ELECTRICAL SYSTEM PROTECTION STRATEGY USING SLIP FREQUENCY CALCULATION
An electrical system includes a plurality of synchronous electrical machines, and an electronic control unit structured to controllably trip a circuit breaking mechanism such as a relay coupling together the electrical machines. The control unit executes control logic to select trip options responsive to slip frequency between the synchronous electrical machines that is induced by a power swing.
1 . A method of protecting an electrical system comprising:
receiving data indicative of an angle of an impedance trajectory defined by an impedance in the electrical system during a power swing;
calculating a value indicative of a frequency of a slip between a first and a second electromagnetic machine M the electrical system that is induced by the power swing, responsive to the data;
asserting a trip mode responsive to the calculated value; and
outputting a trip command to a circuit breaking mechanism in the electrical system according to the asserted trip mode so as to decouple the first and the second electromagnetic machines.
2 . The method of claim 1 further comprising receiving data indicative of a second angle of the impedance trajectory, and wherein the calculating includes calculating the value responsive to the data indicative of the second angle.
3 . The method of claim 2 wherein the first angle and the second angle arc defined by the impedance trajectory at an outer blinder crossing point and an inner blinder crossing point, respectively.
4 . The method of claim 3 wherein the calculating includes calculating the frequency via the equation:
f
slip
=
δ
i
-
δ
o
2
·
π
·
T
oi
where T oi is a travel time of the impedance trajectory from the outer blinder to the inner blinder.
5 . The method of claim 1 wherein the first and second electromagnetic machines include a first and a second electrical generator, and the slip frequency is determined by a difference in rotation frequency between rotors in the first and second electrical generators.
6 . The method of claim 1 wherein the asserting of a trip mode includes asserting one of a trip on-the-way-in mode and a trip on-the-way-out mode, and the outputting further includes outputting the trip command at an earlier time if the trip on-the-way-in mode is asserted and outputting the trip command at a later time if the trip on-the-way-out mode is asserted.
7 . The method of claim 6 further comprising comparing the calculated frequency value with a stored slip frequency value corresponding to an allowable voltage sag time, and asserting the trip on the way in function if the stored slip frequency is less than the stored slip frequency sag time.
8 . An electrical system protection mechanism comprising:
a circuit breaking mecha.nisna positionable within a circuit coupling a first electromagnetic machine to a second electromagnetic machine;
sensing mechanisms structured to monitor parameters of the electrical system indicative of impedance;
an electronic control unit coupled with the sensing mechanisms and in control communication with the circuit breaking mechanism, and the electronic control unit being structured to determine an angle of an impedance trajectory defined by the impedance during a power swing, and to calculate a value indicative of a frequency of a slip between the first and second electromagnetic machines that is induced by the power swing, responsive to the determined angle; and
the electronic control unit further being structured to assert a trip mode responsive to the calculated value, and to output a trip command to the circuit breaking mechanism according to the asserted trip mode so as to decouple the first and the second electromagnetic machines.
9 . The mechanism of claim 8 wherein the circuit breaking mechanism includes a protective relay.
10 . The mechanism of claim 8 wherein the electronic control unit is further structured to determine a second angle of the impedance trajectory, and to calculate the value responsive to the first angle and the second angle.
11 . The mechanism of claim 10 wherein the electronic control unit is further structured to calculate the value responsive to a travel time of the impedance trajectory from an outer blinder having a first resistance setting to an inner blinder having a second resistance setting.
12 . The mechanism of claim 8 wherein the electronic control unit is further structured to compare the calculated value with a stored slip frequency value corresponding to an allowable voltage sag time, and to assert a trip on-the-way-in mode if the calculated slip frequency is less than or equal to the stored slip frequency sag time.
13 . The mechanism of claim 12 wherein the electronic control unit is further structured to assert a trip on-the-way-out mode if the calculated slip frequency is greater than the stored slip frequency sag time.
14 . The mechanism of claim 13 wherein the electronic control unit is further structured to determine a time delay in outputting the trip command in the trip on-the-way-out mode based in part on the slip frequency.
15 . An electrical power system comprising:
a plurality of electromagnetic machines;
electrical circuitry coupling together the plurality of electromagnetic machines;
a circuit breaking mechanism coupled with the circuitry and structured to decouple two of the plurality of electromagnetic machines; and
an electronic control unit in control communication with the circuit breaking mechanism, and being structured to determine an angle of an impedance trajectory defined by an impedance in the electrical circuitry during a power swing, and to determine a frequency of a slip between the plurality of electromagnetic machines that is induced by the power swing, responsive to the determined angle;
the electronic control unit further being structured to output a trip command to the circuit breaking mechanism at a timing that is based at least in part on the determined frequency.
16 . The system of claim 15 wherein the electronic control unit is further structured to determine the impedance responsive to data indicative of voltage amplitude and current amplitude in the electrical system.
17 . The system of claim 15 wherein the electronic control unit is further structured to determine a second angle of the impedance trajectory, and to calculate the value indicative of the slip frequency based on both of the first angle and the second angle and a travel time of the impedance trajectory.
18 . The system of claim 17 wherein the first angle and the second angle are defined by the impedance trajectory at an outer blinder crossing point and an inner blinder crossing point,point, respectively, where the outer and inner blinders are located at levels of resistance along the impedance trajectory.
19 . The system of claim 18 wherein the electronic control unit is structured to calculate the frequency via, the equation:
f
slip
=
δ
i
-
δ
o
2
·
π
·
T
oi
where T oi is a travel time of the impedance trajectory from the outer blinder to the inner blinder.