Current Transformer Application Guide Rev5

abm/CT application guide.doc

P R O T E C T I O N

S E C T I O N

E N G I N E E R I N G

D E P A R T M E N T

T R A N S M I S S I O N

D I V I S I O N

ENGINEERING WORK INSTRUCTION

CURRENT TRANSFORMER

APPLICATION GUIDE

DOCUMENT NO. PROT-PDEV-SCDA-CTAG-2.0

ENGINEERING WORK INSTRUCTION Protection Section, Engineering Department, Transmission Division

CURRENT TRANSFORMER APPLICATION GUIDE

Doc. No. PROT-PDEV-SCDA-CTAG Revision No. 5.0 Date 4/2/2013 Page 2

DOCUMENT CONTROL

SUBJECT Section/Unit Protection Section, Engineering Department, Transmission Division, TNB

Documentation Type ENGINEERING WORK INSTRUCTION Category

Title CURRENT TRANSFORMER APPLICATION GUIDE Document No. PROT-PDEV-SCDA-CTAG Revision No. 2.0 Release Date 7.02.13 No. of Pages

INITIATORS LIST

Designation Name Initials Date Senior Technical Expert (System

Design / New Technology) Aminuddin bin Musa ABM

Senior Engineer Shyful Bahrin b.Ismail SBI

APPROVAL LIST

Designation Name Signature Date General Manager (Engineering) Mohd Azhar b. Ahamad

Chief Engineer (Protection) Dr. Satkunarajah Rajendra

DISTRIBUTION LIST

Designation Name Signature Date Deputy Chief Engineer Zainudin b.Md Yusof ZBY Deputy Chief Engineer Abd.Jalal b. Bakir ABJ Deputy Chief Engineer Fadhilah bt. Ahmad FBA

CHANGE RECORD

Revision Date Responsible Person Description of Change




TABLE OF CONTENTS

1.0 CURRENT TRANSFORMER Page 5 1.1 Purpose Page 5

1.2 Main CT Functions Page 5 1.3 CT Definition Page 5

1.4 Instrument Transformer Definition Page 5

1.5 Introduction to Current Transformer Technical Requirement

Page 6

2.0 CURRENT TRANSFORMER SELECTIONS Page 7

2.1 Current Transformer Selections Page 7 2.2 Application Page 7

2.3 Standards Page 7

2.4 Environmental Conditions Page 7 2.5 Creepage Distance Page 8

2.6 Rated Voltage Withstand Level Page 8 2.7 Short Circuit Ratings and Short Time Ratings Page 8

2.8 System X/R Page 9

2.9 Frequency Page 9 2.10 Rated Continuous Thermal Current Page 9

2.11 Ratio Page 10 2.12 Polarity Page 13

2.13 Accuracy Class Page 14 2.14 Rated Burden in VA Page 16

2.15 Rated Vkp, Ie & Rct Page 17

2.16 Instrument Security Factor FS for measuring CT Page 18 3.0 CURRENT TRANSFORMER APPLICATION Page 19 3.1 General Protection CT Page 19

3.2 Protection CT General Applications

- Main Feeder Protection

Page 20

3.3 Protection CT Applications

- Main Current Differential Protection

Page 22


- Main Distance Protection

Page 23

3.5 Protection CT Applications - High Impedance Protection

Page 24

3.6 Protection CT Applications - Low Impedance Busbar Protection

Page 26

3.7 Protection CT Applications - Transformer Differential Protection

Page 28


- Backup Protection

Page 30

3.9 Measuring CT Applications Page 32

4.0 275KV OVERHEAD LINE APPLICATION Page 34

4.1 Overhead Line Application Page 34

5.0 275/132KV AUTOTRANSFORMER APPLICATION Page 35

5.1 Autotransformer Application Page 35

6.0 275KV BUS COUPLER APPLICATION Page 37




6.1 Bus Coupler Application Page 37

7.0 275KV BUS SECTION APPLICATION Page 38

7.1 Bus Section Application Page 38

8.0 132KV OVERHEAD LINE APPLICATION Page 39

8.1 Overhead Line Application Page 39

9.0 132KV POWER TRANSFORMER APPLICATION Page 40 9.1 Power Transformer Application Page 40 10.0 132KV BUS COUPLER APPLICATION Page 42 10.1 Bus Coupler Application Page 42

11.0 132KV BUS SECTION APPLICATION Page 43

11.1 Bus Section Application Page 43

A.1 - 6 APPENDICES Page 44

1a CURRENT DIFFERENTIAL CT REQUIREMENT FOR 275 KV OVERHEAD LINE APPLICATIONS

Page 44

1b CURRENT DIFFERENTIAL CT REQUIREMENT FOR 132 KV OVERHEAD LINE APPLICATIONS

Page 49

1c DISTANCE BACKUP CT REQUIREMENT FOR 275 kV & 132 kV OVERHEAD LINE APPLICATIONS

Page 54

1d BIAS DIFFERENTIAL & HIGH IMPEDANCE DIFFERENTIAL

CT REQUIREMENT FOR AUTOTRANSFORMER AND POWER TRANSFORMER APPLICATIONS

Page 62

1e DISTANCE BACKUP CT REQUIREMENT FOR BUS SEPARATION SCHEME APPLICATIONS

Page 68

1f HIGH & LOW IMPEDANCE DIFFERENTIAL CT

REQUIREMENT FOR BUSBAR PROTECTION APPLICATION

Page 70

A.2 TNB STANDARD CT CHARACTERISTIC AND APPLICATIONS Page 80




1.0 CURRENT TRANSFORMER

1.1 Purpose This guideline is prepared as a guide or convenient reference for engineering

design and design review. The guideline is also intended to standardize the current transformer requirements for TNB applications of new transmission system projects. This guideline is based on TNB requirements and practices, taking into account the international standards and good engineering practices. The guideline is proposed to be a LIVING guideline and to be updated from time to time. It is also intended as a systematic way of identifying past mistakes and in trying to avoid them in the future.

1.2 Main CT Functions

The current transformer (CT) main functions are To transform the high current values in primary system to values that are

suitable or compatible for direct connection to measuring instruments, meters, protection relays/devices and other similar apparatus,

To isolate or insulate or galvanically separate primary high voltage system from the accessible part of the secondary systems

To provide possibility to monitor large currents at high voltage system with low range equipment

To reproduce an accurate scaled down replica of input quantity, To provide possibility to standardize the relays and instrument to rated

current, i.e. secondary rating: 1A or 5A.

1.3 CT Definition

Current transformer is an instrument transformer in which the secondary current, in normal conditions of use, is substantially proportional to the primary current and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections (IEC 60044-1).

1.4

Instrument Transformer

Definition

Instrument transformer is a transformer intended to supply measuring instruments, meters, relays and other similar apparatus (IEC 60044-1).




