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SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system
Ain Shams Engineering Journal xxx (xxxx) xxx
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Electrical Engineering
SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution systemq Essam A. Al-Ammar a, Azhar Ul-Haq b, Ahsan Iqbal b,⇑, Marium Jalal c, Almas Anjum b a
Department of Electrical Engineering, King Saud University, Riyadh, Saudi Arabia Department of Electrical Engineering, NUST College of E&ME, National University of Sciences & Technology, Rawalpindi, Pakistan c Fatima Jinnah Women University, Rawalpindi; and Lahore College for Women University, Lahore, Pakistan b
a r t i c l e
i n f o
Article history: Received 10 August 2019 Revised 8 September 2019 Accepted 21 September 2019 Available online xxxx Keywords: Dynamic voltage restorer (DVR) Synchronous reference frame (SRF) Fuzzy logic Distribution system
a b s t r a c t Dynamic voltage restorer (DVR) is a power electronics based custom power device (CPD) which is used in distribution system to mitigate voltage sag problem, although the performance of DVR depends mainly on its control technique. In this paper, a synchronous reference frame (SRF) theory based versatile control technique is presented for DVR to mitigate voltage sag problem for sensitive loads in distribution system. The DVR is connected with the load bus in series through injection transformer and filter combination. To improve sag detection in DVR a fuzzy logic based automatic switch is introduced. Proposed control technique has capability to settle balanced as well as imbalanced sag. An efficient strategy to regulate DC link capacitor voltage of DVR is also presented in this paper. Performance of DVR with the proposed control technique is evaluated using Matlab/Simulink. Simulation results show the effectiveness of proposed control technique for DVR to settle voltage sag. Ó 2019 THE AUTHORS. Published by Elsevier BV on behalf of Faculty of Engineering, Ain Shams University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).
1. Introduction The quality of electrical power is as important as the availability of power supply. In terms of load power quality (PQ) is a measure of the clean and regularized power supply. Nowadays, electrical PQ becomes main consideration in electrical distribution system, this is due to many factors like presence of sensitive and non-linear loads in the distribution system, penetration of distributed generation (DG) in the distribution system, and technological
Abbreviations: DVR, dynamic voltage restorer; CPD, custom power device; SRF, synchronous reference frame; PQ, power quality; DG, distributed generation; FACTS, flexible alternating current transmission system; FIS, fuzzy inference system; IRP, instantaneous reactive power; ABC, adaline-based control; LPF, low pass filter; RMS, root mean square. ⇑ Corresponding author. E-mail addresses: [email protected] (E.A. Al-Ammar), syed_ahsan50@hotmail. com (A. Iqbal) q This is an open access article under the CC BY license (http://creativecommons. org/licenses/by/4.0/). Peer review under responsibility of Ain Shams University.
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advancement in power electronics equipment [1]. Power quality problems associated with voltage stability in the distribution system are of main concern. Voltage sag or voltage dip which is defined as a sudden reduction in the amplitude of supply voltage is known as one of the most common PQ problems which damages sensitive loads in distribution system. There are many disadvantages which are studied in recent years due to voltage sag like malfunctioning of electrical equipment, lose in the production line, complete failure of equipment [2–6]. Research has been carried out to get knowledge about the mode of using flexible alternating current transmission system (FACTS) devices as custom power devices (CPD) to maintain stability at the consumer load side. FACTS devices like DVR, STATCOM, D-STATCOM, UPFC are used extensively to protect sensitive loads in the distribution system, among them, DVR is an excellent FACTS device that can be used to compensate voltage related PQ problems for sensitive loads [7–8]. DVR is a power electronics based FACTS device, it mainly consists of a voltage source converter, a DC link capacitor, and a control unit. DVR is connected in series with power distribution line or load bus to protect sensitive loads from voltage sag. The performance of DVR depends on its control strategy and its circuit design, different controls have been developed for efficient working of DVR and presented in [9–28]. The d-q transformation method based HTS-SMES magnet current control strategy for DVR to deal with voltage transient in
https://doi.org/10.1016/j.asej.2019.09.001 2090-4479/Ó 2019 THE AUTHORS. Published by Elsevier BV on behalf of Faculty of Engineering, Ain Shams University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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Nomenclature VL Vsc Zl Vt Cf Rf LC VSCDVR
Load Voltages Supply Voltage Line Impedance Terminal Voltage Capacitor Filter Resistor Filter Coupling Inductor Voltage Source Converter of DVR
[9]. Combination of a repetitive and closed-loop control structure for two operational level DVR to tackle voltage related PQ disturbances in [10]. Using of low voltage ride through (LVRT) ability and ramp function voltage recompense technique to minimize DVR power rating and enhance voltage quality in [11]. A dynamic voltage conditioner (DVC) is designed with a quadrature voltage injection control methodology to mitigate long duration voltage stability problems in the distribution system [12]. The improved d-q theory is implemented for DVR to resolve voltage swell problem in [13], detection of the reference signal is important in this method. A new numerical method for fast detection of voltage sag using DVR in [14], the proposed method is superior in comparison with fourier transformation and PLL procedure. A novel philosophy which is a combination of analytical geometry techniques and elliptical restoration based unique mark method for working of DVR in [15], the performance of passive components associated with DVR are also discussed. Multi-function DVR is presented in [16] to overcome voltage quality problems and to limit fault current, low and medium voltage scenarios are tackled with proposed strategy. The effective scheme depends on d-q method is presented for voltage sag compensation in [17], presented methodology is applied on low voltage distribution system of different lengths. A new JAYA optimization scheme for online tuning of controllers in control theory of compensation device in [18], voltage quality is enhanced using this scheme and controller parameters are tuned automatically based on system performance. A control method for tracking of reference load voltages is developed for accurate and sharp compensation of voltage sag using the compensation device in [19]. Feed forward and feed backward based control is developed with conventional PI controllers to regulate voltage quality using DVR in [20]. PSO technique is implemented with numerical intelligence theory for optimization of controller parameters in DVR [21]. The new design of SRF control theory with multiple new modifications for DVR in [22], voltage quality problems for different categories of loads are compensated. Cost effective control method for strong voltage sag reduction using DVR in [23]. Fuzzy logic and PWM based control system is developed for detection and mitigation of voltage sag/swell problem which occurred due to line-ground faults in [24]. Artificial neural network (ANN) and fuzzy logic based control is presented for reliable working of DVR during voltage sag in [25]. A strong energy storage system is designed for fuzzy logic control based DVR in [26], the voltage quality of load side is secured with presented DVR. SRF control based DVR presented to regulate load side voltages in [27], fuzzy logic control based DVR is used to mitigate voltage quality in solar PV and wind turbine connected distribution system in [28]. While designing the FACTS devices one like DVR, it is very important to built good coordination between the control of DC link capacitor voltage and the main control. The main control of DVR should respond accordingly for any changes in DC link capacitor voltage for timely compensation of voltage sag. Moreover, mostly literature focused on only one type of sag mitigation which is balanced sag
Vtabc Vtd Vrefd Vtq Vrefq Zerocomp Zerocomref
measured terminal voltages measured terminal voltages in d axis reference voltages in d axis measured terminal voltages in q axis reference voltages in q axis Zero Component Reference Zero Component
[9–16,19,23,25–28] their control is completely unable to resolve imbalanced sag problem as there are both types of loads present in practical distribution system named balanced and unbalanced. In this paper, an SRF based versatile control technique is presented for DVR to mitigate balanced as well as imbalanced voltage sag. Fuzzy inference system(FIS) based automatic switch is designed to enhance sag . . .. . . 2. DVR circuit design In the scope of this paper, DVR is designed to secure sensitive loads from the severity of the voltage sag problem. DVR is usually connected with the load bus in series to inject require amount of voltages to resolve voltage sag. DVR circuit design with proposed filter combination is presented in this section. First considered three phase supply to sensitive loads without having DVR device in Fig. 1. Vsca, Vscb, and Vscc are three phase supply voltages representing phase a, b, and c respectively. Zla, Zlb, and Zlc representing line impedance for phase a, b, and c respectively. Vta, Vtb, and Vtc are the terminal supply voltages for three phases and VLa, VLb, and VLc are the load voltages. Sensitive loads either balanced or unbalanced are connected with the load bus, in this situation when DVR is absent then load voltages are equal to terminal supply voltages as represent in (1). Detection capability of DVR. DC link capacitor voltage control circuit is designed and linked with main control to efficiently control DC link voltage of DVR. FIS based automatic switch continuously check for voltage sag at the terminal supply voltages point, it acts as online detector switch. It carries information regarding nominal load voltages value and the threshold for voltage sag value. Once it detects sag in load bus at terminal supply voltage point it quickly activates main control of DVR for compensation of sag. The main control of DVR is designed in a versatile manner to resolve dip and unbalance components in supply voltages by using its d, q, and zero components. The challenge of controlling DC link voltages of DVR is fulfilled by using the output of DVR as feedback in the main control. Proposed SRF based control is less complex than the control which is presented in [22], and [23,24], these controls add complexities in an overall controlling effort which slow down the system performance. The contribution of present research is overall design of self-detection based coordi-
Fig. 1. Diagram of three phase supply circuit without DVR.
