A new control method for Dynamic Voltage Restorer with asymmetrical inverter legs based on fuzzy logic controller
ABSTRACT Dynamic Voltage Restorer (DVR) is used in power distribution system to protect sensitive loads in voltage disturbances. The performance of DVR is related to the adopted configuration and control strategy used for inverters. In this paper, an asymmetrical voltagesource inverter controlled with fuzzy logic method based on hysteresis controller, is used to improve operation of DVR to compensate voltage sag/swell. Simulation results using MATLAB/Simulink are presented to demonstrate the feasibility and the practicality of the proposed novel Dynamic Voltage Restorer topology. Total Harmonic Distortion (THD) is calculated. The simulation results of new DVR presented in this paper, are found quite satisfactory to eliminate voltage sag/swell.

Article: Three Phase Multicarrier PWM Switched Cascaded Multilevel Inverter as Voltage Sag Compensator
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ABSTRACT: This paper presents a cascaded H–bridge multilevel inverter based series active filter intended for installation on industrial and utility power distribution systems. The control strategy based on Synchronous Reference Frame theory is designed so that the voltage injected by active filter is able to mitigate the voltage sag, imbalance in the source voltage and reduce the harmonic content. The active power filter which can be used under the condition of voltage sag and unbalanced or distorted source voltages can compensate the harmonics, reactive and negative sequence currents.Simulations have been carried out on MATLAB/Simulink platform with various types of loads. The analysis and simulation results under unbalanced load and dynamic loading are presented in this paper.AASRI Procedia. 2:282–287.
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A new control method for Dynamic Voltage Restorer with
asymmetrical inverter legs based on fuzzy logic controller
H. Ezoji*, A. Sheikholeslami, M. Rezanezhad, H. Livani
Babol University of Technology, Department of Electrical and Computer Engineering, P.O. Box 484, Babol, Iran
a r t i c l ei n f o
Article history:
Received 10 July 2009
Received in revised form 18 November 2009
Accepted 30 January 2010
Available online 4 February 2010
Keywords:
Dynamic Voltage Restorer (DVR)
Asymmetrical voltagesource inverter
Fuzzy logic
Hysteresis controller
a b s t r a c t
Dynamic Voltage Restorer (DVR) is used in power distribution system to protect sensitive
loads in voltage disturbances. The performance of DVR is related to the adopted configura
tion and control strategy used for inverters. In this paper, an asymmetrical voltagesource
inverter controlled with fuzzy logic method based on hysteresis controller, is used to
improve operation of DVR to compensate voltage sag/swell. Simulation results using MAT
LAB/Simulink are presented to demonstrate the feasibility and the practicality of the pro
posed novel Dynamic Voltage Restorer topology. Total Harmonic Distortion (THD) is
calculated. The simulation results of new DVR presented in this paper, are found quite sat
isfactory to eliminate voltage sag/swell.
? 2010 Elsevier B.V. All rights reserved.
1. Introduction
Due to the advent of a large numbers of sophisticated electrical and electronic equipments, such as computers, program
mable logic, electrical drives etc., power quality problems like voltage sag, voltage swell and harmonic distortion can cause
serious problems to industrial and commercial electrical consumers [1,2].
For example, some special facilities are sensitive to voltage disturbances. Therefore, in such cases using compensator for
the sensitive loads is necessary. There are some solutions to these problems. Installation of Dynamic Voltage Restorer (DVR)
for sensitive loads can be considered as a solution [1–3].
DVR is a custom power device, which is connected to the load through a series transformer. To compensate voltage dis
turbances, series voltage is injected through the transformer by a voltagesource converter connected to dc power source
[1,2].
The first DVR was installed in North Carolina, for the rug manufacturing industry. Another was installed to provide service
to a large dairy food processing plant in Australia [4].
A DVR consists of a voltagesource inverter, a seriesconnected injection transformer, an inverter output filter, and an en
ergystorage device that is connected to the dc link [1–5].
