Patent Publication Number: US-6671151-B2

Title: Network protector relay and method of controlling a circuit breaker employing two trip characteristics

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to network protector relays used to control circuit breakers and, more particularly, to such network protector relays for circuit breakers connecting feeders to low-voltage secondary power distribution networks. The invention also relates to a method of controlling a circuit breaker employing two trip characteristics. 
     2. Background Information 
     Low-voltage secondary power distribution networks consist of interlaced loops or grids supplied by two or more sources of power, in order that the loss of any one source will not result in an interruption of power. Such networks provide the highest possible level of reliability with conventional power distribution and are, normally, used to supply high-density load areas, such as a section of a city, a large building or an industrial site. Each power source is a medium voltage feeder supplying the network and consists of a switch, a transformer and a network protector. The network protector includes a circuit breaker and a control relay. The control relay senses the transformer voltages, the network voltages and the line currents, and executes algorithms to initiate breaker tripping or reclosing action. Trip determination is based on detecting reverse power flow, that is, power flow from the network to the primary feeder. 
     Examples of network protector relays are disclosed in U.S. Pat. Nos. 3,947,728; 5,822,165; and 5,844,781. 
     Traditionally, network protector relays were electromechanical devices, which tripped the circuit breaker open upon detection of power flow in the reverse direction. The electromechanical network protector relays are being replaced. One type of electronic network protector relay mimics the action of the electromechanical relay by calculating power flow. 
     Another type of electronic network protector relay uses sequence voltages and currents to determine the direction of current flow for making tripping decisions. Sequence analysis, upon which such relays are based, generates three vector sets to represent a three-phase voltage or current: (1) a positive sequence vector, (2) a negative sequence vector, and (3) a zero sequence vector. U.S. Pat. No. 3,947,728 discloses a sequence based network protector relay, which uses the positive sequence current and positive sequence voltage vectors to make trip decisions. 
     More recently, digital sequence based network protector relays have been utilized which periodically sample (e.g., 8, 16, 32 times per cycle) the current and voltages. 
     FIG. 1 illustrates a secondary power distribution network system  1 , which includes a low-voltage grid  3  servicing various loads  5 . The secondary network bus or grid  3  is energized by multiple sources in the form of feeders  7   a,    7   b,    7   c,    7   d.  Feeders  7   a  and  7   b  are supplied directly from substations  9   a  and  9   b , respectively. Each of the feeders  7   a - 7   d  respectively includes a feeder bus  11   a - 11   d,  a switch  13   a - 13   d,  a feeder transformer  15   a - 15   d,  and a network protector  17   a - 17   d.  The secondary network system  1  and its components are three-phase wye or delta connected, although FIG. 1 shows these as a single line for clarity. Each of the network protectors  17   a - 17   d  includes network protector circuit breakers  19   a - 19   d  and network protector control relays  21   a - 21   d,  respectively. 
     As disclosed in U.S. Pat. No. 5,822,165, which is incorporated by reference herein, the control relays  21   a - 21   d  each include a microcontroller-based circuit (not shown) which monitors the network phase to neutral voltages Vn (e.g., Van, Vbn, Vcn), the transformer phase to neutral voltages Vt (e.g., Vat, Vbt, Vct), and the feeder currents I (e.g., Ia, Ib, Ic). 
     Typically, control relays include a communication module for communication with a remote station over a communication network (or “communication subsystem” in order to avoid confusion with the secondary network bus  3 ). For example, one or more MPCV control relays, which are marketed by Cutler-Hammer of Pittsburgh, Pa., may be connected to the communication subsystem (e.g., without limitation, INCOM physical communication layer, and PowerNet or IMPACC Series III communication software, as marketed by Cutler-Hammer) to allow remote access to protector measurement data of interest. In turn, the control relays perform breaker trip and reclose functions. 
     Advances in solid-state technology continue to improve the functionality of network protector relays. 
     U.S. Pat. No. 5,822,165 discloses a network protector relay, for example, whereby the flexibility of more powerful processing resources relative to first generation solid-state relays and the even older electromechanical designs allow for providing a more robust and safe low-voltage power distribution network. 
