Modeling and Simulation of Marine PV Inverter System Based on MPPT Algorithm

Under the influence of the policy of low carbon development, the development of new energy ships is receiving wide attention. Aiming at the fluctuation of the ship power grid caused by the intermittent and random nature of the PV power generation system, this paper builds a set of ship PV power generation microgrid system models based on the Matlab/Simulink simulation platform. Under the conditions of ambient temperature, light intensity, and other factors, the front-stage Boost circuit uses the MPPT conductance increment method to obtain the maximum power point of the PV module. The rear-stage inverter uses SPWM (sinusoidal pulse width modulation) inverter control in the rear stage to observe the output effect under different solar radiance, and the system is also designed to automatically switch the DC load when the light intensity is too low, which reduces the system energy loss and improves the operation efficiency of the marine PV system.


Introduction
Photovoltaic power generation can convert abundant solar energy into electrical energy, which has broad development prospects and application value [1] .Making full use of solar energy, wind energy, and other renewable energy to achieve energy saving and emission reduction is the development direction of future ships.Photovoltaic power generation applied to ships can serve as part of the energy source of the ship's power propulsion system, but influenced by the external environment and other factors, photovoltaic power generation is intermittent and volatile, leading to the instability of the output power [2] , which cannot be directly supplied to fishing vessels.Therefore, it is necessary to select a suitable inverter control strategy for PV power generation to provide a high-quality power source for the power-using load.For the current PV power generation system simulation model with complicated data calculation and many parameters, the fishing vessel often has load fluctuations during navigation, so this paper proposes a marine PV power generation model based on Matlab/Simulink, and the simulation results show that this strategy can achieve a more good and stable voltage output of the ship PV power generation system.

Photovoltaic cell model
The solar PV module of Trina Solar is used in this study, and the I-V characteristic curve of the PV module and the P-V characteristic curve under different light intensities are shown in Figure 1 below.The top graph in Figure 1 shows that the short-circuit current and open-circuit voltage of the PV cell are affected by the light intensity and are proportional to each other; the bottom graph shows that the maximum power output of the PV cell is obtained at a certain point, and the maximum power value increases with the increase of irradiance.

MPPT conductivity increment method control
PV arrays can produce different output voltages at different temperatures and solar irradiances.At a certain output voltage value, the output power reaches its maximum value, when the operating point of the PV array reaches the highest point of the output power voltage curve, called Maximum Power Point (MPP).To improve the overall efficiency of the PV power system, the working point of the PV array needs to be adjusted in real-time so that it is near the maximum power point, and this process is called Maximum Power Point Tracking (MPPT) [3][4][5] .
Traditional MPPT control schemes, such as the perturbation observation method and constant voltage method, cannot be accurately stabilized at the maximum power point or lack a voltage regulation mechanism [6] .In this paper, we choose the conductance increment method control, which can achieve fast and stable tracking control at the maximum power point.The biggest advantage of this MPPT control algorithm is that the output voltage of the PV array can smoothly track its change when the irradiance changes and the steady-state oscillation is relatively small [7][8] .
The light intensity S=1, 000 w/m 2 and temperature T=25°C are generally taken as reference values in a standard environment.When the light intensity and temperature are not reference values, the maximum power point is changed and new parameter values need to be recalculated.We suppose that  is the short-circuit current,  is the open-circuit voltage, and the voltage and current at the maximum power point are  and  .Under any light intensity S and any temperature T, when the output voltage of the PV cell is V, its corresponding current is I.

Inverter controller strategy selection
The DC power generated by the solar cell needs to be converted by the inverter to be fed into the AC grid, and the choice of its control strategy determines whether the inverter system can operate efficiently, safely, and stably enough under different environmental conditions.In this paper, we choose the SPWM (sinusoidal pulse width modulation) control strategy, whose basic principle is the principle of area equivalence, that is, through a series of narrow pulse signals of different shapes, the integration of the corresponding time is equal, that is, the area is equal, and its final effect is the same [9] .Figure 2 shows the simulation model of the SPWM control strategy built by the system.
Figure 2. SPWM control strategy model

System modeling and simulation results analysis
The simulation model is built by using Matlab simulation software, as shown in Figure 3, where we set the number of photovoltaic panels in parallel to 4 and in series to 10, the ambient temperature to 25°C, and the simulation time to 2 s.The irradiance is 1, 000 W/m 2 under AM 1.5 standard atmospheric environment [10] , which is the sunlight irradiation to the ground and sea surface on a clear day.Therefore, when we build the model, the solar irradiance setting is changed from 1, 000 w/m 2 to 500 w/m 2 , which is closer to the actual working condition of the ship on the sea surface.The system is also built with variable AC load to verify the output phase voltage under different light intensities.During the simulation, by observing the waveform diagram in Figure 4 and recording the effective value of Table 1 in real-time, it was found that the load voltage had a slight distortion during the process of changing the light intensity to the final stabilization, but it quickly returned to normal.The effective value of the load voltage under different light intensities is always maintained at about 220 V.
The maximum power value and the voltage at the maximum power point of the time-voltage module were tested and recorded respectively in the experiments when the irradiance was 1, 000~500 w/m 2 as shown in Table 2, and the irradiance was within the normal range at this time.When the irradiance is too low (below 400 w/m 2 ), the voltage of the ship's PV power generation system is too low and the variable AC load is switched to 0. To reduce the energy loss caused by the inverter, the PV power generation system is switched to a DC load power supply through the IDEAL SWITCH module after the boost circuit.The DC load resistance value is set to 50 Ω in the system simulation, as shown in Figure 5.The output voltage of the PV system is shown in Figure 6, and it can be seen that when the irradiance changes at 0.6 s and 1.2 s, the voltage amplitude changes greatly, and the quality of the ship's power grid will be seriously damaged by direct grid connection at this time.After using MPPT control, the system always works at the maximum power point.When the external environment changes, the output voltage changes more smoothly and does not produce excessive voltage amplitude fluctuations, which ensures the efficiency of the PV module and improves the power quality.

Concluding remarks
With the technology of the time and environmental awareness, PV power generation will certainly be applied to ships more and more.In this paper, the simulation model of a PV grid-connected system is built, the MPPT conductance increment method control is applied in the Boost circuit, and the maximum power point of the PV array is tracked by adjusting the duty cycle of the trigger signal.The SPWM control strategy is adopted in the inverter part to realize a good and stable voltage output of the PV power generation system, and the system automatically switches to DC load when the light intensity is too low, which saves the whole system.The inverter part adopts the SPWM control strategy to achieve good and stable voltage output of the PV power system and considers the automatic switching of the system to DC load when the light intensity is too low, which saves the energy loss of the whole system and has good practicality.

Figure 1 .
Figure 1.I-V characteristic curve and P-V characteristic curve of PV module under different light intensity

Figure 3 .
Figure 3. Photovoltaic power generation system modelThe model simulates the working time of a real PV power generation system with a simulation time of 2 s.Given the randomness of the actual light intensity change, the solar irradiance is set as follows: initial-600 w/m 2 , 0.6 s-1, 000 w/m 2 , and 1.2 s-600 w/m 2 , and the inverter output voltage and current and input voltage waveforms are observed, as shown in Figure4.

Figure 4 .
Figure 4. Inverter output phase voltage, current, and inverter input voltage waveforms at different light intensities

Figure 5 . 2 Figure 6 .
Figure 5. Simulation of switching DC load section when light intensity is lower than 400 w/m 2

Table 1 .
Inverter output phase voltage, current, and input voltage RMS

Table 2 .
Photovoltaic cell maximum power