Abstract:
The present invention provides an electrical power distribution system that includes a first primary distribution panel (PDP) connected to a first source of electrical power and a second PDP connected to a second source of electrical power. The second PDP is connected to the first PDP by a conductor. A first ELCU receives signals corresponding to the flow of current through the first PDP to a load. A second ELCU receiving signals corresponding to the flow of current between the first PDP and the second PDP and a third ELCU receives signals corresponding to the flow of current between the second PDP and the first PDP. The first, second and third ELCUs protect the electrical power distribution system from a variety of fault conditions.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to electrical power distribution systems, and more particularly, to the control of bus tie breakers in electrical power distribution systems. 
         [0002]    Conventional electrical power systems architecture, such as those used in aerospace applications, usually needs to be reconfigured in the event of a power source failure or interconnect cabling failures. In electrical power systems having multiple power sources, when one power source fails, power may be transferred to the distribution bus of the failed power source from another power source. Bus Tie Contactors (BTCs) are typically used to accomplish this transfer of power between busses fed by different sources of electrical power. 
         [0003]      FIG. 1  shows a conventional electrical power distribution system  10  in accordance with the prior art.  FIG. 1  shows the electrical power system  10  during normal mode of operation. Two electrical power sources  12 ,  14  are connected to electrical loads  16 ,  18  via distribution buses  24  and  26  located in power distribution panels  20 ,  22  respectively. The power sources  12 ,  14  may be, for example, electrical generators. Power bus bars  24 ,  26  connect the electrical power sources to the plurality of electrical loads  16 ,  18  through a plurality of Electrical Load Control Units, e.g., (ELCUs)  28 ,  30 . The ELCUs  28 , as well as circuit breakers (not shown) may be used to provide line protection for each load  16 ,  18 . 
         [0004]    In the electrical power distribution system  10  bus tie contactors (BTCs)  32 ,  34  are used to allow transfer of, or to isolate electrical power between, power bus bars  24 ,  26 . The transfer may be performed by connecting (“tying”) electrical power buses together through electrical connection  36 , which may comprise a cable. The BTCs  32 ,  34  may be used to reconfigure the system under certain fault conditions to ensure the availability of power on both buses  24  and  26 . 
         [0005]    Electrical power system  10  may be a variable frequency or a constant frequency power system. In a variable frequency power system, the power sources  12 ,  14  are not synchronized and power buses cannot be tied together. During normal operation, as shown in  FIG. 1 , each power source  12 ,  14  delivers power to its own bus  24 ,  26  and the BTCs  32 ,  34  are de-activated (opened) to keep the generator channels separated. BTCs  32 ,  34  may be controlled by control logic in a bus power control unit (BPCU) hereinafter referred to as a CONTROL DEVICE  46 , which senses current from current transformers (CTs)  48 ,  50 . 
         [0006]      FIG. 2  shows the state of the electrical power system  10  when the electrical source  12  has failed. Generator control units (GCUs)  38 ,  40  may be used to detect the failure of either the electrical source  12  or  14  respectively. Upon failure of electrical source  12 , the associated GCU  38  will isolate the electrical source  12  by commanding a generator control breaker (GCB)  42  to open, thereby removing the power source  12  from the bus bar  24 . 
         [0007]    To ensure availability of power to the loads  16 , connected to the “dead bus”, the BTCs will be activated (closed) by signals from the CONTROL DEVICE  46 , or by GCUs  38  and  40 , as shown in  FIG. 2 . In this way, the unpowered bus bar  24  will be cross-fed by the active power source  14  which may supply the total power to both power bus bars  24 ,  26 . 
         [0008]    Likewise, in the case of a failure of power source  14 , the associated GCU  40  may sense the failure and may command GCB  44  to open and thereby removing the power source  14  from the bus bar  26 . CONTROL DEVICE  46  would also close both BTCs  32 ,  34  so that power source  12  may supply power to both power bus bars  24 ,  26 . 
         [0009]      FIG. 3  shows the electrical power system  10  in the situation where there has been a subsequent power bus fault. In particular, as shown in  FIG. 3 , power bus bar  24  has failed short-circuited; this led to the disconnection of power source  12  from the bus by its GCU. The BTCs  32 ,  34  may once again be de-activated (opened) to isolate the fault. Power bus bar  24  may be de-energized. The power to all loads  16  supplied by power bus bar  24  will be lost. 
         [0010]    Some present aerospace applications have the control logic of the BTCs  32 ,  34  implemented in the GCUs  38 ,  40 , while most applications have the logic implemented in the CONTROL DEVICE  46 . 
