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SOUTHERN REGIONAL LOAD DESPATCH CENTER POSOCO,POWERGRID, BANGALORE

UI regulations,ATC, congestion regulations and experiences in SR By P.Bhaskara Rao, Addl G M, Shamreena Verghese , Mgr 16 May 2011. SOUTHERN REGIONAL LOAD DESPATCH CENTER POSOCO,POWERGRID, BANGALORE. Overview. Ui_regulations Transfer Capability

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SOUTHERN REGIONAL LOAD DESPATCH CENTER POSOCO,POWERGRID, BANGALORE

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  1. UI regulations,ATC, congestion regulations and experiences in SR By P.Bhaskara Rao, Addl G M, Shamreena Verghese , Mgr 16 May 2011 SOUTHERN REGIONAL LOAD DESPATCH CENTER POSOCO,POWERGRID, BANGALORE

  2. Overview • Ui_regulations • Transfer Capability • Relevance of transfer capability in Indian electricity market • Reliability Margin • Difference between transfer capability and Transmission Capacity • Assessment of Transfer capability • Ratio of transfer capability to transmission capacity • Congestion regulations • overview of constraints and hotspots in SR

  3. Transfer Capability - Definition

  4. North American Electricity Reliability Corporation’s definition of TTC(Total Transfer Capability ) • “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” 06th Oct 2009 NRLDC, POWERGRID 4

  5. 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” 06th Oct 2009 NRLDC, POWERGRID 5

  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. Total Transfer Capability as defined in the 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.

  8. Available Transfer Capability as defined in the 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. Simultaneous TTC Area A Area B 2000 MW 4000 MW Area C 5000 MW 06th Oct 2009 NRLDC, POWERGRID 9

  10. Relevance of Transfer Capability inIndian Electricity Market

  11. 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

  12. 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.

  13. 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.” 13th October 2009 POWERGRID 13

  14. 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 13th October 2009 POWERGRID 14

  15. Reliability Margin

  16. 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 06th Oct 2009 NRLDC, POWERGRID 16

  17. 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” 06th Oct 2009 NRLDC, POWERGRID 17

  18. 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;

  19. 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 49.5Hz 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 August 08, 2007 GSIOAR-2007, IT-BHU, Varanasi 19

  20. 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 4 November 2014 NRLDC, POWERGRID 20

  21. Difference between Transfer Capability and Transmission Capacity

  22. Transmission Capacity Vis-à-vis Transfer Capability 06th Oct 2009 NRLDC, POWERGRID

  23. 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

  24. 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 modelled 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.

  25. Cross border capacity available for trade 1 A B 3 2 “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 13th October 2009 POWERGRID 25

  26. Assessment of Transfer Capability

  27. 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. Confirm to reliability criteria and guides. Accommodate reasonable uncertainties in system conditions and provide flexibility. 13th October 2009 POWERGRID 27

  28. 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 4 November 2014 NRLDC, POWERGRID 28

  29. 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 29 Planning Criteria is strictly followed during simulations 06th Oct 2009 NRLDC, POWERGRID 29

  30. Steady State Voltage Limits From Section 5 of Transmission Planning Criteria 30

  31. 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 31

  32. Input Data and Source 06th Oct 2009 NRLDC, POWERGRID 32

  33. Process for assessment • Base case construction (The biggest challenge) • Anticipated network representation • Anticipated load generation • Anticipated trades • Simulations • Increase generation in exporting area with corresponding decrease in importing area till network constraint observed 06th Oct 2009 NRLDC, POWERGRID 33

  34. Concerns-1 Wide range of permissible frequency band Significant interplay of frequency and voltage in a large grid ‘UI’ considered as an infinite source / sink Limited voluntary participation by utilities for congestion management in real-time Primary response, Reactive Support Availability norm falling short of ensuring Dependability Outage of complete power station Outage of complete EHV substation

  35. Concerns-2 Changes in long-term allocations Uncertainties in planned outages of shared resources Medium-term inadequacies in transmission/generation Open Loops, Switching arrangement Safety net Relay settings/ behavior: Credible N-1 contingency getting converted into simultaneous multiple outages in real-time Frequent large scale contingencies Fog, Widespread rains, Cyclone, Silt Limited support from online tools in case of fast events

