Power-electronic solutions to power quality problems

Electric Power Systems Research 66 (2003) 71 /82 www.elsevier.com/locate/epsr

Review

Power-electronic solutions to power quality problems Ambra Sannino a,*, Jan Svensson b, Tomas Larsson c a

Department of Electric Power Engineering, Chalmers University of Technology, SE-41296 Gothenburg, Sweden b ABB Utilities, Light Competence Center, Gothenburg, Sweden c ABB Utilities, Va ¨ stera˚s, Sweden

Abstract In this paper, an overview of power-electronic based devices for mitigation of power quality phenomena is given. The concept of custom power is highlighted. Both devices for mitigation of interruptions and voltage dips (sags) and devices for compensation of unbalance, flicker and harmonics are treated. The attention is focused on medium-voltage applications. Details about field experience are given and recent research results are reported. It is shown that custom power devices provide in many cases higher performance compared with traditional mitigation methods. However, the choice of the most suitable solution depends on the characteristics of the supply at the point of connection, the requirements of the load and economics. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Power quality; Power electronics; Custom power

1. Introduction Power quality phenomena include all possible situations in which the waveform of the supply voltage (voltage quality) or load current (current quality) deviate from the sinusoidal waveform at rated frequency with amplitude corresponding to the rated rms value for all three phases of a three-phase system. The wide range of power quality disturbances covers sudden, shortduration deviations, e.g. impulsive and oscillatory transients, voltage dips (or sags), short interruptions, as well as steady-state deviations, such as harmonics and flicker [1]. One can also distinguish, based on the cause, between disturbances related to the quality of the supply voltage and those related to the quality of the current taken by the load. To the first class belong, among others, voltage dips and interruptions, mostly caused by faults in the power system. These disturbances may cause tripping of ‘‘sensitive’’ electronic equipment with disastrous consequences in industrial plants, where tripping of critical * Corresponding author. Tel.:
/46-31-772-1631; fax:
/46-31-7721633. E-mail addresses: [email protected] (A. Sannino), [email protected] (J. Svensson), tomas.x.larsson @se.abb.com (T. Larsson).

equipment can bear the stoppage of the whole production, with high costs associated [2]. One can say that in this case it is the source that disturbs the load. To avoid consistent money losses, industrial customers often decide to install mitigation equipment to protect their plants from such disturbances. The second class covers phenomena due to low quality of the current drawn by the load. In this case, it is the load that disturbs the source. A typical example is current harmonics drawn by disturbing loads like diode rectifiers, or unbalanced currents drawn by unbalanced loads. A more complicated case is light flicker, which is caused by voltage fluctuations, in turn caused by rapidly varying loads supplied by a weak network. Customers do not experience any direct production loss related to the occurrence of these power quality phenomena. But poor quality of the current taken by many customers together will ultimately result in low quality of the power delivered to other customers: e.g. both harmonics and unbalanced currents ultimately cause distortion and, respectively, unbalance in the voltage as well. Therefore, proper standards are issued to limit the quantity of harmonic currents, unbalance and/or flicker that a load may introduce, e.g. [3]. To comply with limits set by the standards, customers often have to install mitigation equipment. The need for

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devices for mitigation of the second class of phenomena is thus induced by regulatory effort. For both 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’’ [4,5], able to deliver customized solutions to power quality problems. This paper provides an overview of power-electronic based devices to be installed at medium-voltage level for mitigation of power quality phenomena. According to the above classification, there are two major classes of mitigation equipment. Reactive power compensation and harmonic cancellation devices are mostly connected in shunt at the load bus, with the purpose of injecting a current to correct the current taken by the load. Mitigation of flicker will be treated in Section 2 and harmonic filtering in Section 3. Voltage dips and interruption mitigation devices are normally connected between the supply and the load, in order to correct the supply voltage. These devices are described in Section 4. Conclusions are finally drawn in Section 5.

