Reactive current to lower the voltage at the generator?

[Buoyen]

I had a little discussion with a friend about how reactive current influate voltage drop in a distribusion line.

The case:

If I have a generator on one side of a line and a load on the other side. The load needs a specific voltage. Now, I'd need a bit higher voltage on the side with the generator because of the loss in the line. But let's say there was something close to the generator which couldn't handle the extra voltage (another load. f.ex). That means I can't have (much) higher voltage on this side. Then my friend claims that we could minimize the difference in voltage by making a reactive current flow through the line.

I've had some about impedance, reactive current, i etc.. But I don't have a clue about this one. Anyone can tell me about this?

If something is wrong or unclear, please excuse me, I'll trye to explain as good as possible! :)

http://img341.imageshack.us/img341/5823/54660099.th.jpg [Broken]

 

[Bob S]

I don't think that adding extra reactive current will reduce the voltage drop if the load impedance is real. The actual line current is the quadrature sum of the real and reactive components, which is greater than the real current alone, and the line power loss is I²R. There are some instances when the load is reactive, adding some compensating reactance to the line will help, but not in this case. I would recommend some compensating reactance to the line at the load , like capacitors to correct a VAR, to zero out the reactive current in the line.

There is the special case of the line being a quarter wavelength long, in which case you could make the line a quarter wave transformer, with a low voltage (voltage node) at the generator.

Bob S

 

[m.s.j]

Under the most usual operating conditions, the network is predominantly inductive (the situation also can be inverted at least partially in some cases, e.g., during light load hours, with reactive power generated by lines which are relatively unloaded and increasing voltages from generators to loads) and thus:

-reactive powers absorbed by the network itself (e.g., lines, transformers, terminals of dc links, etc.) must be considered, in addition to reactive powers demanded by loads;
-the transportation of reactive powers from generators to loads would usually imply, in addition to the previously mentioned absorption, unacceptable voltage drops (in the same sense, i.e., from generators to loads) if proper actions are not undertaken within the network as specified in the following.

On the other hand, reactive power injections that can be achieved by generators are generally not sufficient for:

-the matching of the total reactive power demand, because of the limits on generators themselves;
-the accomplishment of an acceptable voltage steady-state, because of the concentration of generators in relatively few “sites” (consistent with technical, environmental constraints etc.).

It is then convenient to intervene also within the transmission and distribution networks and near to loads, for instance by:
- injecting reactive power (usually positive, or possibly negative as mentioned above) using shunt condensers or inductors or, for more general control functions, static or synchronous compensators;
- reducing the total absorbed reactive power using series condensers;
- adjusting voltage levels using tap-changing transformers.

The voltage “support” along a line, aside from that which is intrinsically due to series inductances and shunt capacitances of the line itself:
- may make it possible to have acceptable voltage values at any location (also matching the requirements of possible intermediate loads);
- also can be important in relation to the transfer limit of the active power

Regarding the generic load, the addition of a shunt reactive element to absorb a reactive power Q R, and zero active power, obviously provides the capability to adjust the load voltage and/or the overall required reactive power. As an important additional effect, harmonic and/or flicker filtering can be obtained.
Please refer to attached diagram and formula for determination of end line voltage in addition of a shunt reactive element at a load node case.


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