Voltage fluctuations caused by fast changes of industrial loads have been a major concern for both power companies and customers in the area of power quality. The fast response of the Static Compensator (STATCOM) makes it an efficient solution for improving power quality in distribution systems. This paper describes a simplified model for PWM-based STATCOM used in a distribution system for mitigation of voltage fluctuations produced by an Electric Arc Furnace (EAF).
The analyzed system is modeled using MATLAB/SIMULINK Power System Blockset (PSB), including a complete STATCOM model with its power circuits and its control system. Static and dynamic performance of STATCOM is evalueates and voltage fluctiation mitigation studies are performed and discussed. The voltage fluctuation mitigation is obtained by simulations and according to international stardards.Instituto de Investigaciones Tecnológicas para Redes y Equipos Eléctricos (IITREE

Agüero, Jorge Luis

Issouribehere, Fernando

Servicio de Difusión de la Creación Intelectual

XIICLEEE - 12th Portuguese-Spanish Conference on Electrical Engineering XTICLEEE 1704Voltage Flicker Mitigation using a simplified STATCOM model suitable for long time simulationsJorge L. Aguero, Fernando Issouribehere IITREE-LAT, FI, UNLP Calle 48 y 116, B1900AMF, La Plata, Argentina j la@iitree-unlp. org. arAbstract- Voltage fluctuations caused by fast changes of industrial loads have been a major concern for both power companies and customers in the area of power quality. The fast response of the Static Compensator (STATCOM) makes it an efficient solution for improving power quality in distribution systems. This paper describes a simplified model for a PWM- based STATCOM used in a distribution system for mitigation of voltage fluctuations produced by an Electric Arc Furnace (EAF).The analyzed system is modeled using MATLAB/SIMULINK Power System Blockset (PSB), including a complete STATCOM model with its power circuits and its control system. Static and dynamic performance of STATCOM is evaluated and voltage fluctuation mitigation studies are performed and discussed. The voltage fluctuation mitigation is obtained by simulations and according to international standards.I. In t r o d u c t io nMany loads connected to electric power systems may cause power quality problems at all voltage levels and for very different power ratings due to their unbalanced and non­linear behaviour characteristics. However, the main sources of power quality problems affecting large numbers of customers are the high power industrial loads. The fast large swings in active and reactive power required by such loads, cause fast repetitive voltage variations with appreciable voltage distortion caused by harmonics and unbalance. The residential and commercial customers supplied by the same network are subjected to the impact of these voltage variations that produce disturbances to their equipment and flicker in the light output of their electric lamps.EAF loads can cause serious electrical disturbances on a power system. Low level amplitude modulation of the supply voltage of less than 0.5% can cause annoying flicker in lamps and invoke public complaints when the frequencies lie in the range of 3-10 Hz.The most effective way to control voltage fluctuations and therefore to limit flicker is to compensate the reactive power variations of the fluctuating loads at medium/high voltage levels.This paper presents the voltage flicker measured at a steelwork busbar, where an 8-ton, 2.5 MW EAF is connected.To mitigate the voltage fluctuation phenomenon, a 4 MVAr STATCOM was included in the steelwork busbar.The mathematical model and the control strategy to compensate voltage fluctuations are explained and described in detail hereinafter.The capability of the STATCOM to mitigate voltage fluctuations is demonstrated by digital simulations and experimental results.Fig. 1. Steelwork one-line diagramThe steelwork is supplied directly from public distribution network at 13.2 kV. Measurements of three-phase voltages and currents were performed at point 1 (steelwork supplying point) and point 2 (primary of EAF feeding transformer).III. POWER QUALITY MEASUREMENTSAn EAF consists of a refractory lined shell which holds the charge, usually scrap metal. Three large electrodes are held in special clamps on a swing support structure, which can be swung aside for charging, and which allows each electrode to be moved according to the output of the control system.After the furnace is charged with scrap, operation begins by lowering the electrodes to strike electric arcs between the electrodes and the scrap. The heat generated by the three electric arcs provides the heat for melting the scrap.There are several phases