In the article you will learn about what differential protection is, how it works, what positive qualities it has. It will also talk about what are the shortcomings of the differential protection of power lines. You will also learn practical schemes for protecting devices and power lines.
The differential type of protection is currently considered the most common and fastest. It is able to protect the system from phase-to-phase short circuits. And in those systems that use a solidly grounded neutral, it can easily prevent the occurrence of single-phase short circuits. The differential type of protection is used to protect power lines, high power motors, transformers, generators.
There are two types of differential protection in total:
- With tensions balancing each other.
- With circulating current.
This article willboth of these types of differential protection are considered 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, there is a comparison of parameters at the beginning of the element, the protection of which is carried out, as well as at the end. This scheme is used in the implementation of the longitudinal type and 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 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 implement longitudinal type protection, it is necessary to install the same current transformers at both ends. Their secondary windings must be connected to each other in series with the help of additional electrical wires that need to be connected to current relays. Moreover, 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 be equal both in phase and in magnitude. A slightly smaller value will flow through the electromagnetic current winding of the relay. You can calculate it using a simple formula:
Ir=I1-I2.
Assume that the current dependences of the transformers will completely match. Therefore, the aforementioned difference in current values is close to or equal to zero. In other words, Ir=0 and the protection is not working at this time. The auxiliary wiring that connects the secondary windings of the transformers circulates current.
Scheme of longitudinal type differential protection
This differential protection circuit allows you to obtain equal values of currents that flow through the secondary circuit of transformers. Based on this, we can conclude that this protection scheme was named so because of the principle of operation. In this case, the area that is located directly between the current transformers falls into the protection zone. In the event that there is a short circuit, in the protection zone, when powered from one side of the transformer, the current I1 flows through the winding of the electromagnetic relay. 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 there is a very high resistance in the secondary winding. Therefore, almost no current flows through it. According to this principle, the differential protection of tires, generators, transformers works. In the event that I1 turns out to be equal or greater than Ir, the protection starts to operate, opening the contact group of switches.
Short circuit and circuit protection
In the event of a short circuit inside the protected area, bothsides, a current flows through the electromagnetic relay, equal to the sum of the currents of each winding. 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 exactly the same. Therefore, Ir=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 implemented), then the currents of the secondary circuits will differ, even if the primary ones are absolutely the same. Now we need to consider how the differential protection circuit works in the event of an external short circuit on the power line.
External short circuit
In the presence of an external short circuit, an unbalance current will flow through the differential protection electromagnetic relay. 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 begins to increase. Its value also depends on the time after the start of the fault. Moreover, it should reach its maximum value in the first few periods after the start of the closure. It was at this time that the entire I short circuit flows through the primary circuits of the transformers.
It is also worth noting that at first I short circuit consists of two types of current - direct and alternating. They are also calledaperiodic 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 transformer magnetic system. Consequently, the unbalance potential difference increases sharply. As the short circuit current starts to decrease, the unbalance value of the system also decreases. According to this principle, differential protection of the transformer is carried out.
Sensitivity of protective structures
All types of differential protection are fast acting. And they do not work in the presence of external short circuits, 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 has an extremely low sensitivity. To increase it, you must meet many conditions. Firstly, it is necessary to use current transformers that do not saturate the magnetic circuits at the moment when current flows through the primary circuit (regardless of its value). Secondly, it is desirable to use fast-saturating type electrical appliances. They must be connected to the secondary windings of the elements to be protected. An electromagnetic relay is connected to a fast-saturating transformer (current differential protection becomes as reliable as possible) in parallel with its secondary winding. This is how generator or transformer differential protection works.
Increase sensitivity
Assume that an external short circuit has occurred. In this case, 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 that flows through the primary winding of a fast-saturating transformer. In this case, the aperiodic component of the current significantly saturates the core. Therefore, the transformation of the current into the secondary circuit does not occur. 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 less than in the absence of a fast-saturating transformer. Therefore, you can increase the sensitivity by setting the protection current value 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 is saturated very strongly, 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 it is very important. Therefore, the electromagnetic relay operates and switches off the protected circuit. A very low level of transformation for the first approximately one and a half periods of time slows down the action of the protection circuit. But this does not play a big role in the construction of practical circuit protection circuits.
Transformer differential protection does not work in cases where there is damage to the electrical circuit outside the protection zone. Therefore, time delay and selectivity is not required. The protection response time 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-saturating 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 the electrical circuit when exposed to short circuits from the outside. Also, it is not able to open the electrical circuit when subjected to a strong overload.
Unfortunately, the protection can work if the auxiliary circuit is damaged, to which the secondary winding is connected. But all the advantages of differential protection with circulating current interrupt these minor disadvantages. But they are able to protect power lines of very short length, no more than a kilometer.
They are very often used in the implementation of the protection of wires, with the help of which various devices necessary for the operation of power stations and generators are powered. In the event that the length of the power line is very large, for example, it is several tens of kilometers, protection according tothis circuit is very difficult to perform, since it is necessary to use wires with a very large cross section for connecting electromagnetic relays and the secondary winding of transformers.
If you use standard wires, then the load on the current transformers will be too large, as well as the unbalance current. But as for the sensitivity, it turns out to be extremely low.
Designs of protection relays and scope of circuits
In very long power lines, a circuit is used in which there is a protective relay of a special design. With it, you can provide a normal level of sensitivity, and use standard connecting wires. Transverse differential protection works by comparing the current in two lines in phases and magnitudes.
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 phase-to-phase short circuit. The differential protection is carried out as two-phase due to the fact that the power network with the above operating voltages is not grounded by neutrals. Otherwise, the neutral is connected to ground by means of an arc chute.
Auxiliary wires in the design of protective circuits
Current transformers are in relative proximity to each other. Therefore, the auxiliary wires are rather short. When using small diameter wires ontransformers will be exposed to a relatively low load. 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 selectivity. To prevent false alarms, line auxiliary contacts disconnect the circuit.
Straverse circuit differential protection
Transverse protection is widely used in the design of line systems operating in parallel. Switches are installed on both sides of the line. The bottom line is that such lines are very difficult to protect with simple circuits. The reason is that it is impossible to achieve a normal level of selectivity. To improve selectivity, the time delay must be carefully selected. But in the case of using a transversely directed differential protection, the time delay is not needed, the selectivity is quite high. She has major organs:
- Power direction. Double-acting power direction relays are often used. Sometimes a pair of single-acting differential protection relays are used that operate in different directions of power.
- Starting - as a rule, high-speed relays with the maximum possible current are used in its role.
The design of the system is such that current transformers with secondary windings connected in a circulating current circuit are installed on the lines. But all the current windings are switched on in series, afterwhat they are connected with the help of additional wires to the current transformers. In order for the differential-phase protection to work, voltage is supplied to the relay using the busbars of the installations. It is on them that the entire kit is installed. If you look at the circuit for switching on the secondary circuits of transformers and a protective relay, we can conclude why it is called the “directed eight”. The whole system is made in two sets. There is one set at each end of the line, which provides current differential protection for the power line.
Single-phase relay circuit
Voltage to the protection relay is supplied in reverse phase to what is needed to disconnect one line with damage. In normal operation (including in the presence of an external short circuit), only the unbalance current flows 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 beginning of the short circuit, some current flows in the protection zone of the second line. It is worth paying attention to the fact that:
- Start relay activated.
- On the side of one substation, the power direction relay opens the circuit breaker contacts.
- From the side of the second substation, the line is also disconnected using switches.
- In the power direction relay, the torque is negative, therefore the contacts are open.
In the windings of the first line protection relaythe direction of current flow changes (relative to the first line) during a short circuit. The power direction relay keeps the contact group in the open state. The circuit breakers on the side of both substations open.
Only such line differential protection can function properly only when both lines are running in parallel. In the event that one of them is turned off, the principle of operation of the differential protection is violated. Consequently, further protection leads to non-selective shutdown of the second line during external short circuits. In this case, it becomes a conventional directional current, and it does not have a time delay. To avoid this, the cross-directional protection is automatically disabled during the disconnection of one line by breaking the circuit with the auxiliary contact.
Additional types of protection
The tripping currents of the starting relays must be greater than the unbalance currents during an external short circuit. To avoid false positives when one of the lines is disconnected and the maximum load current passes through the remaining one, it is necessary that it be greater than the unbalance potential difference. If there is a transverse type of differential protection on the line, additional degrees must be provided.
They will allow one line to be protected when the parallel one is switched off. Typically they are used for overcurrent protection during an external short circuit (in this case the differential protection does not react). In addition, additional protectionis a backup to the differential (if the latter failed).
Directional and non-directional current protection, cut-offs, etc. are often used. Cross-directional differential protection is simple in design, very reliable and has been widely used in power networks with voltages of 35 thousand volts or more. This is how differential protection works, its principle of operation is quite simple, but you still need to know at least the basics of electrical engineering in order to understand all the intricacies.
