CT and VT.pptx

‫وترتيب‬ ‫جمع‬
‫مهندس‬
/
‫محمد‬ ‫محمود‬
Mahmoud.ali8585@gmail.com

Objective:
1-power system
2-CT and VT
3-Types of relays
4- Types of Circuit breaker
5-Surge arrestors

What is the meaning of the fault :
Change in voltage and current values
The faults reasons:

Failure in insulation

Short circuit between phase and ground

Short circuit between phase and phase

Short circuit between two phase and ground

Open circuit because of Failure in conductor
Or overload


Specification of the faults:
- For number of phases

Single Line to Ground.

Double Lines to Ground

Three Lines

Phase to Phase

Three Lines to Ground

For Unsymmetrical and Symmetrical Fault :

Unsymmetrical Fault :

Single Line to Ground.

Double Lines to Ground

Three Lines

Phase to Phase

Symmetrical Fault :

Three Lines to Ground


Permanent Faults

Transient Faults

Function of protection system‽

protection system elements:
.1
Protective Relays PR
.2
Circuit Breakers CB
.3
Voltage and Current Transformer CT & VT

protection system elements places ‽

The relation between Protective device and
Circuit Breaker:

Fault Clearing Time
=Relay time (20ms)+CB time (50to 100ms)
What happen if relay detect fault and CB not open ‽


Specification for protective device:
 Reliability
 Selectivity
 Sensitivity
 Adequateness
 Speed
 Stability
 Economics

The main tasks of instrument transformer are:
• Insulate secondary circuits from the primary.
• To transform current, or voltages, from a high value to a value easy
to handle for relays and instruments.
• permit the use of standard current ratings for secondary
equipment.
1
2
2
1
N
N
I
I


APPLICATION
 Current transformers (CT,s) are instrument
transformers that are used to supply a reduced
value of current to protective relays , meters and
other instruments.
 CT,s provide isolation from the high voltage
primary , permit grounding of the secondary
windings for safety , and step down the magnitude
of the measured current to a value that can be
safely handled by the instruments

RATIO
 The most common CT secondary full load current
is 1A or 5A.
 CT ratio are expressed as a ratio of rated primary
current to the rated secondary current .
 Example
a 1000/1 A CT will produce 1A of secondary
current when 1000 A flows through the primary .
As the primary current changes the secondary
current will vary accordingly.

POLARITY
 All CT,s are subtractive polarity .
 Polarity refers to the instantaneous direction of the
primary current with respect to the secondary
current and is determined by the way the
transformer leads are brought out of the case .
 On subtractive polarity transformers the H1
primary lead and the X1 secondary lead will be on
the same side of the transformer.

 Notes:
 CT secondary windings are closed possible
download the CT 125% of rated load for any time
 CT secondary windings are opened for Installation
counter not should increase 10% of rated load for
.5 hour time

 Effect of secondary windings for CT and VT:

 Functions of measurement and protection device:
For protection:
1- following the grid continuous
2- send alarm in case fault
3- send signal to open the C.B
For measurement :
- Use for the counters and indication for Readings

 protection device
 measurement device

 Burden
 Burden = The impedance of the secondary circuit in
ohms and power factor. The burden is usually
expressed as the apparent power (S) in volt-amperes
absorbed at a specified power-factor at the rated
secondary current.

RB
LB
BURDEN
=
VA / I² {
EXTERNAL BURDEN

 The purpose of Burden:
1- Discrimination between CT and VT
2- calculate cable resistance
Secondary winding for measurement device :
15VA =5^2 * (ammeter “R” +counter “R”
+voltammeter “R” +….+ cable “R” )
Secondary winding for protection device :
20VA= 5^2 *(earth leakage + OC relay+….+ cable
“R”)

1 Non saturated - zone
2 intermediate - zone
3 saturated -zone

TEST RESULT
current(I) voltage(V)
0.01 9
0.04 9
0.10 9
0.12 9
0.14 9
0.20 9
0.30 9
0.40 9
40.0 9

