A novel unified single-phase active rectifier topology with advanced operating modes for smart grids

VOLUME XX, 2017 1 Date of publication xxxx 00, 0000, date of current version xxxx 00, 0000. Digital Object Identifier 10.1109/ACCESS.2024.Doi Number A Novel Unified Single-Phase Active Rectifier Topology with Advanced Operating Modes for Smart Grids Vitor Monteiro1, (Senior Member, IEEE), Filipe Costa1, Sergio Coelho1, (Student Member, IEEE) and Joao L. Afonso1, (Senior Member, IEEE) 1 Centro ALGORITMI / LASI, University of Minho, Portugal Corresponding author: Vitor Monteiro (e-mail: [email protected]) This paragraph of the first footnote will contain support information, including sponsor and financial support acknowledgment. For example, “This work was supported in part by the U.S. Department of Commerce under Grant BS123456.” ABSTRACT Active rectifiers are of paramount importance for guaranteeing power quality in smart grids, due to their characteristics of sinusoidal current consumption and unitary power factor operation. In this paper, a novel unified single-phase (USP) front-end converter is proposed for active rectifier applications in smart grids, offering additional advantages and a set of relevant features besides the conventional active rectification. With the proposed USP front-end converter, the following independent operation modes are available: (i) operation as a traditional active rectifier, ensuring sinusoidal AC grid current (with very low total harmonic distortion), unitary power factor, and controlled DC link voltage; (ii) operation as a shunt active power filter, producing a current with controlled harmonic content and phase angle at the fundamental frequency, in order to ensure almost sinusoidal current and a unitary power factor at the AC power grid; (iii) combined operation as an active rectifier and as a shunt active power filter, providing power to the DC load, while simultaneously producing a current waveform with reduced harmonic content and controlled power factor. By controlling these individual operation modes, electrical appliances in smart grids can operate dynamically and collaboratively, improving power quality. In addition, the proposed USP front end converter may operate as a single-phase diode rectifier in case of control system failure or damage of the controlled switching devices. This situation is particularly relevant since it allows to supply the DC load even under such unfortunate circumstances. Along with the paper, the topology of the proposed USP front end converter, its principle of operation, and the control strategies for each operation mode, are separately explained. A 3.6 kW, 230 V – 50 Hz laboratorial prototype of the proposed USP front end converter is presented, and the experimental validation is carried out for all operation modes. INDEX TERMS Active Rectifier, Active Power Filter, Power Quality, Single-Phase, Smart Grid I. INTRODUCTION The pioneering model of smart grids is indisputably influenced by the existence of newfound technologies, e.g., electric mobility and renewables, aiming to address environmental and sustainable matters, but at the same time, they also introduce an unpredictable behavior [1], [2], and pose significant challenges, such as cybersecurity risks [3]. In addition, as an imperative feature given their power profile, such technologies must not negatively impact the power quality [4], [5], even when operating in a multifunctional cooperation [6], or with on-board/off-board EV chargers [7], [8]. Thus, a customary contribution to diminish the poor power quality regarding reactive power and current harmonics, consists in the substitution of the old passive AC-DC rectifiers by active rectifiers [9], [10]. As the name implies, active rectifiers offer controllability regarding the AC current, driving the operation with sinusoidal current and unitary power factor. This is feasible to reach by employing distinct topologies of power converters, where the most widespread and simple to control is the boost configuration [11], [12], however new topologies are identified [13], [14]. Although classified as a front-end converter, it is categorized by the internal constitution of two power stages: a passive AC-DC rectifier and a controlled DC-DC converter. Consequently, all 8 VOLUME XX, 2017 the power delivered to the DC load is processed by these two power stages. Opportunely, other topologies can be used, namely the bridgeless topologies, which are categorized by the absence of the AC-DC passive rectifier [15], [16]. Despite