in the EAF operation, each presenting a different impact on the power system in terms of flicker, namely: Boring, Melting and Refining periods. Fig. 2 shows the measured EAF active and reactive power during a typical furnace heat.II. POWER SYSTEM CONFIGURATIONThe one-line diagram of the steelwork power system is shown in Fig. 1.XI1CLEEE - 12th Portugneae-Spanish Conference on Electrical Engineering XIICLEEE_1704Fig. 2. Active and reactive power measurements during a Furnace Heat.A. Flicker measurementsFlicker measurements were made by the IITREE-LAT with the IEC 61000-4-15 normalized flickermeter [1]. Fig 3 shows the results obtained from P,* measurements in 1-minute interval during a complete furnace cycle. This non normalized measurement was performed to evaluate flicker severity caused by fast voltage fluctuations especially during the first charge o f scrap metal.Fig. 3. P,t measurements during a normalized periodThe furnace cycle depicted in Fig. 3 was 2 and a half hour long and the furnace was recharged twice after the initial charge. It is interesting to note that the perturbation level decays as the metal becomes liquid and the arc becomes shorter and more stable. Fig 4 shows the normalized one- week, 10-minute-interval Pat measurements.Fig. 4. P,t measurement during a normalized period.The statistical results obtained from the normalized flicker measurement are presented in Table I.Table I. Statistical result of Flicker measurementTotal observations of 10 minutes 1008PatS5% 2.21Observations with Pst > 1 288Percent of measurement with P* > 1 28.6%The obtained value o f Pst9j% = 2.21 is above the lim its o f European and Argentinian Standards.B. Flicker compensation ratio and STACOM ratingThe arc furnace apparent power could be determined according to the power factor limit described in [2].IV. STATCOM OPERATING PRINCIPLESThe STATCOM is essentially a voltage source behind a coupling reactance with the corresponding V-I and V-Q characteristics [3]-[5] shown in Fig. 5.This parameter is o f fundamental importance for the determination o f the STATCOM compensator size.From observations o f practical installations the approximate equation for estimating the STATCOM rating for the operation with an EAF is given in [3].According to this value, it is possible to estimate the flicker compensation ratio:With this K factor, it is possible to obtain the maximum flicker that could be emitted by the steelwork, according to Argentinian regulations [21.It is possible now to estimate the ratio between the EAF apparent power and the short circuit level at the steelwork supplying point.XIICLEEE - 12th Portuguese-Spanish Conference on Electrical Engineering XIICLEEE_1704From Fig. 5, it is possible to conclude that:• The STATCOM can be operated all over its full output current range even at very low system voltage levels. In other words, the output current can be maintained independently from the a.c. system voltage.• The maximum Var generation or absorption changes linearly with the a.c. system voltage.• The independence of STATCOM output from equivalent system impedance means that the voltage regulator controlling the STATCOM output can be designed for a faster response rate and can provide stable regulation over the full range of system contingencies.The single line diagram of the STATCOM for reactive power supply to the transmission system is shown in Fig. 6, where U is the voltage of the steelwork busbar and Ec is the controllable output voltage.Fig. 6. STATCOM scheme for reactive power generation.The active and reactive power exchanged between the network is given by:distribution system for the voltage fluctuation mitigation study.See = 300 MVAr * Steelwork feeder,71VaVaSteelwork (pu) la_Pcc Vdc Q (Mvar) P (MW)DV/V (%)□□Measurements[±> Operating Mode VdcPTripNABFig. 8. MATLAB model of Utility/Customer system.The EAF is modeled as three controlled current sources, governed by the measured currents in the EAF during a furnace heat. The arc furnace model is shown in Fig. 9.Controlled Current SourceFromW orkspaceControlled Current Source<2>*~lbo u ruFromW orkspaceControlled Current Sourcerk-<!