1. Electrical Oil Services
Life of A Transformer Seminar
Dublin 2016 1 1

2. On-site Options for Transformer Insulation Management
History
Pressures on utilities and generators
Looking after your paper insulation (water & acidity)
Other parameters
Take action – or not?
In-situ treatment
General welcome and overview of presentation

3. Standards referred to… BS148:1932 – 2009 – today known as “Reclaimed mineral insulating oil for transformers and switchgear – Specification” IEC60296:2012 – Unused mineral insulating oils for transformers and switchgear BS5730 / IEC60422 – Mineral insulating oils in electrical equipment – Supervision and maintenance guide
During the course of this presentation, with regards insulating oil supply and management I will refer to these BS and IEC standards, copies of which are available to look at if you wish, but don’t run off with them.

4. Electrical Oil Services Some History4 4

5. A Little History Before Nationalisation 1926 Central Electricity Board
1947 British Electricity Authority
1954 Central Electricity Authority
1957 Central Electricity
Generating Board
2000 Privatisation
When you have been in this industry for a while, in my case 30 years, you gain the benefits of being able to look back on historical events and put what’s happening today into some context. Now I’m not old enough to remember the Central Electricity Board, the British Electricity Authority but I was employed as a Student Engineer with the Central Electricity Generating Board then as a so called “third engineer” in their transmission division before spending the last 26 years in the electrical oil business but very much working in the UK electricity supply industry.
Historical perspective can be useful when trying to understand how we got to where we are today.
In the early years of the 20th Century in the UK each town or City had one or more local powerstations and distribution networks supplying the local populations, supply voltages and frequencies would be different and there was no large scale transmission system operating at 275kV and 400kV as we have today.
The CEB became the British Electricity Authority then, briefly, the Central Electricity Authority up until 1957 with the Nationalisation of the whole UK ESI into what became the Central Electricity Generating Board.
The CEGB had one reason for being – keeping the lights on or security of supply at pretty much any cost.
Equipment purchased by the CEGB, from rotating plant to distribution switchgear was, in the main British made, employing British workers. Not only was it good for UK jobs and the UK economy (with all those taxpayers) the equipment purchased was, largely designed before CAD systems and using high quality materials and in the case of transformers lots of oil, large clearances and so called margins of safety. Some may say the CEGB were taken advantage of with “over engineered” equipment others would say high engineering standards such as BS171 and BEBS T2 transformer specifications ensured we had sturdy equipment, built to last well over 50 years with well proven designs and standardisation of parts.
In 2000 the CEGB was privatised into a dozen separate Distribution Companies or D.I.S.C.O’s, a separate National Grid Company and numerous Generating Companies.
With privatisation came the rise of the professional purchasing officer who had an obligation to open up the transformer market to world wide competition. UK standards BS171 and BEBSUK were deemed “too high” a barrier to entry for European OEMs to compete to they were discarded and replaced by BS EN in the early 1990s. This was a move that was widely seen as a retrograde step for the UK esi but meant of course that the newly powerful procurement specialists could look overseas for alternative sources of transformers.
Around this time the CEGB “encouraged” lots of experienced and hands on design and operational engineers out of the industry, many were spotted with wheel barrows piled high with money running away as fast as possible never to be seen again – unless they were re-employed as contractors, doing the same work as before on twice the salary.
This “dumbing down” of UK transformer standards together with a significant brain drain led, in my view, to a perfect storm for poorly designed transformers, with cheap construction materials, supplied to customers on a functional basis where perhaps the “function” was not always fully understood.

6. In 1941 “Unused Oil” could be delivered with a TAN
Of 0.2mgKOH/g (measure of acidity neutralisation)
Today the limit is just 0.01mgKOH/g (IEC60296:2012)
Continuing on the historical theme but now focusing on insulating oil, I looked back at my 1941 copy of the J&P transformer book and was reminded that insulating oil , as supplied then would come with an ACID NUMBER of 0.2mgKOH/g compared to the limit of 0.01 we see today

7. A transformer purchaser could specify either “Class A”
Or “Class B” insulating oil, depending on how much sludge
He was prepared to put up with
In addition you could purchase either Class A or Class B oil with class B tending to deposit more sludge into your transformer than class B. There are still some 132kV transformers operating in London from the early 50s and of course they were built with reference to BS171 which in 1941 was called “Electrical Performance of Transformers for Power and Lighting” and cost 3s and 6d if you are interested or 17 ½ pence. It’s a testament to British engineering and the thoughtful nature of the national grid design that such transformers are still in service today.

8. Electrical Oil Services Pressure on Utilities and Generators
This brings us to where we are today – and with that historical perspective in mind
lets move on to consider some of the pressures facing utilities and power stations
when it comes to transformer (and we will focus on transformers) transformer life extension 8 8

9. Pressures on utilities (Distribution Network Operators)
Privatisation of the CEGB
Harmony with Europe
Reduced build quality
Professional Purchasing
OFGEM Regulatory review
Extend asset life
Spending restrictions
Value for consumer
Here we can see a map of the UK and Ireland overlaid with the geographical responsibilities of the Distribution Network Operators, many of the DNOs have merged with their neighbours over the years since privatisation leading to 14 separate regulated areas within the six DNOs.
We have already considered some of the factors that have influenced transformer build quality following the privatisation of the CEGB but the other significant influence comes from the Government regularory body OFGEM. The Office for Gas and Electricity Markets regulate the electricity industry’s marketplace in 5-8 year cycles. The current one began in 2015 and runs till As well as limiting the prices end users have to pay for electricity OFGEM has to approve the spending plans of each DNO according to some strict guidelines. The emphasis for this regulatory cycle, catchily known ad RIO-ED1 (Revenue = Incentives + Innovations + Outputs) is life extension of assets rather than straight replacement of old for new.

10. Pressures on Generators
Privatisation of the CEGB
Harmony with Europe
Reduced build quality
Professional Purchasing
“Dash for Gas”
15 year life of a transformer
Base load – 2 shifting
Corrosive sulphur
As for the Power stations or generators when it comes to asset management of transformers they too have had similar pressures to deal with as the utilities following privatisation. The other and possibly more significant factor, is the so-called “dash for gas”. In the CEGB days gas fired power stations existed but mainly to provide a source of generation in “black start” situations. From a strategic energy supply gas was considered far too rare and valuable to burn, wholesale in a power station.
During the early 1990s with the coal industry in the UK under attack from political and environmental quarters and with the deregulated energy market, the cheap imports of coal from neighbours such as Australia a quick win had to be found and it was realised you could have a CCGT gas station up and running within a few years, gas was no longer rare and valuable but plentiful and cheap and these relatively efficient stations could fill the gaps left by the closure of many smaller coal fired stations as they closed up and down the country. At the time I had many conversations with transformer OEMs and gas station engineers who openly stated that these stations will run for around 15 years as a “stop gap” and build quality of transformers and other kit was not, shall we say, high priority. In many cases transformers were specified for base load or continuous operation and had no forced cooling as a result. If a transformer is to stay on load for days or weeks at a time then there is ample opportunity for the natural cyclic cooling to get going and keep the operating temperature under control. If however the regulated market conditions changed and these power stations had to follow a two shifting pattern with load generation coming on and off twice a day what was needed to efficiently cool the transformers was pumps and fans and these were missing, hot spots from local overheating occurred and so have some failures.

11. Electrical Oil Services Looking after your Paper Insulation (water and acidity)
With OFGEM looking to DNOs to extend transformer life by years and power stations needing to keep going just that little longer there is now a much greater focus on transformer asset management .
How long a transformer will last in service depends on numerous variables, some have already been discussed but ultimately once the paper insulation surrounding the copper conductors has lost all its mechanical strength then a transformers days are numbered. So lets move on to consider some of the factors affecting how quickly paper insulation deteriorates – well begin with water and acidity. 11 11

12. Looking after your paper insulation (Water)
Cellulose loves water
Insulation is dried in the factory
Site drying is costly and lengthy
Don’t let your transformer get wet
Now let me make something clear – transformer insulation systems are dried in the OEMs factory, not on site. There are things you can do to lower water in paper once a transformer is installed but it is far better to keep the transformer dry in the first place.

13. How much water is in a transformer?
145MVA, 132kV HSPT GSU Tx
Oil volume 34,704 litres
Water content 23ppm at 50ºC
Weight of oil = 0.87x34,704 litres
= 30,019kg (paper = 3,019kg)
Water in oil = x 23ppm/10⁶
Or ……0.8 litres in the oil
Assuming equilibrium conditions
This calculation is based on a 132kV HSPT transformer (HSPT is one of the factories that closed in the early 1990s to make way for Australian, German, Austrian, Portuguese Korean and Slovenian units – all of which could be delivered and installed cheaper and quicker than a unit from the factory in Walthamstow – apparaently
I wont work through the maths in detail as its in the paper but suffice to say we are looking at less than a litre of water in solution in nearly 35,000 litres of oil

14. These graphs should always be used with a degree of caution as they assume sufficient time has occurred to allow the water in the oil and the water in the paper to arrive at a suitable equilibrium point – its to do with the partial pressures of water in paper and oil but im sure some of you are out there can explain it to me – later – so assuming, and it is often not a given – assuming you have a representative oil sample and the transformer has been operating under steady temperature conditions for a while – maybe days at least – these curves can be misleading at best.
But for the point of this exercise a water content of 23ppm at 50 degrees equates to a paper water content of 2.3%

15. From a typical paper/oil equilibrium chart
23ppm at 50°C equates to 2.3% water in paper
Assuming weight of paper = 10% weight of oil
Then
Water in paper = 3,019kg x 2.3% = 69.4 litres
That’s over three 25 litre cans of water!
Or just over three 205 litre drums of water.
Speaking as someone who has been on site after an HV contractor inadvertently pumped around 1,000 litres of water into a conservator, and knowing how much damage and cost that incurred the fact that the paper can hold that amount of water is remarkable.
It may not be good for the long term health of the transformer but it is a transformer that is still operable.

16. Install some kind of molecular sieve Attention to breathers Fix leaks
Dry air when oil is out
Don’t leave covers off
So the message is keep your transformers dry and don’t let them get wet in the first place.
If oil is escaping then I guarantee water can get in
If covers are left open and paper exposed I guarantee water will get in
If breathers are neglected then, over time water will get in
New transformers are dried to 0.5% in the factory

17. This is a useful curve from the CEGB days and gives good advice to this day on what is an acceptable water in oil for particular voltage transformers, again it is only applicable in situations where the transformer has been operating under steady state conditions for some time.

18. Looking after your paper insulation (Acidity)
No such thing as an UNINHIBITED OIL
Oxidation of oil leads to acid formation
Acid attacks paper
Transformer life is governed by the paper strength - maybe
Permanent damage begins around 0.1mgKOH/g
We will of course move on to options for treating transformers with a “high” water content but for now lets take a quick look at the effects of acidity in oil on the paper insulation and ultimately the transformer life.
Can I start by saying you could put cooking oil in your transformer providing it was clean and dry. It wouldn’t last very long though, you may be familiar with the discolouration of oil in a deep fryer after you have cooked your French fries (see how European I am) – well this is what would happen to cooking oil if you put it into your transformer, no matter how clean and dry it was. The one property that makes an oil an excellent oil for transformers is its ability to withstand heat in the presence of oxygen in other words it must have a good oxidation stability. A transformer oil only has this when it contains some kind of inhibitor – natural or artificial, it doesn’t matter.
When the oil’s inhibitors are depleted the oil begins to oxidise and early stage acids are formed which if left unchecked will attack the paper insulation and weaken it.

19. It is generally accepted, and reflected in British and European standards, that permanent paper damage begins when acidity in oil reaches 0.1mgKOH/g (remember in the 1940s it was supplied with an acidity of 0.2)
A transformers lifespan will ultimately depend on the health of its paper insulation

20. Electrical Oil Services Other Parameters
So the message so far is keep water and acidity to manageable levels and for those critical transformers take action early rather than late.
But what about those other oil properties we see on a typical oil test report these days – acidity and water most people understand 20 20

21. Dielectric Dissipation Factor (DDF)
And Other Parameters?
Dielectric Dissipation Factor (DDF)
Resistivity
Interfacial Tension
Breakdown Voltage
Here are some others that are commonly tested for these days – breakdown voltage we can put with acid and water measurements and is generally well understood.
DDF, Resistivity and Interfacial Tension were never routine tests in older versions of the British Standard or 5730 but they are listed in todays oil maintenance standards.
“poor” values of DDF and IFT in particular will indicate the presence of dissolved polar contaminants in the oil, likely as a result of advanced oil oxidation. They can be useful on-site tests for determining how effective you are being nduring in-situ oil regeneration processes.

