1. Power Flow in Transmission Line
Presented by
T.S.L.V.Ayyarao
Assistant Professor
GMRIT

2. Power Flow in Transmission Line
Fig 1: A Simple two-machine System
 E1 and E2 are the magnitude of the bus voltages,δ the
angle between two and X the line reactance
 The driving voltage drop EL is phasor difference E1-E2
 The line current I = EL/X and lags EL by 900
 The current flow in the line can be controlled by
controlling EL or X or δ

3. Power Flow in Transmission Line
 The rating of series controller would be a fraction of the
rating of the line
 If the angle δ is small, the current flow
largely represents the active power
 Increase or decrease of line
reactance X will greatly affect Fig 2. Phasor diagram
the active power flow
 It is the cost effective means of controlling the power
 The active power at E1 end is P1 = E1E2 sin δ/X
 Reactive power at E1 end is Q1 = E1(E1-E2 cos δ)/X

4. Power Flow in Transmission Line
 The active power at E2 end is P2 = E1E2 sin δ/X
 Reactive power at E1 end is Q1 = E2(E2-E1 cos δ)/X
 Active power flow increases
up to δ = 900 and then falls to 0
 Control is possible well below
δ = 900
 Sufficient margin is required
for transient and dynamic
stability
 Increase or decrease of X will
raise or lower the curves Fig 3. power angle curves for
different X

5. Power Flow in Transmission Line
 Power flow can be controlled by regulating the
magnitude of E1 or E2
 The driving voltage EL doesn’t
vary by much but its phase
angle does
 The change of magnitude
Fig 4. regulating the magnitudes
has much effect on of voltages
the reactive power than
the active power flow

6. Power Flow in Transmission Line
Fig. 5 (a) quadrature (b) with phase angle
 Current and hence power flow can be controlled by
injecting a voltage in series of the line
 By varying the magnitude and phase angle of the
injected voltage, active and reactive power flow can be
controlled.