>•-IcFrom W orkspaceIf the amplitude Ec of the output voltage phasor (Ec) is increased above the amplitude U of the a.c. voltage (U), then the current phasor leads the voltage phasor and current flows from the converter to the a.c. system and the converter generates reactive (capacitive) power to the system.If the amplitude of the output voltage phasor is decreased below that of the a.c. system voltage phasor, then the reactive current flows from the a.c. system to the converter, and the converter absorbs reactive (inductive) power from the a.c. system. This operation is illustrated in Fig. 7.Fig. 9. Arc Furnace model.VI. STATCOM DETAILED MODELThe ± 4 MVAr STATCOM SIMULINK detailed model is shown in Fig. 10. It contains a PWM IGBT inverter, two 10000 jaF capacitors and a control system. The STATCOM is coupled in parallel with the network through a 0.4/13.2 kV A-Y transformer [6]-[8].B <s>»-c9 k +A aB “ i f " bC v  *  cBC  C a-« a >VdcP-*d>N-«3>Fig. 7. Capacitive and Inductive behaviour of STATCOM.V. UTILITY/CUSTOMER POWER SYSTEM MODELA digital simulation model of the distribution network is implemented by using the MATLAB/SIMULINK Power System Blockset (PSB). Fig. 8 shows the upper level of thePWM ControllerFig. 10. STATCOM block diagram.A PWM pulse generator of 1.65 kHz is used to control IGBT bridge. The modulation scheme used is of sinusoidalXIICLEEE - 12th Portuguese-Spanish. Conference oa Electrical Engineering XHCIEEE_1704type. The controller diagram is shown in Fig. 11. It consists of several subsystems: a phase-locked loop (PLL), a measurement system, an a.c. voltage regulator, a d.c. voltage regulator and a current regulator.The PLL is synchronized to the fimdamental of the transformer primary voltage to provide the synchronous reference required by the abc-dqO transformation. The measurement block computes the d-axis and q-axis components of the voltages and currents.The current regulation consists of two PI controllers that control the d-axis and q-axis currents according to (9)-(10).The controller outputs are the voltages E«i and that the PWM inverter has to generate according to (11)-(12). Then these voltages are converted into phase voltages EM, ECb andE c c .The implementation of the control system is based on d-q compensation theory. The compensator input currents track the reference currents by means of the following current controller equations:Fig. 11. STATCOM Controller.PWM waveforms, we can use instead a voltage source having the same averaged value computed upon a chopping period (Tc = 1/1650) in this case. By using this “Averaged Model”, we can simulate the system operation with a larger step time, resulting in a simulation time reduction or, put in another way, the total simulation time can be increased up to 60 seconds.The Averaged Model can be built based on the energy conservation principle, taking into account that the instantaneous power must be the same on the AC side and the DC side of the inverter (neglecting losses). The DC current in the DC-link capacitor can be then computed from the measured AC instantaneous power and the DC-link voltage Udc as:The STATCOM model mentioned above is described in [6] where is presented a detailed model of the inverter that includes the switching of the inverter power switches. This model requires a very small computing time step to represent correctly the PWM waveforms (Tg = 5 (j.s). The simulation time is thus fairly long and it is not possible to perform simulations longer than 5 seconds due to memory requirements.Vn. STATCOM AVERAGED MODEL If we are not interested to represent the chopping of theFig. 12 shows the SIMULINK implementation of the inverter’s Averaged Model. On the AC side, the inverter is modeled three controlled voltage sources which are determined by three voltages V^c from the control system. On the DC side, it is modeled by the DC-link Model. In this model, a capacitor (represented by an integrator) is charged by a DC current source with a value computed as shown in (15). A Switch block is used to avoid a division by zero at starting when the capacitor has no charge.XIICLEEE - 12th Portuguese-Spanish Conference on Electrical Engineering XIICLEEE_1704Fig. 12. SIMULINK diagram of the PWM inverter Averaged Model.VIII. TESTING THE STATCOM AVERAGED MODEL WITH A DYNAMIC LOADThe ability of a compensator to reduce flicker depends on the compensator size and speed of response. In order to evaluate the speed of response of both STATCOM models, the Three-Phase Dynamic Load from SIMULINK was utilized. The active power was set constant at 2.5 MW, and the reactive power was set at 1.8 MVAr. This load is representative of the active and reactive power levels reached by the arc furnace during a complete heat cycle. As it can be shown in Fig. 13 a positive step change of 1.2 MVAr is applied at time t = 0.4 s then a negative step of -3.4 MVAr is applied at t = 0.7 s, and finally the reactive power recovers its original value at t = 1 s.Fig. 14 shows the obtained results for the busbar voltage where both models of STATCOM were connected.As a result, both models (detailed and averaged) showed a similar response.Fig. 13. Reactive power supplied by both STATCOM models.Fig. 14. Busbar voltage controlled by both STATCOM models.IX. TESTING THE STATCOM AVERAGED MODEL WITH THE EAF MODELDigital simulations were performed to evaluate the flicker mitigation in different phases of operation of the EAF when both STATCOM models were connected to the steelwork busbar. The steelwork busbar voltage without STATCOM and with both STATCOM models is shown in Fig. 15 and Fig. 18. The reactive power supplied by both STATCOM models is shown in Fig. 16 and Fig. 19. And finally, the DC-link voltage of both STATCOM models is shown in Fig. 17 and Fig. 20.