1.5

Introduction to Current

Transformer Technical

Requirement

Protective relays are designed to operate from secondary quantities supplied from current transformers and from voltage (or potential) transformers. The secondary output of these devices is the information used by the relays to determine the conditions existing in the plan being protected. It is necessary, therefore, that the secondary output of current and voltage present a true picture to the relays of the conditions in the primary circuit during faults as well as during normal loads. Or, alternatively, that their performance be known under extreme conditions so that any error in reproduction in the secondary circuit can be partially or completely compensated for in the setting and characteristics of the relay. In many applications, core saturation will almost inevitably occur during the transient phase of a heavy short circuit. The performance of the associated instrument transformers during faults is, therefore, an important consideration in providing an effective relaying scheme. The relays and their associated current transformers must be considered as a unit in determining the overall performance of the protective scheme. Consequently, the characteristic of the current and potential transformers at high currents and low voltage respectively, must be known. In any current transformer the first consideration is the highest secondary winding voltage possible prior to core saturation. This may be calculated from : Ek = 4.44 x B A f N volts Where : Ek = secondary induced volts (rms value, known as the knee-point voltage) N = number of secondary turns f = system frequency in hertz A = net core cross-sectional area in square meters. This induced voltage causes the maximum current to flow through the external burden whilst still maintaining a virtually sinusoidal secondary current. Any higher value of primary current demanding further increase in secondary current would, due to core saturation, tend to produce a distorted secondary current. The relevant circuit voltage required is typically : Vk = If/Ip (Rb + Rct + Rl) Equation 1 Where : If = max. fault current

Ip= primary current of the CT Rb = the connected external burden in ohms ZS = the ct secondary winding impedance in ohms ZL = the resistance of any associated connecting leads In any given case, several of these quantities are known or can usually be estimated in order to predict the performance of the transformers. From the ac magnetization characteristic, commonly plotted in secondary volts versus exciting current, Es can be determined for a minimum exciting current. The equation for the relevant circuit voltage given above then indicates whether the voltage required is adequate. ___________________________________________________________




2.0 CURRENT TRANSFORMER SELECTIONS

2.1 Current

Transformer

Selections

The current transformer selection depends on the followings criteria Applications Standards Environmental Conditions Creepage Distance Rated Voltage Withstand Level Short Circuit Ratings and Short Time Ratings System X/R Frequency Ratio and Polarity Accuracy Class Rated Burden in VA or rated Vkp, Ie & Rct Rated continuous thermal current Instrument Security Factor FS for Measuring CT

2.2

Application

The current transformer is used for the following applications; Protection Control and Instruments Revenue Metering

2.3 Standards

International Standards used as reference are IEC 60044-1 Edition 1.2: Instrument transformer Part 1: Current

Transformer IEC 60044-6: Instrument transformer Part 6: Requirements for Protective

Current Transformer for Transient Performance

2.4 Environmental

Conditions

The environmental condition requirements are specified in IEC 60721. Some of the environmental conditions to be considered are Altitude For TNB normal application, the altitude is considered at less than

1000m from sea level Climate ambient air temperature, humidity




2.5

Creepage Distance

The creepage distance is the length of the surface path from live part to the grounded part of the current transformer. The creepage distance selection depends on the pollution level or the degree of susceptible to contamination. The followings are the TNB standard creepage distance applications in accordance to IEC standards;

Pollution Level Minimum nominal specific

creepage distance mm/kV

Creepage distance Arcing distance

TNB Standard Practices

I Light 16 3.5 Not used II Medium 20 Normal TNB Standard

Applications III Heavy 25 4.0 Close to sea side and

other polluted area such as cement

factory IV Very Heavy 31 Very heavily polluted

area such as next to sea side, cement

factory

Total creepage length is calculated by multiplying the creepage distance with the maximum system voltage. For example: for normal 275kV applications, the total length is 20mm/kV multiply by 300kV = 6000 mm.

2.6 Rated Voltage

Withstand Level

TNB standard rated voltage withstand requirement are

Nominal System Voltage

(phase to phase)

kVrms 500 275 132 33 11

Rated Voltage kVrms 550 300 145 36 12 Power-frequency Withstand Voltage

(1 minute)

kVrms 620 460 275 70 28

Lightning Impulse Withstand Voltage

kVpeak 1550 1050 650 170 75

Switching Impulse Withstand Voltage

kVpeak 1175 850 - - -

2.7 Short Circuit

Ratings and Short Time

Ratings

TNB Standard Short Circuit Ratings and Short Time Ratings are

Nominal System Voltage

(phase to phase)

kVrms 500 275 132 33 11

Rated Short Circuit Withstand Current

kArms 50 40 or 50 31.5 25 20

Short Time Ratings Second 1 3 3 3 3 Minimum Rated

Peak Short Circuit Withstand Current

kApeak 125 100 or 125

80 63 50




2.8

System X/R

Standard TNB system X/R parameters for current transformer protection transient performance are

No. Nominal System Voltage

(kV)

Maximum fault current rating *

System X/R Network Time Constant

= L/R (ms)

1. 500 50kA 30 96 2. 275 40kA

(50kA for substation close to 500kV substation)

15 48

3. 132 31.5kA 10 32 4. 33 25kA - - 5. 11 20kA - -

For substation close to power station, system X/R is higher that the standard value above and has to be referred to TNB.

Network Time Constant = L = XL ms , where XL>>R R 2 f R

The transient period normally ends after 5 time period from the fault inception. Note *: For existing substation, the maximum fault current rating is equal to the maximum short circuit rating of the existing primary equipment.

2.9 Frequency

TNB standard frequency is 50 Hz.

2.10 Rated

Continuous

Thermal Current

The Rated Continuous Thermal Current define as the value of the current which can be permitted to flow continuously in the primary winding, the secondary winding being connected to the rated burden, without the temperature rise exceeding the values specified The rated continuous thermal current Icth is specified to allow for overload of equipment. The rated continuous thermal current is the value of the current which can be permitted to flow continuously in the primary winding, the secondary winding being connected to the rated burden, without the temperature rise exceeding the values specified.

No. Extended Current Rating

Voltage Level

% Of Rated Primary Current

1. IEC Standard Rated Continuous Thermal

Current

- 100, 120, 150, 200

2. TNB Standard 500kV 120 of Busbar CT Rated Primary Current 3. TNB Standard 275kV 150 of Feeder CT Rated Primary Current

120 of Busbar CT Rated Primary Current 4. TNB Standard 132kV 150 of Feeder CT Rated Primary Current

120 of Busbar CT Rated Primary Current




2.11

Ratio

Current transformer transformation ratio is the ratio of rated primary current and rated secondary current. The standard rated primary current and rated secondary current in IEC 60044-1 are

No. Rated Current Standard Values (A) 1. Rated Primary

Current 10, 12.5, 15, 20, 25, 30, 40, 50, 60, 75 and their

decimal multiples or fractions. Preferred values are underlined.

2. Rated Secondary Current

1, 2 or 5

1A current transformer is primarily used in transmission system of 132kV and above. It is also used for location with considerable distance from CT to relay. The advantages of 1A rated secondary current are: Improve transient performance Reduced cable or CT size or CT burden requirement In certain application, reduce voltage stress

The thermal loading limits of equipment in planning and operational timescales are define in table below:

Equipment Planning Operation

Lines No thermal overloading allowed

130% for not more than thirty (30) minutes or an applicable time dependent emergency limit.

Underground cables Strict observation of equipment continuous rating

125% for not more than thirty (30) minutes or an applicable time dependent emergency limit.

Transformer No thermal overloading allowed

150% for Transformer below 100 MVA and 130% for above 100 MVA not more than thirty (30) minutes or an applicable time dependent emergency limit.

The selection of CT ratio for equipment to be protected or current to be measured by the CT, determine the following:

Determine 100% or nominal current rating. Determine overload factor of the equipment and calculate overload rating for

equipment contingency loading Select ultimate current value which is a multiplication of the nominal to

current rating accordance to IEC 60044-1 as mention above. Determine the Icth rated from selection of CT current rating. Calculate Icth which must be equal to or higher than the value overload

rating of equipment.




