MV verus LV generation 2

[maunsell]

Guys
i have had a search on this subject and found some useful stuff, however i need to provide the pros and cons for the following:

4 No. (2MV) LV sets stepped up to 11kv - site load 6MV (LV)N=1
X number of HV sets fed directly onto the MV ring.
and LV embedded generation.

all sets to run in synch, be capable of short-term parallelling and only be used as standby generation. The site is not particulary large, the MV ring approx 2KM. it is a uk healthcare site.

Can anyone direct me to any relevant reports, pointers and will strengthen my case?

Thanks in advance

Mark

 

[dpc]

For a health care facility, there must be some strigent local codes requiring emergency power generation. This may limit your options.

If the primary purpose of the generators is to provide emergency power, you should probably favor low voltage generation, up to some size.

I'm not sure I understand your description. Did you mean to type MW instead of MV?

 

[Bung]

By "paralleling" do you mean in parallel with the grid? If the machines can run in synch, they can run in parallel - it's the same thing. If by paralleling you mean capable of synching with and running in parallel with the grid, your local supply authority will have some requirements that must be met. These may be less stringent for a short time "bumpless" transfer from grid supply to on-site gen supply, but they will still have some.

Bung
Life is non-linear...

 

[maunsell]

Thanks Guys. i do mean a Medium Voltage ring, but the Trust are concerned that the scheme does not reflect any local LV embedded generation. Yes the sets will need to run in parallel with in other and soft start with DNO (only for load test.

moving on the Trust i have since learn are now requiring some dependability study, so as to ascertain the probability of availabilty. This i have some knowledge on in regards to MTTF. However, i have some statics relating to the generators but unsure that these are relevant to standby sets or continusly running sets?

Thanks

 

[sparksski]

Maunsell

You will have to ensure that your system complies with the following HTM documents:

HTM 2007 - Electrical supply & distribution
HTM 2011 - Emergency electrical services
HTM 2020 - Electrical low voltage systems
HTM 2021 - Electrical high voltage systems

There is a lot of info in these documents to guide you. I do not recognise/understand the term "DNO".

Do you have a LV or HV REC incomer at present? What is your current arrangement/proposal - central generation at LV, step up tx to 11kv, 11kv ring distribution, step down 11kv tx to 400V at substation/point of use.

Do the Trust prefer LV or HV distribution? If you can provide any further info I may be able to offer advice/comments.

Regards

Mark

 

[Rodmcm]

I have just installed an 11kV ring with 6 x 2-3MW gensets on an industrial site with a 10MW load. Each genset is on a bus, with buscoulpers ( 2 per switchroom) and cable interties between switchrooms. The site is in the desert and not synched to the grid but the same phiosophy applies. The ring provides the dual redundancy to each bus, we fitted bus zone protection at busses and cable interies. The beauty from the operator/maintenance point of view is they have effectively 3 points of supply to any one bus ( genset, buscoupler, cable intertie). Client is very happy so far.

 

[ScottyUK]

Sparksski,

DNO - Distribution Network Operator. More-or-less interchangeable with REC - Regional Electricity Company in the UK.



------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[maunsell]

Sparksski
yes i am aware of tall the relevant HTM,s.

we have 2No. MV incomers (same location) centralised LV generation is being proposed with step up (to 11kV) to support the MV closed ring. All the generators are to be linked to a synch.Bus.

The problem you noted is that the /trust prefer Local LV generation. The scheme i have described above is what we have offered as an alternative. I am aware of most of the disadvantages, but what about the benefits?

and any improvements that could be added?

Thanks

 

[GerH]

I completed a 4 x 2MW centralised Standby that appears similar to your proposal.

We choose LV gensets stepped up to 20kV because LV 2MW are more standard units. No great problems there.

Big question is how to get this central system up and running on grid failure. Energising distribution transformers is problematic, current transients, loss of sync, risk of tripping. We had a PLC scheme to shed the loads and bring them back in steps.

I think distributed local LV sets would be simpler although it lacks the flexibility of a central plant.

 

[maunsell]

GerH Thanks .
The starting of the standby sets bit is what interests me.i guess we would be looking at step loads and redundant PLC, etc. The Trust are parnoid about loss of supply. Given that its a PFI scheme the client wants the cheapest scheme avaialble therefore we -( the designers are constantly battling)

 

[sparksski]

Maunsell

I am employed in a Trust and about 10 years ago we moved from a 11kv ring (with 2 11KV REC incomers) and local lv generation at each substation to a 11KV ring with 2 central 1MW 11kv gennnys. We removed the existing 415V gennys bar one which was connected into the substation serving most of our wards and theatre areas. The remaining gennys were removed and replaced with connection boxes allowing mobile gennys to be brought in and connected as required for maintenance/shutdown/breakdowns etc. There was a lot of opposition to this from the Trust engineers at the time believing that we had lost supply security and flexibilty. This genny and the connection boxes have been used many times and proved to be worthwhile modifications.

We have found that staff are still complicated with the old system, we have recently had Tx/breakers trip out resulting in a localised blackout. The staff can not understand why the genny did not start like it used to on the old system. You can not get through that the genny will now only start after 2 REC incomers are lost, and the whole site is black!

We have recently had the original 2 gennys upgraded as well as an additional genny fitted as part of a new PFI build. This work involved downing the site gennys for 6 weeks and replacing them with 2 temp mobile gennys and 1 x 2MVA tx. This was manned 24/7 for 6 weeks, which was easier than the proposal for manning 1 LV genny at each of the substations. 1 benefit of the upgraded controls and new genny is that the site supplies are restored after a power failure/test run within 15 secs now as opposed to approx 45 sec previously worst case. Sounds bad but the depts concerned are used to it now and deal with any issues, you suggest those kind of delays to hospitals that NEVER test thier gennys and they dont know how they could cope. At least our contingencys are tested monthly.

Our gennys have always started together/parallelled to a common bus and then the site feeder breakers have closed systematically to restore supplies in a set order. We then reset load shedded plant. We have experienced start problems and failures on test runs, but this has been control glitches rather than big hit full load problems. Those glitches have been resolved for a long time now.

We have always had Trust HV AP's and I assume with an existing 11KV ring this Trust has their own AP's so this should not be a barrier. I know some Trusts employ the local REC to carry out their switching. LV generation may offer more flexibilty but potentially with a site blackout there is more to fail with numerous gennys.

What about the option of central HV generation, HV distribution, rather than the LV generation, HV transformation, HV distribution. I appreciate there are only a handful of trusts with HV generation/HV distribution but many with HV distribution and LV generation.

Not a technical response I agree but there may be something of interest for you there.

Regards

Sparksski

 

[maunsell]

Sparksski

Many thanks

i also work in healthcare arena, on PFI bids. You could prove to be a very good ally