An Innovative Bidirectional DC-AC Converter to Improve Power Quality in a Grid-Connected Microgrid

The management of the electrical energy still raises a huge interest for end-users at the household level. Home electricity management systems (HEMS) have recently emerged both to warrant uninterruptible power and high power quality, and to decrease the cost of electricity consumption, by either shifting it in off peak time or smoothing it. Such a HEMS requires a bidirectional DC-AC converter, specifically when an energy transfer is required between a storage system and the AC-grid, and vice versa. This article points out the relevance of an innovative topology based on sinusoidal waveforms from the generation of sine half-waves. Such a topology is based on a DC-DC stage equivalent to an adjustable output voltage source and a DC-AC stage (H-bridge) which are in series. The results of a complete experimental procedure prove the feasibility to improve the power quality of the output signals in terms of total harmonic distortion (THD-values about 5%). The complexity of the proposed converter is minimized in comparison with multilevel topologies. Finally, wide band-gap semiconductor devices (SiC MOSFETs) are helpful both to warrant the compactness and the high efficiency (about 96%) of the bidirectional converter, whatever its operation mode (inverter or rectifier mode).


Introduction
The effective management of electricity still fulfills an important role in achieving the objectives of the sustainable development strategy, particularly addressed by the European Union's energy policy [1,2]. Distribution systems require two key performance indicators: efficient delivery, and reliability of service [3]. Their monitoring and controllability have been enhanced with the ongoing smart grid development [4]. In particular, the modern smart grid is intended to facilitate load-shifting, to avoid load peaks, and to identify automatically faults or outages [5]. All these issues are of utmost importance because of the huge increase of independent small scale power generation systems, also called microgrids. A microgrid, which must supply power to the electricity consumers, can mainly operate in two modes: grid-connected mode, and off-grid mode [6]. The off-grid mode can be achieved in two ways. The first one considers that the electrical power is not available, for example because of a failure or shutdown of the grid. The second one assumes that the power grid is totally inaccessible (i.e., islanded mode). Over several years, standalone microgrids have been widely discussed in the literature. In particular, these kinds of systems consist of photovoltaic (PV) arrays and/or wind turbines, and energy storage systems (i.e., flywheels, supercapacitors or batteries) both to design voltage regulation policy, and control-based load tracking systems [7,8].
At the moment, power management strategies play an increasingly important role in power quality regulation for micro-grids [9]. In particular, the ultimate challenge is to control power flows 1. Enhance the literature review in the field of bidirectional DC-AC converters dedicated to microgrids. 2. Design and test a bidirectional DC-AC converter used in HEMS that have the following main features: high efficiency (higher than 95%), high power quality (THD of the output signal lower than 5%), and high compactness through the use of wide band gap (SiC) power devices. This manuscript is composed of three main sections. In section 2, a complete literature review is proposed to highlight the limitations of existing DC-AC converters. In section 3, an innovative topology of a bidirectional DC-AC converter with the main characteristics previously quoted is introduced. Section 4 discusses the main experimental results, and proves the relevance of such a topology.

