Voltage and Lamp Flicker Issues: Should the IEEE Adopt

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					Should IEEE adopt IEC Flicker Standards                                                                   Página 1 de 15

    Voltage and Lamp Flicker Issues: Should the IEEE Adopt the IEC Approach?

                                  Authors - Biographies at the end of the paper

                            S. Mark Halpin - Mississippi State University, Starkville, MS

                            Roger Bergeron - Hydro Quebec, Varennes (Quebec), Canada

                              Tom Blooming - Cooper Power Systems, Franksville, WI

                           Reuben F. Burch - Alabama Power Company, Birmingham, AL

                  Larry E. Conrad (Chair,TF on Light Flicker) - Cinergy Corp., Plainfield, IN

Thomas S. Key (Secretary, TF on Light Flicker) - EPRI Power Electronic Applications Center, Knoxville, TN


This provides an overview of the IEC flicker measurement and assessment procedures. The IEC procedures are
correlated with existing IEEE flicker standards to show the benefits of the IEC methodology. Application issues, such as
customer impact assessment, are also discussed. Three case studies are provided to show the correlation of IEC
flickermeter output values with documented customer complaints. The case studies also provide areas where
modifications to the IEC flicker standards may be necessary when developing a new IEEE flicker standard. The Lamp
Flicker Task Force is working to adopt and embrace IEC flicker standards as an IEEE recommended practice.


Flicker is a difficult problem to quantify and to solve. The untimely combination of the following factors is required for
flicker to be a problem: 1) some deviation in voltage supplying lighting circuits and 2) a person being present to view the
possible change in light intensity due to the voltage deviation. The human factor significantly complicates the issue and
for this reason flicker has historically been deemed "a problem of perception." The voltage deviations involved are often
much less than the thresholds of susceptibility for electrical equipment, so major operating problems are only
experienced in rare cases. To office personnel, on the other hand, voltage deviations on the order of a few tenths of one
percent could produce extremely annoying fluctuations in the output of lights, especially if the frequency of repetitive
deviations is 5-15 Hz.

Due to the clear relationship between voltage deviation and light response, the term "flicker" often means different things
to different people with the interpretation primarily governed by the concerns of a particular discussion. In each case, the
deviation may or may not be strictly periodic and is usually expressed as a change (as indicated by the change in rms
value) relative to the steady-state level (expressed as an rms value averaged over some time period). For voltage
variations, the change is usually expressed as ∆V/V. A similar expression for light intensity variations also exists.

From a utility application point of view, voltage fluctuations have usually been of interest, perhaps because voltage
changes are easily measured with existing instrumentation. Historically, these voltage changes have been used in
conjunction with "flicker curves" such as those shown in Figure 1 [1,2,3]. These curves, derived from controlled
experiments, offer thresholds of perception and/or irritability when periodic rectangular voltage fluctuations occur
continuously (only threshold of irritability curves are shown here). Even though the use of a simple curve leaves much to
be desired (including an accepted industry-wide definition of the essential ∆V/V term), it is comforting to note that IEEE
and UIE frequency weightings are very similar. The improvements that are now possible, based primarily on existing
IEC standards, are the subject of this paper and will be presented in later sections.

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                                              Figure 1. Historical Flicker Curve

Standards-making bodies tend to focus on the changes in light intensity output in order to account for the human
observability factor. As standards have evolved, significant attempts have been made to include the years of experience
obtained using the "flicker curve" method described previously. There are, however, a number of degrees of freedom that
must be addressed in the development of a universally-accepted standard including lighting circuit voltage, type of lamp
involved, randomness of voltage fluctuation, and human factors which affect perception.

At this time, there are no widely-accepted flicker standards in the United States and Canada. In Europe and other
countries, however, the International Electrotechnical Commission (IEC) has developed a group of standards which
systematically account for many of the difficulties in the "flicker curve" methods. The IEEE Task Force on Light Flicker
is presently considering modifications to these IEC standards that are required for them to be considered for adoption in
the United States and Canada. The following sections describe the existing IEEE and IEC Standards.

IEEE Flicker Standards

The IEEE publishes voltage flicker limits in the form of recommended practice documents. The two most notable are
IEEE 519-1992 [4] and IEEE 141-1995 [3]. Although intended to be identical, they have very slight differences. Both
display the recommended practice on an xy graph as shown in Figure 1. The graph presents a borderline of visibility and
a borderline of irritation curve with each related to the continuity, the amplitude, and the frequency aspects of the voltage
fluctuations. Combinations of flicker frequency and magnitude below the borderline of irritation are assumed to cause
very few to no complaints.

The research behind the IEEE flicker curves is more than 50 years old. Researchers subjected people to a variety of
flicker magnitude and duration combinations from incandescent light bulbs. They used a variety of bulb wattages, but 60
W bulbs dominate the research. The observers reported their feelings about each flicker dosage. They could report that
they did not see it, that they saw it but were not irritated, or that the flicker dosage was irritable to them. The results have
a statistical nature because observers do not always agree about visibility and irritation. Researchers drew the visibility
and irritation curves at "reasonable" levels. A flicker dosage just slightly below the irritation limit might produce mild
irritation from a very few observers. Increasing flicker dosage to slightly above the irritation line will produce two
results. First, a larger percentage of the population will be irritated. Second, people irritated at the lower dosage will
become more irritated. The percentage of irritable population and irritation level both increase with higher flicker dosage.

