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 u f 1 ( 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 u f 1 (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 I 1 in Figure 9), the grid voltage U 1 does not equal the generator voltage UG. More precisely,

U1=U G−(R+ jX ) I 1

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 level. In case U 1 is the RMS value of U1 and UG is the RMS value of UG, the voltage drop equals

∆U=UG−U 1≈ R I 1cosφ+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 ∆U max (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 ∆U C (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’ T f which describes the time needed to reach the minimum voltage level,- the ‘tail time’ T t 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 T f and T t. 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 t f as

t f=2.3 (F dmax )3.2

5.3: The short-time and the long-time flicker indicator

The larger the flicker impression time t f , the larger the flicker phenomenon. By considering a time span of 10 minutes, for each voltage change characteristic the flicker impression time t f will be calculated. By combining all these flicker impression times t f , one obtains the short-time flicker indicator Pst (st = short time)

Pst=(∑ t fT P

)13.2

Here, T P is the total duration of the time interval expressed in seconds (more precisely, T P=10min¿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 T P,- the shape factor F and dmax increase (giving larger t f 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 P¿ (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 P¿ equals

P¿=3√∑i=112 P st i

3

12

According to the IEC 61000-3-3 standard, P¿ 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,- P¿≤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

P= ρ A v3

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 3 p 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 (3 p 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.