HK1021085B - An electric high voltage ac machine - Google Patents

Description

High-voltage AC motor

The present invention relates to a high voltage AC electrical machine for direct connection to a distribution or transmission network, said machine comprising at least one winding.

Such a generator with a rated voltage up to 36kV is described by Paul R.Siedler in 36kV Generation Art from insulation research, the electric World (electric World), 10 and 15 months 1932, pages 524 and 527. These generators include windings formed of medium voltage insulated conductors in which the insulation is subdivided into layers of different dielectric constants. The insulation material used was formed from various combinations of three components, micaceous foil mica, paint and paper.

In the publication EPRI, EL-3391, 4 months 1984 at the Power Research Institute, a concept is proposed for a generator intended to provide such a high voltage that the generator can be connected directly to the Power network without any intermediate transformer. Such a generator should comprise a superconducting rotor. The magnetizing capability of the superconducting field makes it possible to use an air gap winding of sufficient thickness to withstand the electrical stress. However, the proposed rotor has a complex structure with very thick insulation, which significantly increases the motor size. In addition to this, special measures have to be taken in order to insulate and cool the coil end sections.

According to the invention, by high voltage AC machine is meant a rotating electric machine like a generator used in a power station for generating electric power, such as a two-way fed machine, an external pole machine, a synchronous machine, stages of an asynchronous converter, and a power transformer. In addition to transformers, in order to connect such machines to a distribution or transmission network, in the following generally referred to as a power network, a transformer has hitherto been required to transform the voltage to the network level, i.e. in the range of 130-400 kV.

By manufacturing the windings of these machines for insulating high voltage conductors with a solid insulation of a similar construction to the cables used for power transmission, the voltage of the machine can be raised to such a level that the machine can be connected directly to any power network without an intermediate transformer. This transformer can thus be omitted. A typical operating range for these machines is 30 to 800 kV.

For this type of machine, special attention must be paid to the grounding problem.

Grounding of generator systems and other similar electrical systems means an intentional measure to connect the electrical system to ground potential. When a so-called neutral point of the system is available, it is often grounded directly or via a suitable impedance. Occasionally other points in the system are also grounded. If one point in the system is grounded, the entire system can be grounded as long as the power connection extends.

The grounding principle used is determined by the design of the system. For a system that includes a generator directly connected to a Y-delta connected step-up transformer with the delta winding on the voltage side of the generator, the following ground selection is most common:

high resistance ground

Not to earth

-resonant grounding.

High resistance grounding is typically achieved by connecting a low ohmic resistor in the secondary of the distribution transformer and grounding the primary winding of the transformer from the generator neutral. This prior art grounding method is illustrated in fig. 1, which shows a generator 2 connected to a grid 9 by a Y-delta connected step-up transformer 3. The primary 11 of the distribution transformer is connected between the neutral point of the generator 2 and ground. In the secondary 10 of the transformer, a resistor 12 is connected.

Of course, the same kind of grounding can be obtained by mounting a high-ohmic resistor directly between the generator neutral and ground.

Ungrounded electrical systems do not have any intentional ground connections. Thus, the ungrounded generator has no connection between its neutral point and ground, except for possible voltage transformers used to supply the different relays and instruments.

The resonant grounding is also generally implemented as shown in fig. 1, the resistor 12 being replaced by a reactor 12 a. The reactor reactance is selected so that the capacitive current during a line-to-ground fault is neutralized by equal components of the inductive current provided by the reactor 12 a.

Low resistance or low impedance grounding and effective grounding of the above system are also possible. A low resistance or low impedance ground will generate a lower transient overvoltage and a larger ground fault current, which may cause internal damage to the motor.

Low resistance grounding is achieved by intentionally inserting a resistance between the generator neutral and ground. The resistor may be inserted directly in the connection to ground or indirectly in the secondary of a transformer, the primary of which is connected between the generator neutral and ground, see fig. 1.

