EP2467652B1 - Method for operating a cooling device for cooling a superconductor and cooling device suitable therefor - Google Patents

Description

The invention relates to a method for operating a refrigeration device for cooling a superconductor according to the preamble of claim 1. Such a refrigeration device is eg from the US 5 535 593 A known. The invention further relates to a suitable for carrying out the method of refrigeration device according to claim 8.

In electrical apparatus or machines with superconductors, e.g. Motors, generators or superconducting current limiters, the superconductor must be cooled and this is usually in a cryostat containing a cryogenic refrigerant such. liquid neon or liquid nitrogen. A refrigeration device serves for the recondensation of vaporized refrigerant present in the cryostat. The refrigeration device, often referred to as a refrigerator, usually comprises a closed circuit in which a working means, e.g. Helium gas is compressed in a compressor and relaxed again in a refrigeration unit and thereby gives off cooling capacity to the refrigerant in the cryostat. The refrigeration device can, for example, operate on the principle of Gifford-McMahon, according to the pulse tube principle or according to the Stirling principle.

Electrical apparatuses or machines with superconductors are highly suitable for use in mobile devices, such as in ships or offshore platforms, due to their high power density, small footprint and other specific properties of the superconductor. This is what the DE 10 2004 023 481 A1 and the WO 03/047961 A2 Ship propulsion machines and generators with a runner with a rotating high-temperature superconductor field winding, which is arranged in a cryostat in which neon is at a temperature of 25 K as a refrigerant for the superconductor. The cryostat is connected via a cry-heat pipe to a cold head of a refrigeration device, which also includes a compressor.

From the EP 1 526 625 A1 is a short-circuit current protection system for ships and offshore installations with a superconducting current limiter is known in which the superconductor is arranged in a cryostat, in which there is liquid nitrogen at a temperature of 77 K as a refrigerant for the superconductor. For recondensation of vaporized refrigerant is a cooling device, which includes a protruding into the cryostat cold head and a compressor. The refrigeration device itself is not controllable, but a regulation is indirectly by a counter-heater, which is mounted on the cold head. The back-up heater is turned on and off by a temperature control device so that the temperature of the liquid nitrogen is 77 K at ambient pressure. As a compressor, an oil-free linear compressor is preferably used because of its low maintenance requirements.

For the use of electrical apparatus or machines with superconductors in mobile devices, especially on ships or offshore platforms, it should be noted that the operation of the refrigeration device can be ensured even in the inclined position of the components. For example, for operation on ships, operation must be ensured even at an inclination of 22.5 degrees. According to the reciprocating principle working compressors or screw compressors are not suitable for this, since they are oil lubricated and therefore may not be inclined during operation. On the other hand, oil-free linear compressors are suitable. Such a linear compressor usually has two pistons, of which at least one, preferably both synchronously against each other, by a Linear motor with a frequency and with a stroke is linearly movable to the respective other piston or are.

It is known to control the power of such a compressor manually or automatically by varying the motor voltage and the piston frequency. However, as has been found, such a controller is not shipworthy because it does not take into account, for example, dependencies of the resonant frequency of the pistons on the filling pressure in the circuit and the temperature of the working fluid. Furthermore, an inclination or skew of the compressor leads to shifts in the operating point of the compressor. On the one hand, this means that no defined cooling capacity can be set. On the other hand, this leads to setting operating points at which the refrigeration device operates with a very poor efficiency and has a comparatively high demand for electrical energy. By shifting the operating point, it can also lead to the risk of a stop of the piston on a housing of the compressor and consequent safety shutdown of the compressor.

Proceeding from this, it is an object of the present invention to provide a method for operating a refrigeration device according to the preamble of patent claim 1, with a good efficiency, a defined cooling capacity can be generated, so that the refrigeration device in particular for use in mobile devices, such. Ships, is suitable. It is another object of the present invention to provide a suitable for performing the method refrigeration device.

The solution of the object directed to the method is achieved by a method according to claim 1. Advantageous embodiments of the method are each subject of the dependent claims 2 to 7. The solution of the object directed to the refrigeration device is achieved by a refrigeration device according to claim 8. Advantageous embodiments the refrigeration device are each subject of the subclaims 9 to 12.

