Overview of power electronics technology and applications in power generation transmission and distribution

Overview of power electronics technology and applications in power generation transmission and distribution J. M. MAZA-ORTEGA 1 , E. ACHA 2 , S. GARCI ´A 3 , A. GO ´MEZ-EXPO ´SITO 1 Abstract The main objective of this paper is three-fold. First, to provide an overview of the current status of the power electronics technology, one of the key actors in the upcoming smart grid paradigm enabling maximum power throughputs and near-instantaneous control of voltages and currents in all links of the power system chain. Second, to provide a bridge between the power systems and the power electronic communities, in terms of their differing appreciation of how these devices perform when connected to the power grid. Third, to discuss on the role that the power electronics technology will play in supporting the aims and objectives of future decarbonized power systems. This paper merges the equipment, control techniques and methods used in flexible alternating current transmission systems (FACTS) and high voltage direct transmission (HVDC) equipment to enable a single, coherent approach to address a specific power system problem, using ‘best of breed’ solutions bearing in mind technical, economic and environmental issues. Keywords Flexible alternating current transmission systems (FACTS), High voltage direct transmission (HVDC), Voltage source converter (VSC), Insulated gate bipolar transistor (IGBT), Smart grids 1 Introduction It would be fair to say that many of the ideas upon which the foundations of the Flexible Alternating Current Transmission Systems (FACTS) rest, evolved over a period of several decades, building on the experience gained in the areas of High Voltage Direct Transmission (HVDC) and reactive power compensation equipment, methods and operational control. Nonetheless, FACTS, as an integrated philosophy, was a concept brought to fruition at EPRI in the 1980’s [1,2]. Since that time, many great breakthroughs have taken place in the area of power electronics encompassing new valves, control methods and converter topologies [3,4]. The new developments have been incorporated into the fields of FACTS and HVDC, giving rise to a new generation of power transmission equipment in either AC or DC form, with unrivalled operational flexibility [5]. In the words of the developers of the FACTS initiative, FACTS was not intended to be a direct competitor to HVDC transmission but, rather, an initiative able to provide technical solutions to specific AC power transmission problems at a lower cost, particularly when the AC transmission corridor already existed [1]. Over the past two decades, several publications have appeared which have separately reviewed the state-of-the- CrossCheck date: 14 June 2017 Received: 15 March 2017 / Accepted: 30 June 2017 / Published online: 13 July 2017 The Author(s) 2017. This article is an open access publication &J. M. MAZA-ORTEGA [email protected] E. ACHA [email protected] S. GARCI ´A [email protected] A. GO ´MEZ-EXPO ´SITO [email protected] 1 Department of Electrical Engineering, University of Sevilla, Seville, Spain 2 Department of Electrical Energy Engineering, Tampere University, Tampere, Finland 3 School of Electrical Engineering, Universidad Michoacana, Morelia, Mexico J. Mod. Power Syst. Clean Energy (2017) 5(4):499–514 DOI 10.1007/s40565-017-0308-x art of FACTS [6,7] and VSC-HVDC [8,9], mainly from the vantage of high-voltage power transmission. Nevertheless, no attempt seems to have been made to provide a holistic review, as it is done in this paper, on the application of power electronics converters in the generation, transmission and distribution systems. It ought to be said that the original boundaries between HVDC and FACTS were drawn along the types of solidstate converters employed and their control [4], but these boundaries have become blurred. On the one hand, as time has moved on, new power electronic devices such as the Voltage Source Converter (VSC), are being used in both FACTS and HVDC transmission [10]. For instance, VSCs are used in STATCOM applications, which is a FACTS technology, to provide reactive power support [11]. But, when two such devices are connected in cascade, we have the VSC-HVDC rather different from the classical HVDC transmission using thyristor-based bridges and phase control [12]. On the other hand, a wide range of enabling technologies involving power devices (power electronics, photovoltaic and wind energy systems, battery energy storage systems, electrical vehicles, etc.) and also electronic technologies (advanced protection, control, information and communication systems) have become cost effective [13] and even more powerful and popular than a decade ago [14,15]. As a matter of fact, the introduction of these technologies in an all-encompassed manner within the electrical business provides a solid foundation on which to build the new and smarter energy grid [16]. Within this new paradigm, power electronic technology is a key component because of its ubiquitous nature in all the stages of the power system from generation to distribution and involving applications related to DC and AC systems [17]. For these reasons, according to the current state of the