PE Soltns for PQ Problems

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Review

Power-electronic solutions to power quality problems

Ambra Sanninoa,*, Jan Svensson b, Tomas Larsson c

a Department of Electric Power Engineering, Chalmers University of Technology, SE-41296 Gothenburg, Swedenb ABB Utilities, Light Competence Center, Gothenburg, Sweden

c ABB Utilities, Vasteras, Sweden

Abstract

In this paper, an overview of power-electronic based devices for mitigation of power quality phenomena is given. The concept ofcustom power is highlighted. Both devices for mitigation of interruptions and voltage dips (sags) and devices for compensation of

unbalance, flicker and harmonics are treated. The attention is focused on medium-voltage applications. Details about field

experience are given and recent research results are reported. It is shown that custom power devices provide in many cases higher

performance compared with traditional mitigation methods. However, the choice of the most suitable solution depends on the

characteristics of the supply at the point of connection, the requirements of the load and economics.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Power quality; Power electronics; Custom power

1. Introduction

Power quality phenomena include all possible situa-tions in which the waveform of the supply voltage

(voltage quality) or load current (current quality)

deviate from the sinusoidal waveform at rated frequency

with amplitude corresponding to the rated rms value for

all three phases of a three-phase system. The wide range

of power quality disturbances covers sudden, short-

duration deviations, e.g. impulsive and oscillatory

transients, voltage dips (or sags), short interruptions,

as well as steady-state deviations, such as harmonics and

flicker[1]. One can also distinguish, based on the cause,

between disturbances related to the quality of the supply

v

oltage and those related to the quality of the currenttaken by the load.

To the first class belong, among others, voltage dips

and interruptions, mostly caused by faults in the power

system. These disturbances may cause tripping of

sensitive electronic equipment with disastrous con-

sequences in industrial plants, where tripping of critical

equipment can bear the stoppage of the whole produc-

tion, with high costs associated [2]. One can say that in

this case it is the source that disturbs the load. To a voidconsistent money losses, industrial customers often

decide to install mitigation equipment to protect their

plants from such disturbances.

The second class covers phenomena due to low

quality of the current drawn by the load. In this case,

it is the load that disturbs the source. A typical example

is current harmonics drawn by disturbing loads like

diode rectifiers, or unbalanced currents drawn by

unbalanced loads. A more complicated case is light

flicker, which is caused by voltage fluctuations, in turn

caused by rapidly varying loads supplied by a weak

network.Customers do not experience any direct production

loss related to the occurrence of these power quality

phenomena. But poor quality of the current taken by

many customers together will ultimately result in low

quality of the power delivered to other customers: e.g.

both harmonics and unbalanced currents ultimately

cause distortion and, respectively, unbalance in the

voltage as well. Therefore, proper standards are issued

to limit the quantity of harmonic currents, unbalance

and/or flicker that a load may introduce, e.g. [3]. To

comply with limits set by the standards, customers often

have to install mitigation equipment. The need for

* Corresponding author. Tel.: /46-31-772-1631; fax: /46-31-772-

1633.

E-mail addresses: [email protected] (A.

Sannino), [email protected] (J. Svensson), tomas.x.larsson

@se.abb.com(T. Larsson).

Electric Power Systems Research 66 (2003) 71/82

www.elsevier.com/locate/epsr

0378-7796/03/$ - see front matter# 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0378-7796(03)00073-7
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devices for mitigation of the second class of phenomena

is thus induced by regulatory effort.

For both reasons described above, there is a growing

interest in equipment for mitigation of power quality

disturbances, especially in newer devices based on power

electronics called custom power devices [4,5], able to

deliver customized solutions to power quality problems.This paper provides an overview of power-electronic

based devices to be installed at medium-voltage level for

mitigation of power quality phenomena. According to

the above classification, there are two major classes of

mitigation equipment. Reactive power compensation

and harmonic cancellation devices are mostly connected

in shunt at the load bus, with the purpose of injecting a

current to correct the current taken by the load.

Mitigation of flicker will be treated in Section 2 and

harmonic filtering in Section 3. Voltage dips and

interruption mitigation devices are normally connected

between the supply and the load, in order to correct thesupply voltage. These devices are described inSection 4.