The voltagesource converter is a power electronic device, which can generate a sinusoidal voltage with any required
magnitude, frequency and phase angle. This device employs insulated gate bipolar transistors (IGBT) as switches [5]. This
converter injects a dynamically controlled voltage in series with the supply voltage through the three singlephase trans
formers to correct the load voltage. The main functions of the injection transformer include voltage boost and electrical iso
lation [6]. The DC side of the converter is connected to a DC energystorage device. Energystorage devices, such as batteries
or superconducting magnetic energystorage systems (SMES) are required to provide active power to the load when voltage
1569190X/$  see front matter ? 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.simpat.2010.01.017
* Corresponding author. Tel./fax: +98 111 323 9214.
Email address: hadi.ezoji@gmail.com (H. Ezoji).
Simulation Modelling Practice and Theory 18 (2010) 806–819
Contents lists available at ScienceDirect
Simulation Modelling Practice and Theory
journal homepage: www.elsevier.com/locate/simpat
Page 2
sags occur [7]. In this paper, battery is used as a source of the DC voltage for the VSC. The output of the inverter (before the
transformer) is filtered by Passive filters in order to reject the switching harmonic components from the injected voltage [5].
A typical DVR connected to the distribution system is shown in Fig. 1.
Different control strategies were proposed for DVR. VoltageSpace Vector PWM was implemented in [8]. Estimation of
symmetrical components of voltage to control DVR is used in [9]. Hysteresis voltage control can be adopted to improve volt
age quality of sensitive loads [2,10].
In this paper, a DVR with a new inverter topology is presented to suppress the load harmonics and to compensate the
voltage disturbances. The adopted voltagesource inverter is based on an asymmetrical inverter leg to achieve five voltage
levels in output voltage. This inverter has less voltage harmonics generated on the ac terminal of the inverter compared with
twolevel PWM operation. In the adopted inverter, on the contrary of conventional inverter, no flying capacitor and clamped
diode are used in the circuit configuration. The adopted control scheme is fuzzy logic controller based on hysteresis method.
2. Proposed circuit configuration
VoltageSource Converter (VSC) is one of the main parts of DVR. Commonly, a symmetrical VSC with twolevel output
voltage is utilized in DVR. In this paper, a new asymmetrical VoltageSource Converter is proposed to improve the behavior
of DVR. The single–phase configuration of the proposed inverter for DVR is depicted in Fig. 2.
The voltage stress of power switches Sa2and S0
switches Sa1, S0
and b produce five levels in the out put inverter. If the voltage of two capacitors, Vc1and Vc2are equal, five voltage levels
(Vdc, Vdc/2, 0, ?Vdc, ?Vdc/2) are produced in the output of inverter [11].
a2is equal to half of the dc bus voltage and the voltage stress of active
bis equal to dc bus voltage. In high switching frequency, the power switches placed in arms a
a1, Sband S0
Fig. 1. Typical DVR circuit topology (singlephase representation).
Fig. 2. Adopted singlephase DVR based on asymmetrical inverter legs.
H. Ezoji et al./Simulation Modelling Practice and Theory 18 (2010) 806–819
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To produce the mentioned voltage levels in the output, the switches can be defined as follows:
Sxyþ S0
xy¼ 1
ð1Þ
Therefore, the equivalent circuit of converter can be presented as shown in Fig. 3.
Here gaand gbrepresent the switches in leg a and leg b. The ac side to neutral point voltages can be expressed as:
ma0¼gaðgaþ 1Þ
22
mb0¼gbðgbþ 1Þ
22
mc1?gaðga? 1Þ
mc1?gbðgb? 1Þ
mc1
ð2Þ
mc1
ð3Þ
Fig. 3. Equivalent circuit of the adopted DVR.
Fig. 4. Operating states of the adopted inverter: (a) state 1 (ga= 1, gb= ?1); (b) state 2 (ga= 0, gb= ?1); (c) state 3 (ga= ?1, gb= ?1); (d) state 4 (ga= 1,
gb= 1); (e) state 5 (ga= 0, gb= 1); (f) state 6 (ga= ?1, gb= 1).
