2011 IEEE Electrical Power and Energy Conference
Three-phase Cross-phase Voltage Sag Compensator to Compensate Balanced/Unbalanced Voltage Sags and Phase Outages in Distribution Systems E. Babaei, Member, IEEE, M. Farhadi Kangarlu, Student Member, IEEE
Abstract-- Voltage sags are one of the most important and frequent problems in distribution systems. Voltage sags lead to huge financial losses all over the world which makes it more important for sensitive load centers. In this paper a three-phase voltage sag compensator is proposed. The proposed voltage sag compensator is based on the ac-ac conversion eliminating the energy storage elements and dc link capacitor in the conventional voltage sag compensators such as dynamic voltage restorers (DVRs). The proposed topology in this paper energizes the compensator of each phase from the other two phases. Regarding that the Single-phase voltage sags are the most frequent types of voltage sags, the faulted phase will supplied by the two healthy phases in the most cases of voltage sags. This results in a more successful operation under very deep single-phase voltage sags. Even if one phase is lost, the proposed compensator will continue to supply all three phases of the sensitive load. The proposed voltage sag compensator is validated using digital simulation case studies with PSCAD/EMTDC software. Index Terms-- Voltage sag converter, direct ac/ac converter
compensator,
cross-phase
I. INTRODUCTION As the number of sensitive equipments increases in the distribution systems, the voltage quality becomes more and more important. From the outcome of many power quality surveys it can be concluded that more than 90% of voltage related events are the voltage sags [1-2]. According to definition, the voltage sag is momentary decrease in the amount of voltage between 0.1 and 0.9 of nominal voltage. Voltage sag can cause enormous financial and production losses in industrial and sensitive load centers. Therefore, preventing of the sensitive loads from the voltage sags is a very important issue. Power electronic converter based solutions for voltage sags have become more popular since they offer the advantages of flexibility and high performance. For voltage sag compensation, series connected voltage source inverter (VSI) is the most suitable solution. In this configuration, the output E. Babaei is with the Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran (e-mail:
[email protected]). M. Farhadi Kangarlu is with the Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran (e-mail:
[email protected]).
978-1-4577-0404-8/11/$26.00 ©2011 IEEE
voltage of a VSI supplied by a dc voltage source is injected to the line by means of a series transformer. Static series compensator (SSC) or dynamic voltage restorer (DVR) is the series connected device which is mostly used to compensate for voltage sags. The key idea and principle operation of a DVR is to inject a voltage of proper magnitude and phase angle in series to the grid so that the voltage on the sensitive load side remains almost unchanged. Depending on the control strategy of a DVR, the injected voltage can have any magnitude and phase angle between certain limits. Therefore, a DVR have to exchange active power as well as reactive power with the grid. This is true for most of the cases unless the voltage sag is too shallow and the power factor of the load is too low. As a result, an important issue regarding a series voltage sag compensator is how to acquire the needed active power for compensation. Two approaches can be considered; in the first approach the required active power is taken from the grid its own using a rectifier bridge. This rectifier bridge can be connected to the grid side or load side. In the second approach, an external energy storage is used to store energy and use it when a voltage sag occurs. The energy storage can be any type such as super-capacitor, battery, superconducting magnetic energy storage (SMES), etc. more information and detailed comparison of the mentioned approaches can be found in [3]. The above-mentioned topologies are based on dc link and energy storage. However, storing electrical energy is expensive and the dc link capacitor and the energy storage used in a compensator (such as DVR) have the main contribution of the over all cost of the compensator. This has caused the limited application of the energy storage based compensators [4]. Moreover, when the voltage sag duration is longer than expected the compensator will face a challenging problem so that the compensation will not be possible. In order to overcome the mentioned problems, direct ac/ac converter based voltage sag compensators have been presented [5-11]. These compensators use ac/ac converter which are directly connected to the grid without using dc link and energy storage elements. However, these topologies have some drawbacks. In the topologies, the required compensation power in each phase is taken from the same phase. This implies that more power must be taken from the faulted phase which makes the situation worse. This issue becomes more important considering that most of the voltage sags are
