1:Key technology of energy storage

1.1 Energy storage ontology technology

Energy storage technologies are classified according to storage media, which can be divided into physical energy storage, electromagnetic energy storage, electrochemical energy storage, and fuel cells, etc., all kinds of energy storage technology from the point of view of energy conversion, roughly divided into two categories: One type is an energy storage technology that converts mechanical energy into electrical energy, including pumped storage, flywheel energy storage and compressed air energy storage, etc.; the other type is an energy storage technology that converts chemical energy into electrical energy, such as battery energy storage, Supercapacitor energy storage, etc.

1.2 Grid-connected technology

The energy storage converter is the interface between the power grid and the energy storage device and is an important part of the energy storage system, which is mainly composed of the converter and its control system.

The energy storage converter (PCS) can realize the bidirectional conversion of electric energy: in the charging state, the converter acts as a rectifier to store the electric energy from AC to DC into the energy storage device; In the discharge state, the converter acts as an inverter to change the electric energy stored by the energy storage device from DC to AC.

According to the survey, the energy storage unit is composed of an energy storage converter (PCS), energy storage battery stack (BP), and battery management system (BMS), as shown in Figure 1. BP completes DC/AC transformation through PC and then connects to AC bus to realize energy storage and release. PCS controls the charging and discharging action of BP. In the charging state, PCS acts as a rectifying device to convert the electric energy from AC to DC and store it to BP. In the discharge state, PCS acts as an inverter to transfer the electric energy stored in BP from DC to AC and then to the power grid. BMS can monitor the voltage, current, and temperature of BP in real-time, and coordinate the charge and discharge process of BP by transferring key information to PCS, to avoid the occurrence of overvoltage, under-voltage, and overcurrent problems. At the same time, it has the function of charge and discharges balance management. The different choices of converter booster transformers also have certain limitations on the structure of the converter.

Figure 1.Topological structure diagram of the energy storage unit

1.3 Monitoring technology

The energy storage monitoring system can efficiently coordinate and connect the grid dispatching and energy storage system and monitor the operating data and status of the entire system in real-time to ensure that the system is always in good working condition.

2:The role of the energy storage system in microgrid

2.1 Improve the power supply reliability of microgrids. The energy storage device can be used as an uninterruptible power supply in the microgrid, which can respond quickly in case of accidents and avoid large changes in the frequency and voltage of the microgrid, thus increasing the reliability of the power supply of the microgrid. When the microgrid is switching between the island operation mode and the network operation mode, the energy storage device can ensure that the microgrid can switch smoothly between the two operation modes.

2.2 Improve the power quality of microgrids. When the external large grid fails, the microgrid can enter an independent operating state. At this time, the coordinated use of energy storage devices is needed to ensure the power supply safety of important and sensitive loads in the microgrid, and improve the power supply reliability and power quality in the region where the microgrid is located.

2.3 Improve the effective use of electric energy in the microgrid, the economy of the system, and the operational stability of the microgrid. Because the microgrid uses many intermittent power sources such as wind power and photovoltaics, its equivalent moment of inertia is low, the self-balancing ability of the system is very weak, and the stability of voltage and frequency is difficult to self-control. The energy storage device can control its output and absorbed active and reactive power according to demand, providing a strong guarantee for the stable operation of the microgrid.

3:Application scenarios of energy storage in microgrids in remote areas without electricity/weak electricity

3.1 The application of the energy storage system in the household microgrid is shown in Figure 2. The microgrid of this topology mainly includes photovoltaic micro-sources and small wind turbines. The conversion of electrical energy is realized through the integrated inverter control cabinet, and the storage battery is used for energy storage. Connected in parallel to the DC side of the small microgrid, the electricity load is mainly lighting and household appliances, and the oil machine or agricultural power is used as a cold backup to ensure continuous power supply under extreme conditions.

Figure 2. Application of energy storage in a household microgrid

3.2 Application of energy storage in a village-level microgrid

The application of an energy storage system in a village-level microgrid is shown in Figure 3. As shown in the figure, the microgrid with this topology mainly includes photovoltaic micro source, MPPT controller mainly realizes the tracking control of maximum output power of photovoltaic micro source, inverter, and AC distribution cabinet realize the conversion and distribution of electric energy, battery energy storage is parallel connected to the direct current side of the village level photovoltaic storage microgrid, and the electric load is mainly lighting, household appliances, and public service load, and the main power of operation monitoring system To ensure the stable operation of the microgrid.

Figure 3. Application of energy storage in a village-level microgrid

3.3 Application of energy storage in township-level microgrids

The application of the energy storage system in the township-level microgrid is shown in Figure 4. The microgrid of this topology mainly includes photovoltaic micro-sources. The MPPT controller mainly realizes the tracking control of the maximum output power of the photovoltaic micro-source, inverter, and AC The power distribution cabinet realizes the conversion and distribution of electric energy. The storage battery and charger are connected in parallel on the DC side of the village-level optical storage microgrid. The electricity load is mainly lighting, household appliances, and public service loads. The operation monitoring system mainly guarantees the microgrid. The stable operation of the generator guarantees a continuous power supply under extreme conditions.

Figure 4. Application of energy storage in township microgrid

Concluding remarks

The construction of interconnected and independent microgrids in remote agricultural and animal husbandry areas without electricity/weak current and the reasonable configuration of energy storage devices is an effective way to solve the reliability and quality of power supply in these areas. The application scenarios of the three types of energy storage devices mentioned in the previous article in the microgrid are suitable for most remote agricultural and pastoral areas, and it also has economic advantages for the extension of the large power grid to solve the power supply problems in these areas.