Polarization Current VS Absorption Current 2

[BigJohn1]

When doing insulation resistance testing, I understand there are two different measurement ratios: Dielectric Absorption Ratio and Polarization Index.

I think I have a good understanding of polarization current being measured during the PI test where the dipoles in the conductor insulation are being re-oriented in the presence of an electric field.

However, I can't find any solid explanation of absorption current. Is the measurement made during a DAR simply a different value of the same current as a PI?

If not, what is absorption current? Anyone care to tackle this? Thanks.

-John

 

[petronila]

Hello BigJohn

A modern instrument(Megger) could calculate both: PI and DAR values, you can read the following manual: Guide to modern insulation testing by Metrel.

Regards

Carlos

 

[electricpete]

IEEE 43 identifies three components of current:
1 - Capacitive
2 - Resistive (leakage)
3 - ‘absorption’ and ‘polarization’ currents…both terms have the same meaning.
Capacitive decays away quickest, then aborbption current takes long, resistive of course is constant.

Dielectric Absorption (D.A.) = Ration 1 minute reading / 30-sec reading
Polarization Index (P.I.) = Ratio 10 minute / 1 minute reading.

D.A. and PI both attempt to deduce the same behavior (high resistive current compared to other two components… primarily absorbption.). The only reason to do D.A. instead of P.I. is because it’s easier (particularly with hand-cranked megger where cranking for 10 minutes would be a pain). However the P.I. is a better test and the only one recognized by IEEE 43 (D.A. has fallen out of favor now that very few people use hand-cranked megger).

The values of DA tend to run lower than the values of PI for a given machine/condition, as suggested by following limits given by Megger “Stich in Time”:
Questionable: 1 – 1.25 DA, 1-2 PI
Good: 1.34 – 1.6 DA, 2 – 4 PI
Excellent: > 1.6 DA, > 4 PI


=====================================
(2B)+(2B)' ?

 

[BigJohn1]

Thanks for the replies.

So both tests are fundamentally measuring the exact same physical principals, the only difference being that obviously a PI would be more comprehensive?

Is the DAR called that simply because much of the current increase in that test is also capacitive, so it's not primarily measuring polarization and leakage?

Am I correct in thinking that's the importance of comparing the 30 second and 60 second readings: You're trying to null out the capacitive charging that occurs in the first 30?

-John

 

[electricpete]

So both tests are fundamentally measuring the exact same physical principals, the only difference being that obviously a PI would be more comprehensive?

Yes, exactly.

Is the DAR called that simply because much of the current increase in that test is also capacitive, so it's not primarily measuring polarization and leakage?

There is no difference between absorbtion current and polarization current by IEEE43 terminology. Specificaly IEEE Std 43-2000(R2006) Definition 3.1 treats them equivalently:

"3.1 absorption (polarization) current: A current resulting from molecular polarizing and electron drift, which decays with time of voltage application at a decreasing rate from a comparatively high initial value to nearly zero, and depends on the type and condition of the bonding material used in the insulation system."

So personally, I’d be be inclined to think there is not a lot of significance to those particular two choices of names for those particular two tests.

Am I correct in thinking that's the importance of comparing the 30 second and 60 second readings: You're trying to null out the capacitive charging that occurs in the first 30?

Let’s start with absorption: I’d be inclined to think (from the IEEE43 figures) that the PI has a fairly straightforward meaning:
PI = (Iabs1min + Iresistive) / Iresistive.
Where Iabs1min = 1-minute absorption current.
The capacitive is not included here because it is pretty much gone by 1 minute.
You could also rewrite this as:
(Iabs1min / Iresistive) = PI - 1 which gives imo a nice simple definition.

The DAR is not as straightforward. For one thing, absorption current is present on both measurements (30 sec and 60 sec) and it is of course not the same for both. So already we are working with three values (I resistive, Iabs1min, Iabs30sec where in PI we only had two). If Icap is not completely gone at 30 seconds, it’s even messier. So there is not as simple an interpretation for DA as for PI. But I think the values we use as limits tend to be based on empirical-based thumbrules (rather than theory) anyway, so it’s not a big problem.


=====================================
(2B)+(2B)' ?

 

[electricpete]

But I think the values we use as limits tend to be based on empirical-based thumbrules (rather than theory) anyway, so it's not a big problem.

Perhaps the exception is 1.0 limit. Below 1.0 limit on either test means current is increasing… insulation getting worse due presumably to breakdown.

=====================================
(2B)+(2B)' ?

 

[BigJohn1]

...There is no difference between absorbtion current and polarization current by IEEE43 terminology. Specificaly IEEE Std 43-2000(R2006) Definition 3.1 treats them equivalently....

That was actually one of the reason I posted this question, because I saw that definition in IEEE 43 and was confused why there would be such drastically different names for what appeared to be the same phenomenon.


I appreciate the replies. Y'all cleared it up nicely.

-John

 

[Zogzog]

View attachment before reading, the letters refer to the drawing.

When a high DC voltage is first applied the total current (a) consists of three current components;

1. Leakage current
2. Capacitance charging current
3. Absorption current

Conduction/Leakage Current (b) - This current passes through the surface of the insulation. The magnitude of current flow depends on the resistance of the insulation. Surface leakage is usually not a problem because it can be eliminated through external cleaning.

Capacitive Current (c) - The insulating specimen appears, ideally, as a capacitor. As a DC voltage is applied to a capacitor initial charging current flows until the voltage drop across the component equals the source voltage. As the capacitor charges, its charging current decreases to a minimum. This is called its steady-state value.

Dielectric Absorption Current (d) - This current also appears at the initial application of test voltage the same as capacitance current. This current is required to polarize the insulating medium. In other words it is energy absorbed by the insulating system.

To seperate these different qualities of an insulating system you can use the time they take to decay (As you can see on the attachment). Since the currents are decaying the indicated resistance reading on your display will go higher.

When you test, record the values at 30 seconds, 1 minute, and 10 minutes. The 1 mniute reading divided by the 30 second reading is called your Dielectric Absorbtion Ratio (DAR). The 10 minute reading divided by the 1 minute reading is your Polarization Index (PI). The values of these ratios can be used to determine the condition of the entire insulating system and allow you to do a condition assement of your assets.

 

[BigJohn1]

Hey, I know you!

Thanks for the clarification of terms.

-John