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unbalanced where one phase is affected. Moreover, in the single-phase schemes which are extended to three-phase systems it is not possible to correct phase angle jump. This results in the unbalanced load voltage from the view point of phase angles when the voltage sag is accompanied by a phase jump. It is worth noting that voltage sags are usually accompanied by phase angle jump. The aim of this paper is to introduce a new voltage sag compensator which is based on the ac/ac direct converter. Since the converter in each phase is supplied from the other two phases of the grid, it does not face problem related to the required compensation power. This is especially important when considering that most of the voltage sags in the grid are unbalanced where the voltage sag occurs in one phase. Moreover, the phase angle jump can be easily compensated by the proposed compensator. The next section explains the circuit configuration of the proposed voltage sag compensator. Then the switching procedure of the used converters is discussed. The simulation results are given to verify the proposed topology. II. THE PROPOSED CROSS-PHASE VOLTAGE SAG COMPENSATOR In three-phase systems different types of voltage sags can be considered. From the view point of the number of phases affected, they are categorized to three main types: the singlephase sag, the two-phase sag and the three-phase sag. In the single-phase sag the voltage of one of the phases is affected. In the two-phase sag the voltage of two of the phases face voltage sag and in the three-phase sag the voltage of all three phases experience voltage sag. The type of the voltage sag depends on several factors. It mainly depends on the fault type which causes the voltage sag and the system configuration. Therefore, the voltage sag type is mainly dependent on the fault occurred in the system (single-phase, two-phase or threephase) and the system grounding scheme (transformers connections). It is well-known that most of the faults occur in a system are single-line to ground faults (SLGF). In this type of fault one phase in unhealthy and the other two phases are in healthy condition. It can be concluded that most of the voltage sags are as a result of SLGFs in which one of the phases is in sever condition and the other two phases are still in healthy condition. However, the other types of the voltage sag can occur in the system. Considering this discussion, a new threephase voltage sag compensator is proposed. The proposed compensator is well-suited for several types of voltage sag especially single-phase voltage sag which is the most common voltage sag in the three-phase systems. Fig. 1 shows the proposed cross-phase voltage sag compensator. In each phase of the proposed compensator there is a direct converter, a low-pass LC filter, an injection transformer and a bypass switch. The converters used in the compensator structure are called cross-phase direct converters. The term cross-phase is used as the converter in each phase is connected to the other two phases. In this way, the output voltage of each converter is generated using voltages of the other two phases. For instance, the output voltage of the phase
a converter, vt ,a , is generated using the input voltage of phases b and c (i.e. vb and vc ). Considering the cross-phase converter used in the proposed voltage sag compensator, it can be concluded that the compensation power of each phase is taken from the other two phases. It means that if one phase is affected by fault, which is the most common fault in practical systems, the corresponding phase voltage is corrected by taking extra energy from the other phases which are healthy. It is important to note that boosting more power from the faulted phase might worsen the fault condition and cause overloading of the corresponding line. Most of the previously introduced ac/ac direct converter based compensators have this main problem. The converter used in the proposed topology is directly connected to the incoming grid without any intermediate dc link and energy storage elements. Therefore, the proposed compensator has no need for the dc link and energy storage elements which are the most expensive and physically largest parts of a conventional voltage sag compensator [5]. Whereas the dc link and energy storage elements are eliminated in the proposed compensator, it needs lower installation area and has lower cost in comparison with the conventional VSI based voltage sag compensators such as DVRs. Moreover, the proposed compensator will not face any problem when the voltage sag duration is too long. This problem is a common problem of the energy storage based compensators since the energy storage has limited capacity. Form this point of view the proposed compensator can operate permanently to compensate for under-voltage as well as voltage sag. The ac/ac converter based voltage sag compensators and DVRs can be divided into two structures: • The structures that are basically single-phase [6-7], [911] • The topologies that are basically three-phase [5,8] The single-phase structures can be easily extended to three-phase systems by putting three of the together. However, the problem is that compensator in each phase is only connected to the corresponding phase of supply. This has some disadvantages. Firstly, if the voltage sag is accompanied by a phase angle jump (which is common) the compensator will not be able to correct the phase angle leading to an unbalanced three-phase voltage from the view point of phase angle. Secondly, all of the compensation power is taken from the same phase leading to the line overloading (also extra voltage drop on the line impedance) especially when the voltage sag is really deep. This causes unsuccessful operation of the compensator under deep voltage sags and disoperation of it in the case of interruptions. In the three-phase structures, the converter in each phase of the compensator is connected to all three phases on its input side. However, in the existing three-phase ac/ac converter based compensators or DVRs the main part of the compensation power is still taken from the faulted phase of supply.
