Insulator Q&A: your engineering questions answered

To refresh minds and educate new engineers, Tim Van Remmen, Director of Engineering, Lapp Insulators, regularly runs an ‘Insulators 101’ webinar. In his most recent webinar, he fielded some interesting questions from engineers, and shares his answers here.

If you attended my Insulator 101 webinar, you’ll know that insulator design can be complex. I received many questions from engineers during the session, and some in the chat. I’ve expanded on some answers here. I hope it helps.

Feel free to send us your insulator questions any time and we can answer them for you in the next Q&A.

1. Why do long rods need to use C130?

The porcelain long rod insulator is designed to be used in a constant tension application. That means the core of the insulator is under constant stress.

Stress on a porcelain body with an uncontrolled quartz grain size can lead to micro fracture prorogation. These micro fractures can ‘age’ the porcelain and reduce its life expectancy.

The Alumina C130 body replaces the quartz with alumina and therefore does not have micro fracture propagation issues.

2. Do TR post insulators over 350kV BIL have the same cantilever strength rating for both upright and underhung mounting?

NEMA C29.9 Table 1 lists both ‘upright’ and ‘underhung’ cantilever strength ratings, but Table 2 shows only ‘upright’. It’s best to confirm whether the insulator can be used in the upright and/or underhung positions with the insulator manufacturer. 

RUS Bulletin 1724E-300 (2001) section 4.8.1.6 states that insulators with the same bolt circle on top and bottom are rated for both upright and underhung applications, which doesn’t correlate with ANSI C29.9 Table 2. 

Don’t assume that the insulator can be installed in both positions. To be sure, you should see a rating for upright and underhung on the insulator drawing/cut-sheet.

3. Does the RG Glaze© insulator get sized differently than traditional insulators for contamination purposes?

Typically, no, there’s no special sizing done for an RG Glaze insulator. It would usually have the same strength, size and mechanical ratings of a non-RG-Glaze unit.

What differs is the formulation for RG Glaze, which is dependent on the voltage class and surface area of the insulator. Standard RG Glaze is managed to a target of 1±0.5 milliamps, so it’s not really a matter of sizing differently.

The formulation is then matched to the size of the insulator. So you can’t take a 550KV BIL insulator with RG Glaze and apply it at a lower voltage, because you won’t get enough voltage across it to reach the target current.

We manufacture all RG Glaze to order and we discuss the intended application of that RG with the end user. This way, we make sure that the formulation is appropriate for their specific application.

4. Why is double minor shed profile obsolete now ?

CADD analysis has shown that we can achieve just as much leakage distance with the major-minor design as the major-double minor which was primarily used prior to CADD.

5. Why is isostatic process preferred for large insulators ?

The isostatic process typically allows the creation of larger single-piece units, so where a typical 2050 kV BIL insulator would take 3 pieces to assemble. 

A unit created using an isostatic process could reduce this item to a 2 piece assembly. 

This eliminates 1 connection point and associated hardware.

6. In your view, how can the industry effectively revisit IEEE 605 to align on a widely accepted consensus?

I recently went to Denver to meet the IEEE committee. We discussed how it arrived at where it is in the standard, how to resolve it and what to recommend going forward.

I think the basic recommendation is going to be this: if it’s part of the guide now, there will be data showing that we have to change the testing requirements. It shouldn’t be a test of one, but a test of ten, just like ANNEX X of IEEE 693 so that end users can understand the risks.

ASC 113 references the old IEEE 6O5 (2008) standard.

7. How does the insulator manufacturer typically design the bolt circle, bolt size, and plate thickness for mounting?

EHV line posts can carry large cantilever loads and in my experience the manufacturer does not provide the actual mounting bolts. How can the engineer specifying bolts be sure they’re selecting bolts consistent with the bolt circle design?

The specific designation of the bolts that mount to our insulators is the responsibility of the substation designer. We don’t know what the bolts are, what the insulators are mounting to, or the thickness of the structure or steelwork.

It’s up to the substation designer to do perform the calculations on the bolts to understand the stresses that are going to be applied. They’ll define whether they need A3O7, Grade 5 or Grade A bolts.

That’s on the substation designer, not the insulator manufacturer. We only consider how things are mounted to our insulators, not what’s used to mount to them.

Have a question about insulators and engineering? Get in touch and let us know. We’ll try to answer it in the next Q&A from Tim.