TEMPUS ENERGY: FLICKER
1: Introduction
Fast variations in the amplitude of the grid voltage appear when a load consumes different currents
for different periods of the grid voltage. Figure 1 visualizes such a heating resistor 𝑅𝐿 where the heat
can be controlled by controlling the thyristors. During 𝑚 periods the thyristors are passing current
(closed switch) and during 𝑛 periods the thyristors do not conduct (open switch). The ratio 𝑚⁄𝑛
determines the heat dissipation.
Figure 1: Controlling the heat dissipation in a resistor RL
Figure 2: Flicker due to fast changes in the consumed current
Figure 2 visualizes the grid current in case 𝑚 = 1 and 𝑛 = 1. When the grid supplies current, there is
a voltage drop across the grid impedance visualized in Figure 3 and the grid voltage 𝑢𝑔𝑟𝑖𝑑 (𝑡) will be
smaller than 𝑢𝑓1 (𝑡). When the grid supplies no current, this voltage drop across the grid impedance
does not occur and the grid voltage 𝑢𝑔𝑟𝑖𝑑 (𝑡) will not be smaller than 𝑢𝑓1 (𝑡). This implies the
amplitude of the grid voltage is changing each 20 ms as visualized in Figure 2 i.e. the amplitude of the
grid voltage is fast time varying. Such fast changes of the grid voltage are called flicker. Indeed,
feeding a lamp with such a voltage causes a flickering of the light intensity. Notice this flicker is a
repetitive phenomenon.
Figure 3: Thévenin equivalent circuit of the grid
The flicker phenomenon not only occurs due to loads (like reciproking compressors, laserprinters or
Xerox-printers) which are consuming currents having an amplitude which is fast time varying with
respect to time. Wind turbines can also be an important source of flicker. Due to wind speed
variations, due to the wind gradient (the wind speed is higher with an increasing height above
ground level) and due to tower shadow effects, the power output of the wind turbine is fast time
varying. This implies the amplitudes of the currents injected into the grid by the wind turbine are
varying with respect to time. The changing voltage drop across the grid impedance causes flicker
especially when the grid is weak (a large grid impedance) and when the wind energy penetration in
the power production is high.
2: Consequences of the flicker phenomenon
The grid voltage 𝑢𝑔𝑟𝑖𝑑 (𝑡), which has a time-varying amplitude, feeds other loads. In case the load is a
lamp, the light intensity produced by this lamp might vary i.e. there is “an impression of unsteadiness
of visual sensation induced by a light stimulus, whose luminance or spectral distribution fluctuates
with time” (IEC 61000-3-3). This flicker phenomenon of the light causes consumer annoyance,
tiredness and complaint. In an industrial environment, this can lead to a reduced productivity, a
reduced quality of the produced goods and an increase of the number of work accidents. Even a
relatively small fluctuation of the amplitude of the grid voltage has a noticeable variation in the light
intensity as visualized in Figure 4.
Figure 4: Increased light intensity due to an increased amplitude of the voltage
An incandescent lamp is quite sensitive to the flicker phenomenon, but also other types of lamps are
sensitive to changes in the amplitude of the grid voltage. An overview for a number of types of lamps
is given in Figure 5. For instance a fluorescent lamp with an electronic ballast is less sensitive to
flicker than an incandescent lamp. Notice also low energy light bulbs (Compact Fluorescent Lamps =
CFL) having a magnetic or an electronic ballast and halogen lamps are less sensitive to flicker than
incandescent lamps.
Figure 5: Sensitivity to flicker
Notice also the frequency of the variations of the amplitude of the voltage has as an impact on the
sensitivity of the human eye with respect to flicker. Especially for frequencies close to 10 Hz, there is
a maximum sensitivity (when considering voltage fluctuations having a frequency of 5 Hz or 15 Hz,
the human eye is less sensitive).
3: Causes of the flicker phenomenon
There are a lot of loads causing the flicker phenomenon due to changes in the consumed current.
Figure 6 visualizes the time-varying current consumed by a photocopying machine (variations due to
the flash of light, the heating, moving sheets of paper…).
Figure 6: Current consumed by a photocopying machine
Figure 7: Current consumed by a reciprocating compressor
Figure 7 visualizes the current consumed by a reciprocating compressor. During the retreating stroke
the current is smaller than the current during the advancing stroke. Figure 7 visualizes a three phase
current. When considering the high voltage grid, electric arc furnaces are an important source of
flicker since they consume very large powers (a large current causes a changing voltage drop across
the grid impedance).