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Za
Bypass switch
va
Bypass switch
vb
Zb
Bypass switch
vc
Zc
− vi ,b +
− vi ,a + •
Lf S1a
Cf
Rf
•
N .. • + v c ,b − 1 Lf
Cf
N .. +•vc,c − 1 Lf
+ vl ,c −
+ vl ,a −
+ vl ,b −
Cf
Rf
Rf + vt ,a −
S 2a
− vi ,c +
•
N .. +•vc,a − 1
+ vt ,b −
+ vt ,c −
S 3a
S1b S 2b S 3b S1c S 2c S 3c Fig. 1. The proposed cross-phase voltage sag compensator
III. CONTROL STRATEGY
level. Whenever the reference output voltage, vcref,a , is in its
Fig. 2 shows one phase (phase a) of the cross-phase converter. The converter consists of three bidirectional switches. Output voltage depends on the states of the switches. Table I indicates the output voltage in every permissible operation mode. According to the table, when S1a is turned on the output voltage of the converter is equal to va . By turning on the switch S 2 a , the input voltage vb is conducted to the output. Obviously, as the switch S 3a is turned on, the output voltage is zero. Therefore, in every instant three voltage levels are available. The desired output voltage is produced by proper switching between these voltage levels. S1a vb S 2a vc
+ vt ,a −
S3 a
negative half cycle, the maximum negative voltage (between vb and vc ) is selected to be used for switching. Reversely, when vcref,a is in its positive half cycle, the maximum positive voltage between vb and vc is selected for switching. The switching procedure can be seen clearly from the enlarged view of the selected part of the figure. the switching must be accomplished in a way that the low frequency average of the switched output voltage follow the fundamental frequency reference voltage ( vcref,a ). In order to do this, a carrier based switching method is used. For this method, the carrier is a high frequency triangular waveform. However, the reference voltage is not as simple as a sine wave. The reason is that the reference wave should take into account the variation of the voltages being used for the switching. Therefore, the reference waveform is obtained as follows: ref viref , a = vl , a − v a
Fig. 2. Cross-phase converter used in the proposed voltage sag compensator (phase a)
viref ,a −N vcref,a ref a = envelop vcref,a =
TABLE I. OUTPUT VOLTAGE OF THE CONVERTER FOR DIFFERENT SWITCHING STATES Operation mode Switches state Output voltage ( vt , a ) S1a S 2a S 3a 1
1
0
0
va
2
0
1
0
vb
3
0
0
1
0
(1) (2) (3)
ref in which, viref , a , vl , a and v a are the reference value of the
Fig. 3 shows the switching method of the first phase (phase a) converter used in the proposed compensator. As shown in the figure, the desired output voltage is generated by switching between one of the phase a or b voltage and the zero voltage
injected voltage, reference value of the load voltage and the measured load voltage, respectively. Also, viref , a denotes the reference value of the converter output voltage after filtering which depends on the viref , a . In (3), ref a is the reference voltage which is used for PWM pulse generation. envelop is
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the maximum positive (in positive half cycle) or negative (in negative half cycle) voltage level between vb and vc . Using (1)-(3), the following relation can be obtained for ref a :
vlref , a − va (4) − N (envelop) The reference waveform is compared with a triangular waveform to provide the switching pulses. Fig. 4 shows the control block diagram of the first phase. As shown in the figure, the grid voltage is subtracted from the load reference voltage to obtain the reference injected voltage ( viref , a ). This ref a =
IV. SIMULATION RESULTS The simulation results are presented to support the proposed voltage sag compensator. The PSCAD/EMTDC software has been used for simulation. The main data used in the simulation is given in Table II. TABLE II. THE MAIN PARAMETERS OF THE SYSTEM Parameters Values 220 = 1 pu RMS (V) in normal condition Grid Frequency (Hz) 50 R (ohm) 40 Load L (mH) 55 C f ( μF ) 35
value is divided to − N to get the reference voltage on the converter side of the transformer ( vcref,a ). It is worth noting that the sign of the voltage changes because of the direction of the windings of the transformer (see Fig. 1). The reference value for the PWM comparator ( ref a ) is achieved by dividing vcref,a to envelop stated by (3). The value of envelop is obtained by the " max" block. In the control system, it is necessary to identify which of the input voltages (either vb or vc ) is the envelop . This is done by another comparator which compares the voltage vb with the envelop . If the difference is below a certain value, the voltage vb is equal to envelop otherwise the voltage vc is equal to envelop. The switching is done between the voltage which is equal to envelop and the zero voltage level. Generating of the switching pulses has been stated by logic gates. In the description given above, the switching method was explained for the first phase converter. The same procedure is applied for the other two phases.
