Load Shedding

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One of the challenges with the smaller electrical systems found on most boats is managing the power load. Whenever electrical loads are running on a boat, they are delivered by some power source. It might be the generator, it could be shore power, or it could be the inverter. But, whatever the power source, it has some limit on maximum output.

Different power sources have different bounds, but they all have limits. Our generator can reliably and continuously produce 42A. A common shore power configuration in North America is 50A at 240V. In Europe, 32A and 16A at 240V is common and we’ve seen as low as 6A on some shore power pedestals. Our 240V inverter can produce 25 amps and our 120V inverter can produce 33A (details on our power configuration are at A More Flexible Power System for Dirona).

Some of these bounds are quite low, and all are lower than you would expect to find in most apartments, so the electrical loads have to be managed. It’s difficult to stay within those limits, especially when there are multiple people on the boat and some might not be used to managing electrical loads. That’s why boaters frequently are resetting the breakers on shore power pedestals

Our 240V inverter can produce 25 amps and our generator can reliably and continuously produce 42A

Overloading the power source can range from a minor hassle to a major inconvenience. In the case of the generator, you might stall it, which results in a complete power failure until it’s restarted. Putting excess loads on a generator isn’t good for it, and stalling a generator exposes the all the electrical devices on the boat to rapidly decreasing voltage levels and the increasing current draw that results. Stalling a generator is not good for anything, but it generally can be quickly restarted. In the case of an inverter, it might lockout for a brief period after a load-driven thermal overload.

Most boaters have experience with overloading the shore power source and popping a breaker. Resolving this can be as simple as walking outside to the shore power pedestal and resetting the breaker. But in some marinas the breaker cabinets are locked, and help from the marina staff is required to reset. If this happens in the middle of the night, the shore power might be off until morning when the marina staff becomes available. Generally avoiding power source over-draw is better for everyone on the boat, and all the powered equipment on board.

Plugged into twin 6-amp, 240V connections along Sweden’s Gota Canal

To prevent overloading the power source, rules must be put in place. For example, nobody can run a hairdryer when the kettle is on, the microwave can’t be used at the same time as the kettle and the dryer and HVAC or oven can’t be used together. The loads need to be managed to below that of the power source capacity, which means that those on the boat have to know both the power source maximum output and everyone using power on the boat has to communicate. It’s often the case that when a hair dryer is going to be turned on, the user will yell their intent so others on the boat don’t use high draw appliances.

We don’t like that model, and want Dirona to run as much like an apartment as possible, so we implemented load-shedding. This is an approach to automated load management that applies to the generator, inverter, shore power or any other power source. Load-shedding is an automated system that knows the power source limits and prioritizes all the electrical loads ensuring the most important get the available power. This avoids over-drawing the power source, and removes the burden of load management from those on the boat.

When our 240V system is overloaded, be it the generator or the inverter, our load-shedding system first temporarily disconnects the water heater, then the second charger, then the first charger, then the HVAC system. When the 120V system is overloaded, we shed the microwave and the diesel furnace, and finally the engine room fans (with limits on how long they can be off to avoid engine room temperature problems).

The white ‘W/H’ and ‘Chg#2’ boxes at bottom center indicate that our control system has shed the water heater and charger 2 with the generator delivering its maximum of 42 amps (upper right).

Load-shedding gives the feel of an unbounded power source and takes away the hassle of load management. The system just turns loads off and on to keep the draw within the capabilities of the power source. Because the HVAC system always cycles on and off and the shedding through load peaks are usually short term events (often only seconds to tens of seconds long), people on the boat often don’t notice when the system is at work preventing breaker overloads. It makes the boat feel like it has no electrical bound just as most houses and apartments feel. They all have bounds but, in the cases of houses and apartments in many parts of the world, the limit is so high that it’s rarely if ever encountered. Shedding gives the same feel when power sources are more constrained.

Generally what is required to implement a load-shedding system is to have a sensor to measure how much power is being drawn, a controller to switch loads off and on, and then a set of contractors, relays, or remotely controlled breakers that can be used to control the loads.  Ours is a custom design, but many commercial systems are available.

Maretron N2kView, for example, supports shedding of up to ten different loads across four power sources. The Maretron approach is quite nice and, had this been available when we implemented our system, we probably would have used it instead.

Load shedding also is showing up in heavy volume in the RV market. Usually these systems are referred to as Energy Management Systems (EMS) and you can find a lot of alternatives searching for “EMS and load shedding”.  That query, however, will find some very large commercial systems used for managing multi-megawatt loads on oil rigs. The search for “RV EMS and Load shedding” narrows things down to systems that work well at smaller scale.

To learn more about load-shedding, the article Energy Management in the RV Electrical Tutorial is a good place to start. As an example, they describe an Intellitec system which is fairly broadly used in the RV world.