     The primary responsibility of a network protector relay is to recognize and react to backfeed conditions (i.e., power leaving a low-voltage network grid). In the event that the amount of backfeeding power meets the programmed setpoints of the relay, then the relay trips the network protector circuit breaker and, thus, isolates the feeder circuit. The other major responsibility of the relay is, of course, deciding when the transformer voltage conditions are within programmed parameters relative to the network bus voltages, in order to command a reclosure of the network breaker. 
     Referring to FIG. 2, a phasor diagram  31  shows a traditional network relay trip characteristic. The network voltage phasor reference is shown as vector V N    33  at 0°. A normal, lagging network load current vector is included for reference as vector I LOAD    35 . A network relay should trip the protector circuit breaker on backfeed conditions that will occur when the feeder circuit is faulted or when the feeder circuit is opened. I SC  is shown representing a feeder fault backfeed vector  37 , lagging the 180° reference  39  due to the dominating network transformer leakage inductance and feeder cable inductance combination. For the case of an open feeder, the dominating term is typically the transformer secondary winding magnetizing inductance, as indicated by current vector I M    41 . 
     FIG. 2 shows the network protector relay tripping characteristic region  43  (shown in cross-hatch in FIG. 2) with a +5° counterclockwise tilt  45 , the purpose of which is to trip on backfeeding currents that may be highly leading the 180° reference  39  due to system cable charging currents indicated by I C , which is represented by vector  46 . A corresponding non-trip region  47  is shown above the trip region  43 . The threshold line of the trip region  43  is sloped 5 degrees to compensate for phase shift (e.g., in the network transformers; in the current transformers which measure the currents). This avoids unnecessary tripping in response to temporary reverse current conditions which could be caused for instance by a regenerative load on the network  3 . 
     The described vectors  33 , 35 , 37 , 41 , 46  are not drawn to a particular scale, but are simply graphical representations of various system conditions. RT represents the reverse trip setpoint  48  of the network protector relay. In this regard, the circuit breaker  103  has a rated value of current. Typically, the reverse trip setpoint RT  48  is about 0.2% of the rated value of current and is associated with the positive sequence current vector I SC    37 . The intention is that at every network protector relay location, each relay has all possible system current backfeed conditions fall within the trip region  43  of FIG.  2 . 
     There is room for improvement in network protector relays. 
     SUMMARY OF THE INVENTION 
     These needs and others are satisfied by the invention, which is directed to the trip functionality of a network protector relay, which employs two different trip characteristics by adding a “Gull-Wing” trip region to the existing traditional trip region. 
     As one aspect of the invention, a network protector relay for controlling a circuit breaker connected between a polyphase feeder bus and a polyphase network bus comprises: means for sampling polyphase current flowing through the circuit breaker and polyphase network voltage on the network bus to generate digital polyphase current samples and digital polyphase network voltage samples; and digital processor means comprising: means for generating a positive sequence current vector and a positive sequence voltage vector from the digital polyphase current samples and the digital polyphase network voltage samples, means for tripping the circuit breaker open in response to the positive sequence current vector being in a first trip region of a first trip characteristic with respect to the positive sequence voltage vector, the first trip characteristic defined by a first reverse trip setpoint and a first positive angle, and means for tripping the circuit breaker open in response to the positive sequence current vector being in a second trip region of a second trip characteristic with respect to the positive sequence voltage vector, the second trip characteristic defined by a second reverse trip setpoint and a second negative angle. 
     The first positive angle may be about +5 degrees. Preferably, the second negative angle is about −5 degrees. 
     The digital processor means may include means for storing the second negative angle as a predetermined negative angle, or means for configuring the second negative angle. 
     Preferably, the circuit breaker has a rated value of current between the polyphase network bus and the polyphase feeder bus associated with the positive sequence current vector, and the reverse trip setpoint is about 0.2% of the rated value of current. 
     As another aspect of the invention, a method of controlling a circuit breaker connected between a polyphase feeder bus and a polyphase network bus comprises: sampling polyphase current flowing through the circuit breaker and polyphase network voltage on the network bus to generate polyphase current samples and polyphase network voltage samples; generating a positive sequence current vector and a positive sequence voltage vector from the polyphase current samples and the polyphase network voltage samples; employing a first reverse trip setpoint and a first positive angle to define a first trip characteristic having a first trip region with respect to the positive sequence voltage vector; employing a second reverse trip setpoint and a second negative angle to define a second trip characteristic having a second trip region with respect to the positive sequence voltage vector; tripping the circuit breaker open in response to the positive sequence current vector being in the first trip region of the first trip characteristic; and alternatively tripping the circuit breaker open in response to the positive sequence current vector being in the second trip region of the second trip characteristic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic diagram of a low-voltage secondary power distribution network sourced by feeders incorporating network protector relays. 
