6.pdf

Faculty of Electrical Engineering

Universiti Teknologi Malaysia

VOL. 12, NO. 1, 2010, 7-13 ELEKTRIKA

http://conf.fke.utm.my/elektrika

7

Dynamic Voltage Restorers:

A Literature Review

Ahmet TEKE1* M. Emin MERAL2, Ltf SARIBULUT1 and Mehmet TMAY1

1ukurova University, Department of Electrical and Electronics Engineering, Balcal/Adana, TURKEY

2Yznc Yl University, Department of Electrical and Electronics Engineering, Zeve/Van, TURKEY

*Corresponding author: [email protected] (First A. Author), Phone: +90 322 3386868, Fax: +90 322 3386326

Abstract: Custom power is a concept based on the application of power electronic controllers in distribution system to improve the power quality. Dynamic Voltage Restorer (DVR) is one of the Custom Power devices that can mitigate voltage

sag/swell, unbalance and voltage harmonics originating from supply side. DVR has become very popular in recent years in

both low voltage and medium voltage applications. In this paper, the comprehensive reviews of over 80 articles concerning

DVR are presented. The advantages and disadvantages of each possible configuration and control technique are also

presented. This review will help the researchers to select the optimum control strategy and power circuit configuration for

their DVR applications.

Keywords: A Custom Power, Dynamic Voltage Restorer, Power Quality, Literature Review.

1. INTRODUCTION

Dynamic Voltage Restorer is a Custom Power Device

used to eliminate supply side voltage disturbances. DVR

(also known as Static Series Compensator [1-3])

maintains the load voltage at a nominal magnitude and

phase by compensating the voltage sag/swell, voltage

unbalance and voltage harmonics presented at the point of

common coupling [4-6]. DVR injects the missing voltage

of suitable magnitude and phase in series with the line as

shown in Figure 1. During standby operation, DVR

performs no switching. However, when voltage sag/swell

occurs in the system, DVR starts to inject missing voltage

to the system. Many loads facilitated in industrial plants

such as adjustable speed drives and process control

equipments are able to detect voltage faults as minimal as

a few milliseconds. DVR should react as fast as possible

to inject the missing voltage to the system because these

loads are very sensitive to voltage variations [7].

Power

SupplyLoad

DVR SYSTEM

Control Unit: Disturbance detection, reference signal

generation, gate signal generation, voltage measurement.

Power Circuit: Energy storage unit, DC/AC converter, LC

filter, injection transformer, standby and system protection.

POWER QUALITY IMPROVEMENT

Figure 1. Basic representation of DVR

The alternative solution to DVR can be Uninterruptible

Power Supply (UPS), switched autotransformer [8] or D-

STATCOM [9-11]. DVR costs less compared to the UPS

systems. UPSs have typically been designed for the correction of different types of voltage disturbances,

which may not necessary, fall into the category of voltage

sags. Taking the UPS as an example, this has two major

implications [6]. First, the energy that a UPS is required

to store is based upon the long duration of a typical

voltage outage or blackout, not relatively short duration

voltage sag. Secondly, UPS systems are typically

designed for small loads, such as a computer mainframe

or low power safety critical systems. DVR is smaller in

size and costs less compared to the DSTATCOM. The

amount of apparent power injection required by a DSTATCOM to compensate a given voltage sag is much

higher than that of DVR. The main reason of that is DVR

corrects the voltage sag only on the downstream side

[10].

Another solution may be using an autotransformer

where it only takes care of a limited range of voltage sag.

The autotransformer has poor controllability, slow

response time for mechanical switching units and needs

routine maintenance of its parts.

DVR system consists of power circuit and control unit.

A comprehensive literature review of DVR including

control unit, power circuit and field applications is presented. The remainder of the paper is organized as

follows. Section 2 of this paper presents power circuit

configurations of DVR. The controller algorithms of

DVR are clearly presented in Section 3. Section 4

summarizes the field applications of DVR. Finally, main

points and significant results of the paper are summarized

in Conclusions.

Ahmet TEKE, M. Emin MERAL, Ltf SARIBULUT and Mehmet TMAY / ELEKTRIKA, 12(1), 2010, 7-13

8

2. POWER CIRCUIT OF DVR

DVR can be used for medium voltage [12-14] and low voltage applications [15]. DVR is generally designed as

3-phase 3-wire [16] but there are also 1-phase [17] and 3-

phase 4-wire [18] studies for DVR. Available topologies

for DVR are H bridge [19], multilevel [20], four-leg DVR

[21] and transformerless topologies [22]. Power circuit of

DVR generally consists of DC link, DC/AC converter,

LC filter and injection transformer.

