Power System Vulnerability. Background. Power System Operating States

Transcription

1 GPS Satellte Falure Analyss Informaton & Sensng Vulnerablty Assessment Power System Vulnerablty LEO Satellte Intranet Internet Self Healng Strateges Strategy Deployment Power System Operatng States E = demand suppled I = constrants met Early verson by T. DyLacco Ths verson by L. Fnk, K. Carlsen, IEEE Spectrum, March 1978 Slde by Areva T&D, 2006 E I Restoratve Resynchronzaton E I In extrems Cut losses, Protect Equpment System not ntact Normal Load trackng, cost mnmzaton, system coordnaton System splttng and/or load loss E I Alert Preventve Control E I Emergency Heroc Acton System ntact E I Secure Reducton n reserve margns and/or ncreased probablty of dsturbance Volaton of nequalty constrants Background Electrc power grd s consdered a natonal securty matter The relable operaton of the system s of top prorty to socety. Relablty concerns are amplfed by the utlty s deregulaton, whch ncreases the system s openness whle smultaneously decreasng the appled degree of control.

2 Vulnerablty Assessment Is the assessment of power system s ablty to contnue provdng servce n case of an unforeseen catastrophc contngency. Sources of Vulnerablty Natural calamtes Component falures Protecton and control falures Breaks n communcaton lnks Faults Human errors Inadequate securty margn Gamng n the market Sabotage or ntruson by external agents Mssng or uncertan nformaton Sources of Vulnerablty Internal Sources External Sources Informaton & Decsons Sources of Vulnerablty Network & Protecton Intentonal Human Acts Market Communcaton Systems Natural Calamtes The system s nsecure or vulnerable f any of these contngences lead to a dsrupton of servce to part (outages ) or all (blackouts ) of the system

3 Securty Assessment Categores statc securty assessment transent or dynamc securty assessment. Statc Securty Assessment deals wth the post-dsturbance perod after all transents have ded down and the system reaches a new steady state operatng condton A system s sad to be statcally secure f the new steady state operatng condton does not volate any operatng lmts such as bus voltage or lne current ratngs. Dynamc Securty Assessment deals wth each generator's ablty to mantan synchronsm wth the rest of the system durng the transent perod mmedately followng the dsturbance. The system s secured dynamcally f the dynamcs of the system s ded out and the system operatng pont s vable Example electrcal short-crcut caused by the falure of an nsulator on a hghvoltage transmsson lne. Dsturbances result n a sudden surge of current on the lne severely dsruptng all generators that are electrcally close to the fault.

4 Example Protectve relays send a trp sgnal to crcut breakers nstalled at each end of the lne. The crcut breakers then open, removng the dsturbance. The removal of the transmsson lne causes another dsturbance to the power system. Example The trppng of the lne may requre further correctve acton. Ths s called a cascadng outage These dsturbances can lead to severe consequences such as a blackout. Few Example of Blackouts due to Lack of Dynamc Securty Date Locaton Affected Customers New York, USA Loss of Revenue 7.3 mllon $90 mllon France 12 mllon $250 mllon Tokyo, Japan 2.8 mllon $70 mllon What Securty Assessment Offers Fast, relable securty assessment technques can reduce the rsk of blackouts occurrng by provdng utltes wth a means to quantfy the relatve rsk at varous operatng strateges.

5 Challenges Vulnerablty assessment s a hghly nonlnear problem Closed form detaled models do not exst Power network s large and extensve Operatng condtons are wde Range Topology s contnuously changng The lst of contngences s long Some of the falures are sngle events and some as a sequence of events. Challenges Vulnerablty assessment s computatonally ntensve process Assessments need to be contnually repeated On-lne assessment s a challenge Measurements and operatng condtons are nosy Avalable knowledge s n hstorcal examples Statc Securty Assessment Defnton Ablty of the system to reach a state wthn the specfed safety and supply qualty followng a contngency. The tme perod of consderaton s such that the fast actng automatc control devces have restored the system load balance, but the slow actng controls and human decsons have not responded.

