Differential protection: the principle of operation, device, circuit. Differential protection of the transformer. Longitudinal line differential protection

In the article you will learn about what differential protection is, how it works, what positive qualities it has. Also, it will be told about what are the disadvantages of the defensive line protection. Also you will get acquainted with practical schemes of protection of devices and power lines.

Differential type of protection at the moment is considered the most common and high-speed. It is able to protect the system from phase-to-phase faults. And in those systems in which a dull-earthed neutral is used , it can easily prevent single-phase faults from occurring. Differential type of protection is used to protect power lines, high-power motors, transformers, generators.

There are basically two types of differential protection:

1. With tensions balancing one another.
2. With circulating current.

This article will cover both of these types of defenses to learn as much as possible about them.

Diffuse protection using circulating currents

The principle is that currents are compared. And to be more precise, it compares the parameters at the beginning of the element, which is protected, and also at the end. This scheme is used in the implementation of the longitudinal type and transverse. The first are used to ensure the safety of a single power transmission line, electric motors, transformers, generators. Longitudinal differential line protection is very common in modern electric power industry. The second type of differential protection is used when using power lines operating in parallel.

Longitudinal differential protection of lines and devices

In order to protect the longitudinal type, it is necessary to install the same current transformers at both ends . Their secondary windings should be connected to each other in series by means of additional electric wires, which need to connect current relays. And these current relays must be connected to the secondary windings in parallel. Under normal conditions, as well as in the presence of an external short circuit, the same current will flow in both primary windings of the transformers, which will turn out to be equal both in phase and in magnitude. By the winding of the electromagnetic current, the relay will flow a little less than its value. You can calculate it using a simple formula:

I _r = I ₁ -I ₂ .

Suppose that the current dependencies of the transformers will completely coincide. Therefore, the aforementioned difference in current values is close to or equal to zero. In other words, I _r = 0, and protection at this time does not work. In the auxiliary wiring, which connects the secondary windings of the transformers, current is circulated.

Diagram of the longitudinal type of differential protection

Such a differential protection scheme makes it possible to obtain equal in magnitude the values of the currents that flow through the secondary circuit of the transformers. Proceeding from this, it can be concluded that this protection scheme was named so because of the principle of action. In this case, the area that lies directly between the current transformers enters the protection zone. In the event that there is a short circuit, the current I ₁ flows through the winding of the electromagnetic relay in the protection zone when feeding on one side of the transformer. It is sent to the secondary circuit of the transformer, which is installed on the other side of the line. It is necessary to pay attention to the fact that in the secondary winding there is a very large resistance. Consequently, the current practically does not flow through it. On this principle, differential protection of tires, generators, transformers. In the event that I ₁ turns out to be equal to or greater than I _r , the protection starts to operate, producing the opening of the contact group of the switches.

Short circuit and circuit protection

In the event of a short circuit inside the protected zone, a current flows from both sides through the electromagnetic relay , equal to the sum of the currents of each winding. In this case, protection is also activated, breaking the contacts of the switches. All the above examples assume that all technical parameters of the transformers are completely identical. Therefore, I _r = 0. But these are ideal conditions, in reality, because of small differences in the performance of magnetic systems of primary currents, electrical devices differ significantly from each other, even the same ones. If there are differences in the characteristics of the current transformers (when the differential-phase protection of the structure is realized), the values of the secondary circuit currents will differ, even if the primary currents are absolutely identical. Now we need to consider how the differential protection circuit works with an external short-circuit on the power line.

External short circuit

If there is an external short circuit, an unbalance current will flow through the electromagnetic relay of the differential protection. Its value directly depends on what current passes through the primary circuit of the transformer. In normal load mode, its value is small, but in the presence of an external short circuit, it starts to increase. Its value also depends on the time after the start of the fault. And the maximum value it should reach in the first few periods after the beginning of the closure. It was at this time that the first circuit of the transformers flowed through the first circuit of the transformers.

