dvr springer

7/27/2019 Dvr Springer

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Six Leg DVR Topology for Compensation of Balanced Linear

Loads in Three Phase Four wire SystemPradeep Kumar Niranjan Kumar A.K.Akella

[email protected] [email protected] [email protected]

+91-9905205302 +91-8002857194 +91-9431952816

Department of Electrical Engineering, National Institute of Technology Jamshedpur, Jharkhand 831014(India)

Abstract - This paper presents Dynamic Voltage Restorer (DVR) as a new compensation scheme for power quality

improvements in terms of power factor correction and harmonic mitigation for a three-phase four-wire distribution

system, supplying a star (Y) connected balanced R-L load. The Dynamic Voltage Restorer (DVR) is a custom power

device used to protect sensitive loads in power distribution systems from the most frequent voltage disturbances,

such as sags, swells, imbalances, harmonics and low power factor. Indirect Proportional Integral Controller (IPIC)

and Synchronous Reference Frame Controller (SRFC) are used to generate switching patterns for the Pulse Width

Modulated (PWM) controlled Voltage Source Converter (VSC). The DVR system is implemented through extensive

simulation using MATLAB software with its Simulink and Power System Blockset (PSB) toolboxes. The proposed

Synchronous Reference Frame Controller (SRFC) is compared with the Indirect Proportional Integral controller

(IPIC) scheme and the superior features of this novel approach are established in this research work.

Keywords

Voltage source converter (VSC); Dynamic voltage restorer (DVR); Power quality (PQ); Custom power devices

(CPD); Synchronous reference frame controller (SRFC); Indirect proportional integral controller (IPIC).

1. Introduction

Industrial and commercial consumers of electrical power are becoming increasingly sensitive to the quality of

electrical power supply. When interruptions occur due to poor power quality problems, costs, including downtime,

defects, and loss of production may occur. This would result in losses, in term of money to both commercial and

domestic consumers. Therefore, the study of power quality problem is becoming a popular research topic. It has

already been shown that for customers of large loads (from a high kVA to the low MVA range) good solutions are the

installation of custom power devices (Ghosh and Ledwich,2002;Hingorani,1995).Dynamic voltage restorer (DVR) isone of the custom power devices for the power factor correction and harmonic mitigation. The DVR injects three-

phase compensating voltages in series to the power line through a three-phase series transformer or three single-phase

series transformers. The energy required for the compensation is taken from the dc capacitor (Al-Hadidi et al. 2008;

Gole et al. 2008) or another energy-storage element such as a double-layer capacitor, a superconducting magnet

(Lamoree et al. 1994) , or a lead-acid battery (Ramachandaramurthy et al. 2002) . Otherwise it may be taken from the

power system by the shunt converter (Wang and Venkataramanan, 2009; Saleh et al. 2008). There are many control

techniques that are used to compute the command voltage component, such as in (Vilathgamuwa et al. 2002) showed

the open loop control strategy, closed-loop control (Etxeberria-Otadui et al. 2002; Ghosh and Joshi, 2002; Liu et al.

2002) , multi-loop feedback control (Nielsen et al. 2001), selective harmonic closed loop control (Newman et al.

2003) and vector control (Awad et al. 2003). This paper proposes the Indirect Proportional Integral and Synchronous

Reference Frame control scheme for improving dynamic performance of dynamic voltage restorer (DVR) specially

power factor improvement. The performance of dynamic voltage restorer is illustrated with the help of case study


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comprising a three phase source supplied to the three phase star (Y) connected balanced R-L load (Padiyar, 2007) . A

new topology of dynamic voltage restorer is proposed for a three-phase four-wire distribution system, which is based

on three-leg voltage source converter and six-leg voltage source converter for the Indirect Proportional Integral (PI)

and synchronous reference frame control scheme respectively. The voltage source converter compensates the

harmonic voltage, reactive power, and improves the power factor on source side. The insulated gate bipolar transistor

(IGBT) based voltage source converter is self-supported with a dc bus capacitor. Comparative assessments of the

performance of dynamic voltage restorer (DVR) feeding linear loads, without DVR and with DVR are presented.

After giving introduction in this section 1, section 2 describes different components of DVR System. Section 3

presents control strategies based on Indirect Proportional Integral (IPI) and Synchronous Reference Frame (SRF).

Section 4, discusses a case study for the three-phase four-wire distribution system employing dynamic voltage

restorer (DVR) as a power quality improvement device. In section 5 performances have been analyzed from the

obtained results under the condition for without DVR and with DVR and finally section 6 concludes the paper.2. Dynamic Voltage Restoration

The configuration of a dynamic voltage restorer (DVR) is shown in Fig.1. The DVR can inject a (fundamentalfrequency) voltage in each phase of required magnitude and phase. The DVR has two operating modes

1. Standby i.e. short circuit operation (SCO) mode where the voltage injected has zero magnitude.

2. Boost i.e. DVR injects a required voltage of appropriate magnitude and phase to restore the prefault load bus

voltage.

The power circuit of dynamic voltage restorer (DVR) is shown in Fig. 1 has four components listed below:

Fig. 1. Dynamic Voltage Restorer

2.1 Voltage Source Converter (VSC)

Voltage source converter (VSC) is commonly used to transfer power between a dc system and an ac system or back

to back connections for ac systems with different frequencies, such as variable speed wind turbine systems (HU et al.

2008). A dc capacitor is connected on the dc side to produce a smooth dc voltage. The switches in the circuit are

controllable semiconductors, such as insulated gate bipolar transistor (IGBT) or power transistors.


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2.2 Boost or Injection Transformers

Three single phase transformers are connected in series with the distribution feeder to couple the voltage source

converter (VSC) at the lower voltage level to the higher distribution voltage level. The three single transformers can

be connected with star/open star winding or delta/open star winding. The choice of the injection transformer winding

depends on the connections of the step down transformer that feeds the load.

2.3 Passive Filters

The filtering scheme in the dynamic voltage restorer can be placed either on the high-voltage-side or the converter

side of the series injection transformer. The advantage of the converter-side filter is that it is on the low-voltage side

of the series transformer and is closer to the harmonic source. Using this scheme, the high-order harmonic currents

will be prevented from penetrating into the series transformer thus reducing the voltage stress on the transformer.

2.4 Energy Storage

Energy storage is required to provide real power to the load. The energy storage device used for this work is a DC

capacitor which is connected to the DC side of voltage source converter (VSC), carries input ripple current of the

converter and is the main reactive energy storage element. This capacitor could be charged by a battery source orcould be precharged by converter itself.

3. Control Strategies

The major objectives of these control strategies are to ensure that the load bus voltages remain balanced and

sinusoidal. Since the load is assumed to be balanced and linear, the load currents will also remain balanced and

sinusoidal. An additional objective is to ensure that the source current remains in phase with the fundamental

frequency component of the point of common coupling (PCC) voltage. In this work control strategies of indirect

proportional integral controller (IPIC) and a novel approach of synchronous reference frame controller (SRFC) have

been discussed.

3.1 Indirect PI controller

The controller input is an error signal obtained from the reference voltage and the rms value of the terminal voltage.

Such error is processed by a PI controller; the output is the angle , which is provided to the pulse width modulation

(PWM) signal generator. Fig. 2 shows that an error signal is obtained by comparing the reference (set) voltage with

the rms voltage measured at the load point. The PI controller processes the error signal and generates the required

angle to drive the error to zero, i.e., the load rms voltage is brought back to the reference voltage (Kumar and

Nagaraju, 2007).

Fig.2 . Indirect PI controller


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From Fig. 3 the sinusoidal signal Vcontrol is phase-modulated by means of the angle as:

A

B

C

V Sin( t )

V Sin( t 2 / 3)

V Sin( t 2 / 3)

(1)

Fig. 3 . Phase Modulation of the control angle

The modulated signal Vcontrol is compared against a triangular signal in order to generate the switching signals for the

voltage source converters (VSC) valves. The main parameters of the sinusoidal pulse width modulation (PWM)

scheme are the amplitude modulation index of signal, and the frequency modulation index of the triangular signal.

