Partial Discharge Detection
- A common mode of failure of high voltage cables involves voids or inclusions.
- These cause partial discharge (pd) activity which, if allowed to continue unchecked, may compromise the integrity of the insulation.
- Since traditional methods of monitoring pd activity are only suitable for use within a well-screened laboratory, our work has considered a range of alternatives, with the overall aim of developing a system that can detect and locate pd activity on-line.
- Methods include the use of coupling techniques, radio frequency current transducers, and acoustic emissions.
- Recent research has concentrated on requirements for systems capable of continuously monitoring high voltage cable circuits, and we have developed a pd data transmission system using electro-optic technology.
- In addition we are developing techniques for characterizing pd measurement data using genetic algorithms and pattern recognition techniques.
The parameters and the characteristics of the lightning strokes
- Amplitude of the current, the rate of rise, the probability distribution of them and the wave shapes of the lightning voltages and currents.
Isokeraunic level or thunderstorm days
It is defined as the number of days in a year when the thunder is heard or recorded in a particular location. Often it does not distinguish between the ground strokes and the cloud-to-cloud strokes.
Factors influence the lightning induced voltages on transmission lines
- The leader stroke current
Attenuation and distortion of traveling waves
- The decrease in the magnitude of the wave as it propagates along the line is called attenuation.
- The elongation or change of wave shapes that occur is called distortion.
Over voltages generated in EHV system
Over voltages are generated in EHV systems when there is a sudden release of internal energy stored either in the electrostatic form or in the electromagnetic form.
Causes for power frequency and its harmonic over voltages
•
Sudden loss of loads
•
Disconnection of inductive loads or connection of capacitive loads,
•
Ferranti effect,
•
unsymmetrical faults
•
saturation in transformers
Uses of shunt reactors
•
Used to limit the voltage rise due to Ferranti effect.
•
Used to reduce surges caused due to sudden energizing.
Ground wire
•
Ground wire is a conductor run parallel to the main conductor of the transmission line supported on the same tower and earthed at every equally and regularly spaced towers.
•
It is run above the main conductor of the line.
Use of ground wire
•
It shields the transmission line conductor from induced charges, from clouds as well as from a lightning discharge.
Expulsion gap
Expulsion gap is a device which consists of a spark gap together with an arc quenching device which extinguishes the current arc when the gap breaks over due to overvoltage.
Parts of an expulsion gap.
It consists of a rod gap in air in air in series with a second gap enclosed within a fibre tube.
Protector tube
•
It is a device which consists of a rod or spark gap in air formed by the line conductor and its high voltage terminal. It is mounted underneath the line conductor on a tower.
•
Selecting the risk of failure, the statistical safety factor and by firing the withstand level of any equipment or apparatus corresponding to 90% or 95% of the withstand voltage.
Basic Impulse Level
•
It is defined as the minimum insulation impulse withstands voltage of any power equipment or apparatus.
•
The BIL of a power system is usually chosen as 25% to 30% more than the protective level offered by the protective devices.
Main disadvantages of Zinc Oxide arrester
It is the continuous flow of power frequency current and the consequent power loss.
Various insulation levels in a substation
•
The busbar insulation is the highest to ensure the continuity of supply in a substation.
•
The circuit breakers, isolators, instrument and relay transformers are given the next lower limiting level.
•
The power transformers are the costliest and sensitive device and the insulation level for it is the lowest.
Surge arresters
They are non-linear resistors in series with spark gaps which act as fast switches.
Various types of surge arresters used for EHV and UHV systems
•
Silicon carbide arresters with spark gaps
•
Silicon carbide arresters with current limiting gaps
•
The gapless metal oxide arresters
Equation of surge admittance and surge impedance of the transmission line
Y(S)= C/L ((S+α - β )(S+α + β ))1/2
Z(S)= L/C (( S+α-β)(S+α-β))1/2
Where α is the attenuation constant and β is the wavelength constant.
Gas- insulating medium in electrical apparatus
Most of the electrical apparatus use air as the insulating medium and in a few cases other gases such as nitrogen, carbon dioxide, Freon and sulphurhexafluoride
Breakdown voltage
The maximum voltage applied to the insulation at the moment of breakdown is called the breakdown voltage.
Spark breakdown
Spark breakdown is the transition of a non-sustaining discharge into a self-sustaining discharge.
Theories mechanism for breakdown
•
Townsend theory
•
streamer theory
Mean free path
•
It is defined as the average distance between collisions.
•
The mean free path is expressed as k/p cm, where k is a constant and p is the gas pressure in microns.
Ionization
The process of liberating an electron from a gas molecule with a simultaneous production of a positive ion is called ionization.
