Unscheduled Flow (USF) FAQ
(WECC) Why am I getting E-Tag curtailments from my Reliability
Coordinator during USF Events? When I call them they do not know
anything about it!
The NERC Electronic Tag specifications are very explicit about whom and what
type of entity can issue various tag “requests” such as creations, adjustments,
and curtailments.
The WECC USF Procedure allows the Load Serving Entity to make the decisions
regarding which transactions to curtail and how much to curtail each transaction.
Most Load Serving Entities are not considered for Electronic Tag purposes as
having the authority to issue “Curtailments” They can only request “adjustments”
to transactions. These adjustments are considered market based changes and if
they do not meet specific tagging timelines they are automatically denied.
Adjustments made to comply with the USF Procedure are considered as
reliability based curtailments and it is desired to treat the adjustments made via
the WebSAS application the same as any reliability based curtailment. To
facilitate this, the actual E-Tag request generated must come from an entity that
has tag “curtailment” capability. This could be any Transmission Provider, Control
Area, or Reliability Coordinator.
Using the Reliability Coordinator function was the best choice and the least
complicated method for the following reasons:
There are only 3 Reliability Centers so this minimizes the mapping required.
The curtailments associated with USF would be unique as Reliability
Coordinators in the WECC do not currently have any responsibility for actions
taken on any tag.
Curtailment issued through WebSAS will appear as though it is a curtailment
issued by one of the 3 reliability centers (but is not). It should be complied with
and implemented as any curtailment from any Control Area or Transmission
Provider.
Any Questions about these curtailments should be directed to the Sink Control
Area or the “Load Serving PSE”, not the reliability coordinators.
(USF)*What is Unscheduled Flow?
All energy transferred over an AC transmission system flows to some extent on
every available electrical transmission line between the source of the generation
and the end user. Yet in nearly all cases it is scheduled on only one transmission
path. The portion of energy that is flowing on these parallel paths where it is not
scheduled is Unscheduled Flow or USF. USF is also known as “loop flow,”
“parallel path flow,” or “inadvertent flow.”
(WECC)*How do I know when the USF Procedure is being used?
If you are a webSAS subscriber and you are running webSAS then there will be
an audible alarm whenever a USF Action is confirmed by a Path Operator.
If you are a webSAS subscriber and also subscribe to OATI's ETS application,
you can configure your alarm preferences to provide an audible ETS alarm
whenever there is a USF Action initiated by a Path Operator through ETS so you
do not have to have webSAS open all the time.
If you do not subscribe to webSAS you will receive notification by WECCNet
messaging system.
I heard webSAS is available on the Internet. What is the link to the
application?
webSAS is available by paid subscription only. To obtain information about a
subscription to webSAS contact Jerry Dempsey at OATI
(Jerry.Dempsey@oati.net).
Why doesn't the WECC use TLR like the Eastern Interconnection?
1) The WSCC Unscheduled Flow Mitigation Plan was developed and
implemented before the Eastern Interconnection developed its TLR
procedure. It is an FERC-filed and approved tariff. The TLR has only
schedule curtailment as a tool to mitigate USF, while the WSCC Plan has
phase shifting transformers (PSTs) as well as schedule curtailment. More
than 35 years of experience with schedule curtailments in the west has
proven them to be expensive and minimally effective. The use of the PSTs to
control USF has allowed us to avoid many hours of schedule curtailment over
the life of the Plan, using curtailment only as a last resort.
2) The physical nature of the EHV transmission system in the WECC is quite
different from that of the Eastern Interconnection. The transmission system in
the West is configured in a large “circular” network or “loop” with major
transmission located on the perimeter. This allows a relative handful of PSTs
to control flows on the loop. The Eastern Interconnection is much more web-
like in structure with no single dominant continuous EHV transmission path. It
would require many more PSTs to control USF in the Eastern Interconnection
to the extent that those in the west can control it.
What is webSAS?
webSAS (Security Analysis System) is a proprietary internet based application
that is used by WECC to analyze, initiate, communicate, and provide compliance
reports for implementation of the Unscheduled Flow Reduction Procedure. It is
available by subscription through the vendor to provide notification of USF
Events, calculate and display required relief, and provide a rapid method of
transaction curtailments.
Is my company required to subscribe to webSAS?
