Butt, David (1999) An investigation of harmonic correction techniques using active filtering PhD thesis, University of Nottingham Access from the University of Nottingham repository: http://eprints.nottingham.ac.uk/12981/1/301660.pdf Copyright and reuse: The Nottingham ePrints service makes this work by researchers of the University of Nottingham available open access under the following conditions This article is made available under the University of Nottingham End User licence and may be reused according to the conditions of the licence For more details see: http://eprints.nottingham.ac.uk/end_user_agreement.pdf For more information, please contact eprints@nottingham.ac.uk An Investigation of Harmonic Correction Techniques using Active Filtering David Butt, MEng(lIonours) Submitted to the University of Nottingham for the degree of Doctor of Philosophy, August '99 Acknowledgements I would like to begin by thanking both of my supervisors, Dr M Sumner and Dr JC Clare, for their invaluable guidance and encouragement over the duration of this project I would also wish to thank the members of the PEMC group who contributed advice and the technicians who were very helpful during the construction of the practical rig Gratitude must also be given to those people who, although they have not necessarily assisted me in my work and in certain instances have been downright distracting, have made the last four years very enjoyable: In particular, Gez, Chris H, Nikin, Chris S, Jane, Ben, Ash and the resident meercats in the lab who will remain nameless but know who they are I would especially like to thank my parents and my brother for their love and continual support II] can see the carrot at the end of the tunnel" - Stuart Pearce Contents Introduction: The problem of power system harmonics 1.1 Problem overview 1.2 The effects of system harmonics 1.3 1.4 , Reduction of the effects of harmonics 1.3.1 1) Dealing with the problem at source 1.3.2 2) Dealing with the problem at system level 10 Regulations pertaining to power quality 15 1.4.1 BS lEe 61000-3-4:1998 16 1.4.2 IEEE 519 18 1.5 Aims and objectives of this work 18 1.6 Structure of this work 20 1.7 Terminology and definitions of power 1.8 Summary ii , 22 24 CONTENTS iii Alternatives to the s~andard diode bridge rectifier interface 25 2.1 Introduction 25 2.2 The standard diode bridge rectifier 26 2.3 The standard diode bridge rectifier with added line inductance 30 2.3.1 Summary 32 , 34 2.4 The 'Texas' circuit 2.4.1 Circuit operation 35 2.4.1.1 Circuit operation with a highly inductive load 35 2.4.1.2 Circuit operation with capacitive smoothing of the d.c output voltage 2.5 37 2.4.2 Choice of component values 2.4.3 Simulation " 38 2.4.4 The experimental rig 40 2.4.4.1 Performance of the circuit with a resistive load 40 2.4.4.2 Performance at reduced load powers 49 2.4.4.3 Performance with active load 51 52 2.4.5 Summary , The 'Minnesota' circuit • • • • • • • • • • • • • • • 2.5.1 37 Circuit operation I , • • • • • • • • 54 55 iv CONTENTS 2.5.2 2.6 Summary 58 60 The single-switch rectifier 2.6.1 Circuit operation 60 2.6.2 Summary 61 63 2.7 The 'three-switch' rectifier 2.8 2.9 2.7.1 Circuit operation 63 2.7.2 Summary 67 The six-switch rectifier 68 2.8.1 Circuit operation 68 2.8.2 Summary Summary 70 71 An introduction to the shunt active filter 76 , 76 Introduction 77 3.3 Principle of operation 77 3.4 Circuit structure of the shunt active filter 78 3.5 Derivation of the reference current for the shunt active filter 79 3.6 Current control of the shunt active filter 80 3.1 Review of aims and objectives 3.2 v CONTENTS 3.6.1 3.6.2 Linear current controllers 81 3.6.1.1 Stationary PI controller 81 3.6.1.2 Synchronous PI controller 82 3.6.1.3 Deadbeat controller 84 85 Hysteresis controller , 86 Nonlinear current controllers 3.6.2.1 3.6.3 Summary of the various current controllers of the shunt active filter The synchronous PI controller 4.1 88 89 89 Introduction 4.2 The operation and structure of the synchronous PI controller 90 4.2.1 Following the harmonic reference currents - two possible approaches 92 Design of the active filter components 94 4.2.3 Design of the current control loop 97 99 4.2.2 4.2.4 4.3 Design of the dc-link voltage control loop Simulation with synchronous PI control 101 4.3.1 Introduction to the Saber simulation 101 4.3.2 102 Synchronous PI control working under realistic conditions vi CONTENTS id~al 4.3.3 Synchronous PI control working with conditions 105 4.3.4 Synchronous PI control working with non-negligible dead time 4.3.5 Synchronous PI control working with supply distortion 109 107 4.4 Conclusion 111 113 An improved synchronous PI control structure 5.1 Introduction 113 5.2 Analysis of the synchronous PI control structure 114 5.3 Improving the synchronous PI control structure 119 5.3.1 5.3.2 Feedforward terms to compensate the effects of deadtime 119 5.3.1.1 Ideal PWM generation 5.3.1.2 Practical PWM generation 119 , 119 Feedforward terms to compensate for the effects of supply distortion 127 5.4 Simulation results demonstrating the performance of the improved synchronous PI control structure operating as a sinusoidal frontend 129 5.4.1 Simulation parameters 129 5.4.2 Improved synchronous PI control working with non-negligible deadtime 129 5.4.3 Improved synchronous PI control working