Applications of Switch-Mode Rectifiers on Micro-grid Incorporating with EV and BESS

A switch-mode rectifier (SMR) can provide adjustable and well-regulated DC output voltage from the available AC source with good line drawn power quality. Depending on the input/output voltage transfer characteristics, the schematics, the operation quadrant, and control, SMRs possess many classifications and application. Typical potential application examples include grid powered motor drives, battery chargers, various power electronic facilities, micro-grids, and grid-connected battery energy storage system (BESS), etc. In micro-grids, the SMR can be employed as the AC generator-followed converter to yield better generating efficiency. The SMR operation of its grid-connected inverter let the grid-to-microgrid (G2M) operation be conduct‐ able in addition to the microgrid-to-grid (M2G) operation. As for the electric vehicle (EV), the bidirectional inverter can be arranged to perform G2V/V2G operations in idle case, wherein the SMR operation is made in G2V battery charging. To promote the application potential and improve the operation performance of SMRs, this article presents the operation controls and applications of SMRs in microgrid systems incorporating BESS and EV as supplemental facilities. First, the classifications, operation principle, and some key issues of SMRs are explored. Secondly, the configuration of the studied system is introduced. Third, the controls and operations of SMRs in micro-grid, wind generators, and grid-connected interface power converters are described. Then the ones in BESS (B2G/G2B) and EV are introduced. Finally, some conclusions and suggestions are given. drawn power quality. The latter task is achieved by arranging the inverter to be operated as a three-phase 4-quadrant SMR with proper control. The BESS can compensate all load reactive and harmonic powers. (ii) Plug-in energy harvesting system: Various AC sources and DC sources can be connected to the BESS via the three-phase bridgeless discontinuous current mode (DCM) SMR through proper schematic and control arrangements. This type of SMR is chosen owing to the single-quadrant operation requirement. In addition, the interconnected operations of the BESS to the micro-grid and EV are also applicable.


Introduction
Energy exhaustion, carbon-dioxide emission, and global warming issues have seriously received attention worldwide in recent years. The use of micro-grid [1][2][3][4] incorporating with various distributed and renewable sources and energy storage devices is an effective means to reduce these problems. Compared with the AC micro-grid, the DC micro-grid possesses the merits of simpler interface converters, allowing longer common DC bus length and having fewer losses [1]. For establishing a high-performance micro-grid, many interdisciplinary affairs should be properly treated, such as:

