Generation Switch Improvements
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 Ė 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 meterís LED goes off, and type 3 meters you have to detect when the meterís 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.