Low Inertia Microgrid Fault Stability and Protection
By Jason Philhower, P.E., POWER Engineers
with co-author Thomas Morrell, POWER Engineers
Many electric customers are investing in microgrids while maintaining their present utility connection to provide additional reliability and resiliency during grid disturbances which could interrupt electrical service. Many types of distributed generation (DG) can be found in a microgrid. These types include conventional synchronous generators, induction generators and inverter-based power systems. Microgrids can operate either in one of two modes; grid-connected mode or island mode. When the utility source experiences a disturbance, a microgrid operator has the choice of disconnecting from the utility, creating an electrically isolated power system or “island”. When operating in island mode, the microgrid stability during steady state operation and normal transient loading is managed by several types of technologies. This control can be in the form of a local controller, such as DG unit controllers or possibly a microgrid centralized controller responsible for managing multiple DG units and microgrid loads. If a fault occurs during island operation, a centralized controller’s response-time may be too slow to maintain voltage and frequency stability, and will therefore rely on the microgrid’s protection system. The microgrid’s protection system must be designed to ensure that when a fault is cleared, the microgrid can recover to an acceptable post-fault state. The overall protection of a microgrid is affected by its operating mode as well as other factors such as the type or mix of DG sources. Considerations for the protection system design must include the operating characteristics of both synchronous and asynchronous generators and the voltage and frequency dependencies of loads. The protection system’s security, reliability, and sensitivity must also be acceptable during both grid-connected and islanded operating scenarios. Most microgrids use protection that is actually standard time overcurrent-based, as would be used in a typical medium voltage application without DG sources. The overcurrent schemes may not provide sufficient performance during islanded operation due to a lack of DG inertia. By the time a relay operates to remove the fault, the microgrid DG sources may have collapsed to the point that the fault becomes unrecoverable. This paper explores various fault scenarios a microgrid may experience with a different makeup of synchronous and asynchronous generation and discusses possible solutions for low inertia microgrids.
This paper was presented at the 2019 POWER-GEN International Conference.