IGCTs megawatt power switches for medium-voltage applications

Size: px

Start display at page:

Download "IGCTs megawatt power switches for medium-voltage applications"

Marcia Moore
5 years ago
Views:

1 IGCTs megawatt ower switches for medium-voltage alicatios Desigig equimet to switch megawatts of ower at medium-voltage levels is a difficult task. The iheret characteristics of the two available silico switchig techologies, Gate Tur-Off (GTO) Thyristors ad Isulated Gate Biolar Trasistors (IGBTs), force desig trade-offs that icrease the cost ad comlexity of ower cotrol systems. GTO thyristors ot oly require comlex eriheral circuitry to esure reliable oeratio but also switch at low frequecy. Desigers of IGBT-based systems at medium voltage must deal with high losses ad balace a icrease i the umber of comoets with the eed to esure availability. ABB s ew Itegrated Gate Commutated Thyristor (IGCT) techology overcomes the drawbacks of both the GTO thyristor ad the IGBT, ad icludes all the circuitry required to make the ower device reliable ad easy to desig ito medium-voltage alicatios. A BB maufactures high-ower GTO thyristors ad IGBTs for use where reliability is of aramout imortace. Power cotrol system alicatios iclude idustrial motor drives, trai ad shis roulsio, ad trasmissio lies 1, 2. May of these alicatios are at the medium-voltage levels of 2.3, 3.3, 4.16 ad 6.9 kv. Develomet of ower silico switches ABB has ivested cotiuously i the develomet ad roductio of GTO thyristor ad IGBT techologies, ad the erformace of both devices has imroved steadily as a result 3. I the last few years, a series of liked iovatios has created a latform for the desig ad roductio of a ower silico switch which exteds beyod the IGBT ad GTO thyristor erformace eveloe. Kow as the IGCT (for Itegrated Gate Commutated Thyristor), it ca switch more quickly ad has lower losses tha either the GTO thyristor or IGBT. More imortatly, it has characteristics which eable electrical ower system egieers to shrik the size ad cost of mediumvoltage systems while boostig their efficiecy ad reliability. A better ower switch I the 30 years sice their itroductio, ower silico switches have icreased Harold M. Stillma ABB Cororate Techology steadily i comlexity ad caability. The first silico-cotrolled rectifiers could switch ower off oly at the ed of a AC cycle. Progress came with the GTO thyristor, which ca switch at ay oit i the cycle. The itroductio of IGBTs brought faster switchig, but at reset their switchig losses are accetable oly at low voltage levels. The tedecy over the years has bee for the desigers of all these devices to cocetrate maily o the ower switchig itself, so that little attetio has bee aid to the comlexities ivolved i real-world alicatios. GTO thyristors cosist of thousads of idividual switchig elemets fabricated o a silico wafer. Losses occur i all four coditios of oeratio (o, off, switchig o, switchig off). At medium voltage, GTOs exhibit very low o-state losses ad reasoable tur-off losses. However, due to switchig beig o-homogeeous, exteral subber circuits are ecessary for the switchig oeratio. These subber circuits take u more tha half the volume of the fial equimet ad accout for much of the desig comlexity, costs ad losses. Coversely, IGBTs have comaratively higher coductio losses but switch homogeeously, ie they do ot eed a subber. However, they are ot yet available for direct oeratio at all mediumvoltage levels. To overcome this hadica, desigers must coect low-voltage IGBTs i series, dramatically icreasig the comlexity, icreasig losses ad reducig system reliability. A coverter desiged for oeratio at 4.16 kv, for examle, requires four series-coected 1.8 kv IGBTs er hase. GTO thyristors ca be roduced ecoomically for alicatios at most medium-voltage levels. It is aticiated that 3.3-kV ad 4.5-kV IGBTs will become available ad simlify the desig of medium-voltage ower circuits, but it is kow already today that they will have high losses. To overcome these losses ad the resultig heat build-u, the IGBTs 12 ABB Review 3/1997

