A Series Active Power Filter Controlled by Personal Computer

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1 A Series Actie Power Filter Controlled by Personal Computer M. João Sepúleda Freitas, João L. Afonso, Júlio S. Martins Department of Industrl Electrons Uniersity of Minho Campus of Azurém Guimarães (Portugal) phone: , fax: mjs@dei.uminho.pt, jla@dei.uminho.pt, jmartins@dei.uminho.pt Abstract. This paper descres work that is being done in the design and implementation of a series actie power filter for electral power quality purposes. This type of filter is able to compensate for the following oltage related problems in the power grid: short blackouts for a few cycles, oltage distortion due to harmons on a repetitie basis, oltage unbalance in three-phase systems, oltage flker (subharmons) and momentary oer or under oltages. The main objectie of the work descred in the paper is to build a series actie filter controlled by a personal computer with a standard multifunction data acquisition PCI bus card, because of its relatie low cost and ersatility. This PC based solution presents some diffulties since the control of a series actie filter is a kind of applation whh requires a fast controller whh that does not miss samples and where all real-time deadlines must be met each and eery time, or the system will not operate properly. These characterists imply that this applation falls in the hard real-time control category. In the paper, some results are presented and some conclusions are taken using two different operating systems. Furthermore, a comparison with a mrocontroller based implementation will be made. Keywords Series Actie Filter, p-q Theory, Data Acquisition, Control. 1. Introduction There are seeral causes for oltage distortion, namely, non linear loads, some types of oltage sources and thunderstorms. These problems cause instantaneous and long term effects on electral equipment. The short term effects are malfunctioning, interferences and degradation of the performance of dees or equipments. Effects in the long run are, basally, oerheating and premature aging of the electr dees. If the mains oltage is undistorted, but non linear loads are connected to the electral grid, the current harmons produced will cause oltage distortions in the line impedances, and the oltage at the load terminals will also be distorted. With a distorted oltage, een linear loads absorb distorted currents. Figure 1 illustrates this. S MAINS i S i L 1 R L L i L Fig. 1. Effect of nearby nonlinear load CARGA NON LINEAR NÃO LINEAR LOAD LINEAR LOAD Passie filters can be used to compensate some of the problems mentioned aboe, but they hae some limitations, namely: they only filter the frequencies for whh they hae been preiously tuned, its operation cannot be limited to a certain load, resonances can occur, and the electral system can start to operate with capacitie power factor.. Actie power filters Actie power filters hae seeral adantages oer passie ones: compensation is automat, there is no risk of resonances, unity power factor (or any other desired alue) can be achieed permanently and without disturbing the electral network, they can compensate for phase unbalance, and excellent performance can be achieed. They can also be combined with passie filters (whh may be already installed) in hybrid topologies, in order to diminish its rated power. There are mainly two types of actie power filters: the shunt actie filter, represented in figure, and the series actie filter, shown in figure. Power Source a b c N b c a a b Controller c isa isb a* b* c* i* isc isn Inerter a b c i Shunt Actie Filter Fig.. Shunt actie filter - i a i b i c i n Load 41 RE&PQJ, Vol. 1, No.1, April 00

2 Power Source a b c N sa sb sc ca cb cc a b c Load then used to control the actie filter power electron conerter. 4. The control algorithms a b c Controller ca* cb* cc* Series Actie Filter Inerter Fig.. Series actie filter The shunt actie filter is designed to filter the line currents and the series actie filter is designed to filter the mains oltages. It is also possle to combine both topologies to proide both current and oltage filtering.. PC based actie power filter controller Actie power filters are usually controlled by a mrocontroller or a digital signal processor (DSP) with ery good results. Howeer, here an alternatie for the controller is proposed, based on a personal computer (PC) with a general purpose multifunction data acquisition board included in the PCI bus. The major adantages of this approach are the relatie low cost of the equipment, the high processing capabilities of the personal computer processor and its ersatility, allowing many other tasks, such as data acquisition and logging, remote access and monitoring, integration with other systems, and many other possilities. The personal computer used has a 7MHz Intel Pentium III processor with 51MByte memory. The data acquisition board is the model PCI-MIO-16E-4 manufactured by National Instruments. Multifunction cards usually hae seeral analogue inputs, whh can be used to measure the oltages and currents alues necessary to perform the control tasks, but they usually hae only two analogue outputs, whh can be used to generate the oltage compensation signals. So, it is important to analyse in what conditions the two compensation signals are enough for a three-phase system, because the use of a second multifunction board, if necessary, increases both the system cost and introduces time delays in the control loop. Another limitation of these standard multifunction boards concerns the analogue inputs, since the seeral channels usually aailable are multiplexed and share the same digital to analogue conerter, whh causes the acquisition process to take a ery signifant period of time. The control sequence is as follow: the instantaneous alues of oltages and currents are acquired by the multifunction board, then the are executed by the computer s mroprocessor, and then, the compensating signals are outputted through the digital to analogue conerters of the A/D board. These signals are - There were implemented two types of control algorithms: a based on p-q theory controller and a classal controller. A. Controller Based on p-q Theory The methods applied to control the actie filters are decisie in achieing the goals of compensation, in the determination of the filter power rate, and in their dynam and steady-state performance. Basally, the different approaches regarding the calculation of the compensation currents and oltages from the measured distorted quantities can be grouped into two classes: frequency domain and time-domain. The frequency-domain approach implies the use of the Fourier transform and its analysis, whh leads to a huge amount of, making the control method ery heay. In the time-domain approach, the traditional concepts of circuit analysis and algebra transformations assocted with changes of reference frames are used, simplifying the control task. The control algorithm based on the p-q theory for the series actie filter is proposed. Calculations are performed using the instantaneous alues of the measured oltages and currents [1], []. The controller model executes the p-q theory, according to the equations (1) to (14). First, three phase oltages and currents (fundamental positie sequence components of currents) are conerted to (,, 0) axis: ( ) 0 an (1) i ( 1 1 ) an () ( ) () ( 1 i 1 i 1 i ) 0 an (4) i ( i 1 i 1 i ) an (5) ( ) i i i (6) 4 RE&PQJ, Vol. 1, No.1, April 00

