How to Protect Transmission Lines ? | Electrical Engineering

There are several protective schemes for transmission lines and may be grouped into two groups viz., non-unit type and unit type.

The non-unit type of protection includes time-graded overcurrent protection, current-graded overcurrent protection, and distance protection, while the unit type protection includes pilot-wire differential protection, carrier-current protection based on phase comparison method etc.

Separate protection systems are necessary for ground faults because ground faults are more frequent on overhead transmission lines than phase fault, and ground fault current is different from phase fault current in magnitude.

The selection of a particular scheme of protection depends upon the following factors:

1. Economic justifiability of the scheme to ensure 100% continuity of supply.

2. Types of feeders-radial or ring mains.

3. Availability of pilot wires.

4. Number of switching stations in series between supply end and the far end of the system.

5. System earthing-whether the neutral is grounded or insulated.

Overload Protection of Transmission Lines:

This is the simplest way of protecting a line and is, therefore, widely used. Overload or overcurrent protection owes its application to the fact that the fault current in the event of a short circuit will raise to a value several times that of the maximum load current. The ratio of minimum fault current to maximum load current is chosen as criterion in order to prevent the possibility of mal-operation under normal operating conditions. This form of protection can be applied only to simple systems. The overcurrent protection is provided at the supply end of the line.

Figure 5.1 illustrates the protective scheme against overloading of feeders. Three CTs are mounted, one on each phase of the feeder and are connected across the three relay coils. In case of overloading the solenoid plunger system of the relays operate to close the trip coil circuit, which in turn opens the circuit breaker, thereby disconnecting the protected feeder. The relays need readjustment or even replacement whenever a change in the system is made. The operation times are generally large.

The alternative method for overload or overcurrent protection is the well-known Z-connection which requires only two relays for the protection of a 3-phase circuit. 

Overcurrent and Earth-Fault Protection of Transmission Lines:

The general practice is to employ a set of two or three overcurrent relays for protection against phase-to-phase faults and a separate overcurrent relay for single line-to-ground faults. Separate earth fault relays are generally preferred because they can be adjusted to provide faster and more sensitive protection for single line-to-ground faults than that can be provided by the phase relays.

Earth fault current depends on the type of neutral earthing, i.e., whether solidly earthed, insulated or earthed through some resistance or reactance. Where no neutral point is available, grounding transformer is employed.

Whatever the type of neutral earthing be employed, the earth-fault current will be small as compared to phase- fault currents in magnitude. The relay thus connected for earth fault protection is different from the ones provided for phase-to-phase faults.

In case of resistance earthed or solidly earthed systems, the overcurrent element connected in residual circuit of CTs is preferred. The setting of earth-fault relays may be made less than rated full-load current of the line. The practice followed is to apply relays having a setting range of 10 to 40%. A setting of 20% on 300/5 A CT means the relay operates for primary earth-fault current of 300/5 x 20/100 = 12 A.

In the above protection scheme two IDMT type overcurrent relays are connected in two phases through CTs and one earth-fault relay. In case of phase-to-phase faults or overload the IDMT relays trip the circuit breaker. Under healthy conditions, the sum of all the three currents of CTs is zero and the earth-fault relay remains inoperative. As soon as phase-to-ground fault occurs unbalancing in currents causes the earth-fault relay to operate, which in turn trips the circuit breaker.

The earth-fault elements are with inverse characteristics and time-grading is preferred for earth fault protection of radial feeders.

This scheme is employed for 11 kV and 33 kV systems as main protection and is used as a backup protection for transformers and transmission lines in EHV systems.

Current-Graded Protection of Transmission Lines:

An alternative to time grading or in addition to time grading current grading protection can be applied when the impedance between two substations is sufficient. It is based on the fact that the short-circuit current along the length of the protected circuit decreases with the increase in distance between the supply end and the fault point. If the relays are set to operate at a progressively higher current towards the supply end then the drawback of long time delays occurring in graded time lag system can be partially overcome. This is known as current grading. Current-graded systems normally employ high-speed high-set overcurrent relays.

