Versatile Power Electronic Building Block for

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resonant quasi-Z-source DC-DC converter (SRqZSC) features galvanic isolation .... from the simple single-switch [12] to advanced multi-phase. qZSC [13], the ...

Versatile Power Electronic Building Block for Residential DC Microgrids Dmitri Vinnikov, Andrii Chub, Roman Kosenko, Elizaveta Liivik

Abstract—This paper discusses a novel approach to the power electronic building block (PEBB) for residential power generation systems. The PEBB based on the synchronous resonant quasi-Z-source DC-DC converter (SRqZSC) features galvanic isolation, wide input voltage and load regulation range, and high power conversion efficiency. Thanks to the power circuit versatility, the PEBB can be used as a power electronic interface not only for the power sources with unidirectional power flow (e.g. the photovoltaic panels and fuel cells) but also in the bidirectional applications, such as modular battery storage systems. The paper gives an insight into the SRqZSC technology, explains its general design and control principles and applicability issues in the residential power generation systems. Moreover, the experimental results are discussed in the context of different applications by means of the 300 W prototype of the SRqZSC based PEBB for the residential DC microgrids.

Residential AC Power Grid

Department of Electrical Power Engineering and Mechatronics Tallinn University of Technology, Tallinn, Estonia [email protected]

Keywords—power electronic building block; DC microgrid; photovoltaic systems; battery storage; fuel cell power systems.

I.

INTRODUCTION

With the introduction of the EU directive on energy performance of buildings in 2010 [1], zero energy and resource efficient buildings and districts were established as a topical area all across Europe. Multidisciplinary research activities on the energy efficient buildings have been initiated in different topics ranging from the advanced building envelopes and insulation materials to the energy efficient climate control and power distribution systems (PDS). The latter attracts closer attention of power electronics engineers, which has resulted in the appearance of future energy supply paradigms such as residential microgrids [2], nanogrids [3] or intelligent DC homes [4]. The common feature of such systems is that the distributed energy generators (photovoltaic panels, wind turbines, fuel cells, etc) and energy storages (batteries, flywheels, etc) are interconnected through a common high-voltage (HV) DC bus by the help of dedicated power electronic interfaces (Fig. 1). The typical operating voltage of the residential DC coupled PDS is 380…400 V since this voltage level can meet the industry standard for consumer electronics with the power factor correction circuit at the input side [5]. The main task of the power electronic interface is to ensure the voltage matching and optimal balancing of power flow between the main components of the PDS with special attention to power

Fig. 1. An example of DC coupled PDS system for residential buildings and proposed application scenario of the SRqZSC PEBB.

conversion efficiency, power quality, reliability, and safety. Since the majority of renewable energy sources and storages are low-voltage DC systems (20…40 V DC), DC coupling helps to meet these demands, while the overall system efficiency could be improved by up to 5% above the AC system level [6]. Moreover, it provides a unique opportunity of using modular plug-and-play power electronic interfaces [7], thus saving installation space and reducing installation cost and time, which are all increasingly important for the residential power generation systems. For example, microconverters in the rooftop photovoltaic (PV) installations allow for better system design flexibility, module-level monitoring and diagnostics as well as simplified installation and increased energy yield from the PV module due to the module-level maximum power point tracking (MPPT) [8]. Microconverters are typically mounted to the back frame of the PV modules and connected in parallel at the HV DC bus, which makes the power scaling process much easier and faster than the traditional PV string inverters. A similar modular approach was recently proposed also for the residential battery storage systems [9], where the power electronic interface is embedded with the battery, thus allowing for compact plugand-play 1.2 kWh energy storage with easy installation and scalability. Also, high redundancy of parallel power converters secures electricity supply of a building.

Fig. 2. Generalized structure of the proposed SRqZSC PEBB for residential power generation systems.

This paper presents a novel approach to the power electronic building block (PEBB) for the residential DC coupled PDS. The PEBB is characterized by high versatility and ease of use since it could be paired with all basic types of distributed generators and energy storages, as shown in Fig. 1. Depending on the operating power of the source, the PEBBs could be used either as a single unit system (e.g., the PV microconverter) or cascaded in parallel (e.g., the fuel cell converter). It features the embedded control, communication and auxiliary power systems and depending on the application, can support either uni- or bi-directional power flow without any hardware modifications. II.

