Abstract-- In this paper it will be shown that the switching of the power electronic switches can be a cause (source) of flicker. A new switching technique implemented to control a single-phase tap changed-transformer in order to eliminate the internal flicker problem is investigated. The suggested method was subjected to both theoretical and experimental investigations. Further in this paper a method to mitigate a cyclic voltage flicker using an electronically switched tap changed transformer is investigated in Caspoc. The idea of this method is based on injection of a signal, with the same frequency rate as that of the flicker, into the feedback path of the controller. This method utilizing electronically controlled tap-changed transformer, provides a flicker free secondary voltage.
Index Terms—Smarttrafo, flicker
I. INTRODUCTION
lectric energy providers are facing a lot of challenges to deliver electricity with high quality standards. Non-linear loads are expected among a wide range of industrial fields. Such loads create voltage flicker problems, which is not only annoying to the neighboring customer but also disturbs the sensitive electronic equipment [10]. Many scientific institutes, companies and comities have investigated and classified such phenomena. Different methods were suggested to measure the level of flicker voltage at the point of common coupling PCC [1,2,3]. All these methods and measurements are based on the standard severity curve of IEC 1000-4-15.This curve is constructed based on human eyes-brain reaction to light intensity of 60 watt lamp. Any value of percentage voltage change above threshold is objectionable other is satisfactory [1,2]. The investigation of this paper assuming a constant normal load and one non-linear load, causing flicker of 5Hz, both type of loads are connected to the Smarttrafo as depicted in simulation model in Fig.(1) [6]. Investigation of controlling the secondary voltage of tap-changed transformer started a few years ago and ended with a prototype of a “Smarttrafo” which was tested in the field by KEMA Netherlands. This test showed that when a Smarttrafo is connected to the electric network under a certain load condition, the flicker level at that point has increased. The conclusion was that the Smarttrafo is a source of voltage flicker. The source of voltage flicker in the
P.Bauer, Delft University of Technology, Mekelweg 4 2628CD, Delft (email:P.Bauer@ewi.tudelft.nl)
H.Abdulrahman, Philips Power Solutions Dillenburgstraat 9E 5652AM Eindhoeven (email:Haitham.abdulrahman@philips.com)
P.J.van Duijsen Simulation Research .
grid can be classified into two main types. This classification is based upon the source of flicker and they are:
1. External source where voltage flickers, due to non-linear behavior of some loads, is spreading from this load at the PCC towards the electrical network. 2. Internal source where voltages flicker, due to
implementation of semiconductor devices in their control, is spread from the source at the PCC of electricity towards the network [4].
TIME SIGNAL time DC AC F Phase d SCOPE1 R5 3.15 K3 0.9991 K2 0.9991 C2 8e-6 D5 D6 R3 10 R2 0.0017 L2 0.0000264 K1 0.9991 L1 0.04949 L3 0.0000281 R1 0.076 L4 0.0000281 R4 0.076 D4 D3 VAC50HZ1 14.25e3 + -D1 D2 C1 8e-6 0.5 3000 1 T32 T11 S3 S3 V2 GROUND GROUND T22 T11 S2 S2 GROUND T12 T11 S1 S1
Fig. 1: System configuration of Smarttrafo with two types of load , normal and non-linear load in Caspoc
II. SMARTTRAFO LAYOUT AND OPERATION
The Smarttrafo is a three phase tap-changed distribution transformer of 500 kVA, 10kV/400V with continuously controllable taps [1][2]. In the original design each phase is switching individually; therefore it is satisfactorily to perform investigations and analysis of Smarttrafo using only a single-phase model. As depicted in the Caspoc model in Fig. 1 semiconductor switches, IGBT’s S1, S2 and S3, are the main
rulers to change secondary voltage of a single-phase two-tap winding transformer. These switches are turned on and off according to PWM switching pattern so every switch has a duty cycle value d responsible for the switching period of the tap. These duty cycles, and of course turns ratio of primary, secondary and tap windings, are determining secondary voltage v2 governed by equation (1)[7].
H.Abdulrahman, P.Bauer, P.J.van Duijsen
Simulation of flicker mitigation with the
Smarttrafo
(1) (1) 2 1 1 2 3 2 1 1 1 1 1 2 N v N d d N N N d N N N v tap tap ⋅ + ⋅ + + ⋅ + =
Where: N1,N2 and Ntap are number of turns of
primary, secondary and tap windings respectively.
v1 and v2 are r.m.s value of primary and secondary voltage.
d1 , d2 and d3 are duty cycles of semiconductor switches
S1,S2 and S3 respectively.