  36. 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 13th October 2009 POWERGRID 36

  37. Ratio of transfer capability to transmission capacity

  38. 13th October 2009 POWERGRID 38

  39. Congestion

  40. Congestion in Power System “Congestion is a situation where the demand for transmission capacity exceeds the transmission network capabilities, which might lead to violation of network security limits, being thermal, voltage stability limits or a (N-1) contingency condition.” CIGRE_WG_5.04_TB_301 13th October 2009 POWERGRID 40

  41. Visibility of congestion To be handled before-the fact To be handled in real-time • Visible to the market players • “If for a given interconnection, there is more demand for cross border capacity than commercially available, the interconnection is also treated as congested, meaning no additional power can be transferred. This congestion is visible for market players as a limit on their cross-border transactions.”- CIGRE_WG_5.04_TB_301 • Invisible to the market players • “It is possible that even though the available commercial interconnection capacity is not fully allocated to market players, some lines, being internal or cross-border, become overloaded. This physical congestion is a problem of the System Operator and has to be dealt with by this entity.” CIGRE_WG_5.04_TB_301 13th October 2009 POWERGRID 41

  42. Congestion visible to the market • “The more transactions and the more meshed the network, the higher the chance for mismatch between commercial exchange and physical flows.” CIGRE_WG_5.04_TB_301 • Congestion • Sign of growth and vibrant market • Natural corollary to Open Access • Existing transmission system was not planned with short-term open access in mind • Security margins have been squeezed • ‘Pseudo congestion’ needs to be checked 13th October 2009 POWERGRID 42

  43. Congestion in real-time is a security threat Phenomenon common to large meshed grids Coupling between voltage and frequency accentuates the problem in a large grid 13th October 2009 POWERGRID 43

  44. Real-time Congestion types • Was not experienced • Regional grids were small • Trades were limited Experienced occasionally under - Grid Contingencies - Skewed conditions in grid Aggressive Open Access trades • Internal congestion (Intra-zonal) • Within a single System Operator’s control area • Cross-border (Inter zonal) • Also called seams issue • Several System Operators involved 13th October 2009 POWERGRID 44

  45. Types of congestion in Indian context • 3 / 2 / 1 month (s) ahead – advance • First come first served • Day ahead PX • Day ahead bilateral • Contingency transaction • Real time

  46. Reasons for congestion in India • Fuel / resources related constraints • Long haulage of power • Physical network limitations • Fast growing network, transition, mismatch • Inadequate compliance to reliability standards • Inadequacy in Safety net • Market Design/Interplay and behavior of players 13th October 2009 POWERGRID 46

  47. Causes of congestion • Inadequate transmission – including outages • Inadequate reactive support • Weather diversity, seasonal demand variation • Skewed generation availability – monsoon, planned / forced outages • Uneven purchasing power of utilities in a shortage scenario • Compulsion to meet load at all costs (agriculture, festival, election etc.) – Aggressive buying • Economy (cheaper generation to replace costlier generation) • Inflated sale / purchase requirement – Pseudo congestion • Inter play with UI mechanism – Bids based on anticipated UI price

  48. Regulatory initiatives • Modifications in Grid Code & other regulations • Frequency band tightening • Cap on UI volume, Additional UI charge • Inclusion of new definitions (TTC, ATC, Congestion) • Congestion Charge Regulation • Congestion Charge Value, Geographical discrimination • Procedure for Assessment of Transfer Capability • Procedure for Implementation of Congestion Charge 13th October 2009 POWERGRID 48

  49. Suggestions for improving transfer capability-1 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 based generating stations by R&M and better O & M practices avoiding prolonged outage of generation/transmission elements reduction in outage time of transmission system particularly those owned by utilities where system availability norms are not available

  50. Suggestions for improving transfer capability-1 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 stregthening of protection system strengthening the safety net- Under voltage load shedding schemes, system protection schemes

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