1. Short-Circuited Line – If the line is short-circuited at the receiving end, i.e., Z = 0, then the transmitted and reflected waves arc given as: The unique characteristic of the short-circuit is that voltage across it is zero. When an incident voltage wave E arrives on short-circuit, the reflected voltage wave must be -E to satisfy the condition that the voltage across the short-circuit is zero. The waves are shown in fig. 4. 2. Open-Circuited Line – If the line is open- circuited at the receiving end, i.e., Z is infinite, the transmitted and reflected waves are given as: An open-circuit at the end of a Line demands that the current at that point is always zero. Thus when an incident current wave I arrives at the open-circuit, a reflected wave equal to – I is at read more

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If a travelling wave arrives at a point where the impedance suddenly changes the wave is partly transmitted and partly reflected. Loading points, line-cable junctions and even faults constitute such discontinuities. Independent waves meeting along a line will combine in accordance with their polarity to provide different voltage and current levels at the meeting point. It is convenient to adopt a standard sign convention, and in what follows, forward waves of current and voltage are given the same polarity. If the wave is being reflected the corresponding current and voltage waves are given opposite polarity. This may be illustrated by considering waves of current and voltage being transmitted along a line of characteristic impedance Zc terminated by an impedance Z (fig 2). Let E and I read more

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Consider a voltage wave travelling from the supply source end towards the far end, and the progressive charging of the line capacitances will account for the associated current wave. Assume that in a very small time δt the conditions of a current I and a voltage E are established along a length δx of the line (fig 2). The emf E is balanced by the back emf generated by the magnetic flux which is produced by the current in this length of the line. The inductance of the length δx is L δx, (L is inductance of line per unit length) so that the flux built up is I Lδx and the back emf is the rate of build up viz. I L (δx/δt) So we haveE= I L (δx/δt)=I L v(1) where v is the velocity of propagation of wave. The current I carries a charge I δt in the time δt, and this charge remains on read more

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A transmission line is a distributed parameter circuit and distinguishing feature of such a circuit is its ability to support traveling waves of voltage and current. A circuit with distributed parameters has a finite velocity of electromagnetic field propagation. In such a circuit the changes in voltage and current, owing to switching and lightning do not occur simultaneously in all parts of the circuit but spread out in the form of traveling waves or surges. When a transmission line as shown in fig 1 is suddenly connected to a voltage source by closing of a switch, the whole of the line is not energized all at once (the voltage does not appear instantaneously at the other end). This is due to the presence of distributed constants (inductance and capacitance in a loss free line). When read more

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Valve type arresters incorporate non linear resistors and are extensively used on systems, operating at high voltages. Fig 12 (i) shows the various parts of a valve type arrester. It consists of two assemblies (i) series spark gaps and (ii) non-linear resistor discs in series. The non-linear elements are connected in series with the spark gaps. Both the assemblies are accommodated in tight porcelain container. (i) The spark gap is a multiple assembly consisting of a number of identical spark gaps in series. Each gap consists of two electrodes with fixed gap spacing. The voltage distribution across the gap is linearised by means of additional resistance elements called grading resistors across the gap. The spacing of the series gaps is such that it will withstand the normal circuit read more

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This type of arrester is also called ‘protector tube’ and is commonly used on system operating at voltages up to 33kV. Fig 11(i) shows the essential parts of an expulsion type lightning arrester. It essentially consists of a rod gap AA’ in series with a second gap enclosed within the fiber tube. The gap in the fiber tube is formed by two electrodes. The upper electrode is connected to rod gap and the lower electrode to the earth. One expulsion arrester is placed under each line conductor. Fig11 (ii) shows the installation of expulsion arrester on an overhead line. On the occurrence of an over voltage on the line, the series gap AA’ spanned and an arc is stuck between the electrodes in the tube. The heat of the arc vaporizes some of the fiber of tube walls resulting in the read more

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Fig 10 shows the multigap arrester. It consists of a series of metallic (generally alloy of zinc) cylinders insulated from one another and separated by small intervals of air gaps. The first cylinder (i.e. A) in the series is connected to the line and the others to the ground through a series resistance. The series resistance limits the power arc. By the inclusion of series resistance, the degree of protection against traveling waves is reduced. In order to overcome this difficulty, some of the gaps (B to C in Fig) are shunted by resistance. Under normal conditions, the point B is at earth potential and the normal supply voltage is unable to break down the series gaps. On the occurrence an over voltage, the breakdown of series gaps A to B occurs. The heavy current after breakdown will read more

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Fig 9 shows the horn gap arrester. It consists of a horn shaped metal rods A and B separated by a small air gap. The horns are so constructed that distance between them gradually increases towards the top as shown. The horns are mounted on porcelain insulators. One end of horn is connected to the line through a resistance and choke coil L while the other end is effectively grounded. The resistance R helps in limiting the follow current to a small value. The choke coil is so designed that it offers small reactance at normal power frequency but a very high reactance at transient frequency. Thus the choke does not allow the transients to enter the apparatus to be protected. The gap between the horns is so adjusted that normal supply voltage is not enough to cause an arc across the read more

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