DC and UPS system knowledge Q&A

1. What are the hazards of excessive or under-current floating charge?

A: The size of the floating charge current depends on the self-discharge rate of the battery. The result of the floating charge should just compensate for the self-discharge of the battery. If the floating charge current is too small, the discharge of the battery will not be compensated for a long time, and the plate will be sulfided; at the same time, the voltage of the entire DC bus will be reduced. On the contrary, if the floating charge current is too large, the battery will be overcharged, causing the effective material of the plate to fall off, shortening the service life of the battery, and consuming unnecessary energy, making the operation uneconomical. Therefore, in practical applications, the size of the floating charge current should be well controlled to ensure the safety of the battery.

2. How to choose DC grounding? What should I pay attention to?

A: Select:

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

(2) Inquire whether the machine, furnace and fuel stations are being operated.

(3) Switch the branch that is in operation.

(4) Remove branches with poor insulation or suspected insulation.

(5) Search in order based on weather, environment, and load importance.

(6) Select a floating charging device.

(7) Select the battery and DC bus.

(8) After finding the grounding point, contact the relevant maintenance team for processing.

Note:

(1) The test should be conducted by two persons: one operator and one supervisor.

(2) Contact the relevant units before making a choice.

(3) When searching for grounding, a high resistance voltmeter (2000 ohms/volt or more) must be used. Do not use a light bulb to search.

(4) When disconnecting each dedicated DC circuit, it should be closed immediately regardless of whether the circuit is connected or not.

(5) Before cutting off the DC power to the grid control and generator relay protection, necessary measures should be taken to prevent malfunction of the protection device due to the disappearance of DC power.

(6) During the search, do not cause another point to be grounded.

3. Why is the DC bus voltage not allowed to be too high or too low? Otherwise, what are the hazards? What is the allowable range?

A: When the voltage is too high, it is easy to cause overheating or damage to relays and indicator lights that are energized for a long time; when the voltage is too low, it may cause unreliable operation of circuit breakers and protection. The range of bus voltage being too high or too low is generally ±10%.

4. How to determine the operating mode of the DC loop disconnector? What should be paid attention to during operation?

Answer: The operation mode of the DC loop isolating switch is determined according to the length of the network, the magnitude of the current and the magnitude of the voltage drop. Generally, it operates in an open loop under normal circumstances.

Note:

(1) Before performing a ring-breaking operation, check whether there is any possibility of causing current interruption to the network.

(2) When two points of the same pole of a DC system are grounded, the loop disconnector must not be closed until the cause is identified and the fault is eliminated.

5. What is the function of UPS system?

Answer: The UPS system provides safe, reliable, stable, uninterrupted AC power supply that is not affected by switching operations to the factory's computers, communication equipment and certain important loads that cannot be interrupted under normal, abnormal and accident conditions.

6. What are the hazards of DC system grounding?

Answer: The grounding of the DC system should include two situations: one-point grounding of the DC system and two-point grounding of the DC system.

In a DC system, the positive and negative poles of DC are insulated from the ground. When one pole is grounded, there is no path for the grounding current to flow. However, long-term operation of one pole grounded is not allowed because when another pole is grounded at another location, it may cause incorrect operation of the signal device, relay protection or control circuit. If one pole is grounded and then the other pole is grounded, a DC short circuit will occur.

If the DC positive pole is grounded, it may cause relay protection to malfunction. Because generally the tripping coils (such as the outlet intermediate relay coils and the tripping and closing coils, etc.) are connected to the negative power supply, if these circuits are grounded or poorly insulated, it will cause relay protection to malfunction. The DC negative pole grounding is the same as the positive pole grounding. If there is a point of grounding in the circuit, it may cause the relay protection to refuse to operate, causing the accident to expand beyond the level.

If both poles and two points are grounded at the same time, the tripping or closing circuit will be short-circuited, which may not only cause the fuse to blow, but also burn out the contacts of the relay.

7. What items should be checked for normal battery operation?

Answer: (1) The normal indoor temperature is between 10 and 30°C, and the joints and connecting wires are not loose.

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

(3) Indoor equipment is complete and lighting is normal.

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

(5) The plates are not bent, broken or short-circuited.

(6) Open flames, smoking, and any work that may produce sparks are prohibited in the battery room. If fire is necessary, a fire permit must be obtained.

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

Answer: The magnitude of the battery's electromotive force is related to the electrochemical properties of the active substances on the plates and the concentration of the electrolyte. However, when the active substances on the plates are fixed, the battery's electromotive force is mainly determined by the concentration of the electrolyte.

9. What causes battery self-discharge?

A: There are many reasons for self-discharge, mainly:

(1) The electrolyte or the plate itself contains harmful substances. These impurities are deposited on the plate, causing a potential difference between the impurities and the plate, and between the impurities on the plate.

(2) There is a potential difference between the various parts of the plate itself and between the parts of the plate when the plates are in layers of electrolyte with different concentrations.

