SE521013C2 - Rotary electric machine with winding made of high voltage cable - Google Patents

Abstract

A rotating electric machine of a type with rotating field circuit, intended for direct connection to a distribution or transmission network. At least one electric winding of the machine comprises at least one electric conductor, a first layer with semiconducting properties surrounding the conductor, a solid insulating layer surrounding the first layer, and a second layer with semiconducting properties surrounding the insulating layer. A detecting circuit (16) is also arranged to detect earth faults in the rotating field circuit. Methods of monitoring the resistance of the field winding to earth and of determining the rotor temperature in such a machine is also described.

Description

lO 521 013 97/00875. Further description of the insulated conductor or cable can be found in PCT applications SE 97/00901, SE 97/00902 and SE 97/00903.

Furthermore, in the case of system solutions based on brushless feeders for magnetizing a synchronous machine, the rotor winding of the synchronous machine is normally not monitored for earth faults.

The object of the present invention is to provide such a rotating electrical machine intended for direct connection to Kraftnät with the possibility of detecting earth faults in the rotating field circuit.

SUMMARY OF THE INVENTION This object is achieved with a rotating electric machine of the kind initially stated with the features stated in claim 1.

According to a preferred embodiment of the machine according to the invention, the layers are arranged to adhere to each other even when the insulated conductor or cable is bent.

According to other advantageous embodiments of the machine according to the invention, a magnetization system intended for feeding the field circuit comprises a part rotating with the field circuit and parts of the pre-ground fault detection circuit are arranged in said rotating part. The detection circuit comprises a rotary injection circuit for applying a measuring circuit closed by the impedance between field winding and ground, an injection voltage and a measuring unit for measuring the fault current resulting in the injection voltage in said measuring circuit, wherein rectifier units are arranged to form the rectification values. and a wireless communication unit is arranged to transmit said absolute values to a stationary calculation unit for monitoring the resistance of the field winding to ground. In this way, only two process signals, namely the rectified absolute values of the injection voltage and the fault current, need be transmitted to the stationary part for determining the resistance value to ground. This means a limited signal section between the stationary and the rotating part with less requirements for the slip ring-free transmission. Furthermore, the number of rotating units for injection and 10 521 013 measurement is limited. The calculation unit suitably comprises a computer equipment for implementing the required calculation algorithms.

According to another advantageous embodiment of the machine according to the invention, wherein the excitation system is fed from a feeder with a rotating stator side, the injection circuit is fed from the rotating stator side of the feeder. In this case, voltage variations can be compensated with the help of software functions in the computer equipment. These functions are based on known conditions regarding the phase shift of RC circuits as well as the calculation of both real and imaginary current components and absolute values for determination of limit values.

According to another advantageous embodiment of the machine according to the invention, filter circuits are arranged in said measuring circuit for partly filtering out harmonics and partly direct voltage blocking. The filter time constants for filtering harmonics must in this case correspond to the period time of the injection voltage in order for efficient filtering out of harmonics to be possible.

According to a further advantageous embodiment of the machine according to the invention, scaling units are arranged before a comparator for comparing said absolute values of the fault current with predetermined limit values, which scaling units are arranged to normalize and compensate the measured fault current for variations in the injection voltage before the fault current is supplied to the comparator.

This is important because the injection voltage changes with the magnetization.

According to another advantageous embodiment of the machine according to the invention, the above-mentioned problem is solved by the injection circuit being supplied from a constant voltage source.

According to another advantageous embodiment of the machine according to the invention, a stationary voltage source is arranged to supply the injection circuit via a ring transformer. In this way, earth faults can also be detected at a stationary rotor.

Brief Description of the Drawings In order to explain the invention in more detail, selected embodiments of the machine according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 shows in cross-sectional view the insulated conductor used for windings of the machine according to Figure 2 shows a diagram of the excitation system with circuit for detecting earth faults in the field circuit and with devices for determining the rotor temperature in an exemplary embodiment of the rotary electric machine according to the invention. Figures 3-6 show equivalent diagrams for the pre-earth fault included in the detection circuit. the measuring circuit in the event of various faults, and Figure 7 shows an embodiment of a scaling unit for standardization and compensation of the measuring signal.

