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Three Phase Power Monitoring
IoTaWatt has the capability to measure power in polyphase power systems. In the interest of keeping the user interface simple for the majority of users with single phase power, the details are somewhat disguised and/or hidden. As of this writing, there are only a handful of three phase users, but there's a lot of interest so the user interface will be enhanced as more is learned about what works best and what accuracy can be expected from the various methods.
As explained in the introductory single phase section, a voltage reference is needed to measure real power. The challenge with three phase power is to obtain three different voltage/phase reference signals. IoTaWatt can do this by two different methods, that we will call Direct Reference and Derived Reference.
Direct Reference uses three discrete voltage transformers or VTs, each one plugged into a circuit on a different phase. The primary advantage is accuracy, while the disadvantages are that the extra VTs add cost, require using two of the inputs, require special adapters, and require plugs on each phase in proximity to the IoTaWatt.
Derived Reference uses a single voltage transformer and derives a reference voltage/phase for the other two phases by numerically shifting the phase of the single reference by 120° or 240°. The primary advantage is low cost and convenience of installation. The disadvantage is that large variations in phase or voltage between phases will result in decreased accuracy.