Opportunities and challenges of heterogeneous networks for substations automation in smart grids

The immense development in networking and communication technologies can drastically change electrical substations automation and control in the future smart grid. The International Electrotechnical Commission (IEC) 61850 standard is receiving global acceptance to deploy Ethernet local area networks (LANs) for electrical substations.

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Sep 29, 2017
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Author(s): Irfan Al-Anbagi and Hussein T. Mouftah

Abstract

The IEC 61850 standard is a part of the lEC’s Technical Committee 57 architecture for electric power systems. High data-rate LANs and fibre- based Ethernet networks may present an excellent candidate for the implementation of the IEC 61850 standard. However, deployment cost, mobility issues of wired LANs, in addition to the emergence of the electrical vehicles may inspire communication system engineers and system integrators to consider wireless communication technologies such as wireless LANs (WLANs) or wireless sensor networks (WSNs) as legitimate candidates for implementing the IEC 61850 standard. The authors present an overview of the IEC 61850 standard, highlight its importance, requirements and limitations. In addition to that they present the main challenges of implementing WSNs and WLANs in monitoring and controlling substations in a smart grid environment. Furthermore, they present a heterogeneous wireless network architecture for substations automation and suggest recommendations to align the capabilities of this network with the requirements of the IEC 61850 standard.

Introduction

Electric utilities, continuously encounter the challenge of providing reliable power to end-users at competitive prices. Owing to several reasons such as equipment failures, lightning strikes, accidents and natural catastrophes, power disturbances and outages in substations occur and often result in long service interruptions. Thus, power substations should be properly controlled and monitored in order to take the necessary precautions accurately and timely. In this context, substation automation, which is the creation of a highly reliable, self-healing power system that rapidly responds to real-time events with appropriate actions, ensures to maintain uninterrupted power services to the end users [1].

Ever since the ideas electrical system automation and tele-protection processes were created, telecommunication technologies have been key in their success. Telephone-switching-based remote control units have been used in electrical systems since the 1930’s. In the 1960’s, data acquisition systems were installed automatically collecting measurement data from the substations. In the 1980’s and 1990’s, serial communication was used [2]. Currently, thousands of analogue and digital data points are available in a single intelligent electronic device (IED). The increase of communicating data points and IEDs calls for a standard that regulates the communication and specifies the requirements for substation automation. The International Electrotechnical Commission (IEC) 61850 is the first global standard in the electric utility field. The standard is proposed by the IEC [3]; it is also supported by the American National Standards Institute/IEEE. About 60 experts from Europe and North America have jointly developed the IEC 61850 standard. All parts of the IEC 61850 were published in 2004. The standard influences other electric areas such as wind power plants, hydro power, distributed energy resources, electric vehicle to grid integration etc. Currently, the standard is being implemented by major vendors around the world [4].

The IEC 61850 standard is designed to support communication networks and systems in substations. It specifies data models, services and requirements. One of the main advantages of the standard is that it does not block future development of functions; it specifies no protection or control functions and it supports free allocation of functions to devices. The IEC 61850 standard provides the substation configuration description language (SCL) [extensible mark-up language (XML) syntax]. SCL supports consistent system definition and engineering, details described in IEC 61850-6 [3]. The IEC 61850 uses Ethernet and transmission control protocol/Internet protocol (TCP/IP) for communication. However, it is open for future new communication concepts. This feature is considered as one of the main advantages of the IEC 61850 standard. This feature means that the standard does not restrict the use of certain communication protocol or technology. Furthermore, developments in communication technologies have enabled cost-effective remote control systems which have the capability of monitoring real-time operating conditions and performances of substations. The development of communication technologies allows efficient integration with the IEC 61850 standard. Each communication technology offers unique advantages and disadvantages which requires careful design to integrate various communication technologies for substation automation. To satisfy the requirements of the IEC 61850 standard, a highly reliable, scalable, secure, robust and cost-effective communication network between substations and a remote control centre becomes essential. In addition to that the designed communication network should satisfy strict quality of service (QoS) requirements in terms of latency and reliability.

