1. Field of the Invention
This invention relates to a method and an apparatus for detecting an out-of-step in an electric power system, and more particularly to a method and an apparatus for detecting an out-of-step occurred between generator groups installed at both sides of an interconnecting line in an electric power system. This invention also relates to a method and an apparatus for predicting an out-of-step in an electric power system.
2. Description of the Related Art
When detecting the out-of-step occurred between generator groups installed at both sides of a transmission line in an electric power system, an out-of-step detecting apparatus 101 shown in FIG. 20 is provided. In this FIG. 20, G denotes a generator, BUS is a bus, TL is a transmission line, TR is a transformer, L is a load, 102 is a current transformer and 103 is a voltage transformer.
By the way, when detecting the out-of-step using the out-of-step detecting apparatus 101 as described above, several out-of-step detecting methods are known. One of them is a method to detect the out-of-step by using change in impedance of the electric power system detected by the out-of-step detecting apparatus 101 installed at a terminal of a transmission line. The other one is a method to detect the out-of-step from a phase difference of voltages at both ends of a transmission line. Hereinafter, each of these out-of-step detecting methods will be described.
(1) A method to detect the out-of-step from change in impedance.
This out-of-step detecting method is available in two systems: a double circles system (refer to FIG. 21(a) and a three-zones system (refer to FIG. 21 (b)).
The double circles system is a method to detect the out-of-step from an impedance characteristic of a relay that is installed in the out-of-step detecting apparatus 101 at the terminal of the transmission line TL as shown in FIG. 20. Concretely, in FIG. 21(a), by dividing the impedance viewed by the relay at the earthing point into circles A and B, and three impedance zones Z1, Z2 and Z3 are set in advance. The zone Z1 is the inside of the circle A and is for the impedance viewed by the relay when the system becomes faulty. The zone Z2 is for the impedance present between the circles A and B. The zone Z3 is the outside of the circle B and is for the impedance viewed by the relay in the normal operation of the system.
Generally, the impedance Z viewed by the relay shifts from the zone Z3 to the zone Z2 and to the zone Z1 while changing relatively slowly when the electric power system is out-of-step. The double circles system judges that the electric power system is out-of-step when the impedance shifts to the zone Z1 after stayed in the zone Z2 exceeding a specified time.
In the three zones system, in FIG. 21(b), three zones; Z11, Z12 and Z13 are set in advance in the same manner as in the double circles system. The impedance Z viewed by the relay in this state shifts from the zone Z13 to the zone Z12 and to the zone Z11 while changing relatively slowly. The three zones system judges that the electric power system is out-of-step if the impedance Z shifts to the zone Z12 after stayed in the zone Z13 exceeding a specified time.
(2) A method to detect the out-of-step from a phase difference between voltages at both ends of a transmission line.
In this out-of-step detecting method, two voltage detecting means are provided to detect respective voltages at both ends of the transmission line TL. By exchanging these detected voltages through such a transmission means as a microwave circuit, it is judged that the electric power system is out-of-step if a phase angle difference between voltages at both ends exceeds 180.degree..
The method to detect the out-of-step from change in impedance described above is to detect the out-of-step from a time of the impedance to pass through the zones. But, as the impedance passes through the zones while slowly changing, how to determine zone widths is extremely difficult. In other words, there is a problem that the out-of-step is judged or not judged depending on the determined zone widths, and the detection of the out-of-step is not reliable. Further, although a time of the impedance Z to pass through each zone is predetermined, if in a particular case, the impedance Z changes at a faster speed than this predetermined time, such an out-of-step cannot be detected.
On the other hand, in the method to detect the out-of-step from a phase difference between voltages at both ends of a transmission line, voltages at both ends of the transmission line are transmitted using such a transmission means as a microwave circuit. As a result, there are such problems that cost increase is inevitable and it is difficult to find a point of a transmission line to occur an out-of-step locus where voltage becomes zero.