7/27/2019 Calculation for Pt Selection

1/7

TIRODA THERMAL POWER PROJECT1980 MW (3x660 MW Phase I) Tiroda, Gondia District,

Maharashtra TIRODA( )3X660MW

BASIC DESIGN

CALCULATION FOR PT SELECTION PT

2008 11 12

7/27/2019 Calculation for Pt Selection

2/7

CONTENTS

1. SPECIFICATION OF CALCULATION ....................................................................1

2. PRINCIPLE ON PT SELECTION ..............................................................................1

3 CALCULATION FORMULA.......................................................................................1

3.1 SELECTION FOR RATIO OF PT........................................................................1

3.2 SELECTION OF ACCURACY CLASS OF PT ...................................................2

4. SECONDARY LOAD VERIFICATION.....................................................................3

4.1 SECONDARY LOAD OF PT ...............................................................................3

4.2 PARAMETERS OF PT .........................................................................................4

7/27/2019 Calculation for Pt Selection

3/7

1/5

1. SPECIFICATION OF CALCULATION

This calculation shall be done according to the specifications as follows:

The Guide for selection and calculation of current transformers and voltage transformers

Manual for designing electrical part of electric power engineering (2)

2. PRINCIPLE ON PT SELECTION

2.1 It should meet the requirement of rated primary voltage.

2.2 It should meet the requirement of accuracy class and capacity for measuring meters, protection

equipments and automation equipments

3 CALCULATION FORMULA

3.1 SELECTION FOR RATIO OF PT

According to the contract, when PT is star wiring, its phase to phase voltage of secondary winding

shall be 110V. Secondary voltage of PT shall be selected in accordance with Tab. 1-1.

Tab. 1-1 Selection of Secondary Voltage of PT

Winding Main secondarywinding

Aux. secondary winding for zerosequence voltage

Wiring mode of

winding

Phase to

phase

voltage

phase

voltage

In network where

neutral point is

directly grounded

In network where

neutral point is not

directly grounded

Secondary voltage

(V)

110 110/ 3 110 110/3

7/27/2019 Calculation for Pt Selection

4/7

2/5

Tab. 1-2 Accuracy Class of Secondary Winding of PT

Accuracy Error limits Apply to function condition

Phase displacement

Accuracy classvoltage

error ( %) min crad

Range of

voltage

%

Range of

frequenc

y%

Burden

and

power factor%

0.1 0.1 5 0.15

0.2 0.2 10 0.3

0.5 0.5 20 0.6

1.0 1.0 40 1.2

measure

ment

3.0 3.0

80~120 99~101

3P 3.0 120 3.5 protection 6P 6.0 240 7.0

Residual

voltage

winding

6P 6.0 240 7.0

5~150

or(5~190

)

96~102

25~100

and

0.8(lag)

3.2 SELECTION OF ACCURACY CLASS OF PT

The accuracy class shall be selected according to the contract.

Accuracy class of CT refers to permissible error under definite secondary loads. It meets the

requirements in the following tables.

Tab. 1-1 Accuracy Class of PT and Electrical Measuring Devices

Accuracy class

of meters

Accuracy class of PT

0.5 0.5

1.0 0.5

1.5 1.0

2.5 1.0

7/27/2019 Calculation for Pt Selection

5/7

3/5

Tab. 1-2 Accuracy Class of PT and Energy Metering

Accuracy class of watthour

meters

Accuracy class of varhour

meters

Accuracy class of CT

0.2S 2.0 0.2

0.5S 2.0 0.21.0 2.0 0.5

2.0 3.0 0.5

4. SECONDARY LOAD VERIFICATION

4.1 SECONDARY LOAD OF PT

Rated secondary load of PT (We) shall be determined by its accuracy class, it shall be no more than

the permissible load which is submitted by PT manufacturer.

Secondary load of PT for each phase shall be derived from the formula as follows:

W= P Q2 2+ (1-2)

Where: W--- Secondary load of PT(VA);

P--- Active power load (W);

Q--- Reactive power load (var).

Active power load (P) for each phase shall be derived from the formula as follows:

P= Wm cos (1-3)

Where: Wm--- Actual secondary load of PT for each phase (VA);

--- Phase angle error.

Reactive power load for each phase (Q) shall be derived from the formula as follows:

Q= Wm sin (1-4)

Where: Wm--- Actual secondary load of PT for each phase (VA);

--- Phase angle error.

7/27/2019 Calculation for Pt Selection

6/7

4/5

When the maximum actual secondary load is smaller than permissible load of PT for each phase,

then PT meets the requirement of accuracy class for measuring meters and relays.

4.2 PARAMETERS OF PTAccording to the parameter supplied by manufacturer, burden of different devices shall be as below:

Each set of protection or automation devices:

7/27/2019 Calculation for Pt Selection

7/7

5/5

Capacity: 30/75/75/30VA

PT at HV side of startup transformer

Ratio: /0.11kV

3

0.11/

3

0.11/

3

0.11/

3

220

Accuracy class 0.2/3P/3P/6P

Capacity: 30/75/75/30VA

PT at 400kV busbar

Ratio: 0.11kV/3

0.11/

3

0.11/

3

400

Accuracy class 0.2/3P/3PCapacity: 50/100/100VA

PT at 400kV outgoing lines

Ratio: /0.11kV3

0.11/

3

0.11/

3

0.11/

3

400

Accuracy class 0.2/3P/3P/6P

Capacity: 50/100/100/50VA

PT at 11kV or 6.6kV busbar

Ratio: kV3

0.11/

3

0.11/

3

11or kV

30.11

/3

0.11/

3

6.6

Accuracy class 0.5/6P

Capacity: 100/100VA