SPECIAL REPORT FOR STUDY COMMITTEE B5 (Protection and Automation)

Transcription

1 200 Study Committee B5 Colloquium August 25-31, 2013 Belo Horizonte, Brazil SPECIAL REPORT FOR STUDY COMMITTEE B5 (Protection and Automation) PS2: Experience & Application of Non-Conventional Instrument Transformers and Merging Units to Modern Digital Power System Protection and Control Janez ZAKONJŠEK (Slovenia) Summary This special report reviews 14 papers submitted by authors from 11 countries as contributions to the Preferential subject 2 of the 2013 SC B5 Colloquium Experience & Application of Non-Conventional Instrument Transformers and Merging Units to Modern Digital Power System Protection and Control. The papers provide a quite diverse overview of the subject, starting with presentations of different test installations and proposed testing methods, continuing with possibilities offered by non-conventional instrument transformers to develop completely new protection functions and applications, towards optimized process bus architectures, requirements on transient response of complete voltage and current digital acquisition chain together with technology promises for future development. To aid the discussion the contributions were grouped into four themes, namely: A. Current situation and prospective development of merging units and process bus B. Non-conventional instrument transformers (NCITs) and prospective development of new protection functionality C. Experiences gained with test installations and lessons learned D. New technologies requiring new testing skills, equipment and procedures Twelve questions were raised to facilitate a focused discussion on the subject after a brief summary of each paper. Keywords IEC 61850, Sampled values, Transient response, Merging unit, Non-conventional instrument transformers, Process bus 1. INTRODUCTION Different factors influence development of modern power systems. Worldwide spread of environmental concerns influence all important power system players: generation, transmission and distribution together with consumers on different voltage levels. Increased amount of non-conventional energy sources is getting included to existing power systems in points without strong transmission connections to big consumption areas. On the other hand it is extremely difficult to build new transmission lines between these players. Distributed generation, influenced strongly by non-conventional power sources appears suddenly within the distribution networks, which could not be imagined even only few years ago.

2 Not only primary, but also secondary power systems must cope with all these challenges and IEC International Utility Communication Standard presents in this respect a paradigm shift in the right direction. The standard itself is today rather mature in some of its parts, with thousands of installations worldwide experiencing its positive effects. Not so very fast development occurred within the bay/diameter and the process level. A number of test installations has been built up in the past years, with some of them experiencing very good results, while in some of them difficulties have appeared. Nonconventional instrument transformers (NCITs) have shown many advantages compared to conventional ones (CITs) but their implementation is not as fast as one would expect. Some types are still not enough cost effective. Requirements on technical characteristics of complete digital acquisition chain related especially to transient processes in primary system are under development by different working groups and there are still on going discussions about the most efficient architecture and organization of process bus. During this session we have a great opportunity to discuss current situation and possible development from the four complementary perspectives mentioned before and contribute this way to faster development and application of NCITs and MUs to modern digital power system protection and control. 2. CURRENT SITUATION AND PERSPECTIVE DEVELOPMENT OF MERGING UNITS AND PROCESS BUS Four contributions under this subject present current situation within IEC SV technology promise, open issues and latest development within the international standardisation activities. Development of technical requirements on transient response of complete digital acquisition system, including merging units and their building blocks is today one of the priorities within the field as well as future proof architecture of bay/diameter and complete substation process bus and its communication in order to achieve the highest possible flexibility, availability, and reliability of a complete system. Summaries Paper 203 Transient Behaviour of Current Transformers used as Primary Transducers and Input elements in protection IEDs and stand alone merging units from Sweden and Slovenia presupposes that Conventional Current Instrument Transformers (CCIT) will most probably remain in operation for many more years as Primary Transducers as well as input elements in conventional IEDs and Stand Alone Merging Units (SAMU). The transient behavior of a conventional current transformer (CT) is characterized by the performance before and after CT saturation. Before saturation the transient behavior is defined by the transfer function and the frequency response. The transient behavior related to CT saturation is more or less decided by the time to saturation. Protection IEDs shall be able to be connected to SAMUs produced by different manufacturers and of different design. To be able to guarantee correct operation of all major protection functions, requirements on the transient behavior must be carefully specified. In this respect the transient behavior of input CTs of SAMUs is important and must fulfill specified requirements. Paper 206 Towards Optimized Process Bus Architectures for Power System Protection and Control from Switzerland discusses optimized process bus architectures in order to achieve highest possible flexibility, availability, and reliability. To reach this goal a modular concept of process level IEDs (MUs, breaker IEDs, switchgear IEDs, etc), dedicated to specific primary equipment is proposed. In order to obtain fully redundant system a duplicated process bus is suggested in addition to all duplicated equipment (batteries, cores of instrument transformers, etc). Due to time critical GOOSE and SV services only the process bus with zero recovery time is applicable. The authors recommend in this respect the HSR (High-availability Seamless Redundant Protocol) to build the redundant ring 2

3 between the communicating process level IEDs and avoid use of switches. According to the authors all these measures will secure also the highest possible maintainability and extensibility of the complete system. The presented process bus approach is also prepared for the future if the switchyard devices will include embedded process level IEDs. Paper 212 IEC Sampled Values Technology promise, Open Issues, and the State of International Standardization from USA and Brazil provides good overview on current situation regarding sampled values within IEC In addition to this also overview on Merging Units (MUs) and especially Stand Alone Merging Units (SAMUs) is given with respect to some other standards like IEC , IEC It gives updated information on important parameters like: a) Sampling frequency selection, b) Anti-aliasing filter requirements, c) Time synchronization, d) Switchgear integration, e) SV frame payload variants. Paper 214 Basic Protection Requirements on Transient response of Voltage and Current Digital Acquisition System presented on behalf of CIGRE WG B5.24. The paper presents in a very compressed form some of the most important results obtained by CIGRE WG B5.24. New technologies have appeared with NCITs that do not have standardized analogue outputs. Furthermore, sampled values of measuring quantities that process bus distributes via Ethernet network(s) need to be standardized to ensure the same interoperability as today. Therefore it is important to standardize the characteristics of sampled values during different transient conditions in power system. One of the requirements the WG B5.24 puts on producers of MUs (and NCITs) is to provide technical information on transfer function and frequency response of their equipment in form of Bode diagrams. The second requirement concerns dynamic ranges of measured currents and voltages, which may influence the performance of complete protection functionality. This means that the minimum accuracy data as well as some additional data (e.g. saturation of SAMU conventional input CTs) must be available to the algorithm developer as well. Questions 2.1. Introduction of merging unit concept within IEC standard caused a big change in the protection and control IED development approach. In the past, one protection relay has been developed by only one development team being responsible for its complete functionality, including HW, SW and their interdependency. Today the conventional protection relay is divided into two completely different parts, namely MU (in many cases also internal NCIT electronics) and protection IED, possibly developed independently by different teams. Which are the main technical characteristics and parameters, including those appearing also with some very specific protection devices in different countries, that should be specified by producers of NCITs and merging units (including SAMU) as input information to protection and control algorithm developers? 2.2. IEC standard specifies Device conformance Classes with four specified functionalities for different classes of merging units. Such integration approach allows together with publishing of SV, GOOSE services, and IEC information model self-descriptive capabilities also implementation of local intelligence such as breaker failure protection, breaker automation and control, bay automation, synchronism check, synchrophasor-based phasor measurement units, and so on in a simple MU, located in switchyard. We can learn on the other hand that it is important both for the design and maintenance that the process level or process near IEDs are modularized towards the connected primary equipment. What would be the optimal way between these two possibilities, which would allow for the highest dependability, availability, maintainability and extensibility for future systems? 3

