Corona discharge - this air ionization along the wires under the influence of strong electromagnetic fields.
The theory of air ionization
air ionization noticed long ago, but have not been able to interpret. With the advent of in the middle of the XVIII century the first electrostatic discharge generators has become common. Even had time to try to imagine the brutal action of the Leyden jar. True experiments started with electricity after the invention Volta electrochemical energy source.
The world's first arc received in 1802 the Russian scientist with a catchy name Petrov. He predicted the possibility of using this for lighting purposes. Strong disappointment is the fact that the entire scientific world paid attention to the phenomenon. And it turns clear where actually flowing electric current. After negative carbon electrode tapered under the action of the arc, and at the anode small pit formed. Scientific world saw the truth of Benjamin Franklin: charges increasing negative carbon rod, being positive. It was only the beginning of XX century, when experiments with cathode rays were given the first results, it became clear that 100 years ago, made a big mistake.
When arcing five-sixths of the light flux enables the anode. Its temperature in standard physical experiments is 4000 degrees Celsius. It 1000 is greater than that of the cathode, giving 10% of the luminous flux. Others taken from the arc itself, due to the flickering of the ionized gas. At such high temperatures begin to melt even ceramic and tungsten. Welding was invented much later, with the 80-ies (XIX century) carbon electrode later NG Slavyanov suggest using a metal.
Experience Pavlov repeated Davy, other arc not yet engaged. Since filing his study began discharge gas environment. We found the first line spectra. And a Faraday Wheatstone 30s studied discharge in rarefied gases. Seeing the zeal of the English, a foreign engineer, who took Russian citizenship, Jacobi tried to apply a carbon rod to illuminate the streets of St. Petersburg (1846). But anode quickly burned off by increasing the spark gap, and a lamp died out. The situation decided to apple, it has already happened in 30 years, when the age of coal fuses coming to an end. They are used in narrow areas for a long time, for example, when covering the sky during the Second World War and the reflection of enemy raids.
Induction coil (approximately 1846) finally convinced people that the high voltage can create a spark, and Nikola Tesla demonstrated that by using a Faraday screen, even an ordinary mortal would be able to direct the lightning in the right direction. The flames in the night sky over the Wardenclyffe Tower called the most incredible corona in the history of mankind, with the exception of a great inventor arranged later on the roofs of New York.
Scheme occurrence of corona discharge
Accurate determination of corona discharge does not occur in the literature. For the simple reason reluctance of the authors deal with the subject and an abundance of duplicate information, misses the meaning of the content. Determination of corona discharge, at the beginning of this, too, is hardly physically accurate. Correct interpretation of the majority of readers will not accept because of the presence of specific features. In physics, the passage of current passed through the air divided into three portions, prominent in the chart:
- Origin obeys Ohm's law for subcircuit and straight. Here, the flow of current is possible due to external ionization: flame, ultraviolet, radiation or high frequency radiation. The first two factors were already known Wolte (before opening "animal electricity" Galvani) offered to remove static charge from rubber elektroforusa rays of the sun or a candle.
- The second portion is in the saturation region. Scientists say that the current remains relatively constant, the charges when moving between the electrodes actively recombine. And with the growing difference does not change the potential. Until the voltage reaches the third portion.
- impact ionisation avalanche process begins at a high potential difference. The electrons acquire such a high speed that the knock electrons from the gas molecules. On this site current increases rapidly with increasing potential difference, electrical arc can occur.
Discharge observed visually, and is called a spark occurs after the start of the second growth curve. Initially, there is a quiet discharge is not noticeable to the eye. He is often referred to as non-self, you need an external ionizing factor to support the movement of carriers. undervoltage causes immediate recombination of carriers.
Spark discharge is observed at voltages where avalanche ionization is possible. Sparks skips with a frequency of 400 Hz or higher, which is accompanied discernible noise. After each discharge voltage drops due than the maximum free interval. Visually sparks merge into one. Subspecies said ionization type bits are considered related:
- Carpal discharge similar to palm fairy skeleton. Formed between the tip and the charged surface. Noticeable on converters influence machine, power line insulators. Ionization starts from the tip, at this point the field strength is increased, charges flow into the space than is generated by an avalanche process.
