IEEE !hamactions on Electrical Insulation Vol. 23 No. 6, December 1088 1009

Formation of PCDF and PCDD in Electrical Discharges

T. 0. Rouse, R. E. Koch

General Electric Company, Pittsfield, MA

and F. L. DeRood Bsttelle Laboratories, Columbus, OH

ABSTRACT

The formation of polychlorinated dibeneofurans (PCDF) and dibenao-dioxins (PCDD) in electrical discharges in liquids con- taining polychlorinated biphenyls (PCB) has been examined. Little or no formation of these products has been found in high energy short duration breakdowns, ‘arcs’ in air-saturated triehlorobenaene (TCB), in mixtures of TCB and PCB or in mineral oils containing small amounts of PCB. Low energy par- tial discharges could not be sustained in air saturated liquids. Formation of PCDF and PCDD within electrical equipment does not appear likely.

INTRODUCTION

OLYCHLORINATED biphenyls (PCB) were used wide- P ly in electrical equipment until the late 1970’s. Con- cern for their environmental persistence and biological response led to a ban on additional use and to regulation of disposal and maintenance of PCB in continuing use. Further, PCB were usually mixed with tri- and tetra- chlorobenzene (TCB) to make transformer liquids called ‘askarels’. Attention is now shifting to the products of partial oxidation of PCB [l], polychlorinated dibenzo- furans (PCDF), and to polychlorinated dibenzodioxins

(PCDD), also formed with PCDF in the partial oxida- tion of TCB.

Partial oxidation of an insulating liquid in the fail- ure of a transformer or a capacitor can occur in a t least two ways. One is through expulsion of liquid into the surroundings where it can catch fire itself or become involved in a fire resulting from nearby materials. A second is through direct reaction with residual oxygen in the heated zone of an electrical breakdown within the piece of equipment. The formation of PCDF and PCDD in fires involving PCB is being studied widely.

0018-9367/88/1200-100981.00 @ 1988 IEEE

1010 Rouse et al.: Formation of PCDF and PCDD in electrical discharges

Their formation in the dielectric liquid in the vicinity of an electrical breakdown, with oxygen present, has not been examined.

Electrical breakdown in a transformer is a mal- function which is neither readily controlled nor repro- duced and occasionally cannot be contained. Intention- ally failing actual transformers then is not an accept- able approach to studying PCDF and PCDD forma- tion in an electrical breakdown. This paper describes studies of the formation of PCDF and PCDD in con- trolled simulations of electrical breakdown in transform- ers. Both high-energy short-duration breakdown and long-lived low-energy partial discharge situations (‘arcs’ and ‘sparks’) were examined. Askarels alone and min- eral oil containing low concentrations of PCB and TCB have been examined.

EXPERIMENTAL APPROACH

ARCING RCING tests under oil were performed in small test A cells designed to simulate conditions during an arc-

ing fault in an actual transformer. Test cells were capped, filament wound glass/epoxy fuse tubes (6.8 cm ID x40 cm L) containing 1.35 1 of liquid, with a wrap of 0.23 mm thick electrical insulating Kraft paper inside the tube. Both were half saturated with water and the oil was fully saturated with air. The air space over the oil was 17% of the oil volume. A drawing of a test cell is shown in Figure 1.

Figure 1. Test Cell.

Electrodes were 1.5 cm diameter aluminum alloy (6061) rods with chamfered tips separated by a 1.27 cm gap which was bridged by a small diameter low-carbon steel (0.3% Mn, 0.1% Cu, 0.1% Ni) fuse wire to start the arc. Each cell was provided with a connection to a pres- sure gauge, a pressure transducer, and a gas-sampling container. Current, arc voltage, duration and pressure excursions were recorded: 12t products and arc energies were integrated automatically. Fault current magnitude

and duration were varied in order to produce energy levels which could be confined and still be quite severe.

Askarels tested were electrical grade trichloroben- zene (Hooker Chemical) and Aroclorm 1254 and 1260 (PCB mixtures made by Monsanto Company). TCB and 1:l mixture of TCB and Aroclor 1254 were each treated twice with 3% of their mass of activated fuller’s earth, B normal manufacturing practice in removing po- lar impurities prior to filling askarel transformers. One component usually added to askarels was 0.15% of 3,4 epoxy-cyclo-hexyl-methy 1-( 3’,4’-epoxy-cyclc+hexane) car- boxylate as a scavenger for chlorine or water. The pos- sibility of its halogenation, dehydrogenation, and rear- rangement to form PCDF and PCDD is remote and it was omitted here to simplify the testing. Oil samples were made from commercially available fuller’s earth treated transformer oil and Aroclor 1254 (50 ppm wt/ wt, 500 ppm, 3 ppt (part per thousand), 10 ppt, Aro- clor 1260 (10 ppt) or TCB (10 ppt). PCDF, PCDD and some PCBs were separated from the sample matrices after arcing using silica and activated alumina.