It is important to remember that most of the research assumed 60 W incandescent 120 V bulbs. Lower wattage
incandescent bulbs have shorter time constants. They tend to produce the first complaints especially for flicker
frequencies faster than one per second. Higher wattage bulbs have longer time constants and are less responsive. New
lighting technologies have yet different responses to voltage fluctuations. Recent test results using various lamps are

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discussed in a later section.

It is also important to recognize that the laboratory experiments used sudden voltage changes. Rectangular modulation of
the 60 Hz sine wave was the most popular approach. Slower voltage changes, such as sinusoidal modulation, are less
likely to produce flicker complaints. It is also important to note that most of the experiments used repetitive single point
exposures. The experiments used steady voltages except for the single magnitude and frequency combination. Most
practical applications have combinations of several flicker dosages with multiple amplitudes at various frequencies that
possibly are produced by more than one source. Further, the voltage modulation often is not rectangular and not periodic.

For example, consider the complexity of a typical residential circuit that also happens to serve a large seam welder. The
customer will experience flicker from motor starts and other load changes in his or her home. If they share a common
distribution transformer and secondary, they will also experience flicker from other customer loads on the same
transformer. They also see lamp flicker from the seam welder. The total flicker experience then is a combination of these
individual flicker dosages. The IEEE curves do not address multiple dosage issues.

The situation becomes even more complex if the utility adds an adaptive var compensator for flicker control. A seam
welder typically generates rectangular modulation. However, the single or half cycle var compensator controls add
significant complexity. The system sees the full voltage drop until the compensator responds. The voltage moves to a
new point which is unlikely to match the voltage prior to the weld. Voltage over-shoot is common at the end of the weld
due to compensator response time. This voltage modulation is far more complex than anything anticipated by the IEEE
flicker curves.

The IEEE flicker curves have served the industry well for many years. However, better techniques are available. A
flicker measurement protocol developed by the International Union for Electroheat (UIE) and embraced by the IEC
shows great promise. Cooperative efforts between the IEC, UIE, EPRI and IEEE allow the IEC standard to be modified
for a variety of lighting technologies and a variety of system voltages. This effort promotes one universal standard for

It should be mentioned that there are many significant advances beyond the use of single curves such as those in Figure
1. There are numerous major manufacturers in North America that offer flicker measurement products and each is
different. Furthermore, many of the utilities in North America have their own limits for flicker which are not necessarily
based on a formal measurement process. The large number of approaches presently in use is largely responsible for this
attempt by the Task Force on Light Flicker to standardize the measurement, evaluation, and assessment procedures. The
significant prior experience of the UIE and the IEC appears to offer the most promising starting point for standardization
in North America.

The IEC Flicker Measurement Standard

This section describes the IEC 1000-4-15 Standard (referred to in this paper as 4-15 for brevity) which gives the specifics
of a measurement approach for flicker that can be adapted to a wide variety of situations [5]. (Note that 4-15 has replaced
the well-known IEC 868 flickermeter standard) The block diagram of the flickermeter specified in 4-15 is shown in
Figure 2. The major portions of the flickermeter are 1) input processing, 2) "lamp-eye-brain" response, and 3) output

The input processing blocks are designed to established the "normal" voltage, V, and perform initial waveform
manipulations necessary to extract the frequency content of the voltage deviations from this "normal" value. The primary
steps here are 1) converting the rms value of the measured voltage to a reference level to insure that percentage
deviations are equal regardless of the input rms level and 2) squaring the input to ease the separation of the low-
frequency (0.5-25 Hz) variations from the power frequency components via filtering. This function is referred to as a
"squaring demodulator."

The "lamp-eye-brain" characteristic is obtained from a mathematical derivation of 1) the response of a lamp to a supply
voltage variation, 2) the perception ability of the human eye, and 3) the memory tendency of the human

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                                           Figure 2. IEC 1000-4-15 Flickermeter

brain. This section of the IEC flickermeter is where modifications can be made to fit particular needs. The transfer
function shown in (1) is provided as a reasonable model for the first two of these responses.


The coefficients in (1) are given by the IEC for 230 V, 60 W incandescent lamps. An amendment for 120 V, 60 W
incandescent lamps has been adopted by the UIE and is being circulated for consideration by the IEC. Recent testing on
other 120 V lamps (including magnetic and electronic ballast fluorescent and compact fluorescent lamps) in the United
States has resulted in appropriate lamp transfer functions for a very wide range of lamps. The response characteristic in
(1) can be modified to include different lamp characteristics as shown in (2) and used for virtually any application.


The output processing of the flickermeter translates the output of block 4, called the instantaneous flicker sensation, into
the statistical indices Pst and Plt. The short-term flicker severity index, Pst, is a statistical quantification of the
instantaneous flicker sensation and is derived from a time-at-level analysis of the instantaneous flicker sensation. A
single Pst value is calculated every 10 minutes and Pst>1 corresponds to the level of irritability for 50% of the persons
subjected to the measured flicker. The long-term flicker severity index, Plt, is a combination of 12 Pst values. Practical
flicker limits are typically developed from 95th and 99th percentiles of a series of Pst and Plt values collected over time
periods perhaps as long as one week.

