Download POTF - Alternate Paradigm

Alternate Paradigm
The current operational paradigm for WECC Paths combines operating limits and Transfer Capability
(scheduling limits) into a single parameter used in real-time operations – the historical Operational
Transfer Capability (OTC). In the advent of the mandatory NERC Reliability Standards in 2007, the
decision was made to replace OTC with the NERC Term “SOL”. This paper proposes an alternate
paradigm that may solve many of the issues identified with the current paradigm and the issues
associated with Path Operators in general.
Analysis of the Terms
The following definitions are provided below:





Operational Transfer Capability (OTC)
Transfer Capability
Total Transfer Capability (TTC)
System Operating Limit (SOL)
Interconnection Reliability Operating Limit (IROL)
1. Operational Transfer Capability (OTC) – defined in the retired WECC standard TOP-STD-007-0
The OTC is the maximum amount of actual power that can be transferred over direct or parallel
transmission elements comprising:


An interconnection from one Transmission Operator area to another Transmission
Operator area; or
A transfer path within a Transmission Operator area.
2. Transfer Capability – from the NERC Glossary of Terms
The measure of the ability of interconnected electric systems to move or transfer power in a
reliable manner from one area to another over all transmission lines (or paths) between those
areas under specified system conditions. The units of transfer capability are in terms of electric
power, generally expressed in megawatts (MW). The transfer capability from “Area A” to “Area
B” is not generally equal to the transfer capability from “Area B” to “Area A.”
3. Total Transfer Capability (TTC) – from the NERC Glossary of Terms
The amount of electric power that can be moved or transferred reliably from one area to another
area of the interconnected transmission systems by way of all transmission lines (or paths)
between those areas under specified system conditions.
4. System Operating Limit (SOL) – from NERC Glossary of Terms
The value (such as MW, MVar, Amperes, Frequency or Volts) that satisfies the most limiting of
the prescribed operating criteria for a specified system configuration to ensure operation within
acceptable reliability criteria. System Operating Limits are based upon certain operating criteria.
These include, but are not limited to:




Facility Ratings (Applicable pre- and post-Contingency equipment or facility ratings)
Transient Stability Ratings (Applicable pre- and post-Contingency Stability Limits)
Voltage Stability Ratings (Applicable pre- and post-Contingency Voltage Stability)
System Voltage Limits (Applicable pre- and post- Contingency Voltage Limits)
5. Interconnection Reliability Operating Limit (IROL) – from the NERC Glossary of Terms
A System Operating Limit that, if violated, could lead to instability, uncontrolled separation, or
Cascading outages that adversely impact the reliability of the Bulk Electric System.
Analysis of the Terms
Upon examination of the terms, OTC most closely matches the NERC definition of TTC. However, the
decision was made to replace the OTC term with the SOL term. Perhaps this decision was made not
because of the way OTC was defined, but because of the way OTC was utilized. Historically, OTC served
as both a Transfer Capability measure (scheduling limit) and a real-time operating limit. Thus, to cover
the operating limit aspect of OTC, the decision was made to replace OTC with SOL rather than TTC.
While treating OTC as a real-time operating limit may have had merit at one time, improvements in
technology, tools, information, and information sharing have made this approach obsolete for real-time
operations limitations.
The approach described here in no way minimizes the importance of Transfer Capability and establishing
an accurate TTC. As captured in the NERC definitions, Transfer Capability and TTC contain the word
“reliable” and “reliably” respectively. Correct and frequent determination of Transfer Capability and
calculation of TTC is an essential component in the reliability equation.
Proposed Paradigm – Distinguishing Transfer Capability / TTC from SOL
Under the current paradigm, TTC for WECC Paths is established according to the Path SOL. The Path
Transfer Capability, the Path scheduling limit, and the Path operating limit (SOL) are one and the same.
Transfer Capability, historical reliability analyses, investment protection, contracts, allocations, and
other commercial issues are reflected in the Path SOL.
The alternate paradigm proposes to break OTC/Path SOL concept apart and to distinguish its
components as separate values – TTC and SOL. The NERC definitions of Transfer Capability and TTC are
taken at face value, and the SOL remains as a pure reliability parameter. Commercial considerations are
upheld, but they do not determine the SOL; rather these commercial considerations respect SOLs. TTC
is separate and distinct from SOLs (though overlap may occur for stability limits), but TTC is not the SOL
itself; rather, TTC respects SOLs. This paradigm requires a change in how some entities view and apply
the TTC and SOL terms.
Current Paradigm
Investment
Protection
Transfer
Capability
Allocations
& Contracts
Path SOL
Proposed Paradigm




