How to read a current transformer (CT) nameplate

Hi guys. Today we’ll be looking at how one can read and interpret the information given in a current transformer nameplate.

A CT which is short for a current transformer is an electrical material which is used for metering, over current and deferential protection.

By metering, I mean it is used to read the current flowing through an electric circuit. Also, by protection, it is used to protect devices under its zone of protection.

We’ll be using the nameplate from the image above to explain how the information gotten from the nameplate can be interpreted.

 

S.NO: VTS/MAR/011/111-064

This is short for serial number. The number on the nameplate is its serial number

 

Ratio: 300/1/1/1 A

This ratio is talking about the current transformer ratio which means the current transformation ratio.

Here the circuit contains single primary current input and trio secondary current output.

The ratio 300/1-1-1 A means if the circuit primary carries 300amps then the output of the CT secondary will be 1amps.

The trio 1-1-1 A means the first core used for overcurrent protection is 1A, similarly for the other cores used for metering and differential protection.

As we go further, we’ll see how to know it’s for metering, overcurrent protection and differential protection

Note: The secondary part of the CT is not part of the circuit it is only used to protect and meter the circuit. If 300A goes into the CT, 300A will come out of the CT.

 

Line volt/HSV: 33/36KV

Line volt stands for the nominal system voltage the CT is meant to operate on.

While HSV which is short for highest system voltage is the highest RMS line to line voltage which can be sustained under normal operating condition at any point on the system.

From the information above, the nominal system rating is 33kv while the highest system voltage is 36kv.

 

B.I.L: 36/70/170kv

B.I.L is short for basic insulation level of a transformer.

It is a method used to specify the magnitude of the voltage surge that a transformer can tolerate without any damage to the windings and live parts of the transformer.

The 3 specified voltages are the rated operating voltage of the current transformer for some given time.

The first which is 36kv is similar to HSV which is the rated line to line voltage under normal steady operation.

The second value which is 70kv means the CT can be operated at 70kv for 1mins at 50Hz.

Lastly, the third value which is 170kv means the CT’s maximum withstandable impulse voltage is 170kv.

 

S.T.C: 25KA/3sec.

S.T.C. stands for short time current.

This is the RMS value of the primary current which the CT will withstand both thermally and dynamically without damage of the winding.

This value happens to be 25KA from the nameplate given to us.

It can only withstand this value for 3 seconds with their secondary windings short-circuited.

 

I Dyn: 62.5KA (peak)

I Dyn stands for the rated dynamic current.
This means that the peak value of the primary current which a current transformer will withstand without been damaged electrically from mechanical forces is 62.5KA with the secondary winding short-circuited.

This type of current can be generated when the breaker closes back to an existing fault.

In this case, the current may rise up to 2.5times the short circuit current i.e 25KA \times 2.5=62.5KA and this is in line with the IEE standard.

 

Class of Ins: A

This is talking about the class of insulation. The type of transformer insulation used in this case is of class A. This can withstand a temperature of up to 105℃.

 

Frequency: 50Hz

This means the number of cycles per seconds is 50Hz. The input and output frequency of the transformer is 50Hz.

 

Type: O/D, O/C, live tank

This means outdoor oil-filled live tank current transformer.

In this type, the core and secondary winding are installed from high voltage with an insulating machine and placed in the top tank.

In live tank CT’s, the primary conductor is shorter in length which makes it stronger to withstand high short circuit current.

 

No. of core: Three

It means that the secondary part of the CT has 3 cores.

These cores are for metering, overcurrent and differential protection. We’ll find out how to know soon enough.

 

Year of MFR: 2012

This tells that the CT was manufactured in 2012.

 

Made to: IEC: 60044-1

This means the CT is manufactured and tested in accordance with the IEC: 60044-1 standard.

 

Purpose: MET/PROT/PS

This is the part that tells us what the three cores are for.

MET stands for metering.

PROT stands for overcurrent protection and PS stands for protection special which in this case is differential protection.

 

Burden: 30/30/ – VA

The maximum allowable apparent power from the CT is called burden.

Here, 30VA is the maximum limit of the current transformer output.

These data are used to protect the transformer from overloading.

The 30/30VA means both the core can be loaded up to 30volt Amp.

 

I.S.F: <5

I.S.F. stands for instrument security factor of a current transformer.

I.S.F <5 means the ratio of the instrument limits the primary current to the rated primary current is less than 5.

 

V_{k}. (PS core): = >200v

Protection special (PS) is defined by knee point voltage of current transformer V_{k} and excitation current at \frac{V_{k}}{2}.

V_{k}=>200 means the knee point voltage which is defined as the secondary voltage at which an increase of 10% produces an increase in magnetizing current of 50%.

Also, V_{k} is the secondary voltage above which the CT is near magnetic saturation.

 

R_{ct}. (PS core): 2 to 2\cdot5ohms

This means the secondary winding resistance of the PS core of the CT is between 2 to \cdot5 ohms.

 

I_{ext}(PS core): <=30MA

This means the excitation current used for differential protection at \frac{V_{k}}{2} is less than or equal to 30MA at \frac{V_{k}}{2}.

 

QTY of Oil: 18\pm2ltr

This means the quantity of oil in the CT is 18\pm2ltr

 

Total weight: 110\pm10KG

This means the weight of the manufactured CT is 110\pm10KG

 

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