INSTRUMENTATION & CONTROL
How IEC 61850 can widen the unified control zone1 June 2009
The IEC 61850 standard was drawn up to ease the integration of electrical components into power plant automation systems, irrespective of their manufacturing origin. At first intended for substation automation, IEC 61850 can now be employed to widen the common ground in power plant control systems, enabling their resolution into two ‘chains’: the fieldbus standard for process automation and IEC 61850 for the electrical systems.
While traditional power plant control systems focus on automating the operational processes in order to produce the maximum electrical power output at the lowest possible cost, the production process in today's highly automated power plants run by modern control systems can be controlled and monitored by just one operator from a central control room. In the past, the electrical systems in power plants were also interfaced with the process control system, using conventional cabling, but owing to the high cost of the cabling and engineering this type of interfacing had to be limited to the signals needed for displaying the block diagrams in the central control room. If more signals and functions were needed, more input and output cards and more marshalling rack cabinets were necessary, resulting in higher costs.
With the introduction of intelligent devices for substations, and of a standard protocol, IEC 61850, for their automatic control and management, additional information that does have a significant impact on the cost-efficiency of operation and maintenance of the electrical equipment has become accessible. In the interests of consistency, special interfaces were introduced in substation automation technology in order to provide for unlimited access to the equipment data available. Since traditional control systems do not automatically support such interfaces, however, it is common practice to install separate process control systems for substations in addition to the systems controlling the power plant processes (Figure 1).
A consistent technology
When both control areas are brought together in an integrated system environment, a consistent operating concept can be applied throughout the entire power plant. Operators and electricians then work with the same system and use the same graphic displays (Figure 2). With consistent data representation and operation, it is possible to improve the quality of the operational procedures. Depending on the user access rights established, electrical equipment can be controlled from any operator workplace within the process control system. This way, operational data are available from the electrical systems as well, without limitation, no matter where and when they are needed. Integrated, consistent systems also provide the following advantages:
• Thanks to joint storage of process data and data from the electrical systems, evaluations and reports can be carried out and prepared on a uniform basis
• Owing to plant-wide and chronologically accurate signal recordings, troubleshooting and fault investigation are simplified
• The training required for operators,
maintenance personnel and system administrators is reduced
• The system administration, for example management of user access rights, data backup and maintenance of signal data and graphic displays, is simplified
• IT security measures need to be implemented for only one type of system environment
• Reducing the number of engineering tools prevents multiple data input and also assures a high consistency of the documentation
• Bringing the automation of power plant processes and electrical systems together substantially reduces the number of different spare parts required
• The interfaces between the control equipment and the higher-level data processing systems and company-wide networks for the purpose of accessing office computers, exchanging data with the maintenance system, establishing a connection to central load dispatch and enabling remote maintenance, need to be implemented and maintained only once.
It is plain that the implementation of a consistent solution can reduce system operating costs.
The two chains of integration
Taking a close look at today’s automation market reveals that different standards are being applied in process automation and in station automation. While the Profibus and Foundation Fieldbus standards predominate in process automation, different serial protocols are in use for electrical systems depending on the requirements of station automation. These include standards such as IEC 60870-5-101, -103, -104, DNP3.0, Modbus RTU, Lon and a number of manufacturer-specific protocols (Figure 3).
The challenge of integrating the two control ‘worlds’ is how this large number of protocol standards can best be supported. The interfaces needed for this purpose have to be implemented on a project-specific or device-specific basis. Gateways or protocol converters, however, involve higher hardware, maintenance and engineering costs, as these devices have to be configured, tested and documented. In addition to the large number of communication protocols, the traditional solution carries these drawbacks:
• Manufacturer-specific configuration prog-rammes are needed for the engineering tasks.
• There is no standardised documentation for engineering data of protection and control devices (IEDs). Exchanging data between different engineering tools is complicated.
• For maintenance and servicing, specialists are needed who understand the different tools and communication protocols.
• The devices cannot communicate with each other serially (horizontal communication). Fixed wiring or a superimposed station control system is necessary.
• The existing interfaces and protocols are not powerful enough. Not all protocols support the transfer of time stamps, generate measured values in engineering units, or fulfill the respective long-term requirements.
• For time synchronisation, device-specific solutions need to be created.
