Basic Ideas of Protection Relay

Basic Ideas of Protection Relay:

An electric power system should ensure the availability of electrical energy without interruption to every load connected to the system. When the electric power supply is extended to remote villages the power system would consist of several thousand kilometres of distribution lines. The high voltage transmission lines carrying bulk power could extend over several hundred kilometres. Since all these lines are generally overhead line:, and are exposed, there are many chances of their breakdown due to storms, falling of external objects, damage to the insulators, etc. These can result not only in mechanical damage but also in an electrical fault. One of the sources of trouble to continuous supply is the shunt fault or short-circuit, which produces a sudden and sometimes violent change in system operation. Protection Relay and relaying systems detect abnormal conditions like faults in electrical circuits and operate automatic switchgear to isolate faulty equipment from the system as quickly as possible. This limits the damage at the fault location and prevents the effects of the fault spreading into the system. It is the function of the Protection Relay in association with the switchgear to avert the consequences of faults. The switchgear must be capable of interrupting both normal currents as well as fault currents. The Protection Relay on the other hand must be able to recognize an abnormal condition in the power system and take suitable steps to insure its removal with the least possible disturbance to normal operation.

It should be noted that a Protection Relay does not prevent the appearance of faults. It can take action only after the fault has occurred. It would be most desirable if protection could anticipate and prevent faults, but this is obviously impossible except where the original cause of a fault creates some effect which can operate a Protection Relay. However, there are some devices which can anticipate and prevent major faults,e.g. Buchholz relay, a gas operated device which is capable of detecting the gas accumulation produced by an incipient fault in a transformer.

Nature and Causes of Faults

The nature of a fault simply implies any abnormal condition which causes a reduction in the basic insulation strength between phase conductors, or between phase conductors and earth, or any earthed screens surrounding the conductors. Actually the reduction of the insulation is not considered as a fault until it produces some effect on the system, that is until it results either in an excess current or in the reduction of the impedance between conductors or between conductors and earth to a value below that of the lowest load impedance normal to the circuit.

In a power system consisting of generators, switchgear, transformers, transmission and distribution circuits, it is inevitable that sooner or later in such a large network some failure will occur somewhere in the system, The probability of the failure or occurrence of abnormal condition is more on the power lines, simply because of their greater length and exposure to the atmosphere as already mentioned.

Before proceeding to the examination of the several causes of failures, it would be well to classify them according to the causes of their incidence. These are mentioned below.

Breakdown may occur at normal voltage on account of (i) the deterioration of insulation and (ii) the damage due to unpredictable causes such as the perching of birds, accidental short circuiting by snakes, kite strings, tree branches, etc.

Breakdown may occur because of abnormal voltages, though the insulation is otherwise healthy to withstand normal voltage, This may happen because of either (i) switching surges or (ii) surges caused by

The present day practice is to provide a high insulation level of the order of 3 to 5 times the nominal value of the voltage. But still the pollution on an insulator string which is commonly caused by deposited soot or cement dust in industrial areas and by salt, deposited by wind-borne sea-spray in coastal areas cause the insulation strength to decrease. This will initially lower the insulation resistance, and cause a small leakage current to be diverted, thus hastening the deterioration. Even if the installation is enclosed, such as sheathed and armored cables as well as metal-clad switchgear, deterioration of the insulation occurs because of ageing. Void formation in the insulating compound of underground cables due to the unequal expansions and contractions caused by the the and fall of temperatures is another cause of insulation failure.

The line and apparatus insulation may be subjected to transient over-voltages because of the switching operations. The voltage which rises at a rapid rate, may achieve a peak value which approaches three times phase to neutral voltage. It is for this purpose that a higher insulation level is provided initially. If the insulation levels have been correctly chosen and they have not been impaired in the manner described under (a) above, the system will withstand these routine over-voltages. But if the insulation has developed some form of weakness, it is at the time of switching that failure may be expected.

Lightning produces a very high voltage surge in the power system of the order of millions of volts and thus it is not feasible to provide an insulation which can withstand this abnormality. These surges travel with the velocity of light in the power circuits, the limiting factors are the surge impedance and the line resistance.

Consequences of Faults

The most serious result of a major uncleared fault is fire which may not only destroy the equipment of its origin but may spread in the system and cause total  failure. The most common type of fault which is also the most dangerous one is the short circuit which may have any of the following consequences:

  1. A great reduction of the line voltage over a major part of the power This will lead to the breakdown of the electrical supply to the consumer and may produce wastage in production.
  2. Damage caused to the elements of the system by the electrical arc which almost always accompanies a short circuit.
  3. Damage to other apparatus in the system due to overheating and due to abnormal mechanical forces setup.
  4. Disturbances to the stability of the electrical system and this may even lead to a complete shutdown of the power system.
  5. A marked reduction in the voltage which may sometimes be so great that Protection Relay having pressure coils tend to fail.
  6. Considerable reduction in the voltage on healthy feeders connected to the system having fault. This may cause either an abnormally high current being drawn by the motors or the operation of no-voltage coils of the motors. In the latter case considerable loss of industrial production may result as the motors will have to be restarted.

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