Analysis of operating conditions of ground-fault protection schemes for generator stator winding

This paper presents results of simulation studies of the influence of methods of grounding the stator winding of the unit-connected generator neutral on the operating conditions of a 100-percent ground-fault protection excited by the third-harmonic voltage ratio in the generator neutral and at its terminals. The analysis was performed for a generator with the neutral grounded through a ground-fault neutralizer, a third-harmonic stop filter, a resistor, and a capacitive voltage divider. The influence of the parameters of the particular elements of the grounding circuits on the voltage-feeding measuring element of the protection was determined. The capacitance to ground of the generator breakers was taken into account. The simulations were run for 1110-MVA unit-connected generator at ground faults along the whole length of the generator stator winding. It has been found that the method of grounding the generator neutral and the parameter of the grounding elements significantly influences the operating conditions of this protection. It is shown that through the proper selection of these parameters, the ground-fault protection of whole stator winding, counting from the generator neutral to the terminals is ensured.     

Optimization of operating conditions of third harmonic ground-fault protection system of generator-transformer unit with two-states transverse parameters

Such an optimization analysis of the operating conditions of the ground-fault protection excited by the third harmonic voltage for the two-state system of the generator-transformer unit transverse parameters is presented in this paper. The analysis was made for a unit in which a change in its transverse branch parameters occurs as a result of generator breaker switching operations. The optimization analysis has shown that effective ground-fault protection of the generator stator windings is possible, regardless of the operating conditions of the generator breaker.     

Third harmonic ground-fault protection system of unit-connected generator with two parallel branches per phase

The operating conditions of a ground-fault protection reacting to a difference in 3rd harmonic voltages at the star point and at the output terminals of a unit-connected generator with a dipole stator winding formed by two parallel branches in phase are analysed. The voltage feeding the measuring element of the ground-fault protection in the generator is determined for different ways in which its star point is grounded. The paper, analyse the operating conditions of the above protection systems in the unit-connected generator with two parallel branches in stator windings phase and one pair of poles, on the basis of simulation studies. The simulations were run for real generator of 120 MW equipped with generator breakers with additional capacitances to ground. Realistic values of the breakdown channel in the main isolation of the generator stator windings were taken into account. This resistance was determined in experimental tests conducted in real conditions during arcing ground faults.     

Improvements in ground-fault protection of unit-connected generator stator winding using laboratory test environment

This paper describes an idea of realization of a software–hardware laboratory environment enabling to run research and implementation in the area of ground-fault protection of modern unit-connected generators. There are three basic elements in this environment: simulation model of a unit-connected generator for investigating intermittent arc ground-faults in generator stator winding, PC-based system for ground-fault waveforms synthesis and microprocessor-based hardware for implementation of a new protection algorithm developed by authors.     

A new digital ground-fault protection system for generator–transformer unit

Ground faults are one of most often reasons of damages in stator windings of large generators. Under certain conditions, as a result of ground-fault protection systems maloperation, ground faults convert into high-current faults, causing severe failures in power system. Numerous publications in renowned journals and magazines testify about ground-fault matter importance and problems reported by exploitators confirm opinions, that some issues concerning ground-fault protection of large generators have not been solved yet or have been solved insufficiently. In this paper a new conception of a digital ground-fault protection system for stator winding of large generator was proposed. The process of intermittent arc ground fault in stator winding has been briefly discussed and actual ground-fault voltage waveforms were presented. A new relaying algorithm, based on third harmonic voltage measurement was also drawn and the methods of its implementation and testing were described.     

Ground-Fault Protection of Four-Pole Generators With Two Parallel Branches in Phase Winding

An analysis of the operating conditions of a third harmonic ground-fault protection system for a four-pole generator with two parallel branches in the phase winding was carried out. The voltage exciting the measuring element of the protection system for various neutral grounding configurations was determined. The influence of the neutral grounding configurations on the exciting voltage was examined through simulations. The operating conditions of the protection system were determined taking into account the actual third harmonic distribution along the stator winding, the to-ground capacitances of the generator breakers, and the real ground-fault arc resistance     

Influence of difference system parameters on operating conditions of third harmonic ground-fault protection system of unit-connected generator

In the paper, the influence of unbalance of the measuring element of the unit-connected generator stator ground-fault protection system excited by the ratio of third harmonic voltage to ground in the generator neutral and at the generator terminals on the value of the voltage supplied to the measuring element during ground faults is analyzed. The analysis is made for currently used ground-fault protection systems in which the measuring element is balanced by an autotransformer voltage divider or an impedance bridge.