i.
Interfacing the renewable sources to the system with proper interface converters and controls. Each specific source possesses its own key issues to be adequately handled. For a wind AC generator, the SMR is a natural choice to yield better energy conversion characteristics; ii.
Equipped with proper storage devices and their coordinated controls. If the auxiliary charging from the mains is arranged, the SMR must also be adopted to have good charging performance and line drawn power quality; and iii. Energy management control for the constituted sources, storage devices, and loads. Recently, many droop control approaches [1,5] have been developed to enhance the autonomous operation control characteristics of micro-grids.
Similar to micro-grids, the popularization of electric vehicles (EVs) [6] is also effective in reducing fossil energy consumption and carbon-dioxide emission. Moreover, by regarding EV as a movable energy storage device and performing its interconnected operations to a microgrid (M2V/V2M) [7,8] or utility grid (G2V/V2G) [9][10][11][12][13][14], the effectiveness in achieving these goals will be more prominent. Among the commonly used motors, permanent-magnet synchronous motor (PMSM) is one of the commonly used motors for commercialized EVs [15,16], owing to its many distinguished features. Battery is the major source of an EV, hence its type and ratings should be properly chosen [17][18][19]. Different from the energy type of battery, the super-capacitor belongs to the power type energy storage device that has faster response and lower capacity. By incorporating battery with super-capacitor, one can preserve better total energy utilization characteristics. Then, intermittent battery charging/discharging operations can be avoided to increase battery life. The super-capacitor can be directly paralleled to the battery [20], or they can be interconnected via various configured interface DC/DC converters [21,22].
Battery is an important energy source of various portable and movable appliances, such as laptop computers and electric vehicles [23][24][25]. To effectively use battery storage energy, the study of the use of batteries in emergency application for a laptop computer has been presented in [25,26]. The vehicle-to-grid (V2G)/grid-to-vehicle (G2V) operations have also gradually received attention [27][28][29]. Besides, there are specific battery energy storage systems (BESSs) being developed [30][31][32][33][34][35]. In addition to the autonomous operation of supplying power to critical loads [30][31][32]35], BESSs also possess grid-to-battery charging and battery-to-grid discharging capabilities. The line current in grid-side can also be regulated to be nearly sinusoidal, thanks to its power conditioning control ability. In the studied BESS, its DC-link voltage (V dc = 400V) is established from the Li-ion battery bank (v B = 96V) via an interleaved DC/ DC boost-buck converter that has fault-tolerance capability. For effectively utilizing other renewable sources, a plug-in energy harvesting system is developed to make battery supplementary charging from the possible AC and DC sources. As far as the AC source is concerned, a suitable SMR and its control are needed.
In a DC micro-grid or distributed power system, the DC sources and energy storage devices must be interfaced to its common DC-link using suited DC/DC converter [36,37]. For batteries and supercapacitors, bidirectional DC/DC converters are needed to perform charging and discharging operations. For applications with battery voltage being lower than its interfaced DC-link, one can apply the one-leg half-bridge boost-buck bidirectional DC/DC converter [20].
To possess lower current ripples and fault-tolerance capability, the interleaved DC/DC converter [38] can be adopted.
Basically, SMR is formed by inserting a suited DC/DC converter between the diode bridge rectifier and the output filtering capacitor. The surveys for single-phase and three-phase SMRs can be found in [39][40][41][42][43]. For three-phase plants with less stringent power quality requirement, one can adopt the three-phase single-switch (3P1SW) SMR [44,45]. By operating it under discontinuous conduction mode (DCM), the power factor correction is inherently preserved without current feedback control. If higher efficiency is desired, the bridgeless DCM threephase SMR presented in [35] can be employed. To yield good line drawn current waveform tracking control with the minimum switch number, the three-phase three-switch (3P3SW) Vienna SMR [46,47] is a good choice. However, these two types of SMRs possess only AC-to-DC unidirectional power flow capability. For interfacing the output of a wind AC generator, such as wind permanent-magnet synchronous generator (PMSG), to the common DC-bus in DC micro-grid, a suited AC/DC converter is needed. The de-rated characteristics for various AC/DC followed converters can be found in [48]. From the compromised considerations in switch number, switch voltage stress, conversion loss, operation quadrant number, and control performance, the Vienna rectifier [49][50][51] with three switches is a better choice to be the interface converter of AC wind generators. On the other hand, the standard three-phase six-switch (3P6SW) SMR [35] should be used for conducting bidirectional power transfer operations.
SMR can directly provide adjustable and well-regulated DC output voltage from the available AC source with good line drawn power quality. This article presents the operation controls and applications of SMRs in micro-grid system incorporating BESS and EV as supplemental facilities. The contents of this article mainly include: (i) Exploration of classifications, operation principle, and some key issues of SMRs; (ii) Functional description of the studied system; (iii) Introduction to the controls and operations of SMRs in micro-grid, wind generators, gridconnected interface power converters, BESS, and EV; (iv) Experimental evaluations for the studied plant in various operation cases.
In idling G2V operation mode, the battery bank can be charged by the utility grid. A singlephase boost SMR and a three-phase boost SMR are formed to charge the battery bank through the bidirectional interleaved buck DC/DC converter with satisfactory line drawn power quality from the mains. The interconnected operations of the EV to the micro-grid and BESS are also conductible. Moreover, the V2G/G2V via micro-grid or BESS is achievable.

