Ç.Ü Fen ve Mühendislik Bilimleri Dergisi Yıl:2012 Cilt:27-5
IMPLEMENTATION OF VOLTAGE QUALITY DISTURBER CONTROLLER WITH
DIGITAL SIGNAL PROCESSOR*
Gerilim Kalitesi Bozucusu Kontrolcüsünün Sayısal Sinyal İşlemci ile
Gerçekleştirilmesi*
Bülent IRMAK
Elektrik-Elektronik Müh. Anabilim Dalı
Mehmet TÜMAY
Elektrik-Elektronik Müh. Anabilim Dalı
ÖZET
Elektrik ve güç elektroniği ekipmanları son yıllarda gerilim bozukluklarına
karşı daha hassas olmaya başlamıştır. Bununla beraber, firmalar da, üretim
kayıpları nedeniyle kazançları azaltmakta olduğundan konuya daha hassasiyetle
yaklaşmaktadırlar. Güç kalitesini arttırmak ve kritik yükleri şebeke gerilim
hatalarından korumak için yüksek güçte Özel Güç Donanımları (ÖGD)
geliştirilmiştir. Örnek olarak Dinamik Gerilim İyileştiricisi (DGİ), Aktif Güç Filtresi
(AGF), Statik Transfer Anahtarı (STA) verilebilir.
Tezin temel amacı, Özel Güç Donanımlarının performanslarını test etmek
amacı ile Gerilim Kalitesi Bozucusu (GKB) kontrol sisteminin gerçekleştirilmesidir.
GKB gerilim düşümü, yükselmesi, faz kayması ve istenen harmonik değerinde
gerilim üretme özelliğine sahiptir. Önerilen metodun performansı Matlab/Simulink
programında farklı koşullarda incelenmektedir. Tasarlanan sistem deneysel
uygulamalarla test edilmiştir. Simulasyon ve deneysel çalışmadaki sonuçlar
karşılaştırılarak sistemin performansı incelenmiştir.
Anahtar Kelimeler: Güç Kalitesi, Gerilim Düşümü ve Yükselmesi, Gerilim Kalitesi
Bozucu.
ABSTRACT
Electrical machines and power electronic devices become more sensitive to
power quality variations. Because of the production losses end users have an
increased awareness of power quality issues. Custom power devices (CPD) such
as Dynamic Voltage Restorer (DVR), Active Power Filter (APF) and Static Transfer
Switch (STS) supply a level of reliability and power quality that is needed by
electric power customers sensitive to power quality variations.
The basic purpose of this thesis is modeling of the Power Quality DisturberVoltage in order to evaluate performance of CPDs. Proposed PQD-V has an ability
of producing voltage sag, swell, phase shifted voltage and voltage with defined
harmonic order.
Proposed model is simulated in Matlab/Simulink for different cases.
Experimental study of the model is also performed. Simulation and experimental
results are compared in order to evaluate performance of the system.
Keywords: Power Quality, Voltage Sag and Swell, Power Quality Disturber
* Yüksek Lisans Tezi – Msc. Thesis
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Introduction
In recent years, both industrial and commercial customers of utilities have
reported a rising tide of misadventures related to power quality. The trouble stems
from the increased refinement of today’s automated equipment, whether variable
speed drives or robots, automated production lines or machine tools,
programmable logic controllers or power supplies in computers. They and their like
are far vulnerable to disturbances on the utility system than were the previous
generation of electromechanical equipment and the previous less automated
production and information systems (Hingorani, 1995). A growing number of loads
is sensitive to customers’ critical processes which have costly consequences if
disturbed by either poor power quality or power interruption (Rabinovitz, 2000).
For the reasons described above, there is a growing interest in equipment
for mitigation of power quality disturbances, especially in newer devices based on
power electronics called “custom power devices” able to deliver customized
solutions to power quality problems (Sannino et al., 2003).
The term Custom Power describes the value-added power that electric
utilities and other service providers will offer their customers in the future. The
improved level of reliability of this power, in terms of reduced interruptions and less
variation, will stem from an integrated solution to present problems, of which a
prominent feature will be the application of power electronic controllers to the utility
distribution systems and/or at the supply and of many industrial and commercial
customers and industrial parks (Hingorani, 1995).
Custom Power Devices provides an opportunity to obtain specified levels
of power quality from standard service utility distribution systems. In order to
evaluate the performance of a custom power device it is necessary to test this
device in different power system conditions. Power Quality Disturber-Voltage
(PQD-V) is a device that can generate various power disturbance conditions.
In this study, voltage source inverter is used to generate 3 phase both
balanced and unbalanced sag and swell. Phase shifted and harmonic voltage
waveforms are also produced. Because of the structure of VSI to generate 3 phase
unbalanced voltage wave form is allowed. PWM technique is used for the efficient
control of switching of the inverter components.
