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Stationary energy storage systems connect the sectors, but what connects the energy storage systems?

The significance of electrical connection technology for safety and reliability

Energy storage systems are an essential link for sector coupling. The safety and reliability of these systems depend on the electrical connection technology used. What do we need to consider here?

Mitigating climate change and increasing energy independence are among the most important issues of our time. The concept of an All Electric Society and its associated sector coupling offers possible solutions: It is about networking our society, buildings, production facilities, mobility, and infrastructure through energy and information technology. The All Electric Society achieves this by using renewable and unlimited sources of energy that do not produce harmful emissions.

The role of energy storage systems in the All Electric Society

Wind power and solar power are not always available. However, modern society has a constant energy demand, 24 hours a day. To achieve a stable supply, we need to store surplus energy in sufficient quantities so that it can be made available later when required. Battery energy storage systems (BESS) are one way to do this.

Structure and method of operation

Figure 1 shows the typical BESS structure. Depending on the capacity, these stationary systems can be installed in device housings, control cabinets, or even containers. However, the basic modular structure is always similar.

There are three different types of electrical connections: (i) power connection, (ii) signal connection, and (iii) data connection. Respectively, these are equivalent to the transmission of energy flows, of analog signals for monitoring and control, and of digital signals.

The smallest unit of a battery energy storage system is the battery cell, which is the actual storage element. Many of these cells are combined in a battery module and interconnected electrically (power). An electronic circuit ensures a balanced state of charge/discharge of all cells. This is referred to as cell balancing (signals). Temperature sensors monitor the thermal behavior of the cells (signals).

Several battery modules are combined to create a storage cabinet and connected at the power level (Figure 2). All of the modules exchange information (data) with one another and with the control unit of the storage cabinet, known as the power control unit (PCU). The PCU can be seen positioned at the top of the rack in Figure 2. In addition to power and data connections, both the battery modules and the PCU often have signal interfaces to connect external sensors.

Figure 2 -The battery rack with power control unit at the top and battery modules. (Image: Phoenix Contact)

Modular structure of the energy storage system

In the system, several storage cabinets are interconnected at the three aforementioned levels: power, signals, and data. The higher-level controller represents the system management, illustrated in Figure 1 by the open control cabinet. All the electrical connections of the storage cabinets converge again here. It also includes the external interfaces of the energy storage system. Energy is exchanged with the grid using power electronics. Signals from various auxiliary units, such as the fire extinguishing system, converge in the system management. The system management communicates with the grid operator and service providers, who can access the storage system via corresponding data connections.

Cardiovascular and nervous system of the energy storage system

Figuratively speaking, the electrical connections within the energy storage system can be described as its cardiovascular and nervous systems. Energy flows represent the circulatory system that transports vital energy; signals – such as temperatures, gases, and visual impressions – represent our senses. The data lines are used to exchange information, just like nerves.

If your circulatory system or nervous system aren’t working properly, you become unwell. The same is true in an energy storage system. Faults in the electrical connections result in system malfunctions and failure. In some cases,  they can destroy the entire system.

Examples of faults in the connection technology

Three examples illustrate just how important connection technology is, starting with the power wiring. In many cases, a classic screw connection is used to connect battery modules at the power level. Screw connections can come loose under unfavorable conditions. Improper connections during installation or subsequent maintenance can increase contact resistance. At best, this manifests itself in heat loss; however, in the worst-case scenario, excessive overheating can cause a fire in the storage system and destroy it completely.

A typical signal connection is the connection of a current sensor cable to a printed circuit board. These connections are often soldered directly, which, in conjunction with free cable ends, entails the risk of cold solder joints. This results in incorrect signal information, such as low current measurements. Consequently, excessively high charging and discharge currents would result in significant premature aging of the battery.

Data connections are often created using patch cables or extendable cables. Poor quality here, such as tight dimensional tolerances and high-quality contact surfaces, can lead to faults during data transmission. These result in system malfunctions or even system failure, and at worst. can even damage the hardware.

Figure 3 -Innovative connection technologies for power, signals, and data for use in energy storage systems. (Image: Phoenix Contact)

Recommendations for selecting connection technology

For anyone who operates energy storage systems, the faults described ultimately result in financial losses. So how can you avoid them? For power connections, use plug-in connectors whenever you cannot define installation conditions. This also applies to power connections that must be established during installation and maintenance (Figure 3).

When connecting free cable ends to printed circuit boards, innovative connection technologies are better than soldering. Insulation displacement connections, for example, do not require any pretreatment of the conductor end and – like spring connection technologies – offer a high level of convenience and maximum reliability.

In the case of data connections, the ambient conditions define the requirements. IP-protected data connectors are ideal for high pollution degrees or humid environments, while industrial-grade data connections should be used in harsh mechanical conditions.

Conclusion

When it comes to energy storage systems, skimping on the initial investment can lead to costly operations. In addition, these devices and systems are often operated in system-related applications. Reliability is an important factor in realizing the vision of an All Electric Society. Electrical connections should be given the proper attention they deserve.

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