• Special thermal overload protection• Open Phase protection• Unbalance protection2. Generator protection• 80-95% Stator earth fault protection (measured or calculated 3Uo)• Rotor earth fault protection (with external COMBIFLEX RXTTE4 injection unit)• Underimpedance protection• Voltage Controlled/Restrained Overcurrent protection• Turn-to-Turn & Differential Backup protection (directional Negative Sequence.Overcurrent protection connected to generator HV terminal CTs looking intogenerator)• Stator Overload protection• Rotor Overload protection• Loss of Excitation protection (directional pos. seq. OC protection)• Reverse power/Low forward power protection (directional pos. seq. OC protection,2% sensitivity)• Dead-Machine/Inadvertent-Energizing protection• Breaker head flashover protection• Improper synchronizing detection• Sensitive negative sequence generator over current protection and alarm• Phase or phase-to-phase or Negative/Positive/Zero Sequence over/under voltageprotection• Generator out-of-step detection (based on directional pos. seq. OC)• Inadvertent generator energizing10.1.2.4 Inadvertent generator energization SEMOD151947-4 v2When the generator is taken out of service, and non-rotating, there is a risk that the generatorcircuit breaker is closed by mistake.Three-phase energizing of a generator, which is at standstill or on turning gear, causes it tobehave and accelerate similarly to an induction motor. The machine, at this point, essentiallyrepresents the subtransient reactance to the system and it can be expected to draw from oneto four per unit current, depending on the equivalent system impedance. Machine terminalvoltage can range from 20% to 70% of rated voltage, again, depending on the systemequivalent impedance (including the block transformer). Higher quantities of machine currentand voltage (3 to 4 per unit current and 50% to 70% rated voltage) can be expected if thegenerator is connected to a strong system. Lower current and voltage values (1 to 2 per unitcurrent and 20% to 40% rated voltage) are representative of weaker systems.Since a generator behaves similarly to an induction motor, high currents will develop in therotor during the period it is accelerating. Although the rotor may be thermally damaged fromexcessive high currents, the time to damage will be on the order of a few seconds. Of morecritical concern, however, is the bearing, which can be damaged in a fraction of a second dueto low oil pressure. Therefore, it is essential that high speed tripping is provided. This trippingshould be almost instantaneous (< 100 ms).There is a risk that the current into the generator at inadvertent energization will be limited sothat the “normal” overcurrent or underimpedance protection will not detect the dangeroussituation. The delay of these protection functions might be too long. The reverse powerprotection might detect the situation but the operation time of this protection is normally toolong.For big and important machines, fast protection against inadvertent energizing should,therefore, be included in the protective scheme.The protection against inadvertent energization can be made by a combination ofundervoltage, overvoltage and overcurrent protection functions. The undervoltage function1MRK 505 363-UEN A Section 10Multipurpose protection189Application manual
