JP2006050813A - Digital relay device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To detect abnormality in a bypass circuit efficiently, and to improve reliability when protecting a power system. <P>SOLUTION: In one embodiment, a digital relay device 13 comprises a digital relay 14A, a bypass means 15A for bypassing the digital relay 14A based on signals s2a, s3a from the digital relay 14A, the digital relay 14A, and a bypass means 15B for bypassing a digital relay 14B based on signals s2b, s3b from the digital relay 14A; and a bypass abnormality detection means 16a for detecting the abnormality of the bypass means 15A, 15B based on the input state of the signals s2a, s3a in the bypass means 15A, the bypass state of the digital relay 14A by the bypass means 15A, the input state of the signals s2b, s3b in the bypass means 15B, and the bypass state of the digital relay 14B by the bypass means 15B while being provided inside the digital relay 14A. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a digital relay device used for protection of a power system.

  From the viewpoint of ensuring the reliability of protection of the power system, combinations of digital relays and bypass circuits may be multiplexed.

  FIG. 4 is a circuit diagram showing an example of a conventional digital relay device.

  In the power system, a circuit breaker 1 is provided between the system upstream side A1 and the system downstream side A2.

  The digital relay device 2 includes digital relays 3A and 3B, bypass circuits 4A and 4B, switches 5A and 5B, and a trip command power supply P1.

  The digital relay 3A inputs a current I and a voltage V between the system upstream side A1 and the system downstream side A2 from an ammeter and a voltmeter, respectively. Also, the digital relay 3A performs power system abnormality detection (accident detection) based on the current I and the voltage V. When the digital relay 3A detects an abnormality in the power system, it outputs a trip signal s1a to the switch 5A.

  When the digital relay 3A is bypassed, the digital relay 3A outputs a bypass command signal s2a to the bypass circuit 4A.

  When the abnormality of the digital relay 3A is detected, the digital relay 3A outputs the digital relay abnormality signal s3a to the bypass circuit 4A.

  The digital relay 3B outputs a trip signal s1b to the switch 5B when detecting an abnormality in the power system, and outputs a bypass command signal s2b to the bypass circuit 4B when bypassing the digital relay 3B. Is detected, the digital relay abnormality signal s3b is output to the bypass circuit 4B. Other functions are the same as those of the digital relay 3A.

  The switch 5A and the switch 5B are connected in series. When the trip signals s1a and s1b are input, the switches 5A and 5B are turned on, and are turned off in other cases.

  Therefore, when both the switch 5A and the switch 5B are turned on by the trip signals s1a and s1b, a trip command signal is output from the trip command power supply P1 to the circuit breaker 1. That is, when the logical product (AND) of the trip signals s1a and s1b is obtained by hardware outside the digital relays 3A and 3B and the trip signals s1a and s1b are output from both the digital relays 3A and 3B, Circuit breaker 1 is tripped.

  When receiving the trip command signal, the circuit breaker 1 executes a power system disconnecting operation. As a result, when an abnormality is detected in both of the digital relays 3A and 3B, the circuit breaker 1 is cut off and the protection operation of the power system is executed.

  The bypass circuit 4A includes a power source P2a, a power source P3a, switches 6a to 10a, and switch control units 11a and 12a.

  Similarly, the bypass circuit 4B includes a power source P2b, a power source P3b, switches 6b to 10b, and switch control units 11b and 12b.

  The switch 6a is provided in parallel with the switch 5A.

  The switch 7a is turned on when the bypass command signal s2a is inputted, and is turned off in other cases.

  The switch 8a is turned on when the digital relay abnormality signal s3a is inputted, and is turned off in other cases.

  The switch 7a and the switch 8a are provided in parallel between the power supply P2a and the switch control unit 11a.

  When at least one of the switch 7a and the switch 8a is turned on, the current from the power source P2a is supplied to the switch controller 11a, and the switch controller 11a operates.

  The switch controller 11a turns on the switch 9a when at least one of the bypass command signal s2a and the digital relay abnormality signal s3a is input, and turns off the switch 9a in other cases.

  In addition, the switch control unit 11a sets the switch 10b of the bypass circuit 4B to the OFF state when at least one of the bypass command signal s2a and the digital relay abnormality signal s3a is input, and sets the switch 10b to the ON state in the other cases. To do.

