Aluminum electrolytic capacitors have always been a common choice for power supplies because of their low cost. However, they have a limited lifetime and are susceptible to extreme conditions of high and low temperatures. Aluminum electrolytic capacitors have thin aluminum foil placed on both sides of a paper sheet impregnated with electrolyte. This electrolyte evaporates over the life of the capacitor, changing its electrical properties. If the capacitor fails, it reacts violently: pressure builds up in the capacitor, forcing it to release flammable, corrosive gases.
The rate of electrolyte evaporation is closely related to the capacitor temperature. For every 10 degree Celsius drop in operating temperature, the capacitor life doubles. The rated life of a capacitor is usually the result of the temperature at which it is rated. A typical rated life is 1000 hours at 105 degrees Celsius. When these capacitors are selected for long-life applications such as the LED bulb shown in Figure 1 (LEDs have a life of 25,000 hours), the capacitor life becomes an issue. To achieve 25,000 hours of life, such capacitors require an operating temperature of no more than 65 degrees Celsius. This operating temperature is particularly challenging because the ambient temperature in this application can exceed 125 degrees Celsius. There are some temperature-dependent capacitors on the market, but in most cases, aluminum electrolytic capacitors will be the bottleneck component for LED bulb life.
This lifetime temperature dependence actually affects the way you can reduce the voltage rating of the capacitor. Your first thought may be to increase the capacitor voltage rating to minimize the chance of dielectric failure. However, this will result in a higher equivalent series resistance (ESR) of the capacitor. Since capacitors typically have high ripple current stress, this high resistance introduces additional internal power consumption and increases capacitor temperature. Failure rates increase with temperature. In fact, aluminum electrolytic capacitors are typically used at only about 80% of their rated voltage.
The ESR increases sharply at lower capacitor temperatures. In this case, the resistance increases by orders of magnitude at -40oC. This affects power supply performance in many ways. If the capacitor is used at the output of a switching power supply, the output ripple voltage increases by an order of magnitude. In addition, at the frequency above zero created by the ESR and output capacitor, it increases the loop gain by an order of magnitude, which affects the control loop. This creates an unstable power supply with oscillations. To accommodate this strong oscillation, the control loop usually makes significant compromises in terms of space and operates at higher temperatures.
For all electricians, capacitors are quite common and are used quite frequently. So will also often see some of the problems exposed, and then for these we should how to solve it.
A. Oil seepage (leakage)
Problem: For capacitors, oil seepage is as common as a household routine, the causes are also quite multifaceted, mainly the following points.
1, due to improper handling methods, or to take the porcelain casing resulting in cracks in the flange welding.
2, wiring, due to screwing too much force caused by the porcelain sleeve welding damage.
3、Some defects in the manufacturing process may cause the capacitor to seep or leak oil.
4、After the capacitor is put into operation, the internal pressure will increase due to the drastic temperature change, which will make the oil seepage and leakage phenomenon more serious.
5, due to improper operation and maintenance, the lack of maintenance of the capacitor for a long time leading to the peeling of the shell paint and rusting of the outer skin is also a cause of oil seepage and leakage of the capacitor in operation.
Although more common, but can not be ignored Oh, sometimes a small fault can easily cause big problems. The consequence of capacitor seepage and oil leakage is that the impregnating agent is reduced, and the upper part of the component is easily damaged by moisture and breakdown of the capacitor. Therefore, it is necessary to carry out timely processing.
Solution.
1、When installing capacitors, the wiring of each capacitor is connected to the busbar by a separate soft wire, not by a hard busbar connection, in order to prevent the assembly stress from causing damage to the capacitor sleeve and breaking the seal and causing oil leakage.
2、The capacitor should be placed upright and the sleeve should not be carried; when wiring, the screw should not be screwed too hard and the sleeve should be protected.
3, capacitor case and casing weld oil seepage, can be seepage, leakage at the rust, and then repair with tin brazing solder, repair casing weld should pay attention to the iron can not be too hot to avoid silver layer off, after repair for paint. Oil seepage and leakage is serious to replace the capacitor.
