IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 23, NO. 2, MAY 2008 747

Blocking of Zone 3 Relays toPrevent Cascaded Events

Seong-Il Lim, Member, IEEE, Chen-Ching Liu, Fellow, IEEE, Seung-Jae Lee, Member, IEEE,Myeon-Song Choi, Member, IEEE, and Seong-Jeong Rim, Member, IEEE

Abstract—Defense systems are needed to prevent catastrophicfailures of a power grid due to cascaded events. Cascaded eventscan be attributed to improper operations of protective relays. Themost challenging problem for the design and implementation ofa defense system is the performance in accuracy and speed in areal-time environment. Protective devices are normally designedto operate fast in order to isolate the fault(s). This paper proposesa new methodology to distinguish line overloads from actual faultsfor distance relays. In order to distinguish between line flow trans-fers from a line outage and an actual fault, the line outage distribu-tion factor (LODF) and generation shift factor (GSF)-based powerflow estimation method, and a secure peer-to-peer (P2P) commu-nication structure are adopted. Computer simulations of cascadedevents for a six-bus system and the Korean power grid have beenperformed to establish the feasibility of the proposed scheme.

Index Terms—Blackouts, cascaded events, defense system, dis-tance relay, zone 3 relay.

I. INTRODUCTION

POWER grids are among the most important critical in-frastructures in a modern society. The effects of a major

blackout have become more serious as a result of the wide areainterconnections. Due to the lack of transmission enhancementduring the last decades and the resulting low security margins,the risk of major outages for the power grid has also increased.Power systems should be designed to maintain a normal oper-ating state in case of N-1 and some N-2 contingencies. Majoroutages are possible when a single contingency event evolvesinto N-2 or N-3 contingencies due to incorrect settings of pro-tective relays [1], human errors [2], communication failures ofthe control system, and protective relay malfunctions [3]. It isnot practical to maintain security margins for all N-2 or moreserious N-k contingencies from an economic viewpoint [4]. Offline analysis has indicated that cascaded events can be mitigatedif a small amount of load can be shed quickly. In order to preventcascaded events, research to develop defense systems has beenconducted. Most of defense systems perform real-time data ac-

Manuscript received December 27, 2006; revised August 9, 2007. This workwas supported by the Ministry Science and Technology of Korea and KoreaScience & Engineering Foundation through the ERC (Engineering ResearchCenter) Program. Paper no. TPWRS-00901-2006.

S.-I. Lim is with the Department of Electrical Engineering, Kyungnam Uni-verstiy, Masan, Korea.

C.-C. Liu is with the Department of Electrical and Computer Engineering,Iowa State University, Ames, IA 50011 USA.

S.-J. Lee, M.-S. Choi and S.-J. Rim are with the Next Generation Power Tech-nology Center, Myongji University, Yongin, Korea.

Digital Object Identifier 10.1109/TPWRS.2008.919417

quisition, decision making and remedial control during the evo-lution of cascaded events.

A good summary of the concept of adaptive protection whichis expanded to wide area emergency control using modernsensor, computation and communication systems is given in[5]. A special protection system (SPS) or remedial actionscheme (RAS) is defined as a system that detects an abnormalstate and performs corrective actions [6]. An SPS is effectivefor pre-determined specific scenarios but it is not able to handleunexpected situations that are not the pre-determined events.A more general defense system is needed to deal with possibleevents in general. The strategic power infrastructure defense(SPID) is introduced in [7]. The primary function of SPIDis to prevent major blackouts using self-healing and adaptivereconfiguration capabilities based on vulnerability assessmenttechniques. A self-healing scheme proposes a plan to separatethe power grid into islands at a reduced capacity while gen-eration-load imbalance in each island is minimized [8]. Anoverview of defense systems to mitigate catastrophic failureshas been introduced in some countries, e.g., Brazil [9], France[10] and Canada [11]. A feasibility study to evaluate the ideaof real-time communication to prevent negative effects of relayoperations has been performed in [12].