A simple current-graded protection scheme applied to a radial feeder is shown in Fig. 5.8. It consists of high-set overcurrent relays at A, B and C with settings such that relay at A would operate for faults between A and B, the relay at B for faults between B and C and the relay at C for faults beyond C. The current setting diminishes progressively from the supply end to the remote end of the line.

In practice, however, this protection scheme poses some difficulties which are given below:

1. The relay cannot differentiate between faults which are very close to, but are on each side of B i.e., if a fault is very near to station B in section BC, the relay at A may feel that it is in section AB because there may be very little difference in the fault currents and the relays do not discriminate between the fault in the next section and the end of first section.

This is because:

(i) The difference in the fault currents would be extremely small,

(ii) The magnitude of fault currents cannot be accurately determined, and

(iii) The accuracy of the relays under transient conditions is likely to be different.

Hence for discrimination the relays are set to protect only part of the line, usually 80%. For this reason current grading alone cannot be employed and this protection system should be supplemented by time-graded inverse definite minimum time (IDMT) relay system.

2. The fault currents are different for different types of faults and so a certain difficulty is experienced in relay setting.

3. For ring mains, parallel feeders, interconnected systems, where power can flow to the fault from either direction, a system without directional control is not suited.

Differential Pilot-Wire Protection of Transmission Lines:

The term “pilot” means that between the ends of a transmission line there is an interconnecting channel of some sort over which information can be conveyed. Three different types of such a channel are presently in use, and they are called “wire pilot”, “carrier-current pilot” and “microwave pilot”. A wire pilot consists generally of a two-wire circuit of the telephone-line type, either open wire or cable. A wire pilot is generally economical for distances up to 8 or 15 km, beyond which a carrier-current pilot usually becomes more economical. Microwave pilots are employed when the number of services requiring pilot channels exceeds the technical or economic capabilities of carrier current.

The differential pilot-wire protection is most satisfactory and is widely employed on account of the advantages such as simplicity, flexibility, a high stability ratio, rapid fault clearance (a time varying between 0.1 and 0.5 second according to the “break time” of the circuit breaker).

The differential pilot-wire protection is based upon the principle that the currents compared at each end of the line or feeder by the use of pilot wires should be same under normal operating conditions and the equality is lost only when there is a fault in between the two ends. The system is quite similar to that employed for the protection of alternators and transformers and the difference lies only in the length of pilot wires.

Current Balance Differential Protection of Transmission Lines:

Biased differential protection due to McColl is an example of current balance differential protection.

For understanding its principle let us first consider the protection of single phase feeders. The scheme is illustrated in Fig. 5.9. The current transformers CT₁, and CT₂ are mounted on the two ends of the protected line. The secondaries of the two CTs are connected through the restraining coils R₁ and R₂ and pilot wires P₁ and P₂. The operating coils of the relays are also connected across the secondaries of the CTs through diverting resistances DR₁ and DR₂ as illustrated in Fig. 5.9.

Each of these two diverting resistances is equal to pilot wire resistance which will cause same current to flow through the operating coils O₁ and O₂. For equal number of turns on restraining coil and operating coil equal pull will be exerted on the sides of the beam relay. However, mechanical biasing can also be provided by moving the fulcrum towards the operating coil and such an arrangement can allow 10 to 12 per cent greater current to flow in the operating coil before it can close contacts.

For an earth fault at point F, the current in CT₁ will exceed that in CT₂. The secondary current of CT₁ will be flowing in two parallel paths—one path will be through diverting resistance DR₁ and operating coil O₁ and the second path will be through restraining coil R₁, pilot-wire P₁, diverting resistance DR₂, operating coil O₂, restraining coil R₂ and pilot-wire P₂. Obviously the resistance of second path (consists of 3 coils, 2 pilot-wires and one diverting resistance) is 3 times that of first path. Thus three- fourth of the total current will flow through the operation coil O₁ and one-fourth through the restraining coil R₁. If the current flowing through the operating coil O₁ exceeds the relay setting, the feeder will be disconnected from the supply end.

Figure 5.10 shows the application of biased differential protection to a 3-phase feeder. The CT sets CT₁ and CT₂ are connected in delta formation because the star-connection would require four-pilot wires, the fourth pilot-wire being connected between two star points. The diverting resistances DR₁ and DR₂ need only be one half of the resistance of one pilot-wire, as the return pilot utilised for the fault current will also include a resistor equal to one half of this pilot resistance.

