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Transfer Capacity


Transfer Capacity & Congestion Concepts Abhimanyu Gartia WRLDC NRLDC, POWERGRID ENTSOE definition of Reliability Margin Transmission Reliability Margin TRM is a ... – PowerPoint PPT presentation

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Title: Transfer Capacity

Transfer Capacity Congestion Concepts
  • Abhimanyu Gartia

Learning Objectives
  • What is Transfer Capability?
  • How is transfer capability different from
    Transmission Capacity?
  • Why is transfer capability less than transmission
  • 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
  • Does congestion exist in other countries?
  • How to manage congestion?
  • What are the methods to alleviate congestion?

1. What is Transfer Capability ?
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

North American Electricity Reliability
Corporations 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

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.

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.

  • 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
  • Anchoring at intermediate points and their
    strength would affect the strength of the whole

Total Transfer Capability TTC
Thermal Limit
Power Flow
Stability Limit
Total Transfer Capability
Total Transfer Capability is the minimum of the
Thermal Limit, Voltage Limit and the Stability
14 October 2015
What is the relevance of Transfer Capability in
the Indian Electricity Market ?
Provisions in Electricity Act 2003
  • 28(3)(a) The Regional Load Despatch Centre
  • 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
  • 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

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?????

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
  • (c) Margins available due to in-built spare
    transmission capacity created to cater to future
    load growth

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

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

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

3. How is Transfer Capability different from
Transmission Capacity ?
Extracts from CIGRE_Technical Brochure-235_Adviso
ry Group C1.31Management of Transmission
Capacity and Access Impact on System Development
Extracts from CIGRE Technical Brochure-235,
Advisory Group C1.31Management of Transmission
Capacity and Access Impact on System Development
Transmission Capacity Vis-à-vis Transfer
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
Why is transfer capability less than transmission
capacity ?
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
  • When electric power is transferred between two
    areas such the entire network responds to the

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
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
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

Source Approach Paper for Assessment of Transfer
Capability in the Indian context, August 2007,
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

4. How can transfer capability be brought closer
to the transmission capacity ?
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
  • Avoiding prolonged outage of generation/transmissi
    on 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

5. What is Reliability Margin ?
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 events
    from 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
Risk Management
Unlikely events with unacceptable consequences Likely events with unacceptable consequences
Unlikely events with acceptable consequences Likely events with acceptable consequences
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

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
  • 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

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
  • TRM is determined by unintended load frequency
    regulation deviations and needs for common
    reserves and emergency exchanges
  • Net Transfer Capacity (NTC) TTC- TRM

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

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

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
  • 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

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

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
  • 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

How to assess the 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
  • 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
  • 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.

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
Anticipated Ex bus Thermal Generation
Reliability Margin
Available Transfer Capability
Anticipated Ex bus Hydro generation
Last Year pattern
Operator experience
Operating limits
Planning Criteria is strictly followed during
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 Open
Access (LTOA)
Reliability Margin (RM)
Available Transfer Capability is Total Transfer
Capability less Reliability Margin
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.

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

  • Congestion means a situation where the demand
    for transmission capacity exceeds the Available
    Transfer Capability

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Visibility of congestion
To be handled before-the fact
  • 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

To be handled in real-time
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.
  • 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

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

Real-time Congestion types
  • Internal congestion (Intra-zonal)
  • Within a single System Operators control area
  • Cross-border (Inter zonal)
  • Also called seams issue
  • Several System Operators involved
  • Was not experienced
  • Regional grids were small
  • Trades were limited

Experienced occasionally under - Grid
Contingencies - Skewed conditions in
grid Aggressive Open Access trades
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

Congestion Management
  • Priority based rules
  • Pro-rata rationing
  • Auctioning
  • Explicit Auction
  • Implicit Auction
  • Hybrid
  • Market splitting
  • Market coupling

Congestion Management Lessons learnt in Indian
  • Firmness in STOA schedules
  • Use it or Lose it
  • Valuing transmission instead of pro rata
  • Market splitting

Congestion Alleviation Methods
  • Real-time horizon

Congestion Alleviation instruments
  • Classical
  • Compliance to Standards and Grid Code
  • Topology change
  • Re-dispatch
  • Curtailment
  • Market based
  • Commercial signals (Congestion Charge)
  • Ancillary Market
  • Out of merit generation scheduled to pool
  • Reactive power charge- synchronous condenser

Congestion Alleviation Methods
  • Counter trading
  • Re-dispatching (Out of merit generation)
  • Locational Marginal Pricing (LMP)
  • ?node ? deviation price ?congestion charge
  • Transmission Loading Relief (TLR)
  • All these methods would result in significant
    rise in total cost.
  • Price for system security

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 Charge Regulation
  • Congestion Charge Value, Geographical
  • Procedure for Assessment of Transfer Capability
  • Procedure for Implementation of Congestion Charge