The switches S1 and S2 are controlling the 1st tap windings
and the switches S2 and S3 are controlling the 2nd tap. Let us
define a new factor called tap utility factor D. The value of this factor varies between (0 to n) where n is the number of taps, in our case n =2. For D values from 0 to 1; S1 and S2 are
controlling the secondary voltage v2 by periodically switching
1st tap. Value of voltage v
2 is depending on d1 and d2, duty
cycle values of S1 and S2 respectively. For D from 1 to 2 S2
and S3 are controlling secondary voltage. The criteria govern
duty cycle values is that: the instantaneous sum of d1 and d2 or
d2 and d3 should be one. This is shown in equations (2) and
(3)[7].
d1 + d2 =1 ∀ D ∈ [0 1] (2)
d2 + d3 =1 ∀ D ∈ [1 2] (3)
Theoretically, and with help of (2) and (3) relation between utility tap factor D and duty cycles d1, d2 and d3 can be
determined and depicted in Fig.12.
III. EXTERNAL FLICKER MITIGATION
The secondary voltage having a periodical sinusoidal waveform shape flicker, can be represented as:
)
cos(
)
cos(
)
(
50 50 1 ) max( 2 ) 50 max( 2 2ϕ
⋅
⋅
+
ϕ
+
⋅
⋅
+
=
∑
=t
w
t
w
V
V
t
v
m i f f f where:-V2max(50) : 50Hz component voltage amplitude.
V2max(f) : flicker component voltage amplitude.
w50 : 50Hz component frequency 2πf = 100π rad/sec.
wf : flicker component frequency .
ϕ50 : phase angle of 50Hz component and
ϕf : phase angle of flicker component.
m : order of flicker [6,7].
Injecting a signal with the same amplitude and frequency, as the flicker component, will eliminate the flicker effect totally. The flicker signal will be extracted from the polluted secondary voltage v2 to calculate amplitude and frequency of
the flicker signal to be injected later on into the feedback path of the control circuit, driving pulses path, to correct secondary voltage. Extraction of flicker amplitude and frequency is described in detail in [5]
By changing the turn’s ratio of the Smarttrafo the secondary voltage is controllable. In order to change the turns-ratio the utility tap factor D value should be changed too, which in turns alters the duty cycles of the IGBT switches. This D is minimum during low load condition and maximum when full load is connected.
As depicted in Fig. 2, Smarttrafo compensates the voltage drop ∆V whenever load condition changes thus D becomes eventually a function of the voltage drop ∆V. An assumption is made in this study that two kinds of loads were connected to the Smarttrafo, normal and non-linear load. The latter creates a sinusoidal waveform shaped flicker of 5Hz as frequency and 1% as magnitude of ∆V. The control of the Smarttrafo works as follows: whenever a load is connected to the Smarttrafo a time window of 500msec is activated to test the secondary voltage status. If the load is normal then the control circuit of the Smarttrafo will use the standard feedback path with PI regulator as depicted in Fig. 2. But if a non-linear load is connected then, and within the 500msec time window, a flicker signal will be detected via a flicker meter and extracted from
v2(t) in order to be injected later on. The strategy of Smarttrafo
control is depicted in Fig. 2.
A phasor diagram for the relation between the secondary voltage and voltage drop ∆V before and after flicker signal injection is depicted in Fig. 3 and Fig. 4 respectively.
+
+ V
V2(50)Ref+ _
PI Driving
Pulses IGBT Trafo.
500msec. Time Window Flicker Extract D D Non-linear path
Normal load path
2 Flicker Meter (0) Selector Switch V2(50) V2 V2(50) V2 BPF of 50Hz BPF of 50Hz
Fig. 2: Feedback control circuit in case of non-linear load ,flicker extraction and injection.V2max : envelope of secondary voltage and V2max(50) : amplitude
of 50Hz component.
