Corona Effect Differences in Long and Short Hardware Strings

Interesting topic. I wish I knew more about it to help you out.

Hello,

This is a tricky inquiry....

If I didn't misunderstand the problem my proposal is to make an electrical model of all the most relevant items of the electrical network.

Given:

-The working frequency is very low, hence no capacitance / coil inductance

-The surge current and transient phases don't come into account since they last a very short time

- The maximum wattage / power the network is intended to give to customers

- The worst ambient temperature the network is intended to work in

_ The "rho" parameter of each part of the network along with length and diameter of each part

- The thermal conductance between each part and ambient atmosphere

Yield:

- a mesh intended to modelize the whole network

Calculate and make a simulation:

- the loss of power inside the network and the global temperature reached by the critical part of the network as a function of the above parameters

Your choice:

- The temperature exchange between the ambient air and the critical part must go to a balance and keep the critical part temperature below an "acceptable" loss of power

Thermal and electrical parameters of the most common materials could be downloaded with Google

http://www.reade.com/Particle_Briefings/elec_res.html

Hope that could help

The manufacturer should have all the data and tests results you need. I would ask for the UL reporsts and listing to corrovorate all information received from vendors.

You are correct, the smallest conductor will have the highest surface voltage gradient.

With regard to the line attachment hardware, the strings with the socket eye extension links will be the most critical because they are not closely capacitively-coupled to the phase conductors.

Since you mentioned strings, you are apparently using strings of ball-socket suspension insulators of either glass or porcelain. Although the same principles apply to polymer suspensions, at your system voltages, polymer insulators are normally equipped with line (and ground at 400 kV) corona control rings to protect and grade the phase to ground voltage thereby compensating for the lower insulator capacitance.

Additional considerations are the length and strength ratings of the suspension insulators. The worst case will generally be the shortest strings of the lowest strength insulators in the single string, single conductor 230 kV tangent assemblies because that will result in the highest voltage gradient on the line end units.

The line to ground voltage distribution over an energized insulator string is highly non-linear. In a string without line end control rings, about 20% of the line to ground voltage will be developed across the bottom insulator with about 14% across the second, 10% across the third and so on; resulting in about 50% of the line to ground voltage over the bottom 4 or 5 units in the string. This frequently results in cap lip and pin corona on the bottom 1 or 2 units even in fair weather. If the voltage gradient is greater than about 25 kV rms/cm, the corona discharges will be visible and will generate significant RIV and TVI.

Thanks a lot for your answer Bluestone. It is just right on the spot. Below are a couple of drawings of one of the many string arrangements to test (dimensions in mm). As we manufacture fittings we omitted the real length of the insulator strings. And by the way, yes, they would be glass ANSI 52.5 insulators for 25,000 lbs. these are 400 kv strings and 25 insulators are the usual.

If I understood well, a good result in testing option "A" would cover option "B", providing that the quality of the materials finish is the same?

Of the two assemblies option A will induce more corona. At 400 kV I would suspect both of these assemblies to have substantial corona issues without any grading. To generalize the farther from the conductor or bundle the first insulator bell is, and the more hardware bits you have in the assembly, the greater the corona will be.