C. BESS
The DC-link voltage (400 V) is established from the 96 V battery bank via an interleaved bidirectional interface DC/DC converter with two cells. A multifunctional inverter is established to perform the autonomous and grid-connected operations: Battery bank

a. Autonomous operation
The BESS is independent from the utility grid and supplies power to the load.

b. Grid-connected operation
The BESS with three-phase six-switch bidirectional inverter can be arranged to operate in three modes: (i) battery-to-grid (B2G) discharging mode: the BESS provides all load powers and sends the preset real power to the utility grid; (ii) grid-to-battery (G2B) charging mode: the utility grid supplies the load real power and also charges the battery bank with good line drawn power quality; (iii) floating mode: all load real powers are supplied by the utility grid. The BESS can compensate all load reactive and harmonic powers under all grid connected operation modes.
The BESS energy can be supported from the possible AC sources and DC sources via the developed plug-in energy harvesting system, i.e., the energy-harvester-to-BESS (E2B) operation mode. Various AC sources and DC sources can be connected to the BESS via the threephase bridgeless discontinuous current mode (DCM) SMR through proper schematic and control arrangements. Similarly, the interconnected operations of the BESS to the micro-grid and EV are also applicable. Specifically, the EV can be charged by the BESS employing its harvested renewable energies.

Classification of SMRs
From the viewpoints of schematics and control approaches, the SMRs can be categorized as follows.

Schematics
The SMR schematics can be categorized into: (i) Single-phase or three-phase, the three-phase SMR is a natural choice for the plants with larger ratings; (ii) Non-isolated or isolated, normally, the latter type SMR possesses lower energy conversion efficiency; (iii) Buck, boost, or buck/ boost: the boost-type SMR possesses the best PFC control ability since its AC input current is directly related to the energy storage inductor current. As for the buck and buck/boost SMRs, the input low-pass filter is necessary for the inherently discontinuous current; (iv) Single-stage or multi-stage; (v) One-quadrant or multi-quadrant: the multiple quadrant SMR possesses reverse power flow capability from DC side to AC source to achieve the regenerative braking of a SMR-fed AC motor drive; (vi) Hard-switching or soft-switching; (vii) Standard or bridgeless: the bridgeless SMR possesses slightly larger efficiency for the reduced diode voltage drop; (viii) Single-module or interleaved multi-module: the interleaved SMR may have the advantages of rating enlargement, higher reliability owing to redundancy, and smaller current and voltage ripples.

Control methods
a. Low-frequency control: for the single-phase boost SMR, only v-loop is needed and only one switching per half AC cycle is applied. It is simple but subject to having limited power quality characteristics.
b. High-frequency voltage-follower control: without current control loop, only some specific SMRs operating in DCM possess this feature; see for example, the standard buck-boost SMR and the flyback SMR.
c. High-frequency standard control: it belongs to multiplier-based current-mode control scheme with cascade v-and i-control loops.

Operation and schematics
For a properly designed single-phase SMR, the AC input current i ac can be regulated to be sinusoidal and kept in phase with v ac , then the SMR can be regarded as an emulated resistor with the effective resistance of R e viewing from the utility grid. However, the double line frequency output voltage ripple always exists. It can be derived to obtain the peak to peak value of output ripple voltage: where R dc = equivalent DC load resistance, ω = 2π f 1 and f 1 = 60Hz.
In reality, the operation characteristics of an SMR including DC output voltage and AC input power quality are highly affected by the energy storage inductor current PWM control behavior.
Figs. 3(a) to 3(e) show some typical single-phase boost SMR circuits, including: (a) standard type SMR; (b) bridgeless SMR: its efficiency is increased by reducing one diode voltage drop in each half AC cycle; (c) curret-fed push-pull (CFPP) isolated SMR: the higher voltage boosting ratio is obtained by the duty ratio control and the turn ratio; (d) zero-voltage transition (ZVT) soft-switching SMR: the ZVT soft switching is achieved by adding an auxiliary resonant branch [52]; (e) four-quadrant SMR: the H-bridge converter based SMR possesses four operation quadrants. A bidirectional battery energy storage system [25] is shown in Fig. 3

(e)
The single-phase standard buck SMR is depicted in Fig. 4(a), the input filter is needed for its discontinuous input current. The power factor will become worse for the higher output DC voltage. Figs. 4(b) to 4(c) correspond to the standard, the Cuk continuous, and the flyback isolated buck/boost SMRs, respectively. Except for the Cuk SMR, the input filtering is still necessary for the other two types of buck/boost SMRs. In control aspect, the simple voltagefollower control scheme can be applied for these buck/boost SMRs operated in DCM.

