Explosion proof housings for IGBT module based high power inverters in HVDC transmission application

Markus Billmann 1)

Dirk Malipaard 2)

Dr. Herbert Gambach 3)

1) Fraunhofer Institute of Integrated Systems and Device Technology (IISB) Schottkystrasse 10, 91058 Erlangen, Germany Tel. (+49) 9131 / 761 311, e-mail: markus.billmann@iisb.fraunhofer.de 2) Fraunhofer Institute of Integrated Systems and Device Technology (IISB) Schottkystrasse 10, 91058 Erlangen, Germany Tel. (+49) 9131 / 761 311, e-mail: dirk.malipaard@iisb.fraunhofer.de 3) Siemens AG, Energy Sector, Power Transmission Division, Power Transmission Solutions, Guenther-Scharowsky-Str. 2, 91058 Erlangen, Germany, Tel.: (+49) 9131 / 73 52 49, e-mail: herbert.gambach@siemens.com

Abstract

Worldwide many attempts are taken to improve lifetime and power cycling capability of wire bonded

IGBT modules. But even in very conservative inverter designs overload conditions may occur and

cause damage to the inverter. In addition it is a fact that even the best design will reach its "end of life"

some day.

In medium and high power inverters exploded IGBTs are subject to stop operation of the complete in-

verter. An unscheduled and long lasting service process for cabinet cleaning and checking close by

systems is the consecution.

In high voltage applications using IGBT based multilevel topology up to 600kVdc additional demands

have to be ensured:

1. The main system must continue operation in case of single level fault condition.

2. Parts from exploded IGBTs as well as conductive plasma clouds must not reach neighbourhood mul-

tilevel stages, because otherwise the whole inverter leg might be damaged due to high voltage

driven short circuit arcs.

This paper evaluates the measures that can be taken to guarantee that no particles and no plasma

clouds give impact to nearby inverter stages for energies in the range of several ten kilojoules. Different

protection strategies are discussed.

1 Introduction

HVDC systems based on line-commutated con-verter technology (LCC) are used for decades. The key components of this converter topology are disc-type thyristors that have reached a high degree of maturity and high reliability due to their robust technology. HVDC and FACTS with LCC use power electronic components and conven-tional equipment that can be combined in differ-ent applications to transmit active power or to control reactive power. Due to the low losses compared to other power electronic devices, line-commuted thyristor tech-nology is the preferred solution for bulk power transmission, today and in the future. However, line-commutated converters have some technical disadvantages based on the fact that thyristors can only be switched on, not off via the control gate. This means that the commutation within the converter has to be driven by the AC voltages of the network which needs i.e. a minimum short-circuit power.

In new concepts, the changeover to self-commutated converters has now taken place, i.e. to so-called voltage-sourced converters. Fig. 1 shows a point-to-point HVDC connection.

Fig. 1: HVDC “Classic” and HVDC PLUS Technologies

2 Self-commutated converter technology for HVDC sys-tems

Self-commutated converter technology is well-known for more than 30 years for instance in the field of drive applications. In DC transmission, an

independent control of active and reactive power is possible. Self-commutated converters have the ability to supply weak or even passive networks. They offer excellent dynamic performance i.e. in case of control or protection interventions, mak-ing it easier to deal with fault situations. The fa-miliar commutation faults of line-commutated technology belong to the past. In comparison with classical line-commutated HVDC technology, in some cases less space is needed for a converter station because complex filter and compensation systems can be dispensed with. In many applications, the VSC has become a standard of self-commutated converters and will be used more often in transmission and distribu-tion systems in the future. This kind of converter uses power semiconductors with turn-off capabil-ity such as IGBT (Insulated Gate Bipolar Transis-tors) as a suitable example. Some general bene-fits of VSC technology are shown in Fig. 2.

Fig. 2: General features of VSC technology applied in HVDC transmission systems

To ensure uniform voltage distribution not only statically but also dynamically, all devices con-nected in series in one converter arm have to operate as simultaneously as possible. As a re-sult high and steep voltage steps are present at the AC converter terminals. This causes heavy component stress and a high degree of harmonic distortion at the AC converter terminals. To meet the grid code of the AC network extensive filter-ing measures are required. Fig. 3 shows the principle of the two-level converter technology.

Fig. 3: Two-level converter technology for HVDC applications

Both the extensive voltage gradients and the high degree of harmonic distortion can be reduced dramatically if the AC voltage generated by the converter can be selected in smaller increments than at two or three levels only. Converters with

high number of steps are called multilevel con-verters. Different multilevel topologies were pro-posed in the past and have been discussed many times. A new and different approach is the Modu-lar Multilevel Converter (MMC) technology [1]. Fig. 4 shows this kind of converter topology which is in many terms very suitable for HVDC applications.

