Current Transformer Selection Calculation
Prepared by J. Acharya
1.0 Scope
This calculation is intended for checking proposed Current Transformer (CT) for the Line, Transformer or
Bus. One set of calculations can cover a CT or a group of CTs with identical ratio, accuracy and burden.
2.0 Power System Configuration and Equipment
Nominal system voltage (L-L) VRated Vnom VRated kV 400 kV
(at CT location)
6
MVA 10 V A
LineOrTransformer
- Choose if this study is for Line or Transformer.
2.1 Maximum Load Limit
Transformer: Maximum Capacity in MVA
- For transformers, maximum limit is the highest cooled rating.
TrafoMVA 240MVA - Enter 1.5 times the maximum rating. E.g.160MVA x 1.5=240MVA
TrafoMVA
ITrafo 346.41 A
3 Vnom
Line: Maximum operating limit of conductor Ampere rating (e.g. rated at 85 or 100 deg C)
At conductor temp of 85 deg C and ambient temp of 40 deg C
ICond 875A When maximum loading is limited by other factors than conductor ampacity
rating, enter the corresponding maximum desinged Ampere rating of line.
NumBund 4
IConductor ICond NumBund 3500 A
Maximum loading limis
IMAX IConductor if LineOrTransformer = "Line" 3500 A
ITrafo otherwise
CT Check Dr. J Acharya Page 1 of 7
Bus continuous rating:
It is desirable to meet or exceed bus capacity to allow ultimate load growth. For radial and ring bus, it is
the main bus and for 1-1/2 breaker scheme, use diameter bus (not main bus).
BusRating 4000A
3.0 CT Class and Burden
4000
CT ratio selected: CTR 4000
1
4000
Full winding (max) CT ratio: CTRFull 4000
1
CT Class
CT Class (at max ratio):
CT V_rated CT Class V 800 V
CT continuous thermal CT RF
Verify RF data from CT nameplate
current rating factor, RF:
CT secondary Rct 0.0012 CTR ohm 4.8 Ω
winding resistance:
CT test results are preferred.
If test report not available, assume 0.0012 Ohm/turn at 75°C
Cable length is generally short for line CTs in GIS
One-way CT cable length: LLead 50m sustation and longer for outdoor transformers.
AWG Cable sized used later to calculate its resistance.
CT cable size:
AWG (Americal Wire Gauage) is the US standard
measure of electrical conductors size.
CT remanance factor assumed: FluxResidual 0% Residual flux can be range from zero to as high as
80%. In reality, 0-25% is assumed.
4.0 Fault current information
The fault currents are based on the fault studies using the updated short circuit model. For present fault
levels, use the scenario with maximum fault current and for future use the 20-yr outlook model or apply
25% growth factor.
Note: Always use 3I0 for the LG fault current .
4.1 Maximum Fault Current Flow through CT
Apply a close-in fault and record the fault current contribution through the CT being studied.
Conservatively, substation bus faults can be used if close-in fault results not available.
CT Check Dr. J Acharya Page 2 of 7
3PH fault: I3PH.max 51026A XtoR3PH.max 16
1LG fault: I1LG.max 43310A XtoR1LG.max 15
4.2 Minimum Fault Current through the CT that the connected relay can detect
First REMOVE the strongest source by taking the highest fault contributing line or tie-transformer
out of service and simulate faults as below.
- For line relays, apply remote line-end fault with remote terminal open and suitable fault resistance.
- For bus relays, apply a bus fault. For transformer, apply fault on opposite bus of study side.
L-L fault (smaller of ILL.min 17551A
faulted phase current):
For Line only, use LG with fault
I1LG.min 11401A resistance (5ohm-100ohm)
1LG fault (3I0 current):
4.3 Ultimate Fault Current through the CT
Ultimate (10-yr or 20-yr outlook) fault levels from the Short Circuit Study Report are preferred. If
ultimate fault levels are not available, assume 25% higher than present fault levels.
Ultimate
Select whether ultimate fault levels are available:
If ultimate fault levels are provided, enter the values here: IUlt.3PH 63000A IUlt.1LG 63000A
I3PH.max.ult IUlt.3PH if Ultimate = "Yes" 63000 A
1.25I3PH.max otherwise
I1LG.max.ult IUlt.1LG if Ultimate = "Yes" 63000 A
1.25I1LG.max otherwise
For simplicity, same X/R ratio will be used for short-term and ultimate fault levels.
5.0 CT Ratio Check
5.1 CT Sensitivity Check
Imin min ILL.min I1LG.min 11401 A
Imin
Imin.sec 2.85 A
CTR
CT Check Dr. J Acharya Page 3 of 7
A minimum of (0.1 x Inominal) is needed for a relay to detect faults. Therefore, the minimum fault
current must exceed 0.1 Amps secondary.
ResultMin "Relay cannot detect minimum fault. Decrease CT ratio." if Imin.sec 0.1A
"CT ratio is ok" otherwise
ResultMin "CT ratio is ok"
5.2 CT Continuous Rating Check
In order to avoid overloading the CT, the CT ratio must be selected such that its continous current
rating (including its rating factor) exceeds the maximum load.
Maximum load current (as calculated above):
IMAX 3500 A
BusRating 4000A
ICONT max IMAX BusRating 4000 A
ResultCONT "CT continuous rating is too low. Increase CT ratio." if CTR CTRF 1 A ICONT
"CT ratio is ok" otherwise
ResultCONT "CT ratio is ok"
6.0 Present CT Saturation Check
6.1 CT Ratio Check
The CT ratio shall be selected such that the ideal CT secondary fault current is less than 15 times
rated secondary current. Ultimate is 20 x rated CT secondary current.
