Capacitor bank failure analysis
The capacitor bank adopts the common star connection mode, the three-phase common body casing is connected to the same iron frame, and the frame is grounded. The internal structure of the capacitor is a four-string structure in which a plurality of components are connected in parallel, and the inner fuse is protected. The maintenance personnel and the manufacturer personnel dissect the damaged capacitor, and it is found that the fuses in the A and B phases of the damaged capacitor are both blown. After the outsourcing seal is broken, after careful analysis, it is believed that the fuse of one phase fuses two, causing damage to the outer seal. In the case of injury of the outer seal, the long-term operation develops into a shell breakdown and develops into a single-phase grounding. Since the single-phase grounding is unstable arc grounding, the healthy phase generates overvoltage and the other phase also has two fuses, and the outer casing seal is injured, which causes the shell to break down under the action of overvoltage, thereby forming an interphase short circuit, although protection Reliable action, but the thermal effect caused by the huge short-circuit current still causes a certain degree of damage to the capacitor, causing severe deformation of the capacitor casing.
In addition, due to the large amount of non-linear load in the power grid, the harmonics in the power grid occupy a certain content. In addition to serving as a suburban resident, the 110kV Zhanghe Substation is mainly used for industrial power supply. In addition to several 10kV industrial lines, there are also some small-scale chemical plants, foundries and other industrial users on other 10kV lines, all of which may generate harmonics. Although each household generates very few harmonics, it can be collected into a large harmonic current and fed into the power grid, which increases the harmonic level of the power grid and affects the safe operation of the power grid equipment. Due to the reactive power compensation device of this substation, a series reactor with a reactance of 6 is configured. Although the reactance of 6 can suppress the harmonics of 5 times and above, the series reactor and the compensation capacitor are made under the 3rd harmonic. The impedance is capacitive, and harmonic current amplification occurs, causing the capacitor to be overloaded. Although the bus is dominated by the 5th harmonic, the 3rd harmonic content is not very high, and after the capacitor is installed, the capacitive impedance will amplify the original 3rd harmonic content, which may cause the internal fuse to be blown. Since the total protection is set at 1.3 times the rated current of the four sets of capacitors, the total load of the four sets of capacitors is extremely small. When the harmonic content is high for a certain period of time, the total overcurrent protection cannot be operated, causing the fuse in a certain phase to be blown, and the internal fuse cannot be found in time after being blown, resulting in an accident expansion and a quick trip.
From the perspective of protection configuration, the protection of the internal fault of the capacitor is only provided by the internal fuse protection, and the backup protection that causes the accident to expand is not set up - the unbalanced voltage protection, so that the internal fuse cannot be found in time after being blown, resulting in a quick trip accident. Therefore, the imperfect protection configuration is the main reason for the expansion of capacitor accidents.
In addition, the occasional measurement of capacitance is also one of the reasons for the accident. Since the most direct reaction of the internal device of the capacitor is the change of the capacitance, and the capacitance measurement means is backward. When measuring the capacitance of the capacitor, the measurement method of removing the connecting line is required, which not only measures the trouble but may cause the sleeve to be disassembled and connected. The tube is stressed and the casing leaks. Therefore, since the operation, the maintenance personnel have never conducted the capacitance measurement, and the protection of the internal fault of the reaction capacitor has not been set. When the internal fuse is blown, it cannot be found in time, causing the accident to expand.
Capacitor fault improvement measures
1. Install overload protection in each grouping loop
Since the overcurrent protection is set according to the total input of the four sets of capacitors, the overcurrent phenomenon caused by the grouping harmonic current amplification is unresponsive or even unresponsive. Therefore, overload protection is installed in each grouping circuit because the AC contactor can only be broken. Under normal conditions, the load current cannot break the fault current. Replace the AC contactor with the ZN-28 vacuum circuit breaker. When the harmonic content is high, it acts on the trip to avoid damage to the capacitor caused by harmonics and the internal fuse is blown. .
2. Install triangular voltage protection on each group circuit
When the fuse in one phase of the capacitor is blown, the capacitive reactance changes, and the compatibility with the other two is not equal, causing the fault phase and the healthy phase voltage to be unbalanced. Therefore, a low-voltage voltage relay is installed at the opening triangle of the secondary winding of each group circuit
Voltage Transformer. When the fuse in one phase is blown, an unbalanced voltage appears at the opening triangle, and an alarm signal is issued. The device can accurately reflect the internal fault of the capacitor, and is not affected by the system grounding and system unbalanced voltage, and the injured capacitor is taken out of operation in time.
3. Regular measurement of capacitance
For the difficulty in measuring capacitance, advanced measuring equipment has been purchased. The capacitor cluster is measured regularly by a fully automatic capacitor bridge. The capacitance of a single capacitor does not need to be disconnected. The measurement is simple, fast, accurate and reliable. The maintenance personnel regularly measure the capacitance. When the fuse is blown in a certain phase of the capacitor, the capacitance will change. When the measured capacitance decreases, when it exceeds 3, the injured capacitor will be taken out of operation in time.
Negligence in design and maintenance may cause hidden dangers to the safe operation of capacitors. Therefore, proper protection, periodic measurement of capacitance, and prevention of micro-duration can reduce or even avoid capacitor accidents, increase the availability of capacitors, and extend the use of capacitors. life.
