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Provided is an apparatus and method for providing differential protection for a phase angle regulating transformer having a load side and a source side in a three-phase power system. The method includes calculating a first positive sequence component and a first negative sequence component associate

Provided is an apparatus and method for providing differential protection for a phase angle regulating transformer having a load side and a source side in a three-phase power system. The method includes calculating a first positive sequence component and a first negative sequence component associated with incoming secondary currents detected on the source side, calculating a second positive sequence component and a second negative sequence component associated with outgoing secondary currents detected on the load side, applying a phase angle shift to the second positive sequence component and the second negative sequence component to form a compensated positive sequence component and a compensated negative sequence component, respectively, and determining an operate current value and a restraint current value for each of a positive and negative sequence differential element of the differential protection based on the compensated positive sequence component and the compensated negative sequence component.

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What is claimed is: 1. An apparatus for providing differential protection for a transformer having a load side and a source side in a three-phase power system, the apparatus comprising: a microcontroller including a microprocessor, a memory operatively coupled to the microprocessor, a positive sequ

What is claimed is: 1. An apparatus for providing differential protection for a transformer having a load side and a source side in a three-phase power system, the apparatus comprising: a microcontroller including a microprocessor, a memory operatively coupled to the microprocessor, a positive sequence differential element and a negative sequence differential element, the microprocessor adapted to: calculate a first positive sequence component and a first negative sequence component associated with incoming secondary currents detected on the source side, and calculate a second positive sequence component and a second negative sequence component associated with outgoing secondary currents detected on the load side; apply a phase angle shift to the second positive sequence component and the second negative sequence component to form a compensated positive sequence component and a compensated negative sequence component, respectively, to compensate for a phase angle shift introduced by operation of the transformer; and determine an operate current value and a restraint current value for each of the positive and negative sequence differential elements based on the compensated positive sequence component and the compensated negative sequence component. 2. The apparatus of claim 1, wherein the first positive sequence component and the first negative sequence component are derived from a first set of three phasors extracted from the incoming secondary currents, and wherein the second positive sequence component and the second negative sequence component are derived from a second set of three phasors extracted from the outgoing secondary currents. 3. The apparatus of claim 1, wherein the applied phase angle shift is based on a tap position of at least one tap changer and a predetermined number of degrees per step of the transformer. 4. The apparatus of claim 3, wherein the microprocessor is further adapted to: determine a number of steps from a reference position based on the tap position, the tap position adjustable to maintain a set point power flow in the three-phase power system; and multiply the number of steps by the predetermined number of degrees to yield the phase angle shift. 5. The apparatus of claim 1, wherein the microprocessor is further adapted to: multiply the second positive sequence component by a first compensating phasor to form the compensated positive sequence component, the first compensating phasor having a magnitude equal to one and a first phase angle opposite that of the phase angle shift; and multiply the second negative sequence component by a second compensating phasor to form the compensated negative sequence component, the second compensating phasor having a magnitude equal to one and a second phase angle substantially equal to the phase angle shift. 6. The apparatus of claim 1, wherein the microprocessor is further adapted to: calculate an absolute value of a sum of the first positive sequence component and the compensated positive sequence component to form a positive sequence operating current value; calculate an average of an absolute value of the first positive sequence component and an absolute value of the compensated positive sequence component to form a positive sequence restraint current value; and compare the positive sequence operating current value and the positive sequence restraint current value to a first dual slope differential element characteristic to form a first binary output, the first binary output utilized to indicate a trip condition. 7. The apparatus of claim 6, wherein the first binary output indicates the trip condition when the positive sequence operating current value to the positive sequence restraint current value is above a characteristic line of the first dual slope differential element characteristic, and wherein the first binary output indicates a no-trip condition when the positive sequence operating current value to the positive sequence restraint current value is below the characteristic line of the first dual slope differential element characteristic. 8. The method of claim 1, wherein the microprocessor is further adapted to: calculate an absolute value of a sum of the first negative sequence component and the compensated negative sequence component to form a negative sequence operating current value; calculate an average of an absolute value of the first negative sequence component and an absolute value of the compensated negative sequence component to form a negative sequence restraint current value; and compare the negative sequence operating current value and the negative sequence restraint current value to a second dual slope differential element characteristic to form a second binary output. 9. The apparatus of claim 8, wherein the microprocessor is further adapted to: compare a predetermined percentage of a magnitude of the first positive sequence component to a magnitude of the first negative sequence component to form a third binary output; and apply an AND-function to the second binary output and an inverse of the third binary output to form a fourth binary output, the fourth output utilized to indicate a trip condition. 10. The apparatus of claim 9, wherein the fourth binary output indicates the trip condition when the magnitude of the first negative sequence component exceeds the predetermined percentage of the magnitude of the first positive sequence component, and the negative sequence operating current value to the negative sequence restraint current value is above a characteristic line of the second dual slope differential element characteristic; and wherein the fourth binary output indicates a no-trip condition when the predetermined percentage of the magnitude of the first positive sequence component exceeds the magnitude of the first negative sequence component. 11. The apparatus of claim 9, wherein the predetermined percentage is about ten percent. 12. The apparatus of claim 1, wherein the phase angle shift is based on a fixed phase shift. 