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TTC/ATC Computations and Ancillary Services
in the Indian context
by
Western Regional Load Despatch Centre, Mumbai
PSTI Bengaluru27th April 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 A
Total Transfer Capability (TTC)/
Available Transfer Capability (ATC)
computations
Transfer Capability -
Definitions
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
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”
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.
9
Simultaneous TTC
Area A Area B
Area C
2000 MW 4000 MW
5000 MW
Relevance of Transfer Capability
in
Indian Electricity Market
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
Reliability Margin
16
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.
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”
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;
20
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
21
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
Difference between Transfer Capability and Transmission
Capacity
Transmission Capacity Vis-à-vis Transfer Capability
Transmission Capacity Transfer Capability
1 Declared by designer/ manufacturer Declared by the Grid Operator
2 Is a physical property in isolation Is a collective behaviour of a system
3 Depends on design only Depends on design, topology, system conditions, accuracy of assumptions
4 Deterministic Probabilistic
5 Constant under a set of conditions Always varying
6 Time independent Time dependent
7 Non-directional (Scalar) Directional (Vector)
8 Determined directly by design Estimated indirectly using simulation models
9 Independent of Parallel flow Dependent on flow on the parallel path
Transfer Capability is less than transmission capacity because
• Power flow is determined by location of injection, drawaland 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.
Assessment of
Transfer Capability
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.
27
Courtesy: Transmission Transfer Capability Task Force, "Available Transfer Capability Definitions and Determination", North American Electric Reliability Council, Princeton, New Jersey, June 1996 NERC
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.
30
Total Transfer Capability: TTC
Voltage Limit
Thermal Limit
Stability Limit
Total Transfer Capability
Total Transfer Capability is the minimum of the
Thermal Limit, Voltage Limit and the Stability Limit
Time
Power
Flow
31
Intra-day STOA
Day-ahead STOA
Collective (PX) STOA
First Come First Served STOA
Advance Short Term Open Access (STOA)
Medium Term Open Access (MTOA)
Long Term Access (LTA)
Reliability Margin (RM)
Available Transfer Capability is
Total Transfer Capability less Reliability Margin
TTC ATC
RM
32
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
Total Transfer
Capability
Reliability
Margin
less
Available
Transfer
Capability
equals
Planning Criteria is strictly followed during simulations 32
Ampacity
Conductor Type
Ampacity
More than 10 years of age
65 degree conductor 75 degree conductor
40o ambient 10o ambient 40o ambient 10o ambient
ACSR Bersimis 693 1476 945 1601
ACSR Moose 575 1240 799 1344
ACSR Zebra 527 1071 718 1161
For bundled conductors
ACSR Twin Moose 1150 2479 1598 2687
ACSR Quad Moose 2300 4958 3196 5374
ACSR Quad Bersimis 2773 5905 3779 6403
ACSR Triple Snowbird 1725 3719 2397 4031
Thermal limit derived from ampacity
Conductor Type
Thermal limit in MW at 0.975 pu voltage and unity p.f.
More than 10 years of age
65 degree conductor 75 degree conductor
40o
ambient 10o ambient40o
ambient 10o ambient
400 kV ACSR Twin Moose 777 1675 1079 1815
400 kV ACSR Quad Moose 1554 3349 2159 3630
400 kV ACSR Quad Bersimis 1873 3989 2553 4325
400 kV ACSR Triple Snowbird 1165 2512 1619 2723
220 kV ACSR Zebra 196 398 267 431
35
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
1 Line length 386 in kilometer
2 From end shunt reactor in MVAr at 400 kV 72.56 80 MVAr 420 kV
3 To end shunt reactor in MVAr at 400 kV 72.56 80 MVAr 420 kV
4 Surge Impedance Loading (SIL) 515 in MW
5 Conductor typeACSR Twin
Moose
75o C design conductor
temperature and age >10 years
6 Line reactance (X) 0.0002075 Per unit / kilometer / circuit
7 Line susceptance (B) 0.0055 Per unit / kilometer / circuit
8 Base MVA 100
9Power transfer between adjacent buses at 5 % voltage
regulation and 30 deg angular separation = PB
593 (in MW)
10 Total shunt compensation for the line in MVAr 145 Sl. No. (2) + (3)
11 Line charging MVAr 212Line length X B x Base MVA =
Sl. No. (1) x (7) x (8)
12 Degree of shunt compensation = Dsh
0.68 Sl No. (10)/ (11)
13 Degree of series compensation = Dse
0.35 35 % Fixed compensation
14 Multiplying factor-1 (shunt compensation) = k1
0.56 Sqrt(1-Dsh
)
15 Multiplying factor-2 (series compensation) = k2
1.24 1/ Sqrt (1-Dse
)
16 Multiplying factor-3 (St. Clair’s line loadability) = k3
1.15 PB
/ SIL
17 Permissible line loading PL
414 SIL x k1
x k2
x k3
18 Ampacity of the conductor in summer conditions 1598 at ambient temperature of 40o C
19 Thermal limit (MW) in summer = Pth_summer
1079 at 0.975 pu voltage and unity p.f.
20 Operating limit (in MW) in summer 414 Min of PL
and Pth_summer
Illustration of
calculation of
operating limits
of transmission
line
36
37
Steady State Voltage Limits
Voltage (kV rms)
Nominal Maximum Minimum
765 800 728
400 420 380
220 245 198
132 145 122
38
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
Input Data and Source
39
S No. Input Data Suggested Source
1 Planning Criteria Manual on Transmission Planning Criteria issued by CEA
2 Network Topology Existing network with full elements available
Planned outages during the entire assessment period
New transmission elements expected
3 Transmission line limits Minimum of thermal limit, stability limit and voltage limit
4 Thermal unit availability Load Generation Balance report, Maintenance schedule
Anticipated new generating units
5 Thermal despatch Ex bus after deducting the normative auxiliary consumption
Output could be further discounted by the performance index of generating units of a particular size as compiled by CEA
6 Gas based thermal despatch
Past trend
7 Hydro despatch Peak and off peak actual hydro generation on median consumption day of same month last year
The current inflow pattern to be duly accounted
8 Load Anticipated load
9 Credible contingencies Planning criteria + Operator experience
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
EASTERN
REGION
SOUTHERN
REGION
WESTERN
REGION
NORTHERN
REGION
NORTH-
EASTERN
REGION
2
8
4
16
4
Case 1
WR GridNR
ER
SR Case 2
Case 3
NR
SR
ER
NR
ER
SR
Case4 NR
SR
ER
Case 5
WR Grid
NR
ER
SR Case 6
Case 7
NR
SR
ER
NR
SR
Case 8
NR
ER
Possible scenarios for Western Regional Grid
Low probabilityExport capability to ERImportExportImport8
High probabilityImport capability from ERExportImportExport7
Low probabilityImport capability from SRImportExportExport6
High probability ( Poor monsoon in NR)
Export capability to NRImportImportExport5
Low/medium probability
Simultaneous Import capability of WR
ImportImportImport4
Low probabilityExport capability to SRExportImportImport3
Low probability (Load crash in NR)
Import capability from NRExportExportImport2
High probability
Simultaneous export capability of WR
Export Export Export 1
RemarksWork out from this caseSRERNRSl.NO.
Based on above eight scenarios, TTCs on different corridors could be worked out
Real life vs reel life
N-1 criteria
“Element” in theory “Event” in
practice
45
46
(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
47
• 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
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
FLOWGATES
NR:
Central UP-Western UP
UP-Haryana/Punjab
WR:
Chandrapur-Padghe
Chandrapur-Parli
Bina-Gwalior
Soja-Zerda
SR:
Vijaywada-Nellore
Hossur-Selam
Cadappa-Kolar
Neyvelli-Sriperumbudur
ER:
Farakka-Malda
Malda-Purnea
Talcher-Rourkela
Jamshedpur-Rourkela
Farakka-Kahalgaon
Kolaghat-Baripada-Rengali
Part B
Ancillary Services in the Indian context
Outline
• Definition of ancillary services
• Categories of ancillary services
• Ancillary services in the Indian context
Ancillary services……definitions
• Those services that are necessary to support the transmission of capacity and
energy from resources to loads while maintaining reliable operation of the
Transmission Service Provider's transmission system in accordance with good
utility practice. (From FERC order 888-A.)
