# US 8,542,469 B2 · Methodology for protection of current transformers from open circuit burden > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 8,542,469 B2 - Document ID: US8542469B2 - Status: Granted - Kind: B2 - Application number: 12/871,517 - Filed: 2010-08-30 - Published: 2012-03-01 - Granted: 2013-09-24 - Earliest claimed: 2010-08-30 - Assignees: Honeywell International Inc. - Applicants: Randy Fuller; David Hodge; James Lobsinger - Inventors: Randy Fuller; David Hodge; James Lobsinger - Web page: https://patentgrep.com/patent/US8542469B2 ## CPC - H02H 7/042: {for current transformers} - H02H 3/28: involving comparison of the voltage or current values at two spaced portions of a single system, e.g. at opposite ends of one line, at input and output of apparatus {(for transformers H02H7/045)} - H02H: EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS (indicating or signalling undesired working conditions G01R, e.g. G01R31/00, G08B; locating faults along lines G01R31/08; emergency protective devices H01H) ## Abstract A protected electrical power system may comprise a feeder between a power source and an electrical load. A first current transformer may be positioned on the feeder in a first location. The first current transformer may have a shunt resistor electrically connected across its winding. A second current transformer may be positioned at a second location on the feeder. A control unit may be interposed between the first and second current transformers and may be interconnected with the first and second current transformers on current-monitoring loops independent from the feeder. The control unit may be responsive to a predetermined differential in feeder (DF) current between the first and second current transformers to disconnect the power source from the electrical load. The control unit may have a compensation network for reducing DF error resulting from presence of the shunt resistor in the first current transformer. ## Claims ### Claim 1 An electrical power system comprising: a feeder between a power source and an electrical load; a first current transformer positioned at a first location on the feeder, the first current transformer comprising a first shunt resistor electrically connected across the first current transformer; a second current transformer positioned at a second location on the feeder; and a control unit interposed between the first and second current transformers; the control unit interconnected with the first and second current transformers on current-monitoring loops independent from the feeder; the control unit responsive to a predetermined differential in feeder (DF) current between the first and second current transformers to disconnect the power source from the electrical load; and the control unit including a compensation network to reduce DF error resulting from the first shunt resistor electrically coupled across the first current transformer, the compensation network including a series resistor coupled with the second current transformer, the series resistor having a resistance value that, when added to a resistance value of wiring between the second current transformer and a burden resistor associated with the second current transformer, produces a net resistance value approximately equal to a wiring resistance between the first current transformer and a burden resistor associated with the first current transformer. ### Claim 2 (depends on 1) The power system of claim 1: wherein the power source is an electric machine; and wherein the first current transformer is incorporated in the electric machine. ### Claim 3 (depends on 2) The power system of claim 2 wherein the electric machine is a starter generator of an aircraft. ### Claim 4 (depends on 1) The power system of claim 1: wherein the compensation network comprises a second shunt resistor electrically connected across the second current transformer; and wherein the first and second shunt resistors have equal resistance values. ### Claim 5 (depends on 1) The power system of claim 1 wherein the compensation network comprises: a second shunt resistor, having a resistance value equal to that of the first shunt resistor, electrically connected across the second current transformer. ### Claim 6 (depends on 1) The power system of claim 1 wherein the compensation network is incorporated in the control unit. ### Claim 7 A protection system for a feeder comprising: a first current transformer at a first location on the feeder; a control unit that acts responsively to a predetermined DF to interrupt feeder current; wherein the first current transformer is connected to a first shunt resistor; wherein the control unit interrupts feeder current within a time period T after development of an open fault in a first current-transformer loop; wherein the time T is less than a time period in which a rate of power dissipation within the first shunt resistor reaches a power dissipation rate limit for the first shunt resistor; a second current transformer at a second location on the feeder; and a compensation network connected to the second current transformer to compensate for DF error resulting from presence of the first shunt resistor, wherein the compensation network includes a series resistor coupled with the second current transformer, and wherein the series resistor has a value that, when added to a resistance value of wiring between the second current transformer and a burden resistor associated with the second current transformer, produces a net resistance value equal to a wiring resistance between the first current transformer and a burden resistor associated with the first current transformer. ### Claim 8 (depends on 7) The protection system of claim 7 wherein the compensation network comprises: a second shunt resistor electrically connected across the second current transformer. ### Claim 9 (depends on 8) The protection system of claim 8: wherein the second shunt resistor has a resistance value equal to that of the first shunt resistor. ### Claim 10 (depends on 7) The protection system of claim 7: wherein the first shunt resistor has a resistance that produces an DF inaccuracy no greater than about 0.18%; and wherein the first shunt resistor has a nominal power dissipation rating of no more than about 4 watts (W). ### Claim 11 (depends on 10) The protection system of claim 10 wherein the first shunt resistor has a resistance at least as high as 500 ohms. ### Claim 12 (depends on 7) The protection system of claim 7 wherein the first current transformer and the first shunt resistor are incorporated in a power source that supplies power to the feeder. ### Claim 13 (depends on 12) The protection system of claim 12 wherein the power source is a generator. ### Claim 14 (depends on 13) The protection system of claim 13 wherein the feeder is employed to supply electrical power to electrical loads in an aircraft. ### Claim 15 A method for operating an electrical power system comprising the steps of: passing electrical current on a feeder from a power source to electrical loads through a first current transformer to produce a first monitoring current in a first current-transformer loop; passing a portion of the monitoring current through a first shunt resistor connected across the first current transformer; interrupting power passage on the feeder in the event of an open circuit fault in the first current-transformer loop; passing electrical current on the feeder via a second current transformer to produce a second monitoring current in a second current-transformer loop; monitoring a differential between the first and the second monitoring currents; and passing a portion of the second monitoring current through a series resistor of a compensation network coupled with the second current transformer to reduce error-producing effects of the first shunt resistor, wherein the series resistor has a value that, when added to a resistance value of wiring between the second current transformer and a burden resistor associated with the second current transformer, produces a net resistance value approximately equal to a wiring resistance between the first current transformer and a burden resistor associated with the first current transformer, wherein interrupting power passage on the feeder is performed within a time T after development of an open fault in the first current-transformer loop; and wherein the time T is less that a time period in which a rate of within the first shunt resistor reaches a rated limit for power dissipation rate for the first shunt resistor. ### Claim 16 (depends on 15) The method of claim 15 wherein: the first shunt resistor has a resistance that has an error-producing effect no greater than about 0.18%; the first shunt resistor has a nominal power dissipation rating of no more than about 4 watts (W); the first shunt resistor has a resistance at least as high as 500 ohms; and the time T is less than about 0.1 seconds.