Once cascaded events are initiated for various reasons, thezone 3 elements of distance relays play an important role duringits propagation [13]. If a transmission line experiences overloadas a result of line flow transfer from another outage line, thezone 3 relay may disconnect the line based on the fault clearingscheme. The tripping may trigger more serious line flow trans-fers to other lines and other overloaded lines can also be trippedby zone 3 elements by the same mechanism. Even if a defensesystem can gather information and make a decision to controlthe situation, it is very difficult to interrupt the propagation ofcascaded events because protective relay operations are veryfast.

In this paper, a method to block zone 3 operations of distancerelays is developed to secure time for defense systems to take re-medial actions in order to mitigate catastrophic failures. To dis-tinguish between actual faults on the protection zone with lineflow transfers from other disconnected lines, a real-time powerflow calculation technique and Internet based secure communi-cation are used.

The organization of this paper is as follows. In Section II,previous blackouts are analyzed to clarify the role of distancerelays in cascaded events. Section III describes the software al-gorithm and hardware structure for the proposed defense tech-nique. Simulation cases to establish the feasibility of the methodis reported in Section IV.

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748 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 23, NO. 2, MAY 2008

Fig. 1. Characteristics of a distance relay.

II. ZONE 3 PROBLEM IN CASCADED EVENTS

In order to clarify why distance relays can be a cause of thecascaded events, the characteristics of a distance relay such asthe blinder and power swing blocking (PSB) elements are de-scribed in this section. The role of distance relays in the August2003 blackout in the eastern U.S. is analyzed [14].

A. Characteristics of a Distance Relay

In order to determine whether a fault exists in the protectionzone, a distance relay calculates the impedance using the voltageand current measured at one end of the transmission line. Sincethe impedance calculated by the distance relay is subject to er-rors, three protection zone schemes are widely used in trans-mission line protection. Zone 1 is normally set at 85% of theline impedance and generates the tripping signal without delayin order to isolate the faulted section from the network quickly.The main purpose of zone 2 is to cover any fault on the pro-tected line; zone 2 protection operates with a 15–30 cycle timedelay to ensure its coordination with the zone 1 protection. Zone3 is used as a backup protection for the case when the primaryprotection cannot clear the fault on the next line. Therefore theprotection region of zone 3 is wide and could operate in the pres-ence of heavy load. When lines are isolated for some reason, thepower flow of the faulted line will be transferred to other linesthat are in operation. The Zone 3 element of a distance relaytrips the breaker to clear the overload and sometimes it actuallyaggravates the propagation of cascaded events. In this case, thezone 3 operation is not a mis-operation because it trips the lineaccording to the pre-designed protective scheme. However, itsoperation is considered inappropriate because it may contributeto the causes of a major outage.

On the impedance plane of a distance relay shown in Fig. 1,the impedance locus stays at A under the normal load condi-tion and moves to B along the line as the load increases. Theline C called “blinder” is used as a threshold to distinguish be-tween a fault and a heavy loading condition. The blinder isgenerally set at 80% of transmission line capacity. In the pre-vious major blackouts, the impedance locus of the distance relaymoved from A to B along with the load increase due to the lineflow transfer. Setting of the blinder element is very important forthe prevention of cascaded faults. Another important element isPSB which is represented as line G in the Fig. 1. When the power

Fig. 2. Initial state of 2003 North America blackout.

swing occurs due to the energy mismatch in case of a fault, theimpedance locus moves into the operation region along line D.Movement of the impedance locus in case of a power swing isusually slower than a faulted situation. In order to discriminatebetween a fault and a power swing, the passing time of region Eis used as a criterion. The PSB element blocks the trip signal incase the passing time is lower than the threshold, so a distancerelay may not operate under the swing situation at the beginningof a topology change.

It is difficult to assess whether a relay operation under over-load is correct or not. However, if it is obvious that tripping con-dition is caused by load transfer, blocking the line trip is a betteroption unless the line thermal limit is reached or exceeded.

