145kV ferroresonance

[martechman]

What is meant by ferroresonance on 145 kV air break disconnectors and what is done to limit it? Also what is meant by busbar switching capability on the same thing?

 

[mersin33]

I don't know about the ferroresonance part.

Re busbar switching: especially in double busbar stations, one can transfer from one bus to another without opening the breaker, i.e.

to on-load transfer from BB1 to BB2 when the breaker is closed:
close BB2 disconnect switch
open BB1 disconnect switch

as you see, switches need to have a switching capability.

 

[rbulsara]

Ferroresonance is a condition that could occur in power distribution (even transmission, I guess) system when the long line or cable capacitance coincides with inductance of transformers such that when the 3 phase transformers are single phased while lightly loaded, could cause severe over voltages (that could set off the surge or lightening arresters and blow fuses /operate OCPD).

According to Standard Handbook for Electrical Engineers (McGraw Hill), this is phenomena, which is rare, is not well understood but it seems to be resolvable by making sure that all 3 phases close at the same time. The air break isolator switches that may not close all phases simultaneously could cause problems.

A disclaimer: I am not expert in HV transmission, but the above statement is technically correct. I have come across and resolved one such situation in MV distribution system once.

 

[cuky2000]

Martechman,

Could you please clarify the term “air break disconnector” or describe this device?

The reason of my question is because typical air disconnect switches are designed only for isolation after open the system with current breaker devices (i.e. Circ Bkr, Circ Switcher, load breaker device).

Standard disconnect switches do not have interrupting capacity no even to open a short line or withstand the inrush current of a transformer without the help of additional power device. However, for substation busbars without load, disconnect switches could be used for some isolation or transfer bus operation since there is not significant charging current accumulated in the system.

Ferroresonance, Ferranti effect or other type of severe overvoltages in disconnect switch could lead to catastrophic effect.

Even though the enclose information is not for 145 kV, this enclose video provide a view of the arching current created in a disconnect switch.
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[busbar]

 
IEEE C57.105-1978 §7.1 — Ferroresonance is a phenomenon usually characterized by over-voltages and very irregular wave shapes and is associated with the excitation of one or more saturable inductors through capacitance in series with the inductor. Intentional and unintentional single-pole switching may often be the initiator of ferroresonant incidents. Anecdotally, ferroresonance at subtransmission and transmission voltages seems to be able to effortlessly launch local-bus VTs half way to the moon.