GB2175311A

GB2175311A - Fluid-insulated switchgear

Info

Publication number: GB2175311A
Application number: GB08611961A
Authority: GB
Inventors: Fred Berry Waddington; Andreas Kallinicos
Original assignee: MICANITE AND INSULATORS COMPAN
Current assignee: MICANITE AND INSULATORS COMPAN
Priority date: 1985-05-17
Filing date: 1986-05-16
Publication date: 1986-11-26
Also published as: GB2175311B; GB8611961D0; DE3662016D1; EP0202878B1; EP0202878A1; GB8512488D0

Description

1 GB 2 175 311 A 1

SPECIFICATION

Fluid-insulated switchgear The present invention relates to switchgear, and in particular relates to f luid-insulated tap-changers.

Fluid-insulated tap-changers are employed for light-duty switching in a variety of applications, such as distribution transformer installations, underground railways and the like, where repeated on-load switching of cu rrents of up to about 500A at voltages of typical ly a few kV is required. Typically a tap-changer may perform 20,000 to 30,000 make and break cycles annually, and a working life of a minimum of 20 years is normally expected. Owing to their frequent make and break cycles, tap-changer electrodes are subject to considerable 10 wear in use, and forthe same reason the dielectricf luid in a tap-changer quickly becomes contaminated with carbon and other arcing products, which in turn tend to cause the electrodes to wear more rapidly.

Consequently fluid-insu lated tap-chagers are distinguished f rom fluidinsulated switchgear in general bythe requirementto renew their dielectric liquid atfrequent intervals throughout their working life and also bythe requirement that the dielectric liquid employed should be compatible with an absolute minimum of el6ctrode 15 wear. The problem of electrode wear is less severe in heavy duty switchgearfor example, even though such switchgear may switch currents of perhaps several kA at several tens of M Thus although a fairly wide range of dielectric liquids have been used for switchgear in general the only insulating fluid found suitable fortap-changers and similar light-duty switching has been petroleum oil to BS1 48 insulating specification. However, BS1 48 petroleum oil has the following disadvantages; a) it is highly 20 inflammable b) it can have poor lubricating properties c) it can freeze at-20'C and d) being a natural petroleum productthe molecular structure can contain various aromatic structures of questionable toxicity or carcinogeneticy. Also in operation in a tap-changer or switch, petroleum oil evolves large quantities of hydrogen and otherflammable gases, and produces large quantities of colloidal carbon which requires periodic removal. Up to recently attempts to replace petroleum oil with an alternative less flammable fluid has 25 not metwith complete success. Dielectrics based on halogenated oils such as polychlorinated biphenyls fluorinated oils or mixtures of halogenated chlorofluoro compounds have resulted in the formation of large quantities of toxic or corrosive products such as hydrochloric acid or hydrolysable fluorides. Experience has also shown that there is in addition a marked increase in contact erosion with such fluids. Otherfluids based on silicone materials such as dimethylsiloxane and phenylmethyl siloxanes have also been found to be unsatisfactory due to the formation of silica and gelatinous silicone polymers on arcing during switching which effectively abrade electrodes/contacts and can cause insulating films of fused silica to be formed on the contacts.

According to the present invention, a liquid insulated tap-changer incorporates a dielectric liquid which includes as a major constituent one or more monomeric or polymeric esters of the general formula:

0 R-C-R 0 wherein Ware the same or different alkyl groups having five to ten carbon atoms, and Rare independently H or 45 Me 0 11 50 orEtor-CH2-U---l or (in the casethatthe ester is 0 polymeric) -CH2-0 CH2-1o- 11 U 1 11 C1-12-O-C-R' The dielectric liquid may comprise a single said ester ora mixture of such esters.

Preferably in an ester employed in a dielectric liquid according to the invention each R is 2 GB 2 175 311 A 0 Ii C12-U---rl;or one R is ethyl and U 2 the other R is CH2-0-C- R'. R' is preferably hexyl (C6), heptyl (C7) or octyl (C8). R' maybe a straight chain or a branched chain a lkyl group. Preferred esters for use in accordance with the invention are esters (particu I arly tetra-esters) of pentaerythritol (C (CH20HW and heptanoic or octanoic (particularly the n-heptanoic and n-octanoic) acid. Esters oftrimethylol propane (particularly 1, 1, 1 trimethylol propane), di-trimethylol propane10 and dipentaerythritol ((CH20H)3C-CH2-0-CH2-C(CH2OH)3) are also suitable, particularly when the acid is heptanoic oroctanoic acid.

The use of certain aliphaticesters as insulating fluids in fluidinsulated electrical apparatus in general is disclosed in our U.K. Patent No. 1602092, which is hereby incorporated by reference. Specifically, this patent disclosedihe esters trim ethyl o [-p ropa n e tri he ptanoate, trimethyl ol-p ro pane trica pryl ate and pentaerythritol 15 tetrapelargonate and listed their viscosities, boiling points, electrical loss tangents (tan 8), resistivities, permittivities, electrical breakdown strengths,fire points and autoignition temperatures. Howeverthis patent did notsuggestthat any of the esters of the present invention were suitablefor use in tap-changers and in particulardid notdisclosethe amount of contact erosion orcarbon production which arises when the esters are used in tap-changers. Thesetwo parameters are very important in tapchanger applications butare much 20 less important in most othertypes of switchgear applications.

