Research On Low Voltage Through Control Strategy For Full Power Variable Speed Operation Hydro-Generator Units

Full-power variable speed is one of the main ways of variable-speed operation of hydro-generator units.Because of its control system,variable-speed operation range,power response speed,etc.,it has more advantages than doubly-fed variable-speed hydro-generator units,which has attracted widespread attention in the industry.The use of converters requires corresponding low-voltage ride-through capability.Therefore,this article focuses on the low-voltage ride-through control strategy of a full-power variable-speed hydro-generator unit,and analyzes the relationship between the energy flow of the unit and the traditional unloading circuit.Based on this,a low-voltage ride-through control strategy using rotor energy storage and coordinated control of active and reactive power is proposed,and an analytical method for the maximum increase in the speed of the hydro-turbine generator unit during low-voltage ride-through under this strategy is given.The research results can be Improve the low voltage ride-through capability of the full-power variable-speed hydro-generator unit and provide an effective reference for the design of the full-power variable-speed hydro-generator unit.The details are as follows:(1)The mathematical model of a full-power variable-speed hydro-generator unit is established.The speed regulator is used to control the speed of the unit,the machine-side converter controls the active and reactive power,and the grid-side converter controls the traditional control strategy of constant DC bus voltage.The voltage drop characteristics of the unit are analyzed,and the unloading circuit is used to realize the low voltage ride-through of the unit.It is verified by simulation,but the unloading circuit adds hardware,the system is more complex,the cost is high,and the versatility of the converter is reduced.(2)According to the characteristics of strong rotating energy storage capacity,wide speed operation range,and flexible adjustment of turbine power through speed regulator,the generator-side converter controls the DC bus voltage,and the grid-side converter controls the unit.The low voltage ride through control strategy of the rotor energy storage of the rotor energy storage of the speed regulator controlling the speed of the unit.This strategy cancels the unloading circuit,transfers the unbalanced power to the generator,uses the generator rotor to increase the speed to store the unbalanced power during the low voltage ride-through period,and delivers a certain reactive current to the grid according to the magnitude of the grid voltage drop.The simulation results show that the strategy effectively limits the DC bus voltage to within 1.1 pu,reduces the DC bus voltage fluctuations,and the unit speed rises to 1.12pu without affecting the stable operation of the unit.The grid-side converter transmits to the grid The reactive current of 1.05pu gradually decreases to 0 with the recovery of the grid voltage,which improves the low voltage ride-through capability of the unit.(3)When using the rotor energy storage strategy to achieve the unit’s low voltage ride-through,in order to quickly obtain the maximum value of the unit’s speed increase,this paper considers the two situations of the guide vane action and the guide vane inoperative,and the system is reasonably simplified,and the rotation rate is derived.Numerical analytical model of the maximum speed increase.Through simulation verification,this analytical model can better simulate the dynamic process of speed increase,and obtain the maximum speed increase and the influence of unit parameters on the maximum speed increase.It is the low voltage ride through of variable speed units.The selection provides a basis.