Distance protection Scheme: An easy comprehensive Guide 101

Distance protection

It is basically a non-unit form of protection in the power system. It is economical and offers technical benefits. Unlike the phase, neutral overcurrent protection the key advantage of distance relay protection is that the fault coverage of the protected circuit is virtually independent of source impedance variations.

The basic principle

The basic principle of distance protection involves the division of the voltage at the relaying point by the measured current, i.e., the impedance. The apparent impedance so calculated is compared with the reach point impedance. If the measured impedance is less than the reach point impedance then it is perceived that a fault exists in the line between the relay and the reach point.

Let us suppose there is a transmission line AB having a source at A end only. Let us extend the protection zone to B. Let the distance relay be at A end, where the local current and voltage are fetched from CT and PT at A. Assume that the voltage-to-current ratio is 1 for simplicity.

The reach of the relay is thus made to be Z_set. The line is modeled as an R-L circuit for the purpose of relaying without much loss of accuracy.

Let’s consider three faults: an internal fault F1, an external fault F3, and a fault at reach point fault F2. Now, let us compare the relay voltage VR with the product of relay current IR and Z, for all three faults.

F₃, External current is I_R3 and voltage is V_R3, therefore V_R3 > I_R3 x Z_set

F₂, External current is I_R2 and voltage is V_R2, therefore V_R2 = I_R2 x Z_set

F₁, External current is I_R1 and voltage is V_R1, therefore V_R1 = I_R1 x Z_set

Now the desired response of the relay is to restrain from clearing the fault in case of F₃, on the verge of tripping in case of F_2, and will generate the trip signal in case of F₁

The reach point of a relay is the point along the line impedance locus that is intersected by the boundary characteristic of the relay.

Thus the trip logic for the above can be if V_R < I_R x Z_set then trip else restrain. The equation V_R < I_R x Z_set can also be written as V_R/I_R < Z_set. However, the ratio of V_R/I_R can be written as Z_R

The relay, therefore, somehow, must compute the impedance as seen from its location and compare it with a set value to make the trip decision. Because of the simple series model of the faulted line, the line impedance is directly proportional to the distance to the fault. Hence the name distance relay. Such a relay is called an under-impedance relay. In practice, however, the word under is dropped and the relay is simply called impedance relay.

Further, most of the faults involve an arc. The arc is resistive in nature. The arc resistance is a function of the spark over distance “S” in feet. the wind velocity “u” in mph, time “t” in seconds, and the current “I” in amperes. It is given by Warrington’s formula as:

R_arc = 8750 x (S + 3ut) / I^1.4

When we consider the arc resistance, the fault characteristic of the transmission line gets modified from a straight line with a slope to an area as shown.

Relay Performance

Distance protection relay performance is defined in terms of reach accuracy and operating time.

Reach accuracy

Reach accuracy is a comparison of the actual ohmic reach of the distance protection relay under practical conditions with the relay setting value in ohms. Reach accuracy particularly depends on the level of voltage presented to the relay under fault conditions. The impedance measuring techniques employed in relay designs also have an impact.

Operating time

Operating times can vary with fault current, with fault position relative to the relay setting, and with the point on the voltage wave at which the fault occurs. Depending on the measuring techniques employed in a particular relay design, measuring signal transient errors, such as those produced by Capacitor Voltage Transformers or saturating CTs, can also adversely delay relay operation for faults close to the reach point. It is usual for electromechanical and static distance relays to claim both maximum and minimum operating times. However, for modern digital or numerical distance relays, the variation between these is small over a wide range of system operating conditions and fault positions.

Zones of Distance protection:

In order to provide reliability, the distance protection relay is divided into several zones:

Zone 1

It is intended to cover 80% of the protected line and is set to operate instantaneously with no intention of time delay. For digital/numerical distance relays, settings of up to 85% may be safe. The resulting 15-20% safety margin ensures that there is no risk of the Zone 1 protection over-reaching the protected line due to errors in the current and voltage transformers, inaccuracies in line impedance data provided for setting purposes, and errors in relay setting and measurement. Otherwise, there would be a loss of discrimination with fast operating protection on the following line section. As 80-85 % is covered under zone 1, it is an under-reach element.

