Power System Security Introduction
•
Major functions of power system security
•
Operating states of power system
•
FACTORS affecting on power system system security
•
Security analysis
•
Contingency analysis
•
Sensitivity factor
•
AC Power flow method
•
Contingency relaxation
•
Introduction •
•
•
•
System security involves practices suitably designed to keep the system operating when components fail. If the process of cascading failures continues, the system as a whol wh olee or it itss ma majo jorr pa part rtss may co comp mple lete tely ly co coll llap apse se.. Th This is is normally referred to as a System blackout. blackout. A pa parrti ticu cula larr sy syst stem em st stat atee is sa said id to be se secu cure re on only ly wi with th reference to one or more specific contingency cases, and a given set of quantities monitored for violation. Most power systems are operated in such a way that any single con onttingency will not leave othe herr comp mpo one nent ntss heavily overloaded, so that cascading failures are avoided.
Major Fun unc ct ion ons s of Pow ower er Syst yst em Security I.
Syst System em Moni Monito tori ring ng
II. II.
Conti Conting ngen ency cy Anal Analy ysis sis
III.. Correc III Correctiv tivee Action Action Analy Analysis sis
Syst em Moni M onitt or oriing •
•
•
•
System System monitor monitoring ing suppli supplies es the power power system system operat operators ors or dispatchers with up to date information on the conditions of the power system on real time basis as load and generation change. Telemet elemetry ry system systemss measur measure, e, monitor monitor and transmi transmitt the data, data, volt voltag ages es,, curr curren ents ts,, line line flow flows, s, stat status us of circ circui uitt brea breake kers rs,, frequency, generator outputs and transformer tap positions in every substation in a transmission network. Digital compute uters in a control center then hen process the telemeter data and place them in a data base form and inform the operators in case of an overload ov erload or out of limit voltage. Alarm or warnings must be given if required.
Contingency Analysis •
•
•
Many of the problems that occur on a power system can cause ser serious ious trou troubl blee with within in such such a quic quick k time time peri period od tha that the the operator could not take action fast enough. This is often the case with cascading failures. Because use of this aspec ectt of sys systems ope operation, mode odern operations computers are equipped with contingency analysis prog progra rams ms that that model model poss possib ible le syst system em troub trouble less befo before re they they arise. These programs are based on a model of the power system and are used to study outage events and alarm the operators to any potential overloads or out of limit voltages.
Corr or rect ect ive Act ion Ana nallysi ysis •
•
•
•
Corrective action analysis permits the operator to change the oper operat atio ion n of the the powe powerr syst system em if a cont contin inge genc ncy y anal analy ysis sis prog progra ram m pred predic icts ts a seri seriou ouss prob proble lem m in the event vent of the the occurrence of a certain outage. Thus this provides preventive and post contingency control. A simp simple le exam exampl plee of corr correc ecti tive ve ac acti tion on is the the shif shifti ting ng of generation from one station to another. another. This This ma may y re resu sult lt in chan chang ge in powe powerr flows lows and ca caus usin ing g a change in loading on overloaded lines.
Operat perat ing st at e of pow power er syst yst em •
•
•
I.
Equality constraints:- Real and Reactive power balance at each node. Inequality Inequality constraints constraints::- Lim imit itat atio ions ns of phy physi sica call equi equipme pment nt such su ch as cu curr rren ents ts an and d vo volt ltag ages es mu must st no nott ex exce ceed ed ma maxi ximu mum m limits. Five operating states of power system Norm No rmal al st stat atee
II.. Al II Aler ertt st stat atee III. Emergency state IV. Extrem Extremis is state state V. Res Restor torati ative ve sta state. te.
Cont inue………… •
•
•
•
:- Al Alll eq equa uali lity ty an and d in ineq equa uali lity ty con const stra rain ints ts ar aree Normall sta Norma state te:satisfied. Generation is adequate to supply the existing load demand and no equipment is overloaded. Alert state:state:- The security level is below some threshold of adequacy. This implies that there is a danger of violating some of the inequality constraints when subjected to disturbances. Emergency state:state:- Due to severe disturbance, the system can enter emergency state. Here inequality constraints are violated. The system would still be intact, and emergency control action could be initiated to restore the system to an alert state. Extremis state:state:- Here, both equality and inequality constraints are violated. The violation of equality constraints implies that parts of the system load are lost. Emergency control action should be directed at avoiding total collapse.