Introducing IEC 61850 to substation automation systems was a major step towards simplifying the integration of electrical systems. It does not replace the fieldbuses of process automation, but it can effectively reduce the multitude of protocols used for electrical systems. With the help of this standard, power plant control systems can be based on two ‘columns’: the fieldbus standard for process automation and the IEC 61850 standard for the electrical systems.
Power plant electrical systems
Figure 4 shows a simplified block diagram of a conventional thermal power plant. Depending on the type of plant, some specific components, eg static startup systems for gas turbines or emergency power-diesel generators, are not shown in this example. The integration concept for electrical systems is defined on the basis of the requirements for the typical elements of the block diagram (Table 1).
At the present time, there is no common communication standard for all areas in a power plant. The market availability of products supporting the specific communication standards must be taken into account. For example, protection and control devices for low-voltage motors and intelligent inverters are available only with fieldbus interfaces. Only the devices typically used in MV and HV switchbays presently support the IEC 61850 standard.
Special characteristics of IEC 61850
The IEC 61850 standard is far more than simply a protocol and has been drawn up for the communication of all functions in a substation.
Its main objective is interoperability. Protection and control devices from one or several manufacturers would be able to exchange information with each other and to utilise this information for their own functions. The standard uses an object-specific data model for this purpose. The data or functions grouped in the data model are referred to as logical nodes and are defined for all relevant substation components (eg a circuit breaker) and functions (eg an overcurrent protection device). The logical nodes and the data attributes contained in them are assigned designations based on standardised, and therefore binding, semantics.
In order to achieve this, the SCL description language is used as it can cover all aspects of substation automation. SCL is a formal, standardised and extendable description of a substation and is based on the general XML description language. An SCL file describes the structure of the substation along with its various voltage levels, the complete block diagram and the protection and control devices by means of their respective logical nodes, the relation of the logical nodes to the block diagram, the internal communication between the individual logical nodes, and the communication network. The SCL description of a substation can be used as a complete system documentation, for example, for maintenance purposes. The SCL format enables easy data exchange among configuration tools from different manufacturers.
The IEC 61850 standard supports all communication requirements in a substation. For vertical communication among the protection and control devices as well as with the operating
and monitoring level, this standard uses the TCP/IP and MMS (Manufacturing Message Specification) standards. For horizontal communication between protection and control devices, the IEC 61850-GOOSE (Generic Object Oriented Substation Event) specification, that can fulfil the requirements in terms of a quick transfer of protection or interlocking signals via the Ethernet station bus, is used. Consequently, this standard largely replaces the fixed wiring between the protection and control devices and the process control system. The standard separates the functionality of substation automation and the data model from the assigned communication services, while taking into account that the development in communication technology is advancing at a faster pace than technological development in the area of substation automation, including that of electrical protection systems. The IEC 61850 standard thus protects the capital investments made in substation automation.
Impact on system structure
Introducing the IEC 61850 standard when electrical systems are to be integrated will automatically lead to simplified system structures. Figures 5 and 6 show the possible steps for introducing the standard. Figure 5 shows a first stage where the serial protocols for vertical communication of the protection and control devices with the operating and monitoring level of the process control system are replaced with the IEC 61850 standard. The exchange of information among the devices (horizontal communication) still takes place via fixed wiring. But the new standard can also be used for vertical communication purposes. In this case, communication between the protection and control devices (eg for interlocking, protection commands, etc) as well as data exchange with the automation level of the process control system (eg turbine protection/controller), as shown in the Figure 6, are also Ethernet based.
Today's state of the art makes it possible for the automation of the process and of the electrical systems in a power plant to be integrated into a consistent system concept. This way, it is no longer necessary to provide for a separate operating and monitoring system for the electrical components. With this integrated power plant automation concept, substantial cost savings can be realised with respect to the engineering, operation and maintenance activities. Control system concepts must take into account that no common communication standards are available for the entire power plant, not today, nor in the near future.
The IEC 61850 standard has been accepted in the substation automation market and can also be applied to the auxiliary power supply systems in power plants. The portfolio of the leading manufacturers of electrical systems covers the entire scope of applications. Some limitations exist regarding applications that have not yet been defined by the IEC 61850 standard. The generator excitation system is such an application. But extensions of the standard have either been released, or are under way for, hydro power plants (IEC 61850-7-420 or IEC 62344), wind power facilities (IEC 61400-25) and distributed power generation units (IEC 62350). These extensions can also be utilised for conventional power plants. It is therefore safe to say that the IEC 61850 standard is the solution of the future.