While much work remains to be done to properly model the flicker sensitivity of various lamps, the opportunities
suggested by (2) form the basis for the possible adoption of the IEC flickermeter specifications by the IEEE. The
continuous-time methodology of the IEC flickermeter avoids the complications associated with existing IEEE flicker
curves. In addition to incorporating various lamps, the IEC flickermeter automatically incorporates the effects of multiple
flicker frequencies and non-standard (i.e. not square or sine wave) modulating waveforms. For pre-evaluations of
potential flicker-producing customers, the IEC provides shape factors that can be used to translate many typical
modulation waveforms into equivalent square or sine wave modulating waveforms so that flicker estimates can be made
based on pre-determined Pst=1 curves.

With all standards, updates are regularly requested. The 1986 IEC 868 standard was drafted for an analog flickermeter

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designed during the 1970s. For the last 10 years, analog flickermeters have been gradually replaced by digital versions
which emulate each analog function. Very few North American digital flickermeters are available and each new digital
flickermeter that has been developed has been designed based on some interpretations of each analog function described
by the IEC. These interpretations are not obvious and justify a new digital specification that could be addressed by IEEE.

The digital specification should not be considered trivial. Signal processing issues related to sampling rate, quantization
effects, and windowing (to list only three) are relevant factors which must be considered. At this time, there are no
uniform specifications covering these concerns.

Recent Lamp Test Results

In recent months, EPRI's Power Electronics Applications Center has performed flicker tests on several types of modern
lighting. It was observed during these tests that the lamp's amplifying characteristic, or gain factor, is the important
consideration for flicker due to voltage fluctuations. Additional tests show how interharmonics (non-integer harmonics)
and phase-shifting harmonics on the power line can cause fluorescent lamps to flicker, despite their having low gain
factors when compared to incandescent lamps. Gain factor is defined and calculated by measuring relative changes in
light level while inducing controlled voltage fluctuations. By controlling the magnitude and the frequency of voltage
fluctuations, the lamp's flicker response can be determined using a photometer to measure the lamp output. If the
percentage of relative light fluctuation is greater than the percentage of voltage fluctuation, the lamp is said to have an
amplifying effect, or a gain factor greater than unity. Figure 3 shows an example of incandescent and fluorescent lamp
gain factor.

Incandescent lamp gain drops off at higher frequencies because of the thermal inertia of the filament. Similarly, when
voltage fluctuations change gradually, as in a sine wave, rather than instantly, as in a rectangular wave, a different flicker
response is observed. For infrequent changes (1 Hz) a rectangular-shaped voltage change causes about twice the level of
flicker of a sine-wave shaped change of the same magnitude. As the frequency of modulation increases, thermal inertia
of the lamp filament begins to mask these shape differences. Furthermore, a 230-V lamp, typical in Europe, has a thinner
filament than a 120-V lamp used in North America and therefore allows illumination changes to occur more quickly,
thus exhibiting more flicker.

                                   Figure 3. Gain Factor Variations for Different Lamps

In contrast, fluorescent lamps have very little thermal inertia and respond even faster. While the time constant for a 120
V incandescent lamp is about 28 ms and a 230V lamp about 19 ms, a typical fluorescent lamp has a time constant of less
than 5 ms. Consequently, fluorescent lamps are more sensitive to different voltage wave shapes, harmonic-related phase
shifts, and more rapid voltage fluctuations. Using gain factor to predict the flicker performance of a variety of different
type lamps has been effective in lab tests. More than 50 different lamps, including incandescent, compact and 4-foot
magnetic and electronic fluorescent, and high intensity discharge types have been evaluated. A wide range of flicker
performance is documented and these results are being used to influence lamp manufacturers to design more flicker-free
products. These test results are shown in Figure 4.

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Lamp dimmers are also believed to play a role in the increased number of flicker-related complaints. The use of
incandescent dimmers in homes substantially increases lamp susceptibility to voltage changes. A typical electronic
dimmer will nearly double the change in light output for a typical voltage change compared to the same lamp with no
dimmer. Figure 5 shows test results for lamps with various percentages (0, 25, 50, and 75%) of dimming.

One relatively new source of flicker study is the unique voltage distorting characteristics of different types of arc
furnaces. Simple harmonic distortions of the voltage usually do not play a significant role in flicker

                                          Figure 4. Flicker Response Test Results

                                      Figure 5. Dimmer Effects on Lamp Gain Factor

severity. Recently, however, special cases of harmonic distortion and harmonic phase shifting were found to be a direct
cause of lamp flicker in fluorescent lamps. To better understand how higher frequency harmonics can cause low
frequency flicker, tests were performed using a 5%, 185-Hz interharmonic component added to the fundamental. This
interharmonic alters the waveshape and effects the voltage peaks much more than the rms value, which remains fairly
constant on a cycle-by-cycle basis. The 185 Hz non-integer harmonic causes a cyclical "beat" of 5 Hz and also cyclically
changes the phase angle of the voltage peaks by a few degrees. It was found that while incandescent lamps exhibited a
slight amount of flicker, certain fluorescent lamps responded to the interharmonic voltage by flickering at the "beat"

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frequency much more noticeably. Figure 6 shows test results of flicker caused by interharmonic voltages.

Utilities have stated that when investigating a flicker complaint, it is very helpful to understand the flicker characteristics
of different lamps. In some cases, it is possible to reduce the flicker level by simply re-lamping. This can be
demonstrated to utility customers complaining of flicker by providing sources of flicker-free lamps (i.e. gain factors less
than 1.0). The other solutions to flicker include installing mitigation equipment or properly addressing service
requirements to flicker-producing loads. A critical step in utilizing the IEC methodology is the acceptance of the
correlation between Pst and Plt values (being greater than 1.0) and the

Figure 6. Lamp response to a 5%, 185-Hz inter-harmonic voltage and a 5%, 3rd harmonic voltage with ±90° phase shift.

occurrence of customer complaints and the ability to use these concepts to develop useable limits. The application
methodology needed to base flicker limits on the IEC flicker measurement procedure is discussed in the following

Pst and Plt: Application Methodology for the IEC Flickermeter

Values of Pst and Plt are directly available from the IEC flickermeter. As such, it is directly possible to define flicker
limits based on these values for equipment that is already in service. In many instances, however, it is necessary to
evaluate the flicker emissions of a potential customer before service is provided. Due to the wide variety of equipment,
operating voltages, and service designs (e.g. radial or mesh), the IEC has established three different categories of limits
for 1) low-voltage equipment with rated current less than 16 A, 2) low-voltage equipment with rated current greater than
16 A, and 3) medium and high voltage equipment. Limits are given for both the statistical parameters Pst and Plt as well
as maximum rms voltage deviations. The following overviews of the relevant IEC standards are intended to show how
the use of the Pst and Plt concepts can help to overcome the problems that have plagued North American utilities for
years when trying to apply simple "flicker curves." The reader is strongly cautioned to consult the actual standards before
conducting any flicker assessment.

IEC Standard 1000-3-3

IEC Standard 1000-3-3 [6] gives limits and evaluation procedures for low-voltage equipment with current ratings less
than 16 A. Table 1 provides the different possible methods for evaluating Pst for limit compliance evaluations.

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As shown in Table 1, any voltage fluctuation can be assessed using the flickermeter measurement procedure. Direct
measurement is obviously most appropriate for

Table 1. Methods of Evaluating Pst (IEC 1000-3-3)

 Types of voltage fluctuations               Methods of evaluating Pst

 All fluctuations                            Direct measurement
 Voltage fluctuations with                   Simulation; direct
 known U(t)                                  measurement
 Voltage fluctuation waveforms Analytical method;
 corresponding to existing "shape simulation; direct
 factor" curves                   measurement
 Rectangular voltage changes at
                                             Use Pst=1 "flicker curve"
 known frequency

loads already connected to a supply. If the rms voltage variation waveform, U(t), is known, computer simulation
(including a flickermeter simulation algorithm) can be used. If the waveform U(t) is not known, but the potential load is
known to produce rms voltage variations of a certain type (e.g. motor starting), then existing "shape factors" defined in
1000-3-3 can be used to estimate Pst analytically. Only when the rms voltage variations are known to resemble square
waves can the traditional "flicker curve" approach be used to estimate Pst values. Using the curve methodology, if a given
voltage variation at a given frequency locate a point above the curve, the resulting Pst will be greater than 1.0. Regardless
of the evaluation approach used, each Pst value is to be determined over a ten minute observation window. As defined by
the IEC, one Plt value can be calculated based on N=12 successive Pst values using (3).


Care should be taken in any evaluation other than direct measurement (under actual operating conditions) to match the
duty cycle of the equipment. IEC 1000-3-3 specifies limits of Pst£1 and Plt£0.65 for low-voltage equipment with a current rating less than
(or equal to) 16 A. Furthermore, this equipment shall not produce a maximum relative rms voltage fluctuation of more than 4%.

IEC Standard 1000-3-5

IEC Standard 1000-3-5 [7] gives limits and evaluation procedures for low-voltage equipment with current ratings greater
than 16 A. The limits in 1000-3-5 are those given in 1000-3-3. It is recognized, however, that a lower supply impedance
will be required to meet these requirements for larger equipment. In addition, 1000-3-5 recognizes equipment that may
produce voltage fluctuations at a rate of less than one per hour. In these cases, the limits of Pst and Plt are not applicable.
The maximum rms voltage deviation is limited to 1.33 times the 4% limit of 1000-3-3. IEC 1000-3-5 specially
recognizes that low-voltage equipment with a current rating greater than 75 A should be evaluated based on the actual
supply impedance at the connection point. Pst can then be estimated based on the relative size of the load VA and the
supply transformer VA rating. The Plt limit is set equal to 0.65Pst for equipment with current ratings >75A.

