ATC/TTC Basics Transmission Customer Forum Doug McLaughlin 9/22/06 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 Line Miles Substations Real-time Contingencies 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 ASCR on 75 0 F day = 1627 amps ASCR on 95 0 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 I nterrupters 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 This is Total Transfer Capability (TTC) This is the cable Rating. B A

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 Cable B is the Constraint or Limiting Element. B A 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 B Neighbor BlueNeighbor Green Generator Line Ratings (MVA) Load 100 Worst Contingency Flow of 100 MW (Offline)

Concept: TTC depends upon direction. 100 A C B Neighbor BlueNeighbor Green 100 Off Worst Contingency TTC A-B = ? ATC A-B = ? TTC B-A = ? ATC B-A = ? Assume C Off Constraint A-B Assume C Off What is the Contingency ? Constraint? TTC can be higher in cooler weather. Why? Constraint B-A Base flow of

Base flow of Loop flow TTC A-B = TTC A-B = w/Loop Flow Concept: Loop Flows impact TTC (Contract Path) 100 A C B Neighbor BlueNeighbor Green 100 Off B1 B2 L2 L1 Long way Can a deal from Blue to Green impact our TTC? What if our Loop Flow was 60? Point of Receipt (POR) = B2Point of Delivery (POD) = L = -10 Curtail needed! Redispatch locally or call a Transmission Loading Relief (TLR) 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 B 1716 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: Miles Estimated Total Time: 3 hours, 59 minutes Start: Cole Ln, Atlanta, GA End: Brief Rd, Charlotte, NC Total Distance: 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 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?