Engineering, Technology & Applied Science Research Vol. 12, No.

2, 2022, 8300-8305 8300

Power Quality Improvement using Dynamic Voltage

Restorer with Real Twisting Sliding Mode Control

Muhammad Shahzaib Shah Tahir Mahmood

Electrical Engineering Department Electrical Engineering Department
University of Engineering and Technology University of Engineering and Technology
Taxila, Pakistan Taxila, Pakistan
mshahzaib.Shah@students.uettaxila.edu.pk tahir.mehmood@uettaxila.edu.pk

Asad Ur Rehman Muhammad Qasim Manan

Electrical Engineering Department Mechatronics Engineering Department
University of Engineering and Technology Wah Engineering College, University of Wah
Taxila, Pakistan Wah Cantt, Pakistan
asadur.rehman3@students.uettaxila.edu.pk qasim.manan@wecuw.edu.pk

Mian Farhan Ullah

Electrical Engineering Department
Wah Engineering College, University of Wah
Wah Cantt, Pakistan
farhan.ullah@wecuw.edu.pk

Received: 3 January 2022 | Revised: 22 January 2022 | Accepted: 25 January 2022

Abstract- Higher Power Quality (PQ) is a common demand of Keywords-dynamic voltage restorer; power quality issues;
sensitive industrial customers. PQ issues are gaining attention sliding mode control; real-twisting algorithm; voltage sag/swell
from both end-users and electrical utility companies since they
are generating significant economic losses to sensitive industrial I. INTRODUCTION
loads. Voltage sags/swells are the most significant and usually Power Quality (PQ) is a key problem of today's power
occurring PQ issues in a secondary distribution system. Dynamic systems, since it may have an impact on sensitive loads and
Voltage Restorer (DVR), is a fast, flexible, effective, and dynamic utilities [1-2]. Most of the industrial devices are typically based
Custom Power Device (CPD), that can be used to eliminate
on electronic devices like the programmable logic controller,
voltage sags and swells. Its performance is mostly determined by
microprocessors, computers, and adjustable speed drives that
the control strategy established for switching Voltage Source
Converters (VSCs). This research work develops a fused control
are very sensitive to variations such as voltage sags/swells and
method for VSC of DVR based on the Real-Twisting Algorithm harmonics [3]. As the Distribution System (DS) is the weakest
(RTA) and Sliding Mode Control (SMC) that successfully link in a power system, these PQ problems have a higher
eliminates the impacts of voltage sags/swells. RTA along with the impact on it [4-5]. To mitigate these issues, Custom Power
conventional SMC reduce the effect of chattering, which is a Devices (CPDs) like the Dynamic Voltage Restorer (DVR), the
disadvantage of SMC while retaining its additional qualities like Active Filter (AF), the Unified Power Quality Conditioner
robustness, quicker response time, and insensitivity to load (UPQC), and the Distribution Static Synchronous Compensator
variations. To evaluate the performance of the proposed control (DSTATCOM) are some of the most often utilized power
approach, the MATLAB/Simulink SimPower System toolbox was devices [6]. DVR, due to its superior performance, is regarded
employed. According to the simulation findings, the Real as the best solution for minimizing the PQ issues. It is a fast,
Twisting Sliding Mode Controller (RTSMC) for DVR can detect dynamic, and efficient technology that is employed to
and mitigate voltage sags/swells within 2.5ms which is much effectively mitigate voltage magnitudes [7]. Several control
lower than the allowable limit of 20ms as per semiconductor schemes, e.g. state feedback, self-tuning, Instantaneous
industrial equipment voltage sag immunity standard (SEMI F-47 Reactive Power (IRP) theory, direct quadrature (dq), reference
