Issues with the Path SOL Concept in the Operations Horizon
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.”
Mr. Cauley is asking the West to challenge existing paradigms and consider alternative approaches to
the Path SOL concept and the union of scheduling limits and operating limits.
The current operational paradigm in the West combines operating limits and scheduling limits into a
single parameter used in real-time operations – the historical Operational Transfer Capability (OTC). In
the advent of the NERC Reliability Standards, the decision was made to replace OTC with the NERC Term
“SOL”. Because of this term replacement, SOLs in the West have generally been viewed as a term
applicable only to WECC Paths that describes the amount of power that can be reliably transferred over
a WECC Path – for both scheduling and real-time operations purposes. This Path SOL concept has
resulted in a host of issues for TOPs within WECC and for the WECC RC. This paper seeks to identify
these issues.
Issues with the Path SOL concept and the union of scheduling limits and operating limits:
1. The Path SOL Concept muddies the waters between reliability limitations and commercial
limitations.
a. Under the current Path SOL paradigm, Transfer Capability, scheduling limitations,
allocations, commercial considerations, and historical reliability assessments performed
in years past are all rolled up together into a parameter that is monitored in real-time
operations as an SOL. In the West, Path SOLs are the WECC Path Ratings unless studies
indicate a lower Path SOL value.
b. While WECC Path Ratings have a basis in reliability studies performed in the planning
horizon, the Path Rating process and the granted Path Rating exist primarily to ensure
protection of investments. It is often unclear whether the Path SOL is an actual
reliability constraint or whether it exists to accommodate scheduling allocations among
the owners or other commercial issues.
c. When OTCs are used in real-time as System Operating Limits confusion abounds in the
following manner:
i. The SOLs do not take into account actual system conditions, thus leading to
confusion by operators due to the disagreement between tools and the SOL;
inquiries from regulators, lawsuits, or any other legal proceeding is difficult to
defend when the tools don’t support the actions taken by operators;
ii. Who’s responsibility it is to mitigate when the appropriate contracts or
procedures are not in place. Often the question comes up from TOPs – “Are you
(RC) going to compensate us for adjusting our generation to fix someone else’s
problem?”
2. Path SOLs do not take into consideration – and often conflict with – real-time tools and
information. (See RC presentation)
a. Thermal and steady state voltage limited Path SOLs are established and operated to
despite the information provided by real-time tools and assessments. It is common for
thermal and voltage Path SOLs to outright conflict with real-time tools and information.
For example, it is a common occurrence to see a thermal or voltage Path SOL being
exceeded, but real-time tools show no indication of a thermal or voltage reliability
problem pre- or post-contingency. The opposite is also true. Real-time tools may show
thermal or voltage reliability problems, but the Path SOL is not being exceeded. While it
may not always be easy to determine acceptable system performance for stability issues
with real-time tools and information, it is very easily accomplished for thermal and
steady state voltage concerns.
b. Establishing a thermal or voltage Path SOL ahead of time and treating it as a real-time
operating limit almost guarantees that the Path SOL will be too high or too low most of
the time – either too conservative or not conservative enough. Path SOLs that are too
low result in unnecessary and costly mitigation actions (redispatch, schedule
curtailments, load shedding). Path SOLs that are too high might result in reliability
issues that are missed or disguised. Real-time tools and information expose these
inaccuracies for thermal and voltage Path SOLs specifically.
c. If no other limitation is identified through studies, the Path SOL defaults to the WECC
Path Rating. Many of these WECC Path Ratings are based on studies performed many
years ago, yet they are treated as real-time operating limits despite information
provided by current information and real-time tools.
3. The Path SOL paradigm potentially disguises other critical limitations (such as stability limits)
that are not the “most limiting criteria”.
a. A Path Operator performs transfer analysis on a path and establishes a thermally limited
Path SOL of 500 MW to prevent loss of X from resulting in Y exceeding its short-term
Facility Rating. The thermal Path SOL of 500 MW is communicated to TOPs and the RC.
b. Performing studies that stress the path to 550 MW would result in voltage collapse of a
wide area of the power system.
c. The stability limitation may be overlooked since it is not the most limiting criteria for the
Path. TOPs may simply stop studying at the 500 MW level since a limitation was
encountered at that transfer level.
d. If the TOP actually did uncover the 550 MW voltage collapse limitation in studies, it
might not be communicated to neighboring TOPs and the RC because of the presence of
the more restrictive thermal limitation of 500 MW.
e. This approach results in a situational awareness gap for significant reliability issues.
4. The Path SOL paradigm results in “Chasing the SOL” – a needless exercise in futility
a. Scenario - - a Thermal Path SOL of 500 MW is established for a sample Path. There are
no known associated stability issues in the area. Something changes on the system – a
transmission or generation Facility trips offline. Following the change on the system,
flows jump to 450 MW but stay below the previously established SOL of 500 MW. 25
minutes later the TOP determines a new SOL of 425 MW and communicates the new
SOL to the RC. Since the SOL has been exceeded already for 25 minutes, the RC will
immediately issue a directive to alleviate the SOL exceedance, up to and including the
shedding of firm load. All the while real-time tools show that the thermal postcontingency constraint (SOL) is not being exceeded. Since the Path SOL is considered an
SOL, TOP-004-2 R4 requires that SOLs be known and established or else the TOP is
operating in an “unknown operating state”. The 30-minute aspect to this requirement
could deem a condition as an “emergency” that truly is not an emergency. This false
“emergency” could drive inappropriate and costly operator action.
In summary, TOP engineers chase down the thermal Path SOL, and TOPs and the RC
operate to it despite information provided by real-time tools. Anytime anything of
significance changes on the system, a new thermal Path SOL needs to be recalculated.
This approach may be necessary to some degree for stability concerns, but it is
unnecessary for thermal and voltage concerns. Real-time tools and information can
address system performance for real-time operations.
b. Path SOLs have an underlying contingency, monitored element, and limiting condition
that describe the Path SOL. Adequate tracking and communication of the Path SOL and
all of the necessary underlying information for the Path SOL is an administrative burden
for TOPs and the RC, especially when an SOL is being “chased” as described above.
5. The Path SOL paradigm results in increased TOP compliance risk
a. The current Path SOL paradigm equates Transfer Capability to SOL. Many NERC
Reliability Standards contain requirements for SOLs, but there are no Reliability
Standards that mandate that Paths have SOLs. The RC’s SOL Methodology determines
what an SOL is and what an SOL is not. Quite simply, if Transfer Capability was not
considered to be an SOL, then there would be a significantly decreased compliance risk
for TOPs. Effectively a change of paradigm decreases TOP’s compliance exposure across
the board.
b. How much time does a TOP have to return within an SOL? If it is a TOP-007-WECC-1
Path, the time is clear (30 minute) yet it gives no flexibility to assess current system
conditions so we train our operators to get within the SOL in no more than 30 minutes.
If it is not a TOP-007-WECC-1 Path/SOL, then the answer is not so clear. Some TOPs
would say they have 30 minutes for any Path SOL. The RC currently will direct action to
alleviate the SOL exceedance within 30 minutes. Arguably the NERC standards don’t
require 30 minute mitigation for all SOL exceedances. There is no consistency on how
this is treated across the Western Interconnection.
c. The use of Path SOLs give TOPs a false sense of operating with acceptable system
performance. Without real-time tools such as Real-Time Contingency Analysis (RTCA),
it is almost impossible to really understand how the system is performing.
6. The Path SOL paradigm pre-supposes the need for monitoring WECC Paths for operating
reliability.
a. The Path SOL paradigm (and TOP-007-WECC-1) necessitates monitoring WECC paths in
operations, but if there are better ways of ensuring acceptable system performance in
operations, we should not pre-suppose that all WECC Paths need to be monitored in
operations to ensure reliability. Do we monitor WECC Paths because that’s what we’ve
always done? Do we monitor WECC Paths because they are scheduled against? Do we
monitor Paths because of an absence of adequate real-time tools and information? Do
we monitor Paths as a proxy for other reliability constraints? While some WECC Paths
may be critical for system monitoring, there may be some WECC Paths that may add
little to no value for real-time monitoring and reliability assessment, especially in light of
real-time tools and information.
b. The 2013 Path Catalog contains a list of 82 Paths, 18 Phase III projects, and 9 Phase II
projects. Are the 82 Paths listed in the Path Catalog the only Paths that have Path SOLs
associated with them? Beyond those Paths listed in the Path Catalog, are there any
other “Paths”? Which of these other Paths have SOLs and which do not? What makes a
path a “Path”? Who decides? When does a Path need an SOL? It is unclear what
criteria are used to determine the existence of a Path and whether a Path should have
an SOL for operations monitoring. This ambiguity results in confusion as it relates to
determining how operators should monitor the power system for acceptable system
performance.
7. The Path SOL concept is extraneous and duplicative in light of the revised SOL Methodology that
will be issued in August.
a. This 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.”
b. Note the inclusion of WECC Path Ratings and the phrase “Unless more restrictive
limitations are present”. These inclusions are necessary in light of the current Path SOL
paradigm and the TOP-007-WECC-1 Reliability Standard. Regardless, the SOL
Methodology clearly states that SOLs are Facility Ratings, voltage limits, voltage stability
limits, and transient stability limits, any of which can be the SOL at any point in time in
real-time operations.
c. The SOL Methodology contains specific criteria for acceptable system performance for
the pre- and post-contingency state. These criteria essentially require that the BES
remain within Facility Ratings, voltage limits, voltage stability limits, and transient
stability limits in the pre- and post-contingency state. Operating within these limits
ensures operational reliability. Operating within these limits AND within Path SOLs is
duplicative and unnecessary for reliability. Acceptable system performance for realtime operations (operational reliability) can be achieved by using the former without the
latter.
8. The Path Operators and the TOPs have limited ability to manage Path SOL exceedances
a. There are no automated tools to manage congestion in the Western Interconnection. It
is left to real-time operators to hash out what manual actions are going to happen to
relieve the SOL exceedance. In the “Path SOL” paradigm, we have an abundance of
these SOLs that are arguably unnecessary thus leading to more operator requirements
to manage the problems encountered.