     FIG. 2 is a plot of a traditional network protector relay trip characteristic. 
     FIG. 3 is a plot showing a backfeed problem of a network protector relay. 
     FIG. 4 is a block diagram of a network protector relay in accordance with the present invention. 
     FIG. 5 is a plot of a “Gull-wing” trip characteristic in accordance with the present invention. 
     FIG. 6 is a block diagram showing the logical implementation of the traditional network protector relay trip of FIG.  2  and the “Gull-wing” trip of FIG. 5 in accordance with the present invention. 
     FIG. 7 is a flow chart and block diagram of a routine utilized by the network protector relay of FIG. 4 for the trip functions of FIGS.  2  and  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     There have been several recently reported episodes where network protector relays failed to trip the network circuit breaker during a feeder fault event. Investigation into these occurrences has led to a theory that suggests that with newer, more efficient network transformers, relatively higher X/R ratio ratings (e.g., 8 to 12) are more commonplace. With such transformers, the resulting backfeed current under fault conditions, especially faults in close proximity to the transformer, potentially yield backfeed currents (I SC ) that are more lagging (i.e., clockwise shifted) than what a lower X/R transformer would produce. In the worst case, the backfeed current can approach a purely inductive 90° lag. With present day transformer technology, it is less likely that an open feeder magnetizing current backfeed I M  would approach a purely inductive 90° lag, but it is still a plausible concern. 
     FIG. 3 shows a scenario whereby a feeder fault backfeed vector  37 ′ (I SC ) and a current vector  41 ′ (I M ) both could fail to meet the traditional 5° tilt trip criteria (e.g., trip region  43 ), thereby resulting in no trip action from the network protector relay when it should command a breaker trip. Unfortunately, the 5° tilt  45 , which is deployed to guarantee a protector trip should the feeder system exhibit dominating cable capacitance for the case of an open feeder, reduces, and may even eliminate, the effective trip margin at the left plane region boundary. 
     Referring to FIG. 4, a block diagram of a network protector relay  100  is illustrated. The relay  100  includes a sampling circuit  101  and a microcontroller  102  which monitor the network phase to neutral voltages Van, Vbn, Vcn (e.g., Vn of FIGS.  1  and  4 ), the transformer phase to neutral voltages Vat, Vbt, Vct (e.g., Vt of FIGS.  1  and  4 ), and the feeder currents Ia, Ib, Ic (e.g., I of FIG. 1) through current transformers (not shown). The relay  100  controls a circuit breaker (CB)  103 , which is connected between a polyphase feeder bus  104  and a polyphase network bus  105 . A single exemplary 11-bit plus sign analog to digital (A/D) converter  106  digitizes the polyphase currents and voltages for input to the microcontroller  102 . Since a single A/D converter  106  is employed, the voltages and currents are sequentially fed thereto by analog multiplexers  107 ,  108  under the control of the microcontroller  102 . 
     As the range of currents can vary widely from reverse magnetization currents of a few ten thousandths per unit to forward overcurrents of about fifteen per unit, a programmable gain amplifier  109  adjusts the gain (e.g., 1, 2, 4, 8, 16) applied to the analog input  110  for application at the input  112  of the A/D converter  106 . As disclosed in U.S. Pat. No. 5,822,165, the microcontroller  102  utilizes the sensed currents and voltages in algorithms which generate a circuit breaker trip signal (i.e., signal  140  of FIG. 6) in response to detection of reverse current flowing out of the network bus  105  into the feeder bus  104  (e.g., out of network  3  into one or more of the feeders  7   a,    7   b,    7   c,    7   d  of FIG.  1 ), and also in response to forward currents which exceed a preset current/time characteristic. In turn, the control relay  100  performs breaker trip and reclose functions. 