4 2.3. One of the important reasons to introduce NCITs together with MUs is also the expected economical effect, caused by extremely reduced lengths and amount of remaining copper cables. This will of course happen only if MUs and other primary equipment related IEDs will be located as close as possible to the corresponding primary equipment outside in the switchyard. This definitely requires rugged design of all these equipments, especially when it comes to external atmospheric conditions like temperature and humidity. Are these requirements specified today for protective relays (e.g. operating ambient temperature in rugged range between -40 O C and +55 O C) satisfactory or they should be extended to more extreme values? What would be the expected (and accepted) lifetime of these equipments if the requirements will remain on the same level? 3. NON-CONVENTIONAL INSTRUMENT TRANSFORMERS AND PROSPECTIVE DEVELOPMENT OF NEW PROTECTION FUNCTIONALITY Development of different new technologies permitted also development and later on efficient production of NCITs, based on rather long known laws of physics. NCITs have generally much better technical characteristics than CITs. Some of them can operate correctly in wide frequency range from DC current on one side to very high frequency currents (and voltages) on the other side, which makes it possible to introduce completely new protection functionality as well as to use them in installations where it was for different reasons not possible to use CITs. Summaries Paper 201 New Protection Relay for Variable Speed DFI Motor Generators from United Kingdom, France and Switzerland discusses new protection functionality for Variable Speed Double Fed Induction machines, which are getting an important part of modern pump storage hydro power plants. Traditional protections do not cover all possible operating conditions of these DFI machines, especially not in cases when the rotor and stator currents are characterized with very low frequency. Non conventional VTs and CTs with their outputs based on IEC standard are used for development of new protection functionality, which provides all necessary functionality mainly during the start-up and run-down of the machine. Paper 205 Operating Performance of Rogowski Coils versus Conventional Iron-core Current Transformers for Protective Relaying Purposes from USA presents basic characteristics of Rogowski coil based transducers and compares them with characteristics of conventional, iron-core current transformers. The inherent Rogowski Coil performance characteristics include high measurement accuracy and a wide operating current and frequency range allowing the use of the same device for both metering and protection. In addition, Rogowski Coils make protection schemes possible that were not achievable by conventional CTs because of saturation, size, weight, and/or difficulty encountered when attempting to install current transformers around conductors that cannot be opened. Paper 211 Relay Protection Solutions based on Non-conventional Current Sensors in Actual Industry / Utility Applications jointly prepared by authors from Portugal, United Kingdom and USA. The paper presents novel solutions for protection, metering, and control in electric power system using Rogowski coil as non-conventional current sensors. Current differential protection of electric arc furnace transformers is presented as typical innovative industrial application, where conventional current transformers cannot be used. Dedicated differential protection of power cables in combined OHL cable transmission circuits is an example for utility application where Rogowski coil based current sensor is installed at both ends of protected cable, making it this way possible to distinguish strictly between faults in cable and on overhead line section. The second utility application is advanced differential protection of capacitor banks, which has already showed some good practical results. 4

5 Questions 2.4. All three papers covering this theme present new possibilities for improved protection of power system, starting with introduction of completely new protection principles applied to Variable Speed DFI Motor Generators, continuing over differential protection of electric arc furnace transformers and cable section of combined cable overhead line towards advanced differential protection of capacitor banks. What other new functionality, beneficial for more dependable operation of power system could be developed in future for all voltage levels, if we consider excellent transfer characteristics of NICTs? Could, in this respect, the state of current technology used within the complete Digital Substation represent some obstacles for their implementation? 2.5. Most of articles presented lately during different occasions deals on the current measuring side with Rogowski coil and Faraday effect based sensors while on the voltage side RC dividers and very seldom also Pockel s effect based sensors have been mentioned. What are on one side the prevailing advantages of NCITs primarily used today and disadvantages of other known methods which prevent them to become more applicable? What can we expect in this respect for the future? 2.6. Discussion about Digital Substation is generally oriented towards EHV and HV AC substations without any special attention to MV installations. The power system society discusses today very much not only HVDC lines, but also HVDC systems and all related FACTS devices. How can development of NCIT technology and IEC standard as a whole influence the development and design of future HVDC systems and other FACTS devices on one side and MV installations on the other one? 4. EXPERIENCES GAINED FROM TEST INSTALLATIONS AND LESSONS LEARNED Three contributions report on test installations comprising between others also NCITs, MUs, and process buses. The lessons learned are generally positive, especially in installations with one vendor only. Generally it is possible to split such tests in three different groups: a) Site testing of newly developed primary equipment with corresponding secondary elements, b) A single vendor installation, and c) Multivendor test installation. A paper from Germany presents such project together with a number of experiences gained. Summaries Paper 202 Acquisition, Transmission and Processing of Process data in a 400kV bay with Process Bus acc. to IEC LE an Experience Report from Germany. This report gives an overview on commissioning and operation experience from Intelligent Substation project of the German TSO AMPRION. The authors have faced the main challenges residing in the interoperation of devices from different vendors and in the application of new methods for testing the process bus technology. The system has been operated under specific network conditions and the observed behaviour has been analyzed. NCITs got installed as part of the project. Their output signals got transferred via Optically Powered data Link (OPDL) to the merging units, which generate sampled values according to IEC protocol and fed them into the Ethernet process bus. Within the project the transmission of SV to various protection devices of different manufacturers has been established. The substation is fully operational since commissioning. Paper 210 Movement to the Fully Digital Substation prepared by authors from United Kingdom, France and Brazil. Digital devices such as numerical protection relays and digital 5