For TNB system, the selected rated secondary current values are 1A or 5A (for 11 kV system). The selected ratios depend on the primary equipment rated current carrying capacity as follows; Note: Icth thermal continuous rating for CT line 1.5 ICT ratio or 1.2 for 275 kV above and for CT busbar = 1.2 ICT ratio

Voltage System

(kV)

Equipment Type Primary Equipment Rated Current Carrying

Capacity

Overload OHL = 1.30 Irated

TX = 1.3 1.5 Irated

Cable = 1.25 Irated

Selected CT Ratio

Icth thermal continuous

rating

500 Overhead Line 500kV, 4 X 500 mm2

(Curlew)

2800 MVA 3233.26 A

4203.2 A 4000/1 4800 A

500 Autotransformer 500/275kV 750MVA

750 MVA 866 A

1126 A 1000/1 1200 A


1050 MVA 1212 A

1576 A 1500/1 1800 A

500 Tie Bus and Busbar 4000A

4000 A 4000/1 4800 A


750 MVA 1575 A

2048 A 2000/1 2400 A


1050 MVA 2204 A

2865.2 A 2500/1 3750 A


(Zebra)

683 MVA 1433.93 A

1864.1 A 1500/1 2250 A

275 Overhead Line 275kV, 3 X 400 mm2 (Zebra), (1000MVA)

1000 MVA 2099.5 A

2729.3 A 2500/1 3750 A


240 MVA 504 A

655.2 A 600/1

900 A


180 MVA 378 A

491.4 A 400/1 600 A

275 Tie Bus and Busbar 4000A

4000A

4000/1 4800 A


240 MVA 1050 A

1365 A 1200/1 1800 A


180 MVA 787.3 A

1023.5 A 800/1 1200 A


(Batang)

282 MVA 1233.43 A

1603.5 1200/1 1800 A


(Batang)

141 MVA 616.7 A

801.7 600/1 900 A

132 Underground Cable 132kV, 150MVA

150 MVA 656 A

820 A 900/1 1350 A

132 Underground Cable 132kV, 100MVA

100 MVA 437 A

546.3 A

450/1 675 A

132 Transformer 132/33kV 90MVA

90 MVA 394 A

591 A 400/1 600 A


45 MVA 196.8 A

295.2 A 200/1 300 A




Voltage System

(kV)

Equipment Type Primary Equipment Rated Current Carrying

Capacity

Overload OHL = 1.30 Irated

TX = 1.3 1.5 Irated

Cable = 1.25 Irated

Selected CT Ratio

Icth thermal continuous

rating


30 MVA 131 A

196.5 A 150/1 225 A


15 MVA 65.6 A

98.4 A 75/1 112 A

132 Busbar 3150A 3150 A

3000/1 3600 A


90 MVA 1575 A

2362.5 A 1600/1 2400 A


45 MVA 787.5 A

1181.3 A 800/1 1300 A

33 Underground Cable

600/300/1

900 A

33 Overhead Line

600/300/1

900 A

33 Busbar 2000A 2000 A 2000/5 2400 A


30 MVA 1575 A

2362 A 1600/1

2400 A


15 MVA 787.5 A

1181.3 A 800/1

1200 A

11 Underground Cable 600/300/5

11 Busbar 2000A 2000 A 2000/5

2400 A

11 Local Transformer 300kVA

16 A 24 A 300/5 450 A




2.12

Polarity

Current transformer primary winding is connected in series with power system circuit. The primary winding terminal markings are P1, P2. The secondary winding terminal markings are S1, S2 and S3 (for double ratio CT). As a convention, if primary current flows from P1 to P2, secondary current will flow out of S1.

P2P1

S2S1

Ip

Is Normally, for TNB standard practice, the P1 marking is placed towards the source or circuit breaker or busbar. Precaution needs to be taken for extension work where the existing P1 marking may be towards the load side.

-Q1

-Q2

-Q0 P1

-Q8

-Q9

-TU

P2

-FV

Core 2

Core 3

Core 1

Core 4

Core 5

Core 6

The differential protection current transformer star point is located towards the protected circuit or equipment. For other applications, the current transformer star point is located towards the load side. Current transformer is earthed at one point only. For TNB standard practice, the current transformer locations for overlapping zone of protection (except for bus coupler bay) are normally at one side of the circuit breaker due to economical reason and space limitation.

-Q0 -Q0

Current transformer may be located at both side of the circuit breaker for better protection coverage for fault at the blind zone, i.e. between the current transformer and the circuit breaker.




2.13 Accuracy Class

Main categories of current transformer accuracy class used in TNB are

No Category Class Standards TNB Reqt. Class

Remarks

1. Protection Current

Transformer

5P 10P

IEC 60044-1 5P The protection current transformer accuracy class is designated by highest permissible composite error at rated accuracy limit primary current.


Transformer for Special Purpose

Application

PX IEC 60044-1

PX Low leakage reactance CT. Equivalent of Class X as in BS 3938 standard, expressed in term of rated knee-point voltage Vkp, maximum excitation current Ie at rated knee-point voltage, maximum secondary winding resistance RCT (secondary winding at 75C), and turn ratio. The turns ratio error shall not exceed 0.25%.


Transformer for Transient Performance

TPS IEC 60044-6 TPS A closed core CT with low leakage flux current transformer for which performance is defined by the secondary excitation characteristics and turn ratio error limits. No limit for remanence flux. The error limits do not exceed 0.25% at turns ratio.



TPX IEC 60044-6 TPX Accuracy limit defined by peak instantaneous error during specified transient duty cycle. No limit for remanent flux.



TPY IEC 60044-6 TPY Accuracy limit defined by peak instantaneous error during specified transient duty cycle. Remanent flux not exceeds 10% of the saturation flux.

6. Measuring Current

Transformer

0.1 0.2 0.5 1 3 5

IEC 60044-1 0.2 0.5

The measuring current transformer accuracy class is designated by highest permissible percentage current error at rated current. Class 0.5 is normally used for instrumentation and control purposes. Class 0.2 is used for revenue or energy metering purpose.

Note: For current transformer with multi-ratio with tapping on the secondary winding, the accuracy class and the rated burden requirements should refer to the highest transformation ratio. The detailed selections of the current transformer class selection for TNB system are highlighted in Section 3.




The error for both measuring current transformer and protective current transformer are as follows:

Accuracy Class

Percentage current (ratio) error at percentage of rated

current shown below

Percentage displacement at percentage of rated current shown below

Minutes Centiradians

5 20 100 120 5 20 100 120 5 20 100 120 0.1 0.4 0.2 0.1 0.1 15 8 5 5 0.45 0.24 0.15 0.15 0.2 0.75 0.35 0.2 0.2 30 15 10 10 0.9 0.45 0.3 0.3 0.5 1.5 0.75 0.5 0.5 90 45 30 30 2.7 1.35 0.9 0.9 1.0 3.0 1.5 1.0 1.0 180 90 60 60 5.4 2.7 1.8 1.8

Accuracy is stated at 10%-120% of rated current and with 25%-100% rated burden. At small load current, CT with high rated VA is less accurate than CT with small VA ratings.

Limit of current error and phase displacement for measuring current transformer (IEC 60044-1)

Accuracy Class Current error at rated primary

current %

phase displacement at rated primary current

Composite error at rated accuracy limit primary current % minutes centiradians

5P 1 60 1.8 5 10P 3 - - 10

Accuracy are stated at 100% of rated current to rated accuracy limit primary current

Limit of current error for protective current transformer (IEC 60044-1)

Class At rated primary current At accuracy limit condition

Ratio error % Phase displacement Maximum peak instantaneous error

% Minute Centirad

TPX 0.5 30 0.9 10 TPY 1.0 60 1.8 10 TPZ 1.0 180 18 5.3 0.6 10

NOTE: For some applications, deviation from the above values may be necessary. Similarly, the absolute value of the phase displacement may in some cases be of less importance than achieving minimal deviation from the average value of a given production series.