In the method according to the invention, the stroke of the at least one movable piston is regulated to a, preferably predetermined, desired value. The stroke of a piston is here understood to mean the distance traveled by the piston from a first dead center (reversal point) of its reciprocating movement to a second dead center (reversal point). By such a control of the stroke, a fixed operating point of the refrigerating device can be independent of the temperature, the filling pressure of the working fluid and other influences such as e.g. be set an inclination of the compressor. On the basis of the piston stroke and the frequency, an accurate conclusion on the generated cooling capacity is possible. It is thus possible to set an operating point in which a defined efficiency, in particular a predetermined, cooling capacity is generated with good efficiency. Such a refrigeration device operated is thus particularly suitable for use in mobile devices, such. Ships, suitable.

For a precise and high-performance drive of the or each movable piston, the refrigeration device comprises in each case an electric motor and a frequency converter for supplying the motor with electric current with a predeterminable voltage and frequency.

Thus, the refrigerating device comprises two movable pistons, which are each driven by a frequency converter of an electric motor with frequency synchronous voltage, the motors are designed as two-phase AC motors and the frequency converter as a three-phase inverter with a voltage intermediate circuit, the inverter on the input side connected to a three-phase network and the output side are connected via two phases to the respective motor, and wherein an additional capacitor is connected in parallel to the voltage intermediate circuits.

According to an advantageous embodiment of the setpoint for the stroke is derived from a setpoint for the cooling capacity and the control of the stroke to a predetermined setpoint, the cooling capacity is controlled to this target value and / or regulated.

In the case of two pistons reciprocating in a linearly reciprocating manner relative to one another, a mean value from the stroke of the two pistons can also be used as a controlled variable for the control of the piston stroke.

If the or each movable piston is driven by a respective motor, the control of the piston stroke can be done very accurately that is used as a control variable for the control of the piston stroke, the voltage applied to the respective motor.

In the control of the piston stroke while the frequency of the reciprocating motion can be fixed.

According to a particularly advantageous embodiment, however, a resonance frequency of the reciprocating motion is determined in the control of the piston stroke and set the frequency of the reciprocating motion of the at least one movable piston to this resonance frequency. As a result, an operating point with optimum efficiency can be set automatically during operation.

The resonance frequency can be determined particularly easily by means of a phase shift between a motor current and a motor voltage. Alternatively, the resonance frequency can also be determined via the control value for the control of the piston stroke.

Advantageously, in two synchronously reciprocating linearly reciprocating piston in the control of the piston stroke deviations and irregularities with respect to a zero position of the piston, for example, due to an inclined position of the compressor compensated. In an inclined position of the compressor, in which the reciprocating motion not only has a component in the horizontal direction, but also a component in the vertical direction, the motor forces also have both a horizontal and a vertical component. The vertical component interacts with gravity. As a result, at one point in time, one of the pistons causes the engine power to act with gravity, while the other piston acts counter to gravity. As a result, in one of the pistons a lower driving force is necessary to reach the dead center, as with the other piston. Correspondingly, in the case of operation with a constant control value for both motors (eg motor voltage), the paths of the pistons would change, resulting in a zero offset of the pistons, which reduces the maximum cooling capacity. This can eg by a specifically different control of the two motors, for example For example, in the form of an offset in their control variables (eg by a DC voltage component in the motor voltage) are compensated.

A cooling device according to the invention for cooling a superconductor comprises a linear compressor for compressing a working fluid and a refrigeration unit for delivering a cooling power to a cryogenic coolant of the superconductor by relaxing the working fluid, wherein the linear compressor has two pistons, of which at least one, preferably both synchronously against each other, with a frequency and with a stroke is linearly movable to the respective other piston or are. In this case, the refrigerating device comprises a regulating device, which is set up in such a way that it regulates the stroke of the at least one movable piston to a desired, preferably predefinable, desired value.

Preferably, data are stored in the control device, which describe a relationship between the cooling capacity and the piston stroke.

According to a particularly advantageous embodiment, the refrigeration device comprises a higher-level control and / or regulating device for controlling and / or regulating the refrigeration capacity to a predetermined desired value by controlling the piston stroke.

For measuring the piston stroke of the at least one movable piston, the regulating device may comprise a measuring device, preferably a magnetic field sensor or an optical sensor.

