Research And Improvement Of Control And Protection Strategy Of Static Var Compensation Device

With the continuous development of modern power technology,the scale of the power grid is getting bigger and bigger,and the voltage level is also becoming higher and higher.The high-voltage and large-capacity power transmission system has an increasing demand for reactive power.As a typical reactive power compensation device,static reactive power compensation device(SVC)plays a significant role in increasing the system voltage level,improving power quality and enhancing power transmission capacity,so its performance is of great significance for the safe and stable operation of power grid.In this paper,the research and improvement of the control and protection strategy of static var compensation was carried out.The main research content is as follows:(1)In order to more effectively control the TSC as a reactive power reserve branch,based on the relationship between the thyristor trigger angle of TCR and the reactive power demand of system,a new TSC control strategy based on the average value of TCR three-phase trigger angles was designed.Meanwhile,in order to ensure the fast and accurate switching of the capacitors in the TSC,a trigger control method that enables both positive and negative phase thyristors at the zero-crossing point of the thyristor terminal voltage in the TSC was also designed.(2)Aiming at the problem that the SVC needs to continuously adjust the output in order to maintain the voltage at the target value during the steady state of the system,which causes more operating losses,a voltage dead zone auxiliary control was added to the voltage control strategy.That is,when the voltage is within the allowable error range,the SVC is locked,and outside the range,the SVC is released to adjust the voltage to the range.On the basis of voltage dead zone control,a voltage reference value which can be changed according to the target voltage is also designed accordingly,which can expand the adjustable range of SVC while making SVC have more reactive power reserve.(3)Aiming at the problem that the coupling transformer of SVC will be saturated when the second harmonic distortion of system voltage occurs,the TCR balance auxiliary control was designed.That is,by combining the fast Fourier transform and PI adjustment to modulate the trigger angle,the anti-parallel thyristors can achieve different conduction times in the positive and negative directions,thereby reducing the DC current component to zero and suppressing the influence of the second harmonic on the device.(4)Aiming at the problem of high voltage in the transient process of SVC connected power system after the fault is cleared,a new SVC low-voltage protection control strategy to suppress voltage overshoot was designed.That is,when the system is running normally,the susceptance reference value output by the main PI regulator and the trigger angle of the conversion are firstly sampled,delayed and recorded,after the fault they are locked and the original input and output of the main PI regulator are also shielded.Then the locked trigger angle is input into the trigger control link of TCR to prepare the pulse signal,and controls the TCR to output inductive reactive power to offset the capacitive reactive power that increases suddenly to suppress voltage overshoot after the fault is cleared.At the same time,the equivalent error is calculated according to the locked susceptance reference value,and it is input to the main PI regulator to change its output.When the SVC exits the low voltage protection control,the output of the main PI regulator is equal to the value before the fault,so as to avoid oscillation during switching back to the constant voltage main control.(5)Aiming at the overvoltage problem of the SVC installed on the low-voltage side of the transformer when the system is under heavy load operation,an improved control strategy for SVC access side overvoltage protection was proposed.That is,by using proportional integral control instead of conventional simple integral control,the access side overvoltage value can be reduced quickly and steadily.By changing the voltage reference value,the susceptance reference value output by the SVC voltage controller is equal to the minimum susceptance value after the limiter is limited,which can avoid the impact on the SVC voltage control performance.By increasing the gain coefficient of the SVC voltage controller,the capacitive reactive power surge caused by the lag effect of SVC when the system exits the heavy-load operation and the voltage recovers quickly can be alleviated,and the secondary overvoltage caused by this can also be suppressed.The simulation results in PSCAD/EMTDC software combined with actual engineering projects show that the above-designed SVC protection and control strategy can achieve the expected results,is feasible and effective,and has certain practical significance for improving the performance of SVC.