Ethernet local area networks (LANs) may be the preferred tool of choice for implementing the standard in substations. Ethernet offers several advantages for such applications. Ethernet is simple to set-up, it is relatively inexpensive, can offer noise immunity if cables are properly shielded. However, Ethernet has many disadvantages such as it is difficult to change which means that any changes to the network will result in ‘down time’ as the bus must be broken and a new section spliced in at the point of the break, this is specifically difficult in a substation environment. Ethernet is fault intolerant, which means that if device or cable section connected to the network fails, it could cause the entire network to fail. Ethernet is relatively difficult to troubleshoot, it is not easy to determine what node or cable section is causing a problem. Furthermore, the use of wired Internet and TCP/IP may not satisfy the requirements set by the IEC 61850 standard, since data communication in Internet is based on best effort service. In addition, using Ethernet in substation environments where there are thousands of distributed IED devices is a challenging issue. The main challenge is associated with installation and maintenance costs. The cost drastically increases as the number of IEDs increase. Another important aspect is related to actual installation and routing of cables and wires in the substation to reach different substation components such as transformers and capacitor banks to monitor and control these devices. Recent advances and cost reduction in deployment of wireless communication technologies such as wireless LANs (WLANs) and wireless sensor networks (WSNs), worldwide interoperability for microwave access (WiMAX) and cellular enable them to be a cost-efficient and reliable tool for implementing the IEC 61850 standard in substations.

In this article, we present an overview of the IEC 61850 standard, highlight its importance, requirements and limitations. We present the advantages and features of using wireless communication technologies in substation automation. In addition to that, we present the main challenges of implementing WSNs and WLANs in monitoring and controlling substations in a smart grid environment. Furthermore, we propose a heterogeneous wireless network architecture for substations automation and suggest recommendations to align the capabilities of this architecture with the requirements of the IEC 61850 standard in the smart grid.

‘Background’ section presents a background on the literature, ‘Overview of the IEC 61850 standard’ section presents an overview of the IEC 61850 standard. ‘Challenges of SAS’ section presents wireless communication technologies in substation automation, and finally ‘Wireless communication technologies for substation automation’ section concludes this article.

Background

In the literature, there have been a few studies that survey communication technologies in smart grid environment, in general and in substations in specific [5–7]. There have been several studies that consider using wireless communication for substation automation and control [8–11]. The majority of the work presents the conceptual design of either WSNs [12–16] or WLANs [17] for substation automation applications without considering the requirements of the IEC 61850 standard. There are a few studies which consider the design and development communication networks for substation in IEC 61850 environment [18,19]. However, the literature is limited in terms of the application wireless communication technologies for substation automation with IEC 61850 standard.

Thonet and Deck [8] have developed an integrated communication platform: namely, e-Breaker-for medium-voltage switch gears. Their platform is based on wireless technology and complies with the IEC 61850 standard. They have shown that their platform targets a 50% cut in engineering costs. Brown et al. [9] have proposed the use of wireless applications for differential protection and monitoring of air-core inductors even at 150 kbps for protection within the substation. Smith et al. [10] have discussed using recloser controls with Ethernet communications and 900 MHz Ethernet radios to communicate with each other using peer-to-peer communications. The authors have discussed smart trip coordination and fault isolation using this available communication technology. In [11], the authors have developed a document describing the application of protective relays using spread spectrum radio communication for power-system protection schemes. They have presented background information, bibliography and recommendations. Furthermore, the authors have discussed spread spectrum radio communication technologies and topologies that may be applicable for use in protective relay schemes.

Tommila et al. [20] have applied short message service functionality of the digital cellular network to remotely control and monitor substations. Lowrey [21] has utilised the control channel of the cellular network in some alarm-based substation monitoring cases. Both of these communication technologies are suited to the applications that send a small amount of data and cannot provide QoS requirements of the IEC 61850 standard.

Sun et al. [12] have proposed to use a private wireless network dedicated for power distribution system monitoring. In [13], Al-Anbagi et al. have introduced a medium access scheme, delay-responsive, cross-layer (DRX) data transmission that addresses delay and service differentiation requirements of the smart grid. In [14], an experimental study on the statistical characterisation of the wireless channel in different electric-power-system environments was presented. Faria [15] has proposed the projection, implementation and evaluation of a WSN acquiring of non-critical data on distribution substations.

Al-Anbagi et al. [22] have presented a QoS differentiation scheme for high priority data in multihop WSNs based on an optimisation scheme for monitoring partial discharge activities in high-voltage transformers (VTs). Gao and Wang [16] have proposed a hierarchical clustering WSN based on multi-agent system. They have appointed sustainable power-supplied nodes as clustering heads and the nodes measuring temperatures of the joints are ordinary nodes belonging to certain cluster and they interact only with the belonged head.

