13.8 KV incomer and bus-tie major failure

[junior tech]

good day guys,
I badly in need of your comments with regards to electrical protection system. The issue took a place when we tried to run a motor with rated power of 600 KW, 13.8 KV and rated current of 30 Amps. This motor fed from ABB switchgear with tow incomers and normally closed bus tie between. What happen is when the motor received start signal, the main incomer which motor connected to has tripped along with bus tie causing all other equipment to trip as well. When motor's feeder events been checked, it shows that the motor tripped due to instantaneous over-current around 2900Amps which is very large value. And the main 13.8KV incomer’s events show that the vacuum circuit breaker VCB open due to under-voltage with differential relay of bus PT activated too. I’d reviewed the protection set points of both motor feeder and main incomer. Motor over current set point is 7.6 times motor rated current with time delay of 0.05 second, incomer set point of under-voltage low and high respectively 80% and 90% of rated voltage with time delay of 3 seconds for both high and low settings. My concern, why did the incomer and bus-tie open? Is it because motor draw high current and cause under-voltage at main incomer? If yes, so why did it open before the period of its time delay completed since the over-current was instantaneously?
Please I need a your experience to clarify this issue…. Thanks

 

[Knobhead]

Is the motor DOL starting? Sounds like it, even so it should only be 200A max.
Have any tests been done on the motor to check it’s integrity? From the figures you give it looks like the VCB closed on to a dead short.

My biggest worry is your competence to do this work.

 

[junior tech]

Dear tony,
tis motor is delta and according to its datsheet the starting current is 650% of its rated current. the motor had a flash over. and ofcoures we had test motor's winding insulation and resistances test and found windings already been cut off. anyhow, my request is trying to analyse what went wrong and what protection system suppose to be activated.

many thanks to you

 

[Knobhead]

You need to look at why the motor VCB didn’t take care of the fault. For the feeder to go out on U/V with 3 seconds delay says there was prolonged overload.
I would seriously consider replacing the motor VCB vacuum bottles, I have had them weld closed under fault. What was the duration of the O/L shown on the feeder? I assume 3 seconds.

Do you know what caused the motor to fail?

 

[junior tech]

Dear Tony,
you did not got my point.. the motor feeder as I mentioned has been tripped... and this is my question... why did the main incomer trip since motor feeder already tripped?.. regarding your question about motor itself, the motor windings found shorted and that the cause of instantenous O/C not overload as you said

 

[johnboy6083]

I'm not experienced in very high power systems like the one you describe, but to me it sounds like your protection devices have no short circuit discrimination. They may well discriminate for over current, but not for short circuit.

All based on what you described, and all In My Opinion. Get an engineer to. Heck the charts for the protective devices.

 

[Engineer54]

Need a lot more information than you have so far given. What ratings are your main incomer VCB's and bus coupler VCB, and what settings have been applied for long and short time settings instantaneous trip, along with any other protection function parameters that have been set on these VCB's ?? Same information for the motor feeder VCB!! What other protection relay's are on this MV switchboard, you have mentioned under voltage (27), differential (87) do you also have a lockout relay (86).

Has this switchboard been in service a long time, or is it a relatively new installation?? Has a protection relay study been undertaken on this switchboard (there should have been one before this switchboard was put into service)!! If there is one available, check that all the actual set point parameters against those given in the study report (an inexperienced someone, may have altered the set points for one reason or another) . To be honest, without anything else to go on, apart from what you've mentioned above, I'm suspecting your protection hasn't been correctly set-up/graded. Don't totally rely on ''Event recorders'', on catastrophic failures they can be very misleading in these conditions to say the least...

I don't particularly like VCB's, on main switchboards, i prefer to see GCB's, though i've never experienced vacuum bottles welding closed, i have heard a good few stories of similar things happening in fault conditions to them!! Especially on normally closed bus sections where the combined fault levels of 2X transformers can be extremely high!!

Where in the world are you based, your obviously not in the UK??

 

[Engineer54]

Having another look at what you have posted, i'd be looking at the set point parameters of the 87 differential relay, which would take out both incomer and bus tie (depending on the controls design) the under voltage relay, wouldn't normally knock out a bus tie, only the main incomer. What about the second incomer, you haven't mentioned if that too was tripped out, though i would suspect it did unless you have 2 X 87 relay's one looking after each side of the bus sections. Then again you did say that this fault occurrence, tripped out all the other equipment too. So until you give us a bit more detail to work with, it's all guess work!! lol!!

I don't suppose that you have a single line drawing of this Switchboard that you could post up here?? Things become much clearer when you know what your looking at ...lol!!

 

[junior tech]

Dear engineer, thanks a lot for your elaboration...

I did not said that the under-voltage protection open both incomer and bus tie, it has only opened the incomer which that motor fed from and another incomer did not tripped while it supposed to fed the downstream equipment of the tripped incomer through bus tie since its philosophy is normally closed as I mentioned previously which will prevent any power interruption for downstream equipment. Anyhow, the multi-protection relay used at incomers is ABB REF542+.. and yes there are tow deferential relays as you said and sure if that 87 activated at any incomer for any reason it will activate 86 along with to lock the VCB out.
For the events, did you experienced any unreliable? I hope these are the details you need...

 

[Engineer54]

Protection relays tend to be very reliable units in service, but they do need to be set correctly and periodic checking/testing, hence why it is imperative that a protection relay co-ordination study is performed prior to being put into service. I have trouble opening PDF files from where i am, so i can't see the functions of your multi protection relay, even then you may not be utilising all the protection functions available on this unit...

I know you didn't say that both incomers were knocked out, but it could have been assumed they were, from your previous posts!! The same with the 27 relay, you were thinking along the lines that the under voltage relay was tripped because of the high fault current associated with the motor failure, but that just wouldn't happen, and certainly not for the 3 seconds set on the time delay. The main tripping of one half of this switchboard was because the 87 differential relay saw a fault and tripped out, ... which energised the 86 lockout relay. These basically act as ''Zone'' protection relay's and as you have now stated, that you have two, the same would happen if a fault was sensed on the other side of this switchboard, eg ...Incomer and bus coupler would trip out to isolate the bad section of the board.

As for the events recorder, yes i've been mislead by these unit's in the past, especially where multiple relays have been shown to have operated. Not having experience of your ABB REF 542+ i don't know what the protocol is for event recording. Does it list the operation of relays as in the sequence of events, or just show a list of the operated relays related to a given event??

Tell me what is the fault rating of your MV main switchboard, and what KVA/MVA are the 2 X transformers supplying this board?? Is there any chance you can post a single line drawing for your switchboard ?? You still haven't mentioned in what part of the world, were talking about??

 

[junior tech]

here is the single line diag.

 

[Engineer54]

here is the single line diag.

Thanks for trying, but the file is too small to blow-up, everything is too blurred to read. Any chance of reposting with a larger file??

 

[junior tech]

sorry! I tried but could not...:frown2: