Lateral and longitudinal differential protection: application and principle of operation, advantages and disadvantages

To ensure the long-term operation of electrical equipment, various types of protection are used. Differential protection has become widespread due to its high speed. It is used in networks with a solidly grounded neutral for the safe operation of power lines, electric motors, prefabricated machines, transformers, autotransformers and generators from short circuits, as well as for home use.

Types and features of work

Differential protection is a type of relay protection that is characterized by absolute selectivity and very high response speed. There are such types of differential protection: transverse and longitudinal. The choice of the appropriate differential protection depends directly on the situation, and in order to be able to apply it accurately, you need to know in which cases it is applied, the principle of operation, as well as the main disadvantages and limitations.

Longitudinal protection

Longitudinal differential protection must be installed as the main one for the protection of powerful transformers and autotransformers.

Primary requirements:

1. Single transformers and autotransformers from 6300 kVA.
2. Parallel operating transformers and autotransformers with power from 4000 kVA.
3. Reliable and noise-free communication line between 2 transformers.
4. Transformers and autotransformers with a power of 1000 kVA (current cutoff cannot achieve the required sensitivity in the event of a short circuit at the terminals with high voltage, while the maximum protection should not be more than 0.5 seconds).

Scheme 1 - Longitudinal differential protection of the transformer:

The principle of operation of differential protection is reduced to comparing the values ​​of the phase currents flowing through the protected sections of the corresponding lines. Current transformers are used, which are used to measure the current strength in the protected section of the circuit. The secondary windings of these transformers are connected to the current relays, as a result of which the difference in currents falls on the relay winding.

During normal operation, the difference in the value of the currents in the current relay circuit will be zero. However, in the event of a short circuit, not the difference will enter the relay coil, but the sum of the currents. The relay contacts close, and a command is issued to completely disconnect the damaged section of the circuit.

However, this all works great only in theory. In a real case, a non-zero current will flow through the coil of the current relay. This current is called unbalance current.

The main reasons for the appearance of an unbalance current on the current relay winding:

1. The characteristics of current transformers often have slightly different characteristics. At the manufacturing plant, they are produced in pairs, their characteristics are preliminarily checked and adjusted (change the number of turns of the windings to comply with the correspondence of the transformation ratio of the transformer, which is necessary protect).
2. The occurrence of a magnetizing current, which appears in the windings of a protected transformer. In normal mode, the value of this current reaches up to 5% of the nominal. When the transformer is idle, this current for a short time can exceed the rated value by several times.
3. Different connections of the primary and secondary windings of the transformer (star and delta). In this interpretation, the vectors of currents in the primary and secondary windings will be displaced by 30 degrees, which will complicate the selection of the number of turns. This can be easily compensated by connecting the windings properly (delta on the star side and star on the delta side).

It should be taken into account that modern devices based on microprocessors are able to compensate on their own and for this you just need to specify in the settings of this device.

Lateral protection

Applies to high voltage lines only. Transverse differential protection selects and de-energizes one faulty line.

It consists of a current relay for the direction of power, which is connected, as in the longitudinal differential protection, from the corresponding section to the difference in currents.

The current is supplied to the relay through series-connected contacts for automatic output of protection when the problem line is disconnected, in order to avoid its action in case of short circuit (short circuit). The torque of the power direction relay depends directly on the current, voltage, and also on the angle between these vector quantities.

In the event of a short circuit, the current value on one of the lines will be greater than on the other, and the current in the relay will have the same direction as in the first line. Consequently, the relay will close its contact (the current will be enough to attract the core), and the differential protection will turn off the line with a large current value. The same will happen when the value of the rated current in the second line increases, but the other contactor group will open.

Scheme 2 - Transverse transformer differential protection

The principle of operation of the lateral protection is approximately the same as that of the longitudinal protection, but there is a main difference: current transformers should be installed at the ends of the individual lines that are connected to this site.

Advantages and disadvantages

Despite its widespread use due to its high response speed, each type of differential protection has its own pros and cons.

Longitudinal differential protection advantages:

1. Absolute selectivity.
2. Possibility of using with other types of protection.
3. Perfect for short power transmission lines.
4. Disconnection of the emergency section of the network without delay.

The disadvantages of longitudinal protection include:

1. Reduced efficiency in the design of long transmission lines.
2. Devices for monitoring the failure of auxiliary wires are required to correct the differential protection.
3. The emergence of an imbalance current.
4. High cost when using a relay (relay with braking).
5. A very complex implementation (communication lines for current transformers are additionally built).

Advantages of transverse diffraction protection:

1. High selectivity (100%).
2. Does not affect the operation of other relays in the circuit.
3. Instant response.

Disadvantages of lateral protection:

1. There is an increasing need to restart the protection when triggered.
2. It is not used as the main and only protection.
3. It is necessary to take into account the dead zones, of which there are several.
4. Cannot protect line ends and substation busbars.
5. Cannot determine the location of the short circuit.
6. It is not used for power lines, where it is required to disconnect only damaged sections.
7. Not applicable with circuit breakers.
8. It is necessary to completely disconnect the damaged line.

Household use

These types of protection can be used for residential buildings in networks with voltages from 230 to 400 volts, however, these devices are called dip devices. They are of two types: differential circuit breakers and residual current devices. The principle of their operation is based on a consequence of Kirchhoff's law (I law), which implies the following rule: the values ​​of the incoming and outgoing currents must be equal. If a leakage current is generated, then the values ​​do not match, and the protected area is disconnected.

The main causes of leakage current:

1. Touching parts of equipment that are energized by humans or animals.
2. Breakdowns in the insulation of a line of wiring or equipment.

In some cases, the automation (differential device) is triggered when there is no load (connected electricity consumers). The main reason is a malfunction of the device or current leakage in the junction box itself. However, if the device is in good working order, then in this case it is necessary to completely turn off all the machines after the differential device, and all circuit elements are checked for breakdown to the case. To select differential protection, it is necessary to take into account the premises and features of the electrical circuits that are subject to protection.

Differential protection is the best choice for apartments with wiring without grounding. To ensure the highest efficiency, it is necessary to install a 3-level protection (several devices for 10, 30 and 100-300 mA).

To ensure safety, it must be checked by pressing the "Test" button at least 2 times a month, it is advisable to do this regularly.

Difautomats - better protection, which performs the functions of an RCD and a circuit breaker. If there is a generator in the dwelling, which has become widespread, then this type of protection can also be applied to it. The circuit includes a current relay that connects to a current transformer. The relay must be installed on the stator between the star-connected zero points. During normal operation, the protection does not work, but when an interturn short circuit occurs, a difference in the magnetic fluxes of the current relay appears and the protection is triggered.

Differential protection can also be applied and for protection against multiphase short-circuits. To do this, you need to purchase a special differential device for multiphase protection.

Improving the effectiveness of differential protection

Despite a huge number of advantages over other types of protection, differential protection requires an increase in the efficiency of its operation in an emergency during the operation of generators. To do this, you must observe the following rules:

1. Inclusion of additional resistors to the measuring current relays.
2. Minimization of aperiodic values ​​and setting of cutoffs for transient unbalance currents.
3. Application of a relay with a delayed response time.

Thus, differential protection is widely used to ensure the stable operation of electrical equipment and power lines, protection against fires and flares, unforeseen financial costs, as well as to preserve life and health person.