Conclusions are finally drawn inSection 5.

2. Flicker mitigation

Arc furnace operation has traditionally been the cause

of flicker problems on medium-voltage and high-voltage

systems. Lately, wind turbines have also been reported

to cause this phenomenon [6]. For flicker mitigation, a

number of different methods are available, which differ

in performance, feasibility and cost [7]. Electrical

methods for flicker reduction deal with the arc furnacecurrent and can be grouped in direct and indirect

methods. The direct methods include apparatus that

alter the arc furnace current directly and are connected

in series with the furnace. Indirect methods do not

intervene on the arc furnace current, but tackle its

effects indirectly with a mitigating device connected in

parallel with the furnace by injecting compensating

current.

A method to reduce flicker is selecting carefully the

arc furnace power in-feed. Special transformer config-

urations for furnace supply have been used for flicker

reduction [8,9]. Reinforcing the grid is an effectivemeans of flicker mitigation but it is expensive and,

therefore, sometimes adopted when a future expansion

of the arc furnace plant is foreseen. The reinforcement

may, however, not always be possible due to environ-

mental impact.

With a so-called dimmer inserted in series with the

arc furnace, its current can to some extent be controlled

[10]. The dimmer is a series device equipped with anti-

parallel thyristors and a series reactor. The disadvantage

is the slow response time and the high harmonic content.

Due to the high rating of the device, it also becomes

rather costly.

Series capacitors have also been investigated and

installed for this purpose [11]. A capacitor in series

with the arc furnace with a proper value of capacitance

is capable of canceling the reactance between the source

and the arc furnace. However, the risk of sub-synchro-

nous resonance (SSR) for series compensated lines has

contributed to reluctance to install series capacitors.Moreover, the influence of the resistance of the grid is

not taken into account.

By inserting a linear reactor in series with the arc

furnace, the short-circuit current of the furnace is

reduced thanks to the higher total impedance. Further-

more, increased reactance in the circuit means larger

phase shifts between voltage and current and thereby a

more stable electric arc. However, the series reactor

must be selected with care since high values of the series

impedance will reduce the furnace power and thereby

the steel production rate. Saturable reactors in series

have also been used to quickly cut current peaks due tofor instance short-circuit in the arc furnace[12].

However, the most used devices for flicker mitigation

are by far the static var compensator (SVC) and the D-

Statcom, described in the following.

2.1. Static var compensator (SVC)

An SVC can be used for ac voltage control by

generation and absorption of reactive power by means

of passive elements. It can also be used for balancing

unsymmetrical loads. As shown in Fig. 1, it is normally

constituted by one thyristor controllable reactor (TCR)and a number of thyristor switched capacitor (TSC)

branches[13]. The value of the reactance of the inductor

is changed continuously by controlling the firing angle

of the thyristors, while each capacitor can only be

switched on and off at the instants corresponding to the

current zero crossings, in order to avoid inrush currents

in the capacitors. With this arrangement, the SVC can

generate continuously variable reactive power in a

Fig. 1. Principle scheme of an SVC.

A. Sannino et al. / Electric Power Systems Research 66 (2003) 71 /8272


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specified range, and the size of the TCR is limited to the

rating of one TSC branch. Obviously, the size of the

reactor limits the power that can be absorbed in the

inductive range.

The SVC can be found in applications such as power

line compensation[14], compensation of railway feeding

system[15], reducing disturbance from rolling mills[16]and arc furnace compensation [17] (both for reactive

power supply and for flicker mitigation). The ability to

absorb changes in reactive power makes to some extent

the SVC suitable for flicker reduction. In this case, the

SVC normally consists of a TCR branch with a filter (no

TSC). An SVC installed together with an arc furnace not

only reduces the flicker, but also, thanks to the stabilized

ac voltage, increases the steel production and its quality

[18]. However, the ability of the SVC to mitigate flicker

is limited by its low speed of response.

2.2. D-Statcom

Among the advantages of using forced-commutated

converters are the ability of both produce and consume

active and reactive power. The active power can also be

controlled independently of the reactive power and vice

versa. Moreover, using a voltage source converter (VSC)

with pulse-width modulation (PWM) gives a faster

converter control, which is needed for flicker mitigation

purposes.

