RU2437193C1 - Method, system and device of differential protection - Google Patents

Abstract

FIELD: electricity. ^ SUBSTANCE: electrical network includes protected object having two or more ends, device (12, 14) of differential protection as to current and current transformer (11, 13) installed on each end (A, B). According to the method the measured values from the second end of the protected object are obtained on the first end of protected object; changes of the measured values on the first end are compared to the changes of measured values on the second end, and at the stage of comparison of changes of the measured values, which showed different results, there determined is the type of failure occurred in electrical network. ^ EFFECT: improving reliability. ^ 15 cl, 5 dwg

Description

Technical field

The present invention relates to the field of power transmission and distribution systems, in particular to protection methods and protective equipment in such systems.

State of the art

The electric power transmission or distribution system contains protection systems designed to protect, control and control the operation of devices forming part of the energy system. Protective devices detect, inter alia, short circuits, current overloads and voltage overloads in power lines, transformers and other parts of the power distribution system.

In such electrical systems, protection equipment is used for protection and control. This protection equipment detects and isolates faults, for example, on transmission and distribution lines, using breakers, and, after eliminating the fault, restores the flow of current. Alternatively, the protection equipment can be configured to select an alternative energy path during fault detection.

Differential current protection is a relatively new and reliable way to protect power networks. It is based on the idea of comparing currents on both sides of the protected zone or protected block. The protected unit or zone may be any part of the power supply network, for example, a transmission line, a transformer, a generator or a bus of a transformer substation.

Figure 1 schematically shows the principle of differential current protection. Under normal operating conditions, the sum of all currents I ₁ , I ₂ entering and leaving the protected device 1, for example, a transmission line, is zero. If a failure occurs in the protected unit 1, the sum of the currents at the different ends will no longer be zero. Under normal operating conditions, the secondary currents in the current transformers 3a and 3b connected between the protected unit 1 and the current relays 2 and designed to reduce the primary current of the power supply system are also equal, i.e. i ₁ = i ₂ , and no current flows through current relay 2. If a failure occurs in the protected unit 1, the currents will no longer be equal and current will flow through the current relay 2. Differential relay 2 in this case opens the circuit breakers (not shown) located at both ends of the protected unit 1. An open circuit breaker (circuit breakers) thereby isolates the faulty protected unit (s) from the rest of the power supply network.

Currently, local measurements are used to determine if the current transformer circuits are functioning. 2 shows a secure power transmission line L. It is common practice to use two main protective devices for protection, for example, a differential current protection device 4 and a distance protection device 5. To detect failures, the differential current protection device 4 uses the measured values from its current transformer CT1 and the reference value from the current transformer CT2 of the other main protective device 5. That is, the measured values from the local current transformers CT1 and CT2 are used, which requires mixing the currents of the two protective devices 4 and 5. The disadvantage of this solution is that it is difficult to determine whether a detected failure has occurred in a real protected device or in a reference protected device.

In addition, difficulties arise in determining whether the resulting failure is a failure of the primary network or a failure of the secondary network. If there is a failure in the secondary network, for example, failures that occur in the circuits of the current transformer, it is very undesirable to open the breaker, in which the primary transmission lines are disconnected unnecessarily, interrupting the supply of electricity to consumers.

In view of the foregoing, it is desirable to create an improved method of differential protection. In particular, it is desirable to provide a differential protection method in which the current transformer circuits are monitored. Additionally, it is desirable to create an appropriate differential protection system.

Summary of the invention

An object of the present invention is to provide an improved method for protecting a power supply network in which the above-described disadvantages of the prior art are eliminated or at least reduced. In particular, it is an object of the present invention to provide a differential protection method in which a failure within a protection system, in particular in current transformer circuits, can be easily and reliably detected.

Another objective of the present invention is to provide an improved method of differential protection, in which failures in the current transformers of the protection device can be detected independently of the current transformers of other protective equipment.

Another objective of the present invention is to provide a differential protection method in which primary failures can be reliably distinguished from secondary failures.

These and other problems are solved by the methods and systems described in the independent claims.