Series capacitors have also been investigated and installed for this purpose [11]. A capacitor in series with the arc furnace with a proper value of capacitance is capable of canceling the reactance between the source and the arc furnace. However, the risk of sub-synchronous resonance (SSR) for series compensated lines has contributed to reluctance to install series capacitors. Moreover, the influence of the resistance of the grid is not taken into account. By inserting a linear reactor in series with the arc furnace, the short-circuit current of the furnace is reduced thanks to the higher total impedance. Furthermore, increased reactance in the circuit means larger phase shifts between voltage and current and thereby a more stable electric arc. However, the series reactor must be selected with care since high values of the series impedance will reduce the furnace power and thereby the steel production rate. Saturable reactors in series have also been used to quickly cut current peaks due to for instance short-circuit in the arc furnace [12]. However, the most used devices for flicker mitigation are by far the static var compensator (SVC) and the DStatcom, described in the following.

2. Flicker mitigation Arc furnace operation has traditionally been the cause of flicker problems on medium-voltage and high-voltage systems. Lately, wind turbines have also been reported to cause this phenomenon [6]. For flicker mitigation, a number of different methods are available, which differ in performance, feasibility and cost [7]. Electrical methods for flicker reduction deal with the arc furnace current and can be grouped in direct and indirect methods. The direct methods include apparatus that alter the arc furnace current directly and are connected in series with the furnace. Indirect methods do not intervene on the arc furnace current, but tackle its effects indirectly with a mitigating device connected in parallel with the furnace by injecting compensating current. A method to reduce flicker is selecting carefully the arc furnace power in-feed. Special transformer configurations for furnace supply have been used for flicker reduction [8,9]. Reinforcing the grid is an effective means of flicker mitigation but it is expensive and, therefore, sometimes adopted when a future expansion of the arc furnace plant is foreseen. The reinforcement may, however, not always be possible due to environmental impact. With a so-called ‘‘dimmer’’ inserted in series with the arc furnace, its current can to some extent be controlled [10]. The dimmer is a series device equipped with antiparallel thyristors and a series reactor. The disadvantage is the slow response time and the high harmonic content. Due to the high rating of the device, it also becomes rather costly.

2.1. Static var compensator (SVC) An SVC can be used for ac voltage control by generation and absorption of reactive power by means of passive elements. It can also be used for balancing unsymmetrical loads. As shown in Fig. 1, it is normally constituted by one thyristor controllable reactor (TCR) and a number of thyristor switched capacitor (TSC) branches [13]. The value of the reactance of the inductor is changed continuously by controlling the firing angle of the thyristors, while each capacitor can only be switched on and off at the instants corresponding to the current zero crossings, in order to avoid inrush currents in the capacitors. With this arrangement, the SVC can generate continuously variable reactive power in a

Fig. 1. Principle scheme of an SVC.

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specified range, and the size of the TCR is limited to the rating of one TSC branch. Obviously, the size of the reactor limits the power that can be absorbed in the inductive range. The SVC can be found in applications such as power line compensation [14], compensation of railway feeding system [15], reducing disturbance from rolling mills [16] and arc furnace compensation [17] (both for reactive power supply and for flicker mitigation). The ability to absorb changes in reactive power makes to some extent the SVC suitable for flicker reduction. In this case, the SVC normally consists of a TCR branch with a filter (no TSC). An SVC installed together with an arc furnace not only reduces the flicker, but also, thanks to the stabilized ac voltage, increases the steel production and its quality [18]. However, the ability of the SVC to mitigate flicker is limited by its low speed of response.

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was that the active power of the arc furnace was reduced by about 20%. With a VSC with proper control algorithm, a PST equal to 0.65 could be reached. This corresponds to a decrease of more than 80%. Field measurements have been performed to verify the performance [25,26]. In Fig. 2, the reactive current of the arc furnace and the response of the mitigation device are shown for a steel manufacturer operating a 31.5/37.8 MVA electrical arc furnace together with a ladle furnace rated 5.9/7.7 MVA [26]. The current from the device clearly follows the arc furnace current. The result is a consistent mitigation of the flicker (by about 3.5 times) with the device in operation, as shown by the flicker measurements of Fig. 3, carried out in accordance with the IEC method [27].

