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CT > BASIC FUNCTION > Flashcards

BASIC FUNCTION Flashcards

1
Q

What is the primary function of current transformers?

A

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

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2
Q

Current transformers converts values for the following equipment

A

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

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3
Q

Standardized output of CT

A

1A and 5A

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4
Q

Standards for standard requirements for instrument transformers

A

IEEE C57.13

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5
Q

Standard for high-accuracy instrument transformers

A

IEEE C57.13.6

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6
Q

Current transformers standards

A

IEC 60044-1

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7
Q

Requirement for protective current transformers for transient performance

A

IEC 60044-6

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8
Q

Additional requirements for current transformers

A

IEC 61869-2

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9
Q

State two goals of CT

A

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

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10
Q

Construction types of CT

A

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

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11
Q

Polarity marks of current transformers

A

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

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12
Q

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

A

Subtractive

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13
Q

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

A

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

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14
Q

Primary current, load facing direction

A

H2

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15
Q

Example of a multi ratio current transformer

A

600:5

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16
Q

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

A

X2-X3

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17
Q

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

A

X1-X2

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18
Q

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

A

X1-X3

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19
Q

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

A

X4-X5

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20
Q

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

A

X3-X4

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21
Q

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

A

X2-X4

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22
Q

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

A

X1-X4

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23
Q

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

A

X3-X5

24
Q

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

A

X2-X5

25
Q

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

A

X1-X5

26
Q

Available C class ratings

A

C100
C200
C400
C800

27
Q

The 200 of a C200 CT

A

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

28
Q

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

A

C rating

29
Q

Standard burden of C200

A

200V/(5*20) = 2 ohms

30
Q

What does ALF stands for

A

Accuracy limit factor

31
Q

Operating ALF

A

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

32
Q

CT Class according to IEC 60044-6

A

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

33
Q

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

A

TPS

34
Q

Standards for Knee point of CT

A

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

35
Q

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

A

ANSI C57.13

36
Q

To avoid AC saturation (formula)

A

Vx > Is x (Zs + Zb)

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

37
Q

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

A

True

38
Q

What are the three main components of CTs?

A

Winding
Core
Insulation

39
Q

What are the tests required?

A

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

40
Q

Tests for CT circuit check

A

Secondary injection
Primary injection

41
Q

Tests for winding healthiness check

A

Burden measurement
Ratio check

42
Q

Test for core healthiness check

A

Excitation characteristic check

43
Q

Test for polarity marking check

A

Polarity check

44
Q

Test for insulation check

A

IR value measurement

45
Q

Disconnects all CT secondary wiring and secondary earthing of all cores

A

IR value measurement

46
Q

Four types of IR value measurement

A

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

47
Q

Minimum acceptable value of (kV + 1) mega ohms

A

Primary to Earth
Primary to Secondary

48
Q

Minimum acceptable value of 1 megaohms

A

Secondary to earth
Core to core

49
Q

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

A

Polarity check

50
Q

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

A

Ratio check

51
Q

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

A

Burden measurement

52
Q

Difference between primary and secondary injection

A

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

53
Q

Summary of tests for potential transformers

A

IR value measurement
Polarity check
Ratio check
Secondary injection

54
Q

Three types of IR value measurement tests for potential transformers

A

Primary to Earth
Primary to Secondary
Winding to winding

55
Q

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

A

Polarity check

CT (1 decks)

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