Research On The Turn-off Transient Behaviours And Protection Circuits Of IGCT Used In 6kV High Voltage Variable Frequency Drives

With the development of power electronic devices and electronic technology, high power electronic technology has been widely applied to power system in recent years, and it will become one of the main driving forces of technological innovation in power system. High voltage and high-power variable frequency drive (VFD) technology, which is an effective method to resolve the waste of energy of traditional AC drive system, has practical significance to the sustainable development of power system.The first chapter of this paper introduces the importance of Medium Voltage Converter and discusses the use of snubber circuit. The second chapter introduces the compose of the converter and its principle, describes the strongpoint of the common AC-DC-AC converter; and proposes a way to discuss on the Integrated Gate-Commutated Thyristor (IGCT). We know, the turn-on energy loss of the Gate Turn-Off Thyristor (GTO) is low, but it needs external circuitry to protect the GTO's transient process; Insulated-Gate Bipolar Transistor (IGBT) is an advanced semiconductor switch for high frequency, but it has higher turn-on energy loss. IGCT has both strongpoint of IGBT and GTO, it satisfies the conflicting demands of high current, high voltage and fast turn-off through carrier lifetime engineering on the one hand, and the flexible functionality of a standard gate unit on the other; the IGCT has rapidly gained acceptance in major areas of high power electronics. IGCT also is used by the 6kV/2500kW Three Level Neutral Point Clamped (NPC) Medium Voltage Converter. The turn-off voltage of single IGCT used can't satisfy the converter's demand. To achieve higher output voltages the series connection of two IGCTs is necessary. Because of the difference of IGCT, the two IGCTs may not work at the same time: if their turned-on time are not exactly the same, one of them may endure over di/dt which will damage the conductor; if the two IGCTs's turned-off time are not exactly the same, one of them may endure over high transient turn-off voltage which may damage the conductor. Once the IGCTs are damaged, the converter won't work properly.Although the IGCT has gained extensive acceptance in major areas of high power electronics, the experience of how to use it well is very limited; and also the price of IGCT is very high and IGCT must be got abroad for a long period, once it's damaged, it will cost a long time and do a lot of job to get a new one. To get the best use of IGCT, we should know the IGCT's characteristics well; the third chapter of this paper proposes a method to discuss on the turn-off transient behaviors of IGCT, the mathematic simulations and experimental results show that the model proposed in this chapter can give a good description of the turn-off transient behaviors of IGCT. To balance the voltage distribution of series connected IGCTs during switching, the use of external circuit across each of the series connected IGCTs is necessary to realize an equal voltage distribution between the devices. The fourth chapter proves the necessary to get external circuit to protect the IGCTs in series connection; it also compares the two common snubber circuits of RC and RCD; based on the IGCT's transient turn-off model proposed in the third chapter, the fourth chapter discusses the importance of RC snubber circuit in protecting the IGCTs in series connection in detail. The mathematic simulations show that the properly designed snubber circuit is very important in protecting the IGCTs from damage during the transient process. The fifth chapter introduces the management software's function of the 6kV/2500kW Three Level Neutral Point Clamped (NPC) Medium Voltage Converter. At last, the sixth chapter makes a conclusion of this paper and relative spare work has been prospected.