220kV變電站電氣一次初步設(shè)計,kv,變電站,電氣,一次,初步設(shè)計 1 37 ENVIRONMENTALLY FRIENDLY LOW COST HV MV DISTRIBUTION SUBSTATIONS USING NEW COMPACT HV AND MV EQUIPMENT F ILICETO C DI MARIO E COLOMB0 Gestore Rete Trasmissione V COLLOCA G COMO F POZZANA S SCIARRA ENEL Distribuzione Rome University of Rome La Sapienza CESI Milan Nazionale Rome Italy SUMMARY The paper presents a new standardized design for environmentally friendly low cost HV MV distribution substations The new design adopted by ENEL Distribuzione in Italy is based on the following concepts which are discussed in the paper Use of a simplified single line diagram and lay out Use of new equipment types Very simple and compact design Almost complete prefabrication The paper firstly describes the new model applied by ENEL Distribuzione for the development of the MV networks which enables use and takes advantage of the new type of compact HV MV substations Typical applicable simplified substation single line diagrams are reviewed and justification is given of the selected diagram The relevant implementation is presented for i the air SF6 hybrid insulated 170kV switchgear ii the new 24kV metal enclosed air insulated switchgear which is housed in a container and is very compact due to the use of circuit breakers withdrawable with vertical movement The authors believe that the new largely prefabricated HV MV substation will much contribute to improve the MV network topology uprate the power capability and facilitate the extension of distribution networks improve quality of service to consumers and reduce substantially the operation costs Keywords substation switchgear HY MV compact design 1 INTRODUCTION Following the privatization and de regulation of the electric energy market in recent years the technical economic and environmental requirements of network planning and operation have rapidly become more strict as a result of public opinion regulatory authorities and competition On the other hand the availability of modem network components based on new technologies and the need for uprating the power capability of networks and improving the quality of service to consumers with minimal environmental impact has required a drastic revision in system development planning and network operation strategies by power utilities These new requirements are especially the case in Italy due to the growing awareness of the public the valuable landscape of the country and the high population density Distribution power utilities should therefore address the following network development targets i Minimizing the visual impact as well as electromagnetic field and audible noise emissions of HV MV step down substations and MV networks ii Improvement of the quality of service to consumers in keeping with the new Standard issued in Italy in 1999 by the Electricity and Gas Authority Compliance with these regulations with regards to the limitation of interruptions of power supply in particular will require large investments iii Reduction in the capital and operation cost of HV MV distribution systems by limiting the network redundancies reducing construction and maintenance costs of network components and reducing energy losses 2 ENEL S MV NETWORK CONFIGURATION AND OPERATION MODEL The preferential topology of ENEL s MV networks consists of MV feeders originating at one HVMV substation and terminating at an adjacent HV MV substation Each feeder supplies several MV LV transformer stations and is normally operated radially by sectionalizing at an intermediate MV LV station In an emergency fault in a MV line or unavailability of a HV MV station the sectionalizing point s is are relocated A feeder tying two HVMV stations may then have to be supplied from one terminal only Although it is desirable to use un tapped feeders there are cases in which a few MV LV stations are supplied from radial lateral lines originating from intermediate points of a main feeder The HV subtransmission networks are mostly operated at 132kV Northem Italy or 150kV Central Southem Italy There are also substations supplied at 220kV Most of ENEL MV networks are operated at 20kV Most of ENEL HVMV subtransmission stations have been equipped in the past with two step down transformers usually rated at 2x25 MVA or 2x40 MVA ONAN 2x3 1 5 MVA or 2x50 MVA ONAF and in some cases at 2xl6MVA or 2x63MVA or 2x100 MVA These substations were designed and built with conventional technologies with little prefabrication The HV switchyard outside the major towns was an outdoor air insulated station AIS The MV switchgear housed in a building was metal enclosed with cubicle switchgears type according to IEC Standards 298 and withdrawable circuit breakers CBs HV SF6 insulated stations GISs and sometimes also 20kV SF6 insulated switchgears have been used inside the major cities with fixed CBs The installation of two HV MV transformers per substation specified with an ONAF rating providing redundancy of about 50 in normal operation was justified by the