High Voltage & Reasons for Partial Discharge

Introduction

Electric energy is moved from generating site ( source where it is converted from other form of energy) to load (where it will be used ) through high voltage transmission lines. High voltage is used to reduce energy loss, however it produces strong electric field that results in partial discharge when it exceeds the breakdown strength of the insulating material. Partial discharge can result into breakdown of the insulating material and causes energy immersion. There are several reasons for partial discharge like improper installation of cables, adging of wires and manufacturing defects. Engineers consider testing for partial discharge is a good way to find any problems with the cables and evaluate the insulation of power apparatus. That will be discussed in this document, and first the structure of cables will be discussed as it were Pd occur.

High Voltage Power Lines

Components of High Voltage power cableConductor, inner semi-conductor screen, insulator, outer semi-conductor, earth semiconductor screen and protection sheet are the main components of the cable from inside to outside respectively. The conductor is where current is mainly being transferred, and usually made from aluminium or copper as they provide the minimum losses. The inner-semi conductor is a single layer that reduces stress on the insulator which arises due to the electric field by acting as a smooth, homogeneous surface. Insulators wrap electric cables to separate electrical conductors and prevent current through them, while the outer semi-conductor avoid contact between insulator and earth-wise by avoiding voids and gaps that can arise due to mechanical and thermal stress. Earth metallic screen is the protection of insulation and it conducts earth fault current until the system is switched off. Last but not least, is the protection sheet which protects the cable against damage.

Cable Setup. The components mentioned above are what builds up a cable, however there are other components that are used along the cables in the transmission system: joints and terminations. Joints connect to cable together and they are used since the length of a single cable is not enough to cover the long required distances. On the other hand, the terminations interface between the cable and the electric system. Partial discharge occurs usually in joints and terminals as they are installed on sites -unlike cables which are manufactured then transported- and this makes them higher subject to human error. Types of partial discharge:

1. 1. Internal Discharge

Breakdown due to internal charge starts when there is a void or cavity in the insulator that will have a low dielectric strength: electric field strength in voids higher than across dielectrics. So even if the voltage is operating at its average level, the breakdown might occur due to exceeding the breakdown strength.

1. 1. Surface Discharge

Surface discharge appearing on the surface of the insulating material happens when the strength of the material get weaker due to different environmental effects. High stress is what initiate this type of Pd so it usually occur on cable accessories as there is no outer semi-conductive screen.

1. 1. Electric Tree Discharge

Electric tree discharge is initiated by a tree-like path of electrical deterioration through the dielectric body. This tree path keeps till the breakdown occur when electrodes are bridged.

1. 1. Partial Discharge Detection

Early detection of partial discharge is required so that in faults can be improved to prevent dielectric breakdown. Testing for the assistance of partial discharge is either offsite (manufacturing quality control) or onsite which is the tough one due to existence of noise. The testing process basically starts with detection of a signal, transforming the signal and finally Signal acquisition.

Signal Detection

Signal is being detected by a sensor, and a good sensor is the one that has good signal to noise ratio, can be calibrated, and easy connected and transported.Transient earth voltage(TEV) is a way to obtain a signal.This test is based on the proportionality of insulation condition and level of the transient signal, where a transient signal is the electromagnetic pulses which were produced during PD and escape through the joint. These signals are being detected by special capacitance sensors, which are placed on the external surface of switchgear. The received signal is being subjected to different processing techniques, and it got filtered, amplified and converted to an electrical signal as received. The amplitude of the TEV signal carries information about the number of PD pulses, PD pulse width and distance from detection points. Electrical optical modulator is another way to collect data by using polarized laser light is sent via modulater input and the output isreceived via optical receiver and optical fiber are connected to the optical receiver to transmit signals.

Signal transmission

Signal transmission is needed since sensors must be placed near the partial discharge source to ensure accurate results which are either overhead power cables or underground or water. Transmission can be done with coaxial cables. However, it results in the attenuation of signals when used over a long distance. Thus optical fibre is used as it provides less attenuation, no electric interface and isolation of measuring equipment.

Signal interpretation

Signal first needs to be processed, and this is done using both hardware and software tools. The hardware is only responsible for preprocessing while the software does the modulation, digitalisation, and denoising to improve the reliability of measurements. Lastly, PD source is being identified by interpreting data and recognising patterns. For example, it is assumed that Pd close to the sensors will have a smaller pulse width: pulse width is used to identify the location. Moreover, peak amplitude and count rate are used to reveal long-term trends and wavelength analyses help in discriminating errors and keeping only the real PD detection.