A new method for protection zone selection in ... - IEEE Xplore

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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 15, NO. 3, JULY 2000

A New Method for Protection Zone Selection in Microprocessor-Based Bus Relays Bai-Lin Qin, Armando Guzman-Casillas, and Edmund O. Schweitzer, III

Abstract—Use of graph theory simplifies representation of complex bus arrangements in power system stations. This paper presents a new method, based upon graph theory, for selecting bus protection zones in microprocessor-based relays. We use a typical bus arrangement to illustrate the graphical representation of station arrangements, graph operations, and associated matrix operations. We also describe an implementation of the zone selection method and use two examples to demonstrate the advantages of the method. Using the status of switching devices in the station, the zone selection method provides the relay with real-time bus arrangement information. The bus relay uses this information to assign input currents to a differential protection zone and to select which breakers to trip for a bus fault or breaker failure. Index Terms—Busbars, graph theory, microprocessor-based bus relays, protection zone selection, protective relaying.

I. INTRODUCTION

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N POWER system networks, a bus is a connection point for many generation, transmission, or load circuits. If a fault occurs on a bus, all circuits supplying the fault current must trip to isolate the fault. A bus fault may result in considerable loss of service and severe system disturbance. Station arrangements are often designed to minimize the number of circuits that must be opened for a bus fault [1]–[4]. As a result of improved continuity of energy supplies and flexibility of system operations, some power system stations use complex bus arrangements that increase demands for sophisticated bus protection schemes [5]. Protection zone selection must be highly discriminative, such that a bus relay operates only for a protection zone fault. Traditional electromechanical and analog electronic relays have been used widely for bus protection in power systems. Most applications appear as simple bus arrangements, such as single-bus single-breaker, single buses connected with bus-tie, main and transfer bus single-breaker, double-bus double-breaker, double-bus single-breaker, and breaker-and-a-half bus. Electromechanical relay schemes are complicated, expensive, and bulky when applied in the protection of multiple bus arrangements. Previous efforts to protect complex bus arrangements used analog electronic bus relays. Haug and Forster [6] described an electronic bus relay that used directional comparison and current differential schemes to protect complex bus arrangements. This approach allocated specific protection to each bus in station arrangements. Individual protection schemes worked in combination or separately to correspond with the bus configuration Manuscript received June 21, 1999. The authors are with Schweitzer Engineering Laboratories, Inc., Pullman, WA 99163. Publisher Item Identifier S 0885-8977(00)07580-4.

used at the time. According to switching changes in station arrangements, the relay switched current direction signals for the directional comparison scheme. The relay also switched current transformer (CT) secondary current signals for the differential scheme [6]. The CT current switching for protection zone selection may result in hazards from an open CT secondary circuit. Forford and Linders introduced a medium-impedance differential bus relay that avoided these hazards [7], [8]. Switching occurred on the secondary of ratio matching CT’s, eliminating an open circuit in the main CT secondary. Also, switching at this low current level reduced hardware requirements. Forford and Linders also presented a scheme that used a bus differential relay in conjunction with a directional relay for protecting double-bus arrangements. The directional relay identified which of the two buses should trip when a fault occurred on either bus. This scheme still required switching trip circuits, but eliminated CT circuit switching [7]. Royle and Hill introduced another bus relay to protect complex bus arrangements [9]. They included auxiliary relays in the protection scheme to represent the disconnect switches in bus arrangements. Royle and Hill built standardized modular relays to replicate station bus components. The replica framework used screened multi-core cables that could be rearranged to provide varied schemes for different bus arrangements [9]. In typical analog electronic bus relays, the station bus replica accomplished zone selection. Designing, implementing, and testing replicas to suit different bus arrangements have been major drawbacks of analog electronic bus relays [10]. In microprocessor-based bus relays, software can provide bus replicas or mimics [10]–[12]. This paper presents a new application of graph theory to protection zone selection in microprocessor-based bus relays. The proposed method possesses full generality; it is applicable to any bus arrangement in power system stations. We illustrate the graphical representation of station bus arrangements, describe graph operations and associated matrix operations for zone selection, and demonstrate an implementation of the zone selection method in microprocessor-based bus relays. We conclude with two application examples for typical station bus arrangements. II. GRAPHICAL REPRESENTATION OF BUS ARRANGEMENTS In power system stations, major components of bus arrangements usually include buses, breakers, CT’s, disconnect switches, incoming and outgoing lines, etc. Interconnections of these components consist of the following branches: breaker branch, CT branch, breaker-CT branch, and disconnect branch. A breaker branch is an interconnection containing a circuit