CT 1200 / 5 A
Rs 0.0024 ohm/turn
V =
= 1.785 x 240
= 428.4 v
5
1200
*
785
.
1

 Connection of CT with protective devices :

problem of opening of secondary side of CT:
Burden effect on CT Saturation:

 Use CT in protective and measurement devices:

 Connection CT (600/300/5/5):
 Connection in series: 300/5/5:

 Connection in parallel: 600/5/5:

Terminal markings
 GENERAL RULES
The terminal markings shall identify: the primary and
secondary windings; the winding sections, if any; the
relative polarities of windings and winding sections;
the intermediate tapings, if any

GRAPHIC SYMBOLS OF CURRENT
TRANSFORMERS

 Current transformers are divided into two main types:
 1. Indoor type
 2. Outdoor type
Indoor Current Transformer:
 Indoor transformers are used in the high, medium,
and low voltage
 general purpose applications for protection and
measurements

 OUTDOOR CURRENT TRANSFORMERS:
 core and coil are immersed in oil inside insolated
bushing. These bushings may be long or short
depending on the rated voltage.
 The insulation material of such current transformers is
usually oil impregnated paper. Another method for
insulation employs a gas such as Sulfur Hexafluoride (
SF6).

 CT use signals source in some protective devices :
 Non Directional Over Current Protection O/C
 Non Directional Earth Fault Protection E/F
 Differential Protection
 in some protective devices use CT and VT
(information source )
 Directional Earth Fault
 Distance Protection

 Specification for CT:
1. Rated Burden (2.5 , 5.0, 7.5 , 10, 15, 30 VA)
2. Continuous Rated Current
3. Short circuit current and duration time
4. Rated secondry current
5- Turns ratio
6- Classes(5P, 10P, X): 5P use with Instantaneous oc Relay
- 10P use with IDMT oc Relay
- X no need high accuracy

The main characteristics of protection CT cores are:
• Lower accuracy than for measuring transformer .
• High saturation voltage.
5P and 10P The error is then 5 and 10 at the specified ALF and at rated
burden.

No. of primary turns = 1 turn
No. of secondary turns = N turn
Ip = N * Is
Ideal transformer for (Is) to flow through R there must be some
potential Es = The E.M.F
Es = Is * R
Es is produced by an alternating flux in the core.
dt
d
Es



Flux required to produce Es
B
s
CT
s
s z
I
R
I
E *
* 

A
B*


Where
B = Flux density in the core
A = cross-sectional area of core
N
A
f
B
Ek *
*
*
*
44
.
4

 
L
CT
B
s
s z
z
z
I
E 

 Required s
k E
E 
Equ. 1
Equ. 2

CT 2000/5 , Rs =0.31, Imax =40 KA , MaX Flux density =1.6 Tesla
Find maximum secondary burden permissible if no saturation is to occur.
Solution
N=2000/5 = 400Turns
Is max = 40000/400 = 100Amps
From Equ.1
Vk = 4.44*1.6*20*60*(400/10000) = 340 Volt
 Max burden = 340/100 = 3.4 ohms
Max connected burden = 3.4 - 0.31 = 3.09

CT ratio are selected to match the maximum load current requirements.
i.e. the maximum design load current should not exceed the CT rated
current.
The CT ratio should be large enough so that the CT secondary current does
not exceed 20 times rated current under the maximum symmetrical primary
fault current.

It is customary to place CT,s on both sides of the breaker. So that the
protection zones will overlap.
The protection Engineer can determine which side of the breaker is best for
CT location .
All possibilities of fault position should be considered .

The overlap should occur across a C.B, so the C.B lies in both zones for this
arrangement it is necessary to install C.Ts on both sides of the C.B.
C.T,s mounted on both sides of breaker no
unprotected region
 No region un protected

Current transformers mounted on C.B side
only of breaker fault shown not cleared by
bus bar protection.

