ATC/TTC Basics

1. ATC/TTC Basics Transmission Customer Forum Doug McLaughlin 9/22/06

2. 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

3. 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

4. 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 Facility Ratings Rating is higher in cooler weather. Why?

5. Physical Design & condition Ambient (weather) conditions ROW conditions Ratings Considerations Ratings change constantly due to weather and ROW conditions.

6. Potential Equipment Limits (limiting elements) Busses Conductors Hardware Jumpers Transformers Lines Fuses Interrupters Bushings Relaying Switches CTs Breakers

7. Unusual ROW Conditions and Faults Ratings vary based upon ROW conditions

8. 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?

9. B 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.This is the cable Rating. A 0.Due to the N-1Contingency loss of Cable A. 1000.This is Total Transfer Capability (TTC). 1000 – 4(200) = 200. This is Available Transfer Capability (ATC).

10. 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? • What is the ATC? • What is the risk if a fifth 200 lb passenger rides? • What should the operator do? Contingency 900.Cable B is the Constraint or Limiting Element. A 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. Take a Pre-Contingency Action to reduce the load. A Post-Contingency Action could be used to reduce the load.

11. Worst Contingency Flow of 100 MW Very Simple Circuit Example Neighbor Blue Neighbor Green A B 200 100 Line Ratings (MVA) 150 200 Generator 100 100 (Offline) Load C • 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.).

12. Worst Contingency 200 200 Base flow of 100 100 Constraint A-B Constraint B-A Concept: TTC depends upon direction. Neighbor Blue Neighbor Green TTC can be higher in cooler weather. Why? What is the Contingency? Constraint? A B 200 100 150 200 100 Off C Assume C Off Assume C Off TTCA-B= ? ATCA-B= ? 50 TTCB-A= ? ATCB-A= ? 200 50 200

13. Loop Flow = 40 Base flow of 100+ 40 Loop flow Concept: Loop Flows impact TTC (Contract Path) Point of Receipt (POR) = B2 Point of Delivery (POD) = L2 Neighbor Blue Neighbor Green Contract path B2 L2 B1 Long way L1 A B 200 100 What if our Loop Flow was 60? Can a deal from Blue to Green impact our TTC? 150 200 100 Off C TTCA-B= TTCA-B=w/Loop Flow 50 150 - 100 -60 = -10 Curtail needed! Redispatch locally or call a Transmission Loading Relief (TLR). 10

14. 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) TTC and Power Flows are impacted by:

15. 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?

16. Max Simultaneous 2834 A to B with 184 C to B Max C to B1716 MW 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.

17. 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 Firm versus Non-firm ATC CBM and TRM are Zero for exports All TTC offered firm or non-firm Firm and Non-firm commitments. No TRM or CBM

18. 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 How TTC differs from Transmission Service How far is it from Atlanta to Charlotte? • 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)

19. 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) requirespecific 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. TSR Studies 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.

20. 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 If you don’t remember anything else…

21. Questions?