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Sierra Pacific Power Company
OASIS
Sierra Pacific Policy on Transmission
Reliability Margin and Capacity Benefit Margin
Effective Date: 09/24/2007
Date last
Re-evaluated:
09/24/2007
Definitions
According to the Western
Electricity Coordinating Council (WECC) document, Determination of Available Transfer Capability within the
Western Interconnection, the definitions for Transmission
Reliability Margin (TRM) and Capacity Benefit Margin (CBM)
are:
TRM
“TRM is the amount of
transmission transfer capability necessary to provide a reasonable
level of assurance that the interconnected transmission network will
be secure under a broad range of uncertainties in system conditions.
TRM accounts for the inherent uncertainty in system conditions and
system modeling, and the need for operating flexibility to ensure
reliable system operation as system conditions change.”
Further, the Document
specifically addresses possible TRM components. They are:
- Transmission necessary
for the activation of operating reserves
- Unplanned transmission outages (for paths
in which contingencies have not already been considered in
establishing the path rating)
- Simultaneous limitations
associated with operation under a nomogram
- Loading variations due
to balancing of generation and load
- Uncertainty in load
distribution and/or load forecast
- Allowances for unscheduled
flow
CBM
“CBM is the amount of firm
transmission transfer capability reserved by Load Serving Entities
(LSEs) on the host transmission system where
their load and generation resources are located, to enable access to
generation from interconnected systems to meet generation reliability
requirements. CBM is a uni-directional
quantity with identifiable beneficiaries, and its use is intended only
for the time of emergency generation deficiencies.”
Sierra Policy---TRM
Sierra Pacific Power Company
(Sierra) utilizes TRM. Sierra specifically utilizes items
#1 through #5
above. Sierra’s transmission system is quite constrained (dynamically
and post-disturbance voltage deviations) from various single
contingencies. The import/export operating nomograms, addressing item #3
above, posted on the Internet at OATiOasis/Transmission Information/Import Nomograms, fully describe the system-wide import
limit factoring in the “simultaneous limitations associated with operation
under a nomogram.”
The true system simultaneous import limit, determined
by study, is 1010 MW. Sierra also applies additional TRM to account for
minor load variations (item #4) and small errors in load forecast
(item #5).
The total TRM for those items is 10 MW. In application, the total import
capability described in the operating
nomograms is decremented by nomogram component.
In essence, the difference between the
non-simultaneous Sierra import limit (i.e., the arithmetic sum of all
paths Total Transfer Capability (TTC)---1357 MW),
the simultaneous limit (i.e., the simultaneous interaction of all the
paths---1010 MW), and the minor load variations (10 MW) is the total
system TRM. The TRM formula is:
|
Non-Simultaneous Import
Limit (i.e., sum of all path ratings) |
1399 MW |
|
less |
|
|
Operation under a nomogram |
1010
MW |
|
TRM, part A |
389
MW |
|
Plus |
|
|
Loading Variations and
Uncertainty in Load Distribution |
10 MW |
|
Total
TRM |
399
MW |
|
|
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In order to manage Available Transmission
Capacity (ATC) calculations, Sierra has
chosen to create a “virtual path” (to reflect the import limit) in series
with the physical paths. Rather than posting to TRM individual paths, on
OATi OASIS, the “virtual path” has a TTC of 1399
MW and a TRM of 399 MW posted. The difference (1000 MW) is equal to
the import limit allowable for scheduling.
When customers attempt to reserve transmission
capability to serve load within the SPPC system, the ATC on both the
physical path under consideration and the series virtual path are
decremented. Schedules on all physical paths must cross the virtual
path to reach the load.
Previously, the 1000 MW import limit was programmed
as the TTC value for the "import" virtual path. As of today, 9/17/2007 the
"import" path TTC is 1399 MW and the TRM value is 399 MW. Net resulting
ATC calculations yield the same results as before.
TRM for Reserve Sharing
Sierra is a member of the
Northwest Power Pool (NWPP) Reserve Sharing Group. The NWPP exists
to allow member control areas/balancing areas to reduce the contingency
reserves they may carry and allow access to other reserves to meet the
requirements of the NERC Disturbance Control Standard (DCS) detailed in BAL-002-0.
The Sierra TRM for contingency
reserves is variable since it is load based. According to the WECC
Minimum Operating Reliability Criteria (MORC), contingency reserves are
seven percent of the Load Responsibility. Load Responsibility is the
actual control area/balancing area demand less firm imports plus firm
exports. For purposes of defining TRM for reserves, Sierra has
neglected firm exports. Further, all imports are deemed firm to
simplify the calculations. Thus, the contingency reserve requirement
becomes: (Load – import)*0.07.
To meet DCS, all control
areas/balancing areas must have access to reserves equal to at least their
Most Severe Single Contingency (MSSC); the largest generator output.
For Sierra, the generator output is always greater than the internal
contingency reserves. The remainder must be accessible, from the NWPP
entities, on the Midpoint/Humboldt intertie (the
only path available using the NWPP automated reserve sharing
system).
Since load varies, the NWPP
contingency reserves needed by SPPC vary as well. While the load
changes moment-to-moment, Sierra chose to set aside TRM (on the
Midpoint/Humboldt intertie only) based upon the
peak load expected in a calendar quarter. TRM is thus:
TRM = MSSC – (Maximum Quarterly
Load – Expected Import)*.07
The Transmission Provider will
assess the TTC and the ATC of the Transmission System to provide the
service requested as prescribed by FERC regulations and in accordance with
the process detailed in Sections 4 and 5 of the Transmission Provider's
annual FERC Form 715 submittal. In determining the level of capacity
available for new transmission service requests, the Transmission Provider
will take into consideration that capacity needed to meet current and
reasonably forecasted loads of Native Load Customers, existing Network
Customers, existing Firm Point-To-Point Transmission Service Transmission
Customers, pending applications for Firm Point-To-Point Transmission
Service, and other existing contracts.
Sierra Policy---CBM
Sierra Pacific Power
Company does not use CBM.
Re-evaluation
Sierra Pacific Power
Company will re-evaluate TRM needs annually. Currently Sierra Pacific
Power Company does not use CBM, and will re-evaluate CBM as it is
requested by Transmission Customers as well as CBM will be re-evaluated at
a minimum annually.
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