Calculation for Pt Selection

西南电力设计院

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工 程 名 称 TIRODA THERMAL POWER PROJECT 1980 MW (3x660 MW Phase – I) Tiroda, Gondia District, Maharashtra

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印度 TIRODA(提隆达)3X660MW 燃煤电站 概 念 设 计 BASIC DESIGN

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专

业

电 二

计 算 书 名 称 CALCULATION FOR PT SELECTION

PT 选型计算

批

准 ：

月

日

审

核 ：

月

日

校

核 ：

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设计计算 ：

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2008 年 11 月 12 日

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

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

secondary Aux.

winding Wiring winding

mode

of Phase phase

voltage 110

winding

for

zero

sequence voltage to phase voltage

voltage Secondary

secondary

110/√3

In network where In network where neutral

point

is neutral point is not

directly grounded

directly grounded

110

110/3

(V)

1/5

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

Error limits

Apply to function condition

Phase displacement Accuracy

class

voltage error (±%) ±min

%

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

3.0

3.0

protectio

3P

3.0

120

3.5

n

6P

6.0

240

7.0

ment

80~120

6.0

Burden

frequenc

and

y

power

%

factor%

99~101 25~100

0.8(lag) 5~150 or(5~190

6P

Range of

and

Residual voltage

voltage

±crad

0.1 measure

Range of

240

7.0

96~102

)

winding

3.2 SELECTION OF ACCURAC Y 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 Accuracy class of PT of meters 0.5

0.5

1.0

0.5

1.5

1.0

2.5

1.0

2/5

Tab. 1-2 Accuracy Class of PT and Energy Metering Accuracy class of watthour Accuracy class of varhour Accuracy class of CT meters

meters

0.2S

2.0

0.2

0.5S

2.0

0.2

1.0

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 2 + Q 2

Where:

(1-2)

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×cosϕ

Where:

(1-3)

Wm--- Actual secondary load of PT for each phase (VA); ϕ--- Phase angle error.

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

Q= Wm×sinϕ

Where:

(1-4)

Wm--- Actual secondary load of PT for each phase (VA); ϕ--- Phase angle error. 3/5

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 PT According to the parameter supplied by manufacturer, burden of different devices shall be as below: Each set of protection or automation devices: