This document discusses methods for calculating available transfer capability (ATC) in electric power systems. It describes three main categories of ATC determination: 1) methods using multiple load flow and continuation power flow simulations, 2) optimization-based power flow methods, and 3) methods using linear sensitivity factors. For the first method, load flow is run incrementally while checking for violations, and continuation power flow finds the maximum loadability. The second method formulates ATC as an optimization problem. The third method uses distribution factors derived from DC and AC power flow to calculate ATC based on transaction impacts on line flows.

1. “Course Name: Restructured Power System”
“ATC Calculation:Part-I”
Mrs.Pooja Jain,Assistant Professor
Department of Electrical Engineering
Swami Keshvanand Institute of Technology, Management &
Gramothan, Ramnagaria, Jagatpura, Jaipur

2. Introduction
ATC of a transmission network is its unutilized transfer capacity that is available for further
transactions to the market participants. Power transaction between a specific seller bus/area and a
buyer bus/area can be committed only when sufficient ATC is available for that interface to ensure
system security. The information about the ATC is to be continuously updated and made available to
the market participants through the internet based system such as open access same time information
system(OASIS).

3. Methods of ATC Determination:
The methods used for the ATC determination can be classified under
three categories, namely,
• Method based on multiple load flow and continuation power flow
• Method based on Optimization Power Flow
• Method based on linear sensitivity factor

4. Method based on multiple load flow and
continuation power flow
• This method runs ac load flow for each
increment of transaction between an
interface and checks whether any of the
operating conditions such as line flow limit
or bus voltage limit is violated.
• For the increment of transaction ,CPF is
also run to find out the maximum load
ability or voltage instability point. The
minimum out of two critical transaction
values is taken as the TTC for the system
intact condition. This is repeated for each
outage case and worst case is used to
declare the TTC and ATC values. A
procedure to calculate ATC based on this
concept is shown in fig.:-

5. Optimization based Method:
The ATC determination can be formulated as an optimization problem which finds out the maximum
value of the transaction between given interface while satisfying the network power balance and
security constraints such as line flow, voltage limits and voltage instability. Hamoud proposed an
optimization based method using dc load flow. A large fictitious load and a large fictitious generator
with very high cost function were assumed to be connected at the buyer bus and a large but cheap
generator connected to the seller bus. The generation at the seller bus was maximized through ELD.
ATC was computed from the optimal output of the fictitious generator at the seller bus.

6. ATC Determination using Linear sensitivity
factors:
• Linear sensitivity factors offer a great potential for real time calculation of ATC.
• Christe et.al and Ejebe et.al have discussed the use of DCPTDF’sand line outage distribution
factors for the ATC determination.But these factors are derived based on dcload flow assumptions
and hence provides less accurate results.
• Ashwani et.al have proposed a new set of ACPTDF’s,VDF’s and outage distribution factors to
determine ATC more precisely.
• The coefficient of linear relationship between the amount of a transaction and the flow on a line
has been termed Power transfer distribution factor(PTDF’s)

7. DCPTDF’s:
• A transaction is a specific amount of power that is injected into the system at a bus by a generating
company and consumed at another bus by a consumer.
• The linearity property of the dc power flow model has been used to find the transaction amount
that would give rise specific power flow, such as an interface limit.The PTDF defined from dc load
flow is being called DCPTDF.
• Using DC power flow,the real power flow(Pij) in a line between buses i and j can be defined as :

8. Contd…
• For a given real power injection vector [P],the dc load flow can be used to solve the us angle vector
as :
• If Xpq denotes the pq th element of the matrix X, the DCPTDF for a transaction between buses m
and n and flowin line –ij can be expressed as:
• The change in line flow associated with a new transaction is given by :

9. ACPTDF’s:
• The ACPTDF’s are proposed for calculation of ATC were used to find various transmission system
quantities for a change in MW transaction at different operating conditions.
• If we consider bilateral transaction tk between a seller bus m and buyer bus n. Line 1 carries the
part of transacted power and is connected between buses i and j .
• For a change in real power ,transaction among the above buyer and seller by tk MW, if the change
in a transmission line quantity q1 is q1,PTDF can be defined as:

10. Thank You