(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERUION TREATV (PCT)
`
`-副
`
`町田町
`
`----聞ー幽・・・・・"l
`
`ーー-I
`
`l ----聞I l -
` l l i --I l
`l i --------l l
` l -l
`
`(10) Iotematiooal Publicatiou Number
`W O 01/17092 Al
`
`(19) World Iotellectual Property Orgaoizatioo
`
`International Bureau 録勝
`
`(43) Internatiooal Publicatioo Date
`8 March 2001 (08.03.2001)
`
`PCT
`
`(74) Age日ts:HEDBERG， Ake et al.; Aros PatentAB， P.o.. Box
`: H02J 13/00，
`(51) Interoational Patent Classification7
`H02P9i∞， 13/00， GOIK 7/00， H02H 7/00
`1544， S・75145 Uppsala (SE).
`
`(21) IlIteroational Application Number: PCT/SEOOI01606
`
`(22) lnternational Filing Date: 23 August 2似 約 (23.08.20的)
`
`(25) Filiog Laoguage:
`
`(26) Publicatio日La日guage:
`
`Swedish
`
`English
`
`(30) Priority Data:
`9903026-4
`9903037-1
`9904476-0
`
`27 Aogust 1999 (27.08.1999) SE
`27 Aogost 1999 (27.08.1999) SE
`8 December 1999 (08.12.1999) SE
`
`σ1) Applicant (for all designated States excepL US): Ass AB
`[SElSE]; Kopparbergsvagen 2， S-721 83 V泌 teras(SE).
`
`(81) Designated States (national): AE， AG， AL， A M， AT， AU，
`AZ， BA， BB， BG， BR， BY， BZ， CA， CH， CN蚕 CR，CU，CZ，
`DE， DK， D M， DZ， EE， ES， Fl， GB， GD， GE， G H， GM， HR，
`HU， ID， IL， IN， 1S， lP， KE， KG， KP， KR， KZ， LC， LK， LR，
`LS， LT， LU， LV， MA， MD， M G， MK， MN， M W， M X， MZ，
`No.， NZ， PL， PT， Ro.， RU， SD， SE， SG，S1，SK， SL， Tl， T M，
`TR， TT， TZ， UA， UG， US， UZ， VN， YU， ZA， Z W
`
`(84) Designat吋 States(regional): ARIPO patenl (GH， G M，
`KE， LS， M W， MZ， SD， SL， SZ， TZ， UG， ZVl)， Eurasian
`patent (AM， AZ， BY， KG， KZ， :tvID， RU， TJ， TM)， European
`palent (AT， BE， CH， CY， DE， DK， ES， Fl， FR， GB， GR， IE，
`IT， LU， M C， NL， PT， SE)， uAPI patent (BF， BJ， CF， CG，
`C1，α'1， GA， GN， GW， M L， M R，問 ，SN， TD， TG).
`
`(72) Inventors; and
`
`亘亘亘::(σ75句)1凶nve側E凶Itωorsl泊'Ap仰pl“ic伺ar目附B“巾t“s ぴ仰or U.ω's 0仰ni)ω:リ): GERT民.fAR，
`Lar“s [βS1町ν'SE町);Hu肌001同e句:ga糊ta刷a訓n川6，Sふ釘-ヴ7η2226V 詰泌s蹴蜘刷b飽刷刷E創釘J'応l
`-一ii NVSVEEN， A n凶)e[N0.冷，(0.];Vassbunnveien 12， N・l388
`Borgen (Nu). GJERDE， Jan， Ove [NulNu); Nedre
`Silkestra 8， N-0375 uslo (Nu). LOF， Per・Anders
`[SElSE]; Timr色gatan84， Sゅ 16262Vlillingby (SE).
`
`Published:
`- Wilh international search report.
`
`For two-letter codes and olher abbreviations， refor to the "Guid-
`ance Notes on Codes andAbbreviations" appearing at the begin-
`ning of each regular issue of lhe PCT GazetLe.