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
E.A. Al-Ammar et al. / Ain Shams Engineering Journal xxx (xxxx) xxx
Fig. 2. Diagram of three phase supply circuit secured with DVR.
nated control of SRF and fuzzy logic system for DVR which can eliminate power quality problem such as balanced and unbalanced voltage sag in distribution line for sensitive loads, fuzzy logic provide sensing capability to SRF control to eliminate voltage sag whereas, DC link capacitor voltage control technique of DVR used in this paper is distinctive and it is effective enough to regulate DC link capacitor voltage of DVR during sag time. The paper is organized as. DVR circuit design in the distribution system is presented in Section 2. SRF based control technique is presented in Section 3. Simulation results with balanced and imbalanced voltage sag cases are discussed in Section 4. Section 5 highlight the conclusions of this paper.
V La
¼
V ta
V Lb
¼
V tb
V Lc
¼
V tc
9 > = > ;
3
ð1Þ
If terminal supply voltages suffer from sag due to faults and transients then the same voltages travel to load end and destroy sensitive loads. Fig. 2 shows another scenario where sensitive loads are secured from sags using DVR device, in this condition if sag occurs at terminal supply voltages point then it will mitigate using DVR and sensitive loads will be secured as shown in (2), DVR injected voltages add in series with terminal supply voltages. In this state, DVR should be able to inject the accurate difference of voltages which is present between terminal supply voltages and nominal load voltages.
V La
¼
V Lb
¼
V Lc
¼
9 V ta þ V DVRa = > V tb þ V DVRb > ; V tc þ V DVRc
ð2Þ
Fig. 3 shows complete circuit design of DVR device with three phase supply system. DVR with associated components like voltage source converter, DC link capacitor, and passive filters combination connected in series with load bus through series injection trans-
Fig. 3. DVR circuit design.
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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former IT. Combination of Cf and Rf which are present at the secondary side of the series injection transformer act as a filter to filter out unwanted noise signals from injected voltages. LC act as coupling inductor and CDC is the DC link capacitor. VSCDVR is a voltage source converter of the proposed DVR. The power rating of the series injection transformer and voltage source converter depends on the value of injected voltages and line current. Whereas, the numerical value of coupling inductor mainly depends on switching frequency of VSCDVR, modulation index, and allowable ripple in VSC current. Size of noise filter depends on the size of coupling inductor and switching frequency of VSCDVR [7]. In the ideal condition when terminal supply voltages are equal to nominal load voltages then injection transformer IT serve as isolation between VSCDVR and the load bus, in the case when there is sag present in terminal supply voltages then injection transformer IT act as series voltage source between the terminal supply voltage and load voltages. Besides the importance of control theory for DVR performance, the role of the DC link capacitor is also significant. Accurate size of DC link capacitor plays a key part in the operation of voltage sag mitigation. Fig. 4 shows a single phase equivalent circuit diagram of the DVR with power supply sensitive load system, in which all the parameters are same as of Fig. 3 including injection transformer IT, Cf and Rf, coupling inductor LC, voltage source converter VSCDVR, DC link capacitor CDC. 3. Versatile control technique for DVR As it is already mentioned that the performance of DVR largely depends on the design and effectiveness of its control technique. Two factors are most important for any PQ compensation device. 1) Detection of PQ problem when it violates PQ standards. 2) Once a PQ problem is detected then PQ device should compensate that
problem in a minimum period of time. In this paper, as DVR is used to compensate voltage sag problem for sensitive loads, for this purpose a proficient control technique is developed for DVR to resolve voltage sag in a minimum period of time. Fig. 5 shows proposed overall control strategy for DVR, it based on two part control system, a FIS based automatic switch control, and an SRF based main control. FIS based automatic switch is designed to wisely sense voltage sag problem in power distribution load bus, and SRF based main control is designed to effectually mitigate voltage sag in a minimum amount of time. Refer to Fig. 5 first the FIS based control detects voltage sag at the terminal voltage supply point, if there is sag present in line then it provides switching signals to main control of DVR for mitigation of sag. 3.1. Fuzzy based automatic switch Performance of FIS control philosophy is superior to ordinary logic operators when swift judgment capabilities are needed. FIS control is composed of two linked and interdependent parameters named rule define parameter and data define parameter. Both parameters play a key role in controlling capabilities of FIS [29– 31]. Using FIS control, an automatic switch is developed and presented in this section to command main control according to voltage sag sensing at a terminal voltage supply point. The output of FIS always follows numerical rules which are defined in rule define parameter section of FIS control. Fig. 6 shows surface viewer of FIS based control in two dimensions. Fig. 7(a), (b) shows surface viewer in three dimensions. Both are according to defined rules, basically, FIS control is designed here on root mean square (RMS) equivalent of nominal load side voltages. The x-axis shows input data to FIS control and Y-axis shows output data from FIS control, x-axis shows the RMS values of ter-
Fig. 4. DVR circuit design single phase equivalent.
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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Fig. 5. Overall control strategy for DVR.
tently it is important to link control of DC link capacitor voltage to main control of DVR. For this purpose, an efficient sub-control is developed in which first measured value of DC link voltage (VDCm) is passing from LPF and compared with a reference value of DC link capacitor voltage (VDCref), and after applying PI controller the result (Vcv) is add in main control of d-axis, it is shown in (4).