Limitations of Existing DC-AC Topologies
Multilevel converter topologies are still widely used for medium and high voltage applications, such as electrical motor drive or grid connected converters, because they generate very low harmonics. The higher the number of levels, the lower the harmonics. Multilevel converters can be divided into two categories: symmetric topologies, and asymmetric ones. Karimi et al. have recently pointed out that the performances of each category depend on the number of DC sources, and the number of semiconductor devices that must be controlled [25].
The voltage of the DC sources determines the maximum amplitude of the output voltage. The number of DC sources sets the number of voltage levels. As can be seen in Figure 3, it is possible to design a multilevel inverter with approximately the same number of DC sources up to 7 levels. Above this limit asymmetric topologies offer the possibility to use fewer DC sources.  . Symmetric and asymmetric multilevel inverters: variation of the number of DC sources versus the number of levels [25].
Multilevel converters may also require a high number of power semiconductor devices to be controlled, gate drivers, and diodes. As can be seen in Figure 4, asymmetric structures meet their absolute meaning when the number of levels is higher than seven. Thus, it helps to reduce the number of gate drivers, and the complexity of the control strategy.  At the moment, neutral-point clamped (NPC), flying capacitor (FC) also known as capacitor clamped, and cascaded H-bridge (CHB) multilevel converters are the most developed topologies (see Figure 5). NPC converters (see Figure 5 (a)) are currently widely used in industrial applications in the range of 2.3 kV to 4.16 kV [26]. In particular, 3-level NPC converters are deployed as widely as possible [27][28][29]. In recent years, 4-level and 5-level NPC converters have been studied both to improve their power quality and increase their output power [30,31]. Nevertheless, 3-level NPC topologies have most times found industrial applications. The main drawback of a NPC topology is the difficulty to maintain the voltage balance between all DC capacitors. This issue has been solved through the development of new control strategies. However, such a strategy requires a significant computing power.
FC inverters (see Figure 5 (c)) are more commonly dedicated to high voltage and high power applications, because they allow a large serialization of power components [32]. Power semiconductor switches and diodes show some weaknesses in high voltage. The main challenge of FC inverters is to equally share the voltage between their various capacitors. The control strategy of this kind of multilevel converter must take into account the state of charge of all capacitors both to ensure a safe operation, and a high efficiency of the whole converter. However, in high power applications, FC topologies require large capacitor banks [33].
CHB inverters (see Figure 5 (b)) have been widely discussed in the literature because of many advantages such as simple layout (i.e., H-bridge), high modularity, and simple control strategy. Such a topology is a cascaded inverter which is composed of an addition of numerous converters connected in series. However, for PV power plants, the CHB topology exhibits a major drawback. Indeed, the power flow inside the power semiconductor switches is unbalanced, because of the electrical features of a PV module (i.e., the voltage and current from a solar panel are unbalanced too). All multilevel converters require the increase of the number of power devices to improve the output power quality. This necessarily has an impact on the compactness and the cost-effectiveness of the converter, but also on the control strategy complexity. Asymmetrical CHB topologies have been introduced to overcome those issues. Such a topology proposes to use unequal DC sources to reduce the number of power devices. This leads to an unequal power flow inside the various power switches, and unequal voltage stresses. Compared with NPC and FC topologies, CHB inverters require also many DC sources which must be isolated. Table 1 sums up the main performances of such topologies reported in the literature [34][35][36][37][38][39][40][41][42]. The most important requirements stated in the previous section (i.e., efficiency higher than 95%, and THD of the output signals lower than 5%) can be achieved, and particularly with the NPC and FC topologies [34], [39]. Kadam et al. have particularly highlighted that 5-level NPC topology requires a high number of semiconductor devices (i.e., 8 power switches to be controlled, and 4 diodes), and a high number of mandatory capacitors (i.e., 4 capacitors) for the required function [34]. Regarding the FC topology, Lei et al. have highlighted that it is necessary to increase the number of levels (i.e., 7-level inverter structure), the number of semiconductor devices (i.e., 16 power switches to be controlled), and the number of mandatory capacitors (i.e., 6 capacitors) to meet the same performances as described above [39]. As a consequence, this solution increases the complexity of the converter. Some of the above issues can be partially covered by CHB topologies. Indeed, Villanueva et al. have proved that the THD-values of the output signals (i.e., either the current or the voltage) can reach approximately 5%, without increasing the complexity of the topology (e.g., 8 semiconductor devices to be controlled) [40]. However, in this kind of 5-level inverter, the magnitude of the DC voltage is doubled in comparison with the NPC and FC topologies. Finally, in this example, the efficiency of the whole converter is not discussed.  Figure 6 shows the general architecture of the bidirectional DC-AC converter proposed in this article. Such a topology is based on the energy transfer between a DC voltage source and an AC one, and vice versa. The whole converter is composed of a DC-DC stage and a DC-AC stage which are in series.

General Architecture
The aim of the DC-DC stage is to generate a rectified sine wave. The inductance named L (see Figure 6) is calculated to neglect the current ripple (which is the result of the high switching frequency i.e., 300 kHz) in comparison with the low-frequency signal component (i.e., 50 Hz). So, the DC-DC converter is equivalent to a controllable output voltage source. The output current can be controlled by adjusting the voltage from an adaptive filtering stage. It has a major interest, and especially when the output current decreases drastically. In such cases, it is possible to decrease the Vc-voltage. In particular, when the Vc-parameter is higher than the Vac-one (i.e., the voltage from the AC mains), the output current has a positive value, and vice versa. Therefore, the electrical energy transfer is performed from the storage system to the AC-grid (inverter mode) or from the AC-grid to the storage system (rectifier mode). The DC-DC converter is composed of wide band-gap semiconductor devices, i.e. 36 A, 900 V SiC MOSFETs (C3M0065090D, Cree). Those power switches enable, among other things, to increase the switching frequency to optimize the compactness of the whole converter.
The DC-AC stage is in charge of inverting a half sine wave out of two to get a full sine wave output signal. This stage is composed of four 47 A, 500 V MOSFETs (IRFPS43N50KPBF, Vishay). Their switching frequency is equal to 50 Hz. This low frequency enables the use of MOSFETs manufactured on a silicon substrate. The power devices' turning-on and off are performed with zero-crossing of the AC mains to minimize the losses. The use of a DC-DC converter coupled with an H-bridge has many advantages comparing to existing topologies:


Standard DC-DC converter and H-bridge are well-common topologies.  Most of topologies use an H-bridge composed of at least four switches which operate at high frequency. In the topology described in this article, only two power devices (inside the first stage) operate at high frequency (i.e., 300 kHz). All the switches inside the H-bridge operate at low frequency (i.e., 50 Hz).  At high frequency, it is of utmost importance to take into consideration the delay between two switches in the same leg for safety reasons. With only one leg at high frequency, the safety delay is easier to control.  At high frequency, the capacitance used to modulate the Vc-voltage is low (about 13 µF). Many other topologies consider that the use of AC-type capacitor is mandatory.

Solutions to Modulate the Output Voltage of the DC-DC stage
As can be seen in Figure 7, three solutions are proposed to adjust the Vc-voltage of the DC-DC stage. To explain each solution, the whole DC-AC converter is assumed to operate in the inverter mode.
The first solution (see Figure 7, Case 1) consists in using directly the capacitor (named C) of the DC-DC converter, which is equivalent here to a BUCK converter. Its capacitance must not be too low to avoid a high ripple on the Vc-voltage. Even if this solution is simple because it does not use any power device, it has a major drawback. Indeed, a current may flow inside the transistor named T2 (see Figure 6) during the discharge of the capacitor named C. Thus, it can lead to overheating of the transistor.
To avoid the problem described above, as can be seen in Figure 7, Case 2, a resistor coupled with a switch can be used. If the impedance of the load is too high, then the capacitor named C can be discharged through the resistor. It is important to note that this solution is useful when the output power of the inverter is not constant during its operation. However, this solution is particularly penalizing for the efficiency of the DC-DC stage and of course, the whole DC-AC converter.
The last solution (see Figure 7, Case 3) consists in n-quadrupoles in parallel. Each quadrupole is composed of a capacitor and a power switch which are in series. The aim of such a solution is to fix the ripple of the Vc-voltage. In that case, there are 2 n -1 possibilities to adjust the equivalent capacitance of the system. Even if this solution is slightly more complex than the previous ones, an adaptation to much more loads can be achieved. Finally, the efficiency of the DC-DC stage is not so much penalized in that case. In the next sections of the manuscript, three capacitances were used.

Inverter Mode
In the inverter mode, the power flows from the storage system to the AC-grid. In this kind of operation, the most important objective is to control the current injected to the AC-grid by adjusting the output voltage of the DC-DC stage. In that case, the modulation of this output voltage is performed using three quadrupoles in parallel (see Figure 7, Case 3). The values of the three capacitances are equal to 10 µF, 1 µF, and 68 nF, respectively. It is important to remind that the values of those capacitances must be well-designed to minimize the ripple of the Vc-voltage. As can be seen in Figure 8, when the value of the capacitance is low or high, the THD-values of the output signals increase. Figure 9 shows the structure of the control circuit of the MOSFETs inside the first stage. This stage is in charge of generating a half sine output signal. To achieve this objective, the microcontroller adjusts the duty cycle of each power device. The higher the duty cycle, the higher the injected current on the AC-grid.

Rectifier Mode
In the rectifier mode, the power flows from the AC-grid to the storage system. In this kind of operation, the most important objective is to control the absorbed current. This one must have a sinusoidal waveform. As a consequence, a power factor correction (PFC) is mandatory to be compliant with the IEC 61000-3-2 standard. This function is performed thanks to the DC-DC stage.
As can be seen in Figure 10, it is also important to modulate the Vc-parameter by adjusting the equivalent capacitance (see Figure 7, Case 3). In particular, this capacitance must be optimized (about 800 nF) to be sure that the absorbed current is quasi sinusoidal. Figure 11 shows the structure of the control circuit of the MOSFETs inside the DC-DC stage. The control circuit is composed of a current sensor and a voltage sensor which enable to create the PFC strategy.

Experimental Test Setup
The operation modes of the bidirectional DC-AC converter described in this manuscript were validated at low power (i.e., lower than 1.5 kW) through a complete experimental test procedure. Figure 12 gives the illustration of the whole converter. This one is composed of a DC-DC stage, a DC-AC stage, an adaptive filter, and a power supply (i.e., +5 V, and +12 V) to supply the onboard electronics. The whole converter is powered by a STM32F407VG microcontroller. A 3 kW programmable DC power supply (reference: SM300-10, Delta Electronika) was used to simulate the storage system (i.e., the DC input). This kind of power supply has two independent outputs (i.e., each channel can be adjustable from 0 to 300 V for the voltage, and from 0 to 10 A for the current). A resistive load (1,300 W rheostat; from 0 to 11.5 Ω; 10 A; reference: ECO2-106, Langlois) was used in the DC-AC stage to be sure that its main principle of operation was well contained. Of course, in the near future, the final prototype of the whole system will be tested using non-linear loads.
A high voltage differential probe (reference: P5205, Tektronix), and a 15 A AC/DC current probe (reference: TCP202, Tektronix) were used to measure the output voltage and output current, respectively. The output power of the DC-DC stage was measured using a watt meter (reference: PX 110, Metrix). Regarding the measurement of the output power of the DC-AC stage, the same type of electronic meter was used. Finally, the THD of the output voltage (or current) of the inverter was measured using a clamp-on harmonic power meter (reference: F27, Chauvin Arnoux). This kind of power meter can measure a THD-value up to the rank no. 25, with frequencies between 0.5 Hz and 20 kHz.