An automatic adjustment of an operating point with an optimal efficiency is possible in that the control device is set up such that it determines a resonance frequency of the reciprocating motion during the control of the piston stroke and adjusts the frequency of the reciprocating motion to this resonance frequency.

FIG 1

ein Beispiel für einen Schiffsantrieb mit einem Motor mit einem Supraleiter,

FIG 2

einen schematischen Schnitt durch einen Linearkompressor,

FIG 3

ein Diagramm mit einer Darstellung der Abhängigkeit der Kälteleistung vom Kolbenhub,

FIG 4

Komponenten für die Ansteuerung und Regelung des Linearkompressors,

FIG 5

ein Diagramm mit Messwerten für den Hub der Kolben eines Linearkompressors,

FIG 6

ein Blockschaltbild der Regelung,

FIG 7

ein Diagramm mit einer Darstellung der Abhängigkeit der Kälteleistung und des Hubs von der Frequenz,

FIG 8

erfindungsgemäße Form mit zweiphasigen Motoren und dreiphasigen Umrichtern.

The invention and further advantageous embodiments of the invention according to features of the subclaims are explained in more detail below with reference to examples in the figures; show in it:

FIG. 1

an example of a ship propulsion with a motor with a superconductor,

FIG. 2

a schematic section through a linear compressor,

FIG. 3

a diagram showing the dependence of the cooling capacity on the piston stroke,

FIG. 4

Components for the control and regulation of the linear compressor,

FIG. 5

a diagram with measurements for the stroke of the pistons of a linear compressor,

FIG. 6

a block diagram of the scheme,

FIG. 7

a diagram showing the dependence of the cooling capacity and the stroke of the frequency,

FIG. 8

inventive form with two-phase motors and three-phase converters.

An in FIG. 1 Shown and known from the prior art ship propulsion system 1 comprises a high-temperature superconducting motor (HTS motor) 2, which is arranged in a nacelle 3 outside the actual hull and is also referred to as a pod drive. The HTS engine 2 can also be located inside the ship. The HTS motor 2 has a rotor 4 with a rotating high-temperature superconductor field winding 5, which is arranged in a cryostat 6 in which neon with a temperature of 25 K is as a refrigerant for the superconductor. The rotor 4 is surrounded by a stator (stator) 7. In between there is an air gap. The power supply of the HTS motor via electrical lines 8. The HTS motor 2 is connected via a propeller shaft 9 with a propeller 10.

The cryostat 6 is connected via a cryogenic heat pipe 12 to a refrigeration unit 22 of a refrigeration device 20. The refrigeration device 20 comprises a closed thermodynamic circuit 21 for a working medium into which, in addition to the refrigeration unit 22, an oil-free linear compressor 30 and a heat exchanger 24 are connected. In the circuit 21, the working fluid is compressed in the compressor 30, cooled in the heat exchanger 24 and relaxed in the refrigeration unit 22 and thereby gives off cooling capacity to the refrigerant of the superconductor. Refrigerant evaporated in the cryostat 6 is supplied to the refrigeration unit 22 via the cryogenic heat pipe 12 and recondensed on a cooled surface of the refrigeration unit 22.

When the refrigeration device 20 operates according to the Gifford-McMahon principle, the refrigeration unit 22 is a so-called cold head. For example, helium gas is used as the working medium. However, the refrigeration device can also work, for example, according to the pulse tube principle or according to the Stirling principle.

Further details of the linear compressor 30 are shown schematically in FIG FIG. 2 shown. The linear compressor 30 has two pistons 31, which are movable in a housing 34 in the direction indicated by the arrows 32 linearly against each other at a frequency f and with a stroke H to the respective other piston 31. In one variant, one of the two pistons 31 may also be held stationary and only the other piston 31 may be linearly movable with a frequency f and with a stroke H on it.

The drive of the two pistons 31 is effected by a respective linear motor 33. By means of the piston movements, helium gas having a low pressure is sucked in via a feed designated by 35. The sucked helium gas is compressed by the pistons 31 and ejected again via 36 discharges designated.

On the input side of the motors 33 is a two-phase motor voltage U, which causes a motor current I.

