In the article you will learn about what differential protection is, how it works, and what positive qualities it has. It will also be told about the shortcomings of the differential protection of power lines. You will also get acquainted with practical schemes for protecting devices and power lines.
The differential type of protection is currently considered the most common and fastest. He is able to protect the system from interphase faults. And in those systems that use grounded neutral, it can easily prevent the occurrence of single-phase faults. The differential type of protection is used to secure power lines, electric motors of increased power, transformers, generators.
There are two types of differential protection:
- With tensions balancing each other.
- With circulating current.
In this article, both of these types of differential protection will be examined in order to learn as much as possible about them.
Differential protection using circulating currents
The principle is that currents are compared. And to be more precise, the parameters are compared at the beginning of the element that is being protected, as well as at the end. This scheme is used in the implementation of the longitudinal type and the transverse. The former are used to ensure the safety of a single power 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
To protect the longitudinal type, it is necessary to install the same current transformers on both ends. Their secondary windings must be connected to each other in series using additional electrical wires, which must be used to connect current relays. Moreover, these current relays must be connected in parallel with the secondary windings. Under normal conditions, as well as in the presence of an external short circuit, the same current will flow in both primary transformer windings, which will turn out to be equal both in phase and in magnitude. A relay with a slightly lower value will flow through the coil of electromagnetic current . You can calculate it by a simple formula:
I r = I 1 -I 2 .
Suppose that the current dependences of the transformers are completely the same. Therefore, the aforementioned difference in current values is close to or equal to zero. In other words, I r = 0, and the protection does not work at this time. In the auxiliary wiring that connects the secondary windings of the transformers, current is circulated.
Longitudinal differential circuit type
Such a differential protection circuit makes it possible to obtain equal values of the currents that flow through the secondary circuit of the transformers. Based on this, we can conclude that this protection scheme was called so because of the principle of action. In this case, the section that is located directly between the current transformers falls into the protection zone. In the event that there is a short circuit, current I 1 flows through the winding of the electromagnetic relay in the protection zone when powered from 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. Differential protection of tires, generators, transformers works according to this principle. In the event that I 1 turns out to be equal to or greater than I r , the protection starts to work, making the contact group of the circuit breakers open.
Short circuit and circuit protection
In the event of a short circuit inside the protected zone, a current equal to the sum of the currents of each winding flows through the electromagnetic relay on both sides. In this case, protection is also activated by opening the contacts of the switches. All the above examples assume that all the technical parameters of the transformers are completely identical. Therefore, I r = 0. But these are ideal conditions, in reality, due to small differences in the performance of magnetic systems of primary currents, electrical appliances differ significantly from each other, even of the same type. If there are differences in the characteristics of current transformers (when differential-phase protection of the structure is realized), then the values of the currents of the secondary circuits will differ, even if the primary ones are exactly the same. Now we need to consider how the differential protection circuit works with an external short circuit on the power line.
External short circuit
In the presence of an external short circuit, an unbalance current will pass through the differential protection electromagnetic relay. Its value directly depends on what current flows through the primary circuit of the transformer. In normal load mode, its value is small, but in the presence of an external fault it begins to increase. Its value also depends on the time after the start of the short circuit. Moreover, it should reach its maximum value in the first few periods after the start of the circuit. It was at this time that the entire I short circuit flows along the primary transformer circuits.
It is also worth noting that at first I short circuit consists of two types of current - direct and alternating current. They are also called aperiodic and periodic components. The differential protection device is such that in this case the presence of an aperiodic component in the current should always cause excessive saturation of the transformer magnetic system. Therefore, the potential difference of unbalance increases sharply. When the short circuit current begins to decrease, the value of the unbalance of the system decreases. According to this principle, 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-circuit, so it is necessary to choose electromagnetic relays, taking into account the maximum possible unbalance current in the system in the presence of an external short circuit. It is worth paying attention to the fact that this type of protection produces extremely low sensitivity. To increase it, it is necessary to observe many conditions. Firstly, it is necessary to use current transformers in which the magnetic cores are not saturated at the moment when current flows through the primary circuit (regardless of its value). Secondly, it is desirable to use electrical appliances of fast-saturating type. They need to be connected to the secondary windings of the elements that are being protected. An electromagnetic relay is connected to a rapidly saturable transformer (differential current protection becomes as reliable as possible) parallel to its secondary winding. This is how differential protection of a generator or transformer works.
Sensitivity increase
Suppose an external fault occurred. At the same time, a certain current flows through the primary circuits of protective transformers, consisting of aperiodic and periodic components. The same “components” are present in the unbalance current, which flows along the primary winding of a rapidly saturable transformer. In this case, the aperiodic component of the current significantly saturates the core. Therefore, the transformation of the current in this case does not occur in the secondary circuit. With the attenuation of the aperiodic component, a significant decrease in the saturation of the magnetic circuit occurs, and gradually a certain current value begins to appear in the secondary circuit. But the maximum level of unbalance current will be much lower than in the absence of a rapidly saturable transformer. Therefore, sensitivity can be increased by setting the value of the protection current to less than or equal to the maximum value of the potential difference of the unbalance.
