CT sec current --> Data recorder 1

[n1mr0d]

Hi, i want to plot the secondary current of a 1500:5 CT on a Yokogama Portable Hybrid recorder. I thought of using a 1 ohm 50W burden resistor on the CT, and then rectifying the voltage over the resistor which would be put in the recorder. The recorder accepts voltages up to 50V dc.

The nameplate of the CT says:

AEP 42 70 996 Ir.23a
10/42 kV 1500A
PDS 5A
150 kA therm 4.8 ohm
- kA dyn Kl 1
50 Hz

Does someone have any suggestions regarding rectification and safety measures etc?

 

[waross]

You will have a form factor error. You may use a form factor correction factor if you have a clean sin wave. The form factor correction factor depends on the wave shape. It is the ratio of the average value of the wave form versus the RMS value of the wave form.
The value for a pure waveform is about 1.1. The difference is sometimes noticable with different wave forms phase to phase versus phase to neutral. If you measure the voltages with a D'arsonval type meter, wich responds to average values rather than RMS values, you will see a noticable deviation from the normal 1.73:1 ratio when you have differing waveforms.
Your system will result in converting RMS values to average values.
If you are tracking trends and can live with an error that may be 20% you can go ahead.
If you need accurate mkeasurements, look for a transducer to convert AC RMS to DC. I usually start looking for something like this at Omega.com
yours

 

[dpc]

My thought would be to use a small clamp-on current transducer on the secondary (5A) CT wiring. The transducer can give you a millivolt output proportional to the current in the secondary (and primary). We do this all the time and it works well. Avoids the necessity of messing with the CT secondary wiring.

 

[dpc]

Check out AEMC Instruments if you decide to go with a clamp-on probe.

 

[n1mr0d]

Are any of these devices of any use?

http://www.chipcatalog.com/Cat/83.htm

 

[Skogsgurra]

Nimrod,

I assume that you mean a Yokogawa recorder. Not Yokogama, which I know nothing about.

Yes, putting an extra burden like 1 ohm should be OK. But you know about opening secondary windings, don't you?

The Yokogawa recorders usually have an RMS function built-in, so you don't have to rectify your signal.

But if you need to rectify it, dont put the rectifier after the resistor. It will give you a voltage drop around 1.5 volts, which means that you will not even see currents below 500 A primary. So, there's more to it than form factor.

The PI/(2xsqrt(2)) is important as waross says, but you should get rid of the -30 percent error before correcting for it. Also, you may want to smooth the rectified signal since the recorder is fast enough to reproduce the ripple of the rectified signal. When you do, you will have a value that is sqrt(2) higher than your RMS value. So, that is what you should correct for.

Now, how do you avoid the two diode drops in the rectifier? Simple, rectify before you connect the resistor. The current transformer is a current source with very high compliance (that's why you get high voltages when you open the secondary) so the diode bridge doesn't infuence current at all. The output is the rectified secondary current - no more, no less.

Safety measures? Good you asked. But if you feel that you need to ask - please be instructed by your fellow engineers.

Re. the RMS/voltage chips in the catalogue. Yes, very good chips, most of them. But there is a big difference between hooking up a resistor (and perhaps a rectifier) and designing a circuit board with power supply, analogue interface and these circuits. Orders of magnitude in complexity. The resistor/rectifier can be done in a few hours - the chip thing needs a week or more if you are unexperienced. And then you don't even have a decent PCB - just a bread-bord. And a badly calibrated one, too.

Gunnar Englund

http://www.gke.org

 

[jghrist]

Considering skogsgurra's caution about open CT secondaries, you might want to consider a higher wattage resistor. A 10000A fault on the primary would put 33.3A on the secondary, or 1111W. If you burn out the resistor, you will have an open circuit and possibly dangerously high voltages.

 

[n1mr0d]

10000A fault?? Is that possible for a 6.4 MVA, 3.3 KV gen with Inom 1175A? Also a 1kW resistor would be huge and expensive. A more readily avaiable 200W resistor can handle up to 4243A primary current.

 

[davidbeach]

The generator could easily supply 6 - 10 times nominal current into a fault. Best to skip the resistor entirely and leave the CT shorted while making your measurements with a clamp on probe as dpc suggested.

 

[dpc]

http://www.aemc.com/products/pdf/2110.71.pdf

These do work pretty well - and could come in handy in the future.

 

[ScottyUK]

nimrod,

Check out the non-contact sensors from LEM-HEME:

http://www.lem.com.

They're almost embarassingly cheap these days, and provide a valuable layer of protection for your expesnive instrument. Leave the CT circuit alone - big CTs are scary things when the secondary goes O/C with an energised primary. Been there, seen it, ran away. (18000/5 Class X on a generator diff. scheme for those in the know).

If you do pursue the resistor idea in spite of the warnings to isolate the measuring instrument, the 1kW would be a short-term rating, not continuous. The 200W resistor would probably be fine. The generator protection should operate very quickly to clear the fault. If it didn't the burnt out resistor will be the least of the problems.

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