Figure 8: Electric arc furnace
3.1: The impact of the grid impedance
As already visualized in Figure 3, the changing current accounts for a changing voltage drop across
the grid impedance. This implies the amplitude of the grid voltage also changes. In case no current is
consumed, the grid voltage equals the generator voltage. Due to the consumed current (denoted as
̅1 does not equal the generator voltage 𝑈
̅𝐺 . More precisely,
𝐼1̅ in Figure 9), the grid voltage 𝑈
̅1 = 𝑈
̅𝐺 − (𝑅 + 𝑗𝑋) 𝐼1̅
𝑈
where 𝑅 + 𝑗𝑋 is the grid impedance. Especially when the current is ohmic-inductive (which is
commonly the case), the ohmic-inductive grid impedance accounts for a decrease of the voltage
̅1 and 𝑈𝐺 is the RMS value of 𝑈
̅𝐺 , the voltage drop equals
level. In case 𝑈1 is the RMS value of 𝑈
∆𝑈 = 𝑈𝐺 − 𝑈1 ≈ 𝑅 𝐼1 𝑐𝑜𝑠𝜑 + 𝑋 𝐼1 𝑠𝑖𝑛𝜑 .
This implies the ohmic component of the current accounts for a voltage drop due to the resistive part
𝑅 of the grid impedance. The inductive component of the current accounts for a voltage drop due to
the inductive part 𝑋 of the grid impedance.
In case of a high voltage grid, the inductive part of the grid impedance is dominant i.e. 𝑋 ≫ 𝑅.
Therefore, in a high voltage grid mainly the variations in the reactive component of the current
account for variations in the grid voltage and flicker (in case of a capacitive current one obtains a
changing increase of the voltage level, in case of an inductive current one obtain a changing decrease
of the voltage current). When considering the electric arc furnace of Figure 8, there are large
variations in the consumed reactive power (and reactive current component) implying flicker. When
considering a low voltage grid, 𝑅 ≫ 𝑋. This implies mainly changes in the consumed active power
account for flicker.
Figure 9: Impact of the consumed current on the grid voltage
4: Mitigation of the flicker phenomenon
Since the repetitive variations of the grid voltage level mainly influence the light production of
electric lamps, it is a good practice to place the lamps far removed from the loads which consume
varying currents. Since the variations of the grid voltage are related with the grid impedance, the
flicker phenomenon is reduced by
-
-
making the grid impedances as small as possible (by making 𝑋 smaller in case of a high
voltage grid and making 𝑅 smaller in case of a low voltage grid, the short circuit currents will
be larger),
placing the loads which consume varying currents as close to the feeding point as possible
(this reduces the grid impedance where this varying current is flowing).
4.1: Reactive power compensation
When considering a high voltage grid feeding an electric arc furnace, the flicker phenomenon can be
solved or reduced by placing a reactive power compensator in parallel with the furnace. In case the
furnace accounts for a variation ∆ 𝑄𝐹 of the consumed reactive power and the compensator
accounts for a variation ∆ 𝑄𝐶 = −∆ 𝑄𝐹 , the total consumed reactive power remains constant. The
voltage drop across the grid impedance remains constant and the flicker problem is solved. Figure 10
visualizes a grid feeding an arc furnace. The reactive power consumed by the reactor is controlled by
the thyristors. In case the voltage level decreases, less reactive power will be consumed by the
reactor which restores the voltage level (due to the control system). In case the voltage level
increases, more reactive power will be consumed by the reactor which decreases/restores the
voltage level.
The reactive compensator can be realized by using thyristors and an inductor as visualized in Figure
10. Notice this type of compensator always consumes reactive power. As visualized in Figure 11, it is
also possible to use a rotating synchronous machine. The synchronous machine is motoring but since
there is no mechanical load, the active power consumption is limited. By controlling the excitation of
the machine, the reactive power can be controlled. This reactive power can be inductive or capacitive
i.e. also the sign of the reactive power can be changed. By measuring the grid voltage level, a control
system is able to control the excitation and the reactive power. A decrease of the grid voltage
indicates the reactive power consumed by the load is increasing which indicates the excitation
current of the synchronous machine must be increased (this implies the consumed reactive power
decreases or the generated reactive power increases) in order to restore the grid voltage level.
