Methods and systems to prevent relay misoperation and enhance relay dependability during ultrasaturation is disclosed. An ultrasaturation detection system may identify an ultrasaturation condition by comparing a differential current to a first and second derivative of the differential current. When an ultrasaturation condition is determined to be present, the ultrasaturation detection system may provide parameters to a relay to prevent misoperation and enhance relay dependability.

A method of current differential protection performed in a control device is disclosed, wherein the control device has a first operate-restrain characteristic with a differential characteristic pick-up setting I.sub.D. The method includes: determining currents of all terminals of a protected object; determining a differential current based on the determined currents; determining direct current, DC, components in the respective determined currents; detecting a fault; and adjusting, for a detected external fault, the operate-restrain characteristics by setting an adjusted differential characteristic pick-up setting I.sub.D.sub._.sub.adj to be equal to the sum of the differential characteristic pick-up setting I.sub.D and the determined DC components, providing an adapted operate-restrain characteristics. Corresponding control device, computer program and computer program product are also disclosed.

A system and method is presented for lowering inrush current to an uninterruptible power supply. During a startup phase, an AC voltage is applied to the secondary winding of a transformer interposed between an input power supply and a rectifier. An active rectifier coupled to the secondary winding of the transformer is operated as an inverter and supplies the voltage to the secondary winding of the transformer during the startup phase. The magnitude of the AC voltage applied to the secondary winding of the transformer is initially less than the magnitude of the input voltage and is increased gradually over time until it reaches the magnitude of the AC input voltage. In this way, the magnetizing flux of the transformer is increased from zero to a steady-state without having the transformer saturate.

Examples of the disclosure include a system for modifying a trip level of a circuit, the system comprising a protection device configured to activate responsive to a differential current exceeding the trip level, a first current transformer coupled to an input of the circuit, a second current transformer coupled to the output of the circuit, at least one measurement circuit coupled to the first current transformer and to the second current transformer, the at least one measurement circuit being configured to obtain a first current measurement from the first current transformer, obtain a second current measurement from the second current transformer, determine a bias current based on the first current measurement and the second current measurement, and modify the trip level of the protection device based on the bias current.

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 disclosure relates to a method for preventing a maloperation of differential protection of an optical fiber caused by saturation of a single Current Transformer (CT) of 3/2 connection. By applying a combination of the differential judgment and the sub-CT current judgment, reliable identification of internal and external faults is ensured, and the problem of misjudging of the internal fault as the external fault can be prevented as well.

A system for high-impedance differential protection for power systems using a variety of signals from the power system. A fault may be detected using filtered phase current or filtered phase voltage magnitudes from current transformers (CTs) connected in parallel by phase. A fault may be detected using filtered phase voltages and raw phase voltages from the CTs. A fault may be detected using raw phase currents and raw phase voltages from the CTs. A fault may be detected using filtered phase currents and raw phase currents. A fault may be detected using raw phase currents and raw neutral currents. The embodiments herein maintain dependability and security of a differential element even when low-class CTs are used. The embodiments herein may allow users to optimize pickup settings even when low-class CTs are used.

An apparatus includes a transient voltage suppression (TVS) device configured to be coupled in parallel with a burden resistor. The apparatus also includes a switch configured to couple the TVS device across the burden resistor. The apparatus further includes a comparator configured to detect a peak output voltage provided by a circuit, determine whether the peak output voltage meets or exceeds a threshold, and control the switch based on whether the peak output voltage meets or exceeds the threshold.