— ST AT CO M-detailed — STATCO M- ave raged2.5 3 3.5 4 4.5 5Time [s]Fig. 16. Reactive power supplied by both STATCOM models.2.5 3 3.5 4 4.5 5Time [sjFig. 15. Steelwork busbar voltage.XIICLEEE - 12th Portuguese-Spanish Conference oa Electrical Engineering XHCIEEE_1704Fig. 18. Steelwork busbar voltage.Fig. 19. Reactive power supplied by both STATCOM models.Fig. 20. DC-link voltage of both STATCOM models.As it can be seen in the previous figures, both STATCOM models presented a very similar response.Finally, Fig. 21 presents the Flicker level (Pst95%) in the steelwork busbar voltage, during a complete furnace cycle. The Flicker level was obtained by digital IEC 61000-4-15 Flickermeter model implemented in MATLAB-SIMULINKenvironment. The Pst values obtained in the case without STATCOM in operation were similar to the ones obtained by field measurements performed in the steelwork. It can be seen in this figure that the Pst values obtained with both STATCOM models were practically the same. The averaged STATCOM models has the important additional advantage of being able to perform simulations up to 60 seconds long, and therefore the Flicker values obtained are more representative of those obtained with the connection of a real STATCOM.TimeFig. 21. Pst evaluation during a complete heat cycle.The ability of a compensator to reduce flicker depends mainly in the speed of response of the control system, and in both STATCOM models the control loop parameters were adjusted to be exactly the same. The minor differences in both responses are due to inefficient filtering of high frequencies generated by commutation of high power IGBT switches, in the case of STATCOM detailed model.Additionally, both STATCOM models reduce Flicker levels under the requirements of International and Argentinian Standards.X. CONCLUSIONSIn this paper two STATCOM models for voltage Flicker compensation has been compared.The first model, called “detailed model” was described in detail by the same authors in [6]. This model has the important advantage of been a physical model that includes all the power electronics associated with the PWM converter. However, this model requires a very small computing time step (Ts = 5 (is) to adequately represent the PWM waveforms. With this model can not be achieved simulation times of more than 5 seconds.The second model, called “averaged model” was described in detail in [7], this model is based on the energy conservation principle and the PWM inverter is replaced by three controlled voltage sources and the DC-link is replaced by a control loop where the capacitor is replaced by an integrator. In this model the computing time step can be increased ten times and simulation times of 60 seconds can be achieved.Both STATCOM models have demonstrated very similar performance in compensating Flicker generated by an EAF, and are valid for Flicker mitigation and propagation studies. However, the 60 second simulations long are suitable forXIICLEEE - 12th Portuguese-Spanish Conference on Electrical Engineering XTICLEEE_1704Flicker studies and for comparison with real Flicker measurements performed every one minute.XI. REFERENCES[1] IEC 61000-4-15. Electromagnetic Compatibility (EMC). Part 4: Testing and measurement techniques. Section 15: Flickermeter. Functional and design specifications.[2] Anexo a la Resolution ENRE 99/97. Base Metodoldgica para el Control de la Emision de Perturbaciones. Etapa 2.[3] Static Synchronous Compensator (STATCOM) for Arc Furnace and Flicker Compensation. WG B 4.19. CIGRE Publication. December 2003.[4] Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems. Narain G. Hingorani, Laszlo Gyugyi. Wiley-IEEE Press 1999. ISBN-0780334558[5] Thyristor-Based FACTS Controllers for Electrical Transmission Systems. R. Mohan Mathur, Rajiv K. Varma. IEEE Press and Wiley & Sons, Inc. 2002. ISBN 0-471-20643-1.[6] “STATCOM Modeling for Mitigation of Voltage Fluctuations caused by Electric Arc Furnaces”. J. Agiiero, F. Issouribehere and P. Battaiotto. 2006 PES General Meeting. 1-4244-0493-2.[7] “Modeling and Simulation of a Distribution STATCOM using Simulink's Power System Blockset”. P. Giroux, G. Sybille, H. L. Huy. Pp. 990-994. 0-7803-7108-9/01.[8] “Voltage Flicker Mitigation Using PWM-Based Distribution STATCOM”. J. Sun, D. Czarkowski andZ. Zabar. Pp 616-621. 0-7803-7519-X/02.