6 Contngency Is an abnormal event (such as a fault) whch could be potentally damagng to the power system operaton. Each contngency manfests tself dfferently, resultng n dfferent types of outages. statc securty assessment (SSA). Contngency The most common are the sngle/multple lne outages and generator outages. Contngency If a contngency causes a volaton of the securty varables, the present power system state s nsecure towards that outage. The process of evaluatng the steady state securty of potentally damagng contngency of a power system s called statc securty assessment (SSA). Securty Varables Bus voltages Lne flows (thermal lmts)

7 SSA Process Contngency defnton (CD) Contngency selecton (CS) Contngency evaluaton (CE) Contngency Defnton (CD) CD s a process by whch a lst of contngences whose probablty of occurrence s deemed suffcently hgh are thought to be of vtal nterest to system securty. Contngency Screenng (CS) the process that shortens the orgnal long lst of contngences by removng the vast majorty of cases havng no volatons. fast and approxmate method of selectng key contngences for a more thorough evaluaton. Contngency Screenng (CS) DC load flow (Actve power contngency screenng) Dstrbuton Factor Performance Index Human expertse Intellgent Technques

8 Contngency Screenng (CS): DC Load Flow Assumptons: Only real power flow s used All voltages have same magntudes (1pu) All voltage angles are small sn cos 1 P Contngency Screenng (CS): DC Load Flow P V G N V k G cos B sn k 1 2 V V V cos B V V sn k k P Contngency Screenng (CS): DC Load Flow N V V G cos B sn P G k k 1 P 2 V V V cos B V V sn N G B k 1 P k B k Contngency Screenng (CS): DC Load Flow Step 1: Solve for P Step 2: Compute lne flow N G B k 1 P B

9 Contngency Screenng (CS): DC Load Flow Any contngency that causes P SM for all s elmnated from the lst Contngency Screenng (CS): DC Load Flow Lmtaton of DC Load Flow Only real power s consdered Voltage volaton s gnored SM =Magntude of maxmum apparent power flow of lne Contngency Screenng (CS): Dstrbuton Factor X ( ) X (0) X Y ( ) Y X(0): Predsturbance states : Contngency X( ): Post dsturbance states Y: Change n system topology X Y : Senstvty of system to change n topology (Transfer matrx) X X Y Y Contngency Screenng (CS): Dstrbuton Factor Assumes lnear relatonshp between pre and post contngency states Requre the dervatve X Y Y could be hard to compute

10 PI Contngency Screenng (CS): Performance Index (PI) W V ( V ref W, W : weghtng factors W V (ref): desred value of V S (max): maxmum ratng of lne S k S ( ) max Contngency Evaluaton (CE) A process by whch a fast ac powerflow s used on successve ndvdual cases n decreasng order of severty. The resultng securty varables are checked for post contngency volatons. Contngency Evaluaton (CE) Assumes the system reaches a steady state CE uses a full ac power-flow to calculate the change n voltages and lne flows after a contngency The system s nsecure from statc pont of vew f any Voltage or power flow volatons exsts substantal load s lost Operator Vew System Dsplay

11 Example Queston If any lne trps, wll another nearby lne be overloaded? Wll a bus voltage be too low? Too hgh? Overloaded Lne

12 Must examne the entre system! Queston: Gven present operaton, would any component outage cause an operatng problem? What outages should be checked? Dffcult Queston How do we check them? System Power Flow Analyss SSA: Issues The objectve of SSA s to detect potental securty volatons before they actually occur. The detecton Warns operator of potental problems gves operator suffcent tme to steer the system away from the nsecure state. SSA: Issues SSA s desgned to performed perodcally at the control center based on the avalable computer resources the level of operatonal sophstcaton of the partcular utlty SSA: Issues For large scale power system, the task of securty assessment s tme consumng and computer ntensve. Large number of potental contngences that have to be analyzed The dfferent load levels Changes n topologes Changes n operatonal strateges

13 SSA: Issues Securty assessment s a classfcaton problem the combnaton of system topologes, states, and contngences determne the securty status of the system Hence, the concept of pattern recognton can be very effectve SSA: Issues The concept of pattern recognton s to capture common underlyng characterstcs between the pre and post-contngency status The captured knowledge can be generalzed to classfy ndependent test data orgnatng from the same statstcal source. Advantages of Pattern Recognton Computatonal speed. Classfers can be developed off-lne Current and future operatng states can be quckly evaluated classfyng a new steady state power system condton nto a secure or nsecure class s trval and does not requre the lengthy computatons of an analytcal soluton. Pattern Recognton s dscussed n Module 8 Dynamc Securty Assessment (DSA)

14 DSA Deals wth the stablty of the system followng a contngency The system s dynamcally secured f Oscllatons damped out and the system reaches a new steady state condton Oscllatons are wthn acceptable range The new steady state s statcally secured Man Challenges to DSA DSA s computatonally ntensve process: DSA s a hghly nonlnear problem Closed form detaled models do not exst Power network s large and extensve Operatng condtons are wde Range Topology s contnuously changng The lst of contngences s long Some of the falures are sngle events and some as a sequence of events. Man Challenges to DSA DSA need to be contnually repeated On-lne assessment s a challenge Measurements and operatng condtons are nosy Avalable knowledge n hstorcal examples DSA Methods Tme doman smulatons Drect stablty methods (Energy functon) Small sgnal analyss (Egenvalues) Crtcal Clearng Angle (Tme) Classfcaton and Pattern Recognton