B. Role of Zone 3 in the 2003 North America Blackout

An analysis of major blackouts in the past shows that prop-agation of the cascaded events may be divided into four steps.The first step is the occurrence of simple faults. Since the powersystem is designed to withstand N-1 contingencies, the systemcan maintain a normal operating state under single contingen-cies. The second step is to evolve into N-2 or even more severecontingencies. Typical causes include failures of protective re-lays, human errors, incorrect analysis of the power system, si-multaneous occurrence of multi-faults and the control systemfailure. In the second step, overloads are observed due to lineflow transfers from a disconnected line to other healthy lines. Inthe third step, the zone 3 elements of distance relays trip over-loaded lines, leading to even more serious line flow transfersto other lines. Complications of the problem can be caused byangle stability, voltage stability, overload, and over- and under-voltages. In the fourth step, the power system can be split intoseveral islands and collapses.

The August 2003 blackout in the eastern U.S. is a good ex-ample of the above classification. On August 14, 2003 at 15:05,three of individual lines were tripped due to tree contacts inOhio. The tripped lines are labeled A, B, C in Fig. 2. Each ofthe faults is within a security tolerance of the system so it isstep 1. Since three single events occurred simultaneously, it canbe regarded as a step 2 condition. Zone 3 element installed inSammis-Star line which is 1 in Fig. 2 trips the overloaded linedue to the line flow transfers from the isolated three lines. All

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LIM et al.: BLOCKING OF ZONE 3 RELAYS TO PREVENT CASCADED EVENTS 749

neighboring lines, 2 to 9, are de-energized sequentially by zone3 trips caused by line flow transfers and this is step 3. Finallythe system isolated and the entire system collapsed in step 4.

From the review of previous events, step 1 and step 2 cannotbe prevented due to the nature of the power system. In step 3,zone 3 of distance relay trips overloaded lines and initiates cas-caded events. Power system analysts indicate that a catastrophicfailure can be mitigated if load shedding has been performed inthe early stage of cascaded events. In order to prevent propa-gation of cascaded events, various defense systems have beenproposed. The most challenging problem of a defense system isto meet the time constraint to acquire data, make decision andtake remedial actions due to the fast operation of distance re-lays. This paper proposes a new method to secure time delaysfor remedial actions of defense systems through blocking of thezone 3 tripping in order to avoid further cascaded events. Themethod to distinguish between load transfer and actual faultswill be discussed in Section III.

III. ZONE 3 BLOCKING METHOD

A zone 3 blocking method called adaptive distance relayscheme (ADRS) is developed in this research. In this section,the overall system configuration is described first. Then, severalcommunication issues such as reliability, speed and cybersecurity are addressed.

A. ADRS Architecture

ADRS consists of one central control unit (CCU) installedin the control center and several regional control units (RCUs)deployed at the substations. The functions of CCU and RCUsare as follows.

— CCU calculates the initial line outage distribution factors(LODFs) and generation shift factors (GSFs) for the en-tire power grid and downloads the results to the RCUs.The topology data is acquired from the energy manage-ment system (EMS) and partially from RCUs if they areavailable at the substations.

— RCUs discriminate between faults with power flow trans-fers and supervise distance relay tripping actions. Localmeasurements at the substation are used by the RCUs.RCUs at different substations communicate with one an-other.

A typical scenario will be used to explain how to prevent cas-caded events based on the proposed ADRS. Fig. 3 shows an ex-ample power system that consists of 12 buses, six RCUs, CCUand EMS. In the normal state, CCU acquires the power systemdata such as topology, breaker status, generations, loads, volt-ages and angles from the EMS and RCUs. CCU calculates theLODF and GSF matrix based on the data and downloads theresults to RCUs. When the fault occurs on line L1, protectiverelays detect the fault and trip breakers BK1 and BK2 to isolatethe faulted section from the system.

RCU1 and RCU3 recognize the outage of line L1 and broad-cast the topological change to every other RCUs. Each RCU isassigned all lines adjacent with the bus where the RCU is lo-cated. After a broadcasted topological change is received by an

Fig. 3. Configuration of the ADRS system.