standard) for sensitive loads. Total Harmonics Distortion (THD) adaptive model, phase shift control, vector template method,
is determined to be less than 5% in all simulated instances. A PCC regulated voltage, instantaneous symmetrical components,
comparative study is also performed between the conventional
DC-link with Proportional-Integral (PI) controller,
SMC and the suggested RTSMC, revealing that the proposed
method outperforms the classical SMC.
Synchronous Reference Frame (SRF) theory, feedback,
feedforward, phase shift control, PI resonant, P + resonant,
Corresponding author: Mian Farhan Ullah
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Artificial Neural Network (ANNs), Fuzzy Controller (FC) are The equivalent circuit diagram of DVR is shown in Figure
used as VSCs of the DVR [8-11]. All these control schemes 2, where load voltage is represented by VL and source voltage is
have advantages and disadvantages, when it comes to represented by Vsource, so the voltage inserted by DVR Vdvr can
generating a clean sinusoidal AC waveform at the DVR's VSC be written as:
output. They're constructed for creating a highly precise
linearized mathematical model of the system that performs well VL = Vsource + Vdvr (1)
under certain operating circumstances. On the other hand, these
control techniques are unable to provide optimal performance Cfc and Lfc are the filter parameter as shown in Figure 2. The
when system parameters change. As a result, an efficient and filter capacitor current ifc can be defined as:
robust control system is required, capable of performing
dVdvr
functions with high precision and stability in dynamic i fc = C fc (2)
conditions. The SMC for DVR overcomes these problems dt
because it is not sensitive to changes in system parameters and
does not require an exact mathematical model of the system,
but it has an important disadvantage, named the chattering
effect [12]. In order to avoid the chattering effect, some
algorithms such as real-twisting, super-twisting, optimal, sub-
optimal, global, integral, and state-observer algorithm have
been used [4, 13-15]. Among these algorithms, the real-
twisting algorithm has the upper hand due to its stability,
robustness, and high tracking accuracy with less chattering
effect.
In this research, an SMC control strategy based on the
second order RTA for VSC of DVR is presented, which can
successfully prevent the impacts of voltage sags/swells in
Fig. 2. Equivalent circuit diagram of DVR.
distribution systems and decrease chattering, which is a
drawback of the traditional SMC. RTSMC and DVRs can Now, KCL is applied at node (al) in the given Figure 2, the
successfully minimize the percentage of THD and voltage result is:
disturbances according to [16], using the MATLAB/
SIMULINK software platform. is − i fl + i fc = 0 (3)
II. MATHEMATICAL MODELING OF DVR IN A DISTRIBUTION In (3), is represents the source current, while ifl and ifc are
SYSTEM
representing the filter inductor and capacitor current
A DVR is a power electronics switching device, connected respectively. Now, putting the value of ifc in (3), gives (4) and
in series with the distribution line in order to inject the after simplification, we get (5):
desirable controlled voltage. Figure 1 depicts the generalized
DVR model as well as the way it is linked to the grid. DVR dVdvr
essentially consists of an energy storage unit and a control
is − i fl + C fc = 0 (4)
dt
system, with a VSC linked in series with grid through a
dVdvr ( i fl − is )
boosting injection transformer.
= (5)
dt C fc
The DVR first state equation is produced in (5). To
generate the second state equation, KVL is applied in Figure 2
at closed loop. So:

