A method for detection of a sub-synchronous oscillation in a power system includes measuring a three-phase measurement signal of an electric system value, analyzing the measurement signal to detect an oscillation component of the measurement signal having an oscillation frequency lower than a system frequency of the power system, deciding whether the detected oscillation component at the oscillation frequency qualifies as a sub-synchronous oscillation, and disconnecting a generator from the power system that might be affected by the sub-synchronous oscillation. To detect sub-synchronous oscillations with low computational effort and good accuracy, an amplitude of each phase of the oscillation component is calculated and compared against a threshold, a sub-synchronous oscillation is detected upon exceeding the threshold during a given time delay, and a fault signal is generated upon detecting a sub-synchronous oscillation. A device having a processing unit is also provided.

Relay devices and methods of use for detecting and reacting to subsynchronous oscillation events in electrical energy generation and transmission systems, such as but not limited to, subsynchronous control interactions between wind-turbine generators and series-compensated transmission lines.

An uninterruptible power supply device provided between a commercial power system and an essential load and supplying AC power to the essential load includes: a power supply part that has an energy storage device and is connected to a power line for supplying power from the commercial power system to the essential load; an open switch for opening/closing the power supply line, the open switch being provided on the power line on the commercial-power-system side of the power supply part; a system abnormality detection part for detecting a system abnormality occurring on the commercial-power-system side of the open switch; and a control part that, when the detected system abnormality is equal to or greater than the tolerance of the essential load or the power supply part against system abnormalities, opens the open switch and supplies AC power from the power supply part to the essential load.

A method for detection of a sub-synchronous oscillation in a power system includes measuring a three-phase measurement signal of an electric system value, analyzing the measurement signal to detect an oscillation component of the measurement signal having an oscillation frequency lower than a system frequency of the power system, deciding whether the detected oscillation component at the oscillation frequency qualifies as a sub-synchronous oscillation, and disconnecting a generator from the power system that might be affected by the sub-synchronous oscillation. To detect sub-synchronous oscillations with low computational effort and good accuracy, an amplitude of each phase of the oscillation component is calculated and compared against a threshold, a sub-synchronous oscillation is detected upon exceeding the threshold during a given time delay, and a fault signal is generated upon detecting a sub-synchronous oscillation. A device having a processing unit is also provided.

A power-on/off command is output to a breaker for switching when a frequency difference between a plurality of electric power supply sources is within a predetermined range and a phase difference between the plurality of electric power supply sources is within a predetermined range, in switching of electric power supply between the plurality of electric power supply sources. A generator drive rotation speed of a transmission device is feedback controlled so that the frequency difference is maintained at a value within the predetermined range and the phase difference is maintained at a value within the predetermined range when the detected frequency difference is within the predetermined range and the detected phase difference is within the predetermined range. A generator rotation speed command is calculated by adding to the rotation speed command of the transmission device an output value obtained by subjecting the detected phase difference to a proportional-integral-control.

A power-on/off command is output to a breaker for switching when a frequency difference between a plurality of electric power supply sources is within a predetermined range and a phase difference between the plurality of electric power supply sources is within a predetermined range, in switching of electric power supply between the plurality of electric power supply sources. A generator drive rotation speed of a transmission device is feedback controlled so that the frequency difference is maintained at a value within the predetermined range and the phase difference is maintained at a value within the predetermined range when the detected frequency difference is within the predetermined range and the detected phase difference is within the predetermined range. A generator rotation speed command is calculated by adding to the rotation speed command of the transmission device an output value obtained by subjecting the detected phase difference to a proportional-integral-control.

Methods and systems for detecting a falling power line in a power transmission or distribution system are described. Electrical measurements at various points in the power distribution system are used to detect changes in a power line indicating that the conductor has broken or become disconnected. Upon detection, control mechanisms are used to deenergize the conductor while the conductor is falling and/or before it contacts the ground.

Methods and systems for detecting a falling power line in a power transmission or distribution system are described. Electrical measurements at various points in the power distribution system are used to detect changes in a power line indicating that the conductor has broken or become disconnected. Upon detection, control mechanisms are used to deenergize the conductor while the conductor is falling and/or before it contacts the ground.

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.