cable-tech pft.pdf

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Conference on Power Cables CABLE TECH 2010 25 & 26 Nov. 2010

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AN INNOVATIVE METHOD FOR FINDING LEAKS IN OIL FILLEDHIGH VOLTAGE CABLES

R.H. Goodwin BSc Pr. Eng. SAIEES.A.I.E.E., H.V. Test (Pty) Ltd, P.O. Box 651287, Benmore, 2010, South Africa.

Email Address: [email protected]

Abstract

Oil filled cables still form a major part of the high voltage (>33kV) cable network inmany countries. In Britain some of the cables were installed in 1933 and are stilloperating 77 years later. These cables are extremely reliable until they are damagedand start leaking. Damage to the underground water is a major problem and anenvironmental nightmare.

Up until 2008, there were very few methods for locating these leaks. The traditional

D.C. drop method which is not very successful because the oil insulates thesurrounding leaking area. The most commonly used method is the FreezingMethod using liquid nitrogen to isolate the various sections. In 1997 Con Edisoncommissioned Brookhaven Laboratories to develop what has become known as thePFT Tracer Location Method.

This paper will discuss the merits of this very innovative tracer method and theexperience gained in the South African Transmission system.

Introduction

Oil filled cables even to this day form the main backbone of transmission cablenetworks for a large number of major utilities throughout the world. The voltagesvary from 33kV to 275kV 3 and single phase. The U.K. have oil filled cables (fluidfilled cables FF cables) which were installed in 1933 and are still operating. In theU.K. it is estimated that there is 4,000 km of oil filled cables. In South Africa thereare 200 circuits varying in length from 500m to 10 km with an estimated total lengthof 600km with voltages from 33 to 132kV. The move away from FF cables is mainlydue to cost and environmental issues.

These FF cables are extremely reliable and, provided the oil pressure is monitoredand maintained, they give very little trouble. Unlike oil impregnated paper cables, oilfilled cables are basically a pipe filled with oil with the paper insulated cores lying inthe pipe. The oil is free to migrate down the pipe and should there be any damageto the outer pipe, this oil will leak out causing a rapid drop in pressure.

FF cables are protected with pressure alarm contacts which then result in the cablebeing disconnected or if the pressure is maintained, by the site head tanks, thepressure will be maintained while the oil leak is located. There have been a numberof leak locating methods introduced over the years but the latest innovative PFTTracer is the most effective and can be done live. All the other methods require thecable to be taken out of circuit.


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Oil Filled Cables

These cables are commonly called Oil Filled Cables but also known as fluid filledcables where the conductors, 3 or single phase, are insulated with insulating papertypes which are impregnated and lying in insulating oil. Unlike the lower voltage

(MV) cables, these cables are held under pressure using Header Tanks. There aretwo types of FF cables.(i) High Pressure Type up to 9 bars and mainly used on the EHV Cables.(ii) Low Pressure Type up to 4 bars.

In the H.P. cables the oil is circulated continuously between two cables eg twoseparate feeders whereas the low pressure FF cables have static header tanks.Typical L.P. cables are shown in Figure 1. The fluid is used for cooling andinsulating. Monitoring of the oil pressure is critical with alarms and tripping functions.These cables are protected using lead or aluminium sheaths, which, in additionprovide an excellent earth fault path. This sheath is then protected by an outer

insulation layer for protection against damage whilst in operation and duringinstallation. Unfortunately, these cables are very expensive to manufacture andthere are very few suppliers left in the world. XLPE cables have superseded theseFF cable

Operating Life Span

In London, there are FF cables that wereinstalled in 1933 and are still operatingtoday 77 years one wonders if thenew XLPE cables will ever match thisoperating life span? What with theproblems with water trees, EHV XLPEcables, operating at 132kV, have a radialelectrical stress of 3kV/mm. With thesestress levels we simply cannot expect thesame life span from these polymeric

cables. Some of the older FF cables have a lead sheath which is perfectly co-entric with an oil pipe for cooling in the middle of the conductor. Quite amazing andto think it was manufactured in 1930.

Cable Oil Leaks

FF cables are extremely reliable until they start leaking. The location of these leakshas been a major problem because of:(i) The environmental damage.(ii) Difficulty in locating these leaks.

Unlike an XLPE cable, FF cable can carry on operating unless the leak is soextensive that the pressure cannot be maintained. Normally, however, these leaks

start small and increase with time.