Inverter Mode
In this section of the manuscript, the aim is to validate the operation of the bidirectional DC-AC converter in the inverter mode. The experimental tests were performed in the following conditions:  Figure 13 shows the output signals of the inverter. In this mode, the converter generates quasi sinewave signals. It is important to note that the inverter was not connected to the AC grid. The resistive load used in this experimental procedure was slightly inductive (i.e., about 360 µH and 285 µH at 120 Hz and 1 kHz, respectively). Thus, the current through the load is smoothed. Figure 14 shows the harmonic spectra at 50 Hz and 50 kHz of the output signals, either the current (see Figure 14 (a) and Figure 14 (b)) or the voltage (Figure 14 (c) and Figure 14 (d)). The main contribution of each spectral decomposition is due to the fundamental, because of the quasi sinewave signals. The harmonics are reduced, whatever the output signal. In the experimental conditions described above, the THD-values of the output signals are about 5%. In comparison with a 5-level topology described in the literature (THD-value about 37%), under approximately the same experimental conditions, the THD-value is here very low [18]. As a consequence, the topology proposed in this article is validated in the inverter mode.

Rectifier Mode
In this section of the manuscript, the aim is to validate the operation of the bidirectional DC-AC converter in the rectifier mode. Above all, the aim was to prove the feasibility to create a power factor correction (PFC). Specifically, the sinusoidal current absorption was carried out thanks to the control of the transistor named T2 (see Figure 6) of the DC-DC stage. The experimental tests were also performed at low power to avoid deteriorating the prototype of the whole converter. Figure 15 highlights the effective operation of the rectifier in the following conditions:  Input voltage (i.e., AC voltage): 100 V (peak value).  Output voltage (i.e., DC voltage): 300 V.  Output power: 120 W. Figure 16 shows the harmonic spectrum of the output current. In particular, these results exhibit the benefits of a sinusoidal current absorption on the magnitude of the harmonics.  Figure 17 shows the evolution of the efficiency of the bidirectional DC-AC converter depending on the output power.

Efficiency of the Bidirectional DC-AC Converter
Regarding the inverter mode (see Figure 17 (a)), the measurements were performed up to 1,300 W. The experimental results exhibit that the efficiency of the inverter can reach 96%. It must be noted that these results depend on the overall topology of the DC-AC converter. Even if the efficiency of the DC-DC stage is very good (i.e., about 98.5%), it would be difficult to improve the efficiency of the overall DC-AC converter, because this topology is composed of two stages in series.
Nonetheless, these experimental results are very encouraging, particularly for one third of the targeted value of the output power.
Regarding the rectifier mode (see Figure 17 (b)), the measurements were performed up to 500 W. The experimental results are very satisfactory, because the efficiency of the rectifier can reach 95.5% at low power.

Conclusions
This article proposed an innovative bidirectional DC-AC converter that can operate in HEMS, particularly when an electrical energy transfer is required between a storage system and the AC-grid, and vice versa. The proposed topology is based on two stages, a DC-DC stage (composed of SiC power MOSFETs that are controlled at 300 kHz) and a DC-AC one (an H-bridge composed of silicon MOSFETs that are controlled at 50 Hz), which are in series. The aim is to generate sine half-waves to optimize the power quality of the whole converter. Such a topology was tested through a complete experimental procedure.
The main achievements of this study are summed up below: In this article, a first prototype of the bidirectional DC-AC converter was designed and tested at low power (i.e., lower than 1.5 kW). Moreover, the whole system was not connected to the AC-grid. It will be interesting to test a grid-connected prototype of the converter. Finally, its operating modes will have to be validated for its targeted nominal power (i.e., 3 kW).
Author Contributions: Sébastien Jacques, Cédric Reymond and Jean-Charles Le Bunetel organized and refined the manuscript. Sébastien Bissey conducted all the experiments and co-authored the article.
Acknowledgments: These research activities are currently supported by "Région Centre Val-de-Loire" (research project number: 2015-00099656). The authors of this manuscript thank our colleagues from this institution who provided insight and expertise that greatly assisted the project.

Conflicts of Interest:
The authors declare no conflict of interest.

Abbreviations
The following abbreviations are used in this manuscript: