Power supplies such as these can result in an unexpectedly high RMS current leading to overloaded switchboards and wiring together with increased utility charges.
Many devices incorporate low cost power supplies that draw narrow current pulses from the mains supply coincident with the voltage peaks. This type of load characteristic has a poor power factor, typically around 0.5, and draws about twice the RMS current that it needs to for its power consumption. When a significant number of such devices are powered off a single circuit the total RMS current demand can become unexpectedly high and may exceed the rated current.
In 4-wire, 3-phase circuits, there is the added problem of the high triplen content (odd multiples of the 3rd harmonic, 3rd, 9th, 15th, 21st etc.) in the phases adding together in the neutral. Overheating and fires can result from the neutral not being oversized appropriately.
The first step of the solution is to explore what the current waveform in each phase looks like if a number of these devices are connected as a balanced phase-to-phase load on a 3-phase auto-transformer.
The taps on the transformer are chosen to produce 240V RMS on each device, rather than the usual 415V RMS phase-to-phase voltage of a 3-phase system.
This phase-to-phase loading of the auto-transformer creates an interesting double current pulse in each of the three incoming phase lines. Splitting the original single current pulse into two immediately improves the power factor.
However, further improvement is possible by adding a balanced phase-to-neutral load in the original single current pulses that are coincident with the voltage peaks. The power factor of this composite current pulse can be 0.9 or higher, nearly halving the RMS current in each phase compared to the usual phase-to-neutral loading of the system.
TechRentals recommends the use of a power quality measurement instrument such as a Hioki 3169 to carry out some final checks and tune-ups, ensuring correct operation of the system.
Power quality measurement instruments can be used to perform:
- Load balance adjustment between the phase-to-neutral and phase-to-phase loads to achieve the best power factor
- RMS input current monitoring for an extended period while looking for low mains events
- Extended period monitoring looking for high mains events to ensure the maximum rated input voltage of the auto-transformer is not exceeded
- Adjusting of the load balance to the worst expected and checking the neutral current to reveal through harmonic analysis how much of the neutral current is due to additive triplens