This invention relates to electric networks, and more particularly it relates to a phase sequence network or filter, responsive to selected symmetrical components of current in a polyphase alternating current electric power circuit.
The theory of symmetrical components is well understood by those skilled in the art of electric power transmission and distribution. In accordance with this theory, the phase currents or voltages in any unbalanced three-phase alternating-current electric circuit can be resolved into three sets of symmetrical, balanced current or voltages vectors known respectively as positive sequence, negative sequence and zero sequences components. The negative and zero sequence components of circuit current and voltage are not present under balanced circuit conditions. A more detailed discussion of the theory of symmetrical components can be found in the textbook, Symmetrical Components, by Wagner and Evans, published by McGraw-Hill in 1933.
It has been known that certain circuits, called sequence networks or filters, can be connected to an electric power system to provide an output signal that is proportional to the magnitude of the negative sequence components of current. These sequence networks or filters are particularly useful in the protective relaying art as they will sense the presence of negative sequence components of current which flow when the power system becomes unbalanced due to abnormal of fault conditions. It has also been known in the protective relaying art that it is desirable to have a sequence network that has a "mixed" response, that is, the output is a function of both the positive sequence components of current and the negative sequence components of current. One of the disadvantages of the prior art sequence networks that have a mixed response is that when the circuit values are adjusted to change the circuit response to the positive sequence components, the circuit's response to the negative sequence components also changes. I have found it desirable to have a sequence network in which the positive sequence response can be selectively varied without affecting the amplitude or phase of the circuit's response to the negative sequence components.
The output of the sequence sensitive circuit is a sine wave which generally must be further processed to derive a d-c control signal. Since the processing of the sine wave signal usually involves some filtering which delays the detection of the fault condition, it is desirable to keep the filter as small as possible.
It is, therefore, one object of this invention to provide a sequence network, responsive to both the positive sequence components and negative sequence components of a three phase alternating current power system, in which the response to the positive sequence components can be selectively varied without affecting the response to the negative sequence components.
It is another object of this invention to provide a mixed sequence responsive circuit in which the positive sequence response is 180.degree. out of phase with the negative sequence response.
Another object of this invention is to reduce the filtering required to derive a control signal from the sine wave output of a sequence sensitive circuit.