Practical Mitigation of Voltage Sag in Distribution Networks by Combining Network Reconfiguration and DSTATCOM Nesrallh Khelef, Student Member, IEEE *, Azah Mohamed, Senior Member, IEEE *
**
, Hussain Shareef
***
UKM University/Department of Electrical Engineering, Bangi, Malaysia. Email:
[email protected] ** UKM University/Department of Electrical Engineering, Bangi, Malaysia. Email:
[email protected] *** UKM University/Department of Electrical Engineering, Bangi, Malaysia. Email: shareef@ eng.ukm.my
Abstract— Voltage sag is considered as one of the most common power quality problems. It may cause sensitive equipment to malfunction and process interruption. This paper presents the application of network reconfiguration and DSTATCOM to mitigate voltage sags in power distribution networks. The proposed method first reinforces the system voltage profile during voltage sag by network reconfiguration. Then DSTATCOMs are placed directly to partially mitigated nodes in order to obtain complete mitigation of voltage sag problem. The simulation results show that it is effective to incorporate network reconfiguration and DSTATCOM to overcome voltage sag problem in practical distribution systems. Keywords — -: voltage sag reconfiguration, DSTATCOM.
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distribution
network,
I. INTRODUCTION Voltage magnitude is one of the major factors that influence the quality of power supply. Loads at distribution systems are usually subjected to frequent voltage sags due to various reasons and it is highly unacceptable for some sensitive loads in high-tech industries. Voltage sag is defined as a decrease in rms voltage or current between 0.1 and 0.9 pu. at the power frequency for durations of 0.5 cycle to 1 min. [1]. Currently a lot of research work is under way to solve the problem of voltage sags in distribution systems. Most of these research works focus on installing voltage sag mitigation devices. The static synchronous compensator (STATCOM) is a shunt-connected reactive compensation equipment which is capable of generating and absorbing reactive power. Its output can be varied so as to maintain specific parameters of electric power system such as the voltage profile [2]. The reactive power compensation by DSTATCOM is acceptable in the presence of linear and nonlinear loads. It also has the ability to mitigate balance and unbalance voltage sags [3]. The installation of these equipments can completely mitigate voltage sag and it may significantly reduce the number of sensitive equipment failures. It also has the capability to mitigate voltage sag for some up and downstream buses [4, 5]. The DSTATCOM can also be used to compensate for a load-induced voltage sags [6]. Some other research works focus on utility efforts in finding feasible solutions to mitigate voltage sags. Since the faults are considered as one of the main causes of voltage sags, utilities try to prevent faults and modify the available fault clearing practice in power systems [7]. To reduce line losses and bus voltage profile improvement in
steady state, some utilities use distribution network reconfiguration. Network reconfiguration can also be an effective approach for power utilities to mitigate voltage sags [8]. It can be done by placing the voltage sag sources as far away as possible from the main power supply [9]. In this paper, network reconfiguration is investigated and presented as a feasible method for mitigating voltage sag in distribution systems. After illustrating that a major part of voltage sag problem can be solved by network reconfiguration, application of DSTATCOM is investigated to overcome the remaining part of voltage sag problem. Finally, the effect of the incorporation of the network reconfiguration and the installation of DSTATCOM devices are presented. II.
NETWORK RECONFIGURATION FOR VOLTAGE SAG MITIGATION The reconfiguration process may reinforce the network against voltage sags by increasing the line impedance during short circuit conditions. A brief overview of network reconfiguration to reinforce against voltage sag propagation is presented below. For this, consider a typical distribution system shown in Fig. 1. In the system, if Vsub is the substation voltage, Vmain is main source voltage and Zs is the Theveninn’s impedance of the source and Zi is the impedance of the feeder i, the substation bus voltage during a fault at bus i can be derived as:
Vsub = ( Zi /( Zi + Zs ))Vmain .
(1)
From (1) it can be seen that Vsub can be improved by finding another higher impedance route between substation and bus i. For example, if Zn > Zi , the bus i can be supplied through feeder n by closing tie switch SWn and opening sectionalizing switch Si shown in Fig. 1. This change in configuration will increase substation bus voltage magnitude. After reconfiguration, the new substation voltage Vsub1 can be written as:
Vsub1 = (Zn /( Zn + Zs))Vmain .
