Energy Storage System Overcurrent Protection Guide

Energy Storage System (ESS) solutions are being paid attention to more than ever. At each step in the grid, from generation to transmission, and from distribution to end users, batteries offer many advantages such as grid stabilization, integration of renewable energy, flexibility, reliability as well as independence. As the need for greener energy grows, so does the importance of energy storage.  While Electrical Energy Storage is not new, the increase of power has brought new constraints and challenges for over-current protection devices. DC fuses must withstand a wide range of constraints such as power cycling, high and low fault currents and coordination with other protective devices. ESS protective schemes are also far from being standardized, resulting in a multitude of protection architectures according to the system or component manufacturer.

The safety of ESS applications has become a major focus. A series of incidents have pointed out that safety has not yet been addressed properly, due certainly to the rapid growth of ESS installations. Smart monitoring systems have allowed mitigating catastrophic failures of ESS installations; however, fuses remain the safest solution once everything else has failed. Today’s ESS installers face the challenge of operating voltages of up to 1500 VDC.  The safety of ESS applications is now being insured as a result of the introduction of specifically designed fuses for ESS applications complimented by test labs simulating actual ESS fault currents and development of new IEC standards for battery usage. The purpose of this document is to guide the reader through the process of selecting the appropriate over- current protecting device from the module up to the container level of their ESS system.

Energy Storage Topology

A typical ESS System consists of several levels of different battery assembly:

Role of Fuses in ESS

A fuse is a device for protecting an electrical system against the effects of overcurrents (excess currents), by melting one or more fuse-elements, thus opening and isolating the faulted circuit. Very fast-acting fuses are widely used for the protection power semiconductors in AC and DC power electronic applications and are now used for battery system protection such as energy storage, UPS, and electric vehicles. ESS fuses provide excellent protection against the potentially damaging effects of short-circuit currents.

ESS fuses achieve this protection by limiting both the magnitude and duration of the fault which limits the amount of energy produced by an overcurrent and the peak current which is allowed to flow.

In ESS, this implies that fuses are not only installed to protect each level of the system from battery short circuits but also protect other over-current protection devices such as contactors and switches from damage when properly selected. In some situations, selective coordination between fuses can be achieved, adding another level of protection.

Fuses Interrupting Ratings and Minimum Breaking Capacity

The primary function of a fuse is to interrupt the over- currents safely, to protect the components and cables of the system from being damaged. However, every fuse has a range of currents it can interrupt safely, and the fuse should not be relied upon to interrupt currents outside of this range.  The Interrupting Rating (IR) and Minimum Breaking Capacity (MBC) are critical parameters defined by international fuse standards that define the range of currents fuses open safely.

  • IR is the maximum prospective current a fuse is tested to safely open at a specific DC voltage and time constant (L/R)
  • MBC is the minimum current a fuse is tested to safely open at a specific DC voltage and time constant (L/R).
  • Therefore “MBC – IR” is the range of currents a fuse can safely open

The fuse MBC is specified at a given voltage and time constant. It is essential to know that the MBC is a function of system voltage and time constant of the circuit where it is used. If the system which the fuse is applied in has a lower voltage and/or time constant, the fuse’s MBC will vary. MBC can vary widely across fuse types. For fuses used in ESS applications, MBC can vary from 3 to 15 times the current rating of the fuse. Contact the fuse manufacturer for additional information on MBC.

While the IR is well-known to users, the MBC is commonly overlooked. In ESS applications, the MBC must be taken into consideration, due to limited short- circuit current generating capabilities of batteries.  Battery racks typically provide a fault current range between 1 to 12kA and ESS systems can go up to 250kA or more when racks or sections are combined in a system.  For the section fuse, the interrupting rating is critical parameter. Nevertheless, it is also important to know the MBC as well, to make sure the fault currents the fuse must interrupt fall within the range of fuse operating range.  For the rack fuse, having a low MBC in the range of 2-3In is highly beneficial to promote coordination with and protection of the contactor. The maximum IR is also critical because when there is an internal fault in the rack, the rack fuse may be exposed to high short circuit currents coming from the cascading faults from the adjacent racks.

Fuse Coordination

The ultimate goal in ESS battery protection is having a solution that safely interrupts the power and can cover the full spectrum of current loads. Coordination of the module/rack/section fuse is an important consideration for proper protection of the system. For additional information contact the manufacturer.

National Electrical Code

The National Electrical Code (NEC) in the United States have established a set of requirements for fuses installed in ESS applications. There are numerous articles in which those requirements are stated:

  • Article 706, Energy Storage Systems
  • Article 480, Storage Batteries
  • Article 690, Solar Photovoltaic (PV) Systems
  • Article 691, Large-Scale Photovoltaic (PV) Electric Power Production Facility
  • Article 692, Fuel Cell Systems
  • Article 694, Wind Electric Systems
  • Article 705, Interconnected Electric Power Production Sources
  • Article 712, Direct Current Microgrids

The most important article for fuses is Article 706.31: Overcurrent Protection 2020.

Battery Protection Standard

A new part of IEC 60269 “Low Voltage fuses” is dedicated to battery protection IEC 60 269-7, Ed.1: Low Voltage Fuses: Supplementary Requirements for fuse-links for the protection of batteries and battery systems

This part defines 2 new utilization categories of fuses: aBat & gBat.

  • “aBat” indicates fuse-links with a partial range d.c. breaking capacity for the protection of batteries and battery systems
  • “gBat” indicates fuse-links with a full-range d.c. breaking capacity for the protection of batteries and battery systems.
  • “gBat fuses shall be able to clear 2 times their rated current, tested at rated voltage (-0 / +5%).

For each one, the breaking capacity shall be higher than 30kA, tested at rated voltage (-0 / +5%).

For the same rated current, the characteristics of gBat or aBat (time current characteristics, clearing I²t, power dissipation) fuses are different. Therefore the fuse selection, among these 2 categories, shall be done according to the electrical specification to ensure safety protection.

    Please contact our NEMA Members for proper selection and sizing for your application.





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