Current cutoff

Current cutoff - a kind of relay protection, consisting in the de-energized circuit in the event of a short circuit on the line. Thanking Shabad MA, proceed.

General Definitions

More Edison fuses used to protect networks against short circuit. Some historians believe the first machines are among his inventions. But the authors were unable to find evidence of this. With regard to the protection relay, detected elementary ignorance of certain people. For example, the responses Mayl.ru man asked what current cutoff different from overcurrent. The definitions are similar, but different purposes!

• Current cutoff is called an immediate disconnection of the protected circuit portion in the event of a short circuit.
• Overcurrent protection is characterized in that the circuit complexity; guards, sometimes branched. Release will be delayed - allowing systems downstream of the line disconnected before. Then overcurrent protection does not take anything. If the situation is heating up, after a preset time is de-energized the entire thread.

This is easily illustrated by the example of panel flats. For example, in a bathroom set outlet (not closer than a predetermined distance from the source of moisture) and protected differential automaton. The apartment is protected against short circuit at the input to the panel. Machine 63 A, for example, if the sensitivity is too high (class A or B), is able to cut room earlier than react on the differential protection. Then the owner will leave without light the whole family. Consequently, at the entrance to the apartment supposed to organize the current protection to enable the machines behind it to make the case, cut down a single room.

In industry, the wise men manage to break the power line, which current cutoff is responsible for its own segment. If a short circuit in the neighborhood, she did not respond. Overcurrent protection is the local alternate embodiment for the apparatus. If you do not run for the local machine, the power is removed after a short delay. This is called remote backup, overcurrent protection devices have the right to be away from the scene of the accident. In the complex are two types of preservative systems are called two-stage overcurrent protection. Both are characterized by a number of qualities:

1. Selectivity - the ability to separately only required to respond to an accident. Sometimes quality is called selectivity.
2. Sensitivity. It relies on the possibility to extend the protection systems along the line. What is not always possible to carry out in respect of extended systems. Due to the remoteness of the sensors do not capture the moment of the accident.
3. Performance is provided to disable the protected area in the shortest time. Given the above the need to allow time for the downstream stages of the system to perform work before.
4. Reliability is treated as non-repudiation.

The executive part

Both types of events are organized with the help of the maximum relays that to protect the theory are divided into:

1. Primary and secondary.
2. Direct and indirect action.

The primary relay called direct acting variety, where the contactor coil and incorporated directly into the protection circuit. Controlled by the current consumption of the equipment and the pluck. direct primary relays are widely used in circuits up to 1 kV. With an increase in voltage class up to 10 kV portion of the secondary relay direct action. This means - measuring transformer is used to relieve the amount of current from the protected circuit. The contactor connected in series with the load. This consumption is greatly reduced, decreased insertion device in reactive power circuit.

Secondary relay indirect used where irrational to try to change the bulky contactor through a low-power current transformer. When large consumer currents and high voltage classes arc extinguished with difficulty, it is necessary to use special measures. The primary winding of the current transformer consists of 1-2 turns or halves without providing strong control signal. We have to use a pointing relay, electromagnetic relay executive team.

Power contactor coil is performed by an additional low-voltage network or battery. Then, the drive current is called operational, is used exclusively for activating the protection circuit.

Overcurrent relays are manufactured with built-in delay or without. In the latter case, no schema refinement are suitable only for the current cut-off can be used in tandem with a timer. And then it becomes possible overcurrent protection. The latter case provides greater flexibility, manufacturers can not predict all the features, therefore, does not determine the correct operation of the delay. Characteristics of such a system is said to be independent from the current, it operates without regard to its magnitude during a short circuit on the line. There is an analogue of the electromagnetic circuit breaker level apartment.

Overcurrent relay Slow action often constructed so that the response time is the smaller, the more will flow in amps circuit. Consequently, characterized dependent characteristic. Modern circuit breakers resemble a combined class of equipment, relays with limited dependent feature. When triggered above a certain threshold is instantaneous, and below it - with delay. For example, A. Zemskov showed that modern machines are able to work for an hour at an overload of 45% before the power is lost.

