breakdown voltage

Breakdown of gas Insulation

Gas ionization

All types of gas ionization method are discuss below with example. Also pictures and sketches are present. An electrically insulating material (or dielectric) is a material that can hold an electrostatic field almost indefinitely. Therefore, these materials are very resistant to the passage of direct current. However, it cannot withstand an infinitely high voltage. If the voltage applied to the dielectric exceeds a critical value, the insulating layer will be damaged. The dielectric can be in gaseous, liquid or solid form.

More about Gas ionization

In fact, a gas dielectric is not without charged particles containing free electrons. Electrons can be cause by radiation or field emission, which can initiate the destruction process. However, these free electrons generate under the action of an electric field are accelerate from the cathode to the anode by applying an electrical stress to the force. When moving in a magnetic field, it gains kinetic energy (1/2 mu2). Since the energy contained is very small, energy is usually expressed in terms of voltage (electron volts, eV, e is the charge of an electron). [Energy Ei = e Vi is expressed in electron volts. 1 e V = 1.6 x 10-19 J]. These free electrons moving toward the anode collide with gas molecules that exist between the electrodes.

In these collisions, some of the electron’s kinetic energy is lost and some is transferring to the neutral molecule. If this molecule gets enough energy (greater than the energy Ei need for ionization), it can be ionized by collisions. The (average) number of ionization collisions of one electron per unit drift in the gap is not constant, but is subject to statistical fluctuations. The electrons that collide with the newly emitted electrons are then accelerate by the electric field, forming an avalanche. As the voltage rises further, an additional ionization process occurs. When these secondary processes occur, ionization increases rapidly with voltage and eventually decays.

In a uniform electric field, ionization below the breakdown voltage is usually very small and does not affect engineering applications. However, at non-uniform electric fields, high stress areas, and voltages well below the breakdown voltage, large amounts of ionization can occur, which constitutes the familiar corona discharge.

Ionization process in gas discharge

Electrical breakdown of gas is cause by various ionization processes. These are gas processes that include electrons, ions, and photons that collide with gas molecules and that occur on or near the electrode surface. [If the stress is 100-1000 kV / cm, electrons may be emitted from the cathode. Field launch].

Ionization is the process of removing electrons from an atom to create a net positive charge (positive ion). The electrons in the outermost orbits are the least attract to the nucleus, so they are most easily remove by the collision process. The energy require to completely remove an external electron from its normal state to a distance well beyond the nucleus is call the initial ionization potential.

A round-trip process in which electrons descend from a long distance to the lowest empty orbit is also possible. In this case, photons with the same energy as the previously absorb energy are emitted.

Ionization due to simple collision

Ionization occur at the electron kinetic energy (1/2 mu²) that colliding with a neutral gas molecule exceeding the ionization energy (Ei = e Vi) of the molecule. (That is, when mu²> Ei)

M + e- (½ acre ²) → M + + 2 e-

Normally, a cation and two slowly moving electrons are produced. For electron energies equal to the ionization energy Ei, the probability of this process is zero, but initially it increases almost linearly and then gradually increases with the electron energy to a maximum.

When a gas molecule is hit by an electron, the collision with a high-energy electron can cause the release of another electron bound to the atom. The ratio of a given electron’s collision with the primary electron mainly depends on the energy of the primary electron. It peaks at about 200-500 eV of primary electron energy. At low energy values, the potential for ionization is small because the energy transferred may not be sufficient to let the electrons escape from the surface of the molecule. At high primary energy values, the energy impacting an electron is sufficient for the electron to penetrate deep into the molecule, thereby reducing the chance of other electrons escaping.

Therefore, as the electron energy increases, the probability of ionization in the air changes, as shown in the figure.

gas ionization method
Ionization probability curve in air

Excitation

In simple collisions, neutral gas molecules are not always ionized under the influence of electrons. In this case, the molecule is in the excited state M * with energy Ee.