with supply distortion132 CONTENTS vii 5.4.4 Synchronous PI control working under realistic conditions 134 5.5 Simulation results demonstrating the performance of the improved synchronous PI control structure with sinusoidal frontend operating as a shunt active filter 136 5.6 5.5.1 Generation of harmonic currents in the dq-frame of reference 136 5.5.2 Test1: Generation of fifth harmonic current 137 5.5.3 Test2: Generation of seventh harmonic current 138 5.5.4 Test3: Generation of fifth and seventh harmonic current 138 5.5.5 Discussion of results 139 Conclusion 142 An advanced synchronous PI control structure using bandpass filters (Method 1) for harmonic signal extraction 144 6.1 Introduction 144 6.2 Analysis of the advanced synchronous PI control structure 145 6.3 Method 1: Application of a bandpass filter to extract the harmonics from a signal 150 6.3.1 Introduction to harmonic signal extraction 150 6.3.2 Implementation of the bandpass filter and design considerations 150 6.3.3 Configuring the bandpass filter to be self-tuning 154 6.4 Simulation results 157 CONTENTS viii 6.4.1 Introduction to the simulations 157 6.4.2 Advanced synchronous PI control working with ideal conditions 158 6.4.3 Advanced synchronous PI control working with non-negligible deadtime 161 6.4.4 Advanced synchronous PI control working with supply distortion162 6.4.5 Advanced synchronous PI control working with pulse-width hmiting 164 6.4.6 Advanced synchronous PI control working under realistic conditions 6.5 Conclusion ',' 167 171 The advanced synchronous PI control structure using low pass filters (Method 2) for harmonic signal extraction 172 7.1 Introduction 172 7.2 Method of harmonic signal extraction 173 7.2.1 Implementation of the method and design considerations 173 7.2.2 Selection of notch filter parameters 175 7.2.3 Comparison of method with method 176 7.2.4 Controller overview 177 7.3 Design of the current controllers , 178 7.4 Performance with 'ideal' conditions 179 , Id Without compensation With compensation DC 2.00 2.00 300Hz Acknl)wledgements 0.11 0.02 Tahle 1: Table showing harmonic spectrum of id over the period 0.51 < Time < 0.53 The author would like to thank the EPSRC fl)r the sponsorship given fl)r this work References The performance of the RML-ATF itself is illustrated in Figure 13 The distorted supply waveform comprises a th fundamental and a hannonic as illustrated The RML-ATF has successfully extracted the fundamental from the distorted waveform - it is difficult to distinguish from the true fundamental itself 300 200 100 ~storted wltaga 5th Harmoric ·100 ·200 -300 0.'-48-0.4 82 =-0.""=464 :-0""=48::'"8-0 648-8 -'0.4'-9-0~.4""'92-0-.4'-94-0.4""'96-0.""'498 '0.5 11mo (s) Figure l3: Example of the RML-tuned filter yielding the fundamental from a signal containing fundamental and 5th harmonic [II H.Akagi, "New trends in active filters", EPE '95, p17-21 (2) S BhattaCharya, A Veltman, DM Divan, and RD Lorenz, "Flux-based active filter controller", IEEE TraIlS Ind Applications, Vol 32, No.3, pp 491-501, 1996 (3) H Pouliquen, P Lemerle, E Plantive, ''Voltage harmonics source compensation using a shunt active filter", E.P.E., Vol 1, ppIl7-122, 1995 [4] S Fukuda and T Endoh, "Control method aIld characteristics of active power filters", EPE, pp l39-145, 1993 [5] Simon, H Spaeth, K-P Juengst, P Komarek, "Experimental setup of a shunt active filter using a superconducting magnetic energy storage device", E.P.E., 1997 [6] Soo-Cbang Pei and Cbein-Chein Tseng, "Real time cascade adaptive notch filter scheme for sinusoidal parameter estimation", Signal Processing, 39, pp1l7130, 1994 [7] PD Evans, PR Close, "Harmonic Distortion in PWM inverter output waveforms", Proc lEE pt B, July 1987, pp 224-232 CONCLUSIONS AND FURTHER WORK D Butt, M Sumner and J C Clare are all with The authors conclude that the effects of process delays, sampling frequency, mains voltage distortion and PWM distortion all have a considerable effect on the performance of the current control in an active shunt filter Each of these effects is quantifiable with suitable current and voltage measurements By using correct teed forward compensation the performance of the current COIltrolloops can be significantly enhanced School of Electrical & Electronic Engineering, University of Nottingham, Nottingham, NG7 2RD United Kingdom h, It has also been shown I that the RML-ATF algorithm can be successfully used '-obtain the fundamental from a distorted signal with a varying frequency, this can then be used to set the centre frequency of other notch filters to obtain the constituent harmonics This information can also be used to identify the harmonics present in the current of a non-linear load and hence provide a reference signal for the active filter Further improvements will still need to be made to obtain true current source performance The next stage of work will include the development of a system impedance measurement system to provide on-line adjustment to the control algorithm Combined power electronic circuit and control loop simulation: why? And how? J C Clare, M Sumner, D Butt and B Palethorpe Power Electronics, Machines and Control Group School of Electrical and Electronic Engineering The University of Nottingham, Nottingham, NG7 2RD Introduction Power electronic systems provide a considerable