Method and apparatus for detecting communication channel delay asymmetry

1. A method for detecting communication channel delay asymmetry between transmission line protection devices, the method, comprising:

calculating, repeatedly, a clock disparity between clocks of the protection devices and communication delays at different paths of a communication channel;

comparing a latest calculated clock disparity and communication delays with previously calculated clock disparities and communication delays, respectively;

determining a channel switching has happened if a change of the calculated clock disparity exceeds a first threshold, or a change of the calculated communication delays for any path exceeds a second threshold; and

determining the channel delays as asymmetrical if a difference between the calculated communication delays of the different paths after the channel switching exceeds a third threshold.

2. The method according to claim 1 , wherein,

the clock disparity and communication delays are calculated based on a difference between the communication delays of data sending and receiving paths; and the method further comprises:

synchronizing signals sampled by protection devices based on the calculated clock disparities and communication delays.

3. The method according to claim 2 , the step of determining the channel delays as asymmetrical comprises:

calculating the communication delays of sending and receiving path after the channel switching, based on the clock disparities calculated before the channel is switched; and

calculating the difference between the communication delays of sending and receiving path after the channel switching.

4. The method according to claim 1 , the step of determining the channel delays as asymmetrical comprises:

calculating the communication delays of sending and receiving path after the channel switching, based on the clock disparities calculated before the channel is switched; and

calculating the difference between the communication delays of sending and receiving path after the channel switching.

5. The method according to claim 4 , the step of calculating the communication delays after the channel switching further comprises:

comparing the calculated communication delays after the channel switching with the calculated communication delays before the channel switching;

identifying a communication path which is un-switched if the change of the calculated communication delays of the path is less than a fourth threshold;

adjusting the clock disparities by the communication delay of the un-switched communication channel, and

adjusting the communication delays of the data sending and receiving path by the adjusted clock disparities.

6. The method according to claim 5 , the step of comparing the calculated communication delays comprises:

calculating a mean value of the calculated communication delays before the channel switching;

calculating a mean value of the calculated communication delays after the channel switching;

comparing the calculated mean value after the channel switching with the calculated mean value before the channel switching.

7. The method according to claim 4 , the step of calculating the difference between the communication delays of sending and receiving paths comprises:

calculating a mean value of the calculated communication delays of sending path and a mean value of calculated communication delays of receiving path; and

calculating the difference between the mean values of the communication delays of sending path and receiving path.

8. The method according to claim 1 , the step of comparing clock disparities and communication delays further comprises:

calculating a mean value of the calculated clock disparities, a mean value of the calculated communication delays of data sending path, and a mean value of the calculated communication delays of data receiving path;

comparing the latest calculated clock disparity with the mean value of calculated clock disparities; and

comparing the latest calculated communication delays with the mean values of calculated communication delays.

9. The method according to claim 1 , wherein:

the first and fourth thresholds depend on an accuracy of oscillators of the protection devices, a length of communication interruption period caused by the channel switching, and a jitter characteristic of communication channels; and

the second and third thresholds depend on the jitter characteristic of the communication channel.

10. A method for detecting communication channel delay asymmetry between transmission line protection devices, wherein the communication channel comprises a first channel and a second channel, the first channel comprises a first path and a second path, and the second channel comprises a third path and a fourth path, the method, comprising:

sending data through the first path of the first channel and receiving the data through the third path of the second channel;

calculating a first delay for sending and receiving the data through the first path of the first channel and the third path of the second channel sequentially;

sending data through the fourth path of the second channel and receiving the data through the second path of the first channel;

calculating a second delay for sending and receiving data through the fourth path of the second channel and the second path of the first channel sequentially;

determining the communication channels as asymmetrical if the difference between the first delay and the second delay exceeds a first threshold;

calculating, at least twice, a third delay for sending and receiving data through the first channel;

calculating, at least twice, a fourth delay for sending and receiving data through the second channel;

obtaining a difference between the at least two third delays and a difference between the at least two fourth delays;

determining the first channel as switched channel if the difference between third delays is more than a second threshold, and determining the second channel as switched channel if the difference between fourth delays is more than the second threshold; and

calculating the communication delays of sending and receiving paths of the switched channel.

11. The detecting method according to claim 10 , wherein:

the first and second thresholds depend on a jitter characteristic of the communication channels.

12. An apparatus for detecting communication channel delay asymmetry between transmission line protection devices, the apparatus, comprising:

a calculating unit, which is configured to repeatedly calculate a clock disparity between clocks of the protection devices and communication delays at different paths of the communication channel,

a comparing unit, which is configured to compare a latest calculated clock disparity and communication delays with previously calculated clock disparities and communication delays, respectively;

a first determining unit, which is configured to determine a channel switching has happened if a change of the calculated clock disparity exceeds a first threshold, or a change of the calculated communication delays for any path exceeds a second threshold; and

a second determining unit, which is configured to determine the channel delays as asymmetrical if a difference between the calculated communication delays of the different paths after the channel switching exceeds a third threshold.