An operating mode control method for photovoltaic (PV) battery hybrid systems

Depending on the PV power, load power, and battery status, the system may operate in different modes. The control loop may have to switch between operating modes. In practice, it is difficult to implement control loop switching because the transition and dynamic process are difficult to control. As a result, this paper presents a generalized mode control method that avoids loop switching across modes. First, system structure and topology are introduced. The operating conditions for both grid-connected and off-grid modes are then divided into six sub-cases. Furthermore, the control architecture, control loop, and reference transition for various scenarios are described. Finally, an experimental platform is built, and the results are presented to verify the proposed method.

1 Introduction

As part of the global green mission, an increasing number of renewable energy sources are being installed. As renewable energy sources gain traction, power electronic converters become increasingly popular (Zhang et al., 2023a). The combination of solar panels and energy storage will be a trend in future energy development, and many experts have conducted extensive research on photovoltaic and energy storage hybrid systems (Zhang et al., 2023b). The system operates in a variety of modes depending on the operational conditions of photovoltaic, energy storage, and the power grid. The seamless transition between modes has also become a research topic (Hmad et al., 2023).

Several papers investigate the transition between grid-connected and offline modes; one of the mode transition control strategies relies on a virtual switch located between two functional modes (Balaguer-Alvarez et al., 2014). During grid-connected mode, the inverter functions solely as a current source, utilizing the current controller loop. When an islanding condition is detected, the virtual switch is assigned to the voltage loop. As a result, the inverter operates as a voltage source (Qinfei et al., 2017). The anti-islanding method has a strong influence on the transition performance. The majority of islanding detection methods rely on continuous monitoring of system characteristics at the point of common coupling (PCC), including current, voltage, frequency, and harmonics (Ahmad et al., 2013). These transition approaches can be classified into three categories: passive, active, and hybrid (Koohi-Kamali and Rahim, 2016; Aillane et al., 2023) presents an improved mode transition approach. During the transition, a super-twisting algorithm approach was used to ensure load voltage and manage the inverter’s nonlinear properties, resulting in a smooth transition with minimal disruption impacts. Ashabani and Mohamed (2014) investigates another switching-based method. Three different approaches are considered: power drooping current controlled, power drooping voltage controlled, and current drooping voltage controlled. Furthermore, to aid smooth mode transitions by dampening power, frequency, voltage, and current signals, an approach based on a supplement controller employing port-controlled hamiltonian (PCH) modeling and control was developed in (Azimi and Lotfifard, 2021). Another approach to achieving a seamless transition is to employ an extra distributed generation (DG) system, which is often a specialized storage unit (Jihed et al., 2019). The premise behind this technique entailed deploying a supplemental energy unit to relieve the transient, which, however, incurs additional expenditures.

The second set of solutions for dealing with the transition problem includes the use of an extra feedforward compensator (Tran et al., 2013). The basic idea behind this control structure group is to include a voltage loop in addition to the current loop when a grid fault occurs. In Hwang and Park (2013), an improved seamless transition based on a phase-locked loop (PLL) mechanism was developed for a three-phase grid-connected inverter. The control strategy entails modifying the PLL depending on the operating mode, synchronizing the output inverter voltage to the grid voltage in grid-connected mode, and generating an angle at the required frequency in off-grid mode. In Harirchi et al. (2015), a similar PLL-based transfer was performed, with feedforward voltage used to mitigate the transition’s negative effects on a three-phase grid-connected PV inverter.

Various research studies have developed transition control without reconfiguring the control structure, also known as unified control (Yi et al., 2018). The overall goal of this technique was to use the voltage control loop as a reference current generator when connected to the grid and as a voltage regulator when off-grid. Unlike previous transition structures, this technique does not necessitate any changes at the control level. Variants of the universal control system have been proposed as a solution to transition issues. In Liu and Liu (2014), the authors developed an indirect current control loop with an applied voltage loop for a three-phase inverter to ensure smooth transfer. In Sowa et al. (2021), a three-phase universal controller for flexible microgrids was presented, ensuring operation in all operating modes without the need for control structure reconfiguration. In Yi et al. (2018), the authors described a unified control and power management system for a hybrid PV-battery application that included both DC and AC charging buses. In Li et al. (2020), a non-linear-simplex method was proposed for determining the optimal controller settings with the goal of reducing voltage variation and achieving a seamless state transition. In the literature (Singh et al., 2017; Jihed et al., 2019), a unified system with droop control approach is investigated. The concept of droop methods has been widely applied to the parallel operation of DG inverters with voltage and/or current control loops. However, one major disadvantage of this method is its poor dynamic performance.