technology, it is proposed to remove the former boundaries originally drawn along FACTS and HVDC to focus on their basic and common underlying power electronic technology. For doing so, it is convenient to adopt a bottom-up approach, starting with the power electronics switches and following with the definition of some basic building blocks, which will be used in specific devices intended for different functionalities. As a consequence, the focus is on the power electronic device irrespective of being either a FACTS or HVDC technology – the aim being to enable the best of breed solutions underpinning the new power-carrying structures that the Smart Grid demands [18,19]. Therefore, and according to this approach, the structure of the paper can be summarized as follows. First, power electronics components are described from scratch beginning with the most widely used power switches and the building blocks that can be formed with them. Second, a review of the power electronic devices, classified according to the way they are connected to the power system, is outlined. Third, the most important applications of these devices to power generation, transmission and distribution are described. Finally, the paper closes with the main challenges of the technology and the main conclusions of the work. 2 Power electronics elemental building blocks The aim of this section is to introduce, following a bottom-up perspective, the basic ‘‘bricks’’ which are used by the various power electronic based devices addressed in the sequel of the paper. 2.1 Power electronic switches A switch is a component that can be open and/or closed through a control signal. Power electronic switches can be broadly classified into two major groups, depending on their controllability (see Fig. 1): 1) Line commutated switches can be turned on when the anode-cathode voltage is positive and an adequate firing signal is sent to the gate. However, once the conduction state is established they can only be turned off when the current is almost zero and the anodecathode voltage is negative. Therefore, these devices are characterized by a single degree of freedom, namely, the firing angle, a(measured from the voltage zero-crossing). The thyristor (or SCR) is the most relevant switch within this group [20]. 2) Self-commutated switches can be turned on, by applying a suitable control signal to the gate, when the collector-emitter voltage is positive, and then turned off just by removing the control signal. This brings an additional degree of freedom compared to line commutated switches. The GTO, IGBT and IGCT are the most relevant switches within this category [20]. 2.2 Basic building blocks From the basic power switches outlined above, the following building blocks can be defined: Fig. 1 Basic power electronic switches 500 J. M. MAZA-ORTEGA et al. 1) AC-switch controllable impedance. Either a capacitor or a reactor is associated in series with an AC switch (see Fig. 2), typically comprising two antiparallel thyristors. In the simplest switched-impedance ON/OFF control scheme, the firing angle is set to a=p/2, leading to a perfectly sinusoidal current, as shown in Fig. 2a for a capacitor. In the inductor case, the amplitude of the current can be optionally regulated by suitable adjustment of the firing angle, a, leading to a non-sinusoidal current, as shown in Fig. 2b. This is referred to as a controlled impedance. In terms of a, the fundamental frequency current and associated variable susceptance are given by [10]:  ITCR ¼jBTCRðaÞ  V BTCRðaÞ¼ 1 2pf2ðpaÞþsin 2a Lp 8 < : ð1Þ where amay lie in the range p/2 \a\p,Lis the rated inductance value,  ITCR is the current phasor,  Vis the applied voltage phasor and fis the nominal frequency. 2) AC/DC Current Source Converter (CSC). The building block shown in Fig. 3, using thyristors, has long been used to inject active power from a three-phase AC system into a DC one (rectifier operation) or vice versa (inverter operation), in a controllable manner, using delay angle control. Note that the DC side in this bridge acts as an almost constant DC current source, owing to the presence of a large smoothing reactor. This device presents only one degree of freedom (the firing angle), which is used to control the transmitted active power. Note that this bridge always absorbs reactive power, which is dependent on its operating point. In addition, low-order harmonics appear in the AC current in accordance to the lower switching frequency. Thus, passive filters are required to mitigate the harmonic distortion and to supply, at least a part, of the reactive power requirements. 