Remote Calibration of Flickermeters
Whilst the calibration of the Flickermeter can be carried-out using the methods outlined above, the calibration of the reference impedance is of equal importance. In many cases, users of flicker test systems request a separate calibration of this impedance which can be carried-out, at rated current conditions, using conventional methods.
One method to overcome these problems is to use a transportable Flickermeter calibrator. An example of a calibrator has been developed by California Instruments Inc. The calibration configuration is shown in Figure 8, the calibrator taking the place of the EUT in Figure 3. The calibrator consists of a set of 'non-inductive' high current resistors. The values of the resistors are arranged that the current drawn will induce a known voltage drop. The extent of the voltage drop corresponds to a pre-selected point on the Flickermeter response curve for squarewave modulation.
Figure 8: Calibration of a Flicker Test System Using a Calibrator.
To give squarewave modulation, the resistor is switched in and out of the circuit using a solid-state relay. The relay is driven by a square wave generator at a rate corresponding to the point on the IEC curve for the drop induced by the resistor. Five different resistors are available in the calibrator allowing the flicker test system to be calibrated at five different amplitudes for all frequency points on the IEC curve.
The calibrator itself needs to be calibrated and its performance assessed which has led NPL to develop a calibration procedure for this system. Tests were carried-out to assess the performance of the calibrator, including the behaviour of the resistors under load. An obvious concern is whether the resistor values undergo any change due to self-heating when under-load. The effect of this would be to cause one of the modulation levels to change with time and thus the calibration test signal to be non-rectangular. Using the computer model of the Flickermeter, it was concluded that the relatively small amount of resistor self-heating observed had a negligible effect on the performance of the system.
In the near future, it is hoped that NPL and other laboratories can use calibrators to calibrate flicker test systems on-site. Such a calibrator would be in effect a travelling standard which would be delivered to a customer's premises. Using a relatively simple procedure, the customer should be able to calibrate the complete flicker system under the same conditions, with the same wiring configuration, that it uses to make its 'every-day' compliance measurements. Using a before and after calibration methodology for the calibrator, it should be possible to issue calibration certificates for the full flicker test system, remotely.
Should remote calibration prove successful, it may be beneficial to further develop this concept. With the help of Flickermeter manufacturers, it should be possible to obtain readings directly from the Flickermeter using a modem or the Internet as a data-transfer medium. Furthermore, it is also possible to control the measurement equipment using these mediums. In this way the calibration procedure can be controlled directly by the calibration laboratory.