Cont inue………… •
nal state in which Restor Restorati ative ve state state::- This is a transitional inequality inequality constraints constraints are met from emergency emergency control actions actions taken but the equality constraints are yet to be satisfied. From this state the system can transmit to either the alert or the normal state depending on the circumstances.
FACTORSaf afffecti ect ing on on power syst yst em security •
I.
As a consequence of many wide spread blackout in interconnected power systems, the priorities for operation of modern power system have evolved to the following: Operat Oper atee the syste system m in such such a wa way y that that power power is deliv deliver ered ed reliably.
II.. Wit II ithi hin n th thee co cons nstr trai aint ntss pla place ced d on th thee sy syst stem em ope opera rati tion on by reliability considerations, the system will be operated most economically. •
•
Engineering groups who have designed the power system’s tran transm smis issi sion on and and gene genera rati tion on syst system emss have have done done so with with reliability in mind. This means that adequate generation has been installed to meet me et th thee lo load ad an and d th thaat ad adeq equa uate te tr tran ansm smis issi sion on ha hass be been en installed to deliver the generated power to the load.
Cont inue………… •
•
•
•
•
If the operation of the system went on without sudden failures or without experiencing unanticipated operating states, we would probably have no reliability problems. However, any piece of equipment in the system can fail, either due to internal causes or due to external causes such as lightning strikes, object hitting transmission towers, or human errors in setting relays. It is highly uneconomical to build a power system with so much redundancy (extra transmission line, reserve generation, etc..) that failures never cause load to be dropped dropp ed on a system. Rather, system are designed so that the probability of dropping load is small. Thus, the power sy syst steems are designe ned d to have suf uffficient redundancy to withstand all major failures events.
Cont inue………… •
There are two major types of failure events,
1. Transm ransmission ission line outag outages es 2. Gener Generation ation unit failur failures es •
•
•
Transmission line failures cause changes in the line flows and voltages voltages on the transmission transmission equipment remaining remaining connected to the system. Ther Theref efore ore,, the the analy analysi siss of tran transmi smiss ssio ion n fa fail ilur ures es re requi quire ress method to predict these flows and voltages so as to be sure they are within their respective limits. Gene Genera rati tion on fa fail ilur ures es ca can n also also ca cause use flow flowss and and volt voltag ages es to chan change ge in the the tran transm smis issi sion on syst system em,, with with the the addi additi tion on of dynamic dynamic problems problems involving involving system frequency and generator generator output.
Security analysis •
System security can be broken down into two major functions:
1. Sec Securi urity ty ass assess essment ment 2. Sec Securi urity ty con contro troll •
•
•
•
The former gives the security level of the operating state. s tate. The The late laterr dete determ rmin ines es the the appr approp opri riat atee secu securi rity ty cons constr trai aine ned d scheduling required to optimally attain the target security level. Systeem se Syst secu curi rity ty as asse sess ssme ment nt is th thee pr proc oceess by wh whic ich h an any y violations are detected. System assessment involves two functions:
1. Sy Syste stem m moni monitor toring ing 2. Contin Contingency gency analy analysis sis
Cont inue………… •
•
•
•
•
•
System monitoring provides the operator of the power system with up to date information on the current conditions of the P.S. P.S. Contingency analysis is much more demanding and normally performed in three states, i.e. Contingency definition, selection and evaluation. Contingency defin Contingency definition ition gi give vess th thee li list st of co cont ntin inge genc ncie iess to be processed whose probability of occurrence is high. This list is in terms of network changes, i.e. branch and/or and/o r injection outages. Thesee co Thes cont ntin inge genc ncie iess ar aree ra rank nked ed in ro roug ugh h or orde derr of se seve veri rity ty employing contingency selection algorithm to shorten the list. Contingency evalua Contingency evaluation tion is th then en pe perf rfor orme med d (u (usin sing g AC pow power er flow) on the successive individual cases in decreasing order of severity. The evaluation process is continue up to the point where no post contingency violations are encountered.
Cont inue………… •
•
The second major function, security control, allows operating personnel to change the power system operation in the event that that a cont contiinge ngency ncy analy nalysi siss prog progra ram m pred predic icts ts a seri seriou ouss problem, should a certain outage occur. occur. Norma mallly it is achieve eved throu hroug gh Optimization(SCO) program.
