Mason C. Russell. The art and science of protective relaying

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DISTANCE RELAYS 71

that opposes a directional element, and it is called an “admittance” or “mho” unit or relay.

In other words, this is a voltage-restrained directional relay. When used with a reactance-

type distance relay, this unit has also been called a “starting unit.” If we let the

control-spring effect be – K

, the torque of such a unit is:

T = K

VI cos (

–

) – K

– K

where

and

are defined as positive when I lags V. At the balance point, the net torque is

zero, and hence:

= K

VI cos (

–

) – K

Dividing both sides by K

VI, we get:

V K

— = Z = — cos (

–

) – ——

I K

If we neglect the control-spring effect,

Z = — cos (

–

)

It will be noted that this equation is like that of the directional relay when the control-

spring effect is included, but that here there is no voltage term, and hence the relay has but

one circular characteristic.

The operating characteristic described by this equation is shown in Fig. 11. The diameter

of this circle is practically independent of voltage or current, except at very low magnitudes

of current or voltage when the control-spring effect is taken into account, which causes the

diameter to decrease.

Fig. 11. Operating characteristic of a directional relay with voltage restraint.

72 DISTANCE RELAYS

The complete reactance-type distance relay has operating characteristics as shown in

Fig. 12. These characteristics are obtained by arranging the various units as described in

Fig. 5 for the impedance-type distance relay. It will be observed here, however, that the

directional or starting unit (S) serves double duty, since it not only provides the directional

function but also provides the third step of distance measurement with inherent

directional discrimination.

The time-versus-impedance characteristic is the same as that of Fig. 8.

Fig. 12. Operating characteristics of a reactance-type distance relay.

DISTANCE RELAYS 73

THE MHO-TYPE DISTANCE RELAY

The mho unit has already been described, and its operating characteristic was derived in

connection with the description of the starting unit of the reactance-type distance relay.

The induction-cylinder or double-induction-loop structures are used in this type of relay.

The complete distance relay for transmission-line protection is composed of three high-

speed mho units (M

, M

, and M

) and a timing unit, connected in a manner similar to

that shown for an impedance-type distance relay, except that no separate directional unit

is required, since the mho units are inherently directional.

The operating characteristic

of the entire relay is shown on Fig. 13.

The operating-time-versus-impedance characteristic of the mho-type distance relay is the

same as that of the impedance-type distance relay, Fig. 8.

By means of current biasing similar to that described for the offset impedance relay, a mho-

relay characteristic circle can be offset so that either it encircles the origin of the R-X

diagram or the origin is outside the circle.

Fig. 13. Operating characteristics of a mho-type distance relay.

74 DISTANCE RELAYS

GENERAL CONSIDERATIONS APPLICABLE TO ALL

DISTANCE RELAYS

OVERREACH

When a short circuit occurs, the current wave is apt to be offset initially. Under such

conditions, distance relays tend to “overreach,” i.e., to operate for a larger value of

impedance than that for which they are adjusted to operate under steady-state conditions.

This tendency is greater, the more inductive the impedance is. Also, the tendency is greater

in electromagnetic-attraction-type relays than in induction-type relays. The tendency to

overreach is minimized in the design of the relay-circuit elements, but it is still necessary

to compensate for some tendency to overreach in the adjustment of the relays.

Compensation for overreach as well as for inaccuracies in the current and voltage sources

is obtained by adjusting the relays to operate at 10% to 20% lower impedance than that for

which they would otherwise be adjusted. This will be further discussed when we consider

the application of these relays.

MEMORY ACTION

Relays in which voltage is required to develop pickup torque, such as mho-type relays or

directional units of other relays, may be provided with “memory action.” Memory action is

a feature that can be obtained by design in which the current flow in a voltage-polarizing

coil does not cease immediately when the voltage on the high-voltage side of the supply-

voltage transformer is instantly reduced to zero. Instead, the stored energy in the voltage

circuit causes sinusoidal current to flow in the voltage coil for a short time. The frequency

of this current and its phase angle are for all practical purposes the same as before the

high-tension voltage dropped to zero, and therefore the relay is properly polarized since,

in effect, it “remembers” the voltage that had been impressed on it. It will be evident that

memory action is usable only with high-speed relays that are capable of operating within

the short time that the transient polarizing current flows. It will also be evident that a relay

must have voltage applied to it initially for memory action to be effective; in other words,

memory action is ineffective if a distance relay’s voltage is obtained from the line side of a

line circuit breaker and the breaker is closed when there is a short circuit on the line.