RCU, the RCU will estimate the power flows on the lines as-signed to this RCU. In this scenario, RCU2 detects an increasein power flow on line L2. The increase is so significant that azone 3 relay there detects that the operating condition is withinthe zone 3 reach. Note that the zone 3 protection here is not forline L1; rather, it is for another line. RCU2 uses the LODF toestimate the power flow following an outage of line L1. The esti-mated values are compared to the actual measurements availablefrom the same substation. Since the power flows are consistent,it is judged that the line being protected by the zone 3 relay isthe cause of the power flow change. Therefore, it is undesirableto trip zone 3 and should be blocked. Once the zone 3 relays areblocked, there is some time available to determine and take re-medial actions such as load shedding. If a load shedding actionis triggered by the system condition, the line flows will changeas a result and the proposed method will monitor the new systemcondition and determine if any zone 3 blocking is still necessary.

According to the line tripping record of August 2003 blackoutin the eastern U.S., the time interval between two line tripsduring the cascaded events varies from 1 second to more than1 minute. Hence, there is time available to block zone 3 trip-ping actions. The purpose of the proposed ADRS is to performthe blocking actions for these zone 3 relays. The line overloadcondition that still persists will rely on remedial actions from adefense system or a similar control system.

B. Communication Issues

ADRS requires peer to peer (P2P) communication to ac-quire the information from other RCUs in the system. A fixeddedicated communication channel with proprietary protocolsis widely used for power system control due to its reliability,speed and cyber security. But P2P communication using a fixedchannel is not only expensive but also inflexible for the futureexpansion. In recent years, the communication technologyhas improved tremendously. Optical fibers have enormousbandwidths and the Quality of Service (QoS) function providesan acceptable reliability [15]. Therefore, it is a good option touse shared communication networks, i.e., Internet for ADRS.Several issues regarding communication such as cyber security,reliability and speed are discussed here.

— Cyber security: virtual private network (VPN) technologyallows only authorized operators to access the data using

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750 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 23, NO. 2, MAY 2008

Fig. 4. Communication structure of the ADRS system.

security mechanisms. The data packet encapsulated at thesecure gateway (SG) and transmitted through a crypto-graph tunnel. Fig. 4 shows an IP security based VPN solu-tion for ADRS to protect unwanted controls from hackers.The encryption time is the primary concern. To address thisissue, the proposed method uses secure gateway which isa dedicated device to encrypt data packet and changes IPaddress in this structure. Therefore the proposed commu-nication method is expected to be sufficiently fast.

— Speed: The data transmission time cannot be guaranteedbecause Internet relies on packet switching but not circuitswitching. ADRS has some time available for communica-tion as the zone 3 operation time is about 1.6 s. A commu-nication within approximately 500 ms would be acceptablefor the ADRS.

— Reliability: The TCP/IP protocol is designed to guaranteecommunication reliability. The TCP layer ensures that alldata arrives at the end of RCUs by checking for possiblemissing packets and initiate retransmission of the data ifthere are missing packets. IP is a routing protocol thatsearches alternative paths in case of a communication net-work trouble.

C. Cascaded Events Discriminating Algorithm

Since the power system model is linear based on the dc powerflow, the voltage angle variation according to the real powervariation could be expressed as

(1)

The line outage distribution factor (LODF) is defined aspower flow increase ratio when the other line on the network isdisconnected. In [16], when line is monitored after an outageon line , LODF is designated and it is defined as

(2)

where

reactance of th line ;

, th element of matrix in (1);

, bus IDs surrounding line ;

, bus IDs surrounding line .

The generation shift factor (GSF) means line power flowchanging ratio with respect to generation changes and is definedby [16]

(3)

where

ID of monitored line;

ID of power injection bus;

reactance of line ;

, th element of matrix in (1);

, bus IDs surrounding line .

The flow of line following an outage on line and generationtrip on bus can be calculated by

(4)

where

flow of line after outage;

flow of line before outage;

generation of bus ;

flow of line .

After the power flow changes take place, the proposed methodmust determine which zone 3 relay(s) to block within 0.5 s, in-cluding computation, communication and control times. As aresult, it is believed that dc power flow is an acceptable com-promise for practical reasons.