To reduce harmonics and switching losses per valve

when using gate turn-off thyristors (GTOs), a number of

possible converter schemes have been proposed, like 12-

pulse, 24-pulse, 36-pulse [19]or even 48-pulse [20]. Forexample, a 12-pulse connection can be achieved with

two converter bridges connected in parallel via two

transformers, one star- and one delta-connected.

A forced-commutated VSC with PWM operation

today seems to be the most suitable apparatus for

flicker mitigation purposes [21]. Recent progresses in

voltage and current ratings of the valves allow using

integrated gate bipolar transistors (IGBTs) with high

switching frequencies, which further improves the speed

of response. A shunt-connected VSC mounting IGBTs

and operated with PWM is normally referred to as

Statcom or D-Statcom, as it is normally installed atdistribution levels [5]. Manufacturers commercializing

this product with different names have realized a

number of successful installations in the latest years

[22,23].

In the study reported in[24], the VSC is found to be

superior to other flicker mitigation methods such as the

SVC and the series saturable reactor. Using an SVC, the

PST (flicker) value of the voltage at the entrance of the

arc furnace plant, initially set to 3.8, could be reduced to

2.4 or approximately 60% of its initial value. Combining

the SVC with a saturable reactor in the studied system,

unity PST could be reached. A disadvantage, however,

was that the active power of the arc furnace was reduced

by about 20%. With a VSC with proper control

algorithm, a PST equal to 0.65 could be reached. This

corresponds to a decrease of more than 80%. Field

measurements have been performed to verify the

performance [25,26]. In Fig. 2, the reactive current of

the arc furnace and the response of the mitigation deviceare shown for a steel manufacturer operating a 31.5/37.8

MVA electrical arc furnace together with a ladle furnace

rated 5.9/7.7 MVA [26]. The current from the device

clearly follows the arc furnace current. The result is a

consistent mitigation of the flicker (by about 3.5 times)

with the device in operation, as shown by the flicker

measurements ofFig. 3, carried out in accordance with

the IEC method[27].

3. Harmonics mitigation

In order to remove current harmonics from the grid,

passive or active shunt-filters are used. Passive filters for

harmonic reduction provide low impedance paths for

current harmonics. Thus, the current harmonics flow

into the shunt filters instead of back to the supply. The

passive filter consists of series LC filters tuned for

specific harmonics, normally combined with a high-

pass filter used to eliminate the rest of the higher-order

current harmonics. The drawbacks with passive filters

are that they are strongly dependent on the system

impedance, which depends on the distribution network

configuration and the loads. Therefore, the system

impedance, which changes continuously, strongly influ-ences the filtering characteristics. In the worst case an

unwanted resonance can occur between the filter and the

system. This may cause the passive filter to act as a

sink for harmonic currents from other sources in the

grid. Therefore, the passive filter can be overloaded by a

current higher than the rated value. Finally, the

capacitors of the passive filter produce reactive power,

which may not necessarily be needed for power factor

correction.

A shunt active filter consists of a controllable voltage

source behind a reactance acting as a current source.

The VSC based shunt active filter is by far the mostcommon type used today, due to its well-known

topology and straightforward installation procedure. It

consists of a dc-link capacitor, power electronic switches

and filter inductors between the VSC and the grid (Fig.

4). The operation of shunt active filters is based on

injection of current harmonics in phase with the load

current harmonics, thus eliminating the harmonic con-

tent of the line current.

When using active filters, it is possible to choose the

current harmonics to be filtered and the degree of

attenuation. The size of the VSC can be limited by

using selective filtering and removing only those current

A. Sannino et al. / Electric Power Systems Research 66 (2003) 71 /82 73


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harmonics that exceed a certain level, e.g. the level set in

IEEE Std. 519-1992 [3]. Together with the active

filtering, it is also possible to control the power factor

by injecting or absorbing reactive power from the load.

A common active filtering method is based on using

the instantaneous active and reactive power [28]. This

method detects all non-sinusoidal components and,

when it is implemented with a hysteresis current con-

troller, allows obtaining good filtering performance.