V + V – V = 0 (6)
dvr fl in
where Vin is the output AC voltage of VSC, Vdvr is the voltage
injected by the DVR, and Vfl is the voltage across the inductor,
given in (7). By putting (7) in (6), we get (8), and, after
simplification, we get (9):

di fl
V fl = L fl (7)
dt
Fig. 1. Generalized single line model of DVR in the distribution network.

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di fl d
Vdvr + L fl − Vin = 0 (8) S = Verr + k Verr (11)
dt dt
To bring the state variable on the sliding surface and to
di fl (V − V )
= in dvr (9) ensure the existence of operation, the given conditions must be
dt Lf satisfied:

Therefore, the state space model of the series connected S = 0 (12)

DVR is shown in (10):
S& = 0 (13)
 −1   1  The switching law can be written as:
 0 L fl  ilf  
 0
L fl   is 
d  i fl   
 = +  +1if S > +c
 Vdvr   −1  Vin 
(10)
dt Vdvr   1 x (t ) =  (14)
 0   0   −1if S < −c
 C fc   C fc 
In (14,) x(t) is the switching control, and c is a constant. If
where ifl and Vdvr are the state variables while is and Vin.are the we get x(t) =+1 then two switches sw1 and sw2 are on. If we
the input variables. get x(t) =-1 then the other two switches, sw3 and sw4, are on.
On the bases of this control law, a gating signal is produced for
III. DESIGNING THE SECOND ORDER RTSMC the VSC. The VSC produced the required magnitude of voltage
Two steps are necessary for the implementation of SMC. and inserted it with the help of the injection transformer. Thus
The first is selecting the sliding surface. The DVR displays the the control strategy used in this paper senses and corrects faults
desired performance when the state trajectory is pushed on the like sags and swells in a short time, i.e. 2.5ms. The state
specified sliding line. The second stage is to drive the system's trajectory in an ideal SMC is aimed at the sliding manifold with
state to reach and remain on the chosen sliding surface in a an infinite switching frequency. On the other hand, real power
finite period. Hence, the difference between the reference converters cannot work at an infinite switching frequency. As a
voltage and the voltage inserted by the DVR is the standard for result, the state trajectory does not go to the origin S=0 and
the suggested procedure for the sliding surface as given in (11). instead follows a discontinuous surface with undesirable
After this, a comparator is applied on the sliding surface (S) oscillations, also known as chattering. The system's un-
with ±c reference quantity. After the comparator, the produced modeled dynamics can be stimulated by such oscillations.
value is passed through the multiplexer. The aim of passing a Therefore RTA is utilized in SMC to remove the chattering
signal via a multiplexer is to apply the law of switching to a effect. The block diagram of SMC along RTA is shown in
single signal, and then to apply the control law to the surface. Figure 3.

Fig. 3. Sliding mode block diagram.

By applying the RTA on the sliding surface, the switching IV. SIMULATION RESULTS AND DISCUSSION
law gives the modified input of control Z as given in (15):
To test the efficiency of the RTSMC for DVR in
• Matlab/Simulink, a test system was developed. The parameter
Z = −n1sign ( s ) − n2 sign( S ) (15) details are given in Table I. Figure 4 depicts the suggested
distribution system that was used to model and simulate the
To remove the undesirable switching components, two DVR using the RTSMC. Three-phase programming source
tuning constants, n 1 and n 2, are used in the control law of RTA. produces voltage sags/swells in the distribution test system
The sliding manifold term sign(s), removes the switching which are then corrected by the DVR. The following analysis is
frequency of the components to increase the life of switches. carried out to evaluate the effectiveness of the suggested
When the designed sliding surface s is greater or less than zero, control approach.
the sign function sign(s) of the sliding manifold gives the +1
and -1 output respectively. The total effect of RTA on SMC • Voltage sag/swell mitigation.
results in less chattering effect, faster response, and robustness • Total harmonic distortion.
against external parameter variations.

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Rs Ls IL Once a fault occurs (voltage sag), the controller will detect it

Vda
Rs Ls
and evaluate its magnitude.
Vdb
Rs Ls
Vdc

Injection Injection Injection

Transformer Transformer Transformer

Ida Cf Idb Cf Idc Cf Three Phase

Lf Load
Lf Lf
(VLa, VLb, VLc)

Three
Phase S1 S3 S5 S9
Voltage S7 S11
Source Vdc
Vdc Vdc
(Vsa, Vsb,
S2 S4
Vsc) S6 S8 S10 S12

DVR

Switching Vda
Signal Vdb(Vla , VLb, VLc)
RTSMC
Vdc

Fig. 4. A three phase system with connected DVR.

TABLE I. PARAMETERS OF THE DISTRIBUTION TEST SYSTEM

Parameter description Values

Grid voltage(phase-phase) 400 V Fig. 5. Voltage sag waveform before and after mitigation: (a) Source
Frequency of the system (fo) 50 Hz voltage with 30% sag, (b) voltage injected by the DVR to mitigate the sag, (c)
Impedance of line (Rs, Ls) 0.8929 Ω, 16.58 mH the compensated load voltage.
Loads Rating (3Ø) Linear Load: P = 10 kW, Q = 1 kvar
Switching constant ±c 0.1
Energy storage(DC) 40v
LC-Filter: L f, Cf 1.8 mH, 5.5 µF
Power rating for coupling transformer 100 kVA
Control action RTSMC
SMC gain γ 0.142 µ
Switching frequency (Fs) 10 kHz
Solver for simulation Ode23tb (stiff/TR-BDF2)
Time of sampling 5 µsec
Filter cutoff frequency 405 Hz
RTSMC tuning gains, n1 and n2 0.5 and 0.5

The RTSMC does not provide any switching signal to run

the DVR when the system voltage does not change (normal
state). When the voltage of the system deviates from its
tolerated range, the controller begins to operate. RTSMC
operates in the following manner:
• Detect voltage sags/swells.
Fig. 6. THD value of the compensated load voltage under voltage sag for
• Compute the voltage sags/swells (percentage). phase A.

