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  1. #1
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520

    Electrical Peak Demand Limiting Controls

    I have a facility control with Tracer Summit and would like to install power monitoring and controls to shed loads when the power exceeds a preset limit. The purpose is to reduce peak 15 minute demand from approx. mid May to July for a smaller utility bill the other 9 months of the year. Last year we hit a peak of 477kw in May some time last year, but the following 3 months we never exceeded 318 kw. That extra 100kw cost us approx. $10k in higher demand pricies for the remainder of the year.

    Here's the HVAC equipment I can control trouch Tracer Summit:

    3 Voyager RTU with gas heat and economizers (no sure on size, I think 10 tons each)
    1 Voyager RTU with electric heat, economizer Varitrac II system (again 10 tons)

    Chilled water system:
    New Trane CGAM 130Ton just installed (4x20 ton and 2x25 ton compressors)

    1 large custom air handler with economizer supply & return fans. (estimate 50 ton capacity, using only 2/3rds capacity on current building design (1 wing left unfinished). VFD on chiller water pump to control discharge temp. Hot water reheat from a steam heat exchanger.
    1 10 ton air handler with 2 pipe hydronic coil. Trol-a-temp - zone control sytem. VFD on blower.
    Couple dozen 2 pipe hydronic fan coils - not on tracer, other than the stema heat exchanger.

    The load on chilled water system at outdoor design conditions I estimate currently is only about 80 tons, but it will run at max for almost 10 minutes when changing over from heating to cooling mode as warm water hits the system. Heat supplied by gas fired steam boilers.

    My thought was to monitor current on the incomming power. If it exceeds lets say 450 amps, then the control starts restricting the RTU's to 1st stage, and either limits RLA, disables the 3rd compressor in each circuit or raises the chilled water setpoint.

    How hard is this to do? Has anyone done this before and what were the results?

  2. #2
    Join Date
    Jan 2012
    Location
    California
    Posts
    88
    We do a bunch of demand limiting and it is and has been part of our standard software for years if not decades. We do it at the setpoint level by relaxing them, and at the AH by decreasing the static setpoint's. I am not familiar with the Tracer programing but if you had a setpoint adjustment you could write to that is what I would do. We typically start at 2 deg f per demand level trip (standards) and make exceptions to the strategy (data space, presidents office...). Hope this helps I'm sure there are other ways to do it but this has worked out well over the years for us.

  3. #3
    Join Date
    Jan 2002
    Posts
    259
    Very easy, done several, very good response from customers. Usually done as part of LEED green building certifications. Give engineers three levels of set points to unload facilities as you get closer to your KW set point. Start looking at KW meters online with bacnet/ip capabilities and talk to your controls contractor about installing/ doing cpl programming in Summit.

  4. #4
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Great responses. I like setpoint control concept. Especially since I think last year's peak occurred after a power loss. Go figure, we lose electric service for a few hours and the power company gets to screw us for another 11 months afterwards.

    Sent from my SGPT12 using Tapatalk 2

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