DC Link Power Capacitor Aluminium Shell High Voltage Capacitor

• Compensate for the difference between the power required by the inverter and the output power of the rectifier bridge that changes by two or six times the power frequency;

 

• Provide input current of switching frequency of inverter or freight frequency converter;

 

• Reduce the current harmonic of switching frequency into the power grid;

 

• Absorb the demagnetization energy of the motor when all power switching devices are turned off in the emergency stop state;

 

• Provide instantaneous peak power;

 

• Protect the inverter or frequency converter from the impact of the instantaneous peak value of the power grid.

 

Protection of Power Capacitors

 (1) Appropriate protection measures should be adopted for the capacitor bank, such as balanced or differential relay protection or instantaneous overcurrent relay protection. For capacitors of 3.15kV and above, a separate fuse can be installed on each capacitor. The rated current of the fuse should be selected according to the characteristics of the fuse and the inrush current when it is turned on. Generally, it is suitable to be 1.5 times the rated current of the capacitor to prevent the capacitor tank from exploding.

 (2) In addition to the above-mentioned forms of protection, the following protections can be made when necessary:

1.If the voltage rise is frequent and long-term, measures should be taken to make the voltage rise not more than 1.1 times the rated voltage.

2.Protection with a suitable current automatic switch, so that the current increase does not exceed 1.3 times the rated current.

3. If the capacitor is connected with the overhead line, a suitable arrester can be used for atmospheric overvoltage protection.

4.In the high-voltage network, when the short-circuit current exceeds 20A, and the short-circuit current protection device or fuse cannot reliably protect the short-circuit to ground, the single-phase short-circuit protection device should be used.

 (3) The correct selection of the protection method of the capacitor bank is the key to ensure the safe and reliable operation of the capacitor, but no matter which protection method is used, it should meet the following requirements:

1.The protection device should have sufficient sensitivity, no matter if a single capacitor in the capacitor bank fails or some components are damaged, the protection device can operate reliably.

2.It can selectively cut off the faulty capacitor, or after the power supply of the capacitor bank is completely disconnected, it is convenient to check the damaged capacitor.

3.In the process of power failure of the capacitor and when the power system has grounding or other faults, the protection device must not malfunction.

4.The protection device shall be easy to install, adjust, test and operate and maintain.

5.Less power consumption and lower operating costs.

 (4) Capacitors are not allowed to install automatic reclosing devices, and the corresponding non-voltage release automatic tripping devices are installed. The main reason is that it takes a certain time for the capacitor to discharge. When the switch of the capacitor bank is tripped, if the capacitor is reclosed immediately, the capacitor will not have time to discharge, and there may be residual charges in the capacitor with the opposite polarity to the reclosing voltage, which will cause the closing. A large inrush current is generated instantaneously, causing the capacitor casing to expand, spray fuel and even explode.

Switching on and off of power capacitors

 (1) Before switching on the power capacitor bank, check the discharge network with a megohmmeter.

 (2) When switching on and off the capacitor bank, the following points must be considered:

1.When the voltage on the busbar (busbar) exceeds 1.1 times the maximum allowable value of the rated voltage, it is forbidden to connect the capacitor bank to the power grid.

2.The capacitor bank shall not be re-connected within 1 minute after it is disconnected from the power grid, except for the automatic repeated connection.

3.When connecting and disconnecting the capacitor bank, a circuit breaker that cannot generate dangerous overvoltage should be selected, and the rated current of the circuit breaker should not be lower.