According to the present invention, there is provided a method of differential protection against a certain type of failure occurring in a power supply network. The power supply network contains a protected object having two or more ends, and a differential current protection device installed at each end. The method comprises the steps of: obtaining on a differential current protection device located at a first end of a protected object, measured values from a differential current protection device installed at a second end of a protected object; in the differential current protection device at the first end, changes in the measured values at the first end are compared with changes in the measured values obtained at the second end; and determine, at the stage of comparing changes in the measured values showing different results, the type of failure that occurred in the power supply network. According to the present invention, since the measured values are obtained at the second end of the protected object, the failure of the current transformer circuit of the current differential protection device can be determined independently of other local protection equipment. Thus, it is possible to avoid undesired mixing of the circuits of two different protective systems and reliably identify failures that occur in a real protection device. Additionally, in the present invention, changes in measured values are compared and the type of failure is determined based on the results of the comparison. This prevents unnecessary opening of the transformer primary lines, since the primary failure can be reliably distinguished from the secondary failure in current circuits. In addition, according to the present invention, increased security is ensured by the fact that opening commands are issued only when necessary. However, such increased security is not provided by reducing the reliability of the system. That is, the reliability of the protected power supply system is maintained, and if a failure occurs in the power supply system, the opening command will be issued with a high degree of reliability. Thus, the present invention provides both increased safety and increased reliability.

According to another embodiment of the present invention, the step of determining the type of failure comprises the step of determining the occurrence of the primary failure if the step of comparing the changes in the measured values indicates changes at two or more ends. If the step of comparing the changes in the measured values shows the changes at only one end, a secondary failure is detected. Thus, a reliable method for detecting and determining the type of failure is proposed.

According to another embodiment of the present invention, the method comprises an additional step in which the breaker is opened in response to the step of comparing the measured values, showing a change in the measured values at two or more ends, i.e. primary failure. The method may also include an additional step in which the response command is blocked if, at the determination step, it is shown that the change in the measured values occurred at only one end, i.e. secondary failure occurred. Thus, unnecessary opening of the primary transmission lines is prevented.

According to another aspect of the present invention, there is provided a differential protection system that provides benefits similar to those described above.

Additional embodiments of the present invention are defined in the dependent claims. Additional options and advantages of the present invention will be apparent from the following detailed description with reference to the attached drawings.

Brief Description of the Drawings

The invention is further explained in the description of the preferred embodiments with reference to the accompanying drawings, in which:

figure 1 depicts a known differential current protection circuit;

figure 2 - diagram of a known differential protection system of the transmission line;

figure 3 is a diagram of a power supply network in which the present invention can be applied;

4 is a diagram of another power supply network in which the present invention can be applied;

5 is a flowchart of the method of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

1 and 2 have already been described in connection with the description of the prior art and do not require additional explanation.

Figure 3 schematically shows an embodiment of the present invention and, in particular, a power supply network 9. The differential protection method of the present invention protects a network entity, which in the example shown is transmission line 10. The current differential protection method is implemented by, for example, appropriate computer hardware and other equipment, and the current differential device 12 is used as an illustrative means implementing the method of the present invention in the following description.

The differential current protection device 12 is connected to the transmission line 10 through a current transformer 11. A current differential protection device 12 is located at the first end, indicated by A, of the transmission line 10. The current transformer 11 reduces the current in a known manner to a level suitable for supplying a current differential protection device 12. At the opposite end, indicated by the position B, of the transmission line 10, there is a differential current protection device 14 together with a corresponding current transformer 13. The distance between the two differential current protection devices 12, 14 may, for example, be from several kilometers to hundreds of kilometers.

The differential current protection devices 12, 14 comprise communication means 19 and 20, respectively, by which two or more communication channels 15 can be created between the differential current protection devices 12, 14. The communication means preferably comprise digital communication means by which it is possible to exchange measurement values in digital format between the different ends of the protected object. The communication channel 15 may be a fiber optic communication channel, i.e. devices for differential current protection 12, 14 are connected via fiber optic cable 15. Means 19, 20 for communication of devices for differential current protection 12, 14 may further comprise, for example, means for fiber optic cable sleeves. It is necessary to create two (or more) communication channels 15, one in each direction, i.e. provide duplex communication.

Communication channels 15 are used to send measured values and other data between the differential current protection devices 12, 14. Therefore, according to the present invention, no local reference measurements are needed. Therefore, mixing circuits of local protection devices can be avoided. The second protective device 16, 17 located on the first end A of the transmission line 10 is thus completely independent of the differential current protection devices 11, 12.

The differential current protection devices 12, 14 measure currents and other parameters. According to the present invention, the current differential protection device 12 at the first end A of the transmission line 10 is configured to compare its measurements with the reference data obtained from the current differential protection device 14 at the second end B of the transmission line 10 to determine if a failure has occurred.

The current values at the end A of the transmission line 10 are compared with the current values at the end B of the transmission line 10. Under normal operating conditions, the currents at both ends A and B are equal.

To perform the above measurements and comparisons, the differential current protection devices 12, 14 comprise processing means 22, 23 suitable for this task. Different ends must have the same logic circuits, and all ends must have the opposite logic.

By comparing the current values or other measured values at both (or several) ends with each other, it is possible to determine the type of failure. An advantage of the present invention is that digital quantities are continuously used, which gives more accurate results than when comparing measured values with predetermined threshold values. If the current does not change in the real protection device, but the current changes at the other end, the opening command at the local end will not be issued, but only a warning will be issued about the malfunction of the remote current transformer.

