Thursday , October 18 2018
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Protection

The modern electric power system consists  of several elements e.g alternators, transformers, station bus bars, transmission lines and other equipment. It is desirable and necessary to protect each element from a variety of fault conditions which may occur sooner or later. As a matter of convenience, here we deals with protection of alternators transformers, bus bars, lines and protection against over voltages.

The most serious faults on alternators which requires immediate attention are the stator winding faults. The major faults on transformers occur due to short circuits in the transformer or their connections. The basic system used for protection against these faults in the differential relay scheme because differential nature of measurements makes this system much more sensitive than other protective systems.

Protection Of Alternators

The generating units, are relatively few in number and higher in individual cost than most other equipments. Therefore, it is desirable and necessary to provide protection to cover the wide range of faults which may occur in the modern generating plant.

Some of the important faults which may occur on an alternator are:

  1. failure of prime mover
  2. failure of field
  3. over-current
  4. over-speed
  5. over-voltage
  6. Unbalanced loading
  7. stator winding faults

Failure Of Prime Mover: When input to the prime-mover fails, the alternator runs as a synchronous motor and draws some current from the supply system. This motoring condition is known as “inverted running

Failure Of Field: The chances of field failure of alternators are undoubtedly very rare. Even if it does occur no immediate damage will be caused by permitting the alternator to run without a field for a short period. It is sufficient to rely on the control room attendant to disconnect the faulty alternator manually from the system bus bars. Therefore, it is a universal practice not to provide automatic protection against this contingency.

Over Current: It occurs mainly due to partial breakdown of winding insulation or due to overload on the supply system. Over current protection for alternators is considered unnecessary because of the following reasons:

  1. The modern tendency is to design alternators with very high values of internal impedance so that they will stand a complete short circuit at their terminals for sufficient time without serious overheating. On the occurrence of an overload, the alternators can be disconnected manually.
  2. The disadvantage of using overload protection for alternators is that such a protection might disconnect the alternators from the power plant bus on account of some momentary troubles outside the plant and therefore, interfere with the continuity of electric service.

Over Speed: The chief cause of over speed is the sudden loss of all or the major part of load on the alternator. Modern alternators are usually provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime mover.

Over Voltage: The field excitation system of modern alternators is so designed that over voltage conditions at normal running speeds cannot occur. However, over voltage in an alternator occurs when speed of the prime mover increases due to sudden loss of the alternator load.

Unbalanced Loading: Unbalanced loading means that there are different phase currents in the alternator. Unbalanced loading arises from faults to earth or faults between phases on the circuit external to the alternator. The unbalanced currents, if allowed to persist, may either severely burn the mechanical fixings of the rotor core or damage the field winding.

Stator Winding Faults: These faults occur mainly due to the insulation failure of the stator windings. The main types of stator winding faults, in order of importance are:

  1. fault between phase and ground
  2. fault between phases
  3. inter turn fault involving turns of the same phase winding

The stator winding faults are the most dangerous and are likely to cause considerable damage to the expensive machinery. Therefore, automatic protection is absolutely necessary to clear such faults in the quickest possible time in order to minimize the extent of damage. For protection of alternators against such faults, differential method of protection (also known as Merz-Price System) is most commonly employed due to its greater sensitivity and reliability.

Differential Protection Of Alternators

The most common system used for protection of stator winding faults employs circulating current principle. In this Scheme of protection, currents at the two ends of the protected section are compared. Under normal operating conditions, these currents are equal but may become unequal on the occurrence of a fault in the protected section. The difference of the currents under fault conditions is arranged to pass through the operating coil of the relay. The relay then closes its contacts to isolate protected section from the system. This form of protection is also known as Merz-Price circulating current scheme.

Balanced Earth Fault Protection

In small size alternators, the neutral ends of the three phase windings are often connected internally to a single terminal. Therefore, it is not possible to use Merz-Price circulating current principle described above because there are no facilities for accommodating the necessary current transformers in the neutral connection of each phase winding. Under these circumstances, it is considered sufficient to provide protection against earth faults only by the use of balanced earth fault protection scheme. This scheme provides no protection against phase to phase faults, unless and until they develop into earth-faults, as most of them will.

