In project X of country X, batteries supply backup power only for 40 seconds after the mains outage occurs and before load low voltage disconnection (LLVD) and BLVD are enabled. The site load current is 900 A. The batteries are manufactured by China and of 3000 Ah and have been installed for about two months.
The possible causes are as follows:
l The loading capacity exceeds the design requirement.
l Batteries are not charged immediately after they discharge completely.
l Multiple batteries have lower voltages than the battery string voltage.
l The voltage drop on the battery loop exceeds the design requirement.
Check the causes one by one as follows:
1. Calculate the loading capacity and battery capacity, and rule out the first cause.
2. Before the batteries discharge, the power system generates no alarms but displays FC (floating charging). The battery voltage is only 49.2 V. This shows that the batteries are not fully charged.
3. Check the voltage of each battery and find no batteries with lower voltages. Then rule out the third cause.
4. The battery voltage is about 49.2 V, and the voltage of the battery busbar in the DC power cabinet is about 45 V. The voltage drop is 4.2 V.
The calculation shows that the load current on the loop is 900 A. Since the battery room is far away from the power system, the voltage drop on the loop greatly exceeds the design requirement (about 0.8 V). This poses the following phenomena:
l Batteries cannot be charged properly. The equalized charging voltage is 56.4 V. If the power system charges batteries in this voltage, the voltage reaching batteries is only 52.2 V. However, the power system considers that the batteries are fully charged, so it charges the batteries in float charging mode in 53.5 V. In fact, the batteries are not fully charged.
l When a mains outage occurs, the battery voltage becomes only 49.2 V after multiple times of discharge, and the voltage reaching the load is even lower than 45 V, which cannot bear the high load current (900 A). Therefore, LLVD and BLVD are triggered at 44 V and 43.2 V respectively.
Persuade the customer to move the batteries towards the power system, reducing the distance by 40%, and to increase the battery cable cross-sectional area from 240 mm2 to 480 mm2.