This paper evaluates directional and adaptive overcurrent protection schemes in microgrids. A microgrid supported by a centralised Battery Energy Storage System (BESS) is chosen for the study. The stringent PQ controller of BESS will not allow it to dissipate into a fault, during its charging mode, causing the conventional directional schemes to mal-operate. A direction estimation scheme using magnitude and angle of superimposed positive sequence imped. This paper evaluates directional and adaptive overcurrent protection schemes in microgrids. A microgrid supported by a centralised Battery Energy Storage System (BESS) is chosen for the study. The stringent PQ controller of BESS will not allow it to dissipate into a fault, during its charging mode, causing the conventional directional schemes to mal-operate. A direction estimation scheme using magnitude and angle of superimposed positive sequence impedance is proposed to address this issue in BESS. Further, two fault detection techniques are proposed for BESS integrated feeders. The Main Protection Unit (MPU) detects an internal fault when there is a mismatch in the direction of relays at either end of a feeder. The second scheme uses adaptive overcurrent relay settings. Since the fault current is limited with PQ control, the pickup current is calculated dynamically with this control. Whereas a fixed pickup current is chosen, when the BESS inverter is voltage controlled. Simulations have been carried out using MATLAB/SIMULINK software. The proposed schemes work well for bidirectional power flows and in grid connected and standalone modes of microgrid operation.••••Investigates the impact of controllers on fault behaviour of Inverter Interfaced Distributed Generators••Presents a case of maloperation of conventional directional relays with PQ controlled inverters••Proposes a direction estimation technique, main and backup protection schemes for a microgrid••Validates proposed schemes through MATLAB/SIMULINK simulations for various fault and non-fault scenarios on a BESS microgridBESSMicrogridProtectionSuperimposed impedanceDirectionalAdaptiveA desirable feature of a microgrid is that it should have the capability to operate in isolation with the host network for long hours/days. The advancements in energy storage (ES) and distributed generation (DG) have made this possible. However, the LV distribution grid is not yet geared up for large scale integration of ES and DG, mainly due to protection coordination issues. The major impediments in microgrid protection are bidirectional current flow and different fault behaviour of inverter interfaced distributed generators (IIDGs) in different modes of microgrid operation.In grid connected mode (GCM), the voltage and frequency are dictated by the grid and microgrid performs only ancillary services. IIDGs are normally operated in current control (PQ control) in this mode. On the other hand, in islanded mode (IM) of operation, various DGs or a master DG, preferably a dispatchable source, are responsible for maintaining the voltage and frequency. The DGs employ voltage control (V/F or Droop control) in this mode of operation. The current control always tries to keep the output current at the target level by adjusting the internal voltage of inverter. Hence the fault current from PQ controlled inverters will be limited and balanced even for asymmetrical faults. In comparison, the fault current from voltage mode control inverters is similar to that of a synchronous alternator. The fault current from voltage-controlled inve. 2.1. Microgrid topologyThe typical topology of a microgrid, is shown in Fig. 1. It comprises of a Solar Photovoltaic (PV) employing MPPT control, a centralised battery energy storage unit (BESS) and loads. All the components are connected to a 415 V busbar at the Point of Common Coupling (PCC). The switch S facilitates the connection of microgrid to the grid. During the grid-connected mode, all the DGs are operated in current controlled mode. PV uses DC link voltage control to transfer maximum power and BESS employs PQ control to deliver/absorb a preset power to/from the utility grid.2.2. Impact of controllers on fault behaviour of IIDGsThe microgrid is operated in grid connected mode (GCM) up to 2.5 s and after that in islanded mode (IM). The BESS operates as PQ-IIDG in GCM and V/F-IIDG in IM. The battery is in charging mode during the entire operation. L-L faults of duration 0.5 s are applied at t=1.5 s and at t=3 s at location F1 in Fig. 1. The LL fault response of BESS is shown in Fig. 2.2.3. Major concerns.