VERSATILE PEBB FOR RESIDENTIAL POWER GENERATION SYSTEMS

A. Introduction to the PEBB Technology The power circuit of the proposed PEBB (Fig. 2) is based on the synchronous resonant quasi-Z-source DC-DC converter (SRqZSC), which is the advanced version of the quasi-Zsource galvanically isolated DC-DC converter (qZSC). The qZSC was proposed in 2009 as a novel class of high step-up DC-DC converters [10]. Due to its continuous input current and inherent short-circuit withstandability, the qZSC was positioned as a versatile power conversion approach for the renewable and alternative energy systems, where the reduced stress of the power source and high reliability play a vital role. Within the past eight years, focus of many researchers worldwide has been on the above area. As a result, many novel topological derivations of the qZSC have been created [11]. However, from the variety of topologies, ranging from the simple single-switch [12] to advanced multi-phase qZSC [13], the full-bridge topology [14] still keeps the leading role due to its excellent control flexibility and power circuit operation symmetry. With minor modifications, the full-bridge qZSC can support the bidirectional power flow [19], resonant switching [20], multi-mode [15], and topology morphing [21] control, which all make it a strong candidate for a versatile PEBB for the residential PDS.

Fig. 2 shows the generalized structure of the proposed PEBB. It features a fully integrated approach, i.e. the control, protection and communication circuitries are embedded in the power board. In addition, the converter has an integrated series resonant tank formed by the leakage inductance Llk of the hybrid isolation transformer and capacitors C1 and C2 of the voltage doubler rectifier (VDR). The coupled inductor based synchronous quasi-Z-source (qZS) network is implemented at the low-voltage (LV) side of the SRqZSC, which, in combination with the synchronous VDR, supports the reverse power flow and vastly improves the converter performance. B. General Specifications of SRqZSC PEBB In this study, a 300 W experimental prototype of the SRqZSC PEBB for the residential PDS was developed (Fig. 3). The generalized specifications and components used in the PEBB are shown in Table I. The PEBB has a plug-and-play functionality and can be used either in the applications with unidirectional power flow control (PV microconverters or fuel cell power conditioners) or in such reversible systems as battery storages. In the design

Fig. 3. Top view of the developed 300 W SRqZSC PEBB.

of the PEBB, special attention was paid to the minimization of the manufacturing costs. Therefore, 95% out of more than 300 components used are the SMD components. The PEBB utilizes natural convection cooling with specially optimized thermal pads and vias used to move the heat from the parts into the core layers, thus eliminating the hotspots. For the unification purposes, both integrated magnetic components, i.e. the coupled inductor LqZS of the qZS network and the hybrid isolation transformer TX, are wound on the RM14 cores. The control system of the PEBB is based on the microcontroller that utilizes the Cortex-M4 core with a floating-point unit and enhanced PWM peripheral that can be configured to operate with a clock frequency of up to 4 GHz. To realize the control and protection functions, the PEBB features voltage and current sensors (Table I) at the low- and high-voltage sides. The current sensors are both based on the linear Hall circuit. The voltage is sensed by a simple resistive divider at the LV side and a capacitive coupled isolated amplifier with a resistive divider at the HV side. Then, analog signals are converted into a digital form by means of an integrated 12-bit analog-to-digital converter of the microcontroller. The PWM signals generated are fed to the magnetic coupling based MOSFET drivers containing two low-side channels for a lower number of components. Auxiliary synchronous buck converter TI LM46002 stabilizes the internal bus of 5 V, out of which the isolated voltage of

9 V is generated for each high-side MOSFET driver using simple non-regulated push-pull converters driven by TI SN6501. In the following sections, examples of the PEBB applications in a residential PV, battery storage and fuel cell power systems are discussed in more detail. III.