Stator Inter-Turn Protection

Merz-Price circulating current system protects against phase-to-ground and phase-to-phase faults. Stator Inter-Turn ProtectionIt does not protect against turn-to-turn fault on the same phase winding of the stator. It is because the current that this type of fault produces flows in a local circuit between the turns involved and does not create a difference between the currents entering and leaving the winding at its two ends current transformers are applied. However, it is usually considered unnecessary to provide protection for inter-turn faults because they invariably develop into earth-faults. In single turn generator, there is no necessity of protection against inter-turn faults. However, inter-turn protection is provided for multi-turn generators such as hydro-electric generators. These generators have double-winding armatures owing to the very heavy currents which they have to carry. Advantage may be taken of this necessity to protect inter-turn faults on the same winding. The figure shows the schematic arrangement of circulating-current and inter-turn protection of a 3 phase Double wound generator. The relays Rc provide protection against phase-to-ground and phase-to-phase faults whereas relays R1 provide protection against inter-turn faults. Stator Inter-Turn Protection image

The given figure shows the duplicate stator winding S1 and S2 of the one phase only with a provision against inter-turn faults. Two current transformers are connected on the circulating-current principle. Under normal conditions, the current in the stator winding S1 and S2 are equal and so will be the currents in the secondaries of two CTs. The secondary current around the loop then is the same at all points and no current flows through the relay R1. If a short-circuit develops between adjacent turns, say on S1, the currents in the stator winding of S1 and S2 will no longer be equal. Therefore, unequal two currents flows through the relay R1. The relay then closes its contacts to clear the generator from the system.

Protection Of Transformers

Transformers are static devices, totally enclosed and generally oil immersed. Therefore, chances of faults occurring on them are very rare. However, the consequences of even a rare fault may be very serious unless the transformer is quickly disconnected from the system. This fault may be very serious unless the transformer is quickly disconnected from the system. This necessities to provide adequate automatic protection for transformers against possible faults.

Small distribution transformers are usually connected to the supply system through series fuses instead of circuit breakers. Consequently, no automatic protective relay equipment is required. However, the probability of faults on power transformers is undoubtedly more and hence automatic protection is absolutely necessary.

As compared with generators, in which many abnormal conditions may arise, power transformers may suffer only from:

  • Open Circuits
  • Overheating
  • Winding short-circuits e.g earth faults, phase to phase faults and inter-turn faults

Protection System For Transformers

For protection of generators, Merz-Price circulating-current system is unquestionably the most satisfactory. Through this is largely true of transformer protection, there are cases where circulating current system offers no particular advantage over other systems or is impracticable on account of the troublesome conditions imposed by the wide variety of voltages, currents and earthing conditions invariably associated with power transformers. Under such circumstances, alternative protective systems are used which in many cases are as effective as the circulating-current system. The principal relays and systems used for transformer protection are:

  1. Buchholz Devices providing protection against all kinds of incipient faults. i.e. slow-developing faults such as insulation failure of windings, core heating, fall of oil level due to leaky joints etc.
  2. Earth-Fault Relays providing protection against earth-faults only.
  3. Over-current Relays providing protection mainly against phase-to-phase faults and over-loading.
  4. Differential System providing protection against both earth and phase faults.

The complete protection of transformer usually requires the combination of these systems. Choice of a particular combination of systems may depend upon several factors such as

  1. Size of a transformer
  2. Type of cooling
  3. Location of transformer in the network
  4. Nature of load supplied
  5. Importance of service for which transformer is required

Buchholz Relay

Buchholz relay is a gas-actuated relay installed in oil immersed transformers for protection against all kinds of faults. It is used to give an alarm in case of incipient faults in the transformer and to disconnect the transformer from the supply in the event of severe internal faults. It is usually installed in the pipe connecting the conservator to the main tank. It is a universal practice to use Buchholz relays on all such oil immersed transformers having ratings in excess of 750 kVA.

Operation: The operation of Buchholz relay is as follows:

  1. In case of incipient faults within the transformer, the heat due to fault causes the decomposition of some transformer oil in the main tank. The products of decomposition contain more than 70% of hydrogen gas. The hydrogen gas being light tries to go into the conservator and in the process gets entrapped in the upper part of relay chamber. When a pre-determined amount of gas gets accumulated, it exerts sufficient pressure on the float to cause it to tilt and close the contacts of mercury switch attached to it. This completes the alarm circuit to sound an alarm.
  2. If a serious fault occurs in the transformer, an enormous amount of gas is generated in the main tank. The oil in the main tank rushes towards the conservator via the Buchholz relay and in doing so tilts the flap to close the contacts of mercury switch. This completes the trip circuit to open the circuit breaker controlling the transformer.

Combined Leakage And Overload Protection

The core-balance protection suffers from the drawback that it cannot provide protection against overloads. If a fault or leakage occurs between phases, the core-balance relay will not operate. It is a usual practice to provide combined leakage and overload protection for transformers. The earth relay has low current setting and operates under earth or leakage and overload protection for transformers. The earth relay has low current setting and operates under earth or leakage faults only. The overload relays have high current setting and are arranged to operate against faults between the phases.

