Turbo-Electric Marine Power Plant and Method of Regulating the Same

This invention relates to a turbo-electric marine power plant which includes a variable-speed heavy gas turbine, a synchronous propulsion generator coupled mechanically with the gas turbine and a synchronous or asynchronous propulsion motor coupled electrically with the propulsion generator. The propulsion motor drives a fixed propeller by means of a propeller shaft. The drive is effected by frequency regulation down to a minimum turbine rpm and by means of slip regulation below the minimum turbine rpm.

Turbo-electric marine power plants which have a heavy gas turbine as prime mover are known. In such a power plant the turbine is connected mechanically with the synchronous propulsion generator. The synchronous generator supplies current to the propulsion motor which may be of the synchronous or asynchronous type. In such an arrangement the propulsion generator and the propulsion motor constitute a drive substitute in the form of an "electric shaft". Since, on the one hand, the turbine runs continuously with the nominal rpm and is, on the other hand, not reversible, the propeller has to be of the adjustable type to make possible a change in the travelling speed of the ship and a reversal of the propelling force by virtue of changing the angular setting of the propeller blades. In this connection reference is made to the periodical SHIP-BUILDING AND SHIPPING RECORD of Feb. 16, 1973 pages 15 - 19 entitled: "General Electric's Heavy Duty Gasturbine/Electric Propulsion System". It is a disadvantage of such an arrangement that the output is, for econimical consideration, limited to approximately 40,000 HP.

In another known embodiment of such power plant there is used a slip clutch in connection with a fixed propeller; thus, the travelling speed of the ship and the reversal of propulsion are controlled by means of the slip clutch.

It is an object of the invention to provide a turbo-electric marine power plant which, without the use of an adjustable propeller, or a slip clutch, may be reliably controlled in all phases of travel, which is simple and economical in construction and which may deliver high outputs as well.

This object and others to become apparent as the specification progresses, are achieved by the invention, according to which, briefly stated, in the marine power plant which has an rpm-regulatable heavy gas turbine, a propulsion generator driven by the turbine, a propulsion motor driven by the generator and a fixed propeller driven by the motor, the turbine rpm is regulated as a function of the propeller rpm to obtain a delivered turbine output which lies above the minimum output limit of the turbine. With this arrangement any rpm, from standstill up to an rpm corresponding to the minimum turbine rpm, is controlled by a propeller rpm-responsive voltage regulation affecting a slip drive.

The marine power plant designed according to the invention has the particular advantage that for all rpm's and travelling speeds it can be reliably controlled and it is adapted to deliver large outputs.

FIG. 1 is a block diagram of a preferred embodiment of a control circuit for the slip drive.

FIG. 2 is a diagram illustrating outputs as a function of the rpm.

FIG. 3 is a diagram illustrating the ratio of slippage losses to nominal rotor losses as a function of the slippage.

FIG. 4 is a block diagram of another preferred embodiment of a control circuit for the slip drive.

The marine power plant designed according to the invention includes a two-shaft, variable-speed heavy turbine as the prime mover. The rpm regulation of the turbine affects the propeller rpm. Thus, for example, an rpm increase of the turbine causes a frequency increase of the propulsion generator. By means of controlling the field current, the armature voltage of the generator is increased in such a manner that there is set a constant value for the ratio formed of the generator voltage and the generator frequency. This means that in case of full current supply for the propulsion motor, the propeller shaft is subjected to constant torques. This regulation applies to the upper and medium propulsion range down to the minimum rpm limit of the heavy gas turbine.

The minimum turbine rpm limit is approximately one third of the nominal rpm of the turbine, or 0.33 n.sub.Tnom. If, the example, due to an overload, the turbine rpm falls below this limit, the gas turbine stalls. For this reason, the regulation of the propeller rpm is effected in the lower third of the propulsion range by means of the slip drive of the propulsion motor.

For the propulsion drive two critical points have to be observed: first, the rotor slip losses should not significantly exceed the nominal rotor losses of the motor (FIG. 3) and, second, at the gas turbine certain rpm's can be set only if the delivered output is above the minimum output limit (FIG. 2).

In order to thus ensure that the rotor slip losses do not significantly exceed the nominal rotor losses, the propulsion ought to be effected with small turbine rpm's, the is, in the neighborhood of the minimum turbine rpm limit of, for example, 0.33n.sub.Tnom. A regulation to a constant turbine rpm of 0.33n.sub.Tnom for driving the motor at slippage rpm's in the lower third of the propulsion range is, however, as indicated above, not possible because the turbine rpm would drift towards the point on the minimum output limit (of approximately 0.56n.sub.Tnom to 0.33n.sub.Tnom), which, with regard to output, corresponds to the received motor output (which, in turn, equals the sum of the slippage loss and the delivered propeller output).

