Io Residual current in protection relay settings

[sparkview]

Hi all,
I am currently helping a colleague to determine a ground fault that tripped one of our 13.8kV outgoing feeders. This feeder is protected by a VAMP 140 overcurrent and earth fault protection relay. Based on the how the current settings of the relay it seems that the Earth fault protection was not accurately programmed during commissioning. Therefore, we would like to verify the settings and simulate in ETAP. The feeder being protected is a 2/0kcmil, 3/C, 15kV, CU cable with a distance of about 2KM and connected to a 13.8kV/ 480V step-down transformer downstream. What we would like to know of the Earth fault settings are accurate or is requiring changes and how is this value calculated. Below is a single line diagram illustrating the system and current settings in ETAP. Phase CT ratio is 100/1 and Residual Ground CT ratio is 50/1.
In advance many thanks for your support and if additional information is required please advice.
Regards,
Jairo

 

[slavag]

What is a type of neutral grounding of the system?
solid, low impedance, high impedance, compensated?
What is a phase to neutral fault current?

 

[sparkview]

Thhanks for the prompt reply. The grounding is solidly grounded. I dont't understannd your second question. Can you please elaborate?

 

[slavag]

Hello
that means, single phase short circuit current.

In your case, system is solid grounded, its very high level of single phase short circuit current.
Some one make short circuit study of system?

Im not familiar with Vamp terminals.
What is 0.67A ?
Why 0.3PU is 30A?
CT is 50/1

Phase CT is 1000/1

Something not correct.

==========
Trip of line was false trip or right trip?

 

[sparkview]

Hi slavag,

On the single line diagram the CT ratio is not correct. It has been changes to 100/1 and the residual CT is 50/1. A short circuit study has not been made. The settings in my opinion are not correct, therefore should be determined. Is there a way to calculate these settings?

 

[stevenal]

Not sure anyone will give you a simple formula. Might be time to read up on the subject.
I am a bit confused about this I0 residual current, however. Residual current as measured by the residual current CT is equal to 3I0.

 

[waross]

I am confused by the pick-up settings.
As I read it, with a 50:1 CT, a pick-up at 20 Amps primary would be 0.40 PU and a pick-up at 30 Amps primary would be 0.60 PU
The first step is to determine what fault current you want to trip at and what time delay or inverse curve you want to use.
Then it is a matter of reading the relay manual to set the relay.


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

 

[slavag]

Without short circuit study....not possible determinate setting.

What is a setting for overcurrent?

 

[sparkview]

The overcurrent settings are found in the picture below. The CT ratio used is 100/1

 

[sparkview]

The PU provided in the image above is not correct because the CT ratio that was set into the relay is 100/1 which should be 50/1. That is the first thing I have noticed when the settings were downloaded from the relay.

 

[slavag]

OK.
next steps.
1. What is a setting ground fault and overcurrent of upstream breaker?
2. What is a data of 13.8/0.48kV transformer?

( setting of 20A, 1sec and 30A 0.5 sec....is not setting)