Actually, it is a most rare circumstance when a short circuit reduces the relay supply voltage

to zero. The short circuit must be exactly at the high-voltage terminals of the voltage

transformer, and there must be no arcing in the short circuit. About the only time that this

can happen in practice is when maintenance men have forgotten to remove protective

grounding devices before the line breaker is closed. The voltage across an arcing short

circuit is seldom less than about 4% of normal voltage, and this is sufficient to assure

correct distance-relay operation even without the help of memory action.

Memory action does not adversely affect the distance-measuring ability of a distance relay.

Such ability is important only for impedance values near the point for which the operating

time steps from T

to T

or from T

to T

. For such impedances, the primary voltage at the

relay location does not go to zero, and the effect of the transient is “swamped.”

DISTANCE RELAYS 75

THE VERSATILITY OF DISTANCE RELAYS

It is probably evident from the foregoing that on the R-X diagram we can construct any

desired distance-relay operating characteristic composed of straight lines or circles. The

characteristics shown here have been those of distance relays for transmission-line

protection. But, by using these same characteristics or modifications of them, we can

encompass any desired area on the R-X diagram, or we can divide the diagram into various

areas, such that relay operation can be obtained only for certain relations between V, I, and

. That this is a most powerful tool will be seen later when we learn what various types of

abnormal system conditions “look” like on the R-X diagram.

THE SIGNIFICANCE OF Z

Since we are accustomed to associating impedance with some element such as a coil or a

circuit of some sort, one might well ask what the significance is of the impedance expressed

by the ratio of the voltage to the current supplied to a distance relay. To answer this

question completely at this time would involve getting too far ahead of the story. It

depends, among other things, on how the voltage and current supplied to the relay are

obtained. For the protection of transmission lines against short circuits, which is the largest

field of application of distance relays, this impedance is proportional, within certain limits,

to the physical distance from the relay to the short circuit. However, the relay will still be

energized by voltage and current under other than short-circuit conditions, such as when

a system is carrying normal load, or when one part of a system loses synchronism with

another, etc. Under any such condition, the impedance has a different significance from

that during a short circuit. This is a most fascinating part of the story, but it must wait until

we consider the application of distance relays.

At this point, one may wonder why there are different types of distance relays for

transmission-line protection such as those described. The answer to this question is largely

that each type has its particular field of application wherein it is generally more suitable

than any other type. This will be discussed when we examine the application of these

relays. These fields of application overlap more or less, and, in the overlap areas, which

relay is chosen is a matter of personal preference for certain features of one particular type

over another.

PROBLEMS

1. On an R-X diagram, show the impedance radius vector of a line section having an

impedance of 2.8 + j5.0 ohms. On the same diagram, show the operating characteristics of

an impedance relay, a reactance relay, and a mho relay, each of which is adjusted to just

operate for a zero-impedance short circuit at the end of the line section. Assume that the

center of the mho relay’s operating characteristic lies on the line-impedance vector.

Assuming that an arcing short circuit having an impedance of 1.5 + j

ohms can occur

anywhere along the line section, show and state numerically for each type of relay the

maximum portion of the line section that can be protected.

2. Derive and show the operating characteristic of an overcurrent relay on an R-X diagram.

76 DISTANCE RELAYS

3. A current-voltage directional relay has maximum torque when the current leads the

voltage by 90°. The voltage coil is energized through a voltage regulator that maintains at

the relay terminals a voltage that is always in phase with–and of the same frequency as–the

system voltage, and that is constant in magnitude regardless of changes in the system

voltage.

Derive the equation for the relay’s operating characteristic in terms of the system voltage

and current, and show this characteristic on an R-X diagram.

4. Write the torque equation, and derive the operating characteristic of a resistance relay.

BIBLIOGRAPHY

1. “A Comprehensive Method of Determining the Performance of Distance Relays,” by

J. H. Neher, AIEE Trans., 56 (1937), pp. 833-844, Discussions, p. 1515.

2. “A Distance Relay with Adjustable Phase-Angle Discrimination,” by S. L. Goldsborough,

A1EE Trans., 63 (1944), pp. 835-838. Discussions, pp. 1471-1472.

