Working out overcurrent protection settings for 11kV feeders and transformers has not been a problem given the fault levels. But how do you work out protection settings for earth fault? i have just noticed that in our distribution system earth fault is set at between 10% and 20%. Neutral is solidly earthed. Is 10/20% standard or how do you work it out?

 

Robbyk, several question...

1) Is this a new installation?

2) Are you sure about the neutral being solidly-earthed?

3) What type of protective relays are being used for EF detection?

4) Do you have any motors, or generators connected on the 11kV system?

Regards, Phil Corso ([email protected])

 

Phil,

(i)It's an old installation

(ii) 11kV network comes from several 33/11kV substations of which the 33/11kV transformers have solidly earthed neutrals.

(iii) we use electromechanical relays (Reyrolle) with erath fault plug settings ranging between 10% and 80%

(iv) There are no 11kV motors and generatorsin the system.

 

Robbyk can you provide additional detail:

5) What are the kVA sizes of the transformers connected to the 11kV system.

6) Do the 11kV feeders supply only LV transformers?

7) Are the feeders 3x1/C or 1x3/C? If the latter are they sheathed? If so, what is sheath material, lead, steel or aluminum?

8) Is each of the 11kV feeders equipped with an earth-fault relay? Is so, are they residual-connected with the phase relays? Or are they zero-sequence type (Core-type), that is, do they encompass the three phase-conductors?

9) What is the estimated total km length of all 11kV feeders?

Regards, Phil

 

PHIL,

5) Transformers: 33/11kV, 3 X 20MVA, 4 X 15MVA, 4 X 10MVA, 2 X 5MVA, 4 X 2.5MVA. Transformers connected to the 11kV system have KVA sizes ranging between 25kva and 1000kva.

6) all 11kV feeders supply lv transformers (11/0.4kV transformers).

7) feeders are 1 x 3/C lead sheaved

8) Each feeder is equipped with earth fault relays, residual connection

9) Estimated total km length of 11kV feeder is 300km.

Regards,
Robbyk

 

Robbyk,

the time setting of the earth-fault relay is easy. However, I am more concerned about the system's ability to safely clear an earth-fault:

The system described has a short-circuit duty of approximately 720MVA. Earth-fault current in the 11kV system could be well over 100kA (peak!) There is no way the lead sheath can support such current even if maximum fault-clearing time (assuming an instantaneous-element EF relay, like a 50N) were 50 ms! There is a strong possibility that such a fault will cause fusing of the sheath for its entire length.

Thus, I ask again, are you sure the transformers connected to the 11kV system do not have their neutrals earthed through an impedance?

Regards, Phil ([email protected])

 

PHIL,

Network described is not wholesome because sources (generating stations) are hundreds of kilometers away and generation on average is at 18kV, which is stepped up to 330kV for transmission throught the country before being stepped down to 66kV for secondary transmission and further to 33kV for primary distribution. i have picked my network from 33/11Kv substations where we have 11kV as our distribution voltage in our locality.

At 33kV, short circuit level for a symmetrical fault is about 6kA.Neutrals of all transformers are solidly earthed in the 11kV distribution subsystem.

 

Robbyk, reur 02-Jun-09... the network you described includes a substantial impedance between the source and the high-side of the 33/11kV transformers.

Fault-duty on the 11kV bus is now 15.9 kA (rms, sym!) While a reduction over the earlier case, current may still be too high for the lead-sheath of small cables. Can you provide the size (mmq) of the smaller 11kV cables? Also provide installation detail, that is, cable in air, buried in the earth, or in conduit!

BTW, if you, or anyone else, would like to know how to make a quick determination of fault-current duty using the shortcut (excuse the pun) kVA or MVA method, contact me offline.

Regards, Phil Corso (Cepsicon [at] aol.com)