Limit of current error for protective current transformer (IEC 60044-6)




2.14

Rated Burden in VA

Current transformer load or burden is specified in terms of apparent power in VA (at current transformer rated secondary current) or impedance in ohms. The standard rated output burden values (IEC 60044-1) are

No. Burden Standard Values (VA) TNB minimum burden for

measuring CT

TNB minimum burden for backup

protection CT 1. CT Rated

Burden 2.5, 5.0, 10, 15, 30 15 VA 30 VA

For VA calculation purposes, the lead resistance is assumed for 4mm2 cross-section copper multicore conductor as follows:

Voltage Level

Applications Copper multicore conductor

cross-section

Copper multicore conductor

resistance/km

Estimated distance for calculation purposes

One Way Lead

resistance in ohm

132kV General protection

except busbar

protection

4mm2 4.61/km 150m 0.691


except busbar

protection

4mm2 4.61/km 250m 1.1525


except busbar

protection

6mm2 3.067/km 500m 1.53

As a rule of thumb, the copper conductor burden in VA may also be calculated by the following formula;

Burden in VA = (Secondary Current Rating)2 X 2 X Lead Length Conductor Cross Section X 57

As a rule of thumb, the burden in VA may be derived from the resistance () value by the following formula;

Burden VA = Isn2 x R

Where: R = secondary resistance, Isn = Rated secondary current As a rule of thumb, for class PX current transformer, the burden VA may be estimated by the following formula;

Burden VA = Vkp.Isn ALF.Isn2.RCT = Vkp.Isn - Isn

2.RCT ALF ALF

= Vkp - Rct (For Isn = 1A)

ALF Where: Vkp = Knee-point voltage, ALF = Accuracy limit factor (typically ALF=20), Rct = CT secondary internal resistance




2.15

Rated Vkp, Ie & Rct

Knee-point voltage Vkp Knee-point voltage Vkp for Class PX in IEC 60044-1 is defined as a point on the CT magnetising characteristics/curve at which a further increase of 10% of secondary voltage requires a 50% increase in the excitation current Ie. Beyond this point, CT is considered to be in saturation condition where the secondary RMS current value and wave shape are no longer correspond to those of the primary current.

VK

IeK

+10%Vk

+50%Iek

EXCIT

ING VO

LTAG

E (V s

)

EXCITING CURRENT (Ie) As a rule of thumb, for class 5P current transformer, the estimated knee-point voltage Vkp may be calculated by the following formula;

Knee-point voltage Vkp = VA . ALF + ALF . Isn. RCT Isn

In practical, factor 1.2 is introduced in the calculation;

Knee-point voltage Vkp = 1 ( VA . ALF + ALF . Isn. RCT ) 1.2 Isn

Note: The factor 1.2 is derived from ratio 1.8Tesla/1.5Tesla, which is the ratio of the value of the magnetizing flux at saturation to that at the working value on the current transformer magnetizing curve

Where: VA = burden, ALF = Accuracy limit factor (typically ALF=20), Isn = Rated secondary current Rct = CT secondary internal resistance Excitation current Ie Excitation current Ie or magnetising current specified at knee-point voltage or some other point on the CT magnetising characteristics/curve.




2.15

Cont

Maximum CT secondary winding resistance Rct The rated secondary winding resistance Rct is taken at 75C. The Rct must be selected to ensure adequate cross sectional area of the secondary winding conductor to carry rated current including the secondary rated short time current. The secondary winding conductor shall take into consideration the maximum current density of the copper primary winding, corresponding to the rated short-time thermal current Ith of 180A/mm

2. As a rule of thumb, for copper conductor secondary winding, the secondary winding resistance Rct value can be estimated by: Rct [ 0.2 to 0.5 per 100 turns. For TNB calculation purposes, the Rct 0.4 per 100 turns.

2.16 Instrument

Security

Factor FS for measuring CT

Measuring current transformer is intended to limit the secondary current at high fault current values, which may cause damage to instrument devices connected to secondary circuit. The requirement above is defined as the instrument security factors FS and is specified to protect the instruments from damage during short circuit condition. The standard instrument security factors FS in accordance to IEC 60044-1 for TNB application is : 5




3.0 CURRENT TRANSFORMER APPLICATION

3.1 General

Protection CT

The class 5P protection current transformer requirements for TNB application are

TNB Applications Minimum VA Accuracy Class Accuracy Limit Factor ALF

All protection application except high impedance

protection and transformer differential protection

15 5P 20

In addition to the above, the Vkp requirement for the main protection must be verified to ensure current transformer doesnt saturate during fault condition. The current transformer CT requirements for high impedance and transformer differential protection are:

TNB Applications Class Vkp Ie RCT

Transformer high impedance

protection and differential protection

PX To be calculated

< 40mA @ Vkp

Refer to Section 2.15 for detailed.

High impedance busbar

protection

PX

To be calculated

< 10mA @ Vkp


The minimum knee-point voltage of the protective current transformer, depending on the relevant protection relay, must be calculated and specified. The excitation current Ie should be as minimum possible to maintain the effective setting of protection relay/scheme. For 500kV substation, the overhead line current transformer of main and backup protections may require CT with better transient performance in accordance to IEC 60044-6 as follows:

TNB Applications Class Min VA Remarks Main protection PX or 5P 30 Relatively low

remanent flux Backup Protection 5P 30 Relatively low

remanent flux




3.2

Protection CT General

Applications - Main Feeder

Protection

The main feeder protection for transmission overhead line and underground cable are Main optical fibre unit protection using current differential protection or

current comparison protection (for all new TNB installations), and Main distance protection (for existing TNB substation).

TNB standard applications for transmission feeder main protection current transformer are


Main feeder protection 30 5P

20

The current transformer for the main feeder protection are designed To provide accurate transformation for fault in the protected zone up to the

maximum fault current rating To provide accurate transformation for through fault condition up to the

maximum through fault current To ensure accurate transformation under both transient and steady state

conditions without saturation, i.e. transforming both ac and dc offset current component of primary to secondary

High-speed operation of main protection relay must be ensured under transient condition leading potential severe CT saturation due to dc transient current. Ideally, CT shall not saturate even if CT flux reaches the maximum value due to fully offset fault current. To meet this saturation free requirement, the following simple formula based on the system X/R shall be taken into consideration for the current transformer design.

)1)(()()( RXRRR

ratioCTI

V RELAYLEADCTrmsFAULT

rmsknee +++>

However, due to economical factors, the CT saturation may be allowed to happen after the protection system operates. Since the maximum protection operating time is 30msec (in accordance to TNB specification), the protection current transformer is required to provide saturation free current transformation of up to 30ms (preferably 1.5 cycle or more saturation free) during this transient condition. Allowance for the system X/R ratio must be taken into consideration for the current transformer design. To ensure that the current transformer does not saturate during the transient and steady state fault conditions, the knee-point voltage Vkp requirement of the current transformer may be dimensioned as follows: Vkp KX. Isn (RCT + 2RL + RR) Where KX = dimensioning factor, Isn = rated secondary current, Ipn = rated primary current RCT = CT secondary internal resistance RL = one way lead conductor resistance from CT to relay The relay resistance is assumed to be negligible for modern numerical relays.




3.2

cont

The CT dimensioning factor KX depends on the system and CT time constant. The dimensioning factor KX also depends on the type of protection relays used as the main protection. Some protective relay manufacturers have also proposed the dimensioning factor KX. As a rule of thumb, the dimensioning factor at a specified time t is expressed as follow:

1)( 1212

21 +

=

Tt

Tt

X eeTTTT

tK

Where T1 : System time constant (L1/R1) T2 : CT secondary circuit time constant (L2/RT) For any given CT knee point voltage Vknee, the following inequality shall be satisfied to secure saturation free for the specified time (e.g. 30msec) from a fault inception.

)()()()( tKRRRratioCT

IV XRELAYLEADCT

rmspnrmsknee ++>

Class-P CT can satisfy this saturation free condition when the CT is operated at much smaller burden than the rated burden to gain the equivalent transient dimensioning factor. For overhead transmission line with autoreclosing facility, consideration must be taken for the remanence flux after the first isolation of the fault. Current transformer may saturate due to residual flux trapped in CT core after isolation of the first fault, which may not be cleared during the autoreclose dead time before the reclosing of the line. Therefore, the following requirements shall be taken into consideration: At least two successive fault applications to simulate successive

autoreclosing of the line Maximum fault is applied at the worst case of transient condition, which is

at maximum dc offset Autoreclose dead time is taken at 750ms




3.2

cont

For class 5P current transformer, the selections of burden in VA and the accuracy limit factor ALF have to reflect the above knee-point voltage Vkp requirement.