`
`言 言 問 Title:ELE訂 版 PuWERSUPERVISlu N
`
`---一-一-一-
`-一一-
`--一一一-一一置置置置
`--一一一---
`ーーーー-置置置置---一一
`
`冒叫
`
`吋
`
`16
`
`わ-0\ (57) Abstract: The present inve附 onulilises di蹴 tm開 urementofm吋 na1critical quantities in direct connection. to interesting
`FF てp伊 附Oω伽inls隠削山si加n似 肘 附c伊 附w附 e釘rp仰m附 cessesin川 t陶he吋dωi仇陀削削n川t似 I附討巾c伊 W附 e釘ro州切恥蜘eωct州t
`. 圃 difficu叫lltωaccess，such as on high potentia1， or at rotating parts (17). The measuaments give direct infolmation about actual， now
`、 「 叫idopera凶onalmargins， concerning marginal critical quantities and in p.副icularfor quantities that is both margina1 critica1 and
`三 material cri 凶caJ. Data about available mar伊lSis transferred ω other units (32) wit 悩n the network or plant (1). The eleωc 仕肘icP伊owe釘r
`るpμl叩
`(υ10， 12， 14) in the plant (1)， which enables a more efficient operation of the planl (1) or network. The me伺asUl陀芭me叩nt“sg斜I\f'刊ef向u凶3訓口he釘r
`偉n附e側wp戸at山h恥sお伽ford川d制e剖仰I怜 叫6
`b加ot白ha邸ssu叩pp伊or凶tfor Ihe op戸巴ra討副t“ionof the sy戸slωema邸swell aωs for future maintenance， modeJ building etc.
`TEMP 1010
`IPR ofU.S. Pat. Nu. 8，008，804
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`

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`W O 01/17092
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`PCT/SEOO/01606
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`ELECTRIC POWER SUPERVISION
`
`TECHNICAL FIELD
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`5
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`The present invention generally relates to supervision and control of electric power
`
`plants and electric power networks. The invention relates in particular to
`
`determination of actual operational margins for electric power 0同ectsor electric
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`power units that are included in the electric power plant or electric power network.
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`10
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`BACKGROUND
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`Supervision and control is used within the electric power area as designation of
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`measurement value collection， remote supervision and remote control of electric
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`power systems. Electric power networks constitute complex technical systems that
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`15
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`often cover large geographical areas， which often lead to a need for advanced
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`systems for supervision and control of these networks. There was early a need for
`
`introducing central control rooms， so-called control centres， from which the operators
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`could form an opinion about what was happening out in the different parts of the
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`electric power network. The condition in the network can be estimated by functions
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`20
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`for topology handling and state estimation. During the last decades， there have
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`furthermore been considerable interconnections of the regional networks to large
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`interregional electric power systems， which can cover very large geographical areas.
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`The development of larger and increasingly complex electric power systems has
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`resulted in increasing demands on data collection， data processing and control and
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`25
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`supervision of these electric power systems.
`
`The overall aim of the control and supervision systems is that the power system is
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`utilised in an economically optimum way and that operation safety goals are fulfilled.
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`The power system may be in a number of different states depending on outer
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`30
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`circumstances， for instance caused by disturbances， such as production and line
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`tripping. When the power system is in the normal operational state， the economical
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`optimisation of the operation constitutes the main task， while aspects regarding
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`delivery security has to become prioritised at aleはoremergency operation.
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`AII over the world， there is a de-regulation of the electric markets. Impo同ant
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`questions for the reliability in the electric power systems of the future are the origin of
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`new actors and the reduced organisational connection between production and
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`5
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`transmission resources. An additional area that draws more and more attention
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`among the power companies is questions around risk handling. On a de-regulated
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`electric market， there are also difficulties to offer spare capacity， since normally
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`unutilised capacity often costs large amounts to maintain in operationally safe
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`condition. A natural consequence of the development in progress， including the de-
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`regulation of electric markets， is an increased focusing on network operation and
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`operational problems. The need for updated information increases with that different
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`operators have to co・operatewith each other in order to get a total electric power
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`system to operate satisfactorily. Since it is not always obvious that all relevant
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`information is available for all pa同iesin an electric power system， new demands are
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`15
`
`put on the exchange of the information that is available. With the sharpened
`
`competition between different actors even higher demands are put to being able to
`
`utilise existing establishments in an optimum way.
`
`SCADA (Supervisory Control And Data Acquisition) functions constitute the basic
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`20
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`pa同 in the operation management system. SCADA comprises data collection，
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`supervision， control and presentation of different functions in the electric power
`
`system. SCADA is closely connected to the calculation functions that are paはsof
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`EMS (Energy Management System) and DMS (Distribution Management System).
`
`EMS comprises analysis and optimisation of the production and transmission
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`25
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`systems via functions for production planning， production control and power system
`
`analysis， while DMS constitutes corresponding functions for distribution network
`
`applications.
`
`An additional interesting area is DSM (Demand-Side Management)， which comprises
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`30
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`energy measuring and load control， among other things with the aim to influence
`
`load profiles and to reduce load peaks. A relatively new area is business systems，
`
`concerning energy trade and customer service， so-called BMS (Business
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`Management System).
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`The technical base for the hardware that is included in the SCADA and EMS
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`systems can be divided into three part systems: local systems， communication
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`systems and operational centre systems.
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`5
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`It is the local systems that handle data collection via remote terminals， so-called
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`Remote Terminal Units (RTUs)， and ordering of different measures， such as for
`
`instance connection operations in the network. Thus， these systems constitute the
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`interface to the processes in the electric power system. Since the electric power
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`10
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`networks constitute geographic剖Iyvery widespread processes， good communication
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`channels are required between the different units that are included in the control and
`
`supervision systems. Operation centre systems may be said to constitute the actual
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`brain in the systems for control and supervision of electric power networks， which
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`leads to that the activity at different operational centres has to be co-ordinated in that
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`15
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`the computers of the operational centres communicate and co-operate in
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`hierarchically formed structures. The hierarchic demand is a natural result of the
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`operational organisation of the power companies and the geographically spread-out
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`process.
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`20
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`The main tasks for SCADA can be divided into data collection， supervision， control，
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`accounting， planning and follow-up. In the part function of the SCADA system that
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`handles supervision， the treatment of measurement values and indications， the
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`supervision of limit values and indications and event and alarm handling are
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`included. Operationallimits are supervised and passing of an operational limit result
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`25
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`in an alarm in the operational centre. Different methods are used in order to indicate
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`alarms and fault signals， depending on their nature and significance for the operation
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`of the electric power network.
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`The paはfunctionof controlling comprises manoeuvring， set point control， blocking
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`30
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`and sending of control orders and limit values to local regulation equipment. The
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`nominal limits concern in most cases pure electrical quantities， such as voltages，
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`currents， powers and phase conditions， and are typically set after recommendations
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`from the manufacturer of the units， based on the present design. These
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`recommendations are based on tests of the apparatuses and70r-tneofetical
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`calculations of its prope吋ies，and margins that should assure a safe operation also at
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`somewhat different operational conditions. By help of different calculation functions，
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`measurement values that has not been fetched directly from the system can still be
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`5
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`calculated more or less reliably with help of measurement values from the network
`
`by applying theoretical models for the processes.
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`The basic functions that are included in EMS are constituted by the functions for
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`production planning， production control， topology determination， state estimation，
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`10
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`network calculation functions， safety analysis and possible practising simulators. In
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`order to carry out network calculations， a good model of the electric power system is
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`required， which is achieved via the functions for topology determination and state
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`estimation. The module for topology determination builds up an electrical model that
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`describes how the nodes and components of the electric power network are
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`15
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`electric剖Iyconnected to each other. AII measurement values suffer from larger or
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`smaller errors， and certain measurement values are missing due to faults in
`
`equipment or lack of remote terminals for data collection.
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`The fast development of power semiconductors creates new possi削litiesto control
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`20
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`and supervise electric power systems. An increased introduction of power
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`electronics， for instance in the form of so-called FACTS components (Flexible AC
`
`Transmission System) can contribute to change the electric power systems from
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`being substantially passive electrical networks for transmission of electric energy to
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`actively controllable systems， in which power flows can be influenced by automatics
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`25
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`or by measures initiated by operators in management centres.
