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.

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.