808
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The ac terminal voltage Vabis expressed as:
mab¼ ma0?mb0¼ga? gb
If the voltage of capacitors c1and c2are equal, therefore, the voltage variation between two capacitor voltages is zero
(Dv = 0). Then, the Eq. (4) can be written as follows:
mab¼ ma0?mb0¼ga? gb
2
There are three possible values for switching function gaand two possible values for gb. Therefore, five different voltage lev
els, Vdc, Vdc/2, 0, ?Vdc/2 and ?Vdc, can be generated on the ac terminal voltage Vab[10]. Fig. 4 gives six valid operating states in
the adopted inverter to generate five different voltage levels on the ac side of inverter. During the positive mains voltage, the
operating states 1–3 are used to generate three voltage levels. Vdc,Vdc/2and 0, on the ac side to control the inverter. During
the negative mains voltage, the operating states 4–6 are selected to generate another three voltage levels, 0, ?Vdc/2and ?Vdc,
on the ac side of the inverter.
2
mdcþg2
a? g2
2
b
Dm
ð4Þ
mdc
ð5Þ
3. Conventional control strategy
The possibility of voltage sag compensation can be limited by a number of factors including finite DVR power rating, dif
ferent load conditions, and different types of voltage sag. Some loads are very sensitive to phase angle jump and others are
tolerant to phase angle jump. Therefore, the control strategy depends on the type of load characteristics. There are three dis
tinguishing methods to inject DVR compensating voltage, that is, presag compensation method, inphase compensation
method, and minimal energy method [12–14]. In this paper, the adopted control strategy is presag compensation to main
tain load voltage at pre fault value.
3.1. Presag compensation technique
Most nonlinear loads such as thyristorcontrolled loads which use the supply voltage phase angle as a set point are sen
sitive to phase jumps. To overcome this problem, this technique compensates the difference between the sagged and the pre
sag voltages by restoring the instantaneous voltages to the same phase and magnitude as the nominal presag voltage. The
drawback is the capacity limitation of energystorage device for the injection of real power.
Fig. 5 shows the singlephase vector diagrams of the presag compensation where Vs; VL; VDVR; and VL presagmean the
magnitudes of the voltage vectors that are explained in Fig. 5 and Eq. (3). In this method, the load voltage can be restored
ideally. When a fault occurs in other lines, the left hand side voltage of DVR, i.e., Vsdrops and the DVR injects a series voltage,
VDVRthrough the injection transformer as:
VDVR¼ VL? Vs
a ¼ tan?1
ð6Þ
ð7Þ
VpresagsinðdÞ
VpresagcosðdÞ ? Vsag
??
3.2. Inphase compensation technique
In inphase compensation technique shown in Fig. 6, the injected DVR voltage (VDVR) is in phase with measured supply
voltage (VS) regardless of the load current and the presag voltage.
Fig. 5. Presag compensation technique.
H. Ezoji et al./Simulation Modelling Practice and Theory 18 (2010) 806–819
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ILand u are load current and load power angle, respectively. The magnitude of VDVRis so that the magnitude of VLis 1 pu.
VDVR¼ 1 ? Vs
The advantage of this method is that the magnitude of the injected voltage is minimum. Therefore, for a given load cur
rent and voltage sag the apparent power of DVR is minimized.
ð8Þ
3.3. Minimal energy technique
Another existing control strategy is to use as much reactive power as possible to compensate the sag. Therefore, the DVR
voltage is controlled in such a way that the load current is in phase with the grid voltage after the sag. As long as the voltage
sag is quite shallow, it is possible to compensate sag with pure reactive power and therefore, the compensation time is not
limited. Fig. 7 shows the phasor diagram for the minimal energy control strategy. In this diagram, d,a are the angles of VLand
VDVR, respectively. In this case, a can be obtained as:
a ¼p
and the d is calculated by the following equation:
?
If the supply voltage parameters satisfy the following condition then the value of d is feasible.
2?u þ d
ð9Þ
d ¼ u ? cos?1
VL? cosðuÞ
Vs
?
ð10Þ
VL? cosðuÞ ? Vs
Inequality (11) means that the level of voltage sag is shallow sag. Therefore, injected active power of DVR is zero and the
optimuma is obtained from (9). If inequality (11) is not satisfied then level of voltage sag will be deep sag and injected active
power is not zero.
ð11Þ
Fig. 6. Inphase compensation technique.
Fig. 7. Minimal energy compensation technique.