Transfer Capability
Investment Protection
Allocations & Contracts
Other Commercial Issues
respect
SOLs
Change in the Application of the SOL Term
This proposed paradigm requires a change in how the SOL term is generally viewed and applied in the
West. The application of the SOL term discussed here is based on the premise that at any given time,
real-time operations can encounter the following limits in the pre- and post-contingency state: Facility
Ratings, voltage limits, transient stability limits, or voltage stability limits. Remaining within these limits
in the pre- and post-contingency state equates to reliable operations. It can be stated that a primary
goal of engineers and operators are to ensure that Facility Ratings, voltage limits, transient stability
limits, and voltage stability limits are not exceeded in the pre- and post-contingency state. Establishing
an appropriate TTC is one aspect of accomplishing this reliability objective for those portions of the
system that are sensitive to system transfers.
Under the proposed paradigm, SOLs are the limits themselves – the Facility Ratings, voltage limits,
transient stability limits, and voltage stability limits – any of which can be the most restrictive limit at
any point in time pre- or post-contingency. This is not a new concept – these reliability parameters
already are considered SOLs as per the current SOL Methodology for the Operations Horizon version 6.1
that became effective on June 4 2012, which states, “All BES operating limits shall be regarded as SOLs,
and every BES Facility and Transmission Path shall have an SOL.” Under the proposed paradigm, the
task at hand for engineers and operators regarding SOLs becomes:



Ensuring that SOLs are known and established consistent with the RC’s SOL Methodology (i.e.,
that Facility ratings , voltage limits, transient stability limits, and voltage stability limits are
known and established), and
Making sure that SOLs (Facility Ratings, voltage limits, stability limits) are not exceeded in the
pre- and post-contingency state, and
Calculating a TTC that respects SOLs (i.e. calculate TTC such that Facility Ratings, voltage limits,
or stability limits are not exceeded in the pre- or post-contingency state)
The simplicity of this paradigm makes for clear and definable SOLs that are easily understood and are
strictly reliability-based.
In the proposed paradigm, SOLs are:




