Low Voltage Generator Grounding

[gvasiliou]

Hi guys,
I need your opinion in the following issue.

I have implemented a project with one LV Generator (3phase, 1900 KVA - 400 Volts - 50 Hz - Star Connection Solid grounded) working in parallel to the Grid (20 kVolts - Star Connection - Solid Grounding).

Generator is connected parallel to the grid using a two windings step up transformer (2500 KVA - Ynd - 0,4/20kV , Solid Grounded).

My customer wants me to prove that Solid Grounding used for the above mentioned Generator is not dangerous to destroy the generator, meaning that in case of a Short Circuit at Generator terminals (LV 400 Volts), generator must be able to withstand the short circuit fault current fed both by Generator and Grid, till protection devices on the generator LV side to work and isolate the fault (sub transient state) from generator terminals.

Generator Manufacturer (Stamford) has advised the attached "thermal damage curve" but this curve doesn't go bellow 2 seconds, and as Stamford advised there is not such a curve bellow 2 seconds.

The Short Circuit study made according to IEC909 and has come to these results:
3phase Bolted Short Circuit Total Fault Current : 72,4 kA
1phase to Ground Short Circuit Total Fault Current: 86,4 kA

I would like to have your comments on the grounding method of this generator. I mean, do you think that generator is capable to withstand the above faults for i.e 1 or 0,5 seconds?

PS : For sure using an earthing resistor (or reactor) on the star point of the generator would result in much more lower fault currents (Single Line to Ground Fault) but the installation is completed and i want to avoid modifications.

 

[dpc]

As you've found, the line-to-ground fault current from a generator generally exceeds the three-phase fault current.

You need to ask the generator manufacturer if the generator is adequately braced for a bolted ground fault at the generator terminals with contribution from the grid. If it is, you are OK. If not, you have a problem. Some generators are only braced for three-phase faults. Generally, you will want to specify the bracing required when you buy the generator.

The concern is the mechanical stresses on the generator windings during a fault, not the thermal limitations.

 

[gvasiliou]

dpc thanks for the reply.
Do you think that adequate bracing of the particular generator can be "proved" by the datasheet provided by the manufacturer....?
(see datasheet attached in this post)

I have to admit that particular manufacturer in EU doesn't provide the kind of support i would like to have....

BR

 

[dpc]

It may be covered in the standards that the machine was built to. I think you'll have to find out from one of their engineers.

In the US, this is generally a specification item.

 

[gvasiliou]

Ok, i got your point... I suppose that the applicable standards define some kind of limit that alternators must be ablet o withstand.
What really worries me is that if generator works on it's own (and not parallel to the grid) the estimated single line to ground fault is something like 32kA.

If you count the Grid contribution to the fault, total fault currents is rising to a value close to 90kA (300% more!).

And i really don't know if applicable built standards allow that kind of margin in the generator construction....

I have to try one more time with Stamford Engineer ... i might be lucky.

 

[waross]

Are you considering an external fault or a fault internal to the generator?

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

 

[mgtrp]

Please bear in mind when conducting your analysis that for a fault on its terminals, the generator winding will only see the generator's contribution to the fault, not the system contribution as well.

 

[gvasiliou]

@waross : The total fault of roughly 90 kA is 50% from own generator and 50% coming from the Grid

@mgtrp: I have doubts if your approach is correct (sorry - no offense)

See the picture bellow.

If the short circuit location is just one inch after the alternator terminals i.e location F1, then windings will suffer only the fault current supplied by the generator (45 kA)

But if short circuit happens at F2 location then windings will suffer both currents from generator and grid (total about 90kA), or i'm wrong somewhere....?

 

[mgtrp]

If the fault occurs at the F2 location then the fault current will be zero.

 

[mgtrp]

To explain a little better, the voltage to earth decreases as you get nearer the neutral end of the generator winding. Accordingly, the fault current contributions from both generator and system will be reduced as you go down the winding. Once you reach the neutral point, the voltage to earth is zero, as the neutral point is directly connected to earth, and no fault current will flow at all.

 

[gvasiliou]

I'm not sure about that ....
Current is zero only after start connection is made, not before.
Before star point connection current in each phase equals to the load current.

Every time that we have to provide "alternator differential protection" we use the following scheme:

When everything works fine, both currents (before and after winding) are EQUAL and thus differential current is zero.

I can personally confirm the above fact. I have troubleshoot "differential protection" problems on generators a lot of times.

 

[iop995]

Generator and grid are 2 parallel connected power supplies; short-circuit it's an undesired load condition when both power supply feed current in this "load". As long as generator work at nominal parameters there are not any current from grid to it. Such currents may apear in some fault conditions: wrong genearator conection to grid (shift phase and / or high voltage / frequency difference) or when output voltage of generator became zero (prime mover stop and/or excitation dead and by a fault generator is not disconected from grid).

 

[waross]

Assuming that you are concerned with an internal fault:
An arcing internal ground fault will probably destroy a generator. With high resistance grounding it will take longer. Differential protection must be used to detect internal faults and the excitation must be removed and the rotation stopped to stop the progression of damage to the core iron. There is no guarantee that
any protection scheme will be able to prevent core damage in the event of an internal fault.
Compare the costs:
A: A new generator end.
B: A rewound generator end and the cost of differential protection.
I have seen Cat sets at about 1600 KVA that were owned and operated by Cat on a power supply contract where an outage incurred stiff penalties. Differential protection was not used.
The issue with an internal fault is not to prevent damage, but to prevent irreparable arcing damage to the iron core.

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

 

[gvasiliou]

I'm happy to see that you people have some kind of experience with all this generator issues...
Thank you for your usefull comments. Your help was valuable.

I would like to go with your help to answer the initial question of my post:
Do you people think that this particular LV generator, with calculated line to ground short circuit fault current at generator output bus bars (point F1) equals to 86,5 kA (50% by generator - 50% by grid), ignoring point F2 since a short circuit fault (not arcing fault) at point F2 will be zero, can be solidly grounded or must be grounded thru a resistor or reactor?

I admit that an earthing resistor (or reactor) on the star point of the generator would result in much lower fault currents(Single Line to Ground Fault) but the installation is completed and i want to avoid modifications.