Polyphase signal monitoring system

Claim 2

A polyphase signal monitoring system comprising at least first, second, and third input terminals adapted to be coupled to a polyphase line; first circuit means having an input circuit means coupled to said first and second input terminals and having an output circuit means for developing a first signal of a predetermined amplitude; second circuit means having an input circuit means coupled to said third input terminal and having an output circuit means for developing a second signal having an amplitude substantially equal to said predetermined amplitude and being substantially 90* out of phase with said first signal; first phase shifting network means coupled across said output circuit means of said first circuit means for developing a third signal being a predetermined number of degrees out of phase with said first signal; second phase shifting network means coupled across said output circuit means of said second circuit means for developing a fourth signal being a predetermined number of degrees out of phase with said second signal; and, signal monitoring means coupled to said first and second phase shifting network means for providing an output indication of the values of a said third signal and a said fourth signal to thereby provide an indication of the value of parameters of a polyphase signal.

Claim 3

A signal monitoring system as defined in claim 1 wherein said first circuit means includes a transformer having an input circuit coupled to said first and second input terminals and an output circuit means for developing a said first signal.

Claim 4

A signal monitoring system as defined in claim 1 wherein said second circuit means includes a transformer having an input circuit coupled to said third input terminal and an output circuit means for developing a said second signal.

Claim 5

A signal monitoring system as defined in claim 1 wherein said first circuit means includes a transformer having an input circuit coupled to said first and second input terminaLs and an output circuit means for developing a said first signal; and, said second circuit means includes a transformer having an input circuit coupled to said third input terminal and an output circuit means for developing a said second signal.

Claim 6

A signal monitoring system as defined in claim 1 wherein said first phase shifting network means includes a reactive circuit means and a resistive circuit means coupled across said output circuit means of said first circuit means.

Claim 7

A signal monitoring system as defined in claim 1 wherein said first phase shifting network means includes an inductive reactance circuit means and a resistive circuit means coupled across said output circuit means of said first circuit means.

Claim 8

A signal monitoring system as defined in claim 1 wherein said first phase shifting network means includes a capacitive reactance means and a resistive circuit means coupled across said output circuit means of said first circuit means.

Claim 9

A signal monitoring system as defined in claim 5 wherein said reactive circuit means and said resistive circuit means are coupled in series connection with respect to each other across said output circuit means of said first circuit means.

Claim 10

A signal monitoring circuit as defined in claim 5 wherein said signal measuring means provides an output indication of the value of a voltage developed from a junction point between said resistive circuit means and said reactive circuit means, and said second phase shifting circuit means.

Claim 11

A signal monitoring system as defined in claim 1 wherein said second phase shifting network means includes a reactive circuit means and a resistive circuit means coupled across said output circuit means of said second circuit means.

Claim 12

A signal monitoring system as defined in claim 1 wherein said second phase shifting network means includes an inductive circuit means and a resistive circuit means coupled across said output circuit means of said second circuit means.

Claim 13

A signal monitoring system defined in claim 1 wherein said second phase shifting network means includes a capacitive reactance means and a resistive circuit means coupled across said output circuit means of said second circuit means.

Claim 14

A signal monitoring system as defined in claim 10 wherein said reactive circuit means and said resistive circuit means are coupled in series connection with respect to each other across said output circuit means of said second circuit means.

Claim 15

A signal monitoring system as defined in claim 1 including common circuit means coupled between said output circuit means of said first circuit means and said output circuit means of said second circuit means.

Claim 16

A signal monitoring system as defined in claim 1 wherein said signal monitoring means includes circuit means for providing a said output indication representative of the difference between the value of a said third signal and the value of a said fourth signal.

Claim 17

A signal monitoring system as defined in claim 1 wherein said signal monitoring means includes circuit means for providing a said output indication representative of the sum of the value of a said third signal and the value of a said fourth signal.

Claim 18

A signal monitoring system as defined in claim 1 wherein said signal monitoring means includes circuit means for providing a said output indication representative of the ratio of the value of a said third signal and the value of a said fourth signal to thereby provide an output indication of the frequency of the polyphase signal.

Claim 19

A signal monitoring system as defined in claim 15 wherein said signal monitoring means includes second circuit means for providing a second output indication representative of the sum of the values of a said third signal and a said fourth signal.