M + e- (½ acre ²) → M * + e-

The excited molecule emits a photon of frequency v and energy v. Energy is release when an electron jumps from one orbit to another.

M * → M + h v

Where h = Planck constant = 6.624 x 10-34 J s

Ionisation by Double electron impact

If the gas molecule has been raise to the excited state (with energy Ee) by a previous collision, the ionization of the excited molecule can occur by collision with a relatively slow electron. The electron requires less energy than the ionization energy, but that energy must exceed the additional energy needed to obtain the ionization energy.

(Ie ½ acre> Ei-Ee)

M * + e- (1/2 acre ²) → M + + 2 e-

Photoionization – Gas ionization method

A ground-state molecule can be ionized by photons of frequency v if the quantum of the emitted energy h v is greater than the ionization energy of the molecule (via an electron jumping from one orbit to another).

(That is h v> Ei, where h = Bode constant = 6.624 x 10-34 Joules)

M + h v → M ++ + e-

Electronic Attachment

If the gas molecule does not occupy the outermost energy level, the impinging electron occupies one of these energy levels, thereby converting the molecule into the negative ion M-.

M + e- → M-

The negative ions thus formed become excited by excess energy.

Note: Electron attachment reduces the number of free electrons and ionization increases the number of free electrons.

Electron detachment

This occurs when negative ions release excess electrons into neutral molecules.

M- → M + e-

Other ways – Gas ionization method

The above process is of paramount importance for the gas discharge phenomenon. Other possible gas processes are ion-atom collisions, excited atom-molecule collisions, atom-atom collisions. Note that due to the relatively slow interaction time, collisions between ions and atoms rarely cause ionization, which allows them to gradually adapt to conditions where the internal motions of the atomic system change without energy changes. please.

Positive ions must have an energy of at least 2 eV to ionize the neutral, unexcited species. In general, ions and atoms of this energy occur only in high-current arcs and thermonuclear discharges.

Breakdown mechanism are devide in to two mechanisms of gas decomposition are known. Also These are the mechanisms of avalanches and streamers.

Electronic avalanche mechanism (Townsend destruction process)

The Townsend decomposition mechanism is one of the processes considere in the decomposition process. It is based on the production of continuous secondary avalanches to produce faults.Also Suppose there are free electrons in a gas with an electric field (due to some external effect such as radioactivity or cosmic radiation). If the field strength is high enough, gas molecules can be ionize by simple collisions, resulting in two free electrons and one positive ion. These two electrons cause further ionization by collisions, producing four electrons and three positive ions. This process is cumulative and its number continues to grow as free electrons continue to move under the influence of electric fields. The large number of electrons and cations produce in this way is called an avalanche. In addition a few millimeters of space, it can grow to contain millions of electrons.

Electron Avalanche

Mathematical analysis – Breakdown mechanism

When the voltage applied to the pair of electrodes increases, the electric field strength of the electrons emitted from the cathode at a specific voltage value moves through the gas at an average speed determined by its mobility, so the current across the gap slowly increases. I will. Electron impact ionization may be the most important process in gas decomposition, but this process alone is not sufficient to cause decomposition.

n0 = electron emitted from the cathode every second

N x = number of electrons moving from the cathode per second x / n [n x> n0 (due to ionization collision in gap)]

alpha = Average number of ionizing collisions caused by one electron drifting in the direction of the electric field per unit. [Townsend’s first ionization coefficient]

Then 1 /alpha = average distance traveled in the direction of the electric field between ionizing collisions.

Also Consider the layer thickness dx at a distance x from the cathode. The nx electrons entering the laminar flow pass through the laminar flow with the electric field E applied. The ionization collisions that occur in the air gap are proportional to dx and nx.

breakdown mechanism equation
equation

Thereby

breakdown mechanism equation
equation

If the distance between the anode and cathode is x = d, the number of electrons hitting the anode per second is nd

gas ionization equation
equation

Therefore, on average, each electron leaving the cathode produces (nd-n0) / n0 new electrons (and corresponding positive ions) in the gap.

