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Substation CT Dimensioning Guide

This document summarizes the calculations for sizing current transformers (CTs) for differential protection of feeder RPCL-1/2 at the Ghatail 132/33kV substation in Bangladesh. It provides the system data, CT specifications, relay specifications, and cable burden calculations. The calculations show that CTs with ratios of 2000/1A and 1000/1A meet the requirements for effective accuracy limit factor to satisfy differential protection for a maximum fault current of 40kA and power system time constant of 47.511ms. Therefore, the selected CTs are correctly dimensioned for this application.

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471 views4 pages

Substation CT Dimensioning Guide

This document summarizes the calculations for sizing current transformers (CTs) for differential protection of feeder RPCL-1/2 at the Ghatail 132/33kV substation in Bangladesh. It provides the system data, CT specifications, relay specifications, and cable burden calculations. The calculations show that CTs with ratios of 2000/1A and 1000/1A meet the requirements for effective accuracy limit factor to satisfy differential protection for a maximum fault current of 40kA and power system time constant of 47.511ms. Therefore, the selected CTs are correctly dimensioned for this application.

Uploaded by

arafin
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
You are on page 1/ 4

1.5 CT dimensioning for Protection Relay ANSI Code 87BB/50BF: Appl.

Core 5

General Project Data:

Customer: PGCB

Project: DESIGN,SUPPLY,INSTALLATION,TESTING AND


COMMISSIONING OF 132/33KV SUBSTATIONS ON
TURNKEY BASIS(PKG-2.1)
Station: Ghatail 132/33KV NEW AIS SUBSTATION

Feeder: RPCL-1/2(LINE BAY-01)

General System and Substation Data:

Nominal Voltage: 132kV

Nominal Frequency: 50 Hz

Power System Time Constant: 47.511 ms

Rated Short Circuit current of the station: 40 kA

According to IEC 60044-1 the rated secondary e.m.f. is given by the following equation:

𝐸𝑘 = 𝐴𝐿𝐹 × 𝐼𝑠𝑛 × (𝑅𝑛 + 𝑅𝑐𝑡 )


Where,

ALF accuracy limit factor (ALF = 20 for 5P20)

Isn = rated secondary current


𝑉𝐴
Rn = rated burden of C.T ( 𝐼2 )
𝑛

Rct = internal burden


30
𝐸𝑘−2000 = 20 × 1 × ( 12 + 10)
= 800 𝑉
15
𝐸𝑘−1000 = 20 × 1 × ( 12 + 5)
= 400 𝑉

1 | Page
Data of Line CT according to IEC-P Norm:

<For CT Ratio 2000/1A>

Accuracy Class: IEC 5P20

Rated Burden: 30 VA

Rated Primary Current Ipn: 2000A

Rated Secondary Current Isn: 1A

Internal Burden RCT at 75oC: 10.0 Ω

Rated Secondary e.m.f. Ek-2000: 800 V

<For CT Ratio 1000/1A>

Accuracy Class: IEC 5P20

Rated Burden: 15 VA

Rated Primary Current Ipn: 1000A

Rated Secondary Current Isn: 1A

Internal Burden RCT at 75oC: 5.0 Ω

Rated Secondary e.m.f. Ek-1000: 400 V

Data of Relay:

ANSI Code: 87B/50BF

Manufacturer: ABB

Type: REB 500

Rated Relay Current Ir: 1A

Relay Burden, SR: 0.25 VA

2 | Page
Calculation of Cable Burden:

Distance Iwire 170 m


Cable cross section αwire 4.0 mm2
Special resistivity(Cu) ρwire .02171 Ωmm2/m at 750C
Effective wire length kwire 2

𝐼𝑤𝑖𝑟𝑒 𝑥 𝜌𝑤𝑖𝑟𝑒 𝑥 𝑘𝑤𝑖𝑟𝑒


Rwire= 𝛼𝑤𝑖𝑟𝑒

Calculated Cable Burden Rwire 1.845 Ω


With 20% margin RI 2.21442 Ω

CT requirements for REB 500:

CT requirements for differential protection:

The current transformers must have a effective accuracy limit factor n’ that is larger than the value of
equations below:

1 × 𝐼𝑘𝑚𝑎𝑥
𝑛′ ≥ (𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑚𝑒𝑛𝑡 1)
5 × 𝐼𝑝𝑛

n’ ≥ 10 for TN ≤ 120ms

n’ ≥ 20 for 120ms≤ TN ≤ 300ms

Where,

n’ = The effective accuracy limit factor =10 (TN = 47.511ms)

Ikmax = Maximum primary through fault current (KA)

Ipn = Rated primary CT current (A)

TN = Power system time constant

Calculation for differential protection:

For ratio 2000/1

1 × 𝐼𝑘𝑚𝑎𝑥
𝑛′ ≥ (𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑚𝑒𝑛𝑡 1)
5 × 𝐼𝑝𝑛

1 × 40000
𝑛′ =
5 × 2000

3 | Page
n’ = 4 (Selected n’ =10 which is greater than 4)

So. In this case requirement 1 is fulfilled.

For ratio 1000/1

1 × 𝐼𝑘𝑚𝑎𝑥
𝑛′ ≥ (𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑚𝑒𝑛𝑡 1)
5 × 𝐼𝑝𝑛

1 × 40000
𝑛′ =
5 × 1000

n’ = 8 (Selected n’ =10 which is greater than 8)

So. In this case requirement 1 is fulfilled.

Where,

n’ = The effective accuracy limit factor =10 (TN = 47.511ms)

Ikmax = Maximum primary through fault current (KA) = 40KA

Ipn = Rated primary CT current (A)@ 2000/1A = 2000A

= Rated primary CT current (A)@ 1000/1A = 1000A

TN = Power system time constant = 47.511ms

The Current Transformer is correctly dimensioned.

4 | Page

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