Voltage Flicker Mitigation using a simplified STATCOM model suitable for long time simulations

SEDICI - Repositorio de la UNLP

XIICLEEE - 12th Portuguese-Spanish Conference on Electrical Engineering XTICLEEE 1704
Voltage Flicker Mitigation using a simplified STATCOM model 
suitable for long time simulations
Jorge L. Aguero, Fernando Issouribehere 
IITREE-LAT, FI, UNLP 
Calle 48 y 116, B1900AMF, La Plata, Argentina 
j la@iitree-unlp. org. ar
Abstract- Voltage fluctuations caused by fast changes of 
industrial loads have been a major concern for both power 
companies and customers in the area of power quality. The fast 
response of the Static Compensator (STATCOM) makes it an 
efficient solution for improving power quality in distribution 
systems. This paper describes a simplified model for a PWM- 
based STATCOM used in a distribution system for mitigation of 
voltage fluctuations produced by an Electric Arc Furnace 
(EAF).
The analyzed system is modeled using 
MATLAB/SIMULINK Power System Blockset (PSB), including 
a complete STATCOM model with its power circuits and its 
control system. Static and dynamic performance of STATCOM 
is evaluated and voltage fluctuation mitigation studies are 
performed and discussed. The voltage fluctuation mitigation is 
obtained by simulations and according to international 
standards.
I. In t r o d u c t io n
Many loads connected to electric power systems may 
cause power quality problems at all voltage levels and for very 
different power ratings due to their unbalanced and non­
linear behaviour characteristics. However, the main sources of 
power quality problems affecting large numbers of customers 
are the high power industrial loads. The fast large swings in 
active and reactive power required by such loads, cause fast 
repetitive voltage variations with appreciable voltage 
distortion caused by harmonics and unbalance. The 
residential and commercial customers supplied by the same 
network are subjected to the impact of these voltage variations 
that produce disturbances to their equipment and flicker in 
the light output of their electric lamps.
EAF loads can cause serious electrical disturbances on a 
power system. Low level amplitude modulation of the supply 
voltage of less than 0.5% can cause annoying flicker in lamps 
and invoke public complaints when the frequencies lie in the 
range of 3-10 Hz.
The most effective way to control voltage fluctuations and 
therefore to limit flicker is to compensate the reactive power 
variations of the fluctuating loads at medium/high voltage 
levels.
This paper presents the voltage flicker measured at a 
steelwork busbar, where an 8-ton, 2.5 MW EAF is connected.
To mitigate the voltage fluctuation phenomenon, a 4 MVAr 
STATCOM was included in the steelwork busbar.
The mathematical model and the control strategy to 
compensate voltage fluctuations are explained and described 
in detail hereinafter.
The capability of the STATCOM to mitigate voltage 
fluctuations is demonstrated by digital simulations and 
experimental results.
Fig. 1. Steelwork one-line diagram
The steelwork is supplied directly from public distribution 
network at 13.2 kV. Measurements of three-phase voltages 
and currents were performed at point 1 (steelwork supplying 
point) and point 2 (primary of EAF feeding transformer).
III. POWER QUALITY MEASUREMENTS
An EAF consists of a refractory lined shell which holds 
the charge, usually scrap metal. Three large electrodes are 
held in special clamps on a swing support structure, which 
can be swung aside for charging, and which allows each 
electrode to be moved according to the output of the control 
system.
After the furnace is charged with scrap, operation begins 
by lowering the electrodes to strike electric arcs between the 
electrodes and the scrap. The heat generated by the three 
electric arcs provides the heat for melting the scrap.
There are several phases in the EAF operation, each 
presenting a different impact on the power system in terms of 
flicker, namely: Boring, Melting and Refining periods. Fig. 2 
shows the measured EAF active and reactive power during a 
typical furnace heat.
II. POWER SYSTEM CONFIGURATION
The one-line diagram of the steelwork power system is 
shown in Fig. 1.
XI1CLEEE - 12th Portugneae-Spanish Conference on Electrical Engineering XIICLEEE_1704
Fig. 2. Active and reactive power measurements during a Furnace Heat.
A. Flicker measurements
Flicker measurements were made by the IITREE-LAT 
with the IEC 61000-4-15 normalized flickermeter [1]. Fig 3 
shows the results obtained from P,* measurements in 1-minute 
interval during a complete furnace cycle. This non 
normalized measurement was performed to evaluate flicker 
severity caused by fast voltage fluctuations especially during 
the first charge o f scrap metal.
Fig. 3. P,t measurements during a normalized period
The furnace cycle depicted in Fig. 3 was 2 and a half hour 
long and the furnace was recharged twice after the initial 
charge. It is interesting to note that the perturbation level 
decays as the metal becomes liquid and the arc becomes 
shorter and more stable. Fig 4 shows the normalized one- 
week, 10-minute-interval Pat measurements.
Fig. 4. P,t measurement during a normalized period.
The statistical results obtained from the normalized flicker 
measurement are presented in Table I.
