guarantee balanced compensated

currents if the system voltage itself is unbalanced (i.e. it contains a fundamental negative-

sequence component). In [3], this drawback was overcome, by the addition of a positive

sequence voltage detector in the shunt-active filter controller. This control circuit determines

the phase angle, frequency and magnitude of the fundamental positive sequence voltage

component. This new control strategy has been denominated as the “Sinusoidal Fryze

Currents” control strategy.



This work exploits the use of that positive-sequence voltage detector to develop a new

control strategy for series- active filter. It is based on a dual minimization method for voltage

compensation, together with a synchronizing circuit (PLL circuit). The synchronizing circuit is

responsible to detect the fundamental frequency, as well as the phase angle of the positive-

sequence voltage component. The dual minimization method is responsible to accurately

determine the magnitude of this voltage component. This control strategy is denominated here

as the “Sinusoidal Fryze Voltages” control strategy. Further, this paper presents the integration

the “Sinusoidal Fryze Currents” and the “Sinusoidal Fryze Voltages” control strategies into an

UPQC controller. Additionally, the UPQC controller includes an algorithm that provides

damping in harmonic voltage propagation and hinders load harmonic currents to flow into the

network.

II. The UPQC Controller

Fig. 2 shows the complete functional block diagram of the UPQC controller. The

part shown in Fig. 2(a) is responsible to determine the compensating current references for

PWM control of the UPQC shunt converter, whereas the other part shown in Fig. 2(b)

generates the compensating voltage references for PWM series converter. Next, each functional

block of Fig. 2 will be detailed.

Fig 2: The functional diagram of the UPQC Controller

(a). Shunt UPQC Converter, (b). Series UPQC Converter

A. The positive sequence voltage Detector:

A positive-sequence voltage detector is [V+1 voltage detector block in Fig. 2(a)] in

terms of "minimized voltages". A dual principle for voltage minimization together with a

phase-locked loop circuit (PLL circuit), as shown in Fig. 3. The used PLL circuit is detailed in

the next section. In fact, this dual principle of voltage minimization is used here for extracting



instantaneously" the fundamental positive-sequence component ( V +1 in phasor notation, or va1,

vb1, vc1 as instantaneous values, as the outputs of Fig. 3) from a generic three-phase voltage.

The distorted and unbalanced voltages vas, vbs, vcs of the power supply are measured and given

as inputs to the PLL circuit.

As shown in the next section, it determines the signals ia1, ib1, ic1, which are in phase

with the fundamental positive-sequence component ( V +1 ) contained in vas, vbs, vcs. Thus, only

the magnitude of V +1 is missing. The fundamental characteristic of the used PLL allows the use

of a dual expression for determining active voltages in the form

)1(

1

1

1

2

1

2

1

2

1

111 −−−−−

++

++=

c

b

a

cba

cc sbb saa s

c p

b p

a p

i

ii

iii

iviviv

v

vv

As an artifice to extract the V +1 component from vas, vbs, vcs. The reason is that the

signals ia1, ib1, ic1 are three symmetric sinus functions with unity amplitude, which correspond to

an auxiliary fundamental positive-sequence current I +1that is in phase with V +1. Hence, the

average value of the "three-phase instantaneous power", 3V +1 I +1cosØ , is maximum (would be

zero if V +1and I +1are orthogonal), and the average signal Rbar in Fig. 3 comprises the total

amplitude of V +1 . Therefore, it is possible to guarantee that the signals va1, vb1, vc1 are

sinusoidal and have the same magnitude and phase angle of the fundamental positive-sequence

component of the measured system voltage.



B. The PLL Circuit

The used PLL circuit, Fig.4 can operate satisfactorily under high distorted and

unbalanced system voltages. The inputs are vab = vas – vbs and vcb = vcs – vbs. The outputs of the

PLL circuit are ia1, ib1, ic1. The algorithm is based on the instantaneous active three-phase power

expression P3ø= vabia + vcbic.

