Connecting a microgrid to an electric power system (EPS) requires the microgrid and EPS owners to form a legal contract and a technical design that ensure the safe, reliable, and economic operation of both the microgrid and the EPS (EPSs are also known as macrogrids). This article focuses on the most common technical design used in North America for this type of connection.
The point of interconnect (POI) between a microgrid and an EPS commonly requires automatic islanding (separation or decoupling), synchronization (reconnection), and dispatch controls. Figure 1 shows all three of these control systems being performed by a single microprocessor-based protective relay at the POI.
The POI relay accomplishes automatic islanding by opening a circuit breaker (or recloser) at the POI. The relay opens the POI when it detects short circuits, open circuits, or backfeed conditions. The relay is configured by skilled engineers to distinguish internal from external system disturbances and to avoid nuisance tripping. Opening the POI provides a crucial indication to the microgrid protection and control systems. The POI opening commonly initiates fast load shedding or distributed energy resource (DER) runback (curtailment) to prevent the microgrid from blacking out. The POI opening indication is also used to change relay protection settings throughout the microgrid to adapt to reduced fault current levels, and it can also signal DER control systems to change their modes of operation.
The relay accomplishes synchronization at the POI by automatically dispatching multiple DER outputs to reduce slip, angle, and voltage amplitude differences at
the POI. Once the relay identifies acceptable conditions, it automatically closes the POI. The ANSI symbol for manual synchronism-check functionality is 25, so automatic synchronization is commonly referred to as A25 when synchronization includes automatic dispatch. A25 systems must always be human-initiated for safety considerations.
Microgrids with large numbers of DERs often require an additional microgrid controller to assist in complex DER dispatch coordination.
Energy management systems (EMSs) (also known as SCADA or distribution management systems [DMSs]) commonly dispatch a POI via a single communications link straight to the POI protective relay. Active and reactive power (or alternatively power factor) dispatch is performed by POI relay logic, which simultaneously dispatches multiple DERs. This makes a microgrid with multiple DERs look like a single dispatchable generator to a utility. Microgrids with large numbers of DERs often require an additional microgrid controller to assist in complex DER dispatch coordination.
Modern POI protection and controls are essential to meeting contract requirements. These contracts define peak power usage, demand charges, generation limitations, frequency and voltage ride-through conditions, EPS-initiated load reductions, protection requirements, and more. Great care must be taken in the design of POI controls and protection systems. These systems must prevent backfeed conditions that can endanger technicians working on the EPS. Most importantly, the skill level of the engineer configuring the POI relay predicates the success of the POI control scheme.
Scott Manson is the SEL Engineering Services technology director at Schweitzer Engineering Laboratories.