3. Harmonics mitigation 2.2. D-Statcom Among the advantages of using forced-commutated converters are the ability of both produce and consume active and reactive power. The active power can also be controlled independently of the reactive power and vice versa. Moreover, using a voltage source converter (VSC) with pulse-width modulation (PWM) gives a faster converter control, which is needed for flicker mitigation purposes. To reduce harmonics and switching losses per valve when using gate turn-off thyristors (GTOs), a number of possible converter schemes have been proposed, like 12pulse, 24-pulse, 36-pulse [19] or even 48-pulse [20]. For example, a 12-pulse connection can be achieved with two converter bridges connected in parallel via two transformers, one star- and one delta-connected. A forced-commutated VSC with PWM operation today seems to be the most suitable apparatus for flicker mitigation purposes [21]. Recent progresses in voltage and current ratings of the valves allow using integrated gate bipolar transistors (IGBTs) with high switching frequencies, which further improves the speed of response. A shunt-connected VSC mounting IGBTs and operated with PWM is normally referred to as ‘‘Statcom’’ or ‘‘D-Statcom,’’ as it is normally installed at distribution levels [5]. Manufacturers commercializing this product with different names have realized a number of successful installations in the latest years [22,23]. In the study reported in [24], the VSC is found to be superior to other flicker mitigation methods such as the SVC and the series saturable reactor. Using an SVC, the PST (flicker) value of the voltage at the entrance of the arc furnace plant, initially set to 3.8, could be reduced to 2.4 or approximately 60% of its initial value. Combining the SVC with a saturable reactor in the studied system, unity PST could be reached. A disadvantage, however,

In order to remove current harmonics from the grid, passive or active shunt-filters are used. Passive filters for harmonic reduction provide low impedance paths for current harmonics. Thus, the current harmonics flow into the shunt filters instead of back to the supply. The passive filter consists of series LC filters tuned for specific harmonics, normally combined with a highpass filter used to eliminate the rest of the higher-order current harmonics. The drawbacks with passive filters are that they are strongly dependent on the system impedance, which depends on the distribution network configuration and the loads. Therefore, the system impedance, which changes continuously, strongly influences the filtering characteristics. In the worst case an unwanted resonance can occur between the filter and the system. This may cause the passive filter to act as a ‘‘sink’’ for harmonic currents from other sources in the grid. Therefore, the passive filter can be overloaded by a current higher than the rated value. Finally, the capacitors of the passive filter produce reactive power, which may not necessarily be needed for power factor correction. A shunt active filter consists of a controllable voltage source behind a reactance acting as a current source. The VSC based shunt active filter is by far the most common type used today, due to its well-known topology and straightforward installation procedure. It consists of a dc-link capacitor, power electronic switches and filter inductors between the VSC and the grid (Fig. 4). The operation of shunt active filters is based on injection of current harmonics in phase with the load current harmonics, thus eliminating the harmonic content of the line current. When using active filters, it is possible to choose the current harmonics to be filtered and the degree of attenuation. The size of the VSC can be limited by using selective filtering and removing only those current

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Fig. 2. Reactive current of the arc furnace and the compensator (SVC light).

harmonics that exceed a certain level, e.g. the level set in IEEE Std. 519-1992 [3]. Together with the active filtering, it is also possible to control the power factor by injecting or absorbing reactive power from the load. A common active filtering method is based on using the instantaneous active and reactive power [28]. This method detects all non-sinusoidal components and, when it is implemented with a hysteresis current controller, allows obtaining good filtering performance. However, the hysteresis current controller has the disadvantages of a varying switching frequency, which produces a continuous harmonics spectrum. When the instantaneous active and reactive power method is

implemented together with sub-oscillated PWM [29] and vector current control [30], an inferior performance is obtained when the switching frequency is low. This may be desired in order to keep the switching losses down. The low performance is caused by the response time of the current controlled VSC, which leads to a phase shift between the reference currents and the output currents. Various methods for compensating for phase shifts have been presented, e.g. [31]. Selective active filtering can be based on various methods. Some of the proposed selective active filters use bandpass filters to extract individual current harmonics [32]. Unfortunately, this method results in phase

Fig. 3. Flicker levels with and without mitigation with VSC-based compensator for the electrical arc furnace (EAF) in Hagfors, Sweden [26].