However, the hysteresis current controller has thedisadvantages of a varying switching frequency, which

produces a continuous harmonics spectrum. When the

instantaneous active and reactive power method is

implemented together with sub-oscillated PWM [29]

and vector current control[30], an inferior performance

is obtained when the switching frequency is low. This

may be desired in order to keep the switching losses

down. The low performance is caused by the response

time of the current controlled VSC, which leads to a

phase shift between the reference currents and the

output currents. Various methods for compensating

for phase shifts have been presented, e.g. [31].

Selective active filtering can be based on variousmethods. Some of the proposed selective active filters

use bandpass filters to extract individual current har-

monics[32]. Unfortunately, this method results in phase

Fig. 2. Reactive current of the arc furnace and the compensator (SVC light).

Fig. 3. Flicker levels with and without mitigation with VSC-based compensator for the electrical arc furnace (EAF) in Hagfors, Sweden[26].



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shifts, which reduce the filtering performance. A better

solution is to use a Fourier series to determine individual

harmonics [31]. Another method uses synchronously

rotating coordinate system for each of the individual

current harmonics [33]. Several methods have beenanalyzed and compared in[34].

4. Mitigation of voltage dips and short interruptions

Short-duration, shallow dips can be mitigated by

improving equipment tolerance characteristics [35/37].

Long-duration, deep dips and interruptions can be

avoided by changing structure and/or operation of the

power system [38]. However, for industrial customers,

who do not normally have access to system or equip-

ment improvement, the installation of additional miti-gation equipment is often the only option left to achieve

the desired quality of supply at the system-load inter-

face.

Traditional devices described in [2] include motor-

generator sets, which use the rotational energy stored in

a flywheel to provide power to the load during the dip,

and constant voltage, or ferro-resonant, transformers

(CVTs). More modern equipment based on power

electronics will be described here. The static series

compensator (SSC) and the static transfer switch

(STS) are analyzed more in detail. Other devices are

also briefly described.

4.1. Static series compensator

The SSC is a VSC connected in series on the

distribution feeder, which provides a controllable

source, whose voltage adds to the source voltage to

obtain the desired load voltage. Depending on the type

of control implemented, it is possible to use the

additional voltage source to correct supply voltage

unbalance, perform load voltage regulation, compensate

for voltage dips and cancel low-order supply voltage

harmonics. The principle of series injection for dip

compensation is depicted in the phasor diagram of

Fig. 5 where Vsag; Vinj; Vload and Iload are the phasors

of supply voltage affected by dip, injected voltage, load

voltage and current, respectively.

Manufacturers that are pioneering this technology

with the name of dynamic voltage restorer (DVR)

have realized installations, among others, at a yarn

manufacture[39], semiconductor plants[40,41], a utility

feeder serv

ing industrial and commercial customers inCanada [42], a large paper mill in Scotland [43]. The

compensation system is installed at medium voltage

level and protected loads have typical ratings between

0.5 MVA and around 21 MVA [40]. This solution,

although costly, is very attractive for large sensitive

industrial customers, as it allows for protection of the

entire plant through the installation of only one device.

However, being a series device, this compensator has the

obvious disadvantage of not protecting the load against

interruptions [44]. Moreover, sensitive loads inside the

plant will not be protected against dips originating

within the plant.

One possible configuration that realizes the series-

injection principle is shown in Fig. 6. The converter

generates the proper voltage to be injected for compen-

sation, and therefore, will often be required to operate

with unbalanced switching functions for the three

phases. The converter is normally a three-phase two-

level VSC with IGBTs[39], but also the use of a three-

Fig. 4. Principle scheme of active filtering using VSC.

Fig. 5. Phasor diagram of the series injection principle.

Fig. 6. Scheme of series compensator for voltage dip mitigation.



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phase three-level neutral-point-clamped (NPC) conver-

ter mounting integrated gate-commutated thyristors

(IGCTs) has been reported in a DVR by a major

manufacturer [41]. The valves on the three legs of the

converter are switched independently of each other,

normally according to a PWM pattern, with high

switching frequency. This ensures fast response and asmooth voltage waveform. The voltage rating of the

converter dictates the maximum injected voltage, which

is thus the maximum (three-phase) dip magnitude that

can be compensated for. Existing DVRs are usually

sized for 50% maximum voltage injection.

