1 Introduction and Applications of Gas Insulated Substation (GIS) Phil Bolin, Hermann Koch, IEEE Substations Committee IEEE Substations Committee A. Overview Gas-Insulated Substations (GIS) are in use world-wide since more than 30 years with in general very good experiences. GIS are most common in use in Japan, the largest single GIS market in the world, in Europe, the Middle East, and also the Far East and South East Asia. In the USA, where this technology was founded, the success of GIS is a limited one. There are two major reasons , one in that available space did not give high pressure to use GIS, as it was the case in Japan, Europe and places in Asia, and as a second reason some design difficulties at the beginning 30 years ago. Some of this first days equipment is still in service due to the long living type of technology. This tutorial is meant to present and teach about all the improvements done in the design of GIS technology and to report about the excellent experiences with GIS in other countries. Restricted availability of space for new substations and the replacement of old equipment in cities and metropolitan areas indicate that in the near future also in the USA a larger number of GIS will be installed and operated. This tutorial is dedicated to all engineers who are or might be in charge of GIS technology today or in the future. The tutorial was set up by professionals all round the world of leading equipment manufacturer, turnkey project executer, industry consultant, operator and experiences user of GIS. B. Introduction to GIS 1) GIS Reliability IEEE GIS Tutorial This chapter addresses the reliability of GIS and provides a brief comparison to an AIS substation. This information is mostly based on CIGRE publications, in particular the latest one from 1998. The details can be found in the Bibliography. The first GIS’s were put in operation in 1967 in Switzerland and Germany. The GIS in Germany is still in operation, whereas the GIS in Switzerland was recently decommissioned after 35 years of operation without major fault or gas leak. The utility made an assessment of the gas leak over the lifetime of this first GIS and concluded that overall leakage rate was about 0.4% per year. US users were among the early adopters of the emerging GIS technology in the 1970’s, however, bad experience with immature designs resulted in a totally different view of GIS reliability between North America (in particular the United States of America) and the rest of the world. Reliability, the economic advantages of reduced life cycle cost and the physical compactness have resulted in the widespread use of GIS over the last 35 years. Even though today’s GIS technology can be considered mature many users’ approach to its application is still rather unique. GIS is used in specific types of applications only. The reliability of GIS has markedly improved since its introduction 35 years ago. The overall trend shows a reduced failure rate for GIS commissioned after 1985. Consequently, CIGRE distinguishes between GIS commissioned before 1985 and after 1985. Table II of the CIGRE report gives an overview over this data [I-1]. The table below gives the voltage class and corresponding voltage levels. TABLE I CIGRE SURVEY 2000 VOLTAGE CLASSES Voltage class kV 1: 60 ≤ Un < 100 2: 100 ≤ Un < 200 3: 200 ≤ Un < 300 4: 300 ≤ Un < 500 5: 500 ≤ Un <700 6: Un > 700 The outdoor GIS population represents about 43% of the total CB-Bay-Years. Tables III and IV of the CIGRE report provide the major failure characteristics of the GIS. One increasing trend is the involvement of circuit breaker failure for the newer GIS observed at all voltage levels. TABLE II CIGRE SURVEY 2000 MAJOR FAILURE FREQUENCY (FF) - 2ND GIS SURVEY TOTAL POPULATION AND COMPARISON BETWEEN THE 1ST AND THE 2ND SURVEYS RESULTS Voltage Class 1 2 3 4 5 6 1 to 5 TOTAL Voltage Class 1 2 3 4 5 1 to 5 GIS in total 2nd GIS survey 1st GIS survey No. of CB-bayFF CB-bay- FF failures years years 27 56884 0.05 38471 0.13 465 32048 1.45 23845 1.1 138 16040 0.86 12955 1.1 179 6371 2.81 4735 4.3 49 4525 1.08 3453 4.2 12 200 6.00 80 14.0 855 115868 0.74 83459 0.96 867 116068 0.75 83539 0.97 GIS commissioned before 1.1.1985 2nd GIS survey 1st GIS survey No. of CB-bayFF CB-bay- FF failures years years 16 28669 0.06 21304 0.17 351 19504 1.80 16035 1.3 100 10362 0.97 8596 1.5 110 3694 