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

Transcription

1 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, Erlangen, Germany Tel. (+49) 9131 / , markus.billmann@iisb.fraunhofer.de 2) Fraunhofer Institute of Integrated Systems and Device Technology (IISB) Schottkystrasse 10, Erlangen, Germany Tel. (+49) 9131 / , dirk.malipaard@iisb.fraunhofer.de 3) Siemens AG, Energy Sector, Power Transmission Division, Power Transmission Solutions, Guenther-Scharowsky-Str. 2, Erlangen, Germany, Tel.: (+49) 9131 / , 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 inverter. 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 multilevel 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 converter 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 conventional equipment that can be combined in different applications to transmit active power or to control reactive power. Due to the low losses compared to other power electronic devices, line-commuted thyristor technology 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 shortcircuit power. In new concepts, the changeover to selfcommutated 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 systems Self-commutated converter technology is wellknown for more than 30 years for instance in the field of drive applications. In DC transmission, an

2 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, making it easier to deal with fault situations. The familiar 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 distribution systems in the future. This kind of converter uses power semiconductors with turn-off capability such as IGBT (Insulated Gate Bipolar Transistors) as a suitable example. Some general benefits of VSC technology are shown in Fig. 2. high number of steps are called multilevel converters. Different multilevel topologies were proposed in the past and have been discussed many times. A new and different approach is the Modular 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 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 connected in series in one converter arm have to operate as simultaneously as possible. As a result 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 filtering 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 With a high number of levels and this is necessary for HVDC applications anyhow the switching 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 converters for HVDC applications In practically all technical systems, even after extremely thorough engineering and testing, sporadic failures of individual components during operation cannot be avoided. This must not affect operation of an HVDC system. Power transmission must continue, even if a certain percentage of inverter cells failed before the next scheduled shutdown for maintenance. This is why redundant power semiconductors are integrated in HVDC converter systems. In a 400 MW HVDC application about 1500 inverter 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 series 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 experience is confirming this. In self-commutated two-level or three-level converters based on series connection of semicon-

3 ductors with turn-off capability, i.e. IGBTs the same requirement does exist. This is why partly IGBT - based on cost intensive press pack technology - are applied. It is very challenging to realize an appropriate long term short circuit capability with this kind of semiconductor devices. Many efforts were made to increase the explosion withstand capabilities of press pack devices [2, 3, 4, 5, 7]. The structure of modular multilevel converter technology as a basis for HVDC applications leads to further requirements for the power semiconductors respectively their housings. These so-called power modules are operated 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 different 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. countless drive and traction applications and become constantly improved. No special devices, designed for HVDC use only, but standard catalog 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 applications. The challenge is to meet the requirement of a sufficient short circuit capability of the semiconductor device and / or its housing. 4 Challenges for wire bonded IGBT module use The dominating challenge is based on the demand 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 consequence. An outer fail safe mechanical short circuit switch is mandatory and must be activated within milliseconds to avoid that this arcing burns down through the baseplate and heatsink material 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 cropping 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. 5: Principle MMC topology and inverter cell arrangement 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 otherwise 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 series 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 components with high availability. They are applied in 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 comparably low energy impact of 5 kjoule, monitored with a high speed camera at 4,000 frames per

4 second. The vaporized material leads to an extreme 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. Conductive dust and plasma clouds spread out. Without any housing they give impact within a radius 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) 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 several hundred kilo amperes. The resulting mechanical forces can tare the complete construction apart. Fig. 9 shows a typical bridge short circuit 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. Capacitor Current [ka] Time [µs] Fig. 9: Typical VSC short circuit voltage and current waveform V CE Voltage [V] Performing this test with 4.5 kv IGBT and 50 kjoule in the DC link, short circuit currents exceed 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 circumstance. 5 Approaches to dam up these effects 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 exploding, forced by such high energies. For lower energies 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 What measures can be chosen to avoid the impact of this destruction to nearby inverter cells? The basic physical answers show up clearly: Cool down plasma and provide volume to fizzle 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 withstand the current forces. Several of these approaches lead to contradictory demands. A customized optimum must be evaluated. In addition to this, the basic electrical function of the inverter cell must be considered.

5 A low inductive commutation cell is mandatory. To make matters worse insulation coordination must not be achieved by solid insulators, such as laminated bus bars. In HVDC systems there are special insulation requirements 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 necessary to avoid main converter chain reactions due to radiated emission, even at fault condition with heavy surge currents of a close-by cell. A proper strain relief for load terminals together with space-saving geometry that allows a suitable cooling in combination with easy production process are of the same importance as overall system costs. One easy way to handle the explosion of single cells could be based on simply gaining space between all cells. But a typical 400 MW design with 1,500 cells already demands a converter hall volume 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 unaffordable dimensions. is part of the commutation cell, but its body withstands short circuit discharges without outer damage, so there is no need to add an additional housing. One optimum is to cover only the vulnerable 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 labyrinths 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 different, principal arrangements that were evaluated with and without foam layers and different number of bus bar bushings. foam metal fibre reinforced plastic Fig. 11: Different, successful tested structures 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 expected costs can not be handled. Other answers have to be found. The danger of spreading plasma clouds and particles 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 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 either 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. Mechanical design must assure that no gaps are directly exposed to internal plasma. Alternatively it

6 must secure that gaps will be covered when internal 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 protection against debris outside the cover. 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 impact of an explosion on nearby inverter components - 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 explosion. 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 explosion proof housings guarantee a safe use of wire bonded IGBT modules in MMC topologies. If operated in VSC systems with less reliability demands, a major benefit can be found in dramatically reduced cabinet maintenance and cleaning time. There will be no contamination outside a well designed explosion proof housing that covers all vulnerable parts of the commutation cell. 7 Literature 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 cables and an additional path for V CE 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 production is eliminated, while strain relief and easy assembly 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 performed 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. [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 US , Sep [3] P. De Bruyne, L. Niemeyer: Arrangement for Semiconductor Power Components, U.S.Patent US , Jul [4] D. Scholz, H. Gerstenköper: Semiconductor Module, U.S.Patent , May [5] K. Kabushiki: Explosion-proof Semiconductor Device, German Patent, DE , May 1987 [6] M. Billmann, J. Dorn: Power Semiconductor Module Comprising an Explosion Protection System, Patent Application Publication, WO2008/031372, Aug [7] H.Schwarzbauer: Explosion Proof Module Structure for Power Components, Particularly Power Semiconductor Components, and Production Thereof, Patent Application Publication, WO 2008/061980, May 2008 [8] S. Gekenidis, E. Ramezani, H. Zeller: Explosion Tests on IGBT High Voltage Modules, IEEE Power Semiconductor Devices and ICs, May 1999