7/27/2019 OVN 31 4-17-00 Dielectric Strength

1/2

To: Ray Garvey/CSIcc:bcc:

Subject: OilViiew News #31, "Dielectric Constant and Dielectric Strength"

This memo provided to share OilView information among OilView minilab users.

If you know others who want to be added to this news letter, just "reply" to me and let meknow their email address. If you want to be taken off of this news letter mailing, pleaserespond and I will take your name off the list.To get back issues of "OilView News," use the Internet,www.compsys.com, Support, OilView News

Today's subject: "Dielectric Constant and Dielectric Strength"

Hello OilView Users,

Danny White, Condition Based Maintenance Specialists with Georgia Powerrecently asked me a very good question. He asked what the 5100 dielectricmeasurement means in terms of Volts. Here is the reply.

The term Dielectric is applied to two physical properties of oil which arerelated but different: Dielectric Strength and Dielectric Constant.

Dielectric Strength is the voltage level at which the insulating property ofthe oil breaks down and electrical arcing begins. Oil is normally a very goodinsulator, allowing a very high voltage to build before arcing.

Dielectric Constant(also known as "Permitivity" or just "Dielectric") is ameasure of impedance to alternating electric fields. In other words this is a measureof how much the material is heated up when AC electric fields pass through. Forexample your microwave oven produces an AC electric field that passes throughthe space in the oven. If the oven is full of air (dielectric = 1) it doesn't heat much at

all even on high power because there is almost no impedance. Even a paperplate (dielectric = 3) receives almost no heat from high power microwave.However, when you put water in the box (dielectric =75) a lot of energy is taken outof the field.

Oxidized oil and oil with moisture in it both have lower dielectric strengththan new oil. They also have higher dielectric constant. New oil has the highestdielectric strength and the lowest dielectric constant it will ever have. Any insultto the oil will lower the dielectric strength and increase the dielectric constant.

There is an inverse relationship between dielectric strength (standard insulatingoil test) and dielectric constant (5100 test). This is why it makes sense to use the5100 to screen for oil quality in transformer oils.

I know you did not ask about this, but I will add that if it were me, I woulduse the 51PC to test transformer oils. This is the only time I will say dilution is notrecommended for the particle counter. The 51PC includes a color sensor and adielectric sensor which report measurements for undiluted tests. Particle countingis not standard for transformer oils, but I think it is a good idea because whenarcing occurs and when the paper insulation around the copper becomesdegraded, the particle counts should increase.

The most critical measurement for transformer oils is dissolved gas in oil.This is important because it reflects the amount of arcing that has occurred and

7/27/2019 OVN 31 4-17-00 Dielectric Strength

2/2

can even identify the maximum temperature and severity of the problem. Whenarcing occurs in a 40,000 gallon transformer, the oil may show little measurableeffect beyond dissolved gases. Keep in mind that in order to measure dissolvedgases you must use a syringe to extract the sample without exposing it to air.

Best regards,

Ray Garvey