Claim 20

A polyphase signal monitoring system comprising: first circuit means for developing a first control signal having a value representative of the sum of the values of positive and negative phase sequence cOmponents of a polyphase system; second circuit means for developing a second control signal having a value representative of the difference of the values of said positive and negative phase sequence components; summing circuit means for developing a first output signal having a value representative of the value of a positive phase sequence component of a polyphase signal; and, difference circuit means for developing a second output signal having a value representative of the value of a negative phase sequence component of a polyphase signal to thereby provide an indication of the value of parameters of a polyphase signal.

Claim 21

A polyphase signal monitoring system comprising: first circuit means for developing a first control signal having a value representative of the sum of the values of positive and negative phase sequence cOmponents of a polyphase system; second circuit means for developing a second control signal having a value representative of the difference of the values of said positive and negative phase sequence components; summing circuit means for developing a first output signal having a value representative of the value of a positive phase sequence component of a polyphase signal; and, difference circuit means for developing a second output signal having a value representative of the value of a negative phase sequence component of a polyphase signal to thereby provide an indication of the value of parameters of a polyphase signal.

Claim 22

A signal monitoring system as defined in claim 19 wherein said first circuit means includes a transformer having an input circuit adapted to be coupled to two lines of a polyphase line and an output circuit means for developing a said first control signal.

Claim 23

A signal monitoring system as defined in claim 19 wherein said second circuit means includes a transformer having an input circuit adapted to be coupled to at least one line of a polyphase line and an output circuit means for developing a said second signal.

Claim 24

A signal monitoring system as defined in claim 19 wherein said first circuit means includes a transformer having an input circuit adapted to be coupled to two lines of a polyphase supply line and an output circuit means for developing a said first signal; and, said second circuit means includes a transformer having an input circuit adapted to be coupled to a third line of a said polyphase supply line and an output circuit means for developing a said second signal.

Claim 25

A multiphase signal monitoring system comprising first, second, and third input terminals adapted to be coupled to a multiphase supply line; first circuit means having an input circuit means coupled to said first and second input terminals and having an output circuit means for developing a first signal of a predetermined amplitude; second circuit means having an input circuit means coupled to said third input terminal and having an output circuit means for developing a second signal having an amplitude substantially equal to said predetermined amplitude and being substantially 90* out of phase with said first signal; first reactive circuit means coupled across said output circuit means of said first circuit means for providing a third signal being out of phase with a said first signal; first resistive circuit means coupled across said output circuit means of said first circuit means for providing a fourth signal; second reactive circuit means coupled across said output circuit means of said second circuit means for providing a fifth signal being out of phase with a said second signal; second resistive circuit means coupled across said output circuit means of said second circuit means for providing a sixth signal; and, signal measuring means coupled to said first and second resistive circuit means for providing an output indication of the value of a said fourth signal with respect to the value of a said sixth signal to thereby provide an indication of the value of parameters of a polyphase signal.

Claim 26

A multiphase signal monitoring system comprising first, second, and third input terminals adapted to be coupled to a multiphase supply line; first circuit means having an input circuit means coupled to said first and second input terminals and having an output circuit means for developing a first signal of a predetermined amplitude; second circuit means having an input circuit means coupled to said third input terminal and having an output circuit means for developing a second signal having an amplitude substantially equal to said predetermined amplitude and being substantially 90* out of phase with said first signal; first reactive circuit means coupled across said output circuit means of said first circuit means for providing a third signal being out of phase with a said first signal; first resistive circuit means coupled across said output circuit means of said first circuit means for providing a fourth signal; second reactive circuit means coupled across said output circuit means of said second circuit means for providing a fifth signal being out of phase with a said second signal; second resistive circuit means coupled across said output circuit means of said second circuit means for providing a sixth signal; and, signal measuring means coupled to said first and second resistive circuit means for providing an output indication of the value of a said fourth signal with respect to the value of a said sixth signal to thereby provide an indication of the value of parameters of a polyphase signal.

Claim 27

A signal monitoring system as defined in claim 23 wherein said signal monitoring means includes circuit means for providing a said output indication representative of the difference between the value of a said third signal and the value of a said fourth signal.

Claim 28

A signal monitoring system as defined in claim 23 wherein said signal monitoring means includes circuit means for providing a said output indication representative of the sum of the value of a said third signal and the value of a said fourth signal.

