Transformer-Less Unified Power-Flow Controller Using the Cascade Multilevel Inverter

The conventional unified power-flow controller (UPFC) that consists of two back-to-back inverters requires bulky and often complicated zigzag transformers for isolation and reaching high voltage. This paper proposes a completely transformer-less UPFC based on an innovative configuration of two cascade multilevel inverters (CMIs). The unique configuration and control of the two CMIs as a power-flow controller make it possible to independently control active and reactive power flows over a line. The new UPFC offers several advantages over the traditional technology, such as transformer-less, light weight, high efficiency, high reliability, low cost, and fast dynamic response. The transformer-less UPFC is thereby very suited for fast and distributed power flow control, such as wind and solar power transmission. Experimental results based on 13.8-kV/2-MV·A transformer-less UPFC prototype are shown to validate the theoretical analysis and operating principle.

I. Introduction

The conventional ac grid network offers significant benefits in system stability and reliability. However, the network cannot effectively handle the congestion of key transmission lines due to its limited controllability. For instance, some lines are overloaded, but others are lightly loaded. Furthermore, undesirable loop flows are often observed. Not only does this lead to an inefficient operation of energy markets, but also the first line to reach its capacity limits the capacity of the entire system, even though a majority of the lines in the system are significantly below their limits [1].