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Load as a Resource

Integrating controllable demand into real-time, security constrained economic dispatch.

Fortnightly Magazine - August 2012

sound dispatch principles ensure that the dispatcher directs generators to increase or decrease their output based upon their ability to ensure that the frequency is maintained at 60 Hz and there is sufficient transmission capability to deliver power to the load—the “security constrained” portion of SCED. Sound economic principles require that when multiple resources are available to the operator, the least expensive resource available to meet the needs of the system is dispatched first—the “economic dispatch” portion of the rule. In traditional regulated environments, the dispatch order is based on the unit’s marginal cost. In the competitive markets, the dispatch order is based on the bid price offered to the market operator.

In order to perform SCED, grid operators traditionally begin with a day-ahead load forecast. The load forecast is built upon derived experience of similar near-term and historic days. In other words, a mild-temperature weekday load in Philadelphia will look very much like the load pattern of a mild-temperature weekday load one year ago and one week ago. Once the forecast is complete, operators then build a supply profile to meet the demand. The supply forecast is based upon the principle of finding the lowest economic unit or units of generation that are able to physically be delivered to meet the demand. In the RTO markets the dispatch paradigm is based upon the generator’s bid offer, while in regulated markets the generation loading is based on cost of service, and largely reflects the generator’s fuel type and heat rate.

Integrating controllable demand (CD) allows utilities to leverage price elasticity, in contrast to the present situation where demand is treated as relatively inelastic to price. In addition, considering the types of CD available—ranging from building loads, storage systems, and distributed generation—it’s possible to add diversity to this new resource. The following steps allow establishing SCED for controllable demand: 1) Establish a day-ahead (DA) load forecast; 2) Establish a bidding process inclusive of CD, or establish a price for CD; 3) Perform a DA SCED to process and generate market clearing prices (MCP) accordingly on an hourly basis ensuring that demand (load forecast minus CD) equals generation; 4) Post a DA schedule highlighting effective stacking of resources to meet actual demand; and 5) On the following day, similar to what was stated earlier, perform a real time SCED—inclusive of both traditional and CD resources—to process and generate adjustments to the DA schedule based on ongoing corrections to the forecast, with the focus being to achieve a balance that results in lowest-cost or lowest-price dispatch.

To integrate actual load into the system, the grid operator supplements the day-ahead SCED plan with real time conditions. Generators are required to increase or decrease their output for real time load and operating conditions to ensure that supply and demand remain in balance. In real time, SCED principles are also deployed to ensure that the resource that is used to achieve the balance results in lowest-cost or lowest-price dispatch.

Neither of these paradigms typically include variable load as a resource that can be modified based upon price and