CT internalresistance

RCT

One-way conductorlead resistance

from CT to relayRL

Relay Burdenor Resistance

RR

Total Resistance RS = RCT + 2RL+ RR

RR is assumed to be negligible for modernnumerical relay

RL

3.3 Protection CT

Applications - Main Current

Differential

Protection

The general current transformer requirements for main current differential protection are referred to Section 3.2. Additional requirements for main current differential protection are: To ensure relay stability during both transient and steady state through

fault condition To maintain CT saturation free until main protection relay operates since CT

saturation at either one terminal may cause maloperation of the current differential protection (the security is violated)

To consider the system X/R ratio for maximum through fault current for ensuring security against CT saturation

To consider similar current transformer characteristic and identical turn ratio at both ends. Ratio correction for unmatched CT ratio in the numerical relay is not preferred. TNB approval is required for ratio correction for unmatched CT ratio in the numerical relay.




3.4

Protection CT Applications

- Main Distance

Protection

The general current transformer requirements for main distance protection are referred to Section 3.2. Additional requirements for main distance protection are: To ensure that CT must not saturate for fault at the end of Zone 1 reach.

The distance protection Zone 1 reach is typically taken as 80% of the line length. The fault current varies depending on the transmission line length and impedance.

The TNB typical line impedance parameters used for the calculation are:

Voltage Level Conductor Type Line Impedance Line Distance for calculation purposes

500kV 4 X 500 mm2 (Twin Curlew)

0.2654/km, 86.29

20km

275kV 2 X 400 mm2 (Twin Zebra)

0.2887/km, 83.08

10km

132kV 2 X 300 mm2 (Twin Batang)

0.274/km, 80.42

5km

As a rule of thumb, the current transformer working point during the transient periods may be verified or checked by the following formula:

Vkp ; ( 1 + X ) . Ifs . Isn (RCT + 2RL) R

= ( 1 + X ) . Ifp . Isn (RCT + 2RL)

R CTR Where: X/R = System X/R ratio (refer to Section 2.8) Isn = rated secondary current Ifs = secondary fault current at Zone 1 reach, Ifp = primary fault current at Zone 1 reach, CTR = CT ratio. RCT = CT secondary internal resistance RL = one way lead conductor resistance from CT to relay




3.5


- High Impedance

Protection

The high impedance protection is used for the following applications: High Impedance Busbar protection Autotransformer Main 2 High Impedance Protection Restricted Earth Fault Protection 1 CB Stub Protection Lead or Interconnector protection

TNB standard applications for high impedance protection current transformer are


Transformer and Lead High Impedance Protection

PX To be calculated

< 40mA @ Vkp

or < 10mA @

Vkp


High Impedance Busbar

Protection

PX

To be calculated

< 10mA @ Vkp


The current transformer requirements for the high impedance CT applications are All current transformers used for high impedance protection purposes

should have identical turn ratios All current transformer characteristic/performance used for high impedance

purposes should be matched The current transformers should have sufficient performance for external

fault to ensure that the current transformers do not saturate for fault current flowing through the protected zone

The current transformers should provide accurate steady state transformation up to the maximum current rating of the associated main plant.




3.5

Cont

The high impedance protection may tolerate a permissible degree of saturation under transient conditions. It is not essential to provide for accuracy or transient performance. The high impedance protection allows for saturation at maximum fault current. Stabilising resistor is required to reduce relay operating current during external fault. The high impedance protection must be stable during through fault condition. However, during internal faults, the relay is designed to respond properly. The knee-point voltage Vkp requirement of the current transformer for busbar protection shall be dimensioned as follows: Vkp > 2 Vs (2 times is minimum requirements, 3 to 5 times is preferred) Where: Vs = relay setting voltage, set under through fault condition The CT internal resistance shall be fixed at the same values for each CT for high impedance protection. As a rule of thumb, the Rct is fixed as follows: Rct 0.4 Ohms/100 turns The selection of excitation current Ie for current transformer influences the relay setting as follows:

excitation current Ie = (Ieff.set Irelay.set) n

Where: Ie = excitation current Ieff.set = effective fault setting expressed in secondary amps Irelay.set = relay setting current n = number of CT groups forming the protected zone for busbar protection




3.6


- Low Impedance

Busbar

Protection

The low impedance busbar protection is used in TNB system as the main 2 protection to protect the busbar. TNB standard applications for low impedance busbar protection current transformer are:

TNB Applications Class Vkp RCT

Low impedance Busbar

Protection

PX or 5P20 600 Volt at Vk/2 Or

30 VA

04 ohm/100 turn

The current transformer requirements for the low impedance busbar protection CT applications are

The current transformers should provide accurate transformation for fault in the protected zone up to the maximum fault current rating

The current transformers should provide accurate transformation for through fault condition up to the maximum through fault current

The current transformers should ensure accurate transformation under both transient and steady state conditions without saturation, i.e. transforming both ac and dc offset current component of primary to secondary

High-speed operation of main protection relay must be ensured under transient condition with severe CT saturation due to dc transient current. Ideally CT shall not saturate even if CT flux reaches the maximum caused by a fully offset fault current. To meet this saturation free requirement, the following simple formula based on the system X/R shall be satisfied.

)1)(()()( RXRRR

ratioCTI


rmsknee +++>

However, due to economical factors, the CT saturation may be allowed to happen after the protection system operates. Since the maximum protection operating time is 30msec (in accordance to TNB Control & Protection Design Philosophy), the protection current transformer is required to provide saturation free current transformation of up to 30ms (preferably 1.5 cycle or more saturation free) during this transient condition. Allowance for the system X/R ratio must be taken into consideration for the current transformer design. To ensure that the current transformer does not saturate during the transient and steady state fault conditions, the knee-point voltage Vkp requirement of the current transformer may be dimensioned as follows: Vkp KX. Isn (RCT + 2RL + RR) Where KX = dimensioning factor, Isn = rated secondary current, Ipn = rated primary current RCT = CT secondary internal resistance RL = one way lead conductor resistance from CT to relay




3.6

cont The CT dimensioning factor KX depends on the system and CT time constant. The dimensioning factor KX also depends on the type of protection relays used as the main protection. Some protective relay manufacturers has also proposed the dimensioning factor KX. As a rule of thumb, the dimensioning factor at a specified time t is expressed as follow:

1)( 1212

21 +

=

Tt

Tt

X eeTTTT

tK

Where T1 : System time constant (L1/R1) T2 : CT secondary circuit time constant (L2/RT)

For any given CT knee point voltage Vknee, the following inequality shall be satisfied to secure saturation free for the specified time (e.g. 10msec) from a fault inception.


IV XRELAYLEADCT

rmspnrmsknee ++>

Class-P CT may satisfy this saturation free condition when the CT is operated at much smaller burden than the rated burden to gain the equivalent transient dimensioning factor.




3.7


- Transformer Differential

Protection

The transformer differential protection is used in TNB system as the main protection to protect the power transformer. TNB standard applications for transformer differential protection current transformer are


Transformer Differential Protection

PX 300 Volt < 40mA @ Vkp

04 ohm/100 turn

The current transformer requirements for the transformer differential protection CT applications are The current transformer characteristic for HV and LV side of the power

transformers should be matched with the use of internal ratio and vector corrections in the numerical relays. The use of interposing CT as the ratio and vector corrections may be accepted subject to TNB approval.

The current transformer should have sufficient performance for external fault. This is to ensure that the current transformers do not saturate for steady state fault current flowing through the protected zone, i.e. the power transformer.