0885–8977/00$10.00 © 2000 IEEE

QIN et al.: PROTECTION ZONE SELECTION IN MICROPROCESSOR-BASED BUS RELAYS

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Fig. 1. A simplified bus arrangement with sectionalized double-bus and single-breaker.

breaker capable of interrupting load and fault currents. A CT branch is an interconnection containing a current transformer, through which a relay obtains input current signals. A breaker-CT branch is an interconnection representing a combination of circuit breaker and current transformer. A disconnect branch refers to interconnections with disconnect switches only, through which we can change the station configuration when no load current is present. In multiple bus arrangements, a breaker-CT branch may serve as a tie breaker to connect two buses. A disconnect branch may directly connect two or more buses. A single bus may form a bus protection zone. Multiple buses solidly interconnected through disconnect switches may also form a bus protection zone. Microprocessor-based bus relays must be capable of handling all protection zone possibilities for any given station arrangement. Graph theory is a powerful analytical tool in understanding and solving large, complex problems in physical science and engineering applications [13], [14]. Electric network analysis has applied it extensively [13]. This paper uses graphs to represent power system bus arrangements in an intuitive, diagrammatic way. Representing a bus arrangement graphically can simplify understanding and investigation of station configuration, especially for bus protection zone selection. Let us suppose that a graph, symbolically represented as , consists of a set of elements called vertices and another collection of elements . If an edge links vertex called edges and vertex , we can show the relationship as . indiFor a directed graph [13], [14], the ordered pair cates that the edge is directed from vertex toward vertex . Fig. 1 shows a simplified station arrangement with a sectionalized double-bus and single-breaker scheme. There are , , , and . A central unit (CU) four buses labeled relay communicates with seven bay units (BU) installed at and seven breaker-CT branches labeled . The arrangement uses three breaker-CT branches as bus links and , branch links and , ties: branch links and . The other four breaker-CT and branch and connect branches terminate at the buses: branches and/or via disconnect branches; branches and to connect to and/or via disconnect branches. A branch of

TABLE I GRAPH REPRESENTATION OF BUS ARRANGEMENTS

disconnect switch links buses and . If disconnect is open, and there is no bus-to-bus connection switch through disconnect switches, each protection zone covers a is closed, single bus. However, if disconnect switch and . then a protection zone must include both buses Table I shows the relationship between station components and elements in their corresponding graphical representation. In the graph a dot or circle represents each vertex, and a curve or line segment with reference direction represents each edge. For a bus arrangement, construction of the associated graphical representation occurs in three steps. The first step is to select vertices representing busbars, convergence points, and termination points. The second step is to develop edges representing breaker branches, CT branches, breaker-CT branches, and disconnect branches. The final step involves combination of vertices and edges to form an easily understood and manipulated graph. Fig. 2(a) shows vertex assignment for the bus arrangement through represent the four busbars. in Fig. 1. Vertices Vertices through are the convergence points between disconnect branches and breaker-CT branches. Vertices through are termination points of the four breaker-CT branches. Fig. 2(b) shows edges, with reference direction, for the bus arrangement in Fig. 1. The reference direction can be chosen arbitrarily or assigned according to CT polarity for associated through represent the breaker-CT CT branches. Edges represent disconnect branches. branches. Edges through The status of disconnect branches (open or closed condition of through ) determines station bus configuration. Fig. 3

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Fig. 2. A graphical representation of the bus arrangement in Fig. 1: (a) Vertex Assignments. (b) Edge Assignments. (Note: The lighter-colored background image of Fig. 1 is provided as a reference.)

shows a graph with 12 vertices and 16 edges representing the bus arrangement in Fig. 1. Note that Fig. 1 shows neither a separate breaker branch nor a separate CT branch. No edges represent these two branch types in Fig. 3. This paper focuses on edges representing breaker-CT branches and disconnect branches. If a by-pass were to connect to a point between the breaker and CT in a bus arrangement, we would use different edges to represent the breaker branches and CT branches separately. III. GRAPH OPERATIONS FOR ZONE SELECTION In a graph representing a bus arrangement in power system stations such as in Fig. 1, we can classify edges into switching edges and weighted edges. The former represents the disconnect branches, and the latter represents the breaker-CT branches in the bus arrangement. We can classify vertices as bus vertices and nonbus vertices. Bus vertices represent all buses in a station, and nonbus vertices represent other vertices in the associated graph. When used in protection zone selections, graph operation involves operation on switching edges and bus vertices.

