Load Reactive Power Compensation by Using DVR by Changing

Abstract—The dynamic voltage restorer (DVR) is one of the modern devices used in distribution systems to protect sensitive loads against sudden changes in voltage magnitude. The changes may be voltage Sag or Swell; the DVR can effectively compensate if the changes are within a particular limit, and also by proper control of the DVR,this can be utilized to compensate almost all voltage related problems like voltage harmonics, unbalance etc. A novel topology of DVRwith additional feature of utilizing the available capacity of the existing DVR to compensate the reactive power demand of the load is presented in this paper. Moreover the required capacity of the reactive power compensation device is highly reduced in this topology. The necessary mathematical equations and practical implementation of this strategy is discussed in this paper

Keywords— Dynamic Voltage Restorer (DVR), Power Quality, Reactive Power Compensation, Q-DVR

I. INTRODUCTION N modern Power System environment voltage Sag and Swell are the most important power Quality (PQ) problem that

encompasses around 80% of distribution system [1]. According to the IEEE 1959–1995 standard, voltage sag is the decrease of 0.1to 0.9 p.u. in the rms voltage level at system frequency and with the duration of half a cycle to 1 min [2]. Starting large motors short circuits, sudden changes of load, and energization of transformers are the main causes of voltage sags [3].

Voltage Sag or Swell is a transient problem and it can be classified as a low or medium frequency transient event on the basis of nature and frequency of occurrence [2]. In recent years, considering the use of sensitive devices in modern industries, different methods of compensation of voltage sags have been used. One of these methods is using the DVR to improve the PQ and compensate the load voltage

Many more control strategies for DVR is proposed to improve the quality of power in critical loads by compensating sags, swells, voltage imbalance and reducing the voltage source total harmonic distortion (THD) [4],[5]. Essentially DVR is a series compensation device which is falls under the category of custom power devices and which is similar to Static Synchronous Series Compensator (SSSC) which is using in transmission system [6]. But SSSC is mainly focused on improving Active and Reactive power flow, and it do not deal

K.P. Rajesh Kumar, Dept. of Electrical and Electronics, Govt. Engineering College, Thrissur, Thrissur, India. E-mail: [email protected]

with other PQ problems like unbalance, harmonics etc. The series device which is using Distribution side can utilize to compensate almost all voltage related issues. And current related issues like current harmonics, reactive current injection for power factor improvement etc. are done by using shunt compensated devices [7]. Shunt devices may be static capacitor bank, synchronous reactance compensator, and some other power electronics based devices also using for the compensation purpose.

Combined series-shunt device like UPQC can effectively compensate many voltage as well as current related problems simultaneously [6], [7]. But UPQC require very complex control strategy and also it utilizes a shunt and series active filter simultaneously. So the cost is higher and also care must be taken to ensure both inverter works properly.

In this paper an effective method to utilize the series active filter using DVR,to compensate a part of reactive power demand of the load is proposed. Usually voltage sag or swell is not a frequent phenomenon. So remaining time the device kept as idle or may be in self-charging mode [6]-[8]. Through this paper the device made to operate for reactive power compensation during normal operating time. This strategy ensures that no additional increment of rating of the device is requiring achieving this task. This paper is divided in to seven sections.

II. DVR- CONTROLL STRATEGIES The typical DVR topology essentially contains a voltage

source inverter (VSI), an injection transformer connected between the ac voltage line and the sensitive load, and a dc energy storage device (Fig. 1)

Fig. 1: Typical Topology of DVR

Vs – Voltage at PCC/ Source voltage

VLOAD – Voltage at the load terminal

VDVR– Voltage Injected by DVR

L, CL – Filter Inductor and capacitor

Vdc– DC Source Voltage

Load Reactive Power Compensation by Using DVR by Changing Voltage Angle

K.P. Rajesh Kumar

I

Proceedings of International Conference on Materials for the Future - Innovative Materials, Processes, Products and Applications – ICMF 2013 411

ISBN 978-93-82338-83-3 © 2013 Bonfring

The series-connected transformer secondary winding injects the compensating voltages generated by the VSI to mitigate voltage sags or swells of the ac line (Fig. 1).VSI may be two level spwm type or multilevel inverters. Multilevel VSI converters [9] such as diode-clamped converters, multilevel flying capacitor converters, or multilevel cascaded H-bridge converters [10], are able to synthesize voltage waveforms with lower harmonic content than two-level converters and able to operate at higher dc voltages. Previous works have been done on different aspects of DVR performance, and different control strategies have been found. Conversional there are mainly 3 basic control strategies for DVR. These are a) In- Phase Compensation, b) Presag Compensation and c) Minimum Energy injection [11]. The Fig.2 Describes the phasor diagram representation of the basic control modes during voltage sag condition.

In the above three strategies minimum energy injection have need more attention. In this with the sag compensation the DVR makes a change in voltage angle. By series inspection of the phasor diagram we can see that here DVR injecting an amount of reactive power to line. Here zero active power injection is possible, but it is limited only in certain level of sag conditions. But during voltage swell condition this quadrature injection is not effective and this is shown in fig.3 (a). So for effective compensation of voltage swell as well as sag condition the combination of inphase and minimum energy injection is utilized. Fortunately we can find that through this method DVR make a phase angle shift to the voltage and consequently make shift to the current also. So here DVR capable to share some amount of reactive power too.

(a) (b)

(c)

Fig.2 a) Inphase Compensation, b) Presag Compensation,

c) Minimum Energy Injection

(a) (b)

(c)

Fig.3: (a) minimum energy injection during swell, (b) In phase compensation during swell, (c) combined in phase and

minimum energy injection So from the phasor diagrams it is clear that minimum

energy injection alone is not possible to compensate voltage swell conditions. But inphase compensation is capable to compensate voltage sag as well as swell condition. So for achieve our objective combination of these too can take under consideration. Inphase compensation is basically active power injection and minimum energy injection is reactive power injection. So combination will inject active and reactive power simultaneously in most of the conditions. Here i am utilized this concepts for making the DVR as a reactive power compensating device. And for convenience i am calling this concept as Q-DVR concept.

III. Q-DVR CONCEPT As told if DVR inject a voltage which is not in phase or

directly out of phase it will make a phase angle shift to the voltage. Due to this voltage angle shift the current taken by the load is also make a phase shift. Due to this phase shift the reactive power demand seen from the source is considerably reduced. This can be graphically represented by phasor diagram shown in fig.4

Fig.4: Reactive power compensation under normal conditions Vs,Is- Voltage at the point of common coupling (PCC)/

Source voltage, Source current

Vload, IL- Load voltage and current before compensation

Vload’, IL’- Load Voltage and current after compensation, k- Rated voltage magnitude, during normal condition

(a) (b)

Fig.5: a) Compensation during voltage sag condition, b) Compensation during voltage swells condition


ISBN 978-93-82338-83-3 © 2013 Bonfring

In the fig.4 the operation of DVR under normal operation condition is shown here a series voltage of magnitude Vdvr is injected at an angle η. So the voltage seen by the load have got a phase shift of angle δ, so correspondingly current taken by the load also got this phase angle shift. In effect there have a reduction in phase difference between source voltage and current. That is new phase angle between source voltage and current is taken as β and it is equal to the difference between old phase angle, Φ and δ. Hence the reactive power demand from the source is reduced considerably.

But the main function of the DVR is the voltage sag/swell compensation, so whatever the additional function it is doing if there is any sag or swell it must be compensated. The operation during sag and swell is illustrated in fig.5. Here injected voltage must compensate voltage sag/swell and also maintain the same voltage angle shift δ

During Voltage sag voltage magnitude reduce by a fraction, so DVR injected voltage must contain two components. One must compensate voltage magnitude and it is represented by Vdvr” in fig.5(a) and another component which have to maintain the phase angle shift and it is represented as Vdvr’. So actual voltage injection is the phasor some of Vdvr” and Vdvr’ and it is represented as Vdvr. The similar analysis during voltage swell condition is represented in fig.5 (b).

IV. MATHEMATICAL COMPUTATION In order to implement this Q-DVR concept practically it is

necessary to find out the magnitude and angle of the series injected voltage in terms of measurable parameters. For doing this we can take three cases separately. That is normal; voltage sag and voltage swell conditions.

A. Normal condition

Refer Phasor diagram shown in fig.4. We can redraw this phasor as shown in fig.6 for simplicity

From fig.6,

Consider ΔADB, AB= AD cosδ = k. Cosδ

Fig.6: Modified phasor diagram at normal condition

Where AD=k, rated voltage magnitude

Assume k= |Vs|=|Vload| BD = AD sin δ = k. Sinδ

BC= AC- AB = k – k cos δ

Consider ΔDBC, CD =

=

= k. . (1)

∠BCD= 180-η

tan(180-η) = =

η =180 - (2)

From (1) and (2) injected voltage magnitude

Vdvr = k. .

And angle, η =180 -

B. Voltage Sag condition

In voltage sag condition as described already it is necessary to inject two components of voltage. So mathematically it can written as

= (3)

Consider the detailed phasor diagram for voltage sag condition is shown in fig.7. This is a simplified diagram drawn based on fig.5(a)

From (3) and referring fig.5 (a) and fig7. We can write Vdvr∠η =Vdvr’∠η’+ Vdvr”∠0 (4)

Vdvr” have the magnitude equal to the amount of voltage sag

From fig.7, Vdvr = BE = (5) BC = AC – AB (6)

We have known, AB = Vs, AC = AE cosδ

Assume Fraction of voltage reduction due to sag is kf,

Kf= (7)

From equation (6), BC = k cos δ – Vs (8)

Fig.7: developed phasor diagram showing compensation

during voltage sag condition From equation (7) Vs = k (1+kf)

= k. no (9)

Where, no= 1+kf From equation (8), BC = k cos δ – k. no

= k (cos δ – no ) (10)

CE = k sin δ (11)

From (5), (10), (11)

Vdvr =

= k.

= k. (12)

Voltage angle, η can be calculated by considering ΔBCE, from the ΔBCE


ISBN 978-93-82338-83-3 © 2013 Bonfring

= =

tan (180- ) = (13)

From equation (13), η = 180 - (14)

C. Voltage swell condition

During voltage swell condition similar to sag, the injected voltage must consist of two components, one should compensate voltage swell and another should maintain the same voltage angle δ, therefore equation (3) and (4) is also valid in this case, but here Vdvr” is injected in directly phase opposition with that of source voltage. So effective magnitude decreases at the load terminals. For the analysis we can draw the developed diagram of phasor diagram shown in fig.5 (b)

From fig.8, consider ΔBCE,

BE = (15)

BC = AB – AC = Vs – AE cos δ (16)

From (7), (9),(16) CB = k (no - cos δ) (17) From (10),(11) Vdvr = BE

= k.

= k. (18)

Consider ∠ABE, 180 – η = (19)

Fig.8: Developed phasor diagram showing compensation

during voltage swell condition

From (17), (19) η = 180 -

= 180 - (20)

Compare (12) and(18) with (14) and (20) we can see that weather sag or swell the injected voltage, magnitude and angle governing the same equation

During normal condition Vs = k, therefore kf become 0, hence no = 1

From (18) Vdvr = k. (21)

From (20) η = 180 - (22)

Compare (21) and (22) with (1) and(2), we can say that the same equation derived for sag and swell condition is valid for normal condition also.

V. BOUNDARY CONDITIONS From the phasor diagrams shown in fig.4 & fig. 5 it is

clear that the amount of reactive power shared by the DVR is depending the amount of voltage angle shift δ. There should

be limitation for the maximum shift. That is as the δ increases the magnitude of the voltage injected by the inverter is also increases. There might be a maximum possible capacity for the inverter. So we might carefully choose the level of compensation inorder to avoid the overloading of DVR.

Assume Sdvr is the maximum possible power that can be shared by the DVR

And, Sdvr = Vdvr . Is* (23)

Isis the current drawn by the load hence power sharing is depending the series injected voltage Vdvr. Assume Vdvr,maxis the maximum possible voltage injection and it can represent terms of the percentage of this maximum voltage limit in terms of desired load voltage by factor Kd.

From (21) Vdvr, max = k.

Kd. k = k.

= 2. (1- cosδmax)

δmax = (24)

So the maximum possible phase shift can calculate by this equation. This will guarantee there is no additional capacity requires for the DVR to compensate part of the reactive power requirement of the load.

VI. PRACTICALIMPLEMENTATION

(a) (b)

(c)

Fig.9: a) Single line diagram of the compensated system, b) complete phasor diagram of the compensated system, c)Phasor

diagram of system with capacitor compensation alone

By using this proposed Q-DVR concept part of the reactive power can be compensated by the DVR. But for the practical application DVR alone may not be suitable to achieve desired level of compensation. So additional to this any other compensating devices like static capacitor, synchronous condenser etc. can be used for 100 % compensation [12]. Fig.9 below showed the schematic representation of such system. Here a shunt capacitor bank is taken as the additional reactive power compensating device. Fig.9 (a) represents the single line diagram of such system.


ISBN 978-93-82338-83-3 © 2013 Bonfring

Total load reactive power demand,

Qload = Vload. Iload. sinφ (25)

In uncompensated system, Qs = Qload = Vload . Iload sinφ

= Vs. Is.sinφ (26)

For 100 % combensation the whole reactive power supplied by the source must be compensated by the capacitor. Vs and Is is the source voltage and current and φ is the phase angle difference between this voltage and current.

From fig.9(c) , assume Qc is the reactive power shared by the capacitor in capacitor combensated system. Assume Ishunt is the capacitor current.

