4. The hazards of DC circuit short circuit faults and the necessity of installing overcurrent protection near the battery pack
The power supply system in the data center can be divided into two circuits: AC and DC. Compared to AC circuits, the impact of DC circuit failures is greater.
The main reasons are as follows:
(1) The harm of battery short circuit is greater than that of AC power. In general, the primary measure for electrical short circuit and fire is to cut off the power supply. For AC power sources, as the electrical energy comes from the municipal power grid or diesel generator sets from top to bottom, when an electrical short circuit fault occurs, there will always be a first level protection device that will act to promptly cut off the short circuited electrical circuit. When the battery pack is located at the end of the power supply system, the electrical energy is provided from bottom to top. As long as it crosses the protective fuse of the DC main distribution panel or the protective circuit breaker of the battery pack, there will be no other protection. When a short circuit fault occurs, it is often impossible to effectively cut off the short circuited electrical circuit. In addition, unlike AC sine waves, DC current has zero instantaneous electromotive force when there is no zero crossing point. Once an electrical short circuit occurs, it is prone to spread. The impedance after a short circuit only depends on the wire resistance and internal resistance of the battery pack, and the short-circuit current is approximately infinite. Therefore, the harm of DC electrical short circuit in the battery pack is much greater than that of AC electrical short circuit.
(2) Causing network interruption accidents
The DC power supply system in communication power is the core system that ensures uninterrupted power supply to communication network equipment, and the backup battery pack is the emergency power source for communication networks. For DC power supply systems, the battery pack is directly connected in parallel to the DC power supply circuit at the output of the rectifier. It is precisely because of the existence of a backup battery pack that power outages or electrical short circuits on the AC side will not directly cause power outages in the communication network. Similarly, for AC UPS systems, as long as the inverter and subsequent circuits are working properly, the backup battery pack can be activated. However, if the DC power system, especially the battery pack, experiences an electrical short circuit, it will inevitably cause the output voltage of the DC power system to drop instantly, causing the load equipment to lose power and leading to network interruption faults, seriously affecting the smooth flow of information communication.
(3) Causing a fire in the computer room
If the electrical short circuit of the battery pack cannot be detected and cut off in time, it will inevitably cause a fire. The better the quality and sufficient power of the battery pack, the greater the harm.
Therefore, the overcurrent protection of the DC circuit, especially the battery pack, in the AC UPS system of the data center is particularly important. Special attention should be paid to configuring a battery protection box near the battery pack side, with built-in switches of DC (or AC/DC dual-use) circuit breakers or fuses. If multiple battery packs are used in parallel, a main output switch should be set up, and switches should be set up separately for each battery pack for protection and operation.