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BASIC FUNCTION Flashcards

(55 cards)

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?

24
Q

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

25
With a primary current of 600A, what are the secondary terminals needed to get 5A?
X1-X5
26
Available C class ratings
C100 C200 C400 C800
27
The 200 of a C200 CT
The secondary terminal voltage at which the CT must maintain within the C rating
28
Less than 3% ratio error at rated current Less than 10% ratio error at 20 times rated current
C rating
29
Standard burden of C200
200V/(5*20) = 2 ohms
30
What does ALF stands for
Accuracy limit factor
31
Operating ALF
ALF' = ALF x (Pi + Pnb) / (Pi + Pbb)
32
CT Class according to IEC 60044-6
TPX - closed iron core TPY - with air-remanence air-gap TPZ - linear core TPS - closed iron core
33
A special version of closed iron core CT for high impedance protection
TPS
34
Standards for Knee point of CT
British Standard BS3938 - CLASS X IEC 60044-1 Ammendment 2000/07 - CLASS PX
35
It is specified in _____ that the secondary terminal voltage U2 at 20 times rated current and rated burden, error is less than 10%
ANSI C57.13
36
To avoid AC saturation (formula)
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
True or False: Remanence only considered in extra high voltage systems Ktf values acc. To relay manufacturer's guides
True
38
What are the three main components of CTs?
Winding Core Insulation
39
What are the tests required?
IR value measurement Polarity Check Ratio check Excitation characteristic check Burden measurement Secondary injection Primary injection
40
Tests for CT circuit check
Secondary injection Primary injection
41
Tests for winding healthiness check
Burden measurement Ratio check
42
Test for core healthiness check
Excitation characteristic check
43
Test for polarity marking check
Polarity check
44
Test for insulation check
IR value measurement
45
Disconnects all CT secondary wiring and secondary earthing of all cores
IR value measurement
46
Four types of IR value measurement
Primary to Earth Primary to secondary Secondary to earth Core to core
47
Minimum acceptable value of (kV + 1) mega ohms
Primary to Earth Primary to Secondary
48
Minimum acceptable value of 1 megaohms
Secondary to earth Core to core
49
Discharge primarily fully before test. Do not connect the battery cell to the primary for a long time
Polarity check
50
Disconnect CT connection. Short other CT cores not under test.
Ratio check
51
Use to check secondary wiring correctness and provides accurate data for CT sizing check
Burden measurement
52
Difference between primary and secondary injection
Secondary injection removed the secondary wiring from the source and connected to ground. Primary injection is connected to the source.
53
Summary of tests for potential transformers
IR value measurement Polarity check Ratio check Secondary injection
54
Three types of IR value measurement tests for potential transformers
Primary to Earth Primary to Secondary Winding to winding
55
Isolating PT from the main bus, and disconnect PT secondary before test
Polarity check