     The network phase to neutral voltages Van, Vbn, Vcn on the network bus  105  are sensed through the analog multiplexer  107 , with the other analog multiplexer  108  outputting a ground reference G to the negative input of difference buffer  113 , which generates analog polyphase network voltage samples at the analog input  110  of the programmable gain amplifier  109 . Alternatively, the buffer  13  generates analog polyphase phasing voltage samples (i.e., Vp (not shown)), which are the differences between the network voltages Vn and the transformer voltages Vt, and which are the voltages across the open circuit breaker  103 ). The circuit  101  samples polyphase current Ia, Ib, Ic flowing through the circuit breaker  103  and polyphase network voltage Van, Vbn, Vcn on the network bus  105  to generate digital polyphase current samples and digital polyphase network voltage samples. 
     The relay  100  also includes a communication module  114  for communications with a remote station, such as personal computer (PC) (not shown), over a communication subsystem  118 . In the exemplary subsystem  118 , an INCOM communication subsystem utilizes a protocol, which is proprietary to Eaton Corporation, although any suitable communication subsystem may be used. For example, the remote PC may send a close command  120  over the INCOM cable  122  to the relay  100 . 
     FIG. 5 shows a plot  130  of the traditional network protector relay trip region  43  and a “Gull-wing” trip characteristic  132  having a trip region  133  (shown in cross-hatch in FIG. 5) in accordance with the present invention. A corresponding non-trip region  134  is shown above the trip regions  43  and  133 . 
     For example, the flexibility and DSP (digital signal processing) power of solid-state designs, such as the MPCV relay or the microcontroller  102  of FIG. 4, can readily deploy such a trip characteristic  132 . The MPCV presently employs DSP representations of the symmetrical components of the network bus voltages Vn and circuit breaker currents I, in order to provide the traditional 5°-tilt characteristic  45 . To logically implement the new “Gull-Wing” region  133 , the firmware code of the microcontroller  102  also checks the boundary for a 5° tilt in the opposite direction (i.e., a −5° tilt  135 ). 
     As shown in FIG. 5, both of the regions  43  and  133  intercept the 180° reference  39  at RT, the reverse trip setpoint  48  of the network protector relay  100 . Although an exemplary −5° tilt  135  is shown, another suitable negative value (e.g., without limitation, about −10°, less than −5°, greater than −5° but less than 0°) may be employed (e.g., without limitation, a value which is hardcoded into the firmware of the microcontroller  102  of FIG. 4, a value which is configured through a microcontroller port  136 ). In this manner, an absolute value of the negative tilt angle may be different from the exemplary 5° tilt value. The microcontroller  102  of FIG. 4 includes a suitable memory (M)  137  for storing the predetermined or configured negative angle. 
     The vector V N  at 0° of FIG. 5 is normal to a reference  50  at 90° and 270°. Those angles, like the 180° reference line  39 , are defined in a counterclockwise direction  51  from the vector V N  at 0°. A first positive angle  52  (e.g., measured counterclockwise with respect to FIG. 5) is defined between the reference  50  and the traditional 5°-tilt characteristic  45 . A second negative angle  54  (e.g., measured clockwise and, thus, opposite the counterclockwise direction  51 ) is defined between the reference  50  and the −5° tilt  135  of the “Gull-Wing” trip characteristic  132 . 
     The characteristic  45  is defined with respect to a first reverse trip setpoint RT  48 . The −5° tilt  135  may be defined with respect to a second reverse trip setpoint RT′ (FIG. 6) or the first reverse trip setpoint RT (FIG.  5 ). 
     FIG. 6 shows the logical implementation of the traditional network protector relay trip signal  140  as determined from the trip region  43  of FIG. 2, and the “Gull-wing” trip signal  142  as determined from the trip region  133  of FIG. 5 in accordance with the invention. The signals  140  and  142  are input by the logical OR function  144 , and a resulting trip command  146  is issued if the feeder fault backfeed vector  37 ′ (I SC ) exceeds either of the tilted trip boundaries (e.g., 5° tilt and −5° tilt) and enters one of the corresponding trip regions  43  and  133 . 
