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Research is done to determine the reliability of different network topologies and how effective different redundancy protocols are. These papers are summarised in Table 2-1 below.

The most common architectures used are star and ring but cascaded and mesh structures are used as well (Sun et al., 2011).

Rapid Spanning Tree Protocol (RSTP), Parallel Redundancy Protocol (PRP) and High Availability Seamless Redundancy (HSR) are mostly used to obtain redundancy in the communication network.

The reliability studies results depend on the Mean Time to Failure (MTTF) of individual components. The outcome of the research will therefore depend on the MTTF value that the researcher uses for the different components in the study. A specific component, like a switch or an IED will have different MTTF values for different suppliers.

The network design must be cost effective. The network design engineer must choose the architecture for the communication network to be reliable and cost effective.

A star connected network has a switch as a single point of failure. The star architecture can be combined with a PRP based SCN architectures to provide a solution to fulfil the availability and performance requirements in SAS for strategic important substations. Practical implementation depends on the components used in the SCN. An example is for the case where an existing station bus network is implemented in a substation and the process bus is required to be added. An IED will require 4 communication ports if the station bus and the process bus are two separate networks and PRP is considered for each network. It may not be economical to replace all the IEDs to implement the process bus.

The IED must support the selected redundancy protocol. The IED of a specific manufacturer may support SNTP and not PRP as an example.

The reliability requirement for a SCN that is used to send GOOSE messages at a station bus level is different than a network that is used to send Sampled Value (SV) messages at a process bus level. The mechanism for sending GOOSE messages allows for messages to be resend. SV messages are sent only once. All the messages are significant for the correct operation of bus zone and differential protection schemes that compares station wide SV messages. A delayed or lost message can cause incorrect protection operation.

Table 2-1 Network Architectures and redundancy protocols

Paper Aim of the paper Network

Architecture

Protocol Result Araujo et al.,2012 A prototype of a HSR node has been

developed and proved over some virtual machines connected in a ring network as the standard states and achieved the target of seamless availability.

Ring PRP and

HSR.

The comparison of PRP and HSR experiments shows solutions to get high availability of the communication with zero time recovery.

Bhardwaj et al,2014

This paper discusses standards IEC 61850 and IEC 61869 used in the communication profile with data availability standard IEC 62439

PRP and HSR

Standards IEC 61850, IEC 61869, and IEC 62439 were discussed with methods IRIG-B and IEEE 1588 to synchronize

Buhagiar et al,2016

The paper describes a French smart substation project, as part of the French government’s plan for smart sub stations.

HSR Extra IED (MPx), connected to the substation buses, to replace the functions inside another IED in the case of failure

Darby et al,2014 The paper discusses the principle of operation of PRP, and the benefits for use in an IEC 61850-based SAS.

PRP Interoperability is proven by extensive multi- vendor interoperability testing and by utility in- service use

Ferrari et al,2103 This paper discusses synchronization systems applied to redundant network infrastructures for substation automation. infrastructures for substation automation: in these systems the reconfiguration of the network after a

PRP & RSTP

PRP performs better as RSTP and guarantees seamless synchronization performance in case of a single fault.

Goraj et al,2012 This paper provides an overview of high availability networks and redundancy protocols for seamless failover critical substation automation applications.

Ring and Star PRP and HSR

PRP and HSR is discussed and compared for substation, station and process bus applications

Kanabar et al,2011 This paper presents the hardware implementation of a process bus

communication network and investigate the proposed corrective measure for Sampled Value (SV) loss/delay.

Cascaded the SV estimation algorithm as a corrective measure for SV loss/delay can enhance the reliability and security of digital protection

functions using an IEC 61850-9-2-based process bus

Kanabar et al,2012 This paper review available standards for Power system protection, control and monitoring and discusses their

applications and current developments.

PRP and HSR

Available standards for Power system PCM are discussed

Kumar et al.,2015 This paper presents simulation results with respect to the delay in GOOSE and SV packets transfer in an Ethernet

environment related to a digital protection scheme.

Ring It was observed that the increase in the SV

frequencies caused higher packet losses for GOOSE messages

Liu et al,2014 This paper proposes a methodology to evaluate the reliability of a Substation Communications Network (SCN) considering different architectures.

Star and Ring RSTP and PRP

The ring architecture station bus is proved to be reliable and cost effective. PRP based SCN architectures is an economical and solution to fulfil the availability and performance

requirements in SAS Meier et al.,2016 Example substations are used to verify

the standardized performance figures

HSR The paper discussed the validity of fault clearance timings for digital substation

defined in IEC 61850, IEC 60044 and IEC 61869 standards.

architectures with reference to timing and performance requirements.

Rahat et al.,2019 A mathematical approach is used to analyse the reliability and availability. A suitable design for efficient operation is proposed.

Ring and Star Advantages and disadvantages of different network topologies are discussed. Simulation results are discussed

Raza et al.,2009 This paper presents analysis and the crucial need for the deployment of Gigabit Ethernet in the substation.

Gigabit Ethernet in the substation is optimized for the current IEC61850 constraints but can also support future considerations smoothly

Stark et al.,2013 This document describes the utilization of the nonconventional measurements and advanced features of IEC 61850 standard for substation automation systems.

PRP and HSR

Non-conventional instrument transformers together with IEC 61850 provide cost-efficient solutions with higher availability compared to traditional instrument transformers

Sun et al.,2011 This paper describes the different Ethernet based process bus

architectures, analysis of the mean time to failure and availability

Cascaded, Star and Ring

The star architecture provides the highest MTTF and availability. The ring and cascade

architecture have the same MTTF and availability.

Younis,2016 The objective of this paper is to evaluate the reliability and availability of different substation communication architectures by using the Reliability Block Diagram (RDB) method.

Cascaded, Star and Ring

Reliability of the specific IEC 61850 SAS has been significant approved when adding redundancy components to the system

Wester and Adamiak,2011

This paper is a tutorial in Ethernet communications and architectures.

Star, Mesh and Ring

The paper address Ethernet fundamentals and cover the most common elements of an Ethernet architecture