Tap Changing (ULTC) Transformers and Reactive Power Control

[powereng]

So I am trying to understand how a ULTC operates in the Reactive Power Control mode (it adjusts taps to control the reactive power in a line). What equation is governing it's operation, and what variables does it adjust in order to increase/decrease reactive flow?
Is it just the simple reactive power flow equation between two buses:

[ tex ] Q_sending = V_sending V_receiving (\frac{V_sending}{V_receiving} - cos(\delta) [ /tex ]

?

And then the transformer just adjusts the sending (or receiving) end voltage, and this modulates the reactive power flow?
Thanks in advance for any help. I've tried looking up ULTC or OLTC reactive power control, but did not come up with much.

 

[zoki85]

Hi,

Voltage regulation and power flow through xfrmr can be infuenced by changing turns ratio of xfrmr; this changes impedance of xfrmr and impedance of load seen by network. If you are familiar with "T" and "π" schematic representations of an ideal xfmr, I believe you can derive equation of depedence of power flow exclusively on turns ratio n=N₁:N₂ (I don't know exact equation from memory). Number of turns of ULTC regulating xfrmrs are changed via taps on higher voltage winding due to smaller currents and better step discretization condition there. Steps are something like 1% and regulating ranges like ±10%. However, reactive powers and voltages in EPS are intimatelly related and we rather talk about Q-V regulation than about these things separately. This is especially important for HV grid where effects of reactive parameters of power transmission lines are unvoidable. Regulating transformers are not enough and reactive powers are regulated to significant extent by other means (synchrounous compensators, inductive shunt reactors, SVCs..)