ring type metering CT wound on the primary 132 kV cable was found defective after high voltage test

consequences :

ring type metering CT was commisioned along with protection cores and found good.

152 kV high voltage test was carried out in the cable. CTs were short cktd and earthed during the test. 152kV was applied in all 3 phases together for 60 minutes. After high voltage test was completed Protection cores was found good. But metering CT shows resistance in Megaohms. but the magnetization characteristic and Knee point voltage as normal.

what will be the cause for the problem; class of metering CT is 0.5FS5

 

Responding to Bala's 19-Apr-09 (13:13) query.

1) Only one metering CT failed? Or one of three CTs failed?

2) What was CT current rating?

3) What was current ratio?

4) What was secondary burden (VA)?

5) How was test performed? Was primary current applied gradually? Or some max value, suddenly?

6) How was test current cut-off?

7) What did you mean by "measured Meg-Ohm?" The secodary was open-circuited?

8) Any visable or audible evidence of failure?

Regards, Phil Corso ([email protected])

 

1) only one metering CT failed

2) 1.2 In continous

3) 600/1 A

4) 15 VA

5) Ratio test was performed by OMIcron CT Analyser (BY voltage method)

6) no sudden drop

7) open circuited

8)no abnormal seen

Regards

Bala

 

Bala,

If it passes the Eye, Ear, Nose test, then the winding failed internally. Maybe resulting from the Ratio-Test.

If you haven't already teken it apart, then send it back to the manufacture for analysis!

Regards, Phil Corso

 

CURRENT TRANSFORMATOR MUST NEVER BE OPEN ON SECONDARY SYDE!!!

 

>Responding to Phil Corso’s query.

> 1) Only one metering CT failed? Or one of three CTs failed? <
1/3

> 2) What was CT current rating? <
75 Amps

> 3) What was current ratio? <
75/5-5-5 Amps

> 4) What was secondary burden (VA)? <
15VA

> 5) How was test performed? Was primary current applied gradually? Or some max value, suddenly? <
secondary Terminals shorted

> 6) How was test current cut-off? <
Short Circuited

> 7) What did you mean by "measured Meg-Ohm?" The secodary was open-circuited? <
20 Meg-Ohm

> 8) Any visable or audible evidence of failure? <
HT Fuse blown out in particular phase

 

Bala... so metering CT had three (3) five Ampere-rated secondaries!

Blown HV fuse is indicative of insulation breakdown between primary and secondary earthed terminal!

What was manufacturer’s conclusion after analysis!

Regards, Phil Corso