V cv ¼ PIðV DCref LPFðV DCm ÞÞ
ð4Þ
(Vd00 )
Fig. 6. FIS surface viewer (two dimensions).
minal voltages which are defined in data defined parameter of FIS control, this is the important part of FIS control as the voltage sag threshold decision is taken on these values of terminal voltages. On the other side, y-axis shows the threshold values for output signal of FIS control which are also defined in data defined parameter. As shown in figures that the output of FIS control is distinguish using signal 1. Output signal (>1) work as ON switching signal for main control and output signal (<1) work as OFF switching signal for main control. When ON signal is delivered to main control then main control activates and compensate sag in line. Following are the defined fuzzy rules in rule define parameter of FIS.
after applying PI controller becomes (Vdcomp). On the other side (Vtq) is first compared with a reference voltage in q-axis (Vrefq). The result (V0q ) is compared with (sqrt((Vrefd)2+(Vd00 )2)), and after applying the PI controller the signal (Vqcomp) is achieved. The effect of (sqrt((Vrefd)2+(Vd00 )2)) helps to force DVR injected voltages to becomes an accurate difference between terminal supply voltages and load voltages. (5), and (6) shows the relation for Vdcomp and Vqcomp.
V dcomp ¼ PIðV cv þ ðV refd LPFðV td ÞÞÞ 2
V qcomp ¼ PIððsqrtððV refd Þ2 þ ðV d 00 Þ ÞÞ ðV refq V tq ÞÞ
ð5Þ ð6Þ
In case if there is imbalanced sag occur then (Zerocomp) along with (Vdcomp) and (Vqcom) force load voltages to become sag free and balanced. To get (Zerocomp), first the zero component of load voltages (Zerocom) is compared with reference of zero component (Zerocomref), and after applying PI controller the result (Zerocomp) is achieved, the relation for (Zerocomp) is given in (7).
1. If (SagI is noSag) then (SagO is off) 2. If (SagI is Sag) then (SagO is on) 3.2. SRF based control Different control theories are implemented for DVR in conducted literature survey including instantaneous reactive power (IRP) theory, synchronous reference frame (SRF) theory or some time called d-q theory, adaline-based control (ABC) algorithm. Control theories like IRP and SRF are used widely in DVR and other FACTS devices, both have advantages and disadvantages. SRF theory is considered more precise, less complex, and more flexible when compared to IRP theory [32–34]. Therefore an SRF theory based versatile control technique is developed for DVR and presented in this section to efficiently mitigate balanced and imbalanced sag to protect sensitive loads in power distribution line. Fig. 8 shows SRF based versatile control technique which is designed here for efficient working of DVR to resolve voltage sag. In this control first terminal voltage measured value (Vtabc) is converted from abc reference frame to direct-quadrature axis rotating reference frame (d-q axis) using park transformation, (3) represents the mathematical relation for park transformation.
32 3 2 3 cosðhÞ cos h 23p v td v ta cos h þ 23p 6 7 6 7 6 7 4 v tq 5 ¼ 4 sinðhÞ sin h 23p sin h þ 23p 54 v tb 5 2
v0
1 2
1 2
1 2
v tc
ð3Þ
After applying low pass filter (LPF) to (Vtd) to filter out higher order components, the (Vtdfil) is compared with reference voltages in d axis named (Vrefd). To control DC link voltages of DVR compe-
Fig. 7. FIS surface viewer (three dimension).
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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Fig. 8. SRF based versatile control technique.
Zerocomp ¼ PIðZerocomref ZerocomL Þ
ð7Þ
To convert d-q-0 rotating reference frame to abc reference frame inverse-park transformation is used, (8) represents mathematical relation for inverse park transformation. Necessary ɵref is provided for park and inverse park transformation. The mathematical operation in d, q, and zero axis is represented with help of (6), (7), and (8) respectively. After apply inverse park transformation the reference signal in abc reference frame (Vabc*) is acheived, SPWM is applied to achieved switching signals for VSCDVR.
2
3
3 32 cosðhÞ sinðhÞ 1 V dcomp 6 7 6 7 76 4 V b 5 ¼ 4 cos h 23p sin h 23p 1 54 V qcomp 5 2p 2p cos h þ 3 sin h þ 3 1 Zerocomp Vc V a
2
4.1. Performance of DVR under balanced sag
ð8Þ
4. Simulation results To evaluate the performance of the proposed control technique for DVR whole system is simulated in Matlab/Simulink environment, the system is designed with parameters given in Table 1.
Table 1 Simulation system parameters. System parameters
Value
Per phase voltages
220 V (1 p.u) 50 Hz R = 0.3 O L = 2.25 mH 2.15 + j1.3 p.u 2.1 + j1.1 p.u 3 + j1.8 p.u 1.15 + j2.2 p.u 1:1 0.65 O 85 lF 0.72 mH 7590 lF
Line frequency Line impedance Balanced load Unbalanced load
Turn Ratio IT Rf Cf Lc DC link capacitor (CDC)
The objective of DVR is to maintain sag free and balanced nominal load voltages at the sensitive load terminal. The sensitive loads which need to be secured are either balanced or unbalanced. Simulations are carried out for two different scenarios named balanced and imbalanced sag condition with balanced and unbalanced loads respectively, these are represented by Figs. 9–12. Simulation results have proved the efficacy of the proposed control technique, unbalanced and imbalanced voltage sag conditions are compensated well in less possible time.
This is the first case when the performance of DVR with proposed control is investigated under balanced sag condition. Nominal supply voltages are 1 p.u, the load impedance is 2.15 + j1.3 p.u, and system frequency is 50 Hz. In this condition, DVR is required to maintain 1 p.u voltages across sensitive load terminal. Fig. 9(a) shows terminal supply point voltages under balanced sag condition without DVR compensation, from 0 s to 0.05 s grid is working under normal operation and maintain 1 p.u voltages. At 0.05 s a balanced sag of 0.4 p.u occur and terminal supply voltages reduced to 0.6 p.u. Fig. 9(b) shows the RMS value of terminal supply voltages under balanced sag condition in p.u. Fig. 9(c) shows balanced sag compensation using DVR, at 0.05 s sag is detected using DVR control and compensate within 5 ms time frame. Fig. 9(d) shows balanced sag compensation results in RMS value. Fig. 9(e) shows the DC link capacitor voltage for DVR, it is seen that at 0.05 s the DC link voltages are disturb as DVR comes in operation at 0.05 s, DC link voltages are stable again in minimum time due to SRF based control of DVR. Fig. 10 shows the results of load active/reactive power under balanced voltage sag condition. Fig. 10(a) Shows active power before DVR compensation, at 0.05 s when balanced sag occurs the active power is reduced from its normal operating value. Fig. 10(b) shows active power recovery at 0.05 s when DVR activate. Fig. 10(c) shows reactive power results before balanced sag compensation when sag occurs at 0.05 s the reactive power is slightly disturbed from its normal operating value. Fig. 10(d) shows the recovery of reactive power when DVR activate at 0.05 s.
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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Fig. 10. Active and reactive power during balanced voltage sag condition (a). Active power before compensation (p.u). (b) Active power after compensation (p.u). (c) Reactive power before compensation (p.u). (d) Reactive power after compensation (p.u).
Fig. 9. Balanced voltage sag condition. (a) Balanced sag at load bus voltages (p.u). (b) Sag RMS (p.u). (c) Balanced sag compensation (p.u). (d) Balanced sag compensation RMS (p.u). (e) DC link capacitor voltage of DVR.
4.2. Performance of DVR under imbalanced sag In this case, the performance of DVR with proposed control is investigated under imbalanced sag condition. Nominal supply volt-
ages are the same 1 p.u, load impedance per phase are 2.1 + j1.1 p. u, 3 + j1.8 p.u, and 1.15 + j2.2 p.u respectively. System frequency is 50 Hz. In this condition, the DVR is required to maintain 1 p.u balanced voltages across sensitive load terminal. Fig. 11(a) shows terminal supply point voltages under imbalanced sag condition without DVR compensation, from 0 s to 0.05 s grid is working under normal operation and maintain 1 p.u voltages. At 0.05 s an imbalanced sag occurs and terminal supply voltages reduced in an unbalanced way. Fig. 11(b) shows the RMS value of terminal supply voltages under imbalanced sag condition in p.u. Fig. 11(c) shows imbalanced sag compensation using DVR, at 0.05 s sag is detected using DVR control and compensate within 5 ms time frame. Fig. 11(d) shows imbalanced sag compensation results in RMS value. Fig. 11(e) shows the DC link capacitor voltage for DVR, it is seen that at 0.05 s the DC link voltages are disturb as DVR comes in operation at 0.05 s, DC link voltages are stable again in minimum time due to SRF based control of DVR.
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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Fig. 12. Active and reactive power during imbalanced voltage sag condition (a). Active power before compensation (p.u). (b) Active power after compensation (p.u). (c) Reactive power before compensation (p.u). (d) Reactive power after compensation (p.u).
Fig. 11. Imbalanced voltage sag condition (a). Imbalanced sag at load bus voltages (p.u). (b) Sag RMS (p.u). (c) Imbalanced sag compensation (p.u). (d) Imbalanced sag compensation RMS (p.u). (e) DC link capacitor voltage.
Fig. 12 shows the results of load active/reactive power under imbalanced voltage sag condition. Fig. 12(a) Shows active power before DVR compensation, at 0.05 s when imbalanced sag occurs the active power is reduced from its normal operating value.
Fig. 12(b) shows active power recovery at 0.05 s when DVR activate. Fig. 12(c) shows reactive power results before imbalanced sag compensation when sag occurs at 0.05 s the reactive power is slightly disturbed from its normal operating value. Fig. 12(d) shows the recovery of reactive power when DVR activate at 0.05 s. For the DVR, Table 2 shows a comparison of controls of different papers with the control of the presented paper in terms of ability to compensate balanced and imbalanced sag and control complexity (control computations). There are fewer papers whose control is able to compensate for both balanced and imbalanced sag. In [17], presented control is less complex however there is no sag detection method is presented, control is switched automatically to compensate sag. The controls in [18,20,21,23], and [24] are more complex and take more computational steps to compensate sag. The control presented in this paper is less complex in terms of
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
E.A. Al-Ammar et al. / Ain Shams Engineering Journal xxx (xxxx) xxx Table 2 DVR Control Comparison with Ability to Compensate Balanced & Imbalanced Sag. Paper #
Compensate balanced sag
[9] [10] [11] [12] [13] [14] [15] [16] [17]
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] This Paper
Compensate imbalanced sag
Control complexity when compensate both sag
Less complex (no sag detection technique) More complex
More complex More complex
More complex More complex
Less complex with sag detection technique
computations and it has the ability to compensate balanced and imbalanced sag. Presented control has also automatic sag detection technique, therefore, presented control for DVR is superior to the other controls in the literature survey. 5. Conclusion Voltage sag is a major power quality problem which damages sensitive loads in the distribution system, second generation FACTS devices can be used as CPD to compensate power quality problems. In this paper, a nifty control structure is designed for DVR to secure sensitive loads in distribution system from voltage sag. Presented control structure of DVR consist of two sub control units an SRF based control and fuzzy logic based control. SRF based control of DVR is responsible for effective compensation of balanced as well as imbalanced voltage sag, DC link capacitor voltage control technique of DVR is integrated into SRF based control for efficient working of DVR. On the other side, fuzzy logic based control is designed for voltage sag detection in distribution power line, it performs online switching of DVR according to presence of sag in line. Simulation results refer to Fig. 9 and Fig. 11 shows the detection and compensation capabilities of proposed DVR control to compensate balanced and imbalanced voltage sag. In the future, authors are planning to design DVR control structure based on fuzzy logic control and sliding mode control (SMC). Declaration of Competing Interest The authors declared that there is no conflict of interest. References [1] Ganesan J, Krishnaveni A. A review on basic concepts and important standards of power quality in power system. Int J Sci Eng Appl 2015;4(5). [2] de Almeida A, Moreira L, Delgado J. Power quality problems and new solutions. RE&PQJ 2003;1(1). [3] Ghahderijani Mohammad Moradi, Castilla Miguel, de Vicuna Luis Garcia, Camacho , Martinez Javier Torres. Voltage sag mitigation in a PV-based industrial microgrid during grid faults. In: Published in: industrial electronics (ISIE), 2017 IEEE 26th international symposium on. p. 19–21.
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[4] Ramasamy Agileswari K, Iyer Rengan Krishnan, Ramachandaramuthy Vigna K, Mukerjee RN. Dynamic voltage restorer for voltage sag compensation. In: IEEE PEDS. p. 1289–94. [5] Thirukkovai S, Venkatesan J, Girirajkumar DrSM. Voltage sag/swell mitigation using UPQC. Int J Sci Eng Res 2014;5(4). [6] SaiRam I, Amarnadh J, Vasishta Kumar KK. Sag/swell migration using multi converter unified power quality conditioner. Int J Adv Eng Technol 2011;1 (5):437–40. [7] Singh B, Chandra A, Haddad KA. Power quality: problems and mitigation techniques. London: Wiley; 2015. [8] Arindam Ghosh, Gerard Ledwich. Power quality enhancement using custom power devices. Kluwer Academic Publishers, ISBN 1-4020-7180-9. [9] Zheng Zixuan, Xiao Xianyong, Chen Xiao Yuan, Huang Chunjun, Xu Jiao. Performance evaluation of a MW-class SMES-based DVR system for enhancing transient voltage quality by using d–q transform control. IEEE Trans Appl Supercond 2018;28(4):1–5. 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[20] Amalorpavaraj RA, Kaliannan P, Padmanaban S, Subramaniam U, Ramachandaramurthy VK. Improved fault ride through capability in DFIG based wind turbines using dynamic voltage restorer with combined feedforward and feed-back control. IEEE Access 2017;5:20494–503. https://doi. org/10.1109/ACCESS.2017.2750738. [21] Kassarwani Neelam, Ohri Jyoti, Singh Alka. Performance analysis of dynamic voltage restorer using improved PSO technique. Int J Electron 2019;106 (2):212–36. [22] Kanjiya Parag, Singh Bhim, Chandra Ambrish, Al-Haddad Kamal. ‘‘SRF Theory Revisited” to control self-supported dynamic voltage restorer (DVR) for unbalanced and nonlinear loads. IEEE Trans Indust Appl 2013;49(5): 2330–40. [23] Rauf Abdul Mannan, Khadkikar Vinod. An enhanced voltage sag compensation scheme for dynamic voltage restorer. IEEE Trans Indust Electron 2015;62 (5):2683–92. [24] Teke A, Bayindir K, Tümay M. Fast sag/swell detection method for fuzzy logic controlled dynamic voltage restorer. 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Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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[31] Butakova Maria A, Chernov Andrey V, Shevchuk Petr S, Vereskun Vladimir D. Neural fuzzy adaptive control for mobile smart objects. Published in: 2018 international symposium on consumer technologies, 2018. [32] Singh B, Solanki J. A Comparison of control algorithm for dstatcom. IEEE Trans Ind Electron Jul. 2009;56(7):2738–45. [33] Zhong Qing, Yao Wenlin, Lin Linxue, Wang Gang, Xu Zhong. Data analysis and applications of the power quality monitoring. Published in: 2018 international conference on power system technology (POWERCON), 2018. [34] Meng Lexuan, Guerrero Josep M. Optimal power quality service in multi-bus microgrid systems. Published in: 2017 IEEE power & energy society general meeting, 16-20 July, 2017.
