Research On Test Bench For Medium Voltage Converters

In recent years,medium voltage converters are widely used in many industrial applications.In the case of high power,medium voltage converters have the advantage of high power density and low cable cost.Testing of medium voltage converters has many challenges.On the one hand,the capacity of grid often can not meet the needs of high power converters.On the other hand,assembling and testing of the large motor brings high cost.In this paper,the test platform of medium voltage converter is studied as follows:Pump-back topology is often used in industry to deal with the weak grid.In chapter II,the performance of DC-coupling and AC coupling pump-back topologies is analyzed from several aspects,such as equipment complexity,control complexity,efficiency and network side performance.In order to reduce the cost of motor,a converter and filter is used to simulate the performance of motor.And different filter schemes are analyzed for hardware design.DC-coupling pump-back test platform with inductor has low common mode impedance,so it is not able to test the operating condition of common mode injection.In chapter III,this paper proposed a method to improve common mode impedance,which combine proportional integral resonance(PIR)control and steady-state control.In this method,resonance control is used to suppress the low frequency component of common mode current,and steady-state component is injected to suppress the high frequency component of common mode current.Common mode voltage can be injected in the test bench by using this method,which makes test condition more reasonable and widens test scope.Current ripple of DC-coupling pump-back test platform is different from real conditions,which may bring problems of thermal and current sampling.In chapter IV,the difference between the test condition and the actual condition is analyzed,and the influence of carrier phase on the amplitude of the ripple is explained.A thermal model is established to evaluate the influence of different ripple on the loss of converter.At the same time,the carrier asynchrony has negative impact on current sampling,which is not conducive to current control.A control method based on dual edge sampling is proposed to solve this problem.Finally,this paper describes the construction of the medium voltage test platform,including hardware design,controller design,fault protection design and other aspects of the design.