3) AC/DC Voltage Source Converter (VSC). This represents an alternative way of moving active power from a three-phase AC system to a DC one and vice versa. In this case (see Fig. 4a) the capacitors make the DC side of the converter to behave as a voltage source. The power switches are IGBTs with antiparallel freewheeling diodes, allowing the flow of bidirectional currents. The switches are driven by PWM-based control signals, with switching frequencies in the kHz range, which are chosen to be odd integer multiples of the fundamental frequency [1,3]. The most relevant feature of the PWM control, as illustrated in Fig. 4b, lies in the fact that it synthetizes a fundamental frequency voltage on the AC side from the DC bus, with controllable amplitude and phase, as given by the basic relationship [20]:  VAC ¼klmaejuVDC ð2Þ where  VAC is the RMS fundamental-frequency component of the VSC’s output line-to-line voltage, uis the phase angle of the complex voltage  VAC relative to the system phase reference, V DC is the DC bus voltage and m a is the inverter’s amplitude modulation index. In the linear range of modulation, m a takes values in the interval: 0\ma\1:Hence, two degrees of freedom, namely active and reactive power, are available with this building block. In this process, the PWM-operated VSC generates highorder harmonics, which appear as side-bands around the Fig. 2 Control of switched capacitor and controlled reactor Fig. 3 AC/DC current source converter (CSC) based on thyristors Overview of power electronics technology and applications in power generation transmission and…501 switching frequency and its multiples [3,20]. The selection of the switching frequency is a trade-off between the switching losses and the filtering requirements. For this reason, in case of high power applications multilevel topologies are a possible alternative to the basic two-level VSC. Multilevel VSC operation is characterized by both low losses (reduced switching frequency) and low harmonic distortion (owing to the use of multiple DC levels). A modular multilevel VSC, as shown in Fig. 5a, stands out from other multilevel topologies because of its ability to easily accommodating any number of levels. This topology stacks simple power units containing two IGBT-valves and a small DC capacitor. In this case, the output voltage is synthetized as shown in Fig. 5b. This multi-level converter technology is offered by at least one vendor, it is termed SVC PLUS  . It provides a nearly ideal sinusoidal-shaped waveform on the AC side. Therefore, there is only little –if any– need for high-fre- quency filtering and no need for low order harmonic filtering. SVC PLUS  uses standard components, such as typical AC power transformers, reactors, capacitors, and industrial class IGBTs that are widely used for traction and industrial drives. 3 Device configurations The different configurations of power electronics devices, based on the above power electronic building blocks, can be classified according to its connection to the power grid. Shunt, series, shunt-series and cascade connections are reviewed in the following subsections. 3.1 Shunt connection 1) Static Var Compensator (SVC). The SVC appeared in the power systems scene at least one decade before the FACTS initiative was put forward [20–22]. The SVC is connected in shunt with the AC system through a step-up transformer. The main function of a SVC is to supply/absorb reactive power to support a specified Fig. 5 AC/DC modular multilevel VSC based on IGBTs Fig. 4 AC/DC VSC based on IGBTs 502 J. M. MAZA-ORTEGA et al. voltage magnitude at the high-voltage side of its connecting transformer. The one-line representation of the SVC is shown schematically in Fig. 6. It comprises a bank of Thyristor Controlled Reactors (TCR) in parallel with a bank of Thyristor Switched Capacitors (TSC). The TCR consumes variable reactive power up to its design limit according to (1). The TCR, however, generates harmonic currents due to the phase control [3]. For this reason, the TCR is normally connected in delta to prevent the triple harmonics from reaching the power system. In addition, passive filtering is required to mitigate the 6 k±1and12k±1gener- ated by the six-pulse and twelve-pulse topologies respectively. The TSC generates reactive power in a variable, discrete manner, with the thyristor pairs operating as switches (ON/OFF); hence, during steady-state operation no harmonic distortion is produced by the TSC. 2) Static Compensator (STATCOM). This is the modern counterpart of the SVC. Just like the SVC, its main function is to supply/absorb reactive power to support a specified voltage magnitude at the high-voltage side of its connecting transformer. Fig. 7shows the oneline schematic circuit of the STATCOM, made up of the VSC, the smoothing inductor, the interfacing transformer and the PWM control system. As shown in the previous section, the VSC is built as a two-level or a multi-level converter [3]. The operational behaviour of the STATCOM is superior to that of the SVC. Being based on a VSC, the STATCOM may be seen as a variable and controlled voltage source, as opposed to a variable shunt susceptance [10]. Basically, it behaves like a fast synchronous condenser without moving parts [1,5]. Therefore the STATCOM does not use bulky inductors and banks of capacitors to absorb and to generate reactive power, respectively [1,3]. The generation/ absorption process is carried out by the action of the VSC voltage control according to (2) to yield either leading or lagging VAR operation to satisfy operational requirements [1]. Note that the current injected by the STATCOM has to be orthogonal to the bus voltage considering a lossless approach because no active power exchange is allowed due to the isolated DC bus of the VSC. The smoothing inductor is used between the VSC and the step-up transformer to eliminate the high-order harmonics. The DC capacitor is used to support and stabilize the DC voltage [1]. An equipment manufacturer has recently patented a new VAR compensation equipment selecting the best attributes of the SVC and the STATCOM [23]. 