1. ERPC, Kolkata

3.  Definition and Concepts  Standards and regulation for TTC calculation  Calculation of TTC  Challenges in calculation of TTC  Way forward

4.  To facilitate non discriminatory open access in transmission  Specifically to ascertain the limit up to which transactions in a given direction can be safely accommodated by the network, with a reasonable degree of accuracy.

6.  “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 a largest unit in the importing control area

7. 750 MW 630 MVA 515 MW Area A Area B 515 MW TTC = 630 MVA TTC = 315 MVA considering N-1 security criteria

8. 500 MW Area A Area B 500 MW1000 MW Gen 500 MW Transfer Capability from Area A to Area B = 1500MW After N-1 contingency=1000 MW Transfer Capability from Area B to Area A = 500MW After N-1 contingency= 0 MW

9.  Thermal Limits  Voltage Limits  Stability Limits Limiting conditions may shift with time as network operating conditions changes Time Horizon Power Flow Thermal Limit Voltage Limit Stability Limit Total Transfer capability

10.  Transmission capacity ◦ Refers to 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 750 MW 630 MVA 1015 MW TTC = 630 MVA

11. Transmission CapacityTransfer Capability 1Is a physical property in isolationIs a collective behaviour of a system 2Depends on design onlyDepends on design, topology, system conditions, accuracy of assumptions 3DeterministicEstimated 4Constant under a set of physical conditions Always varying 5Time independentTime dependent 6Non-directionalDirectional 7Determined directly by designEstimated indirectly using simulation models 8Declared by designer/ manufacturerDeclared by the Grid Operator 9Understood by allOften misunderstood 10Considered unambiguous & sacrosanct Subject to close scrutiny by all stakeholders

12. Sl NoCorridorTransmission Capacity Transfer Capability (For January 2015) 1ER to NR12130 MW3400 MW 2ER to W310690 MW1000 MW 3ER to SR3630 MW2650 MW 4ER to NER2860 MW720 MW 5NR to ER12130 MW2000 MW 6NER to ER2860 MW590 MW 7W3 to ER10690 MW1800 MW

13.  “Transmission Reliability Margin (TRM)” TRM is defined as the amount of transmission transfer capability necessary to provide reasonable assurance that the interconnected transmission system 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.

14.  Need ◦ To take care of uncertainty like  Aggregate Load forecast  Load distribution uncertainty  Forecast uncertainty in transmission system topology  Variations in generation dispatch  Quantification ◦ 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

15.  “Available Transfer Capability (ATC)” 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.  ATC is a prediction of the remaining amount of power that could be transferred on a path between two systems for defined system conditions. ATC = TTC – TRM

16. ATC = TTC – RM Reliability Margin (RM) Long Term Open Access (LTOA) Medium Term Open Access (MTOA) Advance Short Term Open Access (STOA) First Come First Served STOA Collective (PX) STOA Day-ahead STOA Intra-day STOA

17. Standards and Regulatory requirements

18.  CEA Transmission planning criteria ◦ N-1 Contingency ◦ Line loading limits for planning ◦ Voltage limits ◦ Stability limits  Indian Electricity Grid Code.  CERC regulation “Measures to relieve congestion in real time operation” amendment dated 22 nd April 2014 ◦ Methodology for assessment of TTC, TRM and ATC ◦ Power System model to be considered for simulation studies

19. Role of SLDCs :  SLDC shall assess the TTC, TRM and ATC on ◦ Inter-State transmission corridor considering the meshed Intra State corridors for exchange (import/ export) of power with ISTS.  These figures along with the data considered for assessment of TTC would be forwarded to the respective RLDC.  The details of anticipated transmission constraints in the intra State system shall also be indicated separately.

20. 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 one month & 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

21. Role of NLDC :  Assess the TTC, TRM and ATC of inter and intra-regional links/ Corridors respectively for a period of one month & 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

22. 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

23. Method and approach

24.  Setting up power system model  Developing representative scenario to arrive at an initial base case  Separate base cases for calculating export and import capability: both peak and off-peak conditions ◦ Load Generation Balance ◦ Checking of networks  Assessing TTC and TRM with the help of simulation studies

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

26.  Violation of grid voltage operating range or  Violation of transmission element loading limit in n- 1 contingency case or  Violation of emergency limit in the n-1-1 contingency case or  Stability under n-1-1 contingency of a temporary single phase to ground fault on a 765 kV line close to the bus or a permanent single phase to ground fault on a 400 kV line close to the bus  Angular difference of 30 degrees between adjacent buses under n-1 contingency.

27.  TTC/ TRM/ ATC figures to be made available in the website of NLDC as well as RLDCs  Any revisions in these figures also to be published on the website of NLDC & RLDCs, along with reasons  Revision of TTC Changes in anticipated conditions ◦ Commissioning of new element (generator / line / ICT) ◦ Unforeseen change in generating station availability ◦ Forced outage of a transmission element or a generator ◦ Unforeseen change in demand (due to weather fluctuations)

28.  As per CEA transmission planning criteria ◦ The loading limit for a transmission line shall be its thermal loading limit. ◦ Generally, the ambient temperature may be taken as 45°C; however, in some areas like hilly areas where ambient temperatures are less, the same may be taken. ◦ Emergency loading limit of line (to be considered during N-1-1 contingency ) should be considered as 110 % of thermal loading limit.

