Development of Protection Schemes for Microgrids With Integrated Battery Energy Storage

Abstract

Energy storage is a vital component of a resilient microgrid. Though storage will solve all problems related to power mismatches between generation and loads, it brings additional challenges. Due to the bidirectional power flows, control and protection become complicated. The control, as well as the fault behaviour of the microgrid, varies with microgrid mode of operation, demanding adoption of new control and protection strategies. The initial part of the thesis focus on transient behavioural modelling of a LV mi- crogrid with a centralised battery energy storage system (BESS). All the inverters are operated in grid following mode in the grid connected mode of microgrid operation. In the islanded operation, the BESS will act as the grid forming inverter. Consequently, AC fault analysis of the microgrid is carried out. The magnitude and direction of fault current from microgrid feeders are seen to vary and is influenced by the following fac- tors: (i) inverter controller (ii) microgrid mode of operation (grid connected or islanded) (iii) BESS mode (charging or discharging) (iv) fault resistance (v) DERs connected (vi) fault distance (vii) loading level and (viii) fault type. Due to the restrictions imposed by power electronic switches, the fault currents in Inverter Interfaced Distributed Genera- tors(IIDGs) are limited. The non-linear controllers of inverters alter the fault responses in many ways. 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. Hence, legacy protection schemes are not suitable for microgrids. Later chapters attempted to develop protection strategies for AC microgrid feed- ers that are not impacted by the above factors. Adaptive protection strategies and dif- ferential schemes that do not require adaptive settings are proposed in this thesis. The performance of proposed strategies are tested for different fault scenarios by carrying out simulations in MATLAB/SIMULINK software. The final part of this thesis investigated the fault characteristics of a ring type LVDC microgrid. In the transient stage of DC faults, there can be very high currentsand severe dc link voltage variations. The dc link capacitors of the Voltage Source Inverters or DC-DC converters may discharge quickly leading to collapse of dc link voltage. Though the self protection circuits in DC-DC converters can lock the gate pulses to IGBT switches, large currents will freewheel through the antiparallel diodes. This is then followed by grid or source feeding stage (steady state) that will cause grid or other sources to feed large fault currents through antiparallel diodes in the VSI or DC- DC converters. The protection devices (PD) at AC side can access fault currents only at this stage. Hence it is imperative that DC network protection act in the transient stage itself to avoid damages to the converter. A fault localisation scheme based on transient signals is also proposed for ring type DC microgrids. This scheme can interrupt the faulty section accurately in the transient stage itself. The efficacy of proposed schemes are validated by extensive simulations.