It is also worth noting that first I SC consists of two types of current - constant and alternating. They are also called aperiodic and periodic components. The differential protection device is such that the presence of an aperiodic component in the current must always cause excessive saturation of the magnetic system of the transformer. Consequently, the difference in unbalance potentials increases sharply. When the short-circuit current begins to decrease, the unbalance value of the system also decreases. By this principle, the differential protection of the transformer is carried out.

Sensitivity of protective structures

All types of differential protection are high-speed. And they do not work in the presence of external short-circuits, so it is necessary to select electromagnetic relays, taking into account the maximum possible unbalance current in the system in the presence of an external short circuit. It should be noted that protection of this type produces extremely low sensitivity. To increase it, you must meet many conditions. First, it is necessary to use current transformers, which do not saturate the magnetic circuits at the moment when the current flows through the primary circuit (regardless of its value). Secondly, it is desirable to use electric appliances of fast-saturating type. They need to be connected to the secondary windings of the elements, which are protected. The electromagnetic relay is connected to a fast-acting transformer (the differential current becomes maximum reliable) parallel to its secondary winding. This is how the differential protection of the generator or transformer works.

Increased sensitivity

Suppose an external fault occurred. At the same time, some current flowing through the primary circuits of the protective transformers consists of aperiodic and periodic components. The same "components" are present in the unbalance current that flows through the primary winding of the fast-permeable transformer. In this case, the aperiodic component of the current greatly saturates the core. Consequently, the transformation of the current does not occur in the secondary circuit. When the aperiodic component is attenuated, a significant reduction in the saturation of the magnetic circuit occurs, and gradually a certain value of the current begins to appear in the secondary circuit. But the maximum level of unbalance current will be much lower than in the absence of a fast-saturating transformer. Consequently, the sensitivity can be increased by setting the value of the protection current to less than or equal to the maximum value of the unbalance potential difference.

Positive qualities of differential protection

During the first periods, the magnetic circuit saturates very much, transformation practically does not occur. But after the aperiodic component has decayed, the periodic part begins to transform into the secondary circuit. It is worth paying attention to the fact that she has a very great value. Consequently, the electromagnetic relay is activated and turns off the protected circuit. Very low level of transformation, the first approximately one and a half time period slows down the protection circuit. But this does not play a big role in constructing practical circuits for protecting electric circuits.

The differential protection of the transformer does not work in cases where there are faults in the electrical circuit outside the protection zone. Therefore, temporary holding and selectivity is not required. The response time of protection ranges from 0.05 to 0.1 seconds. This is a huge advantage of this type of differential protection. But there is another advantage - a very high degree of sensitivity, especially when using a fast transient transformer. Among the smaller advantages are worth mentioning such as simplicity and very high reliability.

Negative properties

But both longitudinal and transverse differential protection has drawbacks. For example, it is not capable of protecting an electrical circuit when exposed to short circuits from the outside. Also, it is not able to open the electrical circuit under the influence of a strong overload.

Unfortunately, the protection can be activated if the auxiliary circuit is damaged, to which the secondary winding is connected. But all the advantages of diffuse-current protection with a circulating current interrupt these minor deficiencies. But they are able to protect power lines of a very small extent, not more than a kilometer.

They are very often used in the implementation of wire protection, through which a variety of devices necessary for the operation of power plants and generators are fed. In the event that the length of the electric line is very large, for example, several tens of kilometers, it is very difficult to protect this circuit, since it is necessary to use wires with a very large cross section for connecting electromagnetic relays and secondary windings of transformers.

In the event that you use standard wires, the load on the current transformers will be too high, as well as the unbalance current. But as for sensitivity, it turns out to be extremely low.

The design of the protection relay and the scope of the schemes

In electrolynines of a very long length, a circuit is used in which a protective relay is located, having a special design. With its help you can provide a normal level of sensitivity, and the connecting wires apply standard. Transverse differential protection is triggered by comparing the current in two lines in terms of phases and magnitudes.

