Distance protection relay

The distance protection relay is a protection unit in the field of electrical power engineering and is used for the safe operation of power grids . It represents a special form of network protection and is used, for example, in electrical three-phase synchronous machines , power transformers , maximum, high and medium voltage cables and overhead lines.

Basics

In order to guarantee optimal selectivity in a power network with a stable supply at the same time, the determination of the fault location is just as important as the determination of the fault type. The means by which the fault location can be determined depends on the type of network. In the meshed network , fault location can only be achieved through impedance comparison with a directional decision. This is done with distance-dependent impedance relays, commonly referred to as distance protection relays.

The term relay in this context is to be understood historically, as the first distance protection devices were built electromechanically in the first half of the 20th century and included, among other things, special relay designs such as moving coil relays with rectifiers . Except for the name, today's distance protection relays have nothing to do with an electromechanical relay - they are typically digitally operating devices with microcontrollers and various measured value inputs for the voltage and current values, various types of external switching contacts and control and configuration interfaces.

history

Historic electromechanical distance protection relay

In 1923, distance protection relays were first introduced in Germany by AEG and Dr. Paul Neyer AG used. The advantages of these protective devices became apparent very quickly in the course of the constant expansion and the associated meshing of the networks.

The constant further development of the distance protection relays led to different tripping characteristics. Relays with a continuous tripping characteristic have been replaced by relays with a broken characteristic . Later distance protection relays almost without exception had step characteristics acting in one direction. Today's digital protection can vary in each direction with the respective step characteristics. The definition of the distance protection is:

It is a time-graded protection that depends on the resistance and the direction of energy, the command time of which increases in stages as the distance between the relay installation location and the fault location increases.

The advantage of the distance protection relay is that there is no need to lay control and measuring lines to the opposite side of the equipment to be protected. The relay offers optimal operational safety and represents the best technical solution for high-voltage networks.

functionality

Which type of excitation is used depends on the impedance ratio at the installation location of the relay. The impedance ratio is the quotient of the source impedance at the feed location and the short-circuit impedance at the fault location.

Type of suggestion	Impedance behavior	criteria	Settings
Overcurrent	${\ displaystyle Z_ {Q} \ ll Z_ {K}}$	${\ displaystyle I_ {K} \ gg I_ {B}}$	${\ displaystyle I \ geq 2 {,} 0I_ {n}}$
Earth fault		${\ displaystyle U_ {E} \ gg 0}$	${\ displaystyle I_ {E} = 0 {,} 8I_ {n}}$ ${\ displaystyle U_ {E} = 0 {,} 2U_ {n}}$
undervoltage	${\ displaystyle Z_ {Q}> Z_ {K}}$	${\ displaystyle U_ {K} <U_ {B}}$	${\ displaystyle I_ {F} \ geq 0 {,} 7I_ {n}}$ ${\ displaystyle U_ {F} \ leq 0 {,} 6U_ {n}}$
Underimpedance	${\ displaystyle Z_ {Q} <Z_ {K}}$	${\ displaystyle \ phi _ {K} \ not = \ phi _ {L}}$	${\ displaystyle \ phi = \ phi _ {L}}$ ${\ displaystyle Z> Z_ {3}}$

${\ displaystyle Z_ {Q}}$ Source impedance
${\ displaystyle Z_ {K}}$ Short circuit impedance
${\ displaystyle Z_ {Q} / Z_ {K}}$ Impedance ratio

The type of excitation can be taken from the tendency of the impedance ratio. The overcurrent excitation is basically arranged in a distance protection relay, while undervoltage and underimpedance excitation are also used depending on the design. The ground fault excitation is only used in networks with non-effective neutral point treatment . Today's distance protection relays have pickup elements for each conductor including the neutral conductor. When using the types of stimuli, depending on the system configuration, it must be considered how many and which types of stimuli are used. If too many pickup types are activated at the same time, the protection may malfunction.

Distance measurement

Digital distance protection relay

The line impedance is basically dependent on the conductor cross-section, the type and arrangement of the conductors in the network system and the length of the line. The calculated or measured (normal) impedance of the line is set in the distance relay, as well as the (partial) impedances of the individual staggered zones / distances and the tripping times required for these, as well as the voltage levels and transformer ratio. In addition, the earth factor is set, which indicates the ratio of the impedance of the conductor to the impedance of the earth in the area of the line. The earth factor depends on the type of soil, the nature of the soil and the groundwater level. There are also many other setting options.

By measuring the current and voltage, the protection device constantly calculates the current impedance of the line.

If a fault now occurs in the network (e.g. short circuit), the impedance measured by the protective device changes due to the conductor short-circuit and the arcing resistance occurring at the fault location. The impedance measured by the relay is called short-circuit impedance, it is the geometric addition of the line impedance and the arc resistance. The measuring element in the relay compares the short-circuit impedance (correct loop impedance , because the short-circuit loop is now to be considered) with the set line impedance. Thanks to the relay's trigger characteristics, time graduations can be set in several stages depending on the distance to the fault.

In the event of errors that occur in the protection area of the distance protection relay, e.g. B. In the case of a three-pole short circuit with earth contact, the conductor currents and the conductor-earth current (unbalance current) are fed to the relay via current transformers and recorded by a selection circuit. At the same time, the line-to-line voltage and the line-to-earth voltage are recorded by the selection circuit via voltage transformers . Depending on the location of the fault and its direction, the relay compares the short-circuit impedance with the values set according to the step characteristic. Depending on the type of fault, the distance of the fault from the installation location of the relay, the transformation ratios of the current and voltage transformers and the direction of the short-circuit current, the timer integrated in the relay is activated via measuring and direction elements. After the timer has elapsed in the respective preset time stage or when a specified end time is reached, the circuit breaker is switched off and the faulty feeder is selectively disconnected from the network. With a relay type with a three-stage characteristic, three stage times can also be set. Settings for the end time and the limit time are also required. The end time can be set either direction-dependent or direction-independent. The limit time is always direction-independent. The adjustability of the times from 0 to 10s has proven to be sufficient in practice.