Fig. 4: MMC technology for HVDC applications

With a high number of levels – and this is neces-sary for HVDC applications anyhow – the switch-ing frequency of individual semiconductors can be reduced. Since each switching event creates losses in the semiconductors, converter losses can be effectively reduced. A main demand is the use of devices that can be turned off at any time.

3 The choice of semiconductor in self-commutated convert-ers for HVDC applications

In practically all technical systems, even after ex-tremely thorough engineering and testing, spo-radic failures of individual components during operation cannot be avoided. This must not affect operation of an HVDC system. Power transmis-sion must continue, even if a certain percentage of inverter cells failed before the next scheduled shutdown for maintenance. This is why redun-dant power semiconductors are integrated in HVDC converter systems. In a 400 MW HVDC application about 1500 in-verter cells are mounted in one big inverter hall. Each power module has up to several kV offset to nearby inverter cells. If a semiconductor or accessory parts in one se-ries connection fail, the destroyed device has to carry load current for a long time up to the next scheduled shutdown of the transmission system. In line-commutated converters thyristors based on press-pack technology are well-known as very robust semiconductor devices that meet these requirements. A long lasting period of field ex-perience is confirming this. In self-commutated two-level or three-level con-verters based on series connection of semicon-

ductors with turn-off capability, i.e. IGBTs the same requirement does exist. This is why partly IGBT - based on cost intensive press pack tech-nology - are applied. It is very challenging to real-ize an appropriate long term short circuit capabil-ity with this kind of semiconductor devices. Many efforts were made to increase the explo-sion withstand capabilities of press pack devices [2, 3, 4, 5, 7]. The structure of modular multilevel converter technology as a basis for HVDC appli-cations leads to further requirements for the power semiconductors respectively their hous-ings. These so-called “power modules” are oper-ated with their own more or less powerful DC link capacitor (Fig. 5). They have to withstand an enormous short-circuit current and energy that gives impact to both the upper and lower IGBT and / or free-wheeling diode in parallel in case of a failure. The described demand is well-known in many dif-ferent VSC based converter technologies. This requirement can hardly be solved, as in most cases IGBT modules without strong short circuit capability have been designed into the topology. These module semiconductors normally explode in the case of a DC link short circuit event.

Fig. 5: Principle MMC topology and inverter cell arrange-

ment in converter hall

Especially in HVDC applications an explosion of the power semiconductor respectively its housing – the power module – has to be avoided due to two reasons. First power transmission must not be interrupted and must even continue up to the next scheduled shutdown of the transmission system. Second: Parts from exploded IGBT as well as conductive plasma clouds must not reach neighborhood multilevel stages, because other-wise the whole inverter leg might be damaged due to high voltage driven short circuit arcs. Wire bonded IGBT modules therefore might not be the appropriate semiconductor device for se-ries connection in HVDC applications at a first sight. Nevertheless on a second glance they have some very attractive advantages. IGBT modules are well-proven standard industry com-ponents with high availability. They are applied in

countless drive and traction applications and be-come constantly improved. No special devices, designed for HVDC use only, but standard cata-log types at reasonable costs are available and follow a main stream. This steady improvement also leads to a high reliability of the devices. In addition standard modules are much easier to handle during stack assembly. There are many more reasons for the application of standard IGBT modules, even in a series connection of MMC converter technology for HVDC applica-tions. The challenge is to meet the requirement of a sufficient short circuit capability of the semi-conductor device and / or its housing.

4 Challenges for wire bonded IGBT module use

The dominating challenge is based on the de-mand for uninterrupted system operation, even if a single inverter cell gets into fault condition. As the main load current will continue to flow, long term arcing between the remaining fragments on the top of the IGBT´s baseplate will be the con-sequence. An outer fail safe mechanical short cir-cuit switch is mandatory and must be activated within milliseconds to avoid that this arcing burns down through the baseplate and heatsink mate-rial into to the water cooling channel. Besides the load current the identified destruction mechanisms are driven by two major effects.

1. A high pressure pulse that is caused by crop-ping up plasma.

2. Short circuit (magnetic field) current forces.

The high energies stored in the DC link capacitor of the MMC design will very quickly vaporize all aluminum bond wires and silicon chip surfaces.

Fig. 6: Single switch IGBT 1,7kV module; 200µsec and

1msec after destructive turn ON command^

Fig. 6 shows a smaller 1700 V IGBT module and its substrate (Fig. 7) that suffered from a compa-rably low energy impact of 5 kJoule, monitored with a high speed camera at 4,000 frames per

second. The vaporized material leads to an ex-treme pressure pulse inside the IGBT case.