Facility Ratings. In the pre-contingency state, continuous Facility Ratings must not be exceeded.
In the post-contingency state, short-term Facility Ratings must not be exceeded. In either case,
the SOL is the Facility Rating itself.
Voltage limits. The revised SOL Methodology requires TOPs to establish a table that describes
pre- and post-contingency voltage limits for the TOP Area. These voltage limits are SOLs that
must not be exceeded pre- and post-contingency respectively.
Transient stability limits. The revised SOL Methodology version 7.0 effective March 3, 2014
states the following regarding transient stability limits: “To facilitate monitoring, TOPs shall
communicate established transient stability limited SOLs as pre-Contingent MW flow limits on
Transmission Paths/Interfaces consisting of single or multiple transmission elements.”
Voltage Stability limits. The revised SOL Methodology SOL Methodology version 7.0 effective
March 3, 2014 states the following regarding voltage stability limits: “The process for
establishing voltage stability limited SOLs in the WECC RC Area focuses on…identifying the
voltage stability limit (i.e., the maximum pre-contingency power transfer or load level for which a
post-contingency solution can be reached) and applying minimum margins to determine the
corresponding voltage stability-limited SOL…”
Under the proposed paradigm, all of the above are SOLs, but the most restrictive of these is to be used
in real-time operations.
Note that transient stability limits and voltage stability limits are pre-contingent limits that ensure
acceptable post-contingency stability performance. This is not the case for thermal (Facility Ratings) and
voltage limitations. Transient stability limits and voltage stability limits may or may not coincide with
WECC Paths. When these limits coincide with WECC Paths, these limits may equate to the TTC for that
path.
Change in the Application of Transfer Capability / TTC
The current WECC method of establishing TTC for WECC Paths can be summarized as follows: the TOP
performs transfer analyses to determine the Path SOL. Once that Path SOL is established, the TOP
adjusts the TTC to be equal to that Path SOL. The TTC and the Path SOL are one and the same. This
approach is supported by MOD-029 R3 which states, “Each Transmission Operator shall establish the TTC
at the lesser of the value calculated in R2 or any System Operating Limit (SOL) for that ATC Path.”
Under the proposed paradigm, the TTC term is taken at face value. Quite simply, TTC is “the amount of
electric power that can be moved or transferred reliably from one area to another area of the
interconnected transmission systems by way of all transmission lines (or paths) between those areas
under specified system conditions.”
Under the new paradigm TOPs will still need to perform transfer analyses. The same work will need to
be performed as it is today. The key difference is that these TOP studies will be performed with the
objective of establishing a TTC that respects SOLs – not necessarily to determine the SOL itself***. A
TOP will simulate power system transfers and establish a TTC that does not cause Facility Ratings,
voltage limits, transient stability limits, and voltage stability limits to be exceeded in the pre- and postcontingency state.
***Important Caveat – There is an important caveat that must be acknowledged. As noted above,
transient stability limits and voltage stability limits are calculated and expressed as pre-contingent
path/interface flow values. Accordingly, transfer analyses are required to establish transient stability
limits and voltage stability limits. It is possible that transient stability limits and voltage stability limits
may define TTC for certain Paths, rendering TTC and the Path SOL to be the same value. Even still, the
new paradigm is upheld – TTC respects the SOL.
Summary
Under the current paradigm, the TTC is the Path SOL itself – they are one and the same. Under the
proposed paradigm, TTCs and SOLs are separate and distinguished, but they work together to ensure
that reliability is achieved in real-time operations. SOLs become the limits that are not to be exceeded
pre- and post-contingency, and TTCs become a measure of maximum power transfers that respect these
SOLs pre- and post-contingency. There may be cases under the proposed paradigm where TTC may
actually be the SOL. These are instances where a transient stability limit or voltage stability limit exists
on a WECC Path and that limit is the most restrictive limit encountered when calculating TTC.
Proposed Paradigm Consistent With General Industry Direction
The proposed paradigm is consistent with:



The 9/13/2012 NERC CEO (Gerry Cauley) Letter to Mark Maher
The revised SOL Methodology to be issued in August 2013
NERC Project 2012-05 ATC Revisions (MOD A) that seeks to retire several MOD standards
related to TTC/ATC calculation (including MOD-029)
The 9/13/2012 Gerry Cauley Letter
In response to the September 8th blackout event investigations, NERC CEO Gerry Cauley stated the
following in his 9/13/2012 letter to WECC CEO Mark Maher:
“NERC is pleased to see that WECC is holding additional discussions to clarify the
role of Path Operators, including the potential to implement contractual
relationships and make use of RTCA and other tools to improve the accuracy of
system operating limits. As these discussions continue NERC suggests that you
also review the concept of Path Ratings and whether, as the Western
Interconnection has become more highly interconnected, the Path Rating and
Path Operator concept, along with the use of nomograms, still has merit for
real-time operations. Other Interconnections do determine Flowgate limits for
purposes of interchange scheduling, but rely more fully on RTCA for real-time
operating reliability.”
In this letter, NERC endorses the use of Path (flowgate) limits for purposes of interchange scheduling,
but encourages the use of real-time tools and information for real-time operating reliability. The
proposed paradigm (distinguishing TTC from SOL) is aligned with NERC’s recommendations.
Consistent With the Revised SOL Methodology for the Operations Horizon
On August 30, 2013, the WECC RC issued a revised version of the SOL Methodology for the Operations
Horizon. This revised version 7.0 will become effective on March 3, 2014. While the revised SOL
Methodology for the Operations Horizon is written to accommodate the current paradigm, it contains
language that supports the proposed paradigm:

