Transfer Capacity &
Congestion Concepts
Abhimanyu Gartia
WRLDC
Learning Objectives
1. What is Transfer Capability?
2. How is transfer capability different from Transmission
Capacity?
3. Why is transfer capability less than transmission capacity?
4. How can transfer capability be increased?
5. What is Reliability Margin ?
6. How is transfer capability relevant in electricity market?
7. What is the ratio of transfer capability to transmission
capacity?
8. What is congestion in power system?
9. What is the extent of congestion in Indian power system
10. Does congestion exist in other countries?
11. How to manage congestion?
12. 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”
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North American Electricity Reliability
Corporation’s 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”
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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 series
• Anchoring at intermediate points and their strength
would affect the strength of the whole chain.
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Total Transfer Capability: TTC
Thermal Limit
Power
Flow
Voltage Limit
Stability Limit
Total Transfer Capability
Time
Total Transfer Capability is the minimum of the
Thermal Limit, Voltage Limit and the Stability Limit
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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 shall:
• 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 licensee:
• 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
5. 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
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 & Practice
Forum 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.”
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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
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3. How is Transfer Capability
different from Transmission
Capacity ?
Extracts from
CIGRE_Technical Brochure-235_Advisory Group C1.31
Management of Transmission Capacity and Access: Impact on
System Development
Extracts from
CIGRE Technical Brochure-235, Advisory Group C1.31
Management of Transmission Capacity and Access: Impact on System
Development
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
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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 above
• When electric power is transferred between two areas such
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.
Source: NERC
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.
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
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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 capacity.
Source: Approach Paper for Assessment of Transfer Capability in the Indian context,
August 2007, POWERGRID
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
1
A
B
3
2
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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 stations
• Avoiding prolonged outage of generation/transmission 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
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Risk Management
Unlikely events with
unacceptable
consequences
Likely events with
unacceptable
consequences
Unlikely events with
acceptable
consequences
Likely events with
acceptable
consequences
Likelihood
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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
system...
–”
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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 worse…”
– “…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…”
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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”
• TRM is determined by unintended load frequency
regulation deviations and needs for common reserves and
emergency exchanges
• Net Transfer Capacity (NTC) = TTC- TRM
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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
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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”
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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 49.2 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
August 08, 2007
GSIOAR-2007, IT-BHU, Varanasi
37
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 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
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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 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.
Courtesy: Transmission Transfer Capability Task Force, "Available Transfer Capability Definitions and
Determination", North American Electric Reliability Council, Princeton, New Jersey, June 1996 NERC
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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.
Transfer Capability assessment
Trans.
Plan +
approv.
S/D
LGBR
Last
Year
Reports
Weather
Forecast
Last
Year
pattern
Anticipated
Network topology +
Capacity additions
Anticipated
Substation Load
Anticipated
Ex bus
Thermal Generation
Planning
criteria
Credible
contingencies
Simulation
Transfer
Capability
Analysis
less
Brainstorming
Reliability
Margin
equals
Anticipated Ex bus
Hydro generation
Operating
limits
Operator
experience
Planning Criteria is strictly followed
simulations
NRLDC,during
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Available
Transfer
Capability
44
44
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
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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 ?
Total Transfer Capability for import of power in Northern region
5000
4500
Import Capability in MW
4000
3500
3000
2500
2000
1500
1000
500
0
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7000
Transmission capacity versus transfer capability for inter-regional links
during September 2009
Transmission capacity
MW capacity/capability
6000
Forward Transfer Capability
5000
Reverse transfer
4000
93 %
51 %
3000
53 %
2000
36 %
1000
6%
32 %
58 %
20 %
16 %
16 %
0
ER-NR
ER-WR
WR-NR
ER-SR
WR-SR
ER-NER
Inter-regional link
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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
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Congestion
• “Congestion” means a situation where the
demand for transmission capacity exceeds the
Available Transfer Capability”
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Visibility of congestion
• Visible to the market players
To be handled before-the fact
– “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
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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
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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
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Real-time Congestion types
• Internal congestion (Intra-zonal)
– Within a single System Operator’s control area
Was not experienced
-Regional grids were small
-Trades were limited
• Cross-border (Inter zonal)
– Also called seams issue
– Several System Operators involved
Experienced occasionally under
- Grid Contingencies
- Skewed conditions in grid
Aggressive Open Access trades
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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
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Congestion Management
• Priority based rules
• Pro-rata rationing
• Auctioning
– Explicit Auction
– Implicit Auction
– Hybrid
• Market splitting
• Market coupling
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Congestion Management
Lessons learnt in Indian context
• Firmness in STOA schedules
– “Use it or Lose it”
• Valuing transmission instead of pro rata
• Market splitting
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Congestion Alleviation Methods
Real-time horizon
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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 operation
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Congestion Alleviation Methods
• Counter trading
• Re-dispatching (Out of merit generation)
• Locational Marginal Pricing (LMP)
λnode = λ deviation price + λcongestion charge + λlosses
• Transmission Loading Relief (TLR)
All these methods would result in significant rise in
total cost.
“Price for system security”
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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
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