PCDF/PCDD ANALYSIS

PCDF and PCDD were separated from the test matrices using alumina and silica gel chromatography and determined using combined capillary column gas chromatography/high resolution mass spectrometry [2]. Samples were weighed, diluted with hexane and spiked with isotopically labeled 2,3,7,8-tetra-chloro-dibenzo-p- dioxin ( TCDD-13C12), 2,3,7,8-tetra-chlorodibenzofuran (TCD F- l3 C 12) and oc ta-chloro-dibenzo- p-d ioxin ( 0 C D D- 13C12). The solutions were cleaned with concentrated sulfuric acid, washed with water, dried and concentrated. Enriched solutions were prepared in stages of column chromatography. The first was activated alumina; eluted first with hexane, next with hexane/methylene chlw ride (97:3) and finally with hexane/methylene chloride (50:50). The last eluents, after concentrating, were pass- ed through multilayered columns, containing concen- trated sulfuric acid on silica gel and 1 M sodium hy- droxide on silica gel. These columns were eluted with hexane. The hexane eluents, after concentration, were treated with a second activated alumina column. The fi- nal eluents were gently taken to dryness and the residue dissolved in decane.

Analysis was performed using a Carlo Erba Model 4160 gas chromatograph interfaced directly into the ion source of a VG Model 7070 High Resolution Mass Spec- trometer. The mass spectrometer was operated in the

1012 Rouse et al.: Formation of PCDF and PCDD in electrical discharges

I 1 I 1 I I 1 0 1 2 4 6 8 10 12 14

Table 2. PCDF and PCDD Content of Askarels Before and After Arcing.

16

PCDF or FCDD Cont ent - na/a (DDb) 6.0 -

5.0 - 1

4.0 - - E

h

M 3.0 - 2.0 -

1.0 -

Isorncr JCQE Tctra- Penta- Hcxa- Hepta- Octa- Total

POD Tetra- Penta- Hexe- Hcpta- Octa- Total

TCB Tube2

B/A P articles

2 8 0 / 270 100* 2000/2500 990 1700/2000 1500 930/1000 1100 t70/ 150 560

5300/5900 -

33/29 40 11/16 ND

4 1 2 ND m/ND ND ND/ND ND 48/47 -

* - nq P c W q particles

. . 3:l TCB/Aroclor 1254 Tube 4 Tube 5

E/& B/A

370/ 170 2301 96 1850/1740 1200/340 1790/1180 1300/310 2960/1360 S20/ 29

220/ 170 100/ 8 7200/4600 3400/780

21/24 201 6 ND/12 1/ 1 rm/ 2 2 m ND/m N D m ND/ND N D m 21/38 23/ 7

ND - Not Detected.

Hz/CzHz = 0.78 & 0.20 CH4/CzHz = 0.29 f 0.07 CzHd/CzHz 0.46 * 0.07 CzHs/CzHz = 0.04 * 0.01

The concentrations of PCDF and PCDD in the TCB and TCB/1254 mixture before (B) and after (A) arcing are given in Table 2. A residue of finely divided dark carbonaceous particulates was formed in the liq- uids. A portion of this was filtered from a TCB sam- ple, washed with methanol and extracted with benzene. This extract was then analyzed. The results of these analyses are also given in Table 2. The results of analy- sis of the mineral oil samples before and after arcing are given in Table 3.

Neither the Aroclor 1254 itself nor the sample of 10 ppt Aroclor in oil prior to arcing in tube 9 were ana- lyzed directly. The concentrations of PCDF reported in Table 3 for Aroclor 1254 alone were calculated from the average of the analyses of the 1:l mixtures of TCB/1254 (Tubes 4 and 5) and that of the TCB alone (Tube 2) given in Table 2. The concentrations in the unarced Tube 9 sample were in turn calculated from the calcu- lated analysis of the 1254 and the dilution factor. As a check, this same procedure was followed for the 3 ppt 1254 sample (Tube 8) and the calculated and measured values are in reasonable agreement. PCDF content of the 50 and 500 ppm Aroclor 1254 samples were below

I

Figure 2. Arc Energy Versus Pressure In Spiked Oil And Askarel.

the limit of detection both before and after arcing, as was the PCDD concentration in all oil samples.