A low impedance ground, i.e. a low inductance ground, is done in the same way as a low resistance ground, where an inductor is used instead of a resistor. As discussed above, the inductor has a lower ohmic value than required for resonant grounding.

For a system comprising several generators connected to one common feeder or bus with a circuit breaker between the generator terminals and the common bus, a low resistance or low impedance grounding is appropriate.

Effective grounding of the generator neutral point has essentially the same advantages and disadvantages as low resistance or low impedance grounding, with some differences.

The system is said to be effectively grounded if some impedance requirements are met that limit the magnitude of the ground impedance. In an effectively grounded system, the maximum phase-to-ground voltage of the non-faulted phase is limited to 80% of the phase-to-phase voltage in the case of a ground fault.

The power system grid is mainly grounded via a ground connection of the transformer neutral in the system and may comprise no impedance (other than contact resistance), i.e. a so-called direct ground, or have a certain impedance.

For example, in the publication IEEE c62.92-1989, published by the Institute of Electrical and electronics Engineers (Institute of Electrical and electronics Engineers) in new york, usa at 9.1989, for IEEE guidelines for neutral grounding applications in Electrical utility systems, the second part-grounding of synchronous systems, a previously known grounding technique is described.

If the neutral point of the generator is grounded through the low resistance or inductance mentioned above, a path is formed for the third harmonic current from the generator neutral point to ground. If a directly grounded or low impedance grounded transformer winding, or another low impedance grounded generator, is directly connected to the generator, the third harmonic current will normally circulate between them.

In swedish patent applications 9602078-9 and 9700347-9, techniques are described for solving the third harmonic current problem in generator and motor operation of AC machines of the kind to which the present invention relates.

It is an object of the present invention to provide a high voltage AC electrical machine adapted to be connected directly to a distribution or transmission network as described above, the electrical machine being provided with grounding means adapted to different uses and operating conditions of the electrical machine.

This object is achieved by means of a high-voltage AC machine of the kind defined in the introductory part of the description and having the features of the characterizing part of claim 1.

An important advantage of the electric machine according to the invention lies in the fact that the electric field is almost equal to zero in the winding head region outside the second layer having semiconducting properties. There is thus no need to control the electric field outside the winding and no electric field concentrations are formed within the laminations, in the region of the winding ends, in the transition region between them.

According to an advantageous embodiment of the electrical machine according to the invention, at least two adjacent layers have substantially equal coefficients of thermal expansion. In this way, defects, cracks, etc. caused by thermal movements in the winding can be avoided.

According to another advantageous embodiment of the electrical machine according to the present invention, said grounding means comprise low-resistance grounding means for the windings. In this way, the transient overvoltage and the ground fault current can be limited to appropriate values.

According to a further advantageous embodiment of the electric machine according to the invention, the electric machine has a Y-connected winding, the neutral point of which is available, and the high-resistance grounding means comprise a resistor connected in the secondary of a transformer, the primary of which is connected between the neutral point and ground. In this way the resistors used in the transformer secondary have a relatively low ohmic value and a robust construction. Sufficient damping to reduce the over voltage transient to a safe level can be achieved with a resistor of suitable size. In addition, mechanical stress and fault damage are limited by suppression of fault currents during line-to-ground faults. Such an earthing arrangement is also more economical than inserting a high-ohmic resistor directly between the generator neutral and ground.

According to another advantageous embodiment of the electric machine according to the invention, the electric machine has a Y-connected winding, wherein the neutral point is available, and the grounding means comprise a reactor connected in the secondary of a transformer, the primary of which transformer is connected between the neutral point and ground, said reactor having such characteristics that the capacitive current during a ground fault is substantially neutralized by equal components of the inductive current provided by the reactor. Thus, the net fault current is reduced to a lower value by the parallel resonant circuit so formed, and the current is essentially in phase with the fault voltage. The voltage recovery of the faulted phase in this case is very slow and thus the transient behaviour of any earth fault is automatically cancelled in the resonant earthed system.