— Active power is the major concern in discriminating be-tween actual faults and heavy loading conditions. The ef-fect of reactive power is small relative to that of activepower because only the resistance part of impedance isused for the blinder element of a distance relay.

— The power flow calculation in the proposed approach doesnot require a high level of accuracy. In this paper, LODF isused to estimate the effects of a topological change on theneighboring lines. Our interest is in determining whethera fault occurs and not the exact power flow changes.

For a multiple outage case, there is a general method to calcu-late matrix. matrix could be updated using (5) when thereis an outage on the line between bus and [17]

(5)

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LIM et al.: BLOCKING OF ZONE 3 RELAYS TO PREVENT CASCADED EVENTS 751

When a fault occurs on the line and the line is correctly re-moved by the protective relay, the power flow on this line will betransferred to other lines. As a result of an overload on anotherline, zone 3 element of distance protective relay could trip theline, leading to a more serious situation. The important task isto discriminate whether the overload is due to an actual fault orload transfer. In order to identify whether the overload of theselines is caused by line flow transferred from the removed lineor not, the on-line measured post-fault flow can be comparedwith the estimated post-fault flow which can be calculated frompre-fault line flow, LODF and GSF. If the difference is withina tolerance, it is certain that the overload is caused by line flowtransferring. Otherwise, there should be an internal fault. Thecriterion can be presented as follows:

(6)

where

measured post-fault line flow;

estimated post-fault line flow;

error margin.

The error is intended to allow the differences between a mea-sured flow and the (corresponding) computed flow. A measuredflow has intrinsic errors in meter measurements. The computa-tion is based on dc power flow which is also based on a numberof assumptions to simplify the power flow model. If the twonumbers are within the sum of measurement error and compu-tational error, the conclusion of the proposed method about lineoutage is validated.

The proposed method is not intended for zone 2 relays. Theoperating time for zone 2 is much shorter, i.e., 250–500 ms. Itis less certain that the computation, communication and controlactions can be completed within 250 ms. The other considera-tion is that zone 2 reach (impedance) is much smaller than thatof zone 3. It is less likely that heavy loading conditions can enterthe zone 2 reach.

IV. CASE STUDY

In order to study the feasibility of the proposed algorithm,computer simulations have been performed for two differentpower systems. One is a simple six-bus system with the proce-dure step by step together with all data needed for the calcula-tions. The other case here is the Korean power grid that demon-strates the feasibility of the proposed method.

A. Six-Bus System

Fig. 5 shows a system that consists of six buses includingthree generators, three loads and eight lines. The generations,loads and the corresponding MW power flows on the lines in anormal operation state are given in Fig. 5.

Table I provides the settings of blinder elements for the dis-tance relays on each line of the six-bus system. The settings are

Fig. 5. Six-bus system in normal operation condition.

TABLE IBLINDER ELEMENT SETTING IN THE SIX-BUS SYSTEM

TABLE IILINE IMPEDANCES OF THE SIX-BUS SYSTEM

Fig. 6. Six-bus system with outage on the line L1.

calculated by the authors according to the setting guideline ofKorean Electric Power Cooperation (KEPCO).

Table II gives the impedances and admittances of the eighttransmission lines.

Suppose there is a fault on line L1. Protective relays open thebreakers to isolate the fault. Fig. 6 shows the power flows ofother lines following the outage of line L1. The Matlab PowerSystem Tool Box is used to simulate this case. The power flowresults are shown in Fig. 6.

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752 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 23, NO. 2, MAY 2008

TABLE IIIIMPEDANCES SEEN BY DISTANCE RELAYS

TABLE IVLODF MATRIX IN NORMAL OPERATION STATE

TABLE VCOMPARISON OF SIMULATED AND ESTIMATED FLOWS

The impedances seen by distance relays after the line L1outage are calculated by voltages and currents from Matlabsimulations; the results are provided in Table III. Since theimpedance seen by the distance relay protecting line L1 in-stalled in bus B1 is , as shown in Table III. Thisvalue is smaller than its blinder element setting, which is 576.6.As a result, line L2 will also be tripped by the correspondingrelays, initiating a scenario of cascaded events. Line L3, L4,L5, L6 and L7 will also experience overload after line L2 isde-energized. The final result of the cascaded events is a systemcollapse.

