Two-Stage Converter Standalone PV-Battery System Based on VSG Control

Standalone solar PV systems have emerged as potential alternatives to electricity problems in areas where a grid is unavailable. Obtaining full power from a photoelectric system, DC-DC inverter, DC-AC converter, and control system presents great difficulties when building these devices. A standalone two-stage approach is introduced in this work with a boost converter followed by an inverter and a battery with a bidirectional converter. In this paper, a novel virtual synchronous generator (VSG) controller is designed and implemented to adjust the inverter output. The VSG element and the maximum power point tracking (MPPT) used in this study serve the following purposes: to adjust the inverter output and to realize the maximum power of the PV scheme. The new control strategy design was evaluated and validated using extensive MATLAB simulations under different scenarios, including load variations. The system output was evaluated using extensive MATLAB simulations. A hands-on experiment was conducted for the VSG using console testing. Due to the lack of laboratory equipment, we could not experiment with the entire system.


I. INTRODUCTION
PV systems are increasingly being used as distributed generators worldwide because they are environmentally sustainable and clean. The price of PV panels has dropped significantly, and they typically have DC power, which is not always stable. Therefore, before the PV signals can be fed to the output load or linked to the grid power, they require DC-DC or DC-AC conversions. However, since PV power is unstable, standalone PV units need an energy power storage component, usually implemented by a battery bank [1]- [8].
PV supplies change over time because of the source and condition's variable nature under the load or grid specification demand. A single and a two-stage process are two methods used to integrate the solar energy with the load or grid [9]- [11]. The first method is made up of inverters that The associate editor coordinating the review of this manuscript and approving it for publication was Yijie Wang . convert DC to AC according to the load's demand. In the second one, however, the DC voltage supplied by PV cells is first increased and then inverted into AC as needed [12], [13].
Recently, studies on two-stage grid converter structures of standalone applications have been limited. In [14], a twostage single-phase PV system designed to operate in a standalone configuration without batteries was presented. The authors concluded that the device could adequately provide stand-alone power without the use of batteries. Thus, the system costs and maintenance can be reduced if the system is designed practically. Article [15] introduced a proportionalintegral (PI) design to control a two-stage standalone system. The control goals were achieved with satisfaction and success, as per the obtained results.
However, these works do not mention how a VSG control can operate under a two-stage converter. This paper focused on a two-stage converter operating in a standalone mode with a battery based on the VSG control system application.

II. PROPOSED CIRCUIT
A. MAIN CIRCUIT Fig. 1 depicts a schematic diagram of a two-stage converter. A DC-DC, DC-AC, and a storage unit through a bidirectional converter via the PV power source to supply the load are observed. The system also contains the control system of the two stages.

B. THE PV ARRAY
Solar PV units are linked in parallel and series combinations to provide higher power production and create solar PV arrays. This article's PV array has six parallel and four series panels with a combined input capacity of 5 kW. The solar module parameters are shown in Table 1. Fig. 2 presents the solar panel characteristic curves.

C. DESIGN OF THE CONVERTER DC-DC
A boost converter is employed to achieve the required voltage. Fig. 3 presents the design of the frame structure of the DC-DC stage.
The design specifications are defined as follows:   Input voltage V in = 100 to 150 V, switching frequency f sw = 5 kHz, output voltage V out = 500 Vd c , rated power P = 5 kW, current ripple I = 5%, voltage ripple V = 1%. Then, I in = 33 to 50 A and I = 1.65 to 2.5. The inductance capacitance is calculated as:

III. THE CONTROL SYSTEM A. MPPT ALGORITHM CONTROL
The values of PV panel calculations based on PV current and voltage are described in the block diagram shown in Fig. 4. Through this process, converter gate pulses are obtained. Fig. 5 presents the duty-cycle controlled flow chart MPPT algorithm used to verify the PV unit's power on maximum point. The voltage state of the power is determined by multiplying the PV panel current and voltage with the previous one in the duty-cycle control approach. It checks the voltage sampling value of the voltage with the last value in the same way and regulates the duty to obey the power on the maximum point based on the result of the matching power. Fig. 6 shows an inverter block diagram with a control based on the VSG, which consists of the power load and control circuit. The input of this stage is the current and voltage of the load, and its output is the PWM signal used for switching the inverter.    The VSG active and reactive power loops are described in Fig. 7 and Fig. 8, respectively.  The droop equations are expressed as:

B. VSG METHOD
The swing equations are expressed as C. CHARGE-DISCHARGE BATTERY CONTROLLER Fig. 9 presents the battery control for two cases: charge and discharge. The voltage source of the bidirectional converter is the output of the boost converter. The battery parameters are shown in Table 2. V B represents the battery voltage, and I B represents the battery current. The reference charge and discharge current are I B-ref. cha. and I B-ref.dis. , respectively. The S -P is the pulse for the positive switch part of the bidirectional converter, and S -N is the pulse for the negative side.