  The switch 9a and the switch 10a are provided in series between the power source P3a and the switch control unit 12a.

  When both the switch 9a and the switch 10a are turned on, the current from the power source P3a is supplied to the switch control unit 12a, and the switch control unit 12a operates.

  The switch control unit 12a outputs at least one of the bypass command signal s2a and the digital relay abnormality signal s3a from the digital relay 3A to the bypass circuit 4A, and also outputs the bypass command signal s2b and the digital signal from the digital relay 3B to the bypass circuit 4B. When none of the relay abnormality signals s3b is output, the switch 6a is turned on.

  When the switch 6a is turned on, the current from the trip command power supply P1 is supplied to the switch 5B via the switch 6a regardless of the ON / OFF state of the switch 5A. Therefore, the digital relay 3A is bypassed, the digital relay 3A is disconnected from the protection operation of the power system, and the protection operation is performed only by the digital relay 3B.

  Here, at least one of the bypass command signal s2a and the digital relay error signal s3a is output from the digital relay 3A to the bypass circuit 4A, and the bypass command signal s2b and the digital relay error signal s3b are output from the digital relay 3B to the bypass circuit 4B. And at least one of them is output.

  In this case, in the bypass circuit 4A, the switch 10a is turned off, the switch 6a is also turned off according to the switch control unit 12a, and the bypass state of the digital relay 3A by the bypass circuit 4A is released.

  In the bypass circuit 4B, the switch 10b is turned off, the switch 6b is also turned off in accordance with the switch control unit 12b, and the bypass state of the digital relay 3A by the bypass circuit 4B is released.

  As a result, both of the digital relays 3A and 3B that are in the bypass state are again incorporated into the protection operation of the power system.

  The configuration and operation of the digital relay 3B and the bypass circuit 4B are the same as the configuration and operation of the digital relay 3A and the bypass circuit 4A, and thus the description thereof is omitted.

  An outline of the operation of the conventional digital relay device 2 will be described.

  When the digital relays 3A and 3B detect a system abnormality based on the current I and voltage V of the power system, the digital relays 3A and 3B output trip signals s1a and s1b to the switches 5A and 5B, respectively. When both of the trip signals s1a and s1b are output from the digital relays 3A and 3B to the switches 5A and 5B, the digital relay device 2 sets the circuit breaker 1 in a disconnected state.

  When at least one of the bypass command signal s2a and the digital relay abnormality signal s3a is output from the digital relay 3A to the bypass circuit 4A, the bypass circuit 4A bypasses the digital relay 3A, and the digital relay 3A is protected for the power system. Disconnect from. Thereafter, the protection operation of the power system is performed by the other digital relay 3B.

  Similarly, when at least one of the bypass command signal s2b and the digital relay abnormality signal s3b is output from the digital relay 3B to the bypass circuit 4B, the bypass circuit 4B bypasses the digital relay 3B and connects the digital relay 3B to the power system. Disconnect from the protective action.

  At least one of the bypass command signal s2a and the digital relay abnormality signal s3a is output from the digital relay 3A to the bypass circuit 4A, and at least one of the bypass command signal s2b and the digital relay abnormality signal s3b is output from the digital relay 3B to the bypass circuit 4B. When output, both bypass circuits 4A and 4B operate to return the digital relays 3A and 3B to the power system protection operation.

  In the conventional digital relay device 2, the bypass circuits 4A and 4B are configured by hardware.

Patent Document 1 discloses a digital relay device that multiplexes digital relays and enables detection of a power system failure even when one of the digital relays fails.
JP-A-8-149682

  However, in the conventional digital relay device 2, it is not detected whether or not the bypass circuits 4A and 4B are normally executing the bypass function.

  Further, at least one of the bypass command signal s2a and the digital relay abnormality signal s3a is output from the digital relay 3A to the bypass circuit 4A, and at least one of the bypass command signal s2b and the digital relay abnormality signal s3b is output from the digital relay 3B to the bypass circuit 4B. When one of them is output, it is not monitored whether the digital relays 3A and 3B have returned to normal.