• Determine the signal of the switching control. The fault (voltage sag) is corrected in a very small duration
• Generate the switching signal (PWM) for VSC to activate of time (2.5ms) as compared to the allowable limit of IEEE
source and load voltage. standard that is 20ms. Figure 5(b) shows that DVR injects only
the missing value to eliminate the unnecessary high frequency
• Generate the necessary switching signal uninterruptedly to element by utilizing a low pass filter. The pure and sag free
ensure that voltage sags/swells are compensated. compensated system voltage is shown in Figure 5(c). The
compensated voltage THD value is given in Table II and
• Terminate the switching PWM signal, when the voltage Figure 6. The harmonic content which is present in load
sag/swell is resolved. voltage is lower than 5% which is proposed by the IEEE
A. Voltage Sag Mitigation standard 1159-1995.
Due to the sudden switching ON of the sensitive load at the B. Voltage Swell Compensation
supply-side, a three-phase balanced voltage sag of 30% occurs. Due to the switching OFF of sensitive load, a 30% swell is
This sag starts at 0.1s and ends at 0.2s, as displayed in Figure produced in the three-phase source voltage, which starts at 0.1s
5(a). The controller is used to correct the disturbance. During and ends at 0.2s as shown in Figure 7(a). The simulation result
normal operation, if there is no sag then no voltage is inserted.

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shows that the good and quick feedback of sliding mode retains which can successfully prevent the impacts of voltage
the load voltage of the sensitive load according to the ITIC sags/swells in distribution systems and decrease chattering,
standard. Due to the voltage swell, the distortion in the supply- which is a major drawback of the traditional SMC. RTSMC
side voltage is corrected in 2.5ms which is very small time with DVRs successfully minimizes the percentage of THD and
duration, as shown in Figure 7(b). After the compensation, the voltage disturbances as per IEEE standards.
system voltage is shown in Figure 7(c) with a magnitude of
1pu. The compensated voltage THD value is given in Table II In Table III, the proposed RTSMC is compared with the
and Figure 8. This value is less than the IEEE standard value. classical SMC, revealing that the proposed method with SMC
outperforms the classical SMC in terms of voltage sag/swell
recovery, THD, transients, and multimode oscillations.

TABLE III. PROPOSED RTSMC-SMC COMPARISON

Parameter RTSMC SMC

Voltage sag/swell
Excellent Good
recovery (p.u.)
Voltage sag/swell
2.5 ms 4 ms
recovery (s)
THD Between 1% to 4% Between 2% to 8%
Transients Fewer More
Robustness Excellent Excellent
Multi-mode oscillations Fewer Observed

V. CONCLUSION
In this paper, an SMC technique based on RTA for three-
phase DVR is presented. The suggested control mechanism
eliminates chattering, while attains constant switching
frequency. As a result of using RTA in DVR control, a
continuous control input is generated, which can be contrasted
Fig. 7. Voltage swell waveform before and after mitigation: (a) Source to the triangular carrier signal to generate pulse width
voltage with 30% swell, (b) voltage injected by DVR to mitigate swell, (c) modulation signals. To evaluate the performance of the
compensated load voltage.
suggested control approach, the MATLAB/Simulink SimPower
System toolbox is employed. According to the simulation
outcome, the designed RTSMC for DVR effectively corrects
voltage sags/swells and provides the required power within
2.5ms with THD less than 5% for sensitive loads using the
ITIC curve and the SEMI-F-47 standard. The suggested control
approach provides faster response, less disturbances, and better
of sag/swell voltage adjustment. A comparative study was
performed between the conventional SMC and the suggested
RTSMC, revealing that the proposed method outperforms the
classical SMC.
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