Description of preferred embodiments Figure 1 shows a cross-sectional view of an insulated conductor 11, intended for use in at least one of the windings in the machine according to the invention in order to enable direct connection of the machine to the power grid. The insulated conductor 11 comprises a number of strands 35 with a circular cross section of, for example, copper (Cu). These strands 35 are arranged in the middle of the insulated conductor 11. A first semiconducting layer 13 is arranged around the strands 35. An insulating layer 37 is arranged around the first semiconducting layer 13, e.g. PEX insulation. Arranged around the insulation layer 37 is a second semiconducting layer 15. The insulated conductor has a diameter in the range 20-250 mm and a conduit area in the range 80-3000 mm 2.

In the case of the described insulated conductor 11 or cable, the three layers 13, 37, 15 are designed so that they adhere to each other even when the insulated conductor 11 or cable is bent and this flexibility maintains it during its service life. Figure 2 shows the diagram of the excitation system of a rotating electrical machine with one or more windings of the insulated conductor shown in Figure 1 to enable direct connection to the power grid. The excitation system includes both a rotating injection and measuring circuit 16 and a stationary unit 20 for detecting earth faults and for calculating the rotor temperature.

The magnetization system thus comprises a rotating part 1 equipped with a rotating meter G3 which feeds from the rotating stator side a diode or thyristor bridge 12, which is connected with its direct current side to the field winding 14 of the electric machine. Furthermore, an injection and measuring circuit 16 is provided. is used in the detection of earth faults in the field circuit and measuring means 18 for determining the field voltage for temperature calculations. The rotating part 1 further contains measuring devices 5 for feeding the electronic equipment of the rotating part and a communication unit 3. Furthermore, measuring devices 25 are provided for measuring the field current IF.

Wireless communication between the rotating part 1 and stationary equipment 20 takes place by means of the communication unit 3 and a stationary communication unit 4.

By means of an injection circuit, comprising a transformer 8 for voltage adjustment and galvanic separation, a suitable voltage U is applied to the measuring circuit via an injection transformer 9, which voltage is thus taken from the supply side of the supply G3. The measuring circuit contains two parallel RC branches and is closed by the impedance of the field winding 14 to earth. The RC branches serve as current limitation and DC isolation.

The current I generated by the injection voltage U in the measuring circuit is sensed by a sensing circuit 22 via a measuring transformer 11 and converted into a corresponding voltage signal, which is filtered in the filter circuit 24 and rectified in the rectifier 26. The output of the rectifier 26 obtained on the rectifier 26 set the amplitude value for the fundamental tone of the current in the measuring circuit. The injection voltage U is also filtered and rectified in a similar manner in the filter circuit 28 and the rectifier 30, at the output of which a voltage signal UU is obtained, which represents the amplitude value of the fundamental tone of the injection voltage U.

The filter time constants T for filters 24, 28 must correspond to the period time of the injection voltage U and the measuring current l in order to be able to effectively filter out all harmonics.

By means of the communication units 3, 4, the voltage signals UU, U, are transmitted to the stationary part 20 for calculating the resistance of the field winding 14 to ground from these signals in the calculation unit 17.

With the calculation unit 17 it is in this way possible to monitor earth faults of the field winding 14 and when the resistance of the field winding 14 to earth drops below a predetermined level trigger an alarm. 521 013 6 R, denotes the resistance of the field winding 14 to earth, i.e. in practice the resistance to the iron mass of the rotating part, and C, the capacitance of the winding 14 to earth. The resistance R, can in principle vary from infinitely large value to zero. Figure 3 shows an equivalent diagram for the measuring circuit in the case that R 1 - = 0, i.e. the "worst" case with the field winding 14 short-circuited to earth. With known values of the resistance R, the capacitance C and the injection voltage U, the resulting current I1 in the circuit can be calculated, and the appropriate normalization constant can be determined according to the principles described in connection with fi gur 7 below. The absolute value of the current l1 corresponds to the value of the measuring signal U1 which is transmitted to the calculation unit 17, as described above in connection with fi gur 2.

To the right of the equivalent scheme in Figure 3, the sizes and phase positions of the injection voltage U, composed of a resistive component U, and a capacitive component UC, and the current I1 are illustrated. Figure 4 shows the corresponding equivalent diagram in an error-free state, i.e. the transition resistance of the induction to earth is R, - = w. With known values of the injection voltage U, the resistance R and the capacitance C and the measurement of the current l2, the induction 14 capacitance Cjtill ground is determined.