Second, the capacitor shell deformation
Question.
Due to the internal medium of the capacitor under the action of high-voltage electric field free, so that the medium decomposition and precipitation of gas, or due to part of the component breakdown, capacitor electrode to shell grounding discharge and other reasons will make the medium precipitation of gas. These gases in the sealed case will cause the increase of internal pressure, and thus will cause the expansion and deformation of the case. Therefore, the deformation of the capacitor shell is a sign of the capacitor's failure or prefailure.
Solution.
Often the appearance of the operating capacitor set inspection, such as found that the capacitor shell expansion deformation should be timely measures, expansion of serious (100Kvar below the expansion of each side should not be greater than 10mm; 100Kvar and above the expansion of each side is not greater than 20mm) should immediately stop using, and identify the reasons, replace the capacitor. Shell expansion is not serious to take ventilation measures and strengthen the operation of inspection work.
Third, the protection device action
Problems.
1, due to the unbalanced three-phase capacity of the capacitor bank, resulting in three-phase current imbalance, so that the capacitor bank protection device action to jump open the capacitor bank circuit breaker.
2, for capacitors equipped with fuse protection device, due to internal abnormalities of capacitors, capacity changes, pole-to-shell grounding, excessive inrush current and overvoltage, etc., so that the fuse fuse breaks.
3, improper running operation, resulting in the capacitor operating voltage exceeds the specified value, so that the protection device action to jump open the circuit breaker.
Solution.
1, regular measurement of capacitor capacitance value, capacitance value deviation does not exceed the rated value of -5% ~ +10% range, capacitance value should not be less than 95% of the factory value.
2、Before the installation of the capacitor bank, the primary electric capacity should be allocated to make its three-phase capacity balance, and its error should not exceed 5% of the total capacity of a phase; when equipped with relay protection devices should also meet the requirements that the balance current error does not exceed the relay protection action current during operation; after the action of the protection device, the capacitor pole-to-shell insulation resistance should be measured to be not less than 2000MΩ.
3、In order to limit the inrush current and the inflow of high harmonics, the capacitor bank should be equipped with series reactors.
4, the capacitor should be used under the rated voltage, such as the voltage on the grid is too low, the capacitor can not reach the rated output, long-term overvoltage operation to make the capacitor heat, accelerate the insulation aging, easy to cause damage to the capacitor. According to the regulations, when the grid voltage exceeds 10% of the rated voltage of the capacitor for a long time, the capacitor should be withdrawn from operation.
5, the use of fuses for capacitor protection, the choice of fuses to be appropriate, the general fuse rated current should not be greater than 1.3 times the rated current of the capacitor.
6, measurement of capacitor pole to shell insulation resistance should not be less than 2000MΩ.
Four, capacitor porcelain sleeve surface flash discharge
Question.
Capacitor in operation, due to the lack of cleaning and maintenance, its porcelain insulation surface dirt, dirt adsorbed moisture, so that the porcelain sleeve insulation reduced, the surface leakage current increased, resulting in the porcelain sleeve surface flash discharge. In addition, the capacitor porcelain sleeve surface dirty, in the system under the action of a certain overvoltage, resulting in the porcelain sleeve surface flash discharge. The result of the flash discharge, resulting in porcelain sleeve surface porcelain damage, may cause porcelain sleeve insulation breakdown circuit breaker tripping accident.
Solution.
The capacitor set in operation should be regularly inspected and cleaned; take corresponding anti-fouling measures according to the anti-fouling level, and the capacitor should not be installed outdoors in the seriously fouled area.
V. Capacitor explosion
Problem.
Capacitor explosion in operation is a malignant accident, generally in the internal components occur between the poles or to the shell insulation breakdown, and the other capacitors connected in parallel will release a lot of energy to the capacitor, may make the capacitor explode to cause a fire. The causes are.
1, the internal components of the capacitor breakdown: mainly due to poor manufacturing process caused by.