the indubitable benefits of the active rectifiers, they are solely used for ensuring sinusoidal current and unitary power factor on the AC side and controlled DC-link voltage on the DC side. However, by considering the real context of smart grids, it can be valuable to add additional features to the active rectifiers, without jeopardizing its main principle of operation, but providing benefits to the smart grid. Thus, aligned with this thematic, this paper proposes a novel unified single-phase (USP) front-end converter designed to operate in multiple modes: (i) as a traditional active rectifier; (ii) as a shunt active power filter; and (iii) in combined operation, as an active rectifier and as a shunt active power filter. The context and the topology of the proposed USP front-end converter is shown in FIGURE 1, which is composed by two main parts, namely a diode full-bridge converter (formed by the diodes d1 to d4), named as full-wave rectifier (FWR), and by a full-controlled full-bridge converter (formed by the IGBTs g1 to g4), named as bidirectional active rectifier (BAR). The BAR has an intermediary DC-link (C1), while the FWR contains a DC-link interface (C2) for a DC-DC back-end converter or for a direct connection with a DC load. A common-mode inductive filter is used to couple the USP front-end converter with the AC power grid. The possibility of combining different features and technologies, either through several power converters or by integrating them into a single-stage solution, has already been identified and validated in the literature. For instance, a single-phase interface between distributed generation and a shunt active power filter is presented in [17]. Although it relies on two distinct power converters, its main contribution is the implementation of a neural control strategy. A combined three-phase series-shunt active power filter with a power converter connected in the DC-link to interface a solar photovoltaic system is presented in [18], permitting the operation with active/reactive power control through the shunt filter. Nonetheless, the solution requires three distinct power converters and does not provide a DC interface. A similar structure is proposed in [19], but without a power converter to interface the solar photovoltaic system. This reduces hardware complexity but limits the system’s operational flexibility. In contrast, a hybrid DC interface combining battery storage and solar photovoltaic systems is presented in [20], enabling active/reactive power control through the shunt converter. However, this approach requires additional converters and lacks redundancy. An analogous design is proposed in [21] for three-phase applications, but its main drawback lies in the impossibility of controlling the output DC-link voltage. In addition, similar structures can be implemented aiming for the improvement of power quality, while also offering the possibility of operating with bidirectional active power. Therefore, both single-phase and three-phase systems are identified, targeting the compensation or attenuation of current harmonics, the combined mitigation of voltage and current, and in some cases, reactive power compensation. Normally, these solutions employ full-bridge topologies, cascade arrangements, or other innovative power converter structures, as presented in [22]-[29]. Thus, it is recognized the possibility of combining features of active rectifiers with active power filters, which is of utmost importance. A comprehensive comparison is presented in TABLE I, covering the previously discussed approaches. It includes the different types of application considered, the topology of power electronics converters used, their integration context, and the corresponding compensation capabilities. Supported by the analysis of the identified benefits and drawbacks, the distinguished features of the proposed USP front-end converter are described as follows: 1. Operation in three distinct modes, allowing to respond to the power needs of the DC side, as well as to provide add-value functions to the smart grid, namely: (a) operation as a traditional active rectifier, receiving power from the grid and ensuring sinusoidal AC current and unitary power factor; (b) operation as a shunt active power filter, just producing reactive power and current harmonics to the power grid for ensuring sinusoidal current and unitary power factor in the common coupling point; (c) combined operation of both previous cases, providing power to the DC side, while operating as a shunt active power filter, compensating current FIGURE 1. Proposed unified single-phase (USP) front-end converter. L vg a g1 g2 b g3 g4 C1 vdc1 d1 d2 d3 d4 C2 vdc2 iBAR iFWR igild iUSP idc AC/DC Unified Single-Phase (USP) AC DC DC DC iUSP Loads Power Grid 8 VOLUME XX, 2017 harmonics and reactive power of the non-linear loads in the same installation. 