  Therefore, it is difficult to detect abnormality of bypass control by the bypass circuits 4A and 4B, and when abnormality occurs in the bypass circuits 4A and 4B, the bypass control of the digital relays 3A and 3B may not be executed correctly.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a digital relay device that detects an abnormality of a bypass circuit and improves the reliability of protecting a power system.

  The above-described problems include a first digital relay that detects an abnormality of a power system, a first bypass unit that bypasses the first digital relay based on a first signal input from the first digital relay, and an abnormality detection of the power system. A second digital relay; a second bypass means for bypassing the second digital relay based on a second signal input from the second digital relay; an input state of the first signal in the first bypass means; and a first bypass means. The first digital relay bypass state, the second signal input state of the second bypass means, and the second digital relay bypass state of the second bypass means, And a bypass abnormality detecting means provided inside the first digital relay means The location is solved.

  According to the present invention, abnormality of the bypass circuit can be detected, and the reliability of protection of the power system can be improved.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  In the present embodiment, a bypass command signal input to the bypass circuit and a bypass state signal indicating that the digital relay is in a state bypassed by the bypass circuit are taken into the digital relay, and the digital relay relays the bypass circuit. A digital relay device that performs abnormality detection will be described.

  FIG. 1 is a circuit diagram showing an example of a digital relay device according to the present embodiment. Note that the same portions as those in FIG. 4 are denoted by the same reference numerals and description thereof is omitted.

  The digital relay device 13 according to the present embodiment is applied to, for example, a railway receiving and transforming system that supplies electric railway power. The digital relay device 13 may be applied to other devices or facilities such as a substation.

  The digital relay device 13 includes digital relays 14A and 14B, bypass circuits 15A and 15B, switches 5A and 5B, and a trip command power supply P1.

  The digital relay 14A has a feature including a bypass abnormality detection unit 16a, and other functions are the same as those of the digital relay 3A of FIG.

  In the following, the configuration and operation of the digital relay 14A and the bypass circuit 15A that bypasses the digital relay 14A will be mainly described. The same applies to the configuration and operation of the digital relay 14B and the bypass circuit 15B that bypasses the digital relay 14B.

  In the following description, the combination of the digital relay 14A and the bypass circuit 15A is a first system, and the combination of the digital relay 14B and the bypass circuit 15B is a second system.

  When the bypass circuit 15A receives the bypass command signal s2a from the digital relay 14A, the bypass abnormality detection unit 16a inputs the first system command reception signal s4a indicating that fact from the bypass circuit 15A.

  When the digital relay 14A is bypassed by the bypass circuit 15A, the bypass abnormality detection unit 16a inputs a first system bypass state signal s5a indicating that from the bypass circuit 15A.

  When the bypass circuit 15B receives the bypass command signal s2b from the digital relay 14B, the bypass abnormality detection unit 16a inputs a second system command reception signal s4b indicating that fact from the bypass circuit 15B.

  When the digital relay 14B is bypassed by the bypass circuit 15B, the bypass abnormality detection unit 16a inputs a second system bypass state signal s5b indicating that from the bypass circuit 15B.

  And the bypass abnormality detection part 16a is consistency between 1st system command reception signal s4a and 1st system bypass state signal s5a, and between 2nd system command reception signal s4b and 2nd system bypass state signal s5b. The abnormality detection of the bypass circuits 15A and 15B is performed based on the consistency of.

  For example, when both the first system command reception signal s4a and the second system command reception signal s4b are “input” to the bypass abnormality detection unit 16a, both the bypass circuits 15A and 15B are connected to the digital relays 14A and 14B. The bypass is released and the digital relays 14A and 14B are returned.

  For this reason, when both the first system command acceptance signal s4a and the second system command acceptance signal s4b are “input” to the bypass abnormality detection unit 16a, the first system bypass state signal s5a and the second If there is no input for both of the system bypass status signals s5b, it is determined as “normal”, and if at least one of the first system bypass status signal s5a and the second system bypass status signal s5b is “input”, it is determined as “abnormal” To do.

  Further, when both the first system command reception signal s4a and the second system command reception signal s4b are not “input” to the bypass abnormality detection unit 16a, the first system command reception signal s4a and the first system bypass state signal s5a are detected. Is “normal”, and the presence or absence of at least one of the inputs is consistent between the second system command acceptance signal s4b and the second system bypass state signal s5b. If there is no consistency, it is determined as “abnormal”.

