1. Selection and installation of lightning arresters
When selecting arresters, zinc oxide arresters are generally used, and bulky silicon carbide arresters are rarely used. Among zinc oxide arresters, there are some with gaps and some without gaps. The advantage of having a gap is that there is no leakage current under normal conditions and there is no zinc oxide aging problem. The disadvantage is that the gap is prone to change during transportation and installation. The number of lightning arresters is sometimes three, that is, one for each phase, to provide overvoltage protection for each phase, or four, and the neutral point is connected to an arrester to form a combined type. The advantage of this method is that it not only protects the phase-to-ground voltage, but also protects the overvoltage between phases.
When selecting a lightning arrester, be sure to pay attention to the continuous operating voltage value. There is a kind of lightning arrester with a rated voltage of 10kV. It is believed that this arrester can be used in a 10kV neutral point ungrounded system. However, the continuous operating voltage of this kind of arrester is only 8kV. Once one phase is grounded, the voltage of the other two phases will rise to the line voltage, that is, 10kV. This kind of arrester will soon breakdown and explode. For 10kV ungrounded substations and power distribution stations, it is better to use a rated voltage of 17kV and a continuous operating voltage of 13.6kV. This ensures safe operation during normal times and can instantly release lightning current when a lightning wave strikes. Although the residual voltage is as high as 50kV, it is also safe for 12kV switch cabinets because the switch cabinet and the high-voltage side of the transformer can withstand impulse voltages of not less than 75kV.
Some people always feel that for a system with a nominal voltage of 10kV, lightning arresters with a rated voltage of 17kV should be used for lightning protection of lines and distribution devices. The rated voltage is too high, which is a bit unbelievable. They doubt whether it can play a protective role. In fact, this is not the case. Our country stipulates that in medium-voltage systems with ungrounded neutral points, zinc oxide arresters for distribution protection should be 1.3 times of 13kV, that is, 17kV. It seems that the rated voltage of the arrester of 17kV is based on the relevant standard requirements, rather than randomly using excessively high voltage as its rated voltage.
Due to the use of gapless zinc oxide arresters, leakage current is generated under the action of voltage. Under long-term effects, zinc oxide gradually ages, and the leakage current gradually increases, forming a vicious cycle, and the heat generation also increases. This leads to the risk of explosion. For example, if a zinc oxide arrester with an air gap is used, no leakage current is generated at ordinary times, which increases the service life.
If the power distribution room feeds nearby medium-voltage motors or capacitors, the lightning arrester used in the switch cabinet should be selected to be compatible with it. Generally speaking, the residual voltage of the arrester used to protect the motor is much lower than that of the arrester of the transformer or line feeder cabinet. Because the insulation strength of motors is much lower than that of transformers, switchgear or circuits. If the motor is far away from the power distribution room, a special lightning arrester for the motor should be installed on the motor site, and the power distribution room busbar should be equipped with a line or switch cabinet lightning arrester.
The installation location of the arrester is as follows:
(1) There must be a lightning arrester on the busbar in the power distribution room to protect all equipment connected to the busbar.
(2) The outlet circuit of the vacuum circuit breaker must have a lightning arrester. This arrester is generally called an overvoltage protector in order to prevent the circuit breaker's cut-off overvoltage from causing harm. However, the discharge voltage must be compatible with the vacuum circuit breaker cut-off voltage, otherwise it will be useless.
(3) A lightning arrester should also be installed on the side of the power incoming line, especially on the overhead incoming line. Although there is a lightning arrester on the busbar, this can prevent lightning waves from hitting the power incoming line when the power supply is disconnected and causing a jump strike at the break.
2. Selection of live displays, display control devices and digital instruments
1. Charged display
Whether some cabinets are live or not can be checked with a voltmeter. Why is it necessary to install a live monitor? Some people are puzzled by this. You should know that the live indicator not only shows whether the circuit is live, but it also plays an important role. It controls the magnetic lock through its micro-moving contacts, and the electromagnetic lock completes the interlocking of the cabinet door. We know that for the KYN28A-12 switch cabinet, the mechanical interlocking of the grounding switch and the circuit breaker, and the mechanical interlocking of the grounding switch and the cabinet door are completed through the operating lever of the grounding switch. If there is no grounding switch in the cabinet, such as voltage transformer cabinet, isolation cabinet, metering cabinet, busbar lifting cabinet, etc., the cabinet door cannot be opened when the cabinet is electrified. This interlocking function is completed through the electromagnetic lock of the live display.
Some people believe that when the live display fails, it cannot display the live state, causing misjudgment, which may lead to electric shock and casualties. Of course, the above situation may happen, but the probability is very low. If you consider it to be foolproof, you can also choose a live display with a self-checking device.
2. Display control device and digital instrument
There are dynamic simulation lines on the display and control device. The opening and closing positions of the circuit breaker, the closing and closing of the grounding switch, etc. are displayed on the dynamic simulation diagram in real time, which is more eye-catching and intuitive. In addition, the control devices include circuit breaker control transfer switches, manual closing and tripping buttons, temperature and humidity control buttons, cabinet lighting buttons, etc. The author believes that the dynamic simulation diagram has practical uses. The remaining switches and buttons can be installed by the complete set factory on the instrument room panel. As for temperature and humidity control, it is of little use because the heating device is not effective enough and the heater can easily burn nearby components. Moreover, once the cabinet is put into operation, the heat on itself is not only sufficient for dehumidification, but also has some excess heat. For switch cabinets that have been out of operation for a long time, in order to remove moisture, a hair dryer can be used to dry them before putting them into operation.
For digital instruments, if it is a digital tube display, the more accurate the display, the more violent the last bit of the digital tube will beat, making people dazzled. If a liquid crystal display is used and installed on the instrument room panel, it is more than 2 meters away from the ground, making it very difficult for people to observe. If you want to use the communication interface of a digital instrument for remote signaling, it is better to use the communication interface of a microcomputer protection device. In this way, an electromagnetic instrument worth tens of yuan each is enough. The electromagnetic pointer instrument panel is large and can be observed from a long distance. Although the accuracy is not high, it is enough to meet the requirements.
