Electric Vehicle Battery Charging Station based on Bipolar dc Power Grid with Grid-to-Vehicle, Vehicle-to-Grid and Vehicle-to-Vehicle Capabilities

An electric vehicle (EV) battery charging station (EV-BCS) based on a bipolar dc power grid is presented in this paper, which is capable of delivering power to the grid (vehicle-to-grid – V2G mode), and directly exchange power between different EVs connected to the EV-BCS (vehicle-to-vehicle – V2V mode), besides the traditional battery charging operation (grid-to-vehicle – G2V mode). The presented EV-BCS is based on three-level bidirectional buck-boost dc-dc converters and has a modular structure. Simulation results are presented with the aim of validating the aforementioned operation modes, being considered two EVs for simplicity reasons, since it is enough to validate the proposed operation modes. The presented results comprise both balanced and unbalanced operation in terms of power from the EVs viewpoint, with the purpose of considering a real scenario of operation, where a balanced consumption or power injection from the bipolar dc power grid side is always guaranteed.


Introduction
Electric vehicles (EVs) are considered a viable alternative to internal combustion engine vehicles, since they reduce the emission of greenhouse gases from the utilization level point of view, as well as the exploitation of fossil resources [1], [2]. Besides environmental issues, EVs can also have an interesting role regarding smart grids, being combined with renewable energy sources and energy storage systems and rendering ancillary services [3]- [5]. In this regard, the most common operation mode and one of the first ones being proposed was vehicle-to-grid (V2G) [6]- [8], in addition to the primordial battery charging functionality (grid-to-vehicle (G2V)). However, one of the main issues bottlenecking the spread of EVs is the battery charging infrastructure [9], [10]. In the literature can be found some strategies aiming to circumvent this issue, such as battery swapping [11]- [14] and the combination of solar photovoltaic panels and energy storage systems [15]- [17].
As a consequence of fundamental systems for distributed generation and smart grids operating in dc, such as solar photovoltaic panels and energy storage systems, dc power grids are also receiving attention from researchers. The transmission of power in dc can be advantageous over ac, since the former does not present constraints such as harmonic currents, reactive power and skin effect, thereby making dc microgrids an important research topic [18], [19]. Regarding dc power grids, they can either be unipolar or bipolar depending on the number T. J. C. Sousa, et al.
2 of active conductors. While unipolar dc power grids comprise only one active conductor and neutral, originating a single voltage value, bipolar dc power grids contain two active conductors and neutral, giving rise to two symmetrical voltages referenced to neutral or, additionally, one voltage with the double of the base value. Compared to their unipolar counterpart, bipolar dc power grids are advantageous since they allow two voltage values instead of one, besides being more reliable and presenting a higher energy transmission capacity [20]- [22].
In the literature, the application of bipolar dc power grids in EV-BCSs can be found. For instance, an EV-BCS based on a neutral point clamped ac-dc converter is presented in [23], where a bipolar dc power grid is formed by the split dc-link of this converter topology. The dc-dc converters that control the battery charging are connected to the bipolar dc power grid, despite not being addressed in such publication. References [24]- [27] analyze the use of three-level dc-dc converters in bipolar dc power grid based EV-BCSs, although the V2G operation is not covered. In this sense, this paper presents an EV-BCS based on a bipolar dc power grid that, besides the conventional G2V operation, also comprises V2G. Furthermore, a relatively recent operation mode is also considered in this paper, namely vehicle-to-vehicle (V2V) [28]- [31]. In the scope of this paper, only the operation of the dc-dc converters of the EV-BCS is addressed.
The paper is organized as follows: Section 2 presents the EV-BCS, namely the used power converters and their control; Section 3 presents the simulation model and the obtained results; finally, in Section 4 are presented the main conclusions of the paper.

Electric Vehicle Battery Charging Station (EV-BCS)
In this section, the power structure of the EV-BCS is presented, namely the employed dc-dc converter and its respective control system. The referred converter is a three-level two-quadrant buck-boost topology, allowing bidirectional power flow and, consequently, G2V, V2G and V2V operation modes. Moreover, the presence of a split dc-link makes this converter appropriate for bipolar dc power grids [32], [33]. Figure 1 presents the EV-BCS power structure for a number of converters (and EVs) equal to two, where, in theory, any number of equal converters can be connected. It should be referred that only two converters were chosen due to simplicity issues and because it is enough to validate the desired operation modes. In [34], the same power structure is analyzed for smart grids applications, i.e., two three-level two-quadrant buck-boost dc-dc converters sharing the high voltage side.