According to the invention, the stroke of the two pistons 31 is regulated to a predefinable desired value. The setpoint for the stroke is derived from a setpoint for the cooling capacity, which has to be delivered by the refrigeration unit 22 to the refrigerant, here neon, for the superconductor 5. By way of example, the diagram of FIG FIG. 3 the relationship between the cooling capacity K and the stroke H at a constant frequency f of the reciprocating motion of the piston 31. As can be seen, the cooling capacity K increases with increasing stroke H of the piston 31. By controlling the stroke H of the piston 31, the cooling capacity thus controlled to a desired value and / or regulated.

To determine the stroke of the piston 31, a measuring device 37 for determining the stroke of the respective piston 31 is arranged in the interior of the linear compressor 30 on each of the two pistons 31. The measuring device 37 is preferably a magnetic field sensor (eg a Hall sensor) or an optical sensor (eg a laser diode).

Other components of the refrigeration device 20 for the control and actuation of the linear compressor are in FIG. 4 shown. A control device 40 is configured such that it controls the stroke of the piston 31 to a predetermined target value. The control device 40 receives either manually from an operator or from a higher-level control and / or regulating device 50 for controlling and / or regulating the cooling capacity a setpoint value K for the cooling capacity. For this setpoint, target values for the stroke of the pistons 31 and the frequency of the reciprocating motion of the pistons 31 are derived in the control device 40. For this purpose, data 41 are stored in the control device 40, which describe a relationship between the cooling capacity, the piston stroke and the resonance frequency. These relationships may have previously been determined experimentally.

In each case a frequency converter 43 is used to supply the linear motors 33 with a predetermined voltage U of the frequency f _U. A control and / or regulating unit 44 serves to control and / or regulate the frequency converter 43.

As a control variable for the control of the piston stroke, an average value of the stroke of the two pistons 31 is used. For this purpose, the control device 40 detects actual values for the piston positions from the measuring devices 37 via signal lines 42 and determines therefrom an average value of the stroke of the two pistons 31. The output signals of the measuring device 37, e.g. a voltage is applied over at least one period of the stroke, i. a complete float, measured.

The stroke of the two pistons is determined from a difference between the two dead centers of the pistons, in which they reverse their direction of movement, in a period of a reciprocating motion. Exemplary shows this FIG. 5 various Measurements indicating the course of the stroke H over the time t for the two pistons 31 in a period of reciprocation. From these measuring points, the minimum and the maximum of the piston stroke of each piston 31 and thus its stroke per period are calculated.

The average value of the stroke of the two pistons per period gives an actual value H _Im , which is fed to a controller 45 of the control device 40. FIG. 6 The controller 45 determines from the difference between the actual value H _Im for the piston stroke and a setpoint H _S for the piston stroke a control value, here a target value U _s for the motor voltage U , which is supplied from the control device 20 together with a setpoint fs for the frequency of the motor voltage to the control and / or regulating unit 44 of the frequency converter 43. The control and / or regulating unit 44 then controls and / or regulates the output voltage of the two frequency inverters 43 to the required setpoint values Us and fs, the two linear motors 33 being supplied with a frequency-synchronized voltage.

The controller 45 is, for example, an I-controller. The exact structure of the controller 45 is preferably carried out after an evaluation of the step responses of the controlled system and the leadership behavior of the overall system.

As manipulated variables for the control of the piston stroke thus applied to the motors 31 motor voltages U are used. In this case, the frequency of the reciprocating motion can be fixed in the regulation of the piston stroke. However, due to the dependence of the resonance frequency of various operating parameters such as the temperature and the filling pressure, there is a risk that the refrigerating device 20 is operated with a poor efficiency. Exemplary shows FIG. 7 For this purpose, a possible relationship between the stroke H and the cooling capacity K over the frequency f. As can be seen, there is a maximum of the cooling capacity and the stroke in the region of a resonance frequency fo. Preferably, therefore, the resonance frequency of the reciprocating motion is determined by the control device 20 in the control of the piston stroke and the frequency of the reciprocating motion is set to this resonance frequency. As a result, the refrigeration device 20 can be operated at an operating point with optimum efficiency.

The resonance frequency can be determined and controlled on the basis of a relationship between the resonance frequency and the operating parameters (eg the temperature) stored in the control device 40. Preferably, however, the resonance frequency is automatically controlled to an optimum value. For this purpose, the frequency f _{U of} the motor voltage in the direction of larger and smaller frequencies is automatically varied by the control device 40 by changing the set value fs for the frequency of the motor voltage at certain time intervals at a constant predetermined amplitude of the motor voltage U and thereby the phase shift between the motor voltage U and the motor current I determined. The resonance frequency is present when the phase shift is maximal.

For this purpose, the control device 40 receives measured values for the motor voltage U and the motor current I from the frequency converters 43 or the control and / or regulating unit 44 of the converters and determines the phase shift. The determination of the phase shift can also be made directly in the inverters 43 or in the control and / or regulating unit 44 and transmitted to the control device 40.

Alternatively, the resonance frequency can also be determined via the control value for the control of the piston stroke. The resonance frequency is then the frequency at which the control value, here the motor voltage, is the smallest.

Advantageously, by the control device 40 in the control of the piston stroke deviations and irregularities with respect to a zero position of the piston 31, for example due to an inclined position of the compressor 20 taken into account. These can be compensated for example by different setpoint specifications for the two inverters 43 (eg in the form of a DC voltage component in the motor voltage).

In addition, the control device 40 may also include a monitoring that prevents piston stops on the housing walls and excessive motor currents by a setpoint reduction. For this purpose, the extreme values measured by the measuring devices 37 are monitored by the control device 40 for exceeding a predetermined limit value.

The two linear motors 33 can also be fed together by a single frequency converter 43. However, in the control of the piston stroke, the two motors can then compensate for deviations and irregularities with respect to a zero position of the pistons, e.g. at an inclination of the compressor, not be driven differently.

According to the in FIG. 8 shown inventive form, the motors 33 are designed as two-phase AC motors. Since the power grids in larger systems, such as in ships, usually designed as three-phase three-phase networks 60, the frequency 43 are designed as three-phase inverter with a network-side converter 61, a motor-side converter 62 and a voltage intermediate circuit 63 arranged therebetween to a symmetrical load of the network 60.

When using commercially available inverter 43 then there is the danger that they recognize the two-phase load of the intermediate circuit 63 as a phase failure on the network and therefore switch off. A remedy is that the DC link voltages of the two inverters 43 are stabilized via an additional capacitor 64 which is connected in parallel with the intermediate circuits 63 of the two inverters 43.

The cooling power generated by the refrigeration device 20 has now become controllable or controllable by regulating the stroke. This is an enormous savings potential of supplied electrical energy, since the efficiency of a compressor is only about 1%. Commercially available compressors always run under full load, unneeded cooling capacity is compensated or destroyed by counter heating. 1 W of destroyed cooling capacity corresponds to 100 W destroyed power taken from the mains. The control and actuation according to the invention makes it possible to keep the compressor at a fixed operating point without temperature changes or other operational influences (for example slanting of the compressor) leading to shifts in the operating point. Even a striking of the piston and associated safety shutdown of the compressor can be avoided.

A fixed set operating point can also be kept under inclination or skew of the compressor. This is an important requirement for the use of the compressor on ships. Since there are already ship-building versions commercially available for the components used for the control and activation, a refrigeration device according to the invention can thus be carried out fully suitable for shipping.

The operating point of the compressor can be operated by automatically readjusting the operating frequency always close to the resonance point. This can ensure that the compressor is operated at the resonance point at all times, i. has an optimal efficiency.

With the aid of a control device according to the invention, a plurality of compressors, which are operated in a network, can be controlled or regulated in parallel. For example, for a HTS synchronous machine, up to four refrigeration devices (refrigerators) are needed, of which two are provided as redundancy, for example. Instead of two such To run facilities at full load, all four can now be driven at partial load. As a result, all four devices can work in a range that is favorable for the efficiency.

