HRG & Capacitive Charging Current

[doehl1]

Our 13.8kV, 30MVA generator has a neutral HRG (single phase transformer w/ resistor on secondary) that is designed to allow 5A (primary) on the generator neutral during the first phase-ground fault.

Our end user on a new install is advising that their system has a nominal continuous value of charging current =~0.5A, so no issue there, the HRG will allow sufficient charging current under normal conditions.

However, they've also provided us with a calculation that indicates their system charging current rises to ~26A during a ground fault, and remains at this value for the duration of the fault. They've requested 150-200msec delay on our end to allow they're downstream ground fault devices to operate.

What happens in this case during the fault? Does the voltage drop off since the HRG won't allow that much current? Is there possibly insufficient fault current available for their downstream devices to clear the fault? Will the generator 59N protection saturate and immediately trip?

Having difficulty visualizing how this specific case manifests itself...

 

[waross]

First, I think that this is more a word use problem than an electrical problem.
As I understand charging current, this is the sum of capacitive currents and that these are voltage related.
If one line shorts to ground that effectively shorts out 1/3 of the charging current.
However the voltage to ground on the other two lines rises by root 3, or 1.73.
If the charging current is 3 Amps, then a ground fault will cause 3A - 1A + (2 x 1.73) = 3.46A. That is a rise of about a 15% increase.
A rise from 0.5A to 26A is a rise of about 5000%.
Maybe I am going to learn something here.
I would love to see the calculations showing that rise in charging current.
Is a robust tertiary winding backfeeding ground faults?
Does the customer have additional devices connected between the neutral and ground?
And;
I assume that your protection will trip on any fault current above your trip setting, be it 5% over or 5000% over.
Did the customer calculate the backfeed from a tertiary winding based on the rated voltage of the ertiary winding and neglecting the voltage drop due to the winding impedance? That may account for the seemingly unreasonable increase.
And I seem to remember that the single phase impedance of a delta winding is three times the three phase impedance. Is my memory accurate on this?


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

Is your end user connected at 13.8kV level? Is there a transformer between the generator and the end user??
26A capacitive current in end user installation suggests a lot of HV cabling is involved in the subject installation.
HRG system is ideally suited to the generator connected directly to a transformer (Unit transformer connected generator).
When there is a lot of cabling involved at the generator terminals, the large capacitive current means the ground fault current cannot be limited to under 25A.
This means the suitable grounding system shall be be medium resistance type and not HRG any more.

R Raghunath

 

[Kiribanda]

The NGR of a HRG system is typically sized is by estimating the total charging current and multiplying by three.
Then the NGR rated current is taken little higher than that value because that is the actual current flowing during a single phase to ground fault.
Having said above, if one says that the charging current is 26Amps, that means he should prove that the estimated charging current
is at least 8Amps. Typically on a 13.8kV system unless you have at least 2-3km lengths of 15V cables, it is very hard to get a 8 Amp charging current.
Therefore, I believe that 5A NGR is sufficient on your 13.8kV system meaning the estimated charging current is about 2Amps.
If the the NGR value is very high then, during a single phase to ground fault, the 13.8kV system will become an isolated system and
transient/ steady state over voltages lead to insulation failures.