GE P24DM Technical Manual

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Note:When using an IDG Operate characteristic, DT is always used with a value of zero for the Rest characteristic.An additional setting "IDG Time" is also used to set the minimum operating time at high levels of fault current.0123456789101 10 100I/IN>Operating time (seconds)IDG Is Setting RangeIDG Time Setting RangeV00611IDG Time Setting RangeIDG Is Setting RangeOperating time (seconds)Figure 36: IDG Characteristic7.4 DIRECTIONAL ELEMENTIf Earth fault current can flow in both directions through a protected location, you will need to use a directionalovercurrent element to determine the direction of the fault. Typical systems that require such protection areparallel feeders and ring main systems.A directional element is available for all of the Earth Fault stages. These are found in the direction setting cells forthe relevant stage. They can be set to non-directional, directional forward, or directional reverse.Directional control can be blocked by the VTS element if required.For standard earth fault protection, two options are available for polarisation; Residual Voltage (zero sequence) orNegative Sequence.7.4.1 RESIDUAL VOLTAGE POLARISATIONWith earth fault protection, the polarising signal needs to be representative of the earth fault condition. As residualvoltage is generated during earth fault conditions, this quantity is commonly used to polarise directional earthfault elements. This is known as Zero Sequence Voltage polarisation, Residual Voltage polarisation or NeutralDisplacement Voltage (NVD) polarisation.Small levels of residual voltage could be present under normal system conditions due to system imbalances, VTinaccuracies, device tolerances etc. For this reason, the device includes a user settable threshold (IN> VNPol Set),which must be exceeded in order for the DEF function to become operational. The residual voltage measurementprovided in the MEASUREMENTS 1 column of the menu may assist in determining the required threshold settingduring the commissioning stage, as this will indicate the level of standing residual voltage present.Note:Residual voltage is nominally 180° out of phase with residual current. Consequently, the DEF elements are polarised from the"-Vres" quantity. This 180° phase shift is automatically introduced within the device.P24xM Chapter 6 - Current Protection FunctionsP24xM-TM-EN-2.1 111