By Dr. Alexander Apostolov, Areva T&D Inc.
The electric power industry is changing and the design of substations is changing right along with it. Driven by more strict reliability requirements, as well as significant technology developments, a new concept for the substation of the future has emerged.
Designing the substation of the future will require an excellent understanding of primary and secondary equipment in the substation, the transformation of primary system parameters to secondary quantities used by multifunctional intelligent electronic devices (IEDs), and the availability of new types of sensors (such as non-conventional current and voltage instrument transformers) that eliminate many of the issues related to conventional instrument transformers.
The substation of the future will be based on an object-oriented modular approach that will cover the design of the substation primary system, the IEDs providing protection, control, measurements, recording and other functions, as well as their integration in substation automation systems with advanced functionality. It should include solutions for a technically compliant system design that meets new and emerging grid connection requirements and solutions for integrating wind farms or other distributed energy resources in the power grid. These solutions include, but are not limited to, HVAC or HVDC (High Voltage Alternating Current or High Voltage Direct Current) connections; SVCs (Static VAR Compensator) or STATCOMs (an SVC based on Gate-Turn-Off Thyristor technology); switchgear and transformers.
Upgrades of existing substations and construction of new substations in urban areas with limited space availability will also result in an increase in the number of gas insulated substations (GIS) built worldwide.
Considering the fact that more and more substations will be installed in locations that may have extreme climate conditions, both the primary and secondary devices must be able to operate correctly under all weather conditions.
The publication of IEC 61850, the new international standard for substation communications, is an extremely important step in the definition of the “copper-less” substation of the future and will have the greatest impact on future substation design. Distributed substation applications based on high-speed peer-to-peer communications of change of state of breakers, protection and control functional elements or current and voltage sampled values will lead to very efficient and at the same time functionally superior substation solutions.
IEC 61850 defines not only the object models of primary substation equipment, IEDs and functions in a substation automation system, but also the relationship and communications between system components based on the different system requirements. It is very important to understand that just because one can model a function in a device or substation automation system does not mean that the standard attempts to standardize the functions. There are so many different algorithms and characteristics used for different functions (for example a distance protection element), as well as preferences and options, that this will be an extremely difficult task. Instead, the model represents the communications visible attributes and behavior of the device. This is sufficient for the development and implementation of engineering, testing, analysis, integration and other tools that will result in the introduction of real power system engineering automation.
It is important also to remember that the changing technology introduces new methods for interface between the instrument transformers or sensors and the substation IEDs. They need to be able to interface with conventional and non-conventional sensors to allow the implementation of the system in different substation environments.
A simplified diagram with the communications architecture of an IEC 61850 process and substation bus-based substation automation system is shown in Fig. 1 (pg. 23).
The merging unit (MU) interfaces with the process through conventional or non-conventional instrument transformers, generates and multicasts sets of measured sampled values to multiple IEDs in the substation over the substation local area network. In IEC 61850 this is called the “process bus.” Status information for breakers and switches is available through an input/output unit (I/O). In some cases the merging unit and the input/output unit can be combined in a single device that we may call a process interface unit (PIU).
The receiving devices (IEDs or industrial grade computers) then process the data, make decisions and take action based on their functionality. The action of protection and control devices in this case will be to operate their relay outputs or to send a high-speed peer-to-peer communications message over the station bus to other IEDs in order to trip a breaker or initiate some other control function, such as breaker failure protection, reclosing, etc.
All devices and functions in the substation exchange real-time or reporting/analysis data based on the object models defined in IEC 61850. The modeling of complex multifunctional IEDs from different vendors that are also part of distributed functions requires the definition of basic elements that can function by themselves or communicate with each other. These communications can be between the elements within the same physical device or in the case of distributed functions (such as substation protection schemes) between multiple devices over the substation local area network. The basic functional elements defined in IEC 61850 and used in the modeling of the devices in the substation are the logical nodes.
A logical node is “the smallest part of a function that exchanges data.” Multiple instances of different logical nodes become components of different local or distributed protection, control, monitoring and other functions in a substation automation system.
IEC 61850 also allows the development of a new range of protection and control applications that result in significant benefits beyond the conventional hard-wired solutions in today’s typical substation. It supports interoperability between protective relays and control devices from different manufacturers in the substation, which is a necessity in order to achieve substation-level interlocking, protection and control functions and improve the efficiency of microprocessor-based relays applications.
There is consensus in the industry that high-speed peer-to-peer communications between IEDs connected to the substation LAN based on exchange of generic substation event (GSE) messages can successfully replace hard-wiring for different protection and control applications, such as the protection of distribution buses, distributed recording or load-shedding in substations with varying configuration.
Sampled measured values communicated from merging units to different protection and control devices connected to the substation process bus replace the copper wiring between the instrument transformers in the substation yard and the IEDs.
As a result, future substations based on IEC 61850 communications will provide some significant advantages over conventional protection and control systems used to perform the same functions in the substations. They will require reduced wiring, installation, maintenance and commissioning costs; and they will adapt easily to changing bus configuration in the substation.
The substation configuration language allows interoperability and a seamless integration process. The common substation or IED configuration files can be exchanged between different configuration, coordination, analysis or testing tools in a way that significantly improves the efficiency of the engineering process.
Led in large part by IEC 61850, the substation of the future is closer to reality than you might think.
Alexander Apostolov received an MS degree in Electrical Engineering, MS in Applied Mathematics and Ph.D. from the Technical University in Sofia, Bulgaria. He is presently Principal Engineer for Areva T&D Automation. He is a senior member of IEEE and member of the Power Systems Relaying Committee and Substations C0 Subcommittee. He is chairman of the Relay Communications Subcommittee, and serves on many IEEE PES working groups. He is member of IEC TC57 (leader of a Task Force on Power Quality Object Models in IEC 61850) and CIGRE. He is chairman of the technical publications subcommittee of the UCA International Users Group. He holds three patents and has authored and presented more than 220 technical papers.