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H. Ezoji et al./Simulation Modelling Practice and Theory 18 (2010) 806–819
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4. Proposed method
The main considerations for the control system of a DVR include: detection of the start and finish of the sag, voltage ref
erence generation, transient and steadystate control of the injected voltage, and protection of the system. The control sys
tem presented in Fig. 8 is used to control the DVR.
As it is shown in Fig. 8, Vsis the supply voltage used to detect voltage sag and VLis the load voltage which is used as a
feedback of the output voltage.
A new hysteresis voltage control using a user defined fuzzy logic controller in Matlab software is implemented to improve
the DVR performances in fault and abnormal conditions. The following sections, describe the controller unit of DVR in
detailed.
4.1. Voltage sag detection
The essential part for wellperformance of controller in DVR is the sag detection circuit. Voltage sag must be detected fast
and corrected with a minimum of false operations. The voltage sag detection method is based on Root Means Square (RMS)
of the error vector which allows detection of symmetrical and asymmetrical sags, as well as the associated phase jump. The
controller system is presented in Fig. 8.
The threephase supply voltage is transformed from abc to odq frame using Park transformation. Phase Locked Loop (PLL)
is used to track supply voltage phase. The park transformation matrix is shown as follows:
?
33
1ffiffi
h ¼ h0?
mdmqmo
½? ¼
ffiffiffi
2
3
r
cosðhÞ
sinðhÞ
p
cos h ?2p
sin h ?2p
1ffiffi
3
?
cos h ?4p
sin h ?4p
1ffiffi
3
??
????
22
p
2
p
2
64
3
75
ma
mb
mc
2
64
3
75
ð12Þ
Zt
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
0xtdt
jVsj ¼
V2
dþ V2
q
q
ð13Þ
Closed loop load voltage feedback is added, and is implemented in the odq frame in order to minimize any steadystate
error in the fundamental component [2,15,16].
When the grid voltage is normal, the DVR system is held in a null state to lower its losses. When voltage sag is detected,
the DVR switches into active mode to react as fast as possible to inject the required ac voltage. The injection voltage is also
generated according to the difference between the reference load voltage and the supply voltage and it is applied to the VSC
to produce the preferred voltage, using the Hysteresis Voltage Control based on fuzzy logic controller.
4.2. Hysteretic voltage control combined with fuzzy logic controller
There are different methods to produce the signals needed for VSC switching. This part presents a new control approach
which is based on hysteresis voltage control combined with fuzzy logic control method. Comparing with previous ap
proaches the proposed method is able to extend its control capability even to those operating conditions where linear control
techniques fail.
Fig. 8. Control structure of DVR.
H. Ezoji et al./Simulation Modelling Practice and Theory 18 (2010) 806–819
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4.2.1. Hysteretic voltage control
The hysteresis voltage control method is one of the several approaches which have been introduced to produce switching
signals.
A hysteresis band voltage control scheme composed of a hysteresis band around the reference voltage is shown in Fig. 9.
Threephase reference Voltages are obtained by subtraction of presag voltages from threephase detected voltages. This
method is based on the difference between the voltage produced by the converter and the reference voltage [2,17]. The
upper and lower bands are defined by hysteresis band width (HB). As long as the difference between reference voltage
and the produced converter voltage remains between the bands, the switching signal will not change. If the difference
reaches to the upper or (lower) bands, the signal causes the switch to turn off (turn on) [2,17].
The switching frequency of the hysteresis band voltage control method described above depends on how fast the voltage
changes from the upper limit of the hysteresis band to the lower limit of the hysteresis band, or vice versa. Therefore, the
switching frequency does not remain constant throughout the switching operation, but varies along with the voltage refer
ence wave form.
Fig. 9. Hysteresis band voltage control.
Fig. 10. Schematic of implemented method.
Fig. 11. Block diagram of fuzzy controller.
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H. Ezoji et al./Simulation Modelling Practice and Theory 18 (2010) 806–819
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The hysteresis band voltage control is characterized by unconditioned stability, very fast response, needless from any
information about system parameters and good accuracy On the other hand, the basic hysteresis technique also exhibits sev
eral undesirable features; such as uneven switching frequency that causes acoustic noise and difficulty in designing input
filters [2,18].
Table 1
Rule bases of voltage fuzzy controller.
