The
UIE/IEC Flickermeter is the standard for flicker measurement. Its main function is to provide
assessment of the flicker perception caused by voltage fluctuations. Therefore,
the flickermeter should be designed to have the capability of transforming the
input voltage fluctuations into an output parameter proportionally related to
flicker perception. This is done by simulating the process of physiological
visual perception, which is the lamp–eye–brain
chain.
The UIE/IEC flickermeter consists of five blocks, which are defined in IEC 61000-4-15 (Testing and Measurement Techniques – Flickermeter – Functional and Design Specifications). It was originally designed using a 60 W, 230 V, incandescent lamp. Recently though, an extended measurement for 120 V was added to cater such electrical systems just like in the United States (see IEEE 1453-2004).
The
following sections described each block of the UIE/IEC flickermeter with
respect to the signal processing functions provided by the block and to the relationship
corresponding to physiological phenomena.
1.
Independent Voltage Adapter
Block
1 is the input voltage adapter, which scales the input half-cycle RMS voltage
to an internal reference value. The primary function of this block is to allow
flicker measurements be expressed as a percent ratio and become independent of the
input voltage level.
An
automatic gain control circuit with a 10% to 90% step-response characteristic
of one minute gives such functionality. It imitates a well-known characteristic
of human perception wherein moderate-level, constant stimuli to the senses
slowly become unnoticeable.
2.
Square Law or Quadratic Demodulator
Block
2 of the IEC flickermeter is a quadratic demodulator, which squares the scaled
input in order to separate the voltage fluctuation (modulating signal) from the
main voltage signal. This process simulates the behavior of an incandescent
lamp. In other words, the objective of this block is to recapture the
modulating signals while at the same time suppress the mains frequency carrier
signal.
UIE/IEC Flickermeter Block Diagram |
3.
Demodulator and Weighting Filters
Block
3 of the IEC flickermeter consists of three cascaded filters that serve to filter
out components that have frequencies higher than that of the supply voltage and
the DC component, produced from the square law demodulator. It also functions to
weight the input signal according to the incandescent lamp eye-brain response. These
filters are:
a. Demodulator
Filters
- First-order
high-pass (cutoff frequency = 0.05 Hz)
- Sixth-order
low-pass Butterworth (cutoff frequency = 35 Hz)
b.
Weighting Filter
- Band-pass
filter (models the frequency-selective behavior of the human eye)
The
basic transfer function for the weighting filter is shown below.
4.
Nonlinear Variance Estimator
Block
4 completes the eye-brain response model, as it consists of a squaring
multiplier and sliding mean filter. The voltage signal is squared to simulate
the nonlinear eye-brain response, while the sliding mean filter averages the
signal to simulate the short-term storage effect of the brain. The output of
this block is an instantaneous signal proportionally related to the visual sensation of flicker.
This is precisely the purpose of the flickermeter - to provide a measurement method that
would relate voltage fluctuations to units of visual sensation.
5.
Statistical Evaluation
This
part models human irritability due to flicker stimulation. Flicker is bearable
if it occurs infrequently over short intervals. However, tolerance decreases in
the presence of increasing level intensity or event frequency and duration.
Block
5 performs the statistical classification of the instantaneous flicker
sensation, which is the output of Block 4. This output is categorized within an
adequate number of classes. Then, a probability density function is created
based upon each class, and from this a cumulative distribution function can be
formed.
Subsequently, a number of points from the cumulative distribution function are selected to
calculate the short-term flicker severity (PST), as well as the
long-term flicker severity (PLT).
Flicker
Severity
Flicker
level evaluation can be classified into short-term and long-term flicker
severity.
a. Short-term Flicker Severity (PST)
This
is based upon an observation period of 10 minutes, allowing evaluation of disturbances
with a short duty cycle or those that generate continuous fluctuations. PST
can be calculated using the equation shown:
Short-term Flicker Severity |
b. Long-term Flicker Severity (PLT)
On
the other hand, the need for long-term assessment of flicker severity happens
if the duty cycle is long or variable. These include electric arc furnaces or
disturbances on the system that are caused by multiple loads operating
simultaneously. PLT is derived from PST as shown below.
Long-term Flicker Severity |
References:
Baggini,
A. (2008). Handbook of Power Quality
McKim,
J. (1999). UIE Flickermeter Demystified
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