Determination of the Transformer Turns Ratio in a DVR to Reduce the Switches Voltage Rating

Mohammad Farhadi Kangariu, Student Member, IEEE and Ebrahim Babaei, Member, IEEE Faculty of Electrical and Computer Engineering, University ofTabriz

Tabriz, Iran E-mailas : m. farhadi@tabrizu . ac. ir. e-babaei@tabr izu. ac. ir

Abstract- Dynamic voltage restorer (DVR) as one of the most effective custom power devices for voltage sag and swell compensation attracted growing attention in recent years. Various topologies have been presented for DVRs. The topologies are based on both dc/ac inverters and ac/ac direct converters. The ac/ac converter based DVRs address some problems arise in the conventional dc/ac inverter based DVRs. These problems include compensation time limit due to the limited capacity of the energy storage used in the conventional DVR topologies and costly and bulky dc link capacitors. Therefore, in the ac/ac converter based DVRs the installation area and cost can be reduced and compensation

time limitation is eliminated. In this paper, a new algorithm to determination of the transformer turns ratio is given so that the voltage rating of the switches is reduced. On the other hand, as the switches operate with lower voltage, the switching losses and also stress on the switches is mitigated. Moreover, the reduction in the voltage rating as well as required copper for transformer winding leads to reduction in the overall cost of the system. The operation validity of

the system is verified through simulation with the PSCAD/EMTDC software.

Keywords- DVR, ac/ac converter, voltage sag, voltage swell

I. INTRODUCTION

Recently, an increased number of sensitive loads have been integrated in power systems. Consequently, the demand for high power quality (PQ) and voltage stability has been increased significantly [1]. Voltage sags and swells are the most frequent voltage related PQ problems. Voltage sag is a momentary decrease in the rms ac voltage (100/0--90% of the nominal voltage) at the power frequency of duration from 0.5 cycles to a few seconds. Voltage sag is normally caused by short-circuit faults, such as a single-line-to-ground fault in a power system and by the startup of induction motors of large ratings. Voltage swell is defined as a short duration increase in rms supply with an increase in voltage ranging from 1.1 p.u. to 1.8 p.u. of nominal supply. The main reasons for voltage swells are switching large capacitors or the removal of large loads [2].

Occurrence of the voltage sags and swells can lead to very serious economic and technical problems. Therefore, the sensitive loads should be protected against them.

During the past decade, power electronic converter based solutions have been proposed for PQ improvement. The devices are usually called custom power devices [3]. DVR is a series custom power device intended to protect

sensItIve loads from the effects of voltage disturbances such as voltage sags and swells at the point of common coupling (PCC) [1]. Conventional DVR topologies usually include an energy storage connected to a voltage source inverter (VSI) the output of which injected to the grid via an injection transformer after being filtered. Various topologies have been presented for DVRs. Variation in the VSI topologies have resulted in the different topologies with different capabilities [4-11].

Beside the conventional topologies, some topologies have been presented for DVRs which use direct ac/ac converters without need for energy storage elements and intermediate dc link [1-2], [12-13]. This could result in reduction in the physical size and cost of the system. Moreover, the ac/ac converter based DVRs do not face any problem in the case of long duration voltage sags which is a common problem in the energy storage based DVRs.

An ac/ac converter based DVR have been presented in [13]. This topology uses center-tapped transformer. This transformer consists of two outputs which can generally have different values. In other words, the turns ratio of these two parts can be different. This point can be used to determine the transformer turns ratio in a way that the switches and the transformer voltage ratings can be reduced.

In this paper, a new approach is given to reduce the voltage rating of the power electronic switches and the transformer used in a specific topology of DVR with ac/ac converter. Reduction in the voltage rating of the switches and the transformer could result in the reduction in the overall system cost and losses. In the next section, the ac/ac converter based DVR is introduced. Then, the proposed approach is described followed by simulation results.

II. AC/ AC CONVERTER BASED DVR

The ac/ac converter based DVRs use ac/ac converters instead of VSIs to generate the required compensation voltage. Fig. I shows the general configuration of the single-phase DVR with ac/ac converter. Depending on the topology of the ac/ac converter and the transformer structure, different configurations can be achieved that each one has its advantages and disadvantages [1], [13].