It should be noted that IEC 1000-3-3 and 1000-3-5 are considered to be "equipment standards" by which manufacturers
of low-voltage equipment can design their products. However, the limits and evaluation procedures apply equally well to
both the "design" phase and the "operational" phase of low-voltage equipment. The limits set forth serve to limit
fluctuations below thresholds of irritability for users connected to the same supply circuit regardless of whether or not
equipment was designed according to these standards. For this reason, the limits of 1000-3-3 and 1000-3-5 can be
applied in North America where limits are presently not placed on equipment manufacturers. For uses other than
equipment design, however, existing levels of background fluctuations should be taken into account.

IEC Standard 1000-3-7

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IEC Standard 1000-3-7 [8] gives limits and evaluation procedures for equipment connected to medium and high voltage
supply systems. Note that the IEC defines medium voltage (MV) as 1kV<MV£35kV and high voltage as
35kV<MV£230kV; Extra-high voltage (EHV) is considered to be >230 kV. Specific limits are not given that must be
followed; 1000-3-7 recognizes that the limit values for Pst and Plt will vary between utilities depending on the specifics
of the loads served and the supply network. Indicative planning levels, which are the quality targets of a supplying utility,
are given in Table 2.

Table 2. Planning Levels for MV, HV, and EHV Systems

                       Planning Levels
                       MV                    HV-EHV
Pst                    0.9                   0.8
Plt                    0.7                   0.6

These levels are evaluated on a statistical basis. As a general guideline, Pst and Plt should not exceed the planning levels
more than 1% of the time, with a minimum assessment period of 1 week. IEC Standard 1000-3-7 distinguishes between
Pst and Plt values measured throughout a supply system and those associated with a particular fluctuating load. Planning
levels (usually denoted as LPst and LPlt) apply throughout a supply system; the aggregate effects of all fluctuating loads
must be taken into account. Emission limits for individual loads (denoted as EPst and EPlt) must be set so that the
combined effects do not exceed planning levels.

IEC Standard 1000-3-7 presents a three-step procedure for evaluating fluctuating loads. The first step is an "automatic
acceptance" procedure that can be applied to assess the impact of a potential customer without detailed analysis. Table 3
shows the criteria for MV connections which specify the maximum allowable ratio of load power variation, ∆S, to the
available short circuit power, SSC, as a function of the fluctuation rate. Fluctuating loads connected directly to a HV
supply can be accepted without further study provided the ratio Smax/SSC<0.1% where Smax is the maximum load power.

Table 3. Maximum permissible load fluctuations ∆S/SSC for automatic acceptance of MV loads

r (# of variations/minute)       ∆S/SSC (%)

r>200                            0.1
10£r£200                         0.2
r<10                             0.4

Assuming that a particular fluctuating load does not meet the criteria of Table 3, Pst and Plt limits are allocated to each
individual load in a MV system based on each individual load's portion of the total load on the MV system as shown in
(4) for Pst with the quantity GPstMV defined in (5). The factor FMV is used to account for the non-simultaneous nature of
all flicker contributions and will always be less than or equal to 1.0 (more typical values for non-simultaneous
fluctuations are 0.2-0.3). Similar relations apply for Plt.



The following definitions apply to (5):

GPstMV=Maximum global contribution of local loads to the flicker level in the MV system,

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LPstMV=Planning level of the flicker level in the MV system,

LPstHV=Planning level of the flicker level in the supplying (upstream) HV system, and

TPstHM=flicker transfer coefficient from the upstream HV system to the MV system.

The use of a single limit value for the entire MV system under consideration and then allocating contributions to this
total limit among various disturbing loads is a method which allows larger loads (which generate greater revenue) a
greater share of the system's ability to absorb the effects of fluctuating loads. To avoid overly restricting smaller loads,
basic levels of EPst,i=0.35 and EPlt,i=0.25 may be allowed for all loads.

For MV systems with loads that do not individually meet the limits of (4) for EPst and EPlt, a more thorough analysis is
required. In this stage 3 analysis, all available information about present and expected future system and load conditions
should be considered. Any concessions made for an individual load to exceed emission levels determined as in (4) should
not result in the violation of GPstMV and GPltMV limits.

Stage 2 evaluations for loads connected to HV transmission nodes is similar to (4). Equation (6) illustrates the stage 2
evaluation for Pst limits for an individual (the ith) load served at high voltage. StHV represents the total load supplied from
the HV point serving the ith load. In many cases, the total load served at a given point can be difficult to determine. IEC
Standard 1000-3-7 considers various facets of this problem and proposes alternative methods for determining StHV. The
important point is that emission limits for individual loads are provided in terms of the system's total ability to absorb
fluctuations; multiple fluctuating loads are automatically taken into account.


Stage 3 evaluations for HV loads follow the same logic as that proposed for MV loads. In general, detailed studies
incorporating large amounts of system and load data are needed to insure that HV planning levels for LPst and LPlt are not

Case Studies Using the IEC Flickermeter

Three cases studies covering different flicker-producing loads are presented in this section. Where appropriate, flicker
criteria considerations that are appropriate for power contracts are discussed.

Resistance Welding Machines, Case 1

Resistance welding sometimes causes flicker problems for the steel fabrication industry. This is especially true when one
transformer feeds many welders that have random and independent operation. Occasionally too many welders fire at the
same time. The voltage drop at that moment causes numerous cold welds. Large (single) automated welding machines
also sometimes cause flicker problems for electric utilities. This is especially true for welding machines that repeat the
welding one time per second or faster. The frequent repetitive voltage drop from the welder can cause a very noticeable
flicker in lighting.

