Research On Low Voltage Ride Through Technology For Inverter Of A-class Auxiliary Equipment In Thermal Power Unit

Inverter has obvious advantages in soft start,speed control,optimization design,economic operation and so on.But due to voltage fluctuation of the power grid or plant service power,a new problem arises in the use of auxiliary equipment inverter in thermal power unit.The problem is the inverter will trigger a low voltage protection just because of a very short voltage sag,namely the lack of Low-Voltage-Ride-Through(LVRT)ability.In recent years,thermal power unit downtime events caused by short voltage sag happened in succession,which are all because the inverter lack LVRT ability.Normal power production is affected and huge economy loss is caused.So the study of LVRT technology for inverter of A-class auxiliary equipment in thermal power unit has great significance.In this article,a detailed study centred on the key technology of the LVRT problem about A-class auxiliary equipment inverter in thermal power unit is carried,mainly including the LVRT technology of High&Low-Voltage inverter and the voltage sag generator used in the LVRT test.Concrete research content in paper is as follows:Firstly,the LVRT technology of High-Voltage auxiliary inverter is studied.Based on the analysis of the High-Voltage inverter structure and control method,the influence of voltage sag on auxiliary equipment's running state is summarised and concluded.A variable step composite speed tracking restart method as a High-Voltage inverter LVRT technology is proposed,and a MATLAB simulation is carried.The simulation shows that this method can detect the present motor speed more quickly,reduce the restart current and rise the restart success rate.In allusion to the fan system which uses the speed tracking restart method,the modified method of the relevant system control logic is analysed in order to realize the smooth transition of the system.Secondly,the LVRT technology of Low-Voltage auxiliary inverter is studied.The running status of different auxiliary engine under low voltage is analysed.In order to provide reference about choosing LVRT scheme for power plants,the advantages and disadvantages of the applied Low-Voltage inverter LVRT technologies is compared and concluded.An LVRT technology based on the super capacitor and bi-direction DC-DC converter is proposed,which can both solve the high and low voltage ride through.Compared with the existing inverter or LVRT scheme,this scheme is more suitable for short-time voltage support.It has more simple structure,more reliable operation,less energy loss and more function,which provides another feasible way of LVRT problem.Finally,the LVRT test for Low-Voltage auxiliary inverter is studied.An LVRT ability test scheme which uses different types of fault voltage is put forword.A voltage sag generator which can simulate different kinds of voltage sag in power system is designed.Using power electronic building block structure,the generator is composed of a PWM rectifier and a PWM inverter,which composes the back-to-back formation.So its function is expansible and controllable.The MATLAB simulation shows that the scheme can effectively simulate various balanced or unbalanced fault of power grid.According to the above design and simulation,a model machine is produced.The function test for model machine and an LVRT test using the model machine are took.The tests' results indicate that the method described in this article and the design of voltage sag generator can provide the required balanced and unbalanced voltage for the LVRT test flexibly and accurately,determine the LVRT ability of inverter comprehensively and are convenient to carry the spot LVRT test for the A-class auxiliary equipment in thermal power unit by the related organaiztion.