A unified control scheme for harmonic elimination in the front end converter of a 13.8 kV, 100 kVA transformerless intelligent power substation grid tied with LCL filter

Abstract

This paper proposes a simple control scheme to eliminate the lower order harmonics in the line currents of the Front End Converter (FEC) of a 100 kVA Transformerless Intelligent Power Substation (TIPS) connected to the medium voltage (13.8 kV) grid through an LCL filter. Due to medium voltage (13.8 kV) and low power (100 kVA) specification for the FEC, the control technique needs to be specially designed to control very low magnitude of line current (4.2 A rms). Lower order harmonics are present in the grid current due to dead time in the FEC and grid voltage harmonics. Low switching frequency along with the medium voltage and low power levels results in a filter capacitor value that offers low impedance to the lower order harmonic currents. This lower order harmonic current flow through the filter capacitor limits the power that can be delivered by the converter due to lower current rating of the FEC. It also pollutes the control loop and affect system stability. This paper proposes a simple unified control scheme with harmonic current elimination under such conditions. The control scheme eliminates these harmonics in the grid current by regulating the harmonics in the filter capacitor voltage and inductor currents, both on grid side and converter side. System modeling, simulation and experimental results validate the proposed control scheme.