Transfer Capacity & Congestion Concepts

1. Transfer Capacity & Congestion Concepts Abhimanyu Gartia WRLDC

2. Learning Objectives • What is Transfer Capability? • How is transfer capability different from Transmission Capacity? • Why is transfer capability less than transmission capacity? • How can transfer capability be increased? • What is Reliability Margin ? • How is transfer capability relevant in electricity market? • What is the ratio of transfer capability to transmission capacity? • What is congestion in power system? • What is the extent of congestion in Indian power system • Does congestion exist in other countries? • How to manage congestion? • What are the methods to alleviate congestion?

3. 1. What is Transfer Capability ?

4. 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” NRLDC, POWERGRID

5. North American Electricity Reliability Corporation’s definition of TTC • “TTC is the amount of electric power that can be transferred over the interconnected transmission network in a reliable manner based on all of the following conditions • 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” NRLDC, POWERGRID

6. Transfer Capability as defined in the Indian Electricity Grid Code (IEGC) ‘Transfer Capability’ of a transmission network is the ability to transfer electric power when operated as part of the interconnected power system and may be limited by the physical and electrical characteristics of the system considering security aspects of the grid.

7. 2. Total Transfer Capability • “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.

8. Transmission system is a chain of series and parallel elements. • Strength of various links change dynamically and depends on each other also. • Capability of a chain of parallel and series elements would depend on the weakest link in a series • Anchoring at intermediate points and their strength would affect the strength of the whole chain.

9. 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 20 December 2019 NRLDC, POWERGRID 9

10. What is the relevance of Transfer Capability in the Indian Electricity Market ?

11. Provisions in Electricity Act 2003 28(3)(a): The Regional Load Despatch Centre shall: • be responsible for optimum scheduling and despatch of electricity within the region, in accordance with the contracts entered into with the licensees or generating companies operating in the region: 40(c): It shall be the duty of a transmission licensee: • to provide non-discriminatory open access to its transmission system for use by • Any licensee or generating company on payment of the transmission charges; or • Any consumer as and when such open access is provided by the State Commission under sub-section (2) of sec 42, on payment of the transmission charges and a surcharge thereon, as may be specified by the State Commission

12. Provisions in Electricity Act 2003 2 (47) “open access” means the non-discriminatory provision for the use of transmission lines or distribution system or associated facilities with such lines or system by any licensee or consumer or a person engaged in generation in accordance with the regulations specified by the ppropriate Commission Korba Case A?????

13. CERC Open Access Regulations 2004 • Criteria for allowing transmission access: ii) The short term access shall be allowed, if request can be accommodated by utilising: (a) Inherent design margins (b) Margins available due to variation in power flows (c) Margins available due to in-built spare transmission capacity created to cater to future load growth

14. 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.

15. 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.” POWERGRID

16. 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 POWERGRID

17. 3. How is Transfer Capability different from Transmission Capacity ?

18. Extracts from CIGRE_Technical Brochure-235_Advisory Group C1.31Management of Transmission Capacity and Access: Impact on System Development

19. Extracts from CIGRE Technical Brochure-235, Advisory Group C1.31Management of Transmission Capacity and Access: Impact on System Development

20. Transmission Capacity Vis-à-vis Transfer Capability NRLDC, POWERGRID

21. Why is transfer capability less than transmission capacity ?

22. 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 if connection of the customer and the quantum of demand • Other transactions through the area • Parallel flow in the network • Transmission Capacity independent on all of the above • When electric power is transferred between two areas such the entire network responds to the transaction

23. 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. Source: NERC

24. Parallel Flows in the network affect TTC • European Transmission System Operators say • “In a widely interconnected network like for example the UCTE network the power flow through the cross border tie lines between two neighbour areas A and B may be interpreted as superposition of a direct flow, which is related to all the other exchanges in the meshed network and to the location of generations and loads in the several grids. Therefore there would be parallel flow even if all the exchanges in the interconnected systems were set to zero.” Final Report on Definition of Transfer Capacities in Liberalised Electricity Markets, April 2001 NRLDC, POWERGRID

25. 3. Transmission capacity vs transfer capability • For instance we might have an eight-lane expressway between Delhi and Jaipur and a Ferrari driven by a Formula-I racer. Each may have the capacity to operate at a speed of 300 km/hour. Still it would not be possible to cover the 265 km distance between Delhi and Jaipur in less than one hour due to various bottlenecks, road intersections disturbances on the way. In fact it could take as high as three hours giving an average speed of 88 kms/hr (the system capability), which is only 30% of the design capacity. Source: Approach Paper for Assessment of Transfer Capability in the Indian context, August 2007, POWERGRID

26. Cross border capacity available for trade • “Physical capacity connecting zones A and B is sum of 1-3 and 2-3 physical line capacities. However, the cross border capacity available for commercial trade would be less or at most equal to the sum of capacities of cross border lines individually.” CIGRE_WG_5.04_TB_301 1 A B 3 2 POWERGRID

27. 4. How can transfer capability be brought closer to the transmission capacity ?

28. Suggestions for improving transfer capability • Installation of shunt capacitors in pockets prone to high reactive drawal & low voltage • Strengthening of intra-state transmission and distribution system • Improving generation at load centre generating stations • Avoiding prolonged outage of generation/transmission elements • Reduction in outage time of transmission system particularly where system availability norms are not available • Minimising outage of existing transmission system for facilitating construction of new lines • Expediting commissioning of transmission system-planned but delayed execution • Enhance transmission system reliability by strengthening of protection system; Strengthening the safety net- Under voltage load shedding schemes, system protection schemes

29. 5. What is Reliability Margin ?

30. Reliability demands risk management • NERC says • “In the context of electric system reliability, risk is the likelihood that an operating event will reduce the reliability of the interconnection and the consequences that are unacceptable. Because we cannot prevent eventsfrom happening, we plan and operate the electric system so when they do, their effects are manageable, and the consequences are acceptable. So one of the keys to providing a reliable interconnection is managing risks.” ‘Reliability Criteria and Operating Limits Concepts’, Version 4 Draft 8, 2nd May 2007 NRLDC, POWERGRID

31. Risk Management Consequences Likelihood NRLDC, POWERGRID

32. Expert speak ! • Charles Concordia, the power system Guru • “…ties have been said to have two kinds of functions, the economic interchange of energy and the sharing of generation reserve…” • “…if a tie is installed to allow an economic interchange of energy, then it can only be counted upon for reserve support if it has enough margin of capacity at its maximum normal load to withstand a sudden further increase of power flow equal to at least the capacity of, for example, the largest generating unit of the receiving system... • ” NRLDC, POWERGRID

33. Expert speak ! • Charles Concordia, the power system Guru • “…if the import is so great that loss of a generator causes the tie lines to trip, then even more generation is lost, so the situation is made worse…” • “…a tie will make things either better or worse; it cannot remain neutral…” • “…it is the dependable pick-up capacity, rather than the total capacity, that is significant…” NRLDC, POWERGRID

34. 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” • TRM is determined by unintended load frequency regulation deviations and needs for common reserves and emergency exchanges • Net Transfer Capacity (NTC) = TTC- TRM NRLDC, POWERGRID

35. NERC definition of Reliability Margin (RM) • Transmission Reliability Margin (TRM) • Amount of transfer capability necessary to ensure reliable service under a reasonable range of uncertainties in system conditions • Capacity Benefit Margin (CBM) • Amount of transmission transfer capability reserved to ensure access to generation from inter connected system • Reliability Margin is time dependent • In the Indian context • Overdrawal / Underdrawal by constituents resulting from demand forecast error • Sudden outage of a large generator in a control area NRLDC, POWERGRID

36. 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” NRLDC, POWERGRID

37. 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 49.2 to 50.3 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 GSIOAR-2007, IT-BHU, Varanasi

38. Transmission Reliability Margin • h)“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;

39. 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 • The actual power flow only demonstrates the utilization of these margins during real-time and therefore should not be a reason for complain NRLDC, POWERGRID

40. How to assess the Transfer Capability

41. Transfer Capability Calculations must • 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. Courtesy: Transmission Transfer Capability Task Force, "Available Transfer Capability Definitions and Determination", North American Electric Reliability Council, Princeton, New Jersey, June 1996 NERC POWERGRID

42. 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)

43. 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.

44. 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 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 44 Planning Criteria is strictly followed during simulations NRLDC, POWERGRID

45. 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 Open Access (LTOA) Reliability Margin (RM) RM Available Transfer Capability is Total Transfer Capability less Reliability Margin POWERGRID

46. Available Transfer Capability • “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.

47. What is the ratio of transfer capability to transmission capacity in India and other countries ?

48. POWERGRID

49. POWERGRID

50. What is congestion ?