Claim 29

A signal monitoring system as defined in claim 23 wherein said first reactive circuit means and said first resistive circuit means are coupled in series connection with respect to each other across said output circuit means of said first circuit means, and said second reactive circuIt means and said second resistive circuit means are coupled in series connection with respect to each other across said output circuit means of said second circuit means.

Claim 30

A signal monitoring system as defined in claim 23 wherein said first circuit means includes a transformer having an input circuit coupled to said first and second input terminals and an output circuit means for developing a said first signal.

Claim 31

A signal monitoring system as defined in claim 23 wherein said second circuit means includes a transformer having an input circuit coupled to said third input terminal and an output circuit means for developing a said second signal.

Claim 32

A signal monitoring system as defined in claim 23 wherein said first circuit means includes a transformer having an input circuit coupled to said first and second input terminals and an output circuit means for developing a said first signal; and, said second circuit means includes a transformer having an input circuit coupled to said third input terminal and an output circuit means for developing a said second signal.

Claim 33

A polyphase signal monitoring system comprising first, second, and third input terminals adapted to be coupled to a polyphase supply line; first circuit means having an input circuit means coupled to said first and second input terminals and having an output circuit means for developing a first signal which defines a first phasor; second circuit means having an input circuit means coupled to said third input terminal and having an output circuit means for developing a second signal which defines a second phasor having a common midpoint with respect to a said first phasor; first phase shifting network means coupled across said output circuit means of said first circuit means for developing a third signal being a predetermined number of degrees out of phase with said first signal; second phase shifting network means coupled across said output circuit means of said second circuit means for developing a fourth signal being a predetermined number of degrees out of phase with said second signal; and, signal monitoring means coupled to said first and second phase shifting network means for providing an output indication representative of the value of a said third signal with respect to the value of a said fourth signal to thereby provide an indication of the value of parameters of a polyphase signal.

Claim 34

A polyphase signal monitoring system comprising first, second, and third input terminals adapted to be coupled to a polyphase supply line; first circuit means having an input circuit means coupled to said first and second input terminals and having an output circuit means for developing a first signal which defines a first phasor; second circuit means having an input circuit means coupled to said third input terminal and having an output circuit means for developing a second signal which defines a second phasor having a common midpoint with respect to a said first phasor; first phase shifting network means coupled across said output circuit means of said first circuit means for developing a third signal being a predetermined number of degrees out of phase with said first signal; second phase shifting network means coupled across said output circuit means of said second circuit means for developing a fourth signal being a predetermined number of degrees out of phase with said second signal; and, signal monitoring means coupled to said first and second phase shifting network means for providing an output indication representative of the value of a said third signal with respect to the value of a said fourth signal to thereby provide an indication of the value of parameters of a polyphase signal.

Claim 35

A signal monitoring system as defined in claim 30 wherein said first circuit means includes a transformer having an input circuit coupled to said first and second input terminals and an output circuit means for developing a said first signal.

Claim 36

A signal monitoring system as defined in claim 30 wherein said second circuit means includes a transformer having an input circuit coupled to said third input terminal and an output circuit means for developing a said second signal.

Claim 37

A signal monitoring system as defined in claim 30 wherein said first circuit means includes a transformer having an input circuit coupled to said first and second input terminals and an output circuit means for developing a said first signal; and, said second circuit means includes a transformer having an input circuit coupled to said third input terminal and an output circuit means for developing a said second signal.

Claim 38

A signal monitoring system as defined in claim 30 wherein said first phase shifting network means includes a reactive circuit means and a resistive circuit means coupled across said output circuit means of said first circuit means.

Claim 39

A signal monitoring system as defined in claim 30 wherein said first phase shifting network means includes an inductive reactance circuit means and a resistive circuit means coupled across said output circuit means of said first circuit means.

Claim 40

A polyphase signal monitoring system comprising at least first, second, and third input terminals adapted to be coupled to a polyphase supply line; first circuit means havIng an input circuit means coupled to said first and second input terminals and having an output circuit means for developing a first signal which defines a first phasor; second circuit means having an input circuit means coupled to said third input terminal and having an output circuit means having a common midpoint with respect to a said first output circuit means of said first circuit means for developing a second signal which defines a second phasor; first phase shifting network means coupled across said output circuit means of said first circuit means for developing a third signal being a predetermined number of degrees out of phase with respect to a said first signal and having a magnitude substantially independent of the frequency of signals on the polyphase supply line; second phase shifting network means coupled across said output circuit means of said second circuit means for developing a fourth signal being a predetermined number of degrees out of phase with said second signal; and, signal monitoring means coupled to said first and second phase shifting network means for providing an output indication of the value of a said third signal with respect to the value of a said fourth signal to thereby provide an indication of the value of parameters of a polyphase signal.

