Film capacitors have many excellent characteristics, non-polarity, high insulation resistance, excellent frequency characteristics (wide frequency response), and low dielectric loss. Therefore, film capacitors are widely used in 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. At this time, how should we deal with this problem.

Prone failure of film capacitors

Film capacitors are manufactured and polarized similar to the method of manufacturing aluminum electrolytic capacitors, but their storage time is not limited and can work for a long time without DC polarity. The instantaneous reverse voltage generally does not damage the capacitor, while aluminum electrolytic capacitors are different.

In actual applications, there is not always a DC bias voltage. Non-polar tantalum capacitors can also be manufactured, but they are more expensive and may not be 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, namely C/2.

A good-performance film capacitor should have a larger swing at the moment when the power is turned on; the larger the film capacitor's capacity, the greater the swing of its hands. After swinging, the hands can gradually return to the zero position. If the pointer of the multimeter does not swing at the moment when the power is turned on, the capacitor is invalid or open; if the needle keeps indicating the power supply voltage and does not swing, it indicates that the capacitor has been broken down and short-circuited; if the needle swings normally, but does not return to zero , Indicating that the capacitor has leakage current phenomenon.

1. Improper selection of current in the range

Improper selection of the current in the range will produce more positions in the DC maintenance and simple harmonic vibration parts. If the essential current value is greater than the allowable current value of the film capacitor, it will cause the heating effect of the film capacitor. Long-term high temperature operation will cause the service life of the film capacitor to be greatly reduced, and it may burst or even catch fire. In the configuration test, it is allowed to measure the actual peak current required by a dedicated current probe or another method, and then adjust the parameters of the capacitor. It can be configured in the power degradation test to survey the temperature rise of the film capacitor, and determine whether the selection of the film capacitor is suitable according to the allowable parameters of the temperature rise of the film capacitor.

2. Improper wire connection

The improper wire connection is the key to the application of multiple parallel circuits of film capacitors. Because of the wiring method and the different wiring intervals, the shunt of each parallel thin film capacitor in the electronic circuit is different. It is manifested in multiple thin film capacitors connected in parallel, and the temperature rise of each is different. The temperature rise of the film capacitors in some locations is too high, causing accidents of destruction. Therefore, it is necessary to carry out proper wiring and connection for the parallel operation of thin film capacitors, as much as possible, and to increase the service life of thin film capacitors.

3. Voltage beyond the specified range

Beyond the specified range of voltage, the aspect that produces the most is the simple harmonic vibration part. Developers should make preliminary plans after comprehensive consideration based on the configured power, input voltage, circuit topology, load permeability, electronic circuit Q factor and other parameters. After the prototype starts to meet the conditions, actually survey the parameters such as the electric peak value and series resonance on both sides of the film capacitor during the configuration of the output power, and further determine whether the model and parameters of the film capacitor used are accurate.

Analysis of pen-ink capacitor failures and solutions

I once used the PGJ1-5 reactive power compensation screen in the 400V switch. The screen is equipped with 10 BCMJ shunt capacitors, each with a rated output of 16kVar, a rated voltage of 0.4kV, a rated current of 25A, and a temperature category of -25C/ 45C connection. The causes of these two accidents were carefully analyzed and thoroughly dealt with.

cause of issue

High ambient temperature

The reactive power compensation screen is installed in the 400V switch cabinet. There are 8 switch cabinets in the room. The area meter is 30m2, and the opposite is SZ7-800kVA. In the 35kV/0.4 transformer room, the overall ventilation conditions are poor, and the temperature in the hot switchgear room is as high as 48C or more. It can be seen that the high ambient temperature is one of the reasons for the explosion of the capacitor. The compensation screen should be moved to a single ventilated control room, and a wax sheet (temperature indicator sheet) should be posted on the outer side of the capacitor. The personnel on duty can ask the ground to monitor the temperature of the capacitor medium from the displayed temperature.

Very unstable voltage

We start from the formula QC=2π It can be seen from fCV2 that the reactive capacity of the capacitor is proportional to the square of the voltage. When the voltage drops, the reactive power capacity of the capacitor will decrease in proportion to the square of the electric music, that is, the capacity of the capacitor is not fully utilized. When the operating voltage increases, the temperature rise of the capacitor will increase, and even the thermal balance of the capacitor will be destroyed and the capacitor will explode. Therefore, due to standard regulations: capacitors are allowed to operate at 1.1 times the rated voltage for a long time, but the time to operate at 1.15 times the rated voltage within every 24h shall not exceed 30 minutes.

The voltage is extremely unstable, and the voltage fluctuation range is 0.9Ue-1.15Ue (Ue is the rated voltage of 400V), the valley period electricity consumption is often about 450V, and the running time is as long as Th. This is the second reason for the explosion and burnout of the capacitor. Since the SZ7-800kVA power transformer is an on-load voltage regulating instrument, to solve this problem, only one set is required. The KYT-2 on-load voltage regulator controller can always control the voltage to the rated voltage with an investment of less than one thousand yuan.

The presence of harmonic currents

A high-power detectable rectifier is used as the rotating dense DC power supply to operate in parallel with the compensation screen. Because the thyristor device that is connected to the power grid operates, objectively: it acts as a high-order harmonic generator, which will cause the waveform distortion of the circuit voltage and current. The existence of harmonic current often causes abnormal noise in the capacitor, and in severe cases, it causes the capacitor to expand. This is the third reason for the explosion of the capacitor. The main reasons for this occurrence are: (1) The high-order harmonic current is superimposed on the fundamental current to increase the total current of the capacitor: (2) A certain high-order harmonic causes parallel resonance between the inductance of the system and the capacitive reactance of the capacitor, which causes the current flowing into the capacitor to increase faithfully: (3) Local series resonance occurs to a certain high-order harmonic inside the capacitor, which causes overload.

Treatment measures

In order to prevent these situations from happening, an air-core reactor can be connected in series in each phase of the compensation capacitor bank to limit the current. The combined reactance of the capacitor circuit becomes inductive reactance for higher harmonics. In the high-order harmonic quilt, the 3rd harmonic is short-circuited by the delta continuous pressing of the transformer, so this is a measure for the 5th harmonic with 1:1. If the reactance of the series reactor is selected to resonate the 5th harmonic, the 5th harmonic will be short-circuited. For higher harmonics above the 5th harmonic beryllium, the waveform is improved because the capacitor circuit becomes inductive. On the roots: eliminate the possibility of resonance. The reactance of the anti-resonance series air-core reactor can be calculated:

Namely XL "49% XC

Where: L-the inductance of the series reactor, H:

C is the capacity of the compensation capacitor, F:

XL-the inductive reactance of the series reactor, Q:

XC one by one compensates the capacitive reactance of the capacitor, 0.

It can be seen that the reactance of the series reactor is about 4% or more of the capacitive reactance: that's it. Considering that the system frequency is low, the capacity of the capacitor is reduced when there is a $. In fact, the inductive reactance is 5%~6%XC.