Current transformers mounted on bus bar
side only of breaker fault shown not cleared
circuit protection.
C,B will open by line protection but fault
will last.

IEC standard
PROTECTION
 RATIO 2000/5 A
 POWER 20 VA
 CLASS 5P20

IEC standard
MEASURING
 RATIO 2000/5 A
 POWER 20 VA
 CLASS 0.5SF5

CT
class X
 THE FOLLOWING INFRMATION IS REQUIRED
 Turns Ratio
 Knee Point Voltage
 Maximum Excitation Current
 Secondary Circuit Resistance

 General
 As a matter of safety, the secondary circuits of a
current transformer should never be opened under
load, because these would then be no secondary mmf
to oppose the primary mmf, and all the primary
current would become exciting current and thus might
induce a very high voltage in the secondary.

TESTING
Saturation
 The saturation point is reached when there is a rise in the
test current but not the voltage.
Burden

Accuracy Limit Factor ALF:
Example:
CT (200/1 A) ,(ALF=5) Means :

How is read the specification for :
CT (100/1 , 5p10) ‽

 Tests on CT:
 (Turns Ratio Test):

1. Setup the CALIBRATED test instrument in a firm, reasonably level base and dry area.
2. Identify test parameters and terminals for proper connections.
3. The CT must be in de-energized condition and completely isolated from other energized
circuit.
4. For system-installed unit, isolate the burden (cable run to control room).
5. Set-up and effectively ground the test instrument. Supply voltage, 110Vac or 230Vac. Conduct
testing as per the following illustrated test Connections
6. Power ON the Test Instrument and select CTRatio Test Card

7. Properly fill-up test settings:
7.1 Indicate actual name-plate ratio.
7.2 Current injection up to 800A. Indicate actual primary current at Itest.
7.3 For Primary current above 800A, use any current below 800A, or use the current
booster up to 2000A.
10. Note and record all necessary test parameters
11. Evaluate test result as per general standard ratings and accuracy classes for current
transformers
8. Press I/O (test start/stop) push-button to start the test.
9. Save test card for printing.
9.1 For multi-tap core/s, create and save test card for each tap tested

testing as per the following illustrated test Connections:
4. Power ON the Test Instrument and select CTExcitation Test Card
7.1 For multi-tap core/s, create and save test card for each tap tested.
9. Evaluate test result as per general standard ratings and accuracy classes for current transformers

testing as
per the following illustrated test Connections:
4. Power ON the Test Instrument and select PolCheck Test Card
7.1For multi-tap core/s, create and save test card for each tap tested.
8. Note and record all necessary test parameters in standard test forms
9. Evaluate test result as per general standard ratings.

 (Insulation Test) :
1-primary and secondary winding

 2- primary winding and earth

 3- secondary winding and earth:

Current Transformer - Winding Resistance Test
testing as per the following illustrated test Connections:
4. Power ON the Test Instrument and select RWinding Test Card
9. Evaluate test result as per general standard ratings and accuracy classes for current transformers

SATURATION
 Abnormal high primary current
 High secondary burden
 Combination of the above two factors will result in the
creation of high flux density in the current
transformer iron core.
 When this density reaches or exceeds the design limit
of the core , saturation results.

SATURATION
 The accuracy of the CT becomes very poor.
 The output wave form distorted.
 The result secondary current lower in magnitude.
 The greatest dangerous is loss of protective device
coordination

list of CT problems usually found at site:
 • Shorted CT secondaries
 • Open-circuited CT secondaries
 • Miswired CTs
 • CTs that had not been wired
 • CTs installed backwards
 • Incorrect CTs
 • Defective CTs
 • CTs with incorrect ratios or on the wrong taps
 Mind you, this was just at one site and had been