The LODF matrix in the normal operation state is calculatedand shown as Table IV.

When line L1 is de-energized, the MW power flows on alllines are estimated and given in Table V. The second row ofTable V is the time domain simulation results using the Matlabpower system tool box while the third row is calculated based onthe LODF matrix. The estimated power flow on line L2 is 100.8MW, which is not too different from the measured value of 103.2MW. This difference is within the error margin in criteria (6).

Fig. 7. Korean Power System. (a) Overall Korean Power Grid. (b) Transmis-sion lines in North East.

Therefore, RCU will conclude that the overload on line L2 isnot caused by faults but by the line flow transferred from lineL1. The line tripping signal will be blocked by RCU to preventcascaded events.

The overloading condition can be alleviated by load sheddingand/or network reconfiguration. The 10 MW of load sheddingand generation reduction on Bus B1 and B4 are sufficient tomove the impedance locus out of the operation region of thedistance relay for line L2.

In the simulation cases of this paper, Table V shows the dif-ferences between the power flow estimation based on LODFwith time domain simulation using the Matlab power systemtool box for the 6 bus system. The computational error in thistable is about 2.3%. The measurement errors are 10% which isconsisted of 1% CT error, 1% PT error, 3% device errors and5% calculation errors.

B. Korean Power System

In order to establish the practicality of the proposed method,this study case uses the Korean power grid. Simulations usingPSS/E have been performed for the Korean system, which in-cludes 244 generators, 1003 buses and 1520 transmission lines.Fig. 7(a) is an illustration of the power system that shows thelocations of major generations and loads as well as the powerflow directions in a normal operating condition. Fig. 7(b) showsthe north east part of the power grid that is enclosed in a dottedellipse in Fig. 7(a). In Fig. 7(b), solid lines represent double cir-cuit 345-kV lines while dotted lines represent the 154-kV net-work. A large amount of power generated by G1, which has a6000-MW capacity, is delivered from bus B6 to bus B1 and B2through lines L1, L2, L3, L4, L5 and L6. Each line of the 345-kVsystems has a transmission capacity of 2192 MW.

The numerical results are obtained by PSS/E simulations.Suppose there is a double fault on line L1 #1 and line L2 #1.The cases with and without RCU will be compared. Table VIshows the MW flows as a ratio to the line capacity. Supposethat blinder elements for all distance relays are set at 94 ohmwhich corresponds to 100% of the line capacity. In a normal

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LIM et al.: BLOCKING OF ZONE 3 RELAYS TO PREVENT CASCADED EVENTS 753

TABLE VICASCADED EVENTS LEADING TO A SYSTEM COLLAPSE

TABLE VIITRANSFER TIME TO RCU INSTALLATION CANDIDATES

operating condition, the system is strong because all transmis-sion lines are double circuits and most of MW loadings ratiosare less than half of its capacity. However, when the N-2 con-tingency occurs, line L1 #2 is overloaded by flows transferredfrom the faulted lines. The Zone 3 relay installed at bus B6 tripsthe line. As the cascaded events continue, the heavy load on lineL1 #2 is transferred to line L2 #2 and line L2 #2 is also trippedby a zone 3 relay at bus B6. These cascaded events propagate toline L3 and the double circuits of line L3 are tripped by overloadas well by relays at Bus B6. Later, 6000 MW of the large gener-ation G1 is isolated from the network and the remaining powersystem encounters a serious shortage of generation, leading toa system collapse.

A catastrophic failure can be avoided using the ADRS system.When the zone 3 relay generates a tripping signal to open line L1#2, RCU installed at bus B6 is able to know that overload is notdue to faults but load transfers following the proposed scheme.The ADRS system blocks the trip signal of zone 3 relay andrelieves the overload of the line by load shedding. In order toreduce loading of line L1 #2, 300 MW load shedding on loadLD1 and generation decrease on G1 is needed but it is muchbetter than the worst case outcome of a major blackout. Theoutage load can be restored quickly by increased generation ofG4.

