Transcript ATC

ATC/TTC Basics
Transmission Customer Forum
Doug McLaughlin
9/22/06
Goal
• Convey the key concepts of Transfer Analysis.
(Ratings, TTC, Simultaneous TTC, Firm and non-firm ATC, TSRs, etc. )
Topics
• Facility Ratings and Limits
• Total Transfer Capability (TTC) Analysis
• Firm versus Non-firm ATC
• Transmission Service
What is the purpose of the Transmission System?
To safely, reliably, and economically deliver power from generators to customer loads.
Real-time Contingencies
2800+
Lines
400+
Generators
800+
Transformers
13 Million+ Possibilities
29000+ Line Miles
3000+ Substations
Facility Ratings
• The rating is the physical capability of a line or other equipment for a
particular set of conditions, allowing for safe operation.
– Usually in the context of thermal amp limits.
– Voltage and stability constraints may sometimes be more limiting.
1351.5 ASCR on 750 F day = 1627 amps
1351.5 ASCR on 950 F day = 1512 amps
Minimum NESC
Clearance
Rating is
higher in
cooler
weather.
Why?
Ratings Considerations
• Physical Design & condition
• Ambient (weather) conditions
• ROW conditions
Ratings change
constantly due to
weather and ROW
conditions.
Potential Equipment Limits (limiting elements)
Busses
Conductors
Hardware
Jumpers
Lines
Transformers
Fuses
Interrupters
Bushings
Relaying
Switches
CTs
Breakers
Unusual ROW Conditions and Faults
Ratings vary based
upon ROW conditions
Transfer Capability
• Ability to Safely and Reliably move power from one area to another.
– Must be able to survive the loss of the next most critical element.
(Power keeps flowing under N-1 criteria minimum.)
– Total Transfer Capability (TTC) is used to estimate future transfer
capability. (based upon a study of specific system conditions.)
– Not the same as Ratings! Why not?
What is the
minimum number
of paths to have
transfer capability?
Transfer Capability
Think of an elevator with a single 1000 lb cable.
– How much load can it carry?
1000.
This is the cable Rating.
Contingency
– What would be its reliable Transfer Capability?
0.
Due to the N-1 Contingency loss of Cable A.
– Add a second 1000 lb cable. Now what is the
Transfer Capability?
1000.
This is Total Transfer Capability (TTC).
– Now add (4) 200 lb passengers to the elevator. How
much Transfer Capability remains?
1000 – 4(200) = 200.
This is Available Transfer Capability (ATC).
A
B
Transfer Capability
What if Cable B had a 10
minute emergency rating
of 1000 lbs.
Now let’s change some assumptions.
– What if Cable B is only rated for 900 lbs? What is the
TTC?
Contingency
900.
Cable B is the Constraint or Limiting Element.
– What is the ATC?
900 – 4(200) = 100.
– What is the risk if a fifth 200 lb passenger rides?
If Cable A fails, Cable B would fail also.
This would be a System Operating Limit (SOL)
violation and could lead to a Cascading Event.
– What should the operator do?
Take a Pre-Contingency Action to reduce the load.
A Post-Contingency Action could be used to reduce
the load.
A
B
Very Simple Circuit Example
Neighbor Blue
Neighbor Green
A
100
Worst Contingency
200
150
Generator
Flow of
100 MW
Load
B
Line Ratings
(MVA) 200
100
C
100
(Offline)
•Home Area connects with two neighboring systems, Blue and Green.
•Power flows split evenly. Ignore impedance, losses, spinning reserves,
etc. We’ll focus on thermal limits (ignore voltage, stability, etc.).
Concept: TTC depends upon direction.
Neighbor Blue
Neighbor Green
TTC can be
higher in
cooler
weather.
Why?
What is the
Contingency
? Constraint?
Worst Contingency
200
A
B
200
100
150 Base flow 200
Constraint A-B
100
of 100
100
Assume C Off
200
Constraint B-A
Off
C
Assume C Off
TTCA-B= ?
50
TTCB-A= ? 200
ATCA-B= ?
