• Improper synchronizing detection• Sensitive negative sequence generator over current protection and alarm• Phase or phase-to-phase or Negative/Positive/Zero Sequence over/undervoltage protection• Generator out-of-step detection (based on directional pos. seq. OC)• Inadvertent generator energizing12.1.2.4 Inadvertent generator energizationWhen the generator is taken out of service, and non-rotating, there is a risk that thegenerator circuit breaker is closed by mistake.Three-phase energizing of a generator, which is at standstill or on turning gear, causesit to behave and accelerate similarly to an induction motor. The machine, at this point,essentially represents the subtransient reactance to the system and it can be expectedto draw from one to four per unit current, depending on the equivalent systemimpedance. Machine terminal voltage can range from 20% to 70% of rated voltage,again, depending on the system equivalent impedance (including the blocktransformer). Higher quantities of machine current and voltage (3 to 4 per unit currentand 50% to 70% rated voltage) can be expected if the generator is connected to a strongsystem. Lower current and voltage values (1 to 2 per unit current and 20% to 40% ratedvoltage) are representative of weaker systems.Since a generator behaves similarly to an induction motor, high currents will developin the rotor during the period it is accelerating. Although the rotor may be thermallydamaged from excessive high currents, the time to damage will be on the order of afew seconds. Of more critical concern, however, is the bearing, which can be damagedin a fraction of a second due to low oil pressure. Therefore, it is essential that highspeed tripping is provided. This tripping should be almost instantaneous (< 100 ms).There is a risk that the current into the generator at inadvertent energization will belimited so that the “normal” overcurrent or underimpedance protection will not detectthe dangerous situation. The delay of these protection functions might be too long. Thereverse power protection might detect the situation but the operation time of thisprotection is normally too long.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 undervoltagefunction will, with a delay for example 10 s, detect the situation when the generator isnot connected to the grid (standstill) and activate the overcurrent function. Theovervoltage function will detect the situation when the generator is taken intooperation and will disable the overcurrent function. The overcurrent function willhave a pick-up value about 50% of the rated current of the generator. The trip delaywill be about 50 ms.Section 12 1MRK 502 071-UEN -Multipurpose protection450 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062Application manual
