Protective relay

The distance 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, high, high and medium voltage cables and overhead lines.

Basics

To ensure optimum selectivity in a power grid and a stable supply, an error, the fault location is as important as the determination of the fault type. Can be the means by which the fault is determined, depends on the type of network. In a meshed network fault location can only be achieved by impedance compared with directional decision. This is done using distance-based impedance relay, generally referred to as the distance protection relay.

The term relay in this context is to be understood historically, as in the first half of the 20th century first distance protection devices were constructed Electromechanical and included, among other things, specific types of relays such as the Drehspulrelais with rectifiers. Have Today's distance relays, nothing to do except for the name with an electromechanical relay - They are typically digitally working devices with microcontrollers and different measurement inputs for the voltage and current values ​​, various switching contacts to the outside and control and configuration interface.

History

In 1923, distance relays have been used in Germany for the first time. The advantages of these protective devices made ​​very quickly felt in the course of the constant enlargement and the associated interconnection of the networks.

The continuous development of distance relays led to various tripping characteristics. Relay with constant tripping characteristic were replaced by relay with a broken curve. Later distance relays were almost exclusively unidirectional stages characteristic. Today's digital protection may vary with characteristics of the respective steps in each direction. As a definition of the distance protection applies:

The advantage of the distance relay is located in the elimination of the transfer of control and measurement lines to the opposite side of the protected resource. The relay provides optimal reliability and provides the best technical solution for high voltage networks dar.

Operation

What type of excitation is used, depends on the impedance ratio at the location of the relay. The impedance ratio is the ratio of the source impedance at the feed point and the short-circuit impedance at the fault.

• Source impedance
• Short-circuit impedance
• Impedance ratio

From the tendency of the impedance ratio of the kinds of excitation can be removed. The overcurrent pickup is basically arranged in a distance relay, during undervoltage and underimpedance additionally be used depending on the design. The Erdschlussanregung is only used in networks with non-active neutral-point. Today's distance relays have pickup elements for each conductor including neutral. When using the excitation modes has to be considered depending on the system configuration, how many and what types of excitation are used. With too many simultaneously activated excitation modes may lead to failure of protection.

Distance measurement

The line impedance is fundamentally dependent on the wire size, type and arrangement of the wires in the mains system and the length of the line. The calculated or measured (normal) impedance of the line is set in the distance relay, further, the (partial) impedances of the individual relay zones / distances and want for this trip times and the voltage levels and transformer ratio. In addition, the residual compensation is set, this indicates the ratio of the impedance of the conductor to the impedance of the earth in the field of management to. The residual compensation is dependent on soil type, soil and ground water level. There are also many more options available.

By measuring the current and voltage protection device continuously calculates the actual impedance of the line.

Now occurs in the network of a fault ( eg short circuit ), results in the change of the measured by the protection device impedance due to the conductor circuit and the resulting at the fault arc resistance. The now measured by the relay impedance is called short-circuit impedance, it is the geometric addition of the line impedance and arc resistance. The measuring element in the relay compares the short-circuit impedance (real loop impedance, because now the short-circuit loop is considered ) with the set line impedance. Time graduations can thus be adjusted in multiple stages as a function of the distance to fault by the trip characteristics of the relay.

For errors that occur within the scope of distance relays, eg during three -phase short circuit with earth contact, the relay with current transformers are supplied to the phase currents and the phase-earth current ( unbalance ) and detected by a selection circuit. At the same time the phase-phase voltage and the phase-earth voltage are recorded by the selection circuit using voltage transformers. Depending on the location of the fault and its direction now compares the relay, the short-circuit impedance with the values ​​set by the levels characteristic. Depending on the fault, the removal of the error from the installation location of the relay, the ratios of the current and voltage transformers and the direction of the short-circuit current in the integrated relay timer is activated via measurement and directional elements. After the expiration of the timer in the respective preset timer or reaching a specified end time it comes for switching off the circuit breaker, and the faulty feeder is selectively separated from the network. In a relay type with a three-level characteristic three stage times are adjustable. Further settings for the end times and the time limit are necessary. Here, the end time can be set either direction dependent or independent of direction. The time limit is always directional. The adjustability of the time from 0 to 10 seconds has been found in practice to be sufficient.

