CC BY
32
UDC 621.31
https://doi.Org/10.20998/2074-272X.2022.1.09
S.T. Zahra, R.U. Khan, M.F. Ullah, B. Begum, N. Anwar
Simulation-based analysis of dynamic voltage restorer with sliding mode controller at optimal voltage for power quality enhancement in distribution system
Introduction. Nowadays, power quality issues are of considerable interest to both utilities and end users as they cause significant financial losses to the industrial customers. Due to this, power quality assurance in power distribution systems is very important, when considering commercial and industrial applications. Problem Statement. Unfortunately, sudden faults such as sag, transients, harmonics distortion and notching in the power system create disturbances and affect the load voltages. Out of these, voltage sag and harmonics seriously affect sensitive devices. Harmonics in the power system cause increased heating of equipment and conductors, misfires in variable speed drives, and torque pulsations in motors. Harmonics reduction is considered desirable. Methodology. This paper presents an efficient and robust solution to this problem by using dynamic voltage restorer in series with distribution system. Dynamic voltage restorer is economical and effective solution for protecting sensitive loads from harmonics and sag. Control strategy is adopted with dynamic voltage restorer topology and the performance with the proposed controller is analyzed. Novelty. In this research work modelling, analysis and simulation of dynamic voltage restorer with proportional integral controller and dynamic voltage restorer with sliding-mode controller at optimal voltage is used to improve the dynamic voltage restorer performance by reducing total harmonic distortion. Results. The simulation is performed in MATLAB / Simulink software package and comparative analysis of dynamic voltage restorer with different controllers for distribution system is presented. The proposed scheme successfully reduced percentage total harmonics distortion and voltage sag using dynamic voltage restorer with sliding mode controller at optimal voltage which is found to be 0.38 %. References 22, tables 1, figures 19.
Key words: dynamic voltage restorer, sliding-mode controller, total harmonics distortion, voltage sag, power quality improvement.
Вступ. Съогодн питання nKocmi електроенергп викликаютъ значний ттерес як для комунальних тдприемств, так i для пнцевих cnoMMeauie, остлъки вони завдаютъ значних фшансових втрат промисловим споживачам. У зв'язку з цим забезпечення якостi електроенергИ в розподыъних системах е дуже важливим при розглядi комерцшних та промислових застосуванъ. Постановка задет. На жалъ, раптовi несправностi, таю як прогини, перехiднi процеси, спотворення гармотк i витки в енергосистемi створюютъ збурення та впливаютъ на напругу навантаження. З них провали напруги та гармотки серйозно впливаютъ на чутливi пристро!. Гармотки в енергосистемi викликаютъ посилений нагрiв обладнання та провiдникiв, пропуски запалювання в приводах iз змнною частотою обертання, пулъсацп крутного моменту в двигунах. Зниження гармотк вважаетъся бажаним. Методология. У цш статтi представлено ефективне та надшне рiшення це проблеми за допомогою динамiчного адновника напруги по^довно з розподтъною системою. Динамiчний вiдновник напруги - це економне та ефективне рiшення для захисту чутливих навантаженъ вiд гармонт та провисанъ. Прийнято стратегю керування з тоnологiею динамiчного вiдновника напруги та nроаналiзовано ефективтстъ при використанн запропонованого контролера. Новизна. У цш достдницъкт роботi використовуетъся моделювання, аналiз та чиселъне до^дження динамiчного вiдновника напруги з пропорцтним нтегралъним контролером та динамiчного вiдновника напруги з регулятором ковзного режиму при оптималънш наnрузi для тдвищення ефективностi динамiчного вiдновника напруги за рахунок зменшення сумарних гармонтних спотворенъ. Результата. Моделювання виконано в програмному комплека MATLAB /Simulink та представлено nорiвнялъний аналiз динамiчного адновника напруги з рiзними контролерами для розподыъчо! системи. Запропонована схема устшно зменшила вiдсоток спотворення сумарних гармонт i nровалiв напруги за допомогою динамiчного вiдновника напруги з регулятором ковзного режиму при оптималънш наnрузi, який становитъ 0,38 %. Бiбл. 22, табл. 1, рис. 19.