(2)
Although, this change in network configuration improves the voltage magnitude during the fault, sometimes it may cause unacceptable voltage drop in the lines and hence inadequate nominal voltage at various buses during steady state operation. This means that the network configuration must be done in such a way that it
Z1
S2
Z2
1
Vsub1
S1
SW1
IL
VL
1 DC Link
Ish
Vmain
SW2
2 Si
Z1
Load P1 + jQ1
Substation
Substation Main Source
S1
Zi
VSC
i
Sn
Zs
Energy Storage
Zs Zn
Fault Sn
SWn
Zn
Fault
i
n
SWn
Figure 2. DSTATCOM as a shunt compensator
n
Figure 1. Typical distribution system V sub1
does not violate the limits of system voltage profile at steady state. If one considers above condition, the network reconfiguration can only partially improve the voltage profile during voltage sags. Therefore, supplementary voltage sag mitigation techniques are needed in order to obtain the optimal solution. III.
DSTATCOM FOR VOLTAGE SAG COMPENSATION
IL
VL
Z th Is I sh
Load P L + jQ L
Figure 3. Circiut diagram of DSTATCOM connection
DSTATCOM is considered as one of the most famous type of shunt compensation custom power devices that may be used in the field of power quality improvement. These devices are connected directly at load buses in parallel as shown in Fig. 2. It injects a current Ish to correct the voltage sag at the load bus by adjusting the voltage drop across the Theveninn’s equivalent impedance (Zth ) seen from the coupling point. The value of Ish can be controlled by adjusting the output voltage of the voltage source converter (VSC). The circuit diagram of the system shown in Fig. 2 can be reduced to the circuit shown in Fig. 3 for further analysis. According to the Fig. 3, the shunt current Ish can be written as [10]:
The voltage sag correction by the DSTATCOM depends on the short circuit level of the load bus. When the angle between shunt injected current Ish and load bus voltage VL is kept 90º, the desired voltage correction can be achieved without injecting any active power into the system. On the other hand, when the value of Ish is minimized, the same voltage correction can be achieved with minimum apparent power injection into the system. In case where no real power injection from the DSTATCOM is required, the entire load active power (PL) must be provided by the system source. The active power flow through the Theveninn’s impedance Zth as shown in Fig. 3 can be written as:
Ish∠η = IL∠ −θ −Vsub1 / Zth∠(δ + β ) + VL / Zth∠ − β . (3)
PL = (Vsub1VL / Zth ) cos(β − δ ) − (VL2 / Zth ) cos β (8) From (8), the angle δ can be expressed as:
Where IL is the load current, θ is the load power factor angle, VL is the load bus voltage and Vsub1 is the system voltage during fault, η is the angle of shunt injected current Ish and β and δ are the angles of Zth and Vsub1, respectively. Load current and power factor angle can be derived from power expression respectively as:
IL = {(PL + QL) / VL}*
(4)
θ = tan −1 (Q L / P L )
(5)
Furthermore, from Fig. 3 the Theveninn’s equivalent impedance of the system can be obtained as:
Z th = Z sub 1sub1
(6)
Where Zsub1sub1 represents the diagonal element corresponding to Zsub1 in the Z-matrix of the system. Now the injected power of the shunt compensator can be expressed as [10]:
S sh = V L I sh* .
(7)
δ = β − cos−1[(VL /Vsub1 ) cos β + Zth PL /Vsub1VL ] (9) Here, the feasible value of δ is achieved if:
[(VL / Vsub1 ) cos β + Z th PL / Vsub1VL ] ≤ 1 (10) Equation (10) can be rewritten as:
Vsub1 ≥ (VL cos β + Z th PL / VL )
(11)
Thus, when the substation voltage magnitude satisfies (11), the DSTATCOM can correct the voltage sag without injecting any active power into the system. For such a case, the injected complex current and apparent power of the DSTATCOM can easily be found from (3) and (7), respectively. Note that the injected apparent power will have only the reactive component. For a given load current, the magnitude of the injected apparent power of the DSTATCOM depends on the magnitude of the injected current. When the magnitude of the injected current is minimized, the DSTATCOM can correct the voltage sag with minimum apparent power injection into the system. Thus the condition of minimum apparent power injection is:
∂I sh / ∂δ = 0 .
(12)
G R ID
An analytical expression of Ish can readily be obtained from (3), and the solution of (12) provides the other value of the voltage angle δ1 as [10]:
47 46
45
δ1 = tan [(ZthIL sin(β −θ))/(VL + ZthIL cos(β −θ ))]
(13)
28
• Run the load flow and fault analysis for the system after reconfiguration process and record the voltage magnitudes of all the system buses. • Repeat the first step to search for more effective weak area that may be obtained by reconfiguration process. • Classify the buses in already reconfigured distribution network according to voltage magnitudes. A bus is said to be completely mitigated if the bus voltage is > 0.9 p.u. If the bus voltage is between 0.7 and 0.9 p.u it is classified as a partially mitigated bus and the remaining busses are classified as hardly affected buses (Vbus< 0.7 p.u). • Represent the system by looking from each of the above classified buses as Thevinnen’s impedance in series with bus voltage as Thevinnen’s source. • Develop a simplified system model for simulation using Matlab Simulink where the load bus is considered as the common coupling point (PCC). • Use DSTATCOM with an energy storage source at PCC considering both partially mitigated and hard affected buses and record the voltage of bus with and without coupling the DSTATCOM.
