A novel multilevel bidirectional topology for on-board EV battery chargers in smart grids

energies Article A Novel Multilevel Bidirectional Topology for On-Board EV Battery Chargers in Smart Grids Rafael S. Leite, João L. Afonso and Vítor Monteiro * Industrial Electronics Department, School of Engineering, University of Minho, 4800-058 Guimarães, Portugal; [email protected] (R.S.L.); [email protected] (J.L.A.) *Correspondence: [email protected]; Tel.: +351-253-510-392 Received: 31 October 2018; Accepted: 6 December 2018; Published: 10 December 2018   Abstract: This paper proposes a novel on-board electric vehicle (EV) battery charger (EVBC) based on a bidirectional multilevel topology. The proposed topology is formed by an AC-DC converter for the grid-side interface and by a DC-DC converter for the battery-side interface. Both converters are interfaced by a split DC-link used to achieve distinct voltage levels in both converters. Characteristically, the proposed EVBC operates with sinusoidal grid-side current, unitary power factor, controlled battery-side current or voltage, and controlled DC-link voltages. The grid-side converter operates with five voltage levels, while the battery-side operates with three voltage levels. An assessment, for comparison with classical multilevel converters for EVBCs is considered along the paper, illustrating the key benefits of the proposed topology. As the proposed EVBC is controlled in bidirectional mode, targeting the EV incorporation into smart grids, the grid-to-vehicle (G2V) and vehicle-to-grid (V2G) operation modes are discussed and evaluated. Both converters of the proposed EVBC use discrete-time predictive control algorithms, which are described in the paper. An experimental validation was performed under real operating conditions, employing a developed laboratory prototype. Keywords: multilevel converter; electric vehicle; on-board battery charger; power factor correction; power quality; smart grid 1. Introduction The electric vehicle (EV) is considered as the central element to support electric mobility in smart grids, serving to help to address major energy concerns. From a global perspective, different options of EVs can be considered distinguished by the energy storage system, as battery EVs (BEVs) or fuel cell EVs (FCEVs), and by the external interface for the charging process, as plug-in EVs (PHEV) [ 1 – 3 ]. Within the scope of this paper, the final application is for EVs using batteries as the energy storage system, where the main advantage is the capacity of the energy storage system and the main drawback is the required charging time. The relevance of the EV for this purpose is carefully addressed and evaluated in [ 4 – 6 ] in terms of power electronics and control methodologies for the grid-side. As demonstrated in [ 7 , 8 ], since the EV batteries are charged from the power grid (independently of the on-board or off-board technology), power quality is an imperative feature for assuring the grid stability. In this perspective, advanced contributions for the EV controlled action in smart grids, and bearing in mind power quality issues, are presented in [ 9 ]. Additionally, the opportunity to operate in bidirectional mode, as well as to operate in the four quadrants in terms of power quality will also be decisive for contributing to establish energy management strategies in a smart grid perspective. These new contributions for the EV operation in four-quadrants and framed in smart grids, are examined in [ 10 , 11 ]. The flexible incorporation of an EV into the energy management of Energies 2018,11, 3453; doi:10.3390/en11123453 www.mdpi.com/journal/energies Energies 2018,11, 3453 2 of 21 a smart home is presented in [ 12 ], perspective an advanced communication toward to control the charging and discharging processes. Classically, on-board EV battery chargers (EVBC) are projected with twoor three-level topologies for the grid-side coupling converter [ 13 ], however, by increasing the levels, the size of the passive filters can be reduced, as demonstrated in [ 14 , 15 ] for other types of applications, different from EVBC. Nonetheless, the levels cannot be augmented indeterminately. By establishing a tradeoff between power density and required hardware and software, five-level topologies are identified as interesting solutions for different purposes. For instance, considering the grid-side converter, a five-level topology with reduced switching devices and based on the active neutral point clamped in presented in [ 16 ] for grid-tied solar photovoltaic applications; a five-level Vienna-type structure is proposed in [ 17 ] for active rectifiers; a unidirectional five-level based on the T-rectifier topology is analyzed in [ 18 ] for high-speed gen-set applications; an improved five-level topology is presented in [ 19 ] for active rectifiers or grid-tied applications; a unidirectional five-level topology is proposed in [ 20 ] for power factor correction converters, including applications of EVBC; a symmetric cascade five-level topology is proposed in [ 21 ] for grid-tied inverters; based on the previous structure, a bidirectional five-level topology is proposed in [ 19 ], allowing the operation as active converter or as grid-tied converter; a modular unidirectional five-level topology is offered in [ 22 ] for applications of renewable energy sources; a five-level topology for renewables applications is proposed in [ 23 ]; a five-level topology based on the neutral point clamped arrangement is presented in [ 24 ] for motor drivers; a novel five-level topology is proposed in [ 25 ] for unidirectional EVBC; and a five-level topology with reduced switching devices is proposed in [20] for active rectifier applications. In terms of the battery-side converter, two-level topologies are usually employed for the battery-side coupling converter. However, it should be noted that DC-DC multi-level topologies can be applied for other applications, for instance, in [ 26 ] is presented a novel multilevel boost converter for applications of photovoltaics or fuel cell generation systems, in [27] a review of DC-DC fourand three-level topologies is presented, and in [ 28 ] a two-level interleaved and intercoupled boost converter for high power applications is analyzed. The three-level topology is presented in [ 29 ], however, the application is for the voltage balancing of series connected batteries. On the other hand, in this paper, the split DC-link is used as interface for the grid-side converter, where the voltage of the capacitors is always controlled by the grid-side power converter. Moreover, the control algorithm is completely different, in order to obtain a reduced current ripple, where the proposed approach consists in using the converter controlled by current controlling only two switching devices during each operation mode. It should be noted that in [ 29 ] are controlled four switching devices, therefore, controlling only two it is possible to reduce the switching losses and the control complexity (e.g., it is not necessary to deal with any type of dead-time for the switching devices). The above-mentioned five-level topologies for the grid-side converter were validated in the scope of diverse applications, but not all were validated neither compared in the scope of an EVBC. The same occurs with the battery-side converter. Moreover, no EVBC was validated employing multilevel topologies in the grid-side and the