IEC as a flexible tool for electrical systems monitoring

Transcription

1 IEC as a flexible tool for electrical systems monitoring Pau Lloret, Juan L. Velásquez, Lluís Molas-Balada, Roberto Villafáfila, Andreas Sumper Centre d'innovació Tecnològica en Convertidors Estàtics i Accionaments (CITCEA-UPC), Departament d'enginyeria Elèctrica, Universitat Politècnica de Catalunya. EU d'enginyeria Tècnica Industrial de Barcelona, Comte d'urgell, Barcelona, Spain Tel: lloret@citcea.upc.edu Samuel Galceran-Arellano Centre d'innovació Tecnològica en Convertidors Estàtics i Accionaments (CITCEA-UPC), Departament d'enginyeria Elèctrica, Universitat Politècnica de Catalunya. ETS d'enginyeria Industrial de Barcelona, Av. Diagonal, 647, Pl Barcelona, Spain Tel: , Fax: Abstract In the last decades there have been great advances in electronic, information and communication technologies. Up to date, any of these advances have been implemented in electrical systems, but some difficulties have been encountered, particularly regarding to the integration of intelligent electronic devices from different manufactures into a centralized system. With the aim of solving the above mentioned problem, the standard IEC was developed to specify communication networks in substations. In addition to control and protection functions, the information model created around IEC series can help the development of other applications related with electrical monitoring systems. Monitoring can be used to achieve a variety of benefits for utilities, including maintenance prediction, failure prevention, active control, improved commissioning tests and more accurate end of life assessments. Examples of application that could be highly benefited by the adoption of IEC as standard for communications in the power electrical industry are power quality monitoring and condition based maintenance. In this paper, the new standard IEC61850 and the new opportunities that it can offer to the electrical industry are presented. Firstly, a brief presentation of IEC61850 and the main benefits it provides are described. Finally, a more detailed description of the possibilities that IEC61850 can offer in the field of power quality monitoring and condition based maintenance (CBM) is highlighted. Keywords IEC standard; monitoring; power quality; condition based maintenance. I. INTRODUCTION In the last decades there have been great advances in the field of electronic, information and communications technologies that have been introduced progressively in devices and systems from different industry sectors. Over the years, these new technologies have proved its reliability in all operational conditions of high-voltage substations. For that reason, substation automation became accepted worldwide and most of these advances have been implemented. Analogue relays are being substituted by digital devices increasing control, protection, monitoring and communication capacities in substations. Today all new intelligent electronic devices (IEDs) in the market implement monitoring capabilities. At the beginning, substation automation was characterized by manufacturers proprietary solutions, using each one a different proprietary protocol. This procedure encountered some difficulties, particularly regarding to the integration of IED from different vendors into a centralized automation system. It extends integration time and makes necessary the use of protocol gateways that increases system costs if different vendors devices are used. In addition to vendor proprietary solutions, geographically based standards also coexisted at the same time. In the USA, the Electric Power Research Institute (EPRI) developed Utility Communication Architecture (UCA), and then UCA 2.0, which described detailed object models of field devices Endesa s R+D+i awards NOVARE 2005 on Power Quality and Reliability

2 and definitions of the communications behaviour. At the same time in Europe, the International Electrotechnical Commission (IEC) recognised the need to develop a standard interface for the different manufacturer protection IEDs. As a result, the standard Companion standard for the informative interface of protection equipment (IEC ) was created. Faced with the possibility of creating two standards to solve the same problem, members of the IEC and EPRI recognised the need of having only one standard interface to the electrical substation IEDs and concluded that it should be based on the UCA 2.0 data models and services, but harmonised for general use around the world. As a result, the IEC [1] standard series were developed to specify standardized communication networks and systems in substations. Once IEC was published, it has been shown that additional information models could be developed to increase the standard application field. For that reason, nowadays several extensions and actualizations are under way to adapt IEC standard to wind power plants, hydro power plants, distributed energy resources, and also related to power transmission and distribution as well as power quality monitoring. Apart from control and protection functions, IEC series also offers instantaneous information in real-time. This is an important issue for applications that need the use monitoring or metering techniques for their implementation. Monitoring can be used to achieve a variety of benefits for the operators of substation