31
39
SIMULATION RESULTS
A. Simulation Results of Network Reconfiguration A practical distribution system is shown in Fig. 4. The system is composed of 47 buses and 42 lines supplied by a 132KV sub transmission system through four substations which are connected to buses 43, 44, 45 and 46
6
8
17
19
12
SW
SW 13
11
14
22
23 24 25 26 27
SW
SW
21
20
18
SW
7 8 9 10
4
35
40
36
41 32
Figure 4. A practical distribution system.
respectively. The substations 43 and 44 are fed by 132/11KV, 30MVA, while the substations 45 and 46 are fed by 132/33KV, 45MVA and bus 47 is the swing bus. The six tie switches (SWs) between buses 4-41, 11-41, 410, 7-16, 19-23 and 12-23 may be used as alternatives to change the configuration of the system under contingencies. By applying the first step of the proposed methodology, bus 8 is considered as the most sensitive bus in the propagating the voltage sag, where most number of the system buses are affected due to the fault at this bus. Therefore bus 8 is considered as the weak bus of the system. The appointment of the weak area is the significant step in the network reconfiguration. The curve with triangles in Fig. 6 shows the voltage sag distribution of the system buses before reconfiguration. Note that 68% of the whole system is affected by this fault. Based on these results the configuration of the system must be changed in order to place the weak area as far away as possible from the main substation to reduce the fault current and sag propagation. The reconfiguration process has been done and it does not violate the limits of system voltage profile at steady state. Fig. 5 shows the system after reconfiguration, where the process can be summarized as a change in the power supply routes to weak area representing the buses 4, 8,9,10 and 11. This area is supplied by substation 43 through feeder 1 before reconfiguration. To change the previous configuration, weak area buses can be supplied through the substation 41 by closing the tie switches (SWs) 11-41, 4-41 and 4-10 and opening the sectionalizing switches (Ss) 7-8, 3-4, 44GRID 47 44
43
46
45
7
28
5
3
34
34
38
40
33
5 6
4
6
8 Fault
10
9 11 41
8
17
12
21
20 18
SW
7
3 4
35
2
15 16
2
42 39
31
1
1
37
29
30
V.
5
4
16
Fau lt
6
42
2
15
3
38
30
1
1 2
37 33
IV. PROPOSED VOLTAGE SAG MITIGATION METHOD The proposed method can be divided to two stages; the first stage is concerned with network reconfiguration, while the second stage involves the mitigation of voltage sag problem by installing DSTATCOM devices. The proposed method can be summarized by the following steps. • Run short circuit analysis for all system buses in order to appoint which buses causes high level of voltage sag propagation in the distribution system and identify the neighborhood of these buses as weak areas. • Run the load flow and short circuit analysis according to the appointed weak area for the system and record the voltage magnitude of all system buses. • Apply network reconfiguration process using the available tie switches in order to place the weak area as far away as possible from the main substation.
3
34
29
Thus for a given load, the value of δ1 can be easily found from (13). Once the value of δ1 is known, the complex current and apparent power injection of the DSTATCOM can again be obtained from (3) and (7), respectively.
5
SW
−1
44
43 7
19
SW
23 24
SW
13
14
32
Figure 5. A practical distribution system after reconfiguration.
25 26 27 36
Voltage Magnitude. %
120
TABLE II. CHANGES SHORT CIRCUIT ANALYSIS AND TOPOLOGY AFTER NETWORK RECONFIGURATION
100 80
System Status
60
Open Switches
40 Before Rec. After Rec.
20 0 0
10
20 30 No. of Buses
40
Ss 50
Figure 6. System buses voltages magnitudes during voltage sag due to faulted bus No. 8 Before and after reconfiguration.