Normally, the steady state through fault current is taken as 15 to 20 times the transformer full load current

The transformer differential protection must remain stable during through fault condition

In case of the fault at the transformer primary lead, the fault current flowing through the primary CT is equivalent to the primary busbar short circuit current. It is expected that the primary CT may saturate due to the large lead fault current. Therefore, it is imperative that the CT shall not saturate until the transformer biased differential protection operates.

High-speed operation of transformer differential protection as a main protection relay must be ensured under transient condition, which leads to potential severe CT saturation due to dc transient current. Ideally, CT shall not saturate even if CT flux reaches the maximum caused by a fully offset fault current. To meet this saturation free requirement, the following simple formula based on the system X/R shall be satisfied.

)1)(()()( RXRRR

ratioCTI


rmsknee +++>

However, due to economical factors, the CT saturation may be allowed to happen after the protection system operates. Since the maximum protection operating time is 30msec (in accordance to Control & Protection Design Philosophy), the protection current transformer is required to provide saturation free current transformation of up to 30ms (preferably 1.5 cycle or more saturation free) during this transient condition. Allowance for the system X/R ratio must be taken into consideration for the current transformer design.




3.7

cont..

To ensure that the current transformer does not saturate during the transient and steady state fault conditions, the knee-point voltage Vkp requirement of the current transformer may be dimensioned as follows: Vkp KX. Isn (RCT + 2RL + RR) Where KX = dimensioning factor, Isn = rated secondary current, Ipn = rated primary current RCT = CT secondary internal resistance RL = one way lead conductor resistance from CT to relay The relay resistance is assumed to be negligible for modern numerical relays. The CT dimensioning factor KX depends on the system and CT time constant. The dimensioning factor KX also depends on the type of protection relays used as the main protection. Some protective relay manufacturers have also proposed the dimensioning factor KX. As a rule of thumb, the dimensioning factor at a specified time t is expressed as follow:

1)( 1212

21 +

=

Tt

Tt

X eeTTTT

tK

Where T1 : System time constant (L1/R1) T2 : CT secondary circuit time constant (L2/RT)

For any given CT knee point voltage Vknee, the following inequality shall be satisfied to secure saturation free for the specified time (e.g. 10msec) from a fault inception.


IV XRELAYLEADCT

rmspnrmsknee ++>




3.8


- Backup Protection

Backup protection is defined as protection equipment or device that is intended to operate when a system fault is not cleared, or abnormal condition not detected, in the required time because of failure or inability of other protection to operate, or failure of the appropriate circuit breaker(s) to trip. Examples of backup protections are overcurrent protection and distance backup protection. There are three criteria to be taken into account for protective current transformer application for backup protection: The accuracy class The burden in VA The accuracy limit factor

In the case that CT saturates, the backup distance relay may not operate until CT saturation condition is removed. The CT saturation may also cause delay operation of the distance relay. However, since the system time constant is around 0.1 sec or less, typically, the transient performance requirement is not necessary for the backup protection with time delay operation. The current transformer requirement is sufficient for steady state maximum fault current condition. For backup distance protection with delay zone 1 facility the transient performance requirement may be considered for power system level with higher dependability requirement or substation close to power station. For substation close to power station, where the time constant of DC current components are very long, the standard CT requirements and the transient performance may need to be considered in the case that the distance relay zone 1 is normally used in parallel to Main 1 and Main2. This has to be referred to TNB. Accuracy Class and Burden TNB standard applications for protection current transformer are


Backup Protection 30

5P 20

For more than one relays or devices connected to the current transformer secondary circuit, the total burden consists of the combination of relays or devices and secondary lead conductor burden. The total burden of all connected load to the secondary CT circuit, including all lead conductor burden and all relays or devices burden, must be checked against the above minimum required CT burden.




3.8

cont

Lead conductorloop resistance

from CT to relayRlead


Rrelay 1


Rrelay 2

Relay or other devices burden/resistance such as meter,

disturbance recorder, etc. loadTotal Resistance RT = Rlead + Rrelay1 + Rrelay2

L2

As a rule of thumb, the lead conductor resistance may be converted to burden in VA by the following formula;

Burden VA = Isn2 x Rlead

Where: Rlead = Loop lead conductor resistance = 2 x RL, RL = One way lead conductor resistance, Isn = Rated secondary current Accuracy limit factor The current transformer must not saturate during fault condition and the current transformer composite error must be maintained at least up to accuracy limit factor. For example, for current transformer class 5P20 with ratio 100/1A, the composite error shall be less than 5% up to 20 times 100A primary current (2000A). This requirement can be verified by comparing either The CT approximate knee point voltage Vkp with the operating secondary

voltage Vs during maximum fault condition, or The actual accuracy limit factor ALFactual with the operating accuracy limit

factor ALFoperating during maximum fault condition For example, the current transformer knee point voltage must be greater than the secondary voltage Vs developed during short circuit or fault condition: Vs < Vkp, Where: Vs = Ifs x RS, Ifs = secondary fault current = Ifp/CTR, Ifp = primary fault current, RS = RCT + 2RL + Rb. Refer to Section 2.14 for the estimated Vkp if the Vkp value were unknown.




3.9

Measuring CT Applications

There are three criteria to be taken into account for measuring current transformer application: The accuracy class The burden in VA The instrument security factor FS

Accuracy Class and Burden TNB standard applications for measuring current transformer are

No.

TNB Applications Accuracy Class

Percentage current (ratio) error

at percentage of rated current

Minimum VA

1. Ammeter, Instruments including input to SCS BCU,

RTU transducers

0.5 0.5 15

2. Instruments shared with protection CT via interposing

saturation CT (Only for distribution system)

1.0 1.0 15

3. Energy or Revenue Metering 0.2

0.2 15

The measuring transformer operates in normal operating conditions where the accuracy is stated at 10%-120% of rated current and with 25%-100% rated burden.




3.9 cont

The total burden of all connected load to the secondary CT circuit, including all lead conductor burden and all instrument burden, must be checked against the above minimum required CT burden.

Conduct orlead loop

bur den f r omCT t o r elay

VA l ead

Inst r umentbur den

VA1

Inst r ument or ot her devicesbur den such as met er , BCU,

t r ansducer s, et c.

T o ta l B u rd e n V A T = V A le a d + V A 1 + V A 2

Inst r umentbur den

VA2

Instrument security factor The instrument security factor FS is specified for measuring current transformer to protect any instrument devices connected to the secondary circuit from damage during fault condition. Security factor FS5 means during short circuit, where high fault current flows in primary windings, the measuring CT core will saturate at around 5 times its rated value at rated burden. Normally the TNB standard FS factor is 5. For example: 0.5FS5 means Class 0.5 with instrument security factor 5. The security factor FS5 will increase with the actual connected burden. The actual instrument security factor AFS at connected burden should be calculated. Marginal increase of the AFS is allowed provided that the value is not too high which may cause damage to the connected instrument devices. The instrument withstand current rating is also required to be checked against the AFS.




4.0 275KV OVERHEAD LINE APPLICATION

4.1 Overhead

Line

Application

The current transformer requirements for TNB typical 275kV overhead line bay are as follows:

Bay Type CT

Core No.

Device Type CT Requirement Summary

275kV Overhead line

275kV MAIN BUSBAR

275kV RESERVE BUSBAR

-Q1 -Q2

-Q0

=D01

-TI

P1

-Q8-Q9

-TU

P2

Core 1: 1500/ 1 Class 5P20

Core 2: 1500/ 1 Class 5P20

Core 3: 1500/ 1 Class 5P20

Core 4: 1500/ 1 Class 0.5

Core 5: 4000/ 1 Class PX


275/ 0.110kV50VA/ wdg, Class 3P

4000A, 300kV,40kA f or 3sec.

