Hybrid filters for power quality improvement

Hybrid filters for power quality improvement

B. Singh, V. Verma, A. Chandra and K. Al-Haddad

Abstract: Solid-state controllers are widely used to convert AC power for feeding number ofelectrical loads such as adjustable speed drives, furnaces, power supplies etc. Some of thesecontrollers behave as nonlinear loads because they draw nonsinusoidal current from the ACmains.Filter technology for improving power quality of such loads has matured to a reasonable level.Moreover, hybrid filters are considered one of best options for improving power quality for anumber of considerations. A comprehensive review of hybrid filters configurations is given: theircontrol approaches, state of art, design considerations, selection criteria, potential applications,latest trends, future developments and their comparative features. A broad review of the status ofhybrid filters to researchers, design and practice engineers dealing with power quality improvementsis presented. A classified list of more than 150 research publications on the hybrid filters is alsogiven for quick reference.

1 Introduction

Solid-state conversion of AC power using diodes andthyristors is widely adopted to control a number ofprocesses such as adjustable speed drives (ASD), furnaces,chemical processes such as electroplating etc., powersupplies, welding, heating etc. These solid-state convertersare also used in power industries such as HVDCtransmission systems, battery energy storage systems andinterfacing renewably energy electricity generating systems.Some of these solid-state controllers draw harmoniccurrents and reactive power from the AC mains andbehave as nonlinear loads. Moreover, in three-phase ACmains, they also cause unbalance and excessive neutralcurrent resulting in low power factor and poor efficiency ofthe systems. In addition to these, they, also create theproblems of poor utilisation of distribution system, RFIand EMI noise, interference to communication systems,voltage distortion, disturbance to neighbouring consumers,poor power quality at AC source such as notch, sag, swell,noise, spikes, surge, flicker, unbalance, low-frequencyoscillations and malfunction of protection systems. Becauseof such severity of power quality problems, severalstandards have been developed [1–4] and are being enforcedon consumers, manufacturers and utilities. Moreover thepower community has become more conscious of thesepower quality problems and numbers of technology optionshave been reported in the literature and research publica-tions [5–35]. Initially, lossless passive filters (LC) have beenused to reduce harmonics, and capacitors have been chosenfor power-factor correction of these nonlinear loads. Butpassive filters have the demerits of fixed compensation, largesize and resonance with the supply system. Active and filters

(AFs) [17, 19, 21, 26, 31, 32] have been explored in shuntand series configurations to compensate for different typesof nonlinear loads. However, they have the drawback thattheir rating is sometimes very close to load (up to 80%) insome typical applications and thus it becomes a costlyoption for power quality improvement in a number ofsituations. Moreover, a single active filter does not provide acomplete solution for compensation in many cases ofnonlinear loads due to presence of both voltage and currentpower quality problems. However, many researchers [33,34] have classified different types of nonlinear loads andhave suggested various filter options for their compensation.Because of the higher rating of AFs and cost considerations,the acceptability of AFs by users has been limited inpractical situations. In response to these factors, a series ofhybrid filters has been evolved and extensively used inpractice as a cost-effective solution for the compensation ofnonlinear loads. Moreover, these hybrid filters (HFs) arefound to be more effective in providing complete compen-sation of various types of nonlinear loads. Therefore, it isconsidered timely to present a broad perspective on hybridfilter technology for the power community dealing withpower quality issues.

This paper deals with a comprehensive survey on hybridfilters. More than 150 publications [1–155] have beenreviewed and classified into ten categories. The first category[1–35] is on standards, texts, review articles and benchmarkpublications on power quality issues. The second to fourthcategories include publications on single-phase hybrids ofpassive–passive [36–44], passive–active [45–56] and active–active [57–63] filters. The fifth to seventh–categoriespublications are on three-phase, three-wire hybrid filtersof passive–passive [64–73], passive–active [74–117] andactive–active [118–138] combinations. The eighth to tenthcategories of hybrid filters are on three-phase, fourwirepassive–passive [139, 140], passive–active [141–144] andactive–active [145–155] hybrid filters. However, these ninetypes of HFs are further classified into a number of circuitconfigurations. This paper covers the state of the art of HFtechnology, configurations, control approaches, selection ofcomponents and design considerations, comparative fea-tures, selection criteria and potential applications and latesttrends and future developments in HF technology.

B. Singh and V. Verma are with the Department of Electrical Engineering,Indian Institute of Technology, New Delhi 110016, India

A. Chandra and K. Al-Haddad are with the D!ept. de G!enie !Electrique, !ETS,1100, rue Notre-Dame Ouest, Montr!eal, Qu!ebec, Canada, H3C 1K3

E-mail: [email protected]

r IEE, 2004

IEE Proceedings online no. 20045027

doi:10.1049/ip-gtd:20045027

Paperfirst received 04th May 2004

IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 3, May 2005 365

2 State of the art

The technology of power filters is now matured forcompensating different types of nonlinear loads throughcurrent-based compensation and for improving the powerquality of the AC supply through voltage-based compensa-tion techniques such as elimination of voltage harmonics,sags, swell, notches, glitches, spikes, flickers and voltageunbalance and for providing even voltage regulation.Moreover, these filters are also identified according to thenature of the nonlinear loads, such as voltage fed (voltagestiff or voltage source on the DC side of the rectifierthrough the capacitive filter), current fed (current stiff orcurrent source on the DC side of the DC motor drive or thecurrent source for the CSI-fed AC motor drive etc.) and amix of these two types [26, 30–34]. Various topologies suchas passive, active, hybrid filters in shunt, series and both fortwo-wire single-phase, three-wire three-phase and four-wirethree-phase systems have been proposed using currentsource and voltage inverters to improve the power qualityof the AC mains. It has already been mentioned that hybridfilters are considered a better option for power qualityimprovement as a cost-effective and ideal solution for eithercompensation of nonlinear loads or to provide a clean andideal AC supply to a variety of loads. This Section describesthe chronological developments and present status of HFtechnology.

