The wiring mode of the capacitor bank should be selected according to the voltage, protection mode and capacity of the capacitor, usually including delta wiring and star wiring. When the rated voltage of the capacitor is consistent with the rated voltage of the network, the delta connection should be used; when the rated voltage of the capacitor is lower than the rated voltage of the network, the star connection can be used, or series and parallel connection, and then the star connection.
1. Delta wiring
In a 10kV grid, capacitors with rated voltages of 10.5kV and 11kV should be connected in delta. The advantage is that it can reduce the flow into the capacitor bank and the operating overvoltage. Usually, this wiring method can be used for substations and distribution lines with small short-circuit capacity; when the capacity of the capacitor bank is small, the wiring is simple and investment is low. The disadvantage is: if there is a breakdown accident in the capacitor bank, that is, a phase-to-phase short circuit is formed, and the current at the fault point is the phase-to-phase short-circuit current; if the short-circuit capacity of the network is large, the capacitor shell is easy to explode, and even cause a fire, threatening personal safety and the normality of the power grid. run.
2. Star wiring
Capacitors with rated voltages of 6.3kV and 11√3kV should be star-connected; capacitors with rated voltages of 3.15kV and 11/2√3kV should be connected in series and then star-connected.
The advantages of star wiring are:
The voltage that the capacitor is subjected to is the grid phase voltage. When the capacitor breaks through the short-circuit fault, the current at the fault point is 3 times the rated current; when using a star connection with two sections in series per phase, the current at the fault point is only 1.5 times the rated current. It can be seen that the star connection is beneficial to prevent the capacitor from exploding;
After the breakdown of a phase capacitor, it will not cause a short circuit between phases. When the faulty capacitor is cut off by the melt, its full-phase capacitor can continue to run, so as not to interrupt the reactive output of the capacitor bank;
C. It is easy to choose a more perfect protection method. The disadvantage is that if a 6.3kV capacitor is used in a 10kV grid, the capacitor needs to be insulated to ground with an insulator, making the installation operation difficult.
In addition to the above two connection methods, double star or delta connection can also be used to adapt to the situation of large and medium-sized capacitors in series and parallel, which is convenient to cooperate with more perfect relay protection methods. The advantage is that the protection device has high sensitivity and reliable operation. The disadvantage is that the wiring is complicated and the investment is large, and it is only suitable for large and medium-sized capacitor banks.
3. The principle of capacitor bank discharge coil.
1. Discharge coil, English name: dischargecoil, is a commonly used discharge element in capacitor cabinets. The outlet end of the discharge coil is connected in parallel to the two outlet ends of the capacitor bank, and bears the voltage of the capacitor bank during normal operation. The secondary winding reflects the primary transformation ratio, the accuracy is usually 50VA/0.5, and it can run for a long time at 1.1 times the rated voltage. The secondary winding is generally connected to an open delta or phase voltage differential to protect against internal faults in the capacitor bank (the PT on the bus cannot be used).
Second, the capacitor bank open triangle voltage protection, unbalanced voltage protection is actually this kind of protection. According to the requirements of GB-50227, this protection is widely used in capacitor banks with 6kV~66kV single Y connection.
3. Sometimes the discharge coil is replaced by a discharge PT. The discharge coil or voltage transformer of the capacitor mainly depends on the capacity of the capacitor. Generally, the discharge voltage transformer of the small-capacity (1.7Mvar) capacitor bank and the large-capacity capacitor bank (≥1.7Mvar) must be Use a discharge coil, otherwise the voltage transformer will burn or explode.
Capacitor dielectric material.
The dielectric materials used in capacitors are mainly solid and can be divided into two categories: organic and inorganic. Inorganic dielectric materials have microcrystalline ionic structures, amorphous structures and structures (such as ceramics, glass, mica, etc.) according to the molecular structure form. Organic dielectric materials are mainly polymer structures composed of covalent bonds, which can be divided into non-polar (such as polypropylene, polystyrene, etc.) and polar (polyethylene terephthalate, etc.). The dielectrics used in electrolytic capacitors are oxide films and ionic structures grown directly on the anode metal.
The polarization, loss, conduction and breakdown of dielectric materials under the action of an external electric field represent the fundamental properties of the medium, which depend on the composition and molecular structure.
So what is the wiring method of the capacitor bank above? The principle of the discharge coil of the capacitor bank is introduced, and the influence of the change of the connection mode is also introduced. I hope the above related knowledge can be helpful to you~.
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