         [0011]    In applications where the electrical power system  10  has its electrical power generators operating at Constant Frequency (CF) such as that found on the Boeing 747, the system may operate with BTCs  32 ,  34  closed. Thus power sources  12 ,  14  share the burden of supplying power to the downstream loads  16 ,  18 . Hence in a CF power system, it may not be necessary to isolate the channels in normal operation as shown in  FIG. 1 . In the CF power system, power source faults and power bus faults may be handled in a manner similar to the VF power system shown in  FIGS. 1 and 3 . 
         [0012]    There are a number of disadvantages with the BTC control of electrical power system  10  shown in  FIGS. 1-3 . The control of the BTCs  32 ,  34  is relatively complex to insure safe power handling and transfer. 
         [0013]    In more detail, there are two different cases which require these control algorithms. 
         [0000]    Case 1: each power source feeds its own bus, where three control algorithms are needed as follows:
       a) control algorithm for the detection of the transfer condition/request;   b) analysis algorithm for the isolation of the cause of failure; if the generator disconnect was due to an over current fault, the closure of the BTC needs to be inhibited since this points to a bus failure that could propagate to generator  2 ; and   c) protection algorithm (differential fault protection—DP) to inhibit the closure of the BTC in the case a fault to ground is detected on the cable connecting between bus bar  1  and bus bar  2 .
 
Case 2: One generator feeds both busses, where two control algorithms are needed as follows:
   a) control algorithm to isolate an over current fault to the specific bus; this algorithm usually involves the opening of the BTC, monitoring the over current by the GCU; with the assumption that generator  2  feeds both bus bars, if the over current disappears after the opening of the BTC, it means that the fault is on bus bar  1 , therefore the BTC connection must be disabled, if the over current persists, generator  2  must be disconnected from the bus; and   b) protection algorithm (differential fault protection—DP) to open the BTC in the case of a fault to ground is detected on the cable connecting between bus bar  1  and bus bar  2 .       
 
         [0019]    The implementation of the above algorithms requires use of current measurement devices, i.e. current transformers (CT), optimization for the allocation and coordination of control between GCU and CONTROL DEVICE. 
         [0020]    The electric power system  10  shown in  FIGS. 1-3  is a relatively simple example since it addresses a system including only two generating source. In practice, the electrical power system may be more complex, including multiple generators and external power sources. The principle of control remains the same; however, the control algorithms become even more complex. 
         [0021]    As can be seen, there is a need for a simple and efficient way to handle the failure of a power source in electric power systems having multiple power sources. There is also a need for a simple and efficient way to control bus tie contactors during various failure conditions in electrical power systems. 
       SUMMARY OF THE INVENTION 
       [0022]    In one aspect of the present invention, an electrical power distribution system comprises a first primary distribution panel (PDP) connected to a first source of electrical power; a second PDP connected to a second source of electrical power, the second PDP being connected to the first PDP by a ‘cross-tie’ conductor; a first electronic load control unit (ELCU) receiving signals corresponding to the flow of current in the cross-tie conductor through the first PDP; a second ELCU receiving signals corresponding to the flow of current in the cross-tie conductor through the second PDP; and the first and second ELCUs responding to a fault condition in the conductor by interrupting the flow of electrical power between the first and second PDPs. 
         [0023]    In another aspect of the present invention, an electrical power distribution system comprises first source of electrical power connected to a first primary distribution panel (PDP); a second source of electrical power connected to a second PDP; an electrical load; a first electronic load control unit (ELCU) having a first contactor connected to the first source of electrical power, said first contactor having open and closed modes; a second ELCU having a second contactor connected to the second source of electrical power, said second contactor having open and closed modes; a conductor connected to the second contactor and to the electrical load; and the first ELCU sensing a current in a cross-tie conductor from the first source and in response thereto, opening the first contactor and at least one of the first and second ELCUs closing the second contactor to permit electrical power to flow from the first PDP load said cross-tie conductor. 
         [0024]    In accordance with a further aspect of the present invention, a circuit for protecting an electrical power distribution system comprises first power distribution panel (PDP) connected to a first source of electrical power; a second PDP connected to the first PDP by a conductor; a three phase electrical load connected to the first PDP; a first ELCU receiving signals corresponding to the flow of current through each of the three phases of the three phase electrical load; a second ELCU receiving signals corresponding to the flow of current through the second PDP; and at least one of the first and second ELCUs responding to the detection of a phase imbalance between the phases of current through the three phases by disconnecting the first source of electrical power. 