Because of extensive use of solid-state converters, thepollution level in AC supply systems is increasing rapidlyand power quality has become an important area ofresearch. A number of standards, surveys and texts havebeen reported [1–35] for improving the power quality andmaintaining it to the prescribed level through differentapproaches in single-phase, three-phase three-wire andthree-phase four-wire systems. Moreover, hybrid filtershave been developed using passive and active filters withtheir single, two or three units either to improve theirperformance or to reduce the cost of the system comparedto single active or passive filters. Lossless passive filters (LC)have been used for a long time as a combination of single-tuned, double-tuned along with damped highpass filterseither to suck current harmonics through creating harmo-nics valley in shunt with current fed nonlinear loads(thyristor based DC motor drive, HVDC, DC currentsource for CSI etc.) or to block harmonic currents throughcreating a harmonic dam in series with voltage fed nonlinearloads. However, these passive filters have the limitations offixed compensation and resonance with the supply system,which are normally overcome using active filters (AFs). Asingle unit of AF normally has high rating resulting in highcost and even then does not provide perfect compensation.The rating of active filters is reduced through augmenting itby passive filters to form hybrid filters, which reducesoverall cost and in many instances they provide bettercompensation than either passive or active filters. However,if one can afford the cost, then a hybrid of two active filtersprovides the perfect and best solution and thus it is knownas a universal power quality conditioner (UPQC) oruniversal active filter. Therefore, development in hybridfilter technology has been from a hybrid of passive filters toa hybrid of active filters to give a cost-effective solution andperfect compensation.

In a single-phase system, there are a large number ofnonlinear loads, such as fluorescent lamps, ovens, TVs,computers, air conditioners, power supplies, printers,copiers and high-rating traction systems. These loads arecompensated for using a hybrid of passive filters [37, 40, 41]as a low-cost solution and a hybrid of active filters in

traction systems [58]. A major amount of power isprocessed in three-phase three-wire systems, either in ASDswith small rating to reasonable power level, or HVDCtransmission systems in high power rating, and they behaveas nonlinear loads. These loads are also compensated forusing either a group of passive filters or a combination ofactive and passive filters in different configurations depend-ing on the nature of the AC system, such as current fed,voltage fed or a mix of them. Vastly distributed single-phasenonlinear loads have power quality problems in three-phasefour-wire AC systems and are compensated for using anumber of passive filters or active filters or their combina-tion as hybrid filters.

One of major reason for the advance of hybrid filtertechnology consisting active filter elements is due to thedevelopment of fast self-commutating solid-state devicessuch as the MOSFET (metal oxide semiconductor fieldeffect transistor) and IGBT (insulated gate bipolar transis-tor). An improved and low-cost sensor technology is alsoresponsible for reducing the cost and improving theresponse of HFs. Fast Hall-effect sensors and compactisolation amplifiers have made HFs affordable.

Another major factor for the revolution in HFtechnology has been the evolution of microelectronics.The development of low-cost, high-accuracy and fast digitalsignal processors, microcontrollers and application specificintegrated circuits (ASICs) has made possible the imple-mentation of complex control algorithms for online controlat an acceptable price. A number of control theories of HFssuch as instantaneous reactive power theory, synchronouslyrotating frame theory and many more with several lowpass,highpass and bandpass digital filters along with severalclosed-loop controllers such as the proportional–integral(PI) controller, the sliding-mode controller (SMC) etc., havebeen employed to implement hybrid filters. Moreover,many manufacturers are developing hybrid filters even atquite large power ratings to improve the power quality of avast range of nonlinear loads.

3 Configurations

HFs can be classified based by a number of elements intopology, supply system and types of converter used in theircircuits. The supply system can be single-phase (two-wire),three-phase three-wire and three-phase four-wire to feedvariety of nonlinear loads. The type of converters can bevoltage-source inverter (VSI) or current-source inverter(CSI) to realise active filters (AFs) as a part of hybrid filterwith appropriate control. The number of elements in thetopology can either be two, three or more, which may eitherbe active filters (AFs) or passive filters (PFs). Here, the mainclassification is made on the basis of the supply system, withfurther sub-classification on the basis of filter elements.Figure 1 shows the proposed classification of hybrid filtersbased on the supply system with the topology as a furthersub-classification. However, there is common sub-classifica-tion in each case of supply system. Therefore, majorclassification is made on the basis of number (two andthree) and types of elements (passive and active filters) indifferent topologies in each case of supply system. Thesehybrid filters are classified into a hybrid of two passiveelements as shown in Fig. 2 as two circuits followed by ahybrid of three passive elements as shown in Fig. 3 resultingin only two circuits. The main hybrid of two elements, oneactive and one passive filter, has eight valid combinations,as shown in Fig. 4. Similarly the hybrid filters consisting ofthree elements, two passive with one active and one passivewith two active filter elements, each have 18 valid circuits

366 IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 3, May 2005

resulting in a total of 36 circuit combinations, as shown inFigs. 5 and 6, respectively. It is followed by a hybrid of twoand three active filter elements each having only twocircuits, as shown in Figs. 7 and 8, respectively. The hybridfilters of more than three elements are rarely used becauseof cost and complexity considerations and hence are notincluded here. These hybrid filters as a combination of twoand three active and passive elements result in a total of 52practically valid circuit configurations.

However, these 52 circuit configurations of hybrid filtersare valid for each case of supply system of single-phase two-wire, three-phase three wire and three-phase four wire ACsystems. In each case of supply system, four basic elementsof filter circuit as passive series (PFss), passive shunt (PFsh),active series (AFss) and active shunt (AFsh) are required todevelop complete hybrid filter circuit configurations.Figures 9–20 show some of the basic circuits of these fourelements for three cases of supply systems. However, theremay be many more combinations such as active filterelements using current source inverters or reduced devicesvoltage source inverters etc.

Normally each passive filter element employs three tunedfilters, the first two being for the lowest dominantharmonics followed by highpass filter elements. However,in some high-power applications such as HVDC systems,five tuned filter elements are used, having four for the four

lower dominant harmonics and the fifth one as a highpassdamped filter element. In each passive filter element of seriestype (PFss), two lossless LC components are connected isparallel for creating a harmonic dam to block harmoniccurrents. All the three or five components of the passiveseries filter are connected in series, as shown in Figs. 9, 13and 17. However, in the passive shunt filter element (PFsh),two lossless LC components are connected in series, forcreating a harmonic valley to sink harmonic currents. Allthe three or five components of the passive shunt filter(PFsh) are connected in parallel, as shown in Figs. 10, 14and 18 for three supply systems.

Similarly each active filter element employs a VSIpreferably with self-supporting DC bus with electrolyticcapacitor (Cd), and an AC inductor (Lr) along with optionalsmall AC capacitor (Cr) to form a ripple filter to eliminatethe switching ripple. They may also use a CSI with inductiveenergy storage at the DC-link with current control of shuntAC capacitors to form an active filter element. However, aVSI is normally preferred because of various advantages,such as low losses, small size, low noise etc. Depending onsupply system, this VSI-based active filter element may be asingle-phase two-arm bridge, three-phase three-arm bridgeand three-phase four-arm or mid point or three single-phaseto form an active filter element for a four-wire system.These units can be connected in series directly in single

hybrid filters

single − phase three − phase three − wire three − phase four − wire

passive − passive

passive − active passive − active passive − active

passive − passive passive − passive active − active active − active active − active

Fig. 1 Classification of hybrid filters for power quality improvement

isVs

Zs

ACmains PFsh

PFss

nonlinearloads

isVs

Zs

ACmains PFsh

PFss

nonlinearloads

a

b

Fig. 2a Hybrid filter as combination of passive-series (PFss) and passive-shunt (PFsh) filtersb Hybrid filter as combination of passive-shunt (PFsh) and passive-series (PFss) filters

is

is

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Vs

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PFss1 PFss2

PFshnonlinear

loads

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PFsh1 PFsh2nonlinear

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b

Fig. 3a Hybrid filter as combination of passive-series (PFss1), passive-shunt(PFsh) and passive-series (PFss2) filtersb Hybrid filter as combination of passive-shunt (PFsh1), passive-series(PFss) and passive-shunt (PFsh2) filters


phase to reduce the cost [48, 53] or through injunctiontransformers usually with higher turns on the VSI side toform active series filter element (AFss), as shown in Figs. 11,15, 19a, 19band 19c, respectively for two-wire, three-wireand four-wire systems to act as a high active impedance toblock harmonic current and low impedance for funda-mental current. In the same manner, active shunt filterelement (AFsh) may either connected directly or throughstep down transformers to connect the VSI at optimumvoltage to act as an adjustable sink for harmonic currents,as shown in Figs. 12, 16 and 20 for three cases of AC supplysystem, respectively.

The total number of valid basic circuit configurationsof HFs for all three cases of supply system results in156 configurations to suit the majority of applica-tions for improving the power quality of the systemeither having nonlinear loads or polluted AC supply.Moreover, there may be many more variation inactive filter elements or passive filter elements, but thebasic concept of HFs will remain out of these circuitconfigurations.

4 Control approaches

The control scheme is the heart of the HF in which anactive filter (AF) element is involved. Out of 52 configura-

tions, 48 have one, two or three AF elements. Most HFsrequire a control scheme, which has three major stages. Thefirst stage includes the sensing of all essential controlvariables such as AC voltages, AC and/or DC currents,DC voltage etc. to feed in to the digital processor for use inthe control algorithm after signal conditioning to a properlevel for inputting to analogue to digital conversion, eitherinbuilt in the processor or interfaced as a module. In thesecond stage, the control algorithm is implemented throughsoftware in the processor to derive output control signals inthe form of injected voltages for the series active filter (AFss)and currents to act as current sources for the active shuntfilter (AFsh). In the third and last stage, PWM gating signalsare generated either in the processor using a dedicatedPWM generator optimally or complemented in the hard-ware externally. These gating signals are isolated andamplified to feed solid-state switching devices of the AF.

4.1 Signal sensing and conditioningThe control algorithm in its implementation requires anumber of instantaneous current and voltage signals fromthe HF system. These signals are generally AC voltage atthe point of common coupling (PCC), injected voltages inthe AFss element, AC currents in the AFsh element, DC busvoltage or current depending on the use of VSI or CSI inthe implementation of the active filter. The AC voltages are

ACmains

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g

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is nonlinearloads

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PFshvs

is nonlinearloads

f

Fig. 4a Hybrid filter as combination of series-connected passive-series (PFss) and active-series (AFss) filtersb Hybrid filter as combination of parallel-connected passive-series (PFss) and active-series (AFss) filtersc Hybrid filter as combination of passive-shunt (PFsh) and active-series (AFss) filtersd Hybrid filter as combination of active-shunt (AFsh) and passive-series (PFsh) filterse Hybrid filter as combination of active-shunt (AFsh) and passive-shunt (PFsh) filtersf Hybrid filter as combination of series-connected passive-shunt (PFsh) and active-shunt (AFsh) filtersg Hybrid filter as combination of passive-series (PFss) and active-shunt (AFsh) filtersh Hybrid filter as combination of active-series (AFss) and passive-shunt (PFsh) filters


sensed using a potential transformer, isolation amplifieror Hall effect voltage sensors. However, distortedvoltages such as injected voltages in the AFss and DC

voltage are sensed through an isolation amplifier or Halleffect sensors. Similarly current signals are sensed eitherusing current transformers or Hall-effect current sensors,

Acmains

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isVs

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Fig. 5a Hybrid filter as combination of passive-shunt (PFsh), passive-series (PFss) and active-series (AFss) filtersb Hybrid filter as combination of passive-series (PFss) passive-shunt (PFsh), and active-series (AFss) filtersc Hybrid filter as combination of passive-series (PFss1) in series with parallel-connected active-series (AFss) and passive-series (PFss2) filtersd Hybrid filter as combination of passive-shunt (PFsh) and parallel-connected active-series (AFss) and passive-series (PFss) filterse Hybrid filter as combination of passive-series (PFss1), active-shunt (AFsh) and passive-series (PFss2) filtersf Hybrid filter as combination of parallel-connected passive-shunt (PFsh) with active-shunt (AFsh) and passive-series (PFss) filtersg Hybrid filter as combination of active-series (AFss), passive-shunt (PFsh) and passive-series (PFss) filtersh Hybrid filter as combination of series-connected passive-shunt (PFsh) with active-series (AFss) and passive-series (PFss) filtersi Hybrid filter as combination of series-connected passive-series (PFss1) with active-series (AFss) in parallel with passive-series (PFss2) filtersj Hybrid filter as combination of passive-series (PFss) and parallel-connected passive-shunt (PFsh) with active-shunt (AFsh) filtersk Hybrid filter as combination of passive-shunt (PFsh), passive-series (PFss) and active-shunt (AFsh) filtersl Hybrid filter as combination of series-connected passive-series (PFss) with active-series (AFss) and passive-shunt filtersm Hybrid filter as combination of passive-shunt (PFsh1), active-series (AFss) and passive-shunt (PFsh2) filtersn Hybrid filter as combination of passive-series (PFss) and series-connected passive-shunt (PFsh) with active-shunt (AFsh) filterso Hybrid filter as combination of passive-shunt (PFsh1) and series-connected active-series (AFss) with passive-shunt (PFsh2) filtersp Hybrid filter as combination of active-shunt (AFsh), passive-series (PFss) and passive-shunt (PFsh) filtersq Hybrid filter as combination of parallel-connected active-series (AFss) with passive-series (PFss) and passive-shunt (PFsh) filtersr Hybrid filter as combination of passive-shunt (PFsh1) and parallel-connected passive-shunt (PFsh2) with active-series (AFsh) filters


depending on sensing of sinusoidal AC current or distortedAC and DC currents. These variables after sensing arescaled down to a suitable level to be fed to ADC channels,phase lock loop (PLL) input, synchronising signals etc.Sometimes these signals are filtered in hardware or softwareto avoid switching noise or undesired signals with lowpass(LF), bandpass (BF) and highpass (HF) filters.

