Improving the performance of digitally-controlled high power grid-connected inverters

The availability of high speed and high power switching devices, such as the IGBT, has opened the opportunity for an increasing number of grid-connected inverter applications that have historically been unachievable. Recently, the number of inverter applications has surged, with now the focus being on increasing the relative performance and power capability. Such applications include UPSs, dynamic voltage restorers, STATCOMs, frequency converters and distributed grid sources such as solar panels. The inverter switching frequency limits its associated bandwidth and hence performance. Every application can benefit by reduction of the extent of this limitation. While state of the art devices like IGBTs enable such applications, the onus is now on developing high bandwidth digital controllers; the ability to connect multiple devices together to achieve power scaling; and having the confidence that the applications will work with other systems on a grid. Constraints and limitations imposed by the hardware and traditional continuous-time derived controllers are identified. A discrete-time direct design controller is then developed specifically for digital controllers, that for the same inverter configuration, achieves twice the bandwidth of a well-tuned traditional controller. An important feature of a controller is having the configurability of being able to choose inverter bandwidth over stability margin. To provide power scaling above that of a single switching module, investigations are performed on the suitability of actively paralleling inverter modules. Both the use of the developed discrete direct design controller and the identification of potential inter-module instabilities for a particular configuration enables the application of paralleled inverters. The operation is confirmed