SK19896A3 - Arrangement for stator earthing of three-phase current machines - Google Patents

Abstract

In a stator earth leakage detection process, clear differences in the third harmonic in the faultless and in the earth leaking state may be ascertained but the parasitic current is nevertheless shunted and the vibrations is dampened. For that purpose, a three-phase current transformer (2) is connected to the terminals of a three-phase current machine (1), for example a synchronous generator. The three-phase current transformer (2) must have a free magnetic short-circuit in order to transmit a null voltage in the event of an earth leakage of the three-phase current motor (1). The primary winding of the transformer (2) is star connected with an earthed star point. The secondary winding of the transformer (2) is connected so as to form an open triangle and is loaded with the resistance (4) which may be switched off by the switch (5) connected in series. A voltage converter (6) is connected to the earthed star point of the three-phase current machine (1). The secondary voltage of said converter (6) and the voltage at the open triangle winding are supplied to a static earth leakage relay (7) that detects an earth leakage in the generator (1) on the basis of the amplitude and phase position of the third harmonic in said voltages.

Description

CONNECTION OF STATOR EARTHING IN THREE-PHASE MACHINES

Technical field

The invention relates to a stator ground connection in three-phase machines, in particular synchronous generators, wherein the stator winding zero point is grounded through the primary winding of the voltage transformer and a three-phase transformer is connected to the terminals of the three-phase machine.

BACKGROUND OF THE INVENTION

The stator ground connection devices are already known.

A common embodiment utilizes a transformer connected to the machine terminal to create an artificial voltage slip. This is usually achieved by unequal connection of the transformer to the individual phases.

It is also known to form a potential connection of the machine zero point with respect to ground by a voltage applied to the primary side of the three-phase transformer zero point connected to the machine terminal.

On the other hand, AT-PS 326 206 discloses a block connection of a device for protecting the stator winding of synchronous generators against earth faults, which uses an approximately linear resistance between the stator winding and the ground. This resistor can be connected to a zero point or terminal directly by galvanic or through a transformer, this connection being connected, for example, by a low voltage switch. This switch is connected to the secondary circuit of the transformer, the primary winding of which is the connection between the stator winding and the ground.

However, these devices all have the disadvantage that higher costs must be paid, since damage to the stator ground connections without these devices can no longer be reduced.

From IEEE Transactions on Power Apparatus, Volume PPAS-103, No. 4, April 1984, A comparison of 100% stator ground fault protection schemes for JW Pope's stator winding generator and IEEE Transactions on Power Delivery, Volume PWRD-1, Issue 4, October 1986, RJ Marttil's stator ground relay for 100% coverage of the stator winding are also known connections that capture the third harmonic at the zero-to-ground voltage and are used as a criterion for the existence of a ground fault stator. This principle is particularly suitable for determining ground faults near the zero point of a three-phase machine. Otherwise, such ground faults can only be detected by the above-described expensive wiring with artificial zero point slip.

Typically, each of the ground faults that is not near the zero point is captured by measuring the voltage between the zero point and the ground essentially from the base wave or by measuring the zero terminal voltage system on the triangular winding of the voltage transformer system also consisting essentially of the base wave.

In order to dissipate the disturbing current flowing in the electrical grid in the ground connection through the system of single-phase transformers to the machine and to dampen the oscillation during the transient processes, in the existing stator ground harmonic interconnections the third harmonic is located at the zero point of the three-phase machine. resistance. Indeed, both effects affect the ground fault protection function of the ground wave and must be reduced by resistance. Resistance affects the trapping of the third harmonic, since it itself represents a ground fault near the zero point, and so no longer distinguishes the use of a true ground fault from that state.

DE-A 2 002 777 shows the connection of the stator earth fault in three-phase machines, the function of which is based on the comparison of the third harmonic voltage. The zero point of the stator winding is grounded directly through the primary winding of the voltage transformer, the secondary winding of which is connected with a complex resistor and a catch element in series connection. A three-phase transformer is connected to the terminals of the three-phase machine, the primary winding of which is also grounded. The secondary winding of a three-phase transformer connected to an open triangle is terminated by a series connection of another complex resistor and a catch element. Correct dimensioning of both resistors is absolutely essential for the faultless and correct reaction of the catch element in the stator ground fault. However, a corresponding reference to the dimensioning of these resistors is not given in this specification, and it is certainly not common for the average practitioner.