PV-battery hybrid systems operate in a variety of scenarios based on PV power, load power, and battery status, and the control loop may need to switch. Because transition and dynamic processes are difficult to control, it is difficult to implement control loop switching in practice. As a result, this paper proposes a generalized mode control method that avoids loop switching in a variety of scenarios. The main contributions are as follows.

1) This paper describes a generalized operating mode control method. The conditions in grid-connected and off-grid modes are classified into six scenarios based on the values of PV power P_PV, load power P_load, battery state of charge (SOC), and so on. Furthermore, the control architecture, control loop, and reference transition for various scenarios are discussed.

2) In grid-connected mode, the battery-side DC/DC controls the DC-bus voltage, the PV-side DC/DC applies Maximum Power Point Tracking (MPPT) mode to maximize PV output power, and the grid-side DC/AC controls the output power. The control loops for DC/DC and Bi-DC/DC do not need to change in different operating scenarios; only the DC/AC output power reference does.

3) In off-grid mode, the battery-side DC/DC regulates the DC-bus voltage, the PV-side DC/DC uses power point tracking (PPT) mode to track the PV output power reference, and the grid-side DC/AC regulates the output voltage. The DC/AC and Bi-DC/DC control loops do not need to change in different operating scenarios, while only the PV-side DC/DC power reference does.

4) An experimental platform is built, and the results are presented to support the proposed method.

2 System structure and topology

Figure 1 depicts the architecture of a typical PV-battery hybrid system, which is a common DC-bus structure. PVs, batteries, and the grid and load are connected to the DC-bus through DC/DC, bidirectional DC/DC, and DC/AC, respectively.

Figure 1

Figure 1. System structure.

Figure 2 presents the system topology. The Boost topology is used for the PV-side DC/DC. A two-stage structure with LLC plus Buck/Boost is used for the battery-side Bi-DC/DC. The Highly Efficient Reliable Inverter Concept (HERIC) topology is chosen for the DC/AC side.

Figure 2

Figure 2. System topology.

There are two operating modes for the system, which are grid-connected mode and off-grid mode. In the grid-connected mode, the grid is normal, and the system operates in current source mode, feeding power to the grid. In off-grid mode, the grid is not present, and the system operates in voltage source mode to ensure load voltage. Furthermore, depending on PV power, load power, and battery status, the system operates in different scenarios. Furthermore, the control loop may have to switch for different scenarios. Since the transition and dynamic process is difficult to control, it is relatively challenging to implement the control loop switching. Therefore, the following presents a generalized mode control method for avoiding loop switching for different scenarios.

3 System control

Before introducing specific methods, we set basic control prerequisites first. One of them is the priorities of power sources, including three points.

1) PVs are set as the first priority power source. The output power of PV modules should be prioritized to feed the load.

2) Batteries are set as the second priority power source. When there is a surplus or shortage of PV power to the load, the battery is then used to achieve power balance.

3) The grid is set as the third priority power source. When both the PV modules and the batteries reach their limits, the grid is then employed to power the load.

Furthermore, the output power of PV modules, the SOC of batteries, and the charging and discharging power of batteries are restricted as follows.

1) When the PV output power P_PV is less than the set minimum threshold P_{PV_min}, the PV is considered to have no power and the PV side is shut off; otherwise, the PV side should be used to generate power.

2) If the battery SOC is greater than 90%, the battery should not be charged further; if the battery SOC is less than 10%, the battery should not be drained further.

The following will analyze the grid-connected mode and off-grid mode, respectively.

3.1 Grid-connected mode

Based on the values of PV power P_PV, load power P_load, battery SOC, etc., the operating conditions under the grid-connected mode are divided into six scenarios, as shown in Figure 3.

Figure 3

Figure 3. Six different scenarios under the grid-connected mode.

Figure 4 depicts the control architecture in grid-connected mode, where the battery-side DC/DC controls the DC-bus voltage V_dc, the PV-side DC/DC operates in MPPT mode to maximize the PV output power P_PV, and the grid side DC/AC controls the output power P. The advantage of the control architecture shown in Figure 4 is that the control loops for DC/DC and Bi-DC/DC do not need to change in different operating scenarios, with only the DC/AC output power reference P_ref needed to change. This reduces the control loops switching during different scenarios, making it simple to implement.