The non-regulated drifting of the turbine rpm leads, in case of load surges, to undesirable rpm fluctuations and since the minimum output limit does not appreciably vary in the range of approximately 0.3n.sub.Tnom to 0.36n.sub.Tnom, a load surge results, in this range, in a substantial turbine rpm drop. Because of the high turbine time constants, this phenomenon may lead to a drop below the minimum rpm, since the rapid rpm variation can no longer be followed.

The above-discussed fluctuation of the turbine rpm's and a drop below the limit range can be avoided if the turbine rpm is controlled in such a manner as function of the propeller rpm that, on the one hand, the rotor slippage losses do not significantly exceed the nominal rotor losses of the motor and, on the other hand, the delivered turbine output is continuously above the minimum output limit. Thus, to begin with, a turbine rpm n.sub.Tx is selected which is higher than indicated by the minimum output curve and the turbine rpm is regulated as a function of the actual propeller rpm. For this situation the following relationship applies:

In FIG. 2, there is an exemplary manner, selected a value of 0.66n.sub.Tnom as the turbine output rpm. Thus,

[pus] stands for per units and means: values with reference to nominal values.

it is thus seen that the delivered turbine output lies above the minimum output limit. If the desired value for the propeller rpm is fed slowly, the turbine rpm will be realigned without distrubance. FIG. 3 illustrates that in case of the above-described control of the turbine rpm, the rotor slippage losses do not significantly exceed the nominal rotor losses.

In the embodiment according to FIG. 1, there is shown a heavy gas turbine 11 which drives a synchronous propulsion generator 12 by means of a shaft 13. The turbine rpm is sensed by means of a tacho-generator 16. The propulsion generator 12 supplies current to a propulsion motor 21 which may be a synchronous or an asynchronous motor and which drives a fixed propeller 23 by means of a propeller shaft 22. The rpm of the shaft 22 is sensed by a tacho-generator 24.

The turbine output rpm of approximately 0.66n.sub.Tnom (FIG. 2) is set at the regulator (difference former), 31 as a desired value. The rpm of the propeller n.sub.prop is used as an acutal value. The signal n.sub.Tdes'd emitted by the regulator 31 represents the difference between the actual propeller rpm and 0.66n.sub.Tnom. This signal is applied to a regulator (difference former) 32 as the desired turbine rpm. The regulator 32 also receives a signal n.sub.Tactual from the tacho-generator 16, representing the acutal turbine rpm. From a comparision of these two signals there is obtained the regulating signal (that is, the deviation to be eliminated by the regulation) .DELTA.n.sub.T which is applied to a circuit 33. The latter, in turn, emits a setting signal (proportionate to the signal .DELTA.n.sub.T) for altering the fuel supply of the turbine. This is effected in such a manner that with an increase of the actual propeller rpm the turbine rpm changes slowly towards a smaller rpm value from the value of 0.66n.sub.Tnom, but remains above the critical turbine output limit.

The calculation that follows illustrates this process. The propeller rpm is slowly increased from n.sub.prop.actual =0 to, for example, n.sub.prop.actual =0.10. As a result, at the regulator 31 there appears an actual signal value n.sub.prop.acutal =0.10 with a fixed desired value input pf 0.66n.sub.Tnom. Consequently, the regulating signal slowly changes at the regulator output from 0.66 towards 0.56, corresponding to the changed actual value input. For the desired value input of the regulator 32 this means a slow drop. The signal appearing at the output of the regulator 32 is applied to the circuit 33, the setting signal .DELTA.n.sub.T of which now maintains the fuel supply of the turbine in such a manner that the turbine rpm changes slowly above the critical output limit in a regulated manner towards an rpm of, for example, 0.33n.sub.Tnom.

As it has been noted before, the propeller rpm is regulated by means of slippage of the propulsion motor. The actual propeller rpm is, simulataneously with its application to the regulator 31, applied to the propeller rpm regulator (difference former) 34 as an actual value; the desired value input n .sub.prop.des'd is communicated in most cases by engine order telegraph. The regulating signal .DELTA.n.sub.prop is applied to a circuit 35, the emitted setting signal of which serves as a desired value for a voltage regulator 36. From a voltage transformer 25 there is taken, as an actual value, a voltage U.sub.Gactual proportional to the generator voltage and applied to the voltage regulator 36. The regulating signal .DELTA.U.sub.G is supplied to a circuit 37 which emits a setting signal I.sub.E. The latter, which is proportionate to .DELTA.U.sub.G, is applied to a field current rectifier 15 of the field winding 14 of the propulsion generator 12. In this manner the voltage is altered in the armature circuit of the propulsion generator 12 by means of the field current.

