60 Hz.

Joe_B
Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
in Off Grid Solar & Battery Systems #1
Since The long hours of summer sun are here, I have been running my house off grid for the most part. One thing I have noticed is that my digital clocks are all running fast. They are all gaining about 2 minutes a month. So the question I have for all you off grid guys is do you notice any frequency related issues with your inverter systems?
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  • CATraveler
    CATraveler Solar Expert Posts: 98 ✭✭✭✭
    #2
    Re: 60 Hz.

    My RV inverter is not that accurate. But I don't see a issue as your frequency is less that 0.5% off. It will never be as accurate as atomic clocks (1 second in 200 million years) or utility power.

    What is accurate anyway? Recently there was an article about adjusting the clocks by a leap second since the earths rotation had slowed.
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #3
    Re: 60 Hz.

    Hi Joe,

    Two minuter per month is not too bad. Even high quality inverters are not perfect. Most off-gridders that I know tend to use battery operated Quartz clocks and Atomic clocks, and do not depend on AC clocks or timers for accurate time.

    Some of the issues for off grid use are that inverters often drop or perhaps add "cycles" when syncing to a generator for a relatively seamless switch to gensets, which exaggerates the inherent frequency disparity of inverters.
    EDIT: Of course, most gensets are several cycles off (at best) the normal 60/50 Hz grid frequency. Even the inverter gensets are not perfectly accurate frequency sources.

    Atomic and Quartz clocks in use here. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #4
    Re: 60 Hz.

    Your inverter is actually pretty accurate at 2 minutes a month:
    • 2 minute per month * 1/(30 days per month * 24 hours per day * 60 minutes per hour) = 0.000046 = 0.0046% accuracy

    The "instantaneous" accuracy for the grid is typically +/- 0.5 Hz.
    • 0.5 Hz / 60 = 0.008 = 0.8% accuracy

    Of course the grid frequency is usually much more accurate than that--Plus the utility tends to run the frequency slightly slow during the day (during heavy loads) and run a bit fast at night--so the over all 24 hour frequency drift is pretty much zero.

    There are many inverters (if not most) that do not hold frequency accurate at all and running AC powered clocks is pretty much useless (let alone the whole backup generator frequency drift of upwards of 3-5 Hz).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #5
    Re: 60 Hz.
    BB. wrote: »
    Of course the grid frequency is usually much more accurate than that--Plus the utility tends to run the frequency slightly slow during the day (during heavy loads) and run a bit fast at night--so the over all 24 hour frequency drift is pretty much zero.

    -Bill

    My understanding is that there is a standard applied to utility power, at least in the US, which mandates the return to zero offset every 24 hours. The instantaneous frequency has to coordinate across the whole intertied grid unless power is moved from one utility or zone to another via DC transmission. (As is mandated for power moving across state lines into and out of Texas.)
    SMA SB 3000, old BP panels.
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  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
    #6
    Re: 60 Hz.

    Thank you all for the replies. In the winter, my solar yields are too low to go completely off grid as I do in summer. When my inverter is in grid tied mode, it is sync'd to the grid and clocks are super accurate. This summer is the first time I have run for an extended period without being tied to the grid and have noticed the clocks running fast. Guess it is what it is...

    Thanks again.

    Joe
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  • chevenstein
    chevenstein Solar Expert Posts: 100 ✭✭
    #7
    Re: 60 Hz.

    Joe,

    My experience living off grid year round and observing a frequency meter is that 60 is a target, and not the norm. My particular inverter will wander as low as 45Hz to a little over 60 when it is just above the seek load (10 watts), though it is almost spot on under more load. My generators will all over speed with no load and under speed with progressively higher loads, as their governors are not perfect. For example, the LP unit on the house will run as high as 62Hz with little load and bog down to 57 Hz when it is almost loaded to its nameplate capacity.

    NERC (the North American Electric Regulatory Corporation) specifies that the regional grid operators (most of them are ISOs in the US - Independent System Operators) must stay quite close to 60Hz, to the extent that they will add or remove tens or even hundreds of MW (yes, MEGAwatts) of generation in a matter of seconds to keep it balanced. The tendency to be over frequency by some hundredths at one time during the day or under at others is a function of the particular control algorithms in use by your ISO and the ability of the participating generators to react to the signals (usually a 4 second time horizon is used). In the past few years a lot of big generators got new governors and newer equipment replaced older equipment, so the grid frequency is probably as close to 60 as it will get. There was some talk recently about relaxing the standards, but there is too much concern over unknowns.

    For contrast, a business partner of mine does a lot of work in parts of Asia where they tend to not care and the grid and swing around by 1Hz or more (that's a BIG DEAL). But when most people are cooking dinner over open flames there aren't too many people noticing clocks run fast or slow.

    Sorry for the ear full, but this can be an interesting topic.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #8
    Re: 60 Hz.

    If you are talking about your Magnum inverter swinging from 45 to 61 Hz--I would suggest you give the repair center a call and see what they say. That does not sound "normal".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Dill
    Dill Solar Expert Posts: 170 ✭✭
    #9
    Re: 60 Hz.

    my ProWatt SW 1000 subtracts about 7 minutes per week. I don't have a clock in my cabin to check my MorningStar Suresine 300, but that's a moot point since I shut it off when I leave anyways.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #10
    Re: 60 Hz.

    chevenstein,
    i agree with bill that your magnum should not be wandering in frequency by that much and you should be sure it is doing that before you call them up for a possible rma to be fixed.

    as to adding or subtracting power, that really doesn't affect the frequency.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #11
    Re: 60 Hz.

    From the way I understand the control theory behind controlling the grid frequency--It is, very interestingly, all about the total grid power and loads. There is not "one knob" somewhere that a engineer can twist to adjust the grid frequency.

