To Air is Human, To Insulate Divine

By Gary L. Orlove, P.E.
ASNT NDT / PdM Level III

Its winter again, and boy, don't we know it New England. There’s more snow on the ground than we can ever remember having!  So I thought we might take a look at some of low cost ways you can help save thermal energy in your abode.
The contributors to heat loss in a home are: conduction losses to the exterior, radiant losses from the exterior, convection from the exterior, and air infiltration/exfiltration. Here are a few examples of the efficacy of reducing conductive and air movement heat losses with simple but surprisingly effective methods.
Let's take a look at air leaks. Usually infiltration happens on the lower floors of a building and exfiltration occurs at the top floors due to the stack effect (in a heated building, the indoor air is buoyant and applies pressure to the envelope. The pressure increases with the height of the envelope). Air infiltration not only makes a building consume more energy, but it makes it "drafty and cold", especially if you are sitting next to the source of the leakage!
Take a look at the thermal imaging photos below. Here we see an unused air conditioning vent spewing out cold air on the first floor (stack effect). You can't see the air with thermal imaging camera, but you can see the cooling effect the air has on the walls and ceiling. For this retrofit I used a piece of magnetic plastic sheet. You can see in the after thermal image the dramatic reduction in airflow and the effect of adding a bit of insulation to the open areas of the vent. The delta T reduction between the vent and the ceiling was about 30%. Not bad, but I thought I could do better!

   

   

   

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 of insulator is called a post type insulator used for mounting substation open-air switches, these insulators may have 120kV or more across each insulator. The non-ceramic type insulator will have the full rated voltage applied across its length with a fiberglass rod in the center as the structural member and is encased in a ribbed molded cover from connector to connector. The biggest detriment to the non-ceramic insulators is the designer that doesn't put corona rings on above 160 kV. Flash-over damage and the freeze thaw cycles once the cover is penetrated can create arcing paths which will lead to failure.

3. DETRIMENTAL TRANSIENTS

Lightening voltages can create an intense electric field which can make air "sizzle" and can also deliver tremendous fault currents through utility distribution and transmission lines. The next most powerful transients are from capacitor switching, generator synchronizing or accidental faults such as tree branches falling across lines, summer construction severing lines, etc. These transients are coupled from line to parallel line and if high enough voltages and electric fields are involved even to lines running at 90 degrees or by raising the ground plane potential to couple these transients into control signal and analog transducer wiring. The longer the run the higher the potential for induced voltages. Parallel lines that are insulated and become contaminated become capacitors which can discharge creating corona or arcing. Winter icing can create many unintended energized circuits including the fencing surrounding substations.

4. RESOURCES FOR MAINTENANCE

Reliability is locating defective components and preventing the faults that exist from creating an outage. Random outages are also being reduced by squirrel, bird and snake guards. Construction and home-owner safety around distribution voltage electricity education is active in many areas of the country. Predictive maintenance to identify those defective components that will cause an outage uses several types of test equipment including infrared, ultrasound, acoustic, radio frequency, night vision, ultraviolet low-light and daylight multi-spectral corona sensing cameras. Each technology contributes to the knowledge about the condition of the electrical system (motor/generator, MCC, switch gear, distribution system, substation, and transmission line). The technician’s knowledge about the limitations of each technology are also important, the following table is meant to provide basic guidelines, there are variables with different sensors for each technology. The basic device being sensed is a defective pin and cap type insulator as pictured.

5. DEFINING CORONA INDICATIONS

Equipment energized with high voltage has the potential to create ionization of the surrounding air or corona discharge, corona does require energy to produce this chemical reaction. When corona occurs sound is produced from audible through ultrasound, nitric acid is produced when moisture is present, ultraviolet light is emitted, because corona itself can be intermittent the sound can be difficult to detect if moving the sensor too quickly.

Corona in enclosed switch gear can be detected when the nitric acid white or gray powder can be found as a witness to the corona activity. The line-of-sight ultraviolet corona cameras are used to view corona discharge on high voltage electric motor and generator windings, switch gear and substation bus, terminals, linkage, insulators of all types and the daylight corona cameras are especially easy to interpret indications if you remember:

1) Corona is an indicator of conductivity, corona can indicate punctured insulators if present on the insulator cement.
2) Corona can indicate insulator contamination, random, flitting about along the length of the insulator barrel or on the bell.
3) Corona can identify hardware that may suffer from acid attack.
4) Corona can waste power; corona points equals 1 kW per James Booker.
5) Corona can eat the re-enforcing steel cable out of an ACSR cable.
Ultrasound may tell you there is corona present but visibly seeing the exact location and type of corona can identify exactly what the cause and future action should be.

6. DEFINING FLASH-OVER INDICATIONS
At very high voltages flashover can leap across great distances like a stone skipping in a pond and be very deadly. High voltage uses the circumstances offered by partially conductive areas and air that is ionized to bridge gaps that are too long for the applied voltage under a static condition.

7. DEFINING ARCING INDICATIONS
Arcing comes in multiple shapes from arcs across a small gap (a few thousandths) with only a few volts present in electric arc welding to tiny gaps at 220kV producing random arcing, too small to produce heat in the connector's mass, or at 35kV with a loose aluminum wire in a clamp. The ultrasound unless held up to some of these source would not detect them, the corona camera can see a very short arc across the road or a mile away dependent upon the KVA present in the arc which determines the light emitted.

8. DEFINING PARTIAL DISCHARGE
This is the combination of arcing within voids in an insulating material and potential corona discharge in the air over the insulating material. Partial discharge uses a hardwire voltage and current detection method thus it cannot readily discern discharge within a material from airborne corona discharge.

9. SUMMARY TABLES click images below to view full size
The following tabulations summarize the comparative performance parameters of various technologies in terms of specifications, distance performance and suggested applications in detecting and evaluating insulation breakdown corona indications.