Condition monitoring of press-pack IGBT modules for high voltage DC transmission using temperature sensitive mechanical parameter

Abstract

Power electronics is a technology that can efficiently transmit energy to the user, with power semiconductor devices playing a pivotal role as a key component in power conversion. The trend in power semiconductor devices is towards higher power ratings and higher power densities which all lead to higher heat generation inside the device. High thermal stress generated during the operation results in failing power semiconductor devices. Insulated Gate Bipolar Transistors (IGBTs) are the most widely used power electronic switches for medium and high power conversion applications. IGBTs are mostly packaged using bond wires. Bond-wired IGBT modules for high power levels, however, do have reliability issues and often fail open circuit. In High Voltage DC Transmissions (HVDC) power modules that fail open must be avoided. For that reason, in HVDC press pack IGBT (PP IGBT) modules are used which are known to fail short circuit. Although PP IGBTs reduce the risk of module failures by allowing electrical and thermal contact within the module through pressure, instead of through bonding wires they fail eventually over time. Monitoring the health condition of PP IGBT modules is therefore essential and the aim of this work is to develop an on-line measurement technique that can provide an early warning of degrading PP IGBT modules for HVDC applications. This thesis compares different existing online health monitoring techniques and identifies where within a press pack to monitor the health of the module. A new parameter, the collector groove lid deformation, is identified. The relationship between junction temperatures and deformation is shown. The new technique is called Temperature Sensitive Mechanical Parameter (TSMP) in analogy to the well-known Temperature Sensitive Electrical Parameter (TSEP). Sensors suitable for measuring deformation are analyzed and compared, and an FBG based (Fiber Bragg Grating) online monitoring system is proposed, which can monitor multiple series-connected PP IGBT modules in real time through a single optical fibre and can also determine the location of faulty modules and the approximate distribution of chips based on deformation variation. Experimental work is carried out on different PP IGBT modules, and different test requirements are considered by modifying the internal structure of the modules. Practical and simulation work are presented and discussed to verify the effectiveness of the proposed health monitoring system for PP IGBTs