By changing the reactive power in the transmission line, voltage profile also changes.
My question is, how reactive power injection into grid changes the grid voltage? and is there any relation between the grid short circuit level and reactive power injection?
Dear friend,
Please, you refer to "Reactive Power Control in Electric Systems, 1st Edition, by Timothy J. E. Miller (Editor)".
Voltage profile developments in the power system greatly depend on the reactive power (Q). It is all about the phasor shift and the second Kirchhoff's law. The sending end voltage which represent the network connection point, can be reduced by shifting the current phase in front of the receiving end voltage. This is done by the changing the sign of the Q. To put it simply, absorption of Q at node reduces the voltage, and delivery of Q increases it. Now, the relationship to the short circuit levels becomes apparent from the equation S= 3xIU.
Active power (P), W=VICos(phi), when angle phi is zero, would represent resistive circuit and W=VI. Reactive power(Q), is VISin(phi), or when phi is zero Q=zero, or resistive circuit represent(consumes) only active power. When angle phi is 90 degree, W=zero, Qmax=Pure inductive(L) or capecitive(C) effect responsible for maximum voltage rise(C) or voltage reduction(L). Or could be said voltage control could be done with injection of reactive power(L or C effect in circuit) for same active power transfer as (S=P +/- jQ), if +for voltage rise then - for voltage reduction. Practically long transmission line voltage would be raised(C), while for short or medium(L) voltage would reduced than sending end voltage.The increased impedance due to raised reactance could als reduced the fault current...
The following presentation, and pp 227-230 of the book include the basics of the voltage control of power lines,
https://yadi.sk/i/_YTrjJxo-xiBAw
Book Dynamic Security of Interconnected Electric Power Systems - ...
You will easily understand that the reactive power injected from a shunt reactive power source at a given location alters the reactive power flow on the network. As a result, the voltage drop is also changed. In the presentation, it is shown that injecting a capacitive reactive power at the terminals of a lagging power factor load results in reducing the reactive power demand of the load as seen by the network. Consequently, the current flow over the network link is reduced, which results in voltage drop reduction i.e. increase of the voltage magnitude at the load terminals.
The relation you asked for can be expressed through relative voltage change in a given point of the system:
d = (ΔP cosΨ)/Ssc + (ΔQ sinΨ)/Ssc
where ΔP and ΔQ present a load change, Ψ phase angle of the network impedance and Ssc short circuit power at that point in the system.