By Immanuel F. Umenei, NA Vertical Market Manager – Renewable Energy, Littelfuse
Battery-based energy storage systems are in demand but their design is still considered to be in its infancy because the technologies are evolving rapidly. According to the IEEE paper “Arc-Flash in Large Battery Energy Storage Systems — Hazard Calculation and Mitigation,” the design complexity and required technological innovation, as well as the lack of harmonized standards concerning short-circuit current calculations and safety, are fundamentally why accurate calculations of arcing currents in energy storage systems have been difficult to achieve this far.
As the power density of lithium-ion batteries continues to increase, so will the risk of an arc-flash incident. To maximize the capacity of each battery and provide users the longest possible discharge times, storage integrators are working with their suppliers to squeeze more power into a more compact footprint.
Arc-flash mitigation is necessary to preserve a competitive cost per kilowatt-hour. The energy storage industry is poised to dramatically expand, with some forecasts predicting that the global energy storage market will reach 1,095 GW of capacity by 2040. These same forecasts estimate that investments in energy storage will grow to $620 billion over this period.
An arc-flash occurs when fault current flows across an air gap and creates highly ionized gas. Left unmitigated, an arc flash can produce temperatures hotter than the surface of the sun, throw shrapnel that travels more than 700 mph and blast a pressure that exceeds 2,000 psi (higher than a shotgun blast). The danger to workers varies with each piece of equipment, depending on the available fault current, working distance and the opening time of overcurrent protection devices in the electrical circuit.
If a phase-to-phase fault with 20,000 amperes of current occurs in a 480-volt system, then there will be 9.6 million watts of power. Without arc-flash mitigation, an arc that lasts for 200 milliseconds will release 1.92 million joules, based on 480 V x 20,000 A x 200 ms (E = V x I x t).
By way of comparison, TNT releases about 2,175 joules/gram when detonated, so the arc-flash just described corresponds to the detonation of 863 grams of TNT. (One stick of dynamite contains 1,000 grams of TNT.)
According to OSHA, arc-flash incidents are responsible for approximately 80% of electrical injuries and fatalities reported to OSHA among qualified electrical workers. Even when there are no injuries to workers, an arc flash can destroy equipment, requiring costly replacement and system downtime.
The high level of DC power that feeds into inverters from the combined output of the banks of DC batteries is an arc-flash hazard. When the outputs of multiple daisy-chained batteries are brought together in a combiner box, they can also produce enough DC voltage to initiate an arc. Unlike sinusoidal AC power, where the zero crossing helps AC arcs extinguish themselves, DC arcs that arise from batteries are not self-extinguishing.
Calculating arc-flash hazards: Energy storage is different
Almost every type of energy storage system can rapidly release DC fault currents. However, systems that use lithium-ion batteries have a faster energy demand response.
An arc-flash risk’s severity is determined by calculating the potential incident energy. The guide IEEE 1584 – Guide for Performing Arc-Flash Hazard Calculations can be used to determine the arc-flash hazard distance and incident energy.
A revision is forthcoming based on further testing with AC systems. However, DC arc-flash has been less studied and is less understood than arc-flash in AC systems. Arc-flash calculators, such as the Littelfuse Arc-Flash Energy Reduction Calculator, can be used to determine how to reduce incident energy in a facility.
Arc-flash calculations determine the largest possible incident energy. However, a few factors that may not be intuitively obvious can result in higher incident energy levels than would be anticipated if only an overcurrent protective device were used:
- Battery age: As batteries age, their internal impedance increases. This can result in lower arc-flash current, which can in fact lead to higher energy because the overcurrent protection device takes longer to operate.
- State of charge: A partially depleted battery bank may not produce enough short-circuit current to operate a fuse or other overcurrent protection device, yet it may have enough current to create an arc flash.
- Battery banks: An arc flash on one battery bank will be fed from other battery banks that are arranged in the circuit in parallel.
- Battery cabinets tend to direct the energy out of the cabinet door. Because of this, large-scale battery enclosures can expose personnel to more incident energy than a typical enclosure during an arc flash incident, both by containing the fault and by making it more difficult for workers to self-rescue within a typical two-second window.
Arc-flash mitigation in battery-based energy storage systems
To mitigate arc-flash hazards, arc-flash relays detect the light from an emerging arc-flash and send a trip signal to an upstream circuit breaker within a few milliseconds (if it is a high-quality relay). This prevents an arc from developing into a full-blown catastrophic event.
To install an arc-flash relay system, light sensors are placed around the interior of the enclosure that houses the inverter and its associated bus bars that are most likely to be the origin of an arc. The power semiconductor device inside the inverter usually fails-safe, but it or its connectors can potentially fail to ground and cause an arc flash.
Arc-flash relays provide reliable protection even when the state of charge is low. Using an arc-flash relay instead of relying on overcurrent protection devices alone provides a storage system with consistently low incident energy throughout its lifetime.
Battery banks can be protected by monitoring the battery bank with an arc-flash relay that will send a trip signal to a device that disconnects the bank from the bus. Allowing the means for workers to disconnect sections of the battery bank further reduces available incident energy while maintenance is being performed.
Arc-flash relays protect workers who install and maintain batteries and protect energy storage owners from the high cost of worker injuries, including OSHA fines and litigation. For example, an incident reported by OSHA occurred inside a wind turbine and resulted in severe burns to a technician caused by an electrical arc-flash. The proposed fines were $378,000 for the employer. Relays prevent damage that can jeopardize the investment of BESS equipment.
Battery systems do more than power critical facilities when the normal power source fails. Energy storage systems also offer other benefits and ancillary services, including load-leveling, spinning and regulation reserves, transmission and distribution infrastructure deferral, and frequency regulation. These benefits multiply the worth of battery assets to utilities. Investing in arc-flash mitigation is critical to protect these benefits from events that could cause costly catastrophic damage.
Immanuel F. Umenei is the NA vertical market manager, renewable energy for the industrial business unit of Littelfuse. He has a master’s degree in engineering from the University of Illinois, Chicago. With his years of experience in LV circuit protection design and OEM sales and applications engineering, he has specified overcurrent circuit protection components for different electrical systems, particularly solar and energy storage.