EENG 451 HIGH VOLTAGE TECHNIQUES
EXPERIMENT 3
OBSERVING ELECTRICAL DISCHARGES
Done by: Assist. Ahmad ALAhmad 03/11/2019 Objective: To observe the electrical discharges for different electrodes configuration
I. Some useful information
1) What is electrical insulator? An electrical insulator is a material whose internal electric charges do not flow freely; very little electric current will flow through it under the influence of an electric field; offer a very high resistance to the passage of direct currents;
2) What is the breakdown voltage of an insulator? All insulators become electrically conductive when a sufficiently large voltage is applied that the electric field tears electrons away from the atoms.
3) What is Ionization process? The process of liberating an electron from a gas molecule with the simultaneous production of a positive ion is called ionization. In the process of ionization by collision, a free electron collides with a neutral gas molecule and gives rise to a new electron and a positive ion. If we consider a low pressure gas column in which an electric field E is applied across two plane parallel electrodes due to applied voltage between two electrodes. Then, any electron starting at the cathode will be accelerated more and more between collisions with other gas molecules during its travel towards the anode. If the energy (ε) gained by the electron during this travel between collisions exceeds the ionization energy of the atom which is the energy required to liberate an electron from its atomic shell, then Ionization Processes takes place. This process can be represented as 푒− + 퐴 푒− + 퐴+ + 푒− Then the additional electrons will cause ionization by collision again and thus process repeat itself
4) What is partial electrical discharge? A partial discharge within solid insulation. When a spark jumps the gap within the gas-filled void, a small current flows in the conductors
Partial Discharge (PD) is an electrical discharge that does not completely bridge the space between two conducting electrodes. PD Occurs: Anywhere there is a junction between two electrical components. Examples are within solid insulation, across the surface of insulation material, within gas bubbles in liquid insulation and around an electrode surrounded in gas Partial Discharge can happen at any point in the insulation system, where the electric field strength exceeds the breakdown strength of that portion of the insulating material. The discharge may be in a gas filled void in a solid insulating material, in a gas bubble in a liquid insulator, or around an electrode in a gas. When partial discharge occurs in a gas, it is usually known as corona. Partial discharge is generally accepted as the predominant cause of long term degradation and eventual failure of electrical insulation. Studies show that over 85% of disruptive failures in high voltage (HV) and medium voltage (MV) equipment are Partial Discharge related. Partial Discharge action ultimately leads to failure Also, under certain conditions, PD might lead insulation system to breakdown completely due to repetition of partial discharge.
5) What are types of Partial Electrical Discharge Corona - discharge to air Floating Electrode - metal to metal Particle discharge - conductive particles contaminate insulation medium Voids - gaps in solid insulation or gas bubbles in oil Surface discharge - tracking over outside of insulators/bushings
6) What is the negative effect of corona? Corona is responsible for considerable power loss in transmission lines and also gives rise to radio interference. This also leads to deterioration of insulation by the combined action of the discharge ion bombarding the surface and the action of chemical compounds that are formed by the corona discharge. The formation of corona causes the current waveform in the line, and hence the voltage drops to be non- sinusoidal. It also causes a loss of power. There are always some electrons present in the atmosphere due to cosmic radiation etc. When the line voltage is increased, the velocity of the electrons in the vicinity of the line increases, and the electrons acquire sufficient velocity to cause ionization. 7) How does Partial Discharge Occur? Partial discharge occurs in gas filled cavities or defects in the high voltage insulation. These defects can originate in a number of ways; During manufacture – Solid insulators are designed to give an even distribution of electrical stress between the conducting electrodes. However, in practice, defects can arise during manufacture that give rise to small cavities or voids in the insulation bulk. Equipment installation – When electrical equipment is factory assembled or installed on site, mistakes can be made that either damage and therefore weaken the insulation, or cause increased electrical stress across the insulation. Ageing and deterioration – Most insulating materials naturally deteriorate with age as internal chemical bonds break down. This process makes the insulation weaker and less durable when withstanding the electrical stresses arising under normal working conditions. Over stressed in-service – A short circuit fault or lightening impulse is likely to impose stress on the insulation due to a fault current or an overvoltage. Although such events usually have a short duration, the increased electrical stress or heating from the current overload can cause permanent damage to the insulation.
8) Why Partial discharge occurs at normal working voltages The defects or cavities in solid insulation are usually filled with a gas of significantly lower breakdown strength than the surrounding material.
In addition to this, the permittivity of the gas is invariably lower than that of the solid insulation, causing the electric field intensity in the cavity to be higher than that in the surrounding dielectric.