Subscription to webSAS is not required. However due to the large number of
transactions which are in progress at any one time, it is extremely difficult for
many entities to fully comply with the USF Mitigation procedure without some
software application that is linked to the electronic tagging system. Small
companies with limited transactions or infrequent impact on Qualified Paths or
companies that have another entity that agrees to assume responsibility for USF
compliance may receive little benefit from webSAS. Also some large companies
may choose to develop their own application to provide personnel the tools
needed to comply with the USF Procedure.
Why doesn't the Path Operator just curtail schedules until the energy flow
on the path is reduced enough?
All entities scheduling energy on the transmission system contribute to the USF
problem, so all should contribute to the solution. Requiring the Path Operator to
accommodate unlimited amounts of USF would be highly inequitable, shifting the
entire economic burden to those entities scheduling on the Qualified Path. The
Procedure requires the Path Operator to accommodate all USF on the Path
when its own schedules are not being curtailed and up to 5% of Path capability
when actual flow is pushing the limit. This requirement recognizes the fact that
unscheduled flow on an AC network is unavoidable and that everyone benefits
from interconnected operation. So some equitable, coordinated system-wide
procedure is needed to protect the integrity of the Interconnection during
conditions of excessive unscheduled flows. That is why the USF Reduction
Procedure has a significant reliability impact and compliance is included under
the WECC's Reliability Management System.
What is the Unscheduled Flow Reduction Procedure?
The Unscheduled Flow Reduction Procedure is the procedure adopted by the
WECC to relieve actual or potential overload conditions on specific transmission
paths which are susceptible to excessive unscheduled flow. It is applicable for
only seven Qualified Paths at the present time. It can be used only if the cause
of the loading problem is unscheduled flow in the direction for which the Path is
qualified. The Procedure consists of a combination of coordinated actions, which
include use of flow altering devices (Phase Shifting Transformers), schedule
curtailments by parties scheduling on the Qualified Path and ultimately schedule
curtailments by parties scheduling anywhere in the WECC that have an adverse
impact on the Path.
Who is responsible for taking actions per the Unscheduled Flow
Procedure?
All WECC Members have some role in compliance with the USF Procedure.
Path Operators are responsible for initiation of the Procedure and are
ultimately responsible for operation of their respective transmission paths.
The Rocky Mountain Desert Southwest Reliability Coordinator is responsible
for coordinated operation of the phase shifting transformers.
Any WECC Member that is identified as receiving energy (as the identified
LSE on a NERC Transaction Tag) is responsible for compliance with relief
obligations.
Aren’t Control Areas the only members responsible for actions under the
USF Reduction Procedure?
No. At one time Control Areas and a Member’s marketing services operated as
the same entity. Since the advent of De-regulation, Open Transmission Access
and other Electricity Restructuring, the situation has changed. Schedule
curtailment responsibility formerly belonged to what is now the Sink Control Area
(or Balancing Authority). Now it belongs to the Load-Serving Purchasing Selling
Entity. The PSE is in a position to know where its transactions originate, so can
determine the impact of its USF on the Qualified Paths, and it is the only entity
that knows its options for replacing the curtailed energy if necessary.
What is the difference between Unscheduled Flow and Loop Flow?
There is no difference. The term “loop flow” came into use because the Western
Interconnection used to be configured somewhat like a “donut” or loop, with the
northern Nevada, southern Idaho and western Utah area being the “hole in the
donut.” With the addition of transmission lines across this region over the past
twenty years, “loop” is no longer a good description. Other terms sometimes
used to describe the phenomenon are “parallel path flow” and “inadvertent flow.”
They all mean the same thing.
Where can I find a copy of the WECC USF Reduction Procedure and the
USF Mitigation Plan?
Both documents are on the WECC website on the Unscheduled Flow page. On
the WECC home page (www.wecc.biz), click the ”Oft-Requested Documents”
drop-down list and select the Unscheduled Flow link. Both documents are
located near the bottom of the first page, along with a USF Training Document.
I have webSAS. How do I know if I need to adjust a schedule?
Whenever the Path Operator issues a USF Procedure step 1-9, all webSAS
users will receive and audible alarm. The user will open webSAS to the USF
procedure report
What is a TDF?
TDF stands for Transmission Distribution Factor. It is a decimal number,
frequently displayed as a percentage, that indicates what portion of a transaction
between a source and sink will flow over a specific Qualified Path.
For example, for Qualified Path X if a transaction between source A and source
B has a TDF of .45 (45%), then for a 100 MW transaction between A and B, 45
MW will flow on Path X and 55 MW will be distributed among all other parallel
paths.
