dynamic voltage restorer system for power quality … · appearing and disappearing arbitrarily....

M. Balamurugan et al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945

Int J Adv Engg Tech/Vol. VII/Issue III/July-Sept.,2016/108-113

Research Paper DYNAMIC VOLTAGE RESTORER SYSTEM FOR POWER

QUALITY IMPROVEMENT M. Balamurugan1, T.S. Sivakumaran2

Address for Correspondence

1Research Scholar, Department of EEE, Anna University, Chennai, Tamil Nadu, India. 2Professor and Dean, PG Studies, Arunai College of Engineering, Tiruvannamalai, Tamil Nadu, India.

ABSTRACT Dynamic Voltage Restorers (DVRs) are tied to the power grid through power-frequency transformers. In this study, the design of Diode Clamped Multilevel Inverter based dynamic voltage restorer (DVR) is presented and offered to perform the fast fault detection. A novel control method for DVR is aimed with a carrier modulated PWM inverter. The proposed control system is simple to design and has excellent voltage compensation capabilities. The suggested method for voltage sag/swell detection can detect different forms of power disturbances faster than conventional detection methods. To obtain acceptable properties such as transient overshoot, setting time, and steady-state error. By this, it is expected that the result of disruptions on the output of proposed DVR could be contracted for both symmetrical and asymmetrical voltage sag conditions using MATLAB/ Simulink software. To reduce power interruptions, this study offers a new Zeta converter based DVR system. This proposed strategy can quickly access the voltage sag and swell under transient condition. KEYWORDS: Dynamic Voltage Restorer, Power Quality, Voltage Sag/Swell, Zeta Converter, Multilevel Inverter

1. INTRODUCTION Power quality is the measure, analysis, and change of a load bus voltage, to keep up that voltage to be sinusoidal at evaluated voltage and frequency. The phenomena associated with power quality are real stochastic events in the sense that in many cases are appearing and disappearing arbitrarily. Thus, the power quality measure is more than a mere measurement of an electrical parameter; i.e. It is necessary to record them over an enough long time interval. To reduce the enormous amount of data by recording and to analyze several electrical parameters over an extended period, some recording limits are set. If these limits are exceeded, the monitoring instruments evaluate the sensitive data and record just the substantive data of the applicable issues. There are different reasons for monitoring power quality. The most significant cause is the economic damage created by electromagnetic phenomena in critical process loads. Effects on equipment and process operations can include malfunctions, damage, process disruption, and other anomalies.

With distribution power systems, the main financial losses Moreover, power quality problems are connected with voltage sag [1], [2]. Recently, energetic voltage restorer (DVR) systems have demonstrated promise in providing a monetary value-effective resolution for low-voltage (LV) systems [3]. Since insulated gate bipolar transistors have progressed significantly, and at the same time, the cost and physical size of the converters in the DVR structures are reduced [4], [5]. On the other hand, uninterruptible power supplies, which are seen as an alternative to regulating and even off the load voltage, have become very costly [6], [7]. Conventional DVRs are connected to the utility grid through bulky power-frequency transformers. This connection has some demerits such as high cost, large size, relatively high losses, saturation, and start up inrush current [8]–[10]. Besides, the resistance of the transformer causes a voltage drop and increases load voltage harmonics, especially when the load is nonlinear [9]. To reduce the cost and size of the DVRs in LV systems, transformerless DVRs have been proposed [8]. This approach not only adds to the number of switches and needs isolated capacitors for each phase in a three-phase system but also charging circuitry should implement for each phase. These problems cast doubt over the cost-efficient aspect and