Alos At steady state, the number of cations that reach the cathode every second must be exactly equal to the number of newly formed electrons that reach the anode. Therefore, the circuit current is determined by

gas ionization equation
equation

Mathematical analysis – Breakdown mechanism ctd

Also Where I0 is the initial photocurrent at the cathode. Also In the actual breakdown process, electron impact ionization involves a secondary process at the cathode, where free electrons fill the air gap.

Next, let us consider the current growth equation and auxiliary mechanism.

Let

gamma = number of secondary electrons (average) generated by ionization collisions in each cathode gap. [Townsend’s second ionization coefficient]

  1. n 0 = number of primary photoelectrons emitted from the cathode
  2. number of secondary electrons generated at the cathode = n 0 ‘
  3. n 0 “= total number of electrons leaving the cathode

afterwards

gas ionization equation
equation

On average, all electrons leaving the cathode are [ed-1] Collision at the gap, The number of ionization collisions per second in the gap is n0 “(ed-1).

gas ionization equation
equation

Like the main process (only included),

gas ionization equation
equation

Therefore, in steady state, the circuit current I is

gas ionization equation
equation

This equation describes the increase in average current in the gap before the spark breakdown occurs.When the applied voltage increases, eWhen the denominator of the circuit current equation becomes zero and the current I → ∞, d and ed increase to ed → 1. In this case, the current is really only limit by the resistance of the power supply and the conductive gas.

Therefore, the condition can be define as a subdivision and can be written as:

gas ionization equation
equation

This situation is know as the Sparks Townsend standard.

Avalanche breakdown occurs over a relatively long period, typically over a second, and typically does not occur with pulsed voltages.

Determination of the Townsend coefficient – Breakdown mechanism

The Townsend coefficient is determine in the ionization chamber, first evacuated to a vacuum of 10-4 to 10-6 Torr and then filled with the require gas under a pressure of a few Torr. Also The DC voltage apply is approximately 2-10 kV, and the electrode system consists of planar high-voltage electrodes and low-voltage electrodes that are surrounded by protective electrodes to maintain a uniform electric field. Also The low voltage electrode is ground via an electrometer amplifier capable of measuring 0.01 pA to 10 nA of current. UV lamps are use to externally illuminate the cathode to generate the initiating electrons. Then we get the voltage and current characteristics with different gap settings. At low voltage, the current increase is unstable. Then the stable Townsend process is show in Figure.


Typical current growth

According to the Townsend mechanism, the discharge current is

gas ionization equation
gas ionization equation
equation gas ionization
equation

Due to the relationship between ln I and d, the constant and I 0 can be determine from the slope and intercept, respectively, as shown.

Knowing I0 and I0, they can be determined from the points in the upward region of the curve. The experiment can be repeat under different pressures if desired.

Breakdown in electronegative gases

The attachment of electrons to neutral molecules was not considere in the above analysis. Free electrons are remove by the attachment of electrons, which increases the dielectric strength of gas. The gas that causes electron attachment is an electronegative gas.

Similarly, it can be define as the attachment number of each electron drifting in the direction of the electric field. Under these conditions, the equation for average current increase in uniform field can be express as:

breakdown mechanism of gases
equation

The corresponding criteria for spark destruction

breakdown mechanism of gases
equation

Paschen’s law – Gas ionization

When electrons and ions pass through a gas under a uniform electric field E and gas pressure p, their average energy reaches an equilibrium value that depends on the ratio of E / p. More precisely

Alpha / p = f1 (E / p) and gamma = f2 (E / p)

For a uniform electric field gap, the electric field E = V / d. So if you apply the Townsend criterion to spark the decomposition of gas,

breakdown mechanism of gases
equation

You can write according to the following function

breakdown mechanism of gases
equation

This equation shows that the breakdown voltage V is an implicit function of the product of gas pressure p and electrode spacing d.

