The Study On Key Issues Of Current Source Converter For Super-conducting Magnetic Energy Storage System

Super-conducting Magnetic Energy Storage System (SMES) is a product of the combination of the power electronics technology and superconductive technology. It's a perfect component for Flexible AC Trans-mission System (FACTS).It can be used to improve the quality of the modern power network. Power electronics converter or so called power conditioning system (PCS) is the bridge between the superconducting coil and the power network, it's a key parts of the SMES. Comparing with the voltage source converter, current source converter has advantages on technology and economy.Current source converters can be connected directly to get multi-modular converter to reach high current and high power rating. By using carrier phase-shifted SPWM control, the multi-modular converter can get low AC side harmonics even the switch frequency is low. The balancing of the DC current and the characteristic of AC harmonics are the key problem of this kind of converter. This paper studies the property of DC current distributing, analyses the effect of the AC current sideband on the DC current distributing, deems that the carrier phase shifting can result the unbalance of the DC current. This paper studies the characteristic of AC current harmonics on ideal condition and practical condition, especially gives the harmonics of the multi-modular current source converter employing conventional DC current balancing control. Although such conventional DC current balancing control can balance the DC current, the AC harmonics of the converter are still high. This paper presents the carrier swapping method, this method not only can balance the DC current, but also can reduce the AC current harmonics. This paper introduces the principle of this method and gives the experimental results.By connecting the DC side of the current source converter modules in series and the AC side in parallel, also using phase-shifted transformer, the current source converter array can be built. This converter array can reach high current and high voltage rating simultaneously, it suits the application of high power SMES. This paper studies the property of DC current distributing in the current source converter array controlled by modulation signal phase-shifting SPWM, deems that the DC current of each column can not be balanced except the condition that the number of the column of the array is 2 and a equal to 0°. This paper also studies the characteristic of AC current harmonics on ideal condition and practical condition. Although the characteristic of AC harmincs of an N×M array is similar to a quasi 6NM pulse converter on ideal condition, the low order harmonics is still high at practical condition because of the unbalancing of the DC current. A method to solve above problem is presented. By swapping the control sigal in same row, the DC current in each column can be banlanced and the harmonics of the AC current can be reduced. This paper gives the way to implement this method and the experimental results.This paper deduces the relationship between M, a and active power, reactive power in a current source converte controlled by SPWM. A design of the control loop of the active power and reactive power for this converter is given.This paper gives the design for a practical four-module current source converter for a 35kJ/7.5kW-200A high temperature SMES. The configurations of the main circuits, hardware of the control system, DSP and MCU software and the protection circuit are given.At last this paper gives the experimental results of the above practical four-module current source converter and the experimental results of the SMES using this converter. The results are for different conditions, they are signal module, four modules, carrier phase-shifting, without carrier phase shifting, steady state and transient conditions,etc. The experimental results of damping the low frequency oscillation of the generator by SMES was also given.