Table I. Statistical result of Flicker measurement
Total observations of 10 minutes 1008
PatS5% 2.21
Observations with Pst > 1 288
Percent of measurement with P* > 1 28.6%
The obtained value o f Pst9j% = 2.21 is above the lim its o f 
European and Argentinian Standards.
B. Flicker compensation ratio and STACOM rating
The arc furnace apparent power could be determined 
according to the power factor limit described in [2].
IV. STATCOM OPERATING PRINCIPLES
The STATCOM is essentially a voltage source behind a 
coupling reactance with the corresponding V-I and V-Q 
characteristics [3]-[5] shown in Fig. 5.
This parameter is o f fundamental importance for the 
determination o f the STATCOM compensator size.
From observations o f practical installations the 
approximate equation for estimating the STATCOM rating 
for the operation with an EAF is given in [3].
According to this value, it is possible to estimate the 
flicker compensation ratio:
With this K factor, it is possible to obtain the maximum 
flicker that could be emitted by the steelwork, according to 
Argentinian regulations [21.
It is possible now to estimate the ratio between the EAF 
apparent power and the short circuit level at the steelwork 
supplying point.
XIICLEEE - 12th Portuguese-Spanish Conference on Electrical Engineering XIICLEEE_1704
From Fig. 5, it is possible to conclude that:
• The STATCOM can be operated all over its full output 
current range even at very low system voltage levels. 
In other words, the output current can be maintained 
independently from the a.c. system voltage.
• The maximum Var generation or absorption changes 
linearly with the a.c. system voltage.
• The independence of STATCOM output from 
equivalent system impedance means that the voltage 
regulator controlling the STATCOM output can be 
designed for a faster response rate and can provide 
stable regulation over the full range of system 
contingencies.
The single line diagram of the STATCOM for reactive 
power supply to the transmission system is shown in Fig. 6, 
where U is the voltage of the steelwork busbar and Ec is the 
controllable output voltage.
Fig. 6. STATCOM scheme for reactive power generation.
The active and reactive power exchanged between the 
network is given by:
distribution system for the voltage fluctuation mitigation 
study.
See = 300 MVA
r * Steelwork feeder,71
Va
VaSteelwork (pu) 
la_Pcc 
Vdc 
Q (Mvar) 
P (MW)
DV/V (%)
□
□
Measurements
[±> Operating Mode VdcP
Trip
N
A
B
Fig. 8. MATLAB model of Utility/Customer system.
The EAF is modeled as three controlled current sources, 
governed by the measured currents in the EAF during a 
furnace heat. The arc furnace model is shown in Fig. 9.
Controlled Current 
Source
From
W orkspace
Controlled Current 
Source
<2>*~
lb
o u ru
From
W orkspace
Controlled Current 
Source
rk-
<!>•-
Ic
From 
W orkspace
If the amplitude Ec of the output voltage phasor (Ec) is 
increased above the amplitude U of the a.c. voltage (U), then 
the current phasor leads the voltage phasor and current flows 
from the converter to the a.c. system and the converter 
generates reactive (capacitive) power to the system.
If the amplitude of the output voltage phasor is decreased 
below that of the a.c. system voltage phasor, then the reactive 
current flows from the a.c. system to the converter, and the 
converter absorbs reactive (inductive) power from the a.c. 
system. This operation is illustrated in Fig. 7.
Fig. 9. Arc Furnace model.
VI. STATCOM DETAILED MODEL
The ± 4 MVAr STATCOM SIMULINK detailed model is 
shown in Fig. 10. It contains a PWM IGBT inverter, two 
10000 jaF capacitors and a control system. The STATCOM is 
coupled in parallel with the network through a 0.4/13.2 kV 
A-Y transformer [6]-[8].
B <s>»-
c
9 k +
A a
B “ i f " b
C v  *  c
B
C  C a
-« a >
VdcP
-*d>
N
-«3>
Fig. 7. Capacitive and Inductive behaviour of STATCOM.
V. UTILITY/CUSTOMER POWER SYSTEM MODEL
A digital simulation model of the distribution network is 
implemented by using the MATLAB/SIMULINK Power 
System Blockset (PSB). Fig. 8 shows the upper level of the
PWM Controller
Fig. 10. STATCOM block diagram.
A PWM pulse generator of 1.65 kHz is used to control 
IGBT bridge. The modulation scheme used is of sinusoidal
XIICLEEE - 12th Portuguese-Spanish. Conference oa Electrical Engineering XHCIEEE_1704
type. The controller diagram is shown in Fig. 11. It consists 
of several subsystems: a phase-locked loop (PLL), a 
measurement system, an a.c. voltage regulator, a d.c. voltage 
regulator and a current regulator.
The PLL is synchronized to the fimdamental of the 
transformer primary voltage to provide the synchronous 
reference required by the abc-dqO transformation. The 
measurement block computes the d-axis and q-axis 
components of the voltages and currents.
The current regulation consists of two PI controllers that 
control the d-axis and q-axis currents according to (9)-(10).
The controller outputs are the voltages E«i and that the 
PWM inverter has to generate according to (11)-(12). Then 
these voltages are converted into phase voltages EM, ECb and
E c c .
The implementation of the control system is based on d-q 
compensation theory. The compensator input currents track 
the reference currents by means of the following current 
controller equations:
Fig. 11. STATCOM Controller.
PWM waveforms, we can use instead a voltage source having 
the same averaged value computed upon a chopping period 
(Tc = 1/1650) in this case. By using this “Averaged Model”, 
we can simulate the system operation with a larger step time, 
resulting in a simulation time reduction or, put in another 
way, the total simulation time can be increased up to 60 
seconds.
The Averaged Model can be built based on the energy 
conservation principle, taking into account that the 
instantaneous power must be the same on the AC side and the 
DC side of the inverter (neglecting losses). The DC current in 
the DC-link capacitor can be then computed from the 
measured AC instantaneous power and the DC-link voltage 
Udc as:
The STATCOM model mentioned above is described in 
[6] where is presented a detailed model of the inverter that 
includes the switching of the inverter power switches. This 
model requires a very small computing time step to represent 
correctly the PWM waveforms (Tg = 5 (j.s). The simulation 
time is thus fairly long and it is not possible to perform 
simulations longer than 5 seconds due to memory 
requirements.
Vn. STATCOM AVERAGED MODEL 
If we are not interested to represent the chopping of the
Fig. 12 shows the SIMULINK implementation of the 
inverter’s Averaged Model. On the AC side, the inverter is 
modeled three controlled voltage sources which are 
determined by three voltages V^c from the control system. On 
the DC side, it is modeled by the DC-link Model. In this 
model, a capacitor (represented by an integrator) is charged 
by a DC current source with a value computed as shown in 
(15). A Switch block is used to avoid a division by zero at 
starting when the capacitor has no charge.
XIICLEEE - 12th Portuguese-Spanish Conference on Electrical Engineering XIICLEEE_1704
Fig. 12. SIMULINK diagram of the PWM inverter Averaged Model.
VIII. TESTING THE STATCOM AVERAGED MODEL 
WITH A DYNAMIC LOAD
The ability of a compensator to reduce flicker depends on 
the compensator size and speed of response. In order to 
evaluate the speed of response of both STATCOM models, 
the Three-Phase Dynamic Load from SIMULINK was 
utilized. The active power was set constant at 2.5 MW, and 
the reactive power was set at 1.8 MVAr. This load is 
representative of the active and reactive power levels reached 
by the arc furnace during a complete heat cycle. As it can be 
shown in Fig. 13 a positive step change of 1.2 MVAr is 
applied at time t = 0.4 s then a negative step of -3.4 MVAr is 
applied at t = 0.7 s, and finally the reactive power recovers its 
original value at t = 1 s.
Fig. 14 shows the obtained results for the busbar voltage 
where both models of STATCOM were connected.
As a result, both models (detailed and averaged) showed a 
similar response.
Fig. 13. Reactive power supplied by both STATCOM models.
Fig. 14. Busbar voltage controlled by both STATCOM models.
IX. TESTING THE STATCOM AVERAGED MODEL 
WITH THE EAF MODEL
Digital simulations were performed to evaluate the flicker 
mitigation in different phases of operation of the EAF when 
both STATCOM models were connected to the steelwork 
busbar. The steelwork busbar voltage without STATCOM and 
with both STATCOM models is shown in Fig. 15 and Fig. 
18. The reactive power supplied by both STATCOM models 
is shown in Fig. 16 and Fig. 19. And finally, the DC-link 
voltage of both STATCOM models is shown in Fig. 17 and 
Fig. 20.
— ST AT CO M-detailed 
— STATCO M- ave raged
2.5 3 3.5 4 4.5 5
Time [s]
Fig. 16. Reactive power supplied by both STATCOM models.
2.5 3 3.5 4 4.5 5
Time [sj
Fig. 15. Steelwork busbar voltage.