Claim 41

A polyphase signal monitoring system comprising at least first, second, and third input terminals adapted to be coupled to a polyphase supply line; first circuit means havIng an input circuit means coupled to said first and second input terminals and having an output circuit means for developing a first signal which defines a first phasor; second circuit means having an input circuit means coupled to said third input terminal and having an output circuit means having a common midpoint with respect to a said first output circuit means of said first circuit means for developing a second signal which defines a second phasor; first phase shifting network means coupled across said output circuit means of said first circuit means for developing a third signal being a predetermined number of degrees out of phase with respect to a said first signal and having a magnitude substantially independent of the frequency of signals on the polyphase supply line; second phase shifting network means coupled across said output circuit means of said second circuit means for developing a fourth signal being a predetermined number of degrees out of phase with said second signal; and, signal monitoring means coupled to said first and second phase shifting network means for providing an output indication of the value of a said third signal with respect to the value of a said fourth signal to thereby provide an indication of the value of parameters of a polyphase signal.

Claim 42

A signal monitoring system as defined in claim 30 wherein said first phase shifting network means includes a capacitive reactance circuit means and a resistive circuit means coupled across said output circuit means of said first circuit means.

Claim 43

A signal monitoring system as defined in claim 36 wherein said signal monitoring means includes circuit means for providing a said output indication representative of the difference between the value of a said third signal and the value of a said fourth signal.

Claim 44

A signal monitoring system as defined in claim 36 wherein said signal monitoring means includes circuit means for providing a said output indication representative of the sum of the value of a said third signal and the value of a said fourth signal.

Claim 45

A signal monitoring system as defined in claim 36 wherein said signal monitoring means includes circuit means for providing a said output indication representative of the ratio of the value of a said third signal and the value of a said fourth signal to thereby provide an output indication of the frequency of the polyphase signal.

Claim 46

A signal monitoring system as defined in claim 38 wherein said signal monitoring means includes second circuit means for providing a second output indication representative of the sum of the values of a said third signal and a said fourth signal.

Claim 47

A signal monitoring system as defined in claim 36 wherein said first circuit means includes a transformer having an input circuit coupled to said first and second input terminals and an output circuit means for developing a said first signal.

Claim 48

A signal monitoring system as defined in claim 36 wherein said second circuit means includes a transformer having an input circuit coupled to said third input terminal and an output circuit means for developing a said second signal.

Claim 49

A signal monitoring system as defined in claim 36 wherein said first circuit means includes a transformer having an input circuit coupled to said first and second input terminals and an output circuit means for developing a said first signal; and, said second circuit means includes a transformer having an input circuit coupled to said third input terminal and an output circuit means for developing a said second signal.

Claim 50

A signal monitoring system as defined in claim 36 wherein said first phase shifting network means includes a reactive circuit means and a resistive circuit means coupled across said output circuit means of said first circuit means.

Claim 51

A signal monitoring system as defined in claim 1 wherein said signal measuring means includes input circuit means coupled to said first phase shifting network, said output circuit means of said sEcond circuit means, said second phase shifting network and said output circuit means of said first circuit means for receiving said first, second, third and fourth signals, and output circuit means for developing an output signal having a value representative of the ratio between the potential difference of said first and third signals and the potential difference of the second and fourth signals to thereby provide an output signal having a value indicative of the frequency of signals on the polyphase line.

