The power DC screen is a high-voltage power distribution equipment, and a DC power supply system of the whole distribution room. Sometimes it can also cooperate with the AC screen according to the situation, and make up the AC and DC power supply system. The DC screen is mainly applied to large hydropower and thermal power plants, all kinds of substations and other users who use DC equipment, to provide DC power supply for signal devices, protection, automatic devices, emergency lighting, emergency power supply and circuit breaker division and closing operation. The DC system is an independent power supply, which is not affected by the mode of generator, power supply and system operation. Under the condition of external AC power interruption, the battery provides DC power, and the excess electricity saved in the battery is used to supply the high voltage cabinet, closing and closing, protecting equipment, emergency lighting system and so on. The reliability and safety of the DC screen directly affect the reliability and safety of power supply. Therefore, the selection of the DC screen is based on the following aspects:
(1) selection of battery
Uninterrupted power supply equipment, DC battery is the core component. The initial DC power supply systems mostly use ordinary lead-acid batteries, which have low overload capacity and easy to produce acid corrosion and other problems. Gradually with the understanding of alkaline nickel cadmium batteries and compared with the original lead-acid batteries, it is concluded that the alkaline nickel cadmium battery has the following advantages:
1, good mechanical strength, small volume, can withstand strong impact and vibration, not easy to damage.
2, the pressure drop is small, and the loss of energy caused by self discharge is small.
3. The overdischarge ability is strong, and the internal short circuit will not be caused by overcharge
4, the discharge voltage is stable and the life is long. If it is used reasonably, it can be up to more than 3000 use cycles.
5, according to the above alkaline nickel cadmium batteries, heavy load, overload may DC system (such as electromagnetic operating mechanism of the operating power supply), the installation site of good environment and the environment temperature is not high and strong maintenance of power situation, you can use alkaline battery cadmium tweezers. On the other hand, the maintenance free lead-acid battery can be selected.
(2) the selection of the charging system
Charging equipment, is currently using more thyristor phase controlled DC charging system, charging of the battery charge and floating charge is divided into two stages, the charge current of large current and strong charge trickle in two ways. The common control methods include thermal sensor method, timing method, negative sequence voltage slope method, volt ampere method and so on. In recent years, many manufacturers of the DC system, mainly in the improvement of control technology, such as integrated circuit technology, PLC technology, computer monitoring, "three remote" communication technology, continuous integration of them, so that the reliability of phase controlled charging equipment system continue to improve, so this kind of DC mesh screen system, reliability and practicability the product can basically satisfactory. However, the phase control charger still has the disadvantages of large harmonic output voltage, large current ripple, and more difficult to improve the precision of steady current. At the same time, in some cases, the reliability of the system is still not enough. So, overseas gradually adopt a high-frequency switching power supply is relatively new in some higher requirements for DC power supply, the overall structure of the module combination, the module principle diagram is as follows, namely the first phase controlled rectifier for three-phase AC DC, then transform into high-frequency alternating current, high frequency AC by full bridge rectifier, and transformer isolation converted to stable DC output filter. The main characteristic of this DC power supply is high precision, small ripple and low harmonic distortion. It is a kind of high quality DC power supply. When the DC system is made up, the DC output can be provided by the combination of N modules, so the degree of backup is high and the overall reliability of the DC system is high. The DC power supply of existing domestic manufacturers, but in the overall appearance and was less than that of traditional phase controlled DC screen, enterprises can compare the trade-offs in the selection of DC power supply.
(3) maintenance of direct current screen system
The following problems should be paid attention to in the use of nickel cadmium batteries:
1) regularly charge and discharge the battery group with high power to activate the chemical substances inside the battery, reduce the battery inertia and restore the battery capacity.
2) regularly check the battery level and adjust the high and low level of the battery level in time when the battery is full.
3) regularly check the concentration of the battery solution, make up the alkaline solution or add water when the specific gravity changes, and reprepare the electrolyte when necessary.
4) check the battery terminal voltage regularly one by one, and "activate" the battery that has a large drop in the individual voltage.
5) to pay attention to the environment temperature of the battery, it is best to make the battery run at the temperature below 40 C, so as to avoid the self discharge of the battery and cause the battery capacity to decrease.
6) keep the battery clean and the environment dry, so as to avoid the battery creeping, resulting in the decrease of the battery capacity and the decrease of the insulation of the DC system. The alkaline solution spilt in the battery should be cleaned with dry cloth in time and at the time of maintenance.
The battery can be flushed with water.
Compared with the nickel cadmium battery, maintenance free lead-acid battery maintenance workload small operation, usually only need to pay attention to check the battery connection intact, stay connected without rust, corrosion; observation of whether there is swelling, cell deformation; sealing valve is in good condition; there are no signs of melting pole and keep clean and dry battery and so on. At the same time check the battery end voltage, discover the battery failure.
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