We have unexpectedly found that esters in accordancewith the invention, when used as dielectric liquids in tap-changers, significantly reduce contact erosion in comprison with tap- changers which utilise petroleum oil (which has previously been considered the best dielectric fluid fortap- changers). Furthermorethe esters of the present invention form only a small fraction of the carbon produced by petroleum oil in tap-changers, andthe 25 carbon produced bythe esters of the present invention is less colloidal and separates more readilyto leave a cleanersystem. Thus, by using the esters of the present invention in tapchangersthe overall life ofthe tap-changer may be increased andthe required maintenance may be reduced.

The esters of the present invention are considered to be of lowtoxicity and havethe following known advantages over petroleum oil:

a) the esters can be formulated to be fire resistant i.e. they can readily comply with national fire specifications fora low flammability insulating fluid (fire point above 300'C).

b) smaller amounts of hydrogen and otherflammable gases are produced on arcing.

An example of one ester mixture employed in a tap-changer in accordance with the invention consists of pentaerythritol tetra-heptanoate and pentaerythritol tetra-octanoate esters having a composition as follows: 35 0 11 48% C(CH20 -, - M-6rl 1 W4 0 11 42%C(CH20 - - - ISC)-7l 1 5k 0 11 10% C(CH20---rl-7"15P4 The advantages of this ester mixture can be seen from the following table which lists comparative data for BS148 petroleum oil, fluoropentane, and the ester mixture.

3 TABLE

8S148 Petroleum Oil C5F12 ester mixture GB 2 175 311 A 3 5 Fire Point CC) 170 310 Pour Point CC) -30 - -50 Carbon/U (g) 0.023 <0.001 0.002 Hydrogen/kj (CM3) 26 6 Contact erosion after 10 type testing at up to 2W (wt.%) 1 - 0.7 Contact erosion after type testing at 20W (mm/kA) 0.04 0.09 0.02 15 In each case, the fire point was determined bythe method specified in the Institute of Petroleum handbook No. 36/63, the pour pointwas determined by the method specified in the Institute of Petroleum Handbook No. 15/67 (ASTM Designation D97-66), and the type testing involved 20,000 make and break operations in a 33kV 500 amp High Speed Resistor Transition Tap-Changer.

It will be appreciated that in practice, commercially available mixtures of aliphatic acids maybe reacted with the alcohol to form the required esters, and that appropriate blending of esters produced from different batches of acid maybe necessary in orderto achieve the desired pour point, fire point, viscosity or other characteristics. In general the precise composition is not critical, but it should be noted thatthe electrical properties, especially the loss tangent and the resistivity, improve with increasing molecular weight and increasing symmetry of the ester molecule.

Claims

1. A liquid insulated tap-changer incorporating a dielectric liquid which includes as a major constituent one 30 or more monomeric or polymeric esters of formula:

0 11 Crl2-U---rl 1 R-C-R 0 wherein Ware the same or different alkyl groups having five to ten carbon atoms, and Rare independently H or Me 0 11 45 orEtor--M2-u-k,-R' or (in the case that the ester is 0 11 polymeric)-CH2-0-CH2-;-i U 1 11 55 CH2-0-C-R'. -

2. A liquid insulated tap-changer according to Claim 1 wherein the dielectric liquid incorporates a said ester, in which each R is 0 11 CH2-U--rl - 4 GB 2 175 311 A 4

3. A liquid insulated tap-changer according to Claim 1 wherein the dielectric liquid incorporates a said ester in which one R is ethyl and the other R is 0 11 Cl-12-U---11.

4. A liquid insulated tap-changer according to Claim 1 wherein the dielectric liquid incorporates a said ester, in which Wis hexyl (C6), heptyl (C7) oroctyl (C8).

1()

5. A liquid insulated tap-changer according to Claim 4wherein Wis a straight chain ora branched chain 10 alkyl group.

6. A liquid insulated tap-changer according to any preceding Claim wherein the dielectric liquid incorporates, as a said ester, an ester of pentaerythritol and heptanoic or octanoic acids.

7. A liquid insulated tap-changer according to anyone of Claims 1 to 5 wherein the dielectric liquid incorporates as a said ester an ester of trimethyl propane, di- trimethylol propane ordipentaerythritol and heptanolcoroctanoic acid.

8. A liquid insulated tap-changer according to Claim 1 wherein the dielectric liquid comprises a mixture of pentaerythritol tetra-heptanoate and pentaerythritol tetra-octanoate esters.

9. A liquid insulated tap-changer according to Claim 1 wherein the dielectric liquid comprises an ester mixture having the following compositions:- 0 11 48% C(CH20---rl-6n1314 0 42% C(CH20---ISO-7r11514 0 11 10% C(CH20---rl-7"1514.

10. A dielectric liquid suitable for use in a liquid insulated tapchanger according to any preceding Claim and comprising an ester or mixture of esters of pentaerythritol and heptanoic or octanoic acid.

11. A dielectric liquid suitable for use in a liquid insulated tapchangeraccording to any preceding Claim and comprising an ester or mixture of esters of trimethyl propane ordipentaerythritol with heptanoicor 40 octanoicacid.

Printed in the UK for HMSO, D8818935, 10186,7102. Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.