  When the logical product (AND) of the trip signals s1a and s1b is obtained by hardware outside the digital relays 14A and 14B and the trip signals s1a and s1b are output from both the digital relays 14A and 14B, the circuit breaker 1 is tripped.

  The bypass circuit 15A includes a power supply P2a, a power supply P4a, switches 6a to 10a, 17a, and 18a, and switch control units (for example, relays) 19a and 20a.

  Similarly, the bypass circuit 15B includes a power source P2b, a power source P4b, switches 6b to 10b, 17b and 18b, and switch control units 19b and 20b.

  In the present embodiment, the switch 7a and the switch 8a are provided in parallel between the power source P2a and the switch control unit 19a.

  The switch controller 19a turns on the switch 9a when at least one of the bypass command signal s2a and the digital relay abnormality signal s3a is input, and turns off the switch 9a in other cases.

  The switch control unit 19a sets the switch 10b of the bypass circuit 15B to the OFF state when at least one of the bypass command signal s2a and the digital relay abnormality signal s3a is input, and sets the switch 10b to the ON state in the other cases. To do.

  Further, the switch control unit 19a turns on the switch 17a when at least one of the bypass command signal s2a and the digital relay abnormality signal s3a is input, and turns off the switch 17a in other cases.

  The switch 9a and the switch 10a are provided in series between the power supply P4a and the switch control unit 20a.

  When both the switch 9a and the switch 10a are turned on, the current from the power source P4a is supplied to the switch control unit 20a, and the switch control unit 20a operates.

  The switch control unit 20a outputs at least one of the bypass command signal s2a and the digital relay abnormality signal s3a from the digital relay 14A to the bypass circuit 15A, and digitally outputs the bypass command signal s2b and the digital signal from the digital relay 14B to the bypass circuit 15B. When none of the relay abnormality signals s3b is output, the switch 6a is turned on and the switch 18a is turned on.

  When the switch 6a is turned on, the digital relay 14A is bypassed, the digital relay 14A is disconnected from the protection operation of the power system, and the protection operation is performed only by the digital relay 14B.

  Here, at least one of the bypass command signal s2a and the digital relay abnormality signal s3a is output from the digital relay 14A to the bypass circuit 15A, and the bypass command signal s2b and the digital relay abnormality signal s3b are output from the digital relay 14B to the bypass circuit 15B. And at least one of them is output.

  In this case, in the bypass circuit 15A, the switch 10a is turned off, the switch 6a is also turned off in accordance with the switch control unit 20a, and the bypass state of the digital relay 14A by the bypass circuit 15A is released.

  In the bypass circuit 15B, the switch 10b is turned off, the switch 6b is also turned off according to the switch control unit 20b, and the bypass state of the digital relay 14A by the bypass circuit 15B is released.

  As a result, both of the digital relays 14A and 14B in the bypass state are again incorporated into the power system protection operation.

  The switch 17a is turned on when at least one of the bypass command signal s2a and the digital relay abnormality signal s3a is output from the digital relay 14A to the bypass circuit 15A.

  Then, the current from the power supply P4a is supplied to the bypass abnormality detection unit 16a of the digital relay 14A and the bypass abnormality detection unit 16b of the digital relay 14B via the switch 17a. Thereby, the first system command acceptance signal s4a is output from the bypass circuit 15A to the bypass abnormality detection unit 16a of the digital relay 14A and the bypass abnormality detection unit 16b of the digital relay 14B.

  When the switch 6a is turned on according to the operation of the switch control unit 20a and the digital relay 14A is set to the bypass state, the switch 18a is also turned on according to the operation of the switch control unit 20a.

  Then, the current from the power supply P4a is supplied to the bypass abnormality detection unit 16a of the digital relay 14A and the bypass abnormality detection unit 16b of the digital relay 14B via the switch 17a.

Thus, the first system bypass state signal s5a is output from the bypass circuit 15A to the bypass abnormality detection unit 16a of the digital relay 14A and the bypass abnormality detection unit 16b of the digital relay 14B.

  The outline | summary of operation | movement of the above digital relay apparatuses 13 is demonstrated.

