CT Sizing

[DanielElias]

Dear Gentlemen
Can anyone please help me.
I have an electrical system with the next data:
X/R: 15.3; If=14500A; Vsys=10kV
The installation will include a MV Switchgear
I'm thinking of using a Feeder Protection Relay with the next:
-CT rating/Maximum Transient: 80xInom=400A for 1sec
-CT Burden<0.25VA@5A
The resistance of the leads from CTs to the relay is 0.026Ohm aprox. Moreover I have 03 Flex Core CTs C200 800:5A With secondary winding resistance 0.706 Ohms@75°C

I'll apreciate so much if you can tell me if my selection is all rigth or CTs will saturate due to 14500A (SC Current)

Thanks in advance

 

[Alex68]

What is the rated current of your load?
What is the actual burden (load) of your CT, including cables and relays?

 

[alehman]

At 14,500A, the CT secondary current would be
14,500 / 800 * 5 = 90.6 amps

The lead voltage would be 90.6 amps * .026 ohms = 2.36 V
The relay burden is 0.25 / 5^2 = 0.01 ohm
0.01 ohm * 90.6A = .906 V
The secondary winding voltage is 90.6 amps * .706 ohms = 64.0 V

The total secondary voltage is therefore 67.3V. A C200 CT will provide up to roughly 200 V at up to 20 times rated current with 10% accuracy, so you should be fine. This assumes that you use the full secondary CT winding. Frequently CT's have reduced ratio taps which have correspondingly reduced saturation voltage.

 

[DanielElias]

Dear Alehman
Thank you for you reply however I have read GE Paper GER3979 (Dimensioning of Current Transformer for protection application). There they include a factor "Ks" (Saturation or Transient Factor) that has to do with other factors (TPX or TPY CT design, Instantaneous operating time "ts" of Protective Relay ts=2*PI*60*T1*T2/(T1-T2)*(Exp(-ts/T1)-Exp(-ts/T2))+1, Where:

ts:Time to saturation (Datum of relay 2cycles)
T2:Secondary Time Constant (=3sec)
T1rimary Time Constant (=L/R=X/(R*2*PI*60=.04)

When I replace all my data by using a Feeder Relay I find that this factor is 9.42 aprox and the Saturation Voltage Vs=14500*5/800*9.42*(.026+.01+.706)=633V

According to your suggestion I wouldn't have any problem, but when using GE Paper the result is diferent.
What is true about CT Sizing?

 

[Kiribanda]

Hi DanielElias,

I have assumed follows.

800/5 C200 C/t winding resistance = 0.706 Ohms
Wire lead resistance (one way) = 0.026 Ohms
Burden reactance = 0.0 Ohms
Relay burden = 0.1 Ohms (resistive)
Per unit off set in primary current (max.assumed) = 1.0
Per unit remanence of the c/t core = 0.2
Sym fault current = 14500 A
C/T ratio = 160
Sat. factor = 22 (worst case)


Above those input parameters show that the c/t will be saturated within 8.3 ms (Time to Saturation). It will be remain saturated for approximately 94 ms (5.6 cycles at 60 Hz). So you have to take care about Inst. Settings as during this period of time the secondary current will not be a correct replica of the primary.

C200 class seems to be ok if we consider steady state conditions only. But it is not correct for transient conditions. We have to satisfy both as per IEEE C37.110.

Therefore my suggestion is a C/T with 1000/5 with C800 class so that it will not saturate at all. C400 is marginal.

Kiribanda

 

[alehman]

Daniel- it would be helpful if you could have stated the specific nature of your question to start with. If DC transients are a concern, you will need to adjust the sizing criteria. I can't locate the specific GE paper you mention.

Ts = -(X/R)/(2*pi*f)*ln(1-(Ks-1)/(X/R))
where
Ks = Vsat/(Is(Rs+Zb))
Rs = winding resistance and Zb = burden impedance