1. Abstract
The second harmonic inrush restraint function of transformerdifferential relays maintains security of the differential protectionduring transformer inrush events. The typical setpoint for thesecond harmonic restraint is the relay manufacturer’s defaultor recommended setting of 20% of fundamental current, withsome adjustment based on operating experience. However,some operating situations may result in levels of secondharmonic current lower than 20% during inrush, and levels maybe as low as 5%. This lower level of second harmonic currentrequires a lower inrush restraint setting that may impact thetripping time of the differential element for fault conditions. Inaddition, inrush restraint is typically performed on a per-phasebasis, so a loss of security is possible if inrush restraint performsincorrectly on only one phase of the protected transformer.This paper provides several examples of actual events whereloss of security occurred due to incorrect settings of the secondharmonic restraint function, or due to mis-application of crossphaseblocking. Based on the information from these events, thepaper directly discusses considerations and recommendationsfor setting the second harmonic restraint to maintain securityduring transformer inrush including a discussion of traditionaland adaptive inrush restraint techniques. The paper alsoincludes recommendations on when to apply cross-phaseblocking techniques such as 1-out-of-3 blocking, 2-out-of-3blocking and average restraint blocking methods.A key message from this paper is the use of the actual inrushcharacteristic of the protected transformer to determineoptimum setpoints for the differential relay. Microprocessorrelays have the capability to, and should, capture waveformsevery time a transformer is energized. This data should beanalyzed to check the adequacy of the existing second harmonicrestraint settings, to ensure no loss of security occurs.
2. Introduction
Florida Power & Light Company (FPL) is in the process ofreplacing existing transformer protection panels at selectlocations on the FPL transmission system. The existing panelsuse electromechanical or solid-state differential relays, andthe new transformer protection panels will use microprocessorrelays with a standard configuration. As part of the designprocess for these protection panels, FPL is also developing astandard for transformer protection settings.The basic protection for these transformers is differentialprotection. Second harmonic restraint is used to block thedifferential element during inrush events for the transformer.The standard protection settings will include a recommendationfor the selection of the inrush restraint function and the settingsfor this function. Also, these settings will determine whether touse the inrush restraint function on a per-phase or multi-phasebasis.To develop the standard settings for the inrush restraintfunction, FPL used an experimental laboratory procedure alongwith actual operating experiences. A relay was configured tosome inrush restraint function, level setting, and cross-phaseblocking method. These settings were then tested againstsimulated and actual fault events, by playing oscillographicrecords back through a test set to the relay. These tests wererepeated using different setpoints and different restraintfunctions until a standard setting that meets FPL’s operatingcriteria was determined. This standard package of settingswas confirmed by capturing transformer in-rush records fromvarious installations and comparing the setpoint levels to actualsecond harmonic levels to confirm there is enough margin.One of the drivers for this process is an effort to eliminatemisoperations of transformer differential relays due to lowsecond harmonics on inrush without sacrificing protectioncapabilities. Misoperations can occur during the energization ofa transformer due to failure of the harmonic restraint function.A digital fault recorder oscillographic record that captures thistype of harmonic restraint failure was used for the testing laterin this paper. This fault record shows a failure due to the lowlevels of harmonic current produced during an energization.An external fault can also trigger a misoperation during thevoltage recovery period, causing the differential relay to operateimmediately after a fault is cleared from the system.
3. Review of differential protectionprinciples
Differential protection is a fast, selective method of protectionagainst short circuits in transformers, and is the standardprotection used by FPL to protect transformers. Differentialprotection is a practical application of Kirchhoff’s current law.The sum of the currents entering the transformer should equalthe sum of the currents leaving the transformer. Differentialprotection adds the measured currents entering and leavingthe transformer to create a differential current


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