In low-voltage power systems, power capacitors are used to increase the power factor of the system and reduce reactive losses. Power capacitors may occur in the event of damage or even explosion during operation, which may damage the power distribution equipment and damage the building and cause fire. The reasons are analyzed below and preventive measures are proposed.
First, the cause of power capacitor explosion
1, the main reason.
A single power capacitor is connected in a delta shape by three capacitors, mounted in a sealed container of transformer oil, and the top end leads to three terminals, as shown in FIG. In the figure, C is a combination of a group of capacitors (two, three or more).
Let a capacitor between A and B be broken down (see Figure 2). Figure 2 is an equivalent circuit between phases A and B. Its R is the equivalent resistance of the breakdown capacitor. Since the breakdown of the capacitor is a gradual process, the equivalent resistance R is a variable dynamic resistance. During capacitor breakdown, the capacitor generates Joule heat.
Since the dynamic resistance of R is from large to small, the longer the time, the more heat is generated. When the capacitor has excessive leakage current or breakdown, the capacitor generates a large amount of heat in a short time. This heat can decompose the oil in the capacitor to generate a large amount of gas. At this time, the capacitor casing cannot withstand such a sharp increase. Pressure, causing damage to the casing or even explosion.
2, secondary reasons.
The capacitor is used as a power factor compensation, and the amount of capacitor retraction is related to the system. If the voltage polarity of the incoming call is exactly opposite to the polarity of the residual charge of the capacitor group, a large current will be generated, which is also the cause of the capacitor damage.
Second, measures to prevent damage to power capacitors.
1. Under normal circumstances, according to the magnitude of the current effective value of each group of phase capacitors, 1.5 to 2 times, with a fast fuse. If the capacitor is broken, the fast fuse will melt and cut off the power supply, protecting the capacitor from continuing to generate heat.
2. Install an ammeter on each phase of the compensation cabinet to ensure that the current difference between each phase does not exceed ± 5%. If an imbalance is found, immediately exit the operation and check the capacitor.
3. Monitor the temperature rise of the capacitor.
4. Strengthen the inspection of the capacitor bank. Excessive leakage current of the capacitor usually has the following phenomenon: oil leakage occurs in the casing part of the lead wire of the capacitor; Some capacitors do not seep oil, and bulging occurs.
If the above situation is found, the capacitor should be taken out of operation to prevent explosion. Capacitor damage is generally prone to occur during the summer heat period, during which time the inspection should be strengthened.
Third, the cause of the capacitor damage and the explosion.
(1) Capacitor internal component breakdown: mainly due to poor manufacturing process.
(2) Damage to the insulation of the capacitor by the capacitor. The high-voltage side lead wire of the capacitor is made of thin steel sheet if the manufacturing process is poor. The edge is not flat with burrs or severely bent, and the tip is prone to corona. Corona will cause oil to decompose, the casing expands, and the oil surface drops to cause breakdown. In addition, when the cover is closed, if the welding time is too long at the corner, the internal insulation is burned and oil and gas are generated, so that the voltage is greatly reduced and damaged.
(3) Poor sealing and oil leakage: due to poor sealing of the assembled casing, moisture enters the interior, which reduces the insulation resistance; or the oil level drops due to oil leakage, causing the pole to discharge in the direction of the shell or breakdown of the components.
(4) bulge and internal free: due to internal corona, breakdown discharge and severe free, the capacitor under the action of overvoltage, the initial free voltage of the component is reduced below the working electric field strength, thereby causing physical and chemical The electrical effect causes the insulation to accelerate aging, decompose, generate gas, form a vicious circle, and increase the pressure of the casing, causing the outer wall of the tank to explode.
(5) Capacitor explosion caused by charge closing: The capacitor group of any rated voltage is forbidden to be energized and closed. Each time the capacitor bank is reclosed, the capacitor must be discharged for 3 minutes with the switch disconnected. 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, the capacitor bank with a capacity of 160 kvar or more is generally required to be equipped with an automatic trip device without pressure, and the switch of the capacitor bank is not allowed to be equipped with an automatic reclosing.
(6) In addition, explosion may occur due to excessive temperature, poor ventilation, excessive operating voltage, excessive voltage harmonic components, or overvoltage.
Fourth, the classical cause analysis and processing of capacitor explosion.
The ambient temperature is high.
Excessive 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 (temperature display) should be placed on the capacitor casing. The duty person can indirectly monitor the temperature of the capacitor medium from the displayed temperature.
The voltage is extremely unstable.
We can see from the formula QC=2πfCV2 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 voltage, ie the capacity of the capacitor will not be fully utilized. When the operating voltage rises, the temperature rise of the capacitor increases, and even the thermal balance of the capacitor is destroyed to cause the capacitor to explode. Therefore, the national standard stipulates that the capacitor is allowed to operate for a long time at 1.1 times the rated voltage, but the operation time at 1.15 times the rated voltage within 24 hours must not exceed 30 minutes.
The presence of harmonic currents.
The existence of harmonic current often causes an abnormal sound of the capacitor, and causes the capacitor to expand when it is severe, which is the cause of the explosion of the capacitor. The main reason for this is
(1) The harmonic current is superimposed on the fundamental current to increase the total current of the capacitor;
(2) A certain higher harmonic causes parallel resonance between the system inductive reactance and the capacitor capacitive reactance, so that the current flowing into the capacitor is multiplied;
(3) A local series resonance occurs in a capacitor for a certain higher harmonic, thereby causing an overload.
Summary: Treatment measures.
To prevent this from happening, an air choke can be connected in series with 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 higher harmonics, the 3rd harmonic is short-circuited due to the Δ connection of the transformer, so this is a measure for harmonics of 5 or more times. If the reactance of the series reactor is selected to resonate the 5th harmonic, the 5th harmonic is short-circuited. For the higher harmonics of the 5th harmonic or higher, the capacitance circuit becomes inductive, so the waveform is improved, and thus the fundamental The possibility of generating resonance is eliminated. The reactance of the anti-resonant series hollow reactor can be calculated by:
That is XL>4%XC
Where: L – the inductance of the series reactor, H;
C——the capacity of the compensation capacitor, F;
XL – the inductive reactance of the series reactor, Ω;
XC – the capacitive reactance of the compensation capacitor, Ω.
From this, it can be seen that the reactance of the series reactor is about 4% or more of the capacitive reactance. Due to the low system frequency, the capacitor capacity is reduced in the event of an accident. In fact, the inductive reactance is 5% to 6% XC.
After doing the above analysis and processing, the reactive power compensation screen achieves the effect of safe and energy-saving operation.
Post time: Dec-17-2018