“Application of the Ohm and Mho Principles to Protective Relays,” by A. R. van C.

Warrington, AIEE Trans., 65 (1945), pp. 378-386. Discussions, p. 491.

3. “The Mho Distance Relay,” by R. M. Hutchinson, AIEE Trans., 65 (1945), pp. 353-360.

WIRE-PILOT RELAYS 77

Pilot relaying is an adaptation of the principles of differential relaying for the protection

of transmission-line sections. Differential relaying of the type described in Chapter 3 is not

used for transmission-line protection because the terminals of a line are separated by too

great a distance to interconnect the CT secondaries in the manner described. Pilot

relaying provides primary protection only; back-up protection must be provided by

supplementary relaying.

The term “pilot” means that between the ends of the 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; frequently, such circuits are

rented from the local telephone company. A carrier-current pilot for protective-relaying

purposes is one in which low-voltage, high-frequency (30 kc to 200 kc) currents are

transmitted along a conductor of a power line to a receiver at the other end, the earth and

ground wire generally acting as the return conductor. A microwave pilot is an ultra-high-

frequency radio system operating above 900 megacycles. A wire pilot is generally

economical for distances up to 5 or 10 miles, beyond which a carrier-current pilot usually

becomes more economical. Microwave pilots are used when the number of services

requiring pilot channels exceeds the technical or economic capabilities of carrier current.

In the following, we shall first examine the fundamental principles of pilot relaying, and

then see how these apply to some actual wire-pilot relaying equipments.

WHY CURRENT-DIFFERENTIAL RELAYING IS NOT USED

Because the current-differential relays described in Chapter 3 for the protection of

generators, transformers, busses, etc., are so selective, one might wonder why they are not

used also for transmission-line relaying. The principal reason is that there would have to

be too many interconnections between current transformers (CT’s) to make current-

differential relaying economically feasible over the usual distances involved in

transmission-line relaying. For a three-phase line, six pilot conductors would be required,

one for each phase CT and one for the neutral connection, and two for the trip circuit.

Because even a two-wire pilot much more than 5 to 10 miles long becomes more costly than

a carrier-current pilot, we could conclude that, on this basis alone, current-differential

relaying with six pilot wires would be limited to very short lines.

5WIRE-PILOT RELAYS

78 WIRE-PILOT RELAYS

Other reasons for not using current-differential relaying like that described in Chapter 3

are: (1) the likelihood of improper operation owing to CT inaccuracies under the heavy

loadings that would be involved, (2) the effect of charging current between the pilot wires,

(3) the large voltage drops in the pilot wires requiring better insulation, and (4) the pilot

currents and voltages would be excessive for pilot circuits rented from a telephone

company. Consequently, although the fundamental principles of current-differential

relaying will still apply, we must take a different approach to the problem.

PURPOSE OF A PILOT

Figure 1 is a one-line diagram of a transmission-line section connecting stations A and B,

and showing a portion of an adjoining line section beyond B. Assume that you were at

station A, where very accurate meters were available for reading voltage, current, and the

phase angle between them for the line section AB. Knowing the impedance characteristics

per unit length of the line, and the distance from A to B, you could, like a distance relay,

tell the difference between a short circuit at C in the middle of the line and at D, the far

end of the line. But you could not possibly distinguish between a fault at D and a fault at

E just beyond the breaker of the adjoining line section, because the impedance between D

and E would be so small as to produce a negligible difference in the quantities that you

were measuring. Even though you might detect a slight difference, you could not be sure

how much of it was owing to inaccuracies, though slight, in your meters or in the current

and voltage transformers supplying your meters. And certainly, you would have difficulties

if offset current waves were involved. Under such circumstances, you would hardly wish to

accept the responsibility of tripping your circuit breaker for the fault at D and not tripping

it for the fault at E.

But, if you were at station B, in spite of errors in your meters or source of supply, or whether

there were offset waves, you could determine positively whether the fault was at D or E.

There would be practically a complete reversal in the currents, or, in other words,

approximately a 180° phase-angle difference.