In the low voltage side, each converter x is connected to an EVx, and, in the high voltage side, to the bipolar dc power grid, where the positive rail voltage (vdcpos) is applied to the upper capacitor (C2x-1) and the negative rail voltage (vdcneg) is inversely applied to the lower capacitor (C2x). Being a bipolar dc power grid, it is true that vdcpos = -vdcneg; hence, the high side total voltage is 2vdcpos. Nevertheless, since the dc-dc converter operates with three voltage levels, each power semiconductor withstands only a maximum voltage of vdcpos.
Regarding buck mode, where power flows from the high voltage side (bipolar dc power grid) to the low voltage side (battery), corresponding to G2V operation, each converter x uses power semiconductors S4x-3 and S4x (S1, S4 in EV1 and S5, S8 in EV2). The voltage produced by each converter x (vcvx) can assume three values (0, vdcpos, 2vdcpos). In equation (1), the possibilities for vcvx for each switching state can be seen in buck mode, where 0 represents the off-state and 1 the on-state. Equation (2) shows the two operating regions for the converter with respect to the voltages vbatx and vdcpos and the duty-cycle (D). In terms of modulation applied to the converters, a phase shift of 180º is used between the two active semiconductors, doubling the vcvx frequency with respect to its switching frequency.  EAI Endorsed Transactions on Energy Web 12 2022 -12 2022 | Volume 9 | Issue 5 | e5 A predictive current control was used for controlling each EV battery current. According to this strategy, for each time instant k, each converter x should produce a voltage vcvx so that each current ibatx follows its reference ibatrefx. In buck mode, where each current ibatx is considered positive, the produced voltage can be calculated as follows: where Lx is the inductance value of dc-dc converter x inductor and fs is the digital control system sampling frequency.
Regarding boost mode, where power flows from the low voltage side (battery) to the high voltage side (bipolar dc power grid), corresponding to V2G operation, each converter x uses power semiconductors S4x-2 and S4x-1 (S2, S3 in EV1 and S6, S7 in EV2). The voltage produced by each converter x (vcvx) can also assume three values (0, vdcpos, 2vdcpos). In equation (4), the possibilities for vcvx can be seen for each switching state in boost mode, where 0 represents the off-state and 1 the on-state. Equation (5) shows the two operating regions for the converter with respect to the voltages vbatx and vdcpos and the duty-cycle (D). In terms of modulation applied to the converters, similarly to buck mode, a phase shift of 180º is used between the two active semiconductors, doubling the vcvx frequency with respect to its switching frequency.
The predictive current control is also used in boost mode, in this case with the current ibatx assuming negative values.
For positive values of ibatrefx, the digital implementation of this current control is given as follows:

Computational Simulations
This section presents the simulation model and results of the EV-BCS for two EVs, being addressed the G2V, V2G and V2V operation modes, as well as a combination of V2V with G2V and V2V with V2G. The computational simulations were performed in the software PSIM v9.1 from Powersim. The adopted battery model, which is the Thevenin model, can be seen in Figure 2, comprised by the open-circuit voltage (vocx), a capacitor to emulate the dynamic behavior of the battery (Cbatx), a parallel resistor to emulate the battery self-discharge (Rpx) and a series resistor that represents the internal resistance of the battery (Rsx). Table 1 presents the parameters of the power converter and batteries of each EV (where it can be seen that the converters are equal), as well as the batteries, presenting different initial voltage values in order to emulate a more realistic scenario, with vbat1 starting with 250 V and vbat2 with 200 V.   Figure 3 shows the usual operation of both EVs at a charging station (G2V), both charging their batteries with the same current value (20 A). The figure shows the battery voltages (vbat1 and vbat2) and currents (ibat1 and ibat2), the currents drawn from the dc power grid, namely in the positive rail (idcpos), neutral rail (idczer) and negative rail (idcneg), and the voltages produced by the converters (vcv1 and vcv2). It can be seen that the battery voltages are slightly higher than their original values (2 V higher), which is due to the internal resistance of the batteries and not due to the energy accumulation process, given that the figure initial instant is 2 ms. It can be seen that both battery currents present the same average value of 20 A, but ibat2 presents a much smaller ripple than ibat1. This is due to the fact that the voltage vbat2 is practically half (202 V) the total dc power grid voltage, making the three-level buck-boost dc-dc converter operate in a region of strong ripple cancelling. This is visible in the voltage produced by this converter (vcv2), presenting a very low duty-cycle between voltage levels 200 V and 400 V (in other words, presenting a duty-cycle slightly higher than 50%). It is noticeable from voltage vcv1 that converter 1 operates with the same voltage levels but with a higher duty-cycle, meaning a higher ripple in ibat1. Regarding the currents absorbed from the dc power grid, it can be seen that idcpos and idcneg are symmetrical, with average values of 23 A and -23 A, respectively, the first one being EAI Endorsed Transactions on Energy Web 12 2022 -12 2022 | Volume 9 | Issue 5 | e5 positive and the second being negative, meaning that the dc power grid is providing power. The current idczer is the negative sum of idcpos and idcneg, therefore presenting a null average value.   Figure 4 shows the operation of both EVs in G2V but with different current values in order to simulate an unbalance situation. EV1 is charging with a current of 20 A, while EV2 is charging with a current of 40 A. The figure shows vbat1, vbat2, ibat1, ibat2, idcpos, idczer, idcneg, vcv1 and vcv2. In this case, vbat2 presents a value of 204 V, showing the effect of the battery internal resistance when higher currents are applied. It can be seen that both ibat1 and ibat2 present the expected average value, with EV1 presenting the same results as the previous case. Despite being a higher current, ibat2 still has a low ripple due to the same reason as previously mentioned, as it can be seen from voltages vcv1 and vcv2. Regarding the currents absorbed from the dc power grid, idcpos and idcneg are symmetrical but with a higher average value than previously (33.6 A), also with the first one being positive and the second being negative. Accordingly, idczer has no average value. Based on this result, it can be perceived that the EV-BCS is able to consume balanced currents from the bipolar dc power grid even with unbalanced battery charging operation. After being presented the G2V operation, both balanced and unbalanced, Figure 5 shows the V2G operation mode for both EVs, discharging their batteries with the same current value (20 A). This figure shows the same variables as the previous ones. It can be seen that both battery currents are negative, meaning that the power flows from the batteries to the dc power grid, as expected in the V2G operation mode. Also, both ibat1 and ibat2present the same average value of -20 A, but ibat2 presents a much smaller ripple than ibat1, which is due to the same reason as aforementioned. In this case, vcv2 presents a very high duty-cycle between voltage levels 0 V and 200 V (in other words, presenting a duty-cycle slightly smaller than 50%). This happens due to the internal resistance of the batteries, which decreases the battery voltage when current is being supplied by the battery, as it can be seen by the 198 V vbat2 value, which is lower than half the total dc power grid voltage. Once again, it can be seen that the dc power grid currents idcpos and idcneg are symmetrical, but with idcpos being negative and idcneg being positive, conversely to the previous cases. This means that the dc power grid is not supplying power but receiving instead, as supposed with the V2G operation mode. The average value of these currents is 22.1 A, with the current idczer presenting a null average value.  Figure 6 shows the operation of both EVs in V2G but with different current values in order to simulate an unbalance situation. EV1 is discharging with a current of 20 A, while EV2 is discharging with a current of 40 A. This figure shows the same variables as the previous ones. As previously, both ibat1 and ibat2 are negative, meaning that the power flows from the batteries to the dc power grid, as supposed to happen in the V2G operation mode. Both ibat1 and ibat2 present the expected average value, with EV1 presenting the same results as the previous scenario. In this case, vbat2 presents a value of 196 V, showing the effect of the battery internal resistance when higher currents are drawn from the battery. Despite being a higher current, ibat2 still has a low ripple due to the same reason as previously mentioned, as it can be seen from voltages vcv1 and vcv2. Regarding the dc power grid T. J. C. Sousa, et al.

EAI Endorsed Transactions on
Energy Web 12 2022 -12 2022 | Volume 9 | Issue 5 | e5 Electric Vehicle Battery Charging Station based on Bipolar dc Power Grid with G2V, V2G and V2V Capabilities 5 currents, it is noticeable that idcpos is negative and idcneg is positive, as in the previous case, meaning that the dc power grid is receiving power instead of supplying it. Moreover, these currents are symmetrical, presenting an average value of 31.5 A and, therefore, the current idczer has a null average value. Hence, this result shows that the EV-BCS is able to handle unbalances in the power injected by the EVs without unbalancing the dc power grid currents.  Figure 6. Simulation results of the V2G operation mode when EV1 is discharging with a current of 20 A and EV2 is discharging with a current of 40 A.
