ATC/TTC BasicsATC/TTC Basics

Transmission Customer Forum

Doug McLaughlin9/22/06

Transmission Customer Forum

Doug McLaughlin9/22/06

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.

29000+ Line Miles

3000+ Substations

Real-time Contingencies

2800+ Lines

400+ Generators

800+ Transformers

13 Million+ Possibilities

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

Jumpers

Breakers

Transformers

CTs

Bushings

Conductors

Fuses

Interrupters

Relaying

Hardware

Lines

Switches

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?

– What would be its reliable Transfer Capability?

– Add a second 1000 lb cable. Now what is the Transfer Capability?

– Now add (4) 200 lb passengers to the elevator. How much Transfer Capability remains?

Contingency

1000 – 4(200) = 200. This is Available Transfer Capability (ATC).

0.Due to the N-1 Contingency loss of Cable A.

1000.This is Total Transfer Capability (TTC).

1000.This is the cable Rating.

BA

Transfer Capability

Now let’s change some assumptions.– What if Cable B is only rated for 900 lbs? What is the

TTC?

– What is the ATC?

– What is the risk if a fifth 200 lb passenger rides?

– What should the operator do?

Contingency

Take a Pre-Contingency Action to reduce the load.

900 – 4(200) = 100.

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.

900.Cable B is the Constraint or Limiting Element.

BA

What if Cable B had a 10 minute emergency rating

of 1000 lbs.

A Post-Contingency Action could be used to reduce the load.

Very Simple Circuit Example

•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.).

100

A

C

B200

150200

Neighbor Blue Neighbor Green

Generator

Line Ratings (MVA)

Load100 100

Worst Contingency

Flow of 100 MW

(Offline)

Concept: TTC depends upon direction.

100

A

C

B200

150 200

Neighbor Blue Neighbor Green

100 Off

Worst Contingency

TTCA-B= ?

ATCA-B= ?

TTCB-A= ?

ATCB-A= ?

Assume C Off

Constraint A-B

50

50 200

200

Assume C Off

What is the Contingency? Constraint?

TTC can be higher in

cooler weather.

Why?

Constraint B-A

Base flow of 100

200

200

100

Base flow of 100+ 40 Loop flow

TTCA-B=

TTCA-B= w/Loop Flow

Concept: Loop Flows impact TTC (Contract Path)

100

A

C

B200

150200

Neighbor Blue Neighbor Green

100 Off

B1B2

L2L1

Long way

Can a deal from Blue to

Green impact our TTC?

What if our Loop Flow was 60?

Point of Receipt (POR) = B2 Point of Delivery (POD) = L2

150 - 100 -60 = -10 Curtail needed! Redispatch locally or call a Transmission Loading Relief (TLR).

10

50

Contract path

Loop Flow = 40

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

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?

What would you post as the number of cars that

can get to downtown Atlanta per hour next

February?

Guidance from the NERC TTC Supplement

Max Simultaneous 2834 A to B with 184 C to B

Max C to B1716 MW

The best choice for TTC postings would be to use

a transfer ratio that approximates the future usage of the interfaces.

Which value should you choose?

Area C

Area B

Area A

Firm versus Non-firm ATC

Firm ATC = TTC – CBM – TRM – Firm Service Commitments

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.

Non-Firm ATC = TTC – All Service Commitments + Postbacks

Postbacks– Committed service that is not scheduled– Redirect service

CBM and TRM are

Zero for exports

Firm and Non-firm

commitments. No TRM

or CBM

All TTC offered firm

or non-firm

How TTC differs from Transmission Service

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)

How far is it from Atlanta to Charlotte?

Start: Atlanta, Georgia, United States

End: Charlotte, North Carolina, US

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

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.

400

100

By Inspection

Needs Study

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?