Using once again the example of actual value input of a propeller rpm of n.sub.prop.actual =0, the difference between the desired value n.sub.prop.desired applied in advance by the engine order telegraph and n.sub.prop.actual =0 means a high desired value input for the voltage regulator 36. Consequently, a large regulating signal is obtained. This means that the field current in the field winding 14 of the propulsion generator 12 is increased which, in turn, results in a voltage increase in the armature circuit of the propulsion generator. In this manner, the slippage in the propulsion motor 21 is altered towards a higher rotor rpm which, in turn, means an increase in the actual propeller rpm. Thus, there is obtained at the regulators 31 and 34 an increase of the actual value input. This affects the turbine rpm through the regulator 31 in such a manner that with a slow change, the turbine rpm also changes slowly and this change occurs above the critical output value, for example, towards 0.33n.sub.Tnom. The increase of the actual value input at the regulator 34 is compared with an again slowly increasing value of the desired value input from the engine order telegraph and the difference is again applied to the voltage regulator 36. In this manner, the armature voltage is affected in the propulsion generator circuit. The propulsion motor is continued to be driven in slip drive until the propeller rpm runs up to 0.33n.sub.prop.nom. The turbine rpm is then decreased, above the critical output limit value, from 0.66n.sub.Tnom to 0.33n.sub.Tnom.

If the vessel is braked, the above-described regulation below the propeller rpm of 0.33n.sub.prop.actual is effected in a reverse sense until the actual propeller rpm attains 0.

A reversal from forward run to reverse run may be effected by means of a propulsion reverser (not shown) by virture of a phase exchange in the armature circuit of the propulsion motor, whereby merely the direction of rotation is reversed.

The selection of a higher turbine rpm of, for example, 0.66n.sub.Tnom makes possible during the propeller start a higher turbine output, so that the initial break-away torque at the propeller shaft is reliably applied. For a value of n.sub.T = 0.33N.sub.Tnom, the available output is P= 0.5P.sub.nom, while for an rpm value of n.sub.T = 0.66n.sub.Tnom, the available output amounts to P = 0.9P.sub.nom.

The above-discussed selection of the desired value input n.sub.T provides a substantial freedom for designing marine power plants of this type. Since, for redundancy reasons, very often the motor drive output is divided between two gas turbine generator sets into two halves, so that in case of a break-down of one aggregate, the remaining operational turbine is capable of driving the propulsion motor, it has to be ensured that in such an emergency situation the sole drive aggregate is capable to accelerate the propulsion motor from a standstill to its cruising rpm. In such a case the sole operational turbine has to furnish, at the beginning of the propeller start, approximately 90% of the nominal output value. It is further possible to so design marine power plants of this type that the motor drive output is not divided in a half-and-half ratio between the two gas turbine generator sets. In such an arrangement a main propulsion aggregate and an auxiliary propulsion aggregate of lesser output is provided, for example, for a "take-home operation", in case the main aggregate has broken down. In such a case, however, it is conceivable that in case the main propulsion aggregate breaks down during the starting of the motor, the auxiliary aggregate drops below the critical output limit and thus a starting of the motor becomes impossible.

To remedy such a situation, a control circuit as illustrated in FIG. 4 may be used. In this case the regulating signal .DELTA.n.sub.prop of the desired propeller rpm and the actual propeller rpm are used as a desired value input for a voltage/frequency regulation. The actual value is obtained by forming the quotient of the actual turbine rpm (which is proportionate to the generator frequency) and the actual generator voltage value (U.sub.G actual /f.sub.G actual). Thus, at the voltage regulator 36 there appears, as the desired value, the regulating signal .DELTA.n.sub.prop. The quotient forming apparatus 38 receives from the tacho-generator 16 the actual turbine rpm value and at the voltage transformer 25 there is obtained the actual generator voltage value. From these two values there is formed the quotient U.sub.G actual /f.sub.G actual which is applied as the actual value signal [u .sub.G/F ]actual to the voltage regulator 36. If there is a danger of turbine stalling during the starting steps, that is, the rpm decreases towards a smaller value, the frequency value will drop. Consequently, the value of the quotient will be greater. This means that at the input of the voltage regulator 36 there appears a greater actual value input and for the regulator output there is obtained a smaller regulating signal which affects, through the field current rectifier 15, the field winding 14 of the propulsion generator 12. In this manner the field current is decreased and thus the voltage in the armature circuit also decreases. In the propulsion motor the slippage is altered in the sense of a smaller rotor rpm and thus load will be removed from the turbine.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.