breakdown voltage

Breakdown voltage of Gases

Factors that affect the vacuum gap

Ideally, a vacuum is the best insulator with a breakdown strength of about 104 kV / c. The breakdown voltage of a high vacuum is such that, with a small increase. The gap maintained at that voltage is destroy indefinitely. However, this definition is not always possible because subdivisions are affect by many factors.

Electrode separation

For vacuum gaps less than about 1 mm. The breakdown voltage is approximately proportional to length and all other parameters remain constant. This gives a constant breaking strength. For these small gaps, the fracture stress is relatively high, around 1 MV / cm. Field emission of electrons can play an important role in the breakdown process.

V = k d d <1mm

Above a gap of about 1 mm, the breakdown voltage does not increase at the same rate, and the apparent breakdown stress of a long gap is significantly reduced to about 10 kV / cm at 10 cm intervals. [Apparent stress is defined as the voltage required to cause a breakdown divided by the distance between the electrodes].

breakdown voltage of vacuum
Breakdown characteristic

Cranberg theoretically shows that the product of breakdown voltage and breakdown stress remains constant for long gaps.

V E = k1. For V. d> 1 mm,

The constant k1 depends on the electrode material and surface conditions.

For a uniform field gap, E = V / d,

equation
equation

Or, in a more general form, both regions can be represented by equations

V = k dx where x = 0.5 means long gap> 1 mm = 1 means gap <1 mm

Electrode effect

conditioning

The breakdown voltage of the gap increases with continuous flashover until it reaches a certain value. Then say that the electrodes have been adjusted. The increase in voltage is due to burnouts caused by small irregularities or sparks in impurities. That may be present at the electrodes. When studying the effects of various factors on breakdown, the electrodes must first be adjusted to obtain reproducible results.

Breakdown characteristics
Breakdown characteristics

The effect of the adjustment is show. Since the adjusted electrode or gap breakdown voltage is very reproducible compare to other cases, the breakdown value usually obtain through experimentation is the electrode’s adjusted breakdown value. The unregulated electrode breakdown value can be as low as 50% of the regulated electrode breakdown voltage.

Material and surface finish

Effect of electrode material on breakdown
Effect of electrode material on breakdown

The electrode surface forms the physical boundary where the ultimate failure occurs. Therefore, it is not surprising to find that the breakdown strength for a given gap size depends strongly on the electrode material. In general, the higher the surface finish, the higher the breakdown voltage.

Surface contamination

The presence of contaminants in the test battery can reduce the breakdown voltage by as much as 50% of the clean electrode value.

Electrode area and configuration

The increased electrode area makes it more difficult to maintain a given breakdown voltage. Therefore, the breakdown voltage decreases slightly as surface area increases. For example, an electrode with an area of ​​20 cm2 provides a breakdown voltage at a 1 mm gap of 40 kV. While an electrode of the same material with an area of ​​1000 cm2 provides a breakdown voltage at the same 1 mm gap of 25 kV. To do.

The electrode surface forms the physical boundary where the ultimate failure occurs. Therefore, it is not surprising to find that the breakdown strength for a given gap size depends strongly on the electrode material. In general, the higher the surface finish, the higher the breakdown voltage.

Temperature – Breakdown voltage of vacuum

The breakdown voltage changes little with temperature, and for nickel and iron electrodes, the strength does not change up to 5000C. Cooling the electrode to liquid nitrogen temperature increases the breakdown voltage.

Frequency of applied voltage – Breakdown voltage of vacuum

It is know that a given gap has a pulse voltage higher than the AC voltage and an AC voltage higher than the DC voltage. However, at small gaps (2 mm), breakdown voltage has been show to be independent of frequencies in the 50 Hz to 50 kHz range.

Vacuum pressure – Breakdown voltage of vacuum

Breakdown characteristic
Breakdown characteristic

For small gaps, increasing the vacuum will increase the breakdown voltage until there is no change below a certain pressure. The vacuum breakdown region is the region where the breakdown voltage becomes independent of the pressure properties of the gap between the electrodes.

However, at large gaps (about 200 mm pitch), it was find that the breakdown voltage begin to fall again below a certain pressure, so actually improving the vacuum could improve breakdown stress at this stage. Is possible.

For example, using a flat cathode and a 1.6 mm diameter sphere opposite the 200 mm gap. When the pressure is below 5 x 10-4 Torr, the breakdown voltage is constant, but when the pressure increases, the breakdown voltage rises to the maximum value of 5 x 10-4 Torr, and when the pressure further increases, the voltage increases. It drops sharply.