15 Tme Doman Methods Tme doman methods seek to solve a set of dfferental equatons descrbng the moton of the generators n the system. Rotor dynamcs (Swng Equaton) Electrc power often reduced to the power curve Tme Doman Methods P m : mechancal power nput P max : maxmum electrcal power output H: nerta constant, n MWs/MVA : rotor angle, n electrcal radans t: tme, n seconds s : synchronous speed of the rotor V E f Pe X sn P sn 2 2H d P P 2 m dt s max max sn Tme Doman Methods By solvng the swng equaton n the tme doman, the stablty of the system can be assessed. If the rotor oscllatons damp out, the system s stable If not, t s unstable. Tme Doman Methods: Stable Tme

16 Tme Doman Methods: Unstable Tme Tme Doman Methods Many factors nfluence the stablty of the system Prefault system topology Generator loadng condtons Duraton of the dsturbance Severty of the dsturbance Rotor nerta Tme Doman Methods: Challenges Tme doman methods uses numercal smulaton (e.g. Euler or Runge-Kutta) Methods could be tme consumng Rotor dynamcs s nonlnear dfferental equaton Interacton between machnes mpose heavy computatonal burden Drect Methods By the Drect Methods, the transent behavor of a power system can be predcted wthout complete tme doman smulaton Drect methods advantages: Compromse between fast and reasonably accurate assessment could be used n an on-lne envronment or as a prescreenng flter for tme doman smulatons Able to rank the severty of a gven contngency n terms of ts energy margn.

17 Drect Methods: Man Steps Step1: Calculate the transent energy at the nstant the dsturbance s cleared (e.g. acceleratng energy E a ) Step2: Determne the crtcal energy for the current dsturbance (maxmum amount of dampng energy that can be absorb by the power system E d max ) Step3: Calculate the dfference (energy margn) Energy Margn= E d max -E a Small Sgnal Analyss The power system s lnearze about the current operatng pont. The egenvalues of the lnearzed equatons are calculated The method s only accurate for small dsturbances Crtcal Clearng Angle It s another ndcaton of the balance between the accelerated Knetc Energy due to a contngency versus the dampng knetc energy of the system. P P m 1 Crtcal Clearng Angle A a max 2 A d mn 3-max The crtcal clearng angle ( cr ) s the maxmum angle for a stable system,.e. when A amax = A d mn cr Crtcal Clearng angle 3 max

18 Classfcaton and Pattern Recognton (CPR) Several off-lne smulatons are made for several loadng condtons assumng a class of contngences The operatng ponts dentfed wth secure and nsecure states are clustered n the nput space The current operatng condton s compared wth the cantrods of the clusters The mnmum dstance to the centrod of the clusters determnes the securty status of the system. B C 1 1 A A 0 1 CPR Subset A Unverse (X) Subset C Subset B Classfcaton and Pattern Recognton (CPR) Merts: Fast for on-lne applcaton Can be made adaptve to system changes Could use hstorcal data n addton to smulatons Challenges: Input space can be very large Features that determne the securty status are hard to obtan Accuracy of clusters depends on the qualty and quantty of data DSA Process System Data Topology States Features Identfcaton Features Selecton Features Extractons DSA DSA Classfcaton DSA Border

19 Why Feature Identfcaton? Features Identfcaton Elmnates curse of dmensonalty. Enhances class separablty. Reduces pattern dmenson Mantans classfcaton accuracy. Reduce tranng tme Reduce computatonal tme for other applcaton Border Identfcaton Feature Selecton Feature Extracton X 1 X 2 X 1 Y 2 Sensors Feature Selector Classfer Sensors Feature Extractor Y k Classfer X n X n X n Most mportant features are selected Technques: Fsher Dscrmnate All features are combned to form a new reduced set of features Technques: Prncpal components, NN

20 Challenges Assessment All DSA methods assess gven operatng condtons Generalzaton s not possble unless data s clustered Data wthn clusters can be assessed wthout queryng power system models (Fast assessment) Data between clustered s unclassfed Securty Assessment Secure Border Identfcaton Insecure Regon of Confuson

21 What s Border Identfcaton Assessment wth Border Identfcaton A method to track the edge of the securty regon Allows users to dentfy securty margns Insecure Secure Securty Margn Operatng Pont 1 Margn of securty Challenges Identfyng a border pont s computatonally ntensve process Identfyng several ponts on the border that are unformly dstrbuted s extremely dffcult to acheve Identfyng the border n hgh dmenson space s extremely dffcult and requres unrealstc computatonal power Securty Border

