Incorrect CT Secondary Current

[Techy]

We have a 3 phase, 4 wire generator with an Earth Fault Relay supplied via a CT on the star point / neutral. Recently, the plant tripped due to activation of the EFR. Let it be noted there is some imbalance across the phases.

The CT fitted has worked with no issues for many years. It is 1600/5A. Upon restart and synchronisation of the generator the current in the secondary circuit was around 11-12A yet the current in the neutral was around 120A. Once the current in the generator calmed down, the current in the neutral was measured with a clamp meter as 45A (primary), with the Secondary CT circuit measuring 4.3A. The plants KWH meter also calculated close to 45A in the neutral.

With the CT being 1600/5A, it means that at 4.3A the primary would be 1376A (which it's not). For the 45A in the primary I would expect to see around 0.15A in the secondary.

The CT was replaced yet the problem persists. The next step is to investigate the CT leads to the relay and the relay itself. My question is what could cause the CT secondary to be so far out of proportion to the primary?

Thanks

 

[Phil Corso]

Techy...

Some detail, please: a) LV, MV; b) Size; c) Neutral-switching involved; d) Design per NEC-regs!

Regards, Phil Corso

 

[Techy]

Techy...

Some detail, please: a) LV, MV; b) Size; c) Neutral-switching involved; d) Design per NEC-regs!

Regards, Phil Corso

A) LV - 400VAC
B) Generator is rated to 1120kW but restricted to 850kW.
C) There is no local neutral switching.
D) The installation is UK based therefore does not fall under NEC, but the IET 17th Edition.

 

[Tony S]

Drop me a PM with e-mail address.

I doubt it comes under BS7671, more likely BS7430

 

[Phil Corso]

Techy...

For Main and Genset sources:

e) Neutral current-magnitude? 50 or 150Hz?

f) 'PE' Current-magnitude? 50 or 150Hz?

g) Phase Current-magnitudes?

Phil

 

[Techy]

Techy...

For Main and Genset sources:

e) Neutral current-magnitude? 50 or 150Hz?

f) 'PE' Current-magnitude? 50 or 150Hz?

g) Phase Current-magnitudes?

Phil

The genset is synced with a 50hz bus.

I am just a little baffled how I can have 45A flowing through the primary of a 1600/5A CT and be able to measure over 4A on the secondary circuit, unless I am missing something completely basic... which is likey.

 

[JoeStillman]

Is the EFR relay the only thing on the CT? Or is the CT in parallel with phase CT's for residual-sensing?

 

[mivey]

The genset is synced with a 50hz bus.

I am just a little baffled how I can have 45A flowing through the primary of a 1600/5A CT and be able to measure over 4A on the secondary circuit, unless I am missing something completely basic... which is likey.

Well it may be a multi-ratio 1600:5 CT with the taps set at 50:5. 1600:5 seems high for a residual tap.

 

[GoldDigger]

Is the EFR relay the only thing on the CT? Or is the CT in parallel with phase CT's for residual-sensing?

+1
It seems strange for the protective relay to be sensing only the neutral current if you have line to neutral loads.
Are you amp clamping the wires right at the CT coil or downstream somewhere?

 

[mivey]

The next step is to investigate the CT leads to the relay and the relay itself. My question is what could cause the CT secondary to be so far out of proportion to the primary?

Could be bad leads changing the ratio.

Also, any recent testing where someone may have put the tap screw in the wrong hole?

 

[Tony S]

Well it may be a multi-ratio 1600:5 CT with the taps set at 50:5. 1600:5 seems high for a residual tap.

For REF (restricted earth fault) all four CT’s are the same size the variable being the bias resistor. If metering is tied in to this then……

 

[big john]

Silly question: Are you measuring the current on the primary and secondary with a clamp-on ammeter, or are you depending on the relay output for your secondary current value?

I don't see any possible way a correctly wired CT operating within it's nameplate ratings would give you that output.

I would recommend double-checking primary and secondary currents with a couple different ammeters.

 

[Techy]

Well it may be a multi-ratio 1600:5 CT with the taps set at 50:5. 1600:5 seems high for a residual tap.

No there are 4 CT's all are 1600/5

Is the EFR relay the only thing on the CT? Or is the CT in parallel with phase CT's for residual-sensing?

The neutral CT and phase CT's are all in parallel. The phases are all reading correctly on their secondary given their primary current. They feed a three phase over current relay.

+1
It seems strange for the protective relay to be sensing only the neutral current if you have line to neutral loads.
Are you amp clamping the wires right at the CT coil or downstream somewhere?

Neutral has been checked on the primary with a clamp meter at the generator (45A) and with a clamp meter and ammeter at the EFR, reading the high secondary current.

Could be bad leads changing the ratio.

Also, any recent testing where someone may have put the tap screw in the wrong hole?

This plant has ran for over 20 years with its current configuration with no issues and no testing has been done recently.

For REF (restricted earth fault) all four CT’s are the same size the variable being the bias resistor. If metering is tied in to this then……

The phase CT's feed a three channel over current relay, analogue ammeters and a digital kilowatt meter. The neutral CT just feeds a single channel over current relay.

Silly question: Are you measuring the current on the primary and secondary with a clamp-on ammeter, or are you depending on the relay output for your secondary current value?

I don't see any possible way a correctly wired CT operating within it's nameplate ratings would give you that output.

I would recommend double-checking primary and secondary currents with a couple different ammeters.

As mentioned above, the primary has been measured with two different clamp meters both giving a value close to 45A. The kWh / Energy meter fitted to the system has calculated the same value of current in the neutral / primary.

Secondary current has has been measured with two clamp meters and an ammeter in series. All read within each other.

I agree, I don't see how this is happening but it is, which brings me to ask the question of why?

The CT has been replaced and the same problem persists, so that only leaves either an issue with the secondary wiring or the relay itself. I just want to understand the principle / theory as to why / how it could cause this.

 

[big john]

Jury rig a ratio test on the CT.

Since you have two identical CTs and you know this isn't a malfunction, just for kicks I would temporarily install the second CT and short-circuit its secondary.

Then reenergize your primary and measure the secondary current on your existing protection CT and your temporary test CT. If you get a 10:1 ratio on your test CT despite a 320:1 nameplate then a call to the manufacturer is in order.

 

[Haji]

The neutral CT may be connected (perhaps during any routine maintenance) with incorrect polarity with respect to phase CT's polarity. Check by changing the connection of the neutral CT.

 

[Bugman1400]

Was the neutral CT tested before being placed in service? I would short the neutral CT at or near the terminals and then use a clamp-on to measure the secondary current again and compare it to the primary current again. If you still have a discrepancy between the two, it most certainly has to be the CT since everything else would have been removed.

 

[mivey]

Techy,

How about some pics of the CT and installation and exactly where the clamps were placed for the readings.

Any onelines? Any chance you are reading ground currents that are in a loop beyond the CT secondary?