A good example of the continuous welding machine flicker problem is a steel fabrication plant in Indiana. The plant has
enjoyed success and growth into new product lines. Calculations for the newest product line showed uncompensated
flicker levels would exceed the borderline of irritation on IEEE Standard 141-1993. Cost estimates clearly favored
adaptive var compensation compared to lowering the system impedance. Unfortunately, the one cycle response of the
compensator has a bit of a doubling effect on the frequency of the voltage fluctuations. It was impossible to use IEEE
141 to account for the complex voltage fluctuations produced by the welder/compensator combination.

The welders and compensator were served from a distribution circuit with residential customers. There are approximately
1,600 customers who experience the point of common coupling voltage fluctuation magnitudes. About 25 of those
customers have complained with some making repeated complaints. The flicker caused some customers to worry about
the wiring in their homes. One customer worried their perception of flickering lights was an early warning sign for a
seizure or heart attack. Some customers seemed satisfied to know the utility was aware of and working on the situation.

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Should IEEE adopt IEC Flicker Standards                                                                  Página 11 de 15

Some took comfort in knowing their house wiring was not the problem.

The first test of the proposed IEC flickermeter showed excellent results. As discussed previously, a principal output of
the flickermeter is short term flicker severity level Pst. The recommended maximum flicker level before complaints is
about Pst =1.0. The flickermeter output ranged from Pst =1.0 to 1.2 during the situation that generated customer
complaints. The number of customer complaints confirmed the "borderline of irritation" interpretation of Pst=1.0.

Motor Starting and Load Torque Variations

Figure 7 shows a plot of rms voltage data collected on a rural distribution feeder in the southeastern U.S. The feeder
supplied several hundred residential customers and a process industry. The system supplying the distribution substation
was relatively weak (35 MVA short circuit capacity at the 12kV distribution bus) and the industrial load with four large
(2-350 and 2-500 hp) induction motors was located at the end of the feeder. Frequent motor starts and variations in load
torque (up to breakdown torque) characterize the plant load behavior.

These measurements were collected in response to numerous complaints to the local utility offices and the Public Service
Commission. While an IEC flickermeter was not available for use at this site, the entire feeder and motor loads were
simulated in EMTP with the IEC flickermeter modeled in TACS. The UIE 120 V, 60 W incandescent bulb characteristics
were used in the TACS flickermeter implementation.

            Figure 7. RMS Voltages (120 V base) Collected on 12 kV Feeder Near Residential Service Points

Figure 8(a) shows the simulated feeder voltage (on 120 V base) for motor start and no-load acceleration followed by a
torque (step) increase from no load to slightly less than breakdown torque. It is clear that the simulated feeder voltage
deviations are within the ranges in Figure 7. Figure 8(b) shows the corresponding IEC flickermeter Block 4 output
(instantaneous flicker sensation). (The long simulation times required eliminate the practicality of obtaining Pst and Plt
values.) These results clearly indicate values of instantaneous flicker sensation significantly greater than 1.0. If
statistically analyzed over several repetitions, these high levels of instantaneous flicker sensation would most likely
indicate Pst>1. Of course, Pst>1 supports the large number of customer complaints.

                                          (a) 12 kV Feeder Voltage (120 V base)

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                                        (b) IEC Flickermeter Block 4 Output Value

         Figure 8. EMTP Simulation of Distribution Feeder with Fluctuating Motor Load and IEC Flickermeter

Several features of the flickermeter output of Figure 8(b) are worth noting. The different "peaks" in the output
correspond to different voltage variations where the "sharper" and "smoother" variations correspond to higher and lower
meter outputs, respectively. It would be difficult to apply conventional flicker curves such as those in IEEE 519 and 141
due to the aperiodic and non-standard nature (i.e. not rectangular fluctuations) of the variations; the IEC shape factors
could be applied easily to predict these results before the customer was initially connected.

Resistance Welding Machines, Case 2

In Evansville, Indiana, a manufacturer of condenser tubes for residential refrigerators purchased a three-phase, 1500-
kVA resistive spot welder to fasten the condensers to steel wire used for heat dissipation and structural support. To fill a
backlog of condenser orders from a major refrigerator manufacturer, the condenser manufacturer installed the welder and
operated it at full capacity, 24 hours a day.

During the first 13 days of operating the new welder, 107 different residential customers of the local electric utility
complained about flickering lamps, with as many as 25 calls in one day. The complaints compelled the utility and the
user to determine ways to remedy the problem without shutting down the welding operation.

Within a few days investigators began looking for a solution. The approach included taking measurements with a UIE
flickermeter configured for 120-V lamps and recording the power system configuration. At the same time factory
personnel experimented with welder set up.