In an embodiment of the present invention, the current at one end is measured and compared with an earlier current value at the same end, di / dt. At the other end (ends) carry out the corresponding calculations. The values obtained at one end are compared with the corresponding values at the other end (s), and the corresponding values are obtained via communication channel 15. For example, the value measured at the end A can be compared with the value obtained at the same end A for one or two cycles of the power supply system before it, for example, for 20-40 ms in a system operating at a frequency of 50 Hz. If a change in current is observed only at one end, for example, at end A of the transmission line, but is not observed at the other end, for example, end B, we can conclude that the change in current is caused by a secondary failure, for example, a fault in the circuit of current transformer 11, for example, short short circuit or open circuit.

The above can be reduced to the following Boolean expressions, in which A and B are the corresponding ends of the transmission line L ₁ , i are discrete instantaneous current values, preferably given in time with clock synchronization at both ends, and L ₁ is the phase 1 transmission line L :

if i (L ₁ A) = i (L ₁ B), then there is no failure;

if i (L ₁ A)> i (L ₁ B) ″ AND ″

i (L ₁ A) unchanged ″ AND ″

i (L ₁ B) has changed, then

block the open command from end A of the transmission line. That is, the failure is not a primary, but a secondary failure at the end of B. For the end of the B transmission line, the corresponding expressions are:

if i (L ₁ A) = i (L ₁ B), then there is no failure;

if i (L ₁ B)> i (L ₁ A) ″ AND ″

i (L ₁ B) unchanged ″ AND ″

i (L ₁ A) has changed, then

block the opening command from end B, since there is no primary failure.

By changing the magnitude of the current at one of the ends of A, B of the transmission line is meant a change from the previous measurement, for example, with a value measured several milliseconds earlier, as described above.

The above conditions can, of course, be realized for a typical three-phase power supply system having three separate current phases.

A specialist in this field, having read this description, will be able to build the logic for implementing this idea, which is illustrated by the above algorithms.

If an initial failure occurs, shown in FIG. 3 by F1, at both ends A, B, of the transmission line 10, a change in the current value will occur. In response to the detection of the primary failure, the current differential protection device 12 issues a command to open the breaker 18. If a secondary failure occurs, for example, in the current transformer circuits, then the current will only change at the end at which the failure has occurred. There are no current changes at the opposite end. With the present invention, thus, primary failures can be reliably distinguished from secondary failures in current circuits. Additionally, the transmission line 10 is not switched off unnecessarily.

Figure 4 shows a three-phase power supply network with three ends. The present invention can be implemented in such multi-terminal systems having three or more ends. The various ends of the power transmission system are connected by appropriate communication means. Therefore, it is possible to exchange data between all ends of such a power transmission system. If an initial failure occurs, such a failure will be detected / detected by detecting two simultaneous changes at two of the ends of the power line. For example, in a power transmission system with three ends, if the current changes at the two ends of the transmission line, we can assume that a primary failure has occurred and it is necessary to issue an open command. Secondary failure is detected only when a change in current occurs at only one end, regardless of the number of ends of the line.

5 shows the steps of a differential protection method 30 according to the present invention. The method is applicable, for example, in the power supply network 9 shown in FIG. 3, and provides a means for determining the type of failure that occurred in the power supply network 9. The power supply network 9 contains a protected object 10, for example, a transmission line having two or more ends A, B. At each of the ends A, B, differential current protection devices 12, 13 and, preferably, corresponding current transformers 11, 13 are installed. At step 31, the current differential protection device 12 located on the first end A of the protected object 10 receives the measured values from the differential protection device 14 installed on the second end B of the protected object 10. These values are transmitted on the communication channel 15. Then, at step 32, the changes in the values from the measurement obtained at the second end B of the current differential protection device 14, in the current differential protection device 12 located at the first end A, are compared with the changes in the values obtained at the first end A. At step 33, the the type of failure that occurred in the network 9, if at step 32 of the comparison, changes in the measured values show different results. Step 33 determining the type of failure contains a determination of the occurrence of the primary failure, if step 32 comparing the changes in the measured values shows changes at two or more ends A, B. If the comparison step 32 shows changes in the measured values at only one end, then it is determined that a secondary failure has occurred, most likely in current transformer circuits, and appropriate action is taken.

In a preferred embodiment, the step of comparing the changes in the measured values comprises the step of comparing, at the first end A, the changes in the synchronized timed measured values at the first end A with the changes in the measured values at the second end B, which are timed measured values synchronized over the same clock frequency as the measured values at the first end of A. Thus, values that are really comparable are obtained.

Method 30 may include additional steps not shown in the drawing. For example, the method preferably comprises an additional step in which the interrupter 18, 21 is opened 34 in response to changes in the measured values at two or more ends A, B resulting from the comparison in step 32.

The method 30 may comprise an additional step of blocking the opening command if the determination step 33 shows a change in the measured values at only one end, i.e. if a secondary failure is detected.

In the above description, a transmission line 10 was used to illustrate an object or network element that can be protected by means of the present invention. However, it should be noted that other network elements, such as transformers, substations, generators, buses, etc., can be protected by the means of the present invention.

To summarize, the present invention provides an improved method of differential current protection. Current circuits in a protected system can be reliably controlled. Failures in current transformer circuits can be detected independently of current transformers connected to other protection equipment.

Claims (15)

1. The method of differential protection in the network (9) power supply to determine the type of failure that occurred in the network (9) power supply containing a protected object (10) having two or more ends (A, B), with each end (A, C) installed devices (12, 14) current differential protection, characterized in that it contains the stages in which: get (31) in the device (12) differential current protection located at the first end (A) of the protected object (10) , measured values from the device (14) of the differential current protection located on watts at the other end (B) of the protected object (10), compare (32) in the current differential protection device (12) installed at the first end (A), changes in the values measured at the first end (A) with changes in the indicated measured values, received from the device (14) of the differential current protection at the second end (B), determine (33) after comparing the changes in the measured values, showing different results, the type of failure that occurred in the power supply network (9). 2. The method according to claim 1, in which the step (33) of determining the type of failure comprises the step of determining the occurrence of the primary failure if, at the stage of comparing the changes in the measured values, changes are detected at two or more ends (A, B). 3. The method according to claim 2, comprising an additional step in which the interrupter (18, 21) is opened (34) if, during the comparison step, a change in the measured values is detected at two or more ends (A, B). 4. The method according to claim 1, in which the step (33) of determining the type of failure comprises the step of determining the occurrence of a secondary failure if, at the stage of comparing changes in the measured values, changes are detected at only one end (A, B). 5. The method according to claim 4, comprising an additional step in which the opening command is blocked if, at the determination step (33), a change in the measured values is detected at only one end. 6. The method according to claim 1, comprising an additional step in which the step of obtaining (31) and the step (32) of comparing the changes in the measured values are repeated, if at the step (32) of the comparison no changes in the measured values are detected. 7. The method according to claim 1, wherein said comparison of the changes in the measured values comprises the step of comparing current changes at two or more ends (A, B). 8. The method according to claim 1, in which the step (31) of obtaining the measured values comprises obtaining digital values via a fiber optic communication line. 9. The method according to claim 1, in which the step of comparing the changes in the measured values contains a comparison in the device (12) of the differential current protection at the first end (A) of the changes in the time-synchronized measured values obtained at the first end (A) with the changes measured values obtained by the device (14) differential current protection at the second end (B), and the measured values are time-set measured values that are synchronized at the same clock frequency as the measured values at the first nce (A). 10. The method according to any one of claims 1 to 9, in which the differential protection devices (12, 14) are connected to the protected object (1) by means of corresponding current transformers (11, 13). 11. The method according to claim 10, in which the protected device comprises a transmission line (10). 12. The differential protection system in the power supply network (9) containing the protected object (10) having two or more ends (A, B), and on each end (A, B) there is a current differential protection device (12, 14) characterized in that it comprises: means (22, 23) for processing in current differential protection devices (12, 14), designed to measure changes in measured values at each end (A, B), means (19, 20) in the device ( 12, 14) current differential protection for receiving measured values from another differential device (12, 14) current protection, the specified means (22, 23) of processing in each of the devices (12, 14) differential current protection, also designed to compare changes in the measured values obtained at the first end (A) with changes in the measured values obtained at the second end (B), and the means (22, 23) in each of the devices (12, 14) of the differential current protection to determine the type of failure that arose in the power supply network (9). 13. The differential protection system according to claim 12, in which the current differential devices (12, 14) are connected to the protected object (1) by means of a corresponding current transformer (11, 13). 14. Device (12, 14) for differential current protection for a power supply network (9) containing a protected object (10) having two or more ends (A, B), characterized in that it comprises: processing means (22, 23) designed to measure changes in measured values; means (19, 20) for obtaining measured values from the other end / ends (A, B) of the protected object (10); said processing means (22, 23), also intended to compare changes in measured values at the first end (A) with changes in measured values obtained at the second end (B), and said processing means (22, 23), also intended to determine the type failure that arose in the power supply network (9). 15. The device (12, 14) of the differential current protection according to claim 14, comprising means for implementing the method according to any one of claims 1 to 11.

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