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Mauricio Subieta, Ph D
Mauricio Subieta, Ph D
 · Posted in Home

FLISR and IEC 61850 GOOSE Communications over LTE networks using QoS and IP/MPLS

The modernization of electric distribution grids is essential for enhancing their reliability, efficiency, and responsiveness. An important key to this modernization is the implementation of advanced communication technologies to support critical distribution automation (DA) operations such as fault location, isolation, and service restoration (FLISR).

The IEC 61850 standards provide the communications foundation for FLISR and other distribution automation applications through the Generic Object Oriented Substation Event (GOOSE) protocol, which is vital for real-time protection and control. GOOSE messaging provides rapid communication between intelligent electronic devices (IEDs) to support protection and control operations.

Achieving the low latency that these operations require necessitates a robust field area network (FAN) communication infrastructure.

A converged FAN, based on LTE technology combined with quality of service (QoS) and IP/MPLS Virtual Private LAN Service (VPLS) to meet GOOSE layer 2 requirements, offers a solid communications solution. This article explores how these technologies can be integrated to meet the stringent latency requirements of FLISR and GOOSE messaging to provide effective protection in electrical distribution grids.

IEC 61850 Standard

The IEC 61850 standard provides a comprehensive framework for communication networks and systems in substations and across the distribution grid. It facilitates interoperability between devices and systems used in the generation, transmission, and distribution of electrical power.

The standard defines communication protocols for data exchange between devices, system configuration and engineering processes, and data modeling and semantic definitions for equipment. The communication protocols include:

  • Manufacturing Message Specification (MMS): A real-time communication and control protocol that facilitates the exchange of data between IEDs and control systems.
  • GOOSE: A multicast communication protocol used for fast transmission of time-critical messages, such as protection and control signals. This paper focuses on this protocol.
  • Sampled Values (SV): A protocol for the transmission of sampled measurement data, primarily used in digital protection and control systems.

While initially focused on substation automation, IEC 61850 has been extended to cover distribution automation applications and other domains, such as hydropower plants (IEC 61850-7-410), distributed energy resources (IEC 61850-7-420) and wind power plants (IEC 61400-25).

GOOSE Messaging

Described in IEC 61850-8-1, GOOSE is a layer 2 protocol that transports messages over Ethernet. It facilitates high-speed, event-driven communication between IEDs in substations and across the distribution grid.

The key characteristics of GOOSE messaging include:

  • Low latency: GOOSE messages require extremely low transmission latency to ensure timely execution of protection and control commands.
  • High reliability: GOOSE communication must be highly reliable to ensure the integrity of protection and control functions, often including redundant paths.
  • Event driven: GOOSE transmits messages based on specific events, ensuring timely responses.
  • Multicast capabilities: GOOSE supports layer 2 multicast transmission, allowing multiple devices to receive the same message simultaneously. This is essential for coordinated protection schemes.

Effective GOOSE communication is essential for the protection and control operations in electrical distribution systems. It requires a communication network that can meet stringent latency and reliability requirements. Table 1 shows the different time requirements for GOOSE messages.

Table 1: GOOSE message types

 

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Overview of FLISR

FLISR is an advanced distribution automation application designed to improve the reliability of the distribution grid. It quickly identifies, isolates, and restores service during fault conditions, significantly reducing outage duration and improving the grid’s resilience. The key components of FLISR include:

  • Fault detection: Identifying faults using sensors and IEDs.
  • Fault location: Determining the precise location of faults.
  • Fault isolation: Automatically isolating the faulted section to prevent further damage and maintain safety.
  • Service restoration: Reconfiguring the network to swiftly restore power to unaffected areas.

FLISR deployment comes in different flavors, as explained in IEC TR 61850-90-6:

  • Centralized control: When a fault occurs, the main breaker trips, recloses one or more times, and then remains open. The system uses information transmitted to locate the fault, and then sends commands to isolate it, reclose the feeder breaker, and restore power to the healthy upstream feeder section.
  • Distributed control: The feeder equipment controller locates and isolates faults and restores service to healthy sections based on information exchanged among the IEDs that control the main breaker in the substation and the switches in the feeders. 
  • Local control: When a fault occurs on a feeder, the switches react to it by autonomously opening or closing according to local overcurrent or voltage measurements. The decisions are made locally, although there are communications with the master station.