22 Border Identfcaton Technques Border trackng Gradent method Projecton technque Intellgent Technques Inverse ntellgence Border sectonalzaton DSA Indces Vulnerablty Index Reflect the level of system strength or weakness relatve to the occurrence of an undesred event The vulnerablty of power system wll change f : The operatng state change Envronmental condtons change System equpment status change We need a quanttatve measures Common Vulnerablty Index Crtcal Clearng Tme (CCT) Good accuracy, relablty, and modelng capablty Requres ntensve computaton tme Energy Margn Avod the tme-consumng computaton Modelng lmtaton, Less accurate than CCT Egenvalues Antcpated loss of load

23 Vulnerablty Index based on Dstance from Border Operatng Pont 2 Vulnerablty Border Vsualzaton After Frst Event Degree of Vulnerablty Vulnerablty Index Q 140 g 13 P 162 g 12 P 18 g 13 Q 159 g 12 1 Margn T Q L P 9 g Q 162 g 11 Q 148 g VI = Operatng Pont P 144 g T P L Vulnerablty Border Q 116 g Vulnerablty Border Vsualzaton After Second Event Vulnerablty Border Vsualzaton After Thrd Event Vulnerablty Index Vulnerablty Index T Q L T Q L P 45 g Q 15 g 9 VI = VI = T P L T P L

24 Crtcal Clearng Tme (CCT) : DSA Margn CCT CT CCT: Crtcal Clearng Tme CT: Actual Clearng Tme Crtcal Clearng Tme (CCT) Advantages Good accuracy Relablty Equal area crteron can be used (smple modelng) Dsadvantages Requres ntensve computaton tme : DSA Margn Energy Margn E ref E E ref : Allowable level of dampng energy E: Actual dampng energy Advantages Faster than CCT Energy Margn Dsadvantages Less accurate than CCT

25 Egenvalues Egenvalues ref Advantages Straght forward process : DSA Margn ref : Allowable real component of egenvalues : Actual real component of egenvalues Dsadvantages Doesn t reflect the nonlnear nature of the power system Tme consumng for large systems Antcpated Loss of Load (ALL) : DSA Margn P ref P P ref : Allowable amount of load to be shed P : Actual load outage Antcpated Loss of Load (ALL) Advantages: Ths ndex s fully applcable n the case of cascadng events Any control actons can be ncluded (frequency sheddng, ) Smple concept Drectly related to utlty objectves (servng customers) Dsadvantages Computatonally extensve

26 Antcpated Loss of Load (ALL) For small System We can search all possble combnatons of load reducton. For realstc sze system The exhaustve search s practcally mpossble. For N loads, to shed each load from 0%-100% n 1% ncrement, there are 100 N possble combnatons of load reductons Possble soluton Reduce N by selectng load to be shed from among the network loads Use ntellgent technque to speed up the computaton Vulnerablty Index (VI) based on antcpated loss of load It s proposed that the antcpated loss of load wth respect to a sequence of events be used as a VI. Ths ndex s fully applcable n the case of cascadng events. Any control actons can be consdered. Ths concept s smple but computatonally extensve. Vulnerablty Index (VI) based on antcpated loss of load (Cont ) Small System We can search all possble combnatons of load reducton. Realstc sze system The exhaustve search s practcally mpossble. If we shed each load from 0% to 100% n 1% ncrement N loads 100 N possble combnatons of load reductons Fast search algorthms are needed for ths technque to succeed. Under Frequency Load Sheddng Frequency declne, rate of frequency declne Actvated by frequency declne rate Actvated by frequency declne 59.5 Hz 59.3 Hz 58.8 Hz 58.6 Hz 58.3 Hz 20 % 5 % 4 % 4 % 10 % 15 %

27 Scenaro kv 345 kv 500 kv The amount of Load Sheddng v Amount of Load Sheddng [MVA] Under Frequency Relay PSO Total Bus Number The amount of Load Sheddng Scenaro 2 77 v 82 Three Phase Fault 86 Amount of Load Sheddng [MVA] Under Frequency Relay PSO wth Islandng PSO wthout Islandng at T = 0 ms Total 119 Bus Number

28 Scenaro 2 77 v 82 Scenaro 2 77 v 82 Lne Trpped at T = 100 ms Addtonal Trppng due to Hdden Falure at T = 100 ms Load Sheddng Control s Actvated at T = 400 ms The amount of Load Sheddng Under Frequency Relay Cannot stablze ths event Amount of Load Sheddng [MVA] Total Bus Number

29 Dstrbuton of the Total Magntude of Load Sheddng for 1500 dfferent operatng condtons Number of Patterns Total amount of Load Sheddng[MVA] The amount of load sheddng n MVA can be used drectly. Ths amount could be normalzed between 0 and 1. The load sheddng percentage the hghest load sheddng case : 3150 / 68398= 4.6 %