4.2 Control algorithmsThere are many control approaches, which are used inimplementation of HFs having active filter elements.Initially there have been two methods, based on fre-quency-domain and time-domain compensation. Controlmethods based on time domains are simple to implementand result in fast dynamic response due to instantaneous

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Fig. 6a Hybrid filter as combination of active-shunt (AFsh), passive-series (PFss) and active-series (AFss) filtersb Hybrid filter as combination of active-serres (AFss), active-shunt (AFsh) and passive-series (PFss) filtersc Hybrid filter as combination of active-series (AFss1) in active-series (AFss2) filtersd Hybrid filter as combination of active-shunt (AFsh), parallel-connected passive series (PFss) with active-series (AFss) filterse Hybrid filter as combination of active-series (AFss1), passive-shunt (PFsh) and active-series (AFss2) filtersf Hybrid filter as combination of active-shunt (AFsh), passive-shunt (PFsh) and active-series (AFss) filtersg Hybrid filter as combination of passive-series (PFssh), active-shunt (AFsh), passive-shunt (PFsh) and active-series (AFss) filtersh Hybrid filter as combination of series-connected active-shunt (AFsh) with passive-shunt (PFsh) and actives-series (AFss) filtersi Hybrid filter as combination of active-series (AFss), passive-series (PFss) and active-series (AFss2) filtersj Hybrid Filter as combination of active-series (AFss), active-shunt (AFsh) and passive-shunt (PFsh) filtersk Hybrid filter as combination of active-shunt (AFsh), active-series (AFss) and passive-shunt (PFsh) filtersl Hybrid filter as combination of active-series (AFss), passive-series (PFss) and active-shunt (AFsh) filtersm Hybrid filter as combination of active-shunt (AFsh1), passive-series (PFss) and active-shunt (AFsh2) filtersn Hybrid filter as combination of series-connected actives-series (AFss), series-connected active-shunt (AFsh) and passive-shunt (PFsh) filterso Hybrid filter as combination of series-connected active-shunt (AFsh1), series-connected active-shunt (AFsh2) and passive-shunt (PFsh) filtersp Hybrid filter as combination of passive-shunt (PFsh), active-series (AFss) and active-shunt (AFsh) filtersq Hybrid filter as combination of parallel-connected passive-series (PFss) with active-series (AFss) and active-sehunt (AFsh) filtersr Hybrid filter as combination of active-shunt (AFsh1), in series with parallel-connected active-shunt (AFsh2) and passive-shunt (PFsh) filters


derivation of compensation commands and therefore,nowadays, they are only used in HFs.

Normally in the implementation of the active series filterelement (AFss), compensating command signals are derivedin the form of injected voltages, and in the active shunt filterelement (AFsh) compensating commands are derived in theform of currents. There are many control approaches, such

as instantaneous p–q theory [76, 77], synchronous rotatingreference frame d–q theory [80], synchronous detectionmethod [86] and many more to derive the control commandsignals. In general, a closed-loop proportional–integral (PI)or sliding-mode controllers are used on the DC bus voltageof the AF to provide a self-supporting DC bus and areaugmented into a control method to draw small active

Zs

Zs

is

is

Vs

VsAFsh

AFss

AFsh

AFss

Acmains

Acmains

nonlinearloads

nonlinearloads

a

b

Fig. 7a Hybrid filter as combination of active-series (AFss) and active-shunt(AFsh) filtersb Hybrid filter as combination of active-shunt (AFsh) and active-series(AFss) filters

nonlinearloads

nonlinearloads

Zs

Zs

is

is

Vs

Vs

AFsh

AFsh1 AFsh2

AFss1 AFss2

AFss

Acmains

Acmains

a

b

Fig. 8a Hybrid filter as combination of active-series (AFss1), active-shunt(AFsh) and active-series (AFss2) filtersb Hybrid filter as combination of active-shunt (AFsh1), active-series(AFss) and active-shunt (AFsh2) filters

nonlinearloads

C3

L3

C5

L5

Ch Rh

Lh

Zs

is

VsPFSSAc

mains

Fig. 9 Single-phase passive-series filter normally used as a unitPFss in a single-phase hybrid filters

nonlinearloads

C3

L3

C5

L5 Lh

Ch

Rh

Zs

is

Vs

PFSh

Acmains

Fig. 10 Single-phase passive shunt filter normally used as a unitPFSh in a single-phase hybrid filters

nonlinearloads

T3

T4T2

T1

Cd

CrLr

Zs

is

Vs

AFSS

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transformers

Fig. 11 Single-phase passive-series filter normally used as a unitAFss in a single-phase hybrid filter

nonlinearloads

T1 T3

T4T2

Cr

CdLr

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is

Vs

AFSh

Acmains

Fig. 12 Single-phase active-shunt filter normally used as a unitAFsh in a single-phase hybrid filter

isa

isb

isc

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Rh

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C7

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C5

L7

C7

Vsa

Vsb

Vsc

Zsb

Zsa

Zsc

PFSSa

PFSSb

PFSSc

three - phasethree - wirenonlinear

loads

Fig. 13 Three-phase three-wire passive-series filter normally usedas unit PFss in three-phase three-wire hybrid filter


