Fault Current

Hello Shruti Dandekar,

It is a function of the supply voltage, supply resistance if DC, Supply resistance and reactance at Mains frequency if AC, the DC resistance of the line, and if AC then the actual impedance at Mains frequency of the line also.

Calculations for networks and network nodes are often quite very complex and protection relays and equipment are normally chosen from standard units per manufacturers Specifications.

Settings of protection relays and equipment are "roughly set first", then are normally modified after extensive on-site Tests are done.

Without knowing the variables in your particular network, it is not really possible to generalise.

If you need further assistance.... ....

Hello,

Yes whatever you have written is correct. But normally when network protection is designed some standard (Max Value) value of the fault current is taken into consideration for designing e.g. in India it is taken as 40,000. Each and everytime network analysis is not carried out.

So my question is what % of this current should be taken at the reach of Zone 1 in case of diatance protection relay ?

Dear Shruti,

Please note that the 40 kA is specified by your Authority for the purpose of equipment design. This means that all equipment must withstand short circuit current of 40 kA for a specified duration 1 or 3 seconds.

Now back to the distance relay, this relay is an impedance relay, which continuously measures current & voltage and calculate the corresponding impedance. The distance relay has three common protective zones namely (Zone 1, Zone 2 & Zone 3).

Zone 1: is required for the "Direct Underreach Transfer Trip & Permissive Underreach Transfer Trip" (DUTT & PUTT) schemes and may be used in the "Permissive Overreach Transfer Trip" (POTT), and Blocking schemes to improve performance. Regardless of the relay scheme, the Zone 1 must be set so that they do not overreach the nearest remote line terminal. Typical settings would be 80-90% of the positive sequence line impedance with instantaneous tripping.

Zone 2: is set to protect the remainder of the line left unprotected by the Zone 1 setting and provide adequate margin. Time delay is to be added to Zone 2 setting for coordination with the relays at the remote bus. Typical minimum setting would be 120% of line impedance with time delay of 20-30 cycles.

Zone 3: although the line is fully protected with Zone 1 & Zone 2 functions, a third forward-reaching zone is often employed. This Zone 3 is applied as backup for Zone 2. Settings of the Zone 3 vary from 110% of the sum of the protected line and remote line impedances, to 200% of the protected line's impedance depending on the system configuration. 60-cycle time delay would be applied for Zone 3, provided that it does not reach beyond any Zone 2 setting of remote station line sections.

From the above we conclude that the settings of a distance relay are only impedance settings. However, some distance relays have integrated feature of "Directional Over Current" as backup protection for the distance relay. In this case current settings are to be applied to the "DOC" element, keeping in mind that this feature "DOC" protects the whole line no matter where the fault location is on the line.

I hope the above has clear some points to you and found useful.

Hi,

Still I have not got answer to my question. It is not about relay setting. Now for a distance protection relay the formula given by the manufacturer for the calculation of Vk is

Vk = Ikmax * (Isn/Ipn)*a* (Rct + 2Rl + Rr ) &

Vk = Ikmaxz1 * (isnIpn)*k*( Rct + 2Rl + Rr )

Forget about 'a' and 'k' , now the question is if I take Ikmax = 40000 A as given by authority in India what should i take the value og Ikmaxz1 which is the fault current at reach of Zone 1

Hi,

The formula indicated in your message has no relation with the fault current at any zone. It calculates the knee point (saturation point) of the current transformer (Vk). This proves that the used current transformer should sustain the 40 kA without saturation to avoid misoperation of the protection system. Accordingly, the max. fault current (40 kA), is to be used in calculating the knee voltage (Vk) of the current transformer.

Ok then what is the value of the current IkmaxZ1 to be taken?

As in the catalogue it is written that IkmaxZ1 is the fault current at the reach of zone 1

40 kA is to be used.

ok

thank you all for the suggestions