DISCUSSION T appears that arcing TCBs and PCBs under the I conditions here does not result in the formation of

PCDF and PCDD. The average deviation of the analyt- ical method used here is about 25% for all isomers. That is; the average deviation for the results of three analy- ses for each PCDF and PCDD isomer, tetra-, penta-, etc, on each of seven samples taken from transformers was about 25%. Within these confines, there do not appear to be any differences in the PCDF/PCDD con- centrations found in the liquids before and after arcing. The data suggests that there may even have been a re- duction in the concentrations in the liquids. This, in part, could be due to adsorption of the pre-existing fu- rans and dioxins on the carbonaceous material forming in the discharge. No major attempt was made to recover all of this finely divided powder. It is a very visible, but minor, amount of material. Extraction with benzene re- leased only small quantities of PCDF or PCDD. Unless the extraction with benzene is very inefficient, the pow- der is not a sink for any major quantity of PCDF and PCDD formed in the discharge. Several factors mitigate

IEEE Transactions on Electrical Insulation Vol. 23 No. 6, December 1988 1013

against PCDF and PCDD formation in high energy dis- charges, both inside an actual transformer and in this experimental configuration. First, mass transport pro- cesses in these fluids are slow and only limited quanti- ties of vaporized chlorinated aromatic hydrocarbons will reach the reaction zone in the time allowed by an arc. By that same token, the supply of oxygen available to the reaction is limited. In a combustion zone outside the transformer, these limitations are less forceful and a fire forming furans and dioxins can occur. Secondly, the rates of formation of PCDF and PCDD are at their highest between 400 and 700%. The rates of destruction (further oxidation to water, carbon dioxide and hydro- gen chloride) of the furans and dioxins begin to domi- nate the overall process as the temperature is increased and they then are destroyed more rapidly then they are formed or replaced. Temperatures in the core of an elec- trical arc, in the main, are far higher than this. Only a limited zone surrounding the arc will be at temperatures favorable for PCDF and PCDD production.

The results. here with partial discharges obviously are not definitive; they do raise the possibility that par- tial discharges cannot be sustained in these liquids when saturated with gas. A combination of higher temper- atures due to discharge and oxygen availability favor- able to furan and dioxin formation simply may not oc- cur. The temperatures in even the hottest regions in a normally operating transformer will never reach lev- els approaching 400’C. This, together with the lim- ited oxygen available in an operating transformer or ca- pacitor, make PCDF and PCDD formation within nor- mally operating equipment by thermal oxidation pro- cesses unlikely. The TCB and Aroclors used here were retained samples from the mid-1970’s. After a labora- tory treatment with fuller’s earth similar to that done on a larger scale in the manufacture of transformers, mea- surable PCDF and PCDD remain (Table 2). It therefore seems probable that, if unused askarel filled transform- ers could be found and sampled, measurable PCDF and PCDD would be found in the insulating liquid. This, in turn, suggests that the interpretation of analyses of samples from transformers presently functioning, from spills from transformers, and from fluids left after fires involving transformers will be complicated by the pres- ence of an unmeasured and variable background.

Table 3. PCDF Content of Mineral Oil Containing PCB Before and After Arcing

FCDF Content - na/a (DDbl Aroclor - ma PCB/o oil/g (wtl

1254 AL25.L Origin Tube 8 Tube 9 Tube 10

FCDF 1000 3 ppt 10 ppt 10 ppt 1 B/AB/A E O,!/kJ (3)/11 1/ 1

(1)/

(311

(SI/

Penta- (1100) W 0 . 5 (io)/ 4 3/ 2

Hexa- (1400) 111 (14 ) / 2 4/ 3

H-td- ( 2 6 0 0 ) 6/ND (2SI/ND 6 1 7 [ a ) /

Octa- (140) ND/i (i)/ND 23/15 ( 0 . 4 ) /

Total I65001 11/2 f64)/ 7 37/29 1/ND (19)/

[ 1 - calculated values , see t e x t . ND-Not Detected.

PCDF and PCDD was probably more complete and it would not be surprising to find their background level to be lower in capacitors.

ACKNOWLEDGMENTS HE Electric Power Research Institute supported this T work. Their project manager, Gilbert Addis, made

a major contribution to its performance.

REFERENCES

R. D. Kimbrough ed, Halogenated Biphenyls, Ter- phenyls, Naphthalenes, Dibenso-dioxins and Re- lated Products, Elsevier NY 1980.

F. L. DeRoos et al., in preparation.

Annual Book of ASTM Standards, Vol. 10.03, Meth- ods D3612-85 and D3613-82.

Treatment with fuller’s earth was intended to re- move polar impurities. These unwanted materials in- crease the dissipation factor of the dielectric liquid. This has a functional impact on capacitors and they were more carefully removed in the manufacture of capaci- tors than of transformers. The concurrent removal of

Present address: Twin Cities Testing Corp., St Paul, MN.

This paper is based on apresentation given at the 1985 EPRI PCB Seminar.

Manuscript was received on 2 Sep 1987, in final form 19 Apr 1988.