Nasipuri et al. [23] have described the design, test and development of vibration sensing nodes for a WSN that is used for health monitoring of HV equipment in power substations. They have investigated the viability of using inexpensive and low-power wireless sensors for power-system monitoring applications. Nasipuri et al. [24] have described the design and deployment of a large-scale WSN for monitoring the health of power equipment in a substation. All nodes communicate over a multihop wireless mesh network that uses a dynamic link-quality-based routing protocol. They have developed effective monitoring applications for the substation using low-cost wireless sensor nodes that can sustain long periods of battery life. The authors have studied the battery consumption in the network and present a transmission scheme that conserves communication cost by enabling the sensor nodes to transmit observation samples only when their values are significantly different from those transmitted previously.

The use of WSN has also been used to indirectly protect substations. For example, Zhuang et al. [25] have presented a WSN and flood detection with 0.02 V/mm sensitivity, for flood monitoring of distribution substations of electrical utilities in low-lying areas.

Abdel-Latif et al. [17] have investigated WLAN [wireless fidelity (Wi-Fi)] with 11 Mbps data rate using laboratory set-up, and demonstrated the successful use of WLAN for line differential protection applications. Oyedapo et al. [26] have presented a precoder-based technique of transmitting data using a group of cooperating nodes to transmit data to the receiver in an ad hoc mode. The authors have employed the two precoding-based techniques max − d min and P-orthogonal spatial multiplexing (OSM) that operate with a closed-loop transmission and both precoders optimise the minimal distance criterion.

There is limited scope for the use of wireless communication. For example, Hunt et al. [18] have analysed pilot protection of distribution system with the help of digital radio technologies. They have shown that digital radio can successfully send the IEC 61850 Generic Object Oriented Substation Event (GOOSE) message within 10–15 ms of the time as a transfer trip signal.

Overview of the IEC 61850 standard

The development of the smart grid and its integration with various existing and emerging technologies and standards such as the IEC 61850 standard requires efficient and reliable unified framework. The growing number of available data points in a single IED and strict communication requirements calls for a carefully designed and cost-efficient structure. The main requirements of the IEC 61850 standard can be summarised below:

  • High-speed communication
  • Guaranteed delivery times
  • High availability
  • Supports voltage and current sampled data
  • Supports file transfer
  • Supports auto configuration
  • Supports interoperability
  • Supports security

To satisfy these requirements, work on different communication architecture started with the development of the Utility Communication Architecture (UCA). Recommended protocols resulted for the various layers Open System Interconnect communication system model. Protocols, data models and service definitions were also defined which are also known as UCA. IEC Technical Committee Number 57 (TC57) based their development on the work done in the UCA. TC57 formed a working group 10 which resulted in the formation of the IEC 61850 for communication networks and systems in substations [27].

Structure of the IEC 61850 standard

The IEC 61850 standard focuses on communications within the substation. The standard document defines different aspects of substation communication network. The standard is divided into ten major sections as shown in Figs 1 and 2 [3].

Fig 1: Structure of the IEC 61850 standard

Fig 2: Network architecture in the IEC 61850 standard

Parts 3 and 4 identify the general, system and project requirements, these requirements identify the services and data models and application protocol required of substation monitoring. Part 5 identifies specific communications requirements, these requirements identify the transport, network, data link, and physical layers that are needed to meet the overall requirements of substation automation. Part 6 defines the SCL which is an XML-based language that promotes system configuration and facilitates the interaction between different parts of the standard requirements. The SCL allows the description of the relations between the substation automation system (SAS) and the substation [3].

Since the IEC 61850 standard adopts an abstract nature, the definition of the data items and the services are independent of any underlying protocols. Part 7 defines the abstract communication services as well as defining the abstract data objects which include status, control, and measurement. This definition aids in developing the concept of common data classes which defines the building blocks for creating the larger data objects. Part 8 defines the mapping of the abstract data object and services into the manufacturing messaging specification. Part 9 defines the mapping of the sample measured values (SMVs) which can be unidirectional point-to-point and bi-directional multipoint accordingly into an Ethernet data frame, which is more generally known as the process bus. Finally, part 10 of the document defines a testing methodology in order to determine ‘conformance’ with the numerous protocol definitions and constraints defined in the document.

