◊ This is part of the ‘Electricity Generation’ series of articles ◊
Supply planning
Supply planning is required to manage long term demand changes or the generation sustainment lifecycle. Existing generation facilities have finite life spans which require a continuous process of constructing, refurbishing and replacement. Generating facilities may take years to go from identifying the need to being in-service. All of this requires planning.
Supply Planning is performed by various organizations depending on geographic jurisdiction. The supply planning authority utilizes load growth forecasting and computer modelling to determine the capacity and type of generation required to meet reliability criteria.
Ontario’s regional planning authority is the Independent Electricity System Operator (the IESO)
A reliability authority defines supply requirements at a functional-level in the form of standards that must be met to ensure bulk power system reliability.
Ontario recognizes the North American Electric Reliability Corporation (NERC) as the standards authority
A political or regulatory process determines the economic and environmental criteria for supply.
Ontario’s regulatory authority is the Ontario Energy Board (the OEB)
Each geographic jurisdiction will have its own organizations designated to provide oversight of grid supply. Interconnection across jurisdictions requires adherence to a common set of functional requirements set by a reliability authority.
Supply scheduling
Having secured the long-term supply capacity required to meet operational needs, the short-term supply must be scheduled days or weeks ahead. Scheduling the availability of supply capacity is overseen by the regional operating authority through a process with generators. The process of scheduling generation is region-specific. A publicly owned, vertically integrated utility like Hydro Quebec will have a different process than Ontario which has investor-owned generation and an electricity market.
Demand forecasting, outage planning and contingencies factor into resource scheduling.
Dispatch and constraints
Supply (generation) must always meet demand – otherwise some immediate intervention is required to avoid a blackout. As demand changes, the supply must respond in real time to maintain voltage and frequency within acceptable limits. Generators have governors, excitation systems and inverter outputs to manage the small fluctuations in load, however, the available supply (including reserve) is managed through formal processes.
Generators that are grid connected operate in accordance with connection agreements and operating instructions to ensure the supply is capable of meeting demand. The mechanism used to manage supply is called dispatch.
Dispatch
In order for supply to meet demand, generation is dispatched by the system operator or balancing authority.
Dispatch is a term for directing generators to either increase or decrease output based on operating needs. The protocols for dispatch are region-specific. In general, a central authority manages the output of individual generators to balance the supply and demand. Generator dispatch instructions may be through verbal, electronic or automated mechanisms. Dispatching may be driven by reliability requirements, system limitations (see constraints), economics, contractual terms or environmental considerations.
Not all generation is dispatchable. Dispatch capability may be a matter of technical or contractual circumstances. Non-dispatchable or self-dispatching generation operates based on their own independent criteria. An example of self-dispatching generation is roof-top solar.
Constraints
Generation on the grid may be constrained by factors associated with the generator itself, the limitations of the electrical network (grid) or reliability criteria. When the grid configuration and generator output cause line segment currents to encroach on their thermal limits or exceed voltage limits, generation may need to be constrained.
Generating capacity that is limited by the delivery infrastructure is called ‘bottled generation’
Unplanned outages can impact power flows at any time, resulting in constraints requiring operator intervention. The regional balancing authority monitors the grid using advanced software applications and issues constraint instructions to generators.
previous article > Fundamentals of supply and demand
next article > Generation by function
Derek