     The “Gull-Wing” trip signal  142  is determined based on measuring reverse current flow, that is, current flow from the network  3  to the feeders  7   a - 7   d . Thus, the network positive sequence voltage V N  is calculated at  148  from the network phase voltages. The positive sequence current I SC  is also calculated at  150  from the phase currents. The reverse current calculation and “Gull-Wing” trip decision is then made at  152 . This process can be visualized by the observing the positions of the positive sequence current I SC  vector  37 ′ and network positive sequence voltage V N  vector  33  in the complex plane as illustrated in FIG.  5 . Real power flow (Watts) from one or more of the feeders  7   a - 7   d  to the network  3  occurs in quadrants I and II of FIG.  5 . On the other hand, real power flow is from the network  3  to the feeder(s) in quadrants III and IV. The trip signal  142  is set active if the positive sequence current I SC  vector  37 ′ lies in the trip region  133 . 
     Referring to FIG. 7, a flow chart and block diagram shows the routine  170  employed by the network protector relay  100  of FIG. 4 for the trip functions of FIGS. 2 and 5. As employed in FIG. 7, reference numbers  139 ,  179 ,  181 ,  183 ,  185 ,  187 ,  189 ,  191 ,  193  and  195  correspond to the same numbered steps of FIG. 8E of U.S. Pat. No. 5,822,165. 
     As disclosed in U.S. Pat. No. 5,822,165, following a pumping routine (not shown), if a remote trip command (not shown) has been received over the communications network  118  of FIG. 4, a trip condition  138  is generated. Otherwise, a determination is made at  139  as to whether a “watt-var” mode has been enabled. If not, the program transfers to the traditional (sensitive) trip algorithm at  173  of FIG.  7 . This algorithm implements the trip characteristic  31  shown in FIG.  2 . Initially, the traditional trip algorithm is run at  173 . If the traditional trip is not satisfied at  175 , that is, the positive sequence current vector I SC    37 ′ is not in the trip region  43  of FIGS. 2 and 5, then the program transfers to the “Gull-wing” trip algorithm at  176  of FIG.  7 . This algorithm implements the trip characteristic of the “Gull-wing” region  133  shown in FIG.  5 . If the “Gull-wing” trip is satisfied at  177 , then step  179  is executed as discussed below. Otherwise, the trip delay time count is reset at  178 . 
     On the other hand, if the traditional trip is satisfied at  175 , and the trip delay time has been set to 0, as determined at  179 , and the overcurrent setpoint is 0 as determined at  181 , then a trip condition  138  is generated. If a traditional plus non-sensitive mode has been selected, which includes a small semi-circular instantaneous trip region (not shown), then a trip condition  138  is generated if the positive sequence current vector I SC    37 ′ is within that region as indicated at  183 . The instantaneous trip condition can occur when the feeder is open and there is reverse current magnetizing the secondary of the feeder transformer. The trip condition  138  is also generated if the positive sequence current vector I SC    37 ′ is more than the overcurrent limit, as indicated at  184 . 
     Returning to block  179 , if the time delay is other than zero and the overcurrent setpoint is equal to zero, as indicated at  185 , then a trip condition  138  is generated. A trip condition  138  is also generated if the overcurrent is exceeded at  187 . If not, and the delay is infinite as determined at  189 , then the trip delay time count is reset at  178 . Otherwise, a check is made at  191  to see if a delayed trip has been generated. If the time has not expired at  191 , then a float condition is indicated at  193 . Otherwise, a trip condition  138  is generated. 
     After  178 , tests are provided to determine if an open circuit breaker can be closed. If the negative sequence phasing voltage is high at  195 , this indicates improper wiring, such as crossed phases on the transformer or the network, and, therefore, a trip condition is generated at  138 . 
     The exemplary network protector relay  100  provides a “Gull-Wing” trip characteristic  132 . This provides enhanced trip functionality over a traditional network protector relay trip characteristic as deployed by electromechanical and first generation solid state devices. The exemplary trip characteristic  132  provides a novel mechanism of addressing all possible network backfeed system conditions including those with implementations of low-loss feeder transformers which could result in sustained system backfeeds if existing conventional network protector relay technology were utilized. 
     Any solution proposals to the described backfeed condition that might suggest tuning the traditional tilt angle  45  to the particular system parameters at a given location are flawed in that for the very worst case, the open feeder system extremes for the resulting backfeed (I M  or I C ) are in theory 180° apart. Therefore, no matter how well the tilt angle is tuned, the 180° boundary remains without any margin. On the other hand, the exemplary “Gull-Wing” trip region  133  encapsulates the 180° theoretical spread and a needed safety margin. 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.