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Topic Title: dno transformer saturation
Topic Summary: a question
Created On: 10 May 2013 07:49 AM
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 10 May 2013 07:49 AM
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fergyt20

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speaking with a top wpd operative who said that most of their transformers in my area were at saturation and couldnt take any more
export.
So what are the signs and symptoms and adverse effects to western powers network ???
 10 May 2013 08:41 AM
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John Peckham

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Not only your part of the world but London also. In some places if you ask for a modest new supply you are told the starting price is £50K plus to re-enforce their network with a new Tx. That way you pay a big chunk of the investment and the DNO can sell the remainder of the capacity. The problem is the original public asset has been sold several times over and on each turn of the ownership the cash has been extracted out of the business with very little investment. Of course to maintain and improve the network costs money so very little profit left if any and no dividends and huge salaries and bonuses for the board of directors. Add to that making the engineering talent in the business redundant and not investing in technical training and you don't need a PPE from Oxford to predict the future failure of the service.

No doubt sooner or later the DNO owners will be asking the government for massive increases of tarif to carry out a huge program of works to re-new the networks. The government will not say, " f**k off you have screwed out all the money from the businesses over the years and neglected to maintain and improve the asset" big business has the politicians of all colours in their back pockets.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 10 May 2013 09:39 AM
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OMS

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Perhaps the other point to make is the insatiable growth in the demand for energy.

Even back when the supply industry was in the hands of the citizens, network engineers often had significant financial constraint imposed on them (often incorrectly).

The outcome of that was methods and practices of load estimation that would make most people faint with shock at just how low individual allocations were and just how far area boards would allow infrastructure to run in cyclic overlaod conditions.

In a previous life, I've designed sections of networks for estates as an example where we were working to perhaps 1kW per property and also allowing transformers and cables to run to at least 130% overload over relatively short periods - often that was compounded with the concept of seasonal thermal ratings which allowed more capacity based on the season (and hence prevaling air and ground temperatures - link here John to ERA reports on sizing of buried cables).

So those assets were put in place with a then understanding of what the growth profile was likely to be. In reality, over say a 30 year period, we've seen the huge reduction in demand from industrial consumers balancd out by the huge increases in personal demands and a move to higher than anticipated commercial demands (principally cooling demands)

So, whilst not defending the actions of the privatised utility, they were starting from a point of inheritance of systems that were designed for particular sets of circumstances - the reality of some of the earlier assumptions and cost constraints differing (sometimes wildly) from theory.

Take as an example, the growth of live/work units during the last decade - area board network engineers back in the 70's couldn't possibly have predicted that a % of households wouldn't follow the pattern of people leaving for another place to work and that the dwelling would now impose a totally different demand profile on the networks. equally, most peoples circumstances show that we live and work in totally different patterns than people did say 2 or 3 decades back. As an example, I work an average week from perhaps 3 different locations - I impose a fairly steady demand, but the network geographically can't always support that as it was never designed for that personal load to be hopping about - it was expected that I'd impose a demand at home and a different demand at work and they would be in defined locations - the reality is much diferent

So, for sure, many years of under investment have brought the systems to breaking point - but they weren't in Rolls Royce condition to start with - for various reasons.

regards

OMS

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Failure is always an option
 10 May 2013 10:22 AM
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broadgage

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To be pedantic, "saturation" implies magnetic saturation of the transformer core which leads to gross overcurrent and prompt failure unless disconnected by protective devices.

What we are talking about here might be more properly called fully loaded, or overloaded, or the absence of any margin for load growth.

Overloading a cable or transformer (unless grossly) seldom causes any near term problem, but it certainly reduces the reliable lifetime of the equipment and increases the risk of random failures.
 10 May 2013 01:48 PM
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statter

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... or is the word 'export' in the OP the real clue. Perhaps the offending items have large solar PV arrays connected and the inverter output may be the issue?

If the core really is close to magnetic saturation then the HV fuses would probably blow. Saturable reactors used to be used for dimming lights in theatres but its a long time since I've seen one.

Edited: 10 May 2013 at 02:02 PM by statter
 10 May 2013 05:15 PM
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fergyt20

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Thanks for the responses so far.

Should have explained a bit better....

So the problem seems to be to much current being thrown at the system by all the wind generators etc etc.....

So what does this do to the sub station ??
 10 May 2013 07:33 PM
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broadgage

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Originally posted by: fergyt20

Thanks for the responses so far.
Should have explained a bit better....
So the problem seems to be to much current being thrown at the system by all the wind generators etc etc.....

So what does this do to the sub station ??


I doubt that the problem is too much wind energy or solar input except perhaps in rare cases.
The problem is that the network is not being upgraded to allow for load growth.
If a substation transformer is rated at say 500KW, any load up to that figure may be supplied, and a bit more in the short term.
If however the load REGULARLY and SIGNIFICANTLY exceeds 500KW, then the transformer and probably the cables will have a shorter and less reliable life.
The voltage drop in the cables and transformer windings will increase and may be a cause of complaint.

If the load is in excess of the transformer capacity then the addition of domestic scale wind or solar input will help to an extent.
500KW transformer loaded to 600KW is a problem if prolonged.
500KW transformer with 600KW of load and 150KW of wind or solar input is no problem as the transformer now only carries the net load of 450KW.
 10 May 2013 08:15 PM
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statter

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Dont forget that these networks were originally designed to distribute power in one direction. Moving power both ways depending on time of day etc is taking them well beyond their initial design spec. Yes they are generally robust and will tolerate some overload for a period but voltage regulation can suffer.