2. Even in the case of a failure of the BAR, it is possible to provide power to the DC side through the FWR, which is an intermediate and preferable compromise solution than not feeding the DC side, as happens in a traditional converter. 3. Controlled current on the AC power grid, but with internal current separation, i.e., the AC current (iUSP) from the AC power grid is divided into two parcels, the FWR current (iFWR) and the BAR current (iBAR), allowing to reduce switching losses. 4. Controlled DC-link voltage in all operation modes, like a traditional active rectifier front-end converter and where it can be connected a DC-DC back-end converter or a DC load. 5. Experimental validation of all the previous cases, both in steady-state and transient-state, with a full developed laboratory prototype, specifically designed for such purpose, and tested directly plugged-in into the 230 V AC power grid without any controlled power source. The characterization of the proposed USP front-end converter framed with its advanced operation modes in smart grids is introduced in section II. An explanation of the proposed control algorithm for the distinct operations is presented in section III. The experimental setup and respective validation of the referred operation modes are exposed in section IV. Finally, conclusions are given in section V. II. PROPOSED UNIFIED FRONT-END CONVERTER: PRINCIPLE OF OPERATION A detailed description of the proposed USP front-end converter is presented in this section, highlighting the different and independent operation modes. FIGURE 2 visually explains the operation of the USP front-end converter across the three different operating modes. FIGURE 2(a) illustrates the operation as a traditional active rectifier, ensuring sinusoidal AC grid current (with very low total harmonic distortion), unitary power factor, and controlled DC link voltage. FIGURE 2(b) illustrates the operation as a shunt active power filter, producing a current with controlled harmonic content and phase angle at the fundamental frequency, to ensure almost sinusoidal current and a unitary power factor at the AC power grid. Finally, FIGURE 2(c) illustrates the combined operation as an active rectifier and as a shunt active power filter, providing power to the DC load, while simultaneously producing a TABLE I. Comprehensive comparison, including the different types of application, the topology of power electronics converters used and respective contextualization of integration, as well as the compensation operation. Application Topology Compensation [17] Distributed generation interfacing renewables through the dc-link Single-phase full-bridge in a shunt power filter Current harmonics and reactive power [18] Interfacing solar PV in the DC-link without additional converters Three-phase full-bridge in a unified power quality conditioner Current harmonics and reactive power [19] Interfacing solar PV in the DC-link with additional DC-DC converter Three-phase full-bridge in a unified power quality conditioner Current harmonics and reactive power [20] Interfacing battery storage and solar PV in the DC-link without additional converters Three-phase full-bridge in a unified power quality conditioner Current harmonics and reactive power [21] EV battery chargers, including an additional DC-DC power converter Three-phase swiss rectifier in a shunt power filter Current harmonics and reactive power [22] Interfacing battery storage and solar PV in the DC-link with additional converters Three-phase full-bridge in a unified power quality conditioner Current harmonics and reactive power [23] Interfacing battery storage and solar PV in the DC-link with additional converters Three-phase full-bridge in a universal active power filter Current harmonics and reactive power [24] Interfacing solar PV in the DC-link with additional DC-DC converter Three-phase full-bridge in a universal active power filter Current harmonics and reactive power [25] Interfacing solar PV in the DC-link without additional DC-DC converter Three-phase full-bridge in a shunt power filter Partial current harmonic compensation [26] Interfacing solar PV in the DC-link without additional DC-DC converter Single-phase full bridge in a series/shunt