The current feedback signals ia( w t) = sin( w t) and ic( w t) = sin( w t – 2 π /3) are built up by

the PLL circuit, just using the time integral of output ω of the PI-Controller. Note that they

have unity amplitude and ic( w t) leads 120º ia( w t). Thus, they represent a feedback from a

positive sequence component at frequency ω . The PLL circuit can reach a stable point of

operation only if the input P3ø of the PI-Controller has a zero average value (φ 3

−

P =0) and has

minimized low-frequency oscillating portions in~

3φ P (~

333 φ φ φ P P P +=

−

). Once the circuit is

stabilized, the average value of P 3ø is zero and, with this, the phase angle of the positive-

sequence system voltage at fundamental frequency is reached. At this condition, the auxiliary

currents ia( ω t) and ic( ω t) = sin( ω t – 2π/3), becomes orthogonal to the fundamental positive-

sequence component of the measured voltages vas, vcs respectively. Therefore, ia1( ω t) = sin( ω t

– π/2) is in phase with the fundamental positive-sequence component contained in vas.

.

Fig 3. The V+1 Voltage Detector.



-

Fig4. The Synchronizing Circuit-PLL Circuit

C. The DC voltage regulator

The dc voltage regulator is used to generate a control signal Gloss, as shown in Fig.

2(a). This signal forces the shunt active filter to draw additional active current from the

network, to compensate for losses in the power circuit of the UPQC. Additionally, it corrects dc

voltage variations caused by abnormal operation and transient compensation errors. Fig. 5

shows the dc voltage regulator circuit. It consists only of a PI-Controller [G(s) = Kp + KI/s],

where for normalized inputs, Kp = 0.50 and KI = 80.

Fig5. The DC Voltage regulator

Fig.6 shows in details that functional block named "Current Minimization" in Fig. 2(a).

It determines the instantaneous compensating current references, which should be synthesized

by the shunt PWM converter of the UPQC. It has the same kernel as the Generalized Fryze

Currents methods widely used, like in [4], [5] and [6]. The inputs of the controller are the load

currents ia1, ib1, ic1, the control voltages va1, vb1, vc1 determined by the V+1 detector, and the DC

voltage regulator signal Gloss.



Fig.6 The Current Minimization control algorithm

The conductance G is determined in Fig. 6 corresponds to the active current of the load.

In other words, it comprises all current components that can produce active power with the

voltages va1, vb1, vc1. A low-pass fifth order Butterworth filter is used to extract the average

value of G, which is denominated as Gbar . Now, since va1, vb1, vc1 comprises only the V+1

component, G bar must correspond to the active portion of the fundamental positive-sequence

component ( I+1) of the load current. The control signal Gcontrol is the sum of Gbar and Gloss,

which, together with the control voltages va1, vb1, vc1, are used to determine the currents iaw, ibw,

icw. These control signals are pure sinusoidal waves in phase with va1, vb1, vc1 and include the

magnitude of the positive-sequence load current (proportional to Gbar ) and the active current

(proportional to Gloss) that is necessary to compensate for losses in the UPQC.

Since the shunt active filter of the UPQC compensates the difference between the

calculated active current and the measured load current, it is possible to guarantee that the



compensated currents ias, ibs, ics drawn from the network are always sinusoidal, balanced and in

phase with the positive sequence system voltages. This characteristic represents a great

improvement done at the “Generalized Fryze Currents” control strategy

D. The Damping Control Algorithm

In a UPQC configuration, instability problems due to resonance phenomena may occur.

In order to enhance the overall system stability, an auxiliary circuit can be added to the

controller of the series active filter. The basic idea consists is increasing harmonic damping, as

a series resistance, but effective only in harmonic frequencies, others than the fundamental one.

This damping principle was first proposed by Peng [7], in terms of components defined in the

pq Theory and used by Aredes [8] and Fujita [9]. This damping control algorithm, now is in

terms of abc variables (in the phase mode), can be seen in Fig. 7.

The inputs to the damping circuit are the source currents ias, ibs, ics (compensated

currents), which are flowing through the series transformers of the UPQC, and the voltages

determined by the V+1 voltage detector va1, vb1, vc1. The active and non-active instantaneous

powers are determined by using the equations (2) and (3);

)2(

111

−−−−++=

++=

cscqbsbqasaq

cscbsbasa

ivivivQ

iviviv P

where

3/)( 11 cbaqvvv −= ., 3/)( 11 acbq

vvv −= ., )3(3/)( 11 −−−−−= bacqvvv

Fig.7. Damping control algorithm in terms of abc variables

Note that the voltages vaq, vbq, vcq are achieved from the fundamental positive-sequence voltages

va1, vb1, vc1. Therefore, it is possible to guarantee that the voltages vaq, vbq, vcq are still sinusoidal

and lag 90º the voltages va1, vb1, vc1, respectively. A conductance G and a susceptance B are



determined from the calculated active and non-active instantaneous powers, as shown in Fig.7.