Some key issues
Some key issues for a boost SMR and a buck/boost SMR are indicated in Fig. 5 and Fig. 6. Some comments are given as: a. The input filter is needed for buck/boost SMRs due to the discontinuous input current.
b. The ripples and ratings of the power circuit constituted components must be properly designed and implemented.
c. For the boost SMR in CCM operation, the standard cascade multiplier-based control scheme with v and i loops must be adopted.
d. The voltage follower control scheme without inner current loop can be applied for the buck/boost SMR in DCM operation owing to its inherent PFC capability.
e. In treating the control affairs, the sensed inductor current and output voltage should be filtered with suited low-pass cut-off frequencies. The feedack controller must first be properly designed considering the desired transient and static performances and the effects of comtaminated noises in sensed variables. Normally, the voltage dynamic response speed is: << (2 f 1 = 120Hz), whereas the inner current dynamic response is set as: << switching frequency, but >> (2 f 1 = 120Hz).

Operation and schematics
Detailed surveys for the existing three-phase SMR circuits can be referred to [41][42][43]. The complexities of schematic and control mechanism depend on the control ability and the desired performances. Some commonly used three-phase boost SMRs shown in Figs. 7(a) to 7(e) include: a. Three-phase, single-switch (3P1SW) DCM SMR ( Fig. 7(a)): by operating it in DCM, the PFC is naturally preserved without applying current PWM control. However, it possesses the following limiations: (i) Having higher input peak current and switch stress; (ii) The input line current contains significant lower-frequency harmonics with the orders of 6n ±1, n=1, 2,..., and the dominant ones are the 5 th and 7 th harmonics. Thus, suitably designed AC-side, low-pass filter is required to yield satisfactory power quality; (iii) The line drawn power quality is limited, typically the power factor is slightly higher than 0.95; (iv) It possesses only one-quadrant capability.
b. Bridgeless, three-phase DCM SMR: As shown in Fig. 7(b) [35], one diode drop is eliminated in each line-current path to increase the efficiency compared to 3P1SW SMR. d. Three-phase six-switch standard SMR (Fig. 7(d)): the standard three-phase six-switch SMR [41,42] possesses four operation quadrants and high flexibility in power conditioning control. For a motor drive equipped with such SMR, it may possess regenerative braking ability. However, the switch utilization ratio of this SMR is low, and its control is complicated.
e. Neutral-point clamped (NPC) three-phase standard SMR: the three-level NPC threephase SMR is shown in Fig. 7(e) where the voltage ratings of its constituted power switches and diodes can be only one-half of the DC-link voltage, rather than the full voltage for the standard SMR shown in Fig. 7(d).

Three-phase single-switch (3P1SW) DCM SMR:
For a well-regulated three-phase single-switch (3P1SW) DCM SMR shown in Fig. 7(a), it can be regarded as a loss-free emulated resistor R e viewing from the phase AC source with line  drawn current having dominant 5th and 7th harmonics [44]. Hence, the three-phase line drawn instantaneous power can be approximately expressed as: where δ p ac = ripple AC power and the average AC power P ac is: The AC charging current flowing the output filtering capacitor is: Thus, one can derive the peak-to-peak output voltage ripple from (4):

Three-phase Vienna SMR and three-phase six-switch standard SMR:
For the Vienna SMR and three-phase six-switch standard SMR with satisfactory current mode control, the three-phase line drawn currents will be balanced without harmonics. Hence, from (2) one can find that the DC output voltage ripple will be nearly zero.