A second-order LC filter is normally inserted in

between the converter and the transformer to cancel

high-frequency harmonic components in the converter

output voltage. Another configuration proposed in-

cludes a line-side filter composed by the leakage

inductance of the injection transformer combined with

a capacitor on the line-side of the transformer [41].However, with this configuration the capacitor must be

sized for the higher system voltage and the series

transformer must be sized for the total rms current,

including harmonics[45].

The controller of the SSC can be designed to

compensate for voltage dips by only providing reactive

power, i.e. by injecting a voltage in quadrature with the

load current. However, it is shown in [46] that the

compensation capability of the device is very limited in

these conditions, especially for high values of the power

factor. Therefore, an energy storage device is normally

connected to the dc bus of the converter to provide the

energy necessary for the compensation. Commerciallyavailable DVRs use large capacitor banks for energy

storage [40]. A SSC unit utilizing superconducting

magnetic energy storage (SMES) technology installed

in 1997 is mentioned in[47]. The device is rated at 750

kVA and provides 2.4 MJ of stored energy. No details

are known on field experience. Experimental results on a

prototype are presented in[48]. The problem with a wide

application of SMES is at present the very high cost.

The capacity of the energy storage device has a big

impact on the compensation capability of the system, as

it ultimately determines the ride-through time for the

load. A method for minimizing the size of the necessaryenergy storage device has been recently proposed in[49].

The availability of high-capacity storage at reasonable

cost is a key factor for widespread application of this

technology.

A controller for the SSC designed in the rotating dq-

reference frame, which can achieve fast response and

handle transients properly, has been proposed in [50].

However, this controller is designed under the assump-

tion of symmetrical supply voltages, i.e. it only works

with balanced dips. The controller proposed in [51]

implements a fast technique for positive and negative

sequence detection and thereby is able to compensate for

unbalanced dips. Moreover, the effect of the converter

output filter, which causes voltage drop and phase shift

in the fundamental component of the injected voltage, is

taken into account and compensated for in the con-

troller presented in [51] by using a back-calculation of

the voltage drop, known the filter parameters. An

alternative is to apply a feedback of the capacitorvoltage, as done for example in [50]. Even better

transient performance is obtained by adding another

loop that controls the current through the converter

valves as in[52]. The principle of series injection can also

be applied to cancellation of low-order supply voltage

harmonics. To do this, the controller must be able to

deal with other frequencies than the fundamental. A

controller based on several rotating frames is proposed

in[53].

An interesting case study presented in[54]involves an

industrial facility that produces microprocessors for the

personal computer industry starting from pure siliconwafers. In 12 months before the installation of the SSC,

the plant had experienced 14 voltage dips that were

capable of adversely affecting production. The semi-

conductor plant is served by three 69 kV lines feeding

two transformers and the total plant load is about 45

MVA. Two independent SSCs, each rated 6 MVA at

12.47 kV with 1800 kJ of stored energy, were installed

inside the semiconductor plant, one on each of the two

12.47 kV feeders. Monitoring performed after installa-

tion revealed that the SSCs compensated for over 72

power quality disturbances that could have caused

process interruptions for the end-user. To demonstrate

the performance, a measured three-phase dip down to65% occurring on the grid and the resulting load voltage

due to SSC operation are shown in Fig. 7.

4.2. Static transfer switch

The STS consists of two three-phase static switches,

each constituted in turn by two anti-parallel thyristors

per phase (Fig. 8). Normally, the static switch on the

primary source is fired regularly, while the other one is

off. In the event of a voltage disturbance, the STS is

used to transfer the load from the preferred source to an

alternative healthy source [55]. This results in a veryeffective way of mitigating the effects of both interrup-

tions and voltage dips by limiting their duration as seen

by the load. The success of the STS is mainly due to its

rather low cost compared with other solutions. A

requirement is that a secondary in-feed, independent

from the main source (e.g. a feeder to another substa-

tion), must be available. Therefore, this solution is

particularly attractive for installations that already

have mechanical transfer systems, where upgrading to

a static system does not require major changes in the

layout of the distribution system. Formerly available

only for low voltages, STS systems are now advertized



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for higher voltages and load ratings, which make them

suitable for high-power industrial applications: they

would be capable to protect loads up to 35 MVA

supplied at a voltage as high as 35 kV [56]. The

application of a 15 kV, 600 A STS to an automotive

component plant is described in detail in [57].