requirement for quick supply restoration from the same substation of all the outgoing MV lines in the case of unavailability of one transformer Actually the supply of all the entire MV feeders from the adjacent HVMV substations was hindered by the following two problems i Too long a time was required by the operators for the manual on site closing of the load interrupters at the intermediate points of several long feeders ii Some long MV feeders connecting adjacent HVMV stations had become heavily loaded due to load growth Supply from one end only would therefore cause high voltage drops andor the violation of the thermal current carrying capacity in some old line stretches equipped with small conductors On the other hand with the above described network configuration each HVMV transformer with a large rating supplies in normal operation from a half MV busbar a large mileage of MV overhead lines which thereby adversely affects the exposure of consumers to voltage dips and interruptions These are currently a major concern with regard to the quality of service required by the new regulations Statistics show that about 90 of CIRED2001 18 21 June 2001 Conference Publication No 482 0 IEE 2001 Authorized licensed use limited to NORTH CHINA ELECTRIC POWER UNIVERSITY Downloaded on April 23 2010 at 00 47 20 UTC from IEEE Xplore Restrictions apply 1 37 cumulative service interruption to consumers is caused by the MV network and MV LV stations About one year ago ENEL Distribuzione S p a ENEL Distribution Corporation started implementing a large project for automation of the MV networks including the remote control of the disconnectors at about 50 000 MV LV transformer stations which is about 25 of existing ENEL MV LV stations The remote control will shorten the time for locating faults on MV lines It will also drastically shorten the time for restoring the supply of all the MV feeders from the adjacent HVMV stations in case of unavailability of a HV MV station thereby solving the problem referred to in item i above On the other hand the construction of new HV MV substations justified by the load growth and by the need to improve the quality of service of the MV networks results in a reduction of the length and loading of the MV lines In most of the cases this makes it technically feasible to provide an emergency supply from the adjacent HVMV stations of all the MV lines which are normally supplied by these new stations The latter can therefore be realized in a simplified manner with only one transformer because their load can be supplied in emergency operation by the neighbouring stations via the MV networks Summarizing ENEL s trend is to change the MV network model as follows Old Model a few conventional HVMV stations with two transformers with large ratings and redundant capacity and very limited reserve from the MV network Many long MV lines supplied by each HVMV station and not provided with remotely controlled sectionalizing points New Model a large number of simplified physically small HVMV stations with only one transformer 16 or 25 MVA ONAN rating and with small redundancy of transformer capacity A reduced number of short MV lines radiating from each new HVMV station provided with remotely controlled load interrupters on average 5 along each feeder linking two HVMV stations The new model considerably up rates the loading capability and quality of service of the existing MV networks However it requires several new feeding points from the HV network In practice the required expansion of the HV network is moderate for the following reasons The HV networks are very extensive and meshed in Italy about 45 000 km of 132 150kV lines Most of the new HVMV small stations can therefore be located just below an existing line within the line right of way ROW Where short lateral 132 150kV lines are required for supplying the new HVMV stations length usually does not exceed 10 15km Where appropriate use can be made of standardized compact double circuit HV lines with insulating crossarms the environmental impact of which is comparable to that of a MV overhead line The simplified prefabricated HVMV substations presented in the following paragraphs have a low cost and are compact and environmentally friendly The economic analysis has shown that the additional investment and operation costs in the HV network are far lower than the savings achieved in the MV networks The insertion of a HV MV simplified new station NS between two existing stations ES is shown in principle in Fig 1 A vast system ideally assumed with a regular modular configuration should gradually evolve in the long term as follows a The total number of new simplified HV MV stations equipped with 1 transformer should eventually be about twice the number of the existing HV MV stations b The length of the MV lines Fig 1 will reduce to 50 60 of original lines and they will double in number c The voltage drop and losses on the MV lines will be consequently lowered to 25 36 for an