Abstract:
A ground fault protection system and method for implementing such is provided for protecting an electrical power distribution system having multiple sources and multiple grounds. A set of current transformers are connected to an interface unit which in turn is connected to a ground fault trip function for a circuit breaker. The interface unit has an output with a low impedance, and the outputs of multiple interface units can be connected in series and feed a single ground fault trip function; thereby tripping the circuit breaker on a ground fault detected by any set of current transformers connected to one of the interface units. Another embodiment utilizes multiple, independent ground fault trip functions in a single circuit breaker. Each ground fault trip function is connected to a set of current transformers, and the circuit breaker will trip if any connected set of current transformers detects a ground fault. This embodiment involves a system whereby one circuit breaker is tripped under ground fault conditions using one signal from either of two or more inputs from different groups of sensor circuits. A method is disclosed that includes the steps of sensing the current at various points in the distribution system, monitoring the sensed current for a ground fault, determining which breakers need to be tripped for a detected ground fault, and tripping those breakers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not Applicable  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not Applicable  
         BACKGROUND OF THE INVENTION  
         [0003]    1. Field of Invention  
           [0004]    This invention relates generally to ground fault protection circuits for electrical distribution equipment. More particularly, this invention pertains to a circuit and implementing method for ground fault protection for electrical power distribution systems having multiple sources and grounds.  
           [0005]    2. Description of the Related Art  
           [0006]    Ground fault protection circuits are commonly used for providing automatic circuit interruption upon detection of undesired short circuit currents that flow as a result of a ground fault condition in electrical power distribution systems. Such ground fault protection circuits ordinarily include means for quickly sensing and individually isolating any faults occurring in a respective branch circuit of the power distribution systems and utilize selective coordination to instantly respond and interrupt power only to the system area where a fault occurs, preventing unnecessary loss of power to other areas.  
           [0007]    [0007]FIG. 1 illustrates a simple co-generation distribution system of the prior art that utilizes a simple ground fault protection system. Each of the circuit breakers is controlled by a ground fault trip function that monitors for fault currents flowing through the breaker. A fault resulting in unbalanced currents flowing through the neutral and phase conductors trips the breaker. With both the main supply breaker and the generator breaker closed, the ground fault protection system configuration depicted in FIG. 1 will isolate all ground faults except for a ground fault on the main bus when power is supplied to the bus from the generator.  
           [0008]    [0008]FIG. 2 illustrates the same co-generation distribution system, but the system utilizes a ground fault protection system as taught in U.S. Pat. No. 5,751,524, issued on May 12, 1998, to Swindler. FIG. 2 shows a ground fault protection system using the main circuit breaker current transformer to provide a trip signal to both the main circuit breaker ground fault trip function and to the generator breaker ground fault trip function. The generator breaker ground fault trip function is coupled to the main circuit breaker current transformer through an auxiliary transformer, which causes the generator breaker to trip when a ground fault is detected on the main bus. The use of auxiliary transformers permits a single set of current transformers to trip more than one circuit breaker. Although having this advantage, the use of auxiliary transformers also has the disadvantage of increasing the number of different types of components in the circuit and of requiring additional wiring between the various breakers. With breakers located in different areas, it is desirable to minimize the wiring between breakers.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    An improved ground fault protection system and method for implementing such is provided for protecting an electrical power distribution system having multiple sources and multiple grounds. In one embodiment, two sets of current transformers, each monitoring a different point in the distribution system, are each connected to separate interface units. The outputs of these two interface units are connected in series and feed the trip function of a circuit breaker, thereby providing the circuit breaker with the ability to trip on the detection of a ground fault sensed by either set of current transformers. Another embodiment connects one set of current transformers to two interface units in series. With the outputs of the interface units connected to different circuit breakers, this embodiment trips two circuit breakers based on a ground fault sensed by the set of current transformers. Still another embodiment of the invention includes a circuit breaker with two independent ground fault trip functions, effectively incorporating the interface units with the ground fault trip function in the circuit breaker.  