  When the digital relays 14A and 14B detect an abnormality in the power system based on the current I and voltage V of the power system, the digital relays 14A and 14B output trip signals s1a and s1b to the switches 5A and 5B, respectively. When both of the trip signals s1a and s1b are output from the digital relays 14A and 14B to the switches 5A and 5B, the digital relay device 13 sets the circuit breaker 1 in a disconnected state.

  When at least one of the bypass command signal s2a and the digital relay abnormality signal s3a is output from the digital relay 14A to the bypass circuit 15A, the bypass circuit 15A bypasses the digital relay 14A, and the digital relay 14A operates to protect the power system. Disconnect from. Thereafter, the power system protection operation is performed by the other digital relay 14B.

  Similarly, when at least one of the bypass command signal s2b and the digital relay abnormality signal s3b is output from the digital relay 14B to the bypass circuit 15B, the bypass circuit 15B bypasses the digital relay 14B and connects the digital relay 14B to the power system. Disconnect from the protective action.

  At least one of the bypass command signal s2a and the digital relay abnormality signal s3a is output from the digital relay 14A to the bypass circuit 15A, and at least one of the bypass command signal s2b and the digital relay abnormality signal s3b is output from the digital relay 14B to the bypass circuit 15B. When output, both bypass circuits 15A and 15B operate to restore the digital relays 14A and 14B.

  The bypass abnormality detection unit 16a of the digital relay 14A receives the first system command acceptance signal s4a and the first system bypass state signal s5a from the bypass circuit 15A, and the second system command acceptance signal s4b and the second system bypass state. The signal s5b is input from the bypass circuit 15B.

  The bypass abnormality detection unit 16a then bypasses the bypass circuit 15A based on the input states of the first system command acceptance signal s4a, the first system bypass status signal s5a, the second system command acceptance signal s4b, and the second system bypass status signal s5b. , 15B is detected.

  Similarly, the bypass abnormality detection unit 16b of the digital relay 14B also receives the first system command acceptance signal s4a and the first system bypass state signal s5a from the bypass circuit 15A, and the second system command acceptance signal s4b and the second system command acceptance signal s4b. System bypass state signal s5b is input from bypass circuit 15B.

  The bypass abnormality detection unit 16b then bypasses the bypass circuit 15A based on the input states of the first system command reception signal s4a, the first system bypass state signal s5a, the second system command reception signal s4b, and the second system bypass state signal s5b. , 15B is detected.

  The abnormality detection results of the bypass circuits 15A and 15B by the bypass abnormality detection units 16a and 16b may be displayed on a display device, or may be output by an alarm sound or sound.

  FIG. 2 shows the input states of the first system command acceptance signal s4a, the first system bypass state signal s5a, the second system command acceptance signal s4b, and the second system bypass state signal s5b and the abnormality detection in the bypass abnormality detection units 16a and 16b. It is a figure which shows an example of the relationship with a result.

  Except when both the first system command reception signal s4a and the second system command reception signal s4b are “input”, the input states of the first system command reception signal s4a and the first system bypass state signal s5a are matched, When the input states of the second system command acceptance signal s4b and the second system bypass state signal s5b match, it is determined as “normal”.

  When both the first system command acceptance signal s4a and the second system command acceptance signal s4b are “input”, both the first system bypass state signal s5a and the second system bypass state signal s5b are “no input”. In this case, it is determined as “normal”.

  In other cases, it is determined as “abnormal”.

  FIG. 3 is a functional block diagram illustrating an example of the digital relay device 13.

  The bypass circuit 15A includes a bypass operation unit 21a, a reception signal output unit 22a, and a state signal output unit 23a.

  When at least one of the bypass command signal s2a and the digital relay abnormality signal s3a is input from the digital relay 14A, the bypass operation unit 21a performs an operation for bypassing the digital relay 14A.

  When at least one of the bypass command signal s2a and the digital relay abnormality signal s3a is input from the digital relay 14A, the reception signal output unit 22a outputs the first system command reception signal s4a to the bypass abnormality detection units 16a and 16b.

  The status signal output unit 23a outputs the first system bypass status signal s5a to the bypass abnormality detection units 16a and 16b when the digital relay 14A is bypassed by the bypass operation unit 21a.

  The bypass circuit 15B includes a bypass operation unit 21b, a reception signal output unit 22b, and a state signal output unit 23b.

