1. Transfer Capacity & Congestion Concepts
Abhimanyu Gartia
WRLDC

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

3. 1. What is Transfer Capability ?

4. 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. 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”
06th Oct 2009
NRLDC, POWERGRID

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. 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.

8. 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.
22 April 2017

9. 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
22 April 2017
NRLDC, POWERGRID 9

10. What is the relevance of Transfer Capability in the Indian Electricity Market ?

11. 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

12. 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?????

13. 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

14. 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.

15. 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.”
13th October 2009
POWERGRID

16. 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

17. 3. How is Transfer Capability different from Transmission Capacity ?

18. Extracts from CIGRE_Technical Brochure-235_Advisory Group C1
Extracts from CIGRE_Technical Brochure-235_Advisory Group C1.31 Management of Transmission Capacity and Access: Impact on System Development

19. Extracts from CIGRE Technical Brochure-235, Advisory Group C1
Extracts from CIGRE Technical Brochure-235, Advisory Group C1.31 Management of Transmission Capacity and Access: Impact on System Development

20. Transmission Capacity Vis-à-vis Transfer Capability1
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
06th Oct 2009
NRLDC, POWERGRID

21. Why is transfer capability less than transmission capacity ?

22. 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

23. 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

24. 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
06th Oct 2009
NRLDC, POWERGRID

25. 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

26. 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
13th October 2009
POWERGRID

27. 4. How can transfer capability be brought closer to the transmission capacity ?

28. 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

29. 5. What is Reliability Margin ?

30. 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
06th Oct 2009
NRLDC, POWERGRID

31. unacceptable consequences acceptable consequences
Risk Management
Unlikely events with
unacceptable consequences
Likely events with
acceptable consequences
Consequences
Likelihood
06th Oct 2009
NRLDC, POWERGRID

32. 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... ”
06th Oct 2009
NRLDC, POWERGRID

33. 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…”
06th Oct 2009
NRLDC, POWERGRID

34. 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
06th Oct 2009
NRLDC, POWERGRID

35. 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

36. 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

37. 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

38. 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;

39. 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
22 April 2017
NRLDC, POWERGRID

40. How to assess the Transfer Capability

41. 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
13th October 2009
POWERGRID

42. Europe Net Transfer Capacity = TTC – TRM
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)

43. 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.

44. 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
Planning Criteria is strictly followed during simulations 44
06th Oct 2009
NRLDC, POWERGRID

45. Available Transfer Capability is
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

46. 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.

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

48. 13th October 2009
POWERGRID

49. 13th October 2009
POWERGRID

50. What is congestion ?

51. 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

52. Congestion
“Congestion” means a situation where the demand for transmission capacity exceeds the Available Transfer Capability”

53. 13th October 2009
POWERGRID

54. 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
13th October 2009
POWERGRID

55. 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

56. 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

57. Real-time Congestion types
Internal congestion (Intra-zonal)
Within a single System Operator’s 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
13th October 2009
POWERGRID

58. 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

59. Congestion Management
Priority based rules
Pro-rata rationing
Auctioning
Explicit Auction
Implicit Auction
Hybrid
Market splitting
Market coupling
13th October 2009
POWERGRID

60. Congestion Management Lessons learnt in Indian context
Firmness in STOA schedules
“Use it or Lose it”
Valuing transmission instead of pro rata
Market splitting
13th October 2009
POWERGRID

61. Congestion Alleviation Methods
Real-time horizon
13th October 2009
POWERGRID

62. 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
13th October 2009
POWERGRID

63. 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”
13th October 2009
POWERGRID

64. 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