Ключовi слова: динамiчний виновник напруги, регулятор ковзного режиму, загальш гармошчш спотворення, провал напруги, покращення якост електроенерт.
1. Introduction. Nowadays, power quality issues are or compensation can be achieved to remove/cancel out the
of considerable interest to both utilities and end users as disturbances at the load side [1, 2]. Number of customized
they cause significant financial losses to the industrial power devices are obtainable each having its own Pros and
customers. Due to this, power quality assurance in power Cons for voltage reduction compensation such as
distribution systems is very important when considering DSTATCOM, Superconducting Magnetic Energy Storage
commercial and industrial applications. Unfortunately, (SMES), Dynamic Voltage Restorer (DVR) and Static VAR
sudden faults such as sag, transients, harmonics distortion Compensation (SVC).
and notching in the power system create disturbances and DVR refers a controllable voltage source usually
affect the load voltages. Out of these, voltage sag and inserted between the sensitive load voltage and network,
harmonics seriously affect sensitive devices. Harmonics in which accurately generates a disturbance that perturbs the
the power system cause increased heating of equipment and sensitive load voltage by inserting the voltage into the
conductors, misfires in variable speed drives, and torque distribution line through a transformer. Lead-acid battery-
pulsations in motors. Harmonics reduction is considered powered DVRs are the best and most attractive technology,
desirable. Voltage total harmonics distortion (THD) also providing superior dynamic voltage restoration compared
plays very important role in a power system and power to shunt-connected devices. The main function of a voltage
qua%. According to standard °f ШЕЕ 519-1"2, fe value source inverter power system equipped with a DVR is to
of THD should be equal or less than 5 % of the iundamental inject the desired three-phase voltage into the load [3, 4]. In
frequency. Sandaid of ШЕЕ 1152-1995 defme te vokage this paper modelling, analysis and simulation of DVR with
sags root mean square ra RMS vokage variation havmg a pi controller and DVR with Sliding-Mode Controller
magnitude in between (°Л-°.9) p.u. of value of nominal (SMC) at optima1 voltage is used to reduce THD to voltage and its duration typically varies from 0.5 cycle to 60 s. Installation of custom power device, voltage mitigation
© S.T. Zahra, R.U. Khan, M.F. Ullah, B. Begum, N. Anwar
improve DVR performance. DVR with SMC at optimal Vdc has successfully reduced THD and voltage sag to 0.38 %.
2. Dynamic Voltage Restorer. DVR is a series connected power electronics switching device and is connected in series with the distribution line to inject the desirable controlled voltage. The DVR includes injection transformer, harmonic filter, voltage source inverter, control unit and DC storage unit [3-7].
The heart of DVR is the control unit, which is mainly used to monitor the presence of drop in voltage in the network and if necessary to compensate/insert the missing voltage after determining its phase and magnitude. Control unit has a reference voltage (Vref) structure for the purpose of comparison. A three-phase reference voltage scheme is used as a reference in a dynamic voltage restorer. However, the Vref three-phase voltage must be needs to synchronize with the load voltage (VL) to correctly inject the missing voltage based on magnitude and phase. Simulink diagram for obtaining the reference waveform of voltage is shown in Fig. 1.
Vq
Vo
where M is as follow
=3 ■ mI ■
(i)
2sin mt 2sinl mt -
2k
T
2k 3
2sinl mt +
2k|
T J Va
2k|
— Vb
3 J
V
,(2)
supply voltages respectively; ZS, IS are source impedance and current respectively; VDVR is the injection voltage by DVR and VSAG is sag voltage.
The equivalent circuit of the power system under study is shown in Fig. 2 and phasor diagram of distribution system with DVR is shown in Fig. 3 [11, 12].
