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There are two different dispatch methods available with PV analysis, Scaling, ATC analysis, and scaling injection groups as part of Time Step Simulation that make use of generator economic MW limits and will connect generators as necessary to achieve a specified dispatch level. One is called Economic Merit Order Dispatch and the other is Merit Order Close Dispatch. Economic Merit Order Dispatch is a method that attempts to dispatch generators so that they are at the same relative point in their economic minimum MW to economic maximum MW range. Merit Order Close Dispatch will dispatch each generator in merit order until it hits its economic MW limit before dispatching another generator.
The similarities in input data and operation are described below as well as the specifics of each of the dispatch methods.
There are two basic requirements for input data in order to properly use these features:
Injection Group Specifying Merit Order
Injection groups must be used to specify the merit order in which generators will be dispatched. The participation factors assigned with each generator as part of the injection group define the merit order. The actual merit order is based on participation factors normalized for all generators participating in the dispatch.
Generators will be excluded from the dispatch if the option to Allow only AGC units to varyis checked and AGC = NO for that generator. Generators will NOT be excluded if their normalized participation factor is less than or equal to zero. The normalized participation is used for ranking only and generators will be dispatched in the order of highest to lowest normalized participation factor. Generators that are offline can also participate in the dispatch as long as they can be energized either by changing the Status of the generator or closing breakers to energize. See the Closing Breakers to Energize Generators section below on more information.
Generator Economic Range Fields
The desired economic range in which generators should be dispatched is defined by the MW Economic Minimum and MW Economic Maximum fields. These fields can be found on the Generator Display. In order for a generator to participate in one of these merit order dispatch methods, MW Economic Maximum must be greater than zero.
Economic Merit Order Dispatch
Specified values will also be swapped to enforce the relationship that Min MW <= MW Economic Minimum <= MW Economic Maximum <= Max MW. The Min MW and Max MW fields are the fields that define the overall minimum and maximum output of a generator. The Min MW limit will always be enforced, but the Max MW limit will only be enforced if the option to Enforce unit MW limits is in use.
Merit Order Close Dispatch
Only MW Economic Minimum and MW Economic Maximum limits will be used during this dispatch. These limits will be enforced regardless of how the Enforce unit MW limits option is set.
Dispatching the Generators
Economic Merit Order Dispatch
Economic merit order dispatch allows specifying how generators are dispatched at various transfer levels. Ideally, generators will maintain their output within an economic range. If at a particular transfer level generators will exceed their economic maximum value, additional generators will be dispatched so that all generators are within their economic range. Generators that are not needed to provide MW output will also not provide Mvar output. If at a particular transfer level, generators will fall below their economic minimum value, generators will be de-energized until all generators are within their economic range.
To determine the economic merit order dispatch for an injection group at a particular injection level, all generators in the group are first assumed to be at 0 MW. Valid generators are dispatched in merit order. If the desired dispatch is below the Min MW value of the first generator, that first generator will be turned on to its Min MWand no additional generators will be dispatched. Additional generators will be energized in merit order if previous generators will exceed their MW Economic Maximum values. All energized generators are dispatched such that they are at the same relative point in their MW Economic Min to MW Economic Max range. If energizing a new generator will cause all energized generators to fall below their MW Economic Min levels, all generators will be dispatched such that they are at the same relative point in their Min MW to MW Economic Min range. If all available generators are dispatched and the desired injection exceeds the total MW Economic Maximum for all generators, all generators will be dispatched such that they are at the same relative point in their MW Economic Maximum to Max MW range. If using the option to Enforce unit MW limits, generators will not be dispatched beyond their Max MW limit. If NOT using this option and the desired injection exceeds the total Max MW for all generators, all generators will be dispatched in proportion to their Max MW limits.
If a generator is not valid for dispatch (i.e. AGC = NO if using option Allow only AGC units to vary or MW Economic Maximum <= 0) or is not needed in the dispatch, that generator will remain de-energized if not already energized or will have its Status set to Open so that it will not provide Mvar output in addition to not providing MW output. If a generator is energized, breakers will not be opened to de-energize the generator. Changing the Status of the generator itself will force the generator to be de-energized. Breakers will only change status if a generator needs to be energized but is not already energized because of open breakers.
An example of a injection group with 5 generators is shown below along with the dispatch at various transfer levels:
Merit Order Close DispatchAdded in version 19, build on Aug. 4, 2016
Injection group generators will be dispatched by moving individual generators to either their MW Economic Maximum or MW Economic Minimum limits as appropriate in succession based on their relative participation factors ordered from highest to lowest. Generator economic MW limits will be enforced for those units participating in the merit order close dispatch regardless of how the Enforce unit MW limits option is set.
If increasing the injection and the next generator in merit order is not already online, the Status of the generator will be set to Closed with an initial Mvar output of 0. Breakers will also be closed if necessary to connect the generator in addition to changing its own status. If changing the status or closing breakers will not actually connect the generator, that generator will be skipped, no changes will be made for that generator, and the next generator in merit order will be processed. See the Closing Breakers to Energize Generators section for more information on how breakers are selected.
If decreasing the injection and generators are being backed down to their minimum economic limit, generators will remain online when they hit their limits and the status of generators and breakers will not change.
Closing Breakers to Energize Generators
Generators can participate in one of these merit order dispatch methods provided they can be energized. If a generator is not presently energized, is the next generator in the merit order ranking, and the case has breakers, an attempt will be made to identify breakers that can be closed in order to energize the generator. This functionality does not require having the Integrated Topology Processing add-on to work.
When selecting which breakers can be closed to energize a particular generator, breakers are only selected if in the process of closing them they would ONLY energize the particular generator. Breakers that would energize additional devices will not be selected as valid for energizing the generator. If no appropriate breakers can be found, a generator will remain de-energized and will not be available for dispatch.
Branches with Branch Device Type of Breaker and Load Break Disconnect are included when searching for switching devices to energize generators. Disconnects are not included.
See the Close Breakers Overview topic for more information on how the close with breakers process works.