CT or PT?

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EEC

Senior Member
Location
Maryland
Can you explain the differences and the uses of each type? I know CT is used for metering purposes and outputs current at some ratio of the primary current.
 

iwire

Moderator
Staff member
Location
Massachusetts
I know PTs are often used to measure voltage.

Say you have a 13.8KV line that you want to monitor the voltage of. Building a volt meter to handle 13.8KV directly would be a problem, so you use the PT which outputs a voltage in a ratio to the monitored voltage.
 

mxstar211

Member
Location
Hawaii
A CT is a current transformer, and a PT is potential transformer. Both of them are used in metering. CT's may also be reffered to as a doughnut in service equipment because they are shaped like a doughnut and go around a wire. CT's are usaully at a 5:1 ratio. PT's reduce the voltage to safe level for metering.

In commercial or industrial applications ct's are needed to lower the amperage so that current level is lower so it can be safely measured at the service. They are also needed for all other kinds of metering along with pt's.

Just remember if you ever open a circuit with a CT to shunt the CT! Otherwise the voltage becomes dangerously high.

CT's are in series with circuit and transform current flow, PT's are in parallel, and transform voltage.

I am up a little late than usaual, so please correct me if I am wrong.
 

72.5kv

Senior Member
The standard CT will usually have secondary output of 5amp when full primary current flowa. You can request 1 amp ouput or less. As mention a ct should never be open circuited when current is flowing in the primary, the scondary must be shorted out. standard 400:5 3000:5 or multi ratio

PT are usually rated for phase to ground voltage, a 13.8kv station would use a 70:1 pt or a 23 kv station 120:1 above to voltage, CCVT are used to measure line voltages
 
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Besoeker

Senior Member
Location
UK
Can you explain the differences and the uses of each type? I know CT is used for metering purposes and outputs current at some ratio of the primary current.

PT = Potential transformer used for voltage measurement.
CT = Current transformer used for current measurement.
 

jdsmith

Senior Member
Location
Ohio
The standard CT will usually have secondary output of 5amp when full primary current flowa. You can request 1 amp ouput or less. As mention a ct should never be open circuited when current is flowing in the primary, the scondary must be shorted out. standard 400:5 3000:5 or multi ratio

PT are usually rated for phase to ground voltage, a 13.8kv station would use a 70:1 pt or a 23 kv station 120:1 above to voltage, CCVT are used to measure line voltages

When using multi-ratio CTs keep in mind that the accuracy class is for the full rating. If you use the lower taps the accuracy is lower as well. As a general design practice you can use multi-ratio CTs, just try to use the full setting tap whenever possible.
 

SG-1

Senior Member
Instrument transformers (PTs & CTs) are used for measurement, protection, & control in power systems. Primary voltage and current magnitudes are too large to be measured directly by an instrument or device & must be scaled down to a low standardized value. Instrument transformers isolate control, measurement, & protection circuits from the high voltage & current that is being measured & controlled. These devices have accurate and predictable characteristics that allow protective relaying to correctly operate during abnormal and/or fault conditions that occur on power systems.


? The PT (Potential Transformer), now being referred to as a Voltage Transformer or VT, is a precision step-down instrument transformer designed & used for continuous voltage measurements in power systems. It steps down the voltage to a safe low level so a voltmeter or a device with voltage elements may be used to monitor the primary system voltage. These meters are usually scaled to directly read the primary values. 120 volts is the most typical secondary voltage for this type of transformer. Their ratio is expressed as the primary voltage to 1. The load on a PT is kept as small as possible to maintain regulation and thus the highest possible accuracy.

? The PT steps voltage down and current up. It cannot be operated with it?s secondary winding shorted or the current will go out of control. PTs are fused on both the primary and the secondary winding. The primary fuse must be able to survive inrush current, so they only provide short circuit protection in case of a primary winding fault. The secondary protection is normally sized to protect the PTs from overload.

? A CT (Current Transformer) is a precision toroidal wound step-down instrument transformer used for continuous current measurement in power systems. They usually step the primary current down to 5 amps so a meter or other device with current elements can safely monitor the primary current. The ratio is usually expressed in terms of the primary current to 5 Amps. It may be expressed as the primary current to 1 Amp for some applications. The CT steps current down & voltage up. It must be operated with it?s secondary winding shorted or the voltage will go out of control by exceeding the volts to turns ratio. This will exceed the insulation rating and arcing will occur that produces explosive gases. Since CTs have no primary or secondary fuses a catastrophic failure will result. A toroidal wound CT is also called a donut or bushing type CT.

? A BYZ or Balanced WYE Zero Sequence CT is a specialized low ratio, usually 50:5 or 100:5 CT used for ground fault detection.



If you are now hopelessly confussed my work here is done.

Steve
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
he PT (Potential Transformer), now being referred to as a Voltage Transformer or VT...
The electrical (construction and industrial) industry has been reluctant to use the term VT for at least the past 30 years. I think the term VT is pretty much only preferred by utility protective relay users and manufacturers.
 

