Simultaneous failure of 2 switchboards 2

[Guidoo]

For checking of a flare header, we have to decide which partial power failures can be disregarded as uncredible.
The electrical system of the refinery consists of two main substations, located in two separate buildings. Each substation consists of two switchboards, each distributing the power to different parts of the refinery.
Question is what the possibility is that two switchboards in a single substation fail simultaneously. Does anyone have figures, or know where to find figures in relation to damage (e.g. car crash, lightning), or if one board fails the other board will be damaged as well.
Also more qualitative considerations (e.g. experiences in your own plant/refinery) are wellcome.

 

[RajT]

I think you are too pessimistic. The very reason for having the arrangement you have allowes for a failure on one sub or one switchboard. The chances of a total simultaneous failure on both subs is ofcourse discounted in the design and if it does happen then obviously one has to accept the consequences.

 

[Guidoo]

RajT, thank you for your quick reply.

Point is that we have to demonstrate that simultaneous failure is indeed unlikely. Especially for the two switchboards in a single substation.

Of course if the substation building is hit by a (large) car, a plane or lightning, chances are that both switchgears are damaged.

I am not a electrical engineer (in fact I am a process engineer), but I can also imagine that failure of one switchgear may electrically affect one or more of the other switchgears, causing the other switchgear(s) to fail as well.

 

[electricpete]

well, the correct line of thinking as you have started to consider would be the common-mode failures (the car hitting the substation).

First and foremost... do those two switchboards ultimately receive their power from the same source (looking back toward utility)? What is the redundancy/protection built into that source? And history of reliability of that utility supply. Do the switchboards share any common external dc power supply? Any common trip signals? Lots more along this line.

Along with redundancy, the nuclear industry has latched onto the concept of diversity. If two redundant pieces of equipment are of the same design/manfuacture and maintained the same way, the chances of them both failing at the same time are increased. Perhaps the lubrication in both sets of breakers will harden at a similar rate to result in both sets of breakers failing when called upon. Don't laugh I have seen that one.

It is surely not a simple subject. One reference might be the IEEE gold book.

 

[RajT]

If you have to "demonstrate" that the simultaneous failure of both switchboards is unlikely or rare, then one can argue the case by stating the following:

The design has allowed for high avaliabilty by having two switchboards (presumably fed from separate independent feeds)in two separate sub stations.

As part of the continous preventative maintenance programme carried out on the switchgear, the risk of simultaneous is significantly reduced.

There has never been a simultaneous failure of both the switchboards in any one sub in the history of the plant in question.

The present arrangement offering high availiabilty is based on modern plant design practice and thus considered acceptable.

You may think of other points to add to the above.

I am engaged in design of elect dist systems within the petrochem industry for the last 20 years and would say that the arrangement you have is most common for the very reason.

You did make a point that all faults "external" to the switchboards like a metorite! or something else hitting it or the upstream supply feeders failing simultanusly have been discounted which is an important fact to note.

Historically, the humble switchboard is an extremely reliable piece of equipment and majority of failures/faults are due to human errors.

 

[RRaghunath]

Guidoo,

The cable vault (if there is one) below the switch board panels is one such common facility. A fire in the cable vault will mean losing both the switchboards in that particular substation.

Similarly, if two switch boards are located in the same room, chances of failure of one affecting the other switch board also increase (such as due to fire in one of the switch boards). Even if the two boards happen to be in different rooms but share the common air conditioning / ventilation ducting, this is true.

Hope you find the above useful.

 

[jbartos]

Suggestion: Beside what has been addressed in the above postings and the figure that you are seeking, it will be better to perform reliability study using suitable reliability software and input appropriate equipment reliability data from the available sources / libraries. You will be dealing with MTBF (Mean Time Between Failure), MTTR (Mean Time To Repair), etc.
Visit

http://www.relexsoftware.com/index.asp

http://www.isograph.com/

etc. for more info.
However, the common-mode or common-fault failure is usually considered most important in the same category of multiple entities.