The amplitude index is kept fixed at 1 pu, in order to obtain the highest fundamental voltage component at the

controller output.

controla

triangular

Vm 1p.u.V

(2)

Where controlV

is the peak amplitude of the control signal

triangularV

is the peak amplitude of the triangular signal

The switching frequency (fs) is set at 1080 Hz. The frequency modulation index is given by,

sf

1

fm 1080 / 50 21.6

f

(3)

Where f1 is the fundamental frequency

The modulating angle is applied to the PWM generators in phase A. The angles for phases B and C are shifted by

1200 and 2400 respectively. It can be seen that the control implementation is kept very simple by using only voltage

measurements as the feedback variable in the control scheme. Fig. 4 shows simulink model of dynamic voltage

restorer (DVR) controller.


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Fig. 4. Simulink model of DVR Controller

3.2 Proposed Synchronous Reference Frame Controller

The Synchronous Reference Frame (SRF) control approach (Padiyar, 2007) is used to generate the reference voltages

for the dynamic voltage restorer (DVR). Fig. 5 shows the block diagram representation of synchronous reference

frame (SRF) control scheme.

Fig. 5. Block diagram of the SRF controller

The point of common coupling (PCC) voltage VPa , VPb and VPc are transformed into d-q components using the

following equations:

Pa

P

Pb

P

Pc

V1 11V 2 22 V3V 3 30 V2 2

(4)

Pd P0 0

Pq P0 0

V Vcos t sin t

V Vsin t cos t

(5)

where 0 is the operating system frequency. The DC components in VPdand VPq are extracted by using a low pass

filter. Thus


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Pd Pd

PqPq

VVG(s)

VV

(6)

PdV and PqV are the DC components and G(s) is the transfer function of low pass filter. From Fig. 5 wederive the reference for the active component of the load voltage (V*Ld) as:

*PdLd Cd

V V V (7)

where VCd is obtained as the output of the DC voltage controller (with a proportional gain Kp ). A second order

butterworth low pass filter is used in the feedback path of the DC voltage controller to filter out high frequency ripple

in the DC voltage signal.

In steady state, V *Pq= 0 , therefore

qK*

Lq Pq V Vs (8)Kqis chosen to optimize the controller response.

From the reference values of V

*Ld and V

*Lq we can obtain the desired load voltages in phase coordinates from the

following equations:

**

0 0 LdL

**

0 0 Lq L

cos t sin t VV

sin t cos t VV

(9)

*

La *

L*

Lb *

L*

Lc

1 0V

V32 1V3 2 2 V

V31

2 2

(10)

Finally, the reference compensated voltages for the dynamic voltage restorer (DVR) are given by:

* *

Ca La PaV V V

* *Cb Lb PbV V V (11)

* *

Cc Lc PcV V V The detailed comparison of the proposed control strategy with the Indirect proportional integral controller (IPIC) is

given in section 5.


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4. Simulation of Dynamic Voltage Restorer (DVR): A Case Study

The voltage source converter based DVR connected to distribution system having a balanced load is taken up for

study (Padiyar, 2007). Table1 depicts system data and system diagram is shown in Fig. 6. A dynamic voltage

restorer (DVR) is connected in series with the linear load to improve the power factor on the source side i.e. at the

point of common coupling. The balanced R-L load is star (Y) connected. Fig. 7(a & b) is the power circuit of

proposed three-leg and six-leg voltage source converter (VSC) based DVR integrated with three phase transformer.

It contains full bridge converters connected to a common DC bus. The DC bus voltage is held by the capacitor Cdc .

The function of dc capacitor Cdc is to produce a smooth dc voltage. The switches in the converter represent

controllable semiconductors, such as Insulated Gate Bipolar Transistor (IGBT) or power transistors. The IGBTs are

connected anti parallel with diodes for commutation purposes and for charging the DC capacitor. For converter the

most important part is the sequences of operation of the IGBTs. Pulse Width Modulation (PWM) scheme is used to

generate the pulses for the firing of the IGBTs. IGBTs are used in this work because it is easy to control the switch

on and off of their gates and suitable for the DVR.