SG-1

Senior Member
Thank you Jim, as I do not particularly care for the term VT. I just seem to be seeing it more often now on the drawings, and did not know why. I though the "next generation" of engineers was changing the name. It may be I am just experiencing more utility customers ?

Steve
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090817-1134 EST

From a practical point of view the output voltage of a current transformer when unloaded can not go to infinity. Nor can it theoretically unless the input current is infinite.

A current transformer is not really different than any other transformer other than use and some design considerations.

Until the current transformer core gets saturated the unloaded output voltage is approximately N times the input voltage where N is the turns ratio. The output current with some load is approximately 1/N times the input current.

The input voltage to the current transformer is input current times the equivalent input impedance. The input impedance of the current transformer is approximately (output impedance) / N^2.

When there is no load on the current transformer secondary, then the input impedance is primarily the primary inductive reactance. High compared to the condition of a loaded secondary.

As primary voltage increases the core goes into saturation. If you drove it hard enough, then it ultimately looks an air core transformer, and only a portion primary the flux links with the secondary.

Another practical consideration is that as the voltage on the secondary increases it causes insulation breakdown of the secondary. This limits maximum voltage.

None of this analysis means you do not get large voltage at the output.

I think a current transformer with back to back silicon diodes built into the unit would be an ideal solution. Now a broken external wire will not cause a transformer failure.

.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
As designed and used, a current transformer tries to maintain its secondary _current_ as a strict proportion of its primary current. It tries to hold this output current no matter what the secondary voltage. There is no danger to shorting the secondary; in this case the target current will flow, without much voltage needed to 'push' it through the shorting bar and secondary resistance.

If you open the secondary circuit, the secondary voltage will get extremely high, in an attempt to maintain the secondary current in proportion to the primary current. In some circumstance the voltage can get high enough to damage insulation or arc between terminals. So for a 'Current Transformer' the safe state is with the secondary shorted.

In basic physics, a CT is just like any other transformer. You have a primary coil, a magnetic core, and a secondary coil. The current flow in both the primary and secondary coils creates a magnetic field, which then induces voltage in _both_ of the coils. The relationship between the induced voltage and the applied voltage determines what the transformer is doing.

In a conventional distribution transformer, or in a PT, the primary and core are designed so that the voltage induced in the primary coil is nearly the same as the applied primary voltage. This induced voltage is what limits the primary current to a reasonable value. Voltage is also induced in the secondary coil. If current is allowed to flow in the secondary coil, it tends to reduce the magnetic field strength, which reduces the voltage induced in the primary, so more primary current flows.

In a CT, the primary and core _do not_ limit current much at all. The primary 'coil' is just a short straight length of wire going through the 'donut' of the core, and the core is just the small donut. The 'primary voltage' of a CT is ideally a few millivolts. In use, this primary voltage is placed in series with the load being metered, and it is the load which determines the current flow through the primary.

You still have primary current flowing, which creates a magnetic field, which induces voltage in both primary and secondary coils. Just as in the transformers mentioned above, secondary current flow will reduce magnetic field strength and thus the voltage induced in the primary. But unlike a distribution transformer, this reduced primary voltage does not cause more primary current to flow. Again: the primary voltage is perhaps a few millivolts, and the primary current is limited by the load being measured.

As mentioned above, the primary is a single 'turn'. The secondary is perhaps 200-2000 turns. So the voltage induced in the secondary is 200-2000x the voltage across the primary. If the secondary is opened, then you get no balancing current reducing the magnetic field strength. The core saturates and the primary voltage rises to the maximum that the small core and single turn can sustain. Because of the small core and low turn count, the primary voltage is still quite low, and the primary current will remain limited by the load. But even if the primary voltage makes it up to a couple of volts, the turns ratio multiplier can make for a very high secondary voltage.

So whatever you do, make sure you have a proper 'burden' on the secondary side of your CTs.

-Jon

(Apparently I was writing while Gar was posting. )
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
To simnplify, and in no particular order:

An ammeter has very little resistance, which is why the CT doesn't get damaged when the meter is in place and the shorting bar removed.

The meter current is a represntation of the actual conductor current, so with, say, 400:5 CT's, the meter sees 1 amp for every 80 amps.

An open CT behaves like a step-up PT. An open 200:5 CT, for example, acts like a 1:40 transformer. The high voltage damages the wire insulation.

The CT itself is an iron ring with wire wrapped around it, through the ring. It's a coiled coil, so to speak. The number of turns determines the ratio.
 

SG-1

Senior Member
Mike01 shorting

Mike01 shorting

The shorting bar built into an inline CT and a shorting terminal block both provide the same function. They are used to short circuit the CT output current before it reaches any devices. The secondary current circuit can then be opened to replace a meter or relay that has gone bad or needs calibration without the risk of a catastrophic failure. A test switch is sometimes designed into the current circuit to shunt the CT output current around a device so it may be serviced while other devices continue to operate normally. A switchgear SCV type CT can explode if the secondary current circuit is opened while current is flowing on the primary. Other type CTs like those found built into a network protector are designed so they cannot explode.

Steve
 
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