Figure 1


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Provided the pressure is maintained by the header tanks, and or additional mobilebackup tanks, the cable can continue operating. In the U.K., the utilities have mobiletank trailers which are parked at the header tanks, in order to maintain the pressureparticularly when the continuity of supply is critical. The hydraulic systems areprotected with pressure alarms and trip signals in order to protect the cable and

personnel. A high point termination which has a sudden loss of oil and the alarmand trips do not operate, becomes a bomb scattering porcelain shrapneleverywhere and resulting in a fire.

Location of these oil leaks has traditionally been done using the Freezing Methodwhere liquid nitrogen is used to freeze the cable oil at various locations and bydetermining on which side the pressure continues to drop. The process The Salamimethod is continued until the leak is discovered. Depending on the leak, thelocation method could take up to 10 freezes. During these freezes the cable is off-line and losing revenue. Each freeze could cost up to US$20,000 per freeze takinginto account the excavation work, cost of freezing, the loss of revenue and damage.

Unfortunately this freezing damages the outer insulating jacket (freezing cracks)which then results in water ingress and corrosion of the metal sheath which in timeproduces further leaks. The other disadvantage of this method is the possibledamage that is done to adjacent cables during excavation. Some leaks can take upto a year to locate with this freezing method because of the low leak rate.

The Freezing Method has the following disadvantages;(i) Damage to the outer jacket due to freezing.(ii) Damage to the cable and adjacent cables during excavation.(iii) Pinpointing a leak is not possible.(iv) Damage to other services during excavation.(v) Due to the large number of freezes, damage to roads and other surfaces

could be prohibitive eg in the middle of London.(vi) Cost of freezing cost of liquid nitrogen and logistics tanker etc.(vii) Extended time for leak location.(viii) Greater environmental damage.(ix) Loss of Revenue.

D.C. Voltage Drop Method

The D.C. Voltage Drop Method is a well known method for locating outer jacket

faults on cables. Here a D.C. Voltage is applied to the metallic sheath at the oneend whilst the other lead is connected to a mother earth in the form of an earthspike. The principle is that the current will flow down the metallic sheath to the pointwhere the outer jacket is damaged and then flow back to the DC Test Instrumentearth spike (see attached sketch). The test voltage can be increased up to 5kV.

Pin Pointing earth spikes are then used in conjunction with a galvometer to locatethe fault. The basic assumption is that the damage to the outer jacket is where thecable leak is. Leaking FF cables unfortunately insulate the soil in the vicinity of theoil leak. A further aggravating factor is that the higher the D.C. voltage is taken, thehigher the burning current and the larger the leak becomes it is self defeating. As

the area is saturated with oil, the D.C. leakage decreases making the methodimpossible.


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The PFT Tracing Leak Location:

In 1988, ConEdition approached EPRI and Brookhaven National Laboratories todevelop a Leak Detector for their FF cables. ConEd have the High Pressure (14 bar)Fluid Filled Cables (HPFF) and the Low Pressure Fluid Filled Cables (LPFF) (4-6bar). They still form a major part of their transmission system (Ref 1.). The HPFFcables normally are 2 pipe circuits where the oil is continuously circulating between

the two pipes providing both cooling and insulation. The oil pressure is continuouslymonitored by a PC. FF cables develop leaks due to:

(i) Contractor excavation damage.(ii) Corrosion.(iii) Outer jacket damage by freezing or third party.(iv) Stray currents due to D.C. traction.(v) Mechanical damage from poor laying techniques.(vi) Road works vibration and compression.

Where the cable pipe (HPFF) or cable sheath is compromised a leak starts and inthe case of the HPFF cables, a large volume is discharged in a very short time. Withthe strict Environmental Laws in the USA, ConEdison were desperate to find a newmethod. Brookhaven National Laboratories (through EPRI) carried out extensiveand very detailed tests. (See Ref. 1). The article describes the development of thismethod as far back as January, 1999. Unfortunately, this method was dedicated toConEd (and EPRI) and it was only in 2006, that a couple of ex ConEdisionemployees started to promote this method outside of New York. Since then anumber of players have entered the market providing a service in the USA andrecently providing the equipment into the world-wide market.

Perfluorocarbon Tracer (so-called PFT)

PFTs are Perfluorocarbon Tracers, which were developed by The TracerTechnology Centre of Brookhaven National Laboratories in New York. These PFTshave unique properties of being liquid at room temperature, totally non-toxic andthermally inert.

Perfluorocarbon Tracers (PFTs) have been widely used in air movement studies (3)going back to 1979.

This air movement research is important to determine where for instance airborne

contaminants such as a nuclear radiation spillage (eg Chernobyl) or nerve gas

Figure 2


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(Japan) may migrate to. PFT has been used in the tracing of Contraband Money (5) and has even been used for medical purposes.PFT is man made and does not exist as a natural gas in the atmosphere. It wasestimated in 2009 that there was 500mt of this gas in the atmosphere whichrepresents by volume 8ppq that is 8 x 10-15(4). Any detector must have the ability to

detect this background concentration level. So the detection of small amounts of thisgas in a specific area is absolute necessity.

