### A Brief Discussion on the Energy Strategy Research of Power Photovoltaic Hybrid Energy Storage Systems
#### Abstract
This article presents an energy management control strategy for photovoltaic hybrid energy storage systems, which primarily apply to power generation networks comprising photovoltaic sources (PV), battery energy storage (BES), and AC loads. The strategy effectively leverages the interconnections within the power system, addressing issues such as overcharging and undercharging in current BES systems. It maintains the charging and discharging currents within a relatively stable range, thereby extending the lifespan of the batteries. Experiments were conducted using a 6kVA power converter in hybrid energy systems with both traditional lead-acid and lithium-ion batteries, confirming the correctness and effectiveness of the proposed energy management strategy.
**Keywords:** Energy management; Photovoltaic; Battery storage; Hybrid storage
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### Introduction
In recent years, the application of renewable energy sources (RES) has become increasingly widespread in both residential and industrial sectors. Photovoltaic (PV) technology has found numerous applications among various RES solutions. Within PV power systems, battery energy storage (BES) units play a crucial role in maintaining a continuous supply of electricity. Traditionally, lead-acid batteries have been used as the primary storage medium in BES units; however, advancements in technology have led to lithium-ion batteries emerging as a new solution, thanks to their higher energy density and better conversion efficiency. Regardless of the type of BES structure used, controlling the power within the system is essential to maintain current levels in the grid, protect the lifespan of BES units, and ensure output stability. Effective power management is vital for achieving optimal system performance.
Compared to conventional power systems, photovoltaic systems offer significant advantages in terms of cost and conversion efficiency. PV systems typically transmit power through direct current (DC) systems, which cannot be directly used in most residential and industrial applications. This necessitates a conversion between DC and alternating current (AC), a process that can lead to energy losses and disrupt normal current levels in the circuit. To address these challenges and enhance the overall stability of the grid, this paper proposes an energy management strategy for photovoltaic hybrid energy storage systems.
### Energy Management Strategy for Photovoltaic Hybrid Energy Storage Systems
PV, BES, and DC load transmission are currently the three most commonly utilized technologies in power systems. To explore energy management strategies in power systems effectively, this paper compares the transmission states of direct current (DC) and alternating current (AC) systems, as shown in Table 1.
**Table 1: Comparison of DC and AC-Centric System Architectures**
From Table 1, it is evident that DC-centric transmission systems provide optimal charging protection for battery energy storage systems. Conversely, AC-centric transmission systems protect the stable operation of BES by minimizing the number of conversions from PV sources to AC loads, thereby ensuring flexible deployment of battery systems.
When the system controller fails to manage the energy flow appropriately, safety hazards arise. If the total generation capacity in a PV power system exceeds the maximum capacity of the BES system, the charging voltage and current may surpass the limits of the battery system, jeopardizing the storage system. The proposed energy management strategy enables the BES system to maintain a stable current variation curve during charging and discharging processes while stabilizing internal current transmission.
To ensure the stable operation of the photovoltaic power system, it is necessary to control the current and voltage levels within the grid. Additionally, to prevent excessive charging and discharging of the storage system, the power converter must meet specific conditions. The reference voltage and current levels for battery charging must depend on temperature (TB), represented by a function F. If the conditions outlined in Equation (1) are not satisfied, uncontrolled energy flows could lead to excessively high battery charging voltages, causing irreversible damage.
To mitigate such adverse effects, the design of the battery converter within the grid has been structured as a multi-variable system: direct adjustments control AC voltage, frequency, and phase angle; indirect adjustments manage DC voltage, current, and state of charge (SOC), stabilizing the charging voltage of the BES system.
**Figure 1** illustrates the control block diagram of a hybrid power transmission system using a battery converter and grid-connected PV inverter. The control structure of the grid-connected PV inverter features maximum power point tracking and power reduction functionalities. The controller of the PV inverter continuously monitors grid parameters, demodulating the transmission mode upon detecting significant changes in the received signals.
Typically, communication between the battery converter (transmitter) and the PV inverter (receiver) can be achieved using three methods:
1. Linear variation of f;
2. CF-based mode changes;
3. Digital modulation transmission.
Using this specific approach, the battery converter modulates a controllable carrier signal CF, which the PV inverter demodulates similarly to analog-to-digital conversion. Changes in CF are communicated in binary format, such as using frequency shift keying (FSK) for phase angle signals. In this scenario, the transmitter (battery converter) introduces variations in the flow of CF, which are then detected and demodulated to observe current changes in the transmission system.
**Table 2** outlines the design requirements for the power management controller in distributed generators using battery converters and PV inverters.
