Challenges of DC data center power distribution protection

The power distribution grid of a DC data center is unique due to the highly capacitive characteristics of both the output of the DC power source (Uninterruptable Power Supply - UPS) and the input of the distributed load (multiple Power Supply Units - PSU). In case of a short-circuit to ground in such a system, oscillations are observed due to discharge of the capacitors through distributed RL network created by cables and bus bars. The detailed values of the frequency and amplitude of the current oscillations are system dependant, but in some cases the oscillation period might be as short as single millisecond, while the amplitude might rise to tens of thousands of Amps. The discharge of the input filters of the PSUs creates additional problem - a short current pulse of high amplitude and reversed polarity, which is noticed by all of the protection devices other than those located on the of the short-circuit current path, creates possible danger of unnecessary tripping of these breakers. Thus it is crucial to ensure that only the one, correct circuit breaker reacts during the fault in shortest time possible. This paper presents results of short-circuit to ground simulation in exemplary DC data center, the challenges of the proper protection system design and discusses zone selectivity as possible solution for fast and exact fault location.