Filter
v
400 200 0 -200 -400
vc
400 200 0 -200 -400
Fig. 3. Switching method of the converter shown in Fig. 2.
v lref,a
+
∑
v ref c,a
v ref i,a
400 200 0 -200 -400
S 3a
ref a Comparator
S1a
−
0.000
AND va vb vc
−N
max
envelop AND Comparator
Fig. 4. Control block diagram of the first phase
S2a
R f (ohm)
0.4 2 2.4 5
The simulation results are presented in different operation conditions including balanced and unbalanced voltage sag and also one phase outage. Fig. 5 shows the simulation results for the symmetric three-phase voltage sag. In this case, each phase of the grid encounters 0.5pu voltage sag from 0.02sec to 0.08sec. Then the voltage recovers to its normal value. As the figure shows, the proposed voltage sag compensator restores the voltage on the load side by injecting proper compensation voltage in each phase so that the load voltage remains in the desired level.
v cref, a vb
1.2
N Injection Transformer Series Inductance (mH) Switching frequency (kHz)
t
ref t ,a
L f (mH)
Va [V]
Vb [V]
Vc [V]
Vi,a [V]
Vi,b [V]
Vi,c [V]
Vl,a [V]
Vl,b [V]
Vl,c [V]
0.020
0.040
0.060
0.080
0.100
Fig. 5. Three-phase symmetric sag compensation by the proposed voltage sag compensator
As discussed before, the proposed voltage sag compensator can compensate unbalanced voltage sag as well. Fig. 6 shows the simulation results in which 0.5pu voltage sag has happened in phase a. In this case the voltage sag is
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accompanied by 30D phase jump. As the figure shows, the compensator produces required compensation voltage and injects in to the grid. As a result, phase a voltage on the load side is fully compensated from both magnitude and phase angle points of view. Form this simulation case it can be concluded that the proposed compensator not only is able to restore the magnitude of the voltage but also it can cope with sudden phase jump. The next simulation case has been conducted to show the performance of the proposed compensator against asymmetric two-phase voltage sag accompanied by phase jump. Fig. 7 shows the results for this case. As clear from the figure, the phases b and c face 0.35pu voltage sag and at the same time they experience 20 D phase jump. This condition can be occurred by a two-phase short circuit. The proposed compensator restores the load voltage by injecting required voltage in the phases b and c. consequently, the proposed compensator can also compensate for two-phase voltage sag which is accompanied by phase jump. As mentioned before, the single-phase ac/ac converter based voltage sag compensators or DVRs can be extended to three-phase systems. However, the extended structures are not able to compensate for unbalanced voltage sags if they are accompanied by phase jumps. In order to show this fact, a simulation has been done where the extended single-phase based compensator [6-7], [9-11] is supposed to compensate for the two-phase 0.35pu voltage sag in phases b and c accompanied by 20 D phase jump. The simulation result for this condition is given in Fig. 8. As the figure shows, even the compensator can compensate for the magnitude of the voltage the phase jump is not compensated leading to unbalanced voltage on the load side from the view point of phase angle.
400 200 0 -200 -400 400 200 0 -200 -400 400 200 0 -200 -400 0.000
Va [V]
Vb [V]
400 200 0 -200 -400 400 200 0 -200 -400 400 200 0 -200 -400 0.000
400 200 0 -200 -400 400 200 0 -200 -400 400 200 0 -200 -400
Vc [V]
Vl,a [V]
0.020
Vi,b [V]
Vl,b [V]
0.040
0.060
Vc [V]
Vi,a [V]
Vi,b [V]
Vi,c [V]
Vl,a [V]
Vl,b [V]
Vl,c [V]
0.020
0.040
0.060
0.080
0.100
Va [V]
Vb [V]
Vc [V]
Vi,a [V]
Vi,b [V]
Vi,c [V]
Vl,a [V]
Vl,b [V]
Vl,c [V]
0.020
0.040
0.060
0.080
0.100
Fig. 8. Performance of the single-phase based voltage sag compensator [6-7], [9-11] extended to three-phase under two-phase voltage sag accompanied by phase jump
Vi,c [V]
Vl,c [V]
0.080
Vb [V]
Fig. 7. Performance of the proposed voltage sag compensator under two-phase voltage sag accompanied by phase jump
0.000 Vi,a [V]
Va [V]
0.100
Fig. 6. Performance of the proposed voltage sag compensator under singlephase voltage sag accompanied by phase jump
One of the common problems in the distribution systems is one phase outage where the voltage of the one of the grid phases falls to zero. The proposed compensator is also able to compensate for one phase outage. Fig. 9 shows the simulation results of this case. As shown in the figure, the second phase voltage goes to zero for a time interval of three cycles. In this period, the second phase of the load is fed by the other two phases of the grid. As a result, the second phase load voltage is restored to its pre-fault condition providing a safe operation condition for the load. This is important especially when the load is a three-phase equipment such as an induction motor. This capability of the proposed compensator as a result of the fact that the required compensation voltage in each phase is produced using the other two phases. Most of the ac/ac converter based voltage sag compensators available in the
132
literature do not have such an ability since they use the same phase to produce the compensation voltage in each phase. 400 200 0 -200 -400 400 200 0 -200 -400 400 200 0 -200 -400 0.000
Va [V]
Vb [V]
Vc [V]
Vi,a [V]
Vi,b [V]
Vi,c [V]
Vl,a [V]
Vl,b [V]
Vl,c [V]
voltage restorer,” Elsevier Journal of Electric Power Systems Research, vol. 80, no. 12, pp. 1413-1420, Dec. 2010. [7] S. M. Hietpas, M. Naden, “Automatic voltage regulator using an ac voltage–voltage converter,” IEEE Trans. Ind. Appl., vol. 36, no. 1, pp. 33-38, Jan./Feb. 2000. [8] 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. [9] J. Pérez, V. Cárdenas, L. Morán, C. Núñez, “Single-phase ac-ac converter operating as a dynamic voltage restorer (DVR),” in Proc. IECON, 2006, pp. 1938-1943. [10] E. C. Aeoliza, P. N. Enjeti, L. A. Moran, O. C. Montero-Hernandez, S. Kim, ‘‘Analysis and design of a new voltage sag compensator for critical loads in electrical power distribution systems,’’ IEEE Trans. Ind. Appl., vol. 39, no. 4, pp. 1143-1150, Jul./Aug. 2003. [11] E. Babaei, and 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.