  When at least one of the bypass command signal s2b and the digital relay abnormality signal s3b is input from the digital relay 14B, the bypass operation unit 21b performs an operation for bypassing the digital relay 14B.

  The reception signal output unit 22b outputs the second system command reception signal s4b to the bypass abnormality detection units 16a and 16b when at least one of the bypass command signal s2b and the digital relay abnormality signal s3b is input from the digital relay 14B.

  The state signal output unit 23b outputs the second system bypass state signal s5b to the bypass abnormality detection units 16a and 16b when the digital relay 14B is bypassed by the bypass operation unit 21b.

  In the present embodiment described above, abnormality detection of the power system is performed, abnormality detection of the digital relays 14A and 14B is performed, and abnormality detection of the bypass circuits 15A and 15B is further performed.

  Therefore, the reliability of protection of the power system by the digital relay device 13 can be improved.

  In the present embodiment, since the bypass abnormality detection units 16a and 16b are provided in the digital relays 14A and 14B, respectively, space cost can be suppressed, wiring can be simplified, and the digital relay device 13 can be used as a power system. On the other hand, it can be easily introduced.

  In the present embodiment, each component can be freely modified as long as the same operation can be realized, the arrangement may be changed, the combinations can be freely made, and the components can be divided. Is possible.

  For example, the power source P2a and the power source P4a may be combined, or the power source P2b and the power source P4b may be combined.

  For example, any one of the bypass abnormality detection unit 16a and the bypass abnormality detection unit 16b may be deleted.

  For example, the functions of the bypass abnormality detection units 16a and 16b may be realized by software.

  The present invention is effective in the field of power system protection equipment.

1 is a circuit diagram showing an example of a digital relay device according to an embodiment of the present invention. The figure which shows an example of the relationship between the input state of a 1st system command reception signal in a bypass abnormality detection part, a 1st system bypass state signal, a 2nd system command reception signal, and a 2nd system bypass state signal, and an abnormality detection result. The functional block diagram which shows an example of the digital relay apparatus which concerns on the same embodiment. The circuit diagram which shows an example of the conventional digital relay apparatus.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Circuit breaker, 2, 13 ... Digital relay apparatus, 3A, 3B, 14A, 14B ... Digital relay, 4A, 4B, 15A, 15B ... Bypass circuit, 5A, 5B, 6a-10a, 6b-10b, 17a, 17b , 18a, 18b ... switch, 11a, 11b, 12a, 12b, 19a, 19b, 20a, 20b ... switch control unit, 16a, 16b ... bypass abnormality detection unit, 21a, 21b ... bypass operation unit, 22a, 22b ... acceptance signal Output unit, 23a, 23b ... status signal output unit

Claims (4)

A first digital relay for detecting an abnormality in the power system;
First bypass means for bypassing the first digital relay based on a first signal input from the first digital relay;
A second digital relay for detecting an abnormality in the power system;
Second bypass means for bypassing the second digital relay based on a second signal input from the second digital relay;
The input state of the first signal in the first bypass means, the bypass state of the first digital relay by the first bypass means, the input state of the second signal in the second bypass means, and the second bypass And a bypass abnormality detecting means provided inside the first digital relay for detecting an abnormality between the first bypass means and the second bypass means based on a bypass state of the second digital relay by the means. A digital relay device. The digital relay device according to claim 1, wherein
The bypass abnormality detecting means is configured to match the input state of the first signal and the bypass state of the first digital relay, and match the input state of the second signal and the bypass state of the second digital relay. Based on this, the abnormality detection of said 1st bypass means and said 2nd bypass means is performed, The digital relay apparatus characterized by the above-mentioned. In the digital relay device according to claim 1 or 2,
The bypass abnormality detection means is provided in both the first digital relay and the second digital relay, and is a digital relay device. The digital relay device according to any one of claims 1 to 3,
The first signal includes a first bypass command signal indicating that the first digital relay is bypassed by the first bypass means, and a first digital relay abnormality signal indicating that an abnormality of the first digital relay is detected. At least one of
The second signal includes a second bypass command signal indicating that the second digital relay is bypassed by the second bypass means, and a second digital relay abnormality signal indicating that an abnormality of the second digital relay is detected. A digital relay device characterized by being at least one of the above.

Images