power filter Current harmonics and reactive power [27] Interfacing EV with additional DC-DC converter Single-phase totem-pole boost power factor correction Current harmonics reduction [28] Interfacing EV and solar PV in the DC-link with additional converters Three-phase full-bridge in a shunt power filter Reactive power compensation [29] Interfacing EV, battery storage and solar PV in the DC-link with additional converters Three-phase Cascade full-bridge in a shunt active power filter Current harmonics and reactive power 8 VOLUME XX, 2017 current waveform with reduced harmonic content and controlled power factor. FIGURE 2. Operation of the USP front-end converter across the three different operating modes: (a) Operation as a traditional active rectifier; (b) Operation as a shunt active power filter; (c) Combined operation as an active rectifier and as a shunt active power filter. A. Operation Mode: Active Rectifier The main purpose of an active rectifier is related to the AC side, forcing the operation with sinusoidal current and unitary power factor. In this mode, the power is provided by the AC power grid to the DC side through the USP front-end converter, which also controls the DC-link voltage, so that the back-end converter can properly control the voltage and current levels for the DC load. In this operation mode, the USP front-end converter operates with sinusoidal current and unitary power factor, however, since non-linear loads are connected in the same electrical installation, the main current in the power grid, which is the sum of the currents, present harmonic content, and the power grid exchanges reactive power. This situation is illustrated in FIGURE 3. Any topology of active rectifier needs to establish a proper synchronization with AC voltage to control the desired current. Consequently, a phase-locked loop (PLL) algorithm is used to extract the fundamental component of the AC voltage [30]. This algorithm is not only used to achieve synchronization but also to accomplish the generation of a sinusoidal AC reference current in the presence of a non-sinusoidal AC voltage. In fact, this corresponds to the actual situation of the AC voltage nowadays, which presents harmonic distortion. By using the fundamental component of the AC voltage instead of its total value, i.e., not including the harmonic content, it is possible to achieve a sinusoidal AC current, thereby the USP front-end converter only operates just with active power. Otherwise, the harmonic content of the AC current would be the same as the harmonic content of the AC voltage, which would cause the converter to operate with harmonic power. FIGURE 3. Operation mode of the USP front-end converter as active rectifier, where it operates with sinusoidal current and unitary power factor (iUSP), but due to the non-linear loads in the same electrical installation (ild), the current in the power grid (ig) has harmonic content. 8 VOLUME XX, 2017 To control the AC current, a reference current generation algorithm based on the Fryze Buchholz Depenbrock (FBD) theory was used [31], [32], which is a suitable control theory for single-phase grid connected power converters. In this theory, the converter is modeled by a conductance, consuming a sinusoidal current in phase with the AC voltage. The amplitude of the AC reference current depends upon the operating power, which contains two main components: the power needed to the DC load (pdc) (which can be a direct DC load or through a DC-DC back-end), and the power required to regulate both DC-link voltage (preg), where a proportional-integral (PI) controller is used. To avoid an oscillation with twice the AC voltage frequency, which is intrinsic to single-phase grid-following power converters, the average values of voltage and current were used for the calculation of the required power. The value of pdc is obtained by the product of the average values of the DC-link voltage (vdc2) with the input current for the DC load (idc) (or DC-DC back-end converter). Therefore, the AC reference current (iUSP*) is obtained according to: 𝑖𝑈𝑆𝑃∗(𝑡)= −2 𝑣𝑝𝑙𝑙(𝑡) 𝑝𝑟𝑒𝑔(𝑡)+ 𝑝𝑑𝑐(𝑡) 𝑉𝑝𝑘𝑝𝑙𝑙(𝑡) , (1) where vpll and Vpkpll correspond, respectively, to the fundamental component and to the peak value of the AC voltage. As previously mentioned, the USP front-end converter has as relevant feature the internal division of currents. Thus, FIGURE 4 depicts the inner current separation of the proposed USP front-end converter when operating just as an active rectifier. As expected, iUSP is sinusoidal, corresponding to