implementation of voltage quality …fbe.cu.edu.tr/tr/makaleler/cild27sayi5-1.pdfload is rated at 3...

Ç.Ü Fen ve Mühendislik Bilimleri Dergisi Yıl:2012 Cilt:27-5

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IMPLEMENTATION OF VOLTAGE QUALITY DISTURBER CONTROLLER WITH DIGITAL SIGNAL PROCESSOR*

Gerilim Kalitesi Bozucusu Kontrolcüsünün Sayısal Sinyal İşlemci ile

Gerçekleştirilmesi* Bülent IRMAK Mehmet TÜMAY Elektrik-Elektronik Müh. Anabilim Dalı 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 Disturber-Voltage 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 single-phase PWM inverter. The configuration is the same for other phases.

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

Figure 3. Model of the single-phase PWM inverter


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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.

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

Vm θ Event

0< 0 Sag

=0.85 0 Normal

>0.85 0 Swell

=0.85 0< Phase shift

0.85< 0< Sag and

Phase shift

>0.85 0< Swell and

Phase shift

Table 1. Events


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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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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. In case (6) phase shift operation is tested. Phase A is shifted by 15

0 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 : 41-

48. 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

implementation of voltage quality …fbe.cu.edu.tr/tr/makaleler/cild27sayi5-1.pdfload is rated at 3...

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