 Voltage Transformer Construction:
 For the lower voltages not exceeding 3.3kV, dry type
transformers with varnish impregnated and taped
windings are quite satisfactory in reasonably dry
locations.
 For high voltage, the immersed core and windings in
oil give good primary winding insulation. Another
type of insulation used is cast resin for a range of 12kV-
36kV. A combination of resin and oil is used for up to
100kV


Comparison between VT and power transformer:

Types of voltage transformer:

Potential transformer

Capacitive Voltage Transformer

Specification of voltage transformer:
 Rated Voltage (11KV)
 Level Voltage( 100 or 110 v)
 Breakdown - Pulse Voltage
 Turns Ratio
 Accuracy Class (.1,.2,.3,.4,.5 …..for measuring )
For protection (3p and 5p)


Capacitive Voltage Transformer:

 Notes:
Why is using fuses in VT (.5 to 6A):
Why is using VT in the incoming only :
 Why is using CT in all the Distributor

 What causes the separation of fuses before testing the
incoming cable:
 Steps separation voltage transformer:


‫الجهد‬ ‫محوالت‬ ‫وقاية‬
:

‫مثل‬ ‫عادية‬ ‫غير‬ ‫لحاالت‬ ‫الجهد‬ ‫محوالت‬ ‫تتعرض‬
:

‫مفاجىء‬ ‫حمل‬
.

‫الثانوية‬ ‫الملفات‬ ‫في‬ ‫قصر‬ ‫حدوث‬
.

‫مرتفعة‬ ‫عابرة‬ ‫جهود‬
.

‫للجه‬ ‫و‬ ‫العادية‬ ‫غير‬ ‫الحاالت‬ ‫ضد‬ ‫الجهد‬ ‫محوالت‬ ‫وقاية‬ ‫عادة‬ ‫يتم‬ ‫لذلك‬
‫ود‬
‫حتى‬
11
‫ك‬
.
‫عالية‬ ‫قطع‬ ‫سعة‬ ‫ذات‬ ‫مصهرات‬ ‫طريق‬ ‫عن‬ ‫ف‬
(
H.R.C
)
‫الجهد‬ ‫لمحول‬ ‫االبتدائي‬ ‫الملف‬ ‫و‬ ‫التغذية‬ ‫مصدر‬ ‫بين‬ ‫توصيلها‬ ‫يتم‬
.

‫المغذاة‬ ‫الثانوية‬ ‫الدوائر‬ ‫لوقاية‬ ‫ثالثية‬ ‫مفاتيح‬ ‫استخدام‬ ‫يتم‬ ‫كذلك‬
‫الملف‬ ‫من‬
‫الجهد‬ ‫لمحول‬ ‫الثانوي‬


‫الجهد‬ ‫محوالت‬ ‫اختبار‬
"
Testing Voltage Transformer

‫الم‬ ‫البيان‬ ‫لوحة‬ ‫من‬ ‫المحول‬ ‫بيانات‬ ‫معرفة‬ ‫يتم‬ ‫االختبارات‬ ‫إجراء‬ ‫قبل‬
‫ركبة‬
‫علية‬
(
Name Plate
)
‫حيث‬ ‫من‬

‫التحويل‬ ‫نسبة‬
.

‫الثانوية‬ ‫الملفات‬ ‫عدد‬
.

‫الثانوية‬ ‫الملفات‬ ‫أطراف‬ ‫رموز‬
.

‫ملف‬ ‫كل‬ ‫عبء‬
(
VA
.)

 Tests on VT:
 1- (Turns Ratio Test):

3. Set-up and effectively ground the test instrument. Supply voltage, 110Vac or 230Vac.
Conduct testing as per the following illustrated test Connections:
4. Power ON the Test Instrument and select VTRatio Test Card
9. Evaluate test result as per general standard ratings.

 3- (Insulation Test):
 1-primary and secondary winding

 3- secondary winding and earth

Voltage Transformer - Winding Resistance Test

CT and VT.pptx

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