A known ground fault protection system of a generator stator winding with the neutral point earthed through a voltage transformer is presented in ABB Network Partner catalogue No. 1 MDB 02012-EN issued in November 1995, page 3, fig. 6. The presented system incorporates a single-phase voltage transformer which is connected between the generator neutral point and earth. To the secondary winding of this transformer there are connected in parallel two resistors connected in series to one another, one of a lower resistance and the other having resistance approximately 4 times higher than the first one. A system containing a voltage source of frequency equal to approximately 1/4 of the generator rated frequency is connected in parallel to the lower-resistance resistor. This system is coupled in parallel with the ground-fault protection unit. A single-phase voltage transformer whose secondary winding is

connected in parallel with the inputs of the ground-fault protection unit is connected in parallel to the resistor which has approximately 4 times higher resistance than the first resistor.

A disadvantage of the presented ground-fault protection system of a generator stator winding is that the value of the ground-fault current passing at the point of the ground-fault, in the case of a ground fault in any part of the generator stator winding, depends mainly upon the value of the resistance of the resistors connected on the secondary side of the single-phase voltage transformer connected between the generator neutral point and earth. These resistors always cause an increase in the value of the ground-fault so that the value of this current at the place of the ground fault always largely exceeds 5 A. If a ground fault occurs in the generator stator winding, this current causes a considerable damage to the stator. Another disadvantage of the presented generator stator winding ground-fault protection system is that the value of the ground- fault current exceeding 5 A restricts the length of the area protected by this ground-fault protection system.

The essence of the ground-fault protection system for the winding of an electrical machine, especially for a generator stator winding, according to the invention, where the ground-fault protection system incorporates a single-phase voltage transformer with earthed primary winding, which transformer is connected to the neutral point of the winding, and where a low-resistance resistor is connected to the secondary winding of this transformer, which resistor is connected in parallel to a source of voltage of frequency equal to'/4 of the rated frequency of the machine, and the source of this voltage is coupled with the ground-fault protection unit whose inputs are connected with the secondary winding of another single- phase voltage transformer, and the primary winding of this single-phase voltage transformer is connected in parallel with the secondary winding of the single-phase voltage transformer connected to the neutral point of the winding, is that in the circuit formed by the primary winding of the second

single-phase voltage transformer, the secondary winding of the single- phase voltage transformer installed in the neutral point of the winding, and the low-resistance resistor, a shunt reactor is connected in parallel to the primary winding of the second voltage transformer. The inductance of the shunt reactor is within 0.01 to 2 millihenries.

The object of the invention is explained as an example embodiment in the drawing which shows a unit diagram of a ground-fault protection system of a generator stator winding operating in a unit with a transformer connected in series.

A generator 1 with neutral point 2 is connected with a transformer 3 through busbars 4. Between the generator neutral point 2 and earth there is connected a single-phase voltage transformer 5 whose primary winding is earthed. To the secondary winding of this transformer there is connected in parallel a shunt reactor 6 which is connected in series with a resistor 7 of low resistance of several hundred ohms. The shunt reactor 6 is connected in parallel with the primary winding of a single-phase voltage transformer 8 whose secondary winding is connected in parallel with a ground-fault protection unit 9. The resistor 7 is connected in parallel with a voltage source of frequency equal to 1/4 of the rated frequency of the generator 1, which is an injecting system 10. This system is coupled in parallel with the ground-fault protection unit 9.

The voltage injected by the injecting system 10 serves to bias the conductors of the machine with respect to ground, with a frequency that differs from the generator frequency. A ground fault causes a current that depends on the nature of the fault and on the injection voltage. The ground-fault protection unit 9 observes this current and the injected voltage, determines if a fault has occurred and what the nature of the fault is. If necessary, the ground-fault protection unit 9 operates a breaker for disconnecting the generator.

In a trouble-free operation condition of the generator 1 connected with the transformer 3, ground-fault capacitance current and current passing through the elements connected into the secondary winding of the single- phase voltage transformer 5 equal zero. Inductance of the shunt reactor 6 connected to the secondary winding of the single-phase voltage transformer 5 is selected to limit the current passing through the main insulation of the winding of the generator 1 stator during ground fault to a value below 5 A. The value of the shunt reactor 6 inductance is preferentially within the range from 0.01 to 2 millihenries. By an adequate selection of the inductance of the shunt reactor 6 a proper operation of the ground-fault protection unit 9 is ensured for faults along the whole length of the generator 1 rotor winding, which reduces, and even totally eliminates damage to the magnetic circuit of the generator 1 stator.

The invention can be applied to protecting not only a generator, but also to another type of electrical machine such as a motor or a transformer.