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 15o. 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.