Current Transformer


In the last article we discussed about the Voltage Transformer(VT). Here we will discuss about the Current Transformer (CT). As we know the voltage and current transformers are used in the electric substations to transform high magnitude voltage and current to low magnitude voltage and current suitable for metering and protection purposes. This article require some basic knowledge about the transformer. It is even better if you go through the previous article about voltage transformer.

In the last article we had written two fundamental relationships between the voltage and current of both sides of the ideal transformer. 

For the current transformer the second equation is the main focus, which is reproduced here for convenience.

 The ampere-turns of both sides of the transformer are equal. Otherwise this equation is also called as the conservation of ampere-turns. So for the ideal transformer,

Ip  Np =  I Ns 



The subscripts 'p' and 's' are used for primary and secondary sides of the transformer respectively. Where the symbol I is used for current and 'N' is for number of turns. From the above formula it is easy to guess that for a transformer as Np and Ns are known values which does not change, so the value of secondary current Is is proportional to the primary current Ip, which is desired for accurate measurement of primary current. This is only true for ideal transformer. In case of actual transformer the primary and secondary side currents are not proportional. (see the equivalent circuit of the transformer in last article)

In an actual transformer a small part of the primary current is used as exciting current of the transformer core. So Ip and Is are no more proportional. Hence in the design of the current transformer the main aim is that the excitation current of the current transformer should be low.

Burden And Error


As said above, error is introduced in the measurement of the secondary current. The error happens both in the magnitude and phase angle of the current. The Error in magnitude is said as Current Ratio Error and the error of phase angle is called as Phase Error. the Curret Ratio Error in percentage is given as:

Current Ratio Error = (Kn * Is - Ip ) * 100 / Ip

Here Ip and Is are the actual primary and secondary currents (rms). 

Where as Kn = Ipr / Isr

Ipr and Isr are the rated primary and secondary currents and Kn is the rated transformation ratio.

Similar to the voltage transformer the load on the secondary of the current transformer is called as burden. The burden is either expressed in volt-amperes (VA) or in Ohms. The burden of the CT is the  sum of the burdens due to all  the equipment connected to the CT plus the burden due to the connecting cables.

The CT secondary current is usually rated at 5 Amp or 1 Amp. The secondary current rating should be chosen judiciously according to the application. Suppose we choose a CT of secondary current 5A. If the equipment for connection to the secondary of this CT requires long cables then the burden due to this connecting cables may be a considerable proportion of the total burden.

Burden Due to Cable = IR

R is the total resistance of connecting cables. I is the secondary current of CT. From the above you can have a feel about the value of burden due to cable for both the cases of 5A and 1A secondary rating.

So for the cases where long connecting leads are required then smaller current rating of 1A should be chosen.


Accuracy of current transformer is defined by different standards . The table below is a part of the IEC standard. The full table may be obtained from their web site.


Current Transformer Accuracy Table
Current Transformer Accuracy Classes (As IEC 60044-1)
(Partial Table)

Accuracy Class
Percent Rated current
Current Ratio Error (%)
Phase Error (Minutes)
Application
0.1
5
+0.4
+15
Precise Measurement
0.1
20
+0.20
+8
Precise Measurement
0.1
100
+0.1
+5
Precise Measurement
0.1
120
+0.1
+5
Precise Measurement
0.2
5
+0.75
+30
Measurement
0.2
20
+0.35
+15
Measurement
0.2
100
+0.2
+10
Measurement
0.2
120
+0.2
+10
Measurement



In the table if the secondary current phasor leads the primary current phasor then the phase difference is positive, so the phase error is positive otherwise the error is negative. If you have drawn the phasor diagram for a basic transformer so that the angle between Ip and Is is nearly 180 degrees, then before doing the comparison the secondary phasor should be reversed. (if the CT is an ideal one then the angle between the two phasors should be zero). The above accuracy class table is valid for the burden between 25% to 100% of rated burden.

Construction


From the constructional point of view the current transformer is broadly available in two types. These two are Ring Type and Wound Core type.

The Wound Primary type has the primary winding of one or more turns over the core of the CT. The Window or Ring type does not have a primary winding. Of course both the types have a secondary winding.  The ring or window type has an opening in the center. The primary side is the single conductor which can pass through the opening (see figure below). 



Another variant of ring type is available which is called as bar type. Here a conductor bar is already occupying the center of the core and so a part of the CT. The terminals of the bar are brought out to be connected in series with the power circuit of which the current to be measured(or for protection). One more variation of ring type CT is the split core type. A spring mechanism allows the core to split and the primary conductor is allowed to the center of the core. Then the core is closed. This type of CT is mainly used for current measurement of low voltage distribution maintenance work below 450 volts.

The ring and bar types are mainly used for medium and low voltage CT where as wound primary type is mainly used for high and extra high voltage application.

The HV current transformers are designed as live tank type or dead tank type. The basic design of a CT for HV use is illustrated in figures below. The two types of CT mostly used in high voltage application are Hair Pin type and Top Core type. The Hair Pin type is so named as its primary side conductor resembles a hair pin.The Hair Pin type is mechanically more stable and robust in comparison to Top Core type. In HV application the core of the Hair Pin type is usually insulated by oil or SF6 (sulphur Hexa Fluoride) gas, which resides in a steel tank at the bottom position of the CT module.



The primary conductor in a Top Core type CT is straight passing through the core. The core is positioned at the top of the CT inside a box. See the above figures illustrating the relative position of different parts for both types of high voltage CT. Also in the figure below is shown a sketch of the Top Core type CT. It should be noted that for simplicity the secondary winding is shown as consisting of 2 or 3 turns concentrated at one place. But the actual CT whose secondary winding is comprised of several turns and the winding turns are evenly distributed along the core.

The CT is connected in series with the circuit of which the current is to be measured. When on the live system, the secondary side of the CT should never be left open. If the secondary is not used then the terminals of the secondary are shorted. If the secondary is left open then excessive high voltage is induced in the secondary which is harmful to the personnel and also to the equipment insulation. Also the CT should not be operated with a high resistance burden. The burden here is the total burden due to all the equipments connected to the CT plus the burden due to the cable.



The requirements of CT for the purpose of metering and protection are different. While for the metering purpose the accuracy within certain limits are very important where as in case of protection use, the CT should be able to give a reasonably proportionate secondary current for a variation of primary current which is many times the rated primary current.   The Accuracy Load Factor (A.L.F)  is the ratio of primary current upto which the CT gives reasonably proportionate secondary current  to the rated current. A.L.F is a number which gives the idea that upto how many times of the rated primary current, the CT gives reasonably proportionate secondary current. It is very essential for protection purposes, which is the requirement at faulted system condition with large fault current.

In this article and the previous article we have covered two very important equipment of a electric substation. Effort is made to present the information in simple and accurate ways. For the actual use of the CT and PT one should consult the manufacturer instruction sheet, guidelines and the relevant international and local standards. In the next article we will discuss little more and compare CT and PT and their connection.


2 comments:

RAJIV BISWAS said...

This is really a very helpful article which students require to really get their concepts clear about being an electrical engg

Chari Ross said...

Very good post on CTs, I also have an article regarding types of CTs on my page if anyone is interested in learning more about Current Transformer Theory. Cheers!

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