Analysis of common faults and solutions for the use of film capacitors

Film capacitors have many excellent characteristics, no polarity, high insulation resistance, excellent frequency characteristics (wide frequency response), and low dielectric loss. Therefore, film capacitors are used in large quantities on analog circuits. The current operation of the film capacitor is very regular, and when using the film capacitor, we will find that the current is abnormal. How do we deal with this problem at this time?

Film capacitors are prone to failures:

Film capacitors are manufactured and polarized similarly to the method of manufacturing aluminum electrolytic capacitors, however their storage time is not limited and can be operated for a long time without DC polarity. The instantaneous counter voltage generally does not damage the capacitor, and the aluminum electrolytic capacitor is different.

In practice, there is not always a DC bias voltage. Non-polar tantalum capacitors can also be manufactured, but they are more expensive and not necessarily used after storage. If two identical tantalum capacitors are connected in series back to back, a non-polar capacitor can be obtained. The total capacitance is half of each series capacitor, which is C/2.

A good performance film capacitor at the moment of power-on, the multimeter's hands should have a large swing; the larger the film capacitor capacity, the greater the swing of the hands, after the swing, the hands can gradually return to zero. If the capacitor does not swing when the power is turned on, the pointer of the multimeter does not swing, indicating that the capacitor is broken or open; if the pin always indicates the power supply voltage without swinging, it indicates that the capacitor has been short-circuited; if the hand swings normally, but does not return to zero , indicating that the capacitor has leakage current.

Improper current selection in the range:

The current in the range is improperly selected, and more positions are generated by the DC sustain and the simple harmonics. If the current value required by the essence is larger than the allowable current value of the film capacitor, the film capacitor heating effect will be formed, and the long-term high-temperature operation will cause the film capacitor to be used for a long time, and the possibility of bursting or even burning will occur. In the configuration test, the peak current of the actual demand is allowed to be measured by a dedicated current probe or another method, and then the parameters of the capacitor are mobilized. It can be configured in the power degradation test to investigate the temperature rise of the film capacitor, and to determine whether the film capacitor is suitable for selection according to the temperature rise allowable parameter of the film capacitor.

Improper wire connection:

Improper wire connection is the key to the use of multiple parallel circuits for film capacitors. Because of the wiring method, the spacing of the traces, etc., the capacitance of each parallel film capacitor is different in the electronic circuit. It is represented by multiple parallel film capacitors, each with different temperature rises. The temperature rise of the film capacitor in some positions is too high, causing an accident of destruction. Therefore, it is necessary to properly wire and connect the parallel operation of the film capacitors, and to achieve averaging as much as possible to improve the service life of the film capacitors.

Exceeding the specified range voltage:

Exceeding the specified range of voltages, the most common aspect is the simple harmonics. Developers should make preliminary plans based on the configured power, input voltage, circuit topology, load permeability, and electronic circuit Q factor. After the prototype starts to reach the condition, the parameters of the film capacitors on both sides of the film capacitor, series resonance and other parameters are analyzed during the output power to further determine whether the model and parameters of the film capacitor used are accurate.

Pen and ink capacitor failure and solution case analysis:

I used the PGJ1-5 reactive power compensation screen in the 400V switch space. The screen is equipped with 10 BCMJ type shunt capacitors, each rated output is 16kVar, rated voltage is 0. 4kV, rated current is 25A, temperature category is -25C/ 45C connection method. Serious analysis and thorough treatment of the causes of these two accidents.

cause of issue:

1. High ambient temperature

The reactive power compensation screen is installed in the 400V switch air. There are 8 switch cabinets in the room, and the area meter is 30m2. The opposite side is the SZ7-800kVA 35kV/0.4 transformer room. The overall ventilation condition is poor, and the hot air switch has a high indoor temperature. 48C is k, which shows that the ambient temperature is too high is one of the causes of capacitor explosion. The compensation screen should be moved to a single ventilation control room, and a wax sheet (temperature display) should be attached to the capacitor outside the party. The duty person can ask the ground to monitor the temperature of the capacitor medium from the displayed temperature.

2. The voltage is extremely unstable

We can see from the formula QC=2π fCV2 that the reactive capacity of the capacitor is proportional to the square of the voltage. When the voltage is reduced, the reactive capacity of the capacitor will be reduced in proportion to the square of the electric music, that is, the capacity of the capacitor is not fully utilized. When the operating voltage rises, the temperature rise of the capacitor increases, and even the thermal T-balance of the capacitor is destroyed to cause the capacitor to explode. Therefore, due to the standard: Capacitors are allowed to operate for a long time at 1.1 times the rated voltage, but the time to operate at 1.15 times the rated voltage within 24 hours must not exceed 30 minutes.

The voltage is extremely unstable, and the voltage fluctuation range is 0. 9Ue - 1.15Ue (Ue is rated voltage 400V), the valley period is often used at 450V, and the running time is up to Th, which is the second cause of capacitor explosion. Because the SZ7-800kVA power transformer is a load-regulating instrument, to solve this problem, you only need to set one KYT-2 on-load voltage regulator controller. The investment can be controlled at less than one thousand yuan. It is rated voltage.

3. The existence of harmonic current

A high-power spoiler rectifier is used as the rotary compact DC power supply and the compensation screen is operated in parallel. Due to the thyristor connected to the grid operation, the objective |: acts as a higher harmonic generator, causing waveform distortion of the circuit voltage and current. The presence of harmonic current often causes an abnormal sound of the capacitor, which causes the capacitor to expand when it is severe, which is the third cause of the capacitor's explosion. The main reasons for this happening are: (1) The higher harmonic current is superimposed on the fundamental current, which increases the total current of the capacitor: (2) A certain higher harmonic is caused between the system inductive reactance and the capacitive reactance of the capacitor. Parallel resonance causes the current flowing into the capacitor to increase in confidence: (3) The capacitor internally generates a local series resonance for a certain higher harmonic, thereby causing an overload.

Treatment

To prevent this from happening, an air choke can be stringed in each phase of the compensation capacitor bank to limit the current. The resultant reactance of the capacitor circuit becomes an inductive reactance for higher harmonics. In the high-order harmonics, the 3rd harmonic is short-circuited due to the Δ-press of the transformer, so this is a measure for 1: harmonics for 5 times. If the reactance of the series reactor is selected to resonate the 5th harmonic, the 5th harmonic is short-circuited, and the higher harmonics of the above-mentioned 5 harmonics are inductive, so the waveform is improved. Root wood: Eliminates the possibility of resonance.

It can be seen that the reactance of the series reactor is about 4% or more of the capacitive reactance: Due to the low frequency of the system, the capacitance of the capacitor is reduced when it is used. In fact, the inductive reactance is 5% to 6% XC.