SPP TRM and CBM Practices
Southwest Power Pool established certain guidelines to be used for determining the need for Transmission Reliability Margin and Capacity Benefit Margin in section 4.0 of its Criteria. At the time this Criteria was written, November 1997, SPP transmission providers were responsible for the calculation and posting of ATC with the exception of seasonal calculations which were coordinated and performed by the SPP staff with review by the membership of SPP. In June of 1998, SPP began administering a regional tariff for all but one of its transmission providers. This tariff covered all non-firm point-to-point service and short-term firm for periods of less than one year.
In conjunction with this tariff, SPP began using a new ATC calculation system which was much more comprehensive and centrally administered. The new system is flow based and calculates ATC with a much greater frequency than the individual providers. The calculator also runs in conjunction with an OASIS Automation System which allows it to update the impacts of new reservations and schedules on all paths on OASIS based on sensitivity factors. This helps capture the simultaneous impacts of parallel flows.
The models used for the calculations are driven by a real-time state-estimator solution and future time points are modified with any known outages or returns to service for facilities as well as projected loads and interchange.
Transmission Reliability Margin
TRM is defined as that amount of transmission transfer capability necessary to ensure that the interconnected transmission network is secure under a reasonable range of uncertainties in system conditions. The factors identified by SPP in section 4.0 of the Criteria included load forecast demand and distribution, variations in generation dispatch, parallel path flows, and the SPP Operating Reserve Sharing program.
- Load Forecast Demand and Distribution
SPP now forecasts hourly load for the next seven days for all of its control areas. These load levels have been shown very reasonable to this point. Beyond seven days, SPP projects a demand based on seasonal peak load models for all of its members. These load levels are the projected peaks for that season and are typically very high. SPP does not currently see a need to add additional margin to its flow gates for TRM based on possible demand error.
- Variations Generation Dispatch
Variations to generation patterns are still a viable concern, however, the near-term models are based on real-time snapshots which should remain somewhat consistent if load levels and maintenance patterns do not change much. In the longer-term horizons, generation dispatch may change considerably. SPP is attempting to gather more economic dispatch information from its members to apply that to the ATC calculations, however, longer-term dispatch is probably unknown to the generation controlling entities themselves except for base-load and must run type units.
- Unaccounted for Parallel Flows
Parallel flows were a larger issue when it was uncertain how pertinent data to the ATC calculations would be shared among the transmission providers and how those deals that had multiple wheeling parties in the middle would be identified. The regional tariff has helped this situation immensely within SPP and the geographical and electrical boundaries of SPP have helped reduce the impacts of transactions within or between other regions. SPP's border to the south is the ERCOT system which is connected with two asynchronous DC ties. The same holds true for the Western border to WSCC. The interconnections to the north are rather sparse now that Associated Electric Cooperative has left SPP. There are two significant 345 kV tie areas. These are the Cooper - Fairport - St. Joe loop north of Kansas City and the Red Willow - Mingo 345 in western Kansas. Both of these comprise joint MAPP/SPP flow gates which were rated by MAPP since they are predominantly located within MAPP. These flow gates do have some additional TRM for operating uncertainty. The flow gates in the Kansas City area are subject to some of the same potential loop impacts for MAPP to MAIN or MAPP to SERC transactions. The owners of these facilities have not justified or do not desire additional TRM on these facilities for parallel flow impacts. Although the SPP interconnections with SERC to the east are extensive, most of the flow problems arise for transfers into SPP which will be accounted for. Again, MAPP to SERC transactions may have some impact on SPP flow gates, however the owners have not justified or do not desire additional TRM to account for this.
- SPP Operating Reserve Sharing
The SPP Operating Reserve Sharing program was instituted to provide both reliability and economic benefits to its members. It reduced the amount of internal operating reserves each entity had to maintain while providing an automated way of allocating resources on a region wide level to ensure the loss of any unit was quickly covered. Transmission facilities must be able to support the automatic implementation of the Reserve Sharing program. To that end, SPP has maintained that TRM on the flow gates should provide enough capacity to withstand the impact of the most critical generation loss to that facility. All generation contingencies are simulated by the Operating Reserve Sharing algorithm to determine the highest impacts to the flow gates. This capacity is maintained as TRM.
Another factor to consider in the SPP TRM process is that for the planning horizon, which is primarily next day and beyond, the counterflow impacts of reservations on the flow gates are backed out completely. It is very rare in real-time when the flow on a regionally impacted transmission facility is not offset by some counter flowing schedules. This provides an inherent margin in the calculation which some providers feel, along with the constant TRM provided by the reserve sharing allocation, is a proxy for the generation variation and parallel impacts mentioned earlier.
To maximize transmission use to the extent reliably possible, SPP has decided to allow non-firm sales into TRM. However, the realization of a contingency or long-term outage to a high impact unit or the critical contingency element of a flow gate may result in the curtailment of non-firm schedules and displacement of non-firm reservations whose impacts result in flows within the margin. This will be done to adjust the system for the next potential contingency.
Capacity Benefit Margin
CBM is defined as the amount of transmission interconnection capability reserved by load serving entities to ensure access to generation from interconnected systems to meet generation reliability requirements.
Recent generation reliability studies on a regional and subregional level have shown the SPP region as a whole to be well within the Loss of Load Expectation standard of 1 day in ten years. The SPP Criteria requires a minimum level of capacity margin to be maintained by all members of SPP. Until this year, this Criteria has been 15.25% unless the entities were able to show through probabilistic analysis that they could reduce this level and still maintain the 1 day in ten year standard. It could then be reduced to 13.1% for steam based systems and 9% for hydro based systems. The SPP internal capacity margin continues to be around 15% and historical studies have shown that the loss of load probability of one day in ten years can be maintained with a 10% - 11% capacity margin. The SPP has changed its capacity margin Criteria effective this year requiring each control area to maintain a minimum of 12% capacity margin for steam-based utilities and 9% for hydro-based utilities. Given the internal capacity margin of each member, the historical reliability indicators of transmission strength into SPP, and the move to the regional tariff which allows greater purchasing options, the transmission providers do not desire an additional margin to the flow gates for CBM. Any additional margin, which would be in addition to the TRM held for Operating Reserve Sharing has not been determined necessary by the transmission providers of SPP.