Figure 1: FLISR operation and components

 

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FLISR operations rely heavily on real-time communication to coordinate the actions of various field devices and central control systems. This requires a communication network that supports low latency and high reliability.

LTE-based FAN Supporting Smart Grid Applications: FLISR

Private wireless networks based on LTE technology provide high-speed wireless communications unencumbered by commercial carriers’ modes of operation and regulations. Since a private wireless network’s design and operation are primarily focused on supporting digitalized data-exchanging applications, utilities can implement features within the LTE specifications and standards to meet requirements specific to their mission-critical applications. The LTE standards define characteristics, depending on the available spectrum, that meet the most stringent requirements of smart grid applications, such as FLISR using GOOSE, including:

  • High data rates: LTE offers sufficient bandwidth for transmitting large volumes of data, with theoretical speeds above 600 Mbps in the downlink and 150 Mbps in the uplink.
  • Low latency: LTE supports latency in the tens of milliseconds, which is suitable for real-time applications.
  • Quality of service (QoS): LTE supports prioritization of critical data traffic ensuring reliability for mission-critical applications
  • Scalability: LTE can extend coverage over large areas, making it ideal for large distribution networks.
  • Reliability: LTE provides robust connectivity with high availability, both of which are essential for the continuous operation of smart grid applications.

Quality of Service

QoS mechanisms in the FAN ensure that critical traffic, such as GOOSE messages supporting FLISR, is prioritized, meeting the necessary latency and reliability requirements. Key aspects of QoS include:

  • Traffic classification: Differentiating between various types of traffic (e.g., mission-critical, business-critical, and low-priority data) and assigning them to appropriate QoS classes.
  • Scheduling: Allocating network resources to ensure that high-priority traffic receives adequate bandwidth and low latency.
  • Packet prioritization: Marking packets with different priority levels to manage congestion and ensure timely delivery.
  • Resource allocation: Dynamically assigning network resources to maintain optimal performance for critical applications.

Figure 2 shows how traffic is processed when QoS is implemented.

Figure 2: QoS operation

 

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QoS relies on the Differentiated Services (DiffServ) architecture, which provides a scalable and straightforward method for classifying and managing network traffic. DiffServ uses Differentiated Services Code Point (DSCP) as the mechanism for classifying and prioritizing the traffic. DSCP values range from 0 to 63, with each value representing a specific level of service. These values are typically grouped into classes to simplify QoS policies:

  • Default Forwarding (DF): DSCP 0, best-effort service without specific QoS guarantees.
  • Class Selector (CS): DSCP values from 8 to 56 in increments of 8 (e.g., CS1 = 8, CS2 = 16).
  • Assured Forwarding (AF): DSCP values that specify four classes (AF1x to AF4x) with three drop precedence levels each (e.g., AF11, AF12, AF13) to support different levels of assurance for delivery.
  • Expedited Forwarding (EF): DSCP 46, which is suitable for low-latency, low-loss, and low-jitter services such as VoIP, as well as for real-time applications.

LTE QoS Mechanisms

LTE networks use specific mechanisms to manage QoS and ensure that the traffic receives the same or equivalent service level as in wired point-to-point networks.

QoS Class Identifiers

LTE introduces the concept of QoS Class Identifiers (QCIs), which are sets of parameters that extend the classifications and differentiation of traffic. Each QCI is associated with specific QoS characteristics such as priority level, packet delay budget (PDB), and packet error loss rate (PELR).

LTE bearers

Bearers in LTE are logical channels that carry traffic between the router (UE) and the packet data network (PDN). There are two bearer types: non-dedicated and dedicated.

Non-dedicated bearers

  • Default bearer: Every LTE device is assigned a default bearer when it attaches to the network. This bearer provides basic connectivity to the PDN and supports non-critical, best-effort traffic. The default bearer does not guarantee specific QoS parameters, only basic connectivity.