VII. Biographies
0.020
0.040
0.060
0.080
0.100
Fig. 9. Performance of the proposed voltage sag compensator under one phase outage
V. CONCLUSIONS This paper studies a new topology for direct converter based voltage sag compensator. The converter which is used in the proposed topology is named cross-phase since the output voltage of each phase is produced using the other two input phases. In the case of unbalanced single-phase voltage sag, this is the most common voltage sag, the compensation power in taken from the two healthy phases. Considering the other previously studied direct converter based voltage sag compensators, the main part of the required compensation power in each phase is taken from the same phase. This worsens the fault condition especially in the weak distribution systems. As the results show, the proposed topology not only can compensate unbalanced voltage sags but also balanced voltage sags and phase outages.
Ebrahim Babaei was born in Ahar, Iran in 1970. He received his B.S. in Electronics Engineering and the M.S. in Electrical Engineering from the Department of Engineering, University of Tabriz, Tabriz, Iran in 1992 and 2001, respectively, graduating with first class honors. He received the Ph.D. degree in Electrical Engineering from the Department of Electrical and Computer Engineering, University of Tabriz, Iran, in 2007. In 2004 he joined the Faculty of Electrical and Computer Engineering, University of Tabriz. He was an Assistant Professor position in University of Tabriz from 2007 to 2011 and has been an Associate Professor since 2011. He is the author of more than 100 journal and conference papers. His current research interests include the analysis and control of power electronic converters, matrix converters and multilevel converters, FACTS devices and power system dynamics.
Mohammad Farhadi Kangarlu (S’09) was born in Kangarlu, East Azarbaijan, Iran in 1987. He received the B.S. and M.S. degrees (with first class honor) both in Electrical Power Engineering from University of Tabriz, Tabriz, Iran in 2008 and 2010, respectively. He is currently working toward the Ph.D. degree in Electrical Power Engineering (Power Electronics and Systems). He has been a member of IEEE and IEEE Industrial Electronics Society since 2009. His research interests include power quality, power electronic converters and custom power devices.
VI. REFERENCES [1] [2]
[3] [4]
[5] [6]
D.M. Lee, “A voltage sag supporter utilizing a PWM-switched autotransformer,” Ph.D. Thesis, Georgia Institute of Technology, Atlanta, April 2004. W.E. Brumsickle, R.S. Schneider, G.A. Luckjiff, D.M. Divan, and M.F. McGranaghan, “Dynamic sag correctors: cost-effective industrial power line conditioning,” IEEE Trans. Ind. Appl., vol. 32, no. 1, pp. 212-217, Jan./Feb. 2001. J. G. Nielsen, F. Blaabjerg, “A detailed comparison of system topologies for dynamic voltage restorers,” IEEE Trans. Ind. Appl., vol. 41, no. 5, pp. 1272-1280, Sep./Oct. 2005. C. Meyer, R. W. De Doncker, Y. W. Li, F. Blaabjerg, ‘‘Optimized control strategy for a medium-voltage DVR—theoretical investigations and experimental results,’’ IEEE Trans. Power Electron., vol. 23, no. 6, pp. 2746-2754, Nov. 2008. 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. E. Babaei, M. Farhadi Kangarlu and M. Sabahi, “Compensation of voltage disturbances in distribution systems using single-phase dynamic
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