Fig. 3: General phasor diagram before flicker injection
Fig. 4: General phasor diagram after flicker injection 2006第五屆台灣電力電子研討會
The phasor concept had been used in the previous paragraph to extract the flicker signal. The same concept will be used here to recognize the flicker frequency. Every time a new load is connected to the smarttrafo the secondary voltage is tested for flicker using the flicker meter. This test lasts for 500msec and within this time a decision must be made to activate or deactivates the non-liner path of the control circuit.
a) 5 Hz square waveform flicker
b) 5Hz sine waveform flicker
d) 10 Hz square waveform flicker
Some simulation results of different types of flicker modes, shapes forms and frequencies, are depicted in Figures on this page, implementing the space vector principal to classify the sort of the voltage flicker that affecting the secondary voltage at a certain repetition rate, frequency.
c) 10 Hz sine waveform flicker
f) Non-periodical random frequency flicker
Simulation results of periodical external mitigation are depicted in Fig.5 to Fig.10, where the idea of extracting and injecting a flicker signal is applied. The secondary voltage waveform of the Smarttrafo with flicker signal injection is depicted in Fig.8, when a non-linear load connected to its terminal.
Fig. 5: Load voltage waveform, without voltage injection
Fig. 6: RMS value of load voltage, without voltage injection
Fig. 7: Flicker level without voltage injection
IV. INTERNAL FLICKER MITIGATION
To investigate the internal flicker a simple one-phase model of the Smarttrafo with two taps was considered in Fig.1 [9].
From Fig. 11 and when D = 1; S1 and S3 are switched off
leaving only S2 on constantly, since their correspondent duty
cycles d1 = d3 = 0 and d2 =1. For 0< D <1 then 1st tap is
switched on/off via S1 and S2 and for 1< D <2 then 2nd tap is
switched by S2 and S3. It is possible to calculate any duty cycle
value after specifying the D value.
Fig. 8: Load voltage waveform, with voltage injection
Fig. 9: RMS value of load voltage, with voltage injection
Fig. 10: Flicker level with voltage injection
Another relation can be concluded between secondary voltage v2 and D; such theoretical relation is depicted in Fig.
12. 2006第五屆台灣電力電子研討會
A. Practical realization
In a practical application; the IGBT needs a certain time to switch from one state to another and this time is called a switching time, (delay + rise) time to switch on and (delay + fall) time to switch off. The time ∆tON at which the IGBT is
100% on, or time ∆tOFF, at which the IGBT is 100% open,
both are function of duty cycle d and switching period T as shown in equations (4) and (5) [9].
∆tON = d * T (4)
∆tOFF = (1-d) * T (5)
Fig. 11: Utility tap factor D versus switches duty cycles d1, d2 and d3
(theoretical operation)
Fig. 12: Characteristic curve describes v2 as a function of D (theoretical
operation)
Fig. 13: Non linear relation between utility tap factor D and secondary voltage
v2 (practical operation)
If ∆tON ≅ sum of (delay +rise) and (delay +fall) times; the
IGBT is no longer working as a switch. Therefore there are minimum and maximum values of the duty cycle to keep the IGBT working as a switch. Such a limitation will create a discontinuity region, the gap marked by bold blotch; between
1st tap area and 2nd tap area as depicted in Fig. 13. In this
figure a practical operation is shown.
B. Internal Flicker Identification
This discontinuity, stated in last paragraph, will be reflected on the characteristic curve of Fig. 13. The relation between secondary voltage and utility tap factor becomes non-linear as depicted in Fig. 13. The IGBT duty cycle value as minimum was 0.1 and as maximum was 0.9. It is clear from Fig. 13; that the secondary voltage cannot be well defined when D is within the interval [0.9 1.1] except at 1. Due to this; the controller of the feedback system cannot fully compensate the drop of the secondary voltage in the discontinuous region. Let us assume that D is 0.9. If an extra load is connected to a tap-changed transformer, the secondary voltage will drop below his rated value.
The controller tries to compensate the drop of v2 by
increasing D to say 0.95 in order to sustain the rated voltage. Unfortunately this can’t be done, because the next higher possible value of D after 0.9 is 1 (there is no more linear relation). At this value of D the secondary voltage is more than the required value. Then the controller will see it as a surplus in v2 and will try to reduce v2 by reducing D to a value as 0.95.
But this can’t be achieved also because of the non-linearity and D can be reduced to 0.9 and not to 0.95. Once again the controller will try to compensate v2 and the same cycle as
previous will be repeated without any success. The repetition of compensation cycle without any success will lead to the secondary voltage fluctuation with a certain rate and frequency. Such a voltage fluctuation in this manner is named a flicker. The frequency depends mainly on load conditions and the time constant of feedback loop. In our case this internal flicker frequency is between 10 to 12 Hz. Next a term called error signal ∆v is defined. This term is the difference between the reference values vref. and measured one vmeasur..
