FKI Industrial Drives has supplied Synchrosil converters rated at 3,500kW for an innovative shaft generator system which supplies electrical power required by refrigerated container vessels.
FKI Industrial Drives, working as part of a Brush Electrical Machines (BEM) project team, has supplied Synchrosil converters rated at 3,500kW for an innovative shaft generator system which supplies the majority of electrical power required by 80,942 tonne refrigerated container vessels while they are at sea, utilising the relatively cheap fuel used by the ship's main engine.
The generator take-off system is the modern equivalent of an old but well proven idea.
However, to the project team's knowledge no system on this scale, with such a high nominal power rating of 3,500kW, had ever been attempted before.
In this case the system design was the corollary of two key demands made at the planning stage for the ships.
These were that they should be able to operate with low manning levels and be capable of cruising at sea with very low operating costs.
The result of these demands is that a total of four ships have been fitted with BEM's innovative shaft generator systems.
All the vessels are modern 300 metre long Post-Panamax container ships, of 80,942 gross tonnage, operated by P and O Nedlloyd.
The ships are each propelled by a single 90,000 hp (65 MW) 100 rev / min diesel engine, the largest in service at the time of their commissioning.
They have accommodation for 6,700 containers, over 700 of which can be refrigerated, representing a substantial load on the ship's electrical supply.
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Each of the ships has a 6.6 kV, 60 Hz main distribution system that is fed by four diesel engine driven, 3.6 MW generators and the shaft generator.
The diesel generators operate when the ships are in port, primarily to keep the containers cool.
They are also used at the start of voyages and during slow-speed manoeuvring.
In terms of fuel and maintenance costs, however, they are relatively expensive to run and their protracted use would be uneconomic on long voyages.
Because of this, the major power source, up to 3.5MW, while the ships are cruising is provided by the BEM shaft generator system.
This features a salient pole generator, which is fitted around the ship's main propeller shaft.
The generator operates at 60 to 110 rev/min depending upon the speed of