Liquids and solids are usually 103 times denser than gases and from Paschen’s law it should follow that they possess much higher dielectric strength of the order by 107 V/cm. It filled the complete volume to be insulated and simultaneously will dissipate heat by convection. Oil is about 10 times more efficient than air or nitrogen in its heat transfer capability when in used. It expected to give very high dielectric strength if the order of 10 MV/cm. Normal, are mixtures of hydrocarbon and weakly polarised. The most important factor that affects the electrical strength of insulating oil is the presence of water in the form of fine droplets suspended in the oil.
The electrical properties that essential in determining the dielectric performance of a liquid dielectric are
1. Capacitance per unit volume or its relative permittivity
2. Resistivity
3. Loss tangent (tan δ) or its power factor which is an indication of the power loss under AC voltage application
4. Ability to withstand high electric stresses
Figure 17 Liquid purification system test cell
Figure 17 above shows the liquid breakdown test, the electrodes used for breakdown voltage measurements are usually spheres of 0.5 to 1 cm in diameter with gap spacing of about 100 to 200 µm. The gap is accurately controlled by using a micrometer. Electrode separation is very critical in measurements with liquids and also the electrode surface smoothness and the presence of oxide films have a marked influence on the breakdown strength. The test voltage required for these test are usually low (50 to 100kV), cause of small electrode spacing. The breakdown strength and DC conductivities obtained in pure liquids are very high (order of 1MV.cm and 10-18 to 10-20 ohm/cm respectively. The conductivity being measured at electric fields (order of 1kV/cm). Below figure 18 and 19 shows different types of conduction current-electric field characteristics of liquids
Figure 18 Conduction current-electric field characteristics in hexane at high fields
Figure 19 Conduction current-electric field characteristics in a hydrocarbon liquid
Figure 20 below shows, the maximum breakdown strengths of some highly purified liquids and liquefied gases
Figure 20 Maximum breakdown strengths of some liquids
Equivalent circuit of liquids partial discharge
Figure 21 Equivalent circuit of partial discharge in oil
Above figure 21, the equivalent circuit for streamer discharge, insulating liquid is represented as capacitances. The streamer is represented by capacitance Ca in parallel with a spark gap. Cb is represented capacitance of the sound of the insulation while the rest of the sample is represented by Cm. As the voltage is applied to the streamer before any discharge taken place is
Therefore, the magnitude for the first discharge can be expressed as
Use simulation software, by running the partial discharge of streamer in liquids insulation and breakdown. This is to generating partial discharge pulses and visualization of partial discharge patters and their pulses sequence. Include ϕ-q-n graph, ϕ-q graph for five consecutive cycle and ϕ-n graph. Figure 22 shows two different simulations that had been simulated below
Figure 22 Typical ϕ-q-n pattern (a), pulse sequences (b) and ϕ-n (c) partial discharge in oil/liquid under sinusoidal and triangular voltage
The prerequisites for positive half cycle are
1. Applied voltage should higher than positive inception voltage.
2. Determine factor representing appearance of early electron should higher than positive threshold.
The prerequisites for negative half cycle are
1. Applied voltage should lower than negative inception voltage.
2. Probability representing appearance of early electron should lower than maximum allowed probability.
The considered liquids to be analyze are mineral oils (MO1/A, MO3/C and MO4/B), synthetic ester (SE1/E) and natural esters (VO1/G, VO2/F, VO3/E, VO4/H). Those oils are treated (filtration under vacuum) even if the influence of particle and humidity is negligible under impulse voltage with point-plane arrangement. Below figure 23 experiment done under impulse voltage of positive and negative with point-plane
Figure 23 Negative streamer samples observed in negative polarity (left) and Positive streamer samples observed in positive polarity (right)
No comments:
Post a Comment