3.2 Series connection 1) Thyristor Switched Series Compensator (TSSC). The TSSC connects in tandem a series of paralleled capacitor-thyristor modules in order to modify the apparent electrical length of the line, as shown schematically in Fig. 8for one phase. A block capacitor becomes active by turning off its associated thyristor valve and inactive otherwise, i.e., when the thyristor valve is on the capacitor is bypassed. Therefore, no harmonic distortion is generated. This arrangement enables controllable series compensation, up to the rated capacity, in a discrete, stepwise manner. The larger the number of active modules, the higher the degree of series capacitive compensation. Fig. 6 SVC schematic representation Fig. 7 STATCOM schematic representation Fig. 8 TSSC schematic representation Overview of power electronics technology and applications in power generation transmission and…503 2) Thyristor Controlled Series Compensator (TCSC). The TCSC performs a similar function to the TSSC but its control is carried out in a continuous manner. In addition to increasing the stability margin of the system, the TCSC has shown to damp Synchronous Series Resonance (SSR) and power oscillations [1]. Like the TSSC, it uses a series of blocks connected in tandem but each block comprises the parallel combination of a capacitor and a TCR, as shown schematically in Fig. 9. Basically, the TCSC would exhibit the behaviour of a variable series reactance governed by the TCR’s firing angle a. Therefore, it should be possible to adjust the reactance of the line to perform an effective regulation of the power flow through the series compensated transmission line. Depending on the firing angle, the TCSC may operate in capacitive or inductive modes. However, there is little incentive for inductive operation since this would increase the electrical length of the transmission line, with adverse consequences on stability margins, and extra losses [3]. Note that in this case the TCR harmonic currents do not tend to escape towards the network; instead, the harmonics are trapped inside the TCSC because of the low impedance of the capacitor compared to the network equivalent impedance [10]. TCSCs have been installed in countries that have long transmission distances, such as the USA, Brazil, China and India. It seems that there is limited scope for using this technology in Europe, with the Nordic countries being the exception. Indeed, Fingrid Oyj, the Finish TSO, has shown recent interest in upgrading some of their series compensated transmission lines to include the dynamic characteristics afforded by the TCSC. 3) Synchronous Series Static Compensator (SSSC). The SSSC is the counterpart of the STATCOM for series devices. As shown in Fig. 10, it is composed of a VSC connected through a series coupling transformer. The SSSC injects a series voltage to the incoming nodal voltage to control the power flow through the power line. However, much like the STATCOM, the series voltage must remain in quadrature with the line current for the DC bus voltage to remain constant and no active power being exchanged (assuming a theoretical lossless condition). Note that the SSSC is a fractional power device that has to be rated to the line current but the rated voltage is only a fraction of the rated line voltage. Therefore, with a small rating, it may have a high impact on the controllability of the power flow, which is one of its main advantages [1]. 4) Solid State Tap Changer (SSTC). This device uses a set of power electronic-based switches, usually made up of two antiparallel thyristors which replace the mechanical switches used in the regulation windings of conventional power transformers, as shown in Fig. 11. The thyristors are operated in an ON/OFF mode, so that no harmonic distortion is generated. Depending on the selected switch, the transformer turns ratio gets modified, with ensuing adjustments to the secondary voltage, to meet operational needs. 3.3 Shunt-series connection 1) Solid State Voltage Regulators. This is the electronic version of conventional voltage regulators, and an alternative to the SSTC analysed in the previous subsection. The device controls the nodal voltage by introducing a series voltage using a series transformer as shown in Fig. 12a. This series voltage is obtained from a shunt exciting transformer which, in addition to a number of thyristor-based AC/AC bridges, conforms a controllable voltage source. The thyristor switches are operated using an ON/OFF control and therefore no harmonic distortion is introduced. Note that Fig. 9 TCSC schematic representation Fig. 10 SSSC schematic representation Fig. 11 Solid state tap changer (SSTC) schematic representation 504 J. M. MAZA-ORTEGA et al. depending on the winding arrangement of the exciting transformer, two types of regulators are obtained. On the one hand, a Solid State Tap Changer (SSTC) is obtained if the transformer has wye-wye windings because it is possible to change the magnitude of the controlled node as shown in Fig. 12b. On the other hand, if a wye-delta arrangement is used in the exciting transformer then a Solid State Phase Shifter (SSPS) is generated. In this case, as shown in Fig. 12c, an orthogonal voltage is introduced leading mainly to a change in the phase, and to a lesser extent, the magnitude of the controlled node. Therefore, this device can be used to control in a discrete manner the power flow between the nodes the series transformer is connected to. Neither SSTC nor SSPS seem to have been used so far in any practical installation, but only conventional versions based on mechanically driven servomotors. 