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. European Network of Transmission System Operators’ definition of Total Transfer Capability (TTC) “TTC is that maximum exchange programme between two areas compatible with operational security standards’ applicable at each system if future network conditions, generation and load patterns were perfectly known in advance.” “TTC value may vary (i.e. increase or decrease) when approaching the time of programme execution as a result of a more accurate knowledge of generating unit schedules, load pattern, network topology and tie-line availability”

7. 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

8. 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.

9. 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)

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

11. TTC affected by transactions

12. 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 ER-NR

13. 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

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. Declaration of Security Limits “In order to prevent the violation of security limits, System Operator SO must define the limits on commercially available transfer capacity between zones.” CIGRE_WG_5.04_TB_301 “System Operators try to avoid such unforeseen congestion by carefully assessing the commercially available capacities and reliability margins.” CIGRE_WG_5.04_TB_301

20. Reliability Margin

21. 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.

22. 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”

23. ENTSOE definition of Reliability Margin • “Transmission Reliability Margin TRM is a security margin that copes with uncertainties on the computed TTC values arising from • Unintended deviations of physical flows during operation due to physical functioning of load-frequency regulation • Emergency exchanges between TSOs to cope with unexpected unbalanced situations in real time • Inaccuracies in data collections and measurements”

24. 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;

25. Distinguishing features of Indian grid Haulage of power over long distances Resource inadequacy leading to high uncertainty in adhering to maintenance schedules Pressure to meet demand even in the face of acute shortages and freedom to deviate from the drawal schedules. A statutorily permitted floating frequency band of 49.5 to 50.2 Hz Non-enforcement of mandated primary response, absence of secondary response by design and inadequate tertiary response. No explicit ancillary services market Inadequate safety net and defense mechanism

26. 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

27. Difference between Transfer Capability and Transmission Capacity

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

29. 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

30. 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

31. Transmission Capacity Vis-à-vis Transfer Capability

32. 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

33. 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.

36. Assessment of Transfer Capability

37. 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.

38. Europe • Increase generation in one area and lower it in the other. • A part of cross border capacity is withdrawn from the market to account for • Random threats to the security of the grid, such as loss of a generating unit. This capacity is called as Transmission Reliability Margin (TRM) • TRM based on the size of the biggest unit in the synchronous area and the domestic generation peak of a control area. • Net Transfer Capacity = TTC – TRM • published twice a year (winter and summer)

39. United States • The commercial capacity available for market players is calculated by deducting Transmission Reliability Margin (TRM) and Capacity Benefit Margin (CBM) from Total Transfer Capability • TRM is set aside to ensure secure operation of the interconnected transmission network to accommodate uncertainties in system operations while CBM is set aside to ensure access to generation from interconnected systems to meet generation reliability requirements.

40. 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

42. Operating Limits Stability Limits • property of a power system that enables it to remain in a state of operating equilibrium under normal operating conditions and to regain an acceptable state of equilibrium after being subjected to a disturbance(small or large) • All generators connected to ac interconnected transmission system operate in synchronism. Immediately following a system disturbance, generators begin to oscillate relative to each other,causing fluctuations in system frequency, line loadings, and system voltages. • oscillations must diminish as the electric systems attain a new, stable operating point. • If stable point is not quickly established, the generators will likely lose synchronism & result of generator instability may damage equipment and lead to widespread loadsheddings

43. 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

44. 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

45. Input Data and Source

46. Model to be considered for simulation studies • Assumption of standard data from CEA manual on Transmission planning criteria • Separate base cases for calculating the export and import capability corresponding to both peak and off- peak load and generation with the likely scenario • Windgeneration also needs to be modelled  forecasts • Reactive capability of units  Actual generator capability curve / CEA manual in Transmission planning • Nodal MW demand  forecast by SLDCs/LGBR of RPCs/past trend • Nodal MVAR demand  SLDC forecast OR CEA Planning criteria : • Normal operating limits for transmission line: CEA planning criteria (detailed calculation methodology  ) • Emergency limit for transmission line  110% of normal operating limit • Continuous Operating limit for ICTs  generally 90% of MCR • Reasonable assumptions in case data NA

47. Data preparation for LF studies • Where actual system is not available data from CEA Manual on transmission planning criteria can be used w.r.t parameters as: • Load Power factor 0.85 lag – peak 0.90 lag – Off-peak OR 0.75 lag-peak 0.85 lag – off-peak [Agricultural] • Reactive limits Qmax = 0.5* Active generation Qmin = -0.5 *Qmax • transformer/ reactance – 14-15% • GT – 12.5% • Where actual system data is not available: • Standard R, X, B parameters(p.u/km/ckt) at 100MVA base may be used • Other typical system data may be used

48. Ampacity

49. Thermal limit derived from ampacity

50. Permissible Line Loading Limits From Sec 4.1 of Transmission Planning Criteria SIL at certain voltage levels modified to account for Shunt compensation k1 = sqrt (1- degree of shunt compensation) Series compensation k2 = 1 / [sqrt (1-degree of series compensation) Variation in line loadability with line length(St.Clair’s curve) K3 Permissible line loading = SIL X k1 x k2 x k3 From Sec 4.2 of Transmission Planning Criteria Thermal loading limits at conductor temperature of 75o Ambient 40o in summer and 10o in winter