These potential differences are equivalent to small local discharges, which form currents through the electrolyte, causing the active materials on the plates to dissolve or electrochemically transform into lead sulfate, resulting in loss of battery capacity.

10. What is the floating charge operation mode of the battery?

A: The normal load is mainly supplied by the charging equipment, and a small current is also used to supplement the self-discharge of the battery. The battery does not supply power normally. Only when the load suddenly increases (such as the circuit breaker is closed, etc.) and the charging equipment cannot meet the needs, the battery will discharge a small amount. This operation mode is called floating charge operation mode.

11. Under what circumstances is it possible for the battery room to explode?

A: When the battery is charging, water is decomposed to produce a large amount of hydrogen and oxygen. If these mixed gases cannot be discharged outdoors in time, they will cause an explosion when encountering sparks.

12. What are battery capacity and rated capacity?

A: The capacity of a battery is a sign of the battery's storage capacity, expressed in ampere hours (AH). The ampere hours of the capacity is the product of the ampere of the discharge current and the discharge time, which is the capacity of the battery. The rated capacity of a battery refers to the capacity of the battery when it is fully charged and discharged at a 10-hour discharge rate.

13. Describe the operation mode of the UPS system of units #3 and #4?

Answer: (1) The UPS system is a single-phase two-wire directly grounded system.

(2) When the UPS system is operating normally, the security section MCC rectifies and supplies power to the UPS. After rectification, the power is sent to the inverter and converted into single-phase AC 220V, 50Hz to supply power to the UPS distribution panel.

(3) The standby UPS output serves as the bypass power supply for the UPS systems of units #3 and #4.

(4) Under normal operation, when the working power fails or the rectifier fails, the 230V DC power supply directly supplies power to the inverter, and the UPS bus will not interrupt power supply during the switching process.

(5) When the UPS device inverter fails, it will automatically switch to the bypass power supply and supply power to the UPS distribution panel through the static switch, or use the bypass switch to supply power to the UPS distribution panel.

(6) After the inverter returns to normal or the UPS maintenance work is completed, the bypass power supply should be shut down and the normal operation of the UPS should be restored.

(7) When the UPS device needs to be shut down for maintenance, a bypass switch can be used to supply power to the UPS distribution panel.

14. Describe the operation mode of 110V DC system?

Answer: (1) The 115V DC system adopts single bus operation mode.

(2) Unit 115V DC system working charger A works with DC bus A, charger B works with DC bus B, and the standby charger serves as a backup for chargers A and B. The two outlet switches of the standby charger are equipped with mechanical locking devices to prevent the two battery groups from running in parallel. When any bus of the 115V DC system of unit #3 is under maintenance, the load carried by the bus can be switched to another bus for operation.

(3) During normal operation, each set of DC busbars must be put into operation, and its corresponding equipment and protection must be put into operation accordingly. No one may withdraw its operation without authorization.

(4) Under normal circumstances, the battery pack and the charger device operate in parallel, using the floating charge mode. In addition to supplying power to the normal continuous DC load, the battery is also float-charged with a small current of 0.8 A. In any case, the charger shall not be used to supply power to each DC working bus separately.

(5) The number of working batteries in a 115V battery pack is 54. Under no circumstances are battery packs A and B allowed to operate in parallel.

(6) Each section of the DC bus is equipped with an insulation monitoring device. During normal operation, the insulation monitoring device should be put into operation.

15. What is the function of the 110V DC system?

Answer: (1) Provide reliable DC power supply for remote operation of switchgear, signal equipment, relay protection and automatic devices.

(2)

16. What effects do overcharging and undercharging have on batteries?

Answer: In lead-acid batteries, overcharging will cause premature damage to the positive plate, and undercharging will cause the negative plate to sulfide and reduce capacity.

The phenomena of battery overcharge are: the colors of the positive and negative plates are brighter, the battery chamber has a strong sour smell, there are more battery bubbles, the battery voltage is higher than 2.2V, and most of the battery shed materials are positive.

The symptoms of under-charged battery are: the color of positive and negative plates is not bright, the acid smell in the battery chamber is not obvious, there are fewer bubbles in the battery, the battery voltage is lower than 2.1V, and most of the battery shed material is from the negative electrode.

17. What causes short circuits or bending of lead-acid battery plates?

Answer: The reasons for plate short circuit are: (1) Serious shedding of effective materials. (2) Damage to the plate or separator. (3) Plate bending causing the lug to short circuit. (4) Metal objects falling into the plate.

The reasons for plate bending are: (1) excessive charging and discharging current. (2) improper installation. (3) harmful substances mixed into the electrolyte.

18. What should I pay attention to when using the KVA41 series charger?

Answer: (1) The regulations must be strictly followed when using the device.

(2) Before closing the AC control switch, the voltage and current potential regulators must be adjusted to zero to prevent the output voltage or current initial value from being set too high, causing overvoltage or overcurrent at the moment of power supply and damaging the rectifier element or other components.

(3) The device is not allowed to operate at full load at a voltage lower than 50% of the rated voltage, otherwise the thyristor rectifier element will be damaged due to excessive temperature rise.