           [0010]    The method for implementing the ground fault protection system includes sensing the current at various points in the distribution system and monitoring for a ground fault. When a ground fault is detected, the circuit breakers necessary to isolate the ground fault are determined and tripped. In order to isolate the ground fault, multiple breakers may need to be tripped based on a single point where a ground fault was sensed. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0011]    The above-mentioned features of the invention will become more clearly understood from the following detailed description of the invention read together with the drawings in which:  
         [0012]    [0012]FIG. 1 is a schematic representation of a representative power distribution system having a ground fault protection circuit according to the prior art;  
         [0013]    [0013]FIG. 2 is a schematic representation of a representative power distribution system having a ground fault protection circuit according to the prior art;  
         [0014]    [0014]FIG. 3 is a schematic representation of a representative power distribution system having a ground fault protection circuit according to an embodiment of the present invention;  
         [0015]    [0015]FIG. 4 is a schematic representation of the power distribution system depicted in FIG. 3 and showing per-unit fault currents in the primary conductors of the system;  
         [0016]    [0016]FIG. 5 is a schematic representation of the power distribution system depicted in FIG. 3 and showing per-unit currents sensed by the ground fault protection circuits as would flow in the secondary of the sensors and to the trip function;  
         [0017]    [0017]FIG. 6 is a schematic representation of the generator circuit breaker ground fault trip function as depicted in FIGS. 3, 4, and  5 ;  
         [0018]    [0018]FIG. 7 is a schematic representation of a circuit breaker with a single ground fault trip function fed by two interface units according to the present invention;  
         [0019]    [0019]FIG. 8 is a schematic representation of a circuit breaker with two ground fault trip functions according to the present invention; and  
         [0020]    [0020]FIG. 9 is a schematic representation of a complex power distribution system having a ground fault protection circuit according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    A ground fault protection system and method for implementing such is provided for protecting an electrical power distribution system having multiple sources and multiple grounds.  
         [0022]    [0022]FIGS. 1 through 5 show a single-line diagram of a simple three-phase four-wire electrical co-generation distribution system. The single-line diagrams use a single line to represent the three-phase power and another line to represent the neutral.  
         [0023]    In FIGS. 1 through 5, an off-site source  102  is shown connected to the distribution bus  120  by a main supply breaker M. A generator  104  is connected to the distribution bus  120  by a generator breaker G, and two loads  106   a ,  106   b  are connected to the distribution bus  120  by load breakers L 1  and L 2 . Each breaker M, G, L 1 , L 2  has an associated current sensor or current transformer (CT),  132 ,  134 ,  136   a , and  136   b , respectively.  
         [0024]    [0024]FIGS. 1 through 5 show each CT  132 ,  134 ,  136  encompassing the three phases  126  and neutral  122 . This is a representation of the actual circuit in which individual current transformers are used for each phase  126  and the neutral  122 , with the current transformer secondaries connected in parallel. The set of current transformers measure the vector sum of the currents flowing through the phase conductors and the neutral conductor. Without a ground fault present, the vector sum is zero. The polarities of the CTs  132 ,  134 ,  136  are indicated by the square black dots (polarity mark) adjacent to the windings. More specifically, when primary current enters a given primary winding through the black dot adjacent to this primary winding, secondary current leaves the associated secondary winding through the black dot adjacent to the secondary winding. When the direction of the primary current is reversed, the direction of the secondary current is correspondingly reversed. The figures show the neutral conductor  122  electrically connected to ground  124 .  
         [0025]    [0025]FIG. 1 shows a simple co-generation distribution system with a simple prior art ground fault protection system. Each breaker M, G, and L has a ground fault trip function  152 ,  154 ,  156  connected to a CT  132 ,  134 ,  136  associated with that breaker M, G, or L. With both the main supply breaker M and the generator breaker G closed, the ground fault protection system configuration depicted in FIG. 1 will isolate all ground faults except for a ground fault on the main bus  120 , because the generator ground fault detection system is incapable of detecting the ground fault by means of the power flow through the generator circuit breaker. With a ground fault on the main bus  120 , the main supply breaker M will trip, but the generator breaker G will remain closed.  
         [0026]    [0026]FIG. 2 depicts a similar distribution system with a ground fault protection system as taught by the Swindler patent. An auxiliary transformer  202  is used to actuate the ground fault trip function  154  for the generator breaker G when a ground fault is detected by the main supply breaker M CT  132 . The ground fault protection system depicted in FIG. 2 will isolate all ground faults, including a ground fault on the main bus  120 .  
