Neutral Earth connection

Do you have 4 pole breakers on transformer inputs and switchboard outputs to the UPS


If so, regardless of the breaker status, the N-E bond is in place.


Presumably the 4 pole devices are used for the generator inputs and the switchboard bus section switch


If the above is true, then provided the switchboard logic isn't abnormal, then it's likely the UPS always sees a N-E bond anyway.


The problem would arise if the TX inputs are 4 pole, or you have 4 pole on the input (normal or bypass) on the UPS


Regards


OMS

Thanks


Issue is the generation is 11Kv, two transformers provide 400v 3Ph on a TN-S, neutral link is at transformer and there are four pole devices on supply breaker. 4 pole devices on the UPS supplies awell. Checked it out and the UPS def runs sans neutral earth link during swaps from utility to generator

OK - not a sensible design then


You need a zero phase shift transformer for the UPS in the bypass line (so the N-E link is always in circuit even when the machines are in bypass). The normal feed need only be 3 pole (no neutral)


Regards


OMS

I'll probably receive howls of outrage but unearthed neutrals are quite common overseas and, logically, it is difficult to see what is wrong with them, especially if, as in this case, they are of short duration.

If you have an unearthed neutral then, on the first live conductor fault, that phase becomes the earthed phase and the other two go to 400V to earth and the neutral sits at 230V to earth, (not nice if you touch it, but you shouldn't be touching it), Within a few minutes your generator comes in, closes a N-E and the faulty circuit trips.

So, realistically, what is the chance of a fault and of someone touching a different live conductor in the few minutes that the UPS is the only source - somewhere pretty close to zero I would suggest.

Argueably an unearthed neutral can be safer as the first fault does not produce a dangerous condition, it just earths the system.

But as a counterpoint, an unearthed floating neutral on a harmonically rich load is going to have a potentially significant offset from true earth  - and the UPS might just decide that the voltage is too high (say 10V) and attempt to go to bypass - which is of course dead in your scenario


So you've not killed anyone but you have lost the critical load  - which perhaps defeats the objective of the UPS


Salient lesson in why 3 pole bad - 4 pole better,  isn't necessarily true.


Regards


OMS

If you wish to allow the system to run with one corner of the 3 phase triangle grounded, so the 2 other lives are at 400V and the neutral at 230V, ever, then all capacitors inside   equipment mains filters need to be able to stand this, generally filters in PC power supplies and similar are designed for 250V AC continuous and rather more for a short duration but it is not good to push your luck.

You also may wish to fuse the neutral.


There is less risk in having multiple NE bonds in some configurations, so long as they can carry the full fault current in any one link, and there is no earth fault relay problem.

It is quite common when say 2 transformers are paralleled that in effect there are 2 NE links. So long as the neutral and earth bonds that connect them are up to the load current it is not dangerous.


Also, be aware that if any of your switching arrangements can ever  leave your transformer secondary unearthed, then in some cases it can become very hazardous -

depending on the transformer internals, capacitive coupling between the HV side and the secondary windings can cause them to float to a significant fraction of the HV phase to ground voltage if not tied down.

Yes looking into this further, it is not a good situation at all. The substation is as said a split substation - there is an automatic bus coupler between the two LV sections. The primary feed comes from an 11kv to 400v transformer, one transformer per side. Its been configured to run with both tansformers closed and the bus coupler open. should there be an outage on the HV side the couplers close and either side of the substation remains powered from the live transformer. 


If the mains is lost totally then all the output circuit breakers on the substation open, the remote generators start up and the HV from the generators is supplied to the substations through the existing network. However when the HV fails the main incomer circuit breakers also open and this removes the neutral earth link as this is located on the transformer, precisely for the reason quoted, should there be no earth reference there then there is a case that the transformer secondary voltages could trouble the insulation of the LV wiring. 