Fig. 3. Graph representation of the bus arrangement with sectionalized double-bus and single-breaker.

The first graph operation for protection zone selection is on switching edges. As shown in Fig. 4(a), if a disconnect is open, the corresponding in switching edge graph operation is removal of the edge (or opening the edge ) from the graph . Removal of produces a . The sub-graph of consists of all vertices sub-graph and edges of the original graph except the edge . Note that the


Fig. 4. e

= (v

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Graph operation on switching edges. (a) Removal of edge ), (b) Contraction of edge e = (v ; v ).

; v

end vertices remain in graph . Fig. 4(b) shows edge contraction. If disconnects in switching edge close, edge in graph contracts. Contraction of refers to the operation of deleting an edge from the graph and merging the two end vertices to constitute a single vertex. In other words, a new vertex replaces the two vertices and , so that all the edges incident at and are now incident at the new vertex. The next graph operation is on bus vertices, specifically to of a bus vertex . An incidence set find an incidence set is a set of all edges incident at a vertex, [14] in namely,

where edges are incident at the vertex . We as being define the reference direction of the incidence set toward vertex . If an edge is incident at the vertex and the direction of the edge is away from vertex , then a negative sign will appear before . On the other hand, if edge is directed toward vertex , then no sign appears (by default, the positive sign is not shown). If a bus vertex is not connected to any other bus vertices via switching edges, vertex will form a single protection zone. . If two This zone contains all edges in the incidence set and are endpoints of an edge bus vertices representing a closed disconnect branch, then a new incidence is obtained by set

where the set operation is called ring sum [13], [14]. The ring and refers to a new set consisting sum of two sets or in but not in both and of all elements in . The ring sum does not consider the direction of the edge. and will form a single protection In this case two buses . zone incorporating all edges in To illustrate, consider the graph in Fig. 3. Suppose that the , disconnects associated with switching edges

Fig. 5. Graph operations for zone selection. Open or closed status of switching edges e8–e16 determines the shape of the graph.

and are open. Fig. 5(a) shows that these open edges would disappear from the graph. If the disconnects associated with , and are closed, then Fig. 5(b) switching edges results from the operation of contracting those closed edges. In this case we can form protection zones easily by using the incidence set of each bus vertex:

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vertex, and columns, one for each edge. The matrix element is defined as if edge is incident at vertex and directed toward vertex If switching edge closes, then the contraction of edge , namely merging bus vertices and in Fig. 5(b), produces Fig. 5(c). In this case a new vertex replaces bus vertices and . The ring sum operation is

and protection zone formation is as follows:

We then proceed to close switching edge . Beis already closed, closing edge cause edge causes vertices and to connect via vertex . As shown in Fig. 5(d), three bus vertices , , and , contract into one vertex. Because the ring sum

protection zone formation is as follows:

;

if edge is incident at vertex and directed away from vertex

;

if edge

.

is not incident at vertex

is the coefficient matrix of The incidence matrix Kirchhoff’s current equations in electric networks [13]. This matrix is of considerable interest in bus differential protection refers to an inapplications. A row of the matrix . This vector represents the incidence set cidence vector of the graph . In other words, each row of indicates a vertex, as well as the incidence set matrix corresponding to the vertex. In matrix operation for zone selection, if a switching edge is open, the column of corresponding to the edge is assigned to zero, or

On the other hand, if a switching edge is closed, the operation on the matrix adds the row representing vertex to the row representing vertex and deletes the row representing vertex from the matrix. The new entry of the resulting . Here the row addition obeys the row of is following rule either for

or

is zero, ;

both for

and

are zero, ;

both for

and

are nonzero, .