Dedicated bearer 

  • Established in addition to the default bearer, dedicated bearers are used for specific types of traffic that require particular QoS characteristics. Each dedicated bearer is associated with a specific QCI to ensure that the traffic it carries receives the necessary priority, latency and reliability. Dedicated bearers can be further categorized into:
    • Guaranteed Bit Rate (GBR) bearers: These bearers guarantee a certain amount of bandwidth for the traffic they carry. They are used for applications such as voice calls and real-time video, where consistent bandwidth and low latency are crucial.
    • Non-Guaranteed Bit Rate (Non-GBR) bearers: These bearers do not guarantee a specific bandwidth but still provide better QoS parameters than the default bearer. They are used for applications that require better-than-best-effort service but do not need guaranteed bandwidth, such as streaming video or online gaming.

The LTE QoS architecture combines QCIs and bearers (GBR and Non-GBR) to provide a mapping that supports the wide area network (WAN) QoS and ensures that traffic delivery achieves the latency and priority required for mission-critical applications, such as FLISR.

Table 2: LTE QoS architecture: QCIs and bearers

 

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Mapping DSCP to QCI values

Table 3 maps DSCP values to QCI values in LTE networks. This mapping helps ensure that the IP-based QoS settings align with the LTE-specific QoS requirements.

Table 3: DSCP to QCI mapping

 

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This mapping ensures that traffic classified with specific DSCP values receives the appropriate QoS treatment in LTE networks, aligning IP-based QoS settings with LTE-specific QoS requirements.

IP/MPLS VPLS in Smart Grids

Utilities have successfully and widely introduced packet-based technologies such as Multiprotocol Label Switching (MPLS) to carry network traffic for different critical applications. MPLS is a connection-oriented technology that defines specific primary and backup paths for packet traffic with predefined paths, low propagation latency and packet prioritization. Utilities have been deploying IP/MPLS networks to support converged WAN and backbone connectivity. These networks primarily support packet-based traffic, but they also support legacy technologies such as TDM and serial connections, along with protection and control applications.

Packet-based technologies are capable of handling much larger communication loads and are more in tune with wider telecommunication requirements. LTE and 5G are IP-based technologies that enhance the reliability, resiliency, and security of in-transit data when they are combined with MPLS.

IP/MPLS networks rely on virtual private network (VPN) services and technologies that enable the LTE network to carry the data of many different applications with separate forwarding tables. Regardless of whether VPNs are IP (L3), Ethernet (L2) or serial (RS-232), they will operate in their own realms, shielded from one another, with each application running as if it were on a separate physical network, as shown in Figure 3.

Figure 3: Mission-critical IP/MPLS WAN

 

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A VPLS is one such VPN service. It provides a scalable and efficient data forwarding mechanism that can deliver the enhanced performance and reliability required for GOOSE messaging. The key characteristics of a VPLS include:

  • Scalability: VPLS supports large-scale networks with numerous nodes and mechanisms that mitigate potential layer 2 weaknesses, including broadcast storms and loops.
  • Traffic engineering: VPLS optimizes data routing and resource utilization to meet latency requirements.
  • Reliability: VPLS ensures high availability and quick rerouting in case the network fails.
  • QoS support: VPLS inherently supports QoS, which allows the prioritization of critical traffic and ensures timely delivery of critical data.

Integrating LTE, QoS, and IP/MPLS VPLS for FLISR and GOOSE Communications

In electric distribution grids, the integration of an LTE-based FAN with QoS and IP/MPLS VPLSs provides a robust communication framework for using GOOSE messaging to support FLISR applications.

Architecture

Figure 4 shows a high-level diagram of the interconnection required to support FLISR and GOOSE messaging:

  • A converged FAN provides wireless connectivity for communication between field devices and central control systems.
  • QoS mechanisms ensure that FLISR and GOOSE traffic receives the highest priority, minimizing latency and ensuring reliable communication with specific QCIs and dedicated bearers.
  • A converged WAN provides the connectivity for the LTE radio access network (RAN) and core, as well as for utility mission-critical applications, such as teleprotection.