The flickermeter, shown in Figure 9, used during the investigation was a portable ensemble of interface module, laptop
computer, and cables, all small enough to fit in a briefcase. The computer software for the flickermeter translates
measurements of the voltage fluctuations into a number, in units of Pst, indicating level of light flickering. The
flickermeter automatically takes measurements in ten-minute intervals. If the Pst measured is above 1.0, then utility
customers connected to the same feeder will likely notice flicker in incandescent lamps (the threshold is higher for
fluorescent lighting as indicated in a previous section). Inside the facility, flicker was observed in fluorescent lamps,
indicating serious voltage fluctuations.

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           Figure 9. UIE/IEC Flickermeter Installed at the Substation Oil Circuit Breaker Serving the Factory

A single-line diagram of the utility and user systems is shown in Figure 10. With the flickermeter at the factory service
entrance (a few hundred electrical feet from the welder transformer, point A in Figure 10) voltage fluctuations were
measured at 2 to 2.4 Pst. The plant power consumption (kVA), voltage distortion (THDV), and current distortion (THDI)
were also monitored and recorded at regular intervals using a three-phase power monitor. On the same day, the
flickermeter was moved to the utility 69-kV substation about two miles away. There, the meter was installed at a 12.47-
kV oil circuit breaker on the feeder serving the condenser factory (point B in Figure 10).

While monitoring the power system voltage, different welder electrical and mechanical configurations were tried. To
determine whether the older welder significantly contributed to the voltage fluctuations, its operating schedule was also
recorded and compared to the flickermeter records. In this way, sufficient data was obtained to guide electrical system
and welder changes that mitigated the fluctuations.

Power system tests included varying the 1200-kVAR reactive compensator at the substation (on or off), the 900-kVAR
reactive compensator at the facility (on or off), and the bus tie at the substation (open or closed). Changes in reactive
compensation and the bus tie did not significantly affect the voltage fluctuations at the substation or service entrance.
Closing the bus tie shown in Figure 10 slightly lowered the Pst at the factory service entrance from 2.4 to 2.0. It also led
to more flicker complaints from newly affected customers now tied to the distribution feeder that served the welder.

          Figure 10. Single-Line Diagram Showing the Utility System, Factory Service, and Monitoring Points.

Meanwhile at the welder, operation was maintained at a high throughput with hope that production levels could be
maintained without producing flicker. Investigators suspected that simultaneous firing of electrode pairs was the main
cause of the light flicker, and changed the firing sequence from two at a time to one at a time and decreased the weld
time from two to one cycle per weld. When the welder was reconfigured to fire electrode pairs individually for a duration
of one cycle, the recorded Pst at the service entrance indeed dropped significantly, from >2.0 to about 0.8. See the
recorded flickermeter output in Figure 11.

Next, the cam speed and weld heat were varied to determine the maximum throughput of high-quality welds. After each
change in welder configuration, investigators tested the weld strength of random condenser samples by pulling on
structural wires using a weld test jig. In this way, the investigators determined that a maximum cam speed of 107 RPM
and a weld heat of 83 percent achieved the greatest throughput of high-quality welds.

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                                      Figure 11. Recorded Flickermeter Output (Pst)

The condenser manufacturer and the utility agreed that changing the firing sequence of the electrode pairs from two at a
time to one at a time and the cam speed from 60 RPM to 107 RPM was the most cost-effective option for reducing
flicker and maintaining a high production level. By doing so, the condenser manufacturer reduced voltage fluctuations at
the substation bus to a level that did not create observable flicker. Reconfiguring the welder also benefited the condenser
manufacturer by decreasing the energy consumption per weld and increasing the welder throughput from a speed of 60
RPM to 107 RPM. After the welder was reconfigured, complaints about flicker in the area ceased, as predicted by the
IEC flickermeter Pst values falling below 1.0. Except for the cost of the investigation, reconfiguring the welder was a
cost-free option that enabled the condenser factory to maintain a high production level and meet the backlog of orders for
refrigerator condensers.


There are many practical situations where existing "flicker curve" methodology can not be applied in a consistent
manner. The IEC has moved toward a standardized measurement technique and has developed flicker and voltage
fluctuation limits based on this technique. Significant experience with the measurement technique has validated the
approach for European power systems and the necessary modifications required to adapt the measurement procedure and
the limits to North American power systems are now available. The case studies presented in this paper demonstrate the
correlation between customer complaints and flickermeter output which indicate that flicker and voltage fluctuation
limits based on the IEC methods can be effectively used in North America.


[1] R.C. Seebald, J.F. Buch, and D.J. Ward, "Flicker Limitations of Electric Utilities," IEEE Transactions on Power
Apparatus and Systems, Vol. PAS-104, No. 9, pp. 2627-2631, September 1985.

[2] M. Sakulin and T.S. Key, UIE/IEC Flicker Standard for Use in North America: Measuring Techniques and Practical
Applications," Proceedings of PQA'97, March 1997, Columbus OH.

[3] IEEE Standard 141-1993: Recommended Practice for Power Distribution in Industrial Plants, IEEE, 1993.

[4] IEEE Standard 519-1992: Recommended Practices and Requirements for Harmonic Control in Electrical Power
Systems, IEEE, 1993.

[5] IEC Publication 868, "Flickermeter: Functional and Design Specifications," CEI, 1986.

[6] IEC Standard 1000-3-3, "Limitation of Voltage Fluctuations and Flicker in Low-Voltage Supply Systems for
Equipment with Rated Current £ 16 A," CEI, 1994.