Figure 10: Reactive power compensator
Figure 11: Compensating reactive power using a synchronous machine
Figure 12 visualizes a TSC (Thyristor Switched Capacitors). By controlling the firing angle of the
thyristors, the capacitors will generate a controllable reactive power. Notice the approaches of Figure
10, Figure 11 and Figure 12 are very similar. By placing a compensator which accounts for a reactive
power variation ∆ 𝑄𝐶 = −∆ 𝑄𝐿 the total consumed reactive power remains constant (∆𝑄𝐿 is the
variation of the consumed reactive power by the load).
Figure 12: Thyristor Switched Capacitors
5: Flicker measurement
5.1: The voltage change characteristic
Figure 13: Time function of the RMS voltage
Due to a repetitive decrease or increase of the amplitude of the grid voltage, a flicker phenomenon
occurs. The changes in the light intensity produced by lamps cause discomfort and tiredness which
include subjective aspects. When measuring flicker, these subjective aspects are taken into account
and it is important to take into account:
-
the depth of the repetitive decreases of the voltage level,
the duration of the repetitive decreases of the voltage level,
the number of decreases of the voltage level (on a time scale of 10 minutes or 2 hours).
When considering such a decrease of the voltage level, the time function of the RMS voltage
evaluated as a single value for each successive half period between zero-crossings of the voltage is
visualized in Figure 13. The maximum voltage change characteristic ∆𝑈𝑚𝑎𝑥 (the difference between
the maximum and the minimum RMS values of the voltage in the voltage change characteristic) and
finally the steady-state voltage change ∆𝑈𝐶 (the difference between two adjacent steady-state
voltages separated by at least one voltage change characteristic) are also visualized in Figure 13. The
shape of the relative voltage change characteristic is visualized in Figure 14.
Figure 14: The relative voltage change characteristic
5.2: The shape factor and the flicker impression time
Based on the shape of the voltage change characteristic, a shape factor F can be determined as
visualized in Figure 15 (in case of a voltage step, 𝐹 = 1). The shape of voltage change characteristic
can be described by
-
the ‘front time’ 𝑇𝑓 which describes the time needed to reach the minimum voltage level,
the ‘tail time’ 𝑇𝑡 which describes the time the voltage needs to recover.
The IEC 61000-3-3 standard provides Figure 15 to determine the shape factor F using 𝑇𝑓 and 𝑇𝑡 .
Notice the shape factor F does not depend on the relative depth 𝑑𝑚𝑎𝑥 . Using the shape factor F and
the relative depth 𝑑𝑚𝑎𝑥 , it is possible to calculate the flicker impression time 𝑡𝑓 as
𝑡𝑓 = 2.3 (𝐹 𝑑𝑚𝑎𝑥 )3.2
5.3: The short-time and the long-time flicker indicator
The larger the flicker impression time 𝑡𝑓 , the larger the flicker phenomenon. By considering a time
span of 10 minutes, for each voltage change characteristic the flicker impression time 𝑡𝑓 will be
calculated. By combining all these flicker impression times 𝑡𝑓 , one obtains the short-time flicker
indicator 𝑃𝑠𝑡 (st = short time)
1
𝑡𝑓 3.2
𝑃𝑠𝑡 = (∑ )
𝑇𝑃
Here, 𝑇𝑃 is the total duration of the time interval expressed in seconds (more precisely, 𝑇𝑃 =
10 min = 600 𝑠𝑒𝑐). In general, 𝑃𝑠𝑡 = 1 is assumed to be the conventional threshold of irritability (in
case 𝑃𝑠𝑡 is larger than 1, irritation occurs). The irritation due to flicker increases when
-
voltage change characteristics occur more frequently during the 10 minutes time span 𝑇𝑃 ,
the shape factor 𝐹 and 𝑑𝑚𝑎𝑥 increase (giving larger 𝑡𝑓 values)
giving a larger 𝑃𝑠𝑡 .
Figure 15: Shape factor F for motor-start voltage characteristics having various front times
Not only the short time flicker indicator 𝑃𝑠𝑡 , but also the long term flicker indicator 𝑃𝑙𝑡 (lt = long
term) is important when considering equipment which is operated for more than 30 minutes in time.