Circulating-Current Scheme For Transformer Protection

Merz-Price circulating-current scheme for the protection of a 3-phase delta/delta power transformer against phase-to-ground and phase-to-phase faults. CTs on the two sides of the transformer are connected in star as shown below. This compensates for the phase difference between the power transformer primary and secondary. The CTs on the two sides are connected by pilot wires and one relay is used for each pair of CTs. Transformer Merz-Price Circulating-current Scheme Protection

During normal operating conditions, the secondaries of CTs carry identical currents. Therefore, the current entering and leaving the pilot wires at both ends are the same and no current flows through the relays. If a ground or phase-to-phase fault occurs, the currents in the secondary of CTs will no longer be the same and the differential current flowing through the relay circuit will clear the breaker on both sides of the transformer. The protected zone is limited to the region between CTs on the high-voltage side and the CTs on the low-voltage side of the power transformer.

It is worthwhile to note that this scheme also provide protection for short-circuits between turns on the same phase winding. When a short-circuit occurs between the turns, the turn-ratio of the power transformer is altered sufficiently, enough differential current may flow through the relay to cause its operation. However, such short-circuits are better taken care of by Buchholz relays.

Bus Bar Protection

Bus bars in the generation stations and sub-stations form important link between the incoming and outgoing circuits. If a fault occurs on a bus bar, considerable damage and disruption of supply will occur unless some form of quick-acting automatic protection is provided to isolate the faulty bus bar. The bus bar zone, for the purpose of protection, includes not only the bus bars themselves but also the isolating switches, circuit breakers and the associated connections. In the event of fault on any section of the bus bar, all the circuit equipments connected to that section must be tripped out to give complete isolation.

The standard of construction for bus bars has been very high, with the result that bus faults are extremely rare. However, the possibility of damage and service interruption from even a rare bus fault is so great that more attention is now given to this form of protection. Improved relaying methods have been developed, reducing the possibility of incorrect operation. The two most commonly used schemes for bus bar protection are:

  • Differential Protection
  • Fault Bus Protection

Differential Protection: The basic method for bus bar protection is the differential scheme in which currents entering and leaving the bus are totalised. During a normal load condition, the sum of these currents is equal to zero. When a fault occurs, the fault current upsets the balance and produces a differential current to operate a relay. Bus-bar's Differential Protection

The single line diagram of current differential scheme for a station bus bar is shown. The bus bar is fed by a generator and supplies load to two lines.The secondaries of current transformers in the generator lead, in line 1 and in line 2 are all connected in parallel. The protective relay is connected across this parallel connection. All CTs must be of the same ratio in the scheme regardless of the capacities of the various circuits. Under normal load conditions or external fault conditions, the sum of the currents entering the bus is equal to those leaving it and no current flows through the relay. If a fault occurs within the protected zone, the currents entering the bus will no longer be equal to those leaving it. The difference of these currents will flow through the relay and cause the opening of the generator, circuit breaker and each of the line circuit breakers.

Fault Bus Protection: It is possible to design a station so that the faults that develop are mostly earth-faults. This can be achieved by providing earthed metal barrier (known as fault bus) surrounding each conductor throughout its entire length in the bus structure. With this arrangement, every fault that might occur must involve a connection between a conductor and an earthed metal part. By directing the flow of earth-fault current, it is possible to detect the faults and determine their location. This type of protection is known as fault bus protection.

Protection Of Lines

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:

  1. 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.
  2. In case the nearest breaker to the fault fails to open, back-up protection should be provided by the adjacent circuit breakers.
  3. The relay operating time should be just as short as possible in order to preserve system stability, without unnecessary tripping of circuits.

The protection of lines presents a problem quite different from the protection of station apparatus such as generators, transformers and bus bars. While differential protection is ideal method for lines, it is much more expensive to use. The two ends of a line may be several kilometers apart and to compare the two currents, a costly pilot-wire circuit is required. This expense may be justified but in general less costly methods are used. The common methods of line protection are:

  • Time-graded over-current protection
  • Differential protection
  • Distance protection

Time Graded Over-Current Protection

In this scheme of over-current protection, time discrimination is incorporated. In other words, the time setting of relays is so graded that in the event of fault, the smallest possible part of the system is isolated. We shall discuss a few important cases:

Radial Feeder: The main characteristic of a radial system is that power can flow only in one direction, from generator or supply end to the load. It has the disadvantage that continuity of supply cannot be maintained at the receiving end in the event of fault. Time-graded protection of a radial feeder can be achieved by using

  • Definite time relays
  • Inverse time relays

Parallel Feeder: Where continuity of supply is particularly necessary, two parallel feeders may be installed. If a fault occurs on one feeder, it can be disconnected from the system and continuity of supply can be maintained from the other feeder. The parallel feeders cannot be protected by non-directional over-current relays only. It is necessary to use directional relays also and to grade the time setting of relays for selective trippings.