Lf

Source Load

Inverter

DC Link

Source Load

Source Load

Source Load

(a) Storage systems with auxiliary supply / Inverter side filtering

(b) Storage systems with grid itself / Voltage source inverter

(c) Storage systems with auxiliary supply / Load side filtering

Injection

Transformer

(d) Storage systems with grid itself / Current source inverter

LC

FilterCf

Lf

Cf

Inverter

InverterRectifier

InverterRectifier

DC Link

Lf

Cf

Figure 2. Various power circuit topologies of DVR

2.1 DC Link

DC link (energy storage unit) supplies the required power

for compensation of load voltage during voltage sag/swell

or harmonics. Electrolytic capacitor bank is usually preferred as an energy source for majority of DVR

applications. The selection of the optimum topology and

DVR ratings is related with the distribution of the

remaining voltage, the outage cost and investment cost.

Storage systems with auxiliary supply: This topology

is applied to increase the system performance when the

grid of DVR is weak. In this type, variable DC link

voltage [23] or constant DC link voltage [24] topologies

are applied (Figures 2a, 2c).

Storage systems with grid itself: The remaining

voltage on supply side [16] or load side [19] is used to

supply necessary power to the system if DVR is connected to the stiff grid (Figures 2b, 2d).

Flywheel Energy Storage: The ywheel as a preferred energy storage device, the system utilizes a single AC/AC

power converter for the grid interface as opposed to a

more conventional AC/DC/AC converter, leading to

higher power density and increased system reliability [25-

27].

Therefore, a new inter-phase AC-AC topology is

presented in [28-29] that needs no storage device.

However, this topology has the following disadvantages

over the storage based voltage sag supporter: the voltage

harmonics cannot be compensated, point of common coupling may experience noise due to AC chopper circuit

and the required power is drawn from the faulty phases in

the case of symmetrical sag compensation. In [30],

voltage control using a Distributed Generation supported

DVR is presented. When the DC link is fed from the

rectifier, the rectifier can be controlled using PI [26], [30]

or hysteresis [31].

2.2 DC/AC Inverter

The inverter circuits convert DC power to AC power. The

types of inverter circuits are voltage source (fed) inverter

(VSI) and current source (fed) inverter (CSI) [32-34].

Current source inverter: It is easy to limit over

current conditions but the value of output voltage varies

widely with changes in load (Figure 2d).

Voltage source inverter: The values of output voltage

variations are relatively low due to capacitor but it is

difficult to limit current because of capacitor (Figures 2a, 2b, 2c). Some types of this inverter are: H bridge [19], 6-

bridge [35], multilevel inverter [20], [36] and cyclo-

converter based [29].

VSIs have its drawbacks, such as a rather slow control

of converters (LC filter) output voltage and current

protection problems. DC bus voltage oscillations can be

observed that make the control of series filter output

voltage more difficult. Such problems can be overcome

using current-source converters [34].

2.3 LC Filter

The filter unit eliminates the dominant harmonics

produced by inverter circuit. Filter unit consists of

inductor (L) and capacitor (C). The design procedure for

LC filter is given in [37]. The harmful effects of

harmonics generated by the inverter can be minimized

using the inverter side [38] and line side filtering [39].

Inverter side filtering scheme: It has the advantage of being closer to the harmonic source thus high order

harmonic currents are prevented to penetrate into the

series injection transformer but this scheme has the

disadvantages of causing voltage drop and phase angle

shift in the fundamental component of the inverter output

(Figures 2a, 2b, 2d).

Line side filtering scheme: Harmonic currents

penetrate into the series injection transformer but the

voltage drops and phase shift problems do not disturb the

system (Figure 2c).


9

2.4 Injection Transformer

For most of the time, DVR is in standby mode and

conduction losses account for the bulk of converter losses

during the operation [40]. The grid voltage level and the

voltage sag depth determine the voltage rating of the

transformer. The injection voltage level multiplied by

rated current gives power rating of each phase [41]. Its

main tasks are coupling the injected compensating

voltages generated by the voltage source converters to the

incoming supply voltage and connecting DVR to the

distribution network via the HV-windings and transforms

[42]. In [20] and [36], DVR is implemented using a

multilevel inverter topology allowing the direct

connection of DVR to the distribution network without

using a bulky and costly injection transformer. In [22],

the use of the transformer is eliminated applying the

voltage boosting functions and a dynamic energy

replenishing charging circuit. This indicates a less costly

voltage restorer of a more compact structure.

