Modern power systems are continually shifting towards greater integration of renewable energy sources and interconnection of existing AC grids that may operate asynchronously with respect to each other. The high-voltage direct current (HVDC) transmission and the associated voltage source converters (VSCs) are considered the most efficient technology for integrating the various energy sources. However, one of the key underlying requirements for the effective control of power flow in these VSC-HVDC systems is the appropriate synchronisation of the VSC converter with its corresponding AC grid, which relies on the accurate information regarding the phase angle of the AC voltage at the point of common coupling. This information is typically obtained using the so called Phase-Locked Loop (PLL). The task of grid synchronisation becomes particularly challenging in the cases where the grid interconnection may be located at a large distance from the area within which its voltage is tightly controlled. This is reflected by a large value of the Thevenin equivalent AC grid impedance and such systems are called âweak gridsâ. Weak grid condition introduces significant stability and dynamic challenges since, due to the large grid impedance, both the phase and the magnitude of the voltage at the point of common coupling, which itself is used for both grid synchronisation and active/reactive power control, experience severe fluctuations as a result of variations in the active/reactive power flow. Therefore, this thesis addresses the effect of the grid strength on the operation of the PLL utilised by the VSC converter. The results show that the previously proposed impedance conditioned PLL (IC-PLL) has the ability to increase the upper bound on the achievable power transfer achieved by the VSC converter connected to the weak grid. The effectiveness of this approach is examined by comparing its steady-state power transfer capability and the dynamic performance with that of VSC-HVDC that relies on conventional synchronous reference frame PLL (SRF-PLL) based synchronisation method. Furthermore, the challenge of the variable grid strength is also addressed in this thesis, whereby proposing the adaptive IC-PLL (AIC-PLL), which estimates virtual impedance by utilising feedback signal. The results show that the converter that relies on AIC-PLL has the ability to transfer power that is approximately equal to the theoretical maximum power whilst maintaining satisfactory dynamic performance.
Analysis and Design of Synchronisation Methods for Power Converters Connected to Weak Grid
Hamood, M. (Author). 16 Sept 2020
Student thesis: Phd