This operation corresponds to the ring sum in previous graph operations. To illustrate the matrix operation, consider the graph in Fig. 3. The incidence matrix of the graph is given by

IV. MATRIX OPERATIONS FOR ZONE SELECTION The structure of a graph can be characterized also by its incidence matrix [13], [14]. For a graph with vertices and edges, the incidence matrix, denoted by the symbol , of the graph is a matrix that has rows, one for each

where the incidence vectors are shown at the bottom of the page. Refer to Fig. 5 for a sequence of operations on switching edges. To open switching edges associated with , and , simply set the elements of the columns representing those edges to zero. To contract the


switching edge , first add the row representing to the row representing vertex , then delete the vertex . Similarly, perform a sequence row representing vertex , of contractions on the switching edges , and . The resulting matrix reflects the protection zone selection. Here the resulting matrix is part of the incidence matrix of the graph in Fig. 5(b), with , incidence vectors associated with the bus vertices (see the first matrix at the bottom of the page). and Note that incidence vectors of bus vertices are only related to the weighted edges ( through ), where the current signal is available from CT’s for bus protection. The rest of the matrix ) are elements representing the switching edges ( through all zeros. is closed. Next consider when switching edge and are also If switching edges closed, matrix operations cause the row representing the conand to be added to the row representing . In nected this case the resulting matrix is the second matrix shown at the bottom of the page. Referring to the incidence set of bus vertices in Fig. 5(d), note that both graph operation and matrix operation provided identical protection zones. V. IMPLEMENTATION IN BUS RELAYS Microprocessor-based bus relays have three common protection functions: bus protection, breaker failure protection, and protection zone selection. Zone selection is a basic function for both busbar and breaker failure protection. Accurate zone selection ensures that relays operate according to Kirchhoff’s current law in choosing input currents for differential protection. Zone selection also chooses the necessary circuit breakers to trip in the event of a bus fault or an associated breaker failure. Using the graph concept, we can use either graph operation or matrix operation to select bus protection zones for any station arrangement. Graph operation is a tool for step-by-step graph manipulation, providing a clear picture for analysis of power

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system station maneuvers. Matrix operation is suitable for computer implementation. This paper uses matrix operation to implement the protection zone selection in microprocessor-based bus relays. Fig. 6 shows a block diagram the relay follows to select the protection zone. Main processing steps involved are • Input of incidence vector for all vertices • Input of logic equations for all edges • Processing for zone selection • Zone supervision and check zones A. Input of Bus Arrangement Information Suppose that NBB denotes the number of busbars and NBU denotes the number of bay units at breaker-CT branches in a power system station. The first NBB vertices and the first NBU edges depict busbars and breaker-CT branches, respectively, in the graph representing the station bus. The incidence matrix of the associated graph provides information about station arrangements. Each element in the incidence matrix depends not only on the graphic configuration, but also on the switching status of disconnects and breakers. 1) Incidence Vector Construction: From a station layout, the graph provides easy recognition of the connection branch to a specified point. In the incidence vector construction settings we would enter all branches incident at a given vertex, ignoring the status of disconnect switches and circuit breakers. For a station with a total number of vertices (NV), an incidence vector is constructed as list of edges incident at vertex separated by list of edges incident at vertex separated by

list of edges incident at vertex NV, separated by

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the switching condition of each edge. Similarly, for weighted edges, the status of breaker-CT combinations determines the condition of each edge. In a bus relay, Boolean logic equations express the condition of the edge. If all the series-connected switching devices associated with an edge are in a closed state, the edge is considered to be in its closed condition. On the other hand, if any one of the series-connected switching devices associated with an edge is in an open state, the edge is considered to be in open condition. For a graph with a total number of edges (NE), there are the following NE logic equations: logic equation establishing the link between the two endpoints of logic equation establishing the link between the two endpoints of

logic equation establishing the link between the two endpoints of The logic equation consists of Relay Word elements that reflect the status of disconnect switches and circuit breakers. B. Processing for Zone Selection

Fig. 6.

where

Zone selection processing sequence.

in the relay represents the incidence vector represents , and so on. As defined in the incidence matrix, if the direction of is away from the associated vertex, then a negative edge is sign appears before . On the other hand, if the edge directed toward the vertex, then no sign appears (by default, the positive sign is not shown). For any station with NBB busbars and NBU breaker-CT branches, an edge representing the branch of tie breakers can be identified automatically from the incidence matrix. Because the first NBB vertices and NBU edges have been reserved for representing the busbars and breaker-CT branches, respectively, edge is an edge representing a bus-tie branch if index , and . Identification of bus-tie branches is useful for special treatment of tie breakers, such as in “end zone” protection. 2) Logic Equations for Edges: Graphs of power system stations represent disconnect branches as switching edges and breaker-CT branches as weighted edges. For switching edges, the status of disconnects (closed or open) determines

Once the relay receives interconnection information, it internally creates the incidence matrix of the graph. Then, matrix operations within the relay select bus protection zones automatically. The processing for zone selection includes evaluation of edge status logic equations, graph operation based on the incidence matrix, selection of bay units in each protection zone, evaluation of zone supervision logic equations, and output of selected protection zones. Based on the switch status (Relay Word bits), the central unit will determine 1) the bus or buses to be included in each protection zone, and 2) the active bay units to be included in the associated protection zone. The central unit runs the zone selection algorithm every time the switching operation occurs in the protected station. C. Zone Supervision and Check Zones The zone selection scheme incorporates logic control equations for protection zone supervision. These logic equations provide flexibility for protection and control. Zone supervision is useful for complicated bus arrangements because it provides an approach to supervising zone selection by using switch status and other digital input information. The format of the zone supervision logic equations is Logic control equation for Zone 1 supervision Logic control equation for Zone 2 supervision

Logic control equation for Zone

supervision


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VI. EXAMPLES AND DISCUSSION A. Example 1: Sectionalized Double-Bus With Bus-Tie This paper used the bus arrangement of Fig. 1 to show the graphical representation of station arrangements, graph operations, and matrix operations. In this example we use Fig. 1 to illustrate the implementation of zone selection in bus differential protection. In Fig. 3, which graphically represents is linked to edges , and ; bus Fig. 1, bus 1 is linked to edges , and ; and so on. The 2 setting format for the incidence vectors is

Fig. 7.

Zone selection flow chart.

The relay checks the output of logic control equations to supervise zone selection. If a logic output equals logical “one,” the relay applies no supervision to the corresponding zone. If a logic output equals logical “zero,” the relay blocks the associated zone. Check zone is an additional zone setting. We can select check zones that fulfill the differential protection principle, regardless of switch operation status in the station. An active bay unit assignment to the check zone depends upon the bus arrangement. Use one or more check zones as needed. The format of check zone assignment is simple: list of active bay units separated by for check zone 1

Because and , the edges associated , , and with tie breakers are . The relay would apply any “end zone” protection to these tie breakers. To obtain switching status, enter into the relay the following information for all 16 edges:

list of active bay units separated by for check zone 2

list of active bay units separated by for check zone For simple bus arrangements such as a single-bus configuration, we can avoid the settings for incidence vector construction and edge logic equations. For such a simple configuration, we can use check zones to set the preferred zone directly. The relay will base operation of differential protection only on check zones. In summary, the inputs for the zone selection scheme include • the bus arrangement in the protected station, • the status of disconnect switches and circuit breakers, and • the Relay Word bits from other protection and control functions The outputs are • a list of selected bay units in each protection zone, and • a list of selected bay units in the check zones Fig. 7 illustrates the above method of zone selection.

For this bus arrangement, zone supervision is not assigned for any zone in the application. The check zone assignment is

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where , and represent the bay units at , and , respectively. breaker-CT branches Operation Case 1.1: Suppose all circuit breakers are closed, and disconnect switches operate with the following status:


Operation Case 1.3: If Case 1.2, the switch status is

is closed in above Operation

Then, the zone selection outcome is

Then, zone selection processing yields the following:

Operation Case 1.2: If branch is to be transferred from to bus , then buses and are solidly linked by bus and during the transfer period. The switch status is

Then, the zone selection outcome is

In this case two buses, and , are protected in one zone. is During the transfer period, the tie-breaker bay unit 1 not included in the two-bus protection zone.