After being analyzed the operation modes G2V and V2G, Figure 7 shows the V2V operation mode, where EV1 provides power to EV2. EV2 is charging with a current of 20 A, while EV1 provides the necessary current to perform the battery charging of EV2 without using additional power from the dc power grid. This figure shows the same variables as the previous ones. In this operation mode, ibat1 is negative, as happens in V2G, but ibat2 is positive, as happens in G2V. It can be seen that ibat2 has the expected average value of 20 A, with ibat1 presenting an average value of approximately -16.3 A. This difference is due to the voltage mismatch between the battery voltages of both EVs so that the input and output powers are approximately equal. In this case, the voltage vbat2 has a value of 202 V, making the produced voltage vcv2 alternate between voltage levels 200 V and 400 V. Regarding the dc power grid currents, it can be seen that idcpos and idcneg are overlapped and present a null average value, meaning that the dc power grid is neither receiving nor providing power. The current idczer presents a similar waveform and, as in the previous cases, has no average value.  Figure 7. Simulation results of the V2V operation mode when EV1 is discharging with a current of 16.3 A and EV2 is charging with a current of 20 A.
After being addressed the V2V operation mode exclusively, Figure 8 shows the combination of V2V and G2V operation modes, where EV1 provides power to EV2, but the power provided by EV1 is not enough to perform the battery charging of EV2. EV1 is discharging with a current of 20 A, while EV2 is charging with a current of 40 A. This figure shows the same variables as the previous ones. Once again, ibat1 is negative, similar to V2G, but ibat2 is positive, similar to G2V. Regarding the dc power grid currents, it can be seen that idcpos is positive and idcneg is negative, meaning that the dc power grid is providing power. However, the average value of these currents is only 8 A, since the dc power grid only provides the power difference between the power required by EV2 and the power supplied by EV1. As previously, the currents idcpos and idcneg are symmetrical, with idczer presenting a null average value.  Figure 9 shows the combination of V2V and V2G operation modes, where EV1 provides power to EV2, but the power provided by EV1 is more than the power required to perform the battery charging of EV2. EV1 is discharging with a current of 40 A, while EV2 is charging with a current of 20 A. This figure shows the same variables as the previous ones. Once again, ibat1 is 6 negative, similar to V2G, but ibat2 is positive, similar to G2V. Regarding the dc power grid currents, it can be seen that idcpos is negative and idcneg is positive, contrarily to the previous case, meaning that the dc power grid is receiving power. The average value of these currents is only 14.4 A, since the dc power grid only receives the power difference between the power supplied by EV1 the power required by EV2. As in the previous case, the currents idcpos and idcneg are symmetrical, with idczer presenting a null average value.  Figure 9. Simulation results of the combination of V2V and V2G operation modes when EV1 is discharging with a current of 40 A and EV2 is charging with a current of 20 A.
In order to provide an overview of the obtained simulation results, Table 2 shows the average values of the main variables for each case, i.e., ibat1, ibat2 and idcpos.
The average values of idcneg and idczer are not presented since the average value of idcneg is always symmetrical with respect to idcpos, while the average value of idczer is always null, as expected and previously explained.

Conclusions
This paper presented an electric vehicle (EV) battery charging station (EV-BCS) based on a bipolar dc power grid with vehicle-to-grid (V2G) and vehicle-to-vehicle (V2V) capability, besides the traditional battery charging operation mode (grid-to-vehicle -G2V). The presented EV-BCS is modular and uses three-level bidirectional dc-dc converters. In order to validate all the operation modes (G2V, V2G and V2V, as well as the combination of V2V with G2V and V2G), a case scenario with two converters, and thus two EVs, was considered, being analyzed both balanced and unbalanced operation from the EVs side, in order to emulate a real operation scenario and validate the proper operation of the EV-BCS. The obtained results, based on computational simulations, verify the correct operation of the EV-BCS in all cases, both with balanced and unbalanced current consumption from the EVs, but always with balanced currents from the bipolar dc power grid side.