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Current transformers (CTs) in a few words

Principle of Operation

A current transformer is defined as as an instrument transformerin which the secondary current is substantially proportional to theprimary current (under normal conditions of operation) and differsin phase from it by an angle which is approximately zero for anappropriate direction of the connections. This highlights theaccuracy requirement of the current transformer but also importantis the isolating function, which means no matter what the systemvoltage the secondary circuit need to be insulated only for a lowvoltage.

The current transformer works on the principle of variable flux. Inthe ideal current transformer, secondary current would be exactlyequal (when multiplied by the turns ratio) and opposite to the primary current.

But, as in the voltage transformer, some of the primary current or the primary ampere-turns are utilized formagnetizing the core, thus leaving less than the actual primary ampere turns to be transformed into the secondaryampere-turns. This naturally introduces an error in the transformation. The error is classified into current ratio errorand the phase error.

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Definitions

Typical terms used for specifying current transformer are:

Rated primary current

The value of current which is to be transformed to a lower value. In CT parallence, the load of the CT refers to theprimary current.

Rated secondary current

The current in the secondary circuit and on which the performance of the CT is based. Typical values of secondarycurrent are 1 A or 5 A.

Rated burden

The apparent power of the secondary circuit in Volt-amperes expressed at the rated secondary current and at aspecific power factor.

Composite Error

The RMS value of the difference between the instantaneous primary current and the instantaneous secondarycurrent multiplied by the turns ratio, under steady state conditions.

Accuracy limit factor

The value of primary current up to which the CT compiles with composite error requirements. This is typically 5, 10

or 15, which means that the composite error of the CT has to be within specified limits at 5, 10 or 15 times therated primary current.

Short time rating

The value of primary current (in kA) that the CT should be able to withstand both thermally and dynamically withoutdamage to the windings with the secondary circuit being short-circuited. The time specified is usually 1 or 3seconds.

Class PS/ X CT

In balance systems of protection, CT s with a high degree of similarity in their characteristics are required. Theserequirements are met by Class PS (X) CT s.

Their performance is defined in terms of a knee-point voltage (KPV), the magnetizing current (Image) at the kneepoint voltage or 1/2 or 1/4 the knee-point voltage, and the resistance of the CT secondary winding corrected to75C. Accuracy is defined in terms of the turns ratio.

Knee point voltage

The point on the magnetizing curve where an increase of 10% in the flux density (voltage) causes an increase of50% in the magnetizing force (current).

Summation CT

When the currents in a number of feeders need not be individually metered but summated to a single meter orinstrument, a summation current transformer can be used.The summation CT consists of two or more primarywindings which are connected to the feeders to be summated, and a single secondary winding, which feeds acurrent proportional to the summated primary current.

A typical ratio would be 5+5+5/ 5A, which means that three primary feeders of 5 are to be summated to a single 5Ameter.

Core balance CT (CBCT)

The CBCT, also known as a zero sequence CT, is used for earth leakage and earth fault protection. The concept issimilar to the RVT. In the CBCT, the three core cable or three single cores of a three phase system pass throughthe inner diameter of the CT.

When the system is fault free, no current flows in the secondary of the CBCT. When there is an earth fault, theresidual current (zero phase sequence current) of the system flows through the secondary of the CBCT and thisoperates the relay. In order to design the CBCT, the inner diameter of the CT, the relay type, the relay setting andthe primary operating current need to be furnished.

Interposing CT’s (ICT’s)

Interposing CT’s are used when the ratio of transformation is very high. It is also used to correct for phasedisplacement for differential protection of transformer.

SOURCE: Electric Power Substations Engineering By James C. Burke

About Author //

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high powerbusbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.

Professional in AutoCAD programming and web-design. Present on

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7 Comments

1. Differences between Shunt Reactor and Power Transformer | EEP

Dec 22, 2013

[...] current is based on the load transfer requirement.Detailed view of an iron core divided by airgapsMagnetizing current is small and is negligible value when compared with the secondary rated current.Further, since [...]

(reply)

2. Rogowski Coil Construction | EEP

Dec 13, 2012

[...] Measurements and Protection Figure 1 shows the construction of a Rogowski coil, an air-core currenttransformer that is especially well suited to measuring ripple currents in the presence of a DC component or[...]

(reply)

3. Testing and Commissioning of Current Transformer | EEP

Oct 19, 2012

[...] and specifications.Inspect for physical damage/ defects and mechanical condition.Verify correctconnection of transformers with system requirements.Verify that adequate clearances exist betweenprimary and secondary [...]

(reply)

4. How To Connect Current Transformers? | EEP

Feb 17, 2012

[...] The primary of a current transformer typically has only one turn. This is not really a turn or wrap aroundthe core but just a conductor [...]

(reply)

5. PIX metal-clad switchgear | EEP

Jan 20, 2012

[...] Its position is visible from the front panel, through the inspection window in the cable panel.Currenttransformers, with DIN dimensions, are fitted at the rear of the cubicle.The voltage transformers, fitted at the[...]

(reply)

6. How residual current device (RCD) works? | EEP

Oct 09, 2011

[...] then the load and return currents are equal. In this case the mmfs generated by the load and returncurrent windings are equal; there is no resultant flux in the toroid and the detecting winding does notgenerate any [...]

(reply)

7. Applications and forms of differential relays | EEP

Aug 15, 2011

[...] system element might be a length of circuit, a winding of a generator, a portion of a bus, etc.A currenttransformer (CT) is shown in each connection to the system element. The secondaries of the CTÕs are [...]

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