2) Unified Power Flow Controller (UPFC). This is a compound equipment with two VSCs connected in shunt and series, respectively, and sharing a common DC bus. This device, whose schematic diagram is shown in Fig. 13, has a great operational functionality combining the control capabilities of the STATCOM and the SSSC. It is capable of regulating, simultaneously, the voltage magnitude at the high-voltage node of the shunt connected VSC, the active power flow arriving at the receiving node of the series connected transformer (opposite node to that where the shunt converter is connected) along with the injection of reactive power at that node. Regulation of these parameters is limited by the ratings of the shunt and series converters. Despite its operational flexibility and great expectations when it was first conceptualized, the UPFC has not been so far a commercial success. Only two prototypes are known to exist in the world [1,24] until recently where the world’s first 220 kV UPFC based on MMC VSCs has been installed in Nanjing (China) [25]. 3.4 Cascade connections 1) LCC-HVDC. DC power transmission using two or more of the six-pulse thyristor bridges shown in Fig. 3, suitably connected, is termed classical HVDC transmission. The most basic configuration involves two converters with their DC terminals directly connecting with each other, to form a monopolar, back-to-back HVDC link. One converter station acts as a rectifier and the other acts as an inverter. In high-voltage, highpower applications the converters are twelve-pulse as opposed to six-pulse and are housed in the same building. On the other hand, if the purpose is to transmit power over a long distance then the two converter stations are linked by a DC transmission line or a submarine DC cable depending on the geography separating the two converter stations, which may currently span several thousands of kilometers. These schemes are termed point-to-point HVDC links. In general, HVDC links may be either monopolar or bipolar. Cases of a monopolar, back-to-back and a bipolar, point-to-point HVDC schemes are exemplified, in schematic form, in Fig. 14a and b, respectively. The bipolar link is composed of two mono-polar links, one at positive and one at negative polarity with Fig. 12 Solid state voltage regulator Fig. 13 UPFC schematic representation Overview of power electronics technology and applications in power generation transmission and…505 respect to ground. In fact, each monopolar side can operate on its own with ground return but with the two poles having equal currents then they cancel each other’s ground return to zero. Indeed, the ground path is a valuable resource but limited to emergency period, when one pole is out of service. The use of phase-controlled thyristors as shown in a previous section is characterized by reactive power consumption and low order harmonics. Therefore, in this type of applications passive filtering on the AC and DC sides is required as well as local reactive power compensation to achieve a suitable operation of the link. Notice that the current flow is from the rectifier towards the inverter and so is the power flow when the voltage polarity is positive. Alternatively, the power follows an opposite direction to the current when the voltage polarity reverses, an operational characteristic achieved through firing angle control. Several LCC-HVDCs are in operation around the world [26,27]. However, the technology has intrinsic limitations when applied to multi-terminal HVDC systems because LCC-HVDC is based on current balances. Hence, only serial, multi-terminal HVDC schemes seem to be realisable using this technology. 2) VSC-HVDC. The control functionality of the monopolar, back-to-back VSC-HVDC link is comparable to that of the UPFC but at the expense of using two full rated converters, as illustrated in Fig. 15. Nevertheless, its overall functionality is far greater than the UPFC because asynchronous interconnection between AC systems with different frequencies is enabled [3]. Moreover, the two VSCs do not need to be connected back-to-back but, instead, linked by a cable to transport electrical power with less power loss than an AC transmission line of comparable rating and distance. Such an arrangement is known as mono-polar, point-to-point VSC-HVDC link. However, bipolar schemes are favored over the basic mono-polar link, on grounds of higher power throughputs and reliability [4]. Recent developments of the VSC-HVDC technology are in the arena of the so-called Multi-Terminal VSC-HVDC systems [16,28–30]. The basic representation of the back-to-back VSC-HVDC link shown in Fig. 15 may be expanded to exemplify the case when three VSCs are interconnected on their DC sides through DC cables to make up a three-terminal VSCHVDC system, as shown schematically in Fig. 16. This is a generic