2, damage to the capacitor shell insulation: capacitor high voltage side lead wire made of thin copper sheet, if the manufacturing process is poor, the edge is not flat with burrs or serious bending, its capacity to produce corona, corona will make the oil decomposition, case expansion, oil level drop and cause breakdown. In addition, in the sealing cover when the corner of the burnout time is too long, the internal insulation burns and produces oil and gas to make the breakdown voltage drops greatly and damage.
3, poor sealing and oil leakage: due to poor sealing of the assembly casing, moisture into the internal, so that the insulation resistance is reduced; or due to oil leakage so that the oil surface drops, leading to the shell discharge or component breakdown.
4, drum belly and internal free: mainly due to internal corona, breakdown discharge and serious free, capacitors in the role of overvoltage, will make components start free voltage reduced to the work of electric field strength, thus causing a series of physical, chemical, electrical effects, so that insulation accelerated aging, decomposition, gas generation. Form a vicious circle, resulting in increased pressure in the case, resulting in the drum outside the box wall to explode.
5, the capacitor explosion caused by the capacitor with electricity: any rated voltage capacitor group is prohibited to close with electricity. Capacitor group each time re-close, must be disconnected in the case of the switch will capacitor discharge 3min before. Otherwise, the voltage polarity at the moment of closing may be opposite to the polarity of the residual charge on the capacitor and cause an explosion. For this reason, it is generally stipulated that the capacitor bank with capacity above 160Kvar should be equipped with automatic tripping device when there is no voltage, and the switch of the capacitor bank is not allowed to be equipped with automatic reclosing.
In addition, it may also cause explosion due to high temperature, poor ventilation, high operating voltage, excessive voltage harmonic components or operating overvoltage, etc.
Solution.
Capacitor cast in order to prevent capacitors from bursting accidents, in addition to the requirement to strengthen the operation of the inspection, the main is to install capacitor protection devices, the capacitor will be breached before the bursting accident in time to remove. In operation, such as the capacitor is found to issue a "goo" sound, is a precursor to the collapse of the internal insulation of the capacitor, so should stop running to find the faulty capacitor. After the capacitor burst, the capacitor should be replaced.
Six, the terminal is not firmly installed
Question.
Capacitor terminals are not firmly installed, in the current through the wire, will cause contact resistance increases, sometimes "squeaking" sound of discharge, so that the terminal heat deformation, and discharge sound, serious terminal red melting.
Solution.
Use infrared thermography to measure the temperature of the terminal and the body of the device. If the surface of the terminal has been hot oxidation phenomenon, should polish the terminal contact surface, coated with conductive grease and tighten the screws. If the terminal is seriously heated or melted, the terminal should be replaced.
Seven, capacitor temperature rise
Problem.
The main reason is that the capacitor runs over voltage for a long time, the high harmonic inflow from the nearby rectifier makes the capacitor over current, improper selection of capacitor, too little oil and poor ventilation conditions, etc. In addition, due to the aging of the medium of the capacitor after long-term operation, the increasing dielectric loss (tanδ) may lead to high temperature rise of the capacitor. The temperature rise of capacitor will affect the life of capacitor and lead to the damage of capacitor insulation breakdown.
Solution.
The ambient temperature of the capacitor room should be strictly monitored and controlled in operation. In order to facilitate the monitoring of the ambient temperature in operation, a thermometer should be installed at a place with poor heat dissipation conditions (two-thirds of the height of the capacitor), and the thermometer should be installed at a location convenient for observation. In order to monitor the capacitor's shell temperature, the temperature wax sheet can be pasted on the capacitor shell (near the nameplate). If the room temperature is too high, the necessary ventilation and cooling measures should be taken, and if the room temperature cannot be controlled below 40℃ after taking measures, the operation should be stopped immediately. If the capacitor is the problem, the capacitor should be replaced.
In conclusion, aluminum electrolytic capacitors are usually the cost option. However, you need to determine if their drawbacks will have a negative impact on the application. You need to consider their long life span by their operating temperature. Also, you need to reduce their voltage rating appropriately so that you can achieve temperature operation and thus a long life. You need to understand the ESR range that must be used so that you can properly design the control loop to meet the ripple specification requirements of the design.
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