2 will have to have a larger silico area, ad this will icrease costs. The ideal ower switch would switch like a IGBT ad coduct like a GTO thyristor, ad it would have the low fabricatio costs ad high yields of the GTO thyristors. This is exactly what the IGCT achieves (Table 1). The secret of IGCT techology I IGCT techology, a combiatio of desig iovatios ermits the thousads of idividual ower switchig structures i a modified GTO thyristor to switch fast ad simultaeously. What is more, the low o- ad off-state losses iheret i thyristor desigs are retaied. The first of two key iovatios is the buffer layer desig, which has allowed the o-state ad switchig losses to be reduced by a factor of 2 to 2.5 ad makes the otimum doig rofile of a GTO ad a diode virtually idetical. Previously, itegratig a diode with a GTO has resulted i severe degradatio of the With the IGCTs, equimet desigers ca build more comact ad less costly ower cotrol systems, eg static var comesators for utilities. diode s erformace. Although the idea of the buffer layer is almost as old as the GTO itself, it has ever bee used before for the followig reaso: to reduce the switchig losses, the charge stored i the device i the coductig hase has to be removed quickly at tur-off. I a covetioally desiged GTO this fuctio is erformed by aode shorts, which rovide a ath for the electros to flow out. 1 Table 1: IGCT techology combies the advatages of GTO thyristors ad IGBTs GTO thyristor High-voltage IGBT IGCT advatages Switchig techology Available for most Switches at high Switches at high frequecy medium-voltage levels frequecy Low switchig ad Low o-state losses Low switchig losses o-state losses Subberless Itegrated gate drive Subberless Available for most medium-voltage levels Power circuitry Safe from catastrohic Low arts cout at Safe from catastrohic failure low voltage failure Suitable for arallel ad series coectio at low voltage Prove reliability Itegrated diode ad gate uit for low arts cout Suitable for arallel ad series coectio Equimet desig Prove reliability Modular desig Allows comact, Comact modular equimet desig Reduced cablig ad itercoectio Alicatio-ready modules ABB Review 3/

Aother alicatio for IGCTs i boiler feedum drives 2 The combiatio of aode shorts ad a currets are reduced by almost oe order buffer layer, however, leads to extremely of magitude comared with a

3 Aother alicatio for IGCTs i boiler feedum drives 2 The combiatio of aode shorts ad a currets are reduced by almost oe order buffer layer, however, leads to extremely of magitude comared with a covetioal GTO without buffer. high trigger ad holdig currets. To solve this roblem the aode shorts have The secod desig iovatio addresses the gate cotrol. GTOs ad thy- bee omitted. Istead, the aode is made trasaret, ie ermeable, to the ristors are four-layer () devices. As electros, with the result that the trigger such, they have oly two stable oits i their characteristics o ad off. Every state i betwee is ustable ad results i curret filametatio. The iheret istability is worseed by rocessig imerfectios. This has led to the widely acceted myth that a GTO caot be oerated without a subber. Essetially, the GTO has to be reduced to a stable device (ie, a trasistor) for the few critical microsecods durig turoff. To sto the cathode () from takig art i the rocess, the bias of the cathode - juctio has to be reversed before voltage starts to build u at the mai juctio. This calls for commutatio of the full load curret from the cathode () to the gate () withi about oe microsecod. Thaks to a ew low iductive housig desig, 4,000 A/µs ca be achieved with a low cost 20-V gate uit. Curret filametatio is totally suressed ad the tur-off waveforms ad safe oeratig area are idetical to those of a trasistor (eg, a IGBT). Also, GTOs ca ow switch istatly, without jitter, so that series coectio is o loger a challege. Imrovemet i erformace of the GTO thyristor ad IGBT P Switchig ower 10 8 VA GTO 10 7 IGBT P Fusio of ower device ad circuit desig exeriece The IGCT techology is the result of itese collaboratio betwee device desigers at ABB Semicoductors ad ower circuit desigers at ABB Idustrial Systems. I fact, it was the co-develomet of the ower silico, the ackagig ad the additioal circuitry eeded to make the ower switch suitable for idustrial alicatios that made the IGCT s uique combiatio of characteristics ossible i the first lace. IGCT techology brigs together the ower hadlig device (GCT) ad the device cotrol circuitry (freewheelig diode ad gate driver) i a itegrated ackage 4. By offerig four levels of comoet ackagig ad itegratio 5, 6b, it ermits simultaeous imrovemet i four iterrelated areas: low switchig ad 14 ABB Review 3/1997

