AN ELECTRICAL MACHINE WINDING GROUND-FAULT PROTECTION SYSTEM

Abstract: The object of the invention is a ground fault protection system of a winding of an electrical machine, especially a generator stator winding with unearthed neutral point, applicable during the operation of the machine connected in series with a transformer or connected in parallel with a generator, or directly connected to busbars. The system according to the invention is characterised in that in the circuit formed by the primary winding of the single-phase voltage transformer (8) and the secondary windings of the three-phase voltage transformer (5), between the secondary winding of the three-phase transformer (5) and the low-resistance resistor (7) a shunt reactor (6) is connected in series.

Description: An electrical machine winding ground-fault protection system The object of the invention is a ground fault protection system of a winding of an electrical machine with an unearthed neutral point, especially for a generator stator winding, applicable during the operation of the machine connected in series with a transformer or connected in parallel with another generator, or directly connected to busbars. A known ground fault protection system of a generator stator winding with unearthed neutral point presented in ABB Network Partner catalogue No. 1MDB 02012-EN issued in November 1995, page 3, fig. 5, contains a three-phase voltage transformer connected to the generator output terminals and two resistors, one of a lower resistance and the other having resistance approximately 4 times higher than the first one, the resistors being connected in parallel to the secondary side of the three- phase voltage transformer and being connected in series with one another. A system containing a voltage source of frequency equal to approximately V* 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 with the outputs of the ground-fault protection unit is connected in parallel to the resistor which has BESTATIGUNGSKOPIE 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 generator stator winding, in case of ground faults in any part of the winding, depends mainly upon the value of the resistance of the resistors connected on the secondary side of the three-phase voltage transformer installed at the generator output terminals. In the case of the resistance values of resistors used and required by the protection system, these resistors always cause increase in the value of the ground-fault current passing at the place of the ground fault in the generator stator winding, so that this current largely exceeds 5 A at the place of the ground fault. If a ground fault occurs in the generator stator, 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 three-phase voltage transformer connected to the output terminals of the winding, into whose secondary windings is connected a low-resistance resistor which is connected in parallel to a source of voltage of frequency equal to A of the rated frequency of the machine, and the source of this voltage is coupled with the ground-fault protection unit whose outputs are connected with the secondary winding of a single-phase voltage transformer, and the primary winding of the single-phase voltage transformer is connected in parallel with the secondary winding of the three-phase voltage transformer, is that in the circuit formed by the primary winding of the single-phase voltage transformer and the secondary windings of the three-phase voltage transformer, between the secondary windings of the three-phase voltage transformer and the low- resistance resistor, there is connected in series a shunt reactor. 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 unearthed neutral point N is connected with a transformer 2 through busbars 3. Individual output terminals 4 of the generator 1_ are connected to earthed primary windings of a three-phase voltage transformer 5 into whose secondary winding is connected in parallel a shunt reactor 6 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 V* of the rated frequency of the generator 1_, which is an injecting system 10 which 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 2, ground-fault capacitance current and current passing through the elements connected into the secondary windings of the three- 90 phase voltage transformer 5 equal zero. Inductance of the shunt reactor 6 connected to the secondary windings of the three-phase voltage transformer 5 is selected to limit the current passing through the main insulation of the winding of the generator ^ stator during ground fault to a value less than 5 A. The value of the shunt reactor 6 inductance is 95 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 ^ rotor winding, which reduces, and even totally eliminates damage to the magnetic circuit of the generator 1_ stator. 100 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.

Claims: Claims 1. A ground-fault protection system of a winding of an electrical machine, incorporating a three-phase voltage transformer connected to the output terminals of the machine, to which transformer secondary windings a low-resistance resistor is connected, which resistor is connected in parallel to a voltage source of frequency equal approximately of the rated frequency of the machine, the source of this voltage being coupled with a ground-fault protection unit whose outputs are connected with the secondary winding of a single-phase voltage transformer, and the primary winding of the single-phase voltage transformer is connected in parallel with the secondary winding of the three-phase voltage transformer characterised in that in the circuit formed by the primary winding of the single-phase voltage transformer 181 and the secondary windings of the three-phase voltage transformer 15/, between the secondary winding of the three-phase transformer 151 and the low-resistance resistor 111 a shunt reactor 161 is connected in series. 2. A ground-fault protection system of a winding of an electrical machine according to claim 1 characterised in that the inductance of the shunt reactor 161 is within the range from 0.01 to 2 millihenries. 3. A ground-fault protection system of a winding of an electrical machine according to claim 1 characterised in that the electrical machine is a generator, a motor or a transformer.

Inventors: BERTSCH JOACHIM, FULCZYK MAREK, PFIFFNER MICHAEL, BERTSCH, JOACHIM, FULCZYK, MAREK, PFIFFNER, MICHAEL
Applicants: ABB POWER AUTOMATION AG, BERTSCH JOACHIM, FULCZYK MAREK, PFIFFNER MICHAEL, BERTSCH, JOACHIM, FULCZYK, MAREK, PFIFFNER, MICHAEL
Application year: 2000
Main classification: H02H3
IPC codes: H02H3, H02H7