Research On The Related Issues Of Variable Frequency Drives

With the rapid development of power electronic devices, computer technology and automatic control theory, high power electronic technology has been widely used in the power system this years. In the mechanical and electrical transmission field, the AC drive system will replace the DC drive system. High voltage and high-power variable frequency drive technology, an effective method to resolve the waste of energy of traditional AC drive system, has practical significance to the sustainable development of power system.This paper introduces the buildup and working principle of the high voltage and high-power inverter at first, also several structures widely used now, and points out the advantages and disadvantages of them. At the same time, this paper introduces the basic characteristic and the failure mechanism of the IGCT. Based on the IGCTs, the series connection of power devices is necessary for obtaining 6kV output voltage in neutral point clamped (NPC) three-level high power medium voltage drives (MVDs). As the process of the transient turn-off is very complex for discussing the transient turn-off characteristics of series connected IGCTs, their corresponding turn-off transient current models are developed with the different performances between the connected IGCTs taken into accounts in this paper. According to the current models, the method using differential equations to study the voltage sharing during IGCTs'turn-off transients is proposed. This paper uses a mathematics method to calculate out the turn-off transient voltage of the connected IGCTs. Consequently, the turn-off transient model based on the chopper circuit is deduced to confirm its efficiency, in which the RC-snubbers of IGCT and stray inductance are analysed. Simulation and experimental results show that the model based on the proposed method can well describe the turn-off voltage sharing of series connected IGCTs.In addition, this paper analyses the turn-on and turn-off di/dt of the IGCT when the time constant of the DC-Link is more than or less than the commutated time between two phases. When the time constant of the DC-Link is less than the commutated time between two phases, low turn-on current decay rate can be observed. On the contrary, the turn-on di/dt will be high,which may cause the devices failure. This paper designs a simplified experimental circuit to simulate the switch commutation of the MVDs, the results confirm the analysis in this paper.