Proecion Sraegies for IGBT Curren Source Inverers M. Haberberger 1, F. W. Fuchs 2 1 2 Power Elecronics and Elecrical Drives Chrisian-Albrechs-Universiy Kiel, Germany E-Mail: 1 mkh@f.uni-kiel.de, 2 fwf@f.uni-kiel.de Phone (49) 431 880-6100 Fax (49) 431 880-6103 WWW: hp://www.f.uni-kiel.de/eech/lea Absrac An overview of overcurren and overvolage CSI fauls is presened and sraegies, how o proec a CSI equipped wih IGBT power swiches agains hese failures are developed and explained. The shown sraegies and he relaed circuis are applicable a low expense and require only minor changes in he CSI and is conrol. I. INTRODUCTION Curren source inverers (CSI) wih slow swiching symmerically blocking GTOs are commercially used for many years, especially in high volage and high power applicaions [1]. Wih he availabiliy of fas swiching IGBTs and he possibiliy o increase he swiching frequency, one of he major drawbacks of CSIs, he large and expensive dc link inducor and bulky line and moor side filers [2], have been overcome and CSIs became also aracive in medium power applicaions due o heir sinusoidal inpu and oupu currens and heir inheren abiliy for regeneraive operaion. Bu wih he reducion of he inducive and capaciive componens as well as wih he increased swiching speeds of IGBTs, proecion becomes more difficul because of he shorer ime available o reac on failures. The missing freewheeling diodes as hey can be found e.g. in VSIs (volage source inverers) require exra passive or acive proecion mechanisms in a CSI o provide coninuous conducion pahs for he inducor currens. In lieraure relaed o CSIs only sparse informaion can be found abou proecion agains overcurrens, curren inerrupions and overvolages [3] [4]. The aim of his paper is o show cause and effec of possible faul siuaions in a CSI and o describe sraegies how o preven especially he CSI using IGBTs as power swiches from beeing damaged. In his publicaion a general overview on CSI operaion (II) is given and he failures are divided ino he hree major groups - converer shor circuis, overcurrens and curren inerrupions (III). These fauls are explained in deail and possible proecion measures are shown (IV, V and VI). The analysis is mainly done for he line side converer of an example 22 kw CSI drive. Is elecrical parameers are published in he appendix. The invesigaion applies in adequae manner o he machine side as well as o oher CSI configuraions. I should be noed, ha he invesigaion given in his paper does no cover moor winding shor circuis and earh fauls. II. The basic circui of an IGBT curren source inverer consising of wo hree-phase bridges wih a oal of 12 reverse-blocking swiches (IGBT wih series diode) is shown in figure 1. Boh bridges usually operae in PWM mode. Various modulaion and conrol echniques can be applied depending on he allowed swiching frequency of he IGBTs and he required dynamic response ime of he drive [5]. S1 S3 S5 S2 S4 S6 Line side converer OPERATION OF A CSI S6 S4 S2 S5 S3 S1 Moor side inverer Fig. 1. Basic circui of a CSI drive 3~ moor I is common for all modulaion schemes, ha always four swiches mus conduc o provide a coninuous pah for he dc link curren. This may flow in he dc link only (shored bridge legs) or hrough he line and/or moor phases. Capaciors across he inpu and oupu erminals ensure, ha he currens in he mains and moor inducances can decay, if he relaed bridge leg was acively urned off and he curren has commuaed o he nex phase. III. OVERVIEW OF CSI FAULTS The cause of CSI fauls, ha may damage he converer/inverer are devided ino hree groups: Shor circuis Overcurrens Curren inerrupions There are various condiions, ha could lead o hese fauls. An overview is shown in figure 2. The effec of all failure condiions in a CSI is an overvolage which will be applied o one or more power devices (diodes, IGBT, capacior). As he faul energy, mainly he magneic energy sored in he dc link, mains or moor inducors, is high enough o desroy even more han one device, a chain reacion may resul.