- Corona discharge erupts between several sections of the wire. It caused by the shock air ionization. Peculiar broken teeth like thunderbolts. Their bizarre trajectory of scientists explain the fact that the ionization process extends along the path of least resistance, because of the isotropy of the gas it is impossible to predict the exact path. Crown sometimes smooth and sometimes positive or negative.
Corona discharge leads to loss of energy on the line and transmission line continuously occurs that is distinguishable by ear as a low-frequency hum and crack. In rainy weather resistance of the wire drops may cause the ionized air of languages in the form of small lightning running along the wires or balls. Corona discharge is used in air purification filters (ionizers, chandeliers Chizhevskogo), trapping particles of smoke, dust, causing them to settle.
The above does not accurately understand the arc. At a certain voltage value starts shock air ionization. If the potential difference decreases, the current does not change or grow (see. discharge lamps and fluorescent lamps). This so-called land with negative differential resistance. Process extending between the electrodes, referred to as an arc. The discharge was ignited and the convergence of high-voltage pins, and then goes on their own.
It is known that the welder electrode taps of the details to start impact ionization. Then the electrode is removed, and the arc remains not extinguished. Voltage too low. This is the feature of the arc. This explains why the open lines of power transmission lines do not carry voltage higher than 2 MW. And then begins a corona discharge arc is to put out, we have to work hard.
Tesla Vordenklif built tower, to achieve energy transfer by corona discharge. Create arc instructed to fly to the receiver, and then radiated on, around the globe. As conceived by Tesla wanted to build transmitters, fished languages lightning. Security is provided by a high frequency voltage (radio band).
Summing up, it should be noted that the arc in a different way is called self-discharge, the process can be maintained.
Corona discharge is formed on the geometric fractures due to increased field strength in this area. On that principle work converters and stekateli. The phenomena observed with gas discharge quantitatively described by two coefficients Townsend:
- Alpha is the volumetric ionization rate. Numerically the amount of ionization produced by an electron at a distance of 1 cm.
- Gamma: describes the ionization process at the border of the cathode gas. Here, the electrons leave the surface and begin to march along the field lines. Is the ratio of electrons leaving the cathode to the number here incident ions per unit time.
Both coefficients are growing along with the potential difference. After the non-self-noted avalanche ionization to form between the electrodes a positive charge cloud. This point is related to the emergence of the crown. A further voltage increase leads to disruption of stationarity positive clouds, and current begins to fluctuate around a specific value.
The foregoing called the theory of Rogowski and explains where there is a crown, as the sparks generated. All electrons determined flight and spatial charge distribution. The main sign - no short circuit occurs at the corona discharge as arcing occurs at (momentarily) or arc (permanently).
alpha factor defines distance from the emission electrode. Gamma rather characterizes the geometrical shape and the surface potential difference that led to the appearance of the discharge.
Properties corona discharge
Corona discharge usually occurs in a place with the smallest radius of curvature. If this line is the maximum probability of the formation shown on a mechanical defect. Field of the most frequent occurrence is called corona charging, or corona electrode. Explorer - a positive or negative potential. Accordingly, and distinguished corona same kind (see. above).
Positive and negative discharge different appearance. In the first case, the glow even in the second are the epicenters of the wire surface. Process mechanism between the electrodes:
- In the beginning there is a non-self-discharge. This occurs due to accidental actions: rain, a gust of wind, and so on.
- If the potential difference will continue to grow, it formed a faint glow in the area of the wire, followed by a barely audible crackling. It causing called critical voltage or initial.
- With further increase in potential difference (voltage spark) current grows quadratically, the glow is stronger. Sparks begin to slip with ever-increasing frequency.
- The total increase in the potential difference causes the arc discharge, manifesting itself as a short circuit. Its hard to stop burning.
Important! Critical and spark voltage different for positive and negative crown.
Thus, a corona discharge in a laboratory setting is the predecessor of the spark and the spark - the arc. In practice, at the rated voltage electrical network is not too worried about security. Possible to increase the voltage to 10% without much damage, if in this area there are no frequent bad weather, especially sand storms.
If the distance between the electrodes is too low, the corona discharge is formed: after a non-self comes immediately spark. The wires in the transmission line are trying to spread the distance, use ceramic insulators. Corona discharge is often replaced by brush, if there is a pronounced tip. Both a formal designation of an identical phenomenon.