Southwestern Public Service Company’s Methodology for Computing Available
Transmission Capability
These answers are provided in response to FERC’s “Order Clarifying
Methodology for Computing Available Transmission Capability" in Docket No.
EL99-46-000.
General Information
As a Southwest Power Pool (SPP) member, Southwestern Public Service
Company (SPS) does not currently declare nor subtract any Capacity Benefit
Margin (CBM) from the Total Available Transmission Capability (TTC) on any of
its posted paths when determining Available Transfer Capability (ATC).
Additional information on the SPP use of CBM may be found on the SPP OASIS
or Web site.
The value of export TRM is determined by the SPP staff by using a deterministic
model to simulate the largest single contingency in the SPP. The method of
calculation used by SPP is discussed in the SPP Criteria, Section 4.3.
TRM Reservation by Posted Export Path
SPS has a limited set of interconnects with other utilities. To the west, SPS is
interconnected with Public Service Company of New Mexico (PNM) at Clovis,
New Mexico through a 200 MW High-Voltage Direct-Current (HVDC) facility.
PNM is a member of the Western Electricity Coordinating Council (WECC).
SPS also has another HVDC interconnect with El Paso Electric Company (EPE
and PNM) at Artesia, New Mexico. This 200 MW HVDC facility is owned by EPE
and PNM but operated by SPS under contract. SPS has another HVDC
interconnect with Public Service Company of Colorado (PSCo) at Lamar,
Colorado, that is owned by PSCo and is rated 210 MW. EPE and PSCo are
members of the WECC.
SPS has several interconnects to the east with American Electric Power
Corporation (AEP) in western Oklahoma. These interconnects are with Public
Service of Oklahoma and West Texas Utilities, both of which are operating
companies of AEP.
SPS has two interconnections with Sunflower Electric Power Corporation
(Sunflower) in southwestern Kansas. The first interconnection is between SPS’s
Guymon, Oklahoma facilities and Sunflower’s transmission facilities at Liberal,
Kansas. This interconnect is a single 115 kV line with a phase shifter installed in
the line. SPS also has a single 345 kV interconnect Sunflower at their Holcomb
Plant near Garden City, Kansas.
All of SPS’s eastern tielines are part of import and export flowgates. The TRM
values are computed by SPP across these flowgates. SPP maintains access to
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Southwestern Public Service Company’s Methodology for Computing Available
Transmission Capability
the list of flowgates. Those desiring that list will have to contact SPP to obtain
them. The TRM value is included with the flowgate information provided.
There may also be internal flowgates with TRM reservations on them. Refer to
the SPP for additional information.
TRM Reservation by Posted Import Path
SPS does reserve TRM in the import direction for paths in the SPP region.
SPS’s total import capability with all eastern interconnects in service is
determined by SPP. This value is not determined by company but is the
simultaneous import capability across the AEP, Sunflower, and West Plains
interconnects. SPS’s largest unit is a Tolk Plant unit, rated ~540 MW net. The
sudden loss of this unit requires that all ~540 MW be imported across the
eastern interconnects with AEP, Sunflower, and WestPlains. For all tie lines in
service, SPP calculates the TRM necessary for SPS to survive the loss of a Tolk
unit.
When one of the 345 kV tie lines to AEP or Sunflower is out of service, the TRM
value is raised to 540 MW. SPP will then adjust their ATC calculations to reflect
this situation.
This large reservation of TRM is based on a very short time power import
limitation. On April 16, 1996, SPS experienced a loss of the Tolk plant unit along
with other units. The result was more than 600 MW of power flowed into the
SPS system. Due to the large power flow, the interconnects with AEP opened
on an apparent out of step condition resulting in the cascading loss of the
remaining interconnects with AEP. This event resulted in the complete outage of
approximately 2/3 of the SPS transmission system. This event occurred over a
few seconds, a very short time frame. This short time frame does not allow SPS
enough time to curtail transmission imports through NERC TLR or other
procedure requiring human intervention. Because of the catastrophic effects of
this event, SPS has reserved 540 MW of TRM to be able to withstand the loss of
its largest generating unit, with no cascading outages, when only one 345 kV tieline is in service.
Sale of TRM on Non-Firm Basis
SPS transmission facilities are under the Southwest Power Pool Open Access
Transmission Tariff and any sale of TRM is governed by that tariff and
administered by Southwest Power Pool.
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