Benefits of the IEC 61850

The high cost associated with the substation automation process is considered as of the main challenges of implementing the IEC 61850 standard. However, the numerous advantages of the automation process such as increased power quality and reduced outage response justify the initial investment. Legacy communication protocols as well as TCP/IP can provide the basic electric-power-system capabilities as the serial link version while bringing the advantages of modern networking technologies to the substation. However, these protocols were designed to minimise the amount of transmitted data and do not take advantage of the vast increase in bandwidth that modern networking technologies deliver by providing a higher level of functionality that can significantly reduce the implementation and operational costs of substation automation. The use of the IEC 61850 standard can deliver substantial benefits to users that understand and take advantage of them. These benefits can be summarised below:

  • Interoperability: One of the main issues that face actual implementation of industrial standards is interoperability between devices manufactured by different vendors. In a single substation environment there is a high possibility to find several IEDs manufactured by different vendors. These IEDs will exchange and use information over a common communications media. Data is portable between vendor equipment.
  • Open IED description: The IEC 61850 standard allows the IEDs to be described in a standardised way. This leads to significant reduction in the cost to configure and commission devices. This feature also reduces errors in the configuration process.
  • Communication closer to power equipment: The standard allows the IEDs to have all the required functions to be embedded into the primary equipment. This means that communication with the power equipment can be integrated within the IEDs which is in most cases located very close to the equipment itself.
  • Lower overall cost: High-speed communication from the IED to the control centre allows the replacement of the traditional electrical wiring and control by automated control and configuration process. This could significantly cost for SAS which includes lower installation cost, lower transducer costs, lower commissioning costs, lower equipment migration costs and lower integration costs.
  • Reduction of conventional wiring: The IEC 61850 standard enables devices to exchange data and status using a standardised message format over the station LAN without the need to wire separate links for individual relays. This technique significantly reduces the amount of wires and cables running across the substation which intern reduces the installation cost and time.
  • Future proof: The standard is able to follow the development in communication technology and evolving system requirements.

IEC 61850 communication architecture

Fig 2 shows a typical substation architecture. At the ‘process’ level, data from different sensors (e.g. optical, electronic, voltage and current) in addition to status information is collected and digitised by the merging units (MUs), also known as process interfaces. MUs could be physically located either in the field or in the control units. Data from the MUs is collected through Ethernet links, the collection points can be gigabit Ethernet switches, these switches should be capable of supporting Ethernet priority and Ethernet virtual LAN (VLAN). The use of VLANs allows the Ethernet switch to deliver data to switch ports or IEDs that have subscribed to the data or part of the VLAN.

In the process bus implementations, manufacturers are required to provide the ability to integrate data from existing current transformers (CTs) and VTs with the data from the optical or electronic sensors. At the substation level, a station bus is utilised; this bus is based today on fast Ethernet or gigabit Ethernet. The station bus provides communications between different logical nodes that provide various substation protection, control, monitoring and logging functionalities. In this bus, communications can either work in a connection oriented basis or a connection-less basis (IEC GOOSE) [3]. A redundant communication architecture is recommended as application of IED-to-IED data transmission puts the communication system on the critical path in case of a failure.

Finally, this architecture supports remote network access for all types of data read and write types. As all communication is network enabled, multiple remote ‘clients’ may desire access to the available information. Typical clients would include local human machine interface, operations, maintenance, engineering and planning. The remote access point is one logical location to implement security functions such as encryption and authentication. This implementation alleviates the load from individual IEDs from performing encryption on internal data transfers but still provide security on all external transactions. In addition to that this implementation describes how information is routed between the IEDs. It contains sub-networks, IEDs connected to different sub-networks, access points per IED to sub-networks, addresses and IP addresses of LAN network.

Challenges of SAS

Automation of electrical substations poses a number of challenges in a smart grid environment. The main challenge arises from real-time and reliable response to variations or failures in the substation. These challenges can be summarised as follows:

  • Communication delay: The performance of communication network is measured by the end-to-end delay for latency critical data transmission. As explained in ‘Structure of the IEC 61850 standard’ section, part 5 of the IEC 61850 standard defines specific communication requirements which include the allowable message transmission delay. Fig 3 shows how the message transfer time is defined in the IEC 61850 standard. A piece of information for communications (PICOM) is a piece of information for communication describing an information transfer on a given logical connection with given communication attributes between two logical nodes.

The IEC 61850 standard does not identify how to satisfy these requirements and it does not provide guidance on how to characterise message delivery performance across the entire substation communication network. Communication networks should have the ability to provide priority tagging to satisfy the delay requirements specified in part 5 of the standard. This must be done without affecting the reliability and cost for using expensive network components at the process level. Therefore, different professional network simulation tools can be utilised to evaluate the performance of an SAS network. Although, these simulation tools consider different network topologies, they do not consider process bus functionalities. This requires additional simulation models to consider these functionalities such as IED functionalities.

Fig 3: IEC 61850 transfer time definition

  • Reliability: Part 3 of the standard identifies the requirements for the reliability and states that should not be a single point of failure to cause the substation to be inoperable. In addition to that, the standard states that a failure of any component should not result in an undetected loss of functions nor multiple and cascading component failures. Furthermore, the standard does not support redundant data transmission even for critical applications. Therefore, the balance between reliability and redundancy is a major issue that needs to be addressed in an SAS.
  • Availability: Availability of the SAS is not in the scope of the standard. However, availability is one of the important functional requirements to have successful SAS. One of the main challenges is that the IEC 61850 standard proposes few communication devices with integrated electronic circuits. These devices may be affected by electromagnetic interference which makes it difficult to maintain high levels of availability. Therefore, SAS availability is one of the most challenging issues that need to be addressed within the IEC 61850 framework.
  • Electromagnetic interference: Various electromagnetic interferences such as lightening strikes, switching surges, electrostatic discharges etc. are effects that may take place in a substation especially if the equipment is air insulated. Electromagnetic interference is a major challenge because the general electromagnetic interference immunity requirements used in substations are not sufficient. Part 3 of the IEC 61850 standard outlines the general electromagnetic interference immunity requirements. All the SAS devices such as IEDs, MUs, Ethernet switches, and other communication devices must be in compliance with these requirements.
  • Synchronisation: One of the main functionalities of part 9 of the standard is to define mapping for the SMV. This process involves digitisation of CTs/VTs output at the MUs and communicating these sampled values to bay level IEDs through process bus. These sampled values should be synchronised in time to allow the protection function to utilise various signals from different MUs from various manufacturers. The IEC 61850 standard uses simple network time protocol (SNTP) to implement time synchronisation. SNTP can achieve an accuracy of about 1 ms; this time margin may not be enough for some requirements defined in part 5 such as raw data sampled values. However, precision time protocol (PTP) described in IEEE 1588 can be used to achieve this level of synchronisation. PTP can synchronise distributed clocks with an accuracy of <1 μs via Ethernet networks. The only challenge is that PTP is not integrated with the IEC 61850 standard. Another problem is that the PTP requires a source for external time synchronisation. This issue must be considered in terms of the availability source in the SAS which plays a major role in accuracy of the protection and control functions and complexity of SAS.
  • Environmental requirements: Generally, process bus communication equipment will be exposed to atmospheric conditions for the air insulated substation. Hence, it is important for the outdoor mounted communication devices to meet environmental requirements. Part 3 of the IEC 61850 standard identifies environmental parameters such as temperature, humidity, barometric pressure, mechanical, seismic, pollution and corrosion. Process bus communication must satisfy the communication requirements of SAS while respecting the environmental requirements identified in the standard.
  • SAS architecture: Since the IEC 61850-based SAS architecture supports distributed functions in several modules. If the distributed devices such as IEDs, MUs, Ethernet switch and time synchronisation devices from different vendors may performance issues. This problem becomes more complicated when these devices must follow the IEC 61850 standard and complete the task successfully. This issue increases the complexity of the SAS architecture.
  • Security issues: A security breach within the SAS can cause several issues that might introduce large time delay or data loss even for high priority and critical messages which may cause damages to many substation devices. In addition to that, data security is more critical in when exchanging data beyond the substation (i.e. with the control centre or other substations). Part 3 of the standard identifies the security requirements but does not put details on the security techniques. Firewalls, encryption and authentication can address data security issues in SAS. However, the use of wireless communication adds further complications due to the nature of the open environment.

Wireless communication technologies for substation automation

The current state-of-the-art electric system automation can utilise several wireless communication technologies currently existing for electric system automation [28]. Besides the well-known advantages of using wireless communication technologies for controlling and monitoring substations, wireless communication suffer from some limitations such as susceptibility to electro-magnetic interference, limitations in bandwidth, transmission range and security issues due to the nature of open transmission environment. On the basis of the advantages and limitations of different wireless communication, different technologies can be chosen based on the specific applications requirement and deployment environment.

In the context of choosing a wireless technology for substation automation, existing communication infrastructure such as public network can be utilised, for example, using public cellular networks. Using existing communication infrastructure could be a cost-effective solution. However, there might be other issues related to availability, reliability and real-time monitoring and control applications. Another option is to install a wireless network which is completely owned by the utility. Private wireless networks enable electric utilities to have more control over their communication network and guarantee that their infrastructure is available and can provide the required QoS. However, private wireless networks require a significant installation investment as well as the maintenance cost.

Advantages and disadvantages of wireless communication technologies in SAS

Whether the wireless communication technology is owned by the utility or it is part of a public network, they share some common advantages and disadvantages when they are intended for SAS. These advantages and disadvantages can be briefly summarised below.

Advantages of wireless communications in SAS

Low cost: Wireless communication network enable a cost-effective solution because they do not require the infrastructure and the installation efforts required by wired network. The addition or removal of new nodes from the network is easier in a wireless in environment. In wireless communication, cabling cost is also eliminated.

Rapid installation and commissioning: Since the amount of cabling and infrastructure preparation is much lower in wireless communication, the installation of such systems is faster than that of wired networks. In addition to rapid installation process, wireless communication provides more flexibility compared with wired networks, this is significant in complex environments where laying cables may be very difficult and expensive.

Disadvantages of wireless communications in SAS

Limited coverage: Except for cellular networks and WiMAX, other wireless networks provide a limited coverage, for example, the coverage of WSNs is limited to 10–50 m, whereas the coverage of WLANs is about 100 m. In addition to that, substations may be located in remote locations where access to cellular networks and WiMAX communication may be unavailable because of the unavailability of the infrastructure itself.

Limited bandwidth: The capacity of wireless networks is, in general, lower than wired communication networks. The limited bandwidth combined with radio-frequency interference due to the nature of the environment contributes mainly to the high bit error rate. For example, in WSNs the maximum data rate is 250 kbps; this data rate limits the usability of such networks for delay critical SAS applications. On the other hand, WLANs can provide much higher bandwidth which makes them more suitable for delay critical SAS applications.

Security: One of the main challenges of wireless communication networks is security issues. Security in a wireless environment is a challenge because of the open transmission medium. An efficient and economical solution would be to implement efficient authentication and encryption techniques to provide secure communication.

Wireless networks for SAS

Unlike the information technology (IT) industry, electric utilities do not employ solutions based on advances in technologies. Implementation of any new technology in the electric sector is generally slower than IT industries. In many operating substations, it is common to find equipment that is two or three decades old.

There are several factors that may influence an electric utility to deploy new technologies including wireless communications in automation and control. These factors are: regulatory commitment from authorities, reliability considerations and personnel safety, equipment obsolescence, where replacement parts are no longer available or are prohibitively expensive. In addition, another factor is equipment reliability, where a single point of failure in a component could have drastic consequences in safety and liability. Finally, savings on cost on new investments is another deciding factor.

IEEE 802.11-based WLANs

WLAN has been considered for various smart grid applications such as electrical system protection [9,17,29], demand side management [30], microgrid energy management system [31], automatic distribution reclosing [10] and distribution substation automation and protection [32]. It is expected that there will be a substantial change from traditional methods of substations control and automation. WLAN can offer several advantages and improvements to this process. One of the main advantages offered by WLAN is that it is easy to install a remote terminal, low installation cost and flexibility to add additional devices in future. However, the use of WLAN may not provide a one-size-fits-all solution that will meet the needs of such applications. Another important issue associated with WLANs is the availability of power resources. Although the substation environment can offer several options to power WLAN devices, there might be some challenges associated with availability of low voltages required for these devices. This is especially true in situations where WLAN-based sensors need to be deployed near high-voltage equipment such as high-voltage transformers. Since WLAN devices are relatively power hungry, then powering such devices using batteries is impractical.

IEEE 802.15.4-based WSNs

WSNs-based monitoring systems are favoured due to their unique features and advantages such as enhanced fault tolerance, low-power consumption, self-configuration, rapid deployment and low cost. In addition to that, in substation environments where high voltages are in use, WSN can also provide necessary insulation. WSNs have been proposed for numerous smart applications, for example, power distribution system monitoring [12] and substation monitoring [13,14].

Although IEEE 802.15.4-based WSNs offer several advantages and features over IEEE 802.11-based WLANs, the properties of these networks impose unique challenges if they are intended for SAS. A major challenge is the limited resources, for example, limited energy, limited memory and processing capabilities. These limitations are mainly due to the limited physical size of sensor nodes. On the basis of these limitations, IEEE 802.15.4-based WSNs are tailored for monitoring applications that provide services with high energy efficiency and low data rates. Consequently, WSNs cannot, in general, provide hard QoS guarantees to critical substation monitoring applications. Certain cross-layer design and optimisation techniques can be used to mitigate some of these issues.

To handle high traffic intensities, delay sensitive data transmission and to efficiently integrate with existing SAS an alternative approach can be used for such applications. This approach involves using multiple wireless network technologies. This is done by utilising the advantages and features of WLANs and WSNs to serve the purpose of substation automation. We refer to this hybrid network architecture as a heterogeneous wireless networks for SAS. In the next section, we describe this architecture and explain its main advantages and features.