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Fig. 5. Phasor diagram of the series injection principle.

Fig. 4. Principle scheme of active filtering using VSC.

shifts, which reduce the filtering performance. A better solution is to use a Fourier series to determine individual harmonics [31]. Another method uses synchronously rotating coordinate system for each of the individual current harmonics [33]. Several methods have been analyzed and compared in [34].

4. Mitigation of voltage dips and short interruptions Short-duration, shallow dips can be mitigated by improving equipment tolerance characteristics [35 /37]. Long-duration, deep dips and interruptions can be avoided by changing structure and/or operation of the power system [38]. However, for industrial customers, who do not normally have access to system or equipment improvement, the installation of additional mitigation equipment is often the only option left to achieve the desired quality of supply at the system-load interface. Traditional devices described in [2] include motorgenerator sets, which use the rotational energy stored in a flywheel to provide power to the load during the dip, and constant voltage, or ferro-resonant, transformers (CVTs). More modern equipment based on power electronics will be described here. The static series compensator (SSC) and the static transfer switch (STS) are analyzed more in detail. Other devices are also briefly described.

compensation is depicted in the phasor diagram of Fig. 5 where V¯ sag ; V¯ inj ; V¯ load and I¯ load are the phasors of supply voltage affected by dip, injected voltage, load voltage and current, respectively. Manufacturers that are pioneering this technology with the name of ‘‘dynamic voltage restorer’’ (DVR) have realized installations, among others, at a yarn manufacture [39], semiconductor plants [40,41], a utility feeder serving industrial and commercial customers in Canada [42], a large paper mill in Scotland [43]. The compensation system is installed at medium voltage level and protected loads have typical ratings between 0.5 MVA and around 21 MVA [40]. This solution, although costly, is very attractive for large sensitive industrial customers, as it allows for protection of the entire plant through the installation of only one device. However, being a series device, this compensator has the obvious disadvantage of not protecting the load against interruptions [44]. Moreover, sensitive loads inside the plant will not be protected against dips originating within the plant. One possible configuration that realizes the seriesinjection principle is shown in Fig. 6. The converter generates the proper voltage to be injected for compensation, and therefore, will often be required to operate with unbalanced switching functions for the three phases. The converter is normally a three-phase twolevel VSC with IGBTs [39], but also the use of a three-

4.1. Static series compensator The SSC is a VSC connected in series on the distribution feeder, which provides a controllable source, whose voltage adds to the source voltage to obtain the desired load voltage. Depending on the type of control implemented, it is possible to use the additional voltage source to correct supply voltage unbalance, perform load voltage regulation, compensate for voltage dips and cancel low-order supply voltage harmonics. The principle of series injection for dip

Fig. 6. Scheme of series compensator for voltage dip mitigation.