         [0025]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a block diagram of an electric power system in accordance with the prior art; 
           [0027]      FIG. 2  is a block diagram of the electric power system shown in  FIG. 1  in a first failure mode; 
           [0028]      FIG. 3  is a block diagram of the electric power system shown in  FIG. 1  in a second failure mode; and 
           [0029]      FIG. 4  is a block diagram of an electric power system in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0031]    Broadly, the present invention may be advantageously used in electric power systems, including aerospace electrical primary distribution panels (PDP). Embodiments of the present invention may provide for the protection of outgoing power to system loads and/or other PDPs that may have lost their dedicated source of power. Embodiments of the present invention may use an electric load control unit (ELCU) in place of a Bus Tie Contactor (BTC) commanded contactor. Prior art electric power systems relied on BTCs for this protection function. Also, embodiments of the present invention may use ELCUs for protection of both electrical load feeders and for bus cross tie feeders. Prior art electric power systems only use ELCUs for electrical load feeder protection. 
         [0032]    Embodiments of the present invention may replace the BTC control algorithms located in the Control Device or GCU with the protection algorithms located as standard features in the ELCU, which eliminates additional wiring and complexity. Embodiments of the present invention also may provide an over current algorithm in the ELCU. Prior art power systems used a CONTROL DEVICE having a BTC differential algorithm and had an over current algorithm that was functionally split/implemented between GCUs and BPCU. 
         [0033]      FIG. 4  shows a block diagram of an electric power system  50  which insures the distribution and protection of outgoing power to system loads. The electric power system  50  includes first and second power sources  52 ,  54 , which may comprise electrical generators. Each power source  52 ,  54  may supply electric power to a primary distribution panel (PDP)  56 ,  58 , which in turn may supply electric power to one or more loads  60 . Loads  60  may comprise three phase loads connected to the PDP  56  by three feeder lines  62 ,  64  and  66 . 
         [0034]    Power from the power sources  52 ,  54  may be transferred to power bus bars  68 ,  70  respectively, residing in the PDPs  56 ,  58 . Power bus bar  68  may be connected to a first contactor  78  by means of three lines  72 ,  74  and  76 . Contactor unit  78  may include three contacts  92 ,  94  and  96  connected to lines  72 ,  74  and  76  on an input side and to the feeder line  62 ,  64  and  66  on the output side. 
         [0035]    A first ELCU  80  may be connected to the first contactor  78  though lines  88  and  90  which may carry controls signals that control the state of contactors  92 ,  94  and  96 . First ELCU  80  may also be coupled to the lines  72 ,  74  and  76  through three current transformers (CT)  82 ,  84  and  86 , which sense the current in lines  72 ,  74  and  76 . Likewise, first ELCU  80  may be coupled to lines  62 ,  64  and  66  through CTs  98 ,  100  and  102 , which sense the current in lines  62 ,  64  and  66 . 
         [0036]    First ELCU  80 , (as well as second and third ELCUs  104 ,  106  described below) may be a conventional ELCU. It will be appreciated that ELCUs are common devices used in the aerospace industry for protecting electrical feeders. ELCU  80  may comprise a power contactor, control logic (using discrete components, application specific integrated circuitry or microprocessor) and current sensors (e.g. current transformers or Hall Effect sensors). 
         [0037]    ELCU&#39;s commonly are provided with built-in “intelligence” features such as:
       Overload current protection based on an inverse over-current and time function.   Programmability for over-current protection usually via pins on the ELCU connector.   External command to open circuit allows remote control to by-pass the built-in protection by hardwire or data bus.   Options to implement differential current trip protection via connection to remotely located current sensors.   Phase imbalance protection trip when one of the phase current becomes higher or lower than the rest by a predetermined amount.   Output signal for load current monitoring by hardwire or by data bus.       