Claims (12)

1. Method for operating a cooling device (20) for cooling a superconductor (5), wherein the cooling device (20) comprises a linear compressor (23) for compressing a working medium and a cooling unit (22) for discharging a cooling power (K) to a cryogenic coolant of the superconductor (5) by expanding the working medium, wherein the linear compressor (23) comprises two pistons (31), of which at least one, preferably both synchronously relative to one another, is and/or are able to be moved at a frequency (f) and a stroke (H) in a linear manner relative to the respective other piston, wherein the stroke (H) of the at least one movable piston (31) is regulated at a preferably predeterminable target value, characterised in that each movable piston (31) is driven by one respective motor (33) via one respective frequency converter (43) for supplying the motor (33) with electrical current at a predeterminable voltage and frequency, wherein the voltage (U) applied to the respective motor (33) is used as a manipulated variable for regulating the piston stroke (H), wherein the motors (33) are configured as two-phase AC motors and the frequency converters (43) are configured as three-phase converters with a voltage intermediate circuit (63), wherein the converters (43) on the input side are connected to a three-phase network (60) and on the output side via two phases to the respective motor (33), and wherein an additional capacitor (65) is arranged in parallel with the voltage intermediate circuits (63).
2. Method according to claim 1,
   characterised in that the target value for the stroke (H) is deduced from a target value for the cooling power (K) and by regulating the stroke (H) at a predeterminable target value the cooling power (K) is controlled and/or regulated at said target value.
3. Method according to claim 2,
   characterised in that in two reciprocating pistons (31) moving synchronously relative to one another in a linear manner, an average value from the stroke of the two pistons is used as a controlled variable for regulating the piston stroke.
4. Method according to one of the preceding claims,
   characterised in that when regulating the piston stroke (H) the frequency (f) of the reciprocating movement is fixedly predetermined.
5. Method according to one of claims 1 to 3, characterised in that when regulating the piston stroke (H) a resonance frequency (fo) of the reciprocating movement is determined and the frequency (f) of the reciprocating movement is set to this resonance frequency (fo).
6. Method according to claim 5,
   characterised in that the resonance frequency (fo) is determined via a phase shift between a motor current (I) and a motor voltage (U) or via a manipulated variable for regulating the piston stroke.
7. Method according to one of the preceding claims,
   characterised in that when regulating the piston stroke (H) deviations and irregularities relative to a zero position of the pistons (31) are compensated.
8. Cooling device (20) for cooling a superconductor (5), comprising a linear compressor (23) for compressing a working medium and a cooling unit (22) for discharging a cooling power (K) to a cryogenic coolant of the superconductor (5) by expanding the working medium, wherein the linear compressor (23) comprises two pistons (31), of which at least one, preferably both synchronously relative to one another, is and/or are able to be moved at a frequency (f) and a stroke (H) in a linear manner relative to the respective other piston, further comprising a regulating device (40) which is designed so that it regulates the stroke (H) of the at least one movable piston (31) at a preferably predeterminable target value,
   characterised in that for driving the, or each, movable piston (31) said cooling device comprises in each case an electrical motor (33) and a frequency converter (43) for supplying the motor (33) with electrical current at a predeterminable voltage and frequency and further characterised by two movable pistons (31) which may be driven via one respective frequency converter (43) by one respective electrical motor (33) at a frequency-synchronous voltage, wherein the motors (33) are configured as two-phase AC motors and the frequency converters (43) are configured as three-phase converters with a voltage intermediate circuit (63), wherein the converters (43) on the input side can be connected to a three-phase network (60) and on the output side are connected via two phases to the respective motor (33), and wherein an additional capacitor (65) is arranged in parallel with the voltage intermediate circuits (63).
9. Cooling device (20) according to claim 8, characterised in that data (41) is stored in the regulating device (40) which describes a connection between the cooling power (K) and the piston stroke (H).
10. Cooling device (20) according to one of claims 8 or 9,
    characterised in that it comprises a superimposed control and/or regulating device (50) for controlling and/or regulating the cooling power (K) at a predeterminable target value by regulating the piston stroke (H).
11. Cooling device (20) according to one of claims 8 to 10,
    characterised in that the regulating device (40) comprises a measuring device (37), in particular a magnetic field sensor or an optical sensor, for measuring the piston stroke (H) of the at least one movable piston (31) .
12. Cooling device (20) according to one of claims 8 to 11,
    characterised in that the regulating device (40) is designed so that when regulating the piston stroke (H) it determines a resonance frequency (fo) of the reciprocating movement and sets the frequency (f) of the reciprocating movement to said resonance frequency (fo).

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