Ratings of the dc link capacitor bank of a voltage source converter may have a significant impact on the

IEEE Catalog Number: CFP121IJ-ART ISBN: 978-1-4673-0113-8/12/$31.00 ©2012 IEEE 315

cost and physical size of a conventional DVR. In conventional DVRs, the capacitor is sized for a specified ripple voltage. For reducing the voltage ripple on dc link, the size of the capacitor should be increased. The penalty for making the capacitor large is cost and physical volume. It is a common knowledge that the elimination of the dc link circuit leads to cost reduction as well as improved reliability and longevity. In the ac/ac converter based topologies, elimination of dc link capacitor and energy storage elements leads to considerable reduction in the cost and physical volume. On the other hand in these topologies, the voltage compensation can be accomplished for any long time interval. [14].

Fig. 2 shows one of the topologies which uses ac/ac converter [13]. The topology consists of a center-tapped transformer, an ac/ac converter with two power electronic

switches (S� and SW2 ) and the low-pass LC filter. The

center-tapped transformer is supplied from the grid. At

the output of the transformer two voltage levels, nj v pee and n2 v pee are obtained. If the switch S� is turned on,

the output voltage of the converter before the filter is

equal to nj v pee and in the case that the switch SW2 is

turned on, the output voltage will be equal to -n2 v pee' By

switching between the two voltage levels the required output voltage is generated. The main advantage of this topology is the minimum number of power electronic switches used in its structure. It is worth noting that the switches used in the ac/ac converter are essentially bidirectional. In this paper the common emitter type bidirectional switch is used. This switch is made by two insulated gate bipolar transistors (IGBTs) including their reverse-parallel diodes.

III. PROPOSED METHOD TO REDUCE THE SWITCHES VOLTAGE RATING

Considering Fig. 2, the following equations can be obtained:

Vr, = Vpee + Ve for voltage sag

VL = Vpee - Ve for voltage swell

Ve = njqVpee for voltage sag

(1)

(2)

(3)

Ve = n2qVpee for voltage sag (4)

where, VL, Vpee and Ve are the magnitude of the load

voltage, PCC voltage and the injected voltage, respectively. q is the voltage transfer ratio of the ac/ac

converter.

pee 1---. Adjacent Loads

":"

ac / ac converter, transforme r and filter

Bypass switch

Fig. l. General conf�guration of the DVR with ac/ac converter

v s

pee t---. Adjacent Loads

Bypass switch

+

v pee .., te---:---,

I : �n , �....:.,..' .. ..... --.J(�). Fig. 2. ac/ac converter based DVR using center-tapped transformer

The normalized voltage sag and swell can be written as follows:

VL - Vpee Vsag,PIf =

V L (5)

Vpee - VL Vswell,PIf =

V L (6)

Using (1)-(4), (5) and (6) can be rewritten as follows:

V =� swell,pu 1- n2q

(7)

(8)

In the case of maximum voltage sag and swell, the converter operates in its maximum voltage transfer ration

(qmax)' Taking into mind that qmax = 1 , using (5)-(8),

maximum voltage sag and swell can be obtained as follows:

V max

=�

sag,plt 1 + nj

V max

=�

.,well,plf 1 - n2

(9)

(10)

Equations (9) and (10) are used to determine nj and

n2 in order to compensate for specified maXImum

voltage sag and swell.

Fig. 3 shows variations of (9) versus nj• It can be

seen that as nj increases, the maximum limit of

compensable voltage sag goes up. In other words, with

higher nj deeper voltage sags can be compensated. This

figure can be used to determine nj in a way that up to a

specific value of voltage sag can be compensated. Fig. 4 indicates the maximum limit of compensable

voltage swell as a function of n2• As the figure shows,

for higher n2, higher values of voltage swells can be

compensated. As an example for n2= 0.5 , voltage swells

up to 1 pu can be compensated. This figure is used to

determine n2 in order to compensate for a specific

maximum voltage swell.

IEEE Catalog Number: CFP121IJ-ART ISBN: 978-1-4673-0113-8/12/$31.00 ©2012 IEEE 316

0 .8 ,-------�------�------�--��_,

0.6

0.2

O �------�------�--------�------� o 2 3 4

n1 Fig. 3. Variations of maximum limit of compensable voltage sag versus

0.8

0.6 V�:��.PII

0.4

0.2

O L-______ L-______ L-____ �� ____ �

0.1 0.2 0.3 0.4 0.5

Fig. 4. Variations of maximum limit of compensable voltage swell versus n2

The peak voltage on the switches in their off state can be written as follows:

(11)

where, Vs7:�iff.PII and V/:;:PII are the maximum per-unit

voltage on the switches and the maximum voltage of PCC, respectively.