series diode failure shor circui mains unsymmery conrol failure overcurren modulaion faul overvolage driver, swich failure or pulse off curren inerrupion Fig. 2. Overview of cause and effec of CSI fauls mains blackou In he following he fauls and effecs of hese unnormal converer condiions will be explained and i is described, how hey can be handled by proecive measures o preven he CSI from being damaged or o limi he exen of damage. IV. LINE / MOTOR SIDE SHORT CIRCUIT Shor circui condiions wih very low dynamic impedance, e.g. as hey can occur in VSIs when shoring he dc link capacior, should a firs sigh no be a problem in a CSI, because here is no direc shor circui pah across he only capaciive elemen - he line and moor side filer capaciors. Even if all bridge IGBTs are simulaneously urned on, a leas one of he series diodes is locaed in reverse direcion and blocks he curren. This is no longer rue, if one of he diodes is defecive and remains in a shor circui sae. This is ypically he case afer a diode breaks hrough and was hermally overloaded. Because his ype of faul was experienced in an experimenal CSI seup in he laboraory, he effec of a shored series diode was analyzed in deail and is described now. A. Curren commuaion from phase o wih aniparallel diode. The laer is required o conduc he reverse curren of he relaed series diode during commuaion. I is supposed, ha a he beginning of he invesigaion S3 conducs he dc link curren and is now urned off (figure 4). The commuaion volage is assumed posiive in his momen. Immediaely afer urning V1 on he curren commuaes o V1/D1. When he curren hrough D3 becomes zero, he diode ges he commuaion volage as reverse volage and mus block. Oherwise he inverse IGBT diode would sar conducing. B. Shor circui behaviour wih defecive series diode A shor circui will happen, if D3 is defecive. The commuaion pah wih he filer capacior C 12 as volage source is now saically shored as long as V1 is on. The shor circui curren sars o rise wih a slope only limied by parasiic inducances in he commuaion pah and he semiconducor conducance. This is oulined by i S3,b and i S1,b in figure 4. v / i I d 0 - v / i I d 0 i S3 v S1 i S1 v D3 i S1,b i S3,b v D3,b Fig. 4. Commuaion swich volages and currens v C23 D 1 v D1 D 3 v D3 S 1 S 3 V 1 V 3 v C31 v L l1 L s1 i l1 1 i s1 ~ L s3 i s3 v l2 L l2 C 12 ~ i 2 i S1 Fig. 3. Commuaion pah from phase o (op half bridge) Figure 3 shows a par of he op half bridge of he line side converer wih he swiches S1 and S3 o represen he commuaion pah from phase o. Every swich consiss of a series diode (D 1, D 3 ) and an IGBT (V 1, V 3 ) Fig. 5. Capacior volages and swich curren a shor circui in he commuaion circui (simulaion)
Figure 5 shows a SPICE simulaion resul for he described faul case using ypical values for he elemens in he commuaion pah of a 22 kw drive buil up for CSI measuremens on a 400 V grid wih a nominal phase curren of 35 A (L s1 = L s3 = 150 nh, C 12 = 48 µf). The curren peak in figure 5 resuls of he shor circui of he filer capacior caused by he defecive series diode. Alhough his curren reaches a very high level, his may no necessarily desroy he IGBT, inverse diode or he remaining series diode, because i flows only for a shor, limied ime. Todays IGBTs are capable o wihsand a capaciive shor of he dc link of a VSI for some microseconds wihou damage. Compared o he dc link in VSIs, he sored energy in he line or moor side filer capaciors in a CSI is raher low. Wih he mean pulse power and he ransien hermal response of IGBT and diode (daashee parameer) he juncion emperaure rise can be calculaed using equaion (1). ( ) P d Tj = Z h pulse (1) C. Proecion measures agains shor circui condiions If he calculaed juncion emperaure exceeds he maximum raing of he devices, a shor circui deecion and proecion circui for every IGBT is proposed he same way as hey are used in VSIs, e.g. by supervising he IGBT's collecor-emier volage and acively urning he swich off. Bu even if he devices could wihsand his firs shor circui curren pulse and shor circui proecion measures are eliminaed due o equaion (1) and economical reason, a second effec has o be considered. v C23 v C31 Fig. 6. Behaviour of he line currens a repeiive shor circui in he commuaion circuis (simulaion) I can be derived from he schemaic in figure 3 ha he shor circui condiion does no end afer dissipaing he i i i filer capacior energy. Besides he shor of he capacior, here is also a unidirecional shor beween wo mains phases, which may lead o an almos unlimied power flow and an excessive rise in phase currens ogeher wih heavy unsymmeries in inpu volages and currens. Figure 6 shows he capacior volages and phase currens, if he CSI coninues pulsing and he capacior C 12 is repeiively shored. The volage drop causes he currens, especially he curren hrough phase in he shown example, o rise beyond heir nominal values. To preven he CSI from furher damage, wo sraegies are proposed depending on he cycle ime of he digial conrol, which characerizes he ime delay beween occurance of he faul and possible deecion. I should be noed, ha he cycle ime mus no necessarily be equal o he swiching frequency of he drive. Especially in low swiching CSIs, he conrol cycle ime is ofen much lower o provide a good accuracy for model calculaions. a) Cycle ime < 250 µs The slope of he phase curren is limied by he raher large grid or moor inducances, so in CSIs using high sampling raes, addiional