In the same way as in Figure 3, to the right of the diagram are shown the sizes and phase positions of the injection voltage U, which is composed of a resistive component U, in phase with the current l2, and a capacitive component, consisting of the voltage drop UC across the capacitors C and the voltage drop U, - across the capacitance Cj, and the current l2.

Figure 5 shows a corresponding equivalent diagram at a transition resistance between winding 14 and ground Rj, where 0 between those illustrated in Figures 3 and 4. With known values of the resistances R, the capacitances C, the earthing capacitance CJ., The injection voltage U and the currents l1 and l2 from the cases shown in fi gures 3 and 4, as well as predetermined limit values of the transition resistance to earth R, l3 for alarm and trip, as mentioned in connection with figure 2.

Thus, the impedance Z1 is across the two parallel branches, each containing 2R in series with 2C, 10 521 013 1 z1 = R 3 = U / (Z1 + Z2) To the right of the diagram in Figure 5, sizes and phase positions of voltages and currents are shown in the same way as in Figures 3 and 4. This diagram shows that current l3 is in phase with current l2 in Figure 4. a current component ICJ through the transition capacitance Cj and a current component ln- through the transition resistance Rj, the latter two current components being perpendicular to each other in the diagram, i.e. phase-shifted 90 °.

Figures 3 and 5 show cases with faults on the DC side of the supply of the field winding from the feeder G3, cf. Figure 2. Figure 6 illustrates a situation with faults on the AC side of the rectifier bridge 12. Faults on the alternating current side are characterized by the addition of an additional supply source Uac and the absolute value of the current is composed of two components. One which is driven by the usual injection voltage U and one which is driven by the potential level of the fault point towards ground, represented by the voltage Uac. In the event of a fault on the AC side, the total absolute value of the fault current will therefore exceed the limit values calculated in the case in Figure 5, often by a good margin, with the triggering of an alarm as a result.

The corresponding phase diagram on the right in Figure 6 corresponds to that in Figure 5.

In the case of variations, the injection voltage U must be compensated for these variations by scaling. Alternatively, predetermined limits in a comparator for triggering alarms, etc. must be changed, which is much more cumbersome.

Figure 7 shows a scaling unit 32, 34 which is included in the calculation unit 17 in Figure 2. In this scaling unit 32, 34, the measured value U is represented, representing the absolute value of the current I by multiplying by a normalizing constant K1. The appropriate size of the normalization constant K1 can be determined by a measuring method according to fi Figure 3. In the same way, the measuring signal U. "is compensated for variations in the injection voltage U by scaling with a compensating constant K2, where K2 = UU at the time of normalizing the measuring signal U. The normalized current compensated for variations in the injection voltage U is applied to a comparator 38, in which this current is compared with various predetermined limit values Lim 1, Lim 2, Lim 3 for triggering alarms, emitting a trigger signal, etc.

With the measuring device 18 the field voltage is measured and with the measuring device 25 the field current and corresponding measuring signals UF and IF are transmitted via the wireless communication units 3,4 to a unit 40 in the stationary equipment 20 for calculating the rotor temperature from these measuring signals, see Figure 2. in the measuring device 18, the field voltage signal is filtered with a time constant T1, which must correspond to 0.3 times the idle time constant of the field winding 14. Namely, when the electrical machine is not phased in on the mains, it has a time constant corresponding to the idle time constant and if the machine is connected to the mains, this time constant changes depending on the inductance of the mains by a factor of approximately 0.3.

The unit 40 can in turn be connected to e.g. rotor temperature indicating means, alarms or trip means for activating these depending on the determined value of the rotor temperature.

Numerous modifications of the above-described embodiment of the invention are of course possible within the scope of the invention. Thus, the invention can also be applied to stationary solutions, such as static feeders, and the supply voltage to the injection unit can be transformed into the rotating part by means of a ring transformer, which means that earth faults can also be detected at a stationary machine.

