BASIC FUNCTION

Question 1

What is the primary function of current transformers?

Answer 1

1. To convert primary values to manageable values
2. To isolate the protection, control, and measurement equipment from high voltages of the system

Question 2

Current transformers converts values for the following equipment

Answer 2

Power meters, revenue meters, protective relays, fault recorders, SCADA

Question 3

Standardized output of CT

Answer 3

1A and 5A

Question 4

Standards for standard requirements for instrument transformers

Answer 4

IEEE C57.13

Question 5

Standard for high-accuracy instrument transformers

Answer 5

IEEE C57.13.6

Question 6

Current transformers standards

Answer 6

IEC 60044-1

Question 7

Requirement for protective current transformers for transient performance

Answer 7

IEC 60044-6

Question 8

Additional requirements for current transformers

Answer 8

IEC 61869-2

Question 9

State two goals of CT

Answer 9

1. Should not saturate at max fault levels
2. CTs are required for perform satisfactorily in high fault environment

Question 10

Construction types of CT

Answer 10

Window type, bar type, wound type, split core, hair-pin/tank type, cascade/eye-bolt, top core, combined current-voltage type

Question 11

Polarity marks of current transformers

Answer 11

H1 and H2 - primary terminals
X1 and X2 - secondary terminals

Question 12

What is the polarity with the H1 and X1 on the same side of the current transformer?

Answer 12

Subtractive

Question 13

CT ratio is the ratio of primary current input and secondary current output at full load.
TRUE or FALSE

Answer 13

True, example: 300:5, at full load 300A input on the CT will be converted into 5A output

Question 14

Primary current, load facing direction

Answer 14

H2

Question 15

Example of a multi ratio current transformer

Answer 15

600:5

Question 16

With a primary current of 50A, what are the secondary terminals needed to get 5A?

Answer 16

X2-X3

Question 17

With a primary current of 100, what are the secondary terminals needed to get 5A?

Answer 17

X1-X2

Question 18

With a primary current of 150, what are the secondary terminals needed to get 5A?

Answer 18

X1-X3

Question 19

With a primary current of 200A, what are the secondary terminals needed to get 5A?

Answer 19

X4-X5

Question 20

With a primary current of 250A, what are the secondary terminals needed to get 5A?

Answer 20

X3-X4

Question 21

With a primary current of 300A, what are the secondary terminals needed to get 5A?

Answer 21

X2-X4

Question 22

With a primary current of 400A, what are the secondary terminals needed to get 5A?

Answer 22

X1-X4

Question 23

With a primary current of 450A, what are the secondary terminals needed to get 5A?

Answer 23

X3-X5

Question 24

With a primary current of 500A, what are the secondary terminals needed to get 5A?

Answer 24

X2-X5

Question 25

With a primary current of 600A, what are the secondary terminals needed to get 5A?

Answer 25

X1-X5

Question 26

Available C class ratings

Answer 26

C100 C200 C400 C800

Question 27

The 200 of a C200 CT

Answer 27

The secondary terminal voltage at which the CT must maintain within the C rating

Question 28

Less than 3% ratio error at rated current Less than 10% ratio error at 20 times rated current

Answer 28

C rating

Question 29

Standard burden of C200

Answer 29

200V/(5*20) = 2 ohms

Question 30

What does ALF stands for

Answer 30

Accuracy limit factor

Question 31

Operating ALF

Answer 31

ALF' = ALF x (Pi + Pnb) / (Pi + Pbb)

Question 32

CT Class according to IEC 60044-6

Answer 32

TPX - closed iron core TPY - with air-remanence air-gap TPZ - linear core TPS - closed iron core

Question 33

A special version of closed iron core CT for high impedance protection

Answer 33

TPS

Question 34

Standards for Knee point of CT

Answer 34

British Standard BS3938 - CLASS X IEC 60044-1 Ammendment 2000/07 - CLASS PX

Question 35

It is specified in _____ that the secondary terminal voltage U2 at 20 times rated current and rated burden, error is less than 10%

Answer 35

ANSI C57.13

Question 36

To avoid AC saturation (formula)

Answer 36

Vx > Is x (Zs + Zb) Is = primary current divided by turns ratio Zs = secondary resistance Zb = total burden (lead + devices) connected to the terminal

Question 37

True or False: Remanence only considered in extra high voltage systems Ktf values acc. To relay manufacturer's guides

Answer 37

True

Question 38

What are the three main components of CTs?

Answer 38

Winding Core Insulation

Question 39

What are the tests required?

Answer 39

IR value measurement Polarity Check Ratio check Excitation characteristic check Burden measurement Secondary injection Primary injection

Question 40

Tests for CT circuit check

Answer 40

Secondary injection Primary injection

Question 41

Tests for winding healthiness check

Answer 41

Burden measurement Ratio check

Question 42

Test for core healthiness check

Answer 42

Excitation characteristic check

Question 43

Test for polarity marking check

Answer 43

Polarity check

Question 44

Test for insulation check

Answer 44

IR value measurement

Question 45

Disconnects all CT secondary wiring and secondary earthing of all cores

Answer 45

IR value measurement

Question 46

Four types of IR value measurement

Answer 46

Primary to Earth Primary to secondary Secondary to earth Core to core

Question 47

Minimum acceptable value of (kV + 1) mega ohms

Answer 47

Primary to Earth Primary to Secondary

Question 48

Minimum acceptable value of 1 megaohms

Answer 48

Secondary to earth Core to core

Question 49

Discharge primarily fully before test. Do not connect the battery cell to the primary for a long time

Answer 49

Polarity check

Question 50

Disconnect CT connection. Short other CT cores not under test.

Answer 50

Ratio check

Question 51

Use to check secondary wiring correctness and provides accurate data for CT sizing check

Answer 51

Burden measurement

Question 52

Difference between primary and secondary injection

Answer 52

Secondary injection removed the secondary wiring from the source and connected to ground. Primary injection is connected to the source.

Question 53

Summary of tests for potential transformers

Answer 53

IR value measurement Polarity check Ratio check Secondary injection

Question 54

Three types of IR value measurement tests for potential transformers

Answer 54

Primary to Earth Primary to Secondary Winding to winding

Question 55

Isolating PT from the main bus, and disconnect PT secondary before test

Answer 55

Polarity check