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In this article we will discuss about distance relays and its classification.

**Introduction to Distance Relays:**

Distance protection is the name given to the protection, whose action depends upon the distance of the feeding point to the fault. The time of operation of such a protection is a function of the ratio of voltage and current i.e., impedance. This impedance between the relay and the fault depends upon the electrical distance between them.

Distance-relay group is perhaps the most interesting and versatile family of relays.

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**Principal types of distance relays are:**

(i) Impedance relays

(ii) Reactance relays

(iii) Admittance or mho relays.

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Distance relays differ in principle from other forms of protection in their performance and is not governed by the magnitude of the current or the voltage in the protected circuit but rather on the ratio of these two quantities. Distance relays are actually double actuating quantity relays with one coil energized by voltage and the other coil by current. The current element produces a positive or pick-up torque while the voltage element produces a negative or reset torque. The relay operates only when the V/I ratio falls below a predetermined value (or set value).

During a fault on a transmission line the fault current increases and the voltage at the fault point decreases. The V/I ratio is measured at the location of CTs and PTs. The voltage at PT location depends on the distance between PT and the fault. If the fault is nearer, measured voltage is lesser and if the fault is farther, measured voltage is more. Hence assuming constant fault impedance each value of V/I measured from relay location corresponds to distance between relaying point and the fault along the line. Hence such protection is called the distance protection or impedance protection.

Distance protection is non-unit type protection, the protection zone is not exact. The distance protection is high speed protection and is simple to apply. It can be employed as a primary as well as backup protection. It can be employed in carrier aided distance schemes and in auto-reclosing schemes. Distance protection is very commonly used in protection of transmission lines.

Distance relays are used where overcurrent relaying is too slow and is not so selective. Distance relays are used for both phase fault and ground fault protection and they provide higher speeds for clearing faults than overcurrent relays. Distance relays are also independent of changes in magnitude of the short-circuit currents and hence they are not much affected by changes in the generation capacity and the system configuration. Thus they eliminate long clearing times for faults near the power sources required by overcurrent relays if used for the purpose.

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**Application of Distance Protection: **

Distance protection schemes are widely employed for the protection of high voltage ac transmission lines and distribution lines. They have replaced the overcurrent protection of the transmission lines. The reasons are faster protection, simpler coordination; simpler application, permanent setting without need for readjustments; less effect of amount of generation and fault levels, fault current magnitude; permits the high line loading.

Static distance relays have superior and versatile characteristics and enlist several additional merits of static distance protection schemes.

Distance protection schemes are commonly employed for providing the primary or main protection and backup protection for ac transmission and distribution lines against 3-phase faults, phase-to-phase faults and phase-to-ground faults.

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In some schemes for short lines, the phase-to-ground fault protection sensing may be by distance relay and measurement by overcurrent relays because distance protection for shorter lines are susceptible to errors due to arc fault resistance. In general, the choice of type of distance protection depends on length of line, tripping time required and coordination requirements.

Today trend is toward the use of static distance protection for all types of line faults, main and backup for short, medium and long transmission lines.

**Choice between Impedance Type, Reactance Type and Mho Type Relays: **

The choice between impedance types, reactance type and mho type relays should be properly done with reference to their application to transmission line protection.

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The ground resistance is variable. The ground distance relays must be unaffected by variation in fault resistance. Reactance type relays are preferred for ground fault relaying. For short line sections, reactance type relays are preferred as they are not affected by the arc resistance and more percentage of the line can be protected at high speed. The drawback of reactance relays, however, is that they are likely to operate undesirably on severe synchronising power swings at certain locations unless additional relay equipment is provided.

Mho type relays are best suited for phase fault relaying for long lines and particularly where a severe synchronising power surge may occur. They are most sensitive of all the distance relays. The mho relay is reliable because it combines both the directional and distance measuring functions in one unit.

The impedance type relay is suitable for phase fault relaying for lines of moderate length. The arc affects the impedance relay more than the reactance relay but less than the mho relay. The impedance relay along with the directional unit can be made to change its characteristics and the modified impedance relay may resemble the reactance or mho unit. There is considerable overlap in the application of these relay.

**Classification of Distance Relays****: **

**Distance relays used for the protection of power circuits may be divided into two groups viz.:**

(i) Definite distance relays and

(ii) Time-distance relays.

