| dc.description.abstract | Pumped storage hydropower plants (PSHP) are widely used for bulk energy storage around the world. The variable speed operation of these plants further enables their capability to regulate the power in pump mode as well. In most PSHPs, the variable-speed operation is achieved by the combination of a doubly-fed induction machine (DFIM) and a reversible pump turbine (RPT). The importance of this energy storage system is increasing at the same rate as the penetration of intermittent renewable energy sources like wind and solar into the grid. To better utilize these pollution-free energy sources, the PSHPs with fixed-speed synchronous machine units are being considered for conversion to variable-speed operation.
The variable-speed operation of a synchronous machine in a fixed-speed PSHP can be executed by introducing a full-size converter between the generator transformer and the stator of the machine. The converter decouples the machine rotation from the grid frequency, and thus the speed of the machine can be adjusted to vary the pumping power and to allow the RPT to run at the maximum possible efficiency over a wide range of operations.
There exist several converter topologies for variable speed drives in industrial applications. This thesis focuses mainly on the converter topologies and their control to enable the variable speed operation of the synchronous machine in a fixed-speed PSHP.
First, the converter topologies like neutral point clamped converter, active neutral point clamped converter, and modular multilevel converter have been evaluated and compared for this application based on the application-associated requirements. The analytical loss equations for the semiconductor devices have been derived and used for analyzing the loss at different operating conditions of the converter.
Second, the control method of these converters has been proposed for fast startup of the plant in both pump and turbine operation, fast mode transition from the turbine to pump and vice versa, and during low voltage ride through (LVRT) conditions on the grid terminals. The methods have been tested and verified in the laboratory using a full-size two-level three-phase converter, a synchronous machine, and a hardware-emulated reversible pump turbine.
With the full-size converter, critical operations like the startup in pump mode and the mode transition from the turbine to pump and vice versa can be executed quite faster than the existing DFIM technology. The startup can be accomplished in less than 30 seconds, and the mode transition can be performed in less than a minute which is crucial in following the varying power generation from intermittent renewable energy sources.
The summary of contributions of this PhD research is listed as follows:
1. Analytical loss equations for the PWM modulated ANPC converter and Modular Multilevel Converter have been derived. These equations are a useful tool for loss analysis of these types of converters and determining the rating of the semiconductor devices.
2. A detailed loss analysis of multilevel converters has been carried out and the best possible converter solution to enable the variable speed operation in a fixed speed pumped storage power plant has been proposed. This method can serve as a converter selection method for this technology.
3. A Control strategy to start and stop the power plant in turbine (generation) mode and pump mode has been proposed and verified in a laboratory with a 100 kVA prototype experimental arrangement.
4. Control strategy for the transition of mode from the turbine to pump and vice versa has been proposed and verified in the laboratory. This feature is crucial in adapting intermittently varying power generation from renewable sources like wind and solar. | en_US |
