Nameplate Detail of Current Transformer – A Detail Guide

We all have studied the working of a current transformer in our power system. In this interesting article, we are going to discuss the nameplate parameters of CT (Current transformer). Let’s start!

CT Nameplate Details

A current transformer is present in our power system to measure and protect the system from faults. It has different cores used for different purposes. But here we will only discuss the nameplate parameters in detail one by one.

Here is the list of parameters written on the nameplate of a CT:

• First of all, there will be the name of the manufacturer.
• Then there will be the type of CT (It can be changed from different manufacturers).  
• Then we have the insulation level of CT. In our case of the following CT nameplate by SIEMENS, we have 2 values written in insulation level. These are 460kV (RMS) and the second is 1050kVp (Peak voltage). Here is the nameplate of a CT:

The first value of insulation level voltage is called power frequency voltage. These voltages can rise due to the following reasons:

1. Phase-to-earth Voltages
2. Load Rejection
3. Ferro resonance
4. Ferranti effect

The second value of insulation level voltage is called lightning impulse voltage. These voltages can rise due to the lightning strokes. It means that this CT should withstand these voltages in case of faults for the above reasons.

• Next is the standard according to which the CT is manufactured. In this case, the standard is IEC-61869-2. 

These were basic parameters that are mentioned on the nameplate of CT.

Here are the next parameters of CT nameplates:

• We have rated the primary current that will be flowing normally in CT.
• Then we have the total weight of CT in “kg”.
• Next is the short time current which is mentioned as 40kA/3sec in our above CT nameplate. It indicates that this CT is capable of carrying 40kA current in a fault situation for 3 seconds.
• Next, we have “Idynamic” current or making current. It is mentioned as a 100kA peak. It can be produced when the circuit breaker will close during a fault. Then, in that case, the current in Ct may rise up to 2.5 times the short circuit current.
• Next, we have a frequency of CT for which it is designed.
• Then we have rated the continuous current value. It is 1920A in our case. However, there is a difference between the rated primary and rated continuous current (rated extended continuous current). The rated primary current is normal to stay in CT but the CT is designed to carry more magnitudes of rated primary current. It is capable of carrying up to 120% or 150% of rated primary current.

Hence proved that the rated continuous current is more than the rated primary current. 

• Next is the weight of SF6 gas (in kg). Our case has SF6 gas-operated CT so it is mentioned on its nameplate.
• Next, you structure the diagram of CT in black color in our case. P1 and P2 mentioned the primary of CT. And then in secondary, we have 5 different cores of CT.   
• In the last table, we have some more information.
• We have two ratios in this Ct. First is 1600/1 and next is 800/1. We can use both ratios depending on the primary current flowing in CT.
• Then we have secondary terminals to connect the secondary core with a particular ratio. For 1600Amps we have to connect the CT with terminal 1S1 and 1S3. Similarly, for 800Amps we have to connect the CT with terminals 1S1 and 1S2. 1S1 stands for 1^st core, 2S1 indicates 2^nd core, and 3S1 indicates 3^rd core.
• Next, we have two types of accuracy classes. We have metering and protection classes and both these types have further classes.

For protection class, we have PS or PX class for differential protection. 5P and 10P are present also in the protection accuracy class. Here 5 and 10 represent allowable composite error in CT and P represents the protection class. This type of protection is generally used for over current protection.    

Next, we have metering class according to metering sub-classes as

1. 0.2s –It is the most accurate class among all these classes.
2. 0.2
3. 0.5s
4. 0.5
5. 1
6. Next, we have the VA burden of CT. The burden is anything that is connected along CT secondary. It includes equipment wire and which equipment we are going to connect across CT. The equipment may be a meter or a relay. In our case, we can connect 20VA in our metering core with wire and an accuracy class of 0.2.
7. Next is the instrument safety factor (ISF) and it is applicable for metering class. In some cases, the primary current may increase up to 5 times of primary current. So the equipment may get damaged due to this abnormal current. So the manufacturer should mention the safety factor. Second is the accuracy limit factor (ALF) for the protection class. 10P means the composite error will be within limits if the primary current increases up to 10 times of rated primary current.
8. Next, we have knee point voltage mentioned on the nameplate as “Vk”. These are the voltages applied across the secondary of CT with all other windings of CT open-circuited. Increasing the exciting current in CT will increase the ratio error in the transformer.
9. Next, we have an exciting current mentioned on the nameplate as (30 for 1600 tap and 60 for 800 tap).
10. The last thing is the maximum secondary winding resistance. It is mentioned as 8 ohms for 1600/1 and 4 ohms for 800/1 tap.   

 These are all the parameters of a CT that are mentioned on the nameplate of the CT.

Conclusion:

We have discussed the main parameters of CT’s nameplate in detail. These parameters are written according to standardized values. These are different voltages and current values for a CT. All the information on the nameplate should be tested by the manufacturer before installation of the CT in the actual field.

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