2.5 Standby and System Protection

In standby mode, the injection transformer works like a

secondary shorted current transformer using bypass

switches delivering utility power directly to the load.

Alternatively, during standby operation of DVR, two

upper IGBTs in each phase of the inverter remain turned

off while the two lower IGBTs remain turned on. A short

circuit across the secondary (inverter side) windings of

the series transformer through LF is obtained eliminating the use of bypass switches [19].

Differential current protection of the transformer and

short circuit current on the customer load side are two

examples of many protection functions possibility [42].

The protection of a DVR against voltage surges and short

circuit conditions to prevent its malfunction or

destruction is discussed in [43-44].

3. CONTROL UNIT

The control unit is the most important part of DVR

system. The main considerations for the control system of

DVR include: sag/swell detection, voltage reference

generation for transient/steady state control of the

injected voltage, voltage injection strategies and methods

for generating of gating signals. Source voltage

measurement is generally enough to generate reference

voltage and sag detection signals in most of DVR studies

as shown in Figure 3 [19], [45-46].

3.1 Sag/Swell Detection

Voltage sag/swell must be detected fast and corrected

with a minimum of false operations for three phase

systems. Monitoring of 22qd VV (Vd is the reel part of

the voltage, Vq is the imaginary part of the voltage in dq reference frame) or Vd in a vector controller is the

simplest type of sag/swell detection, which will return the

state of supply at any instant in time and hence, detect

whether or not sag has occurred [45], [47]. To separate

the positive and negative sequence components, low-pass

filters (LPFs) are used after the d-q transformation in the

literature. For effective removal, the cut-off frequency of

the low pass filter must be reduced but it has the side

effect of reducing the controller response time [48]. Other

methods to detect voltage sag/swell are tracking the

positive sequence of the supply voltage [49] applying the

Fourier transform [50], Kalman filtering [51], Neural

Network [52-53] and applying the Wavelet Transform

[54]. Further information about conventional sag

detection method is presented in [55]. In [48], a new control method for DVR system is proposed by detecting

the negative and positive sequence components using

differential controllers and digital filters.

Lf

Cf

Lf

Cf

Vdc

Lf

Cf

G11G10

G9 G12

Supply side Phase to Neutral

Measurement

G7G6

G5 G8

G3G2

G1 G4

L

O

A

D

SupplyPhase_A

Phase_B

Phase_C

Voltage Sag/Swell Detection

Reference Voltage Generation

Switching Signal Generation

Filter

Injection Transformer

Control Unit

4

4

4

Figure 3. Control unit of DVR

There is also single-phase sag detection methods

used in DVR. Soft Phase Locked Loop (PLL) [56],

Mathematical Morphology theory based low-pass filter

[57], Instantaneous Value Comparison Method [58] are

the commonly used methods for single phase sag

detection.

3.2 Reference Signal Generation and Voltage Injection Strategies

Reference voltage generation can be achieved using pqr

[59], dq [33], [48], [60-61], fuzzy logic (FL) [62], sliding

mode [63], artificial neural network [64] and software

PLL [65]. Open-loop feed forward control is generally

preferred in DVR controllers to meet the fast

compensation requirement. However, the presence of the

passive LC filter can introduce voltage oscillations during

transients. These oscillations increase the damping response time of the system as mentioned in [66].

The saturation of the series injection transformer and the

voltage drop across the inductor in steady-state operation

are other factors that affect the performance of DVR in

open-loop control [67]. The load voltage may not be

compensated to the desired value in open-loop feed

forward control. Closed-loop control can reduce the

damping oscillations coursed by the switching harmonic

LC filter and the load voltage can track closer to the

reference load voltage under different load condition.

Multi-loop control and closed-loop state variable control are closed-loop control strategies of DVR [68-69]. The

performances of these control strategies are analyzed with

its dynamic and damping performance. These control

schemes can reduce the damping oscillations, but not

catching up with the fast dynamic response. Other control

strategy is boundary controller [7].


10

DVR should ensure the constant load voltage with

minimum energy dissipation for injection. The

characteristics of the sensitive load determine the types of

control method and the compensation strategy for DVR.

The methods for injection of missing voltage can be

divided into four groups [53], [70-73]: Pre-sag compensation method [74] In-phase voltage injection method [75] Phase advance method [76-77] Voltage tolerance method with minimum energy injection [78]

In pre-sag compensation method, the supply voltage is

continuously tracked and the load voltage is compensated

during fault. On the other hand, in In-phase compensation

method, DVR voltage is always in phase with the

measured supply voltage regardless of the load current

and pre-sag voltage. In phase-advance method,

decreasing the power angle between the remaining voltage and the load current minimizes real power

consumed by DVR. In voltage tolerance method with

minimum energy injection method, the phase angle and

magnitude of corrected load voltage within the area of

load voltage tolerance are changed. The small voltage

drop and phase angle jump on load can be tolerated by

load itself and the size of the energy storage is

minimized.

FL control of DVR for voltage injection is reported in

[62] and [79] in the literature. The control of DVR can be

implemented using Digital Signal Processor (DSP) [80], Field Programmable Gate Array (FPGA) or combination

of them with passive circuits.

3.3 Generating of Gating Signals

The generated reference signal is used to produce gate

switching signals of the inverter. The main modulation

techniques used in DVR are hysteresis [72], pulse width modulation (PWM) [81], deadbeat control [82] and space

vector PWM modulation [83]. The hysteresis control has

the advantages of quick controllability, easy

implementation and variable switching frequency [72].

PWM has a great impact on its transient performance and

higher operating frequency capability [81]. PWM method

is widely used for gate signal generation in custom power

applications. The deadbeat controller has very fast

transient response [82]. The space vector PWM technique

can generate output voltages and/or currents with less

harmonic distortion [83].

4. FIELD APPLICATIONS OF DVR

DVR have been installed in the Semiconductor, Plastic

Extrusion, Food Processing and Paper Mill factories.

Some of DVR applications in service are [84-85]:

i) Orian Rum Company: The first DVR to enter service

was installed by Westinghouse at the Orian Rugs Co.

Plant in the USA. This is a highly automated facility with two main processes. The plant is served by a single

12.47-kV feeder from a 20-MVA substation transformer

four miles away. A 2-MVA DVR, with 660 kJ of Energy

Storage was installed to this plant.

ii) Florida Power Corporation: 2-MVA DVR at

Econlockhatchee was installed as part of Florida Power

Corporations new Power Quality Program.

iii) Bonlac Foods, Australia: The Bonlac load is

approximately 5.25-MVA and the facility is served by a

22-kV feeder from Powercors Kyabram substation 11 miles away. A 2-MVA DVR was installed to this plant.

iv) Caledonian Paper: Scottish Power serves

Caledonian Paper via a 132-kV transmission line which is stepped and the total plant load is 47-MVA. 4-MVA

DVR, with 800W of energy storage was installed to this

plant.

v) PureWave DVR installations have produced

exceptional results for both power suppliers and energy

users. Here are but a few examples:

Semiconductor Manufacturer: A pair of 6-MVA

PureWave DVRs protects a microprocessor

manufacturing facility in the southwest U.S. Operating at

12.47-kV, DVRs serve a 35-MVA load.

Plastic Extrusion Manufacturer: A rug manufacturer in

the U.S. uses a PureWave DVR to protect the yarn extrusion process from sags. A 2-MVA DVR installed on

the 12.47-kV system corrected over 40 sags in its first

year of full operation and over 100 sags the second year.

Food Processing Plant: Interruptions at a powdered milk

manufacturing plant in Australia not only resulted in

costly cleanups, but also mandated regulatory inspections.

The dairy processor realized savings of over $1 million

annually with the installation of a PureWave DVR on its

22-kV system, protecting a 6-MVA load.

Paper Mill: An 8-MVA machine load required

PureWave DVR protection at a paper mill in Scotland. A 4-MVA DVR placed on the mills 11-kV distribution system provides sag correction, resulting in increased

production by enabling the machine to run at full speed.

5. CONCLUSIONS

DVR can mitigate the some types of power quality

disturbances such as voltage sags/swells, voltage

harmonics and unbalances. In this paper, a

comprehensive review of DVR studies is presented. The advantages and disadvantages of applied topologies and

control techniques for DVR are also presented. This

literature review study gives detailed information on

DVR topologies and control methods which provide more

opportunities for studying DVR and give results

interesting in DVR area. With this study, the findings

about DVR studies in the literature and the application

notes of DVRs in service are presented and thus the

trends of DVR through the years are clearly observed.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge Electrical,

Electronics and Informatics Research Group of the

Scientific and Technical Research Council of Turkey

(Project No: EEEAG-106E188) for full financial support.

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