Field of the Invention
The present invention relates in general to the field of power supplies and, more particularly to fault protection in uninterruptible power sources.
Description of the Related Art
It is known to provide data processing systems such as those that include data storage systems with an uninterruptible power source (UPS). For large scale data processing systems, the UPS can include a plurality of high current batteries, often configured in long strings, to supply energy to the UPS. These batteries are often protected by fuses and/or circuit breakers. However, even with fuses and/or circuit breakers it is desirable to provide additional fault protection for high current batteries such as those configured in long strings of batteries.
External circuit protection is extremely important to battery powered circuits. Batteries by design have as little current limiting as possible, to minimize internal losses. Reliable circuit protection is especially important for long strings of batteries which can supply both high peak DC currents and higher voltage that may drive destructive arcing and burning in the event of a fault condition. Known circuit protection devices (like fuses and circuit breakers) are capable of almost instantly interrupting fault currents that are greater than 250% to 500% of their ratings. However, by selecting a circuit protection device that will never trip at the largest expected operating current of the circuit, the same device will likely not be able to guarantee opening quickly enough under fault currents that are below about 250% of the rated current. These moderate fault currents (so called “soft shorts”) can potentially cause slow temperature rises, melting, arcing and fire if given enough time. For many known circuit protection devices, the time to trip or even to open at all is not necessarily accurately predictable in the range of fault current below about 250% the rating.
Another known electronic or intelligent circuit protection includes a switch in series with the load. The switch is normally conduction then held open when fault currents are detected. Additionally, electronic current limiting and/or electronic over current shutdown features are commonly used by electronic power supplies and power converters. These power supplies already have switches in series with the load current, so little additional hardware is required to implement this type of circuit protection.
It is known to state circuit energy (power×time) as “I squared t” (I2t). There is a trend toward reducing the I2t of normally robust power distribution components to meet cost, weight and size reduction goals. A predictable trip time for battery circuit protection is desirable to protect these lower rated power distribution conductors and interconnection.