Fig. 5 shows the off state voltage of the switches

versus n1 and n2• As the figure indicates, higher n1 and

n2 leads to higher off state voltage on the switches. On

the other hand, from Figs. 3 and 4, it can be understood

that higher values of n1 and n2 results in the extended

operation range of the DVR. Therefore, a tradeoff should be made between the aspects.

Fig. 5. Off state voltage of the switches versus n1 and n2

As (11) indicates, the voltage on the switches depends on the transformer turns ratio and the maximum voltage

of Pce. It is clear that Vp:;:PII is obtained in the case of

the maximum voltage swell that the system experiences. As an example, suppose that the maximum voltage sag

the DVR is expected to compensate is 0.5pu. Using (9) n1

will be equal to l. If the maximum expected voltage swell

is 0.5pu then using (10) n2 will be equal to 0.33. With

these values for n1 and n2, using (11), the per-unit value

of the voltage on the switches in their off state is obtained

as ��7:�ff.pII = 1.33xl.5 = l.995pu. However in [13] n1

and n2 have been considered to be equal. This results in

the higher off state voltage on the switches. On the other hand, more copper is used to make the transformer. to compensate 0.5pu voltage sag, the transformer turns ratio

in [13] have been considered to be 1 (i.e. n1 = n2 = 1 ).

With these values for the turns ratios, the off state voltage

of the switches is obtained as ��7:�off.pII = 2 x 1.5 = 3 pu

which is considerably higher than that of the proposed method (1.995 pu).

IV. SIMULATION RESULTS

The simulation results carried out by PSCAD/EMTDC are given to demonstrate the proposed method validity. Table I summarizes the parameters used for simulations.

Fig. 6 shows the simulation results of voltage sag and swell compensation with the ac/ac converter based DVR using proposed method. As the figure indicates, for a period of three fundamental cycles (0.02-0.08 sec. ) OApu voltage sag occurs in the grid voltage. Also, OA5pu voltage swell happens during 0.1-0.16 sec. in both voltage sag and swell conditions the DVR generates and injects the required compensation voltage so that the load voltage is fully compensated.

Fig. 7(a) and Fig. 7(b) show the off state voltage on

the switch S� for the proposed method and the method

presented in [13], respectively. As the figures show the off state voltage on the switch in the proposed method is considerably lower than that of the presented method in [13]. For the voltage sag condition, the voltage on the switch is 248V for the proposed method and 373V for the method of [13]. During the voltage swell condition, the voltage on the switch is 600V and 902V for the proposed method and the method of [13], respectively. The same

condition is true for the switch SW2• Reduction in the

withstand voltage of the switch makes it possible to chose lower rating switch with lower cost. Moreover, the switch operates with lower stress which could result in possible reduction in the switching losses.

TABLE I SYSTEM PARAMETERS USED IN SIMULATIONS Parameter value

R [n) 20 Load

L [mH) 25 Impedance [n) 0.65+ )5

Transformer Turns ratio n1=I,n2=0.33

Switching frequency [Hz) 5000 System frequency [Hz) 50 Voltage peak in normal condition [V) 220/2 =lpu

IEEE Catalog Number: CFP1211J-ART ISBN: 978-1-4673-0113-8/12/$31.00 ©2012 IEEE 317

&D �==��---r--���������

250 -1--+-:;:----1--,:--+:::--+­

O +-������--­-250 -j--j--"-t-----"+--=-j--+--&D�--�-����-�-�-�-�-�

. �

I 2

2

&D

50� J91\ n rm ptD - 9;T V ..... V 4:r ..... Q 4i:f 1J

-&D i i i i i

0.000 0.040 0.080 0.120 0.160

Fig. 6. Voltage sag and swell compensation using the DVR with the proposed approach, from top to bottom the traces are: PCC voltage,

output voltage of the converter before filter, filtered injected voltage and load voltage

i i 0.000 0.040

i 0.080

(a)

i i 0.120 0.160

. .-=V=��=9 Dn�-. __ � __ ..--.�-..��� l.(l<T

0.5k:-j--jf----+-+--jf----

i i i i I 0.000 0.040 0.080 0.120 0.160

(b) Fig. 7. Off state voltage on the switch SW\ (a) for the proposed method,

(b) for the method presented in [13]