Let there be a fault in the R-phase at point F. The excess current in the secondary of CT₁ will flow to junction a where it will divide into two parallel paths—one path through operating coil O₁ and the diverting resistance DR₁ shown by solid arrows, and another path will be through restraining coil R₁, pilot wire P₁, restraining coil R₂, junction b, operating coil O₂, diverting resistance DR₂, junction c and then to junction d through pilot wire P₃, as shown by dotted arrows.

The advantages of this protective scheme are given below:

1. The relay operating current increases automatically with the increase of through fault current which eliminates the possibility of malfunctioning of the relay.

2. As the pilot capacitive current flows through the restraining coil instead of operating coil, this current adds to restraint.

Merz-Price Voltage Balance System of Transmission Lines:

Probably the best known of the differential systems is the Merz-Price system, which, as applied to feeder protection, utilizes the principle of voltage balance. In 3-phase systems each conductor has its own pair of current transformers and relays. The secondaries of current transformers are connected in series by means of pilot wires. In normal conditions i.e., when there is no fault on the feeder, equal currents flow at the two different ends, so induced voltages in the secondaries of current transformers are equal.

As the secondaries are connected in opposition their secondary emfs are equalised resulting into no circulating current in the relays. But whenever fault occurs, currents differ at two ends, so induced emfs in the secondaries of current transformers will differ and circulating current will flow through the pilot wires and relays and the faulty feeder will be isolated.

It will be clear that the current transformers are critical feature of this system, since they have to be balanced exactly, not only initially but permanently. In order, that the induced voltage shall be proportional to line current, it is essential that the magnetic circuit shall not reach saturation, and this is accomplished by employing distributed air gap current transformers.

To secure initial matching, the CTs are balanced against a standard, and to ensure that there shall be no change of characteristics in service they are enclosed within a magnetic shield which prevents neighbouring iron affecting the distribution of flux. The pilot wires are usually in the form of a 3-core cable of size 7/0.73 mm.

This system has the following advantages and disadvantages:

Advantages:

(i) This system is independent of operating voltage and fault power factor.

(ii) This system can be employed for protection of both, ring mains as well as parallel feeders.

(iii) This system provides instantaneous protection for ground faults, so the possibility of these faults involving other phases is reduced.

(iv) This system provides instantaneous relaying thereby reducing the amount of damage to overhead conductors resulting from arcing faults.

Disadvantages:

(i) The trouble due to capacitance currents in the pilot circuit arises from the fact that, under through-fault conditions, voltages of the order of 1,000 volts or more are impressed on this circuit so that capacitance currents are comparatively heavy, and false operation may take place.

However, this drawback can be overcome by the introduction of the Beard-Hunter compensated pilot cable in which means are provided for diverting the capacitance currents from the relays. This action is achieved by surrounding each pilot wire with an insulated metallic screen or sheath which is divided at the centre of its length so as to form two conductors of equal lengths. When a heavy overload comes on, a high voltage is induced in the current transformer secondaries, but the resulting capacitance current, instead of flowing in the relays, flows in the local circuit formed by the sheath, current transformer and pilot wire.

(ii) This system does not provide backup protection or overload protection.

(iii) Difficulties are experienced in balancing the secondaries of the two current transformers and that is why this system cannot be used beyond 33 kV.

(iv) The system will not operate in case a break in the pilot wire occurs.

(v) This system is very expensive owing to the greater length of pilot wires required.

(vi) There is no time delay.

Translay Protection System:

The name “Translay” is evolved from the fact that the relay embodies a transformer feature. This system can be employed for protection of single phase or 3-phase feeders, transformer feeders, feeders with a tee-off and parallel feeders against both earth and phase faults. This system is based on the established principle of the current entering at one end of the feeder being equal at any instant to that leaving at the other end. A simple form of Translay protection for a single phase feeder is shown in Fig. 5.13. Under healthy conditions the line current transformers CT₁ and CT₂, at opposite ends of the feeder carry equal currents and, therefore, the coils 1 and 1′ connected to them induce equal emfs in the windings 2 and 2′ respectively.

