INTERNATIONAL STANDARD

BASIC EMC PUBLICATION

IEC 61000-4-7

Second edition 2002-08

Electromagnetic compatibility (EMC) –

Part 4-7: Testing and measurement techniques - Section 15: Flickermeter - Functional and design specifications

http://webstore.iec.ch/webstore/webstore.nsf/$$search?openform


1 Score and object

       This section gives a functional and design specification for flicker measuring apparatus intended to indicate the correct flicker perception level for all practical voltage fluctuation waveforms. Information is presented to enable such an instrument to be constructed. A method is given for the evaluation of flicker severity on the output of flicker severity on basis of the output of flicker meters complying with this standard.

        This section is based on specification prepared by the “Disturbances” working group of the International Union for Electroheat and published in 1992. Consequently the flikermeter specifications in this section relate only to measurement of 230 V, 50 Hz inputs; specifications for other voltages and other frequencies are under consideration

        The object of this is to provide basic information for the design and the instrumentation of an analogue or digital flicker measuring apparatus. It does not give tolerance limit values of flicker severity.

2 Normative references.

        The following normative documents which, through reference in this text, constitute provisions of this section of IEC 61000-4. at the time of publication, the editions indicated were valid. All normative documents are subject to revision, and parties to agreements based on this section of IEC 61000-4 are encouraged to investigate the possibility of applying the most recent edition of the normative documents indicated below. Members of IEC and ISO maintain registers of currently valid International Standards.

3. Description of the instrument

3.1 General

        The description given below is based on an analogue implementation.

        The flicermeter architecture is described by the block diagram of figure 1, and can be divided into two parts, each performing one of the following task:
1.simulation of the response of the lamp-eye-brain chain;
2.on-line statistical analysis of the flicker signal and presentation of the result.
The first task is performed by blocks 2,3 and 4 of figure 1, while the second task is accomplished by block 5

3.2 Block 1 – Input voltage adaptor and calibration checking circuit

        This block contains a signal generator to check the calibration of the flickermeter on site and a voltage adapting circuit that scales the mean r.m.s. value of the input mains frequency voltage down to an internal reference level. In this way flicker measurements can be made independently of the actual input carrier voltage level and expressed as a per cent ratio. Taps on the input transformer establish suitable input voltage ranges to keep the imput signal to the voltage adaptor within its permissible range.

        NOTE In digital instruments the voltage adaption may be performed by multiplying the instantaneous input voltage by 230 V divided by the actual input voltage averaged over 60 s.

3.3 Block 2 – Square law demodulator

        The purpose of this block is to recover the voltage fluctuation by squaring the input voltage scaled to the reference level, thus simulating the behavior of a lamp.

3.4 Block 3 and 4 – Weighting filters, squaring and smoothing

        Block 3 is composed of a cascade of two filters and a measuring range selector, which can precede or follow the selective filter circuit. The first filter eliminates the d. c. and double mains frequency ripple components of the demodulator output.

        The second filter is a weighting filter block that simulates the frequency response to sinusoidal voltage fluctuations of a coiled filament gas-filled lamp (60 W-230 V) combined with the human visual system. The response function is based on the perceptibility threshold found at each frequency by 50 % of the persons tested.

        NOTE A reference filament lamp for 100-300 V systems would have a different frequency response and would require a corresponding adjustment of the weighting filter. The characteristics of discharge lamps are totally different, and substantial modifications to this standard would be necessary if they were taken into account.

        Block 4 is composed of a squaring multiplier and a first order low-pass filter. The human flicker sensation via lamp, eye and brain is simulated by the combined non-linear response of blocks 2,3 and 4.

        Block 3 alone is based on the borderline perceptibility curve for sinusoidal voltage fluctuations; The correct weighting of non-sinusoidal and stochastic fluctuations is achieved by an appropriate choice of the complex transfer function for blocks 3 and 4. accordingly the correct performance of the model has also been checked with periodic rectangular signals as well as with transient signals.

        The output of block 4 represents the instantaneous flicker sensation.

3.5 Block 5 - On-line statistical analysis

        Block 5 incorporates a microprocessor that performs an on-line analysis of the flicker level, thus allowing direct calculation of significant evaluation parameters.

        A suitable interface allows data presentation and recoding. The use of this block is related to methods of deriving measurements of flicker severity by statistical analysis. The statistical analysis, performed on line by block 5 shall be made by subdiving the amplitude of the flicker level signal into a suitable number of classes. The flicker level signal is sampled at a constant rate. Every time that the appropriate value occurs, the counter of the corresponding class is incremented by one. In this way, the frequency distribution function of the input values is obtained. By choosing a scanning frequency of at least twice the maximum flicker frequency, the final result at the end of the measuring interval represents the distribution of flicker level duration in each class. Adding the content of the counters of all classes and expressing the count of each class relative to the total gives the probability density function of the flicker levels.

        Front this function is obtained the cumulative probability function used in the time-at-level statistical method. Figure 2 schematically represents the statistical analysis method, limited for simplicity of presentation to 10 classes

        From the cumulative probability function, significant statistical values can be obtained such as mean, standard deviation, flicker level being exceeded for a given percentage of time or, alternatively, the percentage of time that an assigned flicker level has been exceeded.

        The observation period is defined by two adjustable time intervals: Tshort and Tlong

        The long interval defines the observation time and is always a multiple of the short interval:

        For on-long processing, immediately after conclusion of each short time interval, the statistical analysis of the next interval is started and the results for the expired interval are made available for output. In this way, n short time analyses will be available for a given observation period Tlong together with the results for the total interval. Cumulative probability function plots should preferably be made by using a Gaussian normal distribution scale.

3.6 Outputs

        3.6.1 General

        The flickermeter diagram in figure 1 shows a number of outputs between block 1 and block 5. The outputs marked with an asterisk are not essential, but may allow a full exploitation of the instrument potential for the investigation of voltage fluctuation. Further optional outputs may be considered.

        3.6.2 Output 1

        The arm of optional output 1 and its associated r. m. s. voltmeter is to display voltage fluctuation waveform in terms of changes in r. m. s value of the input voltage. This can be achieved by squaring, integrating between zero crossings on each half-cycle and square-rooting the signal.

        In order to observe small voltage changes with good resolution, an adjustable d. c. offset and rectification should be provided.

        3.6.3. Output 2

        Output 2 is optional and mainly intended for checking the response of block 3 and making adjustment.

        3.6.4. Output 3

        Output 3 is optional and gives an instantaneous linear indication of the relative voltage change expressed as per cent equivalent of an 8.8 Hz sinusoidal wave modulation. This output is useful when selecting the proper measuring range.

        3.6.5. Output 4

        Output 4 is optional and gives the 1 min integral of the instantaneous flicker sensation.

        3.6.6. Output 5

        Output 5 is mandatory; it represents the instantaneous flicker sensation and can be recorded on a strip-chart recorder for a quick on-site evaluation, or on magnetic tape for long-duration measurements and for later processing.

        3.6.7 Output 6

        Output 6 in block 5 is mandatory and is connected to a serial digital interface suitable for a printer and a magnetic tape recorder. Analogue plots of the cumulative probability function can be obtained directly from this block by using another digital-to-analogue converting interface.