Assuming no neutral is present, equations (1) and (4) are null, and the real power p and imaginary power q are written as follow: p = p ~ p = i i (7) two following ptures show the results of

3 Assuming no neutral is present, equations (1) and (4) are null, and the real power p and imaginary power q are written as follow: p = p ~ p = i i (7) two following ptures show the results of compensating the three phase distorted mains oltages supplying a linear three phase load. Figure 4 shows the three phase mains oltages distorted due to the presence of harmons 5 th, 7 th and 11 th. q = q q~ = i i (8) The oscillating component of real power is obtained by subtracting the aerage real power from the total real power, like in equation (9): ~ p = p p (9) According to the p-q theory, in order to achiee the desired filtering action, it is necessary to hae only the constant alue of real power, and all the other components should be null. So the compensating oltages are calculated by equations (10) and (11): c c ( i ~ p i q) 1 * (10) i i ( i ~ p i q) 1 * (11) i i Fig. 4. Distorted mains oltages Placing a p-q theory based, series actie power filter between the mains and the load, its oltages become sinusoidal, haing no distortion at all. Figure 5 shows the oltages applied to the load. As the load is linear, the currents it absorbs become undistorted too. Then, these oltages must be conerted again to the three phase system, according to equations (1) to (14). ( 1 ) ca* (1) c* ( 1 1 ) = (1) cb* c* c* ( 1 1 ) = (14) cc* c* c* Now, these calculated alues should be used as references in the power actie filter [1], []. The control circuit performs the of the p-q theory, as they work with instant alues and are relatiely simple. Howeer, there are two distinct ways to apply the p-q theory algorithm: one is to achiee constant power flux and the other is to eliminate oltage harmons. If the filter is designed not to turn power constant, but to eliminate all the harmons from the oltages, a major diffulty is introduced, because it is necessary to determine the positie sequence of the fundamental currents. A MatLab/Simulink [4] model was built to analyse the results of the p-q theory algorithm applied to a series actie power filter, under different circumstances. The B. Classal Controller Fig. 5. Undistorted load oltages Very often, the main goal of the series actie filter is to eliminate the oltage harmons. In this case, the reference oltage is always known. Then, it is possle to generate the reference synchronized with the mains oltage and use another controller algorithm. Another approach to the controller algorithm is proposed for the series actie filter, based on a classal PI (proportional integral) controller. A MatLab/Simulink model was built to analyse this alternatie in a single phase series actie filter, but easily transformed in a three phase system. 4 RE&PQJ, Vol. 1, No.1, April 00

The mains oltage is supposed to be distorted due to the nearby presence of a non-linear load: a rectifier with a capacitor in parallel with a resistance in the output.

