Dielectric Breakdown Voltage Testing Testing – IEC 60156 for New and In-service ineral Ins!lating "il Over the past several weeks, we have been discussing testing insulating insulating oils for dielectric breakdown voltage. Aside from a brief description of the IEC method, we have so far provided information on just the two AS! standard methods, "#$$ and "%#%&. In the final installment regarding this topic, we are going to discuss the IEC method from Standard &'%(&, including values used in IEC standards for unused and in)service mineral oil. *et+s start with a brief review of the differences in how this method is run. Standard &'%(& uses electrodes that are similar geometricall to the -"E electrodes used in AS! AS! "%#%&. he spherical electrodes are spaced spaced .( mm apart, and the rate of voltage increase increase is ,''' volts per second. second. he method in standard &'%(& allows the optional use of an impeller operating in similar fashion to the one described for "%#%& e/cept that it operates at (' to 0'' rpm. he IEC method also allows use of a magnetic stirrer operating at a similar rate if there is no significant chance that magnetic particles will be removed from the oil. he presence of magnetic particles would affect dielectric breakdown in the transformer, so removal of those particles b the stirrer during the analsis would ield unrepresentative values. -alues -alues for dielectric breakdown voltage obtained on purified and installed oil using the IEC &'%(& method are tpicall considerabl considerabl higher than those obtained using the two AS! methods. 1re2uentl, 1re2uentl, for properl maintained transformers, the breakdown voltage from the test ma e/ceed e/ceed the ma/imum reportable b the instrument. !an dielectric meters report a ma/imum voltage of &' k-. k-. his is generall sufficient sufficient for evaluating new oil from a supplier or in)service oil. 1or oil being processed for installation, installation, or if a more precise value is re2uired re2uired for in)service insulating oil, use of a dielectric breakdown voltage instrument that measures to higher values is advisable. Conditions that affect the IEC method dielectric breakdown voltage determination are essentiall the same as those that affect the "%#%& dielectric measurement. measurement. he test is sensitive to moisture, o/idation o/idation of the oil, contamination of the oil m non)dielectric materials, and suspended particles. particles. "issolved gas content of the oil also ma affect the results obtained b this method as is the case with "%#%&, but the depression of the dielectric breakdown voltage values is not as significant for the IEC method. IEC lists &'%(& values in two standards. he international industr specification specification for unused oil is IEC &'3&, 1luids for electrotechnical applications applications 4 5nused mineral insulating oils for transformers and switchgear. In that standard, the specification calls for unused oil, as received in bulk from the supplier, to have a minimum dielectric breakdown voltage value of 0' k- as determined b &'%(&. his standard also indicates that the oil received from the supplier must measure a minimum value of $' k- after vacuum filtration in the laborator. hese values are from the third edition of the standard, dated ''0. here is a working group currentl working working to revise this standard, but changes to the dielectric breakdown voltage values are not e/pected. "ielectric breakdown voltage is also addressed in IEC &'6, !ineral insulating oils in electrical e2uipment 4 Supervision and maintenance maintenance guide. 1or new oil in new e2uipment after filling but prior to energi7ing, energi7ing, the standard lists acceptable values for dielectric breakdown voltage as indicated in this table8
ini#!# Dielectric Breakdown Voltage Val!es $sing 60156
E2uipment
9 $.( k-
-oltage "ielectric k-
: $.( k-
: $' k-
9 %$' k: ((
: &'
: &'
IEC &'6 also lists ;ood, 1air, and
In-service Dielectric Breakdown Voltage Val!es %or ineral "il-filled Transfor#ers $sing 60156 E2uipment
9 $.( k-
-oltage
: $.( k-
: $' k-
9 %$' k-
;ood k-
: 6'
: ('
: &'
1air k-
0')6'
6')('
(')&'
= 0'
= 0'
= &'
1or in)service transformers, IEC &'6 includes recommendations for continuing normal monitoring if &'%(& values are ;ood according to the table above. If values are 1air, more fre2uent sampling and cross checking with other tests are recommended. hose other tests include moisture content, particle count distribution, dissipation factor or resistivit, and acid number. If values are e will start another topic ne/t week.