operation enhancement of the traditional structures. As a solution, a current application for DVR system, where load voltage harmonics are also compensated, has been proposed [9]. Thus, effective usage of the series transformer is obtained [9]. Because this DVR system rejects the fifth and seventh voltage order harmonics, high-frequency harmonics should pass through the grid-connected transformer. This approach did not consider the passband limitation of the grid-connected transformer for high-frequency harmonics. Also, high-order harmonics increase losses in the grid-connected transformer. In [10], an H-bridge cascaded multilevel inverter has been implemented to enhance the voltage level without using a transformer. This topology is relatively complex, and the number of power switches is considerable. So, in this topology, cost increases and reliability decreases. A multilevel cascaded-type DVR, able to connect to the grid without a transformer, is proposed in [11]. The multilevel DVR can support a spread sag duration compared to the two-level DVR. Also, harmonics are studied, regarding the number of voltage levels produced by the DVR system. It has been shown that reduced switching losses at the cost of increasing the number of switches and their associated drive and control systems are obtained. This approach cannot address the requirements of LV application, where low-cost, small-size, and low-weight systems are required. Recently, an interphase AC–AC topology for single-phase sag compensation has been proposed in [12]. This cost-efficient topology enjoys small size since it has no storage device. However, it cannot compensate symmetrical voltage sags. The concept of high-frequency-link DVR for a single-phase system has been proposed [13], [14]. The incorporation of the transformer into the converter brings significant improvements such as substantial saving in size and weight. Therefore, the cost may also be reduced. So, this DVR is suitable for protection of devices such as computers and medical equipment [7]. Further investigations of this approach have been developed in [15] to compensate voltage sag in a three-phase system with a peak voltage of about 175 V. A suitable control strategy has been designed to avoid hard switching, especially in cycloconverter. Unfortunately, the output of each phase cannot be controlled. A new concept of Diode Clamped Multilevel Inverter combined with Zeta



converter formed a DVR is proposed in this paper. The performance of DVR depends on the efficiency of control technique of switching the multilevel inverter and zeta converter. In this article, PI controller and Multicarrier PWM control method are used to compensate voltage sag/swell. 2. DYNAMIC VOLTAGE RESTORER

The series voltage controller is connected in series with the protected load as shown in Fig.1. Normally the connection is established via a transformer, but forms with direct connection via power electronics as well exist. The resulting potential difference at the load bus bar equals the totality of the grid voltage and the injected voltage from the DVR.

Filter

7 Level DCMLI

Zetaconverter

Diode Bridge Rectifier

Controlunit

Load

D

Vactual

DVR

V DVRI

ZL

Supply

Vref

Injection Transformer

Vs

Fig.1 Schematic representation of combined zeta converter and DCMLI based DVR control system The circuit on the left-hand side of the DVR

represents the Thevenin equivalent circuit of the system. The system impedance Zth depends on the fault level of the load bus. When the system voltage (Vth) drops, the DVR injects a series voltage VDVR through the injection transformer so that the desired load voltage magnitude VL can be sustained. The series injected voltage of the DVR can be written as,

thLthLDVR VIZVV (1)

Where, VL is the desired load voltage Zth is the load impedance IL is the load current Vth is the system voltage during fault

condition The load current IL is

L

LLL

V

QJPI (2)

The main use of the DVR is to compensate voltage sags and swells apart performing the chores such as harmonic compensation, reduction of transient in voltage and fault current limitation [16-17]. DVR is a series connected custom power device. Its primary purpose is the protection of sensitive loads from any voltage disturbances except voltage output. The proposed DVR system consists of an injection transformer, zeta converter, filter, voltage source converter and control & protection system. Fig.1 shows the formal representation of proposed zeta converter based DVR control system. 2.1 Zeta Converter A zeta converter is a fourth order nonlinear system being that, about energy input, it can seem as a buck-boost-buck converter and about the output, it can be seen as a boost-buck-boost converter. Similar to the SEPIC DC/DC converter topology, the ZETA converter topology provides a positive output voltage from an input voltage that varies above and below the output voltage. The ZETA converter also needs two inductors and a series capacitor sometimes called a flying capacitor. Unlike the SEPIC converter, which is configured with a standard boost converter, the ZETA converter is set up from a buck controller that drives a high-side NMOSFET. Zeta converters

usually have a high transfer voltage gain and also produce high insulation on both sides. The gain of the Zeta converters always depends on the transformer’s turn ratio N, which can be thousand times. The zeta converter is a transformer based converter with a low-pass filter. Its output voltage ripple value is small [18-19]. The circuit diagram of zeta converter is shown in Fig.2.