Claim 52

A signal monitoring system for measuring signals on a polyphase system comprising at least first, second, and third input terminals adapted to be coupled to the polyphase system; first circuit means having an input circuit means coupled to said first and second input terminals and having an output circuit means for developing a first signal of a given frequency and of a predetermined amplitude; second circuit means having an input circuit means coupled to said third input terminal and having an output circuit means for developing a second signal having a phase which is substantially 90* out of phase with said first signal; first phase shifting network means coupled across said output circuit means of said first circuit means for developing a third signal having an amplitude which is substantially independent of the frequency of said first signal and being a predetermined number of degrees out of phase with said first signal; second phase shifting network means coupled across said output circuit means of said second circuit means for developing a fourth signal having an amplitude which is substantially independent of the frequency of said second signal and being substantially in phase with said third signal when the signals on the polyphase system are in phase balance; and, signal monitoring means coupled to said first and second phase shifting network means for providing an output indication of the values of a said third signal and a said fourth signal to thereby provide an indication of the value of parameters of a polyphase signal.

Claim 53

A signal monitoring system for measuring signals on a polyphase system comprising at least first, second, and third input terminals adapted to be coupled to the polyphase system; first circuit means having an input circuit means coupled to said first and second input terminals and having an output circuit means for developing a first signal of a given frequency and of a predetermined amplitude; second circuit means having an input circuit means coupled to said third input terminal and having an output circuit means for developing a second signal having a phase which is substantially 90* out of phase with said first signal; first phase shifting network means coupled across said output circuit means of said first circuit means for developing a third signal having an amplitude which is substantially independent of the frequency of said first signal and being a predetermined number of degrees out of phase with said first signal; second phase shifting network means coupled across said output circuit means of said second circuit means for developing a fourth signal having an amplitude which is substantially independent of the frequency of said second signal and being substantially in phase with said third signal when the signals on the polyphase system are in phase balance; and, signal monitoring means coupled to said first and second phase shifting network means for providing an output indication of the values of a said third signal and a said fourth signal to thereby provide an indication of the value of parameters of a polyphase signal.

Claim 54

A signal monitoring system for measuring signals on a polyphase system comprising at least first, second, and third input terminals adapted to be coupled to the polyphase system; first circuit means having an input circuit means coupled to said first and second input terminals and having an output circuit means for developing a first signal of a given frequency and of a predetermined amplitude; second circuit means having an input circuit means coupled to said third input terminal and having an output circuit means for developing a second signal having a phase which is substantially 90* out of phase with said first signal; first phase shifting network means coupled across said output circuit means of said first circuit means for developing a third signal being a predetermined number of degrees out of phase with said first signal and having an amplitude which is substantially independent of the frequency of said first signal and of a value substantially equal to one-half the amplitude of said first signal; second phase shifting network means coupled across said output circuit means of said second circuit means for developing a fourth signal having an amplitude which is substantially independent of the frequency of said second signal and being substantially in phase with said third signal when the signals on the polyphase system are in phase balance and of a value substantially equal to one-half the amplitude of said second signal; and, signal monitoring means coupled to said first and second phase shifting network means for providing an output indication of the values of a said third signal and a said fourth signal to thereby provide an indication of the value of parameters of a polyphase signal.

Claim 55

A signal monitoring system for measuring signals on a polyphase system comprising at least first, second, and third input terminals adapted to be coupled to the polyphase system; first circuit means having an input circuit means coupled to said first and second input terminals and having an output circuit means for developing a first signal of a given frequency and of a predetermined amplitude; second circuit means having an input circuit means coupled to said third input terminal and having an output circuit means for developing a second signal having a phase which is substantially 90* out of phase with said first signal; first phase shifting network means coupled across said output circuit means of said first circuit means for developing a third signal being a predetermined number of degrees out of phase with said first signal and having an amplitude which is substantially independent of the frequency of said first signal and of a value substantially equal to one-half the amplitude of said first signal; second phase shifting network means coupled across said output circuit means of said second circuit means for developing a fourth signal having an amplitude which is substantially independent of the frequency of said second signal and being substantially in phase with said third signal when the signals on the polyphase system are in phase balance and of a value substantially equal to one-half the amplitude of said second signal; and, signal monitoring means coupled to said first and second phase shifting network means for providing an output indication of the values of a said third signal and a said fourth signal to thereby provide an indication of the value of parameters of a polyphase signal.

Claim 56

A signal monitoring system as defined in claim 30 wherein said first phase shifting network means includes a capacitive reactance circuit means and a resistive circuit means coupLed across said output circuit means of said first circuit means.

Claim 57

A signal monitoring system as defined in claim 30 wherein said signal monitoring means includes circuit means for providing a said output indication representative of the difference between the value of a said third signal and the value of a said fourth signal.