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phase three-level neutral-point-clamped (NPC) converter mounting integrated gate-commutated thyristors (IGCTs) has been reported in a DVR by a major manufacturer [41]. The valves on the three legs of the converter are switched independently of each other, normally according to a PWM pattern, with high switching frequency. This ensures fast response and a smooth voltage waveform. The voltage rating of the converter dictates the maximum injected voltage, which is thus the maximum (three-phase) dip magnitude that can be compensated for. Existing DVRs are usually sized for 50% maximum voltage injection. A second-order LC filter is normally inserted in between the converter and the transformer to cancel high-frequency harmonic components in the converter output voltage. Another configuration proposed includes a line-side filter composed by the leakage inductance of the injection transformer combined with a capacitor on the line-side of the transformer [41]. However, with this configuration the capacitor must be sized for the higher system voltage and the series transformer must be sized for the total rms current, including harmonics [45]. The controller of the SSC can be designed to compensate for voltage dips by only providing reactive power, i.e. by injecting a voltage in quadrature with the load current. However, it is shown in [46] that the compensation capability of the device is very limited in these conditions, especially for high values of the power factor. Therefore, an energy storage device is normally connected to the dc bus of the converter to provide the energy necessary for the compensation. Commercially available DVRs use large capacitor banks for energy storage [40]. A SSC unit utilizing superconducting magnetic energy storage (SMES) technology installed in 1997 is mentioned in [47]. The device is rated at 750 kVA and provides 2.4 MJ of stored energy. No details are known on field experience. Experimental results on a prototype are presented in [48]. The problem with a wide application of SMES is at present the very high cost. The capacity of the energy storage device has a big impact on the compensation capability of the system, as it ultimately determines the ride-through time for the load. A method for minimizing the size of the necessary energy storage device has been recently proposed in [49]. The availability of high-capacity storage at reasonable cost is a key factor for widespread application of this technology. A controller for the SSC designed in the rotating dq reference frame, which can achieve fast response and handle transients properly, has been proposed in [50]. However, this controller is designed under the assumption of symmetrical supply voltages, i.e. it only works with balanced dips. The controller proposed in [51] implements a fast technique for positive and negative sequence detection and thereby is able to compensate for

unbalanced dips. Moreover, the effect of the converter output filter, which causes voltage drop and phase shift in the fundamental component of the injected voltage, is taken into account and compensated for in the controller presented in [51] by using a back-calculation of the voltage drop, known the filter parameters. An alternative is to apply a feedback of the capacitor voltage, as done for example in [50]. Even better transient performance is obtained by adding another loop that controls the current through the converter valves as in [52]. The principle of series injection can also be applied to cancellation of low-order supply voltage harmonics. To do this, the controller must be able to deal with other frequencies than the fundamental. A controller based on several rotating frames is proposed in [53]. An interesting case study presented in [54] involves an industrial facility that produces microprocessors for the personal computer industry starting from pure silicon wafers. In 12 months before the installation of the SSC, the plant had experienced 14 voltage dips that were capable of adversely affecting production. The semiconductor plant is served by three 69 kV lines feeding two transformers and the total plant load is about 45 MVA. Two independent SSCs, each rated 6 MVA at 12.47 kV with 1800 kJ of stored energy, were installed inside the semiconductor plant, one on each of the two 12.47 kV feeders. Monitoring performed after installation revealed that the SSCs compensated for over 72 power quality disturbances that could have caused process interruptions for the end-user. To demonstrate the performance, a measured three-phase dip down to 65% occurring on the grid and the resulting load voltage due to SSC operation are shown in Fig. 7. 4.2. Static transfer switch The STS consists of two three-phase static switches, each constituted in turn by two anti-parallel thyristors per phase (Fig. 8). Normally, the static switch on the primary source is fired regularly, while the other one is off. In the event of a voltage disturbance, the STS is used to transfer the load from the preferred source to an alternative healthy source [55]. This results in a very effective way of mitigating the effects of both interruptions and voltage dips by limiting their duration as seen by the load. The success of the STS is mainly due to its rather low cost compared with other solutions. A requirement is that a secondary in-feed, independent from the main source (e.g. a feeder to another substation), must be available. Therefore, this solution is particularly attractive for installations that already have mechanical transfer systems, where upgrading to a static system does not require major changes in the layout of the distribution system. Formerly available only for low voltages, STS systems are now advertized

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Fig. 7. Measured three-phase dip on grid side (top) and load voltage (bottom) due to SSC operation, from [5].

for higher voltages and load ratings, which make them suitable for high-power industrial applications: they would be capable to protect loads up to 35 MVA supplied at a voltage as high as 35 kV [56]. The application of a 15 kV, 600 A STS to an automotive component plant is described in detail in [57]. Note, however, that the STS cannot protect against dips originating in the transmission system, which will also affect the alternative supply. Yet, a significant improvement can be achieved in the performance of the industrial system against faults at distribution level, which normally cause long duration dips and short interruptions. The load will still see a disturbance during the interval in which the transfer takes place, therefore, it must be completed so quickly that the duration of the resulting