equipments, including maintenance prediction, failure prevention, active control, improved commissioning tests and more accurate end of life assessments. In this work there is a brief description of the standard IEC and the main benefits it offers. Following, a more detailed description of the possibilities that IEC can offer in the field of electrical systems monitoring such as power quality, in section IV, and condition based maintenance (CBM), in section V, are highlighted. II. INTRODUCTION TO IEC The IEC standard defines communications between IEDs in the electrical substation and the related system requirements. It has been developed by Technical Committee (TC) 57 of the IEC, and the objective was the development of an international standard for communication networks and systems in an automated electric substation. Until the new standard appeared, several proprietary communication protocols coexisted in a substation depending on the quantity of different vendors used. This makes the integration of devices from different vendors in the same substation a hard task. One of the main objectives of the new standard is to achieve interoperability of IEDs from different equipment manufacturers. Under the general title Communication networks and systems in substations, IEC series are divided into 14 parts also divided into 10 main topics. The list of these parts is the following: IEC : Introduction and Overview. IEC : Glossary. IEC : General requirements. IEC : System and project management. IEC : Communication requirements for functions and device models. IEC : Configuration description language for communication in electrical substations related to IEDs. IEC : Basic communication structure for substation and feeder equipment Principles and models. IEC : Basic communication structure for substation and feeder equipment Abstract communication service interface (ACSI). IEC : Basic communication structure for substation and feeder equipment Common data classes. IEC : Basic communication structure for substation and feeder equipment Compatible logical node classes and data classes. IEC : Specific communication service mapping (SCSM) Mappings to MMS (ISO/IEC and ) and to ISO/IEC IEC : Specific communication service mapping (SCSM) Sampled values over serial unidirectional multidrop point to point link. IEC : Specific communication service mapping (SCSM) Sampled values over ISO/IEC IEC : Conformance testing. IEC series defines object modelling and information models that describe in an abstract way data names and power system structures. These information models specify what information or data is exchanged, and are focused on field device characteristics. IEC also defines interface models (Abstract Communication Service Interface, ACSI) that describe in an abstract way the mechanics of how data is exchanged to get the right information to the right destination at the right time. The definition of these standardized information and interface models makes possible to achieve interoperability between diferent kinds of devices independently of the device manufacturer. The abstract models defined divide the substation automation system functions in small entities called logical nodes. Logical nodes are conceptually defined in IEC and then described in IEC Logical nodes are independent of physical devices, so several logical nodes can reside in the same device. Based on their functionality, each logical node contains a list of data and data attributes. All this elements form part of the object name identification and it is structured as is shown in Figure 1. As each signal identification has to be unique and unambiguous, logical node prefixes and

3 instance numbers are used to distinguish between data objects with the same logical node function. Figure 1. Structure of the object name identification [1]. All these abstract models defined in the IEC X series are independent of a concrete implementation in an actual protocol. In IEC X and IEC X these abstract communication services and objects are mapped to actual protocols. All communication use Ethernet (ISO/IEC ) as the basic communication technology. Services commonly used for communication within the whole substation are mapped in IEC to MMS (Manufacturing Message Specification, ISO 9506). MMS is an application layer standard designed to support messaging communications between IEDs in a distributed system environment. It was chosen because it is the only public standard protocol that can support easily the ACSI mapping and information models defined in IEC X series. Additional protocols are defined for those ACSI services that are not mapped to MMS, such as SNTP (Simple Network Time Protocol) for time synchronisation messages, or GOOSE (Generic Object Oriented Substation Event) and GSSE (Generic Substation Status Event) for trips and fast messages. Alternatively, services used for the transmission of sampled values are mapped over serial unidirectional multidrop point to point link in IEC , or mapped directly over Ethernet in IEC An overview of all these mapping used for the standard IEC is shown in Figure 2. An important innovation that offers this standard is a configuration language based in XML called Substation Configuration Language (SCL) described in IEC This language allows a formal description of the substation automation system, the switchyard and the relation between