Vsa

isa

Zsa

Ch

Lh RhL7

C7

L5

C5 Ch

Lh RhL7

C7

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C5 Ch

Lh RhL7

C7

L5

C5

Vsb

isb

Zsb

Vsc

isc

Zsc

PFSha PFShb PFShc


loads

Fig. 14 Three-phase three-wire passive-shunt filter normally used as a unit PFsh in a three-phase three-wire hybrid filter


loads

Vsa

Vsb

Vsc

isc

Zsc

Zsb

Tr

Tr

Tr

CrLr

Cd

Lr

LrCr

Cr

Zsa

isa

isb

AFss

Fig. 15 Three-phase three-wire active-series filter normally used asa unit AFss in a three-phase three-wire hybrid filter

isa

ZsaVsa

Vsb

Vsc

Zsb

Zscisb

Cr

CrCr

Lr Lr Lr

Cd

isc

AFSh


loads

Fig. 16 Three-phase three-wire active-shunt filter normally used asa unit AFsh in a three-phase three-wire hybrid filter

n

n

a

b

c

Vsa

isa

Ch Rh

Lh

Ch Rh

Lh

Ch Rh

Lh

C5C3

L5L3

C5C3

L5L3

C5C3

L5L3

isb

isc

Vsb

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Zsa

Zsb

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PFssb

PFssc

three - phasefour - wirenonlinear

loads

Fig. 17 Three-phase four-wire passive-series filter normally used as a unit PFss in a three-phase four-wire hybrid filter

n

a

b

cn

Vsa

isa

Zsa

Zsb

Zsc

Ch

LhRh

Ch

Lh LhRh

Ch

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C5

L5

C3

L3

C5

L5

C3

L3

C5

L5

isb

isc

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Vsc


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PFsha PFshb PFshc

Fig. 18 Three-phase four-wire passive-shunt filter normally used as a unit PFsh in a three-phase four-wire hybrid filter


power from the system for feeding losses in the AF system.These compensating commands, either in the form ofinjected voltage for active series filter (AFss) or in the formof compensating reference currents for a active shunt filterelement (AFsh) of HFs, are used in PWM control togenerate the gating signals for the solid-state switchingdevices of the VSI of the AF of HFs.

4.3 Generation of gating signalsIn the third or final stage of control of the AF in HFs,different closed-loop PWM controllers are used to generategating signals. The PWM controllers are implementedeither in the processor through optimised software andassociated hardware of the processor or in analoguehardware. Nowadays implementation of these PWMcontrollers is preferred in the software of the processor,which reduces the cost of the system and enhances thereliability due to a reduction in component count. Thederived compensating command signals are compared withsensed feedback signals and any error is processed in thePWM controller and generates digital gating signals. Thereare a number of closed-loop PWM controllers such ascarrier-less PWM controllers (hysteresis, on/off etc.),carrier-based PWM controller such as PI, dead beat,sliding-mode controller (SMC), and many more implemen-ted through software in the processor already used forderiving compensating commands. These digital (low/high)gating signals are buffered, isolated and amplified to feedthe gate of the solid-state switching devices of the AF usedin HFs.

5 Selection of components and designconsiderations

The selection of components of the hybrid filters is animportant aspect to attain a high-level performance. Thereare a number of components in HFs, such as passive filterelements, active filter elements, control scheme employingsensors, processor, isolation amplifier circuits, interfacingcircuits, injection transformers etc.

A passive filter (PF) element consists of several ACcapacitors and inductors, and a small resistor to be usedin the damped highpass filter, as shown in Figs. 9, 10, 13,14, 17 and 18. These inductors must have a quality factor ashigh as possible to reduce the losses in the system andthey must be designed in such a way that they must notsaturate in whole current operating range. In passive filters,the value of the capacitor is decided by the required reactivepower in the system, and then the value of the inductoris calculated by tuning it for a particular harmonicfrequency. The quality factor for calculating the value ofthe resistance is decided by the desired sharpness in thecharacteristics of the filter. Similarly the resistance in thehighpass filter is also calculated based on the sharpness or

n

a

b

c

n

a

b

c

a b c

LrCd

Cd

Cd1

Cd2


loads


loads

LrLr

Lr LrLrLr

LrLr Lr

n

isc

isb

isa

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Zsa

Vsb

Vsc

n

isc

isb

isa

Vsa

Zsb

Zsc

Zsa

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Vsc

n

isc

isb

isa

Vsa

Zsb

Zsc

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Vsc

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Tr

Tr

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Cr

Cr

Tr

Tr

Tr

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AFSS


loads

a

b

c

Fig. 19a Three-phase four-wire active-series filter with capacitor mid pointnormally used as a unit AFss in a three-phase four-wire hybrid filterb Three-phase four-wire active-series filter with four-pole topologynormally used as unit AFss in three-phase four-wire hybrid filterc Three-phase four-wire active-series filter with three-single-phase-bridge used as a unit AFss in a three-phase four-wire hybrid filter

n

a

b

c

n

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Tr

Tr

Tr

Tr

Tr

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Vsa

isa

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isc

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Vsc

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loads

Cd 2

Cd 1

AFsh

Fig. 20 Three-phase four-wire active-shunt filter as a unit AFsh ina three-phase four-wire hybrid filter


