Should we consider protection coordination between incoming and outgoing MV Switchgear feeders in cable sizing calculations? Part 1

Introduction

As you know, the following factors are effective in cable sizing:

1.???? Ampacity

2.???? Voltage drop

3.???? Thermal Short Circuit

4.???? Harmonic

I reviewed above factors in more detail in episode (1) cable sizing calculation of the "The Explorer" video series.

Because our subject is related to the effect of protection coordination in sizing of MV cables, we only consider Thermal short circuit factor and ignore others. The subject that is going to be investigated in this article is only related to MV cables. The reasons for this are as follows:

1.???? Fault clearing time in LV circuit breaker is much less than MV cirduit breaker

2.???? Most of LV circuit breaker is Current limiting

S: Minimum cable cross-section due to short circuit factor

t : Clearing time of protection system

K: A constant related to the specifications and material of the cable

Obviously, according to equation (1) clearing time and short circuit current for LV cable is lower than MV cable and the short circuit factor for LV cables does not change the cross.

Clearing time with considering protection coordination or without it?

As we have seen, fault clearing time is one of the parameters of the short circuit factor. Typical clearing time is about 250-300 ms for Mv outgoing feeders and 500-600 ms for Mv incoming feeders. If for any reason protection of outgoing feeders does not work , protection of the incoming feeders will clear fault in 500 ms, but short circuit will pass through both incoming and outgoing feeders for 500 ms. This is called protection coordination in a MV Switchgear.

So there are 2 methods for designing output feeder cables:

Method A) Considering protection coordination in clearing time so t=500 ms

Method B) ignoring protection coordination in clearing time so t=250 ms

Because of the big difference between method (A) and (B), Probably using method (A) will increase the size of outgoing feeder cables. Method (A) is more reliable but more expensive than method (B). Both of these methods are used in design of cable sizing calculation documnets of consulting engineering companies.

Theoretically, if protection of outgoing feeder cables are sized with a time of 250 ms fail, outgoing feeders will be damaged but if we look more closely at the issue, can find reasons that make method (B) reliable.

I published a training video under the title "episode (2) cable sizing calculation of the The Explorer"?and?examined 3 reasons in detail.

Lessons learned:

Cable sizing of outgoing feeders can be done with two methods as belows:
Method A) Considering protection coordination in clearing time so t=500 ms
Method B) ignoring protection coordination in clearing time so t=250 ms

Each of these methods has its own advantages and disadvantages, which are mentioned in the text of the article and training video, and according to the level of reliability that we consider for design of the industrial plant, can use each of these two methods.        

Which method do you use for cable sizing? leave your comment or experience below and subscribe Industrial Electrical Design Newsletter To be notified of future articles. I'd highly appreciate it!

References:

1.???? IEC 60364-5-52

2.???? IEC 60502-2

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