PFT has the following advantages:(i) In these very small quantities (10-100ppm) that are introduced in the cable as

a tracer, the insulation properties of the oil are not compromised.(ii) The gas is very stable and inert.(iii) Unlike SF6, PFTs do not deplete the Ozone layer (4).(iv) PFTs do not oxidize in the atmosphere (4).(v) PFTs have very low boiling points and as such is an ideal tracer for oil filled

cables. (Whilst under pressure in the cable, the PFT remains in solution

once it is released via the leak (atmospheric pressure) the PFT becomes agas easily permeating the soil into the atmosphere).

Oil Filled Cable Hydraulics

The LPFF cables can withstand pressures up to 8 bar but generally operate at 4.5bar. In order to keep these cables under pressure and to have reserve oil (in theevent of a leak) a cable run will consist of a number of hydraulic sections withreservoirs on both ends. The geographical profile of the cable route will inevitablyresult in one end elevated above the other. Whilst one end may have a pressure of4.5 bar the elevated far end may be at 1.5 bar. Obviously it is critical to keep thiscable under pressure at all times and these cables are protected by alarm and tripsignal coms (radio or hard wired). Should the elevated far end suddenly losepressure due to a leak, the cable must immediately be isolated in order to avoid atermination failure.

Great care should be taken during tagging and pressurising of the cable, for this veryreason. These cables are filled and pressurized using a special Degassifying Oilvan. Some of the older oil vans have been operating for over 50 years these vanswere based on heating or vacuuming cycles in order to remove any moisture anddissolved gases. The latest oil vans are PLC controlled and based on high vacuum

(0.17mBar) (2). The oil is treated in batches and the oil is only released to the cablewhen the moisture and dissolved gases are to specification.

Strict control and monitoring of the oil pumped into the cables is recorded by the PLCand then by GSM and the details are transferred to a Central Recording Station forenvironmental control. The tagging equipment is fitted to these oil vans.

Cable Oil Leaks

In HPFF cables with pressures of more than 14 bar, oil leaks of thousands of litrescan occur in a couple of hours. The potential environmental damage is of major

concern. Utilities have to find these leaks fast in order to limit the environmentaldamage. Not only has the leak to be located in a certain time but all the


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contaminated soil has to be removed and destroyed under strict environmentalconditions.LPFF cables have the advantage of causing less environmental damage because ofthe low operating pressure. Unfortunately this has a negative effect, in that theseleaks are more difficult to locate leaks of 30-50 litres per month are common and

with nitrogen freezing, locating these leaks is impossible and most utilities wait forthe flow to increase before leak location is even attempted. In HPFF cables thetracer concentration can be reduced to a minimum because of the volume of oil thatis released at the leak. In LPFF cables generally the tracer concentration is muchhigher especially if leaks of 1-2 litres per day are to be located.

The density of the soil/road or concrete covering the cable will prevent or delay thePFT gas. Tarred roads can make PFT location difficult. Sea sand on the other handmakes location very easy. Cable ducts, pipes and communication cable ducts canresult in false leak location.

Tracer Tagging of the FF Cable

There are a family of PFT tracers (4) that are available for cable oil tagging. It isimportant to carry out compatibility tests in order to make sure that the PFT will notaffect the insulating and cooling properties of the specific cable oil that is being used.

Extensive tests were conducted by EDF, U.K. on the compatibility of a particular PFTgroup. Their investigation concluded that PFT concentration up to 100ppm wouldnot be detrimental to the cable insulation in these LPFF cables (7). They haveembarked on an extensive operation using 5 Degassifying vans to tag all theircables.

In South Africa, the main utility, ESKOM have invested in the same technology andare busy tagging all of the LPFF cables in the Johannesburg area. JohannesburgsCity Power have also embarked on this technology.

In South Africa, there are 250 LPFF cable circuits with lengths varying from 0.5 to 10kms in length. These FF cables have an oil capacity of between 2-5 litres per metre.Because of the structure of the cable it is estimated that 65% of the oil is mobile.The other 35% is trapped in the insulating papers (these figures have beencorrelated and confirmed during the tagging of numerous cables).