22. Electrical Oil Services Take Action – or Not
So as a transformer owner, confronted with a set of oil test results what do you do? 22 22

23. What next? What do these oil test results mean? Should I do anything?

24. Importance of the transformer?
Other DGA trends?
Availability of spares?
Should I do anything?
Remaining paper “life”?
Lead time for replacement?
Whether you take action or not depends to a large degree on engineering decisions linked to operational and commercial ones
How important is the transformer amongst your population?
How much life is there left in the paper?
If it fails what is the lead time for a replacement?
Are just a few of the questions
Opportunity to do other work?

25. BSEN 60422:2013 Property Voltage (kV) Good Fair Poor Acidity (mgKOH/g)
400/275
<0.1
0.1 – 0.15
>0.15
132
0.1 – 0.2
>0.2
Water (ppm 20ºC)
<5
5 – 10
>10
5 – 15
>15
DDF
0.1 – 0.5
>0.5
IFT (mM/m)
All
>28
22 – 28
<22
The Bible we would refer to in the UK is BSEN60422:2013 titled Mineral insulating oils in electrical equipment – Supervision and maintenance guide.
As well as giving good background info on insulating oil properties for in service oil there are table giving us recommended action limits for each property, they are divided into good, fair and poor. In general some action is recommended once the test results fall into the “fair” range.
This is in stark contrast to the 1979 version where an acidity of 0.5mgKOH was given as the trigger to take action – by which time the transformer paper insulation would be extremely weak.

26. You could decide to take no action
At the end of all this navel gazing you could of course decide to take no action other than perhaps further monitoring. So much depends on your health assessment exercise touched on earlier.

27. Electrical Oil Services In-Situ Treatment
Let’s assume that your important transformer is in need of some treatment for water or acidity for example 27 27

28. BSEN 60422:2013 Property Voltage (kV) Good Fair Poor Acidity (mgKOH/g)
400/275
<0.1
0.1 – 0.15
>0.15
132
0.1 – 0.2
>0.2
Water (ppm 20ºC)
<5
5 – 10
>10
5 – 15
>15
DDF
0.1 – 0.5
>0.5
IFT (mM/m)
All
>28
22 – 28
<22
Remember a good time to take action is when the oil tests in the fair range of BSEN60422

29. In all likelihood if you transformer is “wet” then it probably is oxidised with a rising acid level.
How wet is wet is often a difficult one to call as stated earlier , assessment from oil test results is not always easy.
You can have a crack at drying a “wet” transformer with on site mobile plant. “straight” processing using on-site vacuum reconditioning will dry the oil, then if you continue to add heat, some water will be removed from the outer layers of paper. You wont get to the wet paper that will be close to the windings. With such treatment you will have to go away from site, wait a few months for those equilibrium conditions to re establish then return and have another go. This may need to be repeated several times.
If your transformer tank can withstand a full vacuum, say <1mbar pressure then a heating vacuum cycle can be used as shown in this graph.
On site Low frequency drying is very effective but also requires the oil to be removed and the tank needs to be able to withstand a full vacuum.
Colleagues, its much better to not let your transformer get wet in the first place – perhaps fitting a molecular sieve type device from day 1?
A“wet”
transformer?

30. Oil Change
For acidity?
With rising acidity an oil change was always recommended, normal oil reconditioning won’t of course, remove acids and other oil degradation products.
The process is labour and time intensive and will not remove all the oxidised oil from the paper. With a great effort and much planning you may remove 95-96% of the old oil.
Any residual acid oil will leech out of the paper insulation and act as catalysts for further degredation.

