Modelling of Energy Storage Systems

This rule change aims to amend the Market Clearing Engine (MCE) formulation, such that dispatch schedules can better reflect the unique characteristics of energy storage systems (ESS).

Currently, the MCE assumes that generators are only able to inject energy into the grid and not able to withdraw from the grid. As such, offer quantities from generators can only be positive (for injecting into the grid). However, when ESS intends to charge their batteries from the grid, it  should be reflected as a negative offer quantity (to withdraw from the grid).

Furthermore, existing MCE constraints on scheduling of reserve and regulation quantities are largely based on physical constraints of gas generators. However, these constraints do not apply to ESS, which is able to ramp up/down rapidly, and able to provide reserves and/or regulation even while charging.

Rule modifications were proposed to allow ESS to submit positive and negative offer quantities for energy to indicate its willingness to discharge and charge. Furthermore, a different set of (relatively simplified) constraints for provision of reserve and regulation by ESS was introduced, accounting for ESS’ fast-ramping capability, as well as ESS’ ability to provide reserves and regulation when charging.

Incorporation of State-of-Charge in Market Clearing Engine Modelling of Energy Storage Systems

This rule change aims to further enhance the modelling of ESS by incorporating State-of-charge (SoC) data in the MCE, thus constraining ESS dispatch schedules, improving ESS’ deliverability of scheduled quantities. 

SoC refers to the percentage of stored energy remaining in an ESS, an important parameter which affects an ESS’ ability to deliver energy or ancillary services.

There are currently no SoC-related constraints within the MCE, since existing modelling constraints largely reflect physical constraints for gas generators, which do not have SoC concerns. Thus, there is a risk that ESS facilities may not be able to deliver when scheduled, if actual SoC levels in real-time are not considered during scheduling.

Rule modifications were proposed for the Power System Operator (PSO) to provide real-time SoC data for each ESS to EMC before each dispatch period, and for new SoC-related constraints to be incorporated into the MCE formulation. This ensures that ESS dispatch schedules will be backed by real-time SoC data received.

Enhancement to Fallback Mechanisms for StartGeneration and PriorScheduledPurchase

StartGeneration and PriorScheduledPurchase are input parameters within the MCE, which may indirectly affect dispatch schedules:

• StartGeneration values reflect the MW generation level of each generator as of 10 minutes before each period.
• PriorScheduledPurchase values reflect the MW consumption level of each load as of the start of each period.

Due to facility ramping limits, the MCE requires each facility’s generation or consumption level at the start of the period in order to generate feasible dispatch schedules. In the event that the usual data sources for the two parameters are unavailable, fallback values for StartGeneration and PriorScheduledPurchase are required. 

As such, rule modifications were proposed to augment the fallback mechanism for both parameters, providing more fallback values for each parameter. Additional fallback values are based on the latest forecast schedules generated, which should reasonably reflect physical conditions (i.e., actual generation and consumption levels). This will enhance the robustness of the fallback mechanisms for StartGeneration and PriorScheduledPurchase, thereby improving scheduling resiliency. 

All three rule change proposals above were supported by the RCP and approved by the EMA on 26 July. EMC will update the industry of the effective date for the three changes in due course.

For more details on the rule changes mentioned and for future updates, please visit our website.