attenuation of the higher-order harmonics on one side andreducing the losses on the other side to achieve optimumvalue of the quality factor. Another important componentin the hybrid filter is the active filter element (AF), which isnormally realised using a VSI with ripple filter on the ACside and an electrolytic capacitor to provide a self-supporting DC bus. The ripple filter consists of essentialcomponents such as a capacitor (Cr) and an inductor (Lr) toimprove voltage profile. However, a major component ofthe AF is the solid state switching device, which is aMOSFET for small ratings, an IGBT medium powerratings and a GTO (gate-turn-of thyristor) for exceptionallyhigh power rating to build the VSI of the AFs. These solid-state switching devices are used in modular form, especiallyin low and medium power ratings, as power modules (PMs)or intelligent power modules (IPMs) consisting of severaldevices in one module with gating, protection andinterfacing elements to reduce size, cost and weight of theAF to be used as an element in HFs. One of the majorcomponents is a control scheme, which is implemented intothe processor. First, the voltage and current signals aresensed using PT, CT, Hall-effect sensors and isolationamplifiers. Many manufacturers are developing thesesensors at reasonably low cost. The main and importantcomponent is the processor, which executes an online wholecontrol algorithm after receiving input signals throughADC channels, PLL and synchronised interrupt signals. Italso implements PWM control optimally. All three tasks ofinputting data, computation of algorithm and optimisedPWM generation are carried out concurrently in themodern-day specially designed processors for powerelectronics applications. Many manufacturers are offeringthese processors at give away prices, e.g. Texas Instrumentsseries TMS320F24X, Analog Devices ADMC401, Intel80196 etc. and ASICs for easy and cost-effective imple-mentation of HFs.

6 Comparative features and other options forpower quality improvement

The classified HFs of nine major categories and 156 circuitconfigurations are capable of improving power quality fromthe low-cost HF as a combination of two passive elementsto UPQC (unified power quality conditioner) as perfectcompensation for nonlinear loads as well as providing aclean power supply. Unfortunately, in practice sometimes,most critical loads, such as UPS (uninterruptibte powersupply), behave as nonlinear loads and need clean power. Insuch situations, UPQC is the only right HF to provideperfect compensation. However, it is the costliest HF.

In many situations, active shunt filters can alsocompensate nonlinear loads but their rating and cost aremuch higher compared to an HF of a combination of oneactive and one passive element. In many cases, the rating ofthe AF element in the HF can be reduced to less than 5% ofthe load, resulting in drastic reduction in its cost [76].

Similarly, in some applications, an active series filteralone can provide effective compensation in voltage-fedtypes of loads [33], but its rating can be reduced to less thanone-fifth by adding a passive series filter to it [30]. Therefore,a low rating series AF along with a passive series filter ashybrid filter provides cost-effective compensation. However,there are many methods of power quality improvements insome applications, but HF is one of best alternativemethods of improving power quality. For example, inplace of the three-phase diode rectifier with capacitive filter,one may choose a multipulse AC–DC converter [8], oractive series filter alone [33], but it can easily be observed

that the hybrid filter, as a combination of active series withpassive series filter, is a most appropriate method of powerquality improvement for such a voltage-fed nonlinear load.

In some applications, one can also use a unity powerfactor rectifier for improving power quality. However, inretrofit applications, the replacement of an existing rectifier(AC–DC converter) may be a costly option and may needinterruption of process or plant for a longer period withhigher investment. In such cases, a hybrid filter of propertopology may be the right option and may be brought intoservice within no time. However, with HFs, there may bemany options to the users, but one has to select the mostsuitable HF for a specific application depending on the cost,size, level of performance etc. After, taking into accountthese considerations carefully, one can easily select the bestoption of HF for a particular application.

7 Selection criteria and potential applications

Selection of HFs for a particular application is an importantdecision for users. The following are the some of the factorsfor the selection and design of the right HF configurationfor a specific situation:

� type of supply system (single-phase, three-phase three-wire, three-phase four-wire)

� nature of load (voltage fed, convert fed or mixed)

� rating of load

� compensation required in currents (harmonics, reactivepower, balancing, neutral current)

� compensation required in voltages (harmonics, flicker,unbalance, regulation, sag, swell, surge, spikes, notches etc.)

� pattern of load (fixed, variable, fluctuating)

� level of compensation required (THD, individualharmonic reduction meeting specific standard etc.)

� cost (low cost of passive filters to UPQC)

� size� weight� efficiency� reliability (component counts)

� environmental factors (ambient temperature, altitude,pollution level, humidity, types of cooling etc.).

These are only some of the factors that provide someguidelines for the selection and design of a specific hybridfilter. However, the following are the some additionalcriteria for the selection of HFs.

The selection of HF configuration also depends on thenature of the load. Nonlinear loads can be classified in tothree major categories. The first category is voltage fed orvoltage source or voltage stiff loads, such as dioderectifier with capacitive filter at the DC-link feedingvariable-frequency VSI-based AC motor drive (ASDs),power supplies with front-end diode rectifier with capacitivefilters used in computers and other appliances,battery charger etc. These voltage stiff loads drawdiscontinuous and nonsinusoidal currents from the ACmains resulting in very high THD of the AC current,poor power factor and distortions of the AC terminalvoltage at the PCC. They behave as uncontrolled current-sink-type nonlinear loads. One of the best topologies of HFfor such loads is a combination of active series filter (AFss)with passive series filter (PFss) (Fig. 4a) if there is noproblem of pollution of the supply system. It creates a dam


for current harmonics and provides an adjustable solutionfor varying loads. It also avoids the resonance problem ofpassive filters.