Figure 4

Figure 4. System control architecture under grid-connected mode.

It is worth mentioning that, in this paper, v_pv is PV voltage, v_{pv_ref} is PV voltage reference, i_pv is PV current, i_{pv_ref} is PV current reference, V_{dc_ref} is DC-bus voltage reference, d₁ is output duty cycle of the DC/DC control loop, d₂ is output duty cycle of the Bi-DC/DC control loop, and d₃ is output duty cycle of the DC/AC control loop.

3.1.1 Scenario A1

In this scenario, P_PV > P_Load and SOC > 90%, where the PV power is larger than the load power and the battery cannot be charged. In the control loop, as shown in Figure 5, the DC/AC power reference P_ref is set to P_PV. This can result in that the DC/AC outputs the PV-side power P_PV and the battery power P_B is zero. Furthermore, the PV powers the load P_L, and the extra power (P_PV–P_L) is fed to the grid, and the grid power is P_G.

Figure 5

Figure 5. Scenario A1.

3.1.2 Scenario A2

In this scenario, P_PV > P_Load and SOC ≤ 90%, where the PV power is higher than the load power and the battery can be charged. In the control loop, as shown in Figure 6, the DC/AC power reference P_ref is set to P_L. This can result in that the DC/AC outputs the load power P_L and the power fed into the grid is 0. Furthermore, the battery automatically absorbs the remaining power generated by PV, which is P_PV–P_L.

Figure 6

Figure 6. Scenario A2.

3.1.3 Scenario A3

In this scenario, P_PV ≤ P_Load and SOC > 10%, where the PV power is less than the load power and the battery can be discharged. In the control loop, as shown in Figure 7, the DC/AC power reference P_ref is set to P_L. This can result in that the DC/AC outputs the load power P_L and the power fed into the grid is 0. Furthermore, the battery automatically provides the remaining power P_L–P_PV.

Figure 7

Figure 7. Scenario A3.

3.1.4 Scenario A4

In this scenario, P_PV ≤ P_Load and SOC ≤ 10%, where the PV power is less than the load power and the battery cannot be discharged. In the control loop, as shown in Figure 8, the DC/AC power reference P_ref is set to P_PV. This can result in that the DC/AC outputs the PV power P_PV and the power that the battery provides is 0. Furthermore, the grid provides the remaining load power P_L–P_PV.

Figure 8

Figure 8. Scenario A4.

3.1.5 Scenario A5

In this scenario, P_PV ≤ P_{PV_min} and SOC > 10%, where the PV cannot generate power and the battery can be discharged to feed the load. In the control loop, as shown in Figure 9, the DC/AC power reference P_ref is set to P_L. This can result in that the DC/AC outputs the load power P_L and the power from the grid is 0. Furthermore, the battery automatically provides the load power P_L.

Figure 9

Figure 9. Scenario A5.

3.1.6 Scenario A6

In this scenario, P_PV ≤ P_{PV_min} and SOC ≤ 10%, where the PV cannot generate power and the battery cannot be discharged to feed the load. Thus, the load power can only be provided by the grid. In the control loop, as shown in Figure 10, the DC/AC power reference P_ref is set to 0. This can result in that the DC/AC outputs no power and the load power is from the grid.

Figure 10

Figure 10. Scenario A6.

The summary of six scenarios for the grid-connected mode is shown in Figure 11. From it, a switch S₁ is introduced to assign different DC/AC power references. In scenarios A1 and A4, the switch S₁ is placed to “1”. In scenarios A2, A3, and A5, the switch S₁ is placed to “2”. In scenario A6, the switch S₁ is placed to “3”. The details are presented in Table 1.

Figure 11

Figure 11. Control architecture for six scenarios for the grid-connected mode.

Table 1

Table 1. Summary of six scenarios for the grid-connected mode.

3.2 Off-grid mode

Based on the values of PV power P_PV, load power P_load, battery SOC, etc., the operating conditions under the off-grid mode are divided into six different scenarios, as shown in Figure 12.

Figure 12

Figure 12. Six different scenarios under the off-grid mode.

Figure 13 depicts the control mode in off-grid mode, where the battery-side Bi-DC/DC controls the DC-bus voltage V_dc, the PV-side DC/DC operates in power point tracking (PPT) mode to follow the PV output power P_PV, and the grid-side DC/AC controls the output voltage v_L. The advantage of the control architecture shown in Figure 13 is that the DC/AC and Bi-DC/DC control loops do not need to change in different operating scenarios, with only the PV-side DC/DC power reference P_{PV_ref} needed to change. This reduces the control switching during different scenarios, making it simple to implement.

Figure 13

Figure 13. System control architecture under off-grid mode.

3.2.1 Scenario B1

In this scenario, P_PV > P_Load and SOC > 90%, where the PV power is larger than the load power and the battery cannot be charged. In the control loop, as shown in Figure 14, the PV-side DC/DC power reference P_{PV_ref} is set to the load power P_L. This can result in that the PV side cannot implement MPPT function, and the PV-side power P_PV is limited to P_L. Therefore, the PV provides the load power P_L, and the power from the battery P_B is zero.

Figure 14

Figure 14. Scenario B1.

3.2.2 Scenario B2

In this scenario, P_PV > P_Load and SOC ≤ 90%, where the PV power is higher than the load power and the battery can be charged. In the control loop, as shown in Figure 15, the PV-side DC/DC performs MPPT function. This can result in that the PV-side DC/DC outputs the maximum power P_MPPT. Furthermore, the battery automatically absorbs the extra power generated by PV, which is P_PV–P_L.

Figure 15

Figure 15. Scenario B2.

3.2.3 Scenario B3

In this scenario, P_PV ≤ P_Load and SOC > 10%, where the PV power is less than the load power and the battery can be discharged to the load. In the control loop, as shown in Figure 16, the PV-side DC/DC performs MPPT. This can result in that the PV-side DC/DC outputs the maximum power P_MPPT. Furthermore, the battery automatically provides the remaining power, which is P_L–P_PV.

Figure 16

Figure 16. Scenario B3.

3.2.4 Scenario B4

In this scenario, P_PV ≤ P_Load, and SOC ≤ 10%, where the PV power is less than the load power and the battery cannot be discharged. Therefore, as shown in Figure 17, the system cannot operate.

Figure 17

Figure 17. Scenario B4.

3.2.5 Scenario B5

In this scenario, P_PV ≤ P_{PV_min} and SOC > 10%, where the PV cannot generate power and the battery can be discharged to feed the load. The PV-side DC/DC stops. Furthermore, the battery automatically provides the load power P_L as shown in Figure 18.

Figure 18

Figure 18. Scenario B5.

3.2.6 Scenario B6

In this scenario, P_PV ≤ P_{PV_min} and SOC ≤ 10%, where the PV cannot generate power and the battery cannot be discharged to feed the load. Thus, as shown in Figure 19, the system cannot operate.

Figure 19

Figure 19. Scenario B6.

The summary of six scenarios for the off-grid mode is shown in Figure 20. From it, a switch S₂ is introduced to set different DC/DC power references. In scenarios B1, the switch S₂ is placed to “1”. In scenarios B2 and B3, the switch S₂ is placed to “2”. In other scenarios, PV-side DC/DC stops. The details are presented in Table 2.

Figure 20

Figure 20. Control architecture for six scenarios for the off-grid mode.

Table 2

Table 2. Summary of six scenarios for the off-grid mode.

4 Experimental verifications

A platform is built and Figures 21–23 are the experimental results. Figure 21 depicts the experimental results for the grid-connected mode with no PV and the battery switched between charging and discharging. From Figure 21A, the system operates smoothly when the battery is switched from charging to discharging and the current is adjusted from −120 to +120 A. From Figure 21B, the battery is transitioning from charging to discharging, and the entire process is seamless. The battery current is changed from +120 to −120 A, and the grid current is adjusted correspondingly. The proposed mode transition method ensures a smooth transition between these two operational modes.

Figure 21

Figure 21. Experimental results for the grid-connected mode when there is no PV (A) battery charging to discharging; (B) battery discharging to charging.

Figure 22 depicts the experimental results of the transition between grid-connected mode and off-grid mode. From Figure 22A, the system operates smoothly and the load voltage remains stable when the system switched from grid-connected to off-grid mode. In grid-connected mode, the inverter functions as a current source. In the off-grid mode, however, the inverter functions as a voltage source. In addition, when the grid is normal, the battery is charging. When the grid is offline, the battery is switched to discharging mode to maintain the load voltage.