IFault.Max max I3PH.max I1LG.max 51026 A
IFault.Max
IRelay.Max 12.76 A
CTR
ResultCTR.Present "Secondary fault levels are too high. Increase CT ratio." if IRelay.Max 15A
"CT ratio is ok" otherwise
ResultCTR.Present "CT ratio is ok"
CT Check Dr. J Acharya Page 4 of 7
6.2 CT Saturation Check
Calculate effective CT voltage rating:
CTR
CT Vstd_eff 1 FluxResidual CT V_rated 800 V
CTRFull
Calculated CT secondary circuit burden:
LLead 0.232 AWG 2.32
Rlead e ohm 0.164 Ω
1000ft
0.1
Rrly ohm 0.1 Ω Example: Relay burden (0.1VA @ 1A) per SEL-421 Datasheet
2
1
6.2.1 3-Phase Fault
Calculated CT secondary burden RB.3PG Rct Rlead Rrly 5.064 Ω
for a 3-phase fault:
Maximum fault current and X/R ratio I3PH.max 51026 A XtoR3PH.max 16
CT secondary voltage required to avoid saturation:
I3PH.max
Vmax_eff_3PH 1 XtoR3PH.max RB.3PG 1098.19 V
CTR
6.2.2 Single-Phase-to-Ground Fault
Calculated CT secondary burden RB.1PG Rct 2Rlead Rrly 5.228 Ω
for single phase fault:
Maximum fault current and X/R ratio: I1LG.max 43310 A XtoR1LG.max 15
CT secondary voltage required to avoid saturation:
I1LG.max
Vmax_eff_1LG 1 XtoR1LG.max RB.1PG 905.71 V
CTR
6.2.3 Result
Vmax_eff max Vmax_eff_3PH Vmax_eff_1LG 1098.19 V
ResultSat.Present "CT saturates. Increase CT ratio and/or class." if Vmax_eff CT Vstd_eff
"CT ratio and class are ok" otherwise
ResultSat.Present "CT saturates. Increase CT ratio and/or class."
CT Check Dr. J Acharya Page 5 of 7
7.0 Future ultimate CT Performance Check
7.1 CT Ratio Check
The CT ratio shall be selected such that the ideal CT secondary fault current is less than 20 times
the rated current (i.e. 20 A for 1A CT and 100A for 5A CT.
IFault.Max.Ult max I3PH.max.ult I1LG.max.ult 63000 A
IFault.Max.Ult
IRelay.Max.Ult 15.75 A
CTR
ResultCTR.Ult "Secondary fault levels are too high. Increase CT ratio." if IRelay.Max.Ult 20A
"CT ratio is ok" otherwise
ResultCTR.Ult "CT ratio is ok"
7.2 CT Saturation Check
Effective CT voltage rating. CT Vstd_eff 800 V
CT secondary circuit burden: Rlead 0.164 Ω
Relay burden: Rrly 0.1 Ω
7.2.1 3-Phase Fault
Calculated CT burden for 3PH fault: RB.3PG 5.064 Ω
Maximum fault current and X/R ratio: I3PH.max.ult 63000 A XtoR3PH.max 16
CT secondary voltage required to avoid saturation:
I3PH.max.ult
Vmax_eff_3PH.ult 1 XtoR3PH.max RB.3PG 1355.9 V
CTR
7.2.2 Single-Phase-to-Ground Fault
Calculated CT burden for LG fault: RB.1PG 5.228 Ω
Maximum fault current and X/R ratio: I1LG.max.ult 63000 A XtoR1LG.max 15
CT secondary voltage required to avoid saturation:
I1LG.max.ult
Vmax_eff_1LG.ult 1 XtoR1LG.max RB.1PG 1317.48 V
CTR
7.2.3 Result
Vmax_eff.ult max Vmax_eff_3PH.ult Vmax_eff_1LG.ult 1355.9 V
ResultSat.Ult "CT saturates. Increase CT ratio and/or class." if Vmax_eff.ult CTVstd_eff
"CT ratio and class are ok" otherwise
ResultSat.Ult "CT saturates. Increase CT ratio and/or class."
CT Check Dr. J Acharya Page 6 of 7
8.0 Summary of Results
CT Ratio
Sensitivity Check: ResultMin "CT ratio is ok"
Continuous Current
Rating Check: ResultCONT "CT ratio is ok"
CT Saturation - Present Fault Levels
Ratio Check: ResultCTR.Present "CT ratio is ok"
Saturation Check: ResultSat.Present "CT saturates. Increase CT ratio and/or class."
CT Saturation - Ultimate Fault Levels
Ratio Check: ResultCTR.Ult "CT ratio is ok"
Saturation Check: ResultSat.Ult "CT saturates. Increase CT ratio and/or class."
Some digital relays have algorithms for detecting CT saturation during external faults. In these cases,
moderate CT saturation may be acceptable if the CT performs long enough for the algorithm to operate.
If above calculations indicate potential CT saturation, further investigation is recommended. The IEEE
PSRC's CT Saturation Calculator (Excel file) can be used as a tool to estimate the time the when CT
begins to saturate after fault.
CT Check Dr. J Acharya Page 7 of 7