Qc = Vs. Is. sin φ = Vload Ishunt (27)

From fig.9 (b), assume Qc’ is the reactive power shared by the capacitor in combined DVR and capacitor system. Qc’ = Vs.Is. sin β (28)

β = φ - δ (29)

From fig,9 (b), it is clear that phase angle between source voltage and current is reduced to β. Therefore effective reduction in capacitor size can be represented by this difference in reactive power sharing. and assume it is Qc”.

Qc” = Qc – Qc’ = Vs. Is. sinφ - Vs. Is. sin (φ - δ) (29)

And Qc” amount of reactive power must be shared by the DVR. and from fig.4 and fig.6 reactive power shared by the DVR can be calculated. assume Qdvr is the ractive power supplied by DVR

Qc” = Qdvr = Vdvr. Is. sin η (30)

= k. . Is. (31)

= k. . Is.

= Is.k. sinδ (32)

From (32),δ = (33)

At any instant the reactive power shared by the DVR is the difference between load requirement and reactive power supplied by the shunt capacitor. At constant voltage reactive power share of capacitor is constant and assumed it as Qshunt.

Therefore from (33),

δ = (34)

Equation (34) can be used for instantaneous calculation of δ at any working conditions.

From the fig.10 it is clear that lf the load power requirement is more than the maximum capacity of the capacitor bank DVR will produce a positive shift to the voltage and hence it will deliver reactive power. Consiquently if load demand is less than the shunt combensation level DVR will produce a negative shift and hence it will absobe reactive

power. So this controlled DVR can act as both reactive power source as well as sink.

Fig.10 :Operational characteristics of Q-DVR

VII. CONCLUSION DVR is an effective solution for many voltage related

problems like, voltage sag, swell, harmonics etc. So it must be an essentail equipments where sensitive loads are connected. Through this paper it has mathematicaly proved, additional to the above said features, by proper controlling DVR can act as a reactive power compensating device too. So cost and size of other reactive power compensating device can be reduce effectively.

REFERENCES [1] J. A. Martinez and J. Martin-Arnedo, “Voltage sag studies in distribution

networks- part II: Voltage sag assessment,” IEEE Trans. Power Del., vol. 21, no. 3, pp. 1679–1688, Jul. 2006.

[2] J. A. Martinez and J. M. Arnedo, “Voltage sag studies in distribution networks- part I: System modeling,” IEEE Trans. Power Del., vol. 21, no. 3, pp. 338–345, Jul. 2006.

[3] P. Hcine and M. Khronen, “Voltage sag distribution caused by powersystem faults,” IEEE Trans. Power Syst., vol. 18, no. 4, pp. 1367–1373,Nov. 2003.

[4] F. Mohammad Mahdianpoor, Rahmat Allah Hooshmand, Member, IEEE, and Mohammad Ataei,”IEEETran. Power Delivery, Vol. 26, No. 2, April 2011

[5] SNV Ganesh, Dr. K. Ramesh Reddy, Dr. B.V. Sanker Ram. “Different Control Strategies for Power Quality Improvement Using Dynamic Voltage Restorer” Proceeding of the 2011 IEEE Students' Technology Symposium 14-16 January, 2011, IIT Kharagpur

[6] K.R. Padiyar,”FACTS Controllers in Power Transmission and Distribution”, (First Editon), New Age International Publishers, NewDelhi,2007

[7] V. Khadkikar and A. Chandra, “A new control philosophy for a unified power quality conditioner (UPQC) to coordinate load-reactive powerdemand between shunt and series inverters,” IEEE Trans. Power Del.,vol. 23, no. 4, pp. 2522–2534, Oct. 2008.

[8] V. Khadkikar and A. Chandra,”UPQC-S: A Novel Concept of Simultaneous Voltage Sag/Swell and Load Reactive Power CompensationsUtilizing Series Inverter of UPQC

[9] L. G. Franquelo, J. Rodríguez, J. I. Leon, S. Kouro, R. Portillo, and M. A. M. Prats, “The age of multilevel converters arrives,” IEEE Ind. Electron. Mag., vol. 2, no. 2, pp. 28–39, Jun. 2008

[10] J. Dionísio Barros and J. Fernando Silva,”Multilevel Optimal Predictive Dynamic Voltage Restorer”, IEEE Transactions On Industrial Electronics, Vol. 57, No. 8, August 2010

[11] S. S. Choi, B. H. Li, and D. M. Vilathgamuwa, “Dynamic voltagerestoration with minimum energy injection,” IEEE Trans. Power Syst., vol. 15, no. 1, pp. 51–57, Feb. 2000.

[12] T.J.E Miller,”Reactive Power Control in Electric System”, John Wiley(Newyork), 1982


ISBN 978-93-82338-83-3 © 2013 Bonfring

Documents

Load Reactive Power Compensation by Using DVR by Changing