INTRODUCTION

In order to protect effectively unit-connected generators against the effects of ground-faults in their stator windings the phenomena that accompany these faults must be investigated fully. Overvoltages during arcing ground-faults close to the generator terminals the greatest danger to the main insulation of the generator windings pose [1,2]. Appropriately high overvoltages are accompanied by a significant increase in electrical stresses in the winding main insulation. These stresses weaken the insulation and can cause secondary faults.Since the ground-fault arc has interrupted character, multiple ignitions and quenching accompanied by fast transients occur in the ground-fault process [3]. These phenomena have a marked influence on the ground-fault overvoltage values [4].The voltage-current fast transients during ground-faults in the stator of the unit-connected generator depend on the parameters in its zero-sequence circuit and on the mechanism of the interrupted ground-fault [5]. This applies to mainly the elements grounding the generator neutral [7-10], the elements connected to its terminals, the resistance of the ground-fault are and the mechanism of its quenching [11].In this paper the results of an extensive program of simulation tests of the voltage and current fast transients during arcing ground-faults in the stator of unit connected generators with the neutral grounded through the ground-fault neutraliser are presented. The currents in all the branches of the equivalent system scheme, and the voltage drops on its particular elements were determined and the ground-fault current overvoltage danger was analysed. In the analysis the real changes in the dynamic resistance of the ground-fault arc in the main insulation of the stator winding and the real mechanism of its quenching were taken into account.Knowing that fast transients depend slightly on the neutralisation coefficient, a detailed comparative analysis for two unit-connected generators with the biggest difference in parameters at the neutralisation coefficient equal to 0.6 was carried out. On the basis of the numerical computations the influence of the unit parameters on the voltages and currents during arcing ground-faults in the stators o;

137.5 and 1110 MVA generators was determined.

MODEL AND DATA OF TESTED SYSTEM

The extent of the direct damage to the magnetic circuit and the main insulation of the generator stator winding caused by a ground-fault depends on the energy emitted in the ground-fault arc. Since this energy is determined by the value of the ground-fault current, the generator neutrals are grounded through the ground-fault neutraliser formed by a coil connected into the secondary winding of the distribution transformer. In this way the ground-fault current for this generator is limited to a safe value. Grounding through the ground-fault neutraliser allows one to obtain also the optimum operating conditions for ground-fault protections [14].

Fig. l - One-line diagram of generator system into which ground-fault neutraliser has been incorporated.

A failure in the main insulation of the stator winding can occur at any place, including the generator neutral. It follows from the failure analysis of the unit connected generators that ground-faults occur more frequently in the second part of the winding counting from the generator neutral. Since, from the ground-fault overvoltages point of view, the faults close to the generator terminals are the most dangerous, these faults are analysed in this paper.Ground-faults have always interrupted character independent of the causes of the breakdown in the main insulation of the stator windings. The properties of the ground-fault channel change with time. This is because the organic parts of the insulation carbonize at high temperatures.The real changes in the dynamic arc ground resistance and the real mechanism of its burning are discussed in [2,3,11,13,14]. On the basis of these papers simulation tests of ground-fault overvoltages in the system shown in figure 2 were carried out.Because it was found that the resistance's of the generator winding had an insignificant influence on the analysed phenomena, this parameter was neglected in the further analysis. The parameter values of the unit-connected generators assumed for the simulation analysis are listed in table 1.In order to determine the influence of the unit-connected generator parameters on the ground-fault overvoltages, the simulation tests for the units with the biggest difference in parameters were carried out. This allows the full estimation of the overvoltage danger in actually operated units.

Fig.2 - Simplified equivalent scheme of unit-connected generator used in analysis of voltage and current fast transients.

Parameters in this figure stand for:

• C_N - 3/2 of phase capacitance of generator winding to ground;

• C_t- 1/2 of phase capacitance of generator to ground;

• C_z - equivalent capacitance to ground (C_z=C_p+C_Tb+C_TV):

  • C_p - equivalent phase capacitance of connection between generator and main transformer and unit auxiliary transformer to ground,

  • C_Tb - equivalent phase capacitance of main transformer to ground seen from the generator,

  • C_TV - equivalent phase capacitance of unit auxiliary transformer to ground in generator terms,

L_g=1/3(L₀+L")

• L" - subtransients generator inductance, counted as arithmetic average of direct-axis and quadratic-axis subtransient inductance in saturated conditions,

• L₀ - zero-sequence inductance of generator,

• R - phase resistance of generator stator winding,

• L_k,R_k - equivalent inductance and resistance of ground-fault neutraliser, respectively,

• r_a - dynamic resistance of ground-fault arc.

Table 1 - Parameters of unit connected generator

CONCLUSIONS

1. The oscillatory component of current fast transients in the windings of the generator stator after ground-fault arc ignition is a sum of the two oscillatory currents with a double difference in its frequency and it in ranges from a few to several kHz. These currents are dampened very fast and they practically decay between successive ignition and quenching.

2. The parameters of the ground-fault neutraliser have no significant influence on the fast transients in the circuits of the unit-connected generator during an arcing ground-fault in the stator winding of the generator.

3. The voltage drops in the windings of the generator caused by the oscillatory component of the current transient do not exceed a few percent of the amplitude of the generator phase voltage.

4. The difference in the parameters of the generator for the positive sequence of the subtransient current and for zero-sequence currents can be neglected in the analysis of the voltage fast transients during an arcing ground-fault.

5. The fast transients of the voltage after arc ignition do not pose danger to the main insulation of the windings of the unit-connected generator.

6. The difference in the parameters of particular unit-connected generators does not have a significant influence on the ground-fault overvoltage danger.

BIBLIOGRAPHY