The
resistance of a transmission line is significantly less than its reactance. If
the resistance of the transmission line is very low, its conductance ‘

*G*’ is also very low as compared to the susceptance ‘*B*’. Also under normal operating conditions of the transmission line, the difference in the angles of the voltage at the two buses ‘*i*’ and ‘*j*’ (which is connected by the given transmission line) is usually less than 15^{o}. Now the sine of a very small angle is the angle itself.
Similarly, the
numerical values of the voltage at the two buses are very close to 1.0 p.u. The normal range is
between 0.95 to 1.05 p.u. and the product of these two values is nearly equal
to 1.0 p.u. Hence,
the real power flow in a transmission line is proportional to the circuit
susceptance ‘

*B*’ and the difference in voltage phasor angles.
The
two variables of a DC load flow (DCLF) are the voltage angles and the active
power injections. DCLF gives the estimation of active power flows on AC power
system. It does not consider the reactive power flows. The DCLF is less
accurate than AC load flows. They are used where repetitive and fast load flow
estimations are needed such as the transmission expansion planning.

For
MATLAB coding for DCLF wait for my next blog.