Diffuse high-speed protection is used in transmission lines, in which a voltage in the range of 3-35 thousand volts flows. At the same time, reliable protection against interphase faults is provided. Diffuse protection is performed as two-phase because the mains with the above-mentioned operating voltages are not grounded by neutral wires. Or the neutral is connected to ground by means of an arc suppression coil.

Auxiliary wires in the construction of protective circuits

Current transformers are in relative proximity to each other. Consequently, the auxiliary wires have a rather small length. When using small diameter wires, a relatively low load will be applied to the transformers. As for the unbalance current, it is also small. But the degree of sensitivity is very high. In the event of a disconnection of any line, the differential protection becomes current, there is no time delay and no selectivity. To prevent false alarms, the block-contacts of the lines disconnect the circuit.

Cross-Directional Differential Protection of Circuits

Cross-directed protection is widely used in the development of line systems operating in parallel. On both sides of the line, switches are installed. The bottom line is that such a line design is very difficult to protect with simple schemes. The reason is that it is impossible to achieve a normal level of selectivity. To improve the selectivity, it is necessary to carefully select the time delay. But in the case of using a transversely directed diffractor, the time delay is not needed, the selectivity is quite high. It has the main organs:

1. Direction of power. Often a power direction relay with a two-way action is used. Sometimes a pair of single-acting differential protection relays are used that operate at different power directions.
2. Starting - as a rule, high-speed relays with the highest possible current are used in its role.

The design of the system is such that on the lines the installation of current transformers with secondary windings connected to a circuit with a circulating current is performed. But all the current windings are connected in series, after which they are connected by means of additional wires to the current transformers. In order for differential-phase protection to work, the relay is energized using the busbars of the units. It is on them that the whole kit is assembled. If we look at the scheme of switching on the secondary circuits of the transformers and the protective relay, we can conclude why it is called a "directed figure-eight". The whole system is made in two sets. At each end of the line is one set, which provides a differential current protection of the power line.

Single-phase relay circuit

The voltage to the protection relay is fed back in phase to what is needed to disable one line with damage. In normal operation (including when there is an external short circuit), only the unbalance current passes through the relay windings. In order to avoid false trips, it is necessary that the starting relays have a trip current greater than the unbalance current. Consider the work of protecting two lines.

At the moment of the beginning of a short circuit, a certain current flows in the protection zone of the second line. It is worth paying attention to the fact that:

1. The start relay is activated.
2. On the side of one substation, the power direction relay opens the switch contacts.
3. On the second substation side, the line is also switched off by means of switches.
4. In the power direction relay, the torque is negative, therefore, the contacts are open.

In the windings of the first-line protection relay, the current direction (relative to the first line) changes during a short circuit. The power direction relay keeps the contact group in the open state. Switches from both substations open.

Only such differential line protection can function normally only when both lines operate in parallel. In the event that one of them is turned off, the principle of operation of the differential protection is violated. Consequently, in the future protection results in nonselectivity of switching off the second line during external short circuits. In this case, it becomes a conventional directional current, and it does not have a temporary exposure. To avoid this, the transverse protection during the disconnection of one line is automatically outputted by breaking the block-contact circuit.

Additional types of protection

The actuation currents of the start relays must be greater than the unbalance currents during an external short circuit. To avoid false alarms when one of the lines is disconnected and the remaining maximum load current passes, it must be greater than the unbalance potential difference. If there is a cross-directional type on the line, additional degrees should be provided.

They will allow one line to be protected when disconnected in parallel. Typically, they are used to protect against overcurrent overload during an external short circuit (in this case, differential protection does not respond). In addition, dopzaschita is reserve to differential (in the event that the latter refused).

Often, directional and non-directional current protections, cutoffs, etc. are used. The transversely directed differential protection is simple in construction, very reliable and has been widely used in power networks with a voltage of 35 thousand volts. That's how the differential protection functions, the principle of its operation is quite simple, but you still need to know at least the basics of electrical engineering in order to understand all the subtleties.