According to a further advantageous embodiment of the electrical machine according to the invention, the grounding means comprise a Y-delta grounding transformer or a so-called zigzag grounding transformer connected to the grid side of the electrical machine. The use of such grounding transformers is equivalent to low inductance or low resistance grounding in terms of fault current levels and over voltage transients.

For a more detailed explanation of the invention, an embodiment of an electric machine according to the invention, chosen as an example, will now be described in more detail with reference to fig. 2-11 of the accompanying drawings, in which:

FIG. 1 illustrates a prior art ground for a synchronous generator;

fig. 2 shows an example of an insulated conductor used in a winding of an electrical machine according to the invention;

FIG. 3 shows an earthed three-phase machine in the form of a Y-connected generator or motor connected to an electric power system;

4-13 illustrate different examples of the grounded Y-connected motor of FIG. 3;

figure 14 shows an electrical machine in the form of a delta-connected generator or motor connected to an electrical power system in accordance with the present invention;

fig. 15 illustrates the use of a grounding transformer in the system shown in fig. 14.

In fig. 2, an example of an insulated conductor is shown, which conductor can be used in a winding of an electrical machine according to the invention. Such an insulated conductor comprises at least one conductor 4, the conductor 4 being composed of a plurality of uninsulated and possibly insulated strands 5. There is an inner semiconducting layer 6 around the conductor 4, the inner semiconducting layer 6 being in contact with at least some of the uninsulated strands 5. This semiconducting layer 6 is in turn surrounded by a main insulation of the cable in the form of an extruded solid insulation layer 7. The insulating layer is surrounded by an outer semiconducting layer 8. The conductor area of the cable can be between 80 and 3000mm²While the outer diameter of the cable is between 20 and 250 mm.

Figure 3 schematically shows an ungrounded high voltage AC machine in the form of a Y-connected generator or motor 14 directly connected to a power system 16.

Fig. 4 shows an earthing device in the form of an overvoltage protector, such as a non-linear resistive arrester 18, connected between the neutral point 20 of the Y-connected motor 14 and earth. Such a non-linear resistive surge arrester 18 connected to the neutral point protects the insulated conductors used in the windings of the machine from transient overvoltages, such as those caused by lightning strikes.

Fig. 5 shows an embodiment with a high-ohmic resistor 22 connected in parallel to the non-linear resistive lightning conductor 18. The non-linear resistive lightning arrester 18 functions in this embodiment in the same way as in the embodiment shown in fig. 4 and with the resistor 22, a sensitive detection of the ground fault by measuring the voltage across the resistor 22 is achieved.

Fig. 6 shows an embodiment in which the neutral point 20 is grounded with a high resistance. In this embodiment a technique similar to the prior art described in connection with fig. 1 is used. A resistor 24 is thus connected to the secondary 26 of a transformer whose primary winding 28 connects the neutral point 20 of the electric machine 14 to ground. The resistor 24 has a lower ohmic value and a robust structure compared to a high ohmic resistor which is required to be inserted directly between the neutral point 20 and ground for the same effect. The voltage level of the resistor can be reduced. Also in this case a non-linear resistive arrester 18 is connected in parallel with the primary winding 28. With this embodiment, mechanical stress and fault damage are limited during a line-to-ground fault by suppressing the fault current. The transient overvoltage is limited to a safe level and the grounding means is more economical than the direct insertion of a resistor.

In a similar manner, the resonant grounding of the machine can be achieved by replacing the resistor 24 with a reactor having such characteristics that the capacitive current during a line-to-ground fault is neutralized by substantially equal components of the inductive current provided by the reactor. Thus, the net fault current is reduced by the parallel resonant circuit so formed, and this current will be essentially in phase with the fault voltage. After troubleshooting, the voltage recovery on the failed phase will be very slow and determined by the ratio of the inductive reactance of the transformer/reactor combination to the effective resistance. Thus, in such a resonant grounding system, the transient nature of any ground fault will be automatically eliminated. Such a resonant grounding device thus limits the ground fault current to virtually zero, thereby minimizing mechanical stress. After a fault with respect to earth has occurred, the motor can be allowed to continue to operate further until an orderly shutdown can be scheduled.