Seccurity Constr Se strained ned
Contingency Analysis •
The purpose of contingency analysis is to identify the list of contingencies that if occur would create violations in the system operating states. They are ranked in order of severity. severity.
Cont ontiinge ngency ncy anal analysi ysis
Base case AC line flow
Cont inue………
Post ost Out utag age e AC AC load flow
Cont inue………
Post ost ou outt ag age e AC AC load fl f low
Cont ontiinge ngency ncy anal analys ysiis
Sensitivity Factors •
•
•
•
The pr prob oble lem m of st stud udy yin ing g th thou ousa sand ndss of po poss ssib ible le ou outa tag ges becomes very difficult to solve if it is desired to present the results quickly. It is easy to solve it with linear sensitivity factors. These factors show the approximate change in line flows for changes in generation on the network configuration. con figuration. There are two types;
1. Gener Generation ation Shift Factor Factorss 2. Lin Linee Outag Outagee Distr Distributio ibution n Facto Factors. rs.
Generation Shift Factors •
Where , l= line index, i= bus index
•
•
•
•
= change in megawatt power flow on line l when a change in generation, ∆Pi occurs at bus i. ∆Pi=
change in generation at bus i.
It is assumed that the change in generation , ∆Pi , is exactly comp co mpen ensa sate ted d by an op oppo posi site te ch chan ange ge in ge gene nera rati tion on at th thee reference bus, and that all other generation remain fixed. fix ed. The The fact fa ctor orss th then en re repr pres esen ents ts th thee se sens nsit itiv ivit ity y of th thee fl flow ow on line l to a change in generation at bus i.
Cont inue………….. •
If the the gen genera erator tor was ge gener nerati ating ng
MW and it was los lost, t, the then n
new power flow on each line in the network is given by by,, Where,
= flow on line l after the gener generator ator on bus i fails fails.. = flow before the failure.
•
•
The outage flow on each line can be compared to its limit and those exceeding their limit flagged for alarming. This would tell the operations personnel that the loss of the generator on bus i would result in an overload on line.
Line out outa age dist dist ribut butiion fa fact or •
•
It is ap apply ply to th thee te test stin ing g for ov over erloa loads ds whe when n tr tran ansm smis issi sion on circuits are lost.
Where , = line outage distribution factor when monitoring line l after an outage on line k. = Change in MW flow on line l. = Original flow on line k before it was outage.
The flow on line l with k out ou t can be given by, by,
Cont inue…….. •
Where, ,
= Preout Preoutage age flows on lines l and k, respe respective ctively ly..
= Flow on line line l with with line k out. •
By precalculating the line outage distribution factor a very fast procedure can be set up to test all lines in the network for overload for the outage of a particular line.
AC Power flow method
Continue………. •
•
•
•
This procedure will determine the overloads and voltage limit violations accurately. Drawback ::- Its take more time to execute. ex ecute. If the list of outages has several thousands entries then total time to test for all of the outages can be too long. Solution:-Select contingencies in such a way that only those Solution:-Select that are likely to result in an overload or voltage limit violation will study. study. The other cases will go unanalyzed. u nanalyzed.
Con ontt inu nue… e…..
Continue….. •
There are two sources of error can arise during selecting the bad cases from full outage case list.
1. Placing Placing too many many cases cases on the the short short list. 2. Skipping cases.:-A cases.:-A case that that would would have have shown a problem problem is not placed on the short list and results in possibly having that outage take place and cause trouble without the operators being warned.
Con ontt inge ngency ncy sel selec ectt ion on(1P (1P1Q 1Q)) •
•
To measure how much a particular outage might affect the power system, performance index (PI) is used.
The PI will be small value if all flows are within limit, and it will be large large if one or o r more lines are overloaded.
1. Calc Calcul ulat atee PI value value fo forr eac each h li line ne and order order th them em fro larges largestt value to least. The lines corresponding to the top of the list are then the candidates for the short sho rt list. 2. Simply ordere ordered d the PI table table and then picked picked the the top Nc entries entries from the list and placed them on the short list.
Continue….. •
1P1Q method is used to perform an outage case selection.
•
Decoupled power flow is used.
•
The solution procedure is interrupted after one iteration.
•
Advantage:-
1. Give Give suffici sufficient ent inf inform ormati ation on in the soluti solution on at the end of the first iteration of the decou oup pled power flow to give a reasonable PI. 2. The voltag voltages es can also be included included in the the PI. PI.
