Generation Switch Improvements
This section
of the circuit descriptions should be used as a supplement to the simple Generation Switch circuit
description and its Circuit Diagram.
As is
pointed out in the simple circuit description, no use is made of the Reset
input. The improvements in this
description show how the reset input can be used to turn off the Solid State
Relay (SSR) whenever the total load in the house causes power to be imported.
The basic
Generation Switch only detects when the generated power is greater than a
preset level. Once the preset level is
exceeded a load is turned on via the SSR.
The circuit has no knowledge of the total power being used by the
house. If it has been assumed that the
base load of the house is say 0.5kW and the preset level is set to 3.5kW then a
3kW load can be turned on without importing any power. But if the base load of the house has been
temporarily increased to say 1.5kW (a toaster, vacuum cleaner or other medium
sized load) then there is only 2kW spare and the 3kW load would result in power
being imported.
The
following improvements to the Generation Switch circuit causes the load (3kW in
the example above) to be turned off whenever power is imported. It is likely (as in the above example) that
turning off the load will result in power no longer being imported and the
circuit will cycle back on again.
However there is a 10 second time constant, so it will take ten seconds
before the load is again turned on. If
the base load is still too high, it will immediately be turned off again (in a
fraction of a second) and the process will continue until the base load falls
back to normal. Whilst it would not be a
good idea to cycle on and off a load using a conventional mechanical relay, the
SSR has no mechanical parts and is quite happy to be repeatedly turned on and
off. To calculate how much excess power
is consumed during this cycling consider the following:- if it takes 10 seconds
before the load is turned on and 1/10th second to turn off the load,
then the load is on for 1/100th of the time. So, if the 3kW load caused the entire 3kW power
to be imported, the equivalent power imported would be only 30W (3,000W/100).
The
detection of when power is being imported is complicated by the fact that there
are at least 4 types of import meter.
1. The old-style mechanical meter.
Apart from the rotating disc on the meter there is no other indication
of the direction of the power flow. Some
of these meters stop rotating when power is exported, on others the meter disc
rotates backwards.
2. LED always on when exporting. On
this type of meter there is an LED which flashes whenever power is imported –
the rate of flash depends on the amount of power being imported. When power is exported, the LED ceases to
flash but remains on constantly.
3. LED off when exporting. This
meter is similar to the type 2 except that when power is exported, the LED goes
off.
4. LED flashes whenever power is imported or exported.
The LED on this type of meter is as useful for this design as the
rotating disc of type 1 and requires the same method of resolving the problem.
Types 2 and
3 can be accommodated using an identical phototransistor to the one used for
the generation meter flash detector in the Simple Generation Switch. For type 2 meters you just have to detect
when the meter’s LED goes off, and type 3 meters you have to detect when the
meter’s LED goes on.
Types 1 and
4 require a method of detecting the direction of current flow into or out of
the import meter. The Owl Switch circuit will provide a convenient method
of detecting the direction of the current flow.
You do not need to purchase a complete Owl energy monitor in order to
build the circuit; only the clamp-on current detector is needed. The Owl Switch can be set up so that the LED
on the circuit comes on when power is exported or when power is imported. Depending on how the user has set up this
feature of the Owl Switch, the method of connecting to the Generation Switch
will vary. In both cases a
phototransistor is used to detect the state of the Owl Switch LED.
Consider the
scenario that the LED on the Owl Switch is on whenever power is exported; this
is exactly the same as the LED on the Type 2 meter. When importing power and the Owl Switch LED
goes out, this can be detected in exactly the same way as the modifications for
type 2 meters below.
If the LED
on the Owl Switch is selected to be on when importing (off when exporting), the
circuit modifications for type 3 meters, below, can be applied.