A system and method for determining when an electronic interrupting device will open in response to detecting overcurrent, where the interrupting device protects a transformer in a power distribution network. The method includes obtaining a time/current through fault protection curve that is defined by a plurality of time/current points for the transformer that identifies when the transformer may experience thermal or mechanical damage in response to a certain current flow over a certain time in the transformer windings, selecting a time multiplier, and determining an operating curve for the interrupting device by multiplying the multiplier and a time portion of each of the plurality of time/current points on the through fault protection curve, where the operating curve identifies when the interrupting device will open in response to a certain current flow over a certain time.

A restricted earth fault relay includes: a first differential amount calculation unit that calculates a first differential amount based on a zero-phase current and a neutral point current; a current change detection unit that detects changes in the phase currents; and a first suppression amount calculation unit that calculates a first suppression amount. When the changes in the phase currents are detected, the first suppression amount calculation unit calculates a first maximum value as the first suppression amount, the first maximum value being a maximum value of effective values of the subtraction currents in the phases and an effective value of the neutral point current. The restricted earth fault relay further includes an operation determination unit that outputs a protection signal for protecting a three-phase transformer, when the first differential amount and the first suppression amount are within an operating region.

A restricted earth fault relay includes: a first differential amount calculation unit that calculates a first differential amount based on a zero-phase current and a neutral point current; a current change detection unit that detects changes in the phase currents; and a first suppression amount calculation unit that calculates a first suppression amount. When the changes in the phase currents are detected, the first suppression amount calculation unit calculates a first maximum value as the first suppression amount, the first maximum value being a maximum value of effective values of the subtraction currents in the phases and an effective value of the neutral point current. The restricted earth fault relay further includes an operation determination unit that outputs a protection signal for protecting a three-phase transformer, when the first differential amount and the first suppression amount are within an operating region.

An apparatus for detecting disconnection of a secondary side of a current transformer includes a sampling circuit coupled to the secondary side of the current transformer that samples signals from the secondary side of the current transformer. A pull-up circuit is switchably coupled to the sampling circuit in response to a coupling signal in a diagnostic phase. A processor is coupled to the sampling circuit for receiving sampled signals from the sampling circuit. The processor extracts corresponding signal information from the sampled signals and stores the extracted signal information in a memory. The extracted signal information corresponds to statuses that the secondary side of the current transformer is connected or disconnected to a load. A detector is coupled to the processor and the memory, and accesses the memory using the signal information of the sampled signals.

Systems, devices, and methods for a transformer including: a first drive winding (206) wound on a first core; a second drive winding wound on a second core; a sense winding wound across the first and second cores; and a compensation winding wound across the first and second cores; where one or more utility lines are threaded through a middle of the first and second cores, a common mode current in the one or more utility lines causes one or more pulses to appear on the sense winding, a current on the compensation winding is adjusted until the one or more pulses on the sense winding are cancelled out, and the common mode current on the one or more utility lines is the adjusted current on the compensation winding multiplied by a turn ratio between the compensation winding and the sense winding.

Systems, devices, and methods for a transformer including: a first drive winding (206) wound on a first core; a second drive winding wound on a second core; a sense winding wound across the first and second cores; and a compensation winding wound across the first and second cores; where one or more utility lines are threaded through a middle of the first and second cores, a common mode current in the one or more utility lines causes one or more pulses to appear on the sense winding, a current on the compensation winding is adjusted until the one or more pulses on the sense winding are cancelled out, and the common mode current on the one or more utility lines is the adjusted current on the compensation winding multiplied by a turn ratio between the compensation winding and the sense winding.

The present invention discloses a method for detecting a fault and a current differential protection system thereof. The method for detecting fault includes calculating fault component currents, calculating an operate current and a restraint current of all terminals, and adjusting an operate level by adapting an operate-restraint characteristic according to a fault component. The method has very small operating area for external faults, which makes the method very reliable even for the case with serious CT saturation. A very big operating area for internal faults makes the method very sensitive for internal faults even for a heavy load and high resistance fault.

The present invention discloses a method for detecting a fault and a current differential protection system thereof. The method for detecting fault includes calculating fault component currents, calculating an operate current and a restraint current of all terminals, and adjusting an operate level by adapting an operate-restraint characteristic according to a fault component. The method has very small operating area for external faults, which makes the method very reliable even for the case with serious CT saturation. A very big operating area for internal faults makes the method very sensitive for internal faults even for a heavy load and high resistance fault.

Techniques for detecting an inrush current in a power transformer in are disclosed. For example, the presence of an inrush current on a current path in a power transformer may be determined by receiving a signal from a Rogowski coil positioned on a current path of a power transformer, the signal corresponding to a current flowing in the current path; sampling the received signal to produce samples of the received signal; and analyzing the samples of the received signal relative to at least two criteria to determine whether an inrush current is present. When an inrush current is present, operation of a protective relay is blocked.

Techniques for detecting an inrush current in a power transformer in are disclosed. For example, the presence of an inrush current on a current path in a power transformer may be determined by receiving a signal from a Rogowski coil positioned on a current path of a power transformer, the signal corresponding to a current flowing in the current path; sampling the received signal to produce samples of the received signal; and analyzing the samples of the received signal relative to at least two criteria to determine whether an inrush current is present. When an inrush current is present, operation of a protective relay is blocked.