Current Transformer

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

: 07 – 10 – 2010

Date of Submission

: 28 – 10 – 2010

Observations Class

(ii)

CT A

10P10

CT B

0.2

Secondary Resistance Measurement CT A secondary resistance

= 0.1 Ω

CT B secondary resistance

= 0.1 Ω

(iii) Polarity Check Primary current flow is from P2 → P1 Secondary current flow; Switch ON

S2 → S1

Switch OFF

S1 → S2

(iv) Ratio Check

(v)

CT A secondary current

= 0.6 A

CT B secondary current

= 0.6 A

Magnetization Curve

Voltage (V)

Magnetizing Current (mA) CT A

CT B

5

28

54

7

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

(vi) Burden For a current of 50 A in the primary; Current (mA)

Voltage (V)

CT A

650

0.445

CT B

580

0.428

Calculations (a) Magnetization Curves

Magnetization Curve for CT A 25

Secondary Excitation Voltage (V)

20

15

10

5

0 20

30

40

50

60

70

80

90

Secondary Excitation Current (mA)

100

110

120

130

Magnetization Curve for CT B 20

18

Secondary Excitation Voltage (V)

16

14

12

10

8

6

4 0

2

4

6

8

10

12

14

16

Secondary Excitation Current (mA)

18

20

22

24

(b) Knee point voltages The 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 CT Current (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

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 CTs are 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 Classes Accuracy

Class

describes

the

performance

characteristics of a CT and the maximum burden allowable on the CT’s 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.

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 θ = Phase angle error Φ = 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



Why the secondary should never be open circuited in a CT A 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 CT’s must be kept short-circuited.



CT polarity in differential protection Differential 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.