A method for identifying element failure in capacitor banks is provided. Capacitor bank phase current is measured and a zero-sequence or negative-sequence current is calculated or measured. A three-phase voltage is measured from a three-phase bus voltage transformer and a zero-sequence voltage or negative-sequence voltage is calculated. A compensated unbalance current is calculated and compared to a predetermined acceptable range. A failure is identified where the compensated unbalance current is outside the predetermined acceptable range.

An arrangement and method for zero sequence differential protection of a transmission line of a power system are disclosed. A zero sequence differential protection unit is configured to detect an internal fault of the transmission line using current measurements of each phase A, B, C of the transmission line. The zero sequence differential protection unit is configured to initiate a trip when an internal fault is detected. A phase selection unit is configured to determine whether or not any of the phases A, B, C of the transmission line is faulty using a comparison of differential values of the current measurements for each phase. The phase selection unit is configured to, based on the initiation of the trip, determine which of the phases A, B, C is faulty and to finalize the trip for any faulty phase.

An embodiment relates to a switchgear for a single-phase motor and a three-phase motor, the switchgear including a processing unit and a first, second and third current path, the first and third current path each including a current transformer. The processing unit is adapted to detect the current I.sub.1 of the first current path and the current I.sub.3 of the third current path. To provide a cost-effective switchgear for a one-phase motor and a three-phase motor which is adapted to identify the failure of every single phase in the three-phase operation and a phase failure in the one-phase operation, the processing unit is designed such as to detect the currents I.sub.1, I.sub.3 of the first and third current path and to determine, based on the phase shift between the detected currents I.sub.1, I.sub.3 of the first and third current path in which operating mode the switchgear is operated.

A method for detecting faults (4) in a three-phase electrical distribution network comprising determining a zero sequence current (21 C), a first phase current (21A) and a second phase current (21 B) at a location of the three-phase electrical distribution network, determining first filtered currents (22) by removing a frequency component from the determined currents corresponding to a fundamental frequency of the electrical distribution network through filtering out said frequency component, determining directions of the first filtered currents during a first time period (23), and comparing said directions (24) relatively to each other, and, if at least one of the determined directions is opposite with respect to at least one of the other two determined directions, signaling a detection of a fault (25).

A method for protecting a low-voltage distribution network. The low-voltage distribution network includes a low-voltage side of a three-phase distribution transformer that is configured to supply electrical power to at least one single-phase load through a respective distribution line of a plurality of three-phase distribution lines distribution lines. The method includes measuring variations of a periodic neutral-to-ground voltage between a neutral terminal of the three-phase distribution transformer and a local ground node by sampling the variations at a sampling frequency, detecting a fault in the low-voltage power distribution network based on the variations of the periodic neutral-to-ground voltage, and disconnecting the low-voltage side from the low-voltage power distribution network responsive to the fault being detected.

A fault detection system has line current monitors. Each line current monitor couples to a line-phase of an electric power transmission system. Each line current monitor has a phase detector, a loop filter and a controlled oscillator, coupled as a phase locked loop. The phase detector has a rotating frame transform. The phase detector couples to a line-phase and provides in-phase and quadrature signals in a rotating frame, based on in-phase and quadrature signals proportional to current in the line-phase. One or more fault detection modules are coupled to the line current monitors through inter-phase communication of the in-phase and quadrature signals in a time frame rotating at the line frequency. The communication may have electrical isolation.

A fault detection system has line current monitors. Each line current monitor couples to a line-phase of an electric power transmission system. Each line current monitor has a phase detector, a loop filter and a controlled oscillator, coupled as a phase locked loop. The phase detector has a rotating frame transform. The phase detector couples to a line-phase and provides in-phase and quadrature signals in a rotating frame, based on in-phase and quadrature signals proportional to current in the line-phase. One or more fault detection modules are coupled to the line current monitors through inter-phase communication of the in-phase and quadrature signals in a time frame rotating at the line frequency. The communication may have electrical isolation.

An embodiment of the invention relates to a method for ground fault detection for low-voltage three-phase AC circuits having a neutral conductor, in which first to fourth analog current signals of the first to third phase conductors and of the neutral conductor of the three-phase AC circuit are ascertained that each contain the level or an equivalent of the level of the current. The analog current signals are time-division multiplexed, subjected to a/d conversion, and a sequence of time-division-multiplexed first to fourth digital current values present in first to fourth successive time frames form first to fourth current signals. Three of the four digital current signals are interpolated to ascertain interpolated current values. The current value of the time frame of the uninterpolated current signal and the interpolated current values for the time frame of the uninterpolated current signal are used for discovering the ground fault.

A method for detecting faults (4) in a three-phase electrical distribution network comprising determining a zero sequence current (21C), a first phase current (21A) and a second phase current (21B) at a location of the three-phase electrical distribution network, determining first filtered currents (22) by removing a frequency component from the determined currents corresponding to a fundamental frequency of the electrical distribution network through filtering out said frequency component, determining directions of the first filtered currents during a first time period (23), and comparing said directions (24) relatively to each other, and, if at least one of the determined directions is opposite with respect to at least one of the other two determined directions, signaling a detection of a fault (25).

There is provided mechanisms for zero sequence differential protection of a transmission line of a power system. An arrangement comprises a zero sequence differential protection unit configured for, using current measurements of each phase A, B, C of the transmission line, internal fault detection of the transmission line. The arrangement comprises a phase selection unit configured for, using a comparison of differential values of the current measures for each phase, determination of whether any of the phases A, B, C of the transmission line is faulty or not. A trip is caused by the zero sequence differential protection unit when an internal fault is detected by the zero sequence differential protection unit, and the trip starts all the phases A, B, C for the phase selection unit to finalize the trip for said any faulty phase.