Therefore, under the normal working stress of the insulation, the voltage across the cavity may exceed the breakdown value and initiate electrical breakdown, or Partial Discharge in the void.
9) What is flashover?
an electric discharge over or around the surface of an insulator f the surface of porcelain insulator is not properly glazed, moisture can stick over it. This moisture along with deposited dust on the insulator surface, produces a conducting path. As a result, the flash over distance of the insulator is reduced. As the flash over distance is reduced, the chance of failure of insulator due to flash over becomes more.
10) What is solution for flashover? If flash over occurs, the insulator may be over heated which may ultimately result into shuttering of it. SO, Arcing Horn is basically projected conductors used to protect the Insulators in High Voltage Transmission Line from damage during flashover. Over voltage on Transmission Line may occur due to various reasons like lightning strike, sudden load variation, fault etc. Due to this high voltage a flash over may take place which will shatter the Insulator. To prevent Insulator from such an occurrence, it is very important that flashover do not take place through the Insulator. Arcing Horn serves this purpose by providing a bypass flashover the high voltage across the insulator using air as a conductive medium.
The voltage at which an electric discharge occurs between two electrodes that are separated by an insulator. Also known as spark over voltage.
11) Treeing channel
Treeing is an electrical pre-breakdown phenomenon in solid insulation. It is a damaging process due to partial discharges and progresses through the stressed dielectric insulation, in a path resembling the branches of a tree. Treeing of solid high-voltage cable insulation is a common breakdown mechanism and source of electrical faults in underground power cables.
Fig. 2D carbonized electrical trees (or tracking) across the surface of a polycarbonate plate that was part of a trigatron. These partially conducting paths ultimately led to premature breakdown and operational failure of the device
12) Surface discharge at interfaces: Surface discharge is a common electrical discharge that normally occurs on the surface of outdoor insulators and also causes the failure in the electrical insulation system. One of the causes of surface discharge is the presence of high voltage stress.
13) Define uniform and non-uniform electric field Uniform electric field: If an electric field has the same magnitude and same direction everywhere in a given space then this electric field is uniform. The field lines are parallel to each other and equally spaced. Non-uniform electric field: If either the magnitude or direction or both change then it is a non-uniform electric field in that specified space. The field lines tend to be curved and are more concentrated near the charges.
14) What is the effect of uniform and non-uniform electric fields on electrical discharge? In a uniform electric field, a gradual increase in voltage across a gap produces a breakdown of the gap in the form of a spark without any preliminary discharges. If the field is non-uniform, an increase in voltage will first cause a localized discharge in the gas to appear at points with the highest electric field intensity, namely at sharp points or where the electrodes are curved or on transmission line conductors. This form of discharge is called a corona discharge and can be observed as a bluish luminance.
15) Summary:
1. Breakdown: complete electrical discharge through the insulator 2. Flashover: complete electrical discharge jumping around the insulator 3. Partial discharge: partial breakdown on conductors at points with the highest electrical stress
II. Experiments:
Surface discharge Set the needle-plane electrode system with a solid insulator to get a non-uniform field as shown in Fig.1.
Fig.1. Needle-plane electrode system for partial discharge (surface discharge) test
1. Increase a voltage until you can hear a discharge then record the voltage level. 2. Increase voltage until you can see a discharge again record the voltage. 3. Finally, increase voltage until you see breakdown, record the breakdown voltage.
Partial discharge 1. First set rod-plane electrode system in the vessel as shown in Fig.2.
Fig.2. rod-plane electrode system for partial discharge test
2. Increase a voltage until you can hear a discharge if it is possible; then record the voltage level. 3. Increase voltage until you can see a discharge again record the voltage. 4. Interrupt the circuit in case of a breakdown.
Breakdown (complete discharge) 1. First set plane-plane electrode system in the vessel.
Fig.3. plane-plane electrode system for breakdown test
2. Increase a voltage until you can hear a discharge if it is possible; then record the voltage level. 3. Finally, increase voltage until you see breakdown, record the breakdown voltage.
Tests Needle-plane rod-plane Plane-plane Discharge starts at voltage= Breakdown voltage=
Questions:
1. In the case of needle-plane, did the breakdown occur suddenly without any starting electrical discharge on the surface of the insulator? If No, what is the reason in your opinion 2. In which test the electric field can be considered uniform? 3. In which test the electric field can be considered non-uniform? 4. In which test the breakdown suddenly occurs? 5. In which case the breakdown voltage value is higher a) Uniform electric field b) Non-uniform electric field