In this configuration three buses , and merge into one protection zone. At the same time, the operation removes from the zone. tie-breaker bay unit 1 Note that during station switching, as in the above operation cases 1.2 and 1.3, disconnect switches may solidly connect two or more buses in the station. Such connections among buses may cause incorrect bus differential relay operations. The IEEE guide for protective relay applications to power system buses suggests two methods for making these connections without causing misoperations [1]. One method is to disable bus protection before switching. This is not a good practice, because of a great possibility for a switching error and a bus fault during switching. It is critical to have bus protection in service during this time. The second method is to reconnect the relay input currents to form a single temporary protection zone. This method requires insertion and operation of switch contacts in the CT secondary circuits and is, therefore, inconvenient for protection. Power system station switching in complex bus arrangement brings challenges to traditional electromechanical bus relays and analog electronic bus relays. Traditional bus relays use auxiliary switches to connect the secondary windings of CT’s into an appropriate zone and ensure continuous delivery of correct differential current. Traditional bus protection schemes also use auxiliary switches to interconnect the trip circuits and ensure that a bus fault or associated breaker failure trips selected breakers. Compared to the suggested IEEE methods and traditional bus relay schemes, the zone selection method proposed in this paper provides reliable bus protection. The new method provides an advanced protection without disabling bus protection during switching operations. In microprocessor-based bus relays, software logic and numerical processing replace traditional switching operations on CT secondary circuits. Because there is no CT switching, there is no danger of CT open circuit. Relay logic operations and zone selection also replace traditional switching operations on trip circuits, eliminating potential


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Fig. 9. Zone selection considers by-pass switch status in logic equations. Setting inputs to the bus relay are Incidence vectors for vertices

Logic equations for edges

Fig. 8. Double-bus, single-breaker with by-pass line switch.

Zone supervision logic equations

Bay unit assignment for check zones Fig. 9. Graphical representation of the double-bus, single-breaker with by-pass line switch arrangement.

misoperations from switching errors. Zone selection provides accurate information about which breakers need to trip.

Operation Case 2.1: Suppose that all circuit breakers are closed, and that disconnect switches have the following status:

B. Example 2: Double-Bus with By-Pass Line Switch The Fig. 1 substation arrangement, based on one of the common bus arrangements in [15], consists of commonly used components in complex bus designs. These components include multiple busbars, bus ties, and bus sectionalizing disconnect switches. The disadvantage of this bus arrangement is that line breaker problems and associated maintenance require removal of the connected circuit from service. Fig. 8 is a single-line diagram of a double-bus, single-breaker arrangement with by-pass line switch. This arrangement allows greater flexibility for power system operations than does the arrangement in Fig. 1. Either bus provides service to any line, the buses can operate together or independently, and one bus can act as a transfer bus if a line breaker is out of service [3]. Fig. 9 shows a graph representing the station arrangement and seven edges of Fig. 8 using six vertices with reference direction. Vertices and represent the two buses, and . Vertices and are the convergence points between disconnect branches and and are termination breaker-CT branches. Vertices and . Edges points of the two breaker-CT branches, through represent the breaker-CT branches, , and . Edges through represent disconnect branches. Note and from that we excluded by-pass line switches

Then, the outcome of the zone selection algorithm shows that , and zone 2 will protect bus . zone 1 will protect bus

where , and represent bay units at breaker-CT , and , respectively. branches is to be transferred from Operation Case 2.2: If branch to bus , disconnect switches and bus solidly link two buses during the transfer period. The two buses form one zone. At the same time, the operation removes the tie-breaker bay unit from the two-bus zone. The switch status is

and the result is that zone 1 covers both

and

.

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Operation Case 2.3: After the branch the operation is in the following state:

switches to bus

,

and the outcome is Fig. 10. Open tie breaker with one set of CTs.

Operation Case 2.4: Now suppose that branch is on , and that branch is on main bus . For transfer bus to be taken out of service, the operation is in the breaker closes: following condition when

Then, the outcome from matrix operations on the associated graph is:

2)

3)

But the result of zone 2 supervision logic operation is “zero,” resulting in the following final zone selection: 4)