battery-side converters. In this sense, this paper proposes an on-board EVBC multilevel topology. The internal constitution of a conventional on-board EVBC is presented in Figure 1, where is highlighted the bidirectional power flow, between the batteries and the grid, in order to accomplish with the grid-to-vehicle and vehicle-to-grid operations in smart grids. The key contributions of this paper are: (a) a novel EVBC based on a multilevel topology; (b) an analysis of the proposed EVBC in terms of operation targeting smart grids; and (c) an experimental validation using a dedicated developed on-board EVBC. The paper is outlined as follows: A description of the hardware topology of the proposed EVBC is presented in Section 2. The discrete-time predictive control algorithms used for the grid-side converter and for the battery-side converter are presented in Section 3. The foremost experimental results considering diverse operating states for smart grids are presented in Section 4. The main conclusions are discussed in Section 5. Energies 2018,11, 3453 3 of 21 Energies 2018, 11, x FOR PEER REVIEW 3 of 21 Figure 1. Internal constitution of a conventional on-board electric vehicle battery charger (EVBC). 2. EV Battery Charger: Topology Description Figure 2 shows the global electrical schematic of the proposed EVBC. This topology consists of a grid-side converter and a battery-side converter, both with a multilevel characteristic supported by a split DC-link formed by two sets of capacitors (C1 and C2). In terms of other components, the EVBC consists of twelve insulated-gate bipolar transistors (IGBTs) (used as controlled switching devices, eight for the grid-side converter and four the battery-side converter), an inductive coupling filter (L1, L2), and a LC passive filter interfacing the batteries (L3, L4 and C3). Figure 2. Topology of the proposed on-board electric vehicle battery charger (EVBC). The values of the parameters constituting the on-board EVBC, as well as the specifications of the system that was taken into account when choosing components, are given in Table 1. Power Converters Composed by two bidirectional multilevel converters: an ac-dc gridside and a dc-dc battery-side. Communication Interface of communication in order to provide the state of the bidirectional converters Control Algorithms Digital control system of the converters for the grid interface and for the battery interface. on-board EV battery charger (internal constitution) Communication Power Power grid Battery Battery Management System (BMS) dc-dc battery-side converter ac-dc grid-side converter i ev S 3 S 4 S 5 S 6 S 1 S 2 S 7 S 8 v L1 L 1 v g v cv_AC C 2 v dc 2 C 1 v dc 2 S 9 S 10 S 11 S 12 C 3 v cv_DC1 v cv_DC2 v bat i bat v L3 L 3 v L4 L 4 v L2 L 2 Figure 1. Internal constitution of a conventional on-board electric vehicle battery charger (EVBC). 2. EV Battery Charger: Topology Description Figure 2shows the global electrical schematic of the proposed EVBC. This topology consists of a grid-side converter and a battery-side converter, both with a multilevel characteristic supported by a split DC-link formed by two sets of capacitors (C 1 and C 2 ). In terms of other components, the EVBC consists of twelve insulated-gate bipolar transistors (IGBTs) (used as controlled switching devices, eight for the grid-side converter and four the battery-side converter), an inductive coupling filter (L1,L2), and a LC passive filter interfacing the batteries (L3,L4and C3). Energies 2018, 11, x FOR PEER REVIEW 3 of 21 Figure 1. Internal constitution of a conventional on-board electric vehicle battery charger (EVBC). 2. EV Battery Charger: Topology Description Figure 2 shows the global electrical schematic of the proposed EVBC. This topology consists of a grid-side converter and a battery-side converter, both with a multilevel characteristic supported by a split DC-link formed by two sets of capacitors (C1 and C2). In terms of other components, the EVBC consists of twelve insulated-gate bipolar transistors (IGBTs) (used as controlled switching devices, eight for the grid-side converter and four the battery-side converter), an inductive coupling filter (L1, L2), and a LC passive filter interfacing the batteries (L3, L4 and C3). Figure 2. Topology of the proposed on-board electric vehicle battery charger (EVBC). The values of the parameters constituting the on-board EVBC, as well as the specifications of the system that was taken into account when choosing components, are given in Table 1. Power Converters Composed by two bidirectional multilevel converters: an ac-dc gridside and a dc-dc battery-side. Communication Interface of communication in order to provide the state of the bidirectional converters Control Algorithms Digital control system of the converters for the grid interface and for the battery interface. on-board EV battery charger (internal constitution) Communication Power Power grid Battery Battery Management System (BMS) dc-dc battery-side converter ac-dc grid-side converter i ev S 3 S 4 S 5 S 6 S 1 S 2 S 7 S 8 v L1 L 1 v g v cv_AC C 2 v dc 2 C 1 v dc 2 S 9 S 10 S 11 S 12 C 3 v cv_DC1 v cv_DC2 v bat i bat v L3 L 3 v L4 L 4 v L2 L 2 Figure 2. Topology of the proposed on-board electric vehicle battery charger (EVBC). The values of the parameters constituting the on-board EVBC, as well as the specifications of the system that was taken into account when choosing components, are given in Table 1. Energies 2018,11, 3453 4 of 21 Table 1. Parameters and specifications of the on-board EVBC. Parameter Value Unit Inductive Passive Filter L1,L25 mH Inductive Passive Filter L3,L42.5 mH Capacitive Passive Filter C1,C22.24 mF Capacitive Passive Filter C320 µF Nominal Input Voltage (grid-side) vg230 ±10% V Nominal Grid Frequency f50 ±1% Hz Nominal Input Current (grid-side) iev 16 A Total Harmonic Distortion (grid current) - <5% - Total Power Factor - 1 - Nominal DC-link Voltage vdc 400 V Nominal Output Voltage (battery-side) vbat 200–400 V Nominal Output Current (battery-side) ibat 10 A Switching Frequency fsw 20 kHz Sampling Frequency fs40 kHz 2.1. Topology Description: Grid-Side Converter In the development of power electronics systems, electrical grid power quality issues are, more than ever, a major concern. Since the voltage levels produced by grid-side multilevel converters are directly proportional to the quality of the obtained grid current, multilevel converters have emerged as contributors for this concern. The circuit topology of the grid-side multilevel converter proposed for the on-board EVBC is presented in Figure 2. The proposed topology emerged as a derivation of the traditional full-bride rectifier with four devices connected to the split DC-link as the power factor correction (PFC) three-level DC-DC converter. This topology can produce five distinct voltage levels (+v dc , +v dc/2 , 0, − v dc/2 , − vdc) at the terminals of the converter (v cv_AC ). As can be seen, each IGBT is applied to a maximum voltage of +v dc . The configuration of the topology allows to switch necessarily only six of the eight IGBTs, during the full operation as active rectifier or grid-tied inverter, hence, decreasing the switching losses. When the grid side converter operates as active rectifier, Figure 3, during the positive half cycle of the power grid voltage, the IGBTs (S 1 ,S 4 ) are always switched on and the IGBTs (S 2 ,S 3 ) are always switched off. When the IGBT S 6 is switched on, the IGBT S 5 is