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” 06th Oct 2009 NRLDC, POWERGRID 4

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 5

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 considering security aspects of the grid.

system

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.

25 April 2020 8

Power Flow

Total Transfer Capability: TTC

Thermal Limit Voltage Limit Stability Limit Total Transfer Capability

25 April 2020

Time Total Transfer Capability is the minimum of the Thermal Limit, Voltage Limit and the Stability Limit

NRLDC, POWERGRID 9

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 growth available due to in-built spare transmission capacity created to cater to future load

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.”

13th October 2009 POWERGRID 15

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 16

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

1 2 3 4 5 6 7 8 9

Transmission Capacity Vis-à-vis Transfer Capability

Transmission Capacity Declared by designer/ manufacturer Is a physical property in isolation Depends on design only Deterministic Constant under a set of conditions Time independent Non-directional (Scalar) Determined directly by design Independent of Parallel flow Transfer Capability Declared by the Grid Operator Is a collective behaviour of a system Depends on design, topology, system conditions, accuracy of assumptions Probabilistic Always varying Time dependent Directional (Vector) Estimated indirectly using simulation models Dependent on flow on the parallel path

06th Oct 2009 NRLDC, POWERGRID

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

Source: NERC 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.

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 24

•

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 13th October 2009 POWERGRID 26

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

• • • • • • • • Suggestions for improving transfer capability Installation of shunt capacitors low voltage in pockets prone to high reactive drawal & 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, 2 nd May 2007 06th Oct 2009 NRLDC, POWERGRID 30

06th Oct 2009

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...

– ” 06th Oct 2009 NRLDC, POWERGRID 32

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 33

•

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 flow

s 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 34

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 35

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 36

• • • • • • •

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 25 April 2020 NRLDC, POWERGRID 39

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 13th October 2009 POWERGRID 41

• • •

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.

Trans.

Plan + approv.

S/D

Transfer Capability assessment

Planning criteria Credible contingencies Anticipated Network topology + Capacity additions LGBR Last Year Reports Weather Forecast Last Year pattern Anticipated Substation Load Anticipated Ex bus Thermal Generation Anticipated Ex bus Hydro generation Simulation Analysis Brainstorming Operating limits Operator experience Transfer Capability

less

Reliability Margin

equals

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

13th October 2009 POWERGRID 45

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 ?

5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 Total Transfer Capability for import of power in Northern region

13th October 2009 POWERGRID 48

7000 Transmission capacity versus transfer capability for inter-regional links during September 2009 Transmission capacity 6000 Forward Transfer Capability 5000 Reverse transfer 4000

51 % 93 %

3000 2000

53 % 36 % 32 % 58 %

1000

6 % 16 % 20 % 16 %

0 ER-NR ER-WR WR-NR ER-SR Inter-regional link WR-SR ER-NER

13th October 2009 POWERGRID 49

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

POWERGRID 13th October 2009 51

Congestion

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

13th October 2009 POWERGRID 53

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 54

•

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 55

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 56 13th October 2009 POWERGRID

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 57

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 58

Congestion Management

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

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 60

Congestion Alleviation Methods

Real-time horizon 13th October 2009 POWERGRID 61

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 62

Congestion Alleviation Methods

• • • • Counter trading Re-dispatching (Out of merit generation) Locational Marginal Pricing (LMP) λ node = λ deviation price + λ congestion charge + λ losses Transmission Loading Relief (TLR) All these methods would result in significant rise in total cost. “Price for system security” 13th October 2009 POWERGRID 63

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 64