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PT and CT

PT and CT

What is the difference between a potential transformer and a control transformer?Can CT be used instead of a PT?

RE: PT and CT

To answer your question, it depends on your application.  Are you using the transformer simply to measure voltage or to power a control of some sort?  I am assuming by CT you mean control transformer and not current transformer as they are different.

This link may help you

Quote:  Maybe if I try, I can get both feet in my mouth.

RE: PT and CT


'CT' typically stands for 'Current Transformer'

'PT' typically stands for 'Potential Transformer' (A.K.A. 'Voltage Transformer', 'VT')

'CPT' typically stands for 'Control Power Transformer'

RE: PT and CT

Beyond the semantics, you technically can use a CPT as a PT, but it will lack the ability to withstand a fault on the high voltage side as a PT is designed to do. Typically a PT is used ahead of the main protective device and is protected only by the line side fuses. So if there is a fault on the bus and you had used a CPT, it theoretically could become shrapnel before the fuses clear. A PT is designed to be more tolerant of the physical environment it encounters living ahead of a main switch. Also, CPTs tend to have multiple voltage ratings depending on how they are connected. PTs do not, so there is less chance of someone making a mistake which would mess up the metering device.

Conversely, a PT does not make for a good CPT because it it not designed to provide the same current inrush capabilities of a CPT when it may be supplying inductive loads.

RE: PT and CT

Just to state the obvious (one of my specialties), the CPT will probably not have near the accuracy of a metering or relaying PT.  If this is to be used for revenue or billing meter, an accurate metering-class PT should be used.

And just to add a little more obsfuscation, the term Potential Transformer and hence "PT" is deprecated by IEEE.  They now prefer the term "Voltage Transformer", which seems to create more confusion rather than less.

RE: PT and CT

As jraef said, VTs (or PTs) are designed to be accurate within a certain accuracy class.

Control power transformers are generally designed for power output only.

Depending on the power rating and voltage class, CPT's are generally cheaper than VTs, especially when rated 2-3kVA and below. VTs are often used for power applications as they do have a thermal burden rating, which is how much power they can output without overheating, if you don't need the accuracy class performance.

We sell a lot of VTs for power reclosers, automated switches, etc... at distribution level voltages. Mainly because most VTs at this level are dry-type and control power transformers are typically oil-filled.

RE: PT and CT


When you say, "but it will lack the ability to withstand a fault on the high voltage side as a PT is designed to do" I guess you are meaning over voltage withstand capability of PT windings that is defined in IEC 60044 (the voltage in unfaulted phases, in case of earth fault in HV system, can technically go up to 1.8 times the rated).

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