**4. Simulation and Experimental Tests of Novel EV Charger**

The electrical diagram of the simulation linear circuit model of a three-phase EV charger is depicted in the Figure 10. The EV battery is represented with a resistor R1 and voltage source E3. Using different values of sinusoids frequency and its amplitude in the inverter PWM control (SINE1, SINE2 and SINE3 PWM control-Figure 10), it was possible to test the EV charger properties for different values of the modulation factor M and the frequency of modulating voltages. The frequency of the triangular carrier waveform of PWM modulation was set in the TRIANG1 module and *fc* = 3 kHz was used. It is a typical carrier frequency for inverters in the industrial high power drives. The frequency of SINE modules is 300 Hz, and it was the maximum output sinusoidal frequency of industry drive FC used in the laboratory stand. The modulation factor M can be changed between 0 and 1.25 to control output inverter voltage.

The use of the PWM modulator model described by the state graph made it easy to control the inverter IGBT transistors in relation to the sinusoidal PWM pattern. It was assumed that all model elements used in electrical circuits had linearized parameters.

To receive the constant charging current 800 A, the control of the modulation factor value M was used. Figure 11a,b show the obtained results of current and voltage waveforms, which are depended on the modulation factor's value (according to the model from Figure 10, M = 0.6935). The EV battery charging current 1 C was used in the test. The value of the charging current results from the technical specification of tested the Li-ion battery [22]. In the case of continuous modulation (the maximum value of modulation factor is M = 1), the EV battery voltage increased and the current exceeded the permissible charging value, which could destroy the EV battery. Therefore, it is important to choose the appropriate value of the modulation factor M, which allows battery charging with constant current for nominal pack voltages at the level to about 500 V [36].

**Figure 11.** Constant-current battery charging measured in the time interval up to (**a**) 0.5 s, (**b**) 2 ms.

The tests of a diode rectifier powered by an inverter were performed under the following conditions:


In case 1, it was not possible to adjust the value of the rectified voltage. In case 2, the impedance of the LC filter chokes caused an unfavourable significant drop in the rectifier supply voltage. Case 3 made it possible to control the rectified voltage in a wide range. Moreover, the elimination of the capacitors on the DC side of the rectifier did not significantly increase the AC component in the rectified voltage.

The specification of laboratory stand depicted in Figure 12a is presented in Table 1.

**Table 1.** Specification of the laboratory stand from Figure 12a.


When the negative pole of the load (EV battery) is grounded, the CM voltage is absent in the rectified voltage. Therefore, there is no need to filter the inverter CM voltage by using the capacitors (No. 5, Table 1) attached to the DM filter from one side and AC phase voltages from the second side.

**Figure 12.** Laboratory stand: (**a**) equipped with two drive VFCs with built-in rectifying units attached to PWM inverters - the detailed specification is in Table 1, (**b**) with three-phase rectifier unit *In* = 15 A built into the drive VFC, (**c**) programmed value of the maximum output current in 5.5 kW drive VFC.

Given by the VFC drive current limitation, the rectifier load current maintained a constant value thanks to automatically lowering the rectifier supply voltage by the inverter control system. Figure 12b shows the programmed value of the maximum output current 8 A of low voltage (3 × 400 V/50 Hz) and small power (5.5 kW) industrial VFC drive (No. 6, Table 1). The maximum value of the rectified current did not exceed 10 A, which resulted from the power balance (*PAC* = *PDC*).

Figure 12c shows the six-pulse diode rectifier built for drive VFCs of the laboratory stand. Drive VFC outputs (1) were connected to a fast six-diode rectifier (2) with ferrite anti-distortion filter (3) and capacitor bank on the constant voltage side (4) of the rectifier, thus it was possible to charge EV batteries with DC current (5). The DC voltage fluctuations did not depend substantially from the value of the capacitor bank, because the inverter frequency of fundamental voltage harmonic was set to 300 Hz.

The received output DC voltages and DC currents are presented in Figure 13, for the capacitor bank equal to C = 16 μF. By comparing Figure 13a,b, the effective operation of the EV battery charging current stabilizer was visible. The rectifier voltage depends on the value of load resistance. A two-times decrease of the resistance resulted in a decrease of the charging voltage from 500 V to 300 V. In Figure 13a the DC voltage and charging current had a constant value and the output power was about 1 kW. When the rectifier was loaded with the resistance of 30 Ω (Figure 13b), the voltage supplying of the six-pulse diode rectifier automatically decreased. The load current was at the same value of 10 A in accordance with the set point of current limiter in the drive VFC.

**Figure 13.** The DC output voltage and current of EV charger when current limiter in drive VFC is active at different value of rectifier load: (**a**) R = 50 Ω—the current limiter of the inverter is inactive, (**b**) R = 30 Ω—the current limiter of the inverter is active *Ilimit* = 8.0 A.

The operation of the drive VFC could be programmed to perform expected functions of an EV battery charger. The experimental tests done for low power setup confirmed the correctness of performed simulation tests and the possibility of using the drive VFC as the basic DC/DC converter component of EV fast chargers.

#### **5. Discussion**

When charging the EV battery, the voltage inverter does not use freewheeling diodes (they are inactive), because there is a unidirectional energy flow from the DC microgrid to the diode rectifier. Therefore, it should be assumed that the efficiency of charging system will be similar to the efficiency of a drive VFC. The rectifier diodes cause losses similar to those in the inverter freewheeling diodes when supplying an induction motor.

If the drive converter is powered only from the DC microgrid, then it is possible to use one integrated circuit with an inverter and a rectifier to build a DC/DC converter for charging EV batteries.

When building a new converter for battery charging purposes, it is possible to replace sinusoidal modulation, e.g., with triangular modulation. The advantage of using the triangular PWM Figure 14b instead of the sinusoidal PWM (Figure 14a) is the proportional dependence of the value of rectified voltage and modulation factor M. As the amplitude of the triangular of the modulating wave increases linearly (TRIANG11-Figure 14b), there is a directly proportional increase of width modulated pulse. Such proportionality does not occur if the modulating waveform is a sine wave and the modulated waveform is a triangular wave [34]. The spectral analysis of the inverter CM voltage for sinusoidal and triangular modulation shows that there are no significant differences in the CM voltages, in these both kind of PWM modulations as shown in the Figure 14c,d respectively.

**Figure 14.** *Cont*.

**Figure 14.** Comparison of using different modulations: (**a**) sinusoidal modulation, (**b**) triangular modulation, (**c**) harmonics spectrum in the common-mode (CM) voltage using the sinusoidal modulation, (**d**) harmonics spectrum in the CM voltage using the triangular modulation.

#### **6. Conclusions**

The authors proposed a DC 600 V microgrid, which is connected to the intermediate circuits of drive VFC used in the induction motor drives. Thanks to this solution, the efficiency of electric drives has increased, as energy losses on brake resistors for drive converters have been eliminated. The actual efficiency of the converter has not been experimentally tested. The efficiency can be estimated on the basis of the efficiency of the driving frequency converters (VFCs), which reaches values of 98% [21].

In the proposed solution, there are also losses in the rectifier, but the reactive component of the current does not flow via the freewheeling diodes of the inverter while charging the EV battery. The reactive component of the current flows via the freewheeling diodes of the inverter while supplying induction motors. Therefore, the authors conclude that the efficiency of the proposed converter for charging EV batteries will be similar to the efficiency of the drive converter.

The bidirectional converter is an adjustable current source for battery and it was used as an example of fast charging battery converter in this paper. The authors' solution is a converter with adjustable EV battery voltage source. Using of the drive frequency converter as an EV battery charging converter is a novel solution where EV battery is charged via a constant voltage source with regulated value.

The energy supplied by the generator is transferred to the energy storage or other converters connected to the microgrid (load sharing). RES and ES cooperate with the microgrid, which has a hybrid DC converter power supply system for fast charging of EV batteries. Scheduled EV battery charging was used, which is carried out in such a way that when the motor is powered by a frequency converter, it is used to charge an EV battery or a mobile electric work machine. The battery charging converter has been developed through the adaptation of drive VFC, consisting of the attachment of a diode rectifier. The VFC drive is used to set the rectifier DC voltage value. The phase voltage value and frequency are controlled by the PWM drive parameters of the drive VFC inverter.

The use of a microgrid provides the opportunity to integrate a hybrid power supply system for fast EV charging stations in such a way that the battery charging energy does not increase the load of the power system and in addition has an impact on worsening the power quality indicators in the power system.

#### **7. Patents**

There are three patent applications resulting from the work presented in this manuscript:

1. Power electronic converter with the conversion of alternating voltage into regulated direct voltage for fast charging of batteries in electric vehicles. Patent application no. P-434784 dated 24 July 2020.


**Author Contributions:** Conceptualization, J.R.S., M.Z.-M., D.W. and N.P.; methodology and software, J.R.S. and M.Z.-M.; validation and formal analysis, D.W. and N.P.; investigation and resources, J.R.S., M.Z.-M.; data curation, D.W.; writing—original draft preparation, writing—review and editing, D.W. and M.Z.-M.; visualization, M.Z.-M.; supervision, J.R.S.; project administration, J.R.S., M.Z.-M., D.W. and N.P.; funding acquisition, D.W., N.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** In the part of the article related to the modelling of the electric circuit, the ANSYS national scientific software license has been used, which was funded by a computational grant obtained by Kazimierz Pulaski University of Technology and Humanities in Radom, Poland. This work was also supported by Gda ´nsk University of Technology and the Government of the Russian Federation, Grant 08–08.

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