What to consider when selecting a CT ratio for a motor protection? 3

[Dvhez]

I've always seen a 50/5 for almost any load but recently I wanted to understand why my coworkers select that ratio. On what depends? FLA, direct starting (7 times FLA), etc...

For example if a 575V motor has 190 FLA, a 50/5 would be undersized right?

Any help would be great, thanks!

 

[panter]

Transmission ratio of current transformers for motor protection is simply chosen . Nominal current of the motor *190 A* on the primary side side should be in the range of the setting of the protective devices at secondary side * 5 A * . For your motor 200 / 5 A is probably good ratio of current transformer .
In addition, you need to see voltage level and the accuracy class of the current transformer .
First of all you should still take a book and look at the theory about current transformer before any decision .
Good luck .

 

[jraef]

To a smaller extent it also depends on what values you are interested in reading accurately. At the high end of a CT range, you can over drive a meter and at the low end you can lose accuracy.

So while a 200:5 ratio for a 190A motor is fine for reading normal running current, at start-up when that current will be 1140A, that CT will be putting out over 28.5A and even the best meters will be rated for a maximum of 20A. So if you wanted to read that value accurately for some reason, you would need to use a higher ratio on the CT, in this case 300:5 or more.

Conversely if you use a 300:5 CT and are interested in accurately reading current at the extreme low end of the scale, the low load on that CT may affect its accuracy, because CTs are most accurate when they are at or close to saturation. So in that case the 200:5 would be better.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

I have seen a description of an application where the voltage drop across the meter at normal current was below the forward voltage drop of a diode.
Back to back diodes were installed across the meter to shunt the starting current.
I don't know how well it worked.
Comments?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bacon4life]

Meters I am familiar with typically have a continuous rating between 10A and 20 A along with a short duration rating of a few hundred amps. I am curious as to whether Bill saw shunting diodes on digial meters or just on D'Arsonval meters. Here is an example of ratings for an EM132 meter

Current rating: 5A RMS
Maximum continuous current: 10A RMS
Overload withstanding: 15A RMS continuous, 300A RMS for 1 second

A 50:5 CT sounds small but might be OK if it is has a Rating Factor of 4 and the meter inputs are rated at 20 amps continuous. If you need metering quality accuracy, using a smaller ratio with a higher rating factor would be better than using a 200/5 CT.

 

[waross]

It was old D'Arsonval meters.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

50/5A could be core balance CT for the motor feeder to measure earth fault current.
The CT primary doesn't depend on the motor FLA and thus the ratio remains same for motors of different rating.

 

[Sofistioelevib]

Some addiction to the other guys considerations:
in a protection system normally the class precision is relatively important because the primary functionality is to protect the motor (overcurrent i think) and not measure the real current passing through.
Normally the high precision CT have a particular magnetic permeability that can saturated over normal condition, instead the CT for protection system often have no high precision for current measuring but are not so easy to saturate. Because in the protections coordination you have to evaluate the overcurrent (until 10 times In) but also the time restrictions, you must evaluate the real overcurrent position in a coordination graphs and you must measure that current you cannot to do with high precision CT that saturate little after the current dedicated.

 

[che12345]

Dear Mr Dvhez

I have the following opinion for your information

A) I agreed with Mr RRaghunath
" the 50/5A could be core balance CT for the motor feeder to measure earth fault current.
The CT primary doesn't depend on the motor FLA and thus the ratio remains same for motors of different rating"
The (core balance CT) is easily recognized, that it is only [one piece] with [all three phase conductors going through it].

B) I have different opinion with Mr bacon4life
" A 50:5 CT sounds small but might be OK if it is has a Rating Factor of 4 and the meter inputs are rated at 20 amps continuous".
Measurement CT primary current rating usually are selected 1.25 to 1.5 x FLA, it is expected to indicate accurately the occasional overload for a short duration. It is [not] advisable to overload the primary continuously for a long duration. Note: motor starting is limited to a short duration, generally not exceeding 30s.
Most analogue (moving iron/moving coil Class 1) panel meters having compressed scale >2x full scale value. These meters are intended to read generally from say 20% to 120%. Starting current at 6-7 x FLA are not indicated accurately on the compressed scale. The CT is intended to saturate > 1.2 x the primary current rating. There is no value for current ratio accuracy
> 1.2 x primary current rating for measuring CT, per IEC.

C) In many cases, three different types of CTs are used, each for different purposes and with different requirements;
i) One Core balance CT for earth fault protection, see above A),
ii) One CT per phase conductor (total three CTs for three-phase system) for each phase current measurement. Usually a Class 1 [measuring CT] with primary current rating 1.25 to 1.5 x FLA ,
see above B).
iii) One CT per phase conductor (total three CTs for three-phase system) for each phase [over-current protection]. Usually a Class 5P5, 5P10 or 5P20 [protection CT] with the primary current rating 1.5 to 2 x FLA. These protection CTs come into function only when current > FLA and during [over-current faults].

D) Reference IEC 61869-2 for further detail.

Che Kuan Yau (Singapore)

 

[stevenal]

Why isn't the available fault current of concern here?

 

[bacon4life]

stevenal- The thread got a bit off track discussing metering so thanks for bring it back to protection. The SEL paper The Impact of High Fault Current and CT warns again selecting ANSI CTs that will exceed the rated saturation point. However, in a previous forum thread several MVP members seemed unconcerned that the fault current exceeded the CT saturation point by a factor of 5. How would the available fault current impact your recommendation?

Che Kuan Yau- I jumped to the assumption this was in the ANSI/IEEE portion of the world based on the 5 amp secondary. Sounds like the IEC world works a significantly differently than the ANSI world.

 

[stevenal]

bacon4life,

I recommend listening to Mr. Zochol, but need to admit I don't deal much with motor protection. Seemed odd to me the thread ran as long as it did without the subject coming up.