31. In-Situ Regeneration Treats the insulation system
“Deep Cleaning” effect
Removes oxidation products
Tool for helping extend life
No outage necessary
Less stress on paper insulation
DBDS removed
Inhibitor added at the end
The modern technique for treating transformers in-situ is oil regeneration, either carried out with the transformer live and / or carrying load or de energised given suitable conditions. Advantages over oil replacement are clear
Given a thorough treatment that is well planned, with experienced, professional operators this process will get to the parts of the insulation system that a straight oil change just wont get to. There is less stress on the paper insulation as the oil does not have to come out of the transformer.
Other contaminants such as DBDS, introduced into oil to serve as oxidation inhibitors can be removed – and all without either any outages or perhaps short ones at the beginning and the end

32. Site Operation (On-load)
Primary degassed loop
Recondition the oil
Admit oil to adsorbtion beds
Reactivate beds
Repeat till finish
Add inhibitor
A typical regeneration operation is in two stages – firstly traditional vacuum / filtration or reconditioning to lower water levels in the oil and help get transformer temperature up then, usually on day 2 the oil is admitted to the regeneration beds that are usually packed with fullers earth or bauxite – this material attracts oradsorbs polar contaminants in the oil
Eventually, later on in the day the columns become saturated and are reactivated using heat and vacuum. Any burnt oil should always be captured in a secure storage tank.
When the regeneration columns have cooled to ambient temperature the process begins again.
In between regeneration treatment, during column reactivation, the transformer is continually treated using just the reconditioning plant to help lower water and keep the transformer temperature up.

33. A typical live working arrangement is shown here

34. 132kV DNO Transformer (OFGEM)
90MVA
Installed 1963
38,710 Litres of oil
DP of paper 500
Acidity 0.28mgKOH/g
IFT of 13
DDF 0.063mN/m
No history of faults
DGA “normal” for age
Here are a few examples of transformers regenerated for UK DNOs as part of their OFGEM transformer life extension plans
All these examples have starting acidity levels that are a bit on the high side for maximum “bang for your bucks” but given their chronological age they are good candidates for treatment and not ready for the scrap heap just yet.
Again a testament to solid British Engineering of the past

38. This transformer and the next example will be sampled and tested after a year back in service

39. 132kV DNO Transformer (OFGEM)
60MVA
Installed 1959
40,915 Litres of oil
DP of paper 523
Acidity 0.27mgKOH/g
IFT 13
DDF 0.072
No history of faults
DGA “normal” for age

44. 66kV Steel Works Transformer
10MVA
Installed 1959
17,000 Litres of oil
DP of paper 420
Acidity 0.24mgKOH/g
IFT 22
DDF 0.23
No history of faults
DGA “normal” for age
This transformer, in a UK steelworks site is a bit smaller than the others but along with its sisters, critical to the safe operation of the site.
DP of paper is still OK and acidity trends are rising

48. That’s a nod to my childhood and too many Saturday mornings watching cartoons at the Regent Cinema in Great Yarmouth.
So what conclusions can we draw?
Its clear that the days of so-called “over engineered” (I prefer well built) transformers are over. There are many still out there on the UK network and indeed many that would have been scrapped because of their age are now recognised as having value and worth persevering with. It’s a shame that the early privatised ESI in the UK didn’t recognise the similarities with experienced, practical engineers in the early 1990s when so many were let go with their wheelbarrows full of cash.
Today’s transformers are not built to common BS171 / BEBST2 standards but to functional specifications and often, still at lowest cost. It is therefore inevitable that the transformer asset manager will and is looking much closer at solid and liquid insulation management in a way that perhaps wasn’t so necessary 40 years ago.
There is a clear direction of travel within the UK DNOs to make plant last longer and not put on the scrap heap just because it is chronologically old. OFGEM recognise the benefits of in-situ regeneration and accredit it as a tool to help improve a transformers “Health Index”.
But there is much that today’s transformer asset manager can do in addition to treating the oil early – reduce overall operating temperature, ensure leaks are fixed straight away, fit molecular sieves early on after installation to keep paper dry, move to inhibited oil.
The choice is of course yours
That really is all folks

49. Electrical Oil Services
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