Fig.6. System diagram

(a) (b)

Fig. 7. Configuration of DVR (a) Three-leg arrangement (b) Six-leg arrangement


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Table1:System parameters

Parameter Value

AC source voltage and frequency

Line Impedance

Balanced R-L loadFilter parameter

DC-side capacitance,resistance and voltage

Controller Parameter

(Proportional and Integral)

PWM switching frequency

Power Converter

Vs = 415V , f = 50 Hz

Ls = 40 mH, Rs = 1.57

R = 50 , L = 200 mHLf = 9.6 mH, Cf = 4.2 F

Cdc= 5000 F , Rdc = 6000 Vdc = 400V

Kp = -40, Ki = 7

1080 Hz

IGBTs/diodes

5. Simulation Results and Analysis

Fig. 8 (a & b) shows the basic simulation model of dynamic voltage restorer (DVR) system that correlates to the

system configuration shown in Fig. 6 in terms of source, load, DVR, and control blocks. The injection transformer in

series with the load, three-phase source, and the series-connected voltage source converter (VSC) are connected as

shown in Fig. 8 (a & b). These DVR models are simulated with the Indirect PI and Synchronous Reference Frame

Control (SRFC) theory. The models are assembled using the mathematical blocks of SIMULINK block set.

Simulation is carried out in discrete mode at a maximum step size of 110 3 with ode45 (Domand-Prince) solver.

The total simulation period is 1s. There are three numbers of breakers used in these models for the purpose of DVR

in operation with the distribution system. Fig. 9. represents simulink model of SRF controller that is implemented

from the block diagram of the SRF controller as in Fig. 5.

(a) (b)

Fig. 8. MATLAB based model of DVR with (a) Indirect PI Control (b) Synchronous Reference Frame control


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Fig. .9. MATLAB Based Model of Synchronous Reference Frame control Scheme

The main purpose of the simulation is to study two different performances of control aspects: 1) harmonic

compensation and power factor correction by Indirect PI control; 2) harmonic compensation and power factor

correction by Synchronous Reference Frame Control. In addition, the Fast Fourier Transform (FFT) is used to

measure the order of harmonics in the compensated voltage. The Total Harmonic Distortion (THD) of the

compensated voltage is measured without and with DVR for the both cases of control strategies. The system

parameters used in these simulations are given in Table1.

A. Harmonic Compensation and Power Factor Correction by Indirect PI control

The simulation results of the dynamic voltage restorer (DVR) system with the Indirect PI controller are shown in

Fig. 10, in which Vc is the compensated voltage, VL is the load voltage, IS is the supply current and Vdc is the dc

link voltage. From the results it is observed that the compensated voltage Vc is pure sinusoidal indicating absence of

harmonics and load voltage VL remains balanced and sinusoidal because of balanced R-L load. The waveform of

three-phase supply current IS is also sinusoidal in nature and the DC link voltage Vdc remains constant i.e. 1.75 V.

Fig. 10. Steady state response of the DVR with PI controller


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Fig. 11 (a & b) shows the supply current iSa and fundamental frequency component of point of common coupling

(PCC) voltage vPa in phase a. In case of without DVR as shown in Fig. 11(a) the waveform of supply current starts

at t = 0.203 s but PCC voltage starts at t = 0.200 s, indicating a phase difference between supply current iSa and

PCC voltage vPa and hence it is not possible to maintain unity power factor (no power factor correction occurs) but

in case of with DVR, as shown in Fig. 11(b) the waveform of supply current i Sa as well as PCC voltage vPa start for

the same time at t = 0.200 s. Thus there is no phase difference between supply current and PCC voltage and hence it

is possible to maintain unity power factor (power factor correction occurs).

(a) (b)

Fig.11 Fundamental PCC voltage and source current (a) without DVR (b) with DVR

Fig. 12 (a & b) shows the harmonic spectrum of the compensated voltage without and with DVR. It is observed that

the total harmonic distortion (THD) of the compensated voltage is reduced from 27.17% without DVR to 0.00%

with DVR and hence mitigating harmonics in the compensated voltage.

