Available Transfer Capability Determination

1
Available Transfer Capability Determination
Third NSF Workshop on US-Africa Research and
Education Collaboration Abuja, Nigeria, December
13-15, 2004

• Chen-Ching Liu and Guang Li
• University of Washington

2
Overview

• Background of Available Transfer Capability (ATC)
• Definitions of ATC
• Determination of ATC
• Examples of ATC in Nigerian NEPA 330kV Grid
• Optimization Technique to Calculate ATC
• Stability-Constrained ATC Calculation Method
• Conclusions

3
Background

• ATC is the transmission limit for reserving and
  scheduling energy transactions in competitive
  electricity markets.
• Accurate evaluation of ATC is essential to
  maximize utilization of existing transmission
  grids while maintaining system security.

4
Transmission Service Types

• Recallable transmission service Transmission
  service that a transmission provider can
  interrupt in whole or in part.  
• Non-recallable transmission service Transmission
  service that cannot be interrupted by a provider
  for economic reasons, but that can be curtailed
  for reliability.

5
ATC Under Operating Constraints

• Transfer capability must be evaluated based on
  the most limiting factor.

6
Available Transfer Capability (ATC) (North
American Electric Reliability Council)
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Definition of ATC

• ATC TTC TRM Existing Transmission
  Commitments (including CBM)
• Transmission Transfer Capability Margins
• Transmission Reliability Margin (TRM)
• Capacity Benefit Margin (CBM)

8
Transmission Reliability Margin (TRM)

• Uncertainty exists in future system topology,
  load demand and power transactions
• TRM is kind of a safety margin to ensure reliable
  system operation as system conditions change.
• TRM could be 8 or 10 of the TTC

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Capacity Benefit Margin (CBM)

• CBM is reserved by load serving entities to
  ensure access to generation from interconnected
  systems to meet generation reliability
  requirements.
• Intended only for the time of emergency
  generation deficiencies

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State of the Art ATC Methods
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First Contingency Incremental Transfer Capability
(FCITC) First Contingency Total Transfer
Capability (FCTTC)
FCITC
FCTTC
BASE POWER TRANSFERS
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Total Transfer Capability (TTC)

• System Conditions
• Critical Contingencies
• Parallel Path Flows
• Non-Simultaneous and Simultaneous Transfers
• System Limits

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Procedure to Calculate TTC

• Start with a base case power flow
• Increase generation in area A and increase demand
  in area B by the same amount
• Check the thermal, stability and voltage
  constraints.
• Evaluate the first contingency event and ensure
  that the emergency operating limits are met.
• When the emergency limit is reached for a first
  contingency, the corresponding (pre-contingency)
  transfer amount from area A to area B is the TTC.

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Example 1 2-Area NEPA 330kV Grid
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2-Area Base-Case Tie Flow
Single transmission line contingency
Notation
No thermal limit (assumed 120 base case flow)
reached
First thermal limit reached
4.64 MW
Tie Line Flow
21
23
Area 1
Area 2
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Area 1 to Area 2 ATC Calculation
gt 4.64 MW
Increasing Generation DP MW
Increasing Demand DP MW
21
23
Area 1
Area 2
Increased Demand 0.32 MW
Increased Generation 0.32 MW
4.96 MW
7-25
21
23
2-8
FCTTC
Area 1
Area 2
FCITC
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Area 2 to Area 1 ATC Calculation
lt 4.64 MW
Increasing Demand DP MW
Increasing Generation DP MW
21
23
Area 1
Area 2
Increased Generation 0.1 MW
Increased Demand 0.1 MW
4.54 MW
21
23
5-24
7-25
FCTTC
Area 1
Area 2
FCITC
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2-Area ATC Calculation
Direction Area 1 to Area 2 Area 2 to Area 1
Critical Contingency Line 7-25 (Delta-Aladja) Line 7-25 (Delta-Aladja)
Thermal Limit Reached Line 2-8 (Jebba G.S.-Jebba T.S.) Line 5-24 (Alam-Aba)
FCTTC 4.96 MW ?4.54MW
FCITC 0.32 MW 0.1 MW
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Example 2 4-Area NEPA 300kV Grid
AREA 1
AREA 3
AREA 2
AREA 4
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4-Area Base-Case Tie Flows
Area 1
8.24 MW
8.5 MW
16.6 MW
4.64 MW
Area 4
Area 2
Area 3
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Area 3 to Area 1 ATC calculation (Example of
Parallel Path Flows)
Area 1
FCTTC 9.1 8.65 17.75 MW FCITC 17.75 ? (8.5
8.24) 1.01 MW
Increased Demand 1.01 MW
8.65 MW
9.1 MW
17.2 MW
4.64 MW
1-7
7-25
Increased Generation 1.01 MW
Area 4
Area 2
Area 3
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Area 4 to Area 2 Simultaneous ATC with a
Pre-existing Area 3 to Area 1 17.75 MW Transfer
Area 1
FCTTC ?16.99 ? (?17.2) 0.21 MW FCITC 4.85 ?
4.64 0.21 MW
8.65 MW
9.1 MW
Increased Generation 0.21 MW
16.99 MW
4.85 MW
4-10
Increased Demand 0.21 MW
7-25
Area 4
Area 2
Area 3
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Optimization Technique to Calculate ATC
sum of generation in sending area A
Objective
- system dynamic behavior - power flow equations
Subject to
- active power output - thermal limit - voltage
profile - energy margin
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Stability-Constrained ATC
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Second-kick-based energy margin computation
Perform time-domain simulation
- Simulation
Obtain system trajectory following a
pre-specified disturbance sequence
- Trajectory
Compute potential energy of first- and
second-kick trajectories
- Potential energy
Potential energy difference at the respective
peaks of the first- and second-kick disturbances
- Energy margin
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Energy margin sensitivity computation

• Determine the search direction with the
  Broyden-Fletcher-Goldfarb-Shanno (BFGS) method

D is an approximation to the inverse of Hessian
matrix
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Generation adjustment
- Adjustment
- Update
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2-Area Test System
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Stability-Constrained ATC Results
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Conclusions

• ATC provides a reasonable and dependable
  indication of available transfer capabilities in
  electric power markets.
• ATC considers reasonable uncertainties in system
  conditions and provides operating flexibility for
  the secure operation of the interconnected
  network.
• The effects of simultaneous transfers and
  parallel path flows are studied.
• Need for ATC calculation method to incorporate
  voltage, angle stability limits as well as
  thermal limits.

31
References

• 1 North American Electric Reliability Council,
  Available Transfer Capability Definitions and
  Determination, June 1996.
• 2 North American Electric Reliability
  Council,Transmission Transfer Capability, May
  1995.
• 3 S. K. Joo, C. C. Liu, Y. Shen, Z. Zabinsky
  and J. Lawarree, Optimization Techniques for
  Available Transfer Capability (ATC) and Market
  Calculations, IMA Journal of Management
  Mathematics (2004) 15, 321-337.