IV. RESULTS
Simulation studies validated the process model and method presented in Fig. 1. Three scenarios based on the model described in Fig. 1 were simulated to investigate the VSG controller performance. The parameters in Table 3 are used.
The selection range of Table 3 parameters used values available in the lab to conform with the experimental part of the work. Fig. 10 shows the irradiation applied to the PV. The curve, as shown in the figure, contains various irradiations with a maximum value of 1000 W/m 2 . The PV voltage and firststage voltage followed the change in radiation, as shown in Fig. 11 (a) and Fig. 11(b), respectively. At the first stage, the PV rose to 130 V and boosted voltage to 500 V. This is caused by the nature of the MPPT method in PV, and it provided details for the control behavior.

A. SCENARIO A: DIFFERENT IRRADIANCE VALUES
The current results of the PV array and boost converter output are given in Fig. 12(a) and Fig. 12 (b), respectively. The PV current is approximately 50 A, while the boost converter current decreases to 10 A. This decrease depends on the design of the boost converter. Fig. 13 shows the results of the inverter voltage and current for the output load. The inverter side is connected with the load and the output power waveforms shown in Fig. 14    to the new irradiation curve change was plotted for PV voltage and output voltage of the DC-DC stage, as shown in Fig. 16. As seen in Fig. 17, the MPPT tracks the maximum power point (MPP) with 99% efficiency from the PV panel. The control method applied in [16] has been compared with this paper in terms of efficiency. The PV power result is simulated in [16], and the MPPT algorithm tracks a 98% efficiency applied method based on model predictive control. In this paper, the VSG control method was used, as it provides a higher efficiency rate. The advantages of this control method are used for the inverter to behave as a synchronous generator, used for operating inverters in parallel, and for the stability of   a weak power grid or islanding mode [17]- [20]. Fig. 18 shows the PV array and boost converter current.

C. SCENARIO C: BATTERY CHARGE AND DISCHARGE
In the two cases of charging and discharging, a battery bidirectional converter is used. The voltage source ''boost converter output'' enabled the battery to charge with the load    supplied from the source voltage in charging mode, as shown in Fig. 19. The state-of-charge (SOC) is 50%, as shown in Fig. 19 (a). The load voltage is 220 V, and the battery VOLUME 10, 2022   voltage is charged with a value of 237.5 volts, as presented in Fig. 19 (b) and Fig. 19(d). Fig. 19(c) presents the battery reference current IB on this mode selected as −20 A, and the battery charging current is −10 A.
In the discharging mode, the source voltage is disabled, and the battery supplies the load. Fig. 20 shows the discharge operation. The reference current is 20 A, and the battery is discharged with 236 V and 21 A. The battery is connected in This scenario mainly discusses the response to load changes to verify the VSG control technology that has been successfully deployed to be applied effectively for this system. If the time (S) axis in Fig. 21 (a) is observed, it is  noted that at the 0.5 s mark, another load of 1 kW was added spontaneously and suddenly to evaluate the system performance. As seen, a smooth response has been achieved for active power, as shown in Fig. 21(a). Load output current responses under the load changes are depicted in Fig. 21(b), which proves the effectiveness of the controller.

V. EXPERIMENT OF VSG CONTROL RESULTS
This experiment was carried out via a system containing a single inverter, a load in island mode, and its VSG control circuit. The experimental parameters are given in Table 4.

A. SCENARIO A: CHANGE IN VIRTUAL INERTIA, J
This experiment was carried out for different virtual inertia J = 0.2, 0.8, and 1.4, with the load stepped from 100 to 150 . The results are shown in Fig. 22. Every waveform is the VSG frequency or the active load power. As seen, any increase in the virtual inertia considerably improves the frequency dynamics.  Fig. 23 shows the results. The system implemented is shown in Fig. 24.

VI. CONCLUSION
The paper proposes two-stage converter PV systems, including a battery unit. MPPT algorithms and VSG control used in the proposed control strategy. Additionally, a bidirectional converter control is presented. The combined control provided successful tracking under various irradiation scenarios thanks to the proposed method's quick and responsive control capability. In addition to the combined control performance tests and the favorable effects on the whole system, it is seen from the bidirectional battery method that the control of battery modes also has superior control capacity. The MPPT control was subjected to efficiency analysis. The MPPT system has a 99% control efficiency. The experiment proved that the inertia increase greatly improves the frequency dynamics. This means that VSG can truly and very precisely model a synchronous generator. Therefore, it is considered a good controller for this system.