Critical power infrastructure is having to change, with the evolution of both power and communication technologies. There is a requirement for increased real-time monitoring and control to improve reliability and operational efficiency, and to achieve this requires a communications infrastructure that is both highly reliable and deterministic and in addition must be able to interface with equipment in the field of up to 40+ years old.
A recent Engerati webinar focuses on how IP/MPLS (Internet Protocol/Multiprotocol Label Switching) communications – bringing the benefits and flexibility of IP superimposed with the guaranteed levels of performance of MPLS – can meet these demands, with a focus on teleprotection and SCADA and discussion of experiences from the field. [Engerati-Transitioning Experience - SCADA and Teleprotection to IP/MPLS]
“We believe that if we can crack teleprotection over IP/MPLS, then we can support any other utility application,” says Peter Johnson, Utility Markets Vice President at Alcatel-Lucent, who introduces the webinar.
As a protection application, teleprotection is highly safety, and thus highly time, critical, with a key parameter the signal propagation delay requirement, or latency. Current TDM technology offers a latency around 6ms and new solutions must meet or exceed this level. Other requirements pertain to minimizing jitter in the signal and reducing asymmetry in the signal delay.
IP/MPLS performance in critical infrastructure
In the first presentation in the webinar, Steven Blair, research associate at the University of Strathclyde, describes lab tests using IP/MPLS for teleprotection, demonstrating similar and improved performance to the traditional solutions.
For example, in tests of current differential protection over legacy copper DSL links, “the main finding was that MPLS over DSL can meet the delay requirements under the majority of configurations,” says Blair.
Another group of tests, applicable to substation protection applications, included the feasibility of using IEC 61850 sampled values and GOOSE over IP/MPLS for wide-area protection, demonstrating similar tripping times to the conventional IEEE C37.94 but improved tripping times for backup ‘intertrip’ schemes.
Experiences of teleprotection over IP/MPLS
Beyond the validation of IP/MPLS in the Strathclyde lab, the technology has also been widely tested with vendors in Europe and North America and in the field in both differential and distance protection applications in networks in Europe.
Dominique Verhulst, IP Business EMEA Vertical Markets director at Alcatel-Lucent, who reviews global lessons on teleprotection support with IP/MPLS, says that one of the key results from these tests is that the latencies are mostly in the same range, around 3-3.5ms. Among the outliers from these, higher latencies result with use of an analogue interface. On the other hand, with careful engineering of the circuit the latency can be reduced and for example with the Siemens 7SD52 protection relay and E1 interface a latency is achieved of 1.15ms. With Ethernet interfaces that don’t require packetization, the latency can be reduced even further, down to 100μs.
Verhulst highlights the need to understand the impact of packetization, with the jitter buffer size and payload size both having an impact on the resultant latency and bandwidth that is used. For example, at one customer implementation, with a jitter buffer of 6ms and payload of 16 bytes across a microwave link, a latency of 7.4ms is observed. In another application, a 4-leg 220kV 50Hz current differential scheme that has been implemented for the Luxembourg TSO, Creos, achieves latencies from 5.3ms to as low as 1.1ms, depending on the jitter buffer and payload sizes.
Other issues that need to be considered are to manage asymmetry, to put in place the right traffic engineering and quality of service models, the importance of synchronization and the need to use encryption at the appropriate level.
SCADA support with IP/MPLS
In discussing SCADA application, Hansen Chan, marketing manager at Alcatel-Lucent, says that SCADA traffic isn’t as delay sensitive as teleprotection but still requires reliable transport and also requires precise primary reference clock transport over the network for frequency synchronization.
Chan offers four building blocks for migrating SCADA to IP/MPLS. These include terminating the legacy interface, packetizing the TDM traffic for delivery over IP/MPLS, a multi-drop data bridging function to merge the multi-TDM stream from the SCADA remote terminal units, and introduction of the frequency synchronization.
Such a migration should be planned well in advance as any disruption could have a huge economic cost, Chan advises.Disclaimer: Alcatel-Lucent joins Nokia following successful exchange of shares. Find out more