7.1 Summary
In this dissertation, a new high power rating and high dynamic response power electronics system based on the concept of inverter cumulation has been investigated. This high per-formance system extends the physical limitation of current topologies based on single type market-available power electronics devices. Thus, it serves as an effective means of PHiL sys-tems which have higher requirements for its power convention system. As mentioned in Chapter 1, the core idea of theinverter cumulationis that by magnetically coupling different and identi-cal inverters, the final cumulation system accumulates the power and dynamics characteristics from various subunits. The two magnetically coupling methods (magnetic-parallel coupling of identical IGBT inverters and magnetic-series coupling of IGBT and MOSFET inverters) have demonstrated their superior characteristics as the power convention units of two different PHiLs (virtual machine and grid emulator).
The first attempt at the PHiL virtual machine showed success of the parallel coupling inverter modulated by the ’sequential switching’ concept. The cumulation concept increases the system overall modulation frequency while keeping the switching frequency of each device within its physical limitation. The virtual machine is controlled to generate the calculated back-emf voltage according to a reverse machine model. From the EUT point of view, virtual machine behaves like a real machine. The two key issues of the virtual machine are: coupling inductor for the sequential switching of the freewheeling diodes and the reverse model of the induction motor.
This work mainly focuses on the second cumulation topology. One high power IGBT inver-ter is magnetically coupled with another high switching MOSFET inverinver-ter. The two inverinver-ters operate with totally different characteristics (power, DC-link voltage, switching frequency, fun-damental voltage and frequency). Because of the different power rating of the two inverters, it is very critical to ensure the safe operating condition of the smaller power rating sides. The ori-ginal topology neglected the effect of the high amplitude switching components from the IGBT
110 CHAPTER 7. SUMMARY AND FUTURE WORK
inverter, which caused a fatal problem for the MOSFET inverter as a result of its rectifying and recharging working mode. After the first unsuccessful attempt, the author realized that the swit-ching components at the output terminal of the IGBT inverter have to be filtered out. The next key issue of the project is the low pass output filter design and its control. As mentioned before, there are mainly three types of filters: L, LC and LCL filter. Due to the limited filtering effect of the first order L-filter, LC and LCL filters are the two reasonable choices of most industrial applications.
The output of the LC filter is the capacitor voltage, on the other side, the inductor current serves as the output variable of the LCL filter. Choosing a type of filter is determined by the desired output signal. The proposed inverter system is supposed to emulate the behavior of the power grid, therefore the voltage signals generation is the main task. In this case, it performs like a programmable voltage source, thus an LC filter is the right choice. On the other hand, LCL filter is suitable for the system working as a current source.
A later chapter explains the output voltage control of the inverter with an output LC filter.
Three different strategies including the conventional synchronous-frame PI controller, recent popular stationary-frame resonant controller and the state space feedback controller are inves-tigated and implemented. The prototype of the proposedinverter cumulation system was built.
By emulating not only the normal operation, but also the typical fault of the power grid, it demonstrates its high performance as a PHiL grid emulator.
7.2 Outlook
One prospective future work is to investigate new cumulation topologies by going beyond the current coupling method and different inverter topologies. Recently, more and more researchers are focusing on the multilevel inverter in the medium voltage range. There are three main kinds
Figure 7.1: Newinverter cumulationtopology.
of multilevel topologies — neutral point clamped (NPC), flying capacitors (FCs) and cascaded
7.2. OUTLOOK 111 H-bridge (CHB). Among these converter topologies, CHB converters can easily reach higher voltage levels and larger power ratings by standard mature technology components because of its series connection structure of several single phase converters. Unlike the conventional CHB converters, cascading identical single phase inverters with equal dc-link voltage, the hybrid asymmetrical CHB converters combine various different topologies of inverters with unequal DC source voltages. By doing this, the converter can combine the advantages of different topologies, and achieve higher voltage levels with less circuit and control.
The proposed new cumulation topology investigates the possibility of the cascaded coupling of different inverters. Each phase of the system is composed of one Si-based 5-level NPC H-bridge (HV cell) and one SiC-based 3-level H-H-bridge (LV cell). A 15-level phase output voltage waveform will be synthesized by configuring the dc-bus voltages of the two cells in the ratio of 6:1. The recent fast developed SiC switches are considered as ideal candidates for fast switching applications. However due to the complexity of manufacturing, the cost of the SiC-based power devices is still high. The proposed hybrid topology replaces only the low voltage and fast switching cells by using SiC power switches, which increases the performance of the whole system with a reasonable price at same time. The project aims at investigating the realization of hybrid converter systems and implementation of their advanced control methodologies.
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