Through the first stage of the curve about 75 % of the line length between two stations to be covered (eg, btw substations A and B). The other stages of the characteristic cover areas that ignore stations over the next ( n ), so they can register errors and evaluate which may are on other lines. For the plant parts that are behind the circuit breaker ( CB ) of the remote station ( considering the distance relays for the line AB at station A, then that would be the parts of the system that are seen from A behind the LS in station B ), represents the distance relays in A represents a back-up protection 2nd order

This is due to the above- mentioned fact that the impedance zones ( except the first impedance zone ) usually extend beyond the actual line length beyond.

Often two distance protection relays are used for redundancy for important lines or those of high voltage - in which case the main protection relay 1 and the other relay is the backup protection ( 1st order ) for the line AB.

The parameters ( impedance / grading characteristic ) is common to both relays usually identical. (compared with the less important lines often does a simpler definite time relay ( s network protection / DT) as a back-up protection ).

It should be noted to the fact that it was assumed in the previous viewing station A. Forward direction for this relay ( HS and RS- relay) is in the direction of station B. Station B, in turn, an appropriate arrangement, the relay there look in direction A, which corresponds to its forward direction exists. Set the distance relays in both stations a mistake in their forward direction is determined, to ensure that the error on line AB and is not around the next station.

Complex Example

Assumption: This proposal should explain the functional sequence when excited, they do not correspond to the actual building conditions. It is considered here only one voltage level, the lines are connected in the individual stations (eg substations ) to the same busbar. The behavior of the relay is to be regarded only as a possible example, specified times and areas are for the understanding and soft in reality from often. For all relay the same time are assumed for the individual impedance zones. The fault is considered by station D from 10% of the line length DC.

0 s

0.5 s

2.5 s

7 s

When the fault occurs, the error is detected by all the relays except relay 1. The error is too far away for relay 1. The other relays detect the fault as follows:

• Relay 2, 4, 7, 8, 10 and 11 detect errors in the reverse direction.
• Relays 3 and 9 detect errors in forward direction zone 3,
• 5 relay detects fault in the forward direction zone 2,
• 6 relay detects fault in the forward direction zone 1,
• All relays, will detect the failure, begins with the individual zones or the reverse direction allocated time to expire.

Ideally, the following happens:

• 6 relay switches the associated circuit breaker after about 20 ms from (operation time breaker operating time). This recognizes relay 7 no more mistakes and falls off before the set time has expired.
• 5 relay switches its associated power switch in station C after about 0.5 s from ( therefore longer time, because an error was detected in zone 2). Thus, the faulty line CD is off, all other excited relays drop out because they can not see any more mistakes.
• If it is possible relay 5 not turn off the associated LS, switch next relay 3 and 9 from their LS. If relay 3 is unsuccessful, turn next relay 2 and 4 from their associated LS. If relay 9 is unsuccessful, turn next relay from 8 and 10 their associated LS. Upon failure of relay 5 changes to any event, the relay 11 in the reverse direction from its LS. This happens regardless of the behavior of the relay 9 and 3
• If it is possible relay 6 is not to eliminate the associated LS, it switches the relay next 7 in the reverse direction from its LS.
• It is striking here that relay 3 and 9 turn before relay 4, 10 and 11, although the latter are closer to the fault. The reason is that the relay 4, 10 and 11 detect the fault in the reverse direction and the time for fault in the reverse direction in the normal case is set longer than the impedance of the zone in the forward direction. There are also settings in which in the reverse direction is not triggered.

So it can happen under certain circumstances, that lines are switched off, which are not themselves subject to error, but can feed a further error on the bus in substations when other protection devices or LS fail.

The data are again summarized in the table above. The longer times run from only complete when the relay with the shorter times the error does not turn off ( relay or switch failure). Everything works correctly, then fall off the more distant relay before the longer time has elapsed.

Operating limits of the distance protection

The use of the distance protection are technically set specific limitations. These are dependent on the type of the relay ( electro-mechanical, static, or digital), the type of pipe and its line impedance and the ratio of the current transformer. The maximum cable length is determined by the primary line impedance maximum measurable.