Line current differential protection upon loss of an external time reference

A method for providing protection using mixed synchronization modes upon absence or inaccuracy of an external reference, the method comprising:

• selectively operating a plurality of communication channels in a protection system in one of a first synchronization mode and an external reference synchronization mode, wherein the selection is based on characteristics of the respective communication channels, and wherein at least a first communication channel operates in the first synchronization mode and at least a second communication channel operates in the external reference synchronization mode;
• detecting absence or inaccuracy of an external reference;
• in response to the absence or inaccuracy of the external reference, applying at least one fallback strategy for data synchronization to the second communication channel;
• and
• using synchronized data communicated through at least one of the first communication channel and the second communication channel in an analysis to selectively trip a power apparatus.

A method for providing protection upon absence or inaccuracy of an external reference, the method comprising:

• operating a communication channel in a protection system in an external reference synchronization mode;
• estimating, in the external reference synchronization mode, an asymmetry value of the communication channel;
• based on the estimated asymmetry value, determining one or more protection settings;
• detecting absence or inaccuracy of an external reference;
• in response to the absence or inaccuracy of the external reference, switching the communication channel from the external reference synchronization mode to a second synchronization mode;
• applying the protection settings in the second synchronization mode;
• and
• using synchronized data communicated through the communication channel in an analysis to selectively trip a power apparatus.

A method for providing protection upon the absence or inaccuracy of an external reference, the method comprising:

• communicating data between a first relay and a second relay through a communication channel in a protection system;
• detecting absence or inaccuracy of an external reference used to synchronize the data communicated between the first relay and the second relay through the communication channel;
• in response to the absence or inaccuracy of the external reference, entering a master-slave mode wherein the first relay and the second relay inhibit their protection function and become slaves to a third relay that communicates with the first and second relays through other communication channels;
• using an analysis in the third relay, wherein the third relay sends a signal to the first and second relays indicating when to trip a power apparatus based on the analysis;
• and
• tripping the power apparatus in response to the signal sent from the third relay.