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16
how the Energy Logger Works
Voltage
Mains voltage is measured by direct connection to the mains
supply. The data logger samples the voltage signal at 1kHz
for 100ms to calculate RMS voltage and true power.
Current
Current is measured using flexible rogowski coils. They
measure the current passing through the conductor which
they are wrapped around. The current measurement is
relatively immune to interference from currents in conductors
which pass nearby, but not through, the coil.
The data logger samples the current signal at 5kHz for 100ms
to calculate RMS current and true power.
The analogue current signal is filtered before being
sampled. This ensures that the true power figure can be
calculated accurately, even if the current signal has very
sharp spikes. However this filter means that the RMS current
figure displayed and logged by the data logger will be an
underestimate if the current signal has very sharp spikes
above the 6th harmonic.
True Power
At 1ms intervals (1kHz) the data logger multiplies together
the instantaneous current and instantaneous voltage samples.
The average over 100ms (five or six mains cycles) gives true
power. This figure appears on the data logger display.
Three Phase True Power
The data logger can directly measure the current in three
conductors simultaneously, however it can only measure the
voltage of one.
The data logger generates equivalent voltage signals for the
other two phases by delaying the measured voltage signal by
one third and two thirds of a mains cycle. This is effectively
assuming that:
•
The phases are separated by 120°.
•
The phases have the same voltage magnitude.
•
Any voltage harmonic distortion is identical across three
phases.
These assumptions may typically lead to an error of up to 1%.
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Average and Peak
The 100ms burst of readings is repeated every two seconds
(this is reduced to every five seconds if logging on batteries,
to conserve battery power). The Average Power trace shown
in Tinytag Explorer is the average value over the 5 minute
logging interval - an average of 150 true power calculations.
The Maximum Power trace is the largest value calculated
over the same interval. Maximum Current is calculated on the
same basis.
Per-Phase Power
In some configurations Tinytag Explorer will display three
separate traces for the power transferred by each of the three
phases. This is in addition to the Overall Power trace.
The per-phase power traces are calculated assuming that
the load is wye connected. That is, that all the current in
each phase returns through neutral. This assumption is often
entirely true, for example if the three phases power lighting
and office circuits. The per-phase power traces can be used
to judge the balance between those circuits.
If the three phase load is delta connected then it is not
physically possible to break down the overall power into per-
phase components. The per-phase traces are meaningless
and should be ignored. Only the Overall Power trace is
useful.
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