Mountain Backup Power (how the transfer to backup generator works)

This might be more than the Telescope Operator needs to know, but sometimes it helps to know the details.

Eric Buckley wrote: "Below is a detailed explanation written by Jon Aalberg (Sabino Electric) on how our electrical system and specifically, our transfer systems work. Please forward it to any other people in your respective areas that might benefit."

Operation of the Automatic Transfer Switches at the Utility Building

From on 20-Jun-2022: (some typos corrected, and pronouns clarified)

It has come to my attention that there seems to be some confusion on how the ATS's (Automatic Transfer Switches) and backup systems work and the significance of the red light within the LBT (control room ceiling).
I will break this down so it can be understood. First off notice that the words ATS's and systems are plural. There are (2) different ATS's and systems and they are not interconnected.


We know ATS-1 as the PMH-9 with a Micro-AT controller.

ATS-1 is a Medium voltage standalone ATS with its own logic and parameters defined by the Micro-AT.
It is not designed to be interconnected/controlled or influenced by outside sources.
It uses voltage sensors to generate it's own operating power at the milliamp level using the field of the 24,900 volts.
It does not designate or recognize utility power or generated power.
It looks for Preferred source and Secondary source.
It has been programmed by S&C to follow the national utility standards of operation and operated at a high level of interruption (meaning it will ride through a lot of flickers in the system, before reacting). The only exception to the programming is the 10:00 am window of transfer.
It will not power a light bulb but does well with the controller as it was engineered to do.
All the previously installed wiring has been removed for it was not in compliance with the NEC, it also was completely unnecessary.
All interior wiring needs to have an insulation level as the 24,900 volt cabling or be factory installed. Existing additional wiring had a 600 volt rating, passing through the 24,900 volt compartments and was not factory installed.
This is the only piece of equipment we can use to verify whether the site is on generator or connected to the grid. The utility power is the Preferred source and the generator is the Secondary source of power that feed the mountain top.

I need to go into a little detail about the PMH-9 a little bit so the understanding is there for the next part of this.
There are (4) compartments within the PMH-9
Compartment (1) is a fused load that feeds the Vatican, SMT, LBT PMH-12.
Compartment (2) is where the preferred source enters and ties in.
Compartment (3) is a fused load that feeds the Utility Building.
Compartment (4) is where the secondary source enters and ties in.

Now the Utility building is where the generator is housed and feeds the site in case of a preferred source failure. The generator will also feed the Utility building in a local failure as needed. The generator responds to the Utility building and its needs not the Site and PMH-9.
Between the PMH-9 compartment (3) and the Utility switchgear there are several points of possible failure, we have MV cable, oil filled transformer, LV cables and molded case circuit breakers.


We know ATS-2 as the Utility building.

The switchgear in the utility building has (3) key molded case circuit breakers that have the capability of "Talking" with each other.

The (3) key breakers
CB-3 which is the Main Breaker for the switchgear.
CB-1 which is the breaker that ties the generator to the bus.
CB-10 which takes the 480v bus and ties to a step up transformer. This transformer takes 480 volts and steps it up to 24,900 volts which enters the PMH-9 compartment (4).
CB-1 and CB-3 can never close on the bus at the same time, not unlike the compartments (2) and (4) of the PMH-9 and CB-10 will only close when CB-1 is closed.
There is a controller that dictates orders to these breakers as to when to change positions and there are Block outs within the breakers and control wiring that will not allow a breaker to change position unless something has changed to allow.
This controller operates at a much lower level of interruption (meaning it will react quickly to a defined number of flickers in the power) than the Micro-AT in PMH-9. If this controller detects (3) flickers (meaning drops in voltages lower than 440 vac) within 15 seconds, the generator will automatically start up. If the flickers subside within 30 seconds then the generator will go into shut down mode which takes 30 minutes. If the flickers continue (deeming this a possible upcoming preferred source failure) it will transfer the Utility building to generated power, only the utility building for it does not care about the site, that is up to the Micro-AT in PMH-9. Power eventually stabilizes, the Utility building transfers back to commercial power and the generator shuts down.
This action described above will require CB-3 to open and CB-1 to close and the closure of CB-1 will trigger CB-10 to close.
When CB-10 closes, the secondary source at the PMH-9 compartment (4) becomes available.
This does not mean the site is on generator it just means generator power is available, this also allows a much quicker transfer time to site if the Micro-AT in the PMH-9 calls for it.

1-Bird lands on line at Bonita. This usually will send flickering or sags on the line going up the mountain. The Utility building captures (3) Flickers in 15 seconds, the generator starts. Bird falls and clears, the generator will shut down. Bird remains and after 30 seconds of flickering the Utility building transfers. The Commercial line finally clears and stabilizes, the Utility building will transfer back to Utility power and generator will shut down. If the fault remains on the line for more than 3 minutes and has extreme sags, the Micro-AT will recognize this as a fault and transfer the site to generator. You will now be locked on generator until 10:00am the following morning.

2-Hard commercial power outage. The generator will start immediately, Utility building would be the first to come up, there are time delays to allow the Utility building to start before the rest of the site. CB-3 opens, CB-1 closes, CB-10 closes then the Micro-AT will see the secondary source and close on it. You are now locked into generated power until 10:00 am of whatever morning utility power has been restored.

3-These last (2) scenarios do not threaten the site but require the generator. PMH-9 compartment (3) fuse failure causing single phase event, the controller in the Utility building recognizes this immediately, starting and transferring to generator, powering up the Utility building ONLY. Same would go for a faulted line between the PMH-9 and oil-filled transformer or oil-filled transformer and CB-3, transformer failure. This would also activate the secondary source to be available in compartment (4) of the PMH-9.

4- Molded case breaker failure. The generator would start but there would be no automatic transfer and the generator would not automatically shut down. There would be a call for human interaction.

The red light in the LBT (ceiling of the control room). It was decided between Kevin Newton, John Little and I that we would tie this to the generator. When the generator is running, the red light within the LBT will illuminate. Being the Utility building houses the cooling equipment for the LBT we figured this to be a viable option. The only function of this light is to show that the generator is running. I am currently working on a solution to installing a light on the PMH-9 to illuminate when site is on secondary source. It will be a local light at the PMH-9 for we need to keep all low voltage wiring out of the compartments rated for 24,900 vac and being no usable voltages within the control compartment this, sounding so simple, has become quite challenging.

-- JohnHill - 22 Jun 2022
Topic revision: r2 - 24 Jun 2022, JohnHill
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