SUMMARY OF THE INVENTION

The object of the invention is to make a clear difference to the third harmonic in a faultless and ground fault condition, yet to ensure the dissipation of the above-mentioned disturbing currents and to allow the damping of oscillation.

This task is solved by the invention, characterized in that a voltage grounding transformer is connected to the primary relay line input and a primary secondary winding of the three-phase single-phase transformer transformer has a primary secondary winding of this winding connected to a parallel connected resistor. the relay and the conductivity of the resistance relative to the primary side of the three-phase transformer is at least the magnitude of the capacitive conductivity of the stator winding of the three-phase machine relative to ground.

An advantage of the invention is that there is no particular load at the zero point of the three-phase machine with respect to grounding, thereby causing a ground fault near the zero point relative to normal running, a distinct difference in the third harmonic. Resistive currents coming through the transformer are also dissipated by the resistor and damped oscillations with respect to ground. A very large difference in the third harmonic between the no-ground condition and the ground near the zero point is created by the load resistor in the zero terminal voltage system. Indeed, in this type of connection, the third harmonic in the zero terminal voltage system is low in the groundless state. However, if there is a ground fault near the zero point, the third harmonic in this zero system is almost the entire third harmonic produced by the machine as a voltage source with low internal resistance. This virtually generates a strong voltage and does not replace the load with resistance.

The distribution of the third harmonic between the zero point and the terminal side is all the more favorable, the lower the resistance. Equally, its ability to dissipate disturbing currents and damp oscillation increases.

Resistance such as the load impedance mentioned in the error-free state reduces the zero voltage at the terminal side and increases the current voltage at the zero point.

BRIEF DESCRIPTION OF THE DRAWINGS

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a greater part of the known stator grounding engagement variant, and FIG. 2 shows a circuit according to the invention for receiving a stator ground.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 is to the terminals of a three-phase machine 1, which is, for example, a synchronous transformer 2. This generator, connected to a three-phase transformer 2, must have one free magnetic yoke for grounding of the three-phase machine 1, for which a five-core transformer or group of single-phase transformers is suitable .

The primary winding of the three-phase transformer 2 is connected to a star, where the zero point is the three-phase transformer 2 of the triangle and loaded with a resistor 4 located by a switch 5. The grounding of the measuring transformer is grounded. The secondary winding is connected to an open which can be disconnected in a series of points of the three-phase machine 1 is a voltage. The secondary voltage of this voltage transformer 6 and the voltage on the open triangular winding is applied to a static grounding relay 2 which, according to the size and phase position of the third harmonic, intercepts the earth fault with the three-phase machine 1.

In FIG. 2 a single-phase transformer 3 is connected to the grounded zero point of the three-phase transformer 2. · The secondary side of the single-phase transformer 3, which is connected to the grounding relay 7, is connected in series with a resistor 4 with switch 5. as the zero voltage is transmitted by the single-phase transformer 3. · The group connection of the three-phase transformer 2 must allow the passage of the zero current.

Claims (1)

PATENT CLAIMS The stator ground connection of three-phase machines, in particular synchronous generators, where the stator winding zero point is grounded through the primary winding of the voltage transformer and to the terminals of the three-phase machine is connected to a three-phase transformer whose primary winding is connected to the star. the winding of the voltage transformer (6) is connected to the input of the earthing relay (7) and the primary winding of the three-phase transformer (2) includes the primary winding of the single-phase transformer (3) and the secondary winding of this single-phase transformer (3) 4) which is connected to the second input of the grounding relay (7) and the conductivity of the resistor (4) relative to the primary side of the three-phase transformer (2) has at least the capacitance of the stator winding of the three-phase machine (1). T'J