In this type of protection, transmission lines are used to carry protective currents at carrier frequency (30 to 200 kc/s) or at an ultra high frequency (above 900 mega cycles). When the transmission lines carry protective currents at carrier frequency it is called carrier current-pilot protection and when it carries protective current at ultra high frequency its called as microwave pilot wire protection. As no separate pilot wires are used, the transmission lines are used to carry both power current as well as protective carrier currents it causes a great saving. Fig 14 represents a phase blocking system schematic arrangement of the equipment required at both ends of the transmission line. Each end of the transmission line consists of a network which transforms CT output currents into read more

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »

The distance impedance relays are normally used for phase- to-phase faults as for such faults the loop impedance between the phase is obtained which remains constant, but in case of earth fault the loop impedance consists of impedance of one line and the impedance of the earth fault which is a variable factor. Practically it has been estimated that for earth fault loop impedance is approximately 1.5 times the impedance of earth phase (phase and neutral), however for a multiple earth system this factor may be taken 1.25. Hence for earth faults special connections can be made, so the impedance relay may be used for both phase and earth fault From this it appears that the system will require two – impedance relays with voltage restraint elements energized, one for phase faults and read more

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »

When a generating station supplies to three sub-stations radially, it is desired to isolate all those sub-stations beyond the fault point. This can be achieved by the use of time delayed over current relay but the main disadvantage is that if the fault is near the power station it will take much longer time to isolate the system and this may cause serious defects. However with the use of time-distance relay the fault can be cleared much early. Let the impedance of each section of the transmission line be the same. Let the time distance relay installed at the beginning of each section of the line be so adjusted that it provides discrimination with the circuit breaker on the following section which should be about 0.75 sec. At the power station end the relay is adjusted so as to trip in read more

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »

This relay operates at a constant time for all faults independent of the distance of occurrence of fault. For its application consider Fig 10 which represents a power station feeding transforming sub-station. The power transformer is protected independently by inverse-time over current relay. Let, it be required to provide a relay at the input side so that it wilt trip the circuit instantaneously for any fault on the transmission line and at the same time permit the use of time delayed inverse-time over current relay for the transformer This cannot be achieved with the use of instantaneous over current relay set to trip the circuit for a certain minimum short circuit current because the fault current depends on the number of sections in service and the number of sources feeding the read more

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »

Although the protection for cable feeders (only non-pilot wire type) can be used for transmission lines also, but generally such protections for transmission lines are not used as the transmission lines are longer and work at much higher voltage and there are frequent faults. For protection of transmission lines usually distance protections are used, the principle of operation of these relays as explained earlier depends on the fact that under fault conditions the ratio of voltage to current remains constant and is independent of the fault current and its power factor. As, a matter of fact such relays are designed to measure the impedance of the line (in some cases only reactance) or the ratio of voltage to current up to the fault point and it indicates the distance over which the fault read more

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »

The probability of faults occurring on the lines is much more due to their greater length and exposure to atmospheric conditions. This has called for many protective schemes which have no application to the comparatively simple cases of alternators and transformers. The requirements of line protection are: (i)In the event of a short circuit the circuit breaker closest to the fault should open all other circuit breakers remaining in a closed position (ii)In case the nearest breaker to the fault fails to open back up protection should be provided by the adjacent circuit breakers. (iii) The relay operating time should be just as short as possible in order to preserve system stability with out unnecessary tripping of circuits. Differential method is ideal for protection of lines but it is read more

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »

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

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »

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

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »