Chapter 8 Overvoltage Phenomenon & HV Transient Analysis Lightning Phenomena HV Transient Analysis
Classes and shape of overvoltages and standard voltage shape Analysis of overvoltages in power system includes a study of their: magnitudes a) shapes b) durations c) frequency of occurrence d)
Transient overvoltages 2 main transient overvoltages: Due to Switching Due to Lightning A typical switching overvoltages shown at point Y. A typical lightning overvoltages shown at point X.
Lightning Phenomena What is Lightning? Lightning is an electric discharge in air, one terminal of which is cloud. During storms, charges are accumulated in clouds and equal charges of opposite polarity are formed in earth. As these charges increases, the voltage gradient in the air adjacent to the charge center in the cloud increases.
Electrical charges in clouds Mason – Latham theory (Ice Splinter Theory) Moisture in the atmosphere combining with precipitating particles which are suspended in the air are forced higher up into the altitude by updraft. For every km high up into the altitude there is a decrease in temperature of about 0.40C. So the moisture experience super cooling and form crystalline ice with double layered ice structures. During the process of super cooling, water (H 2 O) undergoes ionisation and break up into anions (H+) and cations (OH-).
Electrical charges in clouds Mason – Latham theory (Ice Splinter Theory) The temperature gradient causes an imbalance of ion migration with H+ ions, which are more mobile , accumulating at the outer warmer shell. Due to the expansion of water on freezing, an unequal stress is set up and the ice shell splinters.
Electrical charges in clouds Mason – Latham theory Splintering process of the ice to form minute splinters. Wind pushes the charged splinters higher up in the altitude. The existence of temperature gradient in the super cooling moisture structure allows the migration of H+ and OH- ions. However, H+ ions being lighter become more mobile allowing them to move towards the outer shell, leaving the heavier ions OH- in the inner shell of the ice structures.
Con’t When the gradient exceeds the insulation strength of air, a low current streamer starts downward from the cloud and continues to grow. When the streamer makes contact with the earth, it is like closing a switch between the two charges of opposite polarity, one is the earth and the other in the streamer channel and in the clouds . Thus large current flows.
Different type of Lightning Intra-cloud (within the clouds) Cloud to cloud Cloud to ground Cloud to air Positive stroke or “ Blue sky lightning”
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Con’t Lightning Current: Peak Value Typical value for the first stroke is 30 kA. Typical value for the subsequent stroke is 15 kA. Values over 250 kA has been recorded. Lightning Current: Peak time derivative Typical value for the first stroke is 12 kA/µs. Typical value for the subsequent stroke is 40 kA /µs. Values over 250 kA/µs has been recorded
Con’t Stepped Leader It propagates ionizing the virgin air. It comes in steps of length about 50m. It pauses few tens of microseconds before starting the next step. The average speed of a stepped leader is in the order of 105 m/s. In the last few tens of meters the speed increases due to the enhanced field strength in the presence of ground. It brings a charge of about 5 Coulombs to ground.
Con’t Return Stroke The main event of a lightning flash. It takes the earth potential to the cloud. Return stroke drains charge in the channel and then in the cloud into ground. At the channel base, it produces a double exponential current waveform. The zero crossing time of the current is 40 – 60 μs The speed of return stroke is about 1/3 of the speed of light. It produces strong emission of electromagnetic radiation. The electric and magnetic fields of return stroke has different shape at different distances. They induce voltages on conducting wires.
Con’t Lightning Current and Voltage Parameters Current and Voltage amplitude. The rate of current: 7.5 - 25 kA/ μs . Voltage waveform: 1000 - 5000 kV; 1 MV/μs. Voltage rise time: 2 – 10 μs and tail time 20 – 100 μs .
Con’t Direct and Indirect Lightning Strokes Lightning produces high voltages in structures. If the light strikes direct to the structures then high current flows through it. This is called “DIRECT STROKE”. The other way in which lightning produces voltage in a line is by electrostatic induction. This is called “ INDIRECT STROKE ”.
Lightning Protection Some of the methods used for lightning protection are as follow; Lightning Arrestor (Surge diverters). Earth Wires. Auxiliary devices. Grounding system
Con’t Lightning Arrestors Lightning arresters are the devices which are used to protect the power system to being damage from over voltages by diverting their path to the ground. When the voltage surge reaches the arrester, it sparks over at a certain prefixed spark over voltage. Then provides the surge with a low impedance path between the earth and line. Thus voltage of the line remains in the limit, and the power system is being saved.
Grounding Establishing an electrical connection to earth. Establishing a common reference point for all voltage related to a given system. Factors Effecting Grounding Soil Resistivity. The Application Requirement. How the Grounding System is to be used. The number of earth interfaces that are to be provided for one facility. The grounding resistance or surge impedance actually achieved.
Con’t When lightning strikes, the electric charges pass through lightning conductor to the earth without causing any damage to the building.
High Voltage Transient Analysis Surges on transmission line High voltage surges induced on the transmission line due to; Direct stroke Indirect stroke Switching Operation The surge travels along the line at the speed of light. In case a direct stroke occurs over the top of an unshielded transmission line, the current wave tries to divide into two branches, and travel on either side of the line.
Direct Stroke to the transmission line Surge due to lightning strike : Z 0 can be calculated from the distributed inductance L and distributed capacitance C of the distribution feeder as follows:
Example 1 A 3-phase single circuit transmission line is 400 km long. If the line is rated for 220 kV and has the parameters, R = 0.1 ohms/km, L = 1.26 mH/km, C = 0.009 nF/km, and G = O, find (a) the surge impedance Z 0 ; (b) the resulting overvoltages if the line is hit by a 40kA lightning stroke.
Surge propagation in transmission line As they reach the end (termination) or junction of the line, partly are reflected and transmitted.
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Con’t Bewley Lattice Diagram Diagram to show the position and direction of incident, reflected and transmitted wave on the system at every instant of time. Properties; All waves travel downhill because time increases. Position of any wave at any time can be deduced directly from the diagram. Total potential at any point is the superposition of all the waves arrived at that point. History of wave easily traced. Attenuation is included
Con’t Let t is the time taken for a wave travel from one end of line to the other end of the line and k the corresponding attenuation factor.
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Case (i): Open ended transmission line of surge impedance Z Let the voltage of the travelling wave incident on the line be
Case (ii): Short circuited line
Example 2.0 A long overhead line is joined to a 500km cable which is open-circuited at its far end. The overhead line has an impedance of 600Ω while the cable has the parameters, L=1.62mH/km and C=0.027μF/km. If a surge of 200kV travels from the overhead line to the junction, construct the Bewley Lattice diagram for the surge in the cable up to the third reflection.
Example 2.1 What happen to the Bewley Lattice diagram if the cable is connected a transformer of characteristic impedance 950 Ω .
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