Hitless failover is key to the electrification of everything
This blog was co-written with Nokia by Andreas Jahr and Adrej Goerbing from Siemens.
As countries around the world strive to meet the ambitious goal of net-zero emissions by 2050, the “electrification of everything” trend is growing. More and more technologies that run on fuel combustion are being replaced by those powered by electricity — making the stability and reliability of the electric grid more important than ever.
Soon, any power outage could affect nearly every aspect of our daily lives: not just lighting and internet access but also the charging of electric vehicles (including buses and container trucks), how we heat our homes and much more. Differential protection is one of the key technologies used in power grids to prevent those kinds of failures, especially in high- and medium-voltage power systems.
To effectively safeguard the grid, differential protection needs a resilient communications network that is always up and running. That way, trips will happen reliably and consistently during abnormal power line conditions. Nokia has partnered with Siemens to successfully validate an innovative hitless failover mechanism using active/active IP/MPLS network paths — enabling the continuous, uninterrupted supply of energy needed to power the 24/7 electrification of everything.
What is differential protection?
Power lines are like water pipes: what goes in must come out. A differential protection relay measures the current at each end of a power line, exchanging data continuously between the two endpoints over a mission-critical communications network to compare their respective values. Allowing for a proper delay offset, the relay can detect any current that may be “leaking” from the line. If a leak is found, the circuit gets tripped and the line is de-energized to protect the power grid.
Pros and cons of using IP/MPLS for differential protection
To ensure proper line protection without causing a false trip, the communications network has to deliver line current data with consistent delay and delay symmetry as well as utmost resiliency. TDM-based networks have proven able to meet these requirements in the past, but as TDM equipment and services support reach their end of life, power utilities must begin migrating their protection communications to new network technologies.
With its inherent deterministic quality of service (QoS) capability, IP/MPLS is the clear choice to replace TDM-based protection communications. While delivering consistent delay symmetry can be tricky in an IP/MPLS network due to random network jitter, this can be resolved with an innovative QoS mechanism called asymmetrical delay control (ADC). Invented by Nokia and thoroughly tested and deployed in the field, ADC removes the random jitter-incurred wait-time impact within the jitter buffer and restores delay symmetry.
IP/MPLS also has a rich suite of redundancy protection tools to recover from a network fault. But as quick as the network can react, the failover will still affect relay communications in the ballpark of tens of milliseconds. On top of that, the protection path that the traffic switches to will exhibit different end-to-end QoS charactertistics (i.e., delay and delay symmetry), which could disrupt or even break the relay algorithms for fault detection.
An industry-validated innovation to solve the resiliency challenge
To overcome the resiliency barrier, Nokia has created an innovative hitless failover mechanism using active/active IP/MPLS paths. The data from a relay is duplicated over two hop-by-hop active paths rather than one active and one standby data path, ensuring the receiving router always gets the data it needs. This approach is “hitless” in that it eliminates the time typically required to switch from an active to standby path in case of a path failure (at minimum 50 to 60 milliseconds), which in turn eliminates packet data loss during failover.
To keep the same delay and delay symmetry characteristics after failover, a “combiner” is in place in the IP/MPLS router connected to the relay. It ensures the delay difference between the two paths is taken into account to achieve a hitless failover.
After a period of rigorous testing in our own lab, we recently collaborated with industry for additional validation of this new innovation. At the Siemens Teleprotection Systel Lab in Berlin, Germany, two Siemens SIPROTEC 5 7SL87 line differential relays were connected to a three-node Nokia IP/MPLS network with two active paths.
Figure 1. Relay test bed topology
The results were stellar: all seven test cases passed with flying colors. For those who are interested, Siemens and Nokia have published a one-page summary of the test results.
You can also learn more about carrying differential protection and teleprotection traffic over IP/MPLS in our e-book.