Note, however, that the STS cannot protect against

dips originating in the transmission system, which will

also affect the alternative supply. Yet, a significant

improvement can be achieved in the performance of the

industrial system against faults at distribution level,

which normally cause long duration dips and short

interruptions.

The load will still see a disturbance during the interval

in which the transfer takes place, therefore, it must be

completed so quickly that the duration of the resulting

disturbance at the load terminals is short enough not to

cause equipment trips.

The STS can ensure a fast response due to the

phenomenon of fast switching, usually referred to as

Make-Before-Break switching (MBB). This takes

place when the thyristors of the secondary-source static

switch are fired, thus initiating the transfer, before thecurrent through the primary-source switch has reached

zero. Depending on the circuit conditions, a current

starts flowing in the secondary-source switch in a

direction such that it forces the primary-source switch

to turn off very quickly. Transfer times of less than one

quarter of cycle are quite common for MBB transfer

[55].

An example of the dip mitigation performance of the

STS is given inFig. 9(a) which shows the source voltages

together with the response of the detection system,

denoted as fault signal, for a 70% dip (single-phase).

The dip is detected in about 1 ms and the transfer occursimmediately after. As a result, only a notch ofvery short

duration affects the load voltage (Fig. 9(b)).

However, MBB transfer must be inhibited when this

would lead to a circulating current between the two

sources, thus spreading the fault to the healthy portion

of the system. In this case, the control system must

inhibit firing and wait until the current through the

thyristors in the faulted source switch has become zero,

thus realizing a slow or Break-Before-Make transfer

(BBM). This result is accomplished by monitoring the

load current, together with the voltages of the two

sources.

Fig. 7. Measured three-phase dip on grid side (top) and load voltage (bottom) due to SSC operation, from[5].

Fig. 8. Structure of the STS (single-phase scheme).



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The performance of the STS concerning transfer time

has been analyzed in [58,59]. Results presented allow

concluding that in general the speed of operation of a

STS is high enough to realize a seamless transfer for

sensitive loads. Note, however, that the transfer time

increases even more in case of regenerative load, e.g.

induction motors [60,61]. In industrial plants with high

percentage of rotating load, a specific performance

study could be needed to assess the actual improvement

obtained with installation of the STS.

4.3. Other voltage dip/interruption mitigation devices

Electronic tap changers (Fig. 10) can be mounted on a

dedicated transformer for the sensitive load, in order to

change its turns ratio according to changes in the input

voltage [62]. This device, called static voltage regulator

(SVR) [5], is placed between the supply and the load.

Part of the secondary winding supplying the load is

divided into a number of sections, which are connected

or disconnected by fast static switches, thus allowing

regulation of the secondary voltage in steps. This should

allow the output voltage to be brought back to a higher

level than 90% of nominal value, even for severe voltage

dips. Thyristor-based switches, which can only be turned

on once per cycle, are used; therefore, the compensation

is accomplished with a time delay of at least one half-

cycle. No existing installations are known to date.

A standard solution for low-power equipment is

constituted by the UPS (Uninterruptible Power Supply),

which consists of a diode rectifier followed by an

inverter (Fig. 11). The energy storage device is usuallya battery block connected to the dc link. During normal

operation, power coming from the ac supply is rectified

and then inverted. The batteries remain in standby mode

and only serve to keep the dc bus voltage constant.

During a voltage dip or interruption, the energy released

by the battery block maintains the voltage at the dc bus.

Depending on the storage capacity, the battery block

can supply the load for minutes or even hours. Low cost,

simple operation and control have made the UPS the

standard solution for low-power equipment like com-

puters. For higher-power loads the costs associated with

conversion losses and maintenance of the batteriesbecome too high and this solution no longer appears

to be economically feasible.