challenge to CAD packages due to their strong non-linearities and widely varying time constants This is particularly so if an attempt is made to simulate the entire system including the power circuit, load characteristics, control loops and thermal effects Due to the lack of available tools and desktop computing power it has been normal practice to study each of these aspects independently This has limited the effectiveness of CAD for power electronic systems since important interactions are inevitably either overlooked or simplified to the point that they are unrealistic In addition, considerable ingenuity has been required on the part of the user to partition the problem in such a way that meaningful results are obtained in a reasonable computation time The situation is however changing and tools are now becoming available which enable us to get closer to a "complete system" simulation In this paper we concentrate on combined simulations of power electronic circuits (including representations for the load and supply) and their control systems principally for the purpose of control system design and validation prior to proto typing The need for a combined approach is illustrated with reference to the design of an active filter system A "wish list" of desirable features for a CAD package is outlined Some commonly used programs are compared and our experiences in applying them to this problem are discussed Combined simulation: Why? The need for a combined approach is best illustrated with a specific example A problem we have been concerned with recently is the control design for a shunt active power filter (APF) application The principle of the system is conceptually very simple as shown in Figure I An active filter is connected in parallel with a non-linear load (such as a rectifier) and injects a current which exactly balances the harmonic current drawn by the load so as to improve the distortion factor (and hence power factor) of the current drawn from the utility In practice the non-linear load can be a combination of many loads such as variable speed drives for example IF IF + IH Non linear Load L supply IH Shunt Active Filter Figure I Active power filter principle inverter Figure Practical realisation of shunt filter For example, in the control structure shown in Figure the measured supply voltage is injected into the current control loop after the current controller Despite the fact that the current control loop bandwidth is 400Hz (to adequately follow a 7ili harmonic reference) its rejection to disturbances at this point is very poor Consequently any inappropriate Sili or 7th harmonic components injected at this point due to voltage distortion produce uncontrolled harmonic currents The APF thus becomes a source of distortion rather than a sink Inverter interlock produces similar effects Supply distortion can also affect the reference angle for the 3-phase to 2-axis transformations PWM processing delays introduce phase errors which are important in this application since the voltage across the line inductors is small in comparison to the supply voltage and PWM converter terminal voltage Small phase errors can thus cause large current errors Switching frequency limitations mean that the current control loops must be designed using discrete sampled data (z-dornain) techniques to achieve the required bandwidth Sum and Adaptive notch filter~_ _ , Harmonic Current Reference PWM& Inverter ~ lb -1 AxiS ransform Filter (10kHz) L -_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~ ~ Figure Improved control structure An improved control structure that overcomes these problems using feed-forward techniques and interlock compensation is shown in Figure In arriving at this structure and designing its practical implementation, we have made extensive use of computer simulation using various packages and simulation techniques Without discussing the control in further detail it should be apparent that any simulation model which hopes to accurately predict the performance of this system must include a representation of the supply, the power circuit and a detailed representation of the control loop including the effects of discrete control and the time delays inherent in the PWM process A "wish list" of what we would like to be able to with a CAD tool for this and similar problems is as follows: enter the power electronic circuit, supply and load as circuit elements in a topological description using components from a library if possible, have a choice of different levels of sophistication for switching device modelling, enter the control system in standard control terms as one of the following or a combination of these, control block diagrams, transfer functions, control equations (ie difference equations for a digital controller), port control algorithm directly from a simulation directly to a target hardware controller, have efficient simulation so that interactive CAD is possible modulation as well The model is however useful for looking at aspects of the control design and we can study the effects of supply distortion, PWM calculation time and inverter interlock in an approximate way by injecting error signals at the appropriate points Results illustrating these points are given later dc volts DC link Voltage demand voltage controller Capacitor Bank d