4 coductio losses at medium voltage, simlified circuitry for oeratig the ower semicoductor, reduced ower system costs, ad ehaced reliability ad availability. Also, by rovidig re-e- gieered switch modules, IGCT techology eables medium-voltage equimet desigers to develo their roducts faster. + Advatages of medium-voltage IGCT techology Low switchig losses a A advatage of low-loss switchig is that it allows equimet desigers to choose the switchig rate that best matches the ed-alicatio requiremets. Previous ower devices limited oeratio to 250 Hz at full rated curret. IGCT techology ca oerate at u to four times that seed. I motor drive sys- + tems, for examle, a desiger might choose faster switchig to obtai better system efficiecy. Alteratively, the efficiecy of iverter systems is imroved ad their losses reduced by selectig a slower switchig rate for a IGCT. b Reductio of associated circuitry The uique device-level characteristics of the GCT ermit subberless oeratio, with imortat beefits for desig egieers. 6 demostrates this clearly: iverter desigs with subbers are large ad comlex, while subberless iverters, besides exhibitig lower losses, are comact ad have far fewer comoets. Reliability is better as a result. Equimet desigs based o IGCT techology are also simlified by the itegratio of the freewheelig diodes i the GCT structure. This is ossible as the reductio i thickess of the GCT silico (the same reductio that makes low-loss switchig ossible) also ermits a efficiet diode to be fabricated o the same wafer. Comariso of the GCT, with ower switch ad freewheelig diode itegrated o the same wafer, ad the IGBT. The GCT is sigificatly less comlex tha the IGBT. a Cross-sectio through a GTO thyristor (left), diode (ceter) ad GCT (right), showig how the trasaret emitter ad buffer layer of the GCT allows a thier -base layer. This ermits itegratio of the freewheelig diode i the same structure, lus subberless oeratio i combiatio with low iductive ackagig ad gate drive circuitry. b Cross-sectio through a sigle cell of a IGBT (left) ad a diode (right). A comlete IGBT chi cosists of may such moolithic cells i arallel. MOS techology limits the IGBT chi size to about 1 2 cm 2, so that several chis have to be ackaged i arallel. As IGBTs have ot yet reached the blockig voltage ratig of GCTs, they have to be coected i series to obtai the blockig voltage for a articular alicatio. Lower-cost ower systems The cost of ower cotrol equimet ca be reduced by 30 ercet or more through the use of IGCT techology. Several factors accout for this. GCTs ca be fabricated usig existig GTO roductio rocesses. Sice these rocesses are well uderstood ad available equimet ca be used for them, GCT costs are i lie with those of GTO thyristors. Comared with IGBTs, GCTs are less sesitive to rocess variatios i terms of how these affect the tur-off erformace. Cosequetly, the yield is higher ad 4 ABB Review 3/