         [0027]    [0027]FIG. 3 illustrates one embodiment of the present invention, which uses a plurality of interface units  302 ,  304 ,  306  to form the ground fault protection system. Each interface unit  302 ,  304 ,  306  is an impedance matching device in which the input is either directly connected to a CT or forms part of a loop containing any combination of CTs, ground fault trip functions, and other interface units. The output of the interface unit has a low impedance and can directly drive a ground fault trip function, which has a high impedance, or the output can be placed in series with another interface unit to drive a single ground fault trip function, with either interface unit capable of causing the ground fault trip function to trip the circuit breaker. Additionally, in a variation of this embodiment, the interface unit can be adjusted or calibrated by changing a resistor in the interface unit&#39;s output network. As shown, interface units  304 ,  306  are used to communicate a pair of ground fault trip signals to the generator circuit breaker G, eliminating the need for the auxiliary transformer  202  shown in FIG. 2.  
         [0028]    [0028]FIG. 6 illustrates a schematic of main supply breaker M and generator breaker G connected as depicted in FIG. 3. The outputs of two interface units  304 ,  306 , wired in series, are connected to the ground fault trip function  602  of generator breaker G. The input of one of the interface units  304  is in series with another interface unit  302  and a set of CTs  132 . The output of the interface unit  302  is connected to the ground fault trip function  604  of the main supply breaker M. The input of the other interface unit  306  is connected to a set of CTs  134  located near the generator breaker G. A ground fault sensed by the set of CTs  132  near the main supply breaker M will cause both the main supply breaker M and the generator breaker G to trip.  
         [0029]    [0029]FIG. 7 illustrates a schematic of an embodiment where a single circuit breaker  710  can be tripped from either of two sets of current sensors or CTs  732 ,  734 . FIG. 7 shows a common circuit breaker  710  with a single ground fault trip function  702 . A pair of interface units  704 ,  706  have their outputs wired in series and connected to the ground fault trip function  702 . The input of each interface unit  704 ,  706  is connected to a set of CTs  732 ,  734 . The interface units  704 ,  706  have a low output impedance and each interface unit  704 ,  706  can individually trip the ground fault trip function  702 .  
         [0030]    [0030]FIG. 8 illustrates a schematic of an embodiment in which multiple interface units are incorporated into a single circuit breaker  810 . A circuit breaker  810  is constructed with two independent ground fault trip functions  802   a ,  802   b , either of which can trip the breaker  810 . Each ground fault trip function  802   a ,  802   b  is connected to a set of CTs  832 ,  834 . Those skilled in the art will recognize that the ground fault trip function  802  can either mechanically or electrically trip the circuit breaker  810  without departing from the spirit and scope of the present invention.  
         [0031]    [0031]FIGS. 4 and 5 illustrate the steps for analyzing a ground fault protection system. In these two figures, the underlying assumption is that all breakers M, G, L are closed, that there is a ground fault  412  on the bus  120 , and that the ground fault  412  consists of 2 units of current. According to Kirchoff&#39;s first law, if current is flowing from the sources, then current must return to the sources and whatever current returns to the source must equal that which is going out.  
         [0032]    [0032]FIG. 4 illustrates the flow of the fault current resulting from the ground fault  412 . A current of 2 units flows out of the system at ground fault  412  and flows back into the system at the ground connection  124 . From the ground connection  124 , the current splits with 1 unit flowing towards the source  102  and 1 unit flowing down the neutral  122  through the main supply CT  132 , through the generator CT  134 , into the generator  104 , through the generator  104 , through the generator CT  134 , and then flowing out of the system through the ground fault  412 . The 1 unit of current flowing into the neutral  122  from the ground connection  124  flows into the source  102 , returns from the source  102 , through the main supply CT  132 , and then flowing out of the system through the ground fault  412 .  
         [0033]    The ground fault current flowing through the neutral  122  and phase conductors  126  at CT  132  is flowing away from the polarity marks. Accordingly, the current flow through the secondary of the CT  132 , as shown on FIG. 5, is the sum of the two currents, which is 2 units of current, and the current flow is towards the CT  132  from the polarity mark. With respect to CT  134 , the neutral current of 1 unit is flowing away from the polarity mark and the phase conductors current of 1 unit is flowing into the polarity mark. These two currents cancel each other and, as shown on FIG. 5, result in zero current flow in the secondary of CT  134 .  
         [0034]    [0034]FIG. 5 shows the current flowing through the secondaries of the CTs  132 ,  134 . Current transformer  132  has 2 units of current flowing into its secondary winding, and CT  134  has zero current flowing through its secondary. Because there is no current contributed by CT  134 , interface unit  306  will not trip the generator breaker G. However, the 2 units of current generated by the main supply CT  132  flows though the loop formed by the CT  134 , interface unit  304 , and interface unit  302 . This current flow causes interface unit  302  to trip the generator breaker G on a ground fault and causes interface unit  304  to trip the main supply breaker M on a ground fault. With both breakers M, G open, the ground fault  412  is isolated.  
         [0035]    Those skilled in the art will recognize that the analysis described above and illustrated in FIGS. 4 and 5 can be used on more complex electrical power distribution systems and with different assumptions regarding the location of the ground fault  412  and status of the various circuit breakers without departing from the spirit and scope of the present invention. Complex power distribution systems include multiple power sources and tie buses, which are configured such that power can be supplied to any load from various sources. The ground fault protection system, for both a simple and a complex power distribution system, must isolate any ground fault and minimize the disruption of loads.  
         [0036]    [0036]FIG. 9 is an example of a more complex power distribution system. Three power sources  901 ,  902 ,  903  are connected to three buses  921 ,  922 ,  923 , respectively, and feed three loads  905 ,  906 ,  907 , respectively. Each power source  901 ,  902 ,  903  is connected to the buses  921 ,  922 ,  923  with a main supply circuit breaker M 1 , M 2 , M 3 . The buses  921 ,  922 ,  923  are connected with tie breakers T 1 , T 2 , as shown. The three load circuit breakers L 1 , L 2 , L 3  have standard ground fault protection provided by monitoring the breaker L 1 , L 2 , L 3  outputs with CTs feeding the breaker&#39;s ground fault trip function.  
         [0037]    The ground fault protection system for the remainder of the power distribution system uses an embodiment of the present invention. Each of the main supply circuit breakers M 1 , M 2 , M 3  has an associated CT  931 ,  932 ,  933  and each of the tie breakers T 1 , T 2  has an associated CT  941 ,  942 . The ground fault trip function  951  for the first main source circuit breaker MI and the ground fault trip function  961  for the first tie breaker T 1  form two legs of an illustrated star connection that is bridged by the first main source CT  931 . The third leg of the illustrated star connection includes the ground fault trip function  952  for the second main source circuit breaker M 2  and either an interface unit  962  with an output connected to the second tie breaker T 2  or one of two ground fault trip functions  962  in the second tie breaker T 2 . The outboard end of the third leg is connected to the shared connection between the tie breaker CTs  941 ,  942 . The ground fault trip function  953  for the third main source circuit breaker M 3  is in series with either an interface unit  963  with an output connected to the second tie breaker T 2  or one of two ground fault trip functions  963  in the second tie breaker T 2 .  
         [0038]    When two interface units  962 ,  963  are used, , as shown in FIG. 7, the outputs of the two interface units  962 ,  963  are connected in series and feed the ground fault trip function for the second tie breaker T 2 . When two independent and isolated trip functions  962 ,  963  in a single circuit breaker T 2  are used, as shown in FIG. 8, either trip function  962 ,  963  will trip the second tie breaker T 2 .  
         [0039]    The ground fault protection system illustrated in FIG. 9 isolates only that portion of the system necessary to isolate the ground fault. In order to accomplish this, the ground fault protection system uses either two interface units or two independent trip functions to ensure that the tie breakers trip when required.  
         [0040]    In operation, current sensors or CTs are used to sense current flowing at various points in power distribution systems, regardless of whether the power distribution system is simple or complex. These points are typically near circuit breakers, where bus current is sensed, and ground points, where ground current is sensed. Bus current is measured from the vector sum of currents flowing through each phase conductor and through the neutral conductor of the bus. Ground point current is measured by sensing the current flowing through the ground connection.  
         [0041]    The sensed current is monitored for a ground fault, which is sensed by a non-zero current in the bus or the ground connection. Once a ground fault is detected, the ground fault protection system determines which circuit breakers need to be tripped in order to isolate the ground fault and trips the breakers. The determination of the breakers to be tripped is based upon the location of the ground fault and the topology of the power distribution system. An analysis as illustrated in FIGS. 4 and 5 can be used to analyze the ground fault protection system to ensure that the breakers to be tripped are those that can supply power to the ground fault. As illustrated in the figures, this determination requires one or more circuit breakers to be tripped based on a ground fault detected from any of multiple current sensing locations.  
         [0042]    From the forgoing description, it will be recognized by those skilled in the art that an improved ground fault protection system and method is provided for protecting an electrical power distribution system having multiple sources and multiple grounds. This system and method does not require the use of auxiliary transformers.  
         [0043]    While some embodiments have been shown and described, it will be understood that it is not intended to limit the disclosure, but rather it is intended to cover all modifications and alternate methods falling within the spirit and the scope of the invention as defined in the appended claims.