Friday, January 3, 2014

Shielding in 380kV HV Transmission Lines

     380 kV transmission overhead lines seem to be vulnerable to lightning discharges since they are, mostly, the things that have largest height in field. Lightning discharges most likely prefer to breakdown the air through overhead lines in plain fields. This is a very bad condition for power system since it may result temporary overvoltages, insulation failures or partial energy cuts. That's why overhead lines should be protected against lightning discharges. Most of the time, the protection is satisfied using lightning rods, especially in substations. Actually, sometimes it is inevitable for HV transmission lines to be exposed to lightning discharges. Therefore, the excessive energy should be directed to a point which causes much less damage to power system. Shielding is a kind of protection against the harmful effects of lightning discharges, which is always used in high voltage transmission lines unless it is protected by lightning rods or any other conductor that has higher altitude. Shielding requires to direct the lightning strokes to ground wire instead of phase wires. This aim is satisfied by simply placing the ground wire at a higher place than phase wires so that the nearly all of the strokes are directed towards ground wire. The shielding position of wires can be determined using shielding angle, theta, which is defined as the angle between tower direction and the line passing through ground wire and outermost phase wire. Figure 1 is showing an effective shielding. 

Figure 1: Effective Shielding

     The most important thing to take into consideration for effective shielding is determining the shielding angle. While high shielding angle may result to missing some strokes to phase wires, low shielding angle may result to extra unnecessary tower cost and construction handicaps. Let me try to explain the method for choosing the best procedure for determining shielding angle. 

     Lightning Stroke History in Field
     Provide the lightning stroke historical data for determining the characteristics of the strokes to be protected. Then determine the maximum protection level ,in other words, determine the stroke energies that are most likely to happen and eliminate the extreme (rarely happened) events. If we try to protect the line for even extreme cases, the construction cost will increase dramatically. By using the historical data for lightning strokes we can determine the equipotential lines. Please note that the higher the electrical charges collected in a stroke the the higher the shielding angle is. In other words, if the charge enclosed in a lightning stroke is high, the stroke starts at a longer height. Thus, high shielding angle is enough. However if the charges in a lightning stroke is less which means the lightning stroke starts discharging at the distances closer to earth, then the low shielding angle is needed which can be possible only with high tower length. Please take a look at the graphs shown in figure 2, 3 and 4, they give the relations between shielding angle and lightning strokes.

Figure 2: Percentage of Strokes vs Lightning (kA)

Figure 3: Protective Angle vs Lightning (kA)
Figure 4: Percent Lightning Strokes vs Angle

     It had better to represent a noneffective shielding, figure 5 represents a non effective shielding. 

Figure 5: Noneffective Shielding

Figure 6 shows a practical application of shielding in a thermal plant in Turkey.

Figure 6: Practical Shielding in Kemerköy Thermal Plant, Muğla / TURKEY

Thursday, January 2, 2014

Reducing Corona in HV Transmission Lines

     Corona phenomenon in HV transmission lines can be defined simply as a partial ionization of air in vicinity of conductor which is exposed to high voltage. Actually, corona is a natural cause of high voltage and is inevitable for the high voltage overhead lines in interconnected transmission line system. One can identify the corona phenomenon by its very distinctive audible noise and emitted light that can not be seen directly by human eye. Since there is sound energy and ionization of air, someone can easily conclude that there should be energy loss due to corona. This loss is proportional to the magnitude of the voltage on the conductor.
     It had better to continue with the benefits and harms of corona to electrical system and society. I prefer start with the benefits of corona since it has very few indeed. Since corona can be represented as a loss that is proportional to voltage magnitude in power system, it has a damping effect on temporary over voltages or lightning discharges. By this way harmful effects of over voltages or lightning discharges reduced significantly especially for the long transmission lines. Another benefit of corona can be seen as an audible noise, although it is a harmful feature. Any person who is working on high voltages can identify the high voltage by its corona noise and it can be a warning signal especially if the line of sight or illumination is weak. Let me continue with the harms of corona, this part is much more dominant on benefits. As said before, corona is a source of loss in power system and has very high values for high voltages, V > 200kV. This loss should be reduced since it reduces the efficiency of power system network significantly. Also, the audible noise has a psychological effect on creatures, including human society, living vicinity of HV transmission line. In addition to these harms, corona is a source of electromagnetic waves, which has a negative effect on electronic devices and human body, which is not proved but is under investigation. Let me continue with how can we reduce corona in high voltage transmission lines.
     There are a couple of ways that is effective for reducing the corona, which are reducing the electrical field per unit area and conductor bundling. Ionization of air starts in a point that has the highest electric field per unit area. If the conductor has singularities (sharp ends, wedge shaped ends, burr), corona become effective at smaller voltages. The electrical field per unit area at these singularities is increased since the differentiation of the surface area is undefined, by this way the corona becomes effective even at low voltages if the conductor has sharp ends. Using this information it can be concluded that we should make the conductor surface and conductor ends smooth and eliminate the burrs on conductor in order to reduce the corona. Please note that none of the manufactured high voltage element has singularities on its surface, their surface is made smooth on purpose. You can check the conductor or busbar ends by your hand so that you can confirm that there is no burrs or sharp points on it. The other method for reducing the corona is bundling. Corona is effective for the transmission line voltages higher than about 200 kV, so bundling should be applied to higher voltages like 380 kV interconnected system. Bundling is a method that requires more than one conductor on a phase, by this way the current is shared equally among bundled conductors and the equivalent radius (Geometric Mean Radius GMR) is increased and electric field per unit area decreases. This provides reduced corona in HV transmission lines. The above figure,1, represents the bundling process.

Figure 1: (1) Single Conductor , (2) Bundled Conductor

Conductors (1) and (2) have the same cross sectional area for current conduction but the three bundled conductors (2) have reduced corona effect. The above photography, figure 2, shows the practical usage of bundled conductors in 380 kV transmission line network. 

Figure 2: Two-bundled conductor in 380 kV connection of  Kemerköy Thermal Plant Muğla/TURKEY