PEBB BASED PV MICROCONVERTER

A. Generalized Operation and Control Principle In PV applications, the PEBB operates as the unidirectional step-up DC-DC converter with enhanced MPPT performance. The combination of properties of the voltageand current-source converters gives the SRqZSC an excellent opportunity of the ultrawide input voltage and load regulation, at the same time maintaining the continuous input current. In this study, the MPPT window was set to 18…45 V, which is sufficient to track the maximum power of the majority of 60cell PV modules even in the partial shading conditions. To achieve better tradeoff between the utilization of the components and the power conversion efficiency, the multimode control [15] was implemented, where, depending on the operating conditions of a PV module (irradiation, temperature, shading, etc.), the PEBB can be smoothly switched between three distinct operating modes (Fig. 4): • boost mode with shoot-through pulse-width modulation (ST-PWM) is active for the PV module voltage of 18 V to 33 V. The MPPT here is performed by the variation of the shoot-through duty cycle of the inverter bridge at the LV side of the PEBB (Fig. 5a). Synchronous switches of the qZSnetwork and VDR are conducting only during the active states of the inverter. In this mode, the SRqZSC operates similarly to the boost full-bridge converter [16]. However, due to the properties of the qZS-network, it features higher voltage gain for the same control variables and requires no additional clamping circuits to damp the inductive overvoltages. • normal mode with PWM control corresponds to the maximum power point of the PV module at the most common operating conditions (VLVr = 33 V in the given case study). In this operating point, the duty cycle of the LV side inverter switches is close to 0.5 (Fig. 5b). The synchronous switch of the qZS-network (SqZS) is constantly turned ON but the synchronous switches of VDR are conducting only during the active states of the LV inverter. In this operating point, the behavior of SRqZSC is similar to that of the traditional series resonant converter operating at the resonant frequency.

Input voltage, V

TABLE I. GENERAL SPECIFICATIONS OF THE SRQZSC PEBB Operating parameters Rated voltage of LV port, VLVr 33 V Operating voltage range of LV port, VLV 18…45 V Maximum current of the LV port, ILV 12 A Rated voltage of HV port, VHVr 400 V Operating voltage range of HV port, VHV 380…420 V Switching frequency, fSW 110 kHz Dead-time of LV port switches 70 ns Dead-time of HV port switches 100 ns Operating power range, P 25…300 W Components S1...S4, SqZS Infineon BSC035N10NS5 S 5, S 6 Wolfspeed C3M0280090D LlkqZS 0.6 µH LmqZS 12 µH Llk 24 µH Lm 1 mH n 6 MLCC 26.4 µF (12 × KEMET CqZS1, CqZS2 C1210C225K1R in parallel) Foil 43 nF (33 nF Epcos C 1, C 2 B32672Z6333K and 10 nF Vishay MKP1840310104) El. cap. 150 µF Epcos Cf B43501A6157M and foil 0.47 µF Epcos B32653A6474K Control, driving and measurement Microcontroller ST STM32F334 Allegro LV side current sensor ACS716KLATR-25CB-T LV side voltage sensor Resistive divider 1:20 Allegro HV side current sensor ACS712ELCTR-05B-T HV side voltage sensor TI AMC1200-Q1 Analog Devices ADuM3223 MOSFET drivers (2 low-side channels)

45 42 39 36 33 30 27 24 21 18

PSM controlled buck mode Normal mode

50W 100W 200W 180°

90° variation of ϕ

ST-PWM controlled boost mode

0

0.5 0.2 variation of DST

Fig. 4. Operating modes and corresponding control variables of the PEBB based PV microconverter at different operating powers for the selected input voltage regulation range.

(a)

(b)

(c)

Fig. 5. Modulation methods of the SRqZSC PEBB in the boost (a), normal (b), and buck (c) modes.

PEBB BASED BATTERY CONTROLLER

45 25% load 50% load 100% load

43 41 39 37 35 33 0

20

40

60

80

100 120 140 160 180

Phase shift angle φ, deg

Fig. 6. Input voltage regulation curves of the proposed SRqZSC PEBB in the PSM controlled buck mode at different load conditions. 350

98

300

97

250

96

200

95

150

94

100

93 Operating power profile Efficiency

50

Efficiency, %

B. Efficiency The operating power profile and corresponding power conversion efficiency of the proposed PEBB in the PV microconverter application is depicted in Fig. 7. In the range from 18 V to 25 V, the converter is designed to operate with the maximum input current of 12 A. This is a typical output voltage range of the partially shaded PV module and depending on the operating conditions and type of the module, the power conversion efficiency lies in the range from 92.2%

IV.

Operation of the SRqZSC PEBB with a battery requires no changes in the switching patterns presented in Fig. 5a. The converter is optimized for 24 V lithium batteries. As a result, the SRqZSC PEBB operates in the boost mode when discharging the battery, which is similar to the operation in the PV application described in the previous section. Hence, only operation in the battery charging mode is described below.

Input voltage, V

Fig. 4 plots the experimental control variables of the PEBB based PV microconverter within the predefined input voltage range obtained at different operating powers. It is remarkable that the proposed PEBB features not only the buck-boost functionality within a single switching stage but also has the hybridization of properties. Thus, in the boost mode (yellow region in Fig. 4), it operates as a non-resonant converter having almost linear control characteristics with the minor dependence on the operating power. Apparently, in the PSM controlled buck mode (blue region in Fig. 4), the converter has pure resonant behavior, with its DC gain strictly depending on the load of the PEBB since the latter is directly proportional to the Q-factor of the integrated series resonant tank (Fig. 6). The multi-mode operation with smooth transition between the operating modes could be realized by a single PI-regulator, which is one of the advantages of the proposed converter [15].

to 95.4% (Fig. 8a). In the rest of the input voltage range, the converter is designed to operate with rated power of 300 W featuring its maximum efficiency of 97.4% in the nominal mode, which corresponds to the input voltage of 33 V (Fig. 7). In this operating point, the converter features its highest CEC weighted efficiency, which is close to 97%, including the auxiliary power and control system losses (Fig. 8b).

Operating power , W

• buck mode with phase-shift modulation (PSM), when the voltage of a PV module lies within the range from 45 V to 33 V. The MPPT within this range is performed by the variation of the phase shift angle φ between the leading and lagging legs of the inverter bridge at the LV side. Similar to the normal mode, the synchronous switch of the qZS-network is constantly turned ON but synchronous switches of the VDR are conducting only during the active states of the LV inverter. In this mode, the SRqZSC could be regarded as a series resonant converter controlled by means of PSM at the resonant frequency. Since the quality factor (Q) of the integrated resonant tank is well below unity, the converter features the discontinuous current through the isolation transformer, thus enhancing the soft-switching performance of the PEBB [17]. As an alternative to PSM, the SRqZSC in this mode can be also controlled by the Asymmetrical PWM (APWM), which can slightly enhance the power conversion efficiency for the cost of additional DC blocking capacitor connected in series with the primary winding of the isolation transformer [18].

92

0

91 18

21

24

27

30

33

36

Input voltage, V

39

42

45

Fig. 7. Operating power profile and experimental efficiency of the proposed PEBB in the PV microconverter application.

98 97 96 95 94 93 92 91 90 89 88

Efficiency, %

Efficiency, %

96.0 95.5 95.0 94.5 94.0 93.5 93.0 92.5 92.0 91.5

18 V

2

3

4

5

25 V

6

7

8

9

10

11

ηCEC=96.9% ηCEC=94.1%

25 V 0

12

100

33 V

200

300

Operating power, W

Input current, A (a)

(b)

Fig. 8. Experimental efficiencies of the SRqZSC PEBB based PV microconverter in different operating conditions: in the region of partial shading of the PV module with gradually changing operating power (a) and in the normal (non-shaded) operating conditions at different power levels (b).

A. Operation in the Battery Charging Mode Integrated series resonant tank of the proposed SRqZSC allows voltage buck in both power directions by help of PSM. The SRqZSC PEBB utilizes PSM to achieve reverse power flow and charges the battery connected at the LV port. Switching pattern shown in Fig. 9 is similar to that in Fig. 5c with a slight difference in the duration of dead-times. To achieve better zero current switching conditions, the dead-time TDT1 should be long enough to accommodate discharging of the leakage inductance current into the output capacitive filter: TDT1 = (t3 − t1) ≥ (t2 - t1).

(1)

As it was mentioned in the previous section, the SRqZSC PEBB was designed to maintain the discontinuous resonant current, i.e., quality factor Q < 1, within the entire voltage and power range predefined in the design. This requirement is easy to satisfy using the leakage inductance as the only inductive component in the resonant tank. Several practical assumptions should be considered: C1 = C2 = Cr; Cr ≪ Cf ; Cr ≪ CqZS1/n2; Cr ≪ CqZS2/n2. Fundamentals of the series resonant converter operation with PSM and low Q were elaborated in [22]. The resonant frequency fr can be derived as:

fr =

ωr 1 = . 2 ⋅ π 2 ⋅ π ⋅ 2 ⋅ Lr ⋅ C r

(2)

where RL is the equivalent LV side load resistance. Also, the duration of current reset interval equals:

t 2 − t1 =

 (1 + Q G − G ) ⋅ sin(ω r t1 )  arctan  . ωr  2 − (1 + Q G − G ) ⋅ cos(ω r t1 )  1