`
`Sekiguchi and Masui have in the European patent application EP 0 853 367 with the
`
`title "Electric Power Control System" described a system for remote supervision and
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`regulation， including protection functions in the form of relay protections， of an
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`30
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`electric power network. The described embodiment comprises besides collection
`
`means for measurement values also processing and memory units， which are
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`connected by a communication network. Data in the described system is handled in
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`digital form and the regulation system for information collection contains a main area
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`(so-called core-area) and a communication area (so-called web-area)， and program
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`modules with possibilities to communication in order to control the relay protections.
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`Data and status concerning different relay protections can be graphically shown at a
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`screen， and new programs can be downloaded into the relay protections via the user
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`5
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`interface and a communication network.
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`In a related patent application EP 0 940 901， "Control system， method of protectively
`
`controlling electric power system and storage medium storing program code"， Shirota
`
`et al. have developed the earlier described system for remote supervision and
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`10
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`regulation to also include an accurate time marking of collected measurement data
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`and a method for obtaining parameters for transmission lines. Collected
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`measurement data can be time marked and are so吋edby means of an accurate time
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`label received from a GPS satellite. The current parameters for impedance values of
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`a transmission line can be obtained based on collected data regarding voltages and
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`15
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`currents in the transmission line.
`
`The two patent applications describe in the first place a method to connect and co・
`
`ordinate protection and control equipment in an electric power network. The control
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`units for protection devices and processing units， which are connected via a
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`20
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`communication network， constitute important pa吋s. The described procedure is
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`based on the use of conventionally available and thereby often utilised information in
`
`the operational data network.
`
`In the European patent EP 0 125 796 with the title "System and protection apparatus
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`25
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`for monitoring and control of a bulk electric power delivery system"， a device is
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`described， which is designed for to be placed at an electric transmission line and for
`
`measurement of parameters associated with a power flow over the line. The device
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`contains besides sensors， which for instance can measure temperature， current and
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`voltage， also a radio transmitter for communication with a receiver. The device can
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`30
`
`be placed at a transmission line without having to make this voltage free. Data
`
`collected via the device on the transmission line is transmitted via radio to the
`
`receiver placed at earth potential， for instance in the lower part of the power-line
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`pylon. The patent describes further a method for supervision of the power flow at a
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`number of transmission lines in a switching station by using a number of the above
`
`described devices placed at different transmission lines in the station. The receiving
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`station at the ground can beside receiver and antenna also include a processing unit，
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`a memory and a communication device for sending collected， and possibly
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`5
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`processed， measurement data on to other units in the electric power system.
`
`The described device measures only parameters directly available on or at an
`
`electrical conductor surrounded by air. The measurements are pe斤Ormed on a
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`passive component in the electric power network， a line， without intending to
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`10
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`measure on an in advance determined critical point， so-called "hot spot". The system
`
`contains a one-way information flow from the receiving station of the measurement
`
`device to a supervision unit and is utilised as a pure overload protection. The
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`receiving station can control several lines in the same way in the same time， by
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`measurements of identical type. Examples on control of the power flow can be by
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`15
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`reconnections in the network or changes of the tap-changer position for transformers
`
`with on-Ioad tap-changers. The control of the power flow through supervised
`
`transmission lines is based on data collected locally within the switching station.
`
`In the European patents EP 0 233 507 with the title "Transmission line sensor
`
`20
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`apparatus"， and EP 0 231 909 with the title "RF-antenna for transmission line
`
`sensor"， devices for information collection from high voltage conductors surrounded
`
`by air and their use according to above patents are described more in detail.
`
`A general problem with supervision of electric power networks according to prior a吋
`
`25
`
`is that the measurement data that is used does not have any close and safe
`
`connection to the actually critical points in the electric power processes in an electric
`
`power network. Control is today to a large extent performed by means of limits based
`
`on more or less static models and calculations of differing accuracy.