Proposal is to use neutral earth contactors on the substation bus to ensure that the neutral remains earthed when the incoming circuit breakers are open. Also in addition to this a neutral earth contactor is included on the UPS input switchboard so that if the supplies to the UPS input switchboard are opened then the neutral will remain referenced to earth. The supply is designed and operated as a TN-S system so that should be maintained. The site is a hospital and the UPS supply is used to support critical equipment so this needs to be kept as stable as it can be. There is further work required as some other issues have been identified with the system, The UPS supply although has two independent feeds these feeds pass through the UPS input switchboard which will then in effect be a single point of failure. Some way to automatically ensure that there can not be a supply loss will be required. There may be a way of doing this and also looking at use of a transformer to provide galvanic isolation between the UPS and supply so a local earth point could be utilised. Looking at data centers it may be worth adopting what they use for critical data and employ an additional UPS along with the relevant static switchers to ensure no break supplies to the super critical loads.


With UPS units and circuit breakers it becomes a mine field to try and ensure that the supply is maintained and remains within its design parameters, especially when the UPS system is used to fully support the Isolated Power Supply IPS systems on site.

Depending on the design of the UPS, your PSSC may not be quite what it appears - electronically derived generation, rather than true rotating magnets, are not simple sources with a constant source impedance - rather the controls try to prop up the voltage to keep it constant as the  load increases, up to some critical point, usually a single figure multiple of the rated full load, when it all drops out of regulation and the volts then collapse rapidly with any small further increase in load.

The upshot is that any breaker that is not 'small' compared to the UPS rating (say larger a 10A breaker on a 100A UPS supply) is not reliably going to do the instant trip thing, and you cannot really trust a loop tester.

Earth fault relays configured as a set of jumbo RCDs to isolate zones in the event of earth current exceeding a few amps not clearing in a short time can sometimes provide a way out, but as you say it quickly gets very complex.

(and any earth fault relay is only going to do its thing correctly if there is at least one NE link upstream of it, which is where we came in.)

Switched NE links may be necessary, but consider what happens if one does not switch, and how that fault condition is detected.

Surely the simple fix is to reconfigure the tele-control to trip all outgoing ACB's on loss of mains, but leave the switchboard input breakers closed, then reclose the output ACB's once you are stable on the HV generation (there won't be load on the HV as the output ACB's are open)


That solves the shorter term problem  - then all you need to do is confirm the bus section is 4 pole and you can refit the N-E from the TX spill box to the switchboard (one half at a time) and swap out the 4P ACB's for 3P


That may well be a lot easier than retrofitting N-E contactors into an existing switchboard.


It sounds like a pretty normal dual fed system with HV generation - generally well executed, but flawed in your case by the introduction of 4P ACB's on the incommers.


As an aside - what's protecting the transformer unloading tails into the switchboard - or are you close coupled (ie Transformer switchboard)


Two feeds via the UPS bypass switchboard isn't that unusual if the board has a reasonable form of separation - it's not credible to lose both on single fault


I'm guessing this is Healthcare facility so ideally you should have duplicate A and B UPS anyway to feed into the duplicate IPS systems (HTM 06-01 will give you a good indication of what you are trying to achieve).


Like Mike - I would absolutely try to avoid solving this with N-E contactors (or mashing an MCCB to act as same), in favour of solving the N-E bonds at the switchboard and eliminating the 4P devices. Without knowing what ACB's you have, it's not that easy to say, but it's entirely possible you have a bit of minor copper reworking and have a 3P plug straight in (from the same range)


Regards


OMS

OMS - it is indeed a close coupled board - cast resin transformer adjacent to the switchboard and RMU on the site of the transformer. I believe four pole ACB's were selected for isolation, to break the neutral, however this gives rise to the issues that were being discussed earlier - as soon as the load shed program comes in both breakers to the UPs switchboard open and the neutral earth reference is lost. TN-S distribution is what the system is designed around and losing that TN-S isnt good for the equipment. the earth wrt the neutral becomes floating and that gives rise to concerns. Its one for the designers, but it should not be run with that neutral earth link out of circuit.


Also would have concerns about if the neutrals linked together between the substations then there could be issues with the sizing of the cables, fault current from two systems would have to be taken into consideration as the system could end up sharing the neutral earth links.