Derated characteristics of a PMSG followed by different AC-DC converters
The total derate factor of a surface-mounted PMSG (SPMSG) and an interior PMSG (IPMSG) followed by various AC-DC converters can be derived to be [48]: where, I as = rms value of armature phase current i as , I as1 = rms value of fundamtal armature phase current i as1 , TH D i = total harmonic distortion of i as , δ = the power angle between the back-EMF e as and the terminal voltage v as , P e = electromagnetic developed power, P f = magnetic field developed power, P r = reluctance developed power. The de-rated characteristics of a wind PMSG followed by various AC-DC converters and operation control modes are shown in Fig. 8. Obviously, the conventional diode rectifier possesses significant derate. The three-phase Vienna SMR is a good candidate for being the followed interface converter of the PMSG from the following compromised considerations: (i) minimum switch number; (ii) winding current PWM control ability; (iii) the commutation shift control ability for an IPMSG; and (iv) only one operation quadrant is required.
• Possible plug-in DC sources: Photovoltaic, fuel cell, battery, or other possible harvested sources. A DC source is employed here as an alternative.

B. Control Schemes
In the DC micro-grid shown in Fig. 2, the control scheme of the wind IPMSG with followed Vienna SMR is shown in Fig. 9(a), and the differential mode (DM) and common mode (CM) control schemes of the 1P3W inverter are depicted in Fig. 9 . 1P3W inverter G2M SMR charging mode: The micro-grid can be supported energy from the mains via the SMR formed using the power devices of the 1P3W inverter with power factor correction by the control scheme as shown in Fig. 9(b). The single-phase SMR with 220V/60Hz input is formed by (Q 7 , Q 8 , Q 11 , Q 12 ). The predictive current control is applied to yield fast tracking response. And the voltage and active power controller are set as:

a. IPMSG with followed Vienna SMR
Using the estimated parameters of the IPMSG and the given torque command T e from the wind power MPPT control mechanism, the relationship of its phase current magnitude command Î asw and the commutation shift angle β w can be found [51]. The developed wind generator torque command is preset as T e = 3.23(N − m), and the corresponding optimal variables found are (β w , Î asw ) = ( − 7.8 , 5.7A). The measured steady state (v dc , θ r , θ r , i as ) at     b. Inter-connected operation between EV IPMSM drive, micro-grid, and utility grid-G2V via micro-grid: The grid-connected micro-grid is connected to the DC-link of the EV, and the 1P3W inverter load resistors in Fig. 2

Grid-connected BESS
The system configuration and schematic of the developed grid-connected BESS is shown in Figs. 1 and 2. It can be operated under floating, discharging, and charging modes. All load reactive and harmonic powers can be compensated by the BESS. In charging mode, the bidirectional inverter is operated in SMR mode to let the BESS be supported energy from the grid (G2B).
In addition, a plug-in energy harvesting system is also equipped for the grid-connected BESS. The harvested AC and DC sources can be inputted to the system via a three-phase bridgeless DCM SMR to establish a 350 V DC-link. Then, it is connected to the developed BESS common DC-link (400 V) via a LLC resonant DC/DC isolated converter.

A. System Components
The constituted system parameters are listed below: ii.

2.
Disturbance and command feed-forward controllers: . BESS battery charging from harvested three-phase AC source (E2B) The harvested three-phase auxiliary AC sources are inputted to the system via the three-phase bridgeless DCM SMR. Through the LLC resonant DC/DC converter, the well-regulated BESS 400 V DC bus voltage is established. And the battery bank is charged from the DC bus via the BESS interleaved DC/DC interface converter in buck mode. The measured (v dc , v d , v an , i an ) and (v B , i L , i L 1 , i L 2 ) are shown in Figs. 14(a) and 14(b). Normal and good operation characteristics of all the constituted power stages can be seen from the results. The corresponding measured steady-state characteristics are: • P in = 721.91, P B =564.25, η = 0.78.      The single-phase AC source and DC source can also be inputted to the BESS via the developed plug-in three-phase DCM SMR. The power circuits and control schemes of these two cases are shown in Fig. 15(a) and Fig. 15(b). The single-phase bridgeless boost SMR formed in Fig By replacing the AC input in Fig. 15(a) with the DC source, the DC source harvesting circuit using the three-phase bridgeless DCM SMR embedded components is formed as shown in Fig. 15(b). Its control scheme is also depicted in Fig. 15(b). The energy storage inductors designed in Fig. 15(a) is also suited in this case.    Figure 15. The schematics and control schemes of the developed plug-in energy havesting system with single-phase AC source and DC source inputs: (a) single-phase AC source input; (b) DC source input.