The secondary voltage waveform and its envelope are obtained from simulation of operation (theoretical operation), as depicted in Fig. 14. Voltage ∆v and D are measured and depicted in Fig. 15. Here the tap utility factor can reach any value between the values 0 to 2 as depicted in previously fig.(11). Next there are simulation results of practical realization of tap changed transformer operation. In Fig. 16, the voltage ∆v versus D and in Fig. 17 the secondary voltage and its envelope are depicted.
Fig. 14: Simulation results from Caspoc represent secondary voltage and its envelope (theoretical operation)
In Fig. 17, the secondary voltage fluctuates with 1% of the rated voltage with a frequency of 11Hz (for this load condition). Comparing these results, shows why there was a flicker problem when a tap changed transformer is connected to electric network.
Fig. 15: Simulation results.1st trace is ∆v. and 2nd is D. Left side scale is for
∆v. Right side scale is for D (theoretical operation)
Fig. 16: Simulation results.1st trace is ∆v. and 2nd is D. Left side scale is for ∆v. Right side scale is for D (practical operation)
Fig. 17: Secondary voltage v2 and its envelope (practical operation)
C. Suggested solution
The solution must solve the discontinuity state when tap changed transformer operation is transferred from 1st tap to the
2nd, i.e. when D is within the [0.9 1.1] interval. The solution is to define duty cycle values other than 1 in this gap; and that means all three switches S1, S2 and S3 should be working
together in such way to keep the same duty cycles criteria of unity as in (6) [7].
d1 + d2 + d3 = 1 (6)
In the previous paragraph it was shown that there is no definition for duty cycle values when D is within the [0.9 1.1] interval, except for 1. In the new suggested solution the
situation is different. When D is 0.9 then d1= 0.1 and d2 = 0.9;
any further increment in D can be achieved e.g. as follows: • Switch S1 at fixed duty cycle, d1= 0.1 for the whole
[0.9 1.1] interval
• Switch S2 at d2 = 0.8 and downward to 0.7
• Switch S3 at d3 = 0.1 and upward to 0.2
In this way the criteria of unity for duty cycle values defined in (6) is achieved. The abovementioned distribution of duty cycle values is depicted in Fig. 18. According to this distribution; the secondary voltage will be in three voltage levels instead of two, since three switches are working in a periodic rate, as depicted in Fig. 19. Different patterns are hereby possible e.g. asymmetrical shown in Fig.19 or symmetrical shown in. Selection of the optimal pattern is a matter of further optimisation.
Fig. 18: D versus duty cycles in the new method
Fig. 19 Simulation results from Caspoc of the new method 1st trace is secondary voltage v
2
2nd , 3ed and 4th duty cycles d
1, d2 and d3 of driving pulses
D. Calculation of duty cycles in the new method
The new method is suggesting, fixing the duty-cycle of one switch and relate the other two according to equation (6). It is possible to extract many other sets by fixing d1 and calculate
the other two; or fixing d3 and calculate the other two. To
reach a certain value of the secondary voltage there are thus numerous solutions.
To obtain simulation results using the new method of internal flicker eliminating, and compare it with old one; the model of tap changed transformer is subjected to the same load conditions and circumstances as in former situation. The results obtained are depicted in Fig. 20 where both the error 2006第五屆台灣電力電子研討會
signal ∆v and utility tap factor D are smooth and stable. Also Fig.21 depicts the secondary voltage and its envelope, which is now a flicker free waveform. Comparing the simulation results shows indeed that the new method of switching is eliminating totally the internal flicker problem even when D is working within the discontinuity region, where D = 1.
Fig. 20: Simulation results from Caspoc of new method.1st trace is ∆v and 2nd is D. Left side scale is for ∆v. Right side scale is for D.
Fig.21: Secondary voltage with its envelope with the new switching technique applied
V. CONCLUSIONS
In the paper it is shown that the injection of a flicker signal into the feedback circuit eliminates the cyclic external flicker in the secondary voltage. This procedure can be followed also with any other non-linear load. Also the new suggested switching and distribution of duty cycles for each IGBT switch totally eliminates the internal flicker problem. With these techniques of handling and controlling the IGBT’s driving pulses, the secondary voltage of the Smarttrafo becomes flicker free. A further study will be carried, out to investigate mitigations due to non-cyclic flicker like that in arc furnaces and welding machines.
VI. ACKNOWLEDGMENT
The authors gratefully acknowledge the contributions of Rob Schoevaars by supporting the experimental work and also for the fruitful discussions
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