Electrical Schematic

[Marxman99]

Hi, I am doing course on electrical engineering. I have an electrical schematic. The scheme uses 1000kva transformer, 500kva back-up gen, 2nr 160kva ups (parallel operation).

I have 3 questions:
1. The schematic shows a main 800A fuse and a separate neutral 400A fuse, why cant the fuse 800A switch all phases and neutral together e.g 4pole?

2. The fuses I refer to in 1 above are in the switchpanel, should there be a fuse at the beginning of the incoming cable (cable from the transformer) to protect the cable?

3. In books 3 switches are shown for the wrap-around bypass, why can’t 2 switches be used.

I would very much appreciate answers in layman terms as I have only just started the course.

 

[Engineer54]

Any chance of you posting this schematic here for us to have a look at??
Make it much easier to access what your talking about

 

[Knobhead]

1. There should be NO fuse on the neutral, are you sure it isn’t a link?
2. A fuse switch is usually mounted at the incoming of the transformer tails, the tails and TX secondary being protected by the HV O/L’s and a restricted earth fault relay.
3. Wrap-around bypass? Sorry I don’t understand this one

You need to post a drawing

 

[Knobhead]

1. There should be NO fuse on the neutral, are you sure it isn’t a link?
2. A fuse switch is usually mounted at the incoming of the transformer tails, the tails and TX secondary being protected by the HV O/L’s and a restricted earth fault relay.
3. Wrap-around bypass? Sorry I don’t understand this one

You need to post a drawing

 

[Marxman99]

I think i have successfully uploaded the schematic.

Tony what is a O/L. And why does the Earth fault relay have to be restricted?

 

[Engineer54]

I think i have successfully uploaded the schematic.

Tony what is a O/L. And why does the Earth fault relay have to be restricted?

I've looked at your schematic drawing, and this is not a typical distribution system. Without knowing what this set-up is supplying, I don't think anyone here can help you unless they are familiar with this set-up!! ...By the way your drawing is showing Breakers not Fuses. Not sure what that ''N'' is signifying on the TX secondary supply to the switchboard, but maybe it isn't a neutral.

Also never seen a generator paralleled up with a supplying TX, usually there would interlocks and/or bus couplers preventing parallel operation, What the three breakers are doing in that ''by-pass'' line i don't understand either, it's a very strange set-up marxman!!!


Tony means that the Secondary supply from the TX is protected on the MV side of the TX. O/L meaning ''overload''. The Restricted earth fault relay, in simple terms senses unbalance, accross the phases and neutral.

Have you anymore information on this set-up, maybe a control drawing?? That would at least show us the operation of how this system is meant to work...lol!!!

 

[Marxman99]

Thanks for you answers so far, the've been helpful.

i'm been discussing this schematic; the breakers are MCCBs, the Tx protection is an ACB, and think same for the generator protection. The 2 breakers on the ups output section are only isolators.

I am thinking that the middle breaker in the mechanical wrap-around bypass should not be there, i.e it is a mistake. I think interlock switches needs to be incorporated, which breakers should be interlocked???? Infact my task is to identify what breakers should be interlocked.

Bearing in mind that G59/1 equipment will be used to parallel the mains with the generator, how would you correct the Tx/generator bus section of the schematic?

I dont have anymore drawings yet. Fyi, the system is to feed distribution boards in a room containing data racks.

 

[Engineer54]

Marxman,


Still looking at your schematic, and trying to make sense of this set-up and what it's doing and why!! ...lol!!! Going by your original post, your calling the generator a ''back-up'' if that's the case, why the need to parallel with the mains supply??
The G59 is as i remember, a 59 relay which is an over voltage and with the suffix ''G'' makes it a ''neutral displacement over-voltage relay''. Actually the relay number should be written first followed by the suffix letter(s) eg 59G. Can't see that relay being the sole basis of paralleling with the mains though!!!! While on relay numbers, ....the restricted earth relay that Tony mentioned in his post is an 87 type relay...

As for that By-Pass line, i can only see a need for one such breaker, being interlocked with the 2 UPS units, open when only the UPS units are supplying power to the 3 essential loads.... I would include a bus coupler on the mains side of that By -Pass line as i'm seeing things at the present. Still need to know what that Breaker arrangement is all about, on the TX side?? What does that C in a circle represent??

Do you know if this schematic is correct, or is correcting this scheme, a part of your assignment?? Have you been given a mode of function for this installation?? You would need that information, to complete your task of breaker interlocking etc!!!

Anyway i'm going to bed now and will look at this with fresh eyes tomorrow...lol!!!!

 

[Knobhead]

From what I can see of the drawing there’s some glaring faults, but I need to enlarge the drawing to see it properly.
But are there really two parallel ACB’s for the TX?