Fig. 2. Equivalent circuit of the power system under study
Fig. 1. Simulink reference voltage diagram
In the control unit, a Park transformation also known as direct-quadrature-zero (dq0) transformation has been used to control the DVR. The 3-phase system is more of a simplified system, and it can easily be controlled after transformation from 3-phase abc voltage to two voltage components i.e. Vd and Vq. The V0 which is zero sequence components are ignored for simplicity
where a> is the angular frequency.
There are two modes of DVR which are given below.
In first injection mode, voltage inject operation has been done using DVR to correct the sag. While in the second mode, monitoring operation is usually done in standby mode [8-10]. So, there is no type of other voltage is being injected. Injected transformer low voltage side is shorted using the voltage source inverter. The sag voltage, DVR voltages and load current are written as
IL = il + JQL )L ; (3)
VDVR = VL +(ZS xIS)-VS; (4)
Vsag = VS -(IL xZS), (5)
where IL is the load current; PL, QL are load active and reactive powers respectively; VL, Vs are the load and
Fig. 3. Phasor diagram of distribution network using DVR
3. DVR with PI controller. The DVR plays a key role in detecting voltage sag events, correcting voltage sag problems, and generating Pulse Width Modulation (PWM) trigger pulses. The control unit generates a three-phase Vref and compares the VL to the Vref value [13]. An error signal is generated when voltage is missing from the power system due to voltage sag. If the difference between Vref missing VL is equal to the error signal, the DVR starts to work and injects the missing voltage into the power distribution network. That is, an error signal is transmitted to the PI controller and then PI controller output is converted back to a Park transformation. The signal is transmitted in a discrete pulse width modulation block. Discrete PWM compares the input three-phase converted signal to a saw tooth wave and the PWM generates a pulse to trigger the PWM VS (voltage source) inverter with the desired firing arrangement. The DVR collects the required direct current voltage from the storage device (e.g. 500 V). A voltage source inverter is used to invert the DC storage unit voltage to an AC voltage, and eventually the missing voltage is injected through a three-phase injection transformer, which is then connected in series with the distribution line. The phase-locked loop for a DVR with PI controller is shown in Fig. 4 and the Simulink model is shown in Fig. 5.
Fig. 4. Phase Locked Loop for DVR with PI controller
Z
L
1
1
1
Fig. 5. Simulink Model of DVR with PI controller
4. Control circuit for DVR with SMC. SMC is a
robust and nonlinear control scheme at which the arrangement of the controller is being modified in reaction to the varying state of the system to obtain a desire result. The control procedure for fast switching is used and the trajectory of the system is forced to move along the selected switching area in the state space [14].
There are three major advantages of SMC i.e. model reduction, performance design and robustness parameters. The sliding phase (S.P.) at which a system slides and properly to approach to zero to make the system stable i.e. S = 0 (S is the sliding surface variable). While a system approaches to sliding phase is known as reaching phase (R.P.) i.e. S ^ 0. Equation (6) gives the main equation of SMC
u = -sign(s) ; (6)
S = x + a • x, (7)
where S = 0 and a is the variable
x = -a • x, (8)
Reachability phase is very simple and it says that any controller which obeys SS < 0 will reach to sliding surface/phase. The system has an inertia due to which the reaching phase will little bit move forward as shown in
Fig. 6.
Sliding to zero Fig. 6. Phase plot for actual SMC control
The control law determination of reaching phase is given below in (9) to maintain the system in the sliding surface and due to system inertia, the sliding mode trajectories often chatter back and forth motion along the sliding surface to reach the system at origin which is a chattering also known as oscillation [14]. The S and S signs are always opposite to each other and S\t\ is the switching function of the voltage source inverter i.e. 1 or -1 f1 (on time) for S < 0;
4 = 1/ ^ • , (9)
1 1 [-1 (off time) for S < 0.