Fig. 7: Single switch substrate and base plate after impact

Any IGBT case will immediately explode and cause debris that is gaining velocity. The impulse of the debris easily reaches a degree that will damage traditional inverter cell housings. Con-ductive dust and plasma clouds spread out. Without any housing they give impact within a ra-dius of one meter and much more. While the standard IGBT case opens up and rip the main terminals apart, attached copper bus bars in the range of 800 mm² cross section are easily bent apart 10 mm and even more. (Fig. 8)

Fig. 8: Bent bus bars and fine grained former 3.3kV IGBT

module after 35 kJoule impact

There is a difference in characteristics when a soft coated or a hard molded module is explod-ing, forced by such high energies. For lower en-ergies the soft coated module may not even show damage at its case, while any hard molded device will at least show cracks in the housing, a known advantage in industrial inverter designs with moderate DC link energy. At heavy energy the hard molded case holds the pressure inside for a longer time, gains more pressure and tends to turn more energy into mechanical force. The

soft coated types spread plasma earlier, the plasma cloud spreads wider. However, the final result of destruction appears very comparable. Fig. 8 shows a former 3.3 kV IGBT module after a destructive energy of 35 kJoule. Only fine grained particles remain after this impact. As the basic commutation cell of any VSC must be low inductive, short circuit currents reach sev-eral hundred kilo amperes. The resulting me-chanical forces can tare the complete construc-tion apart. Fig. 9 shows a typical bridge short cir-cuit current waveform. The first dip in the blue trace relates to the turn ON command of a 3.3 kV IGBT. After current self limitation in the range of 10 kA for about 30 µsec 400,000 A peak current are reached, based on a DC link energy of 22 kJoule.

-40 -30 -20 -10 0 10 20 30 40 50 60 70 80-250

-125

0

125

250

375

500

Capacito

r C

urr

ent [k

A]

Time [µs]

-2000

-1000

0

1000

2000

3000

4000

VC

E V

olta

ge [V

]

Fig. 9: Typical VSC short circuit voltage and current

waveform

Performing this test with 4.5 kV IGBT and 50 kJoule in the DC link, short circuit currents ex-ceed 650 kA. Any of this fault sequences leads into a scenario of heavy destruction. All impact must be kept away from close-by cells under any circum-stance.

5 Approaches to dam up these effects

What measures can be chosen to avoid the im-pact of this destruction to nearby inverter cells? The basic physical answers show up clearly:

• Cool down plasma and provide volume to fiz-zle out the pressure peak.

• Keep distances short to prevent debris from gaining speed and impulse.

• Provide flexible zones that absorb mechanical energy by achieving material deformation.

• Strengthen the mechanical design to with-stand the current forces.

Several of these approaches lead to contradic-tory demands. A customized optimum must be evaluated. In addition to this, the basic electrical function of the inverter cell must be considered.

• A low inductive commutation cell is manda-tory. To make matters worse insulation coor-dination must not be achieved by solid insula-tors, such as laminated bus bars. In HVDC systems there are special insulation require-ments concerning clearance and creepage distances.

• All insulators must be partial discharge free within the range of the operating voltage, as the desired lifetime is 30 to 40 years.

• A neat commutation behavior without coupling effects is mandatory for any VSC design.

• A high level of noise immunity for all on board peripheral PCBs of the inverter cell is neces-sary to avoid main converter chain reactions due to radiated emission, even at fault condi-tion with heavy surge currents of a close-by cell.

• A proper strain relief for load terminals to-gether with space-saving geometry that allows a suitable cooling in combination with easy production process are of the same impor-tance as overall system costs.

One easy way to handle the explosion of single cells could be based on simply gaining space be-tween all cells. But a typical 400 MW design with 1,500 cells already demands a converter hall vol-ume of 15,000 m³. The major part of this space is used for sparking distance between cell groups and clearance to earth. Fig. 5 shows a principal arrangement of cells inside the converter hall. Enlarging their in-between distance from a few centimeters to 1m would inflate the hall to unaf-fordable dimensions.

Fig. 10: Housing with inner foam layer to slow down debris after explosion test

Especially when off-shore wind parks are target application for a HVDC transmission line the ex-pected costs can not be handled. Other answers have to be found. The danger of spreading plasma clouds and par-ticles ban any type of traditional open inverter construction. An encapsulated design must be introduced. But covering the complete inverter leads to high system cost. The dc link capacitor

is part of the commutation cell, but its body with-stands short circuit discharges without outer damage, so there is no need to add an additional housing. One optimum is to cover only the vul-nerable parts of the commutation cells, in terms of wire bonded IGBTs, free wheeling diodes and minimum sections of bus bars.

The explosion proof housing needs increased stiffness to withstand the forces that occur during explosion. If no sufficient internal volume for the expected pressure peaks is provided, a bleeder channel is used. This channel must provide laby-rinths and filter structures to avoid that dust or hot plasma reaches the environment [6]. Fig. 10 shows an enclosure with inner foam layers to slow down debris and cut down housing cost. Synergies are used by introducing solid cooling plates as part of this construction. Insulated bushings are necessary for plus/minus DC link, AC output and gate drivers. Fig. 11 gives differ-ent, principal arrangements that were evaluated with and without foam layers and different num-ber of bus bar bushings.

foam

metal

fibre reinforced plastic

Fig. 11: Different, successful tested structures

Bus bars must be arranged in a way that much energy can be disposed in bending force, without affecting the sealing of the load bushings. Any gaps in the design must be sealed. This can ei-ther be achieved by traditional gap filling, or by covering with compartments. Fig. 12 shows plasma and small particles pouring out of a 0.5 mm gap in a load terminal bushing at 35 kJoule, caught by a high speed camera. Me-chanical design must assure that no gaps are di-rectly exposed to internal plasma. Alternatively it

must secure that gaps will be covered when in-ternal pressure is applied.

Fig. 12: Insufficient sealing of a 0.5 mm gap at 35 kJoule

Covering an existing inverter design with foam only (Fig. 13) already provides remarkable pro-tection against debris outside the cover.

Fig. 13: Foam covered IGBT module during assembly and

after explosion

In the first instance an external driver needs a signal path feed through for the gate-emitter ca-bles and an additional path for VCE desaturation monitoring of the high collector voltage. To avoid two explosion proof feed through paths per IGBT, the driver is divided into two functional parts. One is driver circuit at auxiliary emitter voltage level and the other part is directly attached to the gate terminals of the IGBT, to perform voltage relief close to the IGBT module inside the explosion proof enclosure. As additional benefit a wrong connection of the control cables during produc-tion is eliminated, while strain relief and easy as-sembly are introduced. Fig. 8 also shows this second part of the driver circuit after explosion. Customized molded bushings to connect both IGBT driver sections are used. Depending on energy and voltage levels that have to be handled several of the options named above must be chosen to guarantee an explosion proof design. Verification should always be per-formed in extensive hardware tests. Considering the possible energy only will not lead to feasible results. The maximum DC link voltage level must be additionally taken into consideration, because the peak short circuit current depends on this and superposes additional magnetic force impact.

6 Conclusion

To benefit from the various advantages that wire bonded standard IGBT modules offer to MMC inverter topologies (such as HVDC or static VAR compensation) their main disadvantage - the im-pact of an explosion on nearby inverter compo-nents - must be mastered. Several ten tests within a design process of more than 3 years were performed to evaluate different mechanical designs that are able to protect the close-by environment in case of an IGBT explo-sion. The designs have been successfully tested with 3.3 kV and 4.5 kV IGBT modules. 100 % availability of the main converter can be assured for DC link energies in the range of 20 kJoule up to more than 50 kJoule per single inverter cell. Housings that withstand energies up to 75 kJoule are in evaluation process. The evaluated explo-sion proof housings guarantee a safe use of wire bonded IGBT modules in MMC topologies. If op-erated in VSC systems with less reliability de-mands, a major benefit can be found in dramati-cally reduced cabinet maintenance and cleaning time. There will be no contamination outside a well designed explosion proof housing that cov-ers all vulnerable parts of the commutation cell.

7 Literature

[1] R. Marquardt, A. Lesnicar: “New Concept for

High Voltage-Modular Multilevel Converter”,

PESC 2004 Conference, Aachen, Germany

[2] L. Thomas, H. Zeller: “Explosion Protection

for Semiconductor Modules”, U.S.Patent

US6295205, Sep. 2001

[3] P. De Bruyne, L. Niemeyer: “Arrangement

for Semiconductor Power Components”,

U.S.Patent US4162514, Jul. 1979

[4] D. Scholz, H. Gerstenköper: “Semiconductor

Module”, U.S.Patent 7.221.004, May. 2007

[5] K. Kabushiki: “Explosion-proof Semiconduc-

tor Device”, German Patent, DE3032133,

May 1987

[6] M. Billmann, J. Dorn: “Power Semiconductor

Module Comprising an Explosion Protection

System”, Patent Application Publication,

WO2008/031372, Aug. 2004

[7] H.Schwarzbauer: “Explosion Proof Module

Structure for Power Components, Particu-

larly Power Semiconductor Components,

and Production Thereof”, Patent Application

Publication, WO 2008/061980, May 2008

[8] S. Gekenidis, E. Ramezani, H. Zeller: “Ex-

plosion Tests on IGBT High Voltage Mod-

ules”, IEEE Power Semiconductor Devices

and ICs, May 1999