Time lags of Spark breakdown

Spark lag time lag

In fact, especially in high voltage engineering, breakdown in the pulse field is very important. When a voltage is applied, even if the applied voltage greatly exceeds the voltage that caused the breakdown. It will take some time before it is actually destroyed.

Considering the time difference between applying sufficient voltage to cause a breakdown and the actual breakdown. Two basic processes that need attention are avalanche initiation electrons and static breakdown criteria met. Later is the increase in current.

Time lag of impulse breakdown
Time lag of impulse breakdown

If the electric field changes slowly, it is usually not difficult to find the starting electron from natural sources (cosmic rays, gas ion separation, etc.). However, depending on the size of the gap, for pulses of short duration (about 1 second), applying a voltage is not enough to provide the triggering electrons in the natural source and breaks down without the other source. Does not occur. Figure 1.12 shows the time lag of the pulse voltage waveform.

Spark lag time lag – ctd

A time is that elapses from the application of voltage equal to or higher than a static breakdown voltage (Vs) to the spark gap til the appearance of properly and arrange start electrons is called the statistical time lag of the gap. This occurrence is usually Is the statistical distribution.

The time, tf, required to generate a certain amount of current in the ionization process after the appearance of such an electron is call the formation time lag, and this current can be used to determine the breaking of the gap. As shown in the figure, the sum of tf + ts = t is the total time lag. The ratio V / Vs greater than 1 is called the pulse ratio and obviously depends on the growth rate of ts + tf and the applied voltage.

The breakdown may also occur after the peak value of the voltage pulse (that is, at the wave tail). Depending on the time lag, the wavefront or tail of the pulse waveform may fail.

Statistical time lag ts

The statistical time lag is the average time it takes for an electron to appear in the gap and cause a breakdown.

= Proportion of electrons generated in the gap by external irradiation P 1 = Probability of electrons appearing in the gap causing sparks in the region

P 2 = Sparks caused by such electrons appearing in the gap and causing an explosion Probability of releasing

Then the average time difference

dielectric voltage breakdown

As the dose increases, the dose increases, so ts decreases. Similarly, for a given lighting level, clean work function cathodes will be smaller, resulting in longer time lags.

The type of irradiation used is an important factor in controlling P1. This is because the electrons can appear in the right place and cause breakdown. Of course, the most advantageous location is near the cathode.

If the gap is overvoltage, it is more likely to produce enough current to cause breakdown. Therefore, P2 increases with overvoltage and tends to be unified when the overvoltage is about 10%. When P2 → 1 increases with increasing overvoltage, ts → 1 / betaP1.

Formation time lag – Dielectric voltage breakdown

After a statistical time difference, it can be assumed that the initial electrons are available. This will eventually cause a breakdown. The additional time lag required to form the decomposition process is the formation time lag. A series of uninterrupt avalanches must be perform to generate the required gap current (A) that leads to breakdown, and the rate of ionization depends on the particular secondary process. The value of the formation time lag depends on various secondary ionization processes. Again, increasing the voltage above the static breakdown voltage reduces the formation time delay tf.

Time characteristics – Dielectric voltage breakdown

Time-lag characteristics
Time-lag characteristics

The time lag characteristic is the change in breakdown voltage with breakdown time and can be define for a particular waveform. The figure shows the time lag characteristics based on the pulse waveform.

Typical Time lag characteristics
Typical Time lag characteristics

Time delay characteristics are important when designing isolation. If the pole gap provides secondary protection for the transformer, the breakdown voltage characteristic of the pole gap must always be smaller than that of the transformer (gap i) to protect the transformer from dangerous surge voltages. I won’t. This ensures that the gap always flashes in front of the protected device.

However, for such a rod gap, the gap setting will be lower because the sharpness of the two characteristics is different. Therefore, even the smallest overvoltage can be interrupt frequently and does not actually damage the system. Therefore, the rod gap characteristic is usually slightly higher (gap ii) and the characteristics intersect as shown. In this type of protection is provide only in the region where the rod gap characteristics are lower than a transformer rod gap characteristics. Experience has shown that it is extremely unlikely that a voltage value will occur at this intersection. Of course, another option is to improve the characteristics of the transformer. This significantly increases the cost of the transformer. [This decision is like saying that it is much cheaper to replace one transformer per year than to double the cost of such 100 transformers in the system.

Reference

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