AC500kV Substation Design In ChinaTransmission and Substation Department,CPECCAbstract:Some brief informations about AC500kV substations in China,historical data of their design and essential design principles are presented in this paper.Some technical and engineering problems of AC500kV substation design and construction are discussed.It is emphasized to introduce the500kV substation scales,main electrical connections,selection of equipment and apparatus,insulation coordination,distribution switchyard,control and protection etc.Here we also discuss the policy of project cost control at present time and a prospective view of China AC substations in future.1.IntroductionThe research and design of500kV substations in China were started in1977,and the first substation was put into operation in1981.From that time,because of the rapid development of electricity demand in China,design and construction of500kV substations have a great development.Especially in recent years,due to the increase of capital investment for power system construction from the State Government,500kV power system has developed even faster.Till1998about55substations(500kV)were already put into operation,in which217transformers were installed,and the total capacity reached60410 MVA.At present,there are30substations(total capacity26500MVA)under construction.In addition,it has been planned to construction/extends30substations(33transformers,24750 MVA)in accordance with the power delivery from Three Gorges Power Plant.A complete design system of AC500kV substation in China has been gradually established based on the past design and construction experiences and the use of new technologies.This system includes design procedures,design standards,rules and regulations, design management and so on.The essential design principles are safe and reliable,technically advanced,economical and reasonable,and good quality.2.Size of SubstationsChina is a developing country with broad territory.The economic development is very different from one region to another.In accordance with this fact,the500kV substation size is different at different time and for different region.Generally there are several classes of transformer capacity in substation according to different stages of construction:Initial stage,MVA Final stage,MVAⅠ1×500（750）2×500（2×750）Ⅱ1×7503×750Ⅲ1×750（2×750）4×750（1000，1500）The numbers of500kV and220kV outgoing lines from substation differ from time to ually there are1-4circuits of500kV lines and6-8circuits of220kV lines in the first stage of construction,and finally the number reaches6-10circuits of500kV and12-14 circuits of220kV lines.Whether install or not the high voltage reactor is decided by systematical investigation in over voltage level and line length.Reactive compensation facilities at low voltage side of main transformer are also design in accordance with the system conditions and the principle of“compensation on the spot according to the condition of each voltage level and region”. Usually in first stage of construction only low voltage shunt reactors are installed,reserving the possibility to install shunt capacitors.But some times,especially in recent projects,shunt reactors and capacitors may be installed simultaneously in first stage of construction.3.Electrical Design Features3.1Main Electrical ConnectionsIt is very important to select main electrical connections for safe and reliable operation and project cost control.The most popular500kV electrical connection is one-and-half circuit breaker(1½CB)connection,which is flexible,reliable,relatively and less land used.Double sectionalized bus with transfer bus connection also being employed in some early projects.In the1½CB connection configuration,source and load circuit are usually arranged in pair and in series,with same title circuit arranged in different series.It is permitted to connect the same title circuit to the same bus,2transformer groups into series separately and the other2groups to bus separately.3current transformer groups are installed in every series and single phase capacitive voltage transformers(CVT)are installed on bus.Lighting arresters are installed at line and transformer sides.Special calculations are required to decide if bus arresters to be installed.At initial construction stage outgoing lines and transformer groups used1-2breakers. For1½CB connection,if number of series is equal to or more than3at initial stage,outgoingisolating switches are not installed for the line and transformer.If there are only2series in the initial stage,they should be installed.3.2Equipment SelectionThe short circuit current level is usually computed in accordance with the system development plan within10-15years after substation commissioning.In general cases it is selected as50kA,50kA,40or50kA(31.5kA)for500kV,220kV,35kV(66kV)bus–bar respectively.At present time short circuit current level for500kV side reaches63kA in some substations.The problem,that short circuit current level for220kV side in some substations may reach63kA in future,will be solved by employing control measures in the system,for example,to divide the power supply area into parts.Usually single-phase auto-transformers with forced oil and air cooling are equipped. Special system voltage regulation computation is needed for employing on-load tap-changer transformers.In some projects three-phase transformers are used after economical and technical comparison in transformer including its transportation conditions.500kV circuit breakers are usually ceramic pole type or tank type.In some special cases, for example,high altitude,severe atmospheric pollution regions and/or limited site area,SF6 metal enclosed gas insulated switchgear(GIS)may also be used.Circuit breakers usually have4-3breaks in one phase,or2in recent year.500kV isolating switches are usually single-or double-pole horizontal stretch-and-draw type,which occupies relatively less land area.500 kV current transformers are oil or SF6insulated.35(or66)kV shunt reactors are oil immersed type or aircore dry type.In order to reduce switching over voltage,500kV insulation level and air gaps,high quality SF6circuit breakers and good protection performance metal oxide arresters are employed.In case of relatively long line it may be necessary to equip closing resistors in circuit breakers according to computation study.The external insulation level of electrical equipment is selected according to the regional atmospheric pollution ually the selected pollution class is not lower thanⅡ，that is,the unit leakage distance is not less than20mm/kV for500kV and220kV apparatus. The external insulation level for neutral ungrounded low voltage side should be strengthened to one class higher.The general leakage distance is calculated by using maximum operating voltage.3.3Insulation CoordinationInsulation coordination is determined in accordance with the principles stated in China Power Industry Standard DL/T620-1997“Overvoltage Protection and Insulation Coordination for AC Electrical Installations”.Power frequency overvoltage:1.3Uxg for substation side of line breaker,1.4Uxg for line side.Here Uxg is the maximum operating phase voltage.The coordination factor between nominal lighting impulses withstands voltage of electrical installations and residue voltage of substation arresters under nominal discharge current is adopted as1.4.Coordination factor between nominal phase-to-earth switching impulses withstand voltage of electrical installation internal insulation and protection level of arrester under switching overvoltage should not be less than1.15.3.4SwitchyardUsually the following bus bar types are used in500kV switchyard:double bundled diameter-extended soft buses,supported thick aluminum tube buses,or suspended aluminum tube buses by V type insulator strings.The switchyard may be arranged in one,two or three outgoing line directions types.Two outgoing directions type is usually used,and breaker are arranged in three columns with medium height type.The transformers are connected in series with low frame crossing type,each bay width being28m;there are service paths between phases for repairing the switchgear.3.5Grounding SystemThe service life of grounding system is the same as the designed service life of ground project.In case of high soil resistively and great short circuit current the ground potential may exceed the value limited by regulation even if measures to reduce grounding resistance are adopted.In these cases potential equalization and electrical insulation measures should be used according to the regulations.The typical methods to reduce grounding resistance include deep burying grounding poles and extending grounding.3.6Control SystemIn general three types of control systems are employed:electronic one-to-one control, electronic transfer to power one-to-one direct control and220V power one-to-one direct controls.Mosaic control panel and computer monitoring systems are used.Accordingly,conventional control systems should be adequately simplified,for example,the measuring systems and lighted-letter signals can be simplified or even canceled,and partial or all their functions can be realized by computer systems.At present time in some substations computer monitoring-and-control systems are used, at the same time the conventional monitoring-and-control systems are not used.In case of distributed computer monitoring-and-control systems,small relay rooms near switchgear are set up,in which relays and computer subsystems are installed.Basic manual operation functions are reserved in relay cabinets.3.7General Layout of SubstationGeneral layout of500kV substations is composed of the following four parts:500kV outdoor switchyard,220kV outdoor switchyard,and main transformer with reactive compensation installation,main and auxiliary buildings.The layout is arranged in accordance with the general planning and engineering requirements,talking into account the substation natural conditions and special characteristics of projects,fully satisfying requirement in safe and reliable operation/maintenance of the substation.With these preconditions the following principles are also considered:less land used,compact layout,reduced cost and convenient for future extension.In recent years the territory area within substation boundary walls is usually limited to be about7-9ha,and the administration area in front of substation to be about0.5ha.3.8Civil EngineeringThe ground elevation of the substation site shall be higher than the high flood level of 1%frequency.In case the elevation is lower,the substation area shall be surrounded by flood-prevention dike or other reliable flood-prevention facilities.The general building area in substation designed and/or constructed in recent year is as follows:2700-2900m2in case of centralized relay protection and2100-2300m2if protection apparatus are distributed in vicinity of switchyard.Steel tube frameworks are used for the outdoors switchyard installation with advantage of its relatively low steel consumption,but angle iron structure can also be employed.Water sprayer fire extinguishers are employed for main transformers.4.Prospect of500kV Substation DesignChina will come into“The10th Five-year Plan”at the beginning of21st century,and will intensify the construction of electric power system.So there will be great potential in500kV substation projects,and competitive bidding system will be widely used in procurement.An upsurge of500kV substation constructions in accordance with power delivery for the Three Gorges Power Plant will be seen in the near future.Generally domestic made equipments will be employed in these substations,but it is not exclusive to imported ones.In addition to assuring safety and reliability,500kV substation of next century should be designed in accordance with the following principle:economization of land-use,employment of computer monitoring and control systems,unmanned or only a few on duties in substation, etc..So more and more high and new technologies and hi-tech products will be used.。