When Can I Sell Power?
Prepared by Florida Power & Light
Transmission and Substation
Max capacity:
400 MW
Company A
Max capacity:
400 MW
Max capacity:
400 MW
Company B
Company C
•Assume a system with Companies A, B,
and C.
•The companies are interconnected with
lines that are rated at 400 MW each.
?
Company B
Company A
200 MW
Company C
•Now lets say Company A is currently selling
200 MW to Company C.
•Question: How many MW can A now sell to B?
•If the line is rated 400 MW, shouldn’t that be
the answer?
200 MW + 400 MW = 600 MW
Overload!
Company A
400 MW
Company B
Company C
•Suppose a sale of 400 MW from made from A to B
•If a fault occurs between A and B (meaning that some disaster or accident
takes the line out of service so that it can no longer transfer power)
•The 400 MW that was flowing from A to B must now pass through lines A to
C and C to B to reach its destination
• The line from A to C now has the original 200 MW plus the rerouted 400
MW flowing through it, for a total of 600 MW
• With the line from A to C rated for only 400MW the line is overload which
can damage or destroy the line
200 MW + 200 MW = 400 MW
Company A
200 MW
Company B
Company C
•In order to sell power from A to B , the system must
be “first contingency safe” or be able to withstand the
loss of an single line without overloading any part of
the system.
•So in the above case, A can only sell 200 MW to B
without risking the line from A to C
?
Company B
Company A
200 MW
Company C
•Now lets change our question slightly. What if B
wanted to sell to A? Can they also only send
200 MW?
•Lets look at what happens if the same fault
between lines A and B occurs
200 MW - 200 MW = 0?
Company A
200 MW
Company B
Company C
•The 200 MW now must flow indirectly to A by passing
through C. Is the line between A and C at its 400 MW
capacity since it was carrying 200MW initially?
• Actually, the line would be carrying no power through it.The
reason is, power is dependent on direction, so the 200 MW
from A to C will cancel out the 200 MW from C to A, for a net
flow of 0 MW!
400 MW - 200 MW = 200
Company A
400 MW
Company B
Company C
•This means B can sell more than our original guess of
200 MW to A without fear of overloading any part of the
system if a line faults
• B can safely sell up 400 MW to A
•So even though A can only sell 200 MW to B, B can sell
400 MW to A even though they are connected to the
same lines!
Final Thoughts
The actual power grid is much more
complicated than just three locations. At
Transmission and Substations, we
constantly calculate every scenario for
thousands of lines each day.
 By preventing certain transactions, these
precautions help provide reliable
electricity to FPL customers.
