Two for Review
Voltage-Sourced Converters & Disturbance Analysis
Two books are reviewed in this issue, one on voltage-sourced converter (VSC) technology and the second on disturbance analysis for power systems. The reviewer for the first book commends the authors for their efforts in tackling a very challenging subject. The author of the second book, the reviewer says, has gone to great lengths to produce the comprehensive book.
Voltage-Sourced Converters in Power Systems
By Amirnaser Yazdani and Reza Iravani, IEEE – Wiley, ISBN 9780470521564
This is a much needed book recognizing that VSC technology is rapidly developing as it applies to electric power systems. It is so rapid that it is challenging to keep up with VSC configurations and control that continue to emerge and find applications in power systems. It is important to realize that the conventional thyristor-based converter or line commutated converter (LCC) with its controls and protection requirements are far less technically challenging than VSC technologies applied to wind turbine generators and VSC transmission.
The authors emphasize the basic module of a VSC valve, the half-bridge converter. It is often applied as the building block for VSCs that interface between ac and dc systems. There is also increasing interest and application of the full-bridge converter module, particularly in the configuration of some static compensators (STATCOMs), but it is only dealt with superficially in the text. A full-bridge VSC module is also known as an H-bridge and a chain link bridge.
The configurations ap–plied to the study of three-phase applications in some depth in the text are the two-level, three-phase VSC and the three-level, three-phase, neutral point clamped (NPC) VSC; both have found applications in power systems. A key factor in the study of these VSC configurations is the relatively well-known pulse-width modulation (PWM). The study by the authors recognizes that there are many PWM techniques. The most common strategy discussed compares a high–frequency periodic triangular waveform, the carrier signal, with a slow-varying waveform known as the modulating signal that instigates VSC valve switching. How this is achieved with a two-level converter is covered basically. Digital simulations of the various switched three-phase converter configurations with PWM slow down the speed of solution. An averaged model is presented that represents the averaged ac side terminal power and the averaged dc side power of the PWM three-phase converter in a simplified model that avoids switching in simulation, thus speeding up the solution. The average VSC model is said to be adequately accurate for dynamic analysis and control design.
The control strategies put forward by the authors are based on the average VSC model, presumably in an attempt to keep the explanations and analysis simple. The initial ac system model that is applied is a fixed dc voltage at the ac side terminal, presumably representing but not equal to the instantaneous rms voltage of the ac terminal. The per-unit system is presented in an appendix. The initial dc system model is just as simple, a fixed dc voltage where the dc capacitors of a two-level converter are located. The interface reactor in between the ac terminal point and the VSC is represented as a series inductor L with its reactor resistance R. Using this simplified model, a control strategy is developed first as a single-phase, two-level converter and then as a two-level, three-phase converter. These simplified models allow the authors to introduce the concept of feed-forward compensation, where the dc signal representing the instantaneous rms ac terminal voltage is fed into the control stage after the current controller that controls ac current (again a dc signal) through the interface reactor. Feed-forward compensation represented in this way eliminates VSC start-up transients.
Although this simplified model and control strategy demonstrate an apparent satisfactory stabilizing procedure, it is far from the reality of VSC transmission converter control. Fortunately the text moves forward and more general methods for controlling a VSC are presented. Two axis controls based on the familiar dq frame or transformation are presented. In the dq frame of reference for the VSC control, the signals are closer to dc in steady-state operation, allowing simpler control functions to be applied. A realistic control system such as the use of the dq transformation better accommodates the VSC operation from a three-phase ac termination point that may not be perfectly symmetrical and balanced and may contain harmonics.
Controlling the three-level, neutral-point clamped VSC is well described, including the propensity for common-mode, third-harmonic generation and compensation of dc side volt drift in the converter capacitors. A method for compensating the voltage drift is presented, but it is not the only way it can be achieved. Common-mode, third-harmonic generation is easiest limited by ensuring that common-mode current paths are avoided or prevented. The authors present an application using PWM with a third-harmonic injection to reduce third-harmonic content in dc current. And importantly, third-harmonic injection allows a lower ac voltage for a given dc voltage. A case is presented that the three-level NPC and the two-level VSC can be equivalent. Although their circuit structures and switching strategies are quite different, they exhibit basically identical dynamic behavior based on averaged ideal three-phase VSC models. Advantage is taken of this property in the text so that the loops to control the terminal currents/voltages can be similar for either converter configuration. The two basic control modes of current and voltage are presented, where the important fact is emphasized and demonstrated that current control mode prevents converter overload whereas voltage control mode does not. In either case real and reactive power control at the ac terminals can be achieved.
If the frequency of the ac terminating voltage changes or is prone to drift, which is the usual case in electric power systems, the phase-locked loop (PLL) is introduced. The PLL acts as a buffer between the three-phase ac terminal voltage subject to its frequency drifts and ac voltage unbalance. The authors present function designs for the PLL to minimize error between the three-phase voltage of the actual ac power system and what the VSC controls operate from. This analysis by the authors, although enlightening, is still based on the simplified model of the converters, ac system and dc side system. The application of the PLL is made challenging as the ac system weakens with respect to the rating of the VSC where, under realistic conditions, control instabilities, nonlinearities, harmonics, and ac system resonant impedances have to be reckoned with. This is not dealt with in the text.
