Advantages of the shaft alternator systems

OF STN SYSTEMTECHNIK NORD

Arrangement and drive of the shaft alternator

Shaft alternator systems with frequency converter supply three-phase current of constant voltage and frequency to the mains at variable main engine speed. The useful speed range of the shaft alternator can be defined on the basis of the requirements of ship operations control. For example, it is possible to select a shaft alternator speed range of 60% to 100% with constant system output and 60% to 30% with reduced system output.

Shaft alternator systems of STN Systemtechnik Nord have a number of special characteristics which are of advantage both to shipping company and to shipyard:

High flexibility as regards arrangement and drive of the shaft alternator

All shaft alternator systems have one thing in common: The active power required for the mains is generated by the main engine. However, the shaft alternator can be arranged and can be driven by the main engine in very diverse ways:

· Arrangement of the shaft alternator in the shaft line between low-speed main diesel engine and propeller. This configuration with a large air gap of 7.5 mm between stator and rotor and without additional bearings has proven very successful and is the most frequent configuration used. It is particularly simple and sturdy and requires little mainte­nance. By contrast with certain other arrangements, tor­sional vibration problems relating to design and operation are not anticipated with this configuration

· Drive of the shaft alter­nator by the power take-off of a reduction gear which is arranged between medium-speed main diesel engine and propeller. This is the usual and appropriate configura­tion for medium-speed main diesel engines.

In addition, other arrange­ments of the shaft alter­nator are also possible in principle:

· Drive directly from the crankshaft or via a reduc­tion gear on the front end of the main diesel engine.

· Drive via a gear unit arranged in the shaft line between low-speed main diesel engine and propel­ler.

3. Drive via a power take-off of the main diesel engine with integrated gear reducer.

Closed-loop control

Closed-loop control of the shaft alternator system maintains the mains voltage and the mains frequency constant. The automatic voltage regulator is incorporated in the compensator and operates in the same way as at a normal mains alternator.

The frequency controller acts on excitation of the shaft alter­nator. If the mains frequency is too low, as the result of a load increase or engine-speed reduction, the excitation of the shaft alternator is increased and it supplies a higher active current at constant voltage. The higher active power fed into the mains as the result of this causes a rise in frequency until a state of balance is reached between power fed in and power con­sumed, dependent upon the set frequency set value. In the reverse case, excitation of the shaft alternator is reduced if the mains frequency is too high so that the shaft alternator sup­plies a lower active current at constant voltage and so that the mains frequency drops accordingly.

The frequency set value can be set manually as on a diesel generator set or by the superordinated active-power load con­troller of an automatic power supply system. The active power between diesel generator sets operating in parallel can then be distributed in such a way that the shaft alternator, as the most economical source of energy, is fully loaded first and only the power required over and above this is supplied by the auxiliary diesel generator sets. In this case, the actual load does not drop below the minimum load required for the auxiliary diesel engines.

2000 kW, 840-1400 rpm shaft alternator on the polar research vessel „Polarstern", driven via the power take-off of a reduction gear

Fitting the poles of a shaft alternator on a propeller shaft

Shaft alternator systems of STN Systemtechnik Nord can operate unrestrictedly in parallel with other mains generator sets. Voltage and frequency droop are provided for this, as is also conventional on diesel generator sets. This voltage reduc­tion as a function of reactive power for reactive load distrib­ution and frequency reduction as a function of active power for active load distribution is adjustable and can thus be adapted to other mains generator sets.

Parallel operation of two shaft alternator systems on ships with two propellers is also possible unrestrictedly and such a configuration has already been implemented in several cases.

Advantages at a glance

Using shaft alternators is a particularly economical and envi­ronment-friendly method of generating electrical power. For this reason, more and more ships are being equipped with such systems.

The use of shaft alternators affords many advantages:

■ Reduction in fuel costs:

Main diesel engines are operated with heavy diesel oil (and not with more expensive diesel oil as are most diesel generator sets). In addition, they operate with a far better efficiency. By comparison with power generation using auxiliary diesel engines, we thus obtain a substantial saving in fuel costs.

■ Reduction in maintenance and lubricant costs:

The operating time of the diesel generator sets is reduced as a result of sole operation of the shaft alternator at sea and, in some cases, even during estuary navigation.

■ Safety for ship and crew:

Shaft alternators are less susceptible to malfunctions than internal-combustion engines. The availability of the power-generation equipment is thus improved.

■ Savings in personnel:

Personnel levels can be reduced thanks to the simplification of ship machine operation and powering the mains solely from the shaft alternator.

■ Low-noise power generation:

By comparison with diesel generator sets, shaft alternators are extremely quiet since they produce only few additional noise. This means that shaft alternators are also advisable from the point of view of ecology.

All requirements of a ship's mains are met unrestrictedly during shaft alternator operation:

■ Generation of the required active power and reactive power.

■ Selective tripping of short-circuits without failure of the overall system.

■ Starting and shut-down of large consumers without inadmissible voltage and frequency fluctuations.

■ Constant voltage and frequency when subject to main engine speed variations as the result of heavy seas and when maneuvering.

■ Unrestricted parallel operation with other mains generator sets in the entire shaft speed range and in heavy seas.

■ Operation, including synchronization, in the same way and with the same operating controls as on diesel generator sets.

■ Simple integration in automated power generation systems.

■ Automatic changeover to diesel generator operation if the actual shaft speed drops below the speed limit.

TURBOGENERATOR

A turbo-generator comprising a shaft, on which is positioned an axial ventilator, a cover that divides an inflow chamber for a cooling gas transported by the axial ventilator from an outflow chamber for this cooling gas, and a cylindrical channel segment that is positioned coaxially to the axial compressor and surrounds the axial compressor radially, whereby the inflow chamber comprises an asymmetrical cooling gas supply, and whereby in the inflow chamber means for homogenizing the cooling gas supply to the axial ventilator are provided. In order to improve the cooling, and therefore the efficiency of the machine, the inflow chamber forms a rotation-symmetrical annular chamber in the region of an inflow opening of the channel segment. A cylindrical annular collar is positioned coaxially to the axial ventilator within this annular chamber. The annular collar projects axially into the annular chamber and forms a completely surrounding, radial throttling gap at the inflow opening.