(a) (b)

Fig. 12. THD of compensated voltage (a) without DVR (b) with DVR


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B. Harmonic Compensation and Power Factor Correction by Synchronous Reference Frame Control

The dynamic voltage restorer (DVR) is tested for harmonic reduction and power factor correction by connecting

a balanced linear load. The waveforms of the compensated voltage (Vc), load voltage (VL), supply current (IS) and

dc voltage (Vdc) are presented in Fig. 13 to demonstrate the filtering performance of the DVR. It is observed that

compensated voltage Vc is not sinusoidal and also indicates availability of small harmonics. Load voltage VL

remains balanced and sinusoidal. The waveform of three-phase supply current IS is also a pure sinusoidal wave. The

DC link voltage Vdc remains constant near 398V.

Fig. 13. Steady state response of the DVR with SRF controller

Fig. 14 (a & b) shows the supply current iSa and fundamental frequency component of point of common coupling

(PCC) voltage vPa in phase a. In case of without DVR as shown in fig. 14(a) the waveform of supply current iSa

starts at t = 0.2031 s but PCC voltage vPa starts at t = 0.2000 s. This indicates a phase difference between supply

current iSa and PCC voltage vPa and hence it is not possible to achieve unity power factor (no power factor correction

occurs) but when the DVR is in operation with the distribution system, the waveform of supply current iSa as well as

PCC voltage vPa start for the same time at t = 0.2000 s as shown in Fig. 14(b). Thus there is no phase difference

between supply current iSa and PCC voltage vPa and hence it is possible to achieve unity power factor (power factor

correction occurs).


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(a) (b)

Fig.14. Fundamental PCC voltage and source current (a) without DVR (b) with DVR

Fig. 15 (a & b) shows the compensated voltage spectrum without and with DVR. The corresponding total harmonic

distortion (THD) of the compensated voltage is reduced from 32.92% without DVR to 3.84% with DVR.

(a) (b)

Fig.15. THD of Compensated Voltage (a) without DVR (b) with DVR

Thus the fast fourier transform (FFT) analysis of the DVR confirms that the total harmonic distortion (THD) of the

compensated voltage is less than 5% for the both cases of control strategies that are in compliance with IEEE-519

and IEC 61000-3 harmonic standards.

Table2:Measured THD under various Control strategies

Control strategy Without DVR (%) With DVR(%)

Indirect PI

Synchronous Reference

Frame

27.17

32.92

0.00

3.84


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From Table 2 it is clear that with DVR, the THD of compensated voltage is 0.00% for the case of Indirect PI Control

(IPIC), indicating completely elimination of harmonic, which is not practically possible but in case of Synchronous

Reference Frame Control (SRFC), THD of compensated voltage is 3.84% indicating less than 5% of THD value,

which is acceptable for three phase four wire distribution system hence proposed Synchronous Reference Frame

Controller (SRFC) gives better performance in terms of harmonic compensation as compared to Indirect PI

Controller (IPIC).

6. Conclusion

The modeling and simulation of a new topology of dynamic voltage restorer integrated with three phase linear

transformer has been carried out and the performances have been demonstrated for power factor correction and

harmonic reduction. The dynamic voltage restorer is implemented with pulse width modulated controlled voltage

source converter. The converter switching patterns are generated from Indirect PI controller and Synchronous

Reference Frame controller. It has been shown that the system has a fast dynamic response and is able to keep the

total harmonic distortion of the compensated voltage below the limits specified by the IEEE 519 and IEC 61000-3

harmonic standards. The DVR employing the proposed control strategies have been found suitable for starconnected balanced R-L load. The results from the system simulation demonstrate the effectiveness of the DVR in

providing balanced, sinusoidal voltages at the load bus.

Acknowledgments

The authors are thankful to Ministry of Human Resources Development (MHRD), New Delhi, Govt. of India and All

India council of Technical Education (AICTE), for providing financial assistants and supports to do the research work.

References

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