Digital Controller Design for Cascaded-Multilevel-Converter Based STATCOM Systems
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Date
2007-02-26
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Abstract
The purpose of the research has been to develop digital controllers for the Cascaded-Multilevel-Converter based STATCOM systems. STATCOM is one of the most important shunt FACTS controllers, which is designed to support the voltage and improve stability of the power system. Cascaded Multilevel Converter (CMC) is increasingly used at high power area due to its direct high voltage output with no need of transformer, which makes it as a good topology for STATCOM. Also because of the identical structure of each cell (H-bridge), CMC is the best candidate to be modularized.
In this thesis, two different digital controller developments are presented. First digital controller has conventional centralized controller structure. It uses a DSP plus FPGA as central controller. The DSP performs the control functions while FPGA is behaving like a bridge between DSP and peripheral devices. This topology is widely used in industry due to its simple and straightforward structure. Off-line simulation and hardware-in-the-loop real-time simulations are carried out to verify the design. Based on this topology a digital controller system has been developed and implemented in a three level STATCOM system.
However, the conventional centralized controller has some disadvantages. The central controller has direct connections with all converters. For high voltage and power rating converters, it is true that the power converters will consist of many modular blocks and these blocks will be placed at some distance from the controller. In this case, digital switch signals must be sent to the converters via optical fiber to improve reliability. However, the required fiber connections are too many that increase cost and the risk of fault. Moreover, the analog signals from the sensors such as the voltage and current are usually sending back to the central controller through analog wire connections that has low electromagnetic interference (EMI) susceptibility.
To solve these problems, a modular digital controller topology is proposed. It has one central controller plus multiple local controllers. Every module converter has its own local controller which is placed very close to the modular converter. The local controller work as a "brain" for the converter. This "brain" realizes all the sensor signals of the module converter and sends them to central controller via asynchronous series communication protocol. Central controller performs the close loop algorithm and generates switching states. These switching states are sent to local controllers also through asynchronous series communication protocol. All the long distance data transmission is through optical links, which greatly increase the EMI susceptibility. The number of the fibers is reduced due to the series communication protocol. This modular controller is a little bit more complicated than the conventional centralized structure. But it can truly achieve "modularization", which means improved reliability, isolation and increased expansion flexibility. This modular controller is built and verified through 50V 5A experiments.
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Keywords
STATCOM, digital controller, Cascaded-Multilevel-Converter
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Degree
MS
Discipline
Electrical Engineering