switched to state the voltage levels 0 and +v dc/2 . On the other hand, when the IGBT S 5 is switched off, the IGBT S 6 is switched to state the voltage levels +v dc/2 and +vdc. During the negative half cycle of the power grid voltage, the IGBTs (S 2 ,S 3 ) are always switched on and the IGBTs (S 1 ,S 4 ) are always switched off. When the IGBT S 5 is switched on, the IGBT S 6 is switched to state the voltage levels 0 and − v dc/2 . Finally, when the IGBT S 6 is switched off, the IGBT S 5 is switched to state the voltage levels − v dc/2 and − v dc . When the grid side converter operates as an inverter, Figure 4, during the positive half cycle of the power grid voltage, the IGBTs (S 1 ,S 4 ) are always switched on and the IGBTs (S 2 ,S 3 ) are always switched off. When the IGBT S 8 is switched off, the IGBT S 7 is switched to state the voltage levels 0 and +v dc/2 . On the other hand, when the IGBT S 7 is switched on, the IGBT S 8 is switched to state the voltage levels +v dc/2 and +v dc . During the negative half cycle of the power grid voltage, the IGBTs (S 2 ,S 3 ) are always switched on and the IGBTs (S 1 ,S 4 ) are always switched off. When the IGBT S 7 is switched off, the IGBT S 8 is switched to state the voltage levels 0 and − v dc/2 . Finally, when the IGBT S 8 is switched on, the IGBT S7is switched to state the voltage levels −vdc/2and −vdc. Energies 2018,11, 3453 5 of 21 Energies 2018, 11, x FOR PEER REVIEW 5 of 21 Figure 3. Operation stages for the grid-side converter during the operation as active rectifier: (a) vcv_AC = 0; (b) vcv_AC = +vdc/2; (c) vcv_AC = +vdc; (d) vcv_AC = 0; (e) vcv_AC = −vdc/2; (f) vcv_AC = −vdc. Figure 4. Operation stages for the grid-side converter during the operation as grid-tied inverter: (a) vcv_AC = 0; (b) vcv_AC = +vdc/2; (c) vcv_AC = +vdc; (d) vcv_AC = 0; (e) vcv_AC = −vdc/2; (f) vcv_AC = −vdc. A predictive current control technique, with a fixed switching frequency of 20 kHz, was applied to obtain a sinusoidal EVBC current and in phase with the grid voltage (or phase opposition in vehicle-to-grid mode). Since this current control is identified as a linear current control, the modulation technique is applied individually. In this sense, Figure 5 shows the pulse-width modulation (PWM) modulation technique arrangement used for the grid-side converter. The proposed PWM modulation technique requires only one carrier signal and two reference signals to control eight IGBTs. This figure shows the adapted voltage reference (ref1, ref2), the triangular carrier signal (vcarrier), the PWM signals of the IGBTs, the voltage levels produced by the converter (vcv_AC) and the power grid voltage (vg). The pulses signal of the IGBTs (S1, S2, S3, S4) are exclusively dependent of the instantaneous value of the power grid voltage. During the operation as grid-tied inverter, the IGBTs (S7, S8) have an opposite a command signal as the IGBTs (S5, S6), respectively. In the modulation strategy, the voltages references (ref1, ref2) are adapted, based on a digital codification, from the modulating signal sM established by the Equation (1), where vcv_AC are the voltage levels produced by the converter and ma the amplitude modulation index. The voltage levels produced by the converter are obtained in the control algorithm, explained in Section 3.1. = 2 _ . (1) Figure 3. Operation stages for the grid-side converter during the operation as active rectifier: ( a )v cv_AC = 0; (b)vcv_AC = +vdc/2; (c)vcv_AC = +vdc; (d)vcv_AC = 0; (e)vcv_AC =−vdc/2; (f)vcv_AC =−vdc. Energies 2018, 11, x FOR PEER REVIEW 5 of 21 Figure 3. Operation stages for the grid-side converter during the operation as active rectifier: (a) vcv_AC = 0; (b) vcv_AC = +vdc/2; (c) vcv_AC = +vdc; (d) vcv_AC = 0; (e) vcv_AC = −vdc/2; (f) vcv_AC = −vdc. Figure 4. Operation stages for the grid-side converter during the operation as grid-tied inverter: (a) vcv_AC = 0; (b) vcv_AC = +vdc/2; (c) vcv_AC = +vdc; (d) vcv_AC = 0; (e) vcv_AC = −vdc/2; (f) vcv_AC = −vdc. A predictive current control technique, with a fixed switching frequency of 20 kHz, was applied to obtain a sinusoidal EVBC current and in phase with the grid voltage (or phase opposition in vehicle-to-grid mode). Since this current control is identified as a linear current control, the modulation technique is applied individually. In this sense, Figure 5 shows the pulse-width modulation (PWM) modulation technique arrangement used for the grid-side converter. The proposed PWM modulation technique requires only one carrier signal and two reference signals to control eight IGBTs. This figure shows the adapted voltage reference (ref1, ref2), the triangular carrier signal (vcarrier), the PWM signals of the IGBTs, the voltage levels produced by the converter (vcv_AC) and the power grid voltage (vg). The pulses signal of the IGBTs (S1, S2, S3, S4) are exclusively dependent of the instantaneous value of the power grid voltage. During the operation as grid-tied inverter, the IGBTs (S7, S8) have an opposite a command signal as the IGBTs (S5, S6), respectively. In the modulation strategy, the voltages references (ref1, ref2) are adapted, based on a digital codification, from the modulating signal sM established by the Equation (1), where vcv_AC are the voltage levels produced by the converter and ma the amplitude modulation index. The voltage levels produced by the converter are obtained in the control algorithm, explained in Section 3.1. = 2 _ . (1) Figure 4. Operation stages for the grid-side converter during the operation as grid-tied inverter: ( a )v cv_AC = 0; ( b )v cv_AC = +v dc /2; ( c )v cv_AC = +v dc ; ( d )v cv_AC = 0; ( e )v cv_AC = − v dc /2; ( f )v cv_AC = − v dc . A predictive current control technique, with a fixed switching frequency of 20 kHz, was applied to obtain a sinusoidal EVBC current and in phase with the grid voltage (or phase opposition in vehicle-to-grid mode). Since this current control is identified as a linear current control, the modulation technique is applied individually. In this sense, Figure 5shows the pulse-width modulation (PWM) modulation technique arrangement used for the grid-side converter. The proposed PWM modulation technique requires only one carrier signal and two reference signals to control eight IGBTs. This figure shows the adapted voltage reference (ref 1 ,ref 2 ), the triangular carrier signal (v carrier ), the PWM signals of the IGBTs, the voltage levels produced by the converter (v cv_AC ) and the power grid voltage (v g ). The pulses signal of the IGBTs (S 1 ,S 2 ,S 3 ,S 4 ) are exclusively dependent of the instantaneous value of the power grid voltage. During the operation as grid-tied inverter, the IGBTs (S 7 ,S 8 ) have an opposite a command signal as the IGBTs (S 5 ,S 6 ), respectively. In the modulation strategy, the voltages references (ref 1 ,ref 2 ) are adapted, based on a digital codification, from the modulating signal s M established by the Equation (1), where v cv_AC are the voltage levels produced by the converter and m a the amplitude modulation index. The voltage levels produced by the converter are obtained in the control algorithm, explained in Section 3.1. sM=2vcv_ACma. (1) Energies 2018,11, 3453 6 of 21 Energies 2018, 11, x FOR PEER REVIEW 6 of 21 Figure 5. Modified pulse-width modulation (PWM) strategy for the grid-side converter during the operation as active rectifier or as grid-tied inverter. 2.2. Topology Description: Battery-Side Converter The circuit topology of the battery-side converter implemented in the on-board EVBC is presented in Figure 2. The topology consists in four IGBTs connected to the split DC-link and a passive LC filter interfacing the batteries. Furthermore, this topology can produce three distinct voltage levels (+vdc, +vdc/2, 0) at the terminals of the converter (vcv_DC). When the converter operates as a buck-type converter, the energy power flows of the DC-link to the batteries during the charging process. In this mode, the IGBTs (S9, S10) and the anti-parallel diodes of the IGBTs (S11, S12) are used. When only one of the IGBTs (S9, S10) is switched on, the inductors (L3, L4) and the batteries stores energy from the split DC-link (C1 or C2). When both IGBTs (S9, S10) are switched on, the inductors (L3, L4) and the batteries store energy from the DC-link (C1, C2). When both IGBTs (S9, S10) are switched off, the stored energy in the inductors (L3, L4) is released to the batteries. On the other hand, when the converter operates as boost-type converter, the energy power flows of the batteries to the DC-link, during the discharging process. During this mode, the IGBTs (S11, S12) and the anti-parallel diodes of the IGBTs (S9, S10) are used. When both IGBTs (S11, S12) are switched on, the inductors (L3, L4) stores energy from the batteries. When one of the IGBTs (S11, S12) is switched on, the split DC-link (C1 or C2) stores energy from the batteries and the inductors (L3, L4). Finally, when both IGBTs (S11, S12) are switched off, the DC-link (C1, C2) stores energy from the batteries and the inductors (L3, L4). Figures 6 and 7 show the operation stages for the battery-side converter during the operation as buck-type and boost-type, respectively. 0.00 0.01 0.02 0.03 0.04 0.05 Time (s) +v dc +v dc/2 -v dc/2 -v dc 0 v g v carrier S 6 S 5 ref 2 ref 1 v cv_AC S 1,4 S 2,3 S 8 S 7 Figure 5. Modified pulse-width modulation (PWM) strategy for the grid-side converter during the operation as active rectifier or as grid-tied inverter. 2.2. Topology Description: Battery-Side Converter The circuit topology of the battery-side converter implemented in the on-board EVBC is presented in Figure 2. The topology consists in four IGBTs connected to the split DC-link and a passive LC filter interfacing the batteries. Furthermore, this topology can produce three distinct voltage levels (+vdc, +vdc/2, 0) at the terminals of the converter (vcv_DC). When the converter operates as a buck-type converter, the energy power flows of the DC-link to the batteries during the charging process. In this mode, the IGBTs (S 9 ,S 10 ) and the anti-parallel diodes of the IGBTs (S 11 ,S 12 ) are used. When only one of the IGBTs (S 9 ,S 10 ) is switched on, the inductors (L 3 ,L 4 ) and the batteries stores energy from the split DC-link (C 1 or C 2 ). When both IGBTs (S 9 ,S 10 ) are switched on, the inductors (L 3 ,L 4 ) and the batteries store energy from the DC-link (C 1 ,C 2 ). When both IGBTs (S 9 ,S 10 ) are switched off, the stored energy in the inductors (L 3 ,L 4 ) is released to the batteries. On the other hand, when the converter operates as boost-type converter, the energy power flows of the batteries to the DC-link, during the discharging process. During this mode, the IGBTs (S 11 ,S 12 ) and the anti-parallel diodes of the IGBTs (S 9 ,S 10 ) are used. When both IGBTs (S 11 ,S 12 ) are switched on, the inductors (L 3 ,L 4 ) stores energy from the batteries. When one of the IGBTs (S 11 ,S 12 ) is switched on, the split DC-link (C 1 or C2) stores energy from the batteries and the inductors (L3,L4). Finally, when both IGBTs (S11,S12) are switched off, the DC-link (C 1 ,C 2 ) stores energy from the batteries and the inductors (L 3 ,L 4 ). Figures 6 and 7show the operation stages for the battery-side converter during the operation as buck-type and boost-type, respectively. Energies 2018,11, 3453 7 of 21 Energies 2018, 11, x FOR PEER REVIEW 7 of 21 Figure 6. Operation stages for the battery-side converter during the operation as a buck-type converter: (a) The stored energy in L3, L4 is released to the batteries; (b) L3, L4 and the batteries stores energy from C1; (c) L3, L4 and the batteries stores energy from C2; (d) L3, L4 and the batteries store energy from C1, C2. Figure 7. Operation stages for the battery-side converter during the operation as a boost-type converter: (a) L3, L4 stores energy from the batteries; (b) C2 stores energy from the batteries and L3, L4; (c) C1 stores energy from the batteries and L3, L4; (d) C1, C2 stores energy from the batteries and L3, L4. Similar to the algorithm applied in the grid-side converter, in the battery-side converter was also applied a predictive current control technique and a modulation technique with a fixed switching frequency of 20 kHz, to control the current and voltage in the batteries during the charging or discharging processes. In the modulation technique, two 180° phase-shifted carriers were used, in order to reduce the ripple of the EV battery current and, hence, the frequency EV battery current is held twice of the switching frequency. (b)(a) (d)(c) C2vdc 2 C1vdc 2 S9 S10 S11 S12 C3 vcv_DC1 vcv_DC2 vbat ibat C2vdc 2 C1vdc 2 S9 S10 S11 S12 C3 vcv_DC1 vcv_DC2 vbat ibat C2vdc 2 C1vdc 2 S9 S10 S11 S12 C3 vcv_DC1 vcv_DC2 vbat ibat C2vdc 2 C1vdc 2 S9 S10 S11 S12 C3 vcv_DC1 vcv_DC2 vbat ibat v L3 L3 vL4 L4 v L3 L3 vL4 L4 vL3 L3 vL4 L4 vL3 L3 vL4 L4 (b)(a) (d)(c) C 2 v dc 2 C 1 v dc 2 S 9 S 10 S 11 S 12 C 3 v cv_DC1 v cv_DC2 v bat i bat C 2 v dc 2 C 1 v dc 2 S 9 S 10 S 11 S 12 C 3 v cv_DC1 v cv_DC2 v bat i bat C 2 v dc 2 C 1 v dc 2 S 9 S 10 S 11 S 12 C 3 v cv_DC1 v cv_DC2 v bat i bat C 2 v dc 2 C 1 v dc 2 S 9 S 10 S 11 S 12 C 3 v cv_DC1 v cv_DC2 v bat i bat v L3 L 3 v L4 L 4 v L3 L 3 v L4 L 4 v L3 L 3 v L4 L 4 v L3 L 3 v L4 L 4 Figure 6. Operation stages for the battery-side converter during the operation as a buck-type converter: ( a ) The stored energy in L 3 ,L 4 is released to the batteries; ( b )L 3 ,L 4 and the batteries stores energy from C 1 ; ( c )L 3 ,L 4 and the batteries stores energy from C 2 ; ( d )L 3 ,L 4 and the batteries store energy from C 1 ,C 2 . Energies 2018, 11, x FOR PEER REVIEW 7 of 21 Figure 6. Operation stages for the battery-side converter during the operation as a buck-type converter: (a) The stored energy in L3, L4 is released to the batteries; (b) L3, L4 and the batteries stores energy from C1; (c) L3, L4 and the batteries stores energy from C2; (d) L3, L4 and the batteries store energy from C1, C2. Figure 7. Operation stages for the battery-side converter during the operation as a boost-type converter: (a) L3, L4 stores energy from the batteries; (b) C2 stores energy from the batteries and L3, L4; (c) C1 stores energy from the batteries and L3, L4; (d) C1, C2 stores energy from the batteries and L3, L4. Similar to the algorithm applied in the grid-side converter, in the battery-side converter was also applied a predictive current control technique and a modulation technique with a fixed switching frequency of 20 kHz, to control the current and voltage in the batteries during the charging or discharging processes. In the modulation technique, two 180° phase-shifted carriers were used, in order to reduce the ripple of the EV battery current and, hence, the frequency EV battery current is held twice of the switching frequency. (b)(a) (d)(c) C2vdc 2 C1vdc 2 S9 S10 S11 S12 C3 vcv_DC1 vcv_DC2 vbat ibat C2vdc 2 C1vdc 2 S9 S10 S11 S12 C3 vcv_DC1 vcv_DC2 vbat ibat C2vdc 2 C1vdc 2 S9 S10 S11 S12 C3 vcv_DC1 vcv_DC2 vbat ibat C2vdc 2 C1vdc 2 S9 S10 S11 S12 C3 vcv_DC1 vcv_DC2 vbat ibat v L3 L3 vL4 L4 v L3 L3 vL4 L4 vL3 L3 vL4 L4 vL3 L3 vL4 L4 (b)(a) (d)(c) C 2 v dc 2 C 1 v dc 2 S 9 S 10 S 11 S 12 C 3 v cv_DC1 v cv_DC2 v bat i bat C 2 v dc 2 C 1 v dc 2 S 9 S 10 S 11 S 12 C 3 v cv_DC1 v cv_DC2 v bat i bat C 2 v dc 2 C 1 v dc 2 S 9 S 10 S 11 S 12 C 3 v cv_DC1 v cv_DC2 v bat i bat C 2 v dc 2 C 1 v dc 2 S 9 S 10 S 11 S 12 C 3 v cv_DC1 v cv_DC2 v bat i bat v L3 L 3 v L4 L 4 v L3 L 3 v L4 L 4 v L3 L 3 v L4 L 4 v L3 L 3 v L4 L 4 Figure 7. Operation stages for the battery-side converter during the operation as a boost-type converter: ( a )L 3 ,L 4 stores energy from the batteries; ( b )C 2 stores energy from the batteries and L 3 ,L 4 ; ( c )C 1 stores energy from the batteries and L3,L4; (d)C1,C2stores energy from the batteries and L3,L4. Similar to the algorithm applied in the grid-side converter, in the battery-side converter was also applied a predictive current control technique and a modulation technique with a fixed switching frequency of 20 kHz, to control the current and voltage in the batteries during the charging or discharging processes. In the modulation technique, two 180 ◦ phase-shifted carriers were used, in order to reduce the ripple of the EV battery current and, hence, the frequency EV battery current is held twice of the switching frequency. Energies 2018,11, 3453 8 of 21 3. EV Battery Charger: Control Algorithms This section presents the specifications and the methodology used for the control algorithms implementation, both for the grid-side and the battery-side converters. The control algorithm was designed for a digital platform, based on a Texas Instruments digital signal processor (DSP) F28335 (Texas Instruments, Inc., Dallas, TX, USA) and considering a sampling frequency of 40 kHz, obtained with a timer interruption. 3.1. Control Algorithm: Grid-Side Converter Based on the voltages shown in Figure 2, Equation (2) can be established, where v g represents the instantaneous value of the grid voltage, v L1 and v L2 the instantaneous value of the inductance voltage, and v cv_AC is the instantaneous value (i.e., the voltage produced during each sampling period of the DSP) of the voltage produced by the converter: vg=vcv_AC +vL1+vL2. (2) It should be noted that, as represented in Figure 2, it was used a mutual coupling inductance. Therefore, replacing the inductance voltage by its intrinsic equation, and rewriting the equation as a function of the voltage produced by the converter, it is obtained: vcv_AC =vg−L1 diev dt −L2 diev dt . (3) Applying the progressive Euler method, illustrated in Equation (4), the derivative component of the current can be approximated by considering a very low ∆ tin order to obtain a good prediction of the system behavior: diev(t) dt =iev(t+∆t)−iev(t) ∆t. (4) Applying the Equation (4) in the Equation (3), and assuming a sampling frequency of fs =1/T s , results in the digital control Equation (5), where the term krepresents the current sample and [k+ 1] represents the next sample: vcv_AC[k]=vg[k]−(L1+L2)iev[k+1]−iev[k] Ts. (5) Since the law of predictive control consists of a closed-loop control and, if the reference current at time [k+ 1] is to be equal to the current produced by the converter at time [k], the equation that translates the current control implemented can be defined by: vcv_AC[k]=vg[k]−(L1+L2)iev∗[k]−iev[k] Ts. (6) Since, the EVBC is proposed to operate with a sinusoidal current and unitary power factor in the grid-side converter, the instantaneous value of the power grid voltage is directly proportional to the EVBC current. However, aiming to prevent the inclusion of the harmonic distortion of the grid voltage into the current, it is used a phase-locked loop (PLL). Thus, instead of the grid voltage, it is used the output signal from the PLL, resulting in: iev =Gevvpll, (7) where v pll is in phase with the power grid voltage and G ev represents the equivalent conductance of the EVBC from the grid-side point of view, which can be defined according to the mean value of active power (Pev) and the rms value of the power grid voltage (Vg): Energies 2018,11, 3453 9 of 21 Gev =PevVG −2. (8) Applying the Equation (8) in the Equation (7), the reference of the EVBC current is obtained according to: iev ∗=PevVG −2vpll. (9) The active power of the EVBC can be divided in two parts, namely the power to regulate the DC-link voltage and the power to regulate the batteries. Furthermore, because of the split DC-link, two proportional-integral (PI) are used to regulate the DC-link voltage independently in both capacitors (p dc1 ,p dc2 ). Therefore, during the G2V operation mode, the reference of the EVBC current can be defined as: iev ∗=(pdc1+pdc1+pbat)vpllVG −2. (10) On the other hand, during the V2G operation mode, the reference of the EVBC current is established according to Equation (11), where i bat *represent the reference of current to discharge the batteries. iev ∗=(pdc1+pdc1+ibat ∗vbat)vpllVG −2. (11) 3.2. Control Algorithm: Battery-Side Converter During the process of charging the batteries, the battery-side converter operates as buck converter, controlling the charging current or the charging voltage for the batteries. In this way, based on the representations of the currents and voltages between the DC-link and the batteries (cf. Figure 2), it is possible to establish the Equation (12), where v cv_DC represents the voltage produced by the converter (i.e., the sum between v cv_DC1 and v cv_DC2 ), v L3 and v L4 represent the voltages in the inductances (L3and L4), respectively, and vbat represents the voltage in the batteries: vcv_DC =vbat +vL3+vL4. (12) Since the current in the inductance L 3 is the same as that in the inductance L 4 , the Equation (12) can be rewritten, replacing the voltage in the inductance by its intrinsic equation: vcv_DC =vbat +(L3+L4)diL3,L4 dt . (13) Applying the progressive Euler method, the Equation (13) can be established in discrete time as: vcv_DC[k]=vbat[k]+(L3+L4)(iL3,L4[k+1]−iL3,L4[k])Ts −1. (14) Since it is desired that the reference current at time [k+ 1] should be equal to the current produced by the converter at time [k], it is obtained: vcv_DC[k]=vbat[k]+(L3+L4)(iL3,L4 ∗[k]−iL3,L4[k])Ts −1. (15) When the same converter operates as buck converter, but controlling the charging voltage of the batteries, the reference voltage is established as: vcv_DC[k]=vbat ∗[k]−(L3+L4)(iL3,L4[k]−iL3,L4[k−1])Ts −1. (16) The aforementioned equations were defined for the battery-side converter operating as buck converter, i.e., charging the batteries from the grid (G2V mode). On the other hand, a set of equations should also be defined for the battery-side converter operating as boost converter, i.e., discharging the batteries to the grid (V2G mode). Based on the representations of the currents and voltages in Figure 2, Energies 2018,11, 3453 16 of 21 Energies 2018, 11, x FOR PEER REVIEW 15 of 21 The experimental result shown in Figure 17 was obtained in x-y mode in order to identify the DC-link voltage regulation and to clearly identify the five distinct voltage levels (+vdc, +vdc/2, 0, −vdc/2, −vdc) produced by the grid-side converter. Thus, the DC-link voltage ripple (∆vdc1, ∆vdc2) and the voltage levels (vcv_AC), used in the y-axis, are a function of the grid voltage (vg), used in the x-axis. In order to keep the DC-link regulated and balanced, during the positive half-cycle of the grid voltage, the voltage of the capacitor C1 is regulated, and during the negative half-cycle of the grid voltage, the voltage of the capacitor C2 is regulated. Figure 17. Experimental results in x-y mode showing the DC-link voltage ripple (∆vdc1: 5 V/div, ∆vdc2: 5 V/div) (y-axis) and the voltage levels produced by the grid-side converter (vcv_AC: 50 V/div) (y-axis), both in function of the power grid voltage (vg: 100 V/div) (x-axis). Using the Fluke power quality analyzer, in Figure 18a,b shows the harmonic spectrum of the power grid voltage and the EVBC current, measured total harmonic distortion (THD%) of 3.5% and 2.8%, respectively. In power electronics systems, the thermal characteristic is a factor that directly affects the performance of it. So, to analyse the thermal conditions of the EVBC, the experimental results of temperature measurements during the G2V operation mode are presented in Figure 19. Figure 19a shows the overall thermal distribution of the implemented EVBC, Figure 19b shows the measured temperature of IGBT S9 (switched at a fixed frequency of 20 kHz), where was registered a temperature value of 47.8 °C, and Figure 19c shows the measured temperature of the IGBT S11 (only the antiparallel diode is used in this context), where was registered a temperature value of 36.8 °C. Figure 18. Experimental results in G2V mode of the total harmonic distortion and spectral analysis: (a) power grid voltage (vg); and (b) EVBC current (iev). Figure 18. Experimental results in G2V mode of the total harmonic distortion and spectral analysis: (a) power grid voltage (vg); and (b) EVBC current (iev). Energies 2018, 11, x FOR PEER REVIEW 16 of 21 Figure 19. Experimental results of the temperature measurements during G2V operation mode: (a) overall thermal distribution of the developed EVBC prototype; (b) temperature at the IGBT S9; and (c) temperature at the IGBT S11. 4.2.2. Experimental Results: Vehicle-To-Grid (V2G) Operation The developed EVBC was also validated during the V2G operation mode. Furthermore, once the IGBTs S7 and S8 of the grid-side converter has a fixed switching frequency of 20 kHz, to validate the modulation technique applied in these IGBTs, Figure 20 presents the reference signal adapted and the gate-emitter voltage of the respective IGBT. This voltage is a resulting signal of the comparison between the carrier signal and the reference signal. In this result, the reference signal was acquired using an external DAC. Figure 20. Experimental results showing the modulation technique applied to the IGBTs S7 and S8, namely, the reference signals adopted for the modulation (refs7: 1 V/div, refs8: 1 V/div) and the voltage gate-emitter of the respective IGBT (vge_s7: 5 V/div and vge_s8: 5 V/div). Similar to the G2V operation mode, Figure 21 shows the switching states of the grid-side converter according to Table 2 during the V2G operation mode. In this result, the IGBTs S1, S2, S3, and S4, as well as the IGBTs S7 and S8, have a fixed switching frequency of 50 Hz and 20 kHz, respectively. In this operation mode, the IGBTs (S5, S6) are always switched off, reason why they are not represented in this figure. Figure 22 shows the V2G operation mode during a steady operation of the EVBC grid-side current (iev), the grid voltage (vg), the voltage levels assumed by the grid-side converter (vcv_AC), and the DC-link voltage of both capacitors (vdc1, vdc2). The EVBC grid-side current is sinusoidal, but in phase opposition with the grid voltage, meaning that the power follows from the batteries to the Figure 19. Experimental results of the temperature measurements during G2V operation mode: ( a ) overall thermal distribution of the developed EVBC prototype; ( b ) temperature at the IGBT S 9 ; and (c) temperature at the IGBT S11. 4.2.2. Experimental Results: Vehicle-To-Grid (V2G) Operation The developed EVBC was also validated during the V2G operation mode. Furthermore, once the IGBTs S 7 and S 8 of the grid-side converter has a fixed switching frequency of 20 kHz, to validate the modulation technique applied in these IGBTs, Figure 20 presents the reference signal adapted and the gate-emitter voltage of the respective IGBT. This voltage is a resulting signal of the comparison between the carrier signal and the reference signal. In this result, the reference signal was acquired using an external DAC. Similar to the G2V operation mode, Figure 21 shows the switching states of the grid-side converter according to Figure 3during the V2G operation mode. In this result, the IGBTs S 1 ,S 2 ,S 3 , and S 4 , as well as the IGBTs S 7 and S 8 , have a fixed switching frequency of 50 Hz and 20 kHz, respectively. In this operation mode, the IGBTs (S 5 ,S 6 ) are always switched off, reason why they are not represented in this figure. Figure 22 shows the V2G operation mode during a steady operation of the EVBC grid-side current (i ev ), the grid voltage (v g ), the voltage levels assumed by the grid-side converter (v cv_AC ), and the DC-link voltage of both capacitors (v dc1 ,v dc2 ). The EVBC grid-side current is sinusoidal, but in phase opposition with the grid voltage, meaning that the power follows from the batteries to the grid. Furthermore, the five distinct voltage levels (+v dc , +v dc/2 , 0, − v dc/2 , − vdc) produced by the grid-side converter can be seen in this figure. Energies 2018,11, 3453 17 of 21 Energies 2018, 11, x FOR PEER REVIEW 16 of 21 Figure 19. Experimental results of the temperature measurements during G2V operation mode: (a) overall thermal distribution of the developed EVBC prototype; (b) temperature at the IGBT S9; and (c) temperature at the IGBT S11. 4.2.2. Experimental Results: Vehicle-To-Grid (V2G) Operation The developed EVBC was also validated during the V2G operation mode. Furthermore, once the IGBTs S7 and S8 of the grid-side converter has a fixed switching frequency of 20 kHz, to validate the modulation technique applied in these IGBTs, Figure 20 presents the reference signal adapted and the gate-emitter voltage of the respective IGBT. This voltage is a resulting signal of the comparison between the carrier signal and the reference signal. In this result, the reference signal was acquired using an external DAC. Figure 20. Experimental results showing the modulation technique applied to the IGBTs S7 and S8, namely, the reference signals adopted for the modulation (refs7: 1 V/div, refs8: 1 V/div) and the voltage gate-emitter of the respective IGBT (vge_s7: 5 V/div and vge_s8: 5 V/div). Similar to the G2V operation mode, Figure 21 shows the switching states of the grid-side converter according to Table 2 during the V2G operation mode. In this result, the IGBTs S1, S2, S3, and S4, as well as the IGBTs S7 and S8, have a fixed switching frequency of 50 Hz and 20 kHz, respectively. In this operation mode, the IGBTs (S5, S6) are always switched off, reason why they are not represented in this figure. Figure 22 shows the V2G operation mode during a steady operation of the EVBC grid-side current (iev), the grid voltage (vg), the voltage levels assumed by the grid-side converter (vcv_AC), and the DC-link voltage of both capacitors (vdc1, vdc2). The EVBC grid-side current is sinusoidal, but in phase opposition with the grid voltage, meaning that the power follows from the batteries to the Figure 20. Experimental results showing the modulation technique applied to the IGBTs S 7 and S 8 , namely, the reference signals adopted for the modulation (ref s7 : 1 V/div, ref s8 : 1 V/div) and the voltage gate-emitter of the respective IGBT (vge_s7: 5 V/div and vge_s8: 5 V/div). Energies 2018, 11, x FOR PEER REVIEW 17 of 21 grid. Furthermore, the five distinct voltage levels (+vdc, +vdc/2, 0, −vdc/2, −vdc) produced by the grid-side converter can be seen in this figure. Figure 21. Experimental results during vehicle-to-grid (V2G) operation mode: Gate-emitter voltage of the grid-side IGBTs (S1, S2, S3, S4, S7, S8: 5 V/div), and output digital signal of the PLL (vPLL: 150 V/div). Figure 22. Experimental results in V2G operation mode showing the EVBC current (iev: 5 A/div), the power grid voltage (vg: 50 V/div), the voltage levels produced by the grid-side converter (vcv_AC: 100 V/div) and the DC-link voltage of both capacitors (vdc1: 20 V/div, vdc2: 20 V/div). For further details, Figure 23 presents an experimental result, during a time interval of 50 ms, of the EVBC grid-side current (iev), the grid voltage (vg), and the DC-link voltage ripple in both capacitors (∆vdc1, ∆vdc2). With the detail of the current zero-crossing, it is possible to state that during these results the EVBC operates with unitary power factor. As aforementioned, during the positive half-cycle of the power grid voltage, the voltage of the capacitor C1 is regulated and during the negative half-cycle of the power gird voltage, the voltage of the capacitor C2 is regulated, which are controlled by the grid-side converter. Moreover, as it can be seen in this figure, the DC-link voltage has a voltage ripple of 3%. Figure 21. Experimental results during vehicle-to-grid (V2G) operation mode: Gate-emitter voltage of the grid-side IGBTs (S 1 ,S 2 ,S 3 ,S 4 ,S 7 ,S 8 : 5 V/div), and output digital signal of the PLL (v PLL : 150 V/div). For further details, Figure 23 presents an experimental result, during a time interval of 50 ms, of the EVBC grid-side current (i ev ), the grid voltage (v g ), and the DC-link voltage ripple in both capacitors ( ∆ v dc1 , ∆ v dc2 ). With the detail of the current zero-crossing, it is possible to state that during these results the EVBC operates with unitary power factor. As aforementioned, during the positive half-cycle of the power grid voltage, the voltage of the capacitor C 1 is regulated and during the negative half-cycle of the power gird voltage, the voltage of the capacitor C 2 is regulated, which are controlled by the grid-side converter. Moreover, as it can be seen in this figure, the DC-link voltage has a voltage ripple of 3%. Energies 2018,11, 3453 18 of 21 Energies 2018, 11, x FOR PEER REVIEW 17 of 21 grid. Furthermore, the five distinct voltage levels (+vdc, +vdc/2, 0, −vdc/2, −vdc) produced by the grid-side converter can be seen in this figure. Figure 21. Experimental results during vehicle-to-grid (V2G) operation mode: Gate-emitter voltage of the grid-side IGBTs (S1, S2, S3, S4, S7, S8: 5 V/div), and output digital signal of the PLL (vPLL: 150 V/div). Figure 22. Experimental results in V2G operation mode showing the EVBC current (iev: 5 A/div), the power grid voltage (vg: 50 V/div), the voltage levels produced by the grid-side converter (vcv_AC: 100 V/div) and the DC-link voltage of both capacitors (vdc1: 20 V/div, vdc2: 20 V/div). For further details, Figure 23 presents an experimental result, during a time interval of 50 ms, of the EVBC grid-side current (iev), the grid voltage (vg), and the DC-link voltage ripple in both capacitors (∆vdc1, ∆vdc2). With the detail of the current zero-crossing, it is possible to state that during these results the EVBC operates with unitary power factor. As aforementioned, during the positive half-cycle of the power grid voltage, the voltage of the capacitor C1 is regulated and during the negative half-cycle of the power gird voltage, the voltage of the capacitor C2 is regulated, which are controlled by the grid-side converter. Moreover, as it can be seen in this figure, the DC-link voltage has a voltage ripple of 3%. Figure 22. Experimental results in V2G operation mode showing the EVBC current (i ev : 5 A/div), the power grid voltage (v g : 50 V/div), the voltage levels produced by the grid-side converter (v cv_AC : 100 V/div) and the DC-link voltage of both capacitors (vdc1: 20 V/div, vdc2: 20 V/div). Energies 2018, 11, x FOR PEER REVIEW 18 of 21 Figure 23. Experimental results in V2G operation mode showing the EVBC current (iev: 5 A/div), the grid voltage (vg: 50 V/div), a detail of zero crossing between the current and the voltage (iev, vg), and the DC-link voltages ripple in both capacitors (∆vdc1: 2 V/div, ∆vdc2: 2 V/div). Regarding the battery-side converter, the same modulation technique implemented in the G2V operation mode was used, namely the application of two 180° phase-shifted carrier signal. This strategy was adopted to reduce the ripple amplitude of the batteries current. In this sense, Figure 24 shows the current ripple in the inductor L3 according to the gate-emitter voltages, vge_S11 and vge_S12, of the IGBTs S11, S12. It is important to note that during this operation mode, the IGBTs S9 and S10 are always off, reason why they are not shown in the figure. As it can be seen, the measured current ripple in the inductor L3 was 0.13 A for a frequency of 40 kHz, which is twice of the switching frequency. According to this result, when the IGBT S9 or the IGBT S10 is on, the inductor stores energy and during the state transition of one the IGBTs, the inductor releases this energy. Using the power quality analyzer, in Figure 25a,b, the harmonic spectrum of the power grid voltage and the EVBC current is shown, measured THD% of 4.2% and 3.5%, respectively. These figures were obtained employing a Fluke 435 power quality analyzer, and the high value of THD in the power grid voltage is caused by distorted voltage drop in the line impedance, which is produced by distorted current consumed by several nonlinear loads connected to the electrical installation. Figure 24. Experimental results showing the current in the inductor L3 (iL3: 0.1 A/div) and the gate-emitter voltage of the IGBTs S11 and S12 (vge_s11: 5 V/div and vge_12: 5 V/div) during a time interval of 100 µs. Figure 23. Experimental results in V2G operation mode showing the EVBC current (i ev : 5 A/div), the grid voltage (v g : 50 V/div), a detail of zero crossing between the current and the voltage (i ev ,v g ), and the DC-link voltages ripple in both capacitors (∆vdc1: 2 V/div, ∆vdc2: 2 V/div). Regarding the battery-side converter, the same modulation technique implemented in the G2V operation mode was used, namely the application of two 180 ◦ phase-shifted carrier signal. This strategy was adopted to reduce the ripple amplitude of the batteries current. In this sense, Figure 24 shows the current ripple in the inductor L 3 according to the gate-emitter voltages, v ge_S11 and v ge_S12 , of the IGBTs S 11 ,S 12 . It is important to note that during this operation mode, the IGBTs S 9 and S 10 are always off, reason why they are not shown in the figure. As it can be seen, the measured current ripple in the inductor L 3 was 0.13 A for a frequency of 40 kHz, which is twice of the switching frequency. According to this result, when the IGBT S 9 or the IGBT S 10 is on, the inductor stores energy and during the state transition of one the IGBTs, the inductor releases this energy. Using the power quality analyzer, in Figure 25a,b, the harmonic spectrum of the power grid voltage and the EVBC current is shown, measured THD% of 4.2% and 3.5%, respectively. These figures were obtained employing a Fluke 435 power quality analyzer, and the high value of THD in the power grid voltage is caused by distorted voltage drop in the line impedance, which is produced by distorted current consumed by several nonlinear loads connected to the electrical installation. Energies 2018,11, 3453 19 of 21 Energies 2018, 11, x FOR PEER REVIEW 18 of 21 Figure 23. Experimental results in V2G operation mode showing the EVBC current (iev: 5 A/div), the grid voltage (vg: 50 V/div), a detail of zero crossing between the current and the voltage (iev, vg), and the DC-link voltages ripple in both capacitors (∆vdc1: 2 V/div, ∆vdc2: 2 V/div). Regarding the battery-side converter, the same modulation technique implemented in the G2V operation mode was used, namely the application of two 180° phase-shifted carrier signal. This strategy was adopted to reduce the ripple amplitude of the batteries current. In this sense, Figure 24 shows the current ripple in the inductor L3 according to the gate-emitter voltages, vge_S11 and vge_S12, of the IGBTs S11, S12. It is important to note that during this operation mode, the IGBTs S9 and S10 are always off, reason why they are not shown in the figure. As it can be seen, the measured current ripple in the inductor L3 was 0.13 A for a frequency of 40 kHz, which is twice of the switching frequency. According to this result, when the IGBT S9 or the IGBT S10 is on, the inductor stores energy and during the state transition of one the IGBTs, the inductor releases this energy. Using the power quality analyzer, in Figure 25a,b, the harmonic spectrum of the power grid voltage and the EVBC current is shown, measured THD% of 4.2% and 3.5%, respectively. These figures were obtained employing a Fluke 435 power quality analyzer, and the high value of THD in the power grid voltage is caused by distorted voltage drop in the line impedance, which is produced by distorted current consumed by several nonlinear loads connected to the electrical installation. Figure 24. Experimental results showing the current in the inductor L3 (iL3: 0.1 A/div) and the gate-emitter voltage of the IGBTs S11 and S12 (vge_s11: 5 V/div and vge_12: 5 V/div) during a time interval of 100 µs. Figure 24. Experimental results showing the current in the inductor L 3 (i L3 : 0.1 A/div) and the gate-emitter voltage of the IGBTs S 11 and S 12 (v ge_s11 : 5 V/div and v ge_12 : 5 V/div) during a time interval of 100 µs. Energies 2018, 11, x FOR PEER REVIEW 19 of 21 Figure 25. Experimental results in V2G operation mode of the total harmonic distortion and spectral analysis: (a) Power grid voltage (vg); and (b) EVBC current (iev). 5. Conclusions A novel on-board bidirectional EV battery charger (EVBC) was presented. It is constituted by a grid-side converter capable to operate with five voltage levels, and by a battery-side converter capable to operate with three voltage levels. The distinct voltage levels for both converters are obtained using a split DC-link. In order to ensure power quality features, the proposed EVBC operates with grid-side current controlled to improve power factor, and to preserve the battery lifetime the EVBC operates with battery-side controlled current or voltage. Throughout the paper is described the proposed hardware topology, the discrete-time predictive control algorithms used for the grid-side converter and for the battery-side converter, the developed full-scale laboratorial prototype of the EVBC, and the foremost experimental results considering operating modes for smart grids. The obtained results allow validating the key contributions of the paper, mainly, in terms of the bidirectional operation of the novel EVBC based on a multilevel topology. As the EVBC is controlled targeting the EV incorporation into smart grids, the grid-to-vehicle (G2V) and vehicle-to-grid (V2G) operation modes are discussed and evaluated. Author Contributions: All authors contributed equally to the conceptualization and writing of the paper. Funding: This work has been supported by COMPETE: POCI-01-0145-FEDER-007043 and FCT—Fundação para a Ciência e Tecnologia within the Project Scope: UID/CEC/00319/2013. This work is financed by the ERDF—European Regional Development Fund through the Operational Programme for Competitiveness and Internationalisation—COMPETE 2020 Programme, and by National Funds through the Portuguese funding agency, FCT—Fundação para a Ciência e a Tecnologia, within project SAICTPAC/0004/2015—POCI—01-0145-FEDER-016434. This work is part of the FCT project 0302836 NORTE-01-0145-FEDER-030283. Conflicts of Interest: The authors declare no conflict of interest. References 1. Idaho National Laboratory. Charging and Driving Behavior of Nissan Leaf Drivers in the EV Project with Access to Workplace Charging; EV Project; Idaho National Laboratory: Idaho Falls, ID, USA, 2014; pp. 1–4. 2. Matthé, R.; Eberle, U. The Voltec System—Energy Storage and Electric Propulsion. In Lithium-Ion Batteries—Advances and Applications; Elsevier: Amsterdam, The Netherlands, 2014; pp. 151–176. 3. Robledo, C.B.; Oldenbroek, V.; Abbruzzese, F.; van Wijk, A.J.M. Integrating a hydrogen fuel cell electric vehicle with vehicle-to-grid technology, photovoltaic power and a residential building. Appl. Energy 2018, 215, 615–629. 4. Boulanger, A.G.; Chu, A.C.; Maxx, S.; Waltz, D.L. Vehicle Electrification: Status and Issues. Proc. IEEE 2011, 99, 1116–1138. 5. Rajashekara, K. Present Status and Future Trends in Electric Vehicle Propulsion Technologies. IEEE J. Emerg. Sel. Top. Power Electron. 2013, 1, 3–10. 6. Milberg, J.; Schlenker, A. Plug into the Future. IEEE Power Energy Mag. 2011, 9, 56–65. Figure 25. Experimental results in V2G operation mode of the total harmonic distortion and spectral analysis: (a) Power grid voltage (vg); and (b) EVBC current (iev). 5. Conclusions A novel on-board bidirectional EV battery charger (EVBC) was presented. It is constituted by a grid-side converter capable to operate with five voltage levels, and by a battery-side converter capable to operate with three voltage levels. The distinct voltage levels for both converters are obtained using a split DC-link. In order to ensure power quality features, the proposed EVBC operates with grid-side current controlled to improve power factor, and to preserve the battery lifetime the EVBC operates with battery-side controlled current or voltage. Throughout the paper is described the proposed hardware topology, the discrete-time predictive control algorithms used for the grid-side converter and for the battery-side converter, the developed full-scale laboratorial prototype of the EVBC, and the foremost experimental results considering operating modes for smart grids. The obtained results allow validating the key contributions of the paper, mainly, in terms of the bidirectional operation of the novel EVBC based on a multilevel topology. As the EVBC is controlled targeting the EV incorporation into smart grids, the grid-to-vehicle (G2V) and vehicle-to-grid (V2G) operation modes are discussed and evaluated. Author Contributions: All authors contributed equally to the conceptualization and writing of the paper. Funding: This work has been supported by COMPETE: POCI-01-0145-FEDER-007043 and FCT—Fundação para a Ciência e Tecnologia within the Project Scope: UID/CEC/00319/2013. 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