Reliable Grid Condition Detection and Control of Single-Phase Distributed Power Generation Systems

The constant growth of Distributed Power Generation Systems (DPGS) presents an efficient and economic way of generating electricity closer to the load(s). The DPGS can contribute to an efficient and renewable electricity future by potentially: increasing the use of renewable sources of energy; improving the efficiency of the electricity system by reducing transmission and distribution losses; improving the security of the electricity supply through increased diversity of supply and reduced vulnerability to simultaneous system failures. However, the new trend of using DPGS comes also with a suite of new challenges. One of the challenges is the interaction between the DPGS and the utility grid. As a consequence, grid interconnection requirements applying to distributed generation are continuously updated in order to maintain the quality and the stability of the utility grid.
The new upcoming standards addressed to the grid-connected systems will harmonize the combination of the DPGS and the classical power plants. Consequently, the major tasks of this thesis were to develop new grid condition detection techniques and intelligent control in order to allow the DPGS not only to deliver power to the utility grid but also to sustain it.
This thesis was divided into two main parts, namely "Grid Condition Detection" and "Control of Single-Phase DPGS". In the first part, the main focus was on reliable Phase Locked Loop (PLL) techniques for monitoring the grid voltage and on grid impedance estimation techniques. Additionally, a new technique for detecting the islanding mode has been developed and successfully tested. In the second part, the main reported research was concentrated around adaptive current controllers based on the information provided by the grid condition detection techniques.
To guarantee the correct generation of the reference signals and to meet the demands regarding the operation boundaries with respect to voltage amplitude and frequency values required by standards, the grid-connected converters need an accurate and fast detection of the phase angle, amplitude and frequency of the utility voltage. As a consequence, a new voltage monitoring algorithm including an offset rejection technique was proposed in this thesis. Additionally, the discrete implementation of the algorithm was given.
The estimation of the grid impedance can be used by the control of numerous gridconnected systems, such as active filters, islanding detection techniques, non-linear current controllers including hysteresis and predictive control, detection of the operation mode (on-grid or off-grid) of a DPGS, etc. Therefore, estimating the grid impedance can add extra functions into the operation of the grid-connected converters. Hence, three different methods have been developed with relation to the grid impedance estimation. The first method uses the harmonic injection technique; the second method is based on active and reactive power variations and the third proposed method is based on identification techniques.
The islanding detection is an important and necessary safety future of the gridconnected systems to comply with. The islanding should be detected fast in conformity with the standard requirements for the grid interconnection. Accordingly, an accurate and less-disturbing islanding detection algorithm based on PLL technique was developed and tested successfully in this thesis.
The control of a DPGS is mainly designed in accordance with the electrical grid condition at the Point of Common Coupling (PCC). Usually, the parameters of the controllers are tuned for some assumed values of the electrical grid parameters. However, the parameters of the electrical grid such as grid impedance or grid frequency can change. Therefore, the control should be able to follow these changes by using the data provided by the grid condition detection techniques in order to maintain the performance and the robustness of the entire system. Regarding the advance control of DPGS, an active damping technique for grid-connected systems using inductor-capacitorinductor (LCL) filters was proposed in the thesis. The method is based on a notch filter, whose stopband can be automatically adjusted in relation with an estimated value of the grid impedance.