Fig. 7 shows an embodiment with a non-linear resistive surge arrester 18 connected between the neutral point 20 and ground and a grounding transformer 30 connected on the grid side of the electric machine 14. The grounding transformer 30 has a Y-delta configuration with the neutral point of the Y connection grounded and the delta winding isolated. Grounding transformers are commonly used in systems that are ungrounded or have a high impedance ground connection. As a system component, the grounding transformer has no load and does not affect normal system operation. When an unbalance occurs, the grounding transformer provides a low impedance in the zero sequence grid. Thus, a grounding transformer is equivalent to a low inductance or low resistance grounding in terms of fault current levels and transient over-voltages.

The grounding transformer may also be a so-called zigzag transformer with a dedicated winding arrangement, see for example Paul M.Anderson, "analysis of The Fault Power System", The Iowa State university Press/Ames, 1983, page 225-.

For such grounding purposes, it is also possible to use a possible auxiliary transformer.

Fig. 8 shows an embodiment with a low-ohmic resistor 32 connected between the neutral point 20 of the electric machine 14 and ground. The main advantage of such a low resistance ground is the ability to limit transient and temporary overvoltages. However, the current will be larger in case of a single-phase earth fault. The third harmonic current will also be greater in non-interfering operation.

Fig. 9 shows a further embodiment of the machine according to the invention, in which the resistor 32 is replaced by a low-inductance inductor 34 connected between the neutral point 20 and ground. The low inductance ground essentially operates in the same manner as the low ohmic ground. The ohm value of inductor 34 is less than that required for resonant grounding with reference to figure 6.

As an alternative to connecting the resistor 32 or inductor 34 directly between the neutral point 20 and ground, they may be connected indirectly by means of a transformer whose primary is connected between the neutral point 20 and ground and whose secondary contains a resistor or inductor, as described with reference to fig. 6.

In fig. 10, the illustrated embodiment includes two impedances 36 and 38 connected in series between the neutral of the motor 14 and ground, with impedance 36 having a low impedance value and impedance 38 having a high impedance value. The impedance 38 may be shorted by a shorting device 40. In normal operation, the shorting device 40 is open to minimize the third harmonic. In case of a ground fault, the short-circuiting device 40 is controlled to short-circuit the impedance 38 and the potential in the neutral point 20 will decrease and the current to ground will be higher.

In the event of an internal ground fault in the electric machine 14, the impedance 38 is not shorted. As a result the voltage will become high in the neutral point 20 and the current to ground will be limited. In this case, this is desirable, since in this case a large current can lead to damage.

In order to be able to deal with the problems arising from the third harmonic when the AC machine is directly connected to the three-phase power network, i.e. when no step-up transformer is used between the machine and the network, it is possible to use earthing devices in the form of suppression filters 35, 37 tuned to the third harmonic and an overvoltage protector 39, see fig. 11. The filter thus comprises a parallel resonant circuit consisting of an inductor 35 and a capacitive reactance 37. The filters 35, 37 and their overvoltage protectors 39 are sized so that the parallel circuit can absorb the third harmonic from the motor during normal operation, and also limit transients and temporary overvoltages. In the event of a fault, the overvoltage protector 39 will limit the fault voltage so that if the fault is considerable, a fault current flows through the overvoltage protector 39. In the case of a single-phase ground fault, the current is larger because the fundamental wave impedance is small, compared to, for example, the high-resistance ground case.

