This paper presents an autonomous operation of a micro-grid by employing a droop control and PQ control strategy. The local distributed generation (DG) is modeled by a grid-tied inverter with LCL filter. A proportional-resonant controller is adopted to control the inverter. A new control scheme based on a combination of weak droop control and PQ control is proposed in this paper. Droop control aims to share the load change after islanding occurs. Meanwhile, weak droop control equations with a small power-dependent variation are used for generating the reference signal for PQ control after islanding has occurred. These are implemented using code in the Simulink environment. The simulations show that the proposed scheme which combines droop control with PQ control could operates well in both grid-connected and islanded conditions. By employing such a scheme the transition of the micro-grid from grid-connected operation to autonomous operation or vice versa is smooth.

This paper presents a full code-based implementation of breaker operation for power balance after islanding detection that is to sense jump change of harmonic voltage at point of common coupling in Simulink environment. Topology of local distribution generation is modeled by a grid-tied inverter with LCL filter. Proportional resonant controller is applied to control the inverter and narrow band-pass filter is designed to extract the harmonics for islanding detection. Two cases are discussed: one is that the power generated by the inverter is over the rated power of the load and the other condition is that the power generated by the inverter is under the rated power of the load. Two approaches to get power balance after islanding happens are presented. They are power compensation that is to reduce power produced by the inverter and load reduction that is to reduce some loads. Instead of separating the inverter the approaches can make the distributed generation power system work with power balance.