Dr. Essam A. Al-Ammar. Assistant Professor, Department of Electrical Engineering, College of Engineering, King Saud University Riyadh, Saudi Arabia, PhD Arizona State University Tempe, Arizona, USA, 2007, MS.c University of Alabama Tuscaloosa, Alabama, USA, 2003, BS.c King Saud University Riyadh, Saudi Arabia, 1997.
Dr. Azhar Ul Haq. Assistant Professor, Department of Electrical Engineering, College of E&ME, NUST, Rawalpindi, Pakistan.
Ahsan Iqbal MS EE Bahria University, Islamabad, Pakistan, BS EE Comsats University Wah Cantt, Pakistan.
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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Electrical Engineering
SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution systemq Essam A. Al-Ammar a, Azhar Ul-Haq b, Ahsan Iqbal b,⇑, Marium Jalal c, Almas Anjum b a
Department of Electrical Engineering, King Saud University, Riyadh, Saudi Arabia Department of Electrical Engineering, NUST College of E&ME, National University of Sciences & Technology, Rawalpindi, Pakistan c Fatima Jinnah Women University, Rawalpindi; and Lahore College for Women University, Lahore, Pakistan b
a r t i c l e
i n f o
Article history: Received 10 August 2019 Revised 8 September 2019 Accepted 21 September 2019 Available online xxxx Keywords: Dynamic voltage restorer (DVR) Synchronous reference frame (SRF) Fuzzy logic Distribution system
a b s t r a c t Dynamic voltage restorer (DVR) is a power electronics based custom power device (CPD) which is used in distribution system to mitigate voltage sag problem, although the performance of DVR depends mainly on its control technique. In this paper, a synchronous reference frame (SRF) theory based versatile control technique is presented for DVR to mitigate voltage sag problem for sensitive loads in distribution system. The DVR is connected with the load bus in series through injection transformer and filter combination. To improve sag detection in DVR a fuzzy logic based automatic switch is introduced. Proposed control technique has capability to settle balanced as well as imbalanced sag. An efficient strategy to regulate DC link capacitor voltage of DVR is also presented in this paper. Performance of DVR with the proposed control technique is evaluated using Matlab/Simulink. Simulation results show the effectiveness of proposed control technique for DVR to settle voltage sag. Ó 2019 THE AUTHORS. Published by Elsevier BV on behalf of Faculty of Engineering, Ain Shams University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).
1. Introduction The quality of electrical power is as important as the availability of power supply. In terms of load power quality (PQ) is a measure of the clean and regularized power supply. Nowadays, electrical PQ becomes main consideration in electrical distribution system, this is due to many factors like presence of sensitive and non-linear loads in the distribution system, penetration of distributed generation (DG) in the distribution system, and technological
Abbreviations: DVR, dynamic voltage restorer; CPD, custom power device; SRF, synchronous reference frame; PQ, power quality; DG, distributed generation; FACTS, flexible alternating current transmission system; FIS, fuzzy inference system; IRP, instantaneous reactive power; ABC, adaline-based control; LPF, low pass filter; RMS, root mean square. ⇑ Corresponding author. E-mail addresses: [email protected] (E.A. Al-Ammar), syed_ahsan50@hotmail. com (A. Iqbal) q This is an open access article under the CC BY license (http://creativecommons. org/licenses/by/4.0/). Peer review under responsibility of Ain Shams University.
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advancement in power electronics equipment [1]. Power quality problems associated with voltage stability in the distribution system are of main concern. Voltage sag or voltage dip which is defined as a sudden reduction in the amplitude of supply voltage is known as one of the most common PQ problems which damages sensitive loads in distribution system. There are many disadvantages which are studied in recent years due to voltage sag like malfunctioning of electrical equipment, lose in the production line, complete failure of equipment [2–6]. Research has been carried out to get knowledge about the mode of using flexible alternating current transmission system (FACTS) devices as custom power devices (CPD) to maintain stability at the consumer load side. FACTS devices like DVR, STATCOM, D-STATCOM, UPFC are used extensively to protect sensitive loads in the distribution system, among them, DVR is an excellent FACTS device that can be used to compensate voltage related PQ problems for sensitive loads [7–8]. DVR is a power electronics based FACTS device, it mainly consists of a voltage source converter, a DC link capacitor, and a control unit. DVR is connected in series with power distribution line or load bus to protect sensitive loads from voltage sag. The performance of DVR depends on its control strategy and its circuit design, different controls have been developed for efficient working of DVR and presented in [9–28]. The d-q transformation method based HTS-SMES magnet current control strategy for DVR to deal with voltage transient in
https://doi.org/10.1016/j.asej.2019.09.001 2090-4479/Ó 2019 THE AUTHORS. Published by Elsevier BV on behalf of Faculty of Engineering, Ain Shams University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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Nomenclature VL Vsc Zl Vt Cf Rf LC VSCDVR
Load Voltages Supply Voltage Line Impedance Terminal Voltage Capacitor Filter Resistor Filter Coupling Inductor Voltage Source Converter of DVR
[9]. Combination of a repetitive and closed-loop control structure for two operational level DVR to tackle voltage related PQ disturbances in [10]. Using of low voltage ride through (LVRT) ability and ramp function voltage recompense technique to minimize DVR power rating and enhance voltage quality in [11]. A dynamic voltage conditioner (DVC) is designed with a quadrature voltage injection control methodology to mitigate long duration voltage stability problems in the distribution system [12]. The improved d-q theory is implemented for DVR to resolve voltage swell problem in [13], detection of the reference signal is important in this method. A new numerical method for fast detection of voltage sag using DVR in [14], the proposed method is superior in comparison with fourier transformation and PLL procedure. A novel philosophy which is a combination of analytical geometry techniques and elliptical restoration based unique mark method for working of DVR in [15], the performance of passive components associated with DVR are also discussed. Multi-function DVR is presented in [16] to overcome voltage quality problems and to limit fault current, low and medium voltage scenarios are tackled with proposed strategy. The effective scheme depends on d-q method is presented for voltage sag compensation in [17], presented methodology is applied on low voltage distribution system of different lengths. A new JAYA optimization scheme for online tuning of controllers in control theory of compensation device in [18], voltage quality is enhanced using this scheme and controller parameters are tuned automatically based on system performance. A control method for tracking of reference load voltages is developed for accurate and sharp compensation of voltage sag using the compensation device in [19]. Feed forward and feed backward based control is developed with conventional PI controllers to regulate voltage quality using DVR in [20]. PSO technique is implemented with numerical intelligence theory for optimization of controller parameters in DVR [21]. The new design of SRF control theory with multiple new modifications for DVR in [22], voltage quality problems for different categories of loads are compensated. Cost effective control method for strong voltage sag reduction using DVR in [23]. Fuzzy logic and PWM based control system is developed for detection and mitigation of voltage sag/swell problem which occurred due to line-ground faults in [24]. Artificial neural network (ANN) and fuzzy logic based control is presented for reliable working of DVR during voltage sag in [25]. A strong energy storage system is designed for fuzzy logic control based DVR in [26], the voltage quality of load side is secured with presented DVR. SRF control based DVR presented to regulate load side voltages in [27], fuzzy logic control based DVR is used to mitigate voltage quality in solar PV and wind turbine connected distribution system in [28]. While designing the FACTS devices one like DVR, it is very important to built good coordination between the control of DC link capacitor voltage and the main control. The main control of DVR should respond accordingly for any changes in DC link capacitor voltage for timely compensation of voltage sag. Moreover, mostly literature focused on only one type of sag mitigation which is balanced sag
Vtabc Vtd Vrefd Vtq Vrefq Zerocomp Zerocomref
measured terminal voltages measured terminal voltages in d axis reference voltages in d axis measured terminal voltages in q axis reference voltages in q axis Zero Component Reference Zero Component
[9–16,19,23,25–28] their control is completely unable to resolve imbalanced sag problem as there are both types of loads present in practical distribution system named balanced and unbalanced. In this paper, an SRF based versatile control technique is presented for DVR to mitigate balanced as well as imbalanced voltage sag. Fuzzy inference system(FIS) based automatic switch is designed to enhance sag . . .. . . 2. DVR circuit design In the scope of this paper, DVR is designed to secure sensitive loads from the severity of the voltage sag problem. DVR is usually connected with the load bus in series to inject require amount of voltages to resolve voltage sag. DVR circuit design with proposed filter combination is presented in this section. First considered three phase supply to sensitive loads without having DVR device in Fig. 1. Vsca, Vscb, and Vscc are three phase supply voltages representing phase a, b, and c respectively. Zla, Zlb, and Zlc representing line impedance for phase a, b, and c respectively. Vta, Vtb, and Vtc are the terminal supply voltages for three phases and VLa, VLb, and VLc are the load voltages. Sensitive loads either balanced or unbalanced are connected with the load bus, in this situation when DVR is absent then load voltages are equal to terminal supply voltages as represent in (1). Detection capability of DVR. DC link capacitor voltage control circuit is designed and linked with main control to efficiently control DC link voltage of DVR. FIS based automatic switch continuously check for voltage sag at the terminal supply voltages point, it acts as online detector switch. It carries information regarding nominal load voltages value and the threshold for voltage sag value. Once it detects sag in load bus at terminal supply voltage point it quickly activates main control of DVR for compensation of sag. The main control of DVR is designed in a versatile manner to resolve dip and unbalance components in supply voltages by using its d, q, and zero components. The challenge of controlling DC link voltages of DVR is fulfilled by using the output of DVR as feedback in the main control. Proposed SRF based control is less complex than the control which is presented in [22], and [23,24], these controls add complexities in an overall controlling effort which slow down the system performance. The contribution of present research is overall design of self-detection based coordi-
Fig. 1. Diagram of three phase supply circuit without DVR.
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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Fig. 2. Diagram of three phase supply circuit secured with DVR.
nated control of SRF and fuzzy logic system for DVR which can eliminate power quality problem such as balanced and unbalanced voltage sag in distribution line for sensitive loads, fuzzy logic provide sensing capability to SRF control to eliminate voltage sag whereas, DC link capacitor voltage control technique of DVR used in this paper is distinctive and it is effective enough to regulate DC link capacitor voltage of DVR during sag time. The paper is organized as. DVR circuit design in the distribution system is presented in Section 2. SRF based control technique is presented in Section 3. Simulation results with balanced and imbalanced voltage sag cases are discussed in Section 4. Section 5 highlight the conclusions of this paper.
V La
¼
V ta
V Lb
¼
V tb
V Lc
¼
V tc
9 > = > ;
3
ð1Þ
If terminal supply voltages suffer from sag due to faults and transients then the same voltages travel to load end and destroy sensitive loads. Fig. 2 shows another scenario where sensitive loads are secured from sags using DVR device, in this condition if sag occurs at terminal supply voltages point then it will mitigate using DVR and sensitive loads will be secured as shown in (2), DVR injected voltages add in series with terminal supply voltages. In this state, DVR should be able to inject the accurate difference of voltages which is present between terminal supply voltages and nominal load voltages.
V La
¼
V Lb
¼
V Lc
¼
9 V ta þ V DVRa = > V tb þ V DVRb > ; V tc þ V DVRc
ð2Þ
Fig. 3 shows complete circuit design of DVR device with three phase supply system. DVR with associated components like voltage source converter, DC link capacitor, and passive filters combination connected in series with load bus through series injection trans-
Fig. 3. DVR circuit design.
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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former IT. Combination of Cf and Rf which are present at the secondary side of the series injection transformer act as a filter to filter out unwanted noise signals from injected voltages. LC act as coupling inductor and CDC is the DC link capacitor. VSCDVR is a voltage source converter of the proposed DVR. The power rating of the series injection transformer and voltage source converter depends on the value of injected voltages and line current. Whereas, the numerical value of coupling inductor mainly depends on switching frequency of VSCDVR, modulation index, and allowable ripple in VSC current. Size of noise filter depends on the size of coupling inductor and switching frequency of VSCDVR [7]. In the ideal condition when terminal supply voltages are equal to nominal load voltages then injection transformer IT serve as isolation between VSCDVR and the load bus, in the case when there is sag present in terminal supply voltages then injection transformer IT act as series voltage source between the terminal supply voltage and load voltages. Besides the importance of control theory for DVR performance, the role of the DC link capacitor is also significant. Accurate size of DC link capacitor plays a key part in the operation of voltage sag mitigation. Fig. 4 shows a single phase equivalent circuit diagram of the DVR with power supply sensitive load system, in which all the parameters are same as of Fig. 3 including injection transformer IT, Cf and Rf, coupling inductor LC, voltage source converter VSCDVR, DC link capacitor CDC. 3. Versatile control technique for DVR As it is already mentioned that the performance of DVR largely depends on the design and effectiveness of its control technique. Two factors are most important for any PQ compensation device. 1) Detection of PQ problem when it violates PQ standards. 2) Once a PQ problem is detected then PQ device should compensate that
problem in a minimum period of time. In this paper, as DVR is used to compensate voltage sag problem for sensitive loads, for this purpose a proficient control technique is developed for DVR to resolve voltage sag in a minimum period of time. Fig. 5 shows proposed overall control strategy for DVR, it based on two part control system, a FIS based automatic switch control, and an SRF based main control. FIS based automatic switch is designed to wisely sense voltage sag problem in power distribution load bus, and SRF based main control is designed to effectually mitigate voltage sag in a minimum amount of time. Refer to Fig. 5 first the FIS based control detects voltage sag at the terminal voltage supply point, if there is sag present in line then it provides switching signals to main control of DVR for mitigation of sag. 3.1. Fuzzy based automatic switch Performance of FIS control philosophy is superior to ordinary logic operators when swift judgment capabilities are needed. FIS control is composed of two linked and interdependent parameters named rule define parameter and data define parameter. Both parameters play a key role in controlling capabilities of FIS [29– 31]. Using FIS control, an automatic switch is developed and presented in this section to command main control according to voltage sag sensing at a terminal voltage supply point. The output of FIS always follows numerical rules which are defined in rule define parameter section of FIS control. Fig. 6 shows surface viewer of FIS based control in two dimensions. Fig. 7(a), (b) shows surface viewer in three dimensions. Both are according to defined rules, basically, FIS control is designed here on root mean square (RMS) equivalent of nominal load side voltages. The x-axis shows input data to FIS control and Y-axis shows output data from FIS control, x-axis shows the RMS values of ter-
Fig. 4. DVR circuit design single phase equivalent.