the sum of iBAR with iFWR (i.e., iFWR(d1) when iUSP is positive and iFWR(d2) when iUSP is negative). As the connection of both internal parts is performed downstream of the AC coupling inductor, these inner currents are switched at high frequency, i.e., the current of the FWR is pulsed with the resulting switching frequency of the BAR. This approach allows to substantially reduce the RMS value of the FWR input current, which contributes to reduce the power losses. Moreover, in this operation mode the DC-link of the BAR (vdc1) supplies power to the DC-link of the FWR (vdc2), regulating simultaneously the average value of both voltages. Since the FWR is a unidirectional topology, the respective DC-link can only provide power to the DC load or DC-DC back-end converter. However, the arrangement of the BAR and the FWR carries a subtle issue for the integrity of the USP front-end converter when power is being transferred between both DC-links. The operation of the BAR connects in parallel C1 and C2, but the existing dead-time between the IGBTs of the same leg causes the disconnection of the two DC-links. As the DC-links are no longer connected in parallel, their voltages will not be balanced. Therefore, when the IGBT switching establishes a new connection between C1 and C2, high current spikes can flow in the converter due to the parallel connection of non-equal voltage capacitive elements, which may cause damage to the power semiconductors. This phenomenon occurs in a more intense way when using unipolar pulse-width modulation (PWM) due to the zero-level of the produced voltage. To alleviate this problem, a low value inductor can be connected to interface both BAR and FWR, reducing the current stress and allowing a proper operation of the BAR also with unipolar PWM switching scheme. FIGURE 4. Current of the proposed USP front-end converter (iUSP) and the internal currents in the BAR (iBAR) and in the FWR (iFWR). B. Operation Mode: Shunt Active Power Filter In the previous section, the traditional operation using the proposed USP front-end converter as an active rectifier was introduced. Besides that, as presented before, one of the main advantages of the proposed USP front-end converter is the possibility to operate as a shunt active power filter. This new operation mode consists in controlling the USP front-end converter for compensating current harmonics and reactive power on the power grid side. Consequently, in this operation mode, the exchange of active power between the AC power grid and the DC load is not required, representing an essential asset for supporting power quality. The USP front-end converter operates with non-sinusoidal current to compensate the current harmonics of the non-linear loads connected in the same electrical installation, and consequently, the main current in the power grid is sinusoidal and the power grid only provides active power. This situation is illustrated in FIGURE 5. 0 A 40 A -40 A 0 A 40 A -40 A 0 A 40 A -40 A 0.00 s 0.01 s 0.02 s 0.03 s 0.04 s 0.05 s 20 A 21 A 19 A 0 A 40 A -40 A 0 A 40 A -40 A 0.02506 s 0.02496 s iUSP iBAR iFWR (d1) iFWR (d2) 8 VOLUME XX, 2017 FIGURE 5. Operation mode of the USP front-end converter as shunt active power filter, where it operates with non-sinusoidal current (iUSP), and due to its operation, compensating the current harmonic content of the non-linear loads (ild), the current in the power grid (ig) becomes sinusoidal. The AC reference current for the USP front-end converter operating as a shunt active power filter is also obtained with the FBD power theory, which is a largely applied control theory in active power filters. However, in this operation mode, the USP front-end converter cannot be modeled by a conductance, since it must supply the reactive power and harmonic currents demanded by the AC loads connected to the power grid. Instead, the AC loads are modeled by a conductance (meaning the fundamental active power absorbed) connected in parallel with a harmonic current source (meaning the fundamental reactive power and the harmonic currents absorbed by the AC loads). Hence, when operating as a shunt active power filter, the USP front-end converter must operate as a harmonic current source, so the AC power grid is only able to provide fundamental active power (i.e., the harmonic current and reactive power is exchanged just between the USP front-end converter and the AC loads). In this context, the equivalent conductance is also calculated, but since the USP front-end converter does not operate as an active rectifier, the power component pdc is not used. In its place, it is used the active power consumed by the AC loads (pld), which is calculated by a half-cycle average of the instantaneous load power. It is used only a half-cycle of the AC voltage because it is assumed that the AC loads only absorb odd harmonic currents, which makes the instantaneous load power to have twice the power grid frequency. Besides, the utilization of the half-cycle average instead of the full-cycle version, provides a faster transient response towards AC load changing. To generate the AC reference current when operating as a shunt active power filter, the equivalent conductance current should be subtracted from the loads current (ild), which results in the harmonic currents and reactive power absorbed by the loads and provided by the USP front-end converter, according to: 𝑖𝑈𝑆𝑃∗(𝑡)= 𝑖𝑙𝑑(𝑡)− 2𝑝𝑟𝑒𝑔(𝑡) 𝑣𝑝𝑙𝑙(𝑡) 𝑉𝑝𝑘𝑝𝑙𝑙(𝑡) . (2) Once again, the fundamental component of the AC voltage is used so, due to the operation of the USP front-end converter as a shunt active power filter, the grid current can be sinusoidal with unitary power factor. Contrarily to the operation just as an active rectifier, when the USP front-end converter operates just as a shunt active power filter, the FWR is not used, since its DC-link voltage remains constant with a slightly higher value than the DC-link of the BAR, providing no current flow in the FWR diodes. Hence, the proposed USP front-end converter behaves as a regular AC-DC converter. C. Operation Mode: Combination of Active Rectifier and Shunt Active Power Filter The combined operation as an active rectifier and as a shunt active power filter is a relevant operation mode for enhancing power quality in smart grids. The operation mode presented in this section combines the active rectifier process with the shunt active power filter features, i.e., the proposed USP front-end converter operates with a double purpose. In this operation mode, the USP front-end converter operates with non-sinusoidal current to compensate the current harmonics of the non-linear loads connected in the same electrical installation. In addition to the harmonic content, it also presents current in the fundamental frequency of the power grid (50 Hz), ensuring that it operates also with active power. Consequently, the main current in the power grid becomes sinusoidal and the power grid only provides active power, for the loads and for the USP front-end converter. This situation is illustrated in FIGURE 6. To achieve the combined functionalities, the FBD theory should be modified to encompass both power components pdc and pld. Like the shunt active power filter operation mode, the USP front-end converter absorbs current with the harmonic content and reactive power required by the AC loads connected to the AC power grid. The only difference lies in its higher amplitude in the combined operation modes, due to the additional power component required, provided by the USP front-end to the DC loads. Thus, the AC reference current calculation is very similar to the shunt active power filter case, which is given by: 𝑖𝑈𝑆𝑃∗(𝑡) = 𝑖𝑙𝑑(𝑡) − 2𝑣𝑝𝑙𝑙(𝑡)𝑝𝑟𝑒𝑔(𝑡)+ 𝑝𝑑𝑐(𝑡)+ 𝑝𝑙𝑑(𝑡) 𝑉𝑝𝑘𝑝𝑙𝑙(𝑡) (3) 8 VOLUME XX, 2017 FIGURE 6. Operation mode of the USP front-end converter as a combination of active rectifier and shunt active power filter, where it operates with non-sinusoidal current (iUSP), composed by the fundamental frequency of the power grid current (50 Hz) (responsible for ensuring the operation with active power) and by the current harmonics (responsible for compensating the current harmonic content of the non-linear loads (ild)), resulting in a sinusoidal current in the power grid (ig). III. PROPOSED DIGITAL CURRENT CONTROL ALGORITHM The proposed control algorithm, encompassing the distinct operation modes of the proposed USP front-end converter, is presented in this section. A flowchart of the control algorithm is shown in FIGURE 7. As can be seen, the first step consists in reading the different analog to digital converter (ADC) channels. After that, the synchronization with the AC voltage is performed by means of the PLL algorithm and the DC-link voltage is controlled to the established reference average value. According to the operation mode selection (active rectifier; shunt active power filter; or combined active rectifier and shunt active power filter), the AC reference current is established, and the current control is performed through a PWM scheme with a carrier frequency of 20 kHz. The ending process is followed by the start stage to accomplish