In order to verify ADRS can operate within zone 3 delay time,packet transfer test through the internet has been performed.Table VII shows the packet transfer time from Yongin city inKorea, to site candidates for installation of RCUs. From the re-sults, the communication times for the proposed implementationare well within 500 ms.

Two observations from the case study are mentioned here.One is that system connectivity has to be checked before calcu-lating the inverse matrix because isolated power system has asingularity problem of the matrix. The other is that the division

by zero error could disable the software when a radial line isisolated.

In Korea, the voltage levels are 345 kV and then 154 kV. The154 kV is primarily for radial subtransmission lines. The powerflow level on each 154-kV line is not significant from a systempoint of view. Therefore, the proposed method depends only onRCUs for 345-kV lines. In contrast, in other countries, 230-kVlines may be an important part of the transmission grid. Hence,the proposed method will require monitoring of these 230-kVlines as well.

V. CONCLUSIONS

Cascaded events caused by zone 3 elements of distance re-lays due to transmission line overload can be a cause of a powersystem’s vulnerability. A proposed adaptive distance relay al-gorithm in this paper provides a new concept to secure timeto perform remedial controls by a defense system during cas-caded events. In order to distinguish between actual faults andflow transfers, the line outage distribution factor based real-timepower flow estimation method is used. The simulation cases fora six-bus system and the Korean power grid are presented todemonstrate the feasibility of the proposed system.

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754 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 23, NO. 2, MAY 2008

Seong-Il Lim (M’05) received the B.S., M.S., andPh.D. degrees from Myongji University, Yongin,Korea, in 1994, 1996, and 2004, respectively.

He was with Electric Power Research Institute ofKEPCO and then a Visiting Scholar at Iowa StateUniversity, Ames. Currently, he is a Professor atKyungnam University, Masan, Korea. His researchfields are IT applications to power systems andprotective relaying.

Chen-Ching Liu (F’93) received the B.S. and M.S.degrees in electrical engineering from NationalTaiwan University, Taipei, Taiwan, R.O.C., in 1976and 1978, respectively, and the Ph.D. degree fromthe University of California, Berkeley, in 1983.

He served as a Professor at the University ofWashington, Seattle, from 1983–2005. Currently, heis Palmer Chair Professor at Iowa State University,Ames.

Dr. Liu received the IEEE PES Outstanding PowerEngineering Educator Award in 2004 and an Attwood

Associate Award from the U.S. National Committee of CIGRE in 2006. Heserved as Chair of the IEEE PES Technical Committee on Power System Anal-ysis, Computing, and Economics.

Seung-Jae Lee (M’88) received the B.S. and M.S.degrees from Seoul National University, Seoul,Korea, in 1979 and 1981, respectively, and the Ph.D.degree from University of Washington, Seattle, in1988.

Currently, he is a Professor at Myongji University,Yongin, Korea, and Director of the Next-GenerationPower Technology Center. His major research fieldsare protective relaying, substation automation, anddistribution automation.

Myeon-Song Choi (M’96) received the B.S., M.S.,and Ph.D. degrees from Seoul National University,Seoul, Korea, in 1989, 1991, and 1996, respectively.

He was a Visiting Fellow at Pennsylvania StateUniversity, Philadelphia, in 1995. He is currently aProfessor at Myongji University, Yongin, Korea. Hisresearch interests include power system control andprotection.

Seong-Jeong Rim (M’99) received the B.S. degreefrom Seoul National University of Technology,Seoul, Korea, in 1991 and the M.S. and Ph.D.degrees from Soongsil University, Seoul, in 1993and 1998, respectively.

He was with the Electro-Mechanical ResearchInstitute, Hyundai Heavy Industries. Currently, he isa Research Professor at the Next-Generation PowerTechnology Center of Myongji University, Yongin,Korea. His research fields are protective relaying andreliability assessment.

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