Transfer Capacity & Congestion Concepts Abhimanyu Gartia WRLDC.

Transfer Capacity & Congestion Concepts

Abhimanyu GartiaWRLDC

Learning Objectives1. 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 system10. Does congestion exist in other countries?11. How to manage congestion?12. What are the methods to alleviate congestion?

1. What is Transfer Capability ?

06th Oct 2009 NRLDC, POWERGRID 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”

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 5NRLDC, POWERGRID

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.

April 19, 2023 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.

April 19, 2023 NRLDC, POWERGRID 9

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

What is the relevance of Transfer Capability

in the Indian Electricity Market ?

Provisions in Electricity Act 200328(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 20032 (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 & 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.”

13th October 2009 15POWERGRID

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_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

06th Oct 2009 NRLDC, POWERGRID

Transmission Capacity Vis-à-vis Transfer CapabilityTransmission 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 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.

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 inLiberalised 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 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

2

3

A B


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


Risk Management

Unlikely events with

unacceptable consequences

Likely events with

unacceptable consequences

Unlikely events with

acceptable consequences

Likely events with

acceptable consequences

Likelihood

Cons

eque

nces

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...– ”


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…”


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- TRM06th Oct 2009 34NRLDC, POWERGRID


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


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”

August 08, 2007 GSIOAR-2007, IT-BHU, Varanasi 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

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;

April 19, 2023 NRLDC, POWERGRID 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

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.

13th October 2009 POWERGRID 41

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.

44

Transfer Capability assessment

AnticipatedNetwork topology +Capacity additions

Anticipated Substation Load

Anticipated Ex bus

Thermal Generation

Anticipated Ex busHydro generation

LGBR

Last Year

Reports

WeatherForecast

Trans.Plan +

approv.S/D

Last Year

patternOperator

experience

Planning criteria

Operating limits

Credible contingencies

Simulation

Analysis

Brainstorming

Transfer Capability

ReliabilityMargin

less

AvailableTransfer

Capability

equals

Planning Criteria is strictly followed during simulations06th Oct 2009 44NRLDC, POWERGRID


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

TTC ATC

RM

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 ?

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

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

Visibility of congestion• 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 before-the fact

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.” 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


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 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 gridAggressive 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 context

• 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 operation


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”


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


Transfer Capacity & Congestion Concepts Abhimanyu Gartia WRLDC.

Documents

Transcript of Transfer Capacity & Congestion Concepts Abhimanyu Gartia WRLDC.