IGBT series connection under Active Voltage Control with temporary clamp

This paper presents the use of an Active Voltage Control (AVC) technique for balancing the voltages in a series connection of Insulated Gate Bipolar Transistors (IGBTs). The AVC technique can control the switching trajectory of an IGBT according to a pre-set reference signal. In series connections, every series connected IGBT follows the reference and so that the dynamic voltage sharing is achieved. For the static voltage balancing, a temporary clamp technique is introduced. The temporary clamp technique clamps the collector-emitter voltage of all the series connected IGBTs at the ideal voltage so that the IGBTs will share the voltage evenly. © 2012 IEEE

DC-DC CONVERTER FOR POWER COLLECTION IN WIND FARMS

Offshore wind farms have grown rapidly in number in recent years. Several large-scale offshore wind farms are planned to be built at further than 100 km from the United Kingdom coast. While high-voltage high-power installations have addressed the technical issues associated with reactive power flow in AC transmission, reactive power can be avoided by using High-Voltage Direct Current transmission (HVDC). Reactive power causes problems when transmission distances are long, therefore, HVDC transmission is now being considered for wind farm grid connection. However, as wind farms constitute weak systems Line Commutated Converter (LCC) based HVDC is not viable and newer Modular Multilevel Converter (MMC) based Voltage Source Converters (VSC) are needed for the AC-DC conversion. One of the key components in such systems is the DC-DC converter, which is required to act as the interface between the generation, transmission, and distribution voltage levels, and reduces the power conversion stages, avoiding transformers typically used in AC grid integration systems. In addition, there is no high-power Medium-Voltage MV DC-DC converter available for offshore wind farm energy systems at present. The specification requirements of high-power MV DC-DC converters can be set once the output characteristics of the wind turbine generators have been reviewed. An offshore wind farm with MVDC-grid collection does not exist today, but it is a promising alternative, although specification analysis of high power MV DC-DC converters is necessary. The work reported in this thesis aims to introduce two types of high power MV DC-DC converter topologies, for offshore wind farm energy systems, termed single-stage, and multi-stage converters. Ways of reducing losses by soft switching and reduction in the number of components are considered. Both topologies are based on the Marx principle where capacitors are charged in parallel and discharged in series to achieve the step-up voltage transformation. During doldrums, light and calm wind, and for maintenance work, it is necessary to supply the offshore wind farm with auxiliary power. This thesis proposes a novel Bidirectional Modular DC-DC converter (BMDC) and evaluates its performance. The simulation results show that the proposed BMDC allows up to 5% of the wind farm’s power rating to be drown from the onshore substation. This means that the proposed DC-DC converter is capable to provide bidirectional power flow. For offshore wind farm application, BMDC can be inserted between the offshore wind farm and onshore substation. The studies, in this thesis, are based on an input DC collection at 6 kV with the DC to DC converter stepping up the voltage to 30 kV. The proposed system is integrated and simulated with the DC offshore wind farm and a Voltage Source Converter (VSC) in the onshore station. The steady-state simulation results, to transmit the power between two different voltage levels, and the dynamic performance of the proposed converter were investigated. The advantages of the proposed converter include its simple design and that it does not require an AC transformer; hence can easily be implemented in an offshore wind farm since it requires less weight and size on the platform in the sea, which ultimately results in minimal cost. Furthermore, the proposed converter can ride through a fault which complies with the UK Grid code. However, in this case, it is necessary to provide protection systems such as a large chopper resistor for energy absorption or de-loading the wind turbine. Finally, the proposed integrated BMDC converter showed its suitability for offshore wind farms as well as improving their reliability

Electrothermal simulation and characterisation of series connected power devices and converter applications