NLNSZ PSPL
N
Z
P
Sa1, Sa2, S0
Sa1, Sa2, S0
Sa1, Sa2, S0
b
Sa1, S0
Sa1, S0
Sa1, Sa2, Sb
a2, S0
a2, S0
b
Sa1, Sa2, Sb
Sa1, Sa2, Sb
Sa1, Sa2, Sb
Sa1, Sa2, Sb
Sa1, S0
Sa1, S0
S0
S0
S0
a1, Sb
a1, Sb
a1, Sb
bba2, Sb
a2, Sb
b
Fig. 12. Input and output membership functions of voltage controller.
H. Ezoji et al./Simulation Modelling Practice and Theory 18 (2010) 806–819
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4.2.2. Fuzzy logic controller
The switching frequency of hysteresis control method with constant band width is high. This will make the system loss
considerable. In order to produce a five level voltage at the converters output, to reduce the switching losses and to improve
the behavior of DVR, fuzzy controller method is utilized. Fuzzy logic control (FLC) is based on mamdani’s system.
The fuzzy controller has two inputs:
? The difference between the injected voltage and the reference voltage.
? The derivation of the error.
Moreover it has six outputs, the driving signals of switches. Considering the difference between converter output voltage
and reference voltage and its derivation, the controller determines the voltage condition and directly commands the
switches to turn on or off. In conventional hysteresis voltage control, switching signals are determined when the error
reaches to upper or lower hysteresis band but as it is shown in Fig. 9, in this new proposed method, switching commands
are determined according to the error and derivation of the error. The schematic of implemented method is shown in Fig. 10.
The fuzzy logic controller consists of three stages: the fuzzification, rule execution, and defuzzification. In the first stage,
the crisp variables e(k) and de(k) are converted into fuzzy variables E(k) and dE(k) using the triangular membership functions
shown in Fig. 11. Triangular membership functions are chosen to have smooth and constant region in the main points.
E(k) is divided into five fuzzy sets: NL (negative large), NS (negative small), ZE (zero), PS (positive small) and PL (positive
large); and dE(k) is divided into three fuzzy sets: N (negative), ZE (zero), P (positive).
In the second stage of the FLC, the fuzzy variables E and dE are processed by an inference engine that executes a set of
control rules contained in (10) rule bases. The control rules are formulated using the knowledge of the DVR behavior. The
rules are expressed in Table 1:
Different inference algorithms can be used to produce the fuzzy set values for the output fuzzy variables
Sa1; S0
to the maximum of the product of E and dE membership degree.
The inference engine output variables are converted into the crisp values in the defuzzification stage. Various defuzzifi
cation algorithms have been proposed in the literature. In this paper, the centroid defuzzification algorithm is used, in which
the crisp value is calculated as the centre of gravity of the membership function. Fig 12 shows inputs and output member
ship functions.
The definition of the spread of each partition, or conversely the width and symmetry of the membership functions, is gen
erally a compromise between dynamic and steadystate accuracy.
a1; Sa2; S0
a2; Sb; S0
b. In this paper, the max–min inference algorithm is used, in which the membership degree is equal
5. Simulation results
To prove the capabilities of the abovementioned control methods, the test system is modeled with MATLAB/Simulink
(ver. 7) and SimPowerSystem block set. Total Harmonic Distortion (THD) is also calculated to verify the efficiency and
wellperformance of the proposed control method. The supply network is modeled as an ideal voltage source; the injection
transformer has been modeled as a linear element. The transformer winding’s resistances and core saturation effect were
neglected. The inverter is modeled as a typical twopulse inverter with transistors assumed to be ideal switches. Losses
in the inverter were modeled as a resistance connected to the DC side capacitor. The rest of the components have been as
sumed to be ideal ones, and standard SimPower System block set elements were used. The parameters of the case study are
presented in Tables 2 and 3. In the simulated model, the following abnormal conditions are considered:
Table 2
Case study parameters.
ParameterValue
Supply voltage (VL–L)
Vdc, Cf, Rf
Series transformer (VPh–Ph)
ZTrans
RLoad,LLoad
400 V
200 V, 500 lF, 1 O
96/240 V
0.004 + j 0.008
31.84 O, 0.139 H
Table 3
Parameters of induction motor.
ParameterValue
Rated voltage
Nominal supply frequency
Rated out put power
Power factor
Nominal speed, no. of poles
380 V
50 Hz
3 kW
0.82
1430 rpm, 4
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5.1. Resistive load and balanced voltage sag
The first simulation is carried out for a balanced voltage sag and linear resistive load (85 X). The PCC voltage drops to 70%
of its nominal value from 0.1 to 1.8 s as shown in Fig. 13.
The DVR injected voltages and load voltages are shown in Fig. 13a and b.
Fig. 13. Simulation result of DVR response for a balanced voltage sag and linear resistive load.
H. Ezoji et al./Simulation Modelling Practice and Theory 18 (2010) 806–819
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As it can be seen from the results, the DVR is able to produce the required voltage components for different
phases rapidly and help to maintain a balanced and constant load voltage at the nominal value (400 V) during fault
condition.
Fig. 14. Simulation result of DVR response for a balanced voltage sag and induction motor load.
816
H. Ezoji et al./Simulation Modelling Practice and Theory 18 (2010) 806–819
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5.2. Induction motor load and balanced voltage sag
The parameters of the induction motor are listed in Table 3. Similar to the previous case, PCC voltage drops to 30% of its
voltage nominal value from 0.1 s and it is kept until 0.18 s. The PCC sag and DVR injected voltages are shown in Fig. 14. It can
be observed that with starting of induction motors load voltage drops to 30% of its voltage nominal value. The DVR would
inject the compensating voltage immediately after PCC voltage sag is detected; to maintain load voltages at desired level.
Fig. 15. Simulation result of DVR response for a balanced voltage sag and nonlinear load.
H. Ezoji et al./Simulation Modelling Practice and Theory 18 (2010) 806–819
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5.3. Nonlinear load and balanced voltage sag
In this case, the nonlinear load is a diode rectifier bridge with a capacitor bank (200 lF) and resistive load (80 X) con
nected in parallel. Again, the PCC voltage drops to 70% of its nominal value from 0.1 s and lasts for four cycles. The PCC volt
ages and DVR injected voltages are shown in Fig. 15. It can be observed that with nonlinear load connected at downstream,
the PCC voltages, DVR injected voltages, and load voltages become slightly distorted.
Fig. 16. Simulation result of DVR response for a unbalanced voltage sag and linear load.
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As it can be observed from simulation results, DVR is capable to detect the voltage sag quickly and compensate the load
voltage satisfactorily.
5.4. Unbalanced voltage sag
In this case, there is a 30% threephase voltage sag with +30? phase jump in phasea. Voltage is started at t = 0.1 s and it is
kept until 0.18 s. Fig. 16 shows the result of voltage sag compensation using hysteresis voltage control based on fuzzy con
troller. As it can be seen from the results, DVR is able to produce the required voltage for different phases rapidly and a bal
anced and constant load voltage at the nominal value (400 V) is provided.
The calculated THD in all simulation case studies are described in Table 4. According to IEEE standard, the THD in distri
bution networks should be under 5% and as it can be seen in Table 4, the calculated THD satisfy the IEEE standard range.
6. Conclusion
This paper investigates a new control approach which is based on hysteresis voltage control combined with fuzzy logic
control method. All parameters and structures such as study system, and control unit are described in details. The validity of
proposed method is approved by results of the simulation in MATLAB/Simulink for different voltage sag condition. As it can
be seen, the new model of DVR with the presented control method is capable to compensate networks faults and mitigate
their effects on sensitive loads in distribution power systems.
THD is also calculated to evaluate the quality of the load voltage during the operation of DVR. The simulation results show
that the calculated THD in sag conditions fulfill IEEE 519 std. range. The effectiveness of the proposed DVR controller in
rejecting load voltage disturbance is proved by the good performance of the DVR under different loading conditions.
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Table 4
THD for load voltage in all simulation case studies.
Simulation case studies THD (%)
Resistive load and balanced voltage sag
Induction motor load and balanced voltage sag
Nonlinear load and balanced voltage sag
Unbalanced voltage sag
0.12
0.2
0.17
0.16
H. Ezoji et al./Simulation Modelling Practice and Theory 18 (2010) 806–819
819