Many new small generators feed their output into the system through inverters. These have good and bad points. On the plus side they dont add much to network fault levels but they have a significant harmonic output and IMO rely on connection to a reasonable network to maintain harmonics etc within normal tolerances. If you get a lot on one transformer or possibly on one phase I imaging this could cause problems e.g. with voltage rise at times of light demand (I have heard of problems with PFC equipment in this (and harmonics) context).
 11 May 2013 01:13 AM
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alancapon

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It is likely to be as simple as either the distributing main or the substation transformer have reached their capacity. Unfortunately in these scenarios, the next person that needs more power will usually pick up a contribution towards the reinforcement of the network. This will be either additional cables, a larger transformer, or a combination of the two. The issue of embedded generation is a bit of a red herring, as the DNO must be able to supply the entire load without any contribution from the generation. Depending on the network design, there may also be a requirement to retain a percentage of the transformer capacity for backfeeding adjacent substations for fault and / or maintenance activities.

Regards,

Alan.
 11 May 2013 08:31 AM
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psychicwarrior

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yes.....selling off public assets............great idea! not!

as was said earlier, every bit of cash was taken out (profit, bonus, big house, fleet of cares, shareholders etc) and irresponsibly not enough reinvested for the public service it is.

but you watch the spin doctors telling you how much has been invested and its 'our fault' so we have to pay!


ho hum
 11 May 2013 04:59 PM
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CMD

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I would put all the utility companies back under state control like some are in Europe
 11 May 2013 11:15 PM
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ebee

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OMS
Margaret Thatcher would have made you wash your mouth out with soap for that remark!

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 12 May 2013 07:58 AM
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Jaymack

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Originally posted by: ebee
OMS
Margaret Thatcher would have made you wash your mouth out with soap for that remark!

Should have went to Specsavers?

Regards
 12 May 2013 08:26 AM
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ebee

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LOL!



-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 12 May 2013 07:26 PM
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CMD

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Thatcher got it wrong
 12 May 2013 07:36 PM
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OMS

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Originally posted by: CMD

Thatcher got it wrong


I don't think she did actually - it's my belief that it was subsequent policy that generally caused what we now see as problems - to put this into context, it's not often the lights go out to be fair, is it - it's just that we appear to have used up what capacity the older generation of area board engineers put in place

regards

OMS

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Failure is always an option
 12 May 2013 07:44 PM
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statter

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..... And some of the newer generation from the 1990s is also mothballed. Much of the older kit is closing because of LCPD restrictions. At present coal is relatively much cheaper than gas for generation
 12 May 2013 11:50 PM
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Zuiko

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So the problem seems to be to much current being thrown at the system by all the wind generators etc etc.....So what does this do to the sub station ??




Micro-generation back-feeding the distribution network can increase fault levels above its rating.

The previous point about saturation is correct; and it is also correct that many urban LV feeders are running at over 100% rated capacity for significant periods: 315A or 400A fuses protecting a 200A circuit is not uncommon; nor is not uncommon for 11kV/400V secondary substation transformers to be running at 150%.

This does reduce the life of the transformer - it cooks the oil, which in turns breaks down, leading to a feedback loop until the transformer fails. However, transformers are pretty robust and many are very very old indeed, 50+ years old.

Also worth noting that such secondary transformers are sacrificial with very limited protection: it is easier, cheaper and less onerous to allow them to fail, and replace them; than to maintain them.

cheers
W

Edited: 13 May 2013 at 12:00 AM by Zuiko
 14 May 2013 09:49 AM
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joepostle

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This is probably stupid comment / question of the day but the impression I am getting is do the DNOs do very little, if any maintainence (day-to-day or long-term) on any of their lv substations at all? Do they even change the oil? Is there not a general contingency plan to change transformers when they reach a sustained peak load?

My thinking is that this discussion mainly seems to be talking about lv transformers, but at the backbone (ie 400kv/132kv and 132kv/33kv) the same problems must be compounding.
 14 May 2013 02:58 PM
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Zuiko

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All secondary substations are maintained.
Different DNOs may have different maintenance regimes.

Typically:
secondary substations are inspected every six months;
primary substations every three months and prior to cold weather
grid substations every month and prior to cold weather
(super grid, I do not know).


Secondary substation transformers are usually sacrificial. When you consider the sheer number of transformers on the network, it is cheaper, quicker and easier to change them if they fail. Most urban LV feeds can be backfed-and if not generated-whilst the transformer is changed; and it is a rather trivial matter to change a transformer (central London is a nightmare where many are underground).

11kV Switchgear is generally subject to condition monitoring to determine maintenance intervals: this is between 5 years and 20 years depending on the switchgear.

It is much more important to maintan switchgear because the network must be able to be sectioned in case of faults or maintenance.

This will of course mean an oil change (if is is oil-filled gear).

11kV breakers at the primary are maintained every three years; or if oil-filled after the breaker has seen a fault of 3x rated capacity. A typical 11kV breaker is rated at 250MVA and so is maintained when the accumulated fault level approaches 750MVA. Depending on the actual fault levels and the damage the breaker has sustained, this could mean a complete overhaul or just an oil-change.

33kV breakers - same regime but at correspondingly higher fault levels.

You will generally not see any secondary transformers above 1MVA on the distribution network (to keep the magnitude of a single fault as small as possible). Of course, if there are small transformers on the network (some rural pole-mounted TXs are just 50KVA) and if the load is increasing then the transformer will be upgraded; but this is dependent on the network configuration upstream towards the primary - can it take the load? It is not always so easy just to increase the size of the transformer! Cables and OH lines may need to be beefed up, and depending on geography and land-ownership issues, can be very tricky.
IET » Wiring and the regulations » dno transformer saturation

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