Abstract:With the rapid growth of photovoltaic and wind power’s access scale in the power grid, the problem of sub-synchronous oscillation (SSO) in the power system has become increasingly prominent. At present, the design scheme of new energy sub-synchronous damping control (SSDC) is mainly based on the model-driven control method, which relies on the accurate system state equation. However, the power system has time-varying and nonlinear characteristics, and the existing SSDC controller for new energy has limited applications, making it difficult to adapt to complex and changeable operating conditions. Therefore, finding a new strategy to break the barriers in damping control is urgent. At the same time, the existing literature has not explored the multi-machine equivalent model with multiple new energy types. According to previous engineering experience and analysis, the input and output signals of the multi-machine cooperative damping controller are selected. The feedback signal is selected as the grid-connected voltage of the new energy station, and the input position is selected as the inner d-axis of the inverter of the photovoltaic and double-fed wind farms. Then, for a new energy station containing multiple photovoltaic power stations and multiple wind turbines, in the process of designing the SSDC controller, it is similarly equivalent to a multi-input single-output system, and a model-free adaptive control (MFAC) algorithm for multi-machine coordination is designed. The parameters of the simulation system and the controller are designed, and the controller parameters are optimized by the simplex method. Finally, the impedance frequency scan analysis and simulation are carried out in the IEEE 39-node system connected to the new energy station through serial compensation, and the effectiveness of the multi-machine cooperative damping strategy is verified. The following conclusions can be drawn. (1) Impedance frequency analysis shows that the damping control method can effectively reshape the impedance of the new energy station system and significantly improve the positive sequence equivalent resistance of the system in the range of sub-synchronous frequency, reflecting good SSO suppression potential. (2) The numerical example shows that the damping control method has a significant effect on the cooperative control of the equivalent four-machine system of wind power and photovoltaic. A complete control process only requires 24 addition/subtraction calculations and 28 multiplication/division calculations, reducing calculation costs. (3) Considering the input time of the damping controller, the scale of the new energy station, the proportion of wind power out put to solar power output, and the influence of large and small disturbances of the system (changes in wind speed or radiation intensity, the cutting load of the cutting machine), the controller can effectively suppress the sub-synchronous oscillation and is insensitive to the oscillation frequency deviation. In conclusion, the multi-machine cooperative model-free sub-synchronous damping control strategy is adaptable to new energy stations containing multi-PV and multi-DFIG.
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