5 IGCT techology offers four levels of comoet ackagig ad itegratio (a, b ad c, lus Fig. 6b). Equimet desigers ca select ay level as the startig oit of alicatio develomet. Whe the alicatio-ready IGCT stack (Fig. 6b) is chose, much of the time-cosumig work ivolved i desigig the ower circuit ad mechaical systems ca be avoided. 5 a b a Gate commutated thyristor (GCT) Gate commutated thyristor ad diode itegrated o same wafer. Aode desig allows charge to flow i ad out of device quickly. Reduced silico thickess limits stored charge ad ermits fabricatio of diode ad switch o same wafer. Thier silico roduces lower o-state losses. All elemets switch at oce; subber ca be omitted. b Packaged GCT Low iductive housig ermits charge ad gate cotrol currets to flow i ad out of GCT quickly. Press-ack allows freedom of movemet, revetig fatigue failures durig decades of thermal cyclig. Press-ack desig withstads thermal stresses; cotributes rove reliability. c Itegrated gate driver (ot to scale) Low iductive gate drive circuitry ermits cotrol currets to flow i ad out of GCT quickly. Localized, highly itegrated gate drive circuitry reduces stray iductace. Comact size simlifies equimet desig. c costs are lower. I additio, GCT desig simulatio is simler, reducig costs ad seedig u system develomet. GCT techology simlifies ower circuits to the extet that the umber of comoets is reduced by as much as 50 ercet. This is due to the itegratio of the diodes i the GCT ad the reductio i cablig ad itercoectio made ossible by the geerally high level of itegratio. A further reductio i costs is ossible due to the higher oeratig frequecy, which allows some comoets to be made smaller. I additio, the gate drive circuit ower is substatially reduced, allowig less costly comoets to be emloyed. IGCT-based equimet ca be desiged with higher efficiecies tha whe other techologies are used. Losses i the ower circuits ad i the associated circuitry are lower, traslatig ito more comact coolig equimet for a further reductio i costs. Reliability ad availability I a larger cotext, the cost of ower cotrol equimet is small i comariso with the cost of dowtime of the rocess i which it oerates. Equimet availability is therefore aramout. IGCT techology was coceived secifically for use at medium-voltage levels ad esures otimum equimet reliability through: Homogeeous switchig Robust, thyristor-like ackagig techology (o wire bods) Simlified gate cotrol uits Reduced umber of arts Further, i the ulikely evet of a failure, the modular ature of IGCT techology makes relacemet of ower comoets quick ad simle. This reduces sare-arts costs ad kees rocesses o-lie. Alicatios of IGCT techology At the core of the IGCT s erformace advatage is its ability to tur off i 2 microsecods ad coduct like a thyristor. IGCT techology therefore ermits simle iverter desigs with half the losses of alterative techologies. Thaks to IGCTs, reviously imractical circuit toologies rated at u to 100 MW ad requirig series coectio of may devices ca ow be realized. Mediumvoltage equimet based o this techology exhibits very high reliability. For the first time, a ower silico techology has bee matched to mediumvoltage, high-ower alicatios. This eables equimet desigers workig 16 ABB Review 3/1997

a b Comariso of GTO thyristor coverter ad IGCT coverter for the same ower ad voltage levels. IGCT techology allows a dramatic simlificatio that traslates ito lower costs ad imroved reliability.

6 a b Comariso of GTO thyristor coverter ad IGCT coverter for the same ower ad voltage levels. IGCT techology allows a dramatic simlificatio that traslates ito lower costs ad imroved reliability. 6 a GTO thyristor coverter b IGCT coverter, air-cooled (air-coolers are show simlified), with followig features: IGCT elemets easily mouted i comact cofiguratio Desig ermits fast relacemet of IGCTs i ulikely evet of failure with IGCTs to build less costly, more reliable ad more comact ower cotrol systems, icludig: Railway ower suly frequecy chagers Utility iterties Static var comesators for ower factor cotrol 1 Power flow cotrollers for utilities Medium-voltage drives with lie voltages of u to 6.9 kv rms Pum ad fa drives for the chemical, oil ad ower sectors 2 Marie drives/all-electric shis Trasformerless tractio sulies Locomotive drives Iductio heatig resoat iverters Static breakers IGCT techology is available ow ABB is a leadig sulier of high-ower switchig devices to ABB Grou com- aies ad exteral firms. This leadershi is exemlified by the iovatios that have roduced IGCT techology. IGCT-based equimet beefits from lower costs ad high reliability eve at the highest ower levels. Desigers of medium-voltage equimet ca ow choose from three ower silico switch techologies: GTO thyristors, IGBTs ad IGCTs. IGCT techology will be the techology of choice for all alicatios i which the riorities are comactess, high efficiecy, fast develomet ad rove reliability. Refereces [1] H. Grüig et al: High ower hard drive GTO module for 4.5 kv / 3 ka subberless oeratio. PCIM Coferece May 96, Nürberg. [2] H. Grüig, A Zuckerberger: Hard drive of high ower GTOs: better switchig caability obtaied through imroved gate uits. IEEE aer Proceedigs of IEEE Idustry Alicatios Society, 1996 Aual Meetig. Author s address Harold M. Stillma ABB Cororate Techology ABB Asea Brow Boveri AG P.O. box 8131 CH-8050 Zurich Switzerlad Telefax: hal.stillma@abbzh.mail.abb.com ABB Review 3/

Power Semiconductor Devices

Power Semiconductor Devices 1.4. Thyristor (SCR) A thyristor is a four layered semicoductor that is ofte used or hadlig large amouts of ower. It ca be tured ON or OFF, it ca regulate ower usig somethig called hase agle cotrol. It