TTC/ATC Computations and Ancillary Services in the Indian context By National Load Despatch Centre PSTI Bengaluru 10th August 2011
Outline • Part A: TTC/ATC computations • Transfer Capability - Definition • Relevance of transfer capability in Indian electricity market • Difference between Transfer capability and Transmission Capacity • Assessment of Transfer Capability • Ratio of transfer capability to transmission capacity • Congestion • Part B: Ancillary services in the Indian context
Part ATotal Transfer Capability (TTC)/ Available Transfer Capability (ATC) computations
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” 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” 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.
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.
Simultaneous TTC Area A Area B 2000 MW 4000 MW Area C 5000 MW 9
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.]
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.
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.” 13
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 14
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. 16
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” 17
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” 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;
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 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 21
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 the entire network responds to the transaction
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.
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. 27
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)
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.
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 30
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 31
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 32 Planning Criteria is strictly followed during simulations 32
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 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 35
Illustration of calculation of operating limits of transmission line 36
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 38
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 40
NORTHERN REGION NORTH-EASTERN REGION WESTERNREGION SOUTHERN REGION 4 2 8 16 EASTERN REGION 4
WR Grid NR NR ER ER SR Case 1 Case 2 SR NR Case4 NR Case 3 ER ER SR SR
WR Grid NR NR Case 5 ER ER SR Case 6 SR NR NR ER ER Case 7 Case 8 SR SR
Possible scenarios for Western Regional Grid Based on above eight scenarios, TTCs on different corridors could be workedout
Real life vs reel life N-1 criteria “Element” in theory “Event” in practice 45
(n-1)--Element or event ? Difference exists in n-1 criteria in planning and operating horizon Tower collapse/lightning stroke on a D/C Tower. Two main one transfer scheme-Failure of opening of 400 kV Line breaker In practice-Results in multiple loss in elements As per planning criteria- not more than two elements should be affected Coal fired station Fault in 132kV system- may result in loss of power supply to CW system vis a vis tripping of multiple units 46
Non availability/Outage/Non operation of Bus bar protection Results in tripping of all lines from remote stations Weather disturbance or floods Might result in loss of substation/multiple lines in the same corridor Breaker and a half scheme Outage of combination of breakers may result in tripping of multiple line for a fault in one line (n-1)--Element or event ? … contd 47
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 48
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
Suggestions for improving transfer capability-2 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