Auxiliary Components

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Additionally to the Control and Load Boards, several other auxiliary components are required to complete the system.

Dump Loads

Dump loads, sometimes known as ballast loads, are used by the ELC to consume excess power that is produced by the MHP and is not consumed by users. The ELC is able to electronically vary how much power is diverted to the dump loads in order to maintain a constant load on the MHP system. This can be visualized mathematically with the following formula\[ P_{MHP} = P_{consumer} + P_{dump load} \]

\(P_{MHP}\) will remain approximately constant and \(P_{consumer}\) will vary throughout the day as consumers switch on and off appliances, therefore the ELC must vary \(P_{dump load}\) to ensure the equation is satisfied.

Dump Load Types

Heating elements as a ballast load

As the ELC needs to be able to vary the quantity of power diverted to the dump loads, the dump loads must be reasonably simple devices with predictable operation. For this reason the most common choice is to use a rack of either water heating elements or air heating elements.

It would be possible to utilize the power diverted to the dump loads for something more useful than just to be wasted as heat but this is a task for future development.

Air Heaters

Air heaters are a good choice of dump load because unlike water heaters they can operate in air without overheating and therefore don't have the added dangers of being in close proximity to water. Having said this it is important to select air heaters that can function either in stationary air, or provide a suitable fan or similar to keep them within their operating limits. They should also be mounted in a way that ensures the heat can dissipate from them safely, i.e. not mounted close to a wooden wall or ceiling, because whatever your system size, even a few kilowatts produces a LOT of heat!

Water Heaters

Water heaters can be used as dump loads as there is often easy access to running water which can be taken from somewhere in the MHP system. The heaters can be mounted in a water tight container with a steady flow of cool water to ensure they are kept at a suitably low temperature.

The main drawbacks of water heaters are the requirement for everything to be kept dry around the heater connections. Sealant/gaskets must be used to mount the elements the terminals covered to prevent leakage and splashes. If the terminals become wet this can be highly dangerous both in health and safety terms and to the ELC. The other disadvantage of water heaters is that they are more prone to corrosion damage due. This can be limited by keeping the casings properly grounded and the terminals properly sealed.

Selecting Dump Load Size

Choosing your dump load size is relatively simple to do, typically you oversize your dump load by 1 or 2 KW. If you're using heating as suggested, they normally come in sizes of between 500 W to 3000 W. These individual elements are then wired in parallel to give you the desired dump load capacity. For this reason it's normally a good idea to add one extra element E.g. a 10 KW system require 5 x 2 KW = 10 KW heating elements, but add 1 more to give 6 x 2 KW = 12 KW total capacity. This means that the dump load can comfortably cover the system capacity and also provides some redundancy should 1 of the heating elements fail.

Emergency Dump Load

It is recommended that an emergency ballast load be used to provide an extra level of redundancy to the system. The emergency dump load is used in the event that the main dump load fails and the ELC is no longer able to control the system, normally due to heating elements reaching the end of their lives and going open circuit. It is also used in the event of a catastrophic ELC failure such as power failure.

The emergency dump load is fully connected when a catastrophic event occurs and will consume its full rated power. Therefore it should be sized to closely match the total power the MHP produces to prevent either generator overloading or runaway.

Main Contactor

3 Phase contactor

Contactors are used to provide a heavy duty switch to connect/disconnect the consumer to the MHP system. They use an electro-magnet to actuate the connection between their main terminals which can carry the very large currents required. The electro-magnet coil will typically be rated for mains AC voltage (230 V @ 50/60Hz, different ratings will be available based on country standards) and will require a relatively small current to energize (typically a few hundred mA).

Start/Stop Controls

The main contactor should have a "normally open" auxiliary circuit which can be used to keep the coil energized after the "Start" push button is pressed, which should also be a momentary "normally open" switch. The "Stop" push button is momentary "normally closed" and is also wired into this circuit along with the ELC control board connection relay. If the "Stop" button is pressed, or the ELC opens the relay, the coil circuit is broken and the consumer is disconnected from the system. The ELC will open the relay if the electrical supply goes outside voltage or frequency specification. These voltage and frequency limits are fully programmable to suit the users needs.

The Start/Stop electrical circuit can be seen in the system schematics which can be found in the Bluebird ELC Design Files page.

Heat Sinks, Fans and cooling

The switching elements will be dissipating a significant amount of energy so it is important to mount them firmly on a suitable heat sink using thermal paste between the mating surfaces. The recommended TRIAC BTA40-800B has an insulated plate which allows it to be mounted on the heat sink without an external electrically insulating layer. Other components may need to have a layer of thermally conductive but electrically insulating material between the device and the heat sink.

In terms of heat sink specification it is good to keep your switching elements as cool as possible. The TRIACs should dissipate somewhere between 8 and 10 watts each so a heat sink with thermal conductivity of less than 1°C/W and a case fan is recommended.

We've also had success with water cooled heat sinks which have excellent thermal performance but these are more complex to fabricate and must be completely water proof for obvious electrical safety reasons.

Circuit Breakers and Fuses

3 Phase MCB

It is recommended that a circuit breaker is installed between the main contactor and the consumer connection terminals. Normally a miniature circuit breaker(MCB) is sufficient but an appropriate breaker should be selected based on the system size. This will protect the ELC and Generator in the even of an electrical fault on the consumer side.