Design Of Compound Switch In Reactive Compensation Of Low-voltage Network

The development speed of modern power system is accelerating day by day,the system voltage level of the grid continuously enhance,and the scale of the network is also increasing.Accordingly,a large number of inductive loads such as motors and transformers are widely used in the power grid,which will inevitably lead to a great increase in the demand for reactive power in the power system.With the increase of reactive power in the power grid,the current and apparent power in the power grid will increase accordingly,which will lead to the increase of the capacity of inductive equipment such as generator transformer and conductor,and the increase of voltage drop of the line and transformer,resulting in voltage fluctuation and reducing the power supply quality of the power system.To counter the problems above,the most common solution is to make reactive power compensation through reactive power compensation equipment in the appropriate position of the grid.There are only two kinds of switch for traditional reactive power compensation equipment: AC contactor and electronic contactless switch,however these two kinds of switch have some defects when used alone.At the moment when the capacitor is switched on and off,the AC contactor will generate inrush current and easy to produce arc at contact,causing contact to burn.When the electronic contactless switch is on,the power consumption is too high and the heating is serious and easy to be interfered.In order to solve the problems caused by the defects of these two kinds of switches,a kind of compound switch with thyristor and relay in parallel as a reactive power compensation device for capacitor switching has been designed in this thesis,the transient process of capacitor switching is completed by using the voltage zero-crossing conduction characteristic of thyristor and takes advantage of the low power consumption of relay to undertake the steady state process after capacitor input.The whole device has the advantages of small inrush current,low power consumption,strong anti-interference ability and reliable action.This thesis first introduces the basic principle of reactive power compensation and the calculation method of compensation capacity,then through comparing the advantages anddisadvantages of several switches of capacitor,finally adopts the compound switch in parallel form of thyristor and relay as switching element.In addition,analyzes the switching strategy and compensation method of reactive power compensation.Combined with practical applications,determined a switching strategy to find the input zero crossing by detecting the voltage zero at both ends of the thyristor.In terms of hardware,ATmega8 A with high cost performance in AVR series microcontroller is adopted as the controller,the hardware design scheme is made up of zero-crossing detection module,switch driver module,power supply module and logic circuit module.In terms of software,the method of block design is adopted to introduce in detail the specific functions realized by each subroutine.The main program and each subroutine cooperate with each other to realize the overall functions of the compound switch device designed in this thesis.Finally,the function test of the prototype is carried out,and the experimental results show that the function and performance of the device meet the design requirements proposed by the project.