Small-signal Analysis of Active Loads and Large-signal Analysis of Faults in Inverter Interfaced Microgrid Applications

Abstract

Rectifiers and voltage regulators that have characteristics of constant power loads may form a significant percentage of a microgrid's total loads. The real part of the input impedance of a constant power load is negative and it may have control loop dynamics in a similar frequency range to the inverters that are supplying the microgrid. This thesis examines the interactions between an active constant power load and a microgrid for the impact on stability. Participation analysis of the eigenvalues that result from the model of the combined microgrid and active load identified that the low-frequency modes are associated with the voltage controller of the active rectifier and the droop-controllers of the inverters. The analysis also revealed that when the active load dc-voltage controller is designed with large gains, the voltage controller of the inverter becomes unstable but the low frequency modes associated with the droop controller of the inverter remain stable. The transient stability of a microgrid may require that the inverter-interfaced generation remain connected during a fault and return to normal power export once a fault is cleared. For an inverter to supply fault current, the controller of the inverter must current-limit the output and the fault strategy chosen must ensure that the current and voltage limiter do not latch-up and that the controller integrators do not wind-up. This thesis analyses different limiting and reset strategies and concluded that that it is not possible to successfully reset a limiter when using a reset signal from a closed-loop controller within the inverter. In a system where there are cascaded limiters, successful operation is obtained when the inner limit is a saturation limiter and the output limiter is a set-reset limiter. It was found that the transient stability of an inverter interfaced microgrid using a droop control algorithm is dependent on the current limiter and inductance of the network.Open Acces