concept that may comprise nVSCs to link nAC systems of varying sizes, topologies and operational complexity. The DC cables may be overhead, underground or submarine, according to practical requirements. The DC network may also exhibit an arbitrary topology and it may contain DC loads, DC generation and storage systems [19]. Just as in meshed AC transmission systems, there are transmission lines with non-regulated and regulated power flows, power flows sharing between neighboring transmission lines, Fig. 14 HVDC systems Fig. 15 Mono-polar, back-to-back VSC-HVDC Fig. 16 Three-terminal VSC-HVDC system 506 J. M. MAZA-ORTEGA et al. nodes with regulated and non-regulated voltage and so on. Finally, this DC network may even comprise a single node bus where the nVSCs would be sharing a DC capacitor, i.e. multi-terminal back-to-back configuration [31]. 3) DC/DC converters. Power transformers are used in AC systems to suitably interconnect sections of the power grid with different voltage levels. As a matter of fact, power transformers were largely responsible for the great development of the current AC systems, instead of the primitive DC-based ones. However, nowadays the power system has to integrate more and more DC subsystems and components, connected to a diversity of voltage levels, from LV to UHV. This creates a clear need for efficiently performing voltage level conversion at the points of interconnection. DC/DC converters are power electronic devices intended for this application. However, the kind of converters traditionally used for low power applications [32], cannot be directly applied when high powers and high voltages have to be handled. Basically, the most widely adopted solution consists of using DC/AC conversion, with or without transformer, based on twolevel or multilevel VSCs, as shown in Fig. 17 [33,34]. Note that the frequency on the AC side can be much higher than the power frequencies (i.e., 50 or 60 Hz), in order to reduce the transformer size and, hence, that of the DC/DC converter. 4 Applications 4.1 Generation/storage interfacing In the last two decades, increasingly more powerful IGBT-based power converters have found a major application niche as suitable interfaces between the grid and the usually asynchronous and intermittent renewable sources [35–37]. This fruitful relationship started with the incorporation of STATCOMs into older generations of wind farms based on fixed-speed induction machines. The aim was to dynamically provide reactive power support so that those wind generators could satisfy the more stringent lowvoltage ride through capability imposed by new grid codes. Then, more efficient and mechanically more robust variable-speed turbines emerged, namely the doubly-fed induction machine (DFIM) type, in which the rotor current is controlled by a fractional converter, or permanent magnet synchronous machine (PMSM) type, whose stator is fed through a full size converter (Fig. 18). Regarding photovoltaic (PV) power plants, they usually connect to the grid through a DC/DC boost converter and an inverter, as shown in Fig. 19. Old generations of converters, most of them still in use today, were simply able to track the maximum power of the PV modules, while keeping unit power factor at the point of connection. However, the new generation based on VSCs can and are required to perform more sophisticated functions, such as voltage regulation and contribution to frequency stability. 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IEEE Trans Smart Grid 2(2):278–286 [82] Justo JJ, Mwasilu F, Lee J et al (2013) AC-microgrids versus DC-microgrids with distributed energy resources: a review. Renew Sust Energy Rev 24:387–405 [83] Antoniou D, Tzimas A, Rowland SM (2015) Transition from alternating current to direct current low voltage distribution networks. IET Gen Transm Distrib 9(12):1391–1401 J. M. MAZA-ORTEGA received the Electrical Engineering and Ph.D. degrees in Electrical Engineering from the University of Seville, Spain, in 1996 and 2001 respectively. He is with the Department of Electrical Engineering, University of Seville, Seville, Spain, since 1997 where he is currently Associate Professor. His primary areas of interest are power quality, integration of renewable energies and power electronics. E. ACHA received the electrical engineering degree from Universidad Michoacana, Mexico and the Ph.D. degree from the University of Canterbury, Christchurch, New Zealand. He is the Professor of Electrical Power Systems at the Tampere University of Technology (TUT). His areas of research interests are power electronics applications in electrical power systems. S. GARCI ´Areceived the B.Sc., M.Sc. and Ph.D. degrees, all in electrical engineering from Universidad Michoacana, Mexico. She is an Associate Professor at the Faculty of Electrical Engineering of Universidad Michoacana, Me ´xico. Her areas of research interests are power quality and smart grids. A. GO ´MEZ-EXPO ´SITO received the electrical engineering and doctor degrees from the University of Seville, Spain, where he is currently the Endesa Industrial Chair Professor. His primary areas of interest are optimal power system operation, state estimation, digital signal processing and control of flexible ac transmission system devices. 514 J. M. MAZA-ORTEGA et al.