Also need a method of operation

 

[Marxman99]

Engineer54, to avoid even the slightest break in supply to the loads, when the generator is running, at some point the main supply will come back on (hopefully), for a short instance the 2 supplies (gen and Tx) will be running together (to avoid a break of supply to loads). The gen will then be taken of line. The G59/1 relay will ensure that the 2 supplies are in synchronisation (frequency, voltage, phase) with each other. G59/1 is a recommendation document by the utility company, equipment that achieves this recommendation sometimes are referred to as G59/1 relay.

A text book used 3 switches in the bypass line, and the online doc in Fig 1 has switches cb4, cb5, cb6. Explanation of the switches purpose is on page 2. See: http://www.powerlogic.com/literature/SystemGrounding-GroundFaultProtectforUPS_10302006.pdf
(haven’t read the doc yet, just now found it). The arrangement prevents back-feed into the UPS. I just wasn’t sure which switches to make interlocked???

I agree with your position of a bus coupler.

Where you are unsure of the breaker arrangement on the Tx side, are you referring to the breakers on the primary side of the Tx?

Don’t know what the ‘c’ in the circle is for.

To develop and identify the interlock switches is my task. There may well be mistakes on the schematic.

Tony, I have now been given the switchpanel drawing done by the manufacturer. It is attached. It would seem that the neutral breaker is an MCCB as oppose to a ACB. Making any sense now??? Not to me yet!

 

[Engineer54]

Marxman,

Although the document explains a few things as to the operation of such UPS systems in relation to means of grounding etc, it doesn't explain the operation of your system. I still can't see what function this earthing breaker is doing within your system either. Most UPS systems that i have worked with, (None i might add, had such a convoluted set-up as this system, ...lol!!!) ...have for want of a better term, an instantaneous change-over, on both taking the connected load and dropping the connected load. That's why they are called an ''uninterruptible'' power supply.

The inclusion of that neutral breaker on the TX output could very well negate the possibility of a bus coupler, depending on it's function within your system. So it would be down to interlocking the two supply ACBs. The UPS would take up the load during any change over, and open when a stable supply is available. Operation of UPS is normally instantaneous on loss of primary supply and a time lag/delay on reinstatement of primary supply. None of the systems i've worked with, had or allowed paralleling of primary utility power and stand-by generator supply

Are you employed or training with the switchboard company by any chance, or are you with the company where this system is installed or will be installed?? If the latter, surely the controls will be already incorporated within the panel.

As both Tony an i have stated, we need a method of operation, or a control drawing of your system to work through.

Edit..... All the breakers in one form or anther will need auxillary controls/interlocking for automatic operation and safety purposes etc.... not just one or two of them!!!

 

[Knobhead]

Without a method of operation we’re not going to get anywhere. As Engineer says it seems a very convoluted system.
I’m also concerned about the possibility of a utility supply being paralleled to the generator.

 

[Engineer54]

Engineer54, to avoid even the slightest break in supply to the loads, when the generator is running, at some point the main supply will come back on (hopefully), for a short instance the 2 supplies (gen and Tx) will be running together (to avoid a break of supply to loads). The gen will then be taken of line. The G59/1 relay will ensure that the 2 supplies are in synchronisation (frequency, voltage, phase) with each other. G59/1 is a recommendation document by the utility company, equipment that achieves this recommendation sometimes are referred to as G59/1 relay.

Tony,

As the OP has stated here, he is of the belief that his system ''Does'' parallel the utility TX and the Stand-by Gen-set!!!

I don't know if his assumption is correct or not, but i've never had such a situation on any of the projects i've ever been involved with. In fact all possibilities of such paralleling was actively guarded against!!!

Lets see if Marxman can come up with the mode of operation, or a control schematic for his switchboard, ...and see how this set-up is designed to work.

 

[Knobhead]

This is pure conjecture but it’s how I would do it.

• Mains fail
• UPS takes the load
• Generator starts
• Generator and UPS synchronise
• Generator takes load
• UPS shuts down

• Mains restored
• UPS takes load
• Generator unloads
• UPS and mains synchronise
• Mains takes load
• UPS shuts down

I’m sure there are other variants.

 

[Engineer54]

This is pure conjecture but it’s how I would do it.

• Mains fail
• UPS takes the load
• Generator starts
• Generator and UPS synchronise
• Generator takes load
• UPS shuts down

• Mains restored
• UPS takes load
• Generator unloads
• UPS and mains synchronise
• Mains takes load
• UPS shuts down

I’m sure there are other variants.

That sounds pretty much how a typical mains/ gen-set/UPS set-up would operate. I take it you mean by ''synchronise'' is that a stable supply is present at the Input of the UPS, that allows the UPS to change status....