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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Fig. 5. Overall control strategy for DVR.
tently it is important to link control of DC link capacitor voltage to main control of DVR. For this purpose, an efficient sub-control is developed in which first measured value of DC link voltage (VDCm) is passing from LPF and compared with a reference value of DC link capacitor voltage (VDCref), and after applying PI controller the result (Vcv) is add in main control of d-axis, it is shown in (4).
V cv ¼ PIðV DCref LPFðV DCm ÞÞ
ð4Þ
(Vd00 )
Fig. 6. FIS surface viewer (two dimensions).
minal voltages which are defined in data defined parameter of FIS control, this is the important part of FIS control as the voltage sag threshold decision is taken on these values of terminal voltages. On the other side, y-axis shows the threshold values for output signal of FIS control which are also defined in data defined parameter. As shown in figures that the output of FIS control is distinguish using signal 1. Output signal (>1) work as ON switching signal for main control and output signal (<1) work as OFF switching signal for main control. When ON signal is delivered to main control then main control activates and compensate sag in line. Following are the defined fuzzy rules in rule define parameter of FIS.
after applying PI controller becomes (Vdcomp). On the other side (Vtq) is first compared with a reference voltage in q-axis (Vrefq). The result (V0q ) is compared with (sqrt((Vrefd)2+(Vd00 )2)), and after applying the PI controller the signal (Vqcomp) is achieved. The effect of (sqrt((Vrefd)2+(Vd00 )2)) helps to force DVR injected voltages to becomes an accurate difference between terminal supply voltages and load voltages. (5), and (6) shows the relation for Vdcomp and Vqcomp.
V dcomp ¼ PIðV cv þ ðV refd LPFðV td ÞÞÞ 2
V qcomp ¼ PIððsqrtððV refd Þ2 þ ðV d 00 Þ ÞÞ ðV refq V tq ÞÞ
ð5Þ ð6Þ
In case if there is imbalanced sag occur then (Zerocomp) along with (Vdcomp) and (Vqcom) force load voltages to become sag free and balanced. To get (Zerocomp), first the zero component of load voltages (Zerocom) is compared with reference of zero component (Zerocomref), and after applying PI controller the result (Zerocomp) is achieved, the relation for (Zerocomp) is given in (7).
1. If (SagI is noSag) then (SagO is off) 2. If (SagI is Sag) then (SagO is on) 3.2. SRF based control Different control theories are implemented for DVR in conducted literature survey including instantaneous reactive power (IRP) theory, synchronous reference frame (SRF) theory or some time called d-q theory, adaline-based control (ABC) algorithm. Control theories like IRP and SRF are used widely in DVR and other FACTS devices, both have advantages and disadvantages. SRF theory is considered more precise, less complex, and more flexible when compared to IRP theory [32–34]. Therefore an SRF theory based versatile control technique is developed for DVR and presented in this section to efficiently mitigate balanced and imbalanced sag to protect sensitive loads in power distribution line. Fig. 8 shows SRF based versatile control technique which is designed here for efficient working of DVR to resolve voltage sag. In this control first terminal voltage measured value (Vtabc) is converted from abc reference frame to direct-quadrature axis rotating reference frame (d-q axis) using park transformation, (3) represents the mathematical relation for park transformation.
32 3 2 3 cosðhÞ cos h 23p v td v ta cos h þ 23p 6 7 6 7 6 7 4 v tq 5 ¼ 4 sinðhÞ sin h 23p sin h þ 23p 54 v tb 5 2
v0
1 2
1 2
1 2
v tc
ð3Þ
After applying low pass filter (LPF) to (Vtd) to filter out higher order components, the (Vtdfil) is compared with reference voltages in d axis named (Vrefd). To control DC link voltages of DVR compe-
Fig. 7. FIS surface viewer (three dimension).
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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Fig. 8. SRF based versatile control technique.
Zerocomp ¼ PIðZerocomref ZerocomL Þ
ð7Þ
To convert d-q-0 rotating reference frame to abc reference frame inverse-park transformation is used, (8) represents mathematical relation for inverse park transformation. Necessary ɵref is provided for park and inverse park transformation. The mathematical operation in d, q, and zero axis is represented with help of (6), (7), and (8) respectively. After apply inverse park transformation the reference signal in abc reference frame (Vabc*) is acheived, SPWM is applied to achieved switching signals for VSCDVR.
2
3
3 32 cosðhÞ sinðhÞ 1 V dcomp 6 7 6 7 76 4 V b 5 ¼ 4 cos h 23p sin h 23p 1 54 V qcomp 5 2p 2p cos h þ 3 sin h þ 3 1 Zerocomp Vc V a
2
4.1. Performance of DVR under balanced sag
ð8Þ
4. Simulation results To evaluate the performance of the proposed control technique for DVR whole system is simulated in Matlab/Simulink environment, the system is designed with parameters given in Table 1.
Table 1 Simulation system parameters. System parameters
Value
Per phase voltages
220 V (1 p.u) 50 Hz R = 0.3 O L = 2.25 mH 2.15 + j1.3 p.u 2.1 + j1.1 p.u 3 + j1.8 p.u 1.15 + j2.2 p.u 1:1 0.65 O 85 lF 0.72 mH 7590 lF
Line frequency Line impedance Balanced load Unbalanced load
Turn Ratio IT Rf Cf Lc DC link capacitor (CDC)
The objective of DVR is to maintain sag free and balanced nominal load voltages at the sensitive load terminal. The sensitive loads which need to be secured are either balanced or unbalanced. Simulations are carried out for two different scenarios named balanced and imbalanced sag condition with balanced and unbalanced loads respectively, these are represented by Figs. 9–12. Simulation results have proved the efficacy of the proposed control technique, unbalanced and imbalanced voltage sag conditions are compensated well in less possible time.
This is the first case when the performance of DVR with proposed control is investigated under balanced sag condition. Nominal supply voltages are 1 p.u, the load impedance is 2.15 + j1.3 p.u, and system frequency is 50 Hz. In this condition, DVR is required to maintain 1 p.u voltages across sensitive load terminal. Fig. 9(a) shows terminal supply point voltages under balanced sag condition without DVR compensation, from 0 s to 0.05 s grid is working under normal operation and maintain 1 p.u voltages. At 0.05 s a balanced sag of 0.4 p.u occur and terminal supply voltages reduced to 0.6 p.u. Fig. 9(b) shows the RMS value of terminal supply voltages under balanced sag condition in p.u. Fig. 9(c) shows balanced sag compensation using DVR, at 0.05 s sag is detected using DVR control and compensate within 5 ms time frame. Fig. 9(d) shows balanced sag compensation results in RMS value. Fig. 9(e) shows the DC link capacitor voltage for DVR, it is seen that at 0.05 s the DC link voltages are disturb as DVR comes in operation at 0.05 s, DC link voltages are stable again in minimum time due to SRF based control of DVR. Fig. 10 shows the results of load active/reactive power under balanced voltage sag condition. Fig. 10(a) Shows active power before DVR compensation, at 0.05 s when balanced sag occurs the active power is reduced from its normal operating value. Fig. 10(b) shows active power recovery at 0.05 s when DVR activate. Fig. 10(c) shows reactive power results before balanced sag compensation when sag occurs at 0.05 s the reactive power is slightly disturbed from its normal operating value. Fig. 10(d) shows the recovery of reactive power when DVR activate at 0.05 s.
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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Fig. 10. Active and reactive power during balanced voltage sag condition (a). Active power before compensation (p.u). (b) Active power after compensation (p.u). (c) Reactive power before compensation (p.u). (d) Reactive power after compensation (p.u).
Fig. 9. Balanced voltage sag condition. (a) Balanced sag at load bus voltages (p.u). (b) Sag RMS (p.u). (c) Balanced sag compensation (p.u). (d) Balanced sag compensation RMS (p.u). (e) DC link capacitor voltage of DVR.
4.2. Performance of DVR under imbalanced sag In this case, the performance of DVR with proposed control is investigated under imbalanced sag condition. Nominal supply volt-
ages are the same 1 p.u, load impedance per phase are 2.1 + j1.1 p. u, 3 + j1.8 p.u, and 1.15 + j2.2 p.u respectively. System frequency is 50 Hz. In this condition, the DVR is required to maintain 1 p.u balanced voltages across sensitive load terminal. Fig. 11(a) shows terminal supply point voltages under imbalanced sag condition without DVR compensation, from 0 s to 0.05 s grid is working under normal operation and maintain 1 p.u voltages. At 0.05 s an imbalanced sag occurs and terminal supply voltages reduced in an unbalanced way. Fig. 11(b) shows the RMS value of terminal supply voltages under imbalanced sag condition in p.u. Fig. 11(c) shows imbalanced sag compensation using DVR, at 0.05 s sag is detected using DVR control and compensate within 5 ms time frame. Fig. 11(d) shows imbalanced sag compensation results in RMS value. Fig. 11(e) shows the DC link capacitor voltage for DVR, it is seen that at 0.05 s the DC link voltages are disturb as DVR comes in operation at 0.05 s, DC link voltages are stable again in minimum time due to SRF based control of DVR.
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
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Fig. 12. Active and reactive power during imbalanced voltage sag condition (a). Active power before compensation (p.u). (b) Active power after compensation (p.u). (c) Reactive power before compensation (p.u). (d) Reactive power after compensation (p.u).
Fig. 11. Imbalanced voltage sag condition (a). Imbalanced sag at load bus voltages (p.u). (b) Sag RMS (p.u). (c) Imbalanced sag compensation (p.u). (d) Imbalanced sag compensation RMS (p.u). (e) DC link capacitor voltage.
Fig. 12 shows the results of load active/reactive power under imbalanced voltage sag condition. Fig. 12(a) Shows active power before DVR compensation, at 0.05 s when imbalanced sag occurs the active power is reduced from its normal operating value.
Fig. 12(b) shows active power recovery at 0.05 s when DVR activate. Fig. 12(c) shows reactive power results before imbalanced sag compensation when sag occurs at 0.05 s the reactive power is slightly disturbed from its normal operating value. Fig. 12(d) shows the recovery of reactive power when DVR activate at 0.05 s. For the DVR, Table 2 shows a comparison of controls of different papers with the control of the presented paper in terms of ability to compensate balanced and imbalanced sag and control complexity (control computations). There are fewer papers whose control is able to compensate for both balanced and imbalanced sag. In [17], presented control is less complex however there is no sag detection method is presented, control is switched automatically to compensate sag. The controls in [18,20,21,23], and [24] are more complex and take more computational steps to compensate sag. The control presented in this paper is less complex in terms of
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001
E.A. Al-Ammar et al. / Ain Shams Engineering Journal xxx (xxxx) xxx Table 2 DVR Control Comparison with Ability to Compensate Balanced & Imbalanced Sag. Paper #
Compensate balanced sag
[9] [10] [11] [12] [13] [14] [15] [16] [17]
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] This Paper
Compensate imbalanced sag
Control complexity when compensate both sag
Less complex (no sag detection technique) More complex
More complex More complex
More complex More complex
Less complex with sag detection technique
computations and it has the ability to compensate balanced and imbalanced sag. Presented control has also automatic sag detection technique, therefore, presented control for DVR is superior to the other controls in the literature survey. 5. Conclusion Voltage sag is a major power quality problem which damages sensitive loads in the distribution system, second generation FACTS devices can be used as CPD to compensate power quality problems. In this paper, a nifty control structure is designed for DVR to secure sensitive loads in distribution system from voltage sag. Presented control structure of DVR consist of two sub control units an SRF based control and fuzzy logic based control. SRF based control of DVR is responsible for effective compensation of balanced as well as imbalanced voltage sag, DC link capacitor voltage control technique of DVR is integrated into SRF based control for efficient working of DVR. On the other side, fuzzy logic based control is designed for voltage sag detection in distribution power line, it performs online switching of DVR according to presence of sag in line. Simulation results refer to Fig. 9 and Fig. 11 shows the detection and compensation capabilities of proposed DVR control to compensate balanced and imbalanced voltage sag. In the future, authors are planning to design DVR control structure based on fuzzy logic control and sliding mode control (SMC). Declaration of Competing Interest The authors declared that there is no conflict of interest. References [1] Ganesan J, Krishnaveni A. A review on basic concepts and important standards of power quality in power system. Int J Sci Eng Appl 2015;4(5). [2] de Almeida A, Moreira L, Delgado J. Power quality problems and new solutions. RE&PQJ 2003;1(1). [3] Ghahderijani Mohammad Moradi, Castilla Miguel, de Vicuna Luis Garcia, Camacho , Martinez Javier Torres. Voltage sag mitigation in a PV-based industrial microgrid during grid faults. In: Published in: industrial electronics (ISIE), 2017 IEEE 26th international symposium on. p. 19–21.
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Dr. Essam A. Al-Ammar. Assistant Professor, Department of Electrical Engineering, College of Engineering, King Saud University Riyadh, Saudi Arabia, PhD Arizona State University Tempe, Arizona, USA, 2007, MS.c University of Alabama Tuscaloosa, Alabama, USA, 2003, BS.c King Saud University Riyadh, Saudi Arabia, 1997.
Dr. Azhar Ul Haq. Assistant Professor, Department of Electrical Engineering, College of E&ME, NUST, Rawalpindi, Pakistan.
Ahsan Iqbal MS EE Bahria University, Islamabad, Pakistan, BS EE Comsats University Wah Cantt, Pakistan.
Please cite this article as: E. A. Al-Ammar, A. Ul-Haq, A. Iqbal et al., SRF based versatile control technique for DVR to mitigate voltage sag problem in distribution system, Ain Shams Engineering Journal, https://doi.org/10.1016/j.asej.2019.09.001