Power electronics is undergoing significant changes both at the device and at the converter level. Wide bandgap power devices like SiC MOSFETs are increasingly implemented in automotive, grid and industrial drive applications with voltage ratings as high as 1.7kV now commercially available although much higher voltages have been demonstrated as research prototypes. In high power applications where high DC bus voltages are used, as is the case in voltage source converters for industrial drives, marine propulsion and grid connected energy conversion systems, it may be necessary to series connect power devices for OFF-state voltage sharing. In high power applications, before the advent of multi-level converters, series connection of IGBT power modules was commonplace especially for HVDC-voltage source converter applications. However, with the advent of the modular multi-level converter, where the AC voltage waveform is synthesized by discrete voltage steps, the need for series connected is obviated. Most HVDC-VSC applications are now implemented by modular-multi-level converters. However, in some applications like VSCs for distribution network power conversion, there can be a combination between series connection of power devices and multi-level converter. Traditionally, voltage balancing in series connected power devices was achieved using snubber capacitors for dynamic voltage sharing and resistors for static voltage sharing. However, the use of snubber capacitors reduces the switching speed of the converter thereby defeating the purpose of using SiC power devices especially in power converters with high switching frequencies. To avoid this, active gate driving techniques that avoid the use of snubber capacitors during switching are under intensive research focus. This involves intelligent gate drivers capable of dynamically adjusting the gate pulse during switching. To use these gate drivers, it is necessary to explore the boundaries of static and dynamic voltage imbalance in series connected power devices. For example, it is necessary to understand how differences in device junction temperature and gate driver switching rates affects voltage divergence between series connected devices and how this differs between silicon IGBTs and SiC MOSFETs. This is similarly the case between series connected silicon PiN diodes and SiC Schottky diodes. Since silicon IGBTs and PiN diodes respectively exhibit tails currents and reverse recovery during turn-OFF, the dynamics of voltage divergence between series devices will differ from unipolar SiC power devices. Furthermore, the leakage current mechanisms determine the OFF-state voltage balancing dynamics and since Si IGBTs have different leakage current mechanisms from SiC devices, OFF-state voltage balancing in series connected devices will be different between the technologies. The contribution of this thesis is using finite element and compact device models backed by experimental measurements to investigate static and dynamic voltage imbalance in series connected power devices. Starting from the fundamental physics behind device operation, this thesis explores how the leakage currents and tail currents affects voltage divergence in series silicon bipolar devices compared to SiC power devices. This analysis is compared with how the switching dynamics peculiar to fast switching SiC devices affects voltage balancing in series connected SiC devices. Simulations and measurements show that series connected SiC power devices are less prone of excessive voltage divergence due to the absence of tail currents compared to series connected silicon bipolar devices where voltage divergence due to tail currents is evident. Reduced leakage currents due to the wide bandgap in SiC also ensures that it is less prone to voltage divergence (compared to silicon bipolar devices) under static OFF-state conditions. This means the snubber resistances can be increased thereby reducing the OFF-state power dissipation in series connected SiC devices. In the analysis of voltage sharing of series connected devices during the static ON-state and OFF-state it was shown that the zero-temperature coefficient of the power devices determines the voltage sharing and loss distribution in the ON-state while the leakage current and switching synchronization is critical in the OFF-state. Simulations and measurements in this thesis show that the higher ZTC points in silicon bipolar devices compared to SiC unipolar devices means that ON-state voltage divergence depends on the load current. The dominant failure mode for series connected power devices is failure under dynamic avalanche which occurs in cases of extreme uncontrolled voltage divergence. In the investigations of the switching transient behaviour of series connected IGBT and SiC MOSFETs during turn-OFF, it was shown that the voltage imbalance for Si IGBT is highly dependent on the carrier concentration in the drift region during switching while for SiC MOSFET it depends on the switching time constant of the gate voltage and the rate that the MOS-channel cuts the current. The thesis also explores the limits of power device performance under dynamic avalanche conditions for both series silicon bipolar and SiC unipolar devices. In the analysis of SOA of series connected devices it was discussed that the SOA is reduced by increased switching rates and DC link voltages. Finally, the thesis explores the 3L-NPC converter and how the power factor of the load on the AC side of the converter alters the power dissipation sharing between the devices. The results show that loss distribution between the devices in the converter is not just affected by the load power factor but also by the switching frequency