Available Transfer Capability Determination

Available Transfer Capability Determination

Chen-Ching Liu and Guang LiUniversity of Washington

Third NSF Workshop on US-Africa Research and Education Collaboration

Abuja, Nigeria, December 13-15, 2004

TECHNOLOGY &

ENVIRONMENT

ENERGY &

EDUCATION

TECHNOLOGY &

ENVIRONMENT

ENERGY &

EDUCATION

Third US-Africa Research and

Education Collaboration WorkshopAbuja, Nigeria, December 13-15, 2004

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.

Time

Voltage Limit

Stability LimitPower Flow A to B (MW)

Thermal Limit

Total Transfer Capability



6

Available Transfer Capability (ATC) (North American Electric Reliability Council)

MW A->B

TimeOperating Horizon Planning Horizon

Nonrecallable Scheduled

Recallable Scheduled

Nonrecallable Reserved

Recallable Reserved

Total TransferCapability (TTC)Transmission

Reliability Margin

TRM

TRM

Nonrecallable Reserved

Nonrecallable Available Transfer

Capability

Nonrecallable ATC

ATCRecallable

Recallable ATC



7

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



9

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



10

State of the Art: ATC Methods

ATC MethodsATC Methods DescriptionDescription

Linear Approximation MethodDC Power Flow Model,

Thermal Limit Only

Optimal Power Flow Method AC Power Flow Model, Thermal Limit + Voltage Limit

Continuation Power Flow MethodAC Power Flow Model,

Thermal Limit + Voltage Limit (Voltage Collapse)

Stability-Constrained ATC Method Time Domain Simulations with Dynamic Model



11

First Contingency Incremental Transfer Capability (FCITC) & First Contingency Total

Transfer Capability (FCTTC)

FCTTC

FCITC

BASE POWER TRANSFERS



12

Total Transfer Capability (TTC)

• System Conditions

• Critical Contingencies

• Parallel Path Flows

• Non-Simultaneous and Simultaneous Transfers

• System Limits



13

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.



14

Example 1: 2-Area NEPA 330kV Grid



15

2-Area Base-Case Tie Flow

21 23

Area 1

4.64 MW

Area 2

No thermal limit (assumed 120% base case flow) reached

Single transmission line contingency

First thermal limit reached

Notation

Tie Line Flow



16

Area 1 to Area 2 ATC Calculation

21 23

Area 1

> 4.64 MW

Area 2

Increasing

Generation

P MW

Increasing

Demand

P MW

21 23

Area 1

4.96 MW

Area 2

7-25

2-8

Increased

Generation

0.32 MW

Increased

Demand

0.32 MW

FCITC

FCTTC



17

Area 2 to Area 1 ATC Calculation

21 23

Area 1

< 4.64 MW

Area 2

Increasing

Demand

P MW

Increasing

Generation

P MW

21 23

Area 1

4.54 MW

Area 2

7-25

5-24

Increased

Demand

0.1 MW

Increased

Generation

0.1 MW

FCITC

FCTTC



18

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



19

Example 2: 4-Area NEPA 300kV Grid

AREA 1

AREA 2

AREA 3

AREA 4



20

4-Area Base-Case Tie Flows

Area 1

4.64 MW

Area 2Area 4

16.6 MW

Area 3

8.24 MW8.5 MW



21

Area 3 to Area 1 ATC calculation (Example of Parallel Path Flows)

Area 1

4.64 MW

Area 2Area 4

17.2 MW

Area 3

8.65 MW9.1 MW

7-25

1-7

Increased Generation 1.01 MW

Increased Demand 1.01 MW

FCTTC = 9.1+ 8.65 = 17.75 MW

FCITC = 17.75 (8.5 + 8.24) = 1.01 MW



22

Area 4 to Area 2 Simultaneous ATC with a Pre-existing Area 3 to Area 1 17.75 MW Transfer

Area 1

4.85 MW

Area 2Area 4

16.99 MW

Area 3

8.65 MW9.1 MW

7-25

4-10

Increased Generation 0.21 MW

Increased Demand 0.21 MW

FCTTC = 16.99 (17.2) = 0.21 MW

FCITC = 4.85 4.64 = 0.21 MW



23

Optimization Technique to Calculate ATC

A area

maxi

iΔP

0),(

),(

0

),(0

),(

maxmin

maxmax

max

yxEM

VVV

FyxFF

PPP

yxg

yxfx

iii

Objective:

Subject to

sum of generation in sending area A

- system dynamic behavior

- power flow equations

- active power output

- thermal limit

- voltage profile

- energy margin



24

Stability-Constrained ATC

Second-Kick based Energy Margin Computation

Second-Kick based Energy Margin Computation

Time Domain Simulation (ETMSP)

System trajectory

No

Energy Margin Sensitivity Analysis with BFGS Method

Energy Margin Sensitivity Analysis with BFGS Method

Generation AdjustmentGeneration Adjustment

(EM = 0) ?(EM = 0) ?

Yes

ATCATC



25

Second-kick-based energy margin computation

Perform time-domain simulation

Obtain system trajectory following a pre-specified disturbance sequence

Compute potential energy of first- and second-kick trajectories

Potential energy difference at the respective peaksof the first- and second-kick disturbances

- Simulation

- Trajectory

- Potential energy

- Energy margin



26

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

)(,

)(1,

)(

)(

)(1

)(

knm

km

k

kn

k

k

P

EM

P

EM

D

S

S

S



27

Generation adjustment

nm

m

pkn

kn

kk

nm

m

P

P

kEM

S

S

P

P

,

1,

1)()(

1)(1

)(1

,

1,

nm

m

oldnm

oldm

newnm

newm

P

P

P

P

P

P

,

1,

,

1,

,

1,

- Adjustment

- Update



28

2-Area Test System

120 110 11

12

13

101 1

3

2

10 20 G1

G2 G12

G11

• Net power transferred from area A to area B in the base case = 453 MW

453 MWArea A Area B



29

Stability-Constrained ATC Results

Power Transfer

(Area AArea B) & Energy Margin

Gen.

453 MW G1 666 1.9008 1.9008 0.5333 3.745EM = 13.35 G2 754 1.4688 1.4688 0.4667 4.242

461 MW G1 669.745 1.1483 1.1483 0.5161 4.935EM = 10.98 G2 758.242 0.9503 0.9503 0.4838 5.59471.6 MW G1 674.68 0.4748 1.4562 0.3757 2.833EM = 5.50 G2 763.832 0.0864 2.136 0.6242 3.208

G1 677.51 -1.417G2 767.04 -1.604

474.6 MW G1 676.094 1.8204 1.8204 0.4809 0.587EM = 2.22 G2 765.436 1.7345 1.7345 0.5191 0.665475.8 MW G1 676.681 2.4251 2.4251 0.5125 0.047EM = 0.22 G2 766.101 2.0364 2.0364 0.4875 0.054

(MW)

1

2

4 477.6 MW EM = N/A

System goes unstable with

the 1st kick disturbance.

(MW)

5

6

Iteration

3

kmP ,

kmP

EM

,

ksk kmP ,



30

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.

Available Transfer Capability Determination

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