Power Electronics Europe April/May Issue 2022

www.power.com/scale-iflex-lt WIND POWER INVERTER DESIGN 29 www.power-mag.com Issue 2 2022 Power Electronics Europe Active Clamping (AC) and Advanced AC (AAC) – is most effective when active monitoring and shutdown circuitry is located near the switch as shown in Figure 3a. Figure 3b compares the clamping performance that can be obtained using a conventional passive detection architecture relying on a Master Control (MC) unit, and that which is obtained using an Integrated Master Control (IMC) that provides isolation and timing signals and Module Adaptive Gate drivers (MAG) with active drive circuitry mounted directly onto each gate driver to minimize the impact of parasitic circuit inductances and propagation delay. In AAC control, the VCE voltage is monitored as the IGBT is turned off. Active drive via a control ASIC is used to modulate the gate drive to limit current slew rate, which in turn prevents the DC- link voltage from rising to excessive levels. Reducing the value of RG(OFF) in the discrete solution would improve clamp control, but it would reduce switching efficiency. The clamping voltage that can be obtained using the fast response of the MAG-based control can be shown to provide 10 % safety margin for an 860 VDC link voltage. By using the appropriate active clamping approach and positioning the detection and protection circuitry on the switch module, a higher nominal DC- link voltage can be selected which delivers >9 % more power in a given system. Current sharing As shown in Figure 2, many wind turbine systems employ multiple IGBT dual modules in parallel. In a perfect system, the gate drives for each module would be identical and the IGBTs themselves would present similar impedance during conduction, resulting in equal current sharing. Total system current could be set at the maximum current per module multiplied by the number of systems in parallel. Any imbalance in actual operation would lead to some modules working harder – creating increased heating in those modules. To prevent damaging overworked modules, the nominal current for each module would need to be reduced. So, using similar reasoning as was applied to the voltage calculation, by ensuring accurate current sharing, the power through each module may be higher, allowing more system power to be provided from a given IGBT arrangement. Switch timing and gate-bias must be precisely matched between each switch module. Again, the provision of local drivers and control of gate voltage allows for close matching. Figure 4a shows the difference in performance between matched gate drives (timing match shown in Figure 4b) and a gate driver configuration that employs passive modular drives. Unbalanced impedance between the drive feeds to each module causes timing mismatches and reducing sharing accuracy. If the current delivered to each module Figure 3: Controlled turn-off of the gate driver - IMC and MAG – SCALE-iFLex™ LT combines an MC module with active MAG drivers mounted on EconoDUAL 3 IGBT modules (a, top); and maximum voltage seen during short- circuit shutdown with a conventional passive gate drive circuit compared to a combination of IMC and MAG modules for low DC-link inductance (b, bottom) Figure 4: Switch timing and gate-bias must be precisely matched between each switch module - comparison of current sharing between adjacent IGBT modules using passive MAGs (left) and actively driven MAGs with bias control (right). Nominal current is 600 A per module (a, top); and timing match for an actively driven MAG (SCALE-iFlex LT) showing timing mismatch across four modules of less than 20 ns. T D(A) shows the time delay for the switching signal induced by crossing the isolation barrier of the IMC (b, bottom)