Current Transformers

[drax]

Can someone help explain to me exactly how ring type Current Transformers work. I have a multi-tap 800/1200/1600:5 ct with a energized bus going thru it. I know the output would be 5 amps if current flows what direction? how are the turns designed with the core. Any help to a mechanical engineer would be great.

Thanks,
Drax

 

[vidima]

A current transformer has a primary and secondary windings. The primary in this case is the bus going thru it. The secondary which has 800/1200/1600:5 is wound on the core which make up the magnetic circuit.Here it about magnetic flux.

On the CT surface check for P1 and P2 markings. P1 is to face the direction your power is flowing from on the bus. On the output terminals check for S1 and S2. In all ,if these are not marked do a polarity check which is a simple test on CTs.

 

[drax]

Thanks for your help. The CT is marked H1 on one side and has 4 wires coming out. How do I determine which one is common and what ratio (can I take a ohm reading to determine without having to test?) and how do I check for polarity?

thanks in advance.

Drax

 

[230842]

Secondary taps should be marked. There have been several ways to identify polarity. If you could read such a marks it could help to give you an answer. Maybe you have to get some covers out.
Otherwise, you should test. A ohm reading will probably give you no significative information since CT secondary winding resistances are very low (some milliohms) Julian

 

[peterb]

H1 is the polarity side of the CT. Contrary to popular belief, the orientation of the CT in the circuit is immaterial, as the polarity marking gives relative polarity only. The secondary winding will most likely consist of evenly distributed turns around the core, with the taps being taken from completely distributed layers. The number of secondary turns will be 160/240/320, corresponding to the 800/1200/1600-5 taps.
The significance of polarity is that when instantaneous current flow is into the marked polarity side of the CT (H1 in this case), the instantaneous secondary current flow is out of the secondary polarity terminal. This determines the relative polarity for directionally sensitive devices connected to the CT secondary, and the trick is in connecting these correctly for the actual installed polarity.
As for the secondary terminals, these should be marked X1-X2-X3-X4, with the lower numbered terminal in use being the polarity terminal relative to the higher numbered terminal.
If the secondary terminals are not marked, then you will have to do a couple of tests to work them out, as follows -
- Do a secondary magnetizing current check on each pair of secondary leads in turn, with the primary open circuited - this determines the saturation voltage of the winding, which will be proportional to the number of turns. The terminals should correspond as follows - X1X4 = 320 turns; X2X4 = 240 turns; X3X4 = 160 turns; X1X2 = 80 turns; X2X3 = 80 turns
- Do a polarity test. This can be a "flick" test, where a DC ammeter is connected to the secondary terminals and a battery is used to energize the primary momentarily. By observing the direction of the ammeter deflection when the supply is put on and removed gives the relative polarity of the winding.

 

[scottf]

If you are in Canada, the way multi-ratio CTs are listed is like 800/1200/1600:5, however, you probably have 5 leads coming out...X1 thru X5, since there are many more ratios in the CSA multi-ratio configuration (shown in a table in the CSA standard).

If for some reason the markings of the secondary leads are not clear, you can put a conductor through the priimary opening of the CT and apply a voltage to it...you can read the ratio of the secondary leads by measuring the voltage in between each set of turns. If you look at the chart in CSA, you can then figure out which lead is which.

 

[busbar]

What about using ~20 volts {CT test set} to excite any two leads on the secondary, and sorting them out as simple voltage ratios from there? [Leaving the primary out of the process saves embarrassing 'oops' saturation.]

 

[scottf]

We typically do by applying voltage on the primary in our factory when the markings happen to "fall-off" CTs. By energizing the secondaries, it makes if difficult (read can be confusing) when you don't know which wire is which (not impossible of course). Apply voltage to the primary is much easier...I don't think saturation should be an issue for protection cores if the voltage used is 10-50 volts (note should use more than 1 primary turn). Make sure the voltage is kept below say 600 V on the full winding.

 

[peterb]

Scottf -
Sounds to me as though the concept of energizing the primary with a voltage source is the hard way of doing a magnetizing test. You would need to energize with somewhere around 1 volt/Turn primary to avoid saturation, which either means a low voltage or many turns - this may be difficult where the CT is installed in switchgear, as in this case.

Much easier, as Busbar suggests, to energize the secondary and take voltage readings. This is the simpler approach to the full-blown mag test that I referred to above, which will definitely identify the correct numbering. Recall that the voltages across the tapped windings will be in proportion to the turns and that if the source is applied to one of the taps, autotransformer action will provide the voltages across the remaining taps.

Of course, you still need to sort out the polarity, although it should follow from identification of the leads.

 

[scottf]

Peterb-

What you say is exacty correct. My point is though, that if the ratios are not known, energizing from the primary is much easier. Either way will work and slight saturation is not that big of concern...remember, you're not taking an excitation curve here, just enducing some voltages. If your voltage reading is off by 5% it still works just fine.

Just a difference of opinion. Most of my experience is with HV CTs, so being in a switchgear is not much of a concern.

 

[peterb]

Scottf -
Difference of opinion, absolutely.
I guess that my approach is that I will do a mag curve check on all CTs anyway, whether HV post type CT, HV bushing CT (in breaker or transformer), MV metalclad switchgear CT, auxiliary CT or whatever. If it's a CT, I want the mag curve to be sure that it is correct. I have seen several cases where troubleshooting revealed that protection maloperation was due to connection to the wrong set of CTs and the circuits tripped consistently for years on through faults.

 

[drax]

Thanks for all of your help. I used the flick test as suggested with a 9 volt battery through the ct. My digital meter showed the polarity on the taps. thanks!

I could not determine the ratio of the taps by energizing the secondary only and taking the readings. I applied about 16 volts AC to the secondary and could not read any difference between the taps to ground.

I may try using the 9 volt battery and applying a resistor on the secondary terminals to draw current and check that way.

thanks
Drax

 

[jbartos]

Suggestion: Reference
1. ANSI/IEEE Std 242-1986
a. The CT winding polarity may be disregarded in one CT application, e.g. clamp on ammeter will read the same amps if you clamp it with scale directed to you or if you reverse clamping with scale directed to someone else away from you.
However, if there are more CTs connected in some specific connection, then the CT winding polarity counts, e.g. for the CT delta connection, y-connection, etc. See Reference 1 Figure 196 on page 432 where the y-connected CTs have all windings beginnings marked. The delta connection does not show any CT markings, which appears to be incorrect in comparison with Fig. 199 where all CTs have their windings marked.

 

[scottf]

Drax...

Exactly what were your connections on the secondary side and primary side?

 

[peterb]

Drax -
When you do the secondary excitation test, read between the taps, not to ground. What is happening is that you are energizing between two taps of what is effectively an autotransformer. The voltage between one of your energized leads and the others will be in proportion to the turns between the taps (recall that X1X4 = 320 turns; X2X4 = 240 turns; X3X4 = 160 turns; X1X2 = 80 turns; X2X3 = 80 turns). You may get better results with a higher voltage. The 9V battery won't help you much in this case.