In an electric power system generators are initially dispatched according to unit commitment schedules made one day in advance. They determine which generators will come online or go off-line at various times during the following day.
– In order to minimize the cost of meeting load, subject to transmission line capacity limits and other security constraints, vertically integrated utilities are dispatching the committed generators with lower marginal operating costs before those with higher costs. Where there are organized wholesale markets, system operators utilize generators’ bids instead of actual costs in determining how units are dispatched.
– The committed generators may fail to meet their dispatch schedule due to unforeseen equipment failures or other contingencies, necessitating the dispatch of reserves. Generators and demand response can provide these operating reserves.
Operating Reserves Categories
- Frequency Response Reserves
The fastest reserves spans from milliseconds to seconds, used to respond to severe contingencies which affect the power system frequency such as the loss of a generator or transmission line. They are activated automatically on individual generators and at control centers, and are rarely explicitly dispatched by the operator.
- Regulating Reserves
It lasts for seconds and used during normal operation to maintain the balance between supply and demand due to random changes in generation or load. They are dispatched by the operator in times that are faster than the clearing periods of energy markets.
- Ramping Reserves
It spans from minutes to hours responding to ramp events, due to unexpected unit commitment schedule. They can ramp their outputs either up or down.
- Load Following Reserves
These are dispatched during normal operations by the operator to maintain the balance between supply and demand due to cyclical ( daily, weekly) changes in demand or generation, on a slower timescale than regulating reserves.
- Supplemental Reserves
The slowest form of reserves spans from tens of minutes to hours. These are deployed during contingencies alongside faster reserves. They are designed to slowly ramp up and replace faster reserves, which are then available to address future contingency events.
Effect of Variable Energy Resources On Reserves
Wind integration studies and experience have shown that the additional variability and uncertainty associated with higher penetrations of wind will increase operating reserve requirements. Herein after highlights of how variable energy resources affect each category of reserves.
- Frequency Response and Supplemental Reserves
All major international studies have concluded that large VER penetrations do not significantly increase the risks for traditional contingencies.
- Regulating Reserves
The uncertainty and the variability of VERs can create fluctuations in production on the order of minutes. Accommodating these fluctuations may require a modest increase in fast-responding regulating reserves required for normal operations.
- Ramping and Load-following Reserves
Ramping and load-following reserves are primarily used to counteract VER forecast errors in the day-ahead scheduling of plants. Day-ahead load forecast errors are typically below 1% mean average error of production. As a result, operators must conservatively operate the system assuming that the actual VER production could be at least 20% lower than forecasted, requiring large quantities of reserves.
The risk of ramp events in most cases comes from the uncertainty of when they will occur and how long they will last. Indeed, predictions of ramp event occurrence and timing are often difficult and can result in large and sudden forecast errors that deplete reserves that are on-line in a time too short to activate reserves that are fully off-line.
How to Reduce the Cost of Associated Reserves With VERs.
- Improve variable energy resource forecasts and situational awareness through greater sharing of meteorological data ,which will improve wind forecast accuracy in regions with high penetrations of VERs.
- Make more frequent decisions closer to real time .
- Cooperate among balancing areas can offer reliability and economic benefits when integrating large amounts of VERs. By aggregating a geographically diverse collection of VERs, rapid changes in the outputs of individual VERs are replaced by the slower output variations of the aggregated resource.
- Ensuring adequate system flexibility to be capable of continuously modifying their output, or “cycling,” to accommodate the variation in the output of the VERs