Isolator interlocking logic 1

[Mbrooke]

Is it typically industry practice, or recommended that motor operated disconnects have interlocking (error) logic or is a switching procedure checklist and strict protocol considered satisfactory?

Second, does an enable switch for the rotary switches controlling the motor operators and breaker sound typical?

Here is an example:

1. Bay isolator 202-89-L can not be operated unless circuit breaker 202-52-CB is open- this prevents the inadvertent operation where the isolator could break full current and voltage.

2. Bus isolator 202-89-A will only open if 202-89-B is in the closed state, and 202-89-B will only open if 202-89-A is closed assuming breaker 202-52-CB is closed.

3. 202-89-A and 202-89-B can only be closed together (or one opened when both are closed) if the bus coupler is also closed. Ie 89-A OR 89-B can close, but once that happens the other can not close unless 52A is asserted on the coupler breaker. If both are closed neither one can open unless BC is closed.


I know this is showing only one bus type as an exmaple, however my question basically applies to all bus types as it is still relevant IMO.

Finally, is CT over current logic with a pickup/drop out timer helping make the open breaker decision a stable marker? Or 52A contact? Or something more extravagant?

 

[crshears]

The utility that employs me has, generally speaking, always used switchgear interlocking such as you describe on equipment that operates at 115 kV and above, precisely for the purposes you describe. Procedure checklists are not perceived as being sufficiently reliable to provide the equivalent equipment protection.

At certain of our facilities there is a notable exception available from specific manufacturers of SF6 insulated switchgear that enables the operator/controller [o/c] to, with discretion, invoke a "Switchgear Interlock System By-Pass." This SIS by-pass defeats all of the logic and position-driven interlocking on the site to provide a means of quickly removing from potential / separating from the grid any equipment that experiences a low SF6 density condition or hydraulic system failure that would block its tripping. When such a condition occurs, the o/c steps through a series of checks to confirm use of the SIS by-pass will not cause a problem worse than the initiating condition.

For example: in a breaker-and-a-half 500 kV switchyard, if the centre breaker in a diameter becomes inoperable from either control or protection, NERC requirements are to remove that breaker from potential within 30 minutes. Conventional switching in such circumstances could involve shedding 800 MW of nuclear generation, which could be undesirable...instead, SIS by-pass is implemented; first, the o/c scrutinizes the switchyard equipment status to ensure at least one solid galvanic loop exists from one side of the inoperable device to the other such that the following operation only opens up a loop, but does not force a switch to interrupt load, merely re-distribute it. Next, SIS bypass is invoked to defeat the interlock, and before it times out the o/c gives an open impulse to the switches on either side of the inoperable device. Voilà! End of problem.

CR

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

 

[Mbrooke]

Do you know what they use in the logic process to make sure a loop is established? Ie voltage, current, 52A? Can this be done via boolean in a standard 411L or 421?

 

[crshears]

That part is entirely human; via supervisory control, the operator / controller checks that all intervening breakers and switches are closed to form at least one loop, more if possible, then invokes SIS bypass and operates the isolating switches.

CR

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

 

[Mbrooke]

Ok so SIS is manually invoked and not dependent any input logic.

Could it then be argued that a MOD enable switch be sufficient? Ie an operator would have to perform at least 2 actions plus the "are you sure" in order to open a mod.

 

[crshears]

Hmmm...

In normal situations: using mouse, [1] select device, [2] select operation, [3] select execute. Normal operation of such interlocks as you suggest is fully in effect.

When use of SIS bypass is required: using mouse, [1] select yard SIS bypass, [2] select ON, [3] select device, [4] select operation, [5] select execute.

Some of the newer equipment has individual SIS poke points associated with each breaker / MOD combination instead of a site/global SIS.

I hope I'm answering your question...

CR

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

 

[stevenal]

Perhaps this little event might have avoided with some sort of interlocking. See page 20 (pdf 30). Not part of the NERC recommendation, though.

 

[crshears]

That report might make good night shift reading...but yes, if the switching agent at the series capacitor had not been able to operate devices out of sequence the whole schlemozzle might not have occurred.

CR

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

 

[Mbrooke]

Question answered I might do it like the SIS, but for all situations. Basically for me its just to increase the number of mouse clicks if I do it the non interlocking logic way.

Now, as for the substations themselves- how is it done? Do you have rotary switches for equipment or is it done via a bay control screen?


@Stevenal: Much thanks- I am open to both sides of the argument.

 

[crshears]

The utility that employs me utilizes remote control from one central location for the province, sufficient for normal day-to-day operation.

On-site control of all circuit breakers and MODs is also available via a Local Control Computer.

About half of all the high-voltage equipment earthing/grounding switches are motorized, and are provided with local device-specific lockable-mechanism-box open-close pushbutton stations. The other half are strictly manual.



CR

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