Switchgear modeling - Inductive loops

[Adam1980]

Dear all,

I've been performing transient over-voltage studies which involve vacuum circuit breakers (VCB).

In order to simulate the actual phenomena that the VCB causes (mainly high frequency re-igntions), a high frequency model of all the equipment should be included.

I am having a problem comprehending the representation that I have came across of the switchgear. The switchgear is represented as inductance pieces between the breakers and the disconnectes. The inductances would be a representation of the inductive loops caused by the construction of the switchgear and the available current.

I am aware of the idea that the current passing through the busbar of the switchgear would cause a magnetic field which is modelled as an inductance. However I would appreciate if somebody could add to this and provide some more explanation of this representation.

Thank you.
Adam

 

[PHovnanian]

I don't know what more to add without specific questions and/or a peak at the model you are working with.

In addition to modeling the switchgear, you may have to build a more extensive model going back to some source. Like your distribution transformer, service entrance equipment, feeder, etc. Some estimation of the source L,C, and R should be made for these parts of the circuit. At the frequencies of the restrike, don't forget to include the phase to phase and phase to ground capacitances.

This sounds like a job for some specialized modeling s/w that can handle busbar geometries to derive L and C at the frequencies of interest.

 

[Adam1980]

Thank you for the reply.

I am asking in general and not for a specific single line diagram.

The modelling of other elements such as a transformer with its surge capacitance and leakage reactance is clear. also the same for modelling cables either by there surge impedance and propagation velocity or high frequency L and C derived from the surge impedance and permittivity.

However for the bus bar it is still not clear to me how to derive the L based on the geometry which you indicated in the last sentence. Is there any literature that refers to this.

Thank you.
Adam

 

[PHovnanian]

The L and C parameters for busbars and switchgear is derived in much the same way as they are for transmission lines or cable runs. The formulas (and tables) for transmission lines and cables make some simplifying assumptions based on round conductors and the relationship between conductor dimensions and spacing. You should search for information on calculating busbar impedances, since they more closely resemble your switchgear, with its rectangular conductor sections. Or look at some electromagnetic modeling applications.