Before Reconfiguration
7, 43-20 and 10-11 respectively. By this way the faulted bus (bus 8) is placed further away from the main power source as shown in Fig. 5. The short circuit analysis simulation results show that there is no weak area after network reconfiguration can affect the system more than 50%.The curve with circles in Fig. 6 shows the voltage magnitudes of the system buses obtained by the fault analysis of the reconfigured system for the same fault at bus 8. It is obvious from the results that the reconfiguration process affects the voltage profile of a large number of the system buses and provides complete mitigation for the most number of system buses while leaving few busses partially mitigated. Buses in the weak areas are still under severe voltage sag. The results of reconfiguration can be analyzed through the Tables I to IV. Table I changes in steady state condition before and after reconfiguration. Note that this change causes total system losses to increase from 2003.9 kW to 2275 kW. Although this is the disadvantages of reconfiguration, the sag affected area during short circuit analysis is reduced from 68.1% to 31.9% as shown in Table II. Table III shows the classification of buses according to the bus voltage magnitudes. According to the voltage magnitudes shown in Table III the voltage sag problem is completely solved for 32 buses while 15 remaining buses still suffers voltage sag after reconfiguration. From these 15 buses nine of them can be classified as partially mitigated buses and the other five buses experience severe voltage sag where Vbus < 0.6 pu. The voltage magnitudes of system buses which observe a little and hard voltage sag are listed in the Table IV. From the tables it is obvious that the voltage sag problem is solved completely for the most number of buses after reconfiguration. However, the buses listed in Table IV
After Reconfiguration
TABLE I. CHANGES IN STEADY STATE CONDITION AFTER RECONFIGURATION System Status
Steady state system losses (kW)
Before Reconfiguration
2003.9
After Reconfiguration
2203.3
Percentage of voltage magnitude at substation 43 101.47 (pre-sag) 101.97 (pre-sag)
62.267 (during sag) 101.198 (during sag)
Non
3-4, 7-8 20-43 44-7 10-11
SWs 4-41, 1141, 13-23, 19-23 1013, 13-23
13-23, 19-23 and 13-23
System Buses during fault No. of Sag buses Affected under Area sag 32 all < 0.65 p.u
68.08 %
15, 8 of them > 0.80 p.u
31.9 %
may need other technique to overcome their voltage sag problem.
TABLE III. CLASSIFICATION OF SYSTEM BUSES ACCORDING TO VOLTAGE MAGNITUDE System Status
Before Rec.
After Rec.
Voltage magnitude of system buses during a fault at bus 8 Vbus. > 0.6< Vbus < 0.7< Vbus < 0.6 0.9 0.7 Vbus.< 0.9 28, 29,30,3 1, 32, 33, 37, 1,2,3, 4,5,6,7,8,9, 38,39, 15,16,17,18, 10,11,12,13, 40, 41, 19,20,23,24, 14 21,22 and 42,45, 25,26, 27,34, Non 35 46, 36,43 and 44 Total=14 and 47 Total=18 buses Total= buses 15 buses
4,8,9,10, 11 and 41 Total=6 buses
Non
30,31,32,3 3,37, 38,39,40 and 42 Total=9 buses
1,2,3,5, 6,7,12 to 29, 34,35, 36,43, 44,45 ,46 and 47 Total= 32 buses
TABLE IV.
1.4 BUS-40 1.2
SYSTEM BUSES NEED OTHER TECHNIQUE FOR VOLTAGE SAG MITIGATION
33 37 38 39
Voltage magnitude (p.u) 0.827 0.899 0.869 0.865
11
0.525
40
0.829
30
0.892
41
0.527
31
0.82
42
0.855
32
0.773
Bus No.
Bus No.
1 Voltage (pu)
4 8 9 10
Voltage magnitude (p.u) 0.385 Fault 0.051 0.084
0.8 0.6
Compensation 0.97 pu.
0.2 0 0
0.1
0.2 0.3 Time(s)
0.4
0.5
Figure 8. Voltage sag and compensation magnitudes at bus No. 40.
1.4
BUS-4
1.2 1 Voltage (pu)
B. Simulation Results Of DSTATCOM Compensation The voltage sag problem in buses identified in Table IV can be tackled by using DSTATCOM devices. These devices can inject the required current into the system to compensate the partially mitigated bus voltage magnitude. A Matlab Simulink model of the DSTATCOM shown in Fig. 7 is used to simulate the impact of these devices to further mitigate the voltage sag. In the controller of the DSTACOM a similar control algorithm developed in [10] is used. The voltage profile before and after installing the DSATCOM at bus no. 40 is shown in Fig. 8. The propagation of voltage sag during a fault at bus 8 affects bus 40’s voltage magnitude and decreases to 82.9% of its nominal value. However, after compensation its voltage magnitude rises up to 97% for same fault. This shows that DSATCOM can effectively mitigate the voltage sag for partially affected buses. Fig. 9 is plotted to illustrate the performance of DSATCOM when installed at a hardly affected buses such as bus 4. Notice that this voltage sag is also mitigated completely. In the same manner the simulation were carried out for all buses which are identified to install DSATCOMs. The results of the simulation are listed in Table V. The results show that all the buses are completely mitigated after applying DSTATCOM as a shunt compensation device.