Heterogeneous wireless networks for SAS

A heterogeneous network can be defined as a network connecting various network elements with different operating systems and/or protocols. In general, heterogeneous wireless networks refer to wireless networks that use different access technologies. A typical example of heterogeneous network is a wireless network which provides a service through a switching between two services such as WLAN and a cellular network or WiMAX network. As mentioned above, the main goal of heterogeneous networks is to enable real-time and reliable data transmission and to provide the necessary QoS guarantees over an integrated heterogeneous dynamic network that is scalable, upgradeable and secure.

According to the IEC 61850 standard, the most important performance metric is the ability to get the needed information in a timely fashion. The key to achieving these requirements is by allowing seamless integration of heterogeneous networks. One of the main challenges of integrating heterogeneous networks is to achieve smooth flow of data despite using different communication modalities that have different characteristics and capacity constraints. In addition to achieving smooth data flow between heterogeneous networks, there must be a support for variation in session requirements, protocol stacks and rate of change of capacity and connectivity. Furthermore, the network should be scalable and be able to evolve as the SAS environment evolves over time.

In general, there are several open research issues on heterogeneous networks, these challenges can apply to various environments including SAS. These issues can be summarised below:

  • Heterogeneous networks which include multiple communication technologies such as terrestrial wireless, optical fibre, satellite communications and free-space optical links are not widely deployed and tested in delay and reliability aware applications.
  • The effects of different channel types and the predictability of change of channel characteristics including bandwidth, delay and error rate needs to be modelled and tested.
  • Gateway design of multiple network technologies, for example, Wi-Fi, ZigBee and WiMAX needs to be studied, modelled and simulated.
  • Allowing seamless integration of heterogeneous services and traffic types, for example, voice, video, file transfer and short message.
  • Integrating messages of different sizes and transfer rates ranging from short messages sent by sensor nodes to large files sent by high-end IEDs needs to be investigated.
  • QoS requirements of heterogeneous networks with different requirements, for example, deadline requirements, tolerable error rates and network availability needs to be considered for multiple heterogeneous networks.
  • Heterogeneous network management and control systems and administrative domains for SAS application is another open research issue that needs to be tackled.

Fig 4 shows our proposed heterogeneous wireless network architecture for substation automation. At the station and bay levels, the architecture follows network architecture defined in the IEC 61850 standard. Parts 3 and 5 of the IEC 61850 standard define the general and the communication requirements which very much depend on the performance of the process bus and the substation bus. Therefore, it is essential that the architecture does not alter the specifications of these two buses. Our proposed architecture basically fits in the process level at the process interface where the actual sensing and last inch communication takes place.

Fig 4: Heterogeneous wireless network for substation automation

The proposed heterogeneous wireless network at the process level consists of a number of wireless personal area networks (WPANs) and a number of WLANs. Each WPAN consists N ZigBee end devices and a single ZigBee coordinator. All of the WPAN devices use the IEEE 802.15.4 standard. These end devices can be equipped with different types of sensors such as optical, acoustic, temperature and current, and placed to measure different parameters from various substation equipment. End device forms a mesh topology and transmit their data to the coordinator. Each coordinator aggregates the collected data from end devices and forwards the aggregated results to Wi-Fi gateway node. Wi-Fi gateway nodes enable multihop communication (in mesh topology) between various substation devices and the process bus.

The reason behind using ZigBee-based devices closest to the monitored equipment is to enable a mean of low-power data transmission from high-voltage equipment to a central coordinator. Low-power consumption is vital in such situations, since laying low-voltage lines around these high-voltage transforms to power up sensor nodes would be impractical due to cost, feasibility and design issues. An efficient and cost-effective solution is to use sensor devices powered by small batteries. On the basis of the characteristics of ZigBee devices, they would be the best candidates for low-power operation, and since Wi-Fi-based devices are power hungry, they cannot be powered by batteries for such application. On the other hand, having Zigbee devices to aggregate all data and use multihop transmission to reach the process bus within the latency requirements of the IEC 61850 would be an impossible task. Therefore, we propose to use Wi-Fi-based nodes to transmit the aggregated data to the process bus directly or use multi-hoping through other Wi-Fi nodes.

Conclusions

Providing reliable power to customers is one of the main challenges that electric utilities are continuously encountering. Substation equipment failures, accidents and natural catastrophes are main contributors to power disturbances and outages. The IEC 61850 standard was proposed to support communication networks and systems in substations to allow proper control and monitoring. The standard relies on high data-rate LANs and fibre-based Ethernet networks to be the backbone of its architecture. However, deployment costs, mobility issues of these LANs in addition to rapid development of wireless communication technologies are determining factors to consider wireless communication technologies for implementing the IEC 61850 standard in the smart grid.