6 systems like substation control are prevalent technology in substations today. The paper presents in a structured way some other substation areas into which digital technology can permeate so that in progressive way the substation can become fully digital. Some most important drivers towards digital substation are described together with its basic architecture. Two examples of commercial projects are presented at the end. The first one includes a number of optical CTs, merging units, and differential relays, which subscribe to the sampled values in a process bus protection scheme. The second one applies analogue merging units towards conventional CTs and VTs within a GIS substation, providing this way all necessary information to relays and bay controllers over an IEC process bus. Paper 213 Examples of the Application of Optical Instrument Transformers to Power System protection and Control Systems in Japan from Japan. Development of power system in Japan required a novel approach to the design of Voltage and Current instrument transformers. As the highest voltage level in Japan will in future be 1000kV a nonconventional voltage transformer, based on Pockel s electro-optic effect has been developed for 500kV system and operates successfully since A Faraday effect based magnetooptical CT for Hokkaido-Honshu HVDC cable link has been developed and successfully tested for more than one year. Results of steady state accuracy test under different system and environmental conditions comply with requirements of corresponding international standards and their dynamic performance is within expected limits as well. In both cases, by exploiting optical technology, the size and weight of optical instrument transformers have been reduced and costs have also been reduced. Questions 2.7. Four test installations in four different countries have been presented within this group of papers. Considering fast development of complete technology around Digital Substation it would be expected that more test installations worldwide is planned to start or are already in operation. What are the main reasons from the utility, vendor and scientific perspective to start such installations and what results have been expected and obtained so far? 2.8. We do not exaggerate by saying that no standard within the electric power community has been accepted by so many countries, organizations, utilities, and vendors as International Utility Communication Standard IEC There are different organizations dealing with conformance testing, but in real life interoperability between different conforming products has not been obtained from the begging and manual adjustment had to be performed before reliable operation has been secured. What could be the reason(s) for such situation and which measures should be taken within the corresponding organizations that the situation will improve significantly in short time? 5. NEW TECHNOLOGIES REQUIRE NEW TESTING SKILS, EQUIPMENT AND PROCEDURES It is a fact that every new technology represents some kind of paradigm shift, which mirrors itself between others also in new testing equipment, testing methods, and especially also in skills required for successful completion of all tasks included. In case of Digital Substation this means testing of each separate unit, connections and interactions between them as well as testing of complete systems. From the test type point of view it is possible to discuss different requirements on laboratory tests, factory acceptance tests and site tests, which again can be commissioning tests, maintenance tests and tests after equipment repair. All these are discussed in four papers presented under this thematic. 6

7 Summaries Paper 204 Testing of Non-conventional Instrument Transformers Based Bus Protection Systems from USA. The authors emphasize throughout their contribution on how testing of the IEC based busbar protection requires in the first place good understanding of the system architecture and the functionality of each component included. It is important that each component of the system (NCIT, different types of MUs, communication, Central unit, etc.) is thoroughly tested as well as the complete system itself. Appropriate test methods are based on the functional hierarchy of the system and presented for testing of different types of protection components. Testing of complete IEC based busbar differential protection requires, according to the authors, also a distributed testing system that can properly simulate the substation environment under different operating conditions. Paper 207 Validation of Instrument Transformer and Bus Process Digitized by Standard IEC from Brazil describes firstly the understanding of Process Bus and the Merging Unit concept. Consequently it presents a practical way of testing a complete acquisition chain including conventional instrument transformers and merging units. Testing is performed in two steps: secondary accuracy testing of merging units and functional testing of complete chain from the primary side of instrument transformers. Paper 208 The Test Method of Conventional Sampling MU from China informs within its Summary that not all ECT/EVT developed and implemented so far have shown satisfactory performance so they still cannot be completely applied in engineering. The operating experiences still need to be accumulated, and related system documents, test standards still need to be built and optimized as well. Hereinafter presents the test methods for Stand Alone Merging Units (SAMU), which convert currents and voltages from conventional instrument transformers into sampled values according to IEC Required accuracy is specified according to the State Grid of China requirements as well as requirements on different aspects of timing accuracy. According to the requirement of State Grid of China, the new digital substations will use MU matched with electromagnetic CT/VT from 2011, so MU will be more and more important, their reliability will directly affect the operation safety status of digital substation. Paper 209 Line Protection Testing using Process Bus IEC with Network Loading from Brazil presents the results of testing the influence of Process Bus loading on operating time of (distance) protection IEDs. A step by step testing approach has been used, starting with direct connection of one MU to a tested distance relay in order to check the correct operational performance. At the end the number of merging units on the same station bus has been increased to 10 and more than 1000 faults have been simulated in order to study the station bus performance and tripping times of connected protective relays. The final results shows no clear evidence that process bus loading influences the general operating time of protection. Average time difference obtained by thorough statistical evaluation was +1,5ms. Questions 2.9. Loading of process bus during system emergent conditions or even during faults within substation causes a lot of questions and concerns to protection engineers. Paper 209 from Brazil presents a test method used and results obtained from testing the influence of process bus loading on operating times of distance protection in substation. It seems that many tests on process bus loading and its influence on operation of relay protection has been performed worldwide. What tests methods have been used for this purpose, what are the results and lessons learned? 7

8 2.10. It seems that Stand Alone Merging Units (SAMU), which will be connected to CITs will be in mass use also in the future. Paper 208 from China describes a newly developed standard testing procedure for SAMUs, securing this way high reliability of protection operation in Digital Substations. Which parameters are of the outmost importance for correct operation of SAMUs connected to CITs and how should in future their required quality be secured? Securing high quality of protection and control system within Digital Substation as well as within complete power system requires a structured and complete approach. This means that each component (NCIT, MU, protection IED, etc.) has to be tested separately, each connection (e.g. process buss) between different components has to be checked as well as the operation of a complete system must be thoroughly tested before it is put in operation. A number of questions appears in this respect as follows: a) How should we structure in a best possible way all tests necessary to secure reliable operation of a complete Digital Substation? Which are the most important requirements on modern testing equipment to be used? How should we test complex protections, connected to different feeders in Digital Substation like transformer differential protection, busbar protection and maybe line current differential protection? Responsible testing in factory and on site testing during commissioning with different kind of testing equipment definitely contributes to higher reliability of complete secondary system within the Digital Substation, but can not cover absolutely all its elements and possible functional interconnections. In the past, power utilities in many countries practiced so called primary (stage fault) tests after all the commissioning work has been done properly. The reason for such tests was to get confirmation in real life that all equipment and the complete substation operate as necessary under real power system conditions. Would it not be advisable to perform such primary tests again on some important digital installations before they would be put in regular operation? 6. FINAL REMARKS IEC International Utility Communication Standard has been implemented in many installations worldwide with good results. Implementation and practical application of a complete Digital Substation with NCITs, corresponding MUs and eventually SAMUs together with process bus architecture and related process level IEDs are still in development and testing stage. New international standards are in preparation, which will contribute faster progress also within this field. A number of practical installations with different types of NCITs proved their superior characteristics compared to CITs, including great possibilities for development of new protection functionality as well as installations, which have not been possible with CITs. The operating results obtained so far are very positive. Experiences gained from test installations in different countries comprising between others also NCITs, MUs, and process bus showed generally positive results. They also open a number of questions and needs for further improvements of standard, mostly related to multivendor applications and equipment interoperability issues. Introduction of new technologies within Digital Substation mirrors itself between others also in new testing equipment, testing methods, and especially also in skills required for successful completion of all tasks necessary for successful completion of installation. It should be mentioned that testing activities in Digital Substation require first of all good understanding of the system architecture and the functionality of each component included. It is also important that each component separately as well as the complete system itself are thoroughly tested. 8