Page 45 - Energize September 2022

TECHNICAL

some conditioning applied. However, this technique can only be If (i) is not true, supply-borne signal artifacts may prevent accurate
considered valid under the following conditions: measurements being made. There are two primary mechanisms
(a) The mains power line is clean and undistorted (i.e., noise and identified which can affect this measurement. Presence of an
interference free, and perfectly sinusoidal). inter-harmonic on the supply will cause a beat with the supply
(b) The UUT will be deployed in making measurements under the fundamental frequency (and any harmonics which may present)
same conditions when calibration is complete. and be interpreted and measured by the flicker meter as flicker.
(c) The UUT and reference present identical loads to the supply. Or, of course, the supply itself may have flicker upon it.
If (ii) is not true, and an imperfect signal source is employed,
If (a) is not true, and the supply is imperfect, different response inaccurate levels of modulation will be developed at the input to
mechanisms to effects such as harmonic distortion may result in the flicker meter (see Figure 3).
different results between the UUT and the reference. Depending
on the time of day, harmonic distortion on the power line varies
as overall loads change, which further compounds potential for
errors.
If (b) is not true, the UUT will effectively be making a different
measurement to the one for which it has been calibrated.
Consider the situation where a kWh meter is calibrated at a meter
shop where the supply used may be close to perfect as in (1)
above, but is then deployed in the field where harmonic distortion
is likely to be relatively high, and extends to high numbers of Figure 3: Inaccurate levels of flicker developed with imperfect source
harmonics. Differences of up to 68% in measured power were
observed by Dr PS Filipski and PW Labaj of the NRC during an Limitations of both approaches
evaluation of four different reactive power meters in the presence There is a growing requirement to characterise and verify the
of high harmonic distortion. 2 performance of power measurement instruments in the presence
If (c) is not true, different currents will flow in the UUT and of differing forms of distortion and anomalies.
reference, in the case of the parallel connection or substitution IEC 61000-4-30 and IEEE P1159-1 both require that
method, and a different potential will be developed across the power quality measurement instruments are calibrated in
UUT and reference in the case of the series connection. In either the presence of simultaneous, multiple forms of anomalous
case there will be measurement error. signal. For example, a flicker measurement is calibrated in the
presence of controlled amounts of harmonic distortion. While
Switched or variable loads the limitations in accuracy described above may be acceptable
This technique is used particularly with flicker meters. Here, a load in some cases, neither of the approaches outlined above is
is switched to develop a potential difference across a reference able to provide the facility for calibration when simultaneous
impedance, in order to simulate the modulation on mains power degradations are present.
supplies classified as flicker (Figure 2). Careful selection of the
reference impedance and the switched impedances yield signals of Calibration through signal sourcing
sufficient accuracy to calibrate flicker meters. This author proposes a solution in the form of a calibration signal
source which resolves the issues outlined above. The system
Problems with this approach is fundamentally a signal source which provides high accuracy
Again, this approach depends on a number of preconditions: phantom power (separate voltage and current) to calibrate power
(i) The mains power line is perfectly clean and undistorted (i.e., measurement devices. In addition to pure sinusoidal signals,
noise and interference free, and perfectly sinusoidal). the system can also provide the capability to supply all currently
(ii) The source impedance is effectively zero. defined forms of mains power supply degradations (frequency
variation, harmonic distortion, inter-harmonics, fluctuating
harmonics, flicker and dips and swells).
The system proposed can be realised through the use of
arbitrary waveform generators and accurate amplifiers under the
control of a digital feedback loop (see Figure 4).

Problems encountered with modulated signals
The use of a digital feedback loop provides a mechanism for
Figure 2: Flicker meter calibration system maintaining accuracy on complex signals, in particular on signals

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