6kV Medium Voltage Drives Based On The IGCT

Recently, energy saving have been the focus of attention, and developing new energy and energy saving are both proposed as the policy of our government, while the energy saving is treated as the primacy. However, there exists huge waste in those areas, including fans, pumps and compressors, which cost about 21% of the total installed capacity of our power grid and need speed adjust for energy saving. The medium voltage drives (MVDs) discussed in this paper can adjust the input frequency of the high voltage motor to achieve the variable speeds, which is one of the most effective energy saving modes.Firstly, the background of the MVDs and the development of the high power high voltage power devices are introduced in this paper. Then, the general topologies of the MVDs, including two-level, three-level and cascaded multi-level inverters, are analyzed, and the diode clamped three-level inverter based on the IGCTs'series connection is emphasized in this paper.Based on the IGCT used in neutral point clamped (NPC) three-level MVDs and the characteristics of the fast recover diode (FRD), the contribution of the DC-Link (including DC-Link choke, DC-Link resistor and clamped capacitor) to the MVDs safe operation is studied in this paper. Through the analysis on the turn-on and turn-off transience of NPC three-level MVD, the corresponding mathematical models are proposed, based on which the DC-Link choke is designed. With the DC-Link choke, switching transient state and device's over-voltage capability taken into accounts, the relationship between the clamped capacitor and inverter's DC over-voltage is studied.Due to the rating voltage of the IGCT, series connections of two IGCTs and its corresponding dynamic and static snubbers are necessary to satisfy the 6kV motor, which is very popular in our country. According to the test waveforms of the IGCTs, it is treated as one variable current source during its turn-off transients, based on which the transient voltage sharing is analyzed. The test results of the chopper circuit and its mathematical simulation results show its efficiency.Through the researches on the diodes neutral point clamped (NPC) three-level inverter, the transient switching states of one phase leg and its corresponding reverse recovery current decay rates through the fast recovery diodes are presented. Consequently, according to the directions of the output voltage and current, switching time and switching operation in three phase legs, the turn-off transient high di/dt are discussed. With the multi-level modulation strategy used and constant U/f control applied to the squirrel asynchronous motor, the contributions of carrier ratio and the working frequency of the MVDs to the turn-off di/dt are deduced. Accordingly, compared to the modulation modes, the multi-level modulation strategy with carrier ratio equal to 6n is recommended aiming at suppressing the high turn-off di/dt. Furthermore, the dead time compensation is proposed for improving the DC voltage utilization ratio.For suppressing the harmonic pollution to the power system, the multi-pulse rectifier is used, and the harmonic contents of the DC voltage are analyzed. Accordingly, the influence of the short-circuit impedance of the multi-pulse rectifier transformer to the DC voltage is described under different operating mode, based on which its corresponding capability of the rectifier transformer is deduced. Then, a high frequency model of the common-mode model of three-level inverter is established, and the method for suppressing the common-mode voltage of the motor is presented.With the research results in this paper, 6kV, 2.5MW and 6kV, 1.0MW MVDs are designed, and 6kV, 2.5MW MVDs has passed the 6kV, large load experiments.Based on the IGCTs used in the 6kV MVDs, one novel topology based on the double three-level inverter proposed in this paper, in which the 6kV outputs can be achieved without the series connection of IGCTs. Without increasing the power devices, the MVDs will be more compact. Furthermore, higher efficiency and better output waveform can be obtained. According to the commutations among the double three-level inverters phase legs, an improved DC-Link of double three-level inverters is proposed to protect the switching devices effectively.