Issues Concerning the Safe Application of UPS Battery Pack in Data Center Communication (Part 4)

5. Analysis and detection of electrical short circuit faults and leakage hazards in lead-acid batteries

In the cause of electrical short circuits in UPS system battery packs, battery leakage causing short circuits or decreased insulation to the battery rack, resulting in indirect short circuits between the positive and negative poles through the battery rack, has always been a difficult and difficult to diagnose and detect fault with high probability, but the consequences are very serious.

At present, there are more or less deficiencies in the preventive measures for such potential faults:

（1）Add a tray at the bottom of the battery – the tray is flammable;

（2）Adding electric board gaskets to the battery rack cannot avoid the diffusion of electrolyte;

（3）Battery rack insulation to electrical ground – difficult to implement and does not comply with safety regulations;

（4）Installation of very early smoke alarm system in battery room – not timely.

The current DC power supply systems that are higher than the safe voltage (such as power operation power supply, 240V DC power supply system for communication, etc.) require the use of DC circuit to ground suspension working mode and are equipped with insulation monitoring function system.

The so-called insulation supervision refers to the testing of the insulation performance between the DC output and ground in the DC power supply system, to determine whether there is a ground fault or a decrease in insulation performance. When a grounding fault or insulation performance degradation occurs, an alarm will be issued.

The insulation monitoring function mainly achieves the detection of ground insulation resistance by detecting the voltage and current in the DC power supply circuit. Among them, voltage detection technology mainly uses insulation monitoring to monitor the ground voltage of positive and negative DC busbars in real time, and calculates the insulation resistance of positive and negative busbars to ground through the ground voltage. When the insulation resistance is lower than the set alarm value, an alarm signal is sent.

Essentially, an electrical short circuit in a battery pack is also an extreme form of decreased insulation between the positive and negative electrodes. For electrical short circuits caused by battery leakage, it is inevitably due to a decrease in insulation between the positive and negative electrodes or their connection to the battery frame (grounding).

According to the working principle of insulation monitoring, as long as the battery pack is suspended to ground, the positive and negative circuits (including charge and discharge circuits) of the battery pack are not grounded. The presence of battery leakage or decreased insulation to ground can be detected by balancing the bridge resistance. Due to the fact that the measurement object of this detection method is the voltage between the positive and negative poles of the battery pack and the ground, regardless of any ground fault or insulation decrease at any point in the battery pack, it will cause changes in the positive pole to ground voltage U1 or the negative pole to ground voltage U2, which can be quickly reflected in the insulation monitoring system. Therefore, with the detection principle of insulation supervision, it is possible to detect the leakage of battery packs.