 
         [0044]    ELCUs are available for the aerospace market from the following companies:
       Leach (Esterline), part numbers WE-X2YN, ZE-X9YN   Hartman (Tyco), part numbers BPE-494   Honeywell (using Hartman as one of the power contactor suppliers), 3B-39-1,-2; 3B-41-1,-3; 3B50-1; 3B50-2-D; 1592944; 1593921-1; 1593921-2; 1593926-0102   Sundstrand, part numbers 946F029-1, 962C526-1   Eaton Aerospace Remote Control Circuit Breakers, part number SM600BA100N1       
 
         [0050]    The function of the first ELCU  80  may be to control the states of the contactor  78  so that power to the loads  60  may be switched on or off depending on processing the current information sensed by CTs,  82 ,  84 ,  86 ,  98 ,  100  and  102 . Electric power system  50  also may include second and third ELCUs  104 ,  106  connected to a control device  108 . Device  108  may be a CONTROL DEVICE or GCU for the purpose of the control of the status (ON/OFF) of the ELCUs, i.e., detection of failure conditions of the power sources that may require the closure of the two ELCUs that operate now, as proposed by the present invention in the role of BTCs. Second ELCU  104  may be located in PDP  56  and may control the state of contactor  116  through control lines  118  and  120 . Second contactor  116  includes contacts  110 ,  112  and  114  and may be connected to the first power bus  68  through lines  122 ,  124  and  126 . Second ELCU  104  may be coupled to L BUS 230VAC via lines  122 ,  124  and  126  and receive line current information through Current Transformers CTs  128   130  and  132 . 
         [0051]    Third ELCU  106  may be located in PDP  58  and may control the state of contactor  140  through control lines  142  and  144 . Third contactor  140  may includes contacts  134 ,  136  and  138  and may be connected to the second power bus  70  through lines  146 ,  148  and  150 . Third ELCU  106  may be coupled to R BUS 230VAC bus via lines  146 ,  148  and  150  and receive line current information through Current Transformers—CTs  152 ,  154  and  156 . The lower side of contactor  116  may be connected, through bus cross-tie feeders  158 ,  160  and  162 , to the lower side of contactor  140 . The second ELCU  104  may sense the state of CTs  152 ,  154  and  156  in the PDP  58  through lines  164 ,  166  and  168 . Likewise the third ELCU  106  may sense the state of CTs  128 ,  130  and  132  in the PDP  56  through lines  170 ,  172  and  174 . It is noted that, as shown in  FIG. 4 , the outputs of the CTs  128 ,  130 ,  132 ,  152 ,  154  and  156  may be connected differentially, allowing their use for differential protection implementation and over current protection. 
         [0052]    The operation of second and third ELCUs  104 ,  106  and control device  108  may be similar to the operation of the CONTROL DEVICE  46  and BTCs  32 ,  34  as shown in  FIGS. 1-3 . In particular, in a normal mode, second and third ELCUs contactors  104  and  106  are open. In this state the two PDPs  56 ,  58  may operate independently. In a failure mode, where power source  52  fails, control device  108  may sense this condition and close ELCU contactors  116  and  140 . In this way, power from source  54  may directed across feeders  158 ,  160  and  162  to PDP  52  where it can supply electrical power to the load  60 . 
         [0053]    ECLUs  80 ,  104  and  106  along with the control device  108  may be provided with the following algorithms as standard features: algorithm (a) differential protection to protect against faults to ground; algorithm (b) over current protection, and algorithm (c) missing phase protection. By the use of the standard algorithms in ECLUs the control of the electric power system is simplified as compared to the prior art. In particular, algorithm (a) may replace the prior art protection algorithm that inhibits the closure of the BTC in the case where a fault to ground is detected on the feeders  158 ,  160  and  162  connecting power bus bar  68  to power bus bar  70  for circuits where each power source feeds its own bus. Algorithm (b) may replace the prior art analysis algorithm for the isolation of the cause of the failure described above, as well as the prior art control algorithm used to isolate an over current fault to the specific bus in cases where one power source feeds both busses. Algorithm (c) may enhance the protection capabilities of the control due to the fact that it does not permit operation of the loads connected to a power bus bar with a missing phase. 
         [0054]    Thus, it may be seen that the present invention may provide a solution for the implementation of the bus bar connections via the use of ELCUs with integral control instead of classic contactors with control allocated to GCUs or CONTROL DEVICE. Also, the present invention takes full advantage of the features already built in as part of ELCU design to simplify the architectures for an aircraft electric power system, and to implicitly optimize the control logic during normal and abnormal modes of operation. The use of a common device, the ELCU commanded contactor in place of a BTC allows the achievement of a modular design. As such a single device, the ELCU can be used to protect all output feeders from a PDP; both the supply feeders and the bus cross tie feeders. The present invention can allow a modular implementation of power distribution panel by using common devices for both, power distribution and cross tie connections. The present invention also can allow for the simplification of control algorithms located in GCUs and CONTROL DEVICEs including the downgrading of the redundancy levels required for implementation of this hazardous functionality. The present invention can enhance the electrical power system protections by adding the missing phase protection of a bus bar. 
         [0055]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.