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`30
`
`SUMMARY
`
`A problem with supervision systems according to prior art is that the control of and/or
`
`limits for controllable parameters are dependent on calculations and/or models of
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`differing accuracy and reliability. This in turn leads to that， in order to get a safe
`
`operation， one has to apply relatively large security margins in the operation of
`
`electric power 0同ectsand electric power plants， whereby the utilisation does not
`
`become absolutely optimum. Another problem with supervision systems according to
`
`5
`
`prior a同 isthat measurements from different types of electric power 0同ectrarely are
`
`used together to utilise the margins in each electric power object.
`
`A general object with the present invention is thus to provide a supervision method
`
`and devices therefore， to utilise already existent resources in electric power plants
`
`10
`
`and electric power networks more efficiently. A further object with the present
`
`invention is to provide a method and devices that with time enables building-up an
`
`experience that further can improve the understanding of complex electric power
`
`systems and the operation thereof. Another 0同ectis to provide methods and devices
`
`to be able， in an earlier stage and/or with higher security， to detect operational
`
`ユ5
`
`disturbances in an electric power system. A further object is to optimise the operation
`
`of the electric power system to minimise the impact on the environment that electric
`
`power systems have.
`
`These and further object are achieved by methods and devices according to the
`
`20
`
`enclosed claims. In general terms， the present invention can be said to utilise direct
`
`measurement of marginal critical quantities in direct connection to interesting point in
`
`electric power processes in the different electric power 0同ects.These points are
`
`generally located at places difficult to access， such as at high potential in limited
`
`spaces， at high potential inside encapsulations with solid or liquid insulation， or at
`
`25
`
`rotating pa同s.The marginal critical quantities have thereby to be measured at these
`
`places difficult to access. The measurements give direct information about actual，
`
`now valid operational margins concerning marginal critical quantities and in pa同icular
`
`quantities that are both marginal critical and material critical. Information about
`
`available margins is transferred to other units within the network or the plant. The
`
`30
`
`electric power plant or electric power network can then be controlled emanating from
`
`such actual operational margins from several different units in the plant， which
`
`enables a more efficient operation of the electric power plant or the electric power
`
`network， and can give decision suppo吋 forextension of networks by identifying
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`bottlenecks. By the direct measurement， a fast feedback is achieved if accomplished
`
`operational changes do not bring about intended result. The measurements give
`
`furthermore new paths for detecting faults in the systems. Furthermore， databases of
`
`operational conditions can be built， which then can be used both as SUppO同 ofthe
`
`5
`
`operation of the system and for future maintenance， model designing etc.
`
`The advantages with this is that one can， without renouncing security， utilise already
`
`existing margins in electric power systems. Overdimensioning and losses can be
`
`minimised， which also has advantages concerning the environment.
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`10
`
`With the deregulation of electric markets， there is a growing interest in utilising the
`
`network and its included components harder and harder， which sometimes is
`
`expressed as a request to "drive" the electric power networks and its included
`
`components closer and closer to their physical limitations. There is thus today an
`
`15
`
`economical request to better utilise available transmission capacity in electric power
`
`networks as well as margins in components. This leads to a need for to better being
`
`abl恰eto determine t肋hepr陪esen川tmar匂gi川nsto st恰ab削H批tylimits for the electric power network
`
`and br川inginformation about the actual and prl陪esentcondition for component， such as
`
`for instance the temperature in the windings of an electrical machine.
`
`20
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The invention and further objectives and advantages that are achieved thereby are
`
`best understood by reference to the description below and the enclosed drawings， in
`
`25
`
`which:
`
`Fig. 1 is an embodiment of an electric power plant according to the present
`
`invention;
`
`Fig. 2 is an embodiment of an electric power network according to the present
`
`invention;
`
`30
`
`Fig. 3 is an embodiment of an electric power network according to the present
`
`invention with different types of electric power units;
`
`Fig. 4 is an embodiment of an electric power network in several levels
`
`according to the present invention;
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`Fig.5a and 5b are schematic drawings and diagram showing how temperature
`
`margins in electric power objects can be utilised by the present invention;
`
`Fig.6a to 6d are schematic diagrams showing how time variations in the
`
`utilisation of electric power objects can be utilised by the present invention;
`
`5
`
`Fig. 7a to 7e are schematic drawings and diagrams showing how operational
`
`margins in electric power plants can vary and be utilised by the present invention;
`
`Fig. 8a to 8e are schematic drawings and diagrams showing how operational
`
`margins in electric power objects can vary and be utilised in electric power plants by
`
`the present invention;
`
`10
`
`Fig. 9 illustrates a communication network and associated information flows
`
`according to the present invention;
`
`Fig. 10 shows a flow diagram for a method according to a first aspect of the
`
`present invention; and
`
`Fig. 11 illustrates an electric power network presenting a so-called botlleneck.