The normalized DC voltage gain calculated numerically for different loading conditions is presented in Fig. 10. Evidently, the control of the converter is more challenging at low power, i.e. low Q values due to a wider flat zone of low senility to the phase shift. B. Case Study with 24V LiFePO4 Battery A typical 24 V lithium iron phosphate (LiFePO4) battery was used in this study. This battery type is preferable in the given application due to its inherent safety from contamination or fire hazard, superior temperature stability of parameters, high number of recharge cycles, tolerance to partial charge or discharge cycles, etc. [23]-[28]. Charge characteristic of the given LiFePO4 battery is shown in Fig. 11. Evidently, the operating voltage range is between 29.2 V at full charge and cut-off voltage of 18 V at critical discharge. S1

0 0

The normalized DC voltage gain of the converter (G) in the case of reverse power flow can be derived similar to [22] through numerical solving of the implicit equation below:

0

(1 + Q G − G ) ⋅ (G − 1 + 2 cos(ωr t1 ) − (G + 1) ⋅ cos(ωr t2 ) ) −

0

0

S3

S4

TDT2

Q=

π ⋅Q 2

=

π ⋅ Zr 2 ⋅ RL ⋅ n 2

=

π ⋅ Lr

S5 Da·TSW

2 ⋅ RL ⋅ n 2 ⋅ 2 ⋅ C r

(4)

S4 t

S6

t S5 t

VC1 t

VTX,HV

ϕ ⋅ TSW

VTX,LV

360

ITX,HV

0 t0

,

t S3 TSW

SqZS

(3)

where ωr ·t1 = 2·π·Da and Q is the normalized load parameter. The latter can be found as follows:

S1

S2

TDT1

VC2

− 2 ⋅ (G + 1) ⋅ (1 − cos(ωr t2 − ωr t1 ) ) = 0,

(5)

t1

t2

t3

t

t4 t5

Fig. 9. Idealized sketch of currents and voltages of the SRqZSC PEBB operating in the battery charging mode.

0,6 Q=0.05 Q=0.1 Q=0.2 Q=0.3

0,4 0,2 0,0 0

30

60

90

120

150

180

Phase shift angle φ, deg.

Fig. 10. Normalized DC voltage gain as a function of the phase shift angle.

The cell voltage of the LiFePO4 batteries is stable during charging and discharging, which results in the flat voltage curve. The battery voltage is within the narrow range of 24 V to 26 V for the battery state of charge values between 15% and 85%. The performance of the SRqZSC PEBB was evaluated experimentally by means of efficiency measurements. Results in Fig. 12 show that the converter is less efficient in the battery charging mode, when higher RMS currents in the components are caused by the PSM-based control. Roundtrip efficiency of this converter is not the only factor limiting the system performance that depends on roundtrip efficiency of the LiFePO4 batteries, which is typically in the range from 78% to 83% [29][30]. V.

PEBB BASED FUEL CELL POWER CONDITIONING UNIT

Fuel cells are an emerging technology that enables autonomous operation of residential and small commercial buildings from environmentally friendly energy sources like

Battery voltage, V

30

A. System Description In this study, the PEM fuel cell stack Horizon H-1000 was considered for integration into the residential DC coupled PDS. Characteristics of this fuel cell stack are presented in Fig. 13. Obviously, it is required to parallel the SRqZSC PEBB in order to handle the fuel cell current [35]. We considered three converter cells cascaded in an input-paralleloutput-parallel (IPOP) configuration to handle the fuel cell currents up to 36 A. At higher current, the fuel cell power drops and the operation of the converter is not advantageous due to high losses. The fuel cell stack features voltage above the converter rated value, which results in the operation of the converter in the buck mode with PSM control. The converter operation shifts closer to the normal mode when the operating power is rising, i.e., the efficiency is increasing with the operating power. Therefore, the converter power loss would be increasing at slower pace than the operating power. The power conditioning unit (PCU) based on the SRqZSC PEBB for the given fuel cell type is presented in Fig. 14. This PCU requires active current sharing control [36] and therefore, communication through I2C interface is implemented. In addition, interleaving control of cells is used to reduce the input current stress. B. Intercell Communication and Synchronization The cells of the PCU are interconnected through the I2C bus due to the possibility to implement cyclic redundancy

28 26 24 0.1·C

22

0.5·C

20 18 0

20

40

60

State of charge, %

80

100

45

1000

40

800

35

600

30

400

25

Buck mode

200

Boost mode

20

Fig. 11. Charging voltage of a battery of capacity C as a function of the state of charge at two charging currents of 0.1·C and 0.5·C.

