Current Unbalance and DC Bus Unbalance Control for Three Level Grid Tied NPC Inverter Fed by DER with DC-DC Boost Converter

Abstract

Presently, Grid tied inverter fed by DER (Distributed Energy Resources) being widely adopted due to Microgrid Concept in order to reduce overhead in present transmission line. Now a days three level inverter topologies like 3-level NPC and 3-level TNPC are widely used due to its smaller filter requirement with lower current THD compared with Two Level Inverter. The disadvantage of a Multilevel Inverter is DC bus voltage imbalance with respect to its midpoint voltage. This unbalance may be due to unbalance in Grid Voltage or from input side DC-DC Converter. If DC bus is unbalanced across midpoint then three phase current fed to grid is unbalanced which is not desirable for gird stability and it may result in DC capacitor failure at DC bus. If DC bus unbalance control is implemented in control loop algorithm to balance DC bus across midpoint, at the same time if grid voltage is also unbalanced then this results in unbalance current fed to the grid. Hence, in case of unbalanced grid voltage only DC bus unbalance control will not be enough, along with it we need to implement grid voltage unbalance control to get desired performance. In this paper, two control loop (i) DC Bus Control and (ii) Grid Unbalance Control is presented. Simulation result for 100kW gird tied inverter is also presented in a paper for various case study.