algorihms in he conrol scheme, ha could deec such an unnormal curren rise as i appears in he shown faul wihin wo o hree cycles, should be sufficien for proecion. No exra hardware is required. The reacion on he deeced faul should be an immediae pulse off of he complee CSI. Even if he defecive series diode could by deeced by special algorihms wihin he conrol scheme, i seems no realisic o power he CSI safely down using only he remaining operaive devices. b) Cycle ime > 250 µs CSIs wih low updae raes especially in combinaion wih a small mains impedance (ohmic and inducive) as i can be found in high power drives may cause he overcurrens o reach already values oo high o pulse off he CSI when he faul was deeced. Laer in he secion abou overvolage proecion i is shown ha a pulse off is a criical sae iself and requires proecive measures. Their size and exen mainly depends on he iniial curren a pulse off, so i is no advisable o wai unil he conrol deecs he faul and he overcurren has reached a high level. An addiional proecion circui in he analog signal pah is suggesed insead. I compares he measured line currens wih maximum limis, ha could be deermined by he capabiliy of he pulse off proecion shown in secion VI.D. The comparaor should bypass he conrol and rigger he pulse off direcly. This allows he CSI o reac much faser on overcurrens and i provides a deerminisic maximum curren for he dimensioning of oher proecion circuis. Figure 7 shows an ouline where o place he overcurren deecion comparaors.
3 3 curren measuremen overcurren deecion A/D conversion D/A conversion curren limis 4 pulse-off digial conrol 6 pulse generaor ime (= ripple period) is consan, he ripple ampliude rises wih a higher curren level, shown in plo a) in figure 8. Because he fundamenal rms value of he line/moor volage remains consan a a consan power flow, he increase in ripple volage also causes an increase in he maximum volage level across he capaciors a he CSI erminals. Fig. 7. Deecion of overcurrens V. OVERCURRENTS Dynamic overcurrens are usually no a criical problem in CSIs, because he raher large inducors on he line side (mains inducance), in he dc link and on he moor side (machine inducance) lead o a limied curren slope, which in mos cases allows overcurrens o be easily handled by he conrol scheme wihou exra proecion hardware. Bu some aspec should sill be kep in mind. A. Filer capacior volage ripple and commuaion overvolages The filer capaciors across he inpu and oupu erminals are designed o ensure a limied volage ripple on he inpu and oupu erminals when he CSI works wihin is operaing range (nominal dc link curren). The ripple volage across he capaciors is direcly relaed o dc link curren. As he dc link and he line/moor currens can be supposed as consan during one PWM period, he ripple has a riangular shape wih is ampliude proporional o he dc link curren level. Figure 8 shows an exemplary simulaion ime plo for wo differen dc link curren levels. v C a) b) - higher commuaion overvolages These are overvolages caused by he parasiic inducances in he commuaion pah. As he swiching speed of an IGBT, resp. he curren rise and fall ime is almos independen from he collecor curren, an increase in dc link curren direcly causes an increase in he curren slope and wih i in he volage drop across he sray inducances in he commuaion pah. These overvolages are mainly arising on he swich being urned off and are also disribued o oher non conducing devices. The sum of he maximum capacior volage and he maximum commuaion overvolage is he maximum forward and/or reverse volage imposed o he IGBTs and series diodes in normal as well as in criical or faul condiions. This volage has always o be below he absolue maximum raings of he devices, e.g. ypically 1200 V for CSIs working on a 400 V grid. Considering his, a maximum allowed dc link curren can be calculaed and he conrol has o ensure he curren below his level, e.g. by reducing he acive or reacive power. B. Line side overcurrens, moor side overcurrens or unsymmeric curren disribuion A fauly conrol or modulaion scheme, special dynamic condiions or unsymmeries in he mains volages may lead o overcurrens or unbalanced inpu or oupu currens. All his could cause an unsymmeric charge of he capaciors, which may resul in inpu/oupu phase-o-phase volages, ha are higher han he nominal value. This behaviour can be considered by using appropriae facors of safey for he maximum allowed curren or as unsymmeries usually have a hisory, he conrol scheme could early deec hese siuaions and reduce he power or power he CSI oally down. In he overcurren case B. as well as in A. he comparaor shown in figure 7 could be helpful o hold he currens below absolue maximum limis under all circumsances. This ensures, ha as a las measure a oal pulse off is sill possible wihou damaging he CSI. VI. CSI CURRENT INTERRUPTIONS Fig. 8. Volage across he inpu/oupu erminals of he CSI a wo differen dc link curren levels (a: I d = 40 A, b: I d = 20 A) An increased dc link curren has wo major effecs: - higher capacior volage ripple The slope of he riangular volage ripple is linearly dependen on he dc link curren. As he pwm cycle A. Dc link curren inerrupions A oal pulse off or any oher inenional or uninenional urn off of one of he four conducing swiches inerrups he dc link curren pah. A simplified equivalen circui diagram wih he relevan componens for he analysis of curren inerrupions in a CSI is shown in figure 9.