Ring Main System: In this system, various power stations or sub-stations are interconnected by alternate routes, thus forming a closed ring. In case of damage to any section of the ring, that section may be disconnected for repairs, and power will be supplied from both ends of the ring, thereby maintaining continuity of supply.

Differential Pilot-Wire Protection

The differential pilot-wire protection is based on the principle that under normal conditions, the current entering one end of a line is equal to that leaving the other end. As soon as a fault occurs between the two ends, this condition no longer holds and the difference of incoming and outgoing currents is arranged to flow through a relay which operates the circuit breaker to isolate the faulty line. There are several differential protection schemes in use for the lines. Two such schemes are:

  1. Merz-Price Voltage Balance System
  2. Translay scheme

Distance Protection

Both time-graded and pilot-wire system are not suitable for the protection of very long high voltage transmission lines. The former gives an unduly long time delay in fault clearance at the generating station end when there are more than four or five sections and the pilot-wire system becomes too expensive owing to the greater length of pilot wires required. This has led to the development of distance protection in which the action of relay depends upon the distance (Impedance)  between the point where the relay is installed and the point of fault. This system provides discrimination protection without employing pilot wires.

Over-Voltage In Power System

There are several instances when the elements of a power system ( generators, transformers, transmission lines, insulators etc. ) are subjected to over-voltages i.e voltages greater than the normal value. These over-voltages on the power system may be caused due to many reasons such as lightning, the opening of a circuit breaker, the grounding of a conductor etc.  Most of the over-voltages are not of large magnitude but may still be important because of their effect on the performance of circuit interrupting equipment and protective devices. An appreciable number of these over-voltages are of sufficient magnitude to cause insulation break down of the equipment in the power system. Therefore, power system engineers always device ways and means to limit the magnitude of the over-voltages produced and to control their effects on the operating equipment.

Causes Of Over-Voltages

The over-voltages on a power system may be broadly divided into two main catagories

Internal Causes

  • Switching Surges
  • Insulation Failure
  • Arcing Ground
  • Resonance

External Causes

  • Lightning

Internal causes do not produce surges of large magnitude. Experience shows that surges due to internal causes hardly increase the system voltage to twice the normal value. Generally, surges due to internal causes are taken care of by providing proper insulation to the equipment in the power system. However, surges due to lightning are very severe and may increase the system voltage to several times the normal value. If the equipment in the power system is not protected against lightning surges, these surges may cause considerable damage. In fact, in a power system, the protective devices provided against over-voltages mainly take care of lightning surges.

Harmful Effects Of Lightning

A direct or indirect lightning stroke on a transmission line produces a steep-fronted voltage wave on the line. The voltage of this wave may rise from zero to peak value in about 1 μs and decay to half the peak value in about 5 μs. Such a steep-fronted voltage wave will initiate travelling waves along the line in both directions with the velocity dependent  upon the L and C parameter of the line.

  1. The travelling waves produced due to lightning surges will shatter the insulators and may even wreck poles.
  2. If the travelling waves produced due to lightning hit the windings of a transformer  or generator, it may cause considerable damage. The inductance of the windings opposes any sudden passage of electric charge through it. Therefore, the electric charges “piles up” against the transformer (or generator). This induces such an excessive pressure between the windings that insulation may break down, resulting in the production of arc. While the normal voltage between the turns is never enough to start an arc, once the insulation has broken down and an arc has been started by a momentary over-voltage, the line voltage is usually sufficient to maintain the arc long enough to severely damage the machine.
  3. If the arc is initiated in any part of the power system by the lightning stroke, this arc will set up very disturbing oscillation in the line. This may damage other equipment connected to the line.

Protection Against Lightning

Transients or surges on the power system may originate from switching and from other causes but the most important and dangerous surges are those caused by lightning. The lightning surges may cause serious damage to the expensive equipment in the power system either by direct strokes on the equipment or by strokes on the transmission lines that reach the equipment as travelling waves. It is necessary to provide protection against both kind of surges. The most commonly used devices for protection against lightning surges are:

  • Earthing screen
  • Overhead ground wires
  • Lightning arresters or surge diverters

Earthing screen provides protection to power stations and sub-stations against direct strokes whereas overhead ground wires protect the transmission lines against direct lightning strokes. However, lightning arresters or surge diverters protect the station apparatus against both direct strokes and the strokes that come into the apparatus as travelling waves.