Why are all the times in Pacific Standard Time? Even during Daylight
Savings time? This is very confusing!
The original specification for the webSAS application stated that time calculations
would be performed in Pacific Standard Time so that a single consistent time was
used. Prior to webSAS all messages were sent out with time specified in Pacific
Prevailing Time. Other time zones had to convert to their local times. The
confusion caused by using Pacific Standard Time was not foreseen and a
change has been initiated to provide messages in Pacific Prevailing Time and to
allow users to display their own time zones on the webSAS application:
The conversion needed if you are not a webSAS subscribers implemented is as
follows:
EST=PST +3
EDT=PST +4
CST=PST +2
CDT=PST +3
MST=PST +1
MDT=PST +2
PDT=PST +1
If a USF Procedure is issued effective 0200 PST and you are currently on CDT
then your effective time is (0200 + 3) = 0500 CDT
Where can I find my Relief Requirement?
If you subscribe to webSAS, the USF Procedure Report. If not, the SUMMARY
OF CURTAILMENT ACTIONS table on page 12 of the Unscheduled Flow
Reduction Procedure, in combination with the TDF matrix for a Qualified Path will
allow you to calculate it. The matrix will tell you the percentage of your schedule
flowing on the Qualified Path. Enter the table with that percentage to see what
percentage reduction in your schedule you are expected to make. The amount
of relief provided will be the amount of curtailment times the TDF. This is the
relief you are expected to provide, whether you choose to curtail the subject
schedule, curtail another schedule by a greater amount, increase a counter
schedule, or use a Controllable Device to reduce the flow on the Qualified Path.
I have a schedule on a DC transmission line. Why does it still have a relief
requirement?
Remember that the energy must flow over the AC system from the generating
source to the DC converter station. Depending on the electrical distance
involved some amount of the transaction may flow over one or more of the
Qualified Paths. If it is enough to qualify for curtailment (i.e., 10% or more), it will
have a relief requirement.
Can I create a new transaction after a USF Event is declared?
Yes, if it has no contribution to USF in the prevailing direction on the Qualified
Path experiencing the event. You may create a transaction whose USF on the
Qualified Path is in the direction opposite the problem, thus unloading the Path.
Or you may implement a schedule on the Qualified Path in the direction opposite
the prevailing flow. But you cannot create a new transaction that contributes to
USF on the Qualified Path in the direction of the problem.
Can I change a transaction after a USF Event is declared?
What is a USF Action? How is it different from a USF Event?
I don't have any thing to do during a Step 1, 2, or 3. Why am I seeing these
messages?
There is a USF Event and I think I have transactions on the Qualified Path.
What should I do?
Be prepared for possible curtailments if the Path Operator must reduce
schedules on the Path to accommodate USF. You will not have to curtail the
transactions to reduce USF because their contribution to the Path flow is
scheduled flow, not unscheduled flow.
My Provided Relief is negative. What should I do?
When your Relief Provided is negative, this indicates you have increased a
contributing schedule and/or curtailed a counter schedule subsequently
increasing USF on the constrained path. If your Relief Provided is negative, you
will need to curtail a contributing schedule equal to or greater than the negative
relief provided.
What is an alternate action?
In the situation where an entities PST is being operated such that actual flow
equals scheduled flow (holding schedule), there will be zero unscheduled flow on
the constrained path and the schedule would be exempt from curtailment.
Another alternative action would be to increase internal generation that would
reduce USF equivalent to the entities relief requirement. A third alternative
option would be to increase a counter schedule reducing USF on the constrained
path equal to the relief required.
What are E-Tag Adjustments?
E-Tag adjustments are adjustments to E-tags outside of webSAS that cause an
increase or decrease in USF on the constrained path.
What are webSAS adjustments?
WebSAS adjustments are E-tag adjustments implemented in webSAS that
effectively increase or decrease USF on the constrained path.
What is the difference between "Accommodation" and "Contributing
Schedule" Curtailments
During those time periods when there is or it is anticipated that there will be a
scheduling limitation due to USF on a Qualified Transfer Path, the Transfer
Path Operator and those Members scheduling across the Qualified Transfer Path
are required to accommodate a minimum level of USF. Such USF
Accommodation will be achieved by ensuring that the Scheduled Flow across the
Qualified Transfer Path is reduced below the then available Transfer Limit by the
amounts specified in the USF Reduction Procedure.