Figure 22

Figure 22. Experimental results between grid-connected mode and off-grid mode (A) grid-connected mode to off-grid mode; (B) off-grid mode to grid-connected mode.

From Figure 22B, the system operates smoothly and the load voltage remains stable when the system is switched from off-grid to grid-connected mode. The inverter switches from a voltage source in off-grid mode to a current source in grid-connected mode. In addition, the battery is in discharging mode to maintain the load voltage when the grid is offline. When the grid resumes to normal operation, the battery is switched to charging mode. The proposed mode transition method ensures a smooth transition between grid-connected mode and off-grid mode.

Figure 23 depicts the experimental results when the load steps. From Figure 23A, the load steps from 0 to 6 kW. When the load power is low, the PV power feeds into the battery and the battery is in the charging mode. When the load power is high, the PV and battery work together to power the load, and the battery is in discharging mode. From Figure 23A, the system operates smoothly when the load steps from 0 to 6 kW.

Figure 23

Figure 23. Experimental results when the load steps (A) the load steps from 0 to 6 kW; (B) the load steps from 6 to 0 kW.

From Figure 23B, the system operates smoothly and the load voltage remains stable when the system steps from 6 to 0 kW. Furthermore, when the load is 6 kW, the PV and battery work together to supply the voltage. When the load drops to 0 kW, the PV feeds the battery and the battery is in the charging mode. The proposed mode transition method ensures a smooth transition between these two operational modes.

5 Conclusion

Depending on PV power, load power, and battery status, the system operates in various scenarios, and the control loop may need to change. Because the transition and dynamic processes are difficult to control, implementing control loop switching can be difficult. As a result, this paper presents a generalized mode control method that avoids loop switching in a variety of scenarios.

First, the system structure and topology are introduced, with the common DC-bus structure being used. The operating conditions in grid-connected and off-grid modes are then classified into six scenarios. Furthermore, the control architecture, control loop, and reference transition for various scenarios are discussed. In grid-connected mode, the battery-side DC/DC controls the DC-bus voltage, the PV-side DC/DC uses MPPT mode to maximize PV output power, and the grid-side DC/AC controls the output power. The control loops for DC/DC and Bi-DC/DC do not need to change in different operating scenarios; only the DC/AC output power reference P_ref does. Furthermore, P_ref equals P_PV in scenarios A1 (P_PV > P_Load and SOC > 90%) and A4 (P_PV ≤ P_Load and SOC ≤ 10%). P_ref equals P_L in scenarios A2 (P_PV > P_Load and SOC ≤ 90%), A3 (P_PV ≤ P_Load and SOC > 10%), and A5 (P_PV ≤ P_{PV_min} and SOC > 10%). P_ref equals zero in scenario A6 (P_PV ≤ P_{PV_min} and SOC ≤ 10%). In off-grid mode, the battery-side DC/DC regulates the DC-bus voltage, the PV-side DC/DC uses PPT mode to track the PV output power reference, and the grid-side DC/AC regulates the output voltage. During operating mode switching, only the PV-side DC/DC power reference P_PV needs to change; the DC/AC and Bi-DC/DC control loops remain unchanged. P_PV equals P_L in scenario B1 (P_PV > P_Load and SOC > 90%). P_PV equals P_MPPT in scenario B2 (P_PV > P_Load and SOC ≤ 90%) and B3 (P_PV ≤ P_Load and SOC > 10%). PV-side DC/DC stops in scenario B5 (P_PV ≤ P_{PV_min} and SOC > 10%). In B4 (P_PV ≤ P_Load, and SOC ≤ 10%) and B6 (P_PV ≤ P_{PV_min} and SOC ≤ 10%) scenarios, the system stops. Finally, an experimental platform is built, and the proposed methodology is validated by the experimental data. According to the experimental findings, there are no surges during mode transitions with the proposed method and seamless transition among operating modes can be achieved.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

WZ: Writing–original draft, Methodology. YW: Conceptualization, Writing–original draft. PX: Software, Writing–original draft. DL: Supervision, Writing–review and editing. BL: Validation, Writing–original draft.

Funding

The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. This work was funded by Zhejiang Jianbing R&D Program (2024C01242(SD2)).

Conflict of interest

Authors WZ, YW, PX, and BL were employed by Research Center, Ginlong Technologies Co., Ltd.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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