Note that, although based on power electronics, the

UPS due to its low-power/low-voltage ratings is not a

Fig. 9. (a) Source voltages and fault signal, (b) load voltage and current for a 70% dip.

Fig. 10. SVR. Fig. 11. Uninterruptible power supply.



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custom power device according to the definition given

in[5], which covers devices installed in 1 kV through 38

kV distribution systems. However, it is listed here for

completeness.

A static shunt compensator, normally used for flicker

mitigation and active filtering purposes, equipped with

an isolation switch for disconnection from the distribu-tion feeder, results in the backup source ofFig. 12, also

called Backup Stored Energy System (BSES) [5]. This

constitutes an alternative to the UPS to avoid high

steady-state losses due to the energy conversions as the

load power increases. As soon as a disturbance is

detected, the sensitive load is isolated from the power

system by a static switch and supplied by the VSC. For

storing the necessary energy, batteries (Transportable

Battery Energy Storage System, TBESS [63]) or small-

size SMES systems can be used [64]. The main advan-

tages of SMES as compared with the batteries are the

reduced size and lower maintenance requirements.It has been pointed out that the necessity of an energy

storage device of adequate capacity is the biggest

limitation for the SSC. An alternative is to h ave a

second converter connected to the line upstream of the

series compensator to supply the dc bus (Fig. 13). In this

way, energy is transferred to the dc bus continuously.

On the other hand, power coming from the line will be

greatly reduced during a dip. This rectifying stage must

be properly designed to operate correctly with reduced

(and possibly unbalanced) input voltage during the dip

(note that this problem is overcome if the rectifier is

placed on the load side as in [42], but at the expenses ofincreased rating of the SSC). By choosing a shunt-

connected VSC instead of a simple diode rectifier for

this purpose, other control and regulation functions, e.g.

active current filtering or flicker mitigation, can be

performed, thus realizing a multi-purpose compensa-

tor. This concept has been proposed with the name of

unified power quality conditioner (UPQC) [65].

The device ofFig. 14is obtained by a combination of

a STS and a SSC in series. Total protection can thus be

obtained against interruptions and voltage dips, with the

STS taking care of interruptions and dips originated by

faults in the distribution system, which are long and

deep [1] and would deplete the energy storage of theSSC. The SSC will instead compensate for the voltage

dips originated by faults in the transmission systems,

which the STS cannot handle. Note that transmission-

system dips are normally short and shallow[1]. Hence,

the size of the energy storage of the SSC can be greatly

reduced, with a consequent reduction of the cost of the

device.

5. Conclusions

In this paper, an overview of the use of powerelectronics for mitigating power quality phenomena

has been given. The concept of custom power has

been highlighted. Advantages and drawbacks of several

custom power devices have been pointed out. Both

shunt devices, protecting the source from the load, and

series devices, protecting the load from the source, have

been covered. Details about field experience have been

given and recent research results have been reported.

It has been shown that custom power devices provide

in many cases higher performance compared with

traditional mitigation methods. However, the choice of

the most suitable solution depends ultimately on theFig. 12. Backup power source.

Fig. 13. Scheme of the unified power quality conditioner (UPQC).

Fig. 14. Single-phase scheme of a combined STS/SSC.



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characteristics of the supply at the point of connection,

the requirements of the load and economics, i.e. the

customer value added by the installation of a power-

electronics based device.

Acknowledgements

The work of Ambra Sannino was supported by a

Marie Curie Fellowship of the European Community

program IHP under contract number HPMF-CT-2000-

00922 and partly also by ELFORSK, Sweden under

Elektra project no. 3378.

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grid-connected converters, wind power and power

quality.

Tomas Larsson received his M.Sc. degree and Ph.D

degree from the Royal Institute of Technology, Stock-

holm, Sweden in 1991 and 1998, respectively. Since

March 1998 he is working for ABB where he mainly has

been involved in system design for projects dealing with

reactive power compensation. His interests include

voltage source converters in power quality applications,

in particular flicker mitigation.


Documents

PE Soltns for PQ Problems