`
`15
`
`DETAILED DESCRIPTION
`
`In the following detailed description， a number of concepts will be used. In order to
`
`avoid interpretations of these notions， which differs from what is intended in the
`
`20
`
`present description， a number of these concepts will first be defined， before the
`
`actual description sta吋s.
`
`An electric oower obiec! is a device in an electric power system， which comprises a
`
`process or a course of events of electric/electromagnetic type. The electric power
`
`25
`
`object is most often controllable， either internally or by external switching devices，
`
`and has via its controllability an influence on electrical parameters in the electric
`
`power system， in which the object is present. The group of electric power 0同ec胎
`
`comprises e.g.: electrical machines (motors and generato陪)， transformers， reactors，
`
`capacitors and power electronics.
`
`30
`
`An electric oower olan! is defined in the present description as a group of
`
`interconnected electric power 0同ects，which a陪 locatedwithin a relatively limited
`
`area and are operated in a co-ordinated manner and which preferably belong to the
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`same operator. The operation of the electric power plant can be influenced by the
`
`inherent behaviour of the objects as well as an active control by a supervision unit.
`
`By ~lectric oower networ1s is in the present description intended a group of electric
`
`5
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`power plants and single electric power objects， if any， interconnected by electrical
`
`lines， typically distributed over a somewhat wider geographical area than an electric
`
`power plant. The operation of the included components are supervised and
`
`controlled in a co-ordinated manner， possible also in co・operationwith superior， side-
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`existing and/or subordinated electric power networks， by a supervision unit. An
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`electric power network can e.g. be included as a part in a superior electric power
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`network.
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`Electric oower uni! is used in the following as a composed concept for electric power
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`object， electric power plant and electric power network.
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`The concept ~lectric oower svster!! is used in the wide sense of a general system of
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`different electric power units.
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`In different electric power objects in electric power systems， many of the critical and
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`most interesting electric/electromagnetic courses of events takes place at "places
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`difficult to access". Parameters concerning these electric/electromagnetic courses of
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`events are therefore normally difficult to obtain. In prior art， one has chosen to meet
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`the insecurity concerning critical parameters with ce吋aindrawn-up safety margins
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`and limits， based mainly on models and calculations. The present invention is based
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`on measurements， which give earlier not utilised information. By "Qlaces difficult to
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`亘cc皇皇室"is in this description intended:
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`at rotating pa同，
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`at high potential in limited spaces， and/or
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`at high potential within encapsulation with solid or liquid insulation.
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`Rotating pa吋sare commonly present in electric power 0同ects，mainly in rotating
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`electrical machines. To measure parameters directly at such parts has earlier not
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`been considered to be applicable in practice. Measurements at high electrical
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`potential， e.g. over 1 kV， or very close to such a high electrical potential， have also to
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`a large extent been avoided earlier. High voltages are present in many electric power
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`o同ectsand space limitations is a common reason to why one does not peげorm
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`measurements close to these. Encapsulations are often used to screen high
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`potentials and constitute also sometimes an obstacle for a simple supervision of
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`different parameters， in pa同icularin cases where solid or liquid insulation is present.
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`Common for these places difficult to access is that measurements at these places
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`demand relatively complex arrangements. In prior art one has therefore in general
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`chosen to relinquish the use of such quantities， and instead chosen to trust models
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`and calculations or limit values. In certain cases， one has measured certain
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`quantities during testing procedures or similar in order to characterise the electric
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`power 0同ects，but has earlier not really realised the benefit of continuous
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`measurements during operation. This is mainly based on the difficulties to peげorm
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`relevant measurements.
