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高压电网继电保护选型配置与整定 赵鹏利,高压电网继电保护选型配置与整定,赵鹏利,高压,电网,保护,选型,配置

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COMPOSITE SYSTEM RELIABILITY EVALUATION INCORPORATING PROTECTION SYSTEM FAILURESM. Chehreghani BozchaluiDepartment of ECE, Faculty of Engineering, University of Tehran, Iran m.chehreghaniece.ut.ac.irM. Sanaye-PasandDepartment of ECE, Faculty of Engineering, University of Tehran, Iran msanayeut.ac.irM. Fotuhi-FiruzabadDepartment of Electrical Engineering, Sharif University of Technology, Tehran, Iran 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 IntroductionRecent 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 IEEECCECE/CCGEI, Saskatoon, May 2005486Authorized 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 & miscalibration Unwanted tripping in absence of any abnormal state Z33*Z3ZPZUnrevealed failure in correctly set Fig.1. Markov model of component and its protection system reference 9 differentiates the two protection failure modes and represents them as two states: “undesired trip” and “failure to trip”. In this paper, we use this model as shown in Fig. 1, where the following states are used: State 1: The current-carrying component and protection system are both good. State 2: The component is good but the protection is at risk for “undesired trip”. State 3: The component is good but the protection is exposed to “failure to trip”. State 4: The component is good and the protection system is being inspected. State 5: The component is failed while the protection system is still under “undesired trip”. State 6: The component is failed but the protection system is good. State 7: The component is failed while the protection system has experienced “failure to trip”. The notations in Fig. 1 are as below: i inspection rate of protective system. I repair rate of protection system. repair rate of component. failure rate of component. 1P failure rate of protection system to exposure to “undesired trip”. 2P failure rate of protection system to state of “failure to operate”. When a component is in “undesired trip” and “failure to trip”, its protection system is suffering from hidden failure and cause malfunction. 3.2Protection System Failure Properties The “failure to operate” phenomenon is not very complicated because it is only related to the protection device itself rather than system operating condition. The probability of state 3 in Fig. 1 is the probability of “failure to operate”. In the case of “undesired trip” in addition to existence of protection hidden failures, system fault and operating conditions are also related to cascading outage. Distance zone 3 and over current protection have the nearest setting value to a faulty parameter; 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 doesnt overlap. 2)Only first-order contingency is considered to initial fault. 3)All failures are mutually independent. 487Authorized 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) ()?=iiiiLCPLOLPwhere iC: Available capacity in simulationi.iL: Load in simulation i ()iiiLCP: 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)?=ikiikLPEENS8760where iP: is the probability of existence of cascading outage in simulation i,kiL: is load curtailment at bus k in simulationiThis 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): ?=ikiikLSLPECI8760 (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 IS2ISIPIIncorrect setting & miscalibrationUnrevealed failure in correctly set 488Authorized licensed use limited to: NORTH CHINA ELECTRIC POWER UNIVERSITY. Downloaded on May 16,2010 at 08:16:44 UTC from IEEE Xplore. Restrictions apply. Table I Component and Associated Protection System Data for RBTS Line Component Protection System ?(1/yr) (1/yr) 1?P(1/yr) 2?P(1/yr) i(1/yr) I(1/yr) 1,6 1.5 876 .046 .083 4 219 2,7 5 876 .0014 .2 4 219 3 4 876 .0118 .422 4 219 4 1 876 .0165 .166 4 219 5 1 876 .0165 .166 4 219 8 1 876 .0165 .166 4 219 9 1 876 .0165 .166 4 219 6.2ResultsSystem wide reliability indices are summarized in Table II. These indices represent the degree of vulnerability and load curtailment when system suffers from cascading. Table II System reliability indices LOLP EENS (MWh) ECI (MWh/MW-yr) Severity Index (minute) 0.00393 684.323 3.754 0.000428 We also calculate the individual reliability indices with the specified faulted lines. The results are shown in Table III. These indices show that which part of system is the weakest. Table III Individual reliability indices Line LOLP EENS ECI 1 1.67E-7 0.1332 7.2E-4 2 2.05E-10 0.0000144 7.78E-8 3 2.36E-10 0.0001756 9.5E-7 4 1.52E-5 5.3296 2.38E-2 5 1.52E-5 5.3296 2.38E-2 6 1.67E-7 0.1332 7.2E-4 7 2.05E-10 0.0000144 7.78E-8 8 4.29E-7 0.4697 2.53E-3 9 3.9E-3 683.28 3.6934 7 Conclusion and Future Work In this paper, based on a markov model of component paired with its protection system, behavior of power system under cascading outage is analyzed. Reliability indices such as LOLE and EENS and ECI are calculated to describe the impact of protection system failure on reliability of composite systems and cascading outages. Also calculated the reliability indices for each line individually, with this indices we could estimate vulnerable links in the system. In this paper we did not include the influence of voltage based hidden failures in cascading outage. Also the stochastic feature of power system events such as fault location, fault clearing probability and loss of load duration at each curtailment can be considered. In the future work by incorporating above factors more accurate indices for the power system can be obtained. 8 References 1 NERC Disturbance Report; 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, “Anat