Modelling And Control Of Variable Speed Pumped Storage

The electric power generation using renewable energy resources is increasing in the world due to many reasons like increasing power demand,deregulated markets,and environmental concerns etc.To address the impacts of intermittent renewable energy resources,variable speed pumped storage(VSPS)system has got a new attention.Implementing VSPS in the power grid brings the prominent advantages including aligning peak generation to peak loads,reducing imbalance in case of scheduling challenges,improving stability control,enabling further penetration of intermittent generating resources,the ability to supply primary frequency and AC-voltage regulation and others.Hence,this dissertation is dedicated to study on modeling,simulation,control and applications of the VSPS power plant.Pumped storage system(PSS)converts grid-interconnected electricity to hydraulic potential energy during the off-peak periods(lower price energy)and then converting it back during the peak periods(higher price energy).VSPS can be achievable in the applications of asynchronous machine,doubly-fed induction machine(DFIM)and permanent-magnet synchronous machine(PMSM)such that the variable-frequency VSC system is implemented to control the machines.However,due to its convenient features,DFIM has been widely applied.Regarding to the converters,even if cycloconverter has been applied in the VSPS implementation,the DC-Iink converter has preferably been widely deployed due to its advantageous characteristics,and particularly,the three-level voltage source converter(VSC)type used in this dissertation.The VSC allows the DFIM a more versatile and flexible operation.Modelling and simulation of power electronic devices in a system are mostly developed using detailed model methods.This technique which must be discretized in a small time-step embraces exhaustive representation of power converters.Nevertheless,due to computational time and limitation of data storage,the employment of a large power system in this simulation method is complex.Besides,it is not suitable for studying transient stability of the large power system and offers little insight into design quantities.Thus,computationally accurate,effective and fast technique in simulating the grid integrated VSPS system is required as such a phasor model simulation is a right choice.This method can be implemented in the study of machine’s transient stability and any linear system,and offers distinct advantages in analysis,simulation,and control.Based on the phasor model technique,the VSPS containing hydraulic turbine,DFIM with three level VSC converter is designed and verified through MATLAB/Simulink platform in this study.The phasor model simulation of the VSPS achieves results with good accuracy as the detailed model does.It provides very accurate descriptions of observed transients of VSPS system.A vector control whose main concept is a DC like operation principle has been widely used in the AC machines.The stator currents of a three-phase AC machine are identified as two orthogonal components that can be visualized with a vector.A dq-transformation technique which is a vector decomposition is used in this study.The VSC having the rotor and grid side converters are dedicated to develop the vector control regulating the active and reactive powers in turn to maintain the frequency and AC voltage into nominal value.However,since the VSPS in this dissertation is designed based on phasor model technique which ignores PLL,the grid frequency is not well controlled,because the frequency is regulated with the power control loop but open loop for the grid frequency.This is the main challenge having been faced.Therefore,droop-based vector control is proposed and provided to feed the active power control loop of the VSPS.Additionally,the AC voltage droop control is provided with the reactive control to support AC voltage regulation in case of contingencies.To study how fast response of the proposed control strategy,the frequency spectrum analysis of the VSPS is implemented.The simulated results reveal that the proposed control strategy is effective for dynamic disturbance and steady state performance.In the simulation,significant difference between the vector control with and without droop control is observed during contingency conditions.The spectrum bandwidth of the VSPS is quite enough and has an implication to respond quickly when power fluctuation occurs in the power grid.In this dissertation,some applications of the VSPS including stability control,wind power fluctuation smoothening,grid frequency and AC voltage control are verified and taken as a case study.The case study is made by considering a 300 MW/18 kV DFIM based VSPS system and a grid consisting of two hydropower and a diesel power plants with synchronous generator,a wind farm and loads.Stability control is a critical issue and the VSPS can address the issues and is capable to improve the stability of the power system.The results of the case study verify that the VSPS performs very well in stability control of regulating voltage,frequency and active power flow operating with their rated and predefined values.Even,the VSPS withstands the collapse of the grid during loss of a generator in the hydropower station.With the case study of wind farm,the VSPS provides adequate power control.The simulated results show that the VSPS is effective to compensate the wind farm power fluctuation and smoothening the power flow in the grid bus.The frequency spectrum analysis also confirms the performance of the VSPS against with the wind power fluctuation.It 1s also validated that frequency and AC voltage controls are ensured with the implementation of the VSPS integrated with the grid.In general,the phasor model simulation is very effective for analysis and control design of large grid integrated DFIM based VSPS system.Implementing frequency spectrum analysis technique and droop-supportive vector control strategy in the DFIM based VSPS system is an ample solution for stability control,smoothening of wind power fluctuations and other intermittent renewable energy resources and supporting the grid frequency and AC voltage control during contingencies,and hence intensive investigations and studies need to be conducted.