the selected sampling frequency of 40 kHz. The PLL algorithm and the DC-link voltage regulation are processed independently of the selected operation mode for the USP front-end converter, since they represent vital elements of the control system of any AC-DC converter connected to the AC power grid. The essential difference between the operation modes consists in the calculated power components used in the equivalent conductance current calculation of the FBD power theory. Hereafter, the current control is processed, so that iUSP tracks the attained AC reference properly. For this purpose, it was implemented a predictive-based current control that calculates the reference voltage vab* (between points a and b identified in FIGURE 1) that is compared with the PWM carrier to obtain the gate-pulse for the IGBTs and, consequently, for producing the current iUSP according to its reference. For each given sampling time instant k, the reference voltage vab* is given by: 𝑣𝑎𝑏∗[𝑘]= 𝑣𝑔[𝑘]+𝐿 𝑇𝑠(2𝑖𝑔∗[𝑘]− 𝑖𝑔∗[𝑘 − 1]+ 𝑖𝑔[𝑘]) , (4) where vg denotes the instantaneous value of the AC voltage, L the inductance value of the grid connected coupled inductor and Ts the sampling period. FIGURE 7. Flowchart of the developed control algorithm for the proposed USP front-end converter. IV. EXPERIMENTAL SETUP AND VALIDATION The viability of the proposed USP front-end converter was verified with a laboratorial prototype, especially developed for such purpose. A digital signal processor (DSP), with the model TMS320F28335, manufactured by Texas Instruments, was used for digital control implementation. In the conceived power converter, IGBTs with the model FGA25N120N, switched at 20 kHz, and diodes with the model DUR6060W, were used. The DC-link (C2) is composed by a set of capacitors performing a total capacitance of 1.12 mF (450 V), and the intermediary DC-link (C1) is composed by a set of capacitors performing a total capacitance of 2.8 mF (450 V). Since the topology of the USP front-end converter allows a bidirectional power flow through the BAR and a unidirectional flow through the FWR, a greater capacity was obtained in the intermediary DC-link (C1). Thus, for the operation mode as an active rectifier and for the combined operation as an active rectifier and as a shunt Start Read ADC PLL Synchronization Preg Calculation Active Rectifier? YY Predictive Control PWM (20 kHz) End NN Pld CalculationPDC Calculation Reference Calculation Active Power Filter? Y N Active Rectifier and Power Filter? PDC, Pld Calculation 8 VOLUME XX, 2017 active power filter, the USP front-end converter has a total DC-link capacity resulting from the sum of C1 and C2, while in the operation mode just as a shunt active power filter, solely the capacity of C1 is considered. The USP front-end converter is connected to the AC power grid by means of a coupled inductor with a total value of 2.2 mH. The power converter of the USP front-end converter has a physical size of 17 cm x 17 cm. The digital control system is incorporated into a 1U rack unit, which includes the DSP, the signal conditioning board with external ADC, the external DAC, the control boards for the drivers, and the power supply module (+15 V, GND, -15 V). The prototype was developed for laboratory validation and, therefore, it can be further optimized from an industrial perspective. The laboratorial setup is shown in FIGURE 8. The experimental results were obtained with a digital oscilloscope (Yokogawa model DL708E) and with a power quality analyzer (Fluke, model 435) for the three distinct operation modes: (i) active rectifier; (ii) shunt active power filter; (iii) and active rectifier combined with the shunt active power filter operation. Although the proposed USP front-end converter was specifically designed for validation under 230 V – 50 Hz conditions, and for a maximum power of 3.6 kW, it has the potential to be adapted for other voltage and power ratings. This would require a revision or replacement of the main components to meet the specific application requirements, which includes the migration for a three-phase version. In such case, it can offer other control advantages to the main power grid, such as the possibility of operating with different values of current harmonics and power factor among the three phases. Regardless of whether a single-phase or three-phase version is used, several USP front-end converters can be combined within the same installation, enabling a modular and scalable solution. This approach can be relevant since it allows the establishment of a dynamic