Protecting the dependent characteristic is often used in circuits with 20 kV voltage class. It is combined with fuses, a short length shows the dependence characteristics. High-voltage line, typically exhibit independent protection. If you need a delay, it is recommended to use the time switch (timer). Current cutoff is constructed so as not to short-circuit work further down the line. If we take the example of the flats shield, the situation is provided by the inclusion of a sequence of two machines:

1. 63 And entering into the faceplate.
2. 16 A outlet.

Obviously, the more sensitive the machine is considered a lower nominal value, triggered earlier. Although the example does not differ high clarity, but gives an idea as to how the selectivity cutoff current systems. At the same time it entered the postulate is impossible to protect the entire line at once.

Current cutoff: the relay of the circuit

In implementing the scheme considering all kinds of short circuits. Sometimes fail to recognize such situations on the current value, then in the course are negative and zero sequence relay. Standard current cutoff circuit used:

1. Incomplete star. It covers by two or three relays only two phase network. Is often used in circuits 35 kV with isolated or compensated star point (where small leakage currents to ground).
2. Full star. The phases are covered by two, three or four relays. Often used in the networks of 110 kV gluhozazemlonnoy neutral and much skew on phases (large currents flowing in the ground).
3. Triangle. The system of two or three relays, measuring line voltages. More common in the transformer star-delta circuits.
4. A two-phase circuit with one relay in practice is rare. Colloquially referred to as the Group of Eight, in old literature - incomplete triangle. It protects engines of small capacity.

Consider, for example, how a star is incomplete (see. Fig.), At which the current transformers included in the two lines - A and C. Possible cases of behavior of the system:

1. Short-circuit all the phases leads to a situation where no current, and in the other branches of the value is large in the reverse wire (PT3). There is an operation.
2. In interphase circuit A and C is similar.
3. Other types of short-circuit causing phase shift, a current flows in the reverse wire. He estimated PT3 relay, giving the command to the power supply gap.

Lack of incomplete stars - it is essentially not able to track the short to ground phase V. As a result, such protection is not acceptable for circuits with large leakage currents to ground. The current cut-off systems are frequent visitors intermediate relay with powerful contactors. When relying turn off the power quickly, it requires special qualities. Most of the overcurrent relay is not able to handle the operational shutdown circuit.

Unlike full Star: track any possible short circuits, phase transfer and leakage to the ground. Common wire here is not called reflux, and zero: contains relays catching currents and neutral earthing main line. With other types of short-circuit the load is low. Full star applied on line voltage of 110 kV class and gluhozazemlonnoy neutral. Established:

1. The chains from 3 to 35 kV to the ground leakage current is small, it makes no sense to break off the power completely. Used incomplete star.
2. For networks of 110 kV and above are often used instead of overcurrent Distance protection. Additional two reasons:

• When isolated in the neutral line 110 kV and current transformers are used for the organization of the differential protection. As a result, the secondary windings are connected to a triangle (not star).
• The second reason for the non-applicability - single-phase ground fault lines do not have to cause tripping. It is not considered an accident, work is ongoing on-site accident repair team.

When the triangle arguments "against" invalid listed above. This scheme is often used particularly for networks with voltages above 35 kV class. Triangle lack of good neutral, large short-circuit currents on the ground are not converted into the protection chain and closed on the perimeter. This is important at a higher voltage. An additional benefit of becoming an increase of 15% sensitivity to the two-phase circuit-proof.

Finally, when odnoreleynoy protection measurement undergo only two phases. Because of this these types of faults are monitored:

1. Any interfacial short-circuiting. Sensitivity to these kinds of accidents differs twice. Depending on the shutting phases.
2. Short circuit to ground of the measured phase (two out of three).
3. Short circuit in all three phases.

It is impossible to keep track of maintenance on the third line of ground where there is no meter. In addition, the sensitivity is 1.7 times lower than in any of the current cutoff schemes given above. Such protection usually not provided with a transformer, secondary windings of which are combined into a triangle, because blocked determining a particular type of two-phase short circuit. The only advantage of the fact becomes economical - uses a single relay. Odnoreleynaya cutoff circuit current from time to time serves to protect the motor voltage class of 1 kV and higher capacity of up to 2 MW.