0 0

10

20

30

Fuel cell current, A

40

Fig. 13. Power profile of the PEM fuel cell Horizon H-1000. Discharging

97

Efficiency, %

Charging

95 93 91 89

24 V 26 V

87 85

-300

-200

-100

0

100

Power at LV port, W

200

300

Fig. 12. Experimental efficiency of the SRqZSC PEBB in battery powered applications.

Fig. 14. PCU of three SRqZSC PEBBs for the Horizon H-1000.

Fuel cell power, W

0,8

Fuel cell voltage, V

Norm. DC voltage gain G

hydrogen or natural gas [31]-[34]. This section describes how the SRqZSC PEBB operating in a unidirectional mode can be connected in parallel to realize the power backup system based on the proton exchange membrane (PEM) fuel cell.

1,0

The ARM microcontroller used to control the PEBB features high-resolution PWM timers (HRTs) that are capable of operation with PWM clock frequency up to 4 GHz. This unit contains six timers as well as a synchronization input and output. Hence, implementation of the interleaved control is enabled at the hardware level. Optical link was implemented for transmission of the synchronization signal from the controller of the master converter to those in the slave converters. As a result, at each switching period of 10 µs, the HRT counters of the slave microcontrollers are preloaded with some value that corresponds to 60° or 120° phase shift. The optical receivers HFBR-2521Z and transmitter HFBR-1521Z from Broadcom were used for synchronization purposes. This resulted in the propagation delay of roughly 250 ns, which is equivalent to 150 counts at the HRT clock frequency of 576 MHz. This propagation delay was corrected in the software developed to achieve proper interleaving control. C. Experimental Results The fuel cell PCU based on three SRqZSC PEBBs was tested according to the power profile presented in Fig. 13. The measured efficiency is shown in Fig. 15. The measurements were performed with classical PSM and employing cycle skipping control. In the latter case, the cells are in the idle mode every other switching period for power levels below 600 W and equal power sharing among the three cells. The converter efficiency is at maximum at the maximum power of the fuel cell stack. Higher efficiency in case of the cycle skipping control utilization results in much lower power losses at light load as shown in Fig. 16. The efficiency variations were reduced from 13% to 3% within the operating range in case of the cycle skipping control. Also, the equal current sharing was achieved with compensation of the propagation delay in the optical synchronization transmission channels, as can be seen from Fig. 17. 96

Efficiency, %

94 92 90 88 w/o cycle skipping with cycle skipping

86

50

Power loss, W

check and, by this means, improve communication stability. The corresponding peripheral I2C unit of the microcontroller is connected to the high power bus buffer P82B96D from TI through digital isolator ISO1540D from TI to withstand switching noise induced in the twisted pair wire connecting the boards. By means of communication, the master converter can read input current values from the remaining two slave converters and calculate the average input current. Based on this, the master converter updates the input current reference in the slave converters.

40 30 20

w/o cycle skipping with cycle skipping

10 0 0

200

400

600

800

1000

Fuel cell power, W Fig. 16. Measured power losses in the PCU according to the power profile of the case study fuel cell stack.

Fig. 17. The input current sharing between three SRqZSC PEBBs.

VI.

CONCLUSIONS

A novel approach to a versatile power electronic building block for the residential DC coupled power distribution systems is proposed and experimentally validated. The PEBB is based on the synchronous resonant quasi-Z-source DC-DC converter, which, due to flexibility of the power circuit and control could be used either in uni- or bi-directional applications without any hardware modifications. The PEBB has several merits such as wide input voltage regulation range and continuous input current at the LV side, galvanic isolation, low voltage ripple at the HV side, integrated control, protection and communication systems, which enable its use in the design of different residential power conditioning systems ranging from PV microconverters to fuel cell and battery controllers. As a result, this PEBB could be standardized to reduce production costs, installation time, staff training expenses and shipping costs, while enabling high efficiency, scalability, smart metering functionality, and reliability. Future research will address issues of autonomous operation of DC microgrid based on the proposed PEBB using emerging control methods without direct communication link, like DC bus signaling. ACKNOWLEDGMENT

84 82 0

200

400

600

800

1000

Fuel cell power, W Fig. 15. Efficiency of the PCU developed measured according to the power profile of the fuel cell H-1000 with and without cycle skipping control.

This research was supported by the Estonian Centre of Excellence in Zero Energy and Resource Efficient Smart Buildings and Districts, ZEBE, grant 2014-2020.4.01.15-0016 funded by the European Regional Development Fund and by the Estonian Research Council (project PUT1443).

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