v line S line 1,3,5 v d S line 2,4,6 L d C Ld +C oes i d S moor 2,4,6 S moor 1,3,5 Fig. 9. Equivalen circui diagram of he CSI simplified for analyzing curren inerrupions v moor The volage sources v line and v moor represen he momenary value of he line and moor side volage applied o he dc link. S line/moor 1,3,5 is an equivalen for he swich currenly conducing in he op half bridge, S line/moor 2,4,6 for he swich of he boom half bridge of he line and moor side bridge. C Ld sands for he parasiic capaciance of he dc link inducor referenced o he line side dc link erminals. C oes is he sum of all applicable oupu capaciances of he line side bridge (IGBTs and diodes). If one of he line side swiches fails or is urned off, he volage across he inducor will sar rising wih a slope only limied by he parasiic capaciances. For he example of he 22 kw CSI drive (C Ld = 5 nf, L d = 30 mh, C oes = 400 pf) a slope of 8.3 kv/µs will resul a he raed dc link curren of 45 A, which is shown by he simulaion ime plo in figure 10. The ime available from deecing such an overvolage (e.g. a 850 V) unil he proecion mus become acive (below he IGBT breakdown volage of 1200 V) is in his example abou 40 ns. -850V -1200V 40ns Fig. 10. Dc link volage slope afer inerruping he inducor curren (400 V iniial dc link volage) B. Proecion measures agains dc link overvolages To provide his shor response ime, a very fas proecion circui is required. Well known from VSI proecion is acive clamping [4] of he urned off swich, wha has almos zero delay and could on principle be also applicable in a CSI. For his, all 12 swiches mus be equipped wih clamping circuis. One major drawback of acive clamping is he high momenary power dissipaion. In an economically dimensioned CSI, an IGBT is able o clamp a his breakdown volage for a maximum of only some en microseconds, because he iniial juncion emperaure is already very high. In he shown 22 kw drive, clamping a 1200 V is required for more han 1 ms. I seems unrealisic o dimension he CSI in a way o be capable of allowing such a long clamping ime. Anoher disadvanage of he clamping sraegy is he fac, ha he urned off IGBT sill conducs and curren flows owards he mains or moor capaciors. This could cause unsymmeric volages and may resul in line or moor side overvolages. And wha also has o be considered is ha clamping could only work, if he urned off IGBT is sill operaive. I will fail, if a defecive IGBT is he cause of he curren inerrupion. Besides acive clamping here are several oher proecion sraegies using passive devices like varisors or zener diodes. These are applicable, bu is has o be considered, ha hey all have a finie v/i characerisic [6], which is no very seep. Devices, ha block below he maximum volage during normal CSI operaion (e.g. 850 V) and a he same ime are able o limi he volage a full dc link curren below he IGBT/diode breakdown volage (e.g. 1200 V) are raher large. Because of all hese drawbacks of exising proecion measures, a novel wo sep sraegy as presened in [7] is proposed agains CSI dc link curren inerrupions. This could provide boh - a fas and "hard" overvolage limiaion and a freewheeling pah o safely dissipae he inducor energy. The proecion works as follows: 1) The dc link is equipped wih an exra freewheeling pah (IGBT + series diode) across he line and moor side dc link erminals. This pah is riggered by an overvolage deecion circui or if he CSI should inenionally be pulsed off, by he conrol circuiry. The freewheeling pah is held on, unil he inducor energy becomes zero. Compared o acive clamping his reduces he momenary power dissipaion a he expense of a longer ime o ge he curren o zero, which allows a raher small IGBT being used. Depending on he size of he inducor, freewheeling is required for a maximum of only some hundred milliseconds. 2) Because he freewheeling swich is off during normal converer operaion and here are no devices available ha could be urned on (delay ime + rise ime) wihin he required response ime, a fas and "hard" limiing volage clamp is insalled o span he ime from overvolage occurance o geing he freewheeling pah acive. The limiaion circui has o wihsand he high power dissipaion during clamping only for he shor ime of some hundred nanoseconds. Devices working in he avalanche breakdown (e.g. MOSFETs) are he bes choice for his purpose. They have a much seeper v/i characerisic compared o varisors or zener diode [4], which allows very small device being used.
