•Voltage (3U0 or U2)•Current (3I0 · ZNpol or 3I2 ·ZNpol where ZNpol is RNpol + jXNpol), or• both currents and voltage,Dual (dual polarizing, (3U0 + 3I0 · ZNpol) or (U2 + I2 · ZNpol)).Normally voltage polarizing from the internally calculated residual sum or an external opendelta is used.Current polarizing is useful when the local source is strong and a high sensitivity is required. Insuch cases the polarizing voltage (3U0) can be below 1% and it is then necessary to use currentpolarizing or dual polarizing. Multiply the required set current (primary) with the minimumimpedance (ZNpol) and check that the percentage of the phase-to-earth voltage is definitelyhigher than 1% (minimum 3U0>UPolMin setting) as a verification.RNPol, XNPol: The zero-sequence source is set in primary ohms as base for the currentpolarizing. The polarizing voltage is then achieved as 3I0 · ZNpol. The ZNpol can be defined as(ZS1-ZS0)/3, that is the earth return impedance of the source behind the protection. Themaximum earth-fault current at the local source can be used to calculate the value of ZN asU/(√3 · 3I0) Typically, the minimum ZNPol (3 · zero sequence source) is set. Setting is in primaryohms.When the dual polarizing method is used it is important that the settingINx>or the product 3I0· ZNpol is not greater than 3U0. If so, there is a risk for incorrect operation for faults in thereverse direction.IPolMin: is the minimum earth-fault current accepted for directional evaluation. For smallercurrents than this value the operation will be blocked. Typical setting is 5-10% ofIBase.UPolMin: Minimum polarization (reference) polarizing voltage for the directional function,given in % ofUBase/√3.I>Dir: Operate residual current release level in % ofIBase for directional comparison scheme.The setting is given in % ofIBase and must be set below the lowestINx> setting, set for thedirectional measurement. The output signals, STFW and STRV can be used in a teleprotectionscheme. The appropriate signal should be configured to the communication scheme block.7.4.3.3 2nd harmonic restrain M15282-90 v5If a power transformer is energized there is a risk that the current transformer core willsaturate during part of the period, resulting in a transformer inrush current. This will give adeclining residual current in the network, as the inrush current is deviating between thephases. There is a risk that the residual overcurrent function will give an unwanted trip. Theinrush current has a relatively large ratio of 2nd harmonic component. This component can beused to create a restrain signal to prevent this unwanted function.At current transformer saturation a false residual current can be measured by the protection.Also here the 2nd harmonic restrain can prevent unwanted operation.2ndHarmStab: The rate of 2nd harmonic current content for activation of the 2nd harmonicrestrain signal. The setting is given in % of the fundamental frequency residual current.HarmRestrainx: Enable block of stepx from the harmonic restrain function.7.4.3.4 Parallel transformer inrush current logic M15282-97 v5In case of parallel transformers there is a risk of sympathetic inrush current. If one of thetransformers is in operation, and the parallel transformer is switched in, the asymmetricinrush current of the switched in transformer will cause partial saturation of the transformeralready in service. This is called transferred saturation. The 2nd harmonic of the inrush currentsSection 7 1MRK 504 158-UEN ACurrent protection106Application manual
