Dear all,

plz give your comments on following.

Three Alternators sychronised with the grid and off course parallel with each other generating some MW and MVAR. Now suppose we raise the excitation of one of the alternators, its internal voltage will rise but the terminal voltage/grid voltage will remain same if grid voltage is not changing(plz correct me if am wrong).

1) Will internal generated voltage of alternator be same in magnitude to field excitation voltage but opposite in phase or polarity?

2) Practically when the alternator is connected to grid, how we will differentiate between terminal voltage and internal voltage of alternator? At which physical location of the alternator winding this internal voltage will be developed?

3) While three alternators in parallel with the grid are giving different MVARs so will there internal voltage will be different and will there be any circulating current flow between the alternators?

4) What will happen to No. 3 query if the alternators are in parallel but not connected to the grid and we have our own home load.

thanks n regards,
Ashish

 

Alternator circulating currents

question 1 - Will internal generated voltage of alternator be same in magnitude to field excitation voltage but opposite in phase or polarity?

take a look at the pic below.

http://www.2shared.com/photo/EjJAliSc/field_voltage.html

From the field voltage the field current is produced. assuming that it is purely inductive, it will lag the field voltage by 90deg. But the phase of the field flux and the field current is the same. Thus the generator internal voltage produced E will lag this field flux by 90 deg. Thus from the vector diagram you can see that the field voltage and generator internal voltage are opposite in phase.

Question 2 - Practically when the alternator is connected to grid, how we will differentiate between terminal voltage and internal voltage of alternator? At which physical location of the alternator winding this internal voltage will be developed?

There is no way to measure the internal voltage. the voltage measured is the terminal voltage. But the internal voltage can be calculated by the formula

E = V + I*xd

where V is the terminal voltage, I the current output vector, Xd the leakage impedance of the generator (this value is supplied by the manufacturer)

Question 3 - While three alternators in parallel with the grid are giving different MVARs so will there internal voltage will be different and will there be any circulating current flow between the alternators?

Well take the look at the pic again. if it is connected in the fashion all in a single bus and all of them have the same impedance then there will be no circulating currents. IT depends on how your system looks like, ie if you can provide a SLD then it is possible to tell. Check up with the impedance values of your generators. On the same area the circulating current problem is more important in parallel operation of transformers. i have not really given it a thought for generators till now.

Question 4 - What will happen to No. 3 query if the alternators are in parallel but not connected to the grid and we have our own home load.

Again i need to see your sld and have some impedance data to answer your question. Try to learn Etap or any power flow program; then you will be able to work it out on your own.

hope that answers the questions

 

PV... I must take issue with your statement there is 'no' circulating current. While in many cases it is immeasurable, it is always present.

One need only observe, via a strobe-light, the small but present oscillation of the torque or power angle in generators that are paralleled.

When unparalleled, the oscillation virtually disappears!

Regards, Phil Corso ([email protected])

 

If the impedances are equal and so are the terminal voltages, i believe that the circulating current will be zero. there may be minor oscillation in terminal voltage and phase angles which i believe for all practical measurement purposes to be zero. But of course you are free to disagree with me .

As i have said never given this topic of circulating current in generators much thought. It is more important in parallel operation of transformers in the distribution side.