them, and the IED configuration. This permits exchange device configurations using SCL files. The standard defines four types of SCL files for different purposes; SSD files (System Specification Description) make a description of the entire system, SCD files (Substation Configuration Description) describe a single substation, ICD files (IED Capability Description) are the description of items supported by an IED, and finally CID files (Configured IED Description) describe a configuration for a specific IED. The use of these files provides self-description of the substation system elements. This selfdescription reduces manual configuration, installation time and the cost of data management, configuration and maintenance. It also reduces down time due to configuration errors. In IEC , general requirements of the communications network are detailed with emphasis on the quality requirements of IEDs such as reliability, maintainability, system availability and security. It also describes climatic, mechanical, and electrical influences and requirements. This section refers to other existing generic standards such as IEC 60870, IEC and IEC 61000, but additional requirements have been elaborated too. As a global communications standard, the IEC series includes standardized conformance test to ensure that suppliers comply with standard s requirements. IEC specifies conformance testing methods and procedures for testing devices of substation automation systems and gives guidelines for setting up test environments. This part also describes documentation of conformance test report and certifications to demonstrate the capability of the device under test to operate with other IEDs according to the IEC series. As IEC introduce the use of Ethernet bus in substation automation, other benefits of IEC include lower communication infrastructure costs using readily available TCP/IP and Ethernet technology, or a reduction of wiring cost for the use of a high speed process bus that enables sharing of instrumentation signals between devices. However, due to these increasing networking of systems and power system equipments, cyber security has become a major issue. For that reason, IEC [2], IEC [3], and IEC [4] were developed. These parts of IEC specify messages, procedures, and algorithms for securing the operation of all protocols based on or derived from the standard IEC Figure 2. Overview of functionality and profiles [1]. In addition to standardized communications and data models that make possible interoperability between devices, uniformed system handling and harmonized general system properties are also needed. For that reason the IEC series covers not only communications, but also unified system configuration language, environmental conditions and quality requirements, and conformance test procedures and techniques. III. EXTENSIONS OF THE IEC INFORMATION MODEL Despite its recent publication, the IEC standard is consolidating as the future international standard for communications in electric substations. In addition, several extensions and actualizations of the standard are under way regarding not only substations but also almost the whole electrical energy supply chain as well. It includes wind power plants, hydro power plants, distributed energy resources, and also power transmission and distribution.

4 The creation of those new publications are going to increase the number of logical nodes, data objects and data attributes that form the IEC series information model. The main objective of those extensions is the definition of only one harmonized standard for the whole electrical power system, complementing the defined at the beginning regarding only electrical substations. Several activities are being done to adapt IEC standard to wind power plants (IEC ) [5]. The part of the series that defines the mapping to communication profiles, IEC , includes mapping of IEC series into IEC among other four protocols. Until now, the only standardized mapping available is the mapping published in the international standard IEC , others are still under preparation. Part that defines information models to be used in wind power plant condition monitoring systems is also in progress. Simultaneously, IEC TC 57 is actually working in extending IEC to communication in hydroelectric power plants (IEC ) [6]. This new extension of the standard will add another 60 hydropower specific logical nodes to the IEC series. Based on the same concept, the IEC TC 57 is also preparing an extension of IEC in the area of distributed energy resources (DER) (IEC ) [7]. Under the common name IEC 61850, several information models are defined or are under way for exchanging information between IEDs, and for the configuration of systems and devices. IEC series has become the base standard for modelling the whole power system information, as it includes not only electrical substations information models. All these new standardized information models have to ensure that no confusion will arise and interoperability is maintained. IV. IEC AND POWER QUALITY Besides control and protection functions, the availability of instantaneous information is one of the properties that IEC series offers. This is an essential issue for applications that need the use of monitoring techniques for their implementation. At present, one of the means used around the world for getting a good