29. Type of ConductorThermal Limit (MVA /Ckt) * 765 KV Hexa ACSR_Zebra D/C4452 765 KV Quad ACSR_Bersimis S/C3880 400 KV Twin ACSR_Lapwing D/C2142 400 KV Quad ACSR_Bersimis S/C2029 400 KV Quad ACSR_Moose D/C1749 400 KV Quad Moose_AAAC S/C1679 400 KV Twin ACSR_Moose D/C874 400 KV Twin Moose_AAAC S/C840 220 KV Twin ACSR_Moose S/C481 220 KV Single ACSR_Moose S/C240 220 KV Single ACSR_Zebra S/C213 132 KV Single ACSR_Panther S/C83 * Assuming 45 deg C ambient temperature & 75 deg C maximum allowable temperature

30. Challenges in calculation of TTC

31.  Load forecast ◦ Carried out by RLDC ◦ Input from LGBR ◦ Historic data ◦ Passive involvement of state ◦ Need to be done more scientifically with involvement of all parties  Network ◦ Input from planned shutdown ◦ Operational Practice  Proper location of changing load & generation

32.  Segregation of corridor ◦ Sharing of ATC among entity utility  NR and Bihar  NER and North Bengal/Sikkim ◦ In some cases, same constraint may come for multiple entities  How to balance between them Source Sink Common Constraint

33.  Taking care of loop flow ◦ Ac power flows through least impedance path  Power pusher from ER-NR part will flow through ER- WR-NR corridor which may cause over loading in WR  No well distinguished method is there to take care this SR ER+ NER WR NR

34.  Planning Criteria vs Operating Criteria ◦ Operation criteria should me more or less stringent than planning criteria ?  Frequency of Revision of TTC ◦ Daily ◦ Hourly ◦ Event triggered

35. 30 revisions in the month of November,14

36. Way forward

37.  More active involvement of state  State declares there ATC and TTC ◦ Delhi, Punjab and UP already started  Finalization of operational criteria  Line loading limits with change in ambient temperature  Input from National Reliability Council for Electricity ◦ Representation from RPC, CEA and academia ◦ Draft Operational Guidelines for calculation of TTC/ATC and RM circulated

38. Thank you Reliability Market TTC

39. Thank you

40. Draft Operational Guidelines for calculation of TTC/ATC and RM

41.  According to CEA transmission planning criteria ambient temperature should be considered as 45° for planning purpose. ◦ As temperature through out the country varies significantly with place and season, thus this will create margin during operation.  If ambient temperature falls from 45° to 40° ampacity of ACSR moose conductor rises from 798 Amps to 874 Amps a rise 0f 9.5 % (as per IEEE standards 738-2006)  CERC amended regulation for detailed procedure for reliving congestion in real time operation it stated that ◦ Permissible Normal and Emergency limits for transmission element shall be as defined in CEA Transmission Planning criteria

42.  In a demonstration smart grid project approved by Federal Electricity Regulatory Commission (FERC), it is mentioned that with drop in ambient temperature by 10°C the Ampacity goes up by 11 % all other factor remaining same.  Since the ambient temperature over the length of line may vary it is proposed considering maximum temperature as forecasted by IMD for the next day, at two ends of the line. ◦ Ampacity of relevant line may be raised by 2% for every 2° temperature lower than ambient of 45°C, considered as based ampacity

43.  Different ambient temperature may be considered for day and night and between seasons.  Before Increased thermal limit on account of fall in ambient temperature is used system Operator may confirm from the owner of the terminal equipment on both side of the line if the terminal equipment is capable of carrying the increased current and also if the relay setting are in accordance with the same.

44.  The SPS must be considered for calculation of TTC and ATC  The relay setting of the SPS for carrying out load shedding should be set with such time delay so that the load tripping takes place, considering the emergency loading limit of the line, to prevent frequent tripping of load. ◦ Emergency loading limit even for old conductor is 40% over load over the thermal limit for 5 minutes. ◦ However keeping a safe margin, the time delay for SPS operation could be set at 10-30 seconds for an overload of up to 40%.

45.  Before Increased thermal limit on account of emergency is used system Operator may confirm from the owner of the terminal equipment on both side of the line if the terminal equipment is capable of carrying the increased current and also if the relay setting are in accordance with the same.  It has been pointed out by POSOCO that some lines developed hot spots and have to be frequently taken out for attending to the same. ◦ It is mentioned that hot spots are result of loose connections and are likely to develop, even when the nominal current is being carried, which however may get aggravated when a higher current is carried

46.  Contradiction with CEA transmission planning criteria ◦ No re-despatch required for N-1 contingency which contradicts with suggestion of enhancement of TTC by SPS operation ◦ Suggest upto 110 % loading in case of N-1-1 contingency which contradicts with suggestion of loading upto 140 %.  How do we get the Temperature data for substations located in rural areas say Ramagundam, Gooty