Reducing the number of must-run, conventional sets in Northern Ireland to enhance RES penetration, while maintaining system stability

(AIPS), is a system separation event, triggered by a double-circuit, tie-line trip and manifesting as the electrical decoupling of the two regions. SONI’s Operating Security Standards (OSS) mandate that system operations remain secure under this event.

Presently, three HVDC interconnectors link the island of Ireland with Great Britain. Of these, the Moyle interconnector connects Northern Ireland and Scotland. Real-time management of the system separation contingency depends upon the monitored, coincidental combination of flows for both the NSTL and Moyle interconnector.

To maintain frequency stability, prevailing operational policy dictates that at least seven conventional generators must be connected across the island, at all times. At least three of these on-load units must be located in Northern Ireland, as instructed by a transmission constraint group (TCG), known as the minimum number of conventional units on (MUON) constraint. In addition, a minimum total inertia floor applies across the island.

The minimum stable generation levels associated with Northern Ireland’s portfolio of conventional plant serve to reduce the generating opportunity that can otherwise be allotted, across day-ahead and intraday market timeframes, to renewable energy sources (RES). This contributes to the problem of “dispatch-down”, whereby the full potential of RES plant is subject to re-dispatch actions, in real-time, through control room co-ordinated, network constraint and system curtailment rules. Guided by the SOs’ joint operational policy roadmap, it is anticipated that the prospective relaxation of the MUON constraint, to two units, will enhance RES utilisation.

This paper commences by introducing the contemporary challenges faced by SOs, globally, in adapting their grids to increasing penetrations of inverter-based resources (IBRs) that displace conventional forms of generation. New operational paradigms are emerging, characterised by lower levels of system inertia.

A methodology for assessing the feasibility of introducing a two-set MUON rule to Northern

Ireland, while maintaining frequency and transient voltage stability, is outlined subsequently.

Several low system inertia, base cases of interest are identified from historical AIPS data. The relevant transient study archives are extracted using the SOs’ shared EMS interface.

A novel automation framework, developed in Python, facilitates manipulation of the base case files for applied variations to generator dispatch and jurisdictional demand and wind levels.

Using this approach, power flows on both the NSTL and Moyle interconnector can be varied in a controlled manner, so that their full range of operational capability is explored.

Hundreds of representative studies are thereby synthesised, executed in simulation and their results analysed. A commercially available, dynamic simulation tool for conducting electromechanical, time-domain analysis is used to undertake the series of transient stability studies. Machine learning is used to define a frequency stability informed, polynomial model for the export capability of the NSTL. Results demonstrate that frequency and voltage stability criteria can continue to be satisfied under lower inertia operating regimes.