The 1st stage of the characteristic curve covers around 75% of the line length between 2 stations (e.g. between substations A and B). The other levels of the characteristic curve cover areas that go beyond the next (n) stations, so they can register and evaluate errors that may occur. U. lie on other lines. The distance protection relay provides the system parts that are behind the circuit breaker (LS) of the opposite station (if you consider the distance protection relay for line AB in station A, then these would be the system parts that are behind the LS in station B as seen from A) in A represents 2nd order backup protection.

The reason for this is the above-mentioned fact that the impedance zones (except for the 1st impedance zone) i. d. Usually extend beyond the actual cable length.

For reasons of redundancy , 2 distance protection relays are often used for important lines or those of the highest voltage - whereby 1 relay forms the main protection and the other relay forms the backup protection ( 1st order ) for the line AB.

The settings (impedance / graduation characteristic) are OK for both relays. d. Usually identical. (for comparison: with less important lines, a simpler UMZ relay often works as backup protection).

It must be pointed out that the previous consideration was based on station A. The forward direction for these relays (HS and RS relays) is in the direction of station B. In station B there is a corresponding arrangement, the relays there look in direction A, this corresponds to their forward direction. If the distance protection relays in both stations detect a fault in their respective forward direction, it is ensured that the fault is on line AB and not behind the next station.

Complex example

Assumption: The sketch should explain the functional sequence when suggested, it does not correspond to the actual structural conditions. Only one voltage level is considered here, the lines in the individual stations (e.g. substations) are connected to the same busbar. The behavior of the relays is only to be regarded as a possible example, given times and zones are only for understanding and often differ in reality. The same times are assumed for the individual impedance zones for all relays. The error is 10% of the line length DC viewed from station D. The circuit breaker operating time is 20 ms.

Scheme of a network section with several lines and assigned distance protection relays.

Error detection and tripping times
	Zone 1 0 s	Zone 2 0.5 s	Zone 3 2.5 s	Backward 7 s
Relay 1
Relay 2				X
Relay 3			X
Relay 4				X
Relay 5		X
Relay 6	X
Relay 7				X
Relay 8				X
Relay 9			X
Relay 10				X
Relay 11				X

If the error occurs, the error is recognized by all relays except relay 1. For relay 1 the error is too far away. The other relays recognize the error as follows:

Relays 2, 4, 7, 8, 10 and 11 detect errors in reverse direction.
Relays 3 and 9 detect errors in the forward direction zone 3,
Relay 5 detects errors in forward direction zone 2,
Relay 6 detects errors in forward direction zone 1,
In all relays that recognize the error, the time allocated to the individual zones or reverse direction begins to run down.

Ideally, this is what happens:

Relay 6 switches off the assigned circuit breaker after approx. 20 ms (tripping time + circuit breaker operating time). Relay 7 then no longer detects an error and drops out before the set time has expired.
- If relay 6 does not succeed in switching off the assigned LS, relay 7 triggers its LS after 7 seconds in reverse direction.
Relay 5 switches off its assigned circuit breaker in station C after approx. 0.5 s (i.e. a longer period of time since faults were detected in zone 2). The faulty line CD is thus switched off, all other activated relays drop out, since they can no longer detect any errors.
- If relay 5 does not succeed in switching off the assigned LS, relays 3 and 9 next switch off their LS. In addition, the relay 11 in any case switches off its LS in the reverse direction.
  - If relay 3 is unsuccessful, relays 2 and 4 next switch off their assigned LS.
  - If relay 9 is unsuccessful, relays 8 and 10 next switch off their assigned LS.

It is noticeable here that relays 3 and 9 switch before relays 4, 10 and 11, although the latter are closer to the fault location. The reason is that relays 4, 10 and 11 recognize the fault in the reverse direction and the time for faults in the reverse direction is normally set longer than that of the impedance zones in the forward direction. There are also settings that do not trigger in reverse.

Under certain circumstances it can happen that lines are switched off that are not faulty themselves, but can continue to feed a fault via the busbar in substations if other protective devices or LS fail.

The data are summarized again in the table above. The longer times only expire completely if the relays with the shorter times do not switch off the error (relay or switch failure). If everything works correctly, the more distant relays drop out before their longer time has expired.

Application limits of the distance protection

There are technical limits to the use of distance protection. These depend on the type of relay (electromechanical, static or digital), the type of line and its line impedance and the transformation ratio of the current transformer. The maximum line length is determined by the maximum measurable primary line impedance.

literature

W. Doemeland: Manual protection technology . Verlag Technik / VDE-Verlag GmbH, Berlin.
H. Koettnitz, G. Winkler, K. Weßnigk: Fundamentals of electrical operating processes in electrical energy systems . VEB German publishing house for basic industry, Leipzig.
G. Ziegler: Digital distance protection . Publicis Corporate Publishing, Erlangen.

Web links

Commons : Protective Relays - collection of images, videos and audio files

Individual evidence

^ Walter Schossig: Distance Protection: The Early Developments , PacWorld Winter 2008

[scho-1] Walter Schossig: Distance Protection: The Early Developments , PacWorld Winter 2008