improvement in the power quality of the electrical systems is by using IEDs that are located in certain points of the network in order to monitor power quality defining parameters (Figure 3. ). Figure 3. Power quality parameters [8]. Although IEC information models of the first edition of the standard do not deal with all power quality parameters, there are some logical nodes that can be used to monitor some power quality parameters. As an example, following there is a list of some logical nodes used for power quality applications: MMXU and MMXN: acquire values from instrument voltage and current transformers in a three-phase and single-phase system respectively. They also calculate RMS values for currents and voltages, power flows and phase impedances. MMTR: provides basic energy meter measurements. MSTA: provides metering statistics, which includes average, minimum and maximum of metered values over a given evaluation period. MSQI: provides the sequence components and imbalances. MHAI and MHAN: calculate harmonics (including subharmonics and multiples) and interharmonics in a three-phase and single-phase system respectively. TC 57 of the IEC has been publishing amendments to the first edition of IEC series. One of this was an amendment to the first edition of IEC , and defines new common data classes used for the power quality models and for the representation of statistical and historical information. These amendments will be merged in the future with the first edition of IEC X series and will be circulated in the form of its second edition. The second edition of , , and parts that are now in preparation includes extensions of the models for power quality metering as well as extensions for handling of statistical data and history of them. This extension of information models to cover power quality monitoring may include a new logical node group (group Q, [9]) with new logical nodes for flickers, RMS voltage variations including sags, swells, or momentary interruptions, transients, or frequency variations [10]. V. IEC AND CONDITION BASED MAINTENANCE Another application field that could be highly beneficed by the adoption of the IEC standard, and the monitoring resources it offers, is the maintenance based on condition. Nowadays, most maintenance actions in power systems are carried out by either corrective or preventive maintenance. The corrective maintenance lets the component or system run until breakdown or fault before maintenance action is considered. For this reason corrective maintenance is also known as breakdown maintenance or run-to-failure maintenance. In contrast, preventive maintenance is carried out at predetermined intervals according to prescribed criteria and intended to reduce the probability of failure or the degradation of the functioning of an item [11]. This is done by repair or component exchange in preset intervals. Preventive maintenance is sometimes called historical, planned, or calendar based maintenance. It has been shown that these traditional maintenance techniques, corrective and preventive, are very costly and

5 inefficient. To try to maintain the correct equipment at the right time, predictive maintenance techniques were introduced. In predictive maintenance, the maintenance intervals are decided according to the condition of the equipment rather than to time service or number of operations. Predictive maintenance is also known as Condition Based Maintenance (CBM). CBM relies on monitoring selected parameters of the equipment in a manner that the ongoing condition can be continuously or periodically assessed and maintenance is initiated based on the present needs of the equipment condition [12]. The main purpose of CBM is to eliminate or minimize breakdowns and prolong the preventive maintenance intervals. As a result, an increase in equipment availability is achieved, and then power availability and quality is increased too. Liberalization and privatization of electric markets have swept the power sector and as a result business environment become more competitive. Maintenance is often one of the biggest controllable expenditure in a company [13]. For that reason, the introduction of the condition based maintenance concept in substations will allow electric power companies to optimize their maintenance and operation costs, while at the same time that will increase the quality and continuity of the electrical supply due to an increase in the efficiency of devices. Condition monitoring is a major component of predictive maintenance. With the appearance of IEC series, condition monitoring and other monitoring tools become easier to implement in automation substation systems. It defines among its information models several information that could help to determine the condition of substation equipment. For example, the logical node used for modelling circuit breakers (XCBR) includes the sum of switched amperes (SumSwARs) and one operation counter (OpCnt) as some of their data attributes. The sum of switched amperes in a circuit breaker, also known as i2t, and the number of switching operations are some of the key monitoring parameters to know the circuit breaker condition. In a similar way, several logical nodes include key data attributes needed to know the condition of power transformers. Examples of logical nodes used to monitor transformer condition are: YPTR: includes the winding