Fernando Matos and RogÃ©rio Paulo, REN S.A. and Efacec S.A., Portugal
IEC 61850 was created in the 1990s to be an internationally standardized communication and integration method that supports systems built from multivendor, microprocessor-based intelligent electronic devices (IEDs) networked to perform protection, monitoring, automation, metering and control (see Figure 1).
Utilities in Europe and Latin America have transitioned from the single vendor system approaches that were common pre-IEC 61850 to improve asset life-cycle benefits.
Most applications of IEC 61850 target intra-station communication networks to drive distributed automation at bay and station functional levels (the station bus). Bay-to-process and external station interfaces must still be addressed in the general case, at least using standard communication profiles.
Physical Network, Logical Architectures
Ethernet has become the dominant communication infrastructure, supporting both real-time and engineering and maintenance data flows. The three most-common types of network topologies are star, ring and multiple-ring (see Figure 2).
Each of the topologies is best-suited for different application, and has different advantages and disadvantages as summarized in Table 1.
Star topologies are used in smaller and less critical systems, usually with limited redundancy options. Single-ring topologies with rapid spanning tree (RSTP) variants constitute the most common option with one or more switches per bay. Besides ring-based redundancy, IED connections to two ring switches may be employed for devices with double network interfaces, or for duplicated devices, mainly gateways or local servers. Large substations apply multiple rings (typically per voltage level) to ensure network performance, functional independence or both. Virtual local area networks (VLANs) or multicast filtering for traffic segregation are not generally used.
Function allocation to devices and the information flows are fully configurable in IEC 61850 and are hence independent of the physical network. In current practice, three major classes can be identified, though an individual system may exhibit a mixed-class architecture. The three common classes are: RTU (centralized automation), distributed automation (vertical communication only) and distributed automation (peer-to-peer communication). (See Figure 3.)
The RTU class is usually applied to existing conventional system upgrades and with independent protection and control. Bay devices operate as data-concentrators, protocol converters and input/output (I/O) devices. The distributed automation architecture is generally used for new systems and system expansions. New systems typically will be peer-to-peer, with inter/intra-bay IED communication via generic object oriented system event (GOOSE) and bay-to-station communication via IEC 61850 client/server (C/S) services. In non peer-to-peer architecture, inter/intra-bay interfaces are most commonly hardwired if they exist at all.
From the defined IEC 61850 C/S service set, only model browsing, controls, polling and event-reporting (buffered and unbuffered) are commonly used for communication between bay devices and station devices or tools (over manufacturing message specification/transmission control protocol/Internet protocol [MS/TCP/IP]). GOOSE is applied between functions hosted at bay devices (more frequently applied between devices of same vendor). Other services or service applications are not frequently used.
Multiple device selection options are possible for any logical architecture classes. Actual systems tend to fall into one of three categories regarding the use of devices from multiple vendors, with increasing level of multivendor integration, as seen in Table 2.
Interoperability is Key
Interoperability is a main requirements for adequate multivendor system integration and performance, and can be defined according to four levels: data communication interoperability, functional interoperability, engineering interoperability and interchangeability.
Engineering, REN Example and Current SAS Architecture
Where engineering is concerned, IEC 61850 feasible mostly in later integration stages, during implementation, validation and testing. Automation engineers tend to focus on point and document-based approaches instead of object or model-based approaches. System specification, design and documentation according to IEC 61850 are not standard practice, although efforts are moving in this direction. IEC 61850’s application to the whole engineering process still faces some barriers, including interoperability of various tool implementations, limited tool support for specification and iterative configuration, limited configurability of many products’ models, a lack of user-targeted (visual) languages, and still-emerging engineering guidelines from CIGRE, IEC and other bodies.
REN, the operator of the RNT (the National Electricity Transmission Grid) in Portugal, has experienced significant system growth since 2004 (25 percent increase in number of substations) and currently operates 70 substations and about 7,500 kilometers of transmission lines (400 kV, 220 kV and 150 kV). To accommodate this growth, REN began a review of its SAS technical specifications in 2003. The review encompassed the architecture, cabinets, performance, international standard references, functional specifications and engineering process certification. One of the specification review’s relevant goals was to obtain gains in simplification and project implementation time.
The current REN SAS architecture is a multi-vendor bay-level IED-based distributed automation system with independent protection and control. Specifications preclude a multiple-ring Ethernet network with independent IEC 61850 station bus per voltage level. GOOSE messaging is used for control purposes only and C/S services are used between all bay-level IEDs and the station level, which includes the local human-machine interface (HMI) and redundant gateways for data interchange with the national supervisory control and data acquisition (SCADA) system (see Figure 5).
REN’s experience in applying this specification has shown that the process simplification achieved by applying more powerful IEDs and engineering tools, as well as the improved communication interoperability, has effectively shortened implementation schedules. Moreover, installed communication technology has reduced failure rates while providing enhanced services. This approach now allows REN to select the appropriate devices for each application regardless of vendor and helps ensure their interoperability.
IEC 61850 is well established in intra-station systems and its anticipated benefits (simplified installation, increased flexibility, cost reduction in cabling and physical infrastructure, fewer communication protocols used, ease of engineering, etc.) have been effective. Systems of all classes have met customer requirements for functionality and performance, which makes utilities base their multivendor choices on anticipated life-cycle cost, product offering and its value-chain position. The expected benefits of IEC 61850 in system maintenance, expansion and evolution have not been verified because experience in these cases is limited.
The intra-station application of IEC 61850 is a major step forward, but the full potential of the standard has not been attained. First, installed IEC 61850 system architectures have not deviated much from those of modern systems using pre-IEC 61850 technologies. In addition, while automation engineering methods have been made more efficient by SCL and new software tools, they have not fundamentally changed. IEC 61850 applications outside the substation are limited, but growing utility-wide high bandwidth communication networks could heighten activity in this area.
As the second edition of the standard reaches industrial readiness, experts expect that significant improvements in technology will become available. These include high precision network-based synchronization (P1588); communication redundancy without service interruption (PRP/HSR); inter-substation protection, interlocking or remedial action schemes; synchrophasor measurements; cyber-security management; improved engineering tools and process bus applications.
The functional standards’ scope also is likely to broaden, including integrated remote condition monitoring and maintenance, distribution automation and integration of dispersed generation, renewable power plants and global system engineering and management.
These anticipated innovations will lead to changes in substation automation systems architectures as well as the engineering processes used in the industry. Raising the independence of logical architecture from physical network infrastructure and simplifying functional and engineering integration are critical for the future; even more in system-wide than in intra-station applications. This is IEC 61850′ true power, and applying it will help ensure that smart grid infrastructure will continue to be built worldwide.
Fernando Matos has been the head of REN’s substation control department since 2004. He worked 13 years as a project engineer and project manager in the substation automation field.
RogÃ©rio Paulo has worked with SCADA product development and power system control and communication systems for more than 10 years. He is a member of IEC TC 57 WG10 and CIGRE SC B5 and has published several papers on power systems automation and engineering.
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