L0

Cf

S1 C1

C0D

+

-

P

N

1 2

Tx

Fig.2. Circuit diagram of zeta converter The output voltage is given by,

inNVk1

koV

(3)

Where N is the turn ratio of the transformer, and k is the conduction duty cycle k= ton/T. 2.2 Diode Clamped Multilevel Inverter The general structure of the multilevel inverter is to synthesize a sinusoidal voltage from several levels of voltages typically obtained from capacitor voltage sources. Multilevel inverters are being considered for an increasing number of applications due to their high power capability associated with lower output harmonics and lower commutation losses. Multilevel inverters have become an efficient and practical solution for increasing power and reducing harmonics of AC load. The most commonly used multilevel topology is the diode clamped inverter, in which the diode is used as the clamping device to clamp the dc bus voltage so as to achieve steps in the output voltage. Thus, the main concept of this inverter is to use diodes to limit the power devices voltage stress. The voltage over each capacitor and each switch is Vdc. An n level inverter needs (n-1) voltage sources, 2(n-1) switching devices and (n-1) (n-2) diodes. By increasing the number of voltage levels, the quality of the output voltage is improved, and the voltage waveform becomes closer to a sinusoidal waveform. Fig. 3 shows the Simulink model of seven level Diode Clamped Inverter.



g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

++

++

g DS

g DS

++

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

g DS

Fig. 3 Simulink model of seven level Diode Clamped Inverter

2.3 Injection Transformer Its basic purpose is to step up the AC low

voltage supplied by the inverter to the required voltage. In the case of three phase DVR type, three single phase injection transformers are commonly employed. The maximum voltage sag the DVR can compensate depends mainly on the rating of the inverter and the injection transformer. The purpose of the injection transformer is to inject the voltage supplied by the filtered zeta converter based VSI [20]. The injection transformer winding ratio will increase the primary side current, depending upon the functioning of the zeta converter based inverter. 2.4 Harmonic Filter

A passive low pass filter consists of an inductor and capacitance. It can be located either on the high voltage side or the inverter side of the injection transformer. It is applied to strain out the switching harmonic components of the injected voltage. By putting the filter on the inverter side, the higher order harmonics are prevented from infiltrating into the transformer; thereby it reduces the voltage stress on the injection transformer. When the filter is located on the high voltage side, since harmonics can penetrate into the high potential side of the transformer, a higher rating transformer is needed. The role of the filter is to strain out the self-generated harmonics generated by DVR, which is composed of power electronic devices. The primary use of the harmonic filter is to preserve the harmonic voltage content to an acceptable level, which is produced by the zeta converter fed multilevel inverter. 2.5 Controller

A proportional-integral (PI) controller drives the plant to be controlled with a weighted sum of the error ( the difference between the actual sensed output and desired set-point) and the integral of that value. An advantage of a proportional plus integral controller is that its integral term causes the steady-state error to be zero for a step input. The PI controller input is an actuating signal which is the difference between VL* and VL. The load voltage magnitude is compared with a reference voltage. Duty cycle control technique is applied for zeta converter switching to generate a DC voltage equivalent to three phase quantity which is to be fed through voltage source inverter at the load ends. The input voltage of zeta converter is taken out from the transmission line and run through the diode bridge rectifier. PI controller is used with zeta converter to

keep the electric potential at the load terminals [21-22]. 3. Pulse Width Modulation Control In the proposed DCMLI topology, a level based multicarrier PWM strategy is implemented to firing the gate terminals of the MOSFET to obtain the current waveform of seven-level diode clamped inverter. Multicarrier PWM strategy is a comparison of a reference waveform, with vertically shifted carrier signals. In multicarrier PWM technique, m-1 triangular carriers are used for m level inverter output voltage or current. In this proposed seven-level topology six triangular carriers are applied. The carrier waveforms have same amplitude AC and frequency fC. Similarly, the reference waveforms have frequency fref and an amplitude Aref. At every instant, the response of the comparison is decoded to generate the correct switching sequences on the output of the inverter. The frequency modulation index (mf) and amplitude modulation index (ma) are calculated in equation (1) and (2) [23-25].

ref

cf

f

fm (4)

c

refa

A)1m(

Am

(5)

In the level shifted Multicarrier PWM Phase Disposition (PD), Phase Opposite Disposition (POD), Alternative Phase Disposition (APOD) strategies are used. 3.1 Phase Disposition (PD)

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.050

1

2

3

4

5

6

Time (S)

Re

f, C

arr

ier

Fig. 4 Multicarrier PWM waveform (PD

waveform Phase Disposition (PD), where all triangular carriers

above and below the zero reference are in phase. Fig. 3 shows the responses of PD with sine reference and Fig. 4 shows the responses of PD with sinusoidal reference. The responses of this strategy shown for ma = 0.9, mf = 40.



4. DVR COMPENSATION Fig.5 shows the operation of the DVR that injects a controlled voltage generated by the zeta converter fed seven level diode clamped inverter in series to the system voltage using an injecting transformer. The zeta converter regulates the DC voltage by varying duty ratio control using PI controller.

L0

Cf

S1C1

C0D

+

-

P

N

1 2

Tx

AC Main

Injection Transformer

DCMLI

Zeta converterDiode Bridge

Rec tifie r

Load

-1.5

-1

-0.5

0

0.5

1

1.5

-1

-0.5

0

0.5

1

Fig.5. Compensation principle of zeta converter based DVR system

Seven level diode clamped inverter used to convert the DC voltage to AC voltage with the elimination of harmonics to compensate the power quality disturbances, such as voltage sag and swell. During normal operating condition, the DVR injects very low voltage to make up for the voltage drop in the injection transformer and device losses and drop. When sag/swell occurs in the distribution system [26], the combined zeta converter and DCMLI based DVR system either injects/absorbs required control voltage to preserve output voltage to the load side.

The switching pattern of the seven level DCMLI can be inverted during sag/swell conditions. The DVR system is capable of generating or absorbing reactive power. In the proposed system the active power injection of the DVR must be provided by combined zeta converter and DCMLI, which is connected to the same system. The response time of DVR is very small, and it is limited by the power electronics devices apart from the voltage sag/swell detection time. 5. SIMULATION RESULTS AND

DISCUSSION MATLAB software is employed for obtaining the simulations. A three-phase, power distribution system configuration is simulated using Math Works Matlab/Simulink to study the effectiveness and response of suggested DVR control strategy undersupply disturbances (Line-ground fault). Combined zeta converter and DCMLI based DVR is connected in series with a line for compensation. The PI controller controls Zeta converter by varying the duty ratio. Here DVR system is connected in series to the distribution network by using an injection transformer. The DVR operation is based on three phase voltage source inverter with LC output filter to remove high-frequency voltage components. An inductive nature R-L load (R=1KΩ, L =1microh) is considered for a rating of the proposed scheme. Fig.6 shows the Simulink model of the proposed system and combined zeta converter and DCMLI based DVR. The simulation results are presented to prove the possibility of the proposed system.

Ra+

C1

+

RL

[iin]

vin

vout

[iout]

[idvra] -T-

v+-

v+-

v+-

Discrete,Ts = 5e-05 s.

powergui

Ph A

Tx

i+ -

i+ -

In1Out1

Ref

Ph BPh C

+

RL1

+

RL2

Rb Rc

i+ -

i+-

v+-

v+-

vout2

vout3

[iin3]

[iin2]i+ -

i+ -

[iout2]

[iout3]

v+-

v+-

vin2

vin3

[idvrb] -T-v+-

Tx1

+

C2

[idvrc] [vdvrc]v+-

Tx2

+

C3

Out2

Subsystem

Measurement

vin

vout

-T-

-T-

Timer

In1Out1

Ref1In1Out1

Ref2

ref

ref1

Out1

Out2

+

-

R

Y

B

N

Subsystem2

In1Out1

Ref3

Controlled

Voltage Source

|u|

Abs

>=

Switch

2.5

sag

-2.5

swell

-K-

Gain2 Fig. 6 Overall Simulink representation of combined zeta converter and DCMLI based DVR

0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-250

-200

-150

-100

-50

0

50

100

150

200

250

Time (S)

Vol

tage

in V

olts

Fig. 7 Simulated seven level DCMLI output voltage



0 0.05 0.1 0.15 0.2 0.25 0.3-300

-200

-100

0

100

200

300

Time (S)

Inpu

t Vo

ltage (

V)

Fig. 8 Simulated input voltage with presence of swell condition

0 0.05 0.1 0.15 0.2 0.25 0.3-300

-200

-100

0

100

200

300

Time (S)

Lo

ad

Vo

ltag

e (

V)

Fig. 9 Simulated load voltage compensated during swell condition

0 0.05 0.1 0.15 0.2 0.25 0.3-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

Time (S)