In fig. 12 an embodiment is shown wherein the grounding means comprises a demodulation switchable third harmonic suppression filter connected in parallel to an overvoltage protector 40. Such filters can be implemented in several different forms. Fig. 12 shows an example including two series-connected inductors 42, 44 and a capacitor 46 connected in parallel to the series-connected inductors 42, 44. In addition, a shorting device 48 is connected across the inductor 44.

The short-circuiting device 48 is controllable to change the characteristics of the filter by short-circuiting the inductor 44 when a danger of third harmonics between the filter and the electric machine 14 and the grid 16 is detected. This is described in more detail in swedish patent application 9700347-9. Thus the third harmonic is strongly limited during normal operation. In the case of a single-phase earth fault, transient and temporary overvoltages are limited and the current is high in the same way as described in connection with fig. 11.

Figure 13 shows an embodiment in which the neutral point 20 of the electrical machine 14 is directly connected to ground at 21. Such direct grounding limits transient and temporary overvoltages, but generates large currents in the event of a ground fault. In normal operation, the third harmonic current flowing from the motor neutral 20 to ground will be large.

As a further alternative, the earthing device of the electric machine according to the invention can comprise an active circuit for providing a neutral-to-earth connection with the desired impedance properties.

In fig. 14, a delta-connected three-phase electric machine 50 is shown connected directly to the distribution or transmission network 16.

In such a case, a grounding transformer of the same type as used in the embodiment shown in fig. 7 can be connected on the grid side of the motor 50.

As in the embodiment of fig. 7, the grounding transformer may be a Y-delta connection transformer with a neutral point of the Y connection ground, or a so-called zigzag grounding transformer, i.e. a Z-0 connection transformer with a Z ground. The grounding transformer will limit the temporary overvoltage.

As in the embodiment of fig. 7, a possible auxiliary power transformer can also be used for this purpose.

Claims (33)

1. A high voltage AC machine for direct connection to a distribution or transmission network (16), said machine comprising at least one winding comprising at least one insulated current-carrying conductor (4), characterized in that a first layer (6) having semiconducting properties is provided around said conductor (4), a solid insulating layer (7) is provided around said first layer, and a second layer (8) having semiconducting properties is provided around said insulating layer; further characterized in that grounding means (18, 21, 22, 24, 26, 28, 30, 32, 34, 35, 36, 37, 38, 39, 40, 42, 44, 46, 48, 52) are provided to ground at least one point of said winding.

2. The machine of claim 1, wherein the potential of the first layer is substantially equal to the potential of the conductor.

3. A machine as claimed in claim 1 or 2, characterized in that said second layer is arranged to form an equipotential surface substantially surrounding said conductor.

4. A machine as claimed in claim 3, characterized in that the second layer is connected to a predetermined potential.

5. The machine of claim 4, wherein the predetermined potential is ground potential.

6. A machine as claimed in claim 1 or 2, characterized in that at least two adjacent layers have substantially equal coefficients of thermal expansion.

7. An electrical machine according to claim 1 or 2, wherein the current carrying conductor comprises a plurality of strands, only a small number of said strands being uninsulated from each other.

8. A machine as claimed in claim 1 or 2, characterized in that each of the three layers is fixedly connected to an adjacent layer along substantially the entire connecting surface.

9. An AC machine having a magnetic circuit for high voltage, the magnetic circuit comprising a magnetic core and at least one winding, characterised in that the winding is formed by a cable comprising one or more current carrying conductors, each conductor having a plurality of strands, an inner semiconducting layer provided around each conductor, an insulating layer of solid insulating material provided around the inner semiconducting layer, and an outer semiconducting layer provided around the insulating layer; and, grounding means are provided to ground at least one point of the winding.

10. An electrical machine according to claim 1 or 9, wherein the grounding means comprises means for direct grounding of the windings.

11. An electrical machine according to claim 1 or 9, wherein the grounding means comprises means for low resistance grounding of the windings.