Claims (25)

10 15 20 25 30 521 013 PATENT CLAIMS A rotating electric machine of a rotating field circuit type, which machine is intended to be directly connected to a distribution or transmission network, characterized in that at least one electrical winding of the machine comprises at least one electrical conductor, a conductor enclosing the first layer with semiconducting properties, a solid insulating layer enclosing the first layer and a second layer enclosing the insulating layer with semiconducting properties and that a detection circuit is arranged to detect earth faults in the rotating field circuit. Machine according to claim 1, characterized in that the potential of the first layer is substantially equal to the potential of the conductor. Machine according to claim 1 or 2, characterized in that the second layer is arranged to form substantially an equipotential surface, surrounding the conductor. Machine according to claim 3, characterized in that the second layer is connected to a predetermined potential. Machine according to claim 4, characterized in that said predetermined potential is ground potential. Machine according to one of the preceding claims, characterized in that at least two adjacent layers of the machine's winding have substantially equal coefficients of thermal expansion. Machine according to one of the preceding claims, characterized in that the conductor comprises a number of strands, at least some of which are in electrical contact with one another. 15 20 30 521 015 10 Machine according to any one of the preceding claims, characterized in that each of said three layers is fixedly connected to adjacent layers substantially the entire abutment surface. Device according to one of the preceding claims, characterized in that the layers are arranged to adhere to one another even when the insulated conductor or cable is bent. 10. the chin is intended to be connected directly to a distribution or transmission network, characterized by a rotating electric machine of a type with a rotating field circuit, which is characterized in that at least one winding of the machine is formed by a cable comprising one or more live conductors, each conductor having a plurality of strands, an inner semiconductor layer arranged around each conductor, an insulating layer of solid insulating material arranged around said inner semiconductor layer, and a detection circuit being arranged to detect earth faults in the rotating field circuit. A machine according to any one of the preceding claims, characterized by a magnetization system intended for feeding the field circuit comprises a part rotating with the field circuit and that an injection and measuring unit for said detection circuit is arranged in said rotating part. The ring circuit comprises an injection circuit for applying a measuring circuit, which is a machine according to any one of the preceding claims, characterized in that the detection circuit is closed by the impedance between field winding and ground, an injection voltage, and a measuring unit for measuring the injection voltage. , and that rectifier units are arranged to form rectified absolute values of the injection voltage and the fault current, and a wireless communication unit arranged to transmit said absolute values to a stationary calculation unit for monitoring the resistance of the field winding to ground. Rotary stator side feeder, characterized in that the injection circuit is a machine according to claim 12, wherein the excitation system is fed from one fed from the rotating stator side of the feeder. 15 20 30 521 013 11 Machine according to claim 12 or 13, characterized in that filter circuits are arranged in said measuring circuit for filtering out harmonics and direct voltage blocking. Comparing said absolute value of the fault current with predetermined limit values Machine according to claims 12-14, characterized in that a comparator is arranged and, depending on the result of the comparison, triggers alarms. The pre-comparator to normalize and compensate the measured fault current for Machine according to claim 15, characterized in that scaling units are arranged variations in the injection voltage before the fault current is supplied to the comparator. 17. are arranged to measure the voltage and current of the field winding and transmit these machines according to any one of the preceding claims, characterized in that measuring means values to a unit for calculating the rotor temperature. The rotor temperature is stagnant and that said voltage and current values Machine according to claim 17, characterized in that the field winding calculation unit is transferable via the wireless communication unit to said calculation unit. The calculation unit for triggering when the temperature exceeds a predetermined Machine according to claim 17 or 18, characterized in that alarms are connected to the limit value. Machine according to claim 12, characterized in that a stationary voltage source is arranged to supply the injection circuit via a ring transformer. Machine according to claim 12, characterized in that the injection circuit is supplied from a constant voltage source. A method of a rotating electric machine, of a rotating field circuit type, the machine being intended to be directly connected to a distribution or transmission network, the at least one electrical winding of the machine comprising at least one electrical conductor, a conductor enclosing first layer with semiconducting properties, a solid insulating layer enclosing the first layer and a second layer having semiconducting properties enclosing the insulating layer, characterized in that an injection voltage is applied to a measuring circuit which is closed by the impedance between field winding and ground, and the resulting fault current in the measuring circuit is measured, whereupon rectified absolute values of the injection voltage and the fault current are formed and transmitted to a calculation unit for monitoring the resistance of the field winding to ground. Method according to Claim 22, characterized in that harmonics in the measuring circuit are filtered out. Method according to claim 22 or 23, characterized in that said absolute value of the fault current is compared with predetermined limit values and alarms are triggered depending on the result of the comparison. Method according to claim 24, characterized in that before the comparison, the measured fault current is normalized and compensated for variations in the injection voltage.