assigned load condition Fig I Insertion of a simplified HV MVstation NS between two existing stations ES The average loading factor of the HV MV transformers in normal operation at peak load will be increased from about 50 for the existing stations to 75 to 85 in new stations having 3 to 6 adjacent substations The mileage of MV lines supplied by each transformer will be reduced to 50 60 of the present mileage In spite of the very little increase in mileage of total MV lines the power distribution capacity of the network will double As most of MV public networks in Italy are operated with un earthed neutral the maximum ground fault current is reduced in proportion to the line mileage of networks Self extinction probability of phase to Gr transient faults in overhead lines is therefore increased Items b e and g above and the implementation of the remote control of load interrupters in part of the MV LV stations will bring along a substantial reduction of the rate of voltage dips as well as of the rate and duration of interruption of supply to consumers The number of MV lines outgoing from the existing stations will no longer increase On the contrary the new stations will supply far fever MV lines max 10 The congestion of MV lines near HVMV stations will therefore diminish CHOICE OF SINGLE LINE DIAGRAM FOR NEW SIMPLIFIED HVMV SUBSTATIONS Fig 2 shows a set of single line diagrams which have been used in various countries for the HV MV subtransmission substations In order to simplify the diagrams only the CBs and HV disconnectors DSs have been represented which are essential for configuring each diagram The reader should consider current transformers CTs in series with each CB surge arresters SAS connected to power transformer terminals capacitive potential transformers CPTs and rod gaps connected to each HV line terminal line traps if PLC communication is used MV CBs withdrawable or provided with DSs grounding switches on line terminals and busbars Diagrams have been drawn in order of decreasing magnitude of ratio R N of HV CBs N of HV feeders in Fig 2 R l for AI diagram R 0 33 for B2 It is assumed that the HV line can supply the substations from either side Diagram A1 is also well known as the H scheme It has been extensively applied in Italy in the HVMV substations equipped with two transformers and with the line in line out loop connection This conventional scheme requires one CB per each HV feeder R l The redundancy of transformer capacity and of HV line capacity in closed loop operation assures continuity of supply of the MV network in case of outage of one transformer and or one line Although the infrequent maintenance of the DS linking the two busbar sections may require a short outage of the substation this DS is installed because it improves operation flexibility and allows maintenance and works on busbar sections and on feeder DSs Authorized licensed use limited to NORTH CHINA ELECTRIC POWER UNIVERSITY Downloaded on April 23 2010 at 00 47 20 UTC from IEEE Xplore Restrictions apply 1 37 C c2 a s MV 82 E2 F G Fig 2 Typical single line diagrams of simplified HV MV subtransmission stations circuit breaker disconnector disconnector for off load transformer switching Diagram A2 may be applied as the initial stage of A I Diagram BI allows saving one CB compared with AI However the permanent faults in a line cause a short outage of one transformer and relevant MV feeders Diagram B2 is very economic presenting a ratio R 0 25 It has been applied for stations with two transformers of redundant capacity tapped from one circuit of a double circuit line Transfer tripping signals of high reliability to line remote ends are required for an effective protection of transformers The HV line non transient faults cause the outage of one transformer Application of diagram B2 subdivides the system in sections consisting of one HV line and one HVMV transformer protected by 3 CBs the local HV CB one CB on MV side of the transformer and an HV CB at remote line terminal Diagrams C1 D El and F have the common feature of 2 CBs for 3 HV feeders i e ratio R 0 66 Diagram C2 has a ratio R 0 5 Diagrams C1 and C2 of North 4merican origin utilize DSs capable of switching on and off the transformer s at no load The rare faults on a transformer are cleared by opening the two line CBs Normally the line faults do not cause the interruption of load supply The use of by pass DSs of line CBs is justified if the interruptions of the HV lines are to be limited as much as possible If a CB is temporarily by passed for maintenance a fault in the associated line will cause the trip out of the other line local CB and outage of transformer s Furthermore when a CB is by passed a transfer tripping signal is recommended for fast tripping of the line CB at the remote terminal in case of intervention of a transformer protection thereby avoiding heavy damage possibly fire to the transformer Use of the by pass DS in diagram D allows switching of transformer with one of the line CBs without outaging the line Transformer protection is assured by a local CB also when the by pass DS is closed thereby eliminating the use of transfer tripping which is desirable with diagrams C1 and C2 On the other hand if the by pass DS is closed a line fault must be cleared at the two remote line ends and reconfiguring of line protection relays is required As in schemes C1 and C2 a transformer fault in normal operation by pass DS open causes line outage Diagram El has been identified as the most suitable for the new simplified HVMV ENEL s substations Use of a CB on transformer HV side facilitates switching and protection of the machine A forced or planned outage of the line terminated only with a DS causes the temporary outage of the transformer and of supplied MV feeders This event will be rare since the HV lines are relatively short and high speed single pole reclosure is expected to avoid transformer supply interruption in most line faults As described in the following paragraphs the HV part of El diagram stations is a prefabricated modulus By adding a second equal modulus the station is extended as shown in diagram E2 The latter is an H diagram as per AI however the two sections of HV busbars are tied by two DSs in series thereby eliminating the above reported maintenance shortcoming of diagram A 1 Diagram F is complementary to El with a A instead of Y scheme Normally one of the two DSs of the transformer is open The scheme then behaves like El in the case of line or transformer faults However the outage of either of lines for maintenance can be arranged without interruption of transformer supply by preconnecting the transformer to the line which remains in service by means of the DSs switching with DSs is possible if line CB is blocked in closed position for a short while The transformer DSs could be used for by passing the line CB for maintenance The layout of a A scheme F diagram is more complex and costly than layout of the Y scheme El diagram Furthermore duplication of scheme F does not realize the H scheme as per Al because the busbar tie DS is not available These considerations have supported the choice by ENEL of the El diagram It is foreseen that up to 5 6 simplified substations with El diagram can be inserted in a HV line terminating at two supply substations The iteration feasibility of scheme El along a line stems from the fact that a line or transformer fault should normally cause the outage of no more than one HV MV substation Diagram G has been used extensively in various countries for small tapped substations along HV lines Ratio R 0 33 i e very low The merits and limitations of G diagram are well known Diagram G is also applied by ENEL in the cases where there is only one simplified station tapped from a line terminating at two supply stations Contrary to scheme El scheme G is not to be iterated along a line in order to avoid the simultaneous outage of more than one HVMV station 4 USE OF NEW EQUIPMENT TYPES 4 1 HV Switchgear An important distinguishing feature of substations is the type of insulation and enclosure of components New equipment using hybrid SF6 air insulation has been preferred due to the following advantages very compact design fast and reliable installation at site no live parts within the reach of maintenance or service personnel easy replacement in case of failure reduced needs for civil works a building is usually not required Stimulated by ENEL s requirements for this new type of substation some major manufacturers have developed a switching equipment design which is simple requires minimal maintenance Authorized licensed use limited to NORTH CHINA ELECTRIC POWER UNIVERSITY Downloaded on April 23 2010 at 00 47 20 UTC from IEEE Xplore Restrictions apply 1 37 and can be more effectively integrated in the HV subtransmission networks Fig 3a shows the circuit schematic of the Y HV hybrid insulation switchgear Compared to the El single line diagram of Fig 2 the only difference is the omission of the DS busbar side of line CB because in the case of failure or major overhaul the affected single phase modulus is replaced with a spare unit Single phase non compartimentalized SF6 enclosures are accepted because in case of internal failure one complete phase is replaced with a spare unit and transported to the factory for repairs Fig 3b shows a 170kV Y switchgear with two CBs The 3 sets of 170kV bushings can be polymeric They form the interface with the incoming and outgoing line and transformer CBs DSs CTs CPTs and grounding switches are combined in a compact SF6 completely pre assembled enclosure Conventional air insulated surge arresters are connected to the transformer HV terminals The Y switchgear includes remotely controlled motor operated DSs with built in earthing switches An interlocking system assures reliable operation transformer line in line out 4 2 MV Switchgear A new design ENEL patent of air insulated compact MV switchgear allowing 68 reduction in volume compared