There are times in the application of VSC transmission where the VSC defines the frequency and voltage of the terminal ac busbar or system. This is the case where the VSC transmission feeds power into a “dead” or islanded load where there is no other generator in the ac system to define frequency. Similarly at the sending or rectifier end of a VSC transmission feeder from a wind farm, it may be the responsibility of the VSC there to provide a stable frequency for the wind turbine generators to operate. The control strategy the authors promote requires a three-phase ungrounded shunt capacitor or ac filter at the ac side of the interface reactor. They recognize that the VSC system may need to operate with the grid-imposed frequency mode, as well as in the controlled frequency mode. This mode change may occur unexpectedly, and the authors correctly state that the change has to be detected to initiate the required change in the controls. For the mode change to grid-imposed control from the controlled frequency mode, the impacted VSC must resynchronize to the changed ac system it is connected to. The authors demonstrate this with minimum transients resulting. The control strategy adopted in the text depends on the simple system model and the three-phase ungrounded shunt capacitor being part of the VSC configuration, which is not always so. The controls for a realistic system in this situation will not be so simple. The detection of the mode change between grid-imposed control and controlled frequency is not straightforward. How this may be accomplished is not dealt with in the text.
The text goes into greater depth where the VSC ac side is connected directly to the stator or rotor of an electrical machine. This requires rotating field coordinates based on a rotating rotor flux observer. This is essential for the doubly fed induction generator (DFIG) where the flux observer is needed to ensure the ultimate supply voltage to the rotor is correctly synchronized and phased for machine torque or speed control. One simple flux observer is presented for the squirrel cage asynchronous machine. Two rotor flux observers are presented but if less rigorous control is acceptable, simpler flux observers are possible. There are other options for the flux observer through references to the literature, but a basic description of additional options for flux observers would have been helpful. It is generally observed that machine flux and torque are controlled, respectively, by d and q axis current in the rotor field coordinates, particular for squirrel cage induction machines. Now the three-phase ac side output of a VSC normally generates controlled ac voltage to achieve the necessary rotating magnetic field and in so doing accomplishes a variable speed rotor.
There is only a brief mention in a subscript that VSC control can directly control the stator current if a hysteresis band of voltage is applied to achieve ac current control. A desired sinusoidal stator current in magnitude and phase angle is generated in the VSC controls for each phase, and so voltage on each phase is switched between its positive and negative polarity limits to create a current close to the desired current. This does result in a variable switching frequency from the VSC with a challenge to its ac filtering for adequate harmonic compensation. This method is applied to some wind turbine generators with added control strategies and is showing up in more applications.
Part II of the book is “Applications.” A specific chapter is prepared for each application that includes STATCOM, back-to-back HVDC conversion system, and variable speed wind power system. The analysis of each application continues with the simplified systems and associated mathematical processes developed in the first part of the book. Two appendices are included. Appendix A is the mathematical development of the phasor representation for symmetrical three-phase electric machines. Appendix B presents per-unit values to be applied for VSC systems. A comprehensive list of references is included for those who wish to dig deeper into VSC technology.
It is feared that the simplified models used by the authors to arrive at their developments for VSC controls will be inadequate for real systems involving electrical machines. However, there is value in the basic but useful approach presented in the book, particularly for the benefit of graduate students who may be more apt at understanding the equations and need to comprehend VSCs. On the other hand, the mathematical justifications in the book would become excessively complex if real-life systems were applied that included nonlinearities, harmonic resonances, and complex ac systems. These can be separately examined using the latest versions of electromagnetic transient’s type programs.
Mathematical symbols used in the text are not always easily understood, despite the fact that they are extracted from valid equations. These symbols are probably adequate for a graduate textbook but many practicing engineers may find it more challenging to understand their significance. Nevertheless, the authors are to be applauded for their efforts in tackling a very challenging subject. Significant advances in VSC and control technology for application in electric power systems have come forward in the short time since this book was released. It is anticipated that these will be presented in due course.
Disturbance Analysis for Power Systems
By Mohamed A. Ibrahim, IEEE – Wiley, ISBN 978-0-470-91681-0
The book, by Mohamed Ibrahim, has three introductory chapters about the need for tools, techniques, and issues involved in disturbance analysis ranging from the functioning of digital fault recorders to symmetrical components and COMTRADE. Even the first three chapters have small case studies to illustrate the point of the section. There is a chapter on phenomena related to faults and one on power system phenomena and their impact on protection. The fault phenomena chapter has complex relaying situations not encountered in standard texts such as section 2.23 “Delayed Clearing of a Pilot Scheme due to a Delayed Communication Signal.” The power system phenomena chapter also has rare protection events such as 3.39 “Mutual Coupling Phenomenon Causing False Tripping of a High Impedance-Impedance Bus Differential Relay During a Line Phase-too-Ground Fault.”
The remaining five chapters contain more than 80 larger case studies based on the author’s 40 years of experience organized by disturbances involving generators, transformers, overhead transmission lines, cables, and breaker failure protection. Each case has substantial evidence of the event itself with one-line diagrams and DFR records. A typical case study is presented in four to ten pages with numerous figures along with a careful analysis of the event, suggested corrective action, and issues raised by the event. It is a tour de force in the true sense of the word. It is not clear there are very many others who could have written the book. Mr. Ibrahim is uniquely qualified to write such a book and has obviously gone to great lengths to produce this encyclopedic book.
The book should be on the shelves of those in the industry that carry on the task of analyzing such disturbances.