Film capacitors have many excellent characteristics, such as non-polarity, high insulation resistance, excellent frequency characteristics (broad frequency response), and very small dielectric loss. Therefore, film capacitors are widely used in analog circuits. The current operation of film capacitors is very regular, and when using film capacitors, we will find that the current is abnormal. How should we deal with this problem at this time.
Film capacitors are prone to failures
Film capacitors are fabricated and polarized in a similar way to aluminum electrolytic capacitors, however they have unlimited storage time and can operate without DC polarity for extended periods of time. Transient reverse voltages generally do not damage capacitors, unlike aluminum electrolytic capacitors.
A DC bias voltage is not always present in practical applications. 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 capacitance is obtained. The total capacitance is half of each series capacitor, or C/2.
When a film capacitor with good performance is connected to the power supply, the needle of the multimeter should have a larger swing; the larger the capacity of the film capacitor, the greater the swing of the needle. After the swing, the needle can gradually return to zero. If the pointer of the multimeter does not swing at the moment when the capacitor is turned on, it means that the capacitor fails or is open circuit; if the pointer keeps indicating the power supply voltage without swinging, it indicates that the capacitor has been broken down and short-circuited; if the pointer swings normally, but does not return to zero , indicating that the capacitor has leakage current phenomenon.
1. Improper selection of current within the range
Improper selection of current in the range will cause more DC maintenance and simple harmonic vibration. If the required current value is larger than the current value allowed by the film capacitor, it will cause the heating effect of the film capacitor, and the 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 demand peak current 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 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
Improper wire connection, the key occurs in the use of multiple parallel circuits of film capacitors. Because of the wiring method and the different wiring interval, the shunt of each parallel film capacitor in the Electronic circuit is different. It is manifested in multiple parallel film capacitors, each with a different temperature rise. The temperature rise of film capacitors in some positions is too high, resulting in accidental destruction. Therefore, it is necessary to carry out proper wiring and connection for the parallel operation of film capacitors, as much as possible to achieve the average, and prolong the service life of film capacitors.
3. Voltage beyond the specified range
Outside the specified range of voltage, the most important aspect is the simple harmonic vibration part. Developers should make preliminary plans after comprehensive consideration according to the configured power usage, input voltage, circuit topology, load permeability, electronic circuit Q factor and other parameters. After the prototype starts to meet the conditions, the parameters such as the electrical peak value and series resonance on both sides of the film capacitor during the configuration of the output power should be substantially surveyed, and further judge whether the type and parameters of the film capacitor used are accurate.
Case Analysis of Pen-ink Capacitor Failures and Solutions
I have used PGJ1-5 reactive power compensation panel in 400V switch space. The panel is equipped with 10 BCMJ shunt capacitors, each rated output is 16kVar, and the rated voltage is 0. 4kV, rated current 25A, temperature class e-25C/45C connection. The causes of the two accidents were carefully analyzed and thoroughly dealt with.
cause of issue
high ambient temperature
The reactive power compensation panel 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. The temperature is as high as 48C to k, which shows 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 ventilation control room, and a wax sheet (temperature indicator) should be posted on the outside of the capacitor, and the on-duty personnel can monitor the temperature of the capacitor medium from the displayed temperature.
Very 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 is reduced, the reactive capacity of the capacitor will be proportional to the square of the electric music will decrease accordingly, that is, the capacity of the capacitor will not be 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, causing the capacitor to explode. Therefore, the standard stipulates that the capacitor is allowed to operate at 1.1 times the rated voltage for a long time, but the time of running at 1.15 times the rated voltage in every 24h shall not exceed 30min.
The voltage is extremely unstable, and the voltage fluctuation range is 0. 9Ue – 1.15Ue (Ue is the rated voltage of 400V), the electricity consumption in the valley period is often around 450V, and the running time is as long as Th, which is the second reason for the explosion and burnout of the capacitor.Because the SZ7-800kVA power transformer is an on-load voltage regulator, to solve this problem, it is only necessary to set up a KYT-2 type on-load voltage regulator, and the voltage can be always controlled with an investment of less than 1,000 yuan. for rated voltage
The presence of harmonic currents
A high-power detectable rectifier is used as the rotating dense DC power supply and the compensation screen is operated in parallel. Due to the thyristor device connected to the power grid, the objective |: plays the role of 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, which is the third reason for the explosion of the capacitor. The main reasons for this happening are: (1) The high-order harmonic current is superimposed on the fundamental current, which increases the total capacitor current: (2) A certain high-order harmonic is caused between the system inductive reactance and the capacitor capacitive reactance Parallel resonance increases the current flowing into the capacitor: (3) Local series resonance occurs in the capacitor for a certain high-order harmonic, which causes overload.
In order to prevent these situations from happening, an air-core reactor can be placed in series in each phase of the compensation capacitor bank to limit the current. The combined reactance of the capacitive circuit becomes an inductive reactance for higher harmonics. In the high-order harmonics, the 3rd harmonic is short-circuited due to the continuous pressing of the transformer, so this is a measure for the 5th and 1: harmonics. If the reactance of the series reactor is selected to resonate the 5th harmonic, the 5th harmonic will be short-circuited. For the higher harmonics above the 5th harmonic beryllium: the capacitor loop becomes inductive, so the waveform is improved. Genkuji: Eliminates the possibility of resonance. The reactance of the anti-resonant series air-core reactor can be calculated by:
i.e. XL > 49% XC
In the formula: L – the inductance of the series reactor, H:
C – the capacity of the compensation capacitor, F:
XL one – inductive reactance of series reactor, Q:
XC—compensating the capacitive reactance of the capacitor, 0.
It can be seen from this that the reactance of the series reactor is about 4% or more of the capacitive reactance: that’s enough. Considering the low frequency of the system, the capacitance of the capacitor decreases when a $ fault occurs. In fact, the inductive reactance is selected to be 5% to 6% XC.