Active and Reactive Power Flow

Active and Reactive Power Flow

AU : May-15, Dec.-16

The flow of active power (P) and the reactive power (Q) through transmission system has influence on voltage magnitudes and phase difference of voltages at terminals and voltage along the line. Now let us consider active and reactive power flow through short transmission line. 

Consider a short transmission line as shown in Fig. 2.18.1. Neglecting shunt admittance and series resistance we get

From the above expressions it can be seen that the transfer of real power P depends only on angle δ called the power angle and not on relative magnitudes of sending end and receiving end voltages. The transmitted power varies approximately to square of voltage level. When δ = 90°, maximum power is transfered.

P_Rmax = P_Smax = | V_S | | V_R | / X

If the system operates with δ = 0 then average reactive power flow over the line is given as,

Thus reactive power flow depends on the difference in squares of voltages. The voltages at sending and receiving end are held within permissible limits by means of compensation of reactive power which helps in improving steady state stability. In absence of reactive power compensation instability in voltage occurs.

Hence in summary it can be stated that active power flow is determined by power angle δ and the reactive power flow determines voltage magnitudes. Both active power flow and magnitudes of Vs and VR determine the stability of transmission line.

Review Question

1. Derive the expression for the real and reactive power flow through transmission lines.

AU : May-15, Dec.-16, Marks 8