Sag 0.82 pu.
0.4
0.8 0.6 0.4 Sag 0.38 pu. Compensation 0.95 pu.
0.2 0 0
0.1
0.2
0.3
0.4
0.5
Time(s)
Figure 9. Voltage sag and compensation magnitudes at bus No. 4. TABLE V. THE ALREADY AFFECTED SYSTEM B USES WITH DSTATCOM FOR VOLTAGE SAG MITIGATION
4 8 9 10
Voltage Magnitude (p.u) With WithDout STAT 0.385 0.95 Fault Fault 0.90 0.051 0.084 0.90
11
0.525
30
0.892
Bus No.
33 37 38 39
Voltage Magnitude (p.u) With WithDout STAT 0.827 0.95 0.899 0.96 0.965 0.869 0.865 0.983
0.96
40
0.829
0.98
0.98
41
0.527
0.96
42
0.855
0.95
31
0.82
0.98
32
0.773
0.961
Bus No.
The summery of the simulation results are also shown in Fig. 10. In Fig. 10, the plot with solid line represents the result of combining network reconfiguration and DSTATCOM. The capability of combining the network reconfiguration and the application of DSATCOMS can be clearly seen from this figure.
Figure 7. Simulink model of DSTATCOM
120
REFERENCES
Voltage Magnitude %
100
80
[1]
60
Before Rec. After Rec. Rec. & STAT
40
20
0 0
5
10
15
20
25
30
35
40
45
50
No. of Buses
Figure 10. System buses voltages magnitudes during voltage sag before reconfiguration, after reconfiguration and DSTATCOM compensation.
VI. CONCLUSIONS The simulation results show that the application of the proposed method is efficient and feasible for improving the bus voltage profile during voltage sag. The first stage of solution is achieved by placing the voltage sag sources as far as possible away from the main power supply. Although the losses are significantly higher after reconfiguration (~10%), the system become more stable especially in case of high frequency of voltage sag. Although the first stage involves just a change is switching status, it solves majority of the problem and reduces the required number of DSTATCOM devices for complete mitigation. By the installation of DSTATCOM devices directly to the partially mitigated buses as well as other hardly affected buses the problem of voltage sag can be completely solved for the whole system.
IEEE Standard, 1159-1995, Recommended Practice For Monitoring Electric Power Quality, IEEE press, New York, 1995. [2] E. Nasr , S. H. Hosseinian, P. Hasanpor, “Optimal placement of multiple STATCOM for voltage stability margin enhancement using particle swarm optimization,” Springer-Verlag. Electr. Eng, Vol. 90, pp.503–510, July 2008. [3] H. Nasiraghdam, A. Jalilian, “Balanced and Unbalanced Voltage Sag Mitigation using DSTATCOM with Linear and Nonlinear Loads,” Int. J. of Elec. Com. Sys. Eng. V. 1 No. 2, pp 86-91 Spring 2007. [4] Huweg A. F . S. M. Bashi, “Application of inverter based shunt device for voltage sag mitigation due to starting of an induction motor load,” C I R E D 18th International Conf. on Electricity Distribution Turin, 6-9th June 2005 [5] M.H. Haque, “Compensation of distribution system voltage sag by DVR and DSTATCOM,” PPT 2001, IEEE Porto Power Technique Conf., Porto, Portugal. 10-13th September 2001. [6] Sensarma, P.S. Padiyar, K.R. Ramanarayanan, V, “Analysis and performance evaluation of a distribution STATCOM for compensating voltage fluctuations,” IEEE Trans. On Power delivery V. 16.2. pp 259-264, April 2001. [7] Sang Y. Y,Jang H. O, Mitigation of voltage sag using feeder transfer in power distribution system, Power engineering society summer meet., IEEE vol. 3 PP 1421 - 1426. Summer 2000 [8] Chen S.L, Hsu S.C, Mitigation of voltage sags by network reconfiguration of a utility power system. IEEE power engineering society transmission and distribution conf., Vol. 3, PP 2067 - 2072 Asia Pacific, 6-10 Oct. 2002 [9] N. Salman, A. Mohammed, H. Shareef , Reinforcement of power distribution network against voltage sags using graph theory, IEEE Student Conf. Research and Development (SCOReD 2009), UPM Serdang, Malaysia 16-18 Nov. 2009, [10] S.V Ravi Kumar , S. Siva Nagaraju, “Simulation of DSTATCOM and DVR in power systems,” ARPN J. of Eng. Applied Sciences, Vol. 2, No. 3, pp 7-13, June 2007.