[7] IEC Standard 1000-3-5, " Limitation of Voltage Fluctuation and Flicker in Low-Voltage Power Supply Systems with
Rated Current > 16 A," Cei, 1994.

[8] IEC Standard 1000-3-7, "Limitation of Voltage Fluctuation and Flicker for Equipments Connected to Medium and

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High Voltage Power Supply Systems," CEI, 1995.

Authors' Biographies

S. Mark Halpin (M 93) received his BEE, MS, and PhD degrees from Auburn University in 1988, 1989, and 1993,
respectively. He is currently an Assistant Professor in the Department of Electrical and Computer Engineering at
Mississippi State University. His teaching interests include power systems, control systems, and network analysis. His
research interests are in the areas of modeling and simulation techniques for large-scale power systems, power system
transients and harmonics, and computer algorithms. He is active in the IEEE Power Engineering Society and Industry
Applications Society, where he serves as chairman of the IEEE-IAS Working Group on Harmonics.

Roger Bergeron (Eng M'74) After graduating with a B.SC. Degree in 1974 in Electrical Engineering from Sherbrooke
University, he spent the next five years building up his knowledge and acquiring experience in private corporations such
as Québec Iron and Titanium. In 1980, he joined Hydro-Québec where he contributed to many research programs
involving worker safety and service quality. He is the taskforce convener of IEEE P1159.1 on "Recorder Qualification
and Data Acquisition Requirements for Characterization of PQ Events", convener for CSA 311.4 committee on the
Canadian standard regarding Electromagnetic Compatibility of Low Voltage and Low Frequency, IEC77A WG1 TF2
and IEC 77A WG9 member on drafting the IEC 1000-4-7 and IEC 1000-4-30, and UIE WG2 secretary which is the UIE
committee on power quality.

T. M. Blooming (S '89, M '94) received a B.S. degree in electrical engineering from Marquette University in 1992, an
M.E. degree in electric power engineering from Rensselaer Polytechnic Institute in 1994, and an M.B.A. degree from
Keller Graduate School of Management in 1998. He is a Power Systems Engineer with the Systems Engineering Group
of Cooper Power Systems, Franksville, WI. His primary responsibility is to perform analytical studies on industrial
power systems. This involves on-site harmonic, flicker, and transient measurements. It also includes system modeling by
applying computer programs to perform harmonic, power flow, stability, short circuit, and protective device coordination
analysis. He is an instructor in the Cooper Power Systems Power Quality and Harmonics, Overcurrent Protection, and
Transformer Application and Protection Workshops. He is active in the IEEE voltage flicker task force.

Reuben F. Burch, IV (M 70, SM 97) was born in Eastman, GA, on August 5, 1948. He received his BEE degree from
Auburn University in 1970. He is a Principal Engineer in Enhanced Power Quality at Alabama Power Co. in
Birmingham, AL where he mainly performs voltage flicker and harmonic analyses. He is a member of IEEE, PES, and
IAS and is a registered professional engineer in Alabama and Georgia.

Larry Conrad (M'74-SM'91) earned his B.S.E.E. in 1974 and his M.S.E.E degree in 1993 from Rose-Hulman Institute of
Technology. He has been employed at Cinergy since 1974 as Engineer, Project Engineer, Senior Engineer, and Technical
Services and Power Quality Manager. He now is Manager of Operations Engineering and responsible for power quality,
the POWER CLINIC(R) power quality services, reliability, and other duties. He has worked with voltage sag issues since
1985. He is generally credited for inventing voltage sag predictive techniques and equipment coordination methods for
IEEE Std. 493 and P1346. Mr. Conrad is a registered Professional Engineer in the states of Indiana and Ohio. He is a
member of the IEEE Industry Applications Society and the Power Engineering Society. Mr. Conrad is an IEEE
representative to ANSI C84 and an advisor to the US National Committee for IEC SC77A and a member of WG2. Mr.
Conrad is Chairman of IEEE voltage flicker project P1453 and Chairman of the IEEE Std 493, Gold Book, Voltage Sag
Working group. He is also a member of the IEEE P1346, ASD and PLC compatibility, Working Group. He also is
responsible for the voltage considerations chapter in the next revision to IEEE Std. 141, Red Book.

Thomas S. Key was born in Indianapolis, Indiana, in 1947. He received a BSEE from the University of New Mexico in
1970 and an ME in power engineering from Rensselaer Ploytechnic Institute in 1974. He is currently the Technical
Director at EPRI's Power Electronics Applications Center in Knoxville, Tennessee, and is responsible for power quality
related research, development, and testing. During the ten years before joining PEAC in 1989, he managed electrical
power system design and power conditioning system applications for renewable sources of energy at Sandia National
Laboratory in Albuquerque, New Mexico. In 1978, while in the Navy, he collaborated with the Computer Business
Equipment Manufacturers Association to create the well-known "CBEMA curve," which is the industry standard voltage
tolerance envelope for information technology equipment. He is a senior member of the IEEE and is active in the
development of standards in the areas of power quality, photovoltaics, and power system design, and 1996 recipient of
the IEEE Outstanding Engineer in Region 3.

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