Positive qualities of differential protection
During the first periods, the magnetic circuit is very saturated, the transformation practically does not occur. But after the aperiodic component decays, the periodic part begins to transform in the secondary circuit. It is worth paying attention to the fact that she has a very great importance. Consequently, the electromagnetic relay trips and trips the protected circuit. The very low level of transformation the first about one and a half time period slows down the action of the protection circuit. But this does not play a big role in building practical circuits protection schemes.
Differential protection of the transformer does not work in cases where there is damage to the electrical circuit outside the protection zone. Therefore, time exposure and selectivity are not required. The response time of the 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-saturable transformer. Among the smaller advantages it is worth noting such as simplicity and very high reliability.
Negative properties
But both longitudinal and transverse differential protection have disadvantages. For example, it is not able to protect an electric circuit when exposed to short circuits from the outside. Also, it is not able to open an electric circuit when exposed to severe overload.
Unfortunately, protection can work if the auxiliary circuit to which the secondary winding is connected is damaged. But all the advantages of differential protection with a circulating current interrupt these minor flaws. But they are able to protect power lines of a very small length, not more than a kilometer.
They are very often used in the implementation of wire protection, with the help of which various devices necessary for the operation of power plants and generators are powered. In the event that the length of the power line is very large, for example, several tens of kilometers, it is very difficult to perform protection according to this scheme, since it is necessary to use wires with a very large cross-section for connecting electromagnetic relays and the secondary winding of transformers.
In that case, if you use standard wires, the load on the current transformers will be too large, as well as the unbalance current. But with regard to sensitivity, it turns out to be extremely low.
Protection Relay Designs and Scope
In very long-distance power lines, a circuit is used in which a protective relay is located, which have a special design. With it, you can provide a normal level of sensitivity, and the connecting wires to use standard. The transverse differential protection is triggered by comparing the current in two lines by phase and magnitude.
High-speed differential protection is used in power lines in which voltage flows in the range of 3-35 thousand volts. This provides reliable protection against interphase faults. Differential protection is performed as two-phase due to the fact that the power supply network with the above-mentioned operating voltages is not grounded by neutrals. Or the neutral is connected to ground by means of an arc suppression coil.
Auxiliary wires in protective circuit design
Current transformers are in relative proximity to each other. Consequently, the auxiliary wires are quite short. When using small wires, transformers will be affected by a relatively low load. As for the unbalance current, it is also small. But the degree of sensitivity is very high. In case of disconnection of any line, the differential protection becomes current, there is no time delay and selectivity. To eliminate false alarms, the line block contacts disconnect the circuit.
Cross directional circuit protection
Cross directional protection is widely used in the development of line systems operating in parallel. Switches are installed on both sides of the line. The bottom line is that such a design line is very difficult to protect with simple schemes. The reason is that it is not possible to achieve a normal level of selectivity. To improve selectivity, it is necessary to carefully select the time delay. But in the case of using a transversely directed differential protection, a time delay is not needed, the selectivity is quite high. She has the main organs:
- Direction of power. Often, two-way power direction relays are used. Sometimes they use a pair of single-acting differential protection relays that operate in different directions of power.
- Launcher - as a rule, in its role use high-speed relays with the maximum possible current.
The system design is such that current transformers with secondary windings connected to a circuit with circulating current are installed on the lines. But all current windings are connected in series, after which they are connected using additional wires to current transformers. In order for the differential-phase protection to work, voltage is supplied to the relay using the busbars of the units. It is on them that the entire kit is installed. If you look at the connection circuit of the secondary circuits of the transformers and the protective relay, then we can conclude why it is called the "directional eight". The whole system is made in two sets. There is one set at each end of the line, thanks to which differential current protection of the power line is provided.
Single Phase Relay Circuit
The voltage to the protection relay is reversed in phase to what is needed to disconnect one line with damage. In normal operation (including in the presence of an external short circuit), only unbalance current passes through the relay windings. To prevent 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 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:
- The start relay trips.
- From one substation, the power direction relay opens the switch contacts.
- From the side of the second substation, the line is also disconnected using switches.
- In the power direction switch, the rotation moment is negative, therefore, the contacts are open.
In the windings of the protection relay of the first line, the direction of current movement (relative to the first line) changes during a short circuit. The power direction relay keeps the contact group open. The switches on the side of both substations open.
Only such differential line protection can function normally only with the parallel operation of both lines. In the event that one of them is turned off, the principle of differential protection is violated. Therefore, further protection leads to the non-selectivity of disconnecting the second line during external short circuits. In this case, it becomes an ordinary directional current, and it does not have a time delay. To avoid this, laterally directed protection during the disconnection of one line is automatically displayed by breaking the block contact circuit.
Additional types of protection
The operation currents of the starting relays must be greater than the unbalance currents during an external short circuit. In order to avoid false alarms when one of the lines is disconnected and the maximum load current passes through the remaining maximum current, it is necessary that it be greater than the potential difference of the unbalance. If there is a transverse directional type of differential protection on the line, additional degrees must be provided.
They will allow the protection of one line when disconnecting in parallel running. , ( ). , ( , ).
, . . , 35 . . , , , .