hotspot temperature. YLTC: includes key attributes to monitor the tap changer condition. ZBSH: provides properties and supervision of bushings as used for power transformers. SARC: includes attributes for monitoring and diagnostics for arcs. SPDC: includes attributes for monitoring and diagnostics for partial discharges. SIML: supervises liquid insulation medium such as oil used in transformers and tap changers, including attributes like relative saturation of moisture (H2O), insulation liquid temperature (Tmp) and measurement of hydrogen concentration (H2). As described in [10], in future all this nodes will be extended and included in logical node group S, specially dedicated to sensors and monitoring. Other logical nodes also useful for condition monitoring could be the described in section IV that belong to metering and measurement logical node group M, such as MMXU, MMXN, MMTR, MSTA, MHAI or MHAN. VI. CONCLUSIONS As it has been seen in this paper, the adoption of the IEC series as the standard not only for the communications in electrical substations but also in the whole electric power system, is an opportunity that the electric sector should not miss. IEC distinctive features described causes great benefits to its users, especially regarding standardization of communications and data models that make possible interoperability between devices. It also provides device selfdescription that reduces manual configuration, installation time and the cost of data management, configuration and maintenance. The proposed use of Ethernet bus in substation automation is the cause of other benefits, e.g. lower communication infrastructure costs using readily available TCP/IP and Ethernet technology, or a reduction of wiring cost for the use of a high speed process bus. In addition to control and protection functions, the extensive information model created around IEC series can help the development of other applications related with electrical monitoring systems. Therefore, applications which a detailed monitoring is needed, such as power quality monitoring or condition based maintenance, can be also highly beneficed by the adoption of IEC series. Due to technical reasons, those kinds of applications have not been yet generally applied in electric power industry. Nowadays, those technical issues are partially solved thanks to advances in electronic, information and communication technologies and standardization of protocols used in these communications. The appearance of IEC and other IEC based standards will help to develop these monitoring based applications in power electric systems in the following years. However, the development of IEC standard family is not completely concluded. It is in continuous actualization and several addendums, extensions and clarifications are being now developed. An example of this continuous actualization is the proposed extensions of the information models regarding power quality and transformer monitoring, or the use of IEC also outside substations. ACKNOWLEDGMENT Nowadays, the concepts introduced in this paper about the use of IEC for condition monitoring are being applied in a project called 'Substation monitoring for predictive maintenance'. This project has awarded with Endesa s R+D+i international prize NOVARE 2005 on distribution networks in the category of Power Quality and reliability.

6 Thanks to manufacturers that have given their equipments to develop this project and which participate in Endesa s Distribution Innovation Circle (CIDE). REFERENCES [1] IEC SER. Communication networks and systems in substations. All parts [2] IEC Power systems management and associated information exchange - Data and communications security - Part 3: Communication network and system security - Profiles including TCP/IP [3] IEC Power systems management and associated information exchange - Data and communications security - Part 4: Profiles including MMS [4] IEC Power systems management and associated information exchange - Data and communications security - Part 6: Security for IEC [5] IEC Control and monitoring of Wind power plants. Parts 1, 2, 3 and 5 published, and parts 4 and 6 in work. [6] IEC Communication networks and systems for power utility automation - Part 7-410: Hydroelectric power plants - Communication for monitoring and control. Work in progress. [7] IEC Communication networks and systems in substations - Part 7-420: Communications systems for distributed energy resources (DER) - Logical nodes. Work in progress. [8] R. Brown, Electric Power Distribution Reliability, MARCEL DEKKER, United States, [9] A. Apostolov, C. Brunner and K. Clinard, Use of IEC object models for power systems quality/security data exchange, Quality and Security of Electric Power Delivery Systems. CIGRE/IEEE PES International Symposium. Page(s): [10] Karlheinz Schwarz, IEC also outside the substation for the whole electrical power system, 15th Power Systems Computation Conference PSCC, Session 13, Paper [11] IEC International Electrotechnical Vocabulary. Chapter 191: Dependability and Quality of Service [12] User guide for the application of monitoring and diagnostic techniques for switching equipment for rated voltages of 72.5 kv and above. Cigré [13] J. C. Fitch, Proactive maintenance can yield more than a 10-fold savings over conventional predictive/preventive maintenance program, Diagentics Inc., 1995.