The PFT tracer is introduced into the cable via the Degassifier oil van. New (orrecycled) oil is degasified by the oil van and then the PFT is introduced into the oilbefore it is pumped into the cable reservoirs and then into the cable. This tagged oilis slowly (60-600 litres per hour) pumped down the cable to the far end reservoir. Ona good cable (no leaks) a small vacuum can be pulled on the far side which will thenincrease the pump rate up to approximately 200 litres per hour. A 1000 metre cablewith 2l/m in capacity would take approximately 6.5 hours (assuming 65% of the cableis mobile).

Great care has to be taken not to lose pressure or allow air to enter the cable

system. We are presently experimenting with tagging of live cables. Extreme care


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has to be taken because of the risk of losing pressure. It is important to test thepressure alarm and trip signals before this operation is carried out.

Leaking FF Cables

These leaking cables are the obvious and urgent cables requiring tagging they arehowever, the most difficultto tag. The leak rate and location determines the length of time to tag. For example,a 5km cable with a capacity of 2 litres per metre with a leak rate of say 100 litres permonth will take 50 months to tag. Obviously this is unacceptable so the trick is to tagyour good cable FIRST.

Recycling of FF Cable Oil

These FF cables have been in operation for up to 40 years in South Africa. The oilsof 40 years ago were very different from todays high tech oils. However, the onus

was on the suppliers of these oils to make sure that the oils that were supplied overthe years were chemically and electrically compatible.

In the writers opinion provided the oil over the years has been electrically andchemically compatible and the cable hasnot been overloaded or over stressed,then there is no reason why the cable oilcannot be recycled back into the cable.The oil has after all, being operatingsuccessfully over the life of the cable.Great care must be taken in collectingthe oil from the far end. Avoidcontaminating the oil and as far aspossible and avoid moisture ingression.Continual testing of the oil for water,dissolved gas and particle count isimportant. Under these conditions, therecycling of the cable oil has majorfinancial implications.

At a cost of US$5.00 per litre the cost of

pumping virgin oil on a 5km (2 litres permetre) cable is US$25,000 which is a major expense. The recycling of the oil istherefore significant and should be carefully investigated and justified.

Tagging Process

In Figure 3 the PFT injection is shown.

The Degassifer Van pumps tagged PFT oil into the Near Side Reservoir. The PFTtracer remains in suspension in the oil and is transported down the cable to the farside. The oil and PFT is under a pressure of 3-4 bar. When the PFT reaches the

leak location, the PFT is released with the oil into the surrounding earth. The PFTthen evaporates (at atmospheric pressure) and evolves as gas permeating the soil

Figure 3


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The cable route and length should be accuratelyknown in order to avoid false leaks. Most ofthese cables have been in service for 40 years ormore and the topographical landscape may well

have changed. Flooding could have washedaway the top soil. For this reason great caremust be taken when drilling bar holes. For pinpointing the leak, rather drill 2-3 metres to theone side of the cable route.

Mobile Leak Detection Unit

In South Africa, the mobile leak detectionconsists of a Test Van containing the PFT detection equipment. The Van should

preferably be a 4 x 4 vehicle if difficult terrain isto be traversed. Inside the Van the following

equipment is required:

Super silent Portable Generator to powerthe equipment and to provide lighting.

Alternatively a battery pack and inverter. Vacuum sampling device.

Gas Concentrator.

Gas Chromatograph and Detector. Carrier Gases.

Control Desk and Lap Top. GPS Satellite Receiver.

(See the attached pictures Figures 5, 6 and 7).

The latest units have GPS satellite location wherethe actual leaks are automatically taggedon the area map whilst the operator is slowly

(3/5 mph) driving along or beside the cable route.See the attached Figure 6 Map with waypoints.The red waypoints indicate where PFT was

detected.

The final pinpointing is done by using bar holesand taking sample bags. The sample bag withthe highest concentration will indicate the exact location of the leak.

The concentrator and GC must be able to detect down to the ambient backgroundPFT level (10 x 10-15) - this is the calibration level and one of the routine checks onthe detection system. The Degassifer truck and the point of connection to the cablesystem, should be free of leaks and spillage in order to avoid having this influencethe PFT Test Van Detector. Spillage at faults and leaks must be carefully recorded GPS co-ordinates and leak volumes.

Figure 4

Figure 5

Figure 6


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A very useful sampling device is the CATS(Capillary Absorption Tube Sampler) canbe used to measure the PFTConcentration. These are small glass

tubes filled with a material which absorbsthe PFT. These CATS can be placed onfence poles, pylons, poles etc. along theroute of the cable. These are thenanalysed by the concentrater and GC .

Mobile Detector

The mobile detector is mounted in a Test Van.