-FV

TI - Core 1:

Main 1: Current Differential Unit Protection

Ratio = 1500/1 Class = 5P20 VA = 30 Rct 6

TI - Core 2:

Main 2: Current Differential Unit Protection


TI - Core 3:

Backup Protection:


TI - Core 4:

Control & Instrumentation:

Ratio = 1500/1 Class = 0.5 VA = 15

TI - Core 5:

Main 1 Busbar Protection Discriminating Zone:

Ratio = 4000/1 Class = PX Vk = 600 V Io 10mA at 300V Rct = 16

TI - Core 6:

Main 1 Busbar Protection Check Zone:


Note: The summary above applies for all relays except relays which declare that the dimensioning factor as (1 + X/R), such as

Schweitzer (SEL) relay The CT dimensions are not applicable for substations near power station. The CT calculation to be submitted, taking into

consideration the actual X/R ratio. The CT dimensions are also not applicable for 275kV substations close to 500kV substation. The CT calculation to be

submitted, taking into consideration the fault current at 50kA @ 3second as referred to TNB technical specifications.




5.0 275/132KV AUTOTRANSFORMER APPLICATION

5.1

Autotransform

er Application

The current transformer requirements for TNB typical 275/132 autotransformer are as follows:

Bay Type CT

Core No.


275/132kV Autotransformer

275kV MAIN BUSBAR


-Q1 -Q2

-Q0

=D02

-TI


Core 2: 1200 / 600/ 1 Class PX

Core 3: 600/ 1 Class 5P20

Core 4: 600/ 1 Class 0.5



4000A, 300kV,40kA f or 3sec.

275/ 132kV240MVA Yy0d1

Autotransformer SGT

-TU

275/ 0.110kV50VA/ wdg, Class

3P

-FV

P1

P2


3PP2

P1

=E02

132kV MAIN BUSBAR


-Q1-Q2

-Q0

3150A, 145kV,31.5kA f or 3sec.

-TI2

Core 1: 1200 / 1 Class PX

Core 2: 1200 / 1 Class PX

Core 3: 1200/ 1 Class 5P20

Core 4: 1200/ 1 Class 0.5



-TU

TI1 - Core 1:

Main 1: Transformer Biased Differential Protection

Ratio = 600/1 Class = PX Vk = 300 V Io 40mA at Vk/2 Rct = 2.4

TI1 - Core 2:

Main 2: Transformer High Impedance Protection

Ratio = 1200/1 Class = PX 1200/1 Vk = 600 V Io 40mA at Vk/2 Rct = 4.8 600/1 Vk = 600 V Io 40mA at 150V Rct = 2.4

TI1 - Core 3:

Backup Protection:

Ratio = 600/1 Class = 5P20 VA = 30 Rct 2.4

TI1 - Core 4:


Ratio = 600/1 Class = 0.5 VA = 15

TI1 - Core 5:



TI1 - Core 6:



TI2 - Core 1:

Main 1: Transformer Biased Differential Protection

Ratio = 1200/1 Class = PX Vk = 300 V Io 40mA at 300V Rct = 4.8

TI2 - Core 2:

Main 2: Transformer High Impedance Protection

Ratio = 1200/1 Class = PX Vk = 600 V Io 40mA at 300V Rct = 4.8

TI2 - Core 3:

Backup Protection:





TI2 - Core 4:


Ratio = 1200/1 Class = 0.5 VA = 15

TI2 - Core 5:


Ratio = 3000/1 Class = PX Vk = 600 Io 10mA at 300V Rct = 12

TI2 - Core 6:










6.0 275KV BUS COUPLER APPLICATION

6.1 Bus

Coupler

Application

The current transformer requirements for TNB typical 275kV bus coupler bay are as follows:

Bay Type CT

Core No.


275kV Bus Coupler

-Q1-Q2

-Q0

=D03

-TI1

-TU1

-TI2

P1

P2

P1

Core 2: 4000/ 1 Class 5P20

Core 1: 4000/ 1 Class 0.5

Core 3: 4000/ 1 Class PXP2

Core 1: 4000/ 1 Class 5P20


-TU2

4000A, 300kV,40kA f or 3sec.

TI1 - Core 1:

Backup Protection:


TI1 - Core 2:



TI2 - Core 1:


Ratio = 4000/1 Class = 0.5 VA = 15 Fs = 5

TI2 - Core 2:

Backup Protection:


TI2 - Core 3:










7.0 275KV BUS SECTION APPLICATION

7.1 Bus

Section

Application

The current transformer requirements for TNB typical 275kV bus section bay are as follows:

Bay Type CT

Core No.


275kV Bus Section

-Q12

-Q11

-Q0

=D04

-TI1

-TU1

-TI2

P1

P2

P1

Core 2: 4000/ 1 Class 5P20

Core 1: 4000/ 1 Class 0.5


Core 1: 4000/ 1 Class 5P20


-TU2

275kV MAIN BUSBAR 1

275kV MAIN BUSBAR 2

4000A, 300kV, 40kA f or 3sec.



4000A, 300kV, 40kA f or 3sec.

TI1 - Core 1:

Backup Protection:


TI1 - Core 2:



TI2 - Core 1:


Ratio = 4000/1 Class = 0.5 VA = 15 Fs = 5

TI2 - Core 2:

Backup Protection:


TI2 - Core 3:










8.0 132KV OVERHEAD LINE APPLICATION Overhead Line Application

The current transformer requirements for TNB typical 132kV Overhead Line bay are as follows:

Bay Type CT Core

No. Device Type CT Requirement Summary

132kV Overhead line

132kV MAIN BUSBAR


-Q1 -Q2

-Q0

=E01

-TI

P1

-Q8-Q9

-TU

Core 1: 1200/ 1 Class 5P20

Core 2: 1200/ 1 Class 5P20

Core 3: 1200/ 1 Class 0.5



132/0.110kV50VA/ wdg, Class

3P

3000A, 132kV,31.5kA f or 3sec.

-FV

P2

TI - Core 1:

Main: Current Differential Unit Protection


TI - Core 2:

Backup Protection: Backup Distance


TI - Core 3:


Ratio = 1200/1 Class = 5P20 VA =15

TI - Core 4:

Busbar Protection Discriminating Zone: or Low Impedance Busbar Differential Protection


TI - Core 5:

Busbar Protection Check Zone:




consideration the actual X/R ratio.




9.0 132KV POWER TRANSFORMER APPLICATION Power Transformer

Application

The current transformer requirements for TNB typical 132kV power transformer bay are as follows:

Bay Type CT Core


132/33kV 90MVA Power Transformer

-Q1 -Q2

-Q0

=E02

-TI1


Core 3: 400/ 1 Class 0.5



132/ 33kV90MVA Yn0d1

Power Transformer

-TU


3P

- FV

P1

P2


3PP2

P1

=F02

33kV MAIN BUSBAR

33kV RESERVE BUSBAR

-Q0

-TI2


Core 3: 1600/ 1 Class 0.2

-TU

31.5kA for 3sec.3000A, 132kV,132kV MAIN BUSBAR



Core 2: 1600/ 1 Class 1/ 5P20

25kA for 3sec.2000A, 33kV,

TI1 - Core 1:

Main: Transformer Biased Differential Protection

Ratio = 400/1 Class = PX Vk = 300 Io 40mA at 300V Rct = 1.6

TI1 - Core 2:

Backup Protection: Overcurrent


TI1 - Core 3:


Ratio = 400/1 Class = 0.5 VA = 15

TI1 - Core 4:



TI1 - Core 5:









Bay Type CT Core


132/11kV 30MVA Power Transformer

-Q1 -Q2

-Q0

=E03

-TI1


Core 3: 150/ 1 Class 0.5



132/ 11kV30MVA Yn0d11

Power Transformer

-TU


3P

-

FV

P1

P2


3P

P2

P1

=F03

11kV MAIN BUSBAR

11kV RESERVE BUSBAR

-

Q0

-TI2


Core 3: 1600/ 1 Class 0.2

-TU

31.5kA for 3sec.3000A, 132kV,

20kA for 3sec.2000A, 11kV,

132kV MAIN BUSBAR



Core 2: 1600/ 1 Class 1/ 5P20

TI1 - Core 1:

Main: Transformer Biased Differential Protection

Ratio = 150/1 Class = PX Vk = 600 Io 40mA at 300V Rct = 1.6

TI1 - Core 2:

Backup Protection: Overcurrent


TI1 - Core 3:


Ratio = 150/1 Class = 0.5 VA = 15

TI1 - Core 4:



TI1 - Core 5:









10.0 132KV BUS COUPLER APPLICATION Bus Coupler

Application

The current transformer requirements for TNB typical 132kV bus coupler bay are as follows:

Bay Type CT Core


Bay Type CT Core No.


132kV Bus Coupler

-Q1-Q2

-Q0

=E03

- TI1

-TU1

- TI2

P1

P2

P1

Core 2: 3000/ 1 Class 5P20

Core 1: 3000/ 1 Class 0.5


Core 1: 3000/ 1 Class 5P20


-TU2

31.5kA for 3sec.3000A, 132kV,

TI1 - Core 1:


Ratio = 3000/1 Class = 5P20 VA = 30 Rct 12 ohm

TI1 - Core 2:



TI2 - Core 1:


Ratio = 3000/1 Class = 0.5 VA = 15

TI2 - Core 2:



TI2 - Core 3:

Busbar Protection Discriminating Zone: Low Impedance Busbar Differential Protection

Ratio = 3000/1 Class = PX Vk = 600 Io 10mA at 300V Rct = 12 Or Ratio = 3000/1 Class = 5P20 VA = 30 Rct 12







11.0 132KV BUS SECTION APPLICATION Bus Section

Application

The current transformer requirements for TNB typical 132kV bus section bay are as follows:

Bay Type CT Core


132kV Bus Section

-Q12

-Q11

-Q0

=E04

-TI1

-TU1

-TI2

P1

P2

P1

Core 1: 3000/ 1 Class 0.5


P2

Core 1: 3000/ 1 Class 5P20


-TU2

132kV MAIN BUSBAR 1

132kV MAIN BUSBAR 2



31.5kA for 3sec.3000A, 132kV,

31.5kA for 3sec.3000A, 132kV,

TI1 - Core 1:

Backup Protection: Overcurrent / Breaker Failure


TI1 - Core 2:



TI2 - Core 1:


Ratio = 3000/1 Class = 0.5 VA = 15

TI2 - Core 2:









APPENDIX 1a: CURRENT DIFFERENTIAL CT REQUIREMENT FOR 275 KV OVERHEAD LINE APPLICATIONS.

A.1 Overhead

Line Application

Current Differential protection (Main 1 Protection) CT requirements for TNB typical 275kV overhead line bay are as follows:

Bay Type Device Type CT Calculation CT Requirement

Summary Remarks

275 kV Overhead Line Main 1: Current Differential Unit Protection Current Differential Nari Model RCS-931

275kV CT: 2500/1A 5P20 30VA for CTR= 2500/1, Rct = 10, Rr = 0.2VA @ 1A= 0.2, RL = 2 x 4.61 x 0.25 =2.305, Rc = 0.1, then Rb = 0.2+2.303+0.1 = 2.605 Rbn = 30VA/12 =30 Kpcf = 50kA/2500 = 20 Kalf [2 x 20 x (10 + 2.605) / (10 + 30)] = 12.605 for CTR= 1500/1, Rct = 6 kpcf = 50kA/1500 =33.33 Kalf [2 x 33.33 x (10 + 2.605) / (10 + 30)] Then, Kalf 21 CT of 5P20 with burden 30 VA is adequate for Ifmax = 50kA

Ratio = 2500/1 1500/1 Class = 5P20 VA = 30 Ifmax = 40kA @ 50 kA Td = 48 ms Rl = 2.305 ohm Rct = 10 (2500/1) Rct = 6 (1500/1)

Using Method 2: Performance Verification for CT of class P and PR

Kalf K * Kpcf * (Rct + Rb) / (Rct + Rbn)

Where

Kalf (Accuracy limit factor) = Ipal / Ipn ;

Ipal =Rated accuracy limit primary current (A) Ipn = Rated primary current (A)

K = 2 (for RCS-931)

Kpcf (Protective checking factor) = Ipef / Ipn Ipef = maximum prospective fault current Rct = CT secondary winding resistance (Ohm) Rb (Real resistance burden) = Rr + 2*RL + Rc Rr = Relay resistance RL= Resistance of single lead from relay to CT Rc= Contact resistance (0.05-0.1) Rbn (Rated resistance burden) = Sn / Isn2 Sn= Rated burden (VA) Isn= Rated secondary current (A)




Bay Type Device Type CT Calculation CT Requirement Summary

Remarks

Main 1: Current Differential Unit Protection Alstom/Scheider P54*

for CTR= 2500/1, Rct = 10, Vk K. In ( Rct + 2RL) 275kV X/R = 15

56.815250040kA0.0740K =

+=

( )( )

V6992.3051018.562RRIKV Lctnkp

+

+

( )[ ]( )[ ]

9.2920

12.110120699ALF

I1.2RIALFVVA snctsnkp

=

=

=

CT of 5P20 with burden 30 VA is adequate for Ifmax = 40kA for CTR= 1500/1, Rct = 6,

7515150050kA0.0740K =

+=

( )V623

2.305618.56Vkp

+

( )[ ]25.38

2011.210120623VA

=

=

CT of 5P20 with burden 30 VA is adequate for Ifmax = 50kA

Ratio = 2500/1 1500/1 Class = 5P20 VA = 30 Ifmax = 40kA @ 50kA Td = 48 ms Rl = 2.305 ohm Rct = 10 (2500/1) Rct = 6 (1500/1)

K= dimensioning factor In = CT nominal secondary current Rct = CT secondary winding resistance RL= one way lead resistance from CT to relay K is a constant depend on: If = maximum value of through fault current for stability X/R = primary system X/R ratio K is determined as follows: For relays set (TNB recommended setting) at Is1=20%, Is2=2In, k1=30%, K2=150% (A) For (If x X/R) 1000In K 40 + {0.07 x (If x X/R)} and K 65, which is the highest K (B) For 1000In (If x X/R) 1600In: K = 107




Bay Type Device Type CT Calculation CT Requirement Summary

Remarks

Main 1: Current Differential Unit Protection Siemens 7SD52*

275kV CT: 2500/1500/1A 5P20 30VA for 1500/1A, Rct = 6

301A

30VAIS

R 22sn

bb ===

Rb = 2(4.61x0.25) + 0.05 = 2.355

86.172.3556

30620RRRRKK

'

bct

bctsscOALF =

+

+=

+

+=

1st condition: KOALF = 86.17 > 33.33 [50000A/1500A] Condition 1 satisfied 2nd condition: KOALF = 86.17 30 Condition 2 satisfied for 2500/1A, Rct = 10

4.7562.35510

301020RRRR

KK'

bct

bctsscOALF =

+

+=

+

+=

1st condition: KOALF = 64.75 >20 [50kA/2500A] 2nd condition: KOALF = 64.75 30 CT satisfied with 1st and 2nd conditions for Ifmax = 50kA

Ratio = 2500/1 1500/1 Class = 5P20 VA = 30 Ifmax = 40kA @ 50 kA Td = 48 ms Rl = 2.305 ohm Rct = 10 (2500/1) Rct = 6 (1500/1)

KoALF = Operating CT accuracy limiting Factor KnALF = Nominal CT accuracy limiting Factor PBC = Connected burden Pi = Internal CT burden PBN = Connected burden RBN = Connected resistance Ri = Internal CTresistance ISN = secondary current CT requirements: 1st condition: KOALF > IFAULT(rms) / Ipn 2nd condition: KOALF 30 or AC cycle saturation free time (5ms for 50Hz)




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Current Transformer Application Guide Rev5

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