For this station arrangement, when by-pass switch or in Fig. 8 is closed, the relay protects only the main bus. The check zone and the transfer bus zone are disabled. The transfer bus will be protected as part of the transmission line. In bus differential protection, an “end zone,” or so-called “dead zone,” is the section between a circuit breaker and the associated CT’s. Bus relays may fail to correctly detect a fault occurring in the end zone. For instance, if a fault occurs at a location between a tie breaker and its associated CT’s, as indicated in Fig. 10, the fault appears to be inside the zone of bus 1, but outside the zone of bus 2. The relay trips bus 1 unnecessarily, but bus 2 does not trip; the bus relay does not clear the fault. The zone selection method this paper describes can handle this situation easily. Simply include the tie breaker status in the logic equation of its associated edge. Then, the differential calculation will exclude the current of the tie breaker branch when the tie breaker opens. Now, for the same fault condition (fault at a location between the tie breaker and its associated CT’s), bus 1 should remain in normal operation while bus 2 experiences an internal fault. The bus relay would then clear this fault. VII. CONCLUSIONS 1) This paper proposes a new method for protection zone selection and its implementation in microprocessor-based

5)

6)

bus relays. Applying basic graph theory concepts, the proposed method possesses full generality; it is applicable to any bus arrangement in power system stations. Zone selection for complex station arrangements is software based. Graph operations and associated matrix operations in the relay simplify the representation of switching procedures in power systems. The proposed zone selection method provides bus protection with the greatest selectivity. There is no station CT switching and no danger of CT open circuit. There is also no need to disable bus protection before any switching operation. As a result, bus protection is more reliable. The proposed method provides accurate information about which breakers need to trip in case of a bus fault or associated breaker failure. Logic operations and zone selection schemes replace switching of the trip circuit, eliminating any hazards from incorrect trip circuit switching. The proposed method also eliminates extra contacts in the trip circuit wiring, improving reliability of the protection scheme. The proposed zone selection design incorporates logic control equations for protection zone supervision. This feature enhances the flexibility of station protection and control. It is also a way to supervise zone selection by using switch status and other digital input information available in the protected station. In bus differential protection, the proposed method easily handles problems from a fault occurring in an “end zone” between a circuit breaker and associated CT’s. With the aid of logic equations, and using the proposed zone selection method, we can add more bus protection features to the relay. The proposed method provides an easy way to represent station arrangements graphically. The method should also prove beneficial in other analysis of the control, automation, and protection of power system stations with complex multiple bus arrangements. REFERENCES

[1] An American National Standard, IEEE Guide for Protective Relay Applications to Power System Buses. New York, NY: IEEE. [2] P. M. Anderson, Power System Protection. New York: McGraw-Hill: IEEE Press, 1999. [3] J. L. Blackburn, Protective Relaying, Principles and Applications, 2nd ed. New York: Marcel Dekker, 1998. [4] A. Wright and C. Christopoulos, Electric Power System Protection. London: Chapman & Hall, 1993. [5] J. J. Wilson, “A review of modern bus-zone protection design and application,” in Proc. 1992 Southern African Conf. Power System Protection, Pretoria, South Africa, November 3–4, 1992. [6] H. Haug and M. Forster, “Electronic bus zone protection,” in CIGRE, Paris, June 10–20, 1968, 1968 Session 31-11.


[7] T. Forford and J. R. Linders, “Application of a high speed differential relay for buses, machines and cables,” in Presented at the 3rd Annual Western Protective Relay Conference, Spokane, WA, October 18, 1976. , “A half cycle bus differential relay and its applications,” IEEE [8] Transactions on Power Apparatus and Systems, vol. 93, pp. 1110–1120, July/August 1974. [9] J. B. Royle and A. Hill, “Low impedance biased differential busbar protection for application to busbars of widely differing configuration,” in Developments in Power System Protection, London, April 11–13, 1989, IEE Conference Publication Number 302. [10] A. Kumar and P. Hansen, “Digital bus-zone protection,” IEEE Computer Applications in Power, vol. 6, no. 4, pp. 29–34, October 1993.

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[11] A. G. Phadke and J. S. Thorp, Computer Relaying for Power Systems. Taunton, Somerset, England: Research Studies Press Ltd., 1988. [12] J. Esztergalyos, J. Bertsch, and M. Ilar, “Performance of a busbar differential protection based on EMTP simulation and digital system tests,” in Presented at the 24th Annual Western Protective Relay Conf., Spokane, WA, October 20–23, 1997. [13] W. K. Chen, Graph Theory and Its Engineering Applications. Singapore: World Scientific, 1997. [14] K. Thulasiraman and M. N. S. Swamy, Graphs: Theory and Algorithms. New York: Wiley, 1992. [15] W. A. Elmore, Protective Relaying Theory and Applications. New York: Marcel Dekker, Inc., 1994.