Analysis of common faults and solutions of film capacitors

Film capacitor characteristics: non-polar, high insulation resistance, excellent frequency characteristics (wide frequency response), and small dielectric loss. Therefore, it is widely used in analog circuits. The current operation of film capacitors is very regular. Therefore, when using a film capacitor, I find that the current is abnormal. What should I do?

Film capacitors are manufactured similarly and are polarized to aluminum electrolytic capacitors. However, its storage time is not limited, and it can work for a long time without DC polarity. Unlike aluminum electrolytic capacitors, instantaneous reverse voltage does not generally damage film capacitors.

In practical applications, there is not always a DC bias voltage. If non-polar tantalum capacitors are manufactured, they are more expensive and are not necessarily used after storage. But two back-to-back tantalum capacitors in series can get non-polar capacitors. The total capacitance is half of each series capacitance, which is C / 2.

When the film capacitor with good performance is powered on, the meter needle of the multimeter should have a large swing. 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 oscillate at the moment when the power is turned on, the capacitor is invalid or disconnected; if the meter hand keeps indicating the power supply voltage without oscillating, it indicates that the capacitor has been punctured and short-circuited; if the meter oscillates normally, but does not return to zero , Indicating that the capacitor has a leakage current phenomenon.

1. Improper selection of current in the range

The current is not selected properly within the range, and the locations where more current is generated are the DC maintenance and simple resonance. The essential current value is larger than the current value allowed by the film capacitor, which will cause the film capacitor to generate heat, and the long-term high temperature operation will cause the service life of the film capacitor to drop significantly. In severe cases, it may explode or fire. In the configuration test, it is allowed to measure the peak current of the actual demand through 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 investigate the temperature rise of the film capacitor, and to determine whether the selection of the film capacitor is suitable according to the temperature rise allowable parameters of the film capacitor.

2. Improper wiring connection

The improper connection of the wires is mainly caused by the use of multiple parallel circuits of film capacitors. Because of the wiring method, different routing intervals, etc., each shunt capacitor in the electronic circuit is caused to shunt differently. Appears in multiple parallel film capacitors, each with a different temperature rise. The temperature rise of the film capacitor in some locations is too high, causing accidents of destruction. Therefore, it is necessary to properly route and connect the parallel operation of film capacitors, try to achieve an average as much as possible, and improve the service life of film capacitors.

3. Voltage outside the specified range

If the voltage exceeds the specified range, the most common aspect is the simple resonance part. Developers should make preliminary plans after comprehensive thinking based on the parameters such as the configured use power, input voltage, circuit topology, load permeability, and electronic circuit Q factor. After the prototype starts to meet the conditions, actually survey the parameters such as the electrical peak value and series resonance of the two sides of the film capacitor configured during the output power, and further determine whether the model and parameters of the film capacitor used are accurate.

Capacitor failure and solution case:

Malfunction: PGJ1-5 type reactive power compensation screen is used in 400V switch, 10 BCMJ type parallel capacitors are installed in the screen, each rated output is 16kVar, rated voltage is 0． 4kV, rated current 25A, temperature category -25C / 45C connection.

Cause of issue:

1. High ambient temperature

The reactive power compensation screen is installed in the 400V switch space. 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. As high as 48C to k, it can be seen that the high ambient temperature is one of the causes of capacitor explosion. The compensation screen should be moved to a single ventilation control room, and a wax sheet (thermometer) should be affixed to the capacitor outside party. The watchman can monitor the capacitor medium temperature from the displayed temperature.

2. Extremely unstable voltage

We can see from the formula QC = 2π fCV2: the reactive capacity of the capacitor is proportional to the square of the voltage. When the voltage decreases, the reactive capacity of the capacitor will be reduced proportionally to the square of the electric music, that is, the capacity of the capacitor will not be fully utilized. When the operating voltage increases, the temperature rise of the capacitor increases, and even the thermal T-balance of the capacitor is destroyed, causing the capacitor to explode. Therefore, due to the standard regulations: capacitors are allowed to run for a long time at 1.1 times the rated voltage, but the time for running at 1.15 times the rated voltage within 24 hours must not exceed 30min. The voltage pole is unstable, and the voltage fluctuation range is 0. 9Ue-1.15Ue (Ue is a rated voltage of 400V), the power consumption in the valley is often around 450V, and the running time is up to Th, which is the second cause of capacitor explosion and burnout. Because the SZ7-800kVA power transformer is an on-load voltage-regulating instrument, to solve this problem, you only need to set up a KYT-2 type on-load voltage-regulating controller. The investment can be controlled under less than one thousand yuan Is the rated voltage.

3. The presence of harmonic currents

A high-power detectable rectifier is used as a rotary dense DC power supply to run in parallel with the compensation screen. Because it is connected to the thyristor device running on the power grid, it objectively plays a role as a high-order harmonic generator, which will cause circuit voltage and current waveform distortion. The existence of harmonic currents often causes abnormal noises in capacitors, which can cause capacitors to swell in severe cases. This is the third cause of capacitor explosion. The main reasons for this are: (1) The higher harmonic current is superimposed on the fundamental current, which increases the total capacitor current: (2) A 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 with confidence: (3) A local series resonance of a high-order harmonic occurs within the capacitor, causing overload.

Measures:

In order to prevent these situations, a hollow reactor can be connected in series in each phase of the compensation capacitor bank to limit the current. The composite reactance of the capacitor circuit is changed to an inductive reactance for higher harmonics. In the high-order harmonic quilt, the third-order harmonic is short-circuited by the △ continuous pressing of the transformer, so this is a measure for the fifth-order 1: harmonic. If the reactor of the series reactor is selected to make the 5th harmonic resonate, the 5th harmonic is short-circuited. For the higher harmonics of 5th harmonic and above, the waveform is improved because the capacitive loop becomes inductive, so that On roots: Eliminates the possibility of resonance. The reactance of the anti-resonance series hollow reactor can be calculated by:

XL> 4% XC

Where: L-series reactor inductance, H:

C-the capacity of the compensation capacitor, F:

XL-Inductive reactance of series reactor, Q:

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

It can be known from this that the reactance of the series reactor is about 4% or more of the capacitive reactance: it is sufficient. Considering that the system frequency is relatively low, the capacitor capacity decreases when there is a failure. In fact: the inductive reactance is selected as 5% to 6% XC.