The second category of loads is current fed or currentsource or current stiff loads, such as thyristor-converter-fedDC motor drives, and thyristor converters used to deriveDC current source for feeding CSI and HVDC systems.These current stiff loads have stiff type of currentwaveforms and have an almost predetermined level ofharmonics. One of the best topologies for compensating forsuch loads is an HF of a combination of active shunt (AFsh)in series with passive shunt filters (PFsh), as shown inFig. 4f, provided that the load is a balanced one in the caseof a three-phase three-wire system. However, if the powersource has power quality problems and one wants tocompensate for reactive power of the load, then a HF ofUPQC type (Fig. 7a) may be the best option. Therefore,depending on the power quality problems involved, one canchoose a lowest cost and rating topology to a costly but bestcompensating topology for the HF.

The third category of loads are a mix of two types; itmeans that they are neither stiff voltage nor stiff currenttype and may have a group of loads of both kinds. In such acase an HF topology of active series with passive shunt filterelements (Fig. 4h) is one of the best, provided that thepower supply is ideal and an adjustable reactive powercompensation is not required. If more power qualityproblems are involved, one has to choose a more complexand costly topology of the HF, such as UPQCs.

These are some of the basic guidelines for the selection ofsuitable HFs. Once the user identifies the types of powerquality problems, they can find much more literature on thespecific type of HFs and consult application notes of severalmanufacturers of HF to select the most suitable HF for therequired specific applications.

One of major potential application of HFs is in ASDs,which behave as several kinds of nonlinear loads such asvariable-frequency VSI-fed AC motor drives as voltage stiffloads, CSI-fed AC motor drives as current stiff loads etc.Therefore, the right topology of HF for two types of ASDapplications such as these is different, as already discussedearlier in detail. Another major application sector is powersupplies and appliances, which behave as either voltage stiffor mixed types of nonlinear loads.

There are a number of applications of solid-statecontrollers, which have severe power quality problems suchas heaters, ovens, arc furnaces, welders, process industriessuch as electroplating, electro mining etc., utility sector suchas HVDC systems, some of the thyristor controlled staticVAR compensators etc. In view of these existing loads andadditional applications of solid-state controllers withassociated power quality problems, HFs will be increasinglyrequired to improve power quality. Moreover, the newerand additional configurations of HFs will further increasetheir use as a more cost-effective and better solution topower quality problems.

8 Latest trends and future developments

HF technology has developed to a mature level and isextensively used in small rating power supplies to largerating ASDs and HVDC systems. There is consistent newdevelopment of additional circuit configurations of HFs toprovide cost-effective and improved performance of HFs.Some of new concepts, such as reducing the rating of activefilter element in shunt and series filters and eliminating thedrawbacks of passive filters such as resonance and fixed

compensation for improved performance of HFs, have beenimplemented for various nonlinear loads.

New topologies of HFs have been developed, which notonly compensate for nonlinear loads but also provide cleanAC power to the critical loads, even the nonlinear nature.Moreover, the appropriate circuit topology of the HF hasreduced the rating of the AF element and provides moreeffective control. A typical example is the case of therecently introduced hybrid filter as a combination of activeseries filter (5%) with passive series filter (20%), whichneeds only 25% rating of load in the case of voltage fedloads [30, 33], whereas the rating of active shunt filter forsuch loads can be as high as 60–80% of the load. Reductionin the rating of the active filter element allows the use of aMOSFET as switching device in the VSI of the AF inreasonably high rating HFs, which operate at very highswitching frequency, thus providing fast response andreduction in the size of ripple filter passive elements andsize of the DC bus capacitor.

Moreover, use of an improved control algorithm reducesthe requirements of sensors and provides the fast responseof the system. Development of dedicated applicationspecific integrated circuits (ASICs) and new series of DSPsand microcontrollers is providing cost-effective and com-pact HFs.

The development of improved switching devices withreduced conduction losses and high permissible switchingfrequency and better gating requirement will improve theHF technology. Moreover, the concept of IPM throughintegration of several devices into one module, theirprotection and control signal isolation and amplification,will reduce the cost, size, losses and component counts ofHFs.

New development in magnetics such as filter magneticmaterials will reduce the losses and size of passive filterelements such as transformers, inductors, etc., and thus costand weight of HFs. The introduction of better quality ofcapacitors is expected to reduce passive filter elements andreduce the failure rate of HFs. Moreover, invention of newadditional configurations of HFs will provide betteralternatives to existing applications and will explore anumber of newer applications.

9 Conclusions

A comprehensive review of HFs has been presented toprovide a wide exposure on various issues of the HFs to theresearchers, designers and users of these filters for powerquality improvement. A classification of HFs into ninecategories with many circuits in each category is expected toprovide an appropriate topology for a particular applica-tion. These hybrid filters can be considered as a betteralternative for power quality improvement owing toreduced cost, simple design and control and high reliabilitycompared to other options of power quality improvement.Some of the circuit configurations of HFs avoid theproblems involved in passive and active filters, and thereforeprovide cost-effective and better solutions for harmonicelimination of nonlinear loads. Moreover, owing to thelarge number of HF circuits, the user can select the mostappropriate topology with required features to suit for aspecific application. It is hoped that this survey of hybridfilters will be beneficial to designers, users, manufacturersand research engineers dealing with power quality improve-ment.


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Hybrid filters for power quality improvement

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