A grid tied solar photovoltaic based off board electric vehicle charger in G2V and V2G mode

There are substantial costs associated with using fossil fuels such as natural gas, oil, and coal that are not accounted for in market prices in terms of the climate, the environment, and human health.^1,2 Researchers have been finding out the ways of utilizing green energy resources to satisfy electricity shortages. Undoubtedly, electric vehicles (EVs) have the potential to become progressively crucial in the transportation sector going forward, aiding in mitigating environmental issues and current energy related concerns.^3,4

Solar energy is abundant among green energy sources to shift conventional energy sources.⁵ Solar energy is available on the earth’s surface and depends on climate, geographic location, and the time of day. The global grid that connects the counties is expected to supply surplus electricity generated in sun shining regions to the regions that are in darkness, which will eliminate the need for storage. The global grid will be powered by solar Photovoltaic (PV) panels connected to the globe.⁶ 

Electricity demand is subject to fluctuations on a seasonal basis. In the monsoon season, demand is comparatively lower than it is in summer. In such seasonal variations, PV can effectively fulfill the peak load demand.^7,8

A global interlinked network can be technologically feasible and economical. Many opportunities have been initiated from a global interlinked network such as the smooth transfer of the renewable energy supply to fulfill electricity demand over the globe, reducing the need for bulk storage, and reducing the volatility of the energy tariffs.⁹ The demand for electric vehicles has increased due to government programs, self-driving EVs, and significant technological breakthroughs, which have resulted in a massive growth of EV charging stations worldwide.

Furthermore, the development of intelligent EV charging systems has advanced due to the grid based usage of renewable energy sources.^10,11

Since the power from photovoltaic (PV) arrays is produced and consumed within the local area, this type of charging station has a benefit.

For high power, this means that upgrading the transmission lines is not necessary. Additionally, in times of high energy costs, the charging station can operate without drawing electricity from the grid. Another benefit of a charging station based on photovoltaic arrays is its location independence. Coordinated operation of the EV and PV array also avoids the issues brought on by the intermittent nature of PV generation and lessens the impact of PV generation on the energy utilities.^12–18 

Due to the fact that these technologies enable electric vehicles to both take electricity from the grid and return extra energy to the grid, grid-to-vehicle (G2V) and vehicle-to-grid (V2G) technology account for more than half of the global market for smart grids that charge EVs.^19,20 Grid managers have an extra tool to control the supply and demand of electricity owing to this bidirectional energy flow.^21–25 

Various operating modes require different converters and controls. Moreover, the charger’s ability to operate is restricted by the availability of the grid (islanded or grid-connected operation), and these factors have an impact on the charger’s overall operational efficiency.²⁶ A lot of work is being performed to create an integrated system that can carry out the aforesaid tasks, which are advantageous for household loads, the grid, and electric vehicles; therefore, off board chargers are more preferred over on board chargers.^27,28

Many researchers have reported PV grid interfacing using voltage source converters (VSC).^29–31 In terms of high power applications, this topology is well-established and widely accepted, but it has certain limitations. Specifically, it needs specialized control to operate converter power switches and generate synchronous output voltage, which necessitates real and reactive power exchange for compensation.^32–34 Phase locked loop (PLL) is used for obtaining synchronizing signals for continuous tracking of power angle and frequency.³⁵ VSC is connected to the ac grid via a series inductive interface, which is crucial to prevent the dc capacitor from short-circuiting and discharging rapidly into a capacitive load like a transmission line. Furthermore, VSC converters need isolation supplies in driver circuits like boost trap circuits.^36,37

When reactive power control is not a problem and the amount of reactive power used by the converter is able to be supplied from system capacitors and/or filters, the Line Commutated Converter (LCC) has a decisive economic advantage over a self-commutated converter. LCC is a well-known technique used at the receiving end of an HVDC station. For harmonics and reactive power compensation, passive tuned filters are used.³⁸ 

A common mode voltage may be produced by the PV array and grid galvanic connection, leading to the generation of common mode currents. These currents have the potential to introduce electromagnetic interference, distort grid currents, and cause additional losses. To mitigate these issues, a transformer operating at line frequency is incorporated into the grid-tied photovoltaic (PV) system. This transformer provides galvanic isolation between the grid and photovoltaic (PV) array, effectively preventing issues such as dc injection and suppressing leakage currents.³⁹ 

This paper realizes the implementation of grid tied solar photovoltaic based off board electric vehicle chargers using 12 pulse LCC, and with 12 pulse LCC, high characteristics harmonics are greatly reduced. In addition, a simplified maximum power point tracking (MPPT) control is proposed that will track the maximum power point (MPP) of the PV array.

The feasibility of the suggested solution is demonstrated through simulation conducted in MATLAB/Simulink. The results validate the efficacy and practicality of the approach in mitigating common mode voltage issues and improving system performance.

The proposed grid tied solar PV system using 12 pulse LCC for off board EV charging is shown in Fig. 1. The proposed solution can provide bidirectional EV charging operation in G2V and V2G mode. A proposed integration scheme utilizes a 12-pulse Line Commutated Converter (LCC) to connect a photovoltaic (PV) system to a grid via a 33 kV/400 V step-down transformer. This configuration enables the solar PV array to deliver 1 MW of power to the grid. The scheme effectively suppresses the dominant 5th and 7th harmonics, significantly reducing the required filter capacity and enhancing power quality delivered to the grid.

When a transformer provides power to a load with a non-sinusoidal current having an RMS value equivalent to the rated current, the dc losses remain consistent. However, the eddy losses in the windings experience an increase as a result of the heightened frequency of harmonics. Consequently, the temperature rise in the winding increases, potentially exceeding temperature limits.

In the proposed scheme illustrated in Fig. 1, the 12-pulse Line Commutated Converter is operated at a triggering angle of 166°. The value is obtained based on the parameters of the PV panel specified in Table I and an inductance of 0.117 mH, which sets a limit for the firing angle α to prevent commutation failure.

The calculation of photovoltaic (PV) system requirements is based on referencing the solar panel datasheet provided in Table I, as utilized in previous research.³⁸ These data are employed to determine the quantity of panels required to produce 1 MW of power. Assuming an Earth temperature of 40 °C and a photovoltaic panel temperature of 85 °C, a series connection of 20 PV panels, referred to as a string, is established to obtain the dc link voltage and deliver the required current. To achieve this, 144 such strings are connected in parallel.

In Fig. 3, the dc–dc converter control is shown. The control diagram shown in Fig. 3 is used in the bidirectional buck boost converter. This converter is used to regulate the battery charging and discharging current. When the battery is charging from the grid, in that case the dc-dc converter is operated in buck mode, and the EV battery is charging as shown in Fig. 3(a). When the EV battery is discharging and sending power into the grid, in that case, the dc–dc converter is operated in buck mode, and the EV battery is discharging as shown in Fig. 3(b).

Based on the power ratings, EV battery chargers can be divided into level 1, level 2, and level 3. Table II describes the three different power levels.

Level 2 chargers above 10 kW are recommended for the proposed design.

The control characteristics of both stations of HVDC are given in Fig. 4. The converter operates in three modes in HVDC transmission. (i) Constant ignition angle α (minimum) mode, (ii) Constant Current (CC) mode, and (iii) extinction angle control. The interaction of two characteristics (point A) determines the mode of operation. Similarly, PV array (like sending end station) and grid connected LCC (like receiving end of HVDC station) characteristics are given in Fig. 5. Operating point A or A′ depends on irradiation, temperature, and solar azimuth angle. As irradiation changes, the voltage corresponding to maximum power also changes by varying the firing angle of LCC. Figure 6 shows the surface plot of I–V characteristics and firing angle intersection at maximum power point. The intersection of firing angle and I–V characteristics justifies the maximum power tracking at each value of solar irradiance is possible, and the operation of 12p-LCC is maintained at maximum power as shown in Fig. 7.

The Perturb and Observe (P&O) and Incremental Conductance algorithms find widespread application, approaches that rely on the “hill-climbing” technique.⁴⁰ 

The disadvantage of both approaches, despite their simplicity and minimal processing power requirements, is that MPP might become lost or tracked in the incorrect direction when air conditions change quickly.

These drawbacks are completely eliminated in the proposed MPPT control, in which hill climbing is automatically performed because there is a ripple present in the dc link voltage of LCC, which hill climbs the PV power and makes PV power variable. There is no need for a dedicated algorithm or controller to track the MPP of the PV array. In addition, during rapidly changing atmospheric conditions, MPP will not be lost or tracked in the wrong direction, and only a comparator is required to achieve the MPP of the PV array.

An algorithm based on the hill climbing concept includes changes in power in steps. It may lead in the wrong direction due to the rapid change in irradiance. In the proposed technique, the converter is operated at a firing angle for which the segment of ac source appears at the converter output, which contains ripple and changes at periodic intervals.

The suggested grid-tied solar PV system employing a 12-pLCC is thoroughly examined through a MATLAB Simulink model. The voltage supplied by the grid is 33 kV with a frequency of 50 Hz. The primary side of the transformer represents the grid supply line voltage, whereas each secondary side of the transformer operates at 440 V.

Table III shows the system parameters considered for the simulation of the proposed methodology.

Figure 8(a) displays the solar PV voltage at the constant irradiance specified. Figure 8(b) illustrates the voltage across the dc link of LCC, while the current flowing through the dc link of LCC is illustrated in Fig. 8(c). Notably, in Fig. 8(b), the dc link voltage is negative due to the triggering angle surpassing 90°, resulting in a negative average dc voltage and requiring a dc source for bridge operation. This indicates power supply to the ac grid, signifying the converter is operating in inverting mode.

In Fig. 9(a), the phase voltage is depicted on the 33 kV ac side, while Fig. 9(b) illustrates the current on the same side. Figure 10(a) illustrates the harmonic profile of the current on the 33 kV ac side without the presence of a capacitor bank and filter, while Fig. 10(b) illustrates the same with a capacitor bank and filter. Comparing the two, it is evident that utilizing filters and a capacitor bank reduces Total Harmonic Distortion (THD) from 11.56% to 3.52%. Figure 11(a) demonstrates active, reactive, and apparent power on the 33 kV grid under constant irradiance. Figure 11(b) shows the same with varying irradiance, indicating negative active power due to unidirectional current flow and negative average dc voltage. Notably, the negative active power in both figures is attributed to unidirectional current flow and a negative average dc voltage.

The simulation results depicting the Grid-to-Vehicle (G2V) and Vehicle-to-Grid (V2G) modes of operation of the EV battery using the proposed methodology are displayed in Figs. 12 and 13, respectively. Here, the EV’s nominal battery voltage is considered as 160 V, and the dc link voltage (Vdc) is set at 400 V. Figure 12(a) shows both grid current and grid voltage are in phase, hence power is received by the EV battery from the grid. Figure 12(b) shows the dc link voltage V_dc, which is maintained at 400 V. This is accomplished with a suitable current control scheme. Figure 12(c) shows the EV battery voltage of ∼172 V and the fully charged voltage of the battery is 186 V. Figure 12(d) shows the EV battery current, which is −20 A as the battery is charging from the grid.

Figure 13(a) shows both grid current and grid voltage are out of phase, hence power is transferred from the EV battery to the grid. Figure 13(b) shows the dc link voltage V_dc, which is maintained at 400 V. This is accomplished with a suitable current control scheme. Figure 13(c) shows the EV battery voltage of ∼172 V and the fully charged voltage of the battery is 186 V. Figure 13(d) shows the EV battery current, which is 20 A as the battery is discharging and sending power to the grid.

A grid tied solar photovoltaic based off board electric vehicle charger using 12p-LCC in G2V and V2G mode is proposed in this paper. The suggested solution eliminates the need for a separate circuit to keep the converter and ac grid in synchronization. Solar PV MPPT tracking is achieved by the 12p-LCC converter triggering controller itself. A 400 V dc link voltage is used to simulate charging a 160 V electric vehicle battery. The feasibility of the proposed scheme is demonstrated by the grid-to-vehicle mode of operation and the vehicle-to-grid mode of operation. Grid current has a THD value of 3.52%, which meets the IEEE-519 standards.

The authors have no conflicts to disclose.

Jyoti M. Kumbhare: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Sumant G. Kadwane: Supervision (equal); Visualization (equal).

The data that support the findings of this study are available within the article.