The CSI wih he deecion/proecion circuis and he freewheeling pah for he line side of he dc link is shown in figure 11. I applies o he moor side as well. line-side converer overvolage limiaion overvolage limiaion overvolage limiaion 1 from conrol for pulse off Fig. 11. CSI wih overvolage proecion circui agains dc link curren inerrupions (shown is only he line side converer) For experimenal verificaion he hree oscilloscope plos shown in figure 12 were aken on curren inerrupion escases applied o he experimenal 22 kw CSI drive. The iniial curren was always he nominal dc link curren of 45 A. C. Line or moor side curren inerrupions Similar o he dc link, he line and moor side of he CSI is highly inducive. Bu even if all swiches are simulaneously urned off during a oal pulse off, he line and moor curren could no be inerruped. The filer capaciors always provide a curren pah. Wha has o be considered is he fac, ha he capaciors are only dimensioned o limi he volage ripple during normal converer operaion. Afer a pulse off, he capaciors alone canno provide a full overvolage proecion, because he energy siuaion in he line/moor inducors leads o heavy volage overshoos as will be shown now. Fig. 13 shows a SPICE simulaion of a randomly pulsed off 22 kw CSI, ha was running wih nominal line curren of 35 A a a grid volage of 400 V. oal bridge pulse off op half bridge pulse off boom half bridge pulse off v d,lim v op,lim v C23 v boom,lim Ch2 500 V M 1 us Ch4 500 V Ch3 500 V Fig. 12. Measuremen resuls for failure cases caused by dc link curren inerrupions (v: 500V/div, : 1µs/div) The figure on he lef shows a oal converer pulse off, where all swiches were urned off simulaneously, e.g. afer deecing a faul condiion or if he power supply fails. The dc link volage rise is here disribued o boh, he op and he boom half bridge, which causes none of heir limiaion circuis o become acive. In his faul case, he limiaion circui across he dc link erminals limis he volage (v d,lim ). In he mid figure only he conducing swich of he op half bridge was urned off o represen an IGBT, diode or driver failure. As he IGBT in he boom half bridge sill conducs, he boom dc link erminal is conneced o one of he mains phases. The proecion chain of he op half bridge herefore sees he rising dc link inducor volage superimposed o he recified phase-o-phase mains volage. This causes he volage across he op proecion circui o be higher han he volage across he dc link proecion. The op limiing circui is herefore acivaed (v op,lim ). Finally he figure on he righ shows he urn off of he conducing swich in he boom half bridge, which causes he boom proecion circui o be issued. v C31 Fig. 13. Volage across a he inpu/oupu erminals of he CSI (capacior volage) in he case of a line or moor side curren inerrupion (e.g. pulse off) If he pwm immediaely sops, an unconrolled oscillaion process in he second-order inpu and oupu filers will sar, acivaed by he remaining energy in he inducors. Depending on he iniial condiions a pulse off, hese oscillaions may reach levels higher han he maximum raings of he semiconducors and capaciors. D. Proecion agains line and moor side overvolages To proec he CSI agains overvolages a he line and moor side caused by curren inerrupions, he use of varisors across he inpu and oupu erminals as shown in figure 14 is well suied o handle he volage overshoos in
a CSI and absorb he remaining inducive energy. This sraegy has already been presened for marix converer proecion [8], which uses similar line and moor side filer circuiry. I was already menioned in secion IV.C.b, ha a pulse off is a very criical sae iself and he dimensioning of he proecion varisors depends mainly on he iniial curren a pulse off. In he case of a pulse off iniiaed by a faul condiion, his curren could be much higher han he CSI's nominal curren, so he varisors have o be chosen according o he absolue maximum appearing curren level. If using he comparaor circui from figure 7, his curren can be well defined. Oherwise an adequae facor of safey has