V. CONCLUSION

In this paper, a new method for determination of the transformer turns ratio used in an ac/ac converter based DVR is proposed. The proposed method results in considerable reduction in the switches and the transformer voltage rating as well as their voltage stress so that the switches are chosen from lower voltage rating switches leading to lower cost. Moreover, operation of the switches in lower voltage could result in their switching losses reduction. The proposed method also reduces the turns ratio of the transformer so that less copper is used for windings of the transformer. As an example, for O.5pu

voltage sag and swell compensation, the voltage rating of the switches in the proposed method in this paper and the presented method in [13] is 600V and 902V, respectively. The simulation results show the effectiveness of the proposed method.

REFERENCES

[I] E. Babaei, M. Farhadi Kangarlu and M. Sabahi, "Compensation of voltage disturbances in distribution systems using single-phase dynamic voltage restorer," Elsevier Journal of Electric Power Systems Research, vol. 80, no. 12, pp. 1413-1420, Dec. 2010.

[2] E. Babaei, M. Farhadi Kangarlu and M. Sabahi, "Mitigation of voltage disturbances using dynamic voltage restorer based on direct converters," IEEE Trans. Power Del., vol. 25, no. 4, pp. 2676-2683, Oct. 2010.

[3] O. Anaya-Lara and E. Acha, "Modeling and analysis of custom power systems by PSCAD/EMTDC," IEEE Trans. Power Del., vol. 17, no.l, pp. 266-272, 2002.

[4] M. Farhadi Kangarlu, E. Babaei, and F. Najaty Mazgar, "Dynamic voltage restorer using push-pull inverter," in Proc. IEEE 2nd Power Electronics and Drive Systems and Technologies Conference, 2011, Tehran, Iran, pp. 274-278.

[5] P. Roncero-Sanchez, E. Acha, "Dynamic voltage restorer based on flying capacitor multilevel converters operated by repetitive control," IEEE Tran. Power Del. Vol. 24, no. 2, pp. 951-960, Apr. 2009.

[6] S.R. Naidu, and DA Fernandes, "Dynamic voltage restorer based on a four-leg voltage source converter," lET Gener. Transm. Distrib., vol. 3, no. 5, pp. 437-447,2009.

[7] S. Torabzad, E. Babaei and M. Kalantari, "Z-source inverter based dynamic voltage restorer," in Proc. PEDSTC, 2010, Iran, pp. 406-411.

[8] 1. D. Barros, 1. F. Silva, "Multilevel optimal predictive dynamic voltage restore," IEEE Trans. Ind. Electron., vol. 57, no. 8, pp. 2747 - 2760, Oct. 2009.

[9] B. Wang, G. Venkataramanan, M. Illindala, "Operation and control of a dynamic voltage restorer using transformer coupled H­bridge converters," IEEE Trans. Power Electron., vol. 21, no. 4, pp. 1053-1061, luI. 2006.

[10] M. Farhadi Kangarlu, S. H. Hosseini, E. Babaei and A. Khoshkbar Sadigh, 'Transformerless DVR topology based on multilevel inverter with reduced number of switches", in Proc. IEEE 1" Power Electronics and Drive Systems and Technologies Conference (PEDSTC), 2010, Tehran, Iran, pp. 371-375.

[11] C. Meyer, C. Romaus, R. W. DeDoncker, "Five level neutral-point clamped inverter for a dynamic voltage restorer," in Proc. European Coriference on Power Electronics and Applications (EPE),2005.

[12] E. Babaei and M. Farhadi Kangarlu, "A new topology for dynamic voltage restorers without dc link", in Proc. IEEE Symposium on Industrial Electronics and Applications (ISIEA), 2009, Malaysia, pp. 1009-1014.

[13] E. Babaei, M. Farhadi Kangarlu, "Operation and control of dynamic voltage restorer using single-phase direct converter," Elsevier Journal of Energy Conversion and Management, vol. 52, no. 8-9, pp. 2965-2972, Aug. 2011.

[14] E. Babaei and M. Farhadi Kangarlu, "Voltage quality improvement by dynamic voltage restorer based on direct three-phase converter with fictitious dc link," lET Generation Transmission and Distribution, vol. 5, no. 8, pp. 814-823, Aug. 20 II.

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