In the proposed hybrid storage control strategy for PV power systems, power management control must connect with multiple AC networks. This approach facilitates multi-stage monitoring of charging states and information synchronization among external generators.
### Frequency Control-Based Energy Balancing and Load Reduction Strategies
The frequency-based power management control proposed in this paper offers a robust method for balancing multi-charge power in BES and reducing load. This method effectively minimizes energy losses within the system and enhances the stability of the entire link, as illustrated in **Figure 2**.
**Figure 2** presents a generic control structure for frequency control of the battery converter circuit. Part (a) shows a grid structure designed to indirectly control battery charging voltage and current by regulating the power output of the PV inverter. The battery voltage and current controllers for frequency control are selected in the form of proportional integral (PI) compensators. Part (b) depicts a battery converter operating in islanded mode, featuring a controllable circuit breaker for distributing AC loads.
#### 2.1 Analysis of the BES Charging Power System
**Figure 3** displays key waveforms for energy flow control within AC-centric power systems equipped with line frequency control and communication capabilities. Parts (a) and (b) demonstrate the multi-charge curves achieved in the battery converter to meet the stable operating requirements of the BES. The battery converter maintains the SOC and state of health (SOH) parameters of the BES within a stable range.
#### 2.2 BES Discharge Load Reduction
Under low illumination conditions, PV energy may be insufficient to meet AC load demands. Load reduction operations for AC loads are illustrated in **Figure 3(c)**, which activates during periods of higher AC load demand. When the battery SOC is low, the battery converter lowers the line frequency (f), causing the frequency alteration to disconnect the AC load sequence. The size and configuration of the battery depend on SOC, charge/discharge rates, depth of discharge, cycle count, and operating temperature. System conversion efficiency must also be considered during selection to determine the effective usable energy from BES to AC ports.
### Acrel-2000ES Energy Management System Solution
#### 3.1 Overview
The Acrel-2000ES energy management system by Acrel offers comprehensive monitoring and management functionalities for energy storage systems. It includes detailed information on system components (PCS, BMS, electric meters, fire protection, air conditioning, etc.), enabling data collection, processing, storage, querying, analysis, visual monitoring, alarm management, and reporting. The system supports energy scheduling with control functionalities like planned curves, peak shaving, demand management, and backup power. It conducts real-time monitoring of battery group performance and historical data analysis, employing intelligent distribution strategies for battery control, thereby optimizing battery performance and extending battery life. The system is compatible with Windows OS and utilizes SQL Server for database management. It can be used for both energy storage integrated cabinets and containers, serving as a dedicated software platform for energy storage equipment management.
#### 3.2 Applicable Scenarios
The system can be applied in urban areas, highways, industrial parks, commercial districts, residential areas, smart buildings, islands, and regions without electricity for monitoring and managing renewable energy systems.
**Four major application scenarios for industrial and commercial energy storage:**
1. Factories and shopping malls: Energy storage can reduce electricity costs through peak shaving and demand management while acting as an emergency backup.
2. PV charging stations: Self-consumption of PV energy for electric vehicle charging stations, mitigating the impact of high-power charging on the grid.
3. Microgrids: Microgrids offer flexibility in grid-connected or islanded operation, primarily in industrial parks, island microgrids, and remote areas, balancing power generation and load demand.
4. New application scenarios: Commercial energy storage is exploring integrated development in various new contexts, including 5G base stations, heavy-duty battery swapping, and port shore power applications.
#### 3.3 System Structure and Functionality
**3.4.1 Real-Time Monitoring**
The microgrid energy management system should feature a user-friendly interface that visually represents the operational status of each electrical circuit in the form of single-line diagrams. It should monitor parameters such as voltage, current, power, power factor, and dynamic states of circuit breakers and isolators, as well as relevant fault and alarm signals.
The system should manage distributed generation from energy storage systems, providing real-time information on power output, revenue, state of charge, and operational power settings.
The monitoring system interface of the microgrid energy management system includes a main dashboard displaying the composition of photovoltaic, wind power, energy storage, charging piles, and overall load, along with revenue, weather, energy-saving information, and relevant electrical parameters.
**Figure 2** depicts the system’s main interface.
### Conclusion
This paper introduces a flexible power management strategy for power systems that can be effectively utilized in systems incorporating battery converters and photovoltaic inverters. The proposed energy management strategy capitalizes on the interconnections within the power system, enabling controlled charging and discharging of batteries, status monitoring, and performance analysis. It also provides real-time protection and alarms for battery temperature and voltage, ensuring the stability and safety of system operations.
**Editor:** Huang Yu
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