• “Ancillary services are those functions performed to support the basic services
of generation, transmission, energy supply and power delivery. Ancillary
services are required for the reliable operation of the power system.”… Para
30, judgment in appeal no.202 dated 13th December 2006, The Appellate
Tribunal for Electricity[4]
• “Ancillary services are those functions performed by the equipment and people
that generate, control, transmit, and distribute electricity to support the basic
services of generating capacity, energy supply, and power delivery.”….Electric
Power Ancillary Service, Eric Hirst and Brendan Kirby[5]
Ancillary services……definitions (2)
• “Ancillary Services” means in relation to power
system (or grid) operation, the services necessary
to support the power system (or grid) operation in
maintaining power quality, reliability and security of
the grid, eg. active power support for load following,
reactive power support, black start,
etc;………………….Indian Electricity Grid Code 2010
Approach Paper on Ancillary Services submitted to CERC in June 2010
by National Load Despatch Centre (NLDC)
Categories of ancillary services
• Frequency Control Services
• Network control Services
• System Restart Services
Frequency Control Services
Deployment times a key factor for categorizing
Governing system
AGC or LFC
Re-dispatch
Frequency ControlServices (2)
Network control services
• Voltage Control services
– Primary…….(AVR)
– Secondary……..centralized automatic
– Tertiary………..Manual optimization
System restart services
• Black start capability of generating units
– Dead bus charging on request
– Ability to feed load
– Frequency control
– Voltage control
– Act on the directions of system operator
61
Drivers for Ancillary Services
• Reliability and Security
• Deregulated Power Systems
• Services to be obtained from Service Providers
• Decoupling with basic energy services
• Regulatory Directives:
– NLDC/RLDCs to identify ancillary services as per clause 11.1 of the amended CERC UI Regulations, 2009
““ b. Providing ancillary services including but not limited to ‘load generation balancing’ during low grid frequency as identified by the Regional Load Despatch Centre, in accordance with the procedure prepared by it, to ensure grid security and safety:”
POSOCO’s Approach Paper…..............(1)
• Approach paper on ‘Ancillary Services in Indian Context’published by POSOCO in June’10
– Submitted to the Commission
– Comments sought from stakeholders
• Proposed services in the approach paper
– Load Generation Balancing Service (LGBS)
• Use of un-despatched surplus, peaking and pumping stations
– Network Control Ancillary Service (NCAS)
• Power Flow Control Ancillary Service (PFCAS)
• Voltage Control Ancillary Service (VCAS)– use of synchronous condensers
– System Restart Ancillary Service (SRAS)
62
POSOCO’s Approach Paper…………….(2)
• Comments received from various stakeholders
• Service identified for immediate implementation
– Frequency Support Ancillary Service (FSAS)
• LGBS renamed as FSAS
– Other services identified to be introduced subsequently, as the market matures
• Petition to be filed by NLDC
– proposing roadmap and mechanism for introducing
FSAS
63
Frequency Support Ancillary Service (FSAS)
• Focus on utilizing idle generation
– High liquid fuel and diesel cost
– Fragmented need of load serving entities/buyers
– Concern with frequent start stop operation
• Utilization of un-despatched generation from
– Liquid fuel based
– Diesel based
– Merchant/ IPPs/ CPPs
• Quantum available under this service could be limited
– frequency may not always be contained in the operation band
64
Implementation of FSAS…….(1)
• Facilitation through Power Exchange
– Separate category of user group
– Bids to be invited after closure of DAM (morning and evening)
– Supplier, bid area, quantum, duration and price to be specified
– NLDC to compile bids as per bid price, area
• Despatch of bids under FSAS
– System Operator to despatch based on anticipated deficit and frequency profile
– Threshold frequency: lower limit in the IEGC band
– Despatch certainty of at least 12 time blocks
– Merit order to be ensured: low cost bids despatched first
65
Implementation of FSAS…….(2)
• Despatch in case of congestion
– ATC limits to be honored
– Downstream bids despatched first
• Scheduling of bids under FSAS
– Directly incorporated in the schedule of sellers
– No matching one-to-one drawal schedule
– Attributed towards drawal of a fictitious entity i.e ‘POOL’
• buyer/ drawee entity to pay back in the form of UI charges
• Consent from sellers before despatch
– To ascertain readiness for despatch
– Agreed quantum scheduled after 6 time blocks
66
Implementation of FSAS…….(3)
• Options for settlement
– ‘Pay-as-bid price’
– ‘Uniform Pricing’
– To be finalised by the Commission
• Ceiling price for despatch of bids
– CERC’s UI vector ceiling price
• Payment settlement through power exchange
• Settlement on post-facto basis
– On (n+1)th day or next working day
• Power exchanges to be paid facilitation charges
67
Ancillary Services Fund
• ‘Ancillary Services Fund’ account to be opened and maintained by NLDC
• Procurement of Ancillary Services
– Funded from the PSDF via Ancillary Services Fund
– Clause 4 of CERC’s PSDF Regulation
– Clause 11 of CERC’s amended UI Regulations
– Estimated amount to be transferred from PSDF to Ancillary Services Fund on a quarterly basis
• Weekly transfer from UI pool account
– Corresponding to quantum of power despatched
– To avoid frequent transfer from PSDF to Ancillary Services Fund
68
Thank you
Discussion………