Then, high-pass, fifth order Butterworth filters are used to extract the oscillating parts of that

conductance and susceptance.

The auxiliary currents iap, ibp, icp and iaq, ibq, icq, are determined as follows:

1.

aoscapvGi = ., 1

.boscbpvGi = ., )4(.

1−−−−=

coscapvGi

aqoscaqvGi .= ., bqoscbq

vGi .= ., )5(. −−−−=cqoscaqvGi

Damping signals (harmonic components still present in the source currents) are determined as

described in (6).

apaqah iii += ., bpbqbh iii += ., )6(−−−−+= pcqch icii

Finally, the multiplication between the damping signals iah, ibh, ich and a gain K determines the

damping voltages vah, vbh, vch that will be added to the compensating voltage references of the

series active filter of the UPQC, as will be explained in the next section. Thus, the gain K acts

as a harmonic resistance to damp resonance phenomena.

E. Compensating Voltages Calculation

The block diagram that determines the compensating voltages vac, vbc, vcc [Fig. 2(b)],

which is synthesized by the series PWM converter, is shown in Fig. 8. The inputs are the

control voltages determined by the V+1 voltage detector: va1, vb1, vc1, the source voltages: vas, vbs,

vcs, and the damping voltages: vah, vbh, vch.

The compensating voltages are:

)(1 ahasaac vvvv +−= ., )(1 bhbsbbc vvvv +−= ., )7()(1 −−−−+−= chbsccc vvvv

Ideally, the compensated voltages delivered to the critical load will comprise only the

fundamental positive-sequence component (va1, vb1, vc1) of the supply voltage vS . The

damping voltages will improve stability and provide harmonic isolation



Fig.8. Compensating voltages calculation.

As conclusion, the UPQC control strategy provides compensated voltages and currents

that are sinusoidal balanced and minimized (in phase). Therefore, the power factor is ideal, the

voltages delivered to the load are sinusoidal and balanced, and it is possible to guarantee that

the source currents will be sinusoidal, balanced and minimized even if under unbalanced and /

or distorted system voltages.

RESULTS

Some of the simulated results are presented based on the literature for the three- phase

six pulse thyristor rectifier, with 0.2 A DC-current (20 %), used as a non-linear load. The

results are based on the per unit bases. Thus, 1V (phase to ground) and 1A (line current) were

used as the basis of the system and a balanced,1V, three-phase, voltage source is used.

The shunt-active filter and the series-active filter start its operation in 0.2s. The total

simulation time is 0.8s. The thyristor rectifier is connected at t = 0.1s. An inductor and a

resistor, whose values correspond to 0.1 % of the system base impedance, compose the source

impedance. In this case, the short-circuit power at the load terminal is equal to 10 p.u. The

small high-pass filters to mitigate switching frequency harmonics at the series and shunt PWM

converters are R=0.6 Ω and C = 170 µ F. Although it seams a high capacitor, it corresponds

to 5% of the system base impedance. A capacitor of 2400µ F is used at the DC link of the

UPQC. The reference voltage is equal to 4.5 V. To give an idea of the capacitor’s dimension,

the unit capacitor constant (UCC) is calculated, by the following equation:



)8(1.1.3

)5.4(4002.2

1v

2

1 22

−−−−== µ

p

cUcc

Fig. 9 shows the load, shunt and source currents ial , iac,ias, before, and after the start of

the shunt-active filter. After the start of the shunt active filter, the source current becomes

almost sinusoidal. It may be noticed, that the time that the source currents take to reach the

steady state is pretty small. This demonstrates that the proportional and integral gains of the DC

voltage regulator are well dimensioned.

Fig.9. Load currents of the shunt active filter and source current.