Fig. 8. Structure of the STS (single-phase scheme).

disturbance at the load terminals is short enough not to cause equipment trips. The STS can ensure a fast response due to the phenomenon of fast switching, usually referred to as ‘‘Make-Before-Break’’ switching (MBB). This takes place when the thyristors of the secondary-source static switch are fired, thus initiating the transfer, before the current through the primary-source switch has reached zero. Depending on the circuit conditions, a current starts flowing in the secondary-source switch in a direction such that it forces the primary-source switch to turn off very quickly. Transfer times of less than one quarter of cycle are quite common for MBB transfer [55]. An example of the dip mitigation performance of the STS is given in Fig. 9(a) which shows the source voltages together with the response of the detection system, denoted as ‘‘fault signal’’, for a 70% dip (single-phase). The dip is detected in about 1 ms and the transfer occurs immediately after. As a result, only a notch of very short duration affects the load voltage (Fig. 9(b)). However, MBB transfer must be inhibited when this would lead to a circulating current between the two sources, thus spreading the fault to the healthy portion of the system. In this case, the control system must inhibit firing and wait until the current through the thyristors in the faulted source switch has become zero, thus realizing a slow or ‘‘Break-Before-Make’’ transfer (BBM). This result is accomplished by monitoring the load current, together with the voltages of the two sources.

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Fig. 9. (a) Source voltages and fault signal, (b) load voltage and current for a 70% dip.

The performance of the STS concerning transfer time has been analyzed in [58,59]. Results presented allow concluding that in general the speed of operation of a STS is high enough to realize a seamless transfer for sensitive loads. Note, however, that the transfer time increases even more in case of regenerative load, e.g. induction motors [60,61]. In industrial plants with high percentage of rotating load, a specific performance study could be needed to assess the actual improvement obtained with installation of the STS. 4.3. Other voltage dip/interruption mitigation devices Electronic tap changers (Fig. 10) can be mounted on a dedicated transformer for the sensitive load, in order to change its turns ratio according to changes in the input voltage [62]. This device, called static voltage regulator (SVR) [5], is placed between the supply and the load. Part of the secondary winding supplying the load is divided into a number of sections, which are connected or disconnected by fast static switches, thus allowing regulation of the secondary voltage in steps. This should

Fig. 10. SVR.

allow the output voltage to be brought back to a higher level than 90% of nominal value, even for severe voltage dips. Thyristor-based switches, which can only be turned on once per cycle, are used; therefore, the compensation is accomplished with a time delay of at least one halfcycle. No existing installations are known to date. A standard solution for low-power equipment is constituted by the UPS (Uninterruptible Power Supply), which consists of a diode rectifier followed by an inverter (Fig. 11). The energy storage device is usually a battery block connected to the dc link. During normal operation, power coming from the ac supply is rectified and then inverted. The batteries remain in standby mode and only serve to keep the dc bus voltage constant. During a voltage dip or interruption, the energy released by the battery block maintains the voltage at the dc bus. Depending on the storage capacity, the battery block can supply the load for minutes or even hours. Low cost, simple operation and control have made the UPS the standard solution for low-power equipment like computers. For higher-power loads the costs associated with conversion losses and maintenance of the batteries become too high and this solution no longer appears to be economically feasible. Note that, although based on power electronics, the UPS due to its low-power/low-voltage ratings is not a

Fig. 11. Uninterruptible power supply.

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‘‘custom power device’’ according to the definition given in [5], which covers devices installed in 1 kV through 38 kV distribution systems. However, it is listed here for completeness. A static shunt compensator, normally used for flicker mitigation and active filtering purposes, equipped with an isolation switch for disconnection from the distribution feeder, results in the backup source of Fig. 12, also called Backup Stored Energy System (BSES) [5]. This constitutes an alternative to the UPS to avoid high steady-state losses due to the energy conversions as the load power increases. As soon as a disturbance is detected, the sensitive load is isolated from the power system by a static switch and supplied by the VSC. For storing the necessary energy, batteries (Transportable Battery Energy Storage System, TBESS [63]) or smallsize SMES systems can be used [64]. The main advantages of SMES as compared with the batteries are the reduced size and lower maintenance requirements. It has been pointed out that the necessity of an energy storage device of adequate capacity is the biggest limitation for the SSC. An alternative is to have a second converter connected to the line upstream of the series compensator to supply the dc bus (Fig. 13). In this way, energy is transferred to the dc bus continuously. On the other hand, power coming from the line will be greatly reduced during a dip. This rectifying stage must be properly designed to operate correctly with reduced (and possibly unbalanced) input voltage during the dip (note that this problem is overcome if the rectifier is placed on the load side as in [42], but at the expenses of increased rating of the SSC). By choosing a shuntconnected VSC instead of a simple diode rectifier for this purpose, other control and regulation functions, e.g. active current filtering or flicker mitigation, can be performed, thus realizing a ‘‘multi-purpose’’ compensator. This concept has been proposed with the name of ‘‘unified power quality conditioner’’ (UPQC) [65]. The device of Fig. 14 is obtained by a combination of a STS and a SSC in series. Total protection can thus be obtained against interruptions and voltage dips, with the STS taking care of interruptions and dips originated by faults in the distribution system, which are long and

Fig. 12. Backup power source.

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Fig. 13. Scheme of the unified power quality conditioner (UPQC).

Fig. 14. Single-phase scheme of a combined STS/SSC.

deep [1] and would deplete the energy storage of the SSC. The SSC will instead compensate for the voltage dips originated by faults in the transmission systems, which the STS cannot handle. Note that transmissionsystem dips are normally short and shallow [1]. Hence, the size of the energy storage of the SSC can be greatly reduced, with a consequent reduction of the cost of the device.

5. Conclusions In this paper, an overview of the use of power electronics for mitigating power quality phenomena has been given. The concept of ‘‘custom power’’ has been highlighted. Advantages and drawbacks of several custom power devices have been pointed out. Both shunt devices, protecting the source from the load, and series devices, protecting the load from the source, have been covered. Details about field experience have been given and recent research results have been reported. It has been shown that custom power devices provide in many cases higher performance compared with traditional mitigation methods. However, the choice of the most suitable solution depends ultimately on the

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characteristics of the supply at the point of connection, the requirements of the load and economics, i.e. the customer value added by the installation of a powerelectronics based device.

Acknowledgements The work of Ambra Sannino was supported by a Marie Curie Fellowship of the European Community program IHP under contract number HPMF-CT-200000922 and partly also by ELFORSK, Sweden under Elektra project no. 3378.

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Biographies Ambra Sannino received the M.Sc. degree and the Ph.D. degree from the University of Palermo, Italy in 1997 and 2001, respectively. From August 1999 to September 2000 she was a guest researcher at the Department of Electric Power Engineering of Chalmers University of Technology, Gothenburg, Sweden, where she is currently working as Assistant Professor. Her interests include applications of power electronics in power systems and power quality. Jan Svensson received his M.Sc. degree and Ph.D. degree in 1991 and 1998, respectively, from Chalmers University of Technology, Gothenburg, Sweden, where he was also Assistant Professor between July 1998 and June 2002. Since July 2002 he is working at ABB Utilities, Light Competence Center in Gothenburg, Sweden, where he deals with design and control of Light-concept devices. His interests include control of

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grid-connected converters, wind power and power quality. Tomas Larsson received his M.Sc. degree and Ph.D degree from the Royal Institute of Technology, Stockholm, Sweden in 1991 and 1998, respectively. Since

March 1998 he is working for ABB where he mainly has been involved in system design for projects dealing with reactive power compensation. His interests include voltage source converters in power quality applications, in particular flicker mitigation.