SEl-351S O/C and SEL-387E settings CT question

[Sn00ze]

Hello dear colleges, .

I am trying to achieve differential protection on a transformer using a SEL-387E but the LV CT will have to be shared through the downstream overcurrent protection (SEL-351S) due to lack of CTs.

The star point of the SEL-351S prot, is loooking away from the TX. So, my question is, If any one know if it would be an issue to reverse the starpoint of this CT ? Since the SEL-351S is only doing 50/51? Is there settings changes due to this? (It would be looking away from the Line)

or if it is possible for the SEL-387E to reverse the element wiring or if this can be done through settings for the LV CT.?



thanks for the insight, my expertise on what these relays can do through settings is limited.

 

[Fritzy92]

If the SEL-351S is ONLY performing 50/51 functions, then there is no issue with physically swapping the polarity of the CT. No relay settings will need to be changed.

It doesn't look like you're able to change the CT polarity through the settings for the SEL-387E.

 

[JG2828]

If the 351S has voltage inputs, your watt metering and current angles will be 180 degrees out of phase. You could always swap the current leads on the 351S relay so it is looking in the correct direction.
The other option is to leave the wiring at the CT alone and swap the polarity at the 387E.

 

[stevenal]

Star point should not be an issue, since each relay has two terminals per input. Star is made up at the CT so that a four conductor cable can be used. The star is unmade at the relay end before hitting the relay inputs. Star the polarity or non-polarity end as you wish.

Polarity is another matter, but the 387 should be able to deal with any polarity. Add or subtract 6 from the winding compensation from the winding in question. If W2CTC were set to 1 (oclock) for a normal polarity connection, set it to 7 for the reverse polarity connection. If your 387 has voltage inputs, you might want to look at the compang setting also to get the signs of the watts and vars the way you wish. Voltage is less flexible to change via wiring, since the star point is within the relay.

A diagram might help this question.

 

[Sn00ze]

Dear all,

Thank you for your responses. Taking your advises along with what i've done in the past and what i've read in the manual. I've come to the conclusion that, yes if it is 50/51 the direction of the CT would not matter, but it is doing Meteting so compensating for the voltage may be necessary.

However, it would be easier to compensate for the polarity of the CT in settings of the SEL 387E. since the metering is being done through the A protection (T60) tehre is no concern with voltage polarity.

I will pitch this to the settings engineer.

Thank you for the insight. It is appreciated.