12. An electrical machine as claimed in claim 11, said machine having a Y-connected winding, the neutral point of which is available, wherein said low resistance grounding means comprises a low resistance resistor connected between the neutral point and ground.

13. An electrical machine as claimed in claim 11, said electrical machine having a Y-connected winding, the neutral point of which is available, characterised in that said low resistance grounding means comprises a resistor connected in the secondary of a transformer, the primary of which is connected between the neutral point and ground.

14. An electrical machine according to claim 1 or 9, wherein the grounding means comprises means for low inductance grounding of the windings.

15. An electrical machine as claimed in claim 14, said machine having a Y-connected winding, the neutral point of which is available, wherein said low inductance grounding means comprises a low inductance inductor connected between the neutral point and ground.

16. An electrical machine as claimed in claim 14, said electrical machine having a Y-connected winding, the neutral point of which is available, characterised in that said low inductance grounding means comprises an inductor connected in the secondary of a transformer, the primary of which is connected between the neutral point and ground.

17. An electrical machine according to claim 1 or 9, wherein the grounding means comprises means for high resistance grounding of the windings.

18. An electrical machine as claimed in claim 17, said machine having a Y-connected winding, the neutral point of which is available, wherein said high resistance grounding means comprises a high resistance resistor connected between the neutral point and ground.

19. An electrical machine as claimed in claim 17, said electrical machine having a Y-connected winding, the neutral point of which is available, wherein said high resistance grounding means comprises a resistor connected in the secondary of a transformer, the primary of which is connected between the neutral point and ground.

20. An electrical machine according to claim 1 or 9, wherein the grounding means comprises means for high inductance grounding of the winding.

21. An electrical machine as claimed in claim 20, said electrical machine having a Y-connected winding, the neutral point of which is available, wherein said high inductance grounding means comprises a high inductance inductor connected between the neutral point and ground.

22. An electrical machine as claimed in claim 20, said electrical machine having a Y-connected winding, the neutral point of which is available, characterised in that said high inductance grounding means comprises an inductor connected in the secondary of a transformer, the primary of which is connected between the neutral point and ground.

23. An electric machine as claimed in claim 1 or 9, said electric machine having a Y-connected winding, the neutral point of which is available, characterized in that said grounding means comprise a reactor connected in the secondary of a transformer, the primary of which is connected between the neutral point and ground, said reactor having such characteristics that the capacitive current during a ground fault is substantially neutralized by equal components of the inductive current provided by the reactor.

24. An electrical machine according to claim 1 or 9, wherein the earthing means comprises means for changing the impedance of the connection to earth in response to an earth fault.

25. An electrical machine according to claim 1 or 9, wherein the grounding means comprises an active circuit.

26. An electrical machine according to claim 1 or 9, wherein the earthing means comprises a Y-delta earthing transformer connected to the grid side of the electrical machine.

27. An electric machine according to claim 1 or 9, characterized in that the earthing device comprises a so-called zigzag earthing transformer connected to the network side of the electric machine.

28. An electrical machine as claimed in claim 1, said machine having a Y-connected winding, the neutral point of which is available, characterized in that said grounding means comprises a suppression filter for nth harmonic regulation.

29. An electrical machine as claimed in claim 1, said machine having a Y-connected winding, the neutral point of which is available, characterized in that said grounding means comprises a switchable suppression filter for nth harmonic demodulation.

30. A machine as claimed in claim 28 or 29, characterized in that said nth harmonic is the third harmonic.

31. An electrical machine as claimed in claim 1 or 9, said electrical machine having a Y-connected winding, the neutral point of which is available, characterised in that said earthing means comprises an overvoltage protector connected between said neutral point and earth.

32. An electrical machine as claimed in claim 17 having a Y-connected winding with a neutral point available, wherein an overvoltage protector is connected in parallel with the earthing device between the neutral point and earth.

33. An electric distribution or transmission network characterized in that it comprises at least one electric machine according to claim 1 or 9.