Inpu

t Curr

en

t (A

)

Fig. 10 Simulated input current with presence of swell condition

0 0.05 0.1 0.15 0.2 0.25 0.3-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

Time (S)

Lo

ad

Cu

rre

nt (

A)

Fig. 11 Simulated load current compensated during swell condition

0.2 0.25 0.3 0.35 0.4 0.45 0.5-300

-200

-100

0

100

200

300

Time (S)

Inp

ut V

olta

ge

(V

)

Fig. 12 Simulated input voltage with presence of sag condition



0.2 0.25 0.3 0.35 0.4 0.45 0.5-300

-200

-100

0

100

200

300

Time (S)

Lo

ad

Vo

ltag

e (

V)

Fig. 13 Simulated load voltage compensated during sag condition

0.2 0.25 0.3 0.35 0.4 0.45 0.5-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

Time (S)

Inp

ut C

urr

ent (A

)

Fig. 14 Simulated input current with presence of sag condition

0.2 0.25 0.3 0.35 0.4 0.45 0.5-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

Time (S)

Lo

ad

Cu

rre

nt (A

)

Fig. 15 Simulated load current compensated during sag condition

0 0.1 0.2 0.3 0.4 0.5-80

-60

-40

-20

0

20

40

60

80

Co

mp

en

satio

n V

olta

ge

(V

)

0 0.1 0.2 0.3 0.4 0.5-0.4

-0.2

0

0.2

0.4

Time (S)

Co

mp

en

satio

n C

urr

en

t (A

)

Fig. 16 Compensated voltage and current generated by DVR for R phase



0 0.1 0.2 0.3 0.4 0.5-80

-60

-40

-20

0

20

40

60

80

Com

pe

nsa

tion

Voltag

e (

V)

0 0.1 0.2 0.3 0.4 0.5-0.4

-0.2

0

0.2

0.4

Time (S)

Com

pe

nsa

tion

Curr

en

t (A

)

Fig. 17 Compensated voltage and current produced by DVR for Y phase

0 0.1 0.2 0.3 0.4 0.5-80

-60

-40

-20

0

20

40

60

80

Co

mp

en

satio

n V

olta

ge

(V

)

0 0.1 0.2 0.3 0.4 0.5-0.4

-0.2

0

0.2

0.4

Time (S)

Co

mp

en

satio

n C

urr

en

t (A

)

Fig. 18 Compensated voltage and current produced by DVR for B phase

Fig. 7 shows the simulated seven level DCMLI output voltage. From the figure 7, it is clear that the diode clamped inverter produced seven level output waveform. Fig. 8 shows the simulated input voltage waveform with the presence of a swell condition, the swell occurred at the period of 0.1 sec to 0.2 sec. Fig. 9 shows the simulated waveform of load voltage compensated during swell condition. The swelled voltage 2.5V is compensated by the DVR system combined with Zeta converter and DCMLI. Similarly, the current waveform during swell condition and compensated current waveform during swell condition are shown in Fig. 10 and 11 respectively. Fig. 12 shows the simulated input voltage waveform with the presence of sag condition; the sag occurred at the period of 0.3 sec to 0.4 sec. Fig. 13 shows the simulated waveform of load voltage compensated during sag condition. The sag

voltage 2.5V is compensated by the DVR system combined with Zeta converter and DCMLI. Similarly, the current waveform during sag condition and compensated current waveform during sag condition are shown in Fig. 14 and 15 respectively. Fig. 16 shows the compensated voltage and current generated by DVR for R phase. Similarly, the compensated voltage and current produced by DVR for Y phase and B phase are shown in Fig.17 and 18 respectively. 6. CONCLUSION In this work, a fast response and cost effective based Dynamic Voltage Restorer (DVR) is proposed for compensating the problems of voltage sag and swell condition in distribution systems, the effectiveness of DVR using PI controller and Multicarrier PWM strategy is established for the nonlinear load. The response of the zeta converter at DC side and the



performance of seven level diode clamped inverter at AC voltage side are performed well. The converted voltage is properly injected into the power system through injection transformer. Other kinds of controllers like the fuzzy controller and adaptive PI-fuzzy controller may be employed in the DVR compensation scheme in future. REFERENCES

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dynamic voltage restorer system for power quality … · appearing and disappearing arbitrarily....

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