production of selected current harmonics and reactive power. Therefore, according to the availability of the individual USP front-end converters, each one of them can produce selected current harmonics and a specific value of reactive power, with the main goal of ensuring sinusoidal current in the power grid and unitary power factor (e.g., one USP front-end converter can produce 3rd order current harmonics and other one can produce just reactive power). This represents an appealing contribution to power quality management in smart grids. In the perspective of industrializing the USP front-end converter, it is important the consideration of other components and materials ensuring high efficiency across varying power levels (e.g., by considering wide band gap semiconductors and amorphous/nanocrystalline cores for the L coupling filters). Among other considerations, this analysis must be performed considering the requirements of the final application, the voltage range, and the adaptability to diverse operating conditions (e.g., for thermal management), in order to stablish a trade-off between various power levels and efficiency. FIGURE 8. Laboratorial setup used to obtain experimental results. A. Operation Mode: Active Rectifier FIGURE 9 shows the experimental results of the proposed USP front-end converter, obtained for the operation as an active rectifier for an active power of 1 kW. For the AC side, the AC grid voltage (vg) and iUSP are visible, and for the DC side, this figure shows both DC-link voltages (vdc1 and vdc2) and the DC current (idc) (in the experimental validation a resistive load was considered). As power quality is an important aspect for the AC power grid, during this operation mode, the proposed USP front-end converter operates with unitary power factor and a low total harmonic distortion (THD% = 3.8%) in the controlled iUSP. The THD and the spectral analysis of this current are shown in FIGURE 10. FIGURE 11 shows an experimental result during a transient-state, highlighting the moment when the USP front-end begins its operation as active rectifier. In the first stage, it operates as a passive rectifier (i.e., diode bridge) and in the second stage as an active rectifier, where it is demonstrated that the current becomes sinusoidal in the second stage. In addition, due to the operation as active rectifier, the DC-link voltage VDC2 also increases. The proposed control strategy for the USP front-end converter guaranties that the AC grid current is sinusoidal independently of the harmonic distortion of the AC grid voltage, exhibiting a relevant characteristic of the proposed USP front-end converter. As aforementioned, the current separation between the FWR and the BAR is one of the main advantages of the proposed USP front-end converter. FIGURE 12 shows the experimental measure of the iBAR and the iUSP, where the BAR current rms value is substantially reduced compared to the AC grid current. 8 VOLUME XX, 2017 FIGURE 9. Experimental results of the USP front-end converter during the operation as active rectifier: Grid voltage (vg); USP front-end converter current (iUSP); DC-link voltages (vdc1 and vdc2); DC current (idc). FIGURE 10. Experimental results of the USP front-end converter during the operation as active rectifier: THD and spectral analysis of the current (iUSP). FIGURE 11. Experimental results of the USP front-end converter during the transient-state from the operation as passive rectifier to the operation as active rectifier: Grid voltage (vg); Grid current (ig); DC-link voltages (vdc1 and vdc2). B. Operation Mode: Shunt Active Power Filter To validate the proposed USP front-end converter for the operation as a shunt active power filter, FIGURE 13 shows the current consumed by a non-linear load connected to the AC power grid. The THD and the spectral analysis of the ild are shown in FIGURE 14. FIGURE 12. Experimental results of the USP front-end converter during the operation as active rectifier: Detail of the BAR current (iBAR) and USP front-end converter current (iUSP). FIGURE 13. Experimental results of the USP front-end converter during the operation as shunt active power filter: Grid voltage (vg); current consumed by a non-linear load connected to the power grid (ild); DC-link voltage vdc1. FIGURE 14. Experimental results of the USP front-end converter during the operation as shunt active power filter: THD and spectral analysis of the current consumed by a non-linear load connected to the power grid (ild). vg iUSP idc vdc1 vdc2 vg iUSP iBAR vdc1 vdc2 vg ild vdc1