That is, V = f (p.d)

The above derivation does not consider the effect of temperature on breakdown voltage. Use gas pressure. Volume = mass. . You can see absolute temperature, pressure = density. . Absolute temperature. Therefore, the correct statement in the above equation is V = f () Person’s law.

Under constant atmospheric conditions, experiments have shown that the breakdown voltage of a uniform field gap can be expressed in the form:

V = A. d + B √d ​​(d is the gap interval)

Paschen’s law ctd

For air, under normal conditions A = 24.4 kV / cm and B = 6.29 kV / cm 1/2.

[For small gaps of the order of 1 cm, the breakdown voltage gradient in a uniform electric field is about 30 kV / cm, and for large gaps of a few meters, the breakdown voltage gradient drops to about 6 kV / cm]

Figure 1.5 shows typical breakdown and spacing characteristics.

Breakdown characteristic

This change in spacing can be change using Paschen’s law and can include the following gas density changes:

breakdown mechanism of gases
equation

Where delta = relative density (or gas density correction factor).

This formula is N.T.P. Correct for gaps larger than 0.1 mm. However, the pressure x interval product is so small that there is a minimum breakdown voltage know as the Paschen minimum. This can be explained as follows.

Decrease pressure from one point to the right of the minimum point, allowing for a fixed pitch gap

Breakdown characteristic

Therefore, the density decreases and the electrons moving in the electric field collide less with gas molecules as they travel toward the anode. Energy loss occurs with each collision, so the lower the electrical stress, the more kinetic energy (1/2 m u2) needed to ionize the electrons due to the collision.

Minimum sparking voltages

Paschen’s law ctd

In Addition Near the property’s minimum, the density is low and the collisions are relatively low. Here, it is necessary to consider that even if the electron energy exceeds the ionization energy, the collision of the electron with the molecule does not necessarily ionize the molecule. Electrons have specific ionization opportunities, depending on their energy. If the density is reduce and the number of collisions is reduced, the breakdown can only occur with increased ionization opportunities, which increases the voltage to the left of the minimum.

Note that if the density is fixed, the breakdown to the left of the minimum is likely to occur at longer distances (that is, at lower voltages). Also The minimum voltage is typically 300 V, which appears as a product of 5 torr mm or p.d, or a gap of about 0.06 mm in N.T.P.

At very low and very high pressures (compared to atmospheric pressure) Paschen’s law fails. Similarly, Paschen’s Law applies to temperatures below 1100C. As the temperature rises further due to thermal ionization, Paschen’s law eventually fails.

The breakdown characteristics show that for a constant gap spacing, the breakdown voltage and therefore the required breakdown stress is much higher than for atmospheric pressure, very high pressure and very low pressure (high vacuum). I will. . Therefore, both high vacuum and high pressure can be use as insulating media. [Vacuum breakdown area is the area where the breakdown voltage does not depend on the gas pressure. ] Normally, it will be on the right side of Paschen’s minimum value.

Streamer mechanism – Gas ionization

This type of breakdown is mainly due to the avalanche space charge field and the additional effects caused by photoionization in the gas volume. Although the discharge patterns predicted by the Townsend mechanism are highly dispersed, in reality many discharges are filamentous and irregular. Also Streamer theory predicts the occurrence of spark discharges directly caused by a single avalanche.

The space charge create by the avalanche causes sufficient distortion of the electric field, these free electrons move to the head of the avalanche, and the avalanche is create in the process and accumulates rapidly. As the electrons move rapidly, the cations stay in the slower moving tail, and then the electric field builds up in front of your head. Just behind the head, the electric field between the electron and the cation is in the opposite direction of the applied electric field, so the resulting electric field is less intense. Also Again, the electric field is strengthened between the tail and the cathode.

Streamer Mechanism
Streamer breakdown

Due to the electric field between the head and the anode increases, the space charge increases, which in turn strengthens the electric field around the anode. This process is very fast, and the positive space charges reach the cathode very quickly, forming streamers. Figure 1.8 shows the disassembly process.

Referance – Gas ionization method

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