Design and implementation of three-phase AC impedance measurement unit (IMU) with series and shunt injection

Electric power systems based on power electronics technology are prone to instability due to the constant power nature of power electronics converters. The impedances defined in synchronous coordinates are useful tools to predict and evaluate three-phase small-signal system stability. This paper presents the design and implementation of an impedance measurement unit used in three-phase systems up to 100kW. It extracts the system impedance online at any specific operating point. The unit utilizes both series and shunt injection modes to perform impedance identification for any load and source conditions, even if the source is very stiff. Experimental results demonstrate the effectiveness of the designed unit with both linear and nonlinear loads.