What is needed at station A, therefore, is some sort of indication when the phase angle of

the current at station B (with respect to the current at A) is different by approximately 180°

from its value for faults in the line section A B. The same need exists at station B for faults

on either side of station A. This information can be provided either by providing each

station with an appropriate sample of the actual currents at the other station, or by a signal

from the other station when its current phase angle is approximately 180° different from

that for a fault in the line section being protected.

Fig. 1. Transmission-line sections for illustrating the purpose of a pilot.

WIRE-PILOT RELAYS 79

TRIPPING AND BLOCKING PILOTS

Having established that the purpose of a pilot is to convey certain information from one

end of a line section to another in order to make selective tripping possible, the next

consideration is the use to be made of the information. If the relaying equipment at one

end of the line must receive a certain signal or current sample from the other end in order

to prevent tripping at the one end, the pilot is said to be a “blocking” pilot. However, if one

end cannot trip without receiving a certain signal or current sample from the other end,

the pilot is said to be a “tripping” pilot. In general, if a pilot-relaying equipment at one end

of a line can trip for a fault in the line with the breaker at the other end closed, but with

no current flowing at that other end, it is a blocking pilot–otherwise it is like a tripping

pilot.

It is probably evident from the foregoing that a blocking pilot is the preferred–if not the

required–type. Other advantages of the blocking pilot will be given later.

D-C WIRE-PILOT RELAYING

Scores of different wire-pilot-relaying equipments have been devised and many are in use

today, where d-c signals in one form or another have been transmitted over pilot wires, or

where pilot wires have constituted an extended contact-circuit interconnection between

relaying equipments at terminal stations. For certain applications, some such arrangement

has advantages particularly where the distances are short and where a line may be tapped

to other stations at one or more points. However, d-c wire-pilot relaying is nearly obsolete

for other than very special applications. Nevertheless, a study of this type will reveal certain

fundamental requirements that apply to modern pilot-relaying equipments, and will serve

to prepare us better for understanding still other fundamentals.

Fig. 2. Schematic illustration of a d-c wire-pilot relaying equipment. D = voltage-restrained

directional (mho) relay; O = overcurrent relay; T = auxiliary tripping relay;

S = auxiliary supervising relay; PW = pilot wire.

80 WIRE-PILOT RELAYS

An example of d-c wire-pilot relaying is shown very schematically in Fig. 2. The relaying

equipments at the three stations are connected in a series circuit, including the pilot wires

and a battery at station A. Normally, the battery causes current to flow through the “b”

contacts of the overcurrent relay and the coil of the supervising relay at each station.

Should a short circuit occur in the transmission-line section, the overcurrent relay will

open its “b” contact at any station where there is a flow of short-circuit current. If the short-

circuit-current flow at a given station is into the line, the directional relay at that station

will close its “a” contact. The circuit at this station is thereby shifted to include the auxiliary

tripping relay instead of the supervising relay. If this occurs at the other stations, current

will flow through the tripping auxiliaries at all stations, and the breakers at all the line

terminals will trip. But should a fault occur external to the protected-line section, the

overcurrent relay at the station nearest the fault will pick up, but the directional relay will

not close its contact because of the direction of current flow, and the circuit will be open

at that point, thereby preventing tripping at the other stations. If an internal fault occurs

for which there may be no short-circuit-current flow at one of the stations, the overcurrent

relay at that station will not pick up; but pilot-wire current will flow through the supervising

auxiliary relay (whose resistance is equal to that of the tripping auxiliary relay), and

tripping will still occur at the other two stations. (The supervising relays not only provide

a path for current to flow so that tripping will occur as just described but also can be used

to actuate an alarm should the pilot wires become open circuited or short circuited.)

Therefore, this arrangement has the characteristics of a blocking pilot where the blocking

signal is an interruption of current flow in the pilot. However, if the overcurrent and the

supervising relays were removed from the circuit, it would be a tripping pilot, because

tripping could not occur at any station unless all the directional relays operated to close

their contacts, and tripping would be impossible if there was no flow of short-circuit

current into one end.

Fig. 3. Schematic illustration of a d-c wire-pilot scheme where information is transmitted

over the pilot. D = voltage-restrained directional (mho) relay; B = auxiliary blocking relay;

O = overcurrent relay; TC = trip coil; PW = pilot wire.