o be considered. CSI 3~ moor Fig. 14. Inpu / oupu overvolage proecion wih varisors E. Special noe on a oal inverer pulse off In secion VI.B and VI.D proecion sraegies are shown o preven overvolages in he dc link as well as on he line and moor side of he CSI. One may argue, ha he exra freewheeling pahs in he dc link proposed in figure 11 are unnecessary, because he legs of he line and moor side bridge could also be used o shor he dc link erminals and provide a pah for he inducor curren. I mus be noed, ha his sraegy is no advisable, because in he case of an overcurren pulse off iniiaed by a defecive series diode, such a bridge leg shor circui could be equal o a saic phase-o-phase shor circui of wo mains or moor phases, depending on wha bridge leg is acivaed and which diode is defecive. The effec of such a shor circui was described in chaper secion IV.C (figures 5 and 6). I is herefore proposed o never urn on a leg of one of he CSI bridges in he case of a deeced faul condiion. Boh line and moor side bridge mus be compleely urned off o avoid such side effecs. VII. CONCLUSION In a CSI, as long as curren flows, all swiches mus be operaive. Differen from IGBT VSIs, where he bes proecion sraegy in he case of a faul condiion is usually o simply urn he converer off, an IGBT CSI has o be sill acively conrolled. Especially if one or more of he main power swiches in he converer bridges are he cause of he faul, i is no possible o safely power down he converer wihou addiional proecion hardware. In his paper an overview of common IGBT CSI fauls caused by shor-circui condiions, overcurrens and curren inerrupions was given and sraegies including proecion circuis o fully proec he CSI agains hese failures were explained. The presened circuis and sraegies are applicable a raher low expense and need only minor changes in he sandard IGBT CSI srucure. The dc link curren inerrupion proecion can even be used compleely independen from he CSI and if is power supply is buffered by rechargable baeries or chemical capaciors, i can even proec he IGBT CSI in he case of a oal grid and power supply faul. APPENDIX TABLE 1 PARAMETERS OF THE 22kW CSI DRIVE Nominal dc link curren I d,nom 45 A Nominal line curren I L,nom 35 A rms Line-o-line volage V LL 400 V rms Toal line inducance (mains + addiional filer inducor) L L 4 mh Filer capacior capaciance C 12, C 23, (dela connecion) C 31 48 µf Dc link inducor inducance L d 30 mh Parasiic inducor capaciance C Ld 5 nf Max. IGBT blocking volage V IGBT,max 1200 V Toal bridge capaciance referenced o dc link C oes 400 pf Parasiic inducance of one swich pah L S 150 nh REFERENCES [1] H. Semmler, High-Power Indusrial Drives, Proc. of he IEEE, 1994, Vol. 82, Issue 8, pp. 1266-1286 [2] F. W. Fuchs, A. Klönne, DC Link and Dynamic Performance Feaures of PWM IGBT Curren Source Converer Inducion Machine Drives wih Respec o Indusrial Requiremens, 4h Inernaional Power Elecronics and Moion Conrol Conference IPEMC 2004, Xi'an, China, Proc. on CD [3] H. Inaba, K. Hirasawa, T. Ando, M. Hombu and M. Nakazao, Developmen of a High-Speed Elevaor Conrolled by Curren Source Inverer Sysem wih Sinusoidal Inpu and Oupu, IEEE Trans. on Indusry Applicaions, Vol. 28, No. 4, July/Aug. 1992 [4] T. Reimann, R. Krümmer and J. Pezold, Acive Volage Clamping Techniques for Overvolage Proecion of MOS-conrolled Power Transisors, EPE'97, Trondheim, 1997, Proc. Vol. 4, pp.43-48 [5] T. Halkosaari, H. Tuusa, Opimal Vecor Modulaion of a PWM Curren Source Converer According o Minimum Disorion Consrains, Proc. of he IPEC, Tokyo, 2000 [6] J. M. Li, X. Tian and D. Lafore, Energy Absorpion Devices for Solid Sae Inerrupion, EPE'95, Sevilla, 1995, Proc. Vol. 2, pp. 229-234 [7] M. Haberberger and F. W. Fuchs, Novel Proecion Sraegy for Curren Inerrupions in IGBT Curren Source Inverers, PESC04, Aachen, 2004 [8] J. Mahlein and M. Bruckmann, Passive Proecion Sraegy for a Drive Sysem Wih a Marix Converer and an Inducion Machine;, IEEE Transacions on Indusrial Elecronics, Vol. 49, 2002, No. 2, pp. 209-303