Claim 58

A signal monitoring system as defined in claim 30 wherein said signal monitoring means includes circuit means for providing a said output indication representative of the sum of the value of a said third signal and the value of a said fourth signal.

Claim 59

A signal monitoring system as defined in claim 30 wherein said signal monitoring means includes circuit means for providing a said output indication representative of the ratio of the value of a said third signal and the value of a said fourth signal to thereby provide an output indication of the frequency of the polyphase signals.

Claim 60

A signal monitoring system as defined in claim 50 wherein said signal monitoring means includes second circuit means for providing a second output indication representative of the sum of the values of a said third signal and a said fourth signal.

Claim 61

A signal monitoring system as defined in claim 36 wherein said first phase shifting network means includes an inductive reactance circuit means and a resistive circuit means coupled across said output circuit means of said first circuit means.

Claim 62

A signal monitoring system for measuring signals on a polyphase system comprising at least first, second, and third input terminals adapted to be coupled to the polyphase system; first phase shifting means coupled to said first and said second input terminals for developing a first signal having a given amplitude and phase angle; second phase shifting means coupled to said third input terminal for developing a second signal having an amplitude which is substantially equal to the amplitude of said first signal and being substantially in phase with said first signal when the signals on the polyphase system are in phase balance; and, signal monitoring means coupled to said first and second phase shifting means for providing an output indication of the values of a said first signal and a said second signal to thereby provide an indication of the value of parameters of the polyphase system.

Claim 63

A signal monitoring system for measuring signals on a polyphase system comprising at least first, second, and third input terminals adapted to be coupled to the polyphase system; first phase shifting means coupled to said first and said second input terminals for developing a first signal having a given amplitude and phase angle; second phase shifting means coupled to said third input terminal for developing a second signal having an amplitude which is substantially equal to the amplitude of said first signal and being substantially in phase with said first signal when the signals on the polyphase system are in phase balance; and, signal monitoring means coupled to said first and second phase shifting means for providing an output indication of the values of a said first signal and a said second signal to thereby provide an indication of the value of parameters of the polyphase system.

Claim 64

A signal monitoring system for measuring signals on a polyphase system comprising; first phase shifting means having an input circuit coupled to a polyphase system for receiving a first signal of a given phase angle and having an output circuit means for developing a second signal having a given amplitude and being a predetermined number of degrees out of phase with respect to said first signal; second phase shifting means coupled to a polyphase system for receiving a third signal which is substantially 90* out of phase with respect to said first signal and having an output circuit means for developing a fourth signal being a predetermined number of degrees out of phase with respect to said third signal having an amplitude which is substantially equal to the amplitude of said second signal and being substantially in phase with said second signal when the polyphase system is in phase balance; and, signal monitoring means coupled to said first and second phase shifting means for providing an output indication of the values of a said second signal and a said fourth signal to thereby provide an indication of the value of parameters of the polyphase system.

Claim 65

A signal monitoring system for measuring signals on a polyphase system comprising; first phase shifting means having an input circuit coupled to a polyphase system for receiving a first signal of a given phase angle and having an output circuit means for developing a second signal having a given amplitude and being a predetermined number of degrees out of phase with respect to said first signal; second phase shifting means coupled to a polyphase system for receiving a third signal which is substantially 90* out of phase with respect to said first signal and having an output circuit means for developing a fourth signal being a predetermined number of degrees out of phase with respect to said third signal having an amplitude which is substantially equal to the amplitude of said second signal and being substantially in phase with said second signal when the polyphase system is in phase balance; and, signal monitoring means coupled to said first and second phase shifting means for providing an output indication of the values of a said second signal and a said fourth signal to thereby provide an indication of the value of parameters of the polyphase system.

Claim 66

An apparatus as defined in claim 56 including circuit means coupling said first and second phase shifting circuit means for referencing the midpoints of said second and fourth signals to a common voltage potential.

Claim 67

An apparatus as defined in claim 56 including circuit means coupling said first and second phase shifting circuit means for referencing the midpoints of said second and fourth signals to a common current level.

Claim 68

A signal monitoring system as defined in claim 57 wherein saId signal monitoring means includes a summing circuit for measuring the positive phase sequence component of the polyphase system.

Claim 69

A signal monitoring system as defined in claim 57 wherein said signal monitoring means includes a difference circuit for measuring the negative phase sequence component of the polyphase system.