Zone 2

To ensure full cover of the line with allowance for the sources of error, the reach setting of the Zone 2 protection should be at least 120% of the protected line impedance. It provides backup for the faults on the adjoining lines. It is also set to cover remote end bus bar faults. Hence it is called an over-reach element.

Zone 3

Primarily set to provide backup against external uncleared faults and hence it is set to cover the longest adjoining line. Remote backup protection for all electrical faults on adjacent lines can be provided by a third zone of protection that is time-delayed to discriminate with Zone 2 protection plus circuit breaker trip time for the adjacent line.

Characteristics of Distance Protection Relay

Impedance Characteristics

The impedance relay measures the impedance of the line at the relay location. On the protected line section, when a fault occurs, the measured impedance is the impedance of the line section between the location of the relay and the point at which the fault has occurred. The impedance is proportional to the length of the line and hence, to the distance along the line. In the distance relaying terminology, the term impedance includes both resistance as well as reactance.

The microprocessor computes line impedance at relay locations using Irms and Vrms. This type of computation can be performed in many ways. One of the techniques utilizes the Idc and Vdc for computation as these are proportional to Irms and Vrms. Vac and Iac are rectified using rectifiers to obtain Vdc and Idc. These rectifiers employ Integrated circuits and diodes. Now line impedance is computed and the microprocessor issues a trip signal to the circuit breaker if the fault point lies within its protected section.

Reactance Characteristics

A reactance relay measures the reactance of the line at the relay location, and variations in resistance do not affect it. Hence, its performance remains unaffected by arc resistance also during the occurrence of a fault. In case of a fault on the line, the measured reactance is the reactance of the line between the relay location and the fault point. Its characteristic on the R-X diagram is a straight line, parallel to R-axis

MHO Characteristics

A MHO relay measures a component of admittance. But its characteristic, when plotted on the R-X diagram, it is a circle, passing through the origin. It has inherently a directional bias as it detects the fault in the forward direction only. It is also called an admittance or angle admittance relay. Since its characteristic is a straight line when plotted on an admittance diagram, it is called an MHO relay.

OHM Characteristics

An angle impedance relay measures a component of the impedance of the line at the relay location. It is also called an ohm relay. Its characteristic on the R-X diagram is a straight line and it is inclined to the R-axis at an angle. The reactance relay is also a particular case of an angle impedance relay. It is also used in conjunction with other relays, for example, it is used to limit the area of the MHO relay on the R-X diagram to make it less sensitive to power surges.

Other functions of Distance protection relay include:

Power Swing Blocking

Distance protection relay operates by detection of impedance in its zone, and sometimes voltage and current in the system are disturbed by fault. System impedance also changes and if the impedance moves into the relay’s zone it will trip. To prevent this situation, the distance protection relay uses PSB. By detecting the rate of change of the impedance, the relay will know which one is a fault and which one is a power swing, and block itself from tripping.

Fuse failure

Distance protection relay calculates impedance by the ratio of voltage to current. If the voltage goes to zero, the impedance will be zero also. Zero impedance means the fault is very close to the distance relay and it should trip the transmission line.

PT fuse blows can make distance relay see ‘zero impedance’ despite no fault in the high voltage system. Distance relays use the ‘zero sequence concept’ to protect themselves from misoperation.

Switch On to Fault (SOTF)

For safety in transmission line maintenance, the line should be grounded for all 3 phases. After finishing the maintenance work, sometimes ground wires are forgotten to be removed from the line. When CB is closed, it closes on to fault. Distance relay use healthy voltage for reference, so when we close into 3-phase faults, no voltage reference will be there at all. It is possible that all the zones are not tripped. Memory feature is now used to make high-speed trips instead. It is the SOTF function of distance protection relay 21.

Fault locator

It measures the distance between the relay location and fault location in terms of Z in Ohms, or length in KM or percentage of line length. This gets the same inputs as the distance relay connected in series with the main relay. The measurement is initiated by a trip signal from distance relays. The fault locator gives the exact location of the fault, thereby reducing the time of restoration.

FAQ’s