G2V operation of the IPMSM EV drive with single-phase SMR based battery charger
In the developed EV IPMSM drive shown in Fig. 2, a single-phase and a three-phase boost SMR based chargers can be formed using its embedded circuit components. By placing the changeover switch at the position ②, a two-stage single-phase SMR based battery charger is formed as shown in Fig. 17(a). It consists of an H-bridge boost SMR formed by the two outer inverter IGBT legs and a followed interleaved buck DC/DC converter based charger.

A. System Components
The constituted system parameters are listed below:

. Experimental Results
In the single-phase H-bridge boost SMR based charging system shown in Fig. 17(a), the AC input 220 V/60 Hz and the DC-link voltage command V dc * = 400 V are set in G2V operation. By setting the constant current charging with i b * to be i bm * = 0.25C = 3.5A, the measured (v AB , i uA ) and iii. Input energy storage inductors: Figure 17. System configuration and control scheme of the established single-phase H-bridge boost SMR based battery charger: (a) schematic; (b) control scheme.

B. Control Schemes of Three-phase SMR
The current and voltage feedback controllers are set as:

C. Experimental Results
Under the conditions of (V ab = 220V/60Hz, V dc * = 400V, i bm * = 0.25C = 3.5A), the measured (v as , Similarly, good operation performance of the established three-phase SMR based charger is also seen from the results.

Conclusions
Switch-mode rectifier can provide adjustable and well-regulated DC voltage with good AC line drawn power quality. Hence, it has been widely applied in many power electronic equipments to yield improved operation characteristics. However, the schematic and control scheme should be properly selected, designed, and implemented in accordance with the specific application. During the past decades, the development and employment of microgrids and EVs have received much attention worldwide for reducing fossil energy comsumption. This article has presented the applications of switch-mode rectifiers on micro-grids incorporating with EV and BESS. After introducing the basics and some key issues of SMRs, the configuration of the studied system is introduced. Then, the applications and performance evalutions of SMRs in micro-grid, BESS, and EV are presented.
Some conclusions and comments for the practical issues of SMRs in the related plants covered in this article are summarized as follows: 1. DC Micro-grid: (i) The 3P3SW Vienna SMR is adopted as the followed converter of wind PMSG. It possesses the advantages of having good compromised characteristics in derate, switch number, single-quadrant operation, current PWM control flexibility, and commutation shifting ability. (ii) Load inverter: the bidirectional 1P3W inverter is adopted for providing the 110 V/220 V AC sources for powering the home appliances. It can successfully perform the M2G and G2M operations. In G2M operation, the single-phase SMR is formed to allow the utility supply power to the micro-grid for energy support or for making the battery supplementary charging. The EV can also perform G2V/V2G operations via the micro-grid interface converters.

EV PMSM Drive:
The developed battery/SC powered EV IPMSM drive possesses G2V/V2H/V2G operation capabilities. In G2V operation, a single-phase boost SMR and a three-phase boost SMR can be formed using the embedded components to charge the battery bank through the bidirectional interleaved buck DC/DC converter with satisfactory line drawn power quality from the mains. The interconnected operations of the EV to the micro-grid and the BESS can also be conducted.

3.
BESS: (i) Grid-connected operation: the three-phase six-switch bidirectional inverter can be arranged to operate in G2B charging mode. The utility grid supplies the load real power and also charges the battery bank with good line drawn power quality. The latter task is achieved by arranging the inverter to be operated as a three-phase 4-quadrant SMR with proper control. The BESS can compensate all load reactive and harmonic powers. (ii) Plugin energy harvesting system: Various AC sources and DC sources can be connected to the BESS via the three-phase bridgeless discontinuous current mode (DCM) SMR through proper schematic and control arrangements. This type of SMR is chosen owing to the single-quadrant operation requirement. In addition, the interconnected operations of the BESS to the micro-grid and EV are also applicable.