Figure 4: LTE and IP/MPLS architecture for FLISR

 

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Integration

To successfully support the FLISR application over IP/MPLS, it is important to understand the limitations of the technology and to design and configure the equipment accordingly.

The design implementation process involves executing several key steps to ensure the mission-critical FLISR traffic availability and latency requirements are met:

  1. Network design (including QoS considerations) 
  • WAN: The connection between the radio units (RAN) and the core needs to be configured to treat FLISR traffic as priority traffic (marked as AF41, AF42, or AF43) while also considering other higher priority traffic.

For FLISR applications that require lower latency (<50 ms)––for example, fallen wire deactivation, also known as Fallen Conductor Protection/Mitigation–– the WAN traffic could be marked as EF.

Typically, an interior gateway protocol (IGP) will be used to interconnect all the routers in the WAN. Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (ISIS) are the most commonly used IGPs, but other protocols such as internal BGP can also be used. These protocols can be complemented by additional mechanisms such as the Resource Reservation Protocol (RSVP) with Traffic Engineering (TE) extensions to ensure that paths created by these routing protocols take bandwidth and latency requirements into consideration.

  • FAN: Traffic from the LTE routers and user equipment (UE) will be handled based on the information configured in the subscriber database (HSS), where a specific bearer type and QCI will be defined on a per-international mobile subscriber identity (IMSI) basis.

For FLISR-supporting devices, the IMSI should be configured with a dedicated GBR bearer, with a QCI mark of 4 (suitable for real-time applications, as shown in Table 3). If the FLISR application requires latencies less than or equal to 50 ms, the QCI marking should be 3.

  1. IP/MPLS VPLS integration 

A layer 2 network with optional multicasting support is required to properly support FLISR applications using GOOSE traffic.

LTE is designed to operate as a layer 3 technology. It uses specific radio network access protocols and tunneling to ensure the bearer establishment. As such, layer 2 services need to be emulated on top of the layer 3 connections. This is why it is so important to support IP/MPLS services in the router.

A utility can configure a VPLS with a hub-and-spoke topology between the LTE routers as end nodes and the IP/MPLS core router with additional QoS parameters to ensure service-level prioritization to support GOOSE-based FLISR applications. The use of multiple core routers will provide redundancy.

  1. System validation and testing 

It is highly recommended that utilities conduct extensive testing to validate the performance and reliability of the integrated system, ensuring it meets the latency and reliability requirements for FLISR and GOOSE communications. A utility may need to adjust QoS parameters and make configuration changes to the IEDs, taking LTE technology limitations into consideration.

Figure 5 shows a sample latency capture of 300 samples taken every 3 seconds (15 minutes) with an operational system that is based on these proposed integration steps and that includes additional traffic sources. Notice the variation on the latency for the QoS versus the non-QoS shaped traffic.

Figure 5: Latency plot for QoS and non-QoS shaped traffic

 

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Challenges and Considerations

The implementation of LTE-based communication systems to support FLISR applications and GOOSE messaging presents some challenges that need to be considered:

  • Coverage and capacity: The latency, capacity and throughput of the system is ultimately constrained by the available spectrum and the number of use cases being supported by the LTE system. Ensuring comprehensive LTE coverage and sufficient capacity to handle critical traffic requires careful, well-considered design.
  • Security: Implementing robust security measures to protect the communication infrastructure from cyber threats and ensure data integrity is paramount in a mission-critical network. Encryption can potentially increase the response time of the system overall, leading to higher latency.
  • Interoperability: In multivendor networks, ensuring compatibility between network elements from different vendors is a key consideration. Although all the topics mentioned in this article are based on industry standards, implementation details need to be carefully analyzed to ensure a proper end-to-end system response.

Conclusion

The integration of LTE, QoS, and IP/MPLS VPLS technologies provides a robust and reliable solution for supporting FLISR and GOOSE communications in electric distribution grids. This approach ensures the timely and efficient transmission of critical data, enabling rapid fault detection, isolation, and service restoration, as well as reliable protection and control operations. By leveraging these technologies, utilities can significantly enhance the reliability and resilience of their distribution systems, and ultimately deliver greater service quality to their customers.