Maretron N2kView load-shedding configuration page

 

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8 comments on “Load Shedding
  1. Barry Goffe says:

    Hi James! That is a great and fascinating explanation. Could you please add how you developed your own system? I assume you wrote some software that runs on a PC. Is that correct? Are you able to use NMEA interfaces to gather all the usage data? What mechanism do you use for controlling each power source and each power consumer? Not trying to build anything but just incredibly curious. Thanks again for sharing your massive wealth of knowledge.

    • Correct, the central system runs on a PC and data is pulled from the NMEA2000 bus (using Kees Verruijt’s CANBOAT open source system), data is pulled from 5 Raspberry Pis doing digital and analog input, and screen scraping systems that don’t have an API (e.g. the KVH V7hts satellite system). All this data is stored once very 5 seconds in a central relational database. Other programs operate on this data and drive alarms, alerts, set indicators, send email, or take other actions including autostart and load shedding. All actions taken by the control system are acted upon by sending data down the NMEA2000 bus (CANBOAT), or sending requests to the 5 Raspberry Pis doing digital output, or sending data up to the website (mvdirona.com).

      You asked how the load shedding system controls the loads? The control system sends requests to Raspberry Pis that act through digital outputs that drive contractors that open and close these loads on devices like: 1) water heater, 2) HVAC, 3) chargers, 4) microwave, etc.

      We’ll eventually do a video on the system to show more detail on how it works.

  2. Julian Buss says:

    Hi,

    I would like to add that Victron Energy Multiplus Loader/Inverter are using another interesting approach: One can tell the Multiplus how much maximum current it may draw from the current power source. If the load requirement on the boat is higher, the missing power is drawn from the batteries.

    As soon as the load falls back below the configured limit the drawn power from the batteries are loaded back.

    Since power peaks are mostly short this approach only puts minor stress to the batteries.

    In addition you can combine multiple Multiplus devices to support the required loads.

    • Good point. We use both Victron and Mastervolt and like them both but we particularly like our Victron inverter. Modern equipment can do a good job of supporting peaks above power source but there are is a limitations that they can’t solve for: you can’t draw over the circuit capability. A second issue is solved for but not in all designs and that is the transition between charging and inverting (to boost power) has to be seamless or no longer than 16 msec in order to avoid computers and other sensitive electronics going down. Some systems do this splendidly but our old Mastervolt inverter does now and the gap between charging and inverter is so long that sensitive loads not behind a UPS will go down and clocks will reset. Consequently we don’t use that feature. New equipment does far better and it’s a good solution.

  3. Steven Coleman says:

    Hello James,
    In an otherwise properly operating system, the only real negative impact of disabling and enabling quickly would be higher than necessary starting current due to bringing the compressor back on before pressures have had a chance to equalize between the high and low side.

    If I understand you correctly, it would be “possible” for a compressor to start and 40 seconds later be shed and restarted in 10 seconds only to be shed after another 40 seconds if competing loads were present.

    Your systems are small with short piping runs so I doubt oil return would ever be an issue however, do that enough times in an hour motor cooling could become an issue as they rely on the suction gas for cooling. Even that isn’t going to be a problem if they have several min.s of run time once in a while between starts.

    So as long as you are not seeing a situation like I described above the only real impact of faster cycling for you is going to be higher than necessary starting current. I would pay close attention to the values of and start or run capacitors and change them when they dropped below their rating without regard to tolerance. (i.e. a 30uf capacitor should read at least 30uf)

    How long it takes for a system like yours to equalize varies but it’s going to be in the 3-5 min. range.

    So while you probably are not effecting the system you might be happier with amperage draw on start up if you could keep them off slightly longer.

    If that isn’t possible with your load shedding software an easy solution would be installing a timer on each unit.

    https://www.amazon.com/ICM-Controls-ICM203-18-240-Height/dp/B000LDKB3W.

    • Great explanation Steve and it helps. The way we are set up, the HVAC is the highest priority device so it’s the last to shed and the first to be brought back on line. Because it’s the first back online, it can happen fairly quickly but we won’t bring it back online without lots of available capacity available so inrush is not an issue and, since we bring compressors online 1 at a time, inrush is even less of an problem. Since the HVAC is the highest priority, it’s the least likely to get shed a second time. Big loads arriving can take everything to shed but as the load is brought back online, only those that fit are brought back online and HVAC is the highest priority to be brought on and the last priority to shed. We let the water heater and chargers take the abuse.

      Thanks for additional data on HVAC cycling and associated risks.

  4. Steven Coleman says:

    Hello James,

    I was wondering if your Heat pumps have anti-recycle timers on them?

    They probably do however your comment on the loads shedding quickly combined with a 10 year old system got me thinking.

    • Hey Steve. It’s always good to hear from you. There are two parameters but both are very short. If the HVAC system is shut off, it will not restart for 10 seconds and won’t see competing loads for 40 seconds so sheds can’t be closer together than about 1 min but it’s far from the 10 min that is common on a anti-recycle timer. I could see it to longer but the obvious downside is it makes the system less responsive for the users. This is the technical limit but it’s rarely more rapid than several minutes and that only happens when the oven or dryer is cycling heat off and on. What’s the potential negative impact of faster cycling?

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