Protection against surges and overvoltages in BESS

Protection against surges and overvoltages in BESS

BESS systems contain AC/DC converters and battery banks implemented in concrete constructions or in metallic containers. These AC/DC converters have sensitive electronics, and the high-capacity batteries with low dielectric strength pose a risk of explosion in case of arcing. Therefore the need for optimized and reliable electrical protection against the influence of lightning and surge events becomes mandatory. A risk assessment per IEC 62305-2 should first be performed to understand better if an external lightning protection system (LPS) is required. The above standard considers the following four scenarios (Table 1), which are also applicable to a BESS, as shown in Figure 1.

The IEC 60364 series of standards are also applicable to fixed-wired installations such as permanently wired (nonmobile) BESS and need to be considered. Specifically, IEC 60364-4-44 addresses the protection of electrical installations and describes measures against voltage disturbances and electromagnetic disturbances, including transient overvoltages transferred via the supply lines. In addition, IEC 60364-5-53 addresses the selection and installation of SPDs for such applications. Beyond these standards, any national standards and applicable installation rules must also be followed.

Required surge ratings of AC SPDs

When an LPS is installed (which is typically the case with a BESS), the AC SPDs shall be selected using Table 2.In the rare event that no LPS is required, the AC SPDs shall berated as Class II/Type 2 (T2) devices.

Attention: According to IEC 60364-5-53, when the lines entering the BESS building/container are overhead, SPDs rated as Class I/Type 1 (or better T1/T2) with an Iimp=5kA per conductor shall be selected. This general rule may differ in national harmonized standards (such as in Germany VDE 0100 543).

Required surge ratings of DC SPDs 

While rules for the selection and installation of DC SPDs for PV applications is well described in IEC 61643-32, the selection of DC SPDs for BESS applications is relatively new, and a dedicated standard is currently not available.One approach to this difficulty is to follow relevant recommendations found in IEC 62305-4. Because an AC/DC converter is always installed, and SPDs are installed at entering lines (AC/PV), parameters S3 can be eliminated. Scenarios S1 and S2, however, do need to be considered relative to the building structure, the grounding system, the physical distance between the AC/DC converter and the battery, and the associated cable routing. The withstand voltage of the AC converter and battery must also be considered. The selected SPDs need to have a voltage protection level that will adequately protect this, usually Up< Uw is used. Generally, the withstand level of the common mode voltages of +DC to ground and -DC to ground need to be carefully evaluated to ensure that no electrical arc to chassis can occur.Attention: According to IEC 60364-5-53, when the lines entering the BESS building/container are overhead, SPDs rated as Class I/Type 1 (or better T1/T2) with an Iimp=5kA per conductor shall be selected. This general rule may differ in national harmonized standards (such as in Germany VDE 0100 543).

The withstand voltage of live parts (e.g., battery poles)to ground is a function of environmental conditions(humidity, salty air etc.), which need to be limited to safe levels using appropriately selected DC SPDs.The following figures illustrate the selection and installation of DC SPDs under each of the possible lightning scenarios.