Definite distance relays operate instantaneously when the impedance (reactance or admittance) falls below a specified value. These relays may be of impedance, reactance or mho type.

Time-distance relays have the time of operation dependent upon the value of impedance (reactance or admittance) i.e., upon the distance of the fault from the relay point. A fault nearer to the relay will operate it earlier than a fault farther away from the relay. These relays may be of impedance, reactance or mho type.

**Reactance Type Distance Relay:**

Reactance type relay is a high speed relay. It consists of two units-an overcurrent element developing positive torque and a current-voltage directional element which either aids or opposes the overcurrent element depending on the phase angle between the current and voltage. This means, a reactance relay is an overcurrent relay with directional restraint. The directional element is arranged to develop maximum negative torque when its current lags behind its voltage by 90°. The induction-cup or double-induction loop structures are best suited for actuating reactance type distance relays.

A typical reactance relay using induction cup structure is shown in Fig. 3.53 (a). It has a 4-pole structure carrying operating, polarizing, and restraining coils, as shown in the figure. The operating torque is developed by the interaction of fluxes due to current carrying coils i.e. interaction of fluxes of poles 2, 3 and 4 and the restraining torque is produced by the interaction of fluxes due to the poles 1, 2 and 4. Thus operating torque will be proportional to I^{2 }while the restraining torque will be proportional V I cos (θ – 90°). The desired maximum torque angle is obtained with the help of resistance-capacitance circuits, as illustrated in the figure.

If the spring control effect is indicated by – K_{3}, the torque equation becomes –

T = K_{1} l^{2} – K_{2} VI cos (θ – 90°) – K_{3} …(3.24)

Where, θ is defined as positive when I lags behind V

or T = K_{1} l^{2} – K_{2} VI sin θ – K_{3 }

At balance point, the net torque is zero, and hence-

K_{1} I^{2} – K_{2} VI sin θ – K_{3} = 0

or K_{1 }I^{2} = K_{2} VI sin θ + K_{3}

or K_{1} = K_{2} V/I sin θ + K_{3}/I^{2}

or V/I sin θ = K_{1}/K_{2} – K_{3}/K_{2}I^{2}

or Z sin θ = X = K_{1}/K_{2} if the spring control effect is neglected.…(3.25)

The characteristic of a reactance relay will be such that for all the positions of the impedance phasors, whose heads lie on this characteristic will have constant X component i.e., the characteristic will be a straight line parallel to R-axis, as shown in Fig. 3.53 (b). X is the reactance of the protected line between the relay location and the fault point.

The significant point about this characteristic is that the resistance component of the impedance has no effect on the operation of the relay, the relay responds solely to the reactance component. Any point below the operating characteristic—whether above or below the R axis, will lie in the positive torque region. If the value of τ, in the general torque equation T = K_{1}l^{2} – K_{2}V I cos (θ – τ) – K_{3}, is made any other than 90°, a straight line operating characteristic will still be obtained, but it will not be parallel to R-axis. Such a relay is called an “angle impedance relay”.

This relay, as can be seen from its characteristic, is a non-directional relay. It is not capable of discriminating when used on transmission lines, whether the fault has taken place in the section where the relay is located or it has taken place in the adjoining section. The directional unit used with the reactance relay will not be same as used with the impedance type relay because the reactance relay will trip even under normal operating conditions at or near unity power factor.

This is so because the restraining reactive volt-ampere in that case will be nearly equal to zero. Therefore the reactance type distance relay needs a directional unit that is inoperative under normal load conditions. The type of the unit employed for this purpose has voltage-restraining element opposing the directional element, and it is called an “admittance” or “mho” unit or relay.

Reactance type relay is very suitable as a ground relay for ground fault because its reach is not affected by fault impedance (i.e., arcing resistance).

**Mho Type Distance Relay****: **

Mho relay is a high speed relay and is also known as admittance relay. It is also sometimes called an angle impedance relay. In this relay operating torque is obtained by the volt-ampere element and the restraining torque is developed due to the voltage element. It means a mho relay is a voltage restrained directional relay.

A typical mho relay using induction cup structure is shown in Fig. 3.54 (a). The operating torque is developed by the interaction of fluxes due to poles 2, 3 and 4 and the restraining torque due to poles 1, 2 and 4.