13. A method for providing differential protection for a transformer having a load side and a source side in a three-phase power system, the differential protection including a positive sequence differential element and a negative sequence differential element, the method comprising: deriving a first set of three phasors from incoming secondary currents detected on the source side and a second set of three phasors from outgoing secondary currents detected on the load side; based on the first set of three phasors, calculating a first positive sequence component and a first negative sequence component; based on the second set of three phasors, calculating a second positive sequence component and a second negative sequence component; determining a phase angle shift based on a tap position of at least one tap changer and a predetermined number of degrees per step of the transformer, the phase angle shift introduced by operation of the transformer; applying the phase angle shift to the second positive sequence component and the second negative sequence component to generate a compensated positive sequence component and a compensated negative sequence component, respectively; and determining an operate current value and a restraint current value for each of the positive and negative sequence differential elements based on the compensated positive sequence component and the compensated negative sequence component. 14. The method of claim 13, wherein determining the phase angle shift comprises: determining a number of steps from a reference position based on the tap position, the tap position adjustable to maintain a set point power flow in the three-phase power system; and multiplying the number of steps by the predetermined number of degrees to yield the phase angle shift. 15. The method of claim 13, wherein applying the phase angle shift to the second positive sequence component and the second negative sequence component comprises: multiplying the second positive sequence component by a first compensating phasor to generate the compensated positive sequence component, the first compensating phasor having a magnitude equal to one and a first phase angle opposite that of the phase angle shift; and multiplying the second negative sequence component by a second compensating phasor to generate the compensated negative sequence component, the second compensating phasor having a magnitude equal to one and a second phase angle substantially equal to the phase angle shift. 16. The method of claim 13, wherein determining the operate current value and the restraint current value for each of the positive and negative sequence differential elements comprises: calculating an absolute value of a sum of the first positive sequence component and the compensated positive sequence component to form a positive sequence operating current value; calculating an average of an absolute value of the first positive sequence component and an absolute value of the compensated positive sequence component to form a positive sequence restraint current value; and comparing the positive sequence operating current value and the positive sequence restraint current value to a first dual slope differential element characteristic of the positive sequence differential element to form a first binary output, the first binary output utilized to indicate a trip condition. 17. The method of claim 16, wherein the first binary output indicates the trip condition when the positive sequence operating current value to the positive sequence restraint current value is above a characteristic line of the first dual slope differential element characteristic, and wherein the first binary output indicates a no-trip condition when the positive sequence operating current value to the positive sequence restraint current value is below the characteristic line of the first dual slope differential element characteristic. 18. The method of claim 13, wherein determining the operate current value and the restraint current value for each of the positive and negative sequence differential elements further comprises: calculating an absolute value of a sum of the first negative sequence component and the compensated negative sequence component to form a negative sequence operating current value; calculating an average of an absolute value of the first negative sequence component and an absolute value of the compensated negative sequence component to form a negative sequence restraint current value; and comparing the negative sequence operating current value and the negative sequence restraint current value to a second dual slope differential element characteristic of the negative sequence differential element to form a second binary output. 19. The method of claim 18, wherein determining the operate current value and the restraint current value for each of the positive and negative sequence differential elements further comprises: comparing a predetermined percentage of a magnitude of the first positive sequence component to a magnitude of the first negative sequence component to form a third binary output; and applying an AND-function to the second binary output and an inverse of the third binary output to form a fourth binary output, the fourth binary output utilized to indicate a trip condition. 20. The method of claim 19, wherein the fourth binary output indicates the trip condition when the magnitude of the first negative sequence component exceeds the predetermined percentage of the magnitude of the first positive sequence component, and the negative sequence operating current value to the negative sequence restraint current value is above a characteristic line of the second dual slope differential element characteristic; and wherein the fourth binary output indicates a no-trip condition when the predetermined percentage of the magnitude of the first positive sequence component exceeds the magnitude of the first negative sequence component. 21. The method of claim 19, wherein the predetermined percentage is about ten percent. 22. A method for providing differential protection for a transformer having a first side and a second side in a three-phase power system, the differential protection including a positive sequence differential element and a negative sequence differential element, the method comprising: calculating a first positive sequence component and a first negative sequence component associated with secondary currents detected on the first side, and calculating a second positive sequence component and a second negative sequence component associated with secondary currents detected on the second side; applying a phase angle shift to the second positive sequence component and the second negative sequence component to form a compensated positive sequence component and a compensated negative sequence component, respectively, to compensate for a phase angle shift introduced by operation of the transformer; and determining an operate current value and a restraint current value for each of the positive and negative sequence differential elements based on the compensated positive sequence component and the compensated negative sequence component. 23. The method of claim 22, wherein determining the phase angle shift comprises: determining a number of steps from a reference position based on the tap position, the tap position adjustable to maintain a set point power flow in the three-phase power system; and multiplying the number of steps by the predetermined number of degrees to yield the phase angle shift. 24. The method of claim 22, wherein applying the phase angle shift to the second positive sequence component and the second negative sequence component comprises: multiplying the second positive sequence component by a first compensating phasor to generate the compensated positive sequence component, the first compensating phasor having a magnitude equal to one and a first phase angle opposite that of the phase angle shift; and multiplying the second negative sequence component by a second compensating phasor to generate the compensated negative sequence component, the second compensating phasor having a magnitude equal to one and a second phase angle substantially equal to the phase angle shift. 25. The method of claim 22, wherein determining the phase angle shift comprises: determining a fixed phase shift of the transformer.