| Frequency stability of power system is one of the important symbols of power quality.Different from the frequency response characteristics of conventional generator,the intermittent,weak control-ability,volatility of wind energy and weak coupling between traditional wind turbines and power system cause the inertia of power system decrease,which is not conducive to the frequency control of the power system and seriously restricts the development of wind turbines.Therefore,this paper study the use of hydraulic energy storage system(HESS)to further improve the frequency responding capability of hydraulic wind turbine(HWT)to enhance the frequency stability of power system under high wind turbines penetration.In this paper,the frequency response characteristics and frequency control methods of HWT are studied,and the energy storage technology is used to improve the frequency control capability of single HWT.The mathematical model of HESS,hydraulic main drive system and the active power adjust of synchronous generator after connected to the grid are all established.The inertia coefficient of the HESS and the frequency responding characteristics are analyzed.Then the frequency control strategy based on virtual inertia compensation is proposed.According to the actual working conditions,the PID adaptive frequency control strategy based on improved BP neural network is presented.For the frequency control strategy based on virtual inertia compensation,the dynamic transmission and distribution characteristics of system load disturbance during load fluctuation are analyzed firstly,and the inertia coefficient of HESS is converted.The configuration of the capacity of the hydraulic bladder accumulator combing with the requirements of wind turbines frequency deviation are analyzed.Then,the dynamic output active power of the bladder accumulator calculation method based on the variable droop coefficient of the bladder accumulator is calculated,and the transfer function between frequency change and the hydraulic pump/motor swing angle is derived.The system frequency control based on the accurate mathematical model is obtained.For the reliance on accurate mathematical model of frequency control strategy based on virtual inertia compensation,the PID adaptive control strategy based on improved BP neural network is further proposed,which is used to calculating the swing angle phase of the hydraulic pump/motor according to the system frequency deviation and the fluctuation amplitude.The frequency control strategy can be adapted to the actual operating conditions of the frequency control parameters.At the same time,an improved BP-algorithm is proposed aiming at the problem of slow learning rate of BP algorithm,which only considers whether each weight calculation is changed signs without considering the change of its specific value.It can easily overcome the inherent shortcomings of the Sigmoid activation function.Finally,the MATLAB software are used to verify the proposed two frequency control strategies.The proposed two frequency control strategies are experimentally verified on the experimental platform of the energy storage system HWT.The simulation and experimental results show that the two frequency control strategies have good control effects. |
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