The equivalent circuit of the DVR has been checked and it is investigated that the DVR can work on the sliding surface and is following the rule of reaching phase SS < 0 to reach at the sliding surface. The tracking error
of injection voltage dynamics, when the system is on sliding mode, is stable and is given in (10) [4-6]
uf =-Vd • sign{S), (10) where Uf is control input to voltage source inverter; Vd is DVR voltage.
So, the voltage source inverter switches DC storage unit voltage according to sliding function. SMC was traditionally defined using the state space formulation and this practice continuous in sliding mode studies but in recent studies, sliding mode can also be attained by relay control system because this approach is very simple [15, 16]. The relay sliding mode controller does not compulsory any knowledge of system states and a complete system model is not required in relay sliding mode controller [17, 18]. SMC has been added at the end of Park transformation or dq0 to abc transformation. The error signal coming from dq0_to_abc transformation will become input of SMC which is being used to switch the inverter. We have tuned the SMC to reduce the THD to make our DVR valuable and effective [19-22]. Phase locked loop for DVR with SMC is shown in Fig. 7 and its Simulink model is shown in Fig. 8.
Fig. 7. Phase locked loop for DVR with SMC
Fig. 8. Simulink model of DVR with SMC 5. Simulation and results. The power system parameters selected for the simulation are given in the Table 1.
Table 1. Power system and DVR parameters
Parameter Value
Line resistance, ^ 1
Line inductance, mH 5
Line frequency, Hz 50
Filter series capacitance, F 100
Filer series resistance, ^ 1
Load phase voltage, V 220
Load power per phase, W 100
Load inductive reactive power per phase, kVar 0.2
DC supply voltage, V 500
Injection transformer turns ratio 1:1
Load capacitive reactive power per phase, kVar 0.5
Saw-tooth carrier wave frequency, Hz 5500
5.1. Test system with DVR for distribution system. Three phase power test system with DVR coupled with injection transformer is shown in Fig. 9.
The DVR coupling boost transformer is connected in delta in a voltage source inverter side. The 13 kV, 50 Hz three phase supply is step up using three phase
transformer star/delta/delta, 13/66/66 kV feeding two transmission lines. Both transmission lines are step down to 380 V to drive the sensitive load and non-sensitive load. The performance of the system with DVR has been simulated and investigated under three phase short circuit fault.
Fig. 9. Test System with DVR
5.2. Distribution system without DVR. The first simulation did not include a DVR and a three-phase fault was applied to one of the transmission lines, creating a voltage sag before the injection transformer through a fault resistor of 2 kQ. A voltage drop event occurred on the transmission line for 0.05 to 0.185 s and the fault reduced the three-phase load voltage, which could disturb sensitive loads and cause system failure. Also, without a DVR, the voltage drop increased THD to 5.16 %. The load voltage obtained in this case is shown in Fig. 10 and the THD without DVR is shown in Fig. 11.
5.3. Distribution system using DVR with PI controller. The second simulation is carried out using DVR based on PI controller which is connected to distribution system through an injection transformer. We have investigated that the missing voltage in the distribution line has been mitigated completely. Furthermore, THD has also been reduced as per power quality standard to 1.03 % with DVR with PI controller is shown in Fig. 12. Load voltage obtained in this case is shown in Fig. 13.
Fig. 10. Load voltage during fault without DVR
Fig. 12. THD during fault using DVR with PI controller
Fig. 11. THD during fault without DVR
Fig. 13. Load voltage during fault with DVR using PI controller
5.4. Distribution system using DVR with SMC.
The third simulation is being carried out using DVR with SMC which is connected to mitigate more voltage sag
margin with reduction in THD. The load voltage obtained using DVR with SMC is shown in Fig. 14 and THD obtained in this case is shown in Fig. 15.