Material and Method
The waveform of reference signal for a inverter circuit determines the
waveform of the produced output voltage. So, the control strategy of the PQD-V
relies on the reference signal. Desired output voltage is adapted to reference
signal. Desired output waveform is obtained by using different control strategies,
which is discussed in the following parts.
The three single-phase H-bridge Pulse Width Modulation (PWM) voltage
source inverters is used. Figure 1 shows the structure of the proposed PWM
inverter. The main advantage of PWM inverter is including fast switching speed of
the power switches. PWM technique offers simplicity and good response. Besides,
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high switching frequencies can be used to improve on the efficiency of the
converter, without incurring significant switching losses (Lara et al., 2002).
Figure 1. Structure of the proposed PWM inverter
The control strategy is based on comparing the carrier wave signal and
reference signal. In Figure 2 sinusoidal control wave is compared with carrier
wave, which has a higher frequency. This comparison produces the output
waveform at the same fundamental frequency with the control voltage.
The basic idea of PWM is varying the on or off periods at a constant
frequency so that the on periods are longest at the peak of the wave. The PWM
modifies the width of the pulses in a pulse train by using control signal. When the
value of control voltage increases, it results wider pulses. The waveform of control
voltage for a PWM circuit will determine the waveform of the produced voltage.
Amplitude of the carrier signal and reference signal have a vital role while
determining the output voltage value.
Figure 2. Switching strategy for PWM inverter
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The single-phase full wave inverter bridge is built using 4 switches (S1, S2,
S3, S4 ) and 4 anti-parallel diodes (D1, D2, D3, D4 ). Figure 3 shows the singlephase PWM inverter. The configuration is the same for other phases.
Figure 3. Model of the single-phase PWM inverter
PWM signals is generated by comparing reference signal and carrier
signal. The relay block allows the output to switch 0 or 1 values. The output of
Relay 2 has inverse value of Relay 1 output. The undefined condition should be
avoided so as to be always capable of defining the ac output voltage. In order to
avoid the short circuit across the dc bus and the undefined ac output voltage
condition, the modulating technique should ensure that either the top or the bottom
switch of each leg is on at any instant.
In this study, carrier signal is chosen triangular wave with a 10 kHz
frequency. Because of the control strategy, signal is fixed and same for all phases.
Amplitude of signal is 1 V.
Different output voltage waveforms are produced. For each waveform
diffrent reference signal is generated. Reference signal is expressed as
Va = Vmsin(ωt+θ)
(1)
where Vm is amplitude of the reference signal, and θ is the phase angle.
Table 1shows the events with respect to values of Vm and phase angle θ.
Vm and θ values are given by user in order to get desired output. If the Vm
is lower than modulation index in normal operation, sag event occurs. If Vm is
higher than modulation index in normal in this case swell event occurs. In this
study modulation index is 0.85 for normal operation. In normal operation phase
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angle θ is equal to zero. If phase shift event is aimed, value of the θ is given
different than zero. The structure of the proposed model has an advantage that
even sag operation is enabled in one phase, another event e.g. swell operation can
also be enabled for other phase at the same time.
Table 1. Events
Vm
θ
Event
0<
0
Sag
=0.85
0
Normal
>0.85
0
Swell
=0.85
0<
Phase shift
0.85<
0<
>0.85
0<
Sag and
Phase shift
Swell and
Phase shift
In order to generate output voltage wave with harmonics, the reference
signal is formed with respect to harmonic number. Harmonic number is expressed
as,
H = pn±1
(2)
where p is the IGBT number and n>0. For the proposed model each phase has 4
switching device.
The proposed PQD-V is built in MATLAB/Simulink and installed into a
simple power system to feed a resistive load. As shown in Figure 4 dc power
supply energizes the proposed system. The DC link voltage is 230 V. The resistive
load is rated at 3 kVA and receives power from power transformer. The secondary
side of the transformer voltage is phase-phase 380 V (rms) and load is fed from
this point.
The system runs at 50 Hz frequency. A 10 µs sample time (10 kHz
sampling frequency) is used for measurements and controller calculations. The
PQD-V uses self-commutating IGBT solid-state power electronic switches to
generate sinusoidal voltage wave form to the system. The voltage controlled three
single-phase full bridge PWM inverters are used to produce desired voltage wave
form. The switching frequency of the inverters is 10 kHz. Three of single-phase
inverters are connected to the common DC voltage source. The DC voltage source
is an external source supplying DC voltage to the inverter for AC voltage
generation. In order to reduce output distortion caused by DC to AC conversion,
LC filter is used. Output voltage is boosted by power transformer. Three single
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phase 1 kVA identical power transformers are used. Transformer has a turn ratio of
220/380.
Figure 4. Matlab/Simulink mdel of the PQD-V
Experimental study of the model is also performed. The experimental setup
is shown in Figure 5. The proposed system consists of an eZdsp F2812 board,
three H- single-phase IGBT inverter circuit, a passive filter circuit, a transformer, a
linear load, DC source, and measurement devices.