    When there is a (massive) load added to the grid, all the of mechanical turbines actually slow down as they absorb the load. It takes a few seconds (if not minutes) to adjust the driving power to the turbine to hold the new load (for nuclear and coal, it can take hours to affect large power swings--i.e., a 50% or so change in supported load by the plant--adjusting the amount of "heat" from the fuel, boiler/turbine steam flow, etc.).

    Natural gas turbines and hydro stations can react much quicker than "base load" plants (nuc and coal).

    http://en.wikipedia.org/wiki/Load_following_power_plant
    Base load power plants operate at maximum output. They shut down or reduce power only to perform maintenance or repair. These plants produce electricity at the lowest cost of any type of power plant, and so are most economically used at maximum capacity. Base load power plants include coal, fuel oil, almost all nuclear, geothermal, hydroelectric, biomass and combined cycle natural gas plants.

    Peaking power plants operate only during times of peak demand. In countries with widespread air conditioning, demand peaks around the middle of the afternoon, so a typical peaking power plant may start up a couple of hours before this point and shut down a couple of hours after. However, the duration of operation for peaking plants varies from a good portion of the waking day to only a couple dozen hours per year. Peaking power plants include hydroelectric and gas turbine power plants. Many gas turbine power plants can be fueled with natural gas or diesel. Most plants burn natural gas, but a supply of diesel is sometimes kept on hand in case the gas supply is interrupted. Other gas turbines can only burn either diesel or natural gas.
    ...
    Load following power plants run during the day and early evening. They either shut down or greatly curtail output during the night and early morning, when the demand for electricity is the lowest. The exact hours of operation depend on numerous factors.
    ...
    Hydroelectric power plants can operate as base load, load following or peaking power plants. They have the ability to start within minutes, and in some cases seconds. How the plant operates depends heavily on its water supply.
    ...

    [h=3]Boiling water reactors[/h] Boiling water reactors (BWR) and Advanced Boiling Water Reactors can use a combination of control rods and the speed of recirculation water flow to quickly reduce their power level down to under 60% of rated power, making them useful for overnight load-following. In markets such as Chicago, Illinois where half of the local utility's fleet is BWRs, it is common to load-follow (although less economic to do so).

    [h=3]Pressurized water reactors[/h] Pressurized water reactors (PWR) use a chemical shim in the moderator/coolant (see nuclear reactor technology) to control power level, and so normally do not load follow. (In most PWRs, control rods are either fully withdrawn or fully inserted - variable control is difficult, partly due to the large bundle sizes.)

    In France, however, nuclear power plants use load following. French PWRs use "grey" control rods, in order to replace chemical shim, without introducing a large perturbation of the power distribution. These plants have the capability to make power changes between 30% and 100% of rated power, with a slope of 5% of rated power per minute. Their licensing permits them to respond very quickly to the grid requirements.

    So we end up with (to make up some numbers) of ~50% of the electric power on the grid being supplied by "base load" plants are running a full (or fixed) output power for days/weeks/months on end with little modulation of the output (cheapest energy--long time constant for power changes--perhaps a day or more).

    Load follower plants that (probably) are programmed to follow the "predicted" load curve for each day. (24 hour graph for Sunday, July 29th, 2012. Gold line is available power (peaker/hot standby/idling capacity). Solid Blue line is actual demand, dotted lines are 24 hour (purple) and 1 hour (light blue) predictions (for something like 80% of the electricity in California):

    ems_small.gif

    And we have peaker plants that can respond with within seconds to minutes.

    All of the "physical turbines" are synchronized in a region (ignoring DC Intertie connections). And the any "local turbine" will increase or decrease power output as local "throttle" is adjusted.

    It is only when all power and all turbines interact together to slowly increase RPM (line frequency) or decrease line frequency as the change in power into the total grid affects "rotational" speed.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • chevenstein
    chevenstein Solar Expert Posts: 100 ✭✭
    #12
    Re: 60 Hz.

    Adding and subtracting load impacts the frequency of the grid in more or less the way that BB described, though the load following group is usually small and is a separate market in deregulated areas (it is considered an ancillary service and usually called regulation). Recently there has been a shift toward using storage devices and other strategies to regulate the grid, (now defunct) Beacon Power was trying to do this with flywheels and published the graphic below in a public FERC (Federal Energy Regulator Commission) filing, which I think illustrates what is going on on a very short time scale pretty well:

    Attachment not found.

    For those really curious, go to one of the ISOs' web sites and see what you can find in their public libraries on operations and generator control, which will include regulation. I can't remember exactly what is and is not public information any more, so it would be best if you seek it out and find it yourself.

    BB, I agree that there may be something wrong with my inverter, but it has performed without (other) issue for almost three years now so I'm inclined to leave it alone and just avoid plugging any clocks into it 8) I have a suspicion that at low loads the inverter may be occasionally going into seek and that my meter simply can't see that change quickly enough so it averages it out and I see 46Hz, when in reality maybe it was 59.9, then 0 for half a second, then 59.9 :confused:
    1hr.jpg 192.7K
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  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
    #13
    Re: 60 Hz.

    Years ago, before I got into solar I read somewhere that the power companies actually did corrections over night to ensure that an exact amount of cycles occured. They would monitor the frequency of the grid during the day and make up the missing cycles over night. I must admit that when I am on grid, the clocks run within a second over they time between resetting them for DST. Most of my clocks are battery based but my TV and other appliances like the microwave are slowly losing time when on the inverter.

    Chevenstein, 46 Hz is way low and might cause overheating issues with devices that contain transformers. I would be cautious.

    Joe
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