Since the windings 2 and 2′ are connected in opposition by means of pilot wires with the operating coils 4 and 4′ in series with them so no forward torque is exerted on the disc. When a fault occurs, the current through one CT is greater than that through the other so a small current circulates through the operating coils and pilot wires and when it attains the preset value the relay is caused to close the tripping circuit and thus disconnect the faulty feeder. The Translay relay employed is quite similar in construction to an overcurrent induction type relay.

The Translay system, employed for a 3-phase circuit has a single-element relay at each end of the feeder which protects against both faults between phases and faults to earth. The connections (omitting trip circuits for sake of clarity) are shown in Fig. 5.14.

The upper magnetic circuit has three windings, two primaries and a secondary. The upper and smaller primary is a phase-to-phase fault winding and is connected across the red and blue protective current transformer while its mid-point is connected to the yellow. The lower and larger primary winding acts as a leakage winding, and is connected between the blue protective current transformer and the star point of the current transformers.

The secondary winding provided on the upper magnetic circuit acts like the opposed-voltage transformer in the Merz-Price system and is connected in opposition to a similar winding by means of two pilot wires, at the other end of the feeder. The secondary windings provided on the lower magnetic circuit are connected in series with the pilot wires. The rotating disc is composed of two sectors. Under normal operating conditions no current flows through the pilot wires as the opposing voltages are equal. On the occurrence of fault, the voltages in the windings differ and so a current flows through the lower elements and pilot wires.

A forward torque is thus exerted on the disc due to interaction of flux produced in the lower magnetic elements with the leakage flux of the upper magnetic elements. The phase relation required is achieved as in an energy meter. The capacitance currents lead the voltages and tend to rotate the disc in the opposite direction because of a closed copper ring near the end of the projecting limb of the upper magnetic circuit. Thus the main disadvantages of Merz-Price system have been avoided.

The Translay relay can be biased by an unsymmetrical phase adjustment, which provides a backward torque when the flux in the upper element is large.

This system has got following advantages over Merz-Price system:

(i) The capacitive currents do not affect the operation of the relays.

(ii) Only two pilot wires are required.

(iii) The current transformers of normal designs i.e., with air gap can be employed.

(iv) The pilot resistance does not affect the operation as major part of energy required to operate the relay is obtained from current transformer.

(v) The closed copper loop provided in the relay prevents the relay from operating for through fault current.

Split-Conductor Protection of Feeders:

This system is another method of securing the benefits of a balanced method of protection without the necessity of using pilot wires. The principle of operation depends on the fact that two conductors of equal length and impedance, when connected in parallel will share the load equally, provided that the insulation of the system is sound. When a fault develops on any one conductor it will carry more current than the other, and this inequality of currents is arranged to operate a relay and thus isolate the faulty line.

In this system of protection each phase of the line is split into two sections having equal impedances. The two sections are lightly insulated from each other. In this system, a single-turn current transformer is inserted at each end of the split conductor. The current transformers consist of laminated iron rings on which a secondary winding is wound all-round the periphery.

Under healthy conditions the current flowing along the two splits is equal and since these are threaded through the current transformers CT₁ and CT₂ in the opposite directions hence the voltage across the terminals of the evenly spread secondary winding is zero. In fault conditions one of the split takes more current than the other, thereby giving rise to an unbalance of the primary side of the current transformers. Due to unbalancing of currents on the primary side of current transformers resultant flux will be set up in the core of the one of the current transformers and so the current will be induced in the evenly spread secondary and the relay coil R will be energised. The relay contacts will be closed and the trip coil will trip the circuit breaker and isolate the fault.

In the best arrangement the splits are carried into the circuit breakers on both sides of the feeder so that the splits are opened by the breakers. This is explained as- Let the splits be not carried into the circuit breakers and a fault develops at the receiving end of a long line. Under these conditions, the impedance of the differential current transformer at this end may be insufficient to cause unbalance between the currents carried by each split conductor. Hence such a fault will not be cleared by the circuit breakers since the relay will not operate.