Figure 5. Experimental setup of the PQD-V
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Experimental setup can be divided into the power and control circuit. The
control circuit is used to generate the required gate signals to the inverter. The
control section is composed by PC, DSP, interface board and the switching device
gate driver. DSP source code is generated in PC by using Code Composer Studio
program. eZdsp F2812 DSP board is used to produce gate signals for the inverter
driver. IGBT driver receives signals from DSP via an interface board and gives
pulse to the gate of switching device. The power circuit section consists of DC
source, H bridge inverter, LC filter, power transformer and load. Power circuit is
energized from DC source. Inverter produces the output voltage according to gate
switching signal. The output waveform quality is improved by LC filter unit
connected to inverter.
Results
In this thesis, PWM technique has been used. PWM technique with a
switching frequency 10 kHz has been employed in VSI. The PQD-V has been
designed with special importance at the control of PWM inverter. The quick
response and high performance have been proposed in the thesis. The traditional
control techniques have three main drawbacks: slow transient response, necessity
of complex mathematical modeling of the system and requirement of the
transformation process.
Proposed system was tested in different cases. Results verified that
proposed model has an ability of producing voltage sag, voltage swell, phase
shifted voltage and voltage with harmonics. The effectiveness of the simulation
results were verified experimentally with various cases. Left side of the figures
show the simulation results and right side of the figures show the experimental
results.
Figure 6. Results for case (1)
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Figure 7. Results for case (2)
Figure 8. Results for case (3)
Figure 9. Results for case (4)
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Figure 10. Results for case (5)
Figure 11. Results for case (6)
Discussion and Conclusion
This study has been performed as simulation and experimental work. In
simulation part Matlab/Simulink software was used in order to simulate the system.
The proposed PQD-V is built in MATLAB/Simulink and installed into a simple
power system to feed a resistive load. Different cases have been realized to verify
the operation and performance of the designed system in MATLAB/Simulink
program. Design specifications were considered carefully. During simulation, the
sampling resolution of the simulated system should not be very different from the
real system sampling resolution. Otherwise, significant errors can be introduced. In
experimental part TMS320F2812 DSP is used in order to generate pulses for
IGBT’s.
The basics of DSP code generation and hardware installation for proposed
system have been presented in detail. The general principles of control circuit and
power circuit design was described. The study is implemented with a fixed point
TMS320F2812 DSP, which offers simple control and fast dynamic response for
real time applications was used to generate PWM signals for three-phase inverter.
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The effectiveness of the simulation results were verified experimentally
with various cases. The very close agreement of experimental and simulation
results illustrate the efficiency, accuracy, and dynamic response of DSP based
PWM inverter design. The main advantage of the proposed system is that control
strategy allows to produce unbalanced voltage waveform.
In case (1) a three-phase balanced voltage sag is tested. Three-phase
balanced fault occurs resulting in 30% decrease from nominal value between the
period 0.1s and 0.2s in all phases. In case (2) single-phase voltage sag event is
tested. Single-phase fault occurs resulting in 30 % decrease from nominal value.
While sag event occurs in phase A, other phases are not effected. Both phase B
and phase C keep their nominal value. In case (3) a three-phase balanced voltage
swell case is tested. Three-phase balanced fault occurs resulting in 15% increase
from nominal value between the period 0.1s and 0.2s in all phases. In case (4)
single-phase voltage swell event is tested. Single-phase fault occurs resulting in
15 % increase from nominal value. While swell event occurs in phase A, other
phases are not effected. Both phase B and phase C keep their nominal value. In
case (5) voltage with harmonics case is evaluated. Output voltage includes the 5th
and 7th order harmonics. Output waveform is distorted because of harmonic
components. Value of the output voltage is higher than value in normal operation.
0
In case (6) phase shift operation is tested. Phase A is shifted by 15 between the
periods 0.1s and 0.2s.
Simulation and experimental results are very close to each other. There
are small differences in the voltage rms values. Differences are caused by
transformer parameters, voltage drop on the circuit elements and loss in copper
cables.
Sag, swell and flicker generator was proposed for the evaluation and the
test of CPDs. In addition, cases for phase jumping sag and swell voltage are
introduced. The operations of the proposed PQD-V were verified through the
computer simulations and the experimental results. I hope the proposed PQD-V
can be useful to test and evaluate all kinds of CPD before installing it in real site.
References
HINGORANI, N.G., 1995. Introducing custom power. IEEE Spectrum, 32, 6 : 4148.
RABINOVITZ, M.,2000. Power Systems of the Future. IEEE Power Engineering
Review, 10-15
SANNINO, A., SWENSSON, J., LARSSON, T., 2003. Power-electronic solutions to
power quality problems. Electric Power Systems Research, 66:71–82.
LARA, O.A., and ACHA, E., 2002. Modelling and Analysis of Custom Power
Systems by PSCAD/EMTDC. IEEE Transactions on Power Delivery, 17,
1:266-272
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