Interrupting capability of high voltage disconnects 1

[rand1234]

Greetings everyone,

Hope everyone is having a good day!

I had a question regarding interrupting capabilities of high voltage disconnects. Per IEEE 37.30.1, this standard provides you with the ability to conduct calculations to determine the maximum amount of capacitive and excitation/resistive currents a disconnect can break in a no-load scenario. In addition, this ties in directly to the required clearances needed for the disconnect (i.e. the more clearance you have, the higher amounts of current you can break [limits to how much current one can break are set by the standard]).

Determining the excitation/resistive current is pretty straight forward as you just take the no-load currents of your banks. Determining the capacitive currents is a bit more tricky. I have two questions:

a) the proposed disconnect installation is located immediately on the high side of the main bank. The load side of the disconnect will travel through underground cable and then transition to overhead cable before it terminates into a position in the substation. In an unloaded scenario where this disconnect is opened to provide clearance, will it see high amounts of capacitive currents due to it being interconnected to the substation where miles of transmission lines take it to the distribution substations?

b) on the high side of the main bank, we have multiple sets of CT's for protection relays and metering. I was thinking just reading the current values off the relays/metering device will tell me how much current the disconnect needs to break in an unloaded scenario. But I would like to see the current segregated into its components (capacitive vs inductance vs resistive). Is there a way I can estimate this with just the current and power factor values gotten off of these device?

Appreciate any feedback you guys can provide. Apologies if my questions don't make sense. Feel free to ask me for clarification.

 

[bacon4life]

a) Transmission cables have very high capacitance compared to overhead lines. A standard load break disconnect is unlikely to handle multiple miles of cable at 100 kV or higher. I may have misunderstood your configuration as it sounds a bit unusual.

b)CT's tend not to be accurate at low currents, and this method would be entirely inadequate for rating your switch. The excitation current for the transformer should be obtained from the transformer test report. The excitation current of transformers can vary widely, so also consider what will happen when the transformer is replaced. The cable capacitance can be calculated based the on the cable specifications. If you have a physical sample of the cable, you can closely estimate the capacitance based on measuring the geometry.

At a certain length of cable, the cable capacitance would offset the transformer inductance, theoretically leaving just resistive current. Do not less this trick you into under sizing the disconnect.

 

[crshears]

At a certain length of cable, the cable capacitance would offset the transformer inductance, theoretically leaving just resistive current. Do not less this trick you into under sizing the disconnect.

Is not this "tuned" or "tank" circuit one of, if not THE major, cause of transmission circuit ferroresonance? Attempting to remove ferroresonant equipment from service with a disconnect has such a high probability of causing catastrophic switch flashover and equipment damage and/or worker injury that my utility has express instructions forbidding even thinking of trying this, and a fitting instruction it is as we have two dozen or so different circuits at various voltage levels that are susceptible to ferroresonance.

Where breaking duty gets severe enough, we use arcing horns, Joslyn whips, interrupting heads, and other such types of devices to increase the rupturing capabilities of naked switches.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]