41707051 current transformer

7/28/2019 41707051 Current Transformer

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EE421

STUDY ON CURRENT TRANSFORMERS

Instructed By: Eng. W.D.A.S. Wijayapala Name : G. R. Raban

Index Number : 070384P

Field : EE

Group : 8

Date of Performance : 07102010

Date of Submission : 28102010


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Observations

Class

CT A 10P10

CT B 0.2

(ii) Secondary Resistance MeasurementCT A secondary resistance = 0.1

CT B secondary resistance = 0.1

(iii) Polarity CheckPrimary current flow is from P2 P1

Secondary current flow;

Switch ON S2 S1

Switch OFF S1 S2

(iv) Ratio CheckCT A secondary current = 0.6 A

CT B secondary current = 0.6 A

(v) Magnetization Curve

Voltage (V)Magnetizing Current (mA)

CT A CT B

5 28 547 35 200

9 43 910

11 48 2.1 A

13 55 4.1 A

15 64 12.6 A

17 72 18.5 A

19 83 24 A

20 92


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(vi) BurdenFor a current of 50 A in the primary;

Current (mA) Voltage (V)

CT A 650 0.445

CT B 580 0.428


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Calculations

(a) Magnetization Curves

0

5

10

15

20

25

20 30 40 50 60 70 80 90 100 110 120 130

SecondaryExcitationVoltage(V)

Secondary Excitation Current (mA)

Magnetization Curve for CT A


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4

6

8

10

12

14

16

18

20

0 2 4 6 8 10 12 14 16 18 20 22 24

SecondaryExcitationVoltage(V)

Secondary Excitation Current (mA)

Magnetization Curve for CT B


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(b) Knee point voltagesThe knee point of an excitation curve is defined as the point at which a further increase of

10% of secondary e.m.f. would require an increment of 50% of exciting current.

From data obtained by graph;

Knee Point Voltage

CT A 20.0 V

CT B 16.6 V

(c) Burden of CTCurrent (A) Voltage (V) Burden (VA)

CT A 0.65 0.445 0.65 0.445 = 0.289

CT B 0.58 0.428 0.58 0.428 = 0.248


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Discussion

Metering Current Transformers (CT)are rated for specified standard burdens and designed

to be highly accurate from very low current to the maximum current rating of the CT. Because of

their high degree of accuracy, these CTs are typically used by utility companies for measuring

usage for billing purposes.

Protection CTsare not as accurate as Metering CTs. They are designed to perform with a

reasonable degree of accuracy over a wider range of current. These CTs are typically used for

supplying current to protective relays. The wider range of current allows the protective relay to

operate at different fault levels.

As explained above, a measuring CT is only required to operate over the normal range of

load currents. A protection CT is employed to give satisfactory protection over a wide range of fault

conditions. This may even be many times the full load current. Therefore, the secondary winding

resistance of a protective transformer must be made as low as possible.

The 'knee-point' of the excitation curve of a CT is defined as the point at which a further

increase of 10% of secondary e.m.f. would require a 50% increment of the exciting current.

Therefore, the knee-point may be regarded as a practical limit beyond which a specified ratio may

not be maintained. A CT is considered to enter saturation beyond the knee-point. In this region

almost all the primary current is utilized to maintain the core flux.

CT Accuracy ClassesAccuracy Class describes the performance

characteristics of a CT and the maximum burden

allowable on the CTs secondary. Depending on their

Accuracy Class, CTs are divided into Metering Accuracy

CTs or Relaying Accuracy CTs (Protection CTs). The

accuracy class of a CT is expressed by three parts: rated

ratio accuracy rating, class rating, and maximum burden

as shown in the figure.

The first part of the CT Accuracy Class is a

number which is the rated ratio expressed as a percentage.


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For example, a CT with an accuracy class of 0.3 is certified by the manufacturer to be accurate to

within 0.3 percent of its rated ratio value for a primary current of 100 percent of rated ratio.

The second part of the CT Accuracy Class is a letter that designates the application for

which the CT is rated. Metering CTs are designated with the letter B. Relaying CTs have several

different letter designations.

The third part of the CT Accuracy Class is the maximum burden allowed for the CT. This is

the load that may be imposed on a transformer secondary without causing an error greater than the

stated accuracy classification.

Phasor Diagram of a Current Transformer

Es = Secondary induced e.m.f

Vs = Secondary output voltage

Ip = Primary Current

Is = Secondary current

= Phase angle error

= Flux

IsRs = Secondary resistance voltage drop

IsXs = Secondary reactance voltage drop

Ie = Exciting current

Ir= Component of Ie in phase with Is

Iq = Component of Ie in quadrature with Is


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Why the secondary should never be open circuited in a CTA CT will be at risk of being destroyed if the secondary is left open circuit and the primary

current is present. The secondary of a CT must always have a burden connected; an open circuited

secondary can result in the development of a dangerously-high secondary voltage. This will cause

the insulation to fail and the winding to be short circuited. Energized but unused CTs must be keptshort-circuited.

CT polarity in differential protectionDifferential protection is a unit scheme that compares the current on the primary side of a

transformer with that on the secondary side. Where a difference exists (other than that due to the

voltage ratio) it is assumed that the transformer has developed a fault and the plant is automatically

disconnected by tripping the relevant circuit breakers. The operating principle employed by

transformer differential protection is the Merz-Price circulating current system as shown below.

Under normal conditions I1and I2 are equal and opposite such that the resultant current through the

relay is zero. An internal fault produces an unbalance or 'spill' current that is detected by the relay,

leading to operation.

Polarity refers to the instantaneous direction of the primary current with respect to the

secondary current. All current transformers are subtractive polarity (i.e. primary and secondary

currents flow in same direction). When installing CTs for differential protection, care should be

taken to employ the right polarity. If a wrong polarity is used, information fed to the relays will be

erroneous; relays will always detect an unbalance in the system.

41707051 current transformer

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