The revised SOL Methodology contains the following statement: “Unless more restrictive
limitations are present, SOLs shall equal the applicable Facility Ratings, voltage limits, voltage
stability limits, transient stability limits, and WECC Path Ratings. The most limiting of these shall
determine the SOL used in real-time operations.”
The revised methodology centers on acceptable system performance with little focus on WECC
Paths. It was purposefully written not to be a “Path-centric” document.

NERC Project 2012-05 ATC Revisions (MOD A)
This NERC project will result in the retirement of several MOD standards, among them being MOD-028,
MOD-029, and MOD-030. In the future state there will be no “WECC Rated Path Methodology” as
stated in a MOD standard per se. These – and other – MOD standards will be replaced by a single
revised MOD-001 reliability standard, which is being drafted according to NERC’s new fast-track process
at the time this paper was written. Under the new MOD-001 standard, TOPs who utilize TTC or TFC will
be required to prepare a methodology document that describes how TTC/TFC is calculated. The
approach taken by the drafting team is well aligned with the proposed paradigm.
R1 of the DRAFT version of MOD-001 addresses TTC calculation. The language in the DRAFT R1 fully
supports the proposed paradigm of distinguishing TTC from SOL. The proposed language of MOD-001
R1 at the time this paper was written (9/4/2013) is as follows:
R1.
Each Transmission Operator that utilizes TFC or TTC shall prepare, keep current, and
implement a methodology document for calculating its TFC or TTC, if: [Violation Risk Factor:
Lower] [Time Horizon: Operations Planning]
1.1.
1.2.
Each methodology within the document shall include:

A statement that the TFC or TTC shall incorporate facility ratings, voltage limits, and
stability limits pre- and post-contingency; and a description of how this is
accomplished;

What criteria is used to select which limits (such as System Operating Limits (SOLs),
reliability constraints, facility ratings, contractual obligation, sum of ties, etc.) are
relevant to the calculation;
(If applicable) each methodology within the document shall address the following
elements of the TFC or TTC calculation:

How simulation of transfers are performed through the adjustment of generation,
Load, or both;

Transmission topology, including, but not limited to, additions and retirements;

Currently approved and projected transmission uses;

Planned outages;

Parallel path (loop flow) adjustments;

Load forecast; and

Generator dispatch, including, but not limited to, additions and retirements.
1.3.
Each methodology within the document shall describe the process for including any
reliability-related constraints that are requested to be included by another Transmission
Operator, provided the constraints are also used in that Transmission Operator’s TFC or
TTC calculation.
1.3.1
1.4.
The Transmission Operator shall use a distribution factor (Power Transfer
Distribution Factor (PTDF) or Outage Transfer Distribution Factor (OTDF)) cutoff
value of five percent or less when determining if these constraints should be
monitored.
Each methodology within the document shall address the periodicity for the Transmission
Operator to provide updated TFC or TTC values to the Transmission Service Provider.
The draft R1 characterizes TTC as a calculation of Transfer Capability that respects Facility Ratings,
voltage limits and stability limits (i.e., SOLs) pre- and post-contingency which takes into consideration:



expected system conditions such as transmission topology, outages, load forecasts, generation
dispatch (1.2)
neighboring TOP reliability constraints (1.3)
contractual obligations (1.1 second bullet)
This draft language is well aligned with the proposed paradigm. TTC determines how much power can
be transferred over Paths, and these TTC calculations must respect SOLs pre- and post-contingency.
While TTC is critical for reliability, TTC is not the SOL itself.
Path Operator Issues Addressed by the Proposed Paradigm
(not yet documented)