Fig. 14. Load voltage during fault using DVR with SMC
Harmonic order
Fig. 15. THD during fault with DVR with SMC
5.5. Selection of optimal Vdc for THD reduction.
The fourth simulation of DVR with SMC is based on the different values of Vdc to get optimal Vdc for THD reduction. The DC storage unit has a main impact on THD, we have simulated the DVR on different Vdc values and analyzed that at 1500 V storage unit, THD has been reduced to 0.44 %. Furthermore, we have changed the reference voltage i.e. 385 V and tuned on/off time of SMC i.e. 0.00001 s and -0.070 s then THD has been reduced from 0.44 % to 0.38 % as shown in Fig. 16. Load voltage obtained in this case is shown in Fig. 17 and THD is shown in Fig. 18.
VOLTAGE STORAGE (VDC] IN VOLTS
Fig. 16. Selection of optimal Vdc for THD reduction
.'.. .'. ■ r-.
Fig. 17. Load voltage using DVR with SMC at optimal Vdc
6. Comparative analysis of DVR with different controllers for distribution system. MATLAB / Simulink software package is employed to get a comparison of all the results offered by DVR with different controllers for distribution system. Initially, three phase fault has been applied in transmission line to create voltage sag. This fault has reduced the three-phase load voltage with increase of THD up to 5.16 %. The second simulation is carried out using DVR with PI controller which is connected to distribution system through an injection transformer. We have investigated that the missing voltage in the distribution line has been mitigated completely and THD has also been reduced to 1.03 % using DVR with PI controller. The third simulation is being carried out using DVR with SMC which is connected to mitigate more voltage sag margin with reduction in THD up to 0.95 %. Finally, DVR with SMC at optimal Vdc gives reduction of THD up to 0.38 %. The performance comparison of DVR with different controllers is shown in Fig. 19.
Fig. 18. THD for selected signal during fault using DVR with SMC at optimal Vdc
Fig. 19. Comparative analysis of DVR with different controllers for distribution system
Conclusions.
In this research paper, dynamic voltage restorer with PI controller and dynamic voltage restorer with sliding mode controller at optimal voltage Vdc is used to enhance the performance of dynamic voltage restorer by reducing THD. The effectiveness and performance of the proposed control scheme under voltage sag condition are examined. Simulation results shows that percentage total harmonic distortion and voltage sag are successfully reduced by using dynamic voltage restorer with sliding mode controller in distribution system under random fault condition. Percentage total harmonic distortion during fault using dynamic voltage restorer with PI controller, dynamic voltage restorer with sliding mode controller and dynamic voltage restorer with sliding mode controller at optimal voltage is 1.03 %, 0.95 % and 0.38 % respectively. It is obvious that dynamic voltage restorer with sliding mode controller at optimal value of voltage Vdc can mitigate the voltage sag very quickly with minimum percentage total harmonic distortion as compared to dynamic voltage restorer with PI controller to keep the voltage balance under fault events circumstances.
Conflict of interest. The authors declare that they have no conflicts of interest.
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Received 02.12.2021 Accepted 07.01.2022 Published 23.02.2022
Syeda Tahreem Zahra1, Lecturer, PhD Scholar, Rizwan Ullah Khan2, Laboratory Engineer, Mian Farhan Ullah2, Lecturer, Balqees Begum2, Graduate Assistant, Naveed Anwar2, Junior Lecturer,
1 Faculty of Electrical Engineering,
Riphah International University, Islamabad, Pakistan, e-mail: tahreem.zahra@riphah.edu.pk
2 Faculty of Electrical Engineering,
Wah Engineering College, University of Wah, Pakistan, e-mail: rizwan.ullah@wecuw.edu.pk, farhan.ullah@wecuw.edu.pk, balquees.begum@wecuw.edu.pk , naveed.anwar@wecuw.edu.pk (Corresponding Author)
How to cite this article:
Zahra S.T., Khan R.U., Ullah M.F., Begum B., Anwar N. Simulation-based analysis of dynamic voltage restorer with sliding mode controller at optimal voltage for power quality enhancement in distribution system. Electrical Engineering & Electromechanics, 2022, no. 1, pp. 64-69. doi: https://doi.org/10.20998/2074-272X.2022.L09.