4 The mains oltage is supposed to be distorted due to the nearby presence of a non-linear load: a rectifier with a capacitor in parallel with a resistance in the output. The reference is synchronized with main oltage. The simulation proides promising results, as shown in the following ptures. The most signifant waeforms are presented: mains oltage (figure 6), the reference (figure 7), the error signal (figure 8) and load oltage (figure 9). Fig. 6. Distorted main oltage The testing programs built for Mrosoft Windows, using manufacturer s dee drier and function lraries, and for Linux, using real-time module RTAI, packages comedi and comedil were programmed in C language. A. Windows and Manufacturer s Dee Drier The acquisition board s manufacturer proides a dee drier for Mrosoft Windows with a large set of functions to perform seeral tasks. Tests made with the manufacturer s dee drier and Mrosoft Windows 000 reealed poor performance: the acquisition times are ery long; the set of functions aailable is not well suited for the applation, leading to the use some software trks; the controller s program is executed in user mode (low priority) instead of kernel mode (high priority); it is impossle to guarantee a constant sampling frequency because the number of cycles per second has a small artion [5], [6], [7]. B. Linux, RTAI, Comedi and Comedil Fig. 7. Reference oltage Fig. 8. Voltage error Fig. 9. Load oltage The load oltage glitches shown in figure 9 are due to the low-pass filter used to filter the switching frequency. The resonance phenomena may be reduced or eliminated by fine tuning the low pass passie filter. In this case, only one analogue input and one analogue output are used, diminishing the ery long acquisition time and allowing the sample rate to be increased. The sampling frequency is constant, because it is possle to use the board s internal clock to establish a fixed sampling rate. 5. control The proposed PC based solution presents some diffulties since the control of a power actie filter requires a fast controller whh does not miss samples and where all real time deadlines must always be met, or the system doesn t operate properly. This means that the applation needs hard real-time control. Better results were achieed by another solution based on the same DAQ card using a real-time Linux kernel extension (RTAI) together with the comedi. The comedi offers an interface to lots of different DAQ cards and it consists of two complementary packages: comedi whh implements the kernel space functionality and comedil whh implements the user space access to the dee drier functionality. 6. Results The following tables summarise the controller performance when using each of the approaches mentioned before: Windows and Linux Real. Also, a comparison is done with a mrocontroller Intel 8096SA based solution, using p-q theory []. A. Windows and Manufacturer s Dee Drier TABLE I. Windows performance p-q theory floating point 64 bit 4µs No Sampling rate 10samp/cycle (frequency) ( 6.7 khz) Controller delay 00µs B. Linux, RTAI, Comedi and Comedil TABLE II. Linux RT performance p-q theory floating point 64 bit 4µs Yes Sampling rate 00samp/cycle (frequency) (10 khz fixed) Controller delay 50µs 44 RE&PQJ, Vol. 1, No.1, April 00

5 C. Intel Based Controller A shunt actie filter has already been built in this department []. The control circuit is based on an Intel 8096 mrocontroller. A summary of this controller s performance of is presented in table III. TABLE III. i8096 performance p-q theory Integer 8 bit 50µs yes Sampling rate (frequency) Controller delay 7. Conclusion 00samp/cycle (15 khz fixed) masked Although personal computers are relatiely low cost pieces of hardware and hae ery fast processors, performing complated and extensie in a ery short time, thy are not optimized for hard real-time control tasks. The main problems are related to the operating systems, and can only be soled by ery skilled programmers. Another problem has to do with the slow input/output system, requiring extra data acquisition boards.. These boards are connected to the PCI bus and do not take adantage of the full processor speed and are usually designed to do data acquisition for monitoring purposes or for process control. They are ery limited when performing hard real-time control. A mroprocessor based controller has much more limited calculation capabilities, but much faster input/output system. The p-q theory is ery effectie and ery easy to use in the shunt actie filter, because the references (desired currents) are unknown and load dependent, and most of the times, the oltages can be considered almost sinusoidal. On the contrary, this assumption is almost neer true in a series actie filter, i.e., the currents rarely are sinusoidal. This implies the calculation of the positie sequence of the fundamental currents, if the p-q theory will be applied to compensate oltage distortion. On the other hand, the references (desired oltages) are known, allowing the use of a simple classal controller whh proides good results. 8. Future work The deelopment of a PCI bus card with suitable specifations to perform this kind of real-time control, together with a set of dee driers for Mrosoft Windows or Linux would be absolutely necessary to proide the best results when building a system based on a personal computer. This board and set of driers would also be the solution to another major diffulty: the manufacturers do not proide full information about their boards, preenting the construction of a more suitable set of dee driers. Acknowledgement The authors would like to thank Dr. Adrno Taares and Dr. Carlos Sila for their help in this work. References [1] H. Akagi, Y. Kanazawa, A. Nabae, Generalized Theory of the Instantaneous Reactie Power in Three-Phase Circuits, IPEC'8 - Int. Power Electrons Conf., Tokyo, Japan, 198, pp [] E. H. Watanabe; R. M. Stephan; M.. Aredes, New Concepts of Instantaneous Actie and Reactie Power in Electral Systems With Gener Loads, IEEE Transactions on Power Deliery, Vol. 8, nº., April 199. [] J. L. Afonso, H. R. Sila and J. S. Martins, Actie Filters for Power Quality Improement, IEEE Porto Power Tech 001, Porto, Portugal, 10-1 Set [4] James B. Daey; Thomas L. Harman, Mastering SIMULINK, Matlab Currulum Series, Prente Hall 1998, ISBN [5] National Instruments Corporation, DAQ DAQ-STC TM Technal Reference Manual System Timing Controller for Data Acquisition, January 1999 Edition, Part Number 4094B. [6] National Instruments Corporation, DAQ NI-DAQ TM User Manual for PC Compatles Version 6.9.1, Data Acquisition Software for the PC, February 001, Edition Part Number 1644K-01. [7] National Instruments Corporation, DAQ PCI E Series Register Leel Programmer Manual Multifunction I/O Boards for PCI Bus, Noember 1999 Edition, Part Number 41079B RE&PQJ, Vol. 1, No.1, April 00

Simulation Results of a Shunt Active Power Filter with Control Based on p-q Theory

Simulation Results of a Shunt Active Power Filter with Control Based on p-q Theory Emílio F. Couto, Júlio S. Martins, João L. Afonso Department of Industrial Electronic University of Minho Campus de Azurém