XIICLEEE - 12th Portuguese-Spanish Conference oa Electrical Engineering XHCIEEE_1704
Fig. 18. Steelwork busbar voltage.
Fig. 19. Reactive power supplied by both STATCOM models.
Fig. 20. DC-link voltage of both STATCOM models.
As it can be seen in the previous figures, both STATCOM 
models presented a very similar response.
Finally, Fig. 21 presents the Flicker level (Pst95%) in the 
steelwork busbar voltage, during a complete furnace cycle. 
The Flicker level was obtained by digital IEC 61000-4-15 
Flickermeter model implemented in MATLAB-SIMULINK
environment. The Pst values obtained in the case without 
STATCOM in operation were similar to the ones obtained by 
field measurements performed in the steelwork. It can be seen 
in this figure that the Pst values obtained with both 
STATCOM models were practically the same. The averaged 
STATCOM models has the important additional advantage of 
being able to perform simulations up to 60 seconds long, and 
therefore the Flicker values obtained are more representative 
of those obtained with the connection of a real STATCOM.
Time
Fig. 21. Pst evaluation during a complete heat cycle.
The ability of a compensator to reduce flicker depends 
mainly in the speed of response of the control system, and in 
both STATCOM models the control loop parameters were 
adjusted to be exactly the same. The minor differences in both 
responses are due to inefficient filtering of high frequencies 
generated by commutation of high power IGBT switches, in 
the case of STATCOM detailed model.
Additionally, both STATCOM models reduce Flicker 
levels under the requirements of International and 
Argentinian Standards.
X. CONCLUSIONS
In this paper two STATCOM models for voltage Flicker 
compensation has been compared.
The first model, called “detailed model” was described in 
detail by the same authors in [6]. This model has the 
important advantage of been a physical model that includes 
all the power electronics associated with the PWM converter. 
However, this model requires a very small computing time 
step (Ts = 5 (is) to adequately represent the PWM waveforms. 
With this model can not be achieved simulation times of more 
than 5 seconds.
The second model, called “averaged model” was described 
in detail in [7], this model is based on the energy 
conservation principle and the PWM inverter is replaced by 
three controlled voltage sources and the DC-link is replaced 
by a control loop where the capacitor is replaced by an 
integrator. In this model the computing time step can be 
increased ten times and simulation times of 60 seconds can be 
achieved.
Both STATCOM models have demonstrated very similar 
performance in compensating Flicker generated by an EAF, 
and are valid for Flicker mitigation and propagation studies. 
However, the 60 second simulations long are suitable for
XIICLEEE - 12th Portuguese-Spanish Conference on Electrical Engineering XTICLEEE_1704
Flicker studies and for comparison with real Flicker 
measurements performed every one minute.
XI. REFERENCES
[1] IEC 61000-4-15. Electromagnetic Compatibility (EMC). Part 4: Testing 
and measurement techniques. Section 15: Flickermeter. Functional and 
design specifications.
[2] Anexo a la Resolution ENRE 99/97. Base Metodoldgica para el Control 
de la Emision de Perturbaciones. Etapa 2.
[3] Static Synchronous Compensator (STATCOM) for Arc Furnace and 
Flicker Compensation. WG B 4.19. CIGRE Publication. December 2003.
[4] Understanding FACTS: Concepts and Technology of Flexible AC 
Transmission Systems. Narain G. Hingorani, Laszlo Gyugyi. Wiley-IEEE 
Press 1999. ISBN-0780334558
[5] Thyristor-Based FACTS Controllers for Electrical Transmission Systems. 
R. Mohan Mathur, Rajiv K. Varma. IEEE Press and Wiley & Sons, Inc. 
2002. ISBN 0-471-20643-1.
[6] “STATCOM Modeling for Mitigation of Voltage Fluctuations caused by 
Electric Arc Furnaces”. J. Agiiero, F. Issouribehere and P. Battaiotto. 2006 
PES General Meeting. 1-4244-0493-2.
[7] “Modeling and Simulation of a Distribution STATCOM using Simulink's 
Power System Blockset”. P. Giroux, G. Sybille, H. L. Huy. Pp. 990-994. 
0-7803-7108-9/01.
[8] “Voltage Flicker Mitigation Using PWM-Based Distribution STATCOM”. 
J. Sun, D. Czarkowski andZ. Zabar. Pp 616-621. 0-7803-7519-X/02.


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Voltage Flicker Mitigation using a simplified STATCOM model suitable for long time simulations

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