The detector consists of a vacuum pump which is sampling the ambient air as thevehicle navigates the route at 4-5 kph: The batched sampled air is thenconcentrated in special traps. These trap concentrators are specially designed totrap the PFT and other gases. The PFT concentration is minimal when comparedto other gases. The gas is then passed on to a Gas Chromatograph and finally to anECD (Electron Capture Detector) using a Radio Active source. This source is of avery low level but has to be registered and controlled by the local Radio ActiveAgencies. The GC software is designed to look for a particular trace which is anindication of the magnitude of that particular PFT gas.

The mounting of these very sensitive pieces of equipment is very important andgreat care should be taken to protect this laboratory equipment.

The software that is used for the analysis is very simple and a sample is shownbelow

The PFT peaks are then used to place waypoints on the GPS Map software whichis done automatically while the Test Van is driving along the route of the cable. See

Figure 6.

Figure 7 : City Power Central to Florida


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Experiences in South Africa using the PFT Leak Detection

In South Africa we havebeen using the PFTmethod of leak location

for the past 2 years. To

date 39 leaks have beenlocated. An analysis ofthe type of leaks is asfollows:

Band Wipes 5Repairs Sleeves 3Cable Wipes 10Pumping Lines 6Barrier 2Pumping Nipple 1Cable Sheath 12

Total 39 (October 2010)

Most of these cables were installed 40 years ago. It is interesting to note that CableSheath leaks are 30% of the total leaks. The wiping problems could possibly be dueto the fact that the skill of the Tradesman has been lost.

It is very important to record exactly the cable oil that is pumped in order todetermine if the entire cable length is tagged. Feeder Standby is -1.

Some typical sheath leaks are shown below.

Figure 9 Figure 10Benefits:

Figure 8 A graphical presentation of the PFT detected at leak 2

(drive by leak detection).


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The following benefits are listed below:

Environmentally safe PFT.

The cable dielectric strength is not compromised.

The leak location can be done live. The cable operating life can be extended.

Reliability of the cable is greatly improved.

No freezing is necessary to find leaks. Less damage to the environment.

Less damage to adjacent cable.

Less damage to the FF cable (by not freezing).

Less excavation work than freezing. Replacement of these very expensive cables is now not necessary.

Excavating to remove or replace these cables in the centre of a busy city is

avoided. Vastly improved environmental conditions. Less cable oil lost in the sub soil.

Reduced sheath fault damage hence less leaks.

No loss of revenue due to Feeder outage.

Conclusion

The PFT Tracer leak location is technically and economically a viable method of leaklocation in Fluid Filled Cables. Other methods are unreliable, time consuming anddamaging to the leaking cable and adjacent cables. The freezing method isexpensive and unreliable. Recycling of the cable oil should be carefully consideredbecause of the financial benefits. The cable reliability should however not becomprised (the authors opinion). Utilities should concentrate on tagging their goodcables rather than concentrating on the leaking cables (unless the volumes are toogreat). The leak rate determines the tagging rate on a leaking cable. On a non-leaking cable, tagging can be done at 200 litres/hour or more (LPFF).

Drive by leak location is possible using the GPS Mapping technique.

Pin Pointing when using the PFT tracer can be within a meter of the actual leak.

Because this method of leak location is not as intrusive as the freezing method, theoperating life of these LPFF and HPFF cables can be extended. Leak location onPFT tagged cables can be done within 4 hours of notification. The reliability of thesecables will increase and the cost of lost cable oil will decrease significantly.

Of more importance and vital to our future is the environmental damage. Tons (notlitres) of cable oil are lost by the Utilities due to leaks into the underground watersystem. Today this is unacceptable and in most countries Environmental Lawsdictate that all of this lost oil is recorded, recovered and destroyed. In practice, thisis not done and Utilities are at risk of hefty fines. The PFT tracer technology for leak

location will reduce the leak location time to hours instead of months and years.


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Fluid Filled Cable Users will now with this technology be able to account for everylitre of cable oil that is lost due to cable leaks.

References

1) Leak Location in Fluid Filled Cables using the PFT Method. R. Ghafurian, R.N.Dietz, T. Rodenburg, J. Dominguez, N. Tai. IEEE Vol. 14 No. 1 Jan. 1999.

2) Balfour Beatty Paper.

3) Experimental Design data of the April 1997 Multitracer Atmospheric Experimentat Idaho Memorial Engineering Laboratory L.A. 7795-M5 Los Alamos ScientificLaboratory 1979. Various Authors.

4) Environmental Research Technology Brookhaven National Laboratories. NewYork.

5) Vapour Detection of Trafficking of Contraband Money. Brookhaven NationalLaboratories. New York.

6) Wasson-ECE Tracer Analyser.

Acknowledgment:

1) EDF Allan Croucher, Paul Williams for the valuable advice.

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