# 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 voltage-source 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.

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**ABSTRACT:**This paper presents a single stage transformer-less grid-connected solar photovoltaic (PV) system with an active and reactive power control. In the absence of active input power, the grid-tied voltage source converter (VSC) is operated in a reactive power generation mode, which powers the control circuitry, and maintains a regulated DC voltage to the VSC. A data-based maximum power point tracking (MPPT) control scheme which performs power quality control at a maximum power by reducing the total harmonic distortion (THD) in grid injected current as per IEEE-519/1547 standards is implemented. A proportional-integral (PI) controller based dynamic voltage restorer (DVR) control scheme is implemented which controls the grid side converter during single-phase to ground fault. The analysis includes the grid current THD along with the corresponding variation of the active and reactive power during the fault condition. The MPPT tracks the actual variable DC link voltage while deriving the maximum power from the solar PV array, and maintains the DC link voltage constant by changing the modulation index of the VSC. Simulation results using Matlab/Simulink are presented to demonstrate the feasibility and validations of the proposed novel MPPT and DVR control systems under different environmental conditions.Frontiers in Energy. 06/2014; 8(2):240-253. - SourceAvailable from: up.shamsipour-ac.ir[Show abstract] [Hide abstract]

**ABSTRACT:**In this paper the control scheme of a three-level three-phase inverter for grid integration of distributed generation (DG) units is investigated. Furthermore, the inverter compensates the reactive power of the load. The presented control scheme is based on instantaneous power theory in order to generate the appropriate inverter reference currents. In the outer DC bus voltage control loop and in the inner current control loop, the fuzzy logic (FL) control and the Proportional-Resonant (PR) control is applied respectively. The switching pattern generation is achieved using Space Vector Pulse Width Modulation technique. The steady state and dynamic response of the proposed electric power system has been evaluated under various operating conditions. Simulation results verify the effectiveness of the proposed control system.Simulation Modelling Practice and Theory 06/2012; 25. · 1.05 Impact Factor - International Journal of Electrical Power & Energy Systems 12/2012; 43(1):573-581. · 3.43 Impact Factor

Page 1

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 voltage-source 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 voltage-source

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 voltage-source 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 voltage-source inverter, a series-connected injection transformer, an inverter output filter, and an en-

ergy-storage device that is connected to the dc link [1–5].

The voltage-source 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 single-phase 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 energy-storage device. Energy-storage devices, such as batteries

or super-conducting magnetic energy-storage systems (SMES) are required to provide active power to the load when voltage

1569-190X/$ - 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.

E-mail 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. Voltage-Space 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 voltage-source 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

two-level 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

Voltage-Source Converter (VSC) is one of the main parts of DVR. Commonly, a symmetrical VSC with two-level output

voltage is utilized in DVR. In this paper, a new asymmetrical Voltage-Source 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 (single-phase representation).

Fig. 2. Adopted single-phase 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

H. Ezoji et al./Simulation Modelling Practice and Theory 18 (2010) 806–819

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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, pre-sag compensation method, in-phase compensation

method, and minimal energy method [12–14]. In this paper, the adopted control strategy is pre-sag compensation to main-

tain load voltage at pre fault value.

3.1. Pre-sag compensation technique

Most nonlinear loads such as thyristor-controlled 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 pre-sag voltage. The

drawback is the capacity limitation of energy-storage device for the injection of real power.

Fig. 5 shows the single-phase vector diagrams of the pre-sag compensation where Vs; VL; VDVR; and VL pre-sagmean 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

Vpre-sagsinðdÞ

Vpre-sagcosðdÞ ? Vsag

??

3.2. In-phase compensation technique

In in-phase 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 pre-sag voltage.

Fig. 5. Pre-sag 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. In-phase 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 steady-state 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 well-performance 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 three-phase 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 o-d-q frame in order to minimize any steady-state

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.

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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.

Three-phase reference Voltages are obtained by subtraction of pre-sag voltages from three-phase 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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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.

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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 steady-state 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 above-mentioned control methods, the test system is modeled with MATLAB/Simulink

(ver. 7) and SimPower-System block set. Total Harmonic Distortion (THD) is also calculated to verify the efficiency and

well-performance 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 two-pulse 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.

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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.

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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.

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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% three-phase voltage sag with +30? phase jump in phase-a. 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