2.98 3287 4.4 32 3252 0.98 2532 3.7 609 65481 0.93 51754 1.18 GIS commissioned after 1.1.1985 2nd GIS survey 1st GIS survey No. of CB-bayFF CB-bay- FF failures years years 11 28215 0.04 9792 0.06 114 12544 0.91 4605 0.6 38 5678 0.67 2636 0.4 69 2677 2.58 970 4.0 17 1273 1.34 654 1.8 246 50387 0.49 18657 0.51 Voltage Class 1 2 3 4 5 1 to 5 Notes : Failure frequency (FF) = No. of Failures per 100CB-bay-years 2 TABLE III CIGRE SURVEY 2000 IDENTIFICATION OF MAIN COMPONENT INVOLVED IN THE FAILURE FROM GIS VOLTAGE CLASS POINT OF VIEW Main component involved in the failure (whole period) Total number of answers (reported failures) Circuit Breaker or Switch Disconnector Grounding Switch Current Transformer Voltage Transformer Bus bars Bus ducts and Interconnecting Parts SF6/Air Bushing Cable Box/Cable Sealing Power Transf. Interface Chamber/Bushing Surge arrester Other GIS in total Class 2 % 801 =100% 43.4 (30.1) 17.9 (19.2) 4.4 0.9 5.6 (7.7) 5.5 (7.3) 11.9 (17.2) % 435 =100% 54.7 17.2 5.3 0.7 6.2 3.7 4.1 Class 3+4+5 % 335 =100% 29.9 18.2 3.6 1.2 4.8 6.9 22.4 3.6 3.5 0.9 0.9 4.4 0.2 6.9 1.8 1.8 0.7 1.7 0.5 2.1 1.2 1.5 Based on published failure rates for AIS and GIS substation by international organizations such as CIGRE it can be shown that the failure rate of a six breaker ring bus in GIS is lower than that of a nine breaker AIS in a breaker-and-a-half arrangement. There are several commercially available software programs available to make these calculations. Below is a comparison of the failure rates of a six-feeder 230 kV GIS substation versus a nine-breaker AIS. 6 Breaker GIS 230kV TABLE IV CIGRE SURVEY 2000 IDENTIFICATION OF MAIN COMPONENT INVOLVED IN THE FAILURE FROM GIS AGE POINT OF VIEW (5 MOST INVOLVED COMPONENTS) Main component involved in the failure (whole period) Total number of answers (reported failures) Circuit Breaker or Switch Disconnector Voltage Transformer Bus bars Bus ducts, Intercon. Parts GIS in total % 801 =100% 43.4 17.9 5.6 5.5 11.9 GIS before 1.1.1985 % 562 =100% 42.2 18.5 4.4 5.7 14.4 GIS after 1.1.1985 % 239 =100% 46.2 16.3 8.4 5.0 5.9 The worldwide experience with GIS as documented by several organizations including CIGRE, IEEE etc. indicates that GIS has a lower failure rate than a comparable AIS substation. Users in North America might dispute this fact. There are still many first generation GIS in operation, which never lived up to their promise. As a consequence GIS is still viewed with skepticism and often not considered as a default choice. It should be noted that also in North America GIS commissioned after 1985 shows similar failure rates as observed by CIGRE during the last worldwide GIS survey. There is no question that a GIS compared with the same configuration in AIS will always be more expensive if looked at first cost only, unless special conditions exist such as land availability and/or cost, soil conditions, environment etc. But as a substation builder one should not compare a GIS and AIS in the same configuration, but define the required availability of the substation first and then look for the substation configuration that meets those requirements. As an alternative, the availability of the AIS substation in a particular configuration can be used. Then the substation builder should evaluate a GIS configuration, which meets this base availability. 9 Breaker AIS 230kV Failure outages OF (1/yr) OD [h/yr] Line 1 0.0214 0.117 Line 2 0.0214 0.117 T1 0.0214 0.117 AIS 1 1/2 T2 0.0214 0.117 cb T3 0.0214 0.117 T4 0.0214 0.117 0.702 Line 1 Line 2 T1 GIS ring T2 T3 T4 0.0081 0.0081 0.0081 0.0081 0.0081 0.0081 0.072 0.072 0.072 0.072 0.072 0.072 0.432 The expected outage frequency of the GIS feeders is 2.5 times less than in AIS AIS feeder: 47 yrs! GIS feeder: 123 yrs. The expected outage duration of the GIS feeders is 1.6 times less than in AIS. The result of this reliability study is shown in the table be- 3 low. Based on the failure rate the 6 breaker GIS ring bus substation is superior to the 9 breaker AIS in a breaker-and-a- half scheme and should be the preferred choice due to all the advantages of GIS. 2) Design Features The use of SF6 within the power delivery system is mainly driven by gas-insulated switchgear. There are single phase and three phase encapsulated designs. For the distribution voltage level up to 145/170 kV mainly three phase enclosures are used. For higher voltage levels single-phase encapsulation is a standard. In the last years the development of SF6 insulated switchgear was mainly aimed at reducing the use of material and costs while maintaining the extremely high reliability. Main steps of the development were as follows: • progress of circuit-breaker technology, which allowed to reduce the number of interrupter units despite increasing breaking capability • progress of casting and machining technology of aluminum casted parts, which allowed the use of minimized shapes and volumes • use of computerized production and testing equipment with high quality standards • design of integrated components with several functions such as combined disconnect and grounding switches within one gas compartment • use of intelligent monitoring and diagnostic tools to postpone maintenance activities and avoid unnecessary tasks As a result very compact GIS substations designs are available on the market with the following changes in comparison to older equipment: • up to 98% of space reduction in comparison to air insulated switchgear • up to 75% reduction of SF6 volume • delivery of completely sealed and tested bays up to 245 kV • leakage rates down to less than 0.5% per compartment and year 110 100 [% ] 90 80 70 60 50 40 30 Size of building Space requirement Packing volume 20 10 0 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 Year Fig. I-14. Progress of GIS Development (145 kV) The mean time between failures according to international statistics (IEC; CIGRE) has reached levels of 400 to 1000 years depending on the kind of switchgear and its voltage level. The importance of quality and reliability of all kind of switchgear equipment has become an ever increasing topic over recent years. Quality and reliability are the result of a complex process which includes design, manufacturing, delivery and erection and after sales service. Quality and reliability have to be already an integral part of development activities. Reliability starts with the right design, followed by the choice of the suitable materials, using the relevant testing procedures and the appropriate manufacturing techniques and all that accompanied by a stringent quality control. Using the very latest procedures for computer aided design, optimization of parts and components and failure mode effective analysis (FMEA), dynamic calculations of arc extinction and drive behavior accompanied by quality checks are well known methods to meet the users’ expectations. After the design stage, materials and components are subjected to thorough development tests. In this respect particular significance is given to long-term strength even at extremely high numbers of operations, as well as resistance to all kinds of environmental influences. Due to the dominance of mechanical failures, major components such as drive mechanisms are tested on a hydraulic test rig independently from the switchgear but using the previously measured loads given as stress level. Thousands of operating cycles will be performed in just a few hours. In this way the components can be subjected intentionally to a higher load than encountered in the switchgear itself during normal operation in order to determine their safety reserves. This method of testing is proven for contact systems, operating rods, drive components etc. and has enabled considerable improvement in their reliability. These development tests will be followed by prototype testing, comprising mechanical, power, dielectric, heat run and environmental tests even such as seismic testing, up to the limits, on several test objects in parallel. Although specified very clearly, type tests were run at different levels. For some manufacturers it is already common practice today to conduct up to 10000 operations or even more on one or several test objects for mechanical type testing. Furthermore, it is also specified by several utilities to extend the number of successful short circuit interruptions beyond the requirements of the applicable standards for power type testing. The testing during development and type tests are performed in the manufacturer’s own certified test fields or, if requested, in independent laboratories. Special attention will be given to the life time behavior of insulating material. Especially for the GIS insulators an extensive test program has been in place ever since the first delivery of GIS. As a result of tests and more than 35 years of service experience it can be concluded that the life time of GIS insulators will reach more than 50 years without a failure, and that they are nearly insensitive to aging. Many years of experience in switchgear design and usage have allowed to optimize test and measurement techniques to the appropriate severity of the duty of the components with 4 respect to their material and functional characteristics. All these activities were accompanied by parallel theoretical calculations and ever more exact definitions of requirements. Manufacturing has also changed. A customer’s order will be entered into an electronic data processing system, which details the substation down to the components and finally into the single parts. The steps of the manufacturing processes are defined by a process map and accompanied by quality assurance milestones to make sure that the same level of reliability will be reached with every single piece of equipment. The interdependence of manufacturing processes in terms of technical specifications, used materials, available machinery, logistics and personnel is highly complex. The optimal point has been reached when the module meets all the technological demands while being produced in few and simple manufacturing steps. The long time service experience and extensive tests have shown that there is no difference between three-pole and single-pole bus arrangements. In general the bus conductors are arranged symmetrically with insulating and supporting elements like spacers made of cast resin. The enclosures nowadays are no longer made of steel but of aluminum alloy, which offers several advantages such as reduced weight, excellent gas tightness due to excellent sealing area surfaces, high corrosion resistance and negligible resistive and eddy current losses. The enclosure design is adequate to withstand the electrical arc. By extensive research and using the new technologies of 3D-CAD and finite element method (FEM) the enclosure has been designed and optimized including testing the worst case scenarios. The result is a level of safety far greater than that required by the IEC and IEEE standards. The design of a modern GIS substation looks similar to the figures given above without significant differences between the individual manufacturers. However, today’s activities hint at future changes. The following paragraphs provide some highlights of the future trends of GIS development. New techniques such as 3D CAD as a computerized design tool, finite-element method for the mechanical safety of enclosures and housings, stereo-lithography for producing test models, and field distribution calculation for the predetermination of the dielectric stress allow optimizing the components of the HV equipment very precisely. With the use of 3D CAD-systems 3-dimensional modeling is simple, and the data can be used basically for mechanical and electrical optimization. The same data is also the basis for the computerized machining process and measurement system of quality control. The finite element method is an ideal tool for complex components or shapes in the design and for the calculation of critical areas of component stressing either on internal loads such as pressure or on external loads such as seismic activities. Stereo-lithography is a method for the manufacturing of testing models based on 3D CAD design. It is made from photopolymer and semi-cured with a UV laser, which pro- duces the contours. This experimental model can be used for mechanical and dielectric tests and eventually for the manufacturing of casting pattern. For the dielectric tests the models are coated with silver. The effective dielectric design receives considerable support from basically two field distribution analyzing programs: the equivalent charge method and the finite element method. Values of field strength along given contours are one result obtained, minimum-maximum distribution the other. The field distribution analysis programs are linked to the CAD system and allow iteration procedures for the optimization of the dielectric design. The development of ever smaller and more compact substations seems to be limited by the needs of the utilities for convenient service and maintenance access. However, such small equipment offers the advantage that it can be shipped as complete, factory assembled and pre-filled double-bays (245 kV) and triple-bays (145 kV) with shortest erection time on site. A further possibility offered by the space saving design is the installation within a container for mobile use. The circuit diagram in following figure describes the feed back loop to development and production and is the basis for an efficient cost benefit optimization between user and manufacturer of high voltage switchgear. Stresses Basic Design Realisation and Verification Application in Service Field Experience Maintenance Monitoring Failure Rates/ Reliability Fig. I-15. Field Experiences - Feed Back for the Development 5 C. Applications 1) Jacksonville, Florida, USA Ratings Ur Ir UBIL Is 138 kV 2500 A 650 kV 40 kA Fig. I-20 Taylor Street 3) Puget Sound, Bow Lake Substation, USA Fig. I-19. Jacksonville, USA The single line shows a ring bus with 4 bus coupling bays and 16 feeder bays. The bus coupling bays allow to separate the ring bus in up to 4 segments. Each feeder bay has a circuit breaker, grounding switches before and after the circuit breaker, towards the ring bus and towards the in- and outgoing line. Disconnectors are placed at each feeder bay towards the ring bus, towards the in- and outgoing line, and on each side of the circuit breaker of the coupling bays. The use of GIS to solve the single line requirements allows a very compact substation as this bird‘s eye view shows. Easy to see the 4 feeder bay sections and the 4 couple bays, two at the end and two in the middle. The local control cubicles are placed at the center walkway. The GIS substation is placed inside buildings which fits nicely into the neighborhood. Beside the overhead lines nothing reminds the public that there is a 138 kV switchyard. The compactness of GIS and its modular structure allows the use of small buildings and offer a space saving solution. 2) Taylor Street, Chicago, USA, ComEd Ratings Ur Ir UBIL Is 362 kV 4000 A 950 kV 63 kA Ratings Ur Ir UBIL Is 145/115 kV 2000 A 650 kV 40 kA Fig. I-21. Puget Sound, USA The Puget Sound Project at Bow Lake Substation in Oregon USA. This outdoor GIS solution is placed on a small spot in the residential and office neighborhood in an often used 4 breakers, two overhead line feeders and two cable feeders. The design voltage is 145 kV and the operation voltage today is 115 kV with a current rating of 2000 A. The Plan View shows the physical arrangement of the GIS ring bus with 6 circuit breakers, two overhead line connections and two cable connections. The size of the installation is about 6 m by 10 m. The outdoor GIS is three phase insulated. In this side view it can be seen how the GIS is positioned on the steel structures and how the overhead line is connected to the three phase insulated GIS. The small size and the metallic encapsulation offer great advantages of GIS in locations like Puget Sound. Safety to the public is given by GIS because of its metallic enclosure. Even 6 if someone (children searching for their play ball) is jumping over the fence he is not in immediate danger. The small size of the GIS equipment also improves the esthetics of this outdoor installation. 4) Sargans Substation - Switzerland Ratings Ur Ir UBIL IS 110 kV 2500 A 650 kV 40 kA Fig. I-23. Termobahia Modules have been selected by customer as optimized solution for retrofitting the existing AIS substation. There is no advanced condition monitoring installed. Potential Transformers have been installed on the module itself. 6) Braintree Ratings Ur Ir UBIL Is 115/13.8 kV 2000 A 550 kV 31,5 kA Fig. I-22. Sargans, Switzerland Mainly due to very small footprint and also for aesthetic purpose. There is no advanced condition monitoring installed. Relative cost is high for 110 kV substation. Transformers are located into the building and connection between GIS and transformer is through indoor SF6/Air bushing. That is an interesting possibility for 115 kV stations in the US as it enables to have the surge arresters mounted conventionally. 5) Termobahia - Rio de Janeiro, Brazil Ratings Ur Ir UBIL IS 245 kV 2000 A 1050 kV 40 kA Fig. I-24. Braintree 7) Barbana Ratings Ur Ir UBIL Is 145 kV 2000 A 650 kV 40 kA 7 Fig. I-25. Barbana The GIS and Transformers are located under an public park. The substation supplies the inner city with energy