50
ATCB-A= ? 200
Concept: Loop Flows impact TTC
(Contract Path)
Point of Receipt (POR) = B2
Point of Delivery (POD) = L2
Neighbor Blue
L2
B2
B1
Long
way
Neighbor Green
Contract path
L1
100
Can a deal
from Blue to
Green impact
our TTC?
Loop Flow = 40
A
200
Base flow
of 100
+ 40 Loop
flow
TTCA-B=
50
TTCA-B=
10
150
B
200
100
Off
What if our
Loop Flow
was 60?
C
150 - 100 -60 = -10
Curtail needed!
w/Loop Flow Redispatch locally or call a
Transmission Loading Relief (TLR).
TTC and Power Flows are impacted by:
•
•
•
•
•
•
Loads (weather is a major driver)
Outages (both transmission and generation)
Generation Dispatch (fuel volatility is a major driver)
Loop Flows (activities in other markets)
Counter Flows
Imports and Exports from other areas
(simultaneous transfers)
Simultaneous Transfer Capability
What would you post as
the number of cars that
can get to downtown
Atlanta per hour next
February?
It depends, doesn’t it?
•Do we balance the flow North, South,
East, West? Or assume more demand in
some directions than others?
•What’s the weather going to be?
•Will there be any maintenance or
construction?
•Will there be any accidents?
•Week day or Weekend? Peak or nights?
Guidance from the NERC TTC Supplement
Area A
Area C
Area B
Which value should
you choose?
The best choice for TTC
postings would be to use
a transfer ratio that
approximates the future
usage of the interfaces.
Max Simultaneous
2834 A to B with
184 C to B
Max C to B
1716 MW
Firm versus Non-firm ATC
Firm ATC = TTC – CBM – TRM – Firm Service Commitments
CBM and TRM are
Zero for exports
Capacity Benefit Margin (CBM)
– Import Transfer Capacity reserved by load serving entities to ensure access to
generation during emergencies to meet generation reliability requirements.
Transmission Reliability Margin (TRM)
– Transfer Capacity reserved by the Transmission Provider to ensure system
security under a reasonable range of uncertainties in system conditions.
Availability of CBM / TRM
– Both CBM and TRM are made available to all eligible transmission customers on
a non-firm basis.
All TTC offered firm or
non-firm
Non-Firm ATC = TTC – All Service Commitments + Postbacks
Postbacks
– Committed service that is not scheduled
– Redirect service
Firm and Non-firm
commitments. No TRM
or CBM
How TTC differs from Transmission Service
Start: Atlanta, Georgia, United States
End: Charlotte, North Carolina, US
How far is it from
Atlanta to Charlotte?
Total Distance: 245.9 Miles
Estimated Total Time: 3 hours, 59 minutes
Start: Cole Ln, Atlanta, GA 30349
End: Brief Rd, Charlotte, NC 28227
Total Distance: 283.6 Miles
Estimated Total Time: 4 hours, 47 minutes
What’s the difference between TTC and Transmission Service?
•TTC is an estimate of transfer capacity based upon general conditions.
•Transmission Service Requests (TSRs) are for specific transmission service.
(specific time, Source, Sink, amount)
TSR Studies
• Many requests (TSRs) can be approved
by inspection without studies.
– ATC is high relative to request.
– Local area known to be capable of
meeting request.
– If Confirmed, decrement ATC and
consider in future models.
• Some requests (TSRs) require
specific studies.
–
–
–
–
ATC is low relative to request.
Unknown impacts in the local area.
Interacts with other requests.
The specific transfer is added to the
latest model and analyzed.
– If Confirmed, new TTC calculated and
posted.
Needs
Study
100
400
By Inspection
Long Term Requests (1 year or longer)
•All requests are studied. Service will not be turned down if a project can be built.
•Projects paid by Transmission Provider not customer. Cost recovery through tariff.
If you don’t remember anything else…
• Total Transfer Capability is a “Snapshot” estimate of the system’s
capability to move power from one area to another based upon
many unknowns in our area and surrounding areas. Unknowns
include:
– Weather
– Loads
– Generation Dispatch
– Market Utilization of Transmission
– Specific Transmission Service Desired
– Loop flows
– Outages
– ROW Conditions
– Time of use
Questions?