TTC/ATC Computations and Ancillary Services in the Indian context

1. TTC/ATC Computations and Ancillary Services in the Indian context

2. Outline • Part A: TTC/ATC computations • Transfer capability-Definitions • Relevance of transfer capability in Indian electricity market • Difference between Transfer capability and Transmission Capacity • Assessment of TTC/TRM/ATC • Method for improving Transfer capability • Concerns • Part B: Ancillary services in the Indian context

3. Part ATotal Transfer Capability (TTC)/ Available Transfer Capability (ATC) computations

4. Transfer Capability - Definitions

5. North American Electric Reliability Corporation’s (NERC) definition of TTC • The amount of electric power that can be moved or transferred reliably from one area to another area of the interconnected transmission systems by way of all transmission lines (or paths) between those areas under specified system conditions……….16-Mar-2007(FERC) • As per 1995 document of NERC, following conditions need to be satisfied: • all facility loadings in pre-contingency are within normal ratings and all voltages are within normal limits • systems stable and capable of absorbing the dynamic power swings • before any post-contingency operator-initiated system adjustments are implemented, all transmission facility loadings are within emergency ratings and all voltages are within emergency limits”

6. Total Transfer Capability as defined in the IEGC and Congestion charge Regulations “Total Transfer Capability (TTC)” means the amount of electric power that can be transferred reliably over the inter-control area transmission system under a given set of operating conditions considering the effect of occurrence of the worst credible contingency. “Credible contingency” means the likely-to-happen contingency, which would affect the Total Transfer Capability of the inter-control area transmission system Outage of single transmission element (N-1) in the transmission corridor or connected system whose TTC is being determined Outage of the largest unit in the importing control area

7. Available Transfer Capability as defined in the IEGC and Congestion charge regulations “Available Transfer Capability (ATC)” means the transfer capability of the inter-control area transmission system available for scheduling commercial transactions (through long term access, medium term open access and short term open access) in a specific direction, taking into account the network security. Mathematically ATC is the Total Transfer Capability less Transmission Reliability Margin.

8. Non Simultaneous & Simultaneous transfer Capability Non-simultaneous Transfer Capability Amount of electric power that can be reliably transferred between two areas of the interconnected electric system when other concurrent normal base power transfers are held constant Determined by simulating transfers from one area to another independently and non-concurrently with other area transfers. Simultaneous Transfer Capability Is the amount of electric power that can be reliably transferred between two or more areas of the interconnected electric system as a function of one or more other power transfers concurrently in effect.” Reflects simultaneous or multiple transfers with interdependency of transfers among the other areas is taken into account. No simple relationship exists between non-simultaneous and simultaneous transfer capabilities The simultaneous transfer capability MAY be lower than the sum of the individual non-simultaneous transfer capabilities. Simultaneous TTC declared by NR, SR, NER Simutanous TTC can be declared for 2 regions combined also( e.g ER/NER)

9. Simultaneous TTC Area A Area B 2000 MW 4000 MW Area C 5000 MW

10. TTC affected by transactions

11. Simultaenous TTC limits to two regions • January’11 TTC figures • N-1 contingency of 400KV FSTPP-Malda • FSTPP-KHSTPP D/C limitation during outage of 400Kv Malda-Purnea D/C • High voltages along Northern corridor • As 400KV FSTPP-Malda serves both NR & ER in case of increase in TTC of ER-NER ER-NR TTC has to be decreased • Thus we could declare a simultaneous TTC of ER-NR & ER-NER combined

12. Simultaneous TTC limits to two regions • February,2011 limits • NER TTC INCREASED DUE TO INCREASED GOI ALLOCATION & REQUIREMENT OF NER(ASSAM) • NR TTC CORRESPONDINGLY DECREASED ER-NR

13. Simultaneous TTC limits to two regions • June, 2011 limits • ER TTC REDUCED DUE TO LOW VOLTAGE IN CHENNAI AREA & HIGH LOADING ON 400 KV VIJAYAWADA – NELLORE I & II. • July, 2011 limits • WR TTC REDUCED TO 800 MW DUE TO COMMISSIONING OF SIMHADRI U # 3.

14. Relevance of Transfer Capability inIndian Electricity Market

15. Open Access in Inter-state Transmission Regulations, 2008 3( 2) The short-term open access allowed after long / medium term by virtue of- (a) inherent design margins; (b) margins available due to variation in power flows; and (c) Margins available due to in-built spare transmission capacity created to cater to future load growth or generation addition.]

16. LT/MT/ Connectivity procedures-2010 ATC checking  MTOA approvals:: • CTU(nodal agency) shall notify TTC on 31st day of March of each year: for 4 (four) years • Revision by CTU due to change in anticipated network topology or change of anticipated generation or load at any of the nodes • Available Transfer Capability (ATC) for MTOA will be worked out after allowing the already approved applications for Long-term access, Medium Term Open Access and Transmission reliability margin • Grant of MTOA shall be subject to ATC ATC checking  LTA approvals • CTU(nodal agency) shall carry out system studies in ISTS to examine the adequacy of the transmission system corresponding to the time frame of commencement of long-term access to effect the desired transaction of power on long-term basis, using the Available Transfer Capability (ATC). • If transmission system augmentation is required LTA would be granted subject to such augmentation • Revision by CTU due to change in anticipated network topology or change of anticipated generation or load at any of the nodes

17. Tariff Policy Jan 2006 7.3 Other issues in transmission (2) All available information should be shared with the intending users by the CTU/STU and the load dispatch centres, particularly information on available transmission capacity and load flow studies.

