Walid Issa

Control of Transient Power During Unintentional Islanding of Microgrids

In inverter-based microgrids, the paralleled inverters need to work in grid-connected mode and stand-alone mode and to transfer seamlessly between the two modes. In grid-connected mode, the inverters control the amount of power injected into the grid. In stand-alone mode, however, the inverters control the island voltage while the output power is dictated by the load. This can be achieved using the droop control. Inverters can have different power set points during grid-connected mode, but in stand-alone mode, they all need their power set points to be adjusted according to their power ratings. However, during sudden unintentional islanding (due to loss of mains), transient power can flow from inverters with high power set points to inverters with low power set points, which can raise the dc-link voltage of the inverters causing them to shut down. This paper investigates the transient circulating power between paralleled inverters during unintentional islanding and proposes a controller to limit it. The controller monitors the dc-link voltage and adjusts the power set point in proportion to the rise in the voltage. A small-signal model of an islanded microgrid is developed and used to design the controller. Simulation and experimental results are presented to validate the design.

Improved Reactive Power Sharing for Parallel-operated Inverters in Islanded Microgrids

The unequal impedances of the interconnecting cables between paralleled inverters in the island mode of microgrids cause inaccurate reactive power sharing when the traditional droop control is used. Many studies in the literature adopt low speed communications between the inverters and the central control unit to overcome this problem. However, the losses of this communication link can be very detrimental to the performance of the controller. This paper proposes an improved reactive power-sharing control method. It employs infrequent measurements of the voltage at the point of common coupling (PCC) to estimate the output impedance between the inverters and the PCC and then readjust the voltage droop controller gains accordingly. The controller then reverts to being a traditional droop controller using the newly calculated gains. This increases the immunity of the controller against any losses in the communication links between the central control unit and the inverters. The capability of the proposed control method has been demonstrated by simulation and experimental results using a laboratory scale microgrid.

A small signal model of an inverter-based microgrid including DC link voltages

In this paper, a small signal model of an islanded microgrid containing any number of parallel inverters is developed. Each inverter is modelled as an ideal voltage source with an equivalent output impedance as the dynamics of the inner voltage loop controller is significantly faster than the dynamics of the power control loop and DC link voltage, which is included in the model. The model, therefore, can be used to study the dynamics of the DC link voltages during unintentional islanding in addition to analysing the dynamics of the power flow controllers. The model is validated using Matlab/Simulink simulations.

Supervisory Control for Power Management of an Islanded AC Microgrid Using Frequency Signalling-Based Fuzzy Logic Controller

In islanded ac microgrids consisting of renewable energy sources (RES), battery-based energy storage system (BESS), and loads, the BESS balances the difference between the RES power and loads by delivering/absorbing that difference. However, the state of charge and charging/discharging power of the battery should be kept within their design limits regardless of variations in the load demand or the intermittent power of the RES. In this paper, a supervisory controller-based on fuzzy logic is proposed to assure that the battery power and energy do not exceed their design limits and maintaining a stable power flow. The microgrid considered in this paper consists of a PV, battery, load and auxiliary supplementary unit. The fuzzy logic controller alters the ac-bus frequency, which is used by the local controllers of the parallel units to curtail the power generated by the PV or to supplement the power from the auxiliary unit. The proposed FLC performance is verified by simulation and experimental results.

Control Strategy for Uninterrupted Microgrid Mode Transfer During Unintentional Islanding Scenarios

This paper presents a microgrid control strategy to unify the control topology for energy storage systems and renewable energy sources inverters in an ac microgrid and to protect the microgrid reliability from unintentional islanding instability using control loops, which use thedc link voltage as a feedback. This bounds the dc link voltage and provides reliable operation in the microgrid. Simulation validates the proposed control strategy, and experiment results extol the concept.