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COMPOSITE SYSTEM RELIABILITY EVALUATION INCORPORATING PROTECTION SYSTEM FAILURES M Chehreghani Bozchalui Department of ECE Faculty of Engineering University of Tehran Iran m chehreghani ece ut ac ir M Sanaye Pasand Department of ECE Faculty of Engineering University of Tehran Iran msanaye ut ac ir M Fotuhi Firuzabad Department of Electrical Engineering Sharif University of Technology Tehran Iran fotuhi sharif edu Abstract Protection system malfunction play a significant role in the sequence of events leading to power system blackouts This paper describes power system reliability evaluation incorporating protection system failures A reliability model is used in order to determine the impact of protection system failure on power system reliability The mechanism and scheme of protection and their hidden failure are analyzed based on their contribution to the cascading outage after occurrence of a fault A number of reliability indices such as LOLP EENS and ECI are calculated to describe the impact of protection system failures on reliability of power system Keywords Power System Reliability Protection Systems Hidden Failure Cascading Outage 1 Introduction Recent studies show that power protection systems have played significant role in the birth and propagation of major disturbances 1 Historically in power systems protection system dependability the ability to trip when required has taken priority over system security the ability to refrain from tripping when not called for On the other hand in the deregulated power systems the ability to transfer power reliably through a network becomes very important Until protection systems prefer dependability over security the probability of their incorrect operation increase According to 2 security and dependability are intertwined in a protection system The existing protection system with its multiple zone of protection is biased toward dependability and is designed to be dependable even at the cost of global system security 3 Hence vast majorities of relay miss operations are unwanted trips and have been shown to propagate major disturbance These miss operations are noted as the hidden failures which remain dormant when every thing is normal and are exposed as a result of other system disturbance 4 In 5 the list of hidden failures is well documented A study by the North American Electric Reliability Council NERC shows that protective relays are involved in about 75 percent of major disturbance 6 Most blackouts are somehow related to the protection system hidden failures Large scale power system blackout is a rare event However when it occurs the impact on the system is catastrophic 7 In spite of its importance the impact of protection system malfunction on overall system reliability has not been well studied In most reliability studies protection systems are generally assumed fully reliable It is therefore necessary to develop reliability study concerning the protection system This paper describes power system reliability evaluation incorporating protection system failures A reliability model is developed in order to determine the impact of protection system failure on power system reliability Some other researches have used Monte Carlo approach 9 In this paper we use state selection method with a heuristic analytical approach 2 Protection Failure Modes Protection systems have two basic failure modes failure to operate and inadvertent or undesired tripping 8 A power system network is in a continually operating state and hence any failure manifests itself immediately A protection system however remains in a dormant state until it is called on to operate Failures which occur in this system during the dormant state do not manifest themselves until the operating request is made when of course it will fail to response These failures have been defined as unrevealed faults So phenomenon of stuck breaker is included in failure to operate mode This type of failure will directly cause at least one bus isolation in the system because the faulted line will be isolated by backup protection Undesired tripping which is due to a spurious signal being developed in the system thus causing breakers to operate inadvertently manifest itself immediately when it occurs Undesired tripping however makes the problem complicated due to various protection system hidden failures 4 There are two types of undesired tripping one is unwanted tripping that occurs in the absence of any abnormal state that can be remedied immediately by auto reclosure and another is tripping for faults outside the protection zone 9 Tripping for faults outside the protection zone is the main cause of cascading outages 3 Analysis Models and Assumptions 3 1Component Protection Model There have been a number of models developed to facilitate reliability evaluation including protection system failure 9 10 11 12 Model of current carrying component paired with its associated protection system which is proposed in 0 7803 8886 0 05 20 00 2005 IEEE CCECE CCGEI Saskatoon May 2005 486 Authorized licensed use limited to NORTH CHINA ELECTRIC POWER UNIVERSITY Downloaded on May 16 2010 at 08 16 44 UTC from IEEE Xplore Restrictions apply Incorrect setting therefore they are more sensitive to fault and abnormal operating conditions Reference 3 proposed a model of hidden failure probability Fig 2 Distance protection failure probability of exposed line PZ distance protection failure probability Z impedance seen by relay Z3 zone 3 impedance setting of tripping the exposed line as a function of impedance seen by distance relay This model with some modifications is shown in Fig 2 It suits for the situation during fault We choose zone 3 impedance setting as 250 of line impedance After the initial fault cleared power flow in the system would change due to the changing system topology This might lead to over loading problem on certain lines which are at risk to trip To present the post fault situations we propose a probability model of an exposed line tripping incorrectly as function of line flow for over current relay This model is shown in Fig 3 In Figs 2 and 3 PZ and PI are the probability of state 2 in the component protection model Fig 1 PZ and PI are protection system failure properties which are used during fault and post fault periods respectively It shows that the probability of exposed line tripping incorrectly depends on the fault and system operating condition Each line has a different probability for incorrect trip 3 3Cascading Outage A cascading outage refers to a series of tripping initiated by one component failure in the power system When a fault occurs the impact to the power system such as over current or voltage dip may cause some protection devices to miss operate The assumption by 6 is that if any line sharing a bus with a transmission line L trips then hidden failure in line L are exposed That is if one line trips correctly then all line connected to its end bus are exposed to the incorrect tripping The probability of such occurrence is small but not negligible and is considered Spread of disturbance is one dimensional in power system 13 hence the case that more than one line trip at sometime rarely happens In the simulation we let one and only one line trip at one time in specific if more than one line might trip the one with higher tripping probability is selected to be next tripping line In simulation we also assume that 1 failure to trip and undesired trip of the protection system failure doesn t overlap 2 Only first order contingency is considered to initial fault 3 All failures are mutually independent 487 Authorized licensed use limited to NORTH CHINA ELECTRIC POWER UNIVERSITY Downloaded on May 16 2010 at 08 16 44 UTC from IEEE Xplore Restrictions apply Fig 3 Over current protection failure probability of exposed line PI over current protection failure probability I current seen by relay IS over current relay setting 4 When the current carrying component is in failure state the protection system does not fail 5 Inspection of protection system dose not leads to component failure 4 Reliability and Vulnerability Indices The following indices are defined and used 8 4 1Loss of Load Probability LOLP i iii LCPLOLP where i C Available capacity in simulationi i L Load in simulation i iii LCP Probability of loss of load on simulationi A power system can withstand one outage without adequacy and security violation LOLP presents the loss of load resulting from protection system failure in simulation the series of outage is stopped as soon as loss of load occurs 4 2Expected Energy Not Supplied EENS i kii k LPEENS8760 where i P is the probability of existence of cascading outage in simulation i ki L is load curtailment at bus k in simulationi This index with units of MWh shows the impact of hidden failure on system reliability If this index is normalized it can be used to compare various power systems Normalized EENS is defined as Energy Curtailment Index ECI i kii k LS LP ECI 8760 MWh MW yr whereLSis the system total load This bulk power energy curtailment index has also been designed as the Severity Index The total energy not supplied in MW Minute is derived is divided by system load in MW severity is therefore expressed in system minute 5 Calculation of Reliability Indices The calculation of reliability indices contains two parts The first one is generating protection system hidden failure probability and second is calculating reliability indices For the first one from Markov chain in Fig 1 with known state transition rates we figure out the hidden failure probability of each protection system For this purpose we form the transition matrix according to Markov chain then using frequency balance concept we solve the generated equation Probability of state 2 undesired tripping PZ and PI is determined using Figs 2 and 3 They will be used in calculation of reliability indices In calculation of reliability indices following steps are performed 1 Select faulted line 2 Determine all lines that are exposed to miss operation 3 From fault calculations compute the impedance seen by relay for the exposed lines 4 Find the probability of tripping for each exposed line using Fig 2 5 Determine which exposed line will trip If no line trips go to 9 6 Update the exposed lines based on the newly tripped line 7 Re array the system topology if necessary and from power flow calculation calculate the current on the exposed lines 8 Find the tripping probability for each exposed lines using Fig 3 go to 5 9 Record the cascading outage and determine the amount of load curtailment Calculate the reliability indices 6 Case Study 6 1Test System The Roy Billinton Test System RBTS is used as the test system Shown in Fig 3 The basic data for RBTS can be found in 14 In addition the transition rates of component and protection systems are listed in Table I Fig 4 Roy Billinton Test System Unwanted tripping in absence of any abnormal state IS2ISI PI Incorrect setting North American Electric Reliability Council New Jersey 1984 1988 2 A G Phadke S H Hortowitz J S Throp Aspects of power system protection in post restructuring era Proc 32nd Hawaii International Conference on System Science Vol 3 Jan 1999 3 J S Thorp K Bae An Importance Sampling Application 179 Bus WSCC System under Voltage Based Hidden Failures and Relay Misoperation Proceeding of the 31st Hawaii International Conference on System Science Vol 3 1998 pp 39 46 4 A G Phadke J S Thorp Expose Hidden Failure to Prevent Cascading Outage IEEE Computer Application in Power Vol 9 No 3 July 1996 pp 20 23 5 A G Phadke S H Horowitz and J S Thorp Anatomy of Power System Blackouts and Preventive Strategies by Rational Supervision and Control of Protection System Power System Technology Program Energy Division Oak Ridge National Laboratory 1995 6 Final Report August 10th 1996 event West System Coordinating Council Oct 1996 7 S Tamronglak A G Phadke S H Horowitz and J S Thor