In this article, we presented an overview of the IEC 61850 standard, highlighting its requirements and challenges. Furthermore, we presented the opportunities and challenges of implementing wireless communication in monitoring and controlling substations within the context of the IEC 61850. We suggested recommendations to align the capabilities of wireless communications with the requirements of the IEC 61850. We also proposed a heterogeneous wireless network architecture for substations automation and presented a case study to test this architecture in different substation environments.

Utilising wireless communication for substation automation is still immature in terms of modelling, design, actual deployment and testing. In addition, there are many open challenges that need to be addressed. These challenges can be addressed by actual deployment and development of test beds to allow smooth integration with SAS in the near future.

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Case study

We test our proposed heterogeneous architecture by considering it in three different substation environments; these environments are outdoor 500 kV substation, indoor main power room and underground transformer vault. From the point of view of wireless communication, these substation environments differ in their path loss and shadowing deviation. We follow [14], Gungor et al.performed experiments with sensor nodes to measure the link quality indicator (LQI) and the received signal strength indicator (RSSI) with certain radio propagation parameters for different electric-power-system environments. We use the different LQI and RSSI values with the topology presented in Fig 4 and simulate it to test the effectiveness of the system in these environments. We use the following values for channel propagation parameters: outdoor substation (path loss = 3.51, shadowing deviation = 2.95), indoor main power room (path loss = 2.38, shadowing deviation = 2.25) and underground transformer vault (path loss = 3.15, shadowing deviation = 3.19). We assume that the channel is having lognormal shadowing model with shadowing mean of 2.25 dB and that all wireless devices are operating in non-line of sight mode.

We assume that each WPAN consists of 20 ZigBee end nodes and that these nodes are deployed to monitor various substation equipment such as high-voltage transformers and capacitor banks. We assume that there are five different WPANs in each WLAN and these WPANs connect using mesh topology via Wi-Fi nodes and that one of these nodes (the root node) connects to the process bus which connects to the substation bus using gigabit Ethernet line. Furthermore, our test topology consists of four non-interfering WLANs, and all are connected to the same process bus.

Both ZigBee and Wi-Fi devices operate in the 2.4 GHz band, to prevent interference between the two types of devices, we carefully allocate ZigBee devices in the unused channel spaces of the Wi-Fi devices. We assume that ZigBee devices are operating at a maximum bit rate of 250 kbps and these nodes receive constant bit rate traffic form the sensors deployed on substation equipment. We set the packet size transmitted from the ZigBee devices to 128 B and to ensure timely delivery of data from Wi-Fi devices and enable aggregation at these devices, we set the Wi-Fi packet size to 512 B.

We use QualNet network simulator [33] to simulate the scenario described above. We run each simulation for 1 h and averaged ten simulations with different speeds to obtain the results. Table 1 shows the end-to-end delay from a ZigBee node to the substation engineering station connected to the substation bus for different substation environments. Table 2 shows that the rate of data packets lost during transmission from a ZigBee end device to the engineering station. It is important to note that the IEEE 802.15.4 standard is not designed for delay and reliability critical applications such as SAS. To enable QoS in these sensor devices, we enable these devices to utilise a cross-layer QoS approach to reduce the end-to-end delay and increase the reliability of sensor devices that are transmitting alarm data. We use the DRX scheme described in [13]. In the DRX scheme the tagged node performs clear channel assessment in 64 μs instead of 128 μs. In doing so, the tagged node will sense the channel and exit channel sensing state and enter into transmission state before other nodes in the WPAN.

Substation environment

IEEE 802.15.4 delay, ms

DRX delay, ms

outdoor 500 kV substation

210

170

indoor main power room

200

165

underground transformer vault

194

150

Table 1: End-to-end delay in different substation environments

Substation environment

IEEE 802.15.4, >#/b###

DRX, >#/b###

outdoor 500 kV substation

5.32

3.25

indoor main power room

3.62

2.57

underground transformer vault

2.18

1.85

Table 2: Data packets loss ratio in different substation environments

By examining the delay and the packet loss ratio results presented in Tables 1 and 2, respectively, we note that both results are in-line with IEC 61850 latency requirements, Type 3 messages (low-speed messages) and Type 5 messages (diagnostic data). According to the standard, Type 1, Type 2 and Type 4 messages are communicated with the IEDs and not the end devices at the substation [3].

Go to the profile of Hussein Mouftah

Hussein Mouftah

Canada Research Chair in Wireless Sensor Networks, and Distinguished University Professor, University of Ottawa

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