132 kV Line Feeder

1.2 CT dimensioning for Metering: Applicable Core: 2

General Project Data:

Customer: PGCB
Project: DESIGN,SUPPLY,INSTALLATION,TESTING
AND COMMISSIONING OF 132/33KV
SUBSTATIONS ON TURNKEY BASIS
(PACKAGE-2.1)
Station: GHATAIL 132/33KV NEW AIS
SUBSTATION
Feeder: RPCL-1/2(LINE BAY-01)

General System and Substation Data:

Nominal Voltage: 132 kV

Nominal Frequency: 50 Hz

Power System Time Constant: 47.511 ms

Rated Short Circuit current of the station: 40 kA

Data of Line CT according to IEC Norm:

<For CT Ratio 2000/1A>

Accuracy Class: 0.2

Rated Burden: 15 VA

Rated Primary Current Ipn: 2000 A

Rated Secondary Current Isn: 1A

Internal Burden RCT at 75oC: 10 Ω

Instrument Security Factor FS: 5

<For CT Ratio 1000/1A>

Accuracy Class: 0.2

Rated Burden: 7.5 VA

Rated Primary Current Ipn: 1000 A

Internal Burden RCT at 75oC: 5Ω

Instrument Security Factor FS: 5

DIGITAL METER IN BACKUP MIMIC DATA:

Manufacturer: ABB

Type: BCM 802

Rated relay current, Ir2 : 1A

Relay burden SR2 : 0.01VA

Permissive Overload for 1sec IPO : 50

ENERGY METER DATA:

Manufacturer: CEWE

Type: Prometer 100

Rated relay current, Ir3 : 1A

Relay burden SR3 (Per phase): 0.7mVA

Permissive Overload for 1sec IPO : 50

Calculation of Cable Burden:

Distance Iwire 170 m

𝐼𝑤𝑖𝑟𝑒 𝑥 𝜌𝑤𝑖𝑟𝑒 𝑥 𝑘𝑤𝑖𝑟𝑒

Calculated Cable Burden Rwire 0.922675 Ω

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CT requirements for metering:

Requirement 1:

The current transformer for metering must have an effective instrument security factor FS’ that is
smaller than the value of the equations below

(𝑅𝑐𝑡 + 𝑅𝑁 )
𝐹𝑆 ′ = 𝐹𝑆 × 𝑆 ≤ 𝐼𝑝𝑜 (Requirement 1)
𝑅𝑐𝑡 +𝑅𝑙 + 2𝑅
𝐼𝑟

Requirement 2:

The ratio of actual burden to rated burden should be the value of equations below:
𝑆
(𝑅𝑐𝑡 +𝑅𝑙 + 2𝑅 )
𝐼𝑟 )
25% ≤ (𝑅𝑐𝑡 + 𝑅𝑁 )
≤ 100% (Requirement 2)

Where,

FS: Instrument Security Factor

RCT: Internal CT burden at 75oC [Ω]

Rl: Secondary wire burden [Ω]

SR: Rated instrument Burden[VA]

Ir: Rated Secondary Current of Relay [A]

Ipo: Rated instrument minimum permissive overload

<For CT ratio 2000/1>

FS: Instrument Security Factor = 5

RCT: Internal CT burden at 75oC [Ω] = 10 Ω

RN: Rated CT burden = 15 Ω

RL: Secondary wire burden = 1.10721 Ω

SR: Rated instrument burden = 0.0121 Ω

Ir: Rated Secondary Current of Relay [A] = 1A

Ipo: Rated instrument minimum permissive overload = 50

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Requirement 1:

10 + 15
𝐹𝑆 ′ = 5 ×
10 + 1.10721 + 0.0121

𝐹𝑆 ′ = 11.24 ≤ 50

Which fulfills requirement 1.

10+1.1072+0.0121
Requirement 2: Ratio =
10+15

= 44.48%

Therefore, Ratio=44.48% and fulfills requirement 2

<For CT ratio 1000/1>

FS: Instrument Security Factor = 5

RCT: Internal CT burden at 75oC [Ω] = 5 Ω

RN: Rated CT burden = 7.5 Ω

RL: Secondary wire burden = 1.10721 Ω

SR: Rated instrument burden = 0.0121 Ω

Ir: Rated Secondary Current of Relay [A] = 1A

Ipo: Rated instrument minimum permissive overload = 50

Requirement 1:

5 + 7.5
𝐹𝑆 ′ = 5 ×
5 + 1.10721 + 0.0121

𝐹𝑆 ′ = 10.2136 ≤ 50

Which fulfills requirement 1.

5+1.10721+0.0121
Requirement 2: Ratio =
5+7.5

= 48.95%

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Therefore, Ratio=48.95 % and fulfills requirement 2

SO, CT IS CORRECTLY DIMENSIONED.

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