18. Open Access Theory & PracticeForum of Regulators report, Nov-08 “For successful implementation of OA, the assessment of available transfer capability (ATC) is very important. A pessimistic approach in assessing the ATC will lead to under utilisation of the transmission system. Similarly, over assessment of ATC will place the grid security in danger.”

19. Reliability Margin

20. NERC definition of Reliability Margin (RM) Transmission Reliability Margin (TRM) The amount of transmission transfer capability necessary to provide reasonable assurance that the interconnected transmission network will be secure. TRM accounts for the inherent uncertainty in system conditions and the need for operating flexibility to ensure reliable system operation as system conditions change. Capacity Benefit Margin (CBM) The amount of firm transmission transfer capability preserved by the transmission provider for Load-Serving Entities (LSEs), whose loads are located on that Transmission Service Provider’s system, to enable access by the LSEs to generation from interconnected systems to meet generation reliability requirements. Preservation of CBM for an LSE allows that entity to reduce its installed generating capacity below that which may otherwise have been necessary without interconnections to meet its generation reliability requirements. The transmission transfer capability preserved as CBM is intended to be used by the LSE only in times of emergency generation deficiencies.

21. Quote on Reliability Margin from NERC document “The beneficiary of this margin is the “larger community” with no single, identifiable group of users as the beneficiary.” “The benefits of reliability margin extend over a large geographical area.” “They are the result of uncertainties that cannot reasonably be mitigated unilaterally by a single Regional entity”

22. Reliability margin as defined in Congestion charge regulations • “Transmission Reliability Margin (TRM)” means the amount of margin kept in the total transfer capability necessary to ensure that the interconnected transmission network is secure under a reasonable range of uncertainties in system conditions;

23. Reliability Margins- Inference Grid Operators’ perspective Reliability of the integrated system Cushion for dynamic changes in real time Operational flexibility Consumers’ perspective Continuity of supply Common transmission reserve to take care of contingencies Available for use by all the transmission users in real time Legitimacy of RMs well documented in literature Reliability Margins are non-negotiable

24. Difference between Transfer Capability and Transmission Capacity

25. Area Despatch- Example of TTC Area A Area B 515 MW 750 MW 515 MW 630 MVA

26. Transfer capability & Transmission capacity – what’s the difference? Transfer capacity Refers to thermal ratings Transfer capability Refers to the system’s capability of transfer-varies considerably with system conditions Can not be arithmetically added for the individual line capacities and ratings Always less than the aggregated transmission interface between two areas 1015 MW 750 MW 630 MVA TTC = 630 MVA

27. TTC is directional Area A Area B 500 MW Gen 1000 MW 500 MW 1000 MW 500 MW Transfer Capability from Area B to Area A = 500MW Transfer Capability from Area A to Area B = 1500MW

28. Transmission Capacity Vis-à-vis Transfer Capability

29. Transfer Capability is less than transmission capacity because • Power flow is determined by location of injection, drawal and the impedance between them • Transfer Capability is dependent on • Network topology • Location of generator and its dispatch • Pont of connection of the customer and the quantum of demand • Other transactions through the area • Parallel flow in the network • Transmission Capacity is independent of all of the above • When electric power is transferred between two areas the entire network responds to the transaction

30. 77% of electric power transfers from Area A to Area F will flow on the transmission path between Area A & Area C Assume that in the initial condition, the power flow from Area A to Area C is 160 MW on account of a generation dispatch and the location of customer demand on the modeled network. When a 500 MW transfer is scheduled from Area A to Area F, an additional 385 MW (77% of 500 MW) flows on the transmission path from Area A to Area C, resulting in a 545 MW power flow from Area A to Area C.

31. Assessment of Transfer Capability

32. Transfer Capability Calculations must Courtesy: Transmission Transfer Capability Task Force, "Available Transfer Capability Definitions and Determination", North American Electric Reliability Council, Princeton, New Jersey, June 1996 NERC Give a reasonable and dependable indication of transfer capabilities, Recognize time variant conditions, simultaneous transfers, and parallel flows Recognize the dependence on points of injection/extraction Reflect regional coordination to include the interconnected network. Conform to reliability criteria and guides. Accommodate reasonable uncertainties in system conditions and provide flexibility.

33. Operating Limits Thermal Limit • Maximum electrical current that a transmission line or electrical facility can conduct over specified time periods before it sustains permanent damage by overheating or before it violates public safety requirements. • Source CBIP Technical Report Voltage limit • To be maintained as per IEGC • Minimum voltage limits can establish the maximum amount of electric power that can be transferred without causing damage to the electric system or customer facilities • Widespread collapse of system voltage can result in a black out of portions or the entire interconnected network • Critical voltage for these nodes may also be different. Thus the proximity of each node to the voltage collapse point may be different(VCPI Index) • 0 < VCPI < 1 0  stability 1  instability • Voltage collapse  credible event

34. Total Transfer Capability: TTC Voltage Limit Thermal Limit Power Flow Stability Limit Total Transfer Capability Time Total Transfer Capability is the minimum of the Thermal Limit, Voltage Limit and the Stability Limit

35. Intra-day STOA Day-ahead STOA Collective (PX) STOA First Come First Served STOA Advance Short Term Open Access (STOA) TTC ATC Medium Term Open Access (MTOA) Long Term Access (LTA) Reliability Margin (RM) RM Available Transfer Capability is Total Transfer Capability less Reliability Margin

36. Input Data and Source

37. Thermal limit derived from ampacity

38. St.Clair’s curve Line loading in terms of SIL of an uncompensated line as a function of Length assuming voltage regulation of 5% and 30 deg angular difference

39. TTC/ATC calculation methodology-As per congestion charge procedures • Total Transfer Capability between two areas would be assessed by increasing the load in the importing area and increasing the generation in the exporting area or vice versa till the constraints are hit for a credible contingency Credible contingencies • Outage of single transmission element (N-1) in the transmission corridor or connected system whose TTC is being determined as defined in IEGC • Outage of a largest unit in the importing control area Station. TTC is limited by:: • Violation of grid voltage operating range OR • Violation of transmission element operating limit in the base case OR • Violation of emergency limit in the contingency case

40. Credible contingencies From Section 3.5 of IEGC Outage of a 132 kV D/C line or Outage of a 220 kV D/C line or Outage of a 400 kV S/C line or Outage of a single ICT or Outage of one pole of HVDC bi pole or Outage of 765 kV S/C line without necessitating load shedding or rescheduling of generation during steady state operation

41. TTC calculation process Considerations for calculation of TRM – as per congestion procedures • Two percent (2%) of the total anticipated peak demand met in MW of the control area/group of control area/region (to account for forecasting uncertainties) • Size of largest generating unit in the control area/ group of control area/region • Single largest anticipated in feed into the control area/ group of control area Data flow TTC calculation SLDC  RLDC  Import/Export TTC of control areas NLDC  RLDC  Fixation of inter-regional export RLDC  NLDC  Preparation of converged base case NLDC  RLDC  Stitching of base cases from all regions & re-consideration for modification if any RLDC  NLDC  Final Base Case NLDC  Final TTCs uploaded to NLDC wesbite & linked from RLDC websites(upto 3 months advance) • NLDC can revise the TTC/ATCs on requests by RLDCs/SLDCs/suo-motto • The TTC/RM/LTA/approved STOA(till date) & path margin available are declared alongwith the rationale/path limiting the TTC

42. Procedure for declaration of TTC, TRM, ATC and anticipated Constraints Role of SLDC Assess the TTC, TRM and ATC on its inter-State transmission corridor, considering its own control area Indicate details of anticipated transmission constraints in the intra State system Forward these figures along with the assumptions made, to the respective RLDC, for assessment of TTC at the regional level

43. Role of RLDCs Consider the inputs provided by SLDCs Assess TTC, TRM and ATC for intra regional corridors (group of control areas) individual control areas within the region (if required) Inter-regional corridors at respective ends for a period of three months in advance. Forward the results along with the input data considered, to NLDC Also indicate the anticipated constraints in the intra-regional transmission system Procedure for declaration of TTC, TRM, ATC and anticipated Constraints

44. Procedure for declaration of TTC, TRM, ATC and anticipated Constraints Role of NLDC Assess the TTC, TRM and ATC of inter and intra-regional links/ Corridors respectively for three months in advance based on The inputs received from RLDCs TTC/ TRM/ ATC notified/ considered by CTU for medium-term open access. Inform the TTC/ TRM/ ATC figures along with constraints observed in inter-regional/ intra-regional corridors to the RLDCs

45. Procedure for declaration of TTC, TRM, ATC and anticipated Constraints Role of NLDC (contd) Revise the TTC, TRM and ATC due to change in system conditions (including commissioning of new transmission lines/ generation), vis-à-vis earlier anticipated system conditions Revise TTC, TRM and, ATC based on its own observations or based on inputs received from SLDCs/ RLDCs

46. Transfer Capability assessment Planning criteria Credible contingencies Trans. Plan + approv. S/D Anticipated Network topology + Capacity additions LGBR Last Year Reports Weather Forecast Simulation Analysis Brainstorming Total Transfer Capability Anticipated Substation Load less Anticipated Ex bus Thermal Generation Reliability Margin equals Available Transfer Capability Anticipated Ex bus Hydro generation Last Year pattern Operating limits Operator experience Planning Criteria is strictly followed during simulations

47. Sample TTC uploaded at NLDC website

50. NORTHERN REGION NORTH-EASTERN REGION WESTERNREGION SOUTHERN REGION 4 2 8 16 EASTERN REGION 4