Fig. 10 shows the supply voltage vas (uncompensated, left side of the UPQC), the

compensating voltage vac of the UPQC, and compensated voltage vaw, delivered to the critical

load, before and after the start of the series active filter. The vaw voltage, after the start of the

series active filter, becomes almost sinusoidal.



Fig.10. Supply voltages, Compensating voltage, and the compensated voltage delivered

to the critical load.

Fig. 11 shows the source currents ias, ibs, ics, the compensated voltages vaw, vbw, vcw, and

the current ias together with voltage vaw repeated in a separated graphic, before and after the

UPQC energization. It may be seen that, when the UPQC start its operation the source

currents, as well the compensated voltages become almost sinusoidal and balanced. The source

current ias and the compensated voltage vaw are almost in phase after the start of the UPQC. It

confirms that the control strategy proposed is useful in a three phase three-wire system, where

the system voltages are unbalanced and distorted and the load currents with high contents of

harmonics.

CONCLUSIONS

A control strategy for Unified Power Quality Conditioner based on instantaneous active and

reactive powers for three-phase three-wire systems is explained. In case of using in three phase

four-wire systems, there is the necessity of compensating the neutral current. In this case, three-

phase four wire PWM converter is necessary. The computational effort to develop this control

strategy is less as compared with pq-Theory- based controllers, since the (o -d-q) transformation



Fig.11 Source currents, Compensated voltages and the compensated voltages vaw together with

the source currents.

is avoided. For three-phase three-wire systems, the performance of the proposed approach is

comparable with those based on the pq Theory, without loss of robustness even if operating

under distorted and unbalanced system voltage conditions.

References

[1] S. Fryze, “Wirk-, Blind- und Scheinleistung in elektrischen Stromkainsen mit

nichtsinusfömigen Verlauf von Strom und Spannung,” ETZ-Arch. Elektrotech., vol. 53, 1932,

pp. 596-599, 625-627, 700-702.

[2] L. Malesani, L. Rosseto, P. Tenti, “Active Filter for Reactive Power and Harmonics

Compensation”, IEEE – PESC 1986, pp. 321-330.

[3] Luís F.C. Monteiro, M. Aredes, “A Comparative Analysis Among Different Control

Strategies for Shunt Active Filters,” Proc. (CDROM) of the V INDUSCON - Conferência de

Aplicações Industriais, Salvador, Brazil, July 2002, pp.345-350.

[4] T. Furuhashi, S. Okuma, Y. Uchikawa, "A Study on the Theory of Instantaneous Reactive

Power," IEEE Trans. on Industrial Electronics, vol. 37, no. 1, pp. 86-90, Feb. 1990.

[5] L. Rossetto, P. Tenti, "Evaluation of Instantaneous Power Terms in Multi-Phase Systems:

Techniques and Application to Power- Conditioning Equipments," ETEP – Eur. Trans. Elect.

Power Eng.,vol. 4, no. 6, pp. 469-475, Nov./Dec. 1994.[6] M. Depenbrock, D. A. Marshall, J. D. van Wyk, "Formulating Requirements

for a Universally Applicable Power Theory as Control Algorithm in Power Compensators,"

ETEP – Eur. Trans. Elect.Power Eng., vol. 4, no. 6, pp. 445-455, Nov./Dec. 1994.

[7] F.Z. Peng, H. Akagi, A. Nabae, “A New Approach to Harmonic Compensation in Power

Systems – A Combined System of Shunt Passive and Series Active Filters,” IEEE Trans. Ind.

Appl , vol.26, no.6, Nov./Dec. 1990, pp. 983-990.

[8] M. Aredes, J. Häfner, K. Heumann, “ A Combined Series and Shunt Active Power Filter,”

IEEE/KTH Stockholm Power Tech Conf., vol. Power Electronics, pp. 237-242, Stockholm,

Sweden, June 1995.

[9] H. Fujita, H. Akagi, “The Unified Power Quality Conditioner: The Integration of Series and

Shunt Active Filters,” IEEE Trans. On Power Electronics, vol.13, No.2, March 1998.

[10] Power system Harmonics fundamentals, Analysis and Filter design, by G.J.Wakileh



Document BySANTOSH BHARADWAJ REDDY

Email: [email protected]

Engineeringpapers.blogspot.com

More Papers and Presentations available on above site



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A Control Strategy for Unified Power Quality Conditioner Based On

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