But when the splits are carried into the circuit breakers the fault current is confined to the faulty split after the sending end circuit breaker has tripped. In the former case, although the sending end circuit breaker trips, the fault current is not confined to the faulty split but it would divide practically equal between the two splits being solidly connected, so the receiving end circuit breaker will not trip. But in the latter case, the fault current is confined to the faulty split, the opening of the receiving end circuit breaker takes place.

The disadvantage of this system is that we have to make use of a special type of cable with the lower limits for the voltages. In the case of overhead feeders, for each phase, a duplicate set of conductors, insulators, etc., have to be employed. The lines having step-up or step-down transformers or voltage regulators cannot be protected by this method.

Microwave Channel Protection of Transmission Lines:

The microwave channels are used for all types of protections otherwise based on power line carrier or pilot-wire. The transmission is generally by line of sight and this must take into account the curvature of earth and topology of the route cover which the transmission takes place. This limits the maximum length of the simplest microwave channel to about 40 to 60 km.

It uses ultra-high frequency (450 MHz to 10,000 MHz) transmitter-receiver system for connecting the relaying equipment located at the terminals of the protected line. The communication channel is space in this case; thus the line need not be fitted with additional equipment. The transmitters and receivers are controlled in the same way as the carrier-current transmitter and receiver.

With radio links (microwave pilots) the signals are communicated by line of light antenna equipment. These are most expensive, but provide fast and reliable service. In USA radio links are used for communication, remote control and protection.

Distance or Impedance Protection of Transmission Lines:

The distance protection provides discrimination protection without making use of pilot wires. Distance protection is widely employed for protection of high voltage ac transmission lines because of its inherent advantages.

Figure 5.22 shows the simplest system consisting of feeders in series such that the power can flow only from left to right. The relays at A, B, C and D are set to operate with impedances less than Z_A, Z_B, Z_C and Z_D respectively. For a short-circuit fault at point F between substations C and D, the fault loop impedances at power station A and substations B and C are (Z_A + Z_B + Z), (Z_B + Z), and Z respectively. It is obvious that only relay at substation C will operate. Similarly for short-circuit faults between substations B and C, and power station and substation B only relays at substation B and power station A respectively will operate.

A system with instantaneous impedance relays, set to act on impedances less than or equal to the impedance of a section, as illustrated in Fig. 5.22 (a), would be difficult to adjust; a fault near the junction of two sections is likely to cause the operation of two relays. Furthermore, if a fault of finite resistance occurs near the end of a section, it is possible that total impedance is greater than that for relay operation. For these reasons it is advantageous to use impedance time relays, the characteristics of which are illustrated in Fig. 5.22 (b), for the power system illustrated in Fig. 5.22 (a).

If a fault occurs on right hand side of a substation B, say, relay at substation B operates in the minimum time t_m and the breaker at substation B operates t_B second later. If t_B is made less than the time difference between consecutive relays, only one relay will operate. Assume that the fault at F has a resistance causing total impedance at substation C represented by the point F’ (the fault resistance being FF’). Relay as substation C operates in time F’C’, whereas in the previous system it would not operate at all.

An impedance-time relay is a deli­cate mechanism and it is considered worth­while to replace it by three simple imped­ance relays with a definite time of opera­tion. The series combination can be ar­ranged to provide a 3-step-time character­istic, as illustrated in Fig. 5.22 (c), which does the same thing as the previous linear characteristic.

Modern practice is to employ definite distance method of protection applied in 3 zones (steps). A number of distance relays are used in association with timing relays so that the power system is divided into a number of zones with varying tripping times associated with each zone. The first zone tripping which is instantaneous is normally set to 80% of the protected section. The zone 2 protection with a time delay sufficient for circuit breaker operating time and discriminating time margins covers the remaining 20% portion of the protected section plus 25 to 50 per cent of the next section. Zone 2 also provides backup protection for the relay in the next section for fault close to the bus. Zone 3 with still more time delay provides complete backup protection for all faults at all locations.

Thus the distance protection provided for line AB (section 1) serves as backup protection for sections 2 and 3, because in case of occurrence of faults in line BC (section 2) or line CD (section 3) it will clear those in their respective zone time from tripping the circuit breaker at end A.

