Circulating Currents in Generator Grounding Resistors

[mrg397cs]

I have 4-13.8 kV generators that have low resitance (150 amps)grounded neutrals. The generators are the same rating, have 2/3 pitch and are from the same manufacturer. Does anyone have a good reference (formulas) to calculate the circulating harmonic current between generators and resistors?

 

[alehman]

If the generators are identical, theoretically 0. Reality is there will always be small differences. The following article may provide some guidance.

http://www.bmcoi.com/CatLit/Power/TECHNICAL PAPERS/GEN. WINDING PITCH - LEKX3115.PDF

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[rbulsara]

Only triple harmonics currents add up in the neutral. 2/3 pitch almost eliminates them. The CAT publication,which Bill provided the link for, discusses harmonics in the generator windings and not the neutral current. (it in fact professes no need for 2/3 pitch windings, but that is a different matter)

Also even with 2/3 pitch, maximum harmonic currents in the neutral flows when the gens are solidly grounded. For 'individually' resistor grounded units, there will be at least two resistors in series in the neutral circuit between the two generators. This will minimize any probable harmonic currents through the neutral. In fact that is one way to deal with neutral harmonic currents for parallels units with winding pitch other than 2/3.

There are other circulating currents that can and do flow between paralleled sources, not necessarily through the neutral and not related to harmonics or winding type, but that again is a separate subject.

As for the formula, you will have to wait for someone else, but you will need to know the harmonic voltage , which usually a small % of the nominal voltage.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

That was Alan's link, not mine Rafiq.
With identical machines, the harmonics will be identical. With identical harmonic profiles in parallel no current will circulate. Circulating currents are the result of dissimilarities between the generators.
You first must determine the actual difference between supposedly identical machines.
If practice it may be easier to just measure the circulating currents.
I know we are supposed to anticipate and calculate everything in advance, but that is not always possible.
What is possible is to spec the maximum allowable differences in harmonic production and to then test and verify that these conditions have been met during acceptance tests.
Remember, although harmonic currents may flow to the load through the neutral, the circulating currents on the ground loop are dependent on the differences between the harmonic production of the generators.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Bill,

I am so sorry. We associate you with Generators so much that I now read your name when there is not!! :-D

Apology to Alan too!


Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Don't forget catserveng. I can get any old junk to work. Catserveng has more experience with never digital controls.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

If there is a transformer connected to this bus that is also grounded, You also need to consider possible circulating current through this path as well.

But as others have said, it is the net voltage in the circuit for each harmonic that determines that circulating current. If all third harmonic voltages in each generator are equal, there will not be circulating third harmonics.

When using resistance grounding, the power rating of the resistor can also be a concern. Most grounding resistor are specified with a 10-second rating. Per IEEE, these resistors have no official published continuous rating.

But power dissipated in grounding resistors by the harmonic currents is often a lot less than imagined.

David Castor

http://www.cvoes.com

 

[waross]

Circulating current in neutral connections is more an issue when generators with different winding pitches are paralleled.
The presence of harmonics indicates that the waveform is not a perfect sine wave. If accurate voltage waveforms of two generators G1 and G2 with different winding pitches are superimposed, at some times the voltage of G1 will be a little above the voltage of G2 and vice versa. When G1 voltage is higher a current will flow from G1 to G2. When the voltage of G2 is higher a current will flow in the opposite direction. Even with equal RMF voltages from each generator.
I am trying to describe instantaneous differences in voltage in a single cycle. One wave form may have slightly wider shoulders and the other wave form may have a slightly higher peak voltage. A spectrum analyzer or a power quality meter will resolve those discrepancies into harmonics for you.


Bill
--------------------
"Why not the best?"
Jimmy Carter