Selection of Epoxy Support Insulators in MV Switchboards
Abhijeet Limaaye
Certified Energy Auditor committed to make the world a Safer and Energy Efficient place
In case of MV Switchboards , it undergoes both electrical and mechanical stress during operation as well as fault conditions. While electrical stress (temperature rise) is the primary criteria for busbar selection, electrodynamic (mechanical) stress influences selection of busbar support insulators.
The support insulators are made from epoxy resin with Brass threaded inserts. Some manufacturers use MS inserts but Brass is a better option. The insulators have strong electrical and mechanical properties. They also have a very high resistance to chemicals. The following properties are attributed to a good support insulator.
- - Good Dielectric Strength
- - Sufficient creep age distance.
- - Good arcing strength.
- - Good bending and tensile strength.
- - Compact size
Most of the reputed manufacturers today use insulators complying with the above criteria. The Standards IS 9431, IEC 60273 and IEC 60660 lay down some basic selection criteria for the same. While there are a host of tests that need to be carried out, following tests are paramount :
- - Power Frequency Voltage withstand test
- - Impulse Voltage withstand test .
- - Fall bending load test (Mechanical bending strength) .
- - Partial Discharge test
- - Water absorption test
The manufacturers complying to the above standards have formulated values corresponding to the above criteria.
In case of any fault, the force on the busbars can be calculated as below :
F = 2 x L/D x Ix Ix 10-4 kN (Where I and I are in kA)
Where I & I are the phase currents, L is the distance between two supports & D is the phase-to-phase distance.
Let us consider a case of 11kV system with 25kA Short time current capacity . The standard distance between two supports is 500 mm and phase-to-phase distance is 130mm. In case of three phase symmetrical fault , I= I = Peak withstand current = 62.5kA
Hence, F = 2 x 500/130 x 62.5 x 62.5 x 10-4 = 3.004 kN
From the figure, we can see that the insulator is subjected to a cantilever force. This can be calculated as,
F= F x [H + (X/2)] /H
Considering 11kV insulator height = 130mm and Busbar width = 80 mm (For 80x 10mm Busbar) F = 3.004 x [130 +(80/2)]/130 = 3.92kN
Hence, mechanical strength (Falling Load) for 25kA STC must be at least 4kN .
However, if the same criteria is used for STC= 31.5 kA i.e I=I= Peak withstand current =80kA with a 100mm x 10 mm busbar ,
F = 4.92kN and F = 6.81 kN
Hence, mechanical strength (Falling Load) for 31.5kA STC must be at least 7.5kN . The same for 40kA STC will be at least 12kN .
Thus , we can understand that in the selection of support insulators, we have to consider the parameters of both creepage as well as mechanical falling load .
