What is Your IQ ? (Insulation Quality)

Dan Ninedorf

Specialized Camera Sales & Services LLC

N4068 Co Rd ZZ

Montello, WI 53949

Tel: 888-322-9368 or (608)589-5510

Fax: (608)589-5509

Email:

cameras@maqs.net

Website:

www.specialcamera.com

ABSTRACT

Electrical power systems use insulation as structural members to isolate the energized conductors from the grounded

cabinets or structures. Electrical insulation has been of many materials over the years leading to the current

polymers, ceramics, bonding cements, fiberglass and epoxy's. Insulation is subjected to fault currents, over-voltages,

contamination, structural overload, and over-heating. Electrical insulation is used to isolate energized conductors

which may have voltages ranging from 2.4 to over 700kVAC and over 400kVDC for generation, transmission,

distribution and through the conversion to another form of power or lower voltages for industrial and residential

uses.

Electrical insulation is tested using various techniques to identify defects during R&D, manufacturing, field start-up,

sometimes on-line monitoring, and off-line predictive maintenance testing. The defects can be internal or external to

the insulation material, the internal defects "growth" can be accelerated by structural loading or electrical over-

voltage and the external defects can be accelerated by moisture, freezing weather and chemical contaminants.

The physical condition of insulation during manufacture is defined by quality control to various applicable standards

of NEMA, IEEE, UL, military standards, ASNT, and other certifications dependent upon the users' application. The

condition of installed insulation is tested on-line using non-destructive inspection (infrared, ultrasound, corona

camera) and for specific equipment defined off-line testing procedures are used to perform quantitive energized

testing for tan-delta (power factor), dielectric resistance, and surge testing for electric motors.

Keywords:

Corona, insulation quality, dielectric breakdown, arcing, grounding, thermography, ultrasound

1.

INTRODUCTION

Insulation quality is defined as being non-conductive through the applied voltage range, plus any tolerance for

transients or over voltage spikes that may occur. Destructive and non-destructive methods are used to evaluate

insulation quality to many standards for various applications. This paper is meant to describe current high voltage

insulator types, the causes of breakdown of modern insulators, elaborate on non-destructive testing, the tools and the

reasons for its use. The conference presentation will include pictures to elaborate on defects observed and testing

methods.

2.

TESTING INSULATION QUALITY

Insulation quality is tested in several ways, the means to test insulation depends upon its shape or configuration. Flat

material insulation as might be found in switch gear as cabinet bus spacers typically is tested for dielectric

breakdown with a dielectric tester. Electric motor winding insulation is tested for withstand to ground or enclosure

with a dielectric tester, then tested for turn-to-turn dielectric strength or inter-turn shorts with a surge tester which is

applied in both directions to stress the end turns. Other types of insulators, suspension insulators used on outdoor

poles and transmission lines are tested using dielectric and magnetic field testers, these insulators may have about

35kV across each one. These insulator strings have higher voltages present on each insulator at the end of the

insulator string, and the redundancy allows some in the string to be failed without causing an outage. Another type