Measurements are performed during 2 hours (2 hours = 12 times a time span of 10 minutes). Each
time span of 10 minutes, the short time flicker indicator 𝑃𝑠𝑡 𝑖 is determined (𝑖 = 1, … 12). The long
term flicker indicator 𝑃𝑙𝑡 equals
3
12
𝑃𝑙𝑡 = √∑
𝑖=1
3
𝑃𝑠𝑡
𝑖
12
According to the IEC 61000-3-3 standard, 𝑃𝑙𝑡 is not allowed to exceed 0.65.
5.4: The emission limits
The IEC 61000-3-3 standard describes the flicker emission limits in case of a public low voltage grid
when considering devices consuming currents smaller than 16A. The emission limits required by IEC
61000-3-3 are (additional details can be found in the IEC 61000-3-3 standard)
-
𝑃𝑠𝑡 ≤ 1,
𝑃𝑙𝑡 ≤ 0.65,
𝑑(𝑡) during a voltage change shall not exceed 3.3% for more than 500 ms,
𝑑𝐶 ≤ 3.3%,
additional limits for 𝑑𝑚𝑎𝑥 shall not be exceeded.
6: Wind turbines
Flicker not only originates from electrical loads consuming currents which are varying with respect to
time. Also generators, for instance wind turbines, can be a source of flicker. This can be a limiting
factor for integrating wind turbines into the grid. Especially when the grid is weak i.e. the grid
impedance is high (e.g. due to long feeder lines), wind turbines can give problems. Notice however,
even in relatively strong grids (having a small grid impedance) high wind power penetrations can give
flicker problems.
Grid connected wind turbines may have considerable fluctuations in output power. The fluctuations
are mainly caused by wind speed variations, variations in the wind gradient and the tower shadow
effect. The power (kinetic energy) of the wind equals
𝜌 𝐴𝑣 3
𝑃=
2
where 𝜌 is the air density, 𝐴 is the cross section area covered by the rotor blades and 𝑣 is the wind
speed (by multiplying this power 𝑃 by the power coefficient of the wind turbine, the generated
power is obtained). This implies the variations in the generated power are much larger than the wind
speed variations as visualized in Figure 16. These large active power variations account for variations
in the voltage drop across the grid impedance which implies variations in the final grid voltage.
For instance due to the tower shadow effect, a wind turbine with three blades will give a power drop
three times per revolution. This frequency is usually referred to as the 3𝑝 frequency. When
considering fixed speed wind turbines equipped with an induction generator, power pulsations up to
20% of the average power will be generated. Figure 17 visualizes the variations in the generated
power of a fixed-speed wind turbine during normal operation (the steady-state power is also
plotted). The changes in the produced power are due to wind speed variations and the tower shadow
effect (3𝑝 frequency).
Figure 16: Wind speed variations and variations of the output power of a wind turbine (source: Sun)
Figure 17: Measured power generated by a fixed-speed wind turbine (source: Larsson)
Variable speed wind turbines have better performance related to flicker emission in comparison with
fixed speed wind turbines. The variable speed operation of the rotor has the advantage that the
faster power variations are not transmitted to the grid but are smoothed by the flywheel action of
the rotor. Figure 18 visualizes the short time flicker indicator 𝑃𝑠𝑡 for a fixed speed wind turbine and
an variable speed wind turbine. Notice the 𝑃𝑠𝑡 values are indeed lower when considering a variable
speed wind turbine.
Figure 18: Short time flicker from a fixed speed and a variable speed wind turbine (source: Larsson)
The short time flicker indicator 𝑃𝑠𝑡 does not only depend on the wind turbine but also on the grid
impedance. The smaller the grid impedance, the larger the short circuit ratio SCR and the smaller the
𝑃𝑠𝑡 value as visualized in Figure 19.
Figure 19: Short time flicker emission of a wind turbine (source: Larsson)
References
European Standard EN 61000-3-3, Electromagnetic compatibility (EMC) – Part 3: Limits – Section 3:
Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems,
for equipment with rated current ≤ 16A per phase and not subject to conditional connection, June
2001.
Larsson A., Flicker Emission of Wind Turbines During Continuous Operation, IEEE Transactions on
Energy Conversion, vol. 17, no. 1, pp. 114 – 118, March 2002.
Sun T., Chen Z. and Blaabjerg F., Flicker Study on Variable Speed Wind Turbines With Doubly Fed
Induction Generators, IEEE Transactions on Energy Conversion, vol. 20, no. 4, pp. 896 – 905,
December 2005.