If the spring control effect is indicated by – K_{3}, the torque equation becomes –

T = K_{1}V I cos (θ – τ) – K_{2}V^{2} – K_{3} … (3.26)

Where, θ and τ are defined as positive when I lags behind V

At balance point, the net torque is zero, and hence

K_{1}V I cos (θ – τ) – K_{2}V^{2} – K_{3} = 0

or K_{1}/K_{2} cos (θ – τ) – K_{3}/K_{2}VI = V/I = Z

or Z = K_{1}/K_{2} cos (θ – τ) ….(3.27)

If the spring control effect is neglected.

The above equation is similar to that of a directional relay when the control-spring effect is taken into account but the difference is that there is no voltage term in the expression. Hence the relay has but one circular characteristic. The operating characteristic described by Eq. (3.27) is given in Fig. 3.54 (b). The diameter of this circle is practically independent of V or I, except at very low magnitudes of current or voltage when the control-spring effect is considered, which causes the diameter to decrease. The diameter of the circle is given as –

K_{1}/K_{2} = Z_{R} = ohmic setting of the relay.

The relay operates when the impedance seen by the relay falls within the circle. It is the fact that the circle passes through the origin which makes the relay inherently directional. The relay because of its inherently directional characteristic needs only one pair of contacts which makes it fast tripping for fault clearance and reduces the VA burden on the CTs. The impedance angle of the protected line is normally 60 to 70° which is shown by line OC in Fig. 3.54 (b).

The arc resistance R, is represented by length AB which is horizontal to OC from the extremity of the chord Z. By making τ equal to, or little less lagging than θ, the circle is made to fit very closely around the faulty area so that the relay is insensitive to power swings and therefore particular applicable to the protection of long or heavily loaded lines.

The Eq. (3.27) can also be written as –

1/Z cos(θ – τ) = K_{2}/K_{1 }

or Y cos (θ – τ) = K_{2}/K_{1} = Constant …(3.28)

where Y is the admittance in mho

For a given relay τ is constant, and the heads of the admittance phasor Y will lie on a straight line. The characteristic of mho relay on admittance diagram is, therefore, a straight line.

Mho relay is suitable for long EHV/UHV heavily loaded transmission lines as its threshold characteristic in Z-plane is a circle passing through the origin and its diameter is Z_{R} i.e., K_{2}/K_{1}. Because of this, the threshold characteristic is quite compact enclosing fault area compactly and hence, there is lesser chance to operate during power swing and also it is directional.

**Example: **

A line section has an impedance of 3.6 + j6 ohms. Show it on R-X diagram as impedance phasor. If the relay is adjusted to operate for zero impedance short circuit at the end of the line section, show on the same R-X diagram operating characteristics of-

(i) An impedance relay

(ii) A reactance relay and

(iii) Admittance or mho relay, used for the purpose.

Assume that the centre of the mho relay operating characteristic lies on the line impedance phasor. If an arcing short circuit occurs having an impedance of 2 + j 0 Ω anywhere along the line, determine for each type of distance relay the maximum portion of the line that can be protected.

**Solution: **

The characteristics of the relays are shown in Fig. 3.56. OA is the impedance phasor of (3.6 + j 6) ohms; OB = 3.6 units and BA = 6 units. The circle with O as centre and OA as radius represents the characteristic of impedance relay. A straight line drawn parallel to the R-axis at a distance of 6 units corresponding to the reactance of the line represents the characteristic of the reactance relay. The circle drawn on OA as diameter represents the characteristic of the admittance or mho relay for the protection of the given line.

If the impedance of short circuit due to arcing is (2 + j 0) ohms, this is represented by taking OD = 2 units and drawing a line parallel to the impedance phasor OA cutting the mho relay characteristic at F and impedance relay characteristic at E. Then draw EM and FN parallel to R-axis to cut impedance phasor OA at M and N respectively.

Then (OM/OA) x 100 gives the percentage of line protected by the impedance relay and (ON/OA) x 100 gives the percentage of line protected by the mho relay.

On measurement the impedance relay is found to protect 82% of the line while the mho relay is found to protect 77% of the line.

The reactance relay is unaffected by the presence of the arc resistance and hence even with arc present, the percentage of the line that is protected by the reactance relay is 100%.