CT Sizing Calculation for 132kV System: GHATAIL 132/33Kv,
2X80/120MVA NEW AIS Sub-Station
1. Overview:
The purpose of this document is to verify the sizing of the Current Transformer (CT) in terms of
knee point voltage and rated burden of relay and meter.
2. Notes for Calculation:
Design inputs such as
1) Instead of specific relays, the generic ANSI codes for relays are used with their
usual burdens as per the reference document of ABB.
2) Technical specification and drawing in contract document.
3) CT requirements and burden by related relays and meters
Design Criteria:
Knee point voltage: Apart from rated short time rating of the system, to arrive at the minimum
knee point voltage, value for time constant or X/R ratio and steady state through fault current
values need to be determined.
Considering the rated capacity of the bus bars of 132kV, the fault level considered is 40kA.
The time constant (τ) for 132kV system is calculated on the basis of the DC breaking capacity of
the breaker. The breaker is capable of breaking DC current of 40% of the symmetrical capacity
(40kA) of the breaker. The breaker breaking time is 60ms.
SL System Particulars Notation Value Unit
1 Nominal System Voltage V 132 kV
2 Nominal Frequency f 50 Hz
3 Rated Short Circuit Current of Station Iscc 40 kA
4 CB DC Current breaking capacity %DC 40 %
5 CB breaking time T 60 ms
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Time Constant of the system is given by the following formula:
𝑇
𝜏 =
√2
𝑙𝑛( %𝐷𝐶 )
100
Where
T = Circuit Breaker Breaking time in ms = 60 ms
%DC = DC Current breaking capacity (%) = 40%
With given system data for 132kV we get
60
𝜏= =47.511ms
√2
𝑙𝑛( 40 )
100
𝑋
And the X/R ratio is given by, 𝑅
=2 × 𝜋 × ƒ × 𝜏
𝑋
𝑅
= 2 × 𝜋 × 50 × 47.511 × 10−3
𝑋
= 14.926
𝑅
Hence all CT selections are being verified for the above X/R ratio of 14.926.
Rated burden selected for CT must be more than the sum of the relay/metering burden connected
across CT.
3. Calculations:
Calculations performed for the CT parameters are enclosed in following sections.
4. Conclusion:
Calculation results show that selected parameters for CT are adequate to meet the minimum
requirement for the connected relays at each bay. Please refer to “Results Summary”
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132 kV Line Feeder
1.1 CT dimensioning for Metering: Applicable Core: 1
General Project Data:
Customer: PGCB
Project: DESIGN,SUPPLY,INSTAKKATION,
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: 132KV
Nominal Frequency: 50HZ
Power System Time Constant: 47.511ms
Rated Short Circuit current of the station: 40KA
Data of Line CT according to IEC Norm:
<For CT Ratio 2000/1A>
Accuracy Class: 0.2
Rated Burden: 10 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: 5 VA
Rated Primary Current Ipn: 1000 A
Rated Secondary Current Isn: 1A
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Internal Burden RCT at 75oC: 5Ω
Instrument Security Factor FS: 5
ENERGY METER DATA:
Manufacturer: CEWE
Type: Prometer 100
Rated relay current, Ir : 1A
Relay burden SR (Per phase): 0.7mVA
Permissive Overload for 1sec IPO : 50
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 1
𝐼𝑤𝑖𝑟𝑒 𝑥 𝜌𝑤𝑖𝑟𝑒 𝑥 𝑘𝑤𝑖𝑟𝑒
Rwire=
𝛼𝑤𝑖𝑟𝑒
Calculated Cable Burden Rwire 0.922675 Ω
With 20% margin RI 1.10721 Ω
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)
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Where,
FS: Instrument Security Factor
RCT: Internal CT burden at 75oC [Ω]
𝑉𝐴
RN: Rated CT burden ( 𝐼2 )[Ω]
𝑛
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 = 10 Ω
RL: Secondary wire burden = 1.10721 Ω
SR: Rated instrument burden = 0.0021 Ω
Ir: Rated Secondary Current of Relay [A] = 1A
Ipo: Rated instrument minimum permissive overload = 50
Requirement 1:
10 + 10
𝐹𝑆 ′ = 5 ×
10 + 1.107212 + 0.0021
𝐹𝑆 ′ = 9 ≤ 50
Which fulfills requirement 1.
10+1.1072+0.0021
Requirement 2: Ratio =
10+10
= 55.55%
Therefore, Ratio=55.55% and fulfills requirement 2
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<For CT ratio 1000/1>
FS: Instrument Security Factor = 5
RCT: Internal CT burden at 75oC [Ω] = 5 Ω
RN: Rated CT burden = 5 Ω
RL: Secondary wire burden = 1.107212 Ω
SR: Rated instrument burden = 0.0021 Ω
Ir: Rated Secondary Current of Relay [A] = 1A
Ipo: Rated instrument minimum permissive overload = 50
Requirement 1:
5+5
𝐹𝑆 ′ = 5 ×
5 + 1.107212 + 0.0021
𝐹𝑆 ′ = 8.1842 ≤ 50
Which fulfills requirement 1.
5+1.10721+0.0021
Requirement 2: Ratio =
5+10
= 61.0931%
Therefore, Ratio=61.0931% and fulfills requirement 2
SO, CT IS CORRECTLY DIMENSIONED.
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