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`The parameters that are of interest are parameters that have a direct or indirect
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`connection to the operational state of the electric power objects. The parameters are
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`of very differing character. Some of the parameters can be denoted as marginal
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`critical quantities and some of the parameters can be denoted as material critical
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`quantities， and some can be denoted as both margin and material critical quantities.
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`By "marginal critical quantities" is intended quantities that are associated with some
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`kind of limit. If this limit is exceeded， it will have a direct influence on the operation of
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`the electric power 0同ect，relatively independent of other quantities. Thus， for these，
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`direct limits can be set-up. If the limit is exceeded， a well-defined damage or
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`influence on the operation is achieved more or less directly. Examples of marginal
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`critical quantities are: temperature， load angle， phase angle，剖ip，current， voltage，
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`frequency and torque. In current and voltage are also phase quantities associated
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`thereby included. A too high temperature can over a certain limit cause a direct
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`material damage. A phase angle in a synchronous machine has for instance an
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`absolute limit， when the stability of the synchronous machine ceases. Practical limits
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`for such quantities have of course to be set with a certain margin. Voltage can be
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`denoted as a marginal critical quantity， since it has a limit associated with a
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`maximum transmission capacity. If one falls below this limit， the voltage stability in
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`the network is lost.
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`By"material critical auantitie~" are intended quantities， which in principle also have a
`limit. This limit can be difficult to determine， since it is dependent of so many
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`surrounding parameters. These limits do often not constitute any really critical limits
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`but can normally at least during a shorter period of time be exceeded without for sure
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`causing a damage or operational disturbance. At longer exceedings are normally a
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`gradually degradation of the material obtained， a shortening of life. Examples of
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`material critical quantities are: temperature， current， paはial discharges (PD)，
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`vibrations， overtones， minus sequence， zero sequence， rotational speed， air-gap flux
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`and speed. These quantities have often rated values or nominal values for a certain
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`electric power object. It is， however， mostly possible to exceed these nominal values，
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`at least for a shorter time， without causing any immediate damages on the electric
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`power object. A current through a winding in a transformer that exceeds the rated
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`current does not always case a damage. If the winding is cold from the beginning，
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`either as a result of that the transformer earlier has been utilised sparsely， or
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`because of that the temperature of the surroundings is very low， the transformer can
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`withstand a temporary overcurrent without being damaged. If on the contrary the
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`overcurrent is present for a certain time， the winding is probably heated up by time
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`and can at a later occasion reach temperatures that are harmful for the material. The
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`material critical quantities are often related to limits for the marginal critical quantities
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`by more or less complex models and relations. Existing static limits for material
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`critical quantities are normally set with certain， mostly relative large， safety margins.
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`Vibrations constitute a rather typical material critical quantity， for which it is difficult to
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`define any type of exact limit， but instead it has a degrading e仔:ecton the material
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`with time. If vibrations are on during a time period， damages will eventually arise， if
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`the level is su仔icientlyhigh， i.e. over a limit. It is not the level itself， but more a
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`combination of level and time that gives rise to problems and damages in the case of
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`material critical quantities.
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`A quantity such as temperature can according to the reasoning above be
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`characterised as both marginal critical and material critical. There are values for e.g.
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`polymers， which can not be exceeded without having the material going through a
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`non-reversible process. At the same time， a gradual degradation of the material， i.e.
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`an ageing phenomenon， can occur if the temperature is over another lower level， and
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`the consequence depends in this context both on the time and the level itself.
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`Current can also be placed in this double assignment， as a result of its close
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`connection to the temperature.
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`Of all these parameters， the temperature is the incomparably most impo吋ant，since
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`all electric power objects have some so同oftemperature dependent margins.
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`In Fig.， 1 a typical example of an electric power plant 1 is illustrated. Two electrical
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`machines 10， which in this case are similar， operate as generators， each of which
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`produces a certain electrical power. The electrical machine 10