Applicable Power Electronic Switching Modules

As highlighted in section 2.9, the topology features a high number of stator phases. The Electronic Commutator devices switch at the machine fundamental frequency which is typically very low, 10 Hz up to 200 Hz range being envisaged. Owing to this low switching frequency, power electronic switching devices optimised for low conduction losses will be ideal since switching losses are minimal. Power electronic devices with, reverse voltage blocking capability (symmetric or asymmetric) and

4.7 Multi-Level Machine & Converter Integration

gate assisted/controlled current turn off capability will be required. Whole wafer Thyristor type switching devices with gate assisted current commutation such as Gate Commutated Thyristors (GTO)and Integrated Gate-Commutated Thyristor (IGCT)are equally applicable to this topology. Other devices such as Insulated-Gate Bipolar Transistor (IGBT), Metal Oxide Semiconductor Field Effect Transistor (MOSFET)are equally applicable when used in series with reverse voltage blocking diodes.

As highlighted earlier for this multilevel machine and converter topology, low voltage devices with very low on-state Ohmic losses can yield potential significant efficiency improvement benefits. Recently IGBTs with reverse blocking capabili- ties have been reported in literature [146–151] and some manufacturers have started commercial production of these devices. Also, with recent advances in power semicon- ductor device and packaging technology, MOSFETs and normally off Junction Field Effect Transistors (JFETs) with very low on-state losses can now be realised [152], [153]. However, due to the reverse voltage blocking requirement for this machine and converter topology, MOSFETs & JFETS in their conventional packaging will not be suitable for this application. If however, the MOSFETs or JFETS can be repackaged with either series voltage blocking diodes or alternatively repackaged in a back to back configuration and operated in synchronous rectification mode, they can be utilised in this topology. This can potentially yield H-bridge cells with sufficiently low on-state losses, desired voltage blocking with inherent fault current limiting capability.

If silicon carbide MOSFET devices capable of operating at much higher junction temperatures [154–157] are employed, the H-bridge cells can be integrated as part of the machine, resulting in a very compact and power dense machine converter assembly. This is highly desirable for applications where space is a premium such as marine, offshore and aviation industries. This can potentially lead to reduced footprint of the overall drive system and consequently smaller overall power system plant. Moreover, the high thermal conductivity makes SiC an excellent candidate for harsh environments. However, there are still some significant challenges that have to be overcome before this technology can be commercially fully exploited for high temperature applications. These include:

4.7 Multi-Level Machine & Converter Integration

Passive Components: In order to fully exploit the high temperature advantages of SiC

devices, the passive auxiliary components required to realise a power electronic circuit such as gating electronic circuits, require passive components that are capable of operating at these high temperatures. These passive components include capacitors and wound magnetic components. Little attention has been paid to high temperature passive components that could enable the full SiC potential. More work is now being done to study, develop and fully characterise the performance of passive components for use in high temperature applications [158–161].

Device Packaging: Current semiconductor packaging has been adapted to the Si tem-

perature limit of 175 degrees Celsius [154, 155] and is unsuitable, conventional plastic package cannot be exposed to such high temperatures. Further work is required in order produce SiC device packaging that is capable of dealing with the higher absolute junction temperatures and cope with higher temperature swings, thermal and power cycling duty. Research activities in semiconductor device packaging for high temperature applications is increasing [162–165] to enable exploitation of the advantages of wide band gap semiconductor devices.

Low Volume Cooling Systems: One key reason for wanting to operate at high am-

bient temperatures is that it can facilitate use of low volume cooling systems. However, device Junction temperatures exceeding 175 degrees Celsius can have a negative impact on converter efficiency due to the consequent rise in device losses with increasing temperature. Owing to the constraints in packaging and heat extraction from the devices, a tradeoff between losses and operating temper- ature may be required as the losses of SiC power semiconductors increase with device temperature, leading to a consequent decrease in converter efficiency [156, 166, 167, 145]. This general trade-off gives rise to an optimization pro- cedure because the power, that can be dissipated by the cooling system, rises linearly with temperature while the losses typically show a steeper increase with temperature. That is, for a given converter at a certain temperature, the device current cannot be increased any more even though the junction temperature increases but must be decreased in order to prevent the device from a thermal

4.7 Multi-Level Machine & Converter Integration

runaway [155]. However, with ongoing developments in high temperature op- eration of the power electronics components, heatsinking and active cooling thermal management strategies can be significantly downgraded, thus reducing the size, volume, and weight of the overall power electronic systems by as much as two orders of magnitude [154].

Cost: . Currently the cost of developing and applying devices and passive components

for such high temperature environments is still prohibitive for commercial exploitation, but will decrease as these technologies become readily available [145].