Carrier Aided Distance Protection of Transmission Lines:

The directional comparison carrier-pilot relay schemes presently employed are built around standard three-zone step-type distance relays. This speeds up fault clearance for internal zone 2 faults. The carrier channel is employed either for transmission of a stabilizing signal preventing tripping of a remote circuit breaker in the event of a local external zone 2 fault, or for providing a tripping signal in the event of an internal zone 2 fault. The principal features of plain 3-zone distance protection schemes.

Carrier signalling is concerned with the end zones of a protected section A A’. Let the faults occur at points F₁ and F₂ respectively. Fault at point F₁ will be seen at end A in zone 1 and at end A’ in zone 2. Similarly a fault at point F₂ will be seen at end A’ in zone 1 and at end A in zone 2.

Transfer trip or intertrip technique is employed for speeding up the fault clearance at the end which clears the fault in zone 2. This is achieved by control of the carrier transmitter and a carrier receive relay by zone 1 contact. For a fault at point F₁ the zone 1 relay at end A initiates a carrier signal in addition to completing the zone 1 trip circuit of this end. Carrier signal on reaching end A’ trips it immediately by shunting the zone 2 timer contacts with the help of a carrier receiver relay. A fault at point F₂ is also cleared in the same way.

Power Swings of Transmission Lines:

Power swings are surges of power due to the oscillations of generators with respect to each other which may occur due to change in load, switching or faults. The presence of power swing does not necessarily mean the instability of the system. So, it is of utmost importance that the relay must distinguish between a fault and a power swing, and respond correctly.

In general distance relays having mho characteristics are less susceptible to power swings because of their narrow characteristic. Generally, during power swings an out-of-slip blocking relay operates. If the measuring element operates within a certain time after operation of blocking relay, then tripping is allowed. Modern distance relays are stable over a wide range of power swings, they do not trip un-effectively, if power swing returns to normal condition fairly soon. If the condition prevails, the relay trips.

Auto-Reclosing of Transmission Lines:

It has been found that most of the line faults on overhead transmission system are transient in nature. Statistical evidence shows that about 90% of the faults are caused by lightning, birds, vines, tree branches etc. These conditions result in such arcing faults that if the fault energy infeed is interrupted for a short period, the arc extinguishes and the line can be re-energised.

This fact is employed as a basis for auto-reclosure schemes. In such schemes, after the relays at both ends of the line have picked up, the circuit breakers are tripped as far as possible at the same time and reclosed after time has been allowed for deionization. The fault disappears if it is transient, and line is fully restored to service after the reclosure. If the fault is not cleared after the first reclosure, a double or triple attempt of isolation and reclosure can be made. If the fault still persists, the breaker may permanently open till it is reset manually.

An auto-reclosure consists essentially of an oil switch or breaker actuated by relays which make it to open when predetermined current values flow through it. Reclosures are usually connected to protect portions of primary circuits and may take the place of line fuses. The switch or breaker is arranged to reclose after a short interval of time and re-open again should the fault or overload responsible for excess current flow persist. The reclosure can be set for 3 or 4 operations before it locks itself open for manual operation.

Oil circuit reclosures are increasingly employed in unattended substations and rural distribution schemes, where the circuit breakers are installed in outlying areas. They obviate the need for an operator to proceed to the point to close the breaker manually every time it trips. Outages are thereby greatly reduced. In case of persisting fault and getting the reclosure locked, necessitating manual resetting, and the technician after investigating and clearing the fault closes the reclosure.

Auto reclosing may be single or three phase type. Mostly single phase auto-reclosing breakers are preferred as most of the transmission faults are single phase to ground faults. Auto-reclosing in single phase also improves stability as the power remains transmitted through the two healthy phases when one phase is interrupted.

Like any other circuit breaker, the rupturing capacity of reclosure should be properly chosen. In large distribution network, reclosures may be provided to look after each separate zone and fuse protection provided for the subsidiary branch lines in each zone.

The breakers may be rapid auto reclosing type (about 20 cycles or 0.4 second), or delayed auto reclosing (5 to 30 s) type. It is not necessary to check synchronism with high speed reclosures while with delayed auto reclosing breakers, it is necessary to check synchronism before reclosing. For this purpose, synchronising relays are employed.