2. Introduction
 The restructuring of the electric industry throughout the world
aims to create competitive markets to trade electricity and
generates a host of new technical challenges to market
participants and power system researchers.
 For transmission networks, one of the major consequences of
the non-discriminatory open-access requirement is a substantial
increase of power transfers, which demand adequate available
transfer capability (ATC) to ensure all transactions are
economical.
 With the introduction of competition in the utility industry, it is
possible for customers to buy less expensive electrical energy
from remote location. As a result, system operators face the
need to monitor and coordinate power transactions taking place
over long distances in different areas. Therefore, it becomes
essential to evaluate multi-area ATC

3. What is ATC
 Available Transfer Capability (ATC) is a measure of the
transfer capability remaining in the physical transmission
network for further commercial activity over and above
already committed uses. Mathematically, ATC is defined
as the Total Transfer Capability (TTC) less the
Transmission Reliability Margin (TRM), less the sum of
existing transmission commitments (which includes retail
customer service) and the Capacity Benefit Margin
(CBM).
ATC = TTC – TRM – CBM - Existing Transmission
Commitments

4. ATC and Related Terms
 ATC and related terms are depicted graphically below. They form the basis
of a transmission service reservation system that will be used to reserve and
schedule transmission services in the new, competitive electricity market.

5. ATC and Related Terms
 Transfer capability is the measure of the ability of interconnected
electric systems to reliably move or transfer power from one area to
another over all transmission lines (or paths) between those areas
under specified system conditions. The units of transfer capability are
in terms of electric power, generally expressed in megawatts (MW).
 Limits to Transfer Capability:
 The ability of interconnected transmission networks to reliably transfer
electric power may be limited by the physical and electrical characteristics of
the systems including any one or more of the following:
 Thermal Limits — Thermal limits establish the maximum amount of
electrical current that a transmission line or electrical facility can conduct
over a specified time period before it sustains permanent damage by
overheating or before it violates public safety requirements.
 Voltage Limits — System voltages and changes in voltages must be
maintained within the range of acceptable minimum and maximum limits.
For example, minimum voltage limits can establish the maximum amount
of electric power that can be transferred without causing damage to the
electric system or customer facilities.
 Stability Limits — The transmission network must be capable of
surviving disturbances through the transient and dynamic time periods
following the disturbance.

6. Some Terminology
 Total Transfer
capability (TTC)
 TTC is the amount of
electric power that can be
transferred over the
interconnected
transmission network in a
reliable manner based on
certain conditions.
 TTC = Minimum of
{Thermal Limit, Voltage
Limit, Stability Limit}

7. Transmission Transfer Capability Margins
 Two types of transmission transfer capability margins includes:
 Transmission Reliability Margin (TRM) — to ensure the secure
operation of the interconnected transmission network to
accommodate uncertainties in system conditions.
 Capacity Benefit Margin (CBM) — to ensure access to
generation from interconnected systems to meet generation
reliability requirements.
 Individual systems, power pools, subregions, and Regions
should identify their TRM and CBM procedures used to
establish such transmission transfer capability margins as
necessary.
 TRM and CBM should be developed and applied as separate and
independent components of transfer capability margin. The
specific methodologies for determining and identifying
necessary margins may vary among Regions, sub-regions, power
pools, individual systems, and load serving entities.

8. TTC & ATC Calculation Techniques
 Linear sensitivity: This method gives fast but low accuracy calculation
of the ATC by using linear sensitivity without solving power flow
solution. The method employs the DC Power Transfer Distribution
Factor (DC-PTDF).
 The repeated power flow method: It is performed by slightly increase
demand at the receiving zone and increase dispatched power
generation from the sending zone to cover increased demand and
losses in the considered power system.
 The continuation power flow method: is used to find the bifurcation
point or point of voltage collapse when load in power system increases
up to a certain amount.
 Optimization method : The conventional optimization methods that are
applied to solve the power flow calculation can be employed to
determine the ATC value with typical considered constraints.
Methods like the heuristic optimization such as Genetic Algorithm
(GA), Tabu Search (TS), Simulated Annealing (SA) etc. can be used
to calculate the ATC associated with the power flow calculation.

9. TTC & ATC Calculation Techniques
 Probabilistic approach: In it, the variation of power generation
is taken into account. Therefore, they give more information of
the practical system which normally has demand and supply
fluctuations.
 Statistic approaches such as Monte Carlo simulation , stochastic
programming and bootstrap technique can be implemented.
 Due to its robustness, speed and ability to deal with incomplete
or noisy data, the Artificial Neural network (ANN) becomes
interesting method for ATC estimation under a certain condition
of power system.
 OPF Method: formulates some objective functions constrained
by equality and inequality equations, so it could take transient
stability constraint and dynamic stability constraint into account
and may optimize the distribution of generation and load
patterns. AC power transfer distribution factors (ACPTDF)

10. Performance Comparisons of ATC
Methods