Online Transformer Drying Solution

Page 1

The MAIN JOURNAL for POWER GRID SPECIALISTS in RUSSIA

3 - 6 J U N E 2 0 1 9

MADRID, SPAIN

The MAIN JOURNAL for POWER GRID SPECIALISTS in RUSSIA

3 - 6 J U N E 2 0 1 9

MADRID, SPAIN

Page 2

EQU

PME

Online
Transformer Drying Solution

Effect on safety

As shown on the graph besides,

the higher the relative water satura-

tion, the lower the breakdown volt-

age (BDV) of the oil. As water mi-

grates between the solid and liquid insulation in

a transformer with changes in load and, there-

fore, temperature, so does the relative water

saturation in oil. Peaks of relative saturation are

usually observed during transformer changes

of state (high to low temperature or opposite).

Moisture reduction is therefore a key eﬀ ort to be

made to increase safety, especially for Trans-

former with short and frequent load changes

Effect on
transformer
life expectancy

The insulation paper mechanical strength is de-

ﬁ ned by the degree of polimerisation, also called

DP, which represents the average length of cel-

lulose chains in the paper. A new transformer typi-

cally has a DP between 1200 and 1000 while the

end of transformer life is considered when the DP

reaches 200. This degradation cannot be stopped

but its speed will depend on the water content in

paper (see picture on the left).

In CIGRE brochure D1.01.10 (2007) “Fallou

showed that the rate of degradation of the paper

at initial value of 4% water content was 20 times

greater than that at 0.5% water content.”

Moisture is having a huge eﬀ ect on the

speed of paper degradation and therefore on its

life expectancy.

THE ISSUE: Moisture is threatening the transformer

Moisture is one of the major cause of failures for power

transformers and one of the main degradation factor for

the insulation paper. It is therefore increasing the risks

of operation failures and shortening the life expectancy

of the asset.

Unfortunately, moisture can appear in a transform-

er from several external or internal sources and it has

a complex dynamic between the oil and paper inside the

transformer.

The use of silica gel breathers, sealed tanks, or

nitrogen blankets allows engineers to avoid ingress

of all or at least the most part of moisture from the

atmosphere to the transformer. However, when the

transformer is energized, the production of water in-

side the power transformer is a natural and inevitable

occurrence over time due to the depolymerization of

the cellulose paper.

100 110

eak

own v

e (kV

/2.5 mm)

% Relave saturaon (%RS)

Fig. 1. Dependency between breakdown voltage and water
content in insulating liquid*

100

1000

e polymeriz

on speed

Moisture content of insulaon paper (%)

80˚C

100C

120C

Fig. 2. Cellulose depolymerisation speed dependence on mois-
ture content in insulation paper for di

ﬀ

erent temperatures**

* CIGRE Moisture measurement and assessment in transformer
insulation – evaluation of chemical methods and moisture capacitive
sensors, page 10
** CIGRE Moisture measurement and assessment in transformer
insulation – evaluation of chemical methods and moisture capacitive
sensors, page 14

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THE SOLUTION: Maintaining a dry transformer

by continuous

ﬁ

ltration

Maintaining a low level of moisture in a transformer pro-

vides signiﬁ cant beneﬁ ts in terms of operations and risk

as it maintains a constant high insulation level. There-

fore it is possible to load the transformer at a higher level

and to make this load vary without risking to damage the

transformer.

Also it has consequent ﬁ nancial beneﬁ ts since it elon-

gates the life of the asset by slowing down the paper

degradation. This degradation creates particles or even

sludges. Finally moisture is also responsible for creation

of acids in the oil. Hence keeping a low level of moisture

will reduce the maintenance costs.

Oil filtration

LFH or similar method

Online drying system

Solution type

Punctual

Continuous

Transformer live

during process

Up to user risk assessment

(oil ﬂ ow >500l per hour)

YES

Dries oil

YES

Dries paper

YES

Improves Breakdown

voltage

Temporarily (months)

YES

Extend life

expectancy

YES

Dissolved Gasses

level maintained

YES

Operator free process

YES

Only continuous

ﬁ

ltration is ef

ﬁ

cient

to remove moisture from a transformer

As moisture is creat-

ed continuously and

as it is one of the major concern for

the transformer safety and life ex-

pectancy, it seems counter intuitive

to apply a punctual solution for this

continuous problem.

Also it is important to note that

more than 98% of the water in

a transformer is in the paper while

a very low amount is dissolved in

oil. The diﬀ usion time of water from

paper to oil is very slow. This is why

punctual ﬁ ltrations are do not solve

the moisture issue.

200

1200

Years

Life

expectancy

gain

water

extracƟon

ConƟnuous

water

extracƟon

Fig. 3. Bene

ﬁ

t of continuous online drying system

FIELD RESULTS: Installations of TRANSEC

Online Drying System in Saudi Arabia

Since more than 15 years, the TRANSEC Online Drying System technology has been installed on almost

3000 transformers worldwide. We’ll detail in the paragraphs below an installation showing the eﬃ ciency

of this solution.

The 25th CIRED Session
Special issue, June 2019

Page 4

Installation on a transformer

for the Oil & Gas industry in Saudi Arabia

Moi

sture PPM

Temp

eratu

re °C

Temp In

Temp out

PPM In

PPM Out

% Moist Paper

Visit our website: www.streamer-electric.com

EQU

PME

A TRANSEC unit has been installed in No-

vember 2017 on 40 MVA General Electric

3 phases transformer in the Dammam region,

Saudi Arabia.

This 115kV/13.2kV transformer from the

1970s was always used at a relatively low

load which explained that moisture level in

oil never reached an alarming level. But the

level of moisture raised very rapidly in 2015-

2016 when a leakage appeared in the gasket.

In February 2016, the PPM of moisture in oil

reached 54 mg/kg; the relative humidity in

oil was 45%; the estimated water content in

the paper was 7% and the breakdown volt-

age dropped to 22 kV. At this point the trans-

former was immediately stopped and a leak

search had been initiated.

Once the leak had been found and re-

paired, the increase of moisture in the trans-

former stopped but an enormous amount was

already in the oil and paper and had to be

removed in order to operate the transformer

safely.

Using an offline solution like Low Fre-

quency Heating was out of question due to

the risk of paper shrinkage during the remov-

al of such a large volume of water. As a result

the end user decided to use an online drying

device.

Once installed (Figure 1), the TRANSEC

started removing moisture from the oil im-

mediately. It provided a positive effect on the

breakdown voltage value. In a second step

water started to get extracted from the pa-

per as the oil got dried out after a few weeks.

Over the 1st year you can see on the 2 graphs

below the PPM inflow and the water content

dropping. Within that year the water content

in paper went from 5% to about 3% (much

more acceptable level).

We can also see that during the second

year of operation the PPM values of inflow

and outflow were almost matching which is

the sign of water saturation in the Transec

unit. This is confirmed by the water content

increasing again to 4% at the end of the sec-

ond year. The saturated TRANSEC extrac-

tion cylinders have been then replaced by

new ones in order to restart the filtering pro-

cess. They had extracted about 12 litres of

water from that transformer.

To conclude, the TRANSEC device

brought the moisture level of this transformer

back to an acceptable level within one year

and still continues the extraction of moisture.

Breakdown voltage came back to a safe level

and the remaining life expectancy of the as-

set was multiplied by 2.

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Moisture is one of the major cause of failures for power transformers and one of the main degradation factor for the insulation paper. It is therefore increasing the risks of operation failures and shortening the life expectancy of the asset.