Q & A on DC and UPS system knowledge (1)

The floating charge current depends on the self discharge rate of the battery. As a result of floating charge, the self discharge of the battery should be compensated. If the floating charge current is too small, the discharge of the battery will not be compensated for a long time, which will make the electrode plate vulcanized; at the same time, the voltage of the whole group of DC bus will be reduced.

1. What is the harm of too large or too small floating charge current?

A: the floating charging current depends on the self discharge rate of the battery. As a result of floating charge, the self discharge of the battery should be compensated. If the floating charge current is too small, the discharge of the battery will not be compensated for a long time, which will make the electrode plate vulcanized; at the same time, the voltage of the whole group of DC bus will be reduced. On the contrary, if the floating charging current is too large, the battery will be overcharged, which will cause the effective material of the plate to fall off, shorten the service life of the battery, and at the same time, it will also consume the electric energy unnecessarily, thus making the operation uneconomical. Therefore, the floating charging current should be well controlled in practical application to ensure the safety of the battery.

2. How to choose DC grounding? What do you pay attention to?

A: choice:

(1) Switch the insulation monitoring device, determine the grounding polarity and check the insulation condition.

(2) Inquire whether the machine, furnace and combustion posts are operated.

(3) Switch the branch with operation.

(4) Cut off the branch with poor insulation or doubt.

(5) Search according to the importance of weather, environment and load.

(6) Select floating charge device.

(7) Select battery and DC bus.

(8) After finding out the grounding point, contact the maintenance team.

matters needing attention:

(1) Two people, one for operation and one for supervision.

(2) Contact relevant units before selection.

(3) The high internal resistance voltmeter (above 2000 Ω / V) must be used to find the ground. It is forbidden to use the bulb search method.

(4) When cutting off each special DC circuit, whether the circuit is connected or not, it shall be switched in immediately.

(5) Before cutting off the DC of network control and generator relay protection, necessary measures shall be taken to prevent the misoperation of protection device caused by DC disappearance.

(6) Do not ground another point during the search.

3. Why is the DC bus voltage not allowed to be too high or too low? What's the harm?

Answer: when the voltage is too high, it is easy to overheat or damage the long-term live relay, indicator light, etc.; when the voltage is too low, it may cause unreliable action of the circuit breaker and protection. The range of bus voltage too high or too low is usually ± 10%.

4. How to determine the operation mode of DC loop disconnector?

Answer: the operation mode of DC loop disconnector shall be determined according to the length of network, the size of current and the size of voltage drop. Generally, it is open-loop operation under normal conditions.

matters needing attention:

(1) It is necessary to find out whether there is the possibility of current interruption to the network before loop breaking operation.

(2) When the DC system is grounded at the same pole and two points, it is not allowed to close the loop disconnector before the cause is not found out and the fault is not eliminated.

5. What is the function of UPS system?

Answer: as a normal, abnormal and accident situation of the whole plant, UPS system provides safe, reliable, stable and uninterrupted AC power supply which is not affected by switching operation to the computer, communication equipment and some important and uninterrupted loads in the plant.

6. What are the hazards of DC system grounding?

Answer: DC system grounding shall include one point grounding of DC system and two points grounding of DC system.

In the DC system, the DC positive and negative pole are isolated from the ground. When one pole is grounded, it will not cause any harm because there is no path to constitute the ground current. However, it is not allowed for one pole to be grounded for a long time. When another ground occurs at another place of the same pole, it can cause incorrect action of the signal device, relay protection or control circuit. The DC short circuit will be caused when one point is grounded and the other is grounded.

For example, DC Positive grounding may cause relay misoperation. Because the general trip coil (such as the outlet intermediate relay coil and the tripping and closing coils) are all connected to the negative power supply, if these circuits are grounded or poorly insulated again, the relay protection will misoperate. DC negative grounding and positive grounding are the same principle. If there is another point of grounding in the circuit, it may cause relay protection to refuse to act, which will expand the accident.

If two poles are grounded at the same time, the tripping or closing circuit will be short circuited, which may not only fuse the fuse, but also burn out the contact of the relay.

7. What items should be checked for normal battery?

Answer: (1) the indoor temperature is normal in the range of 10-30 ℃, and all joints and connecting wires are not loose.

(2) The room is clean and well ventilated, and the battery surface is free of abrasion and leakage.

(3) The indoor equipment is complete and the lighting is normal.

(4) Check the battery once per shift, and check that the specific gravity is within the specified value. The color of electrolyte is normal and the liquid level is within the range. The battery terminal voltage is normal.

(5) The plates shall be free of bending, fracture and short circuit.

(6) It is forbidden to open fire, smoke or work that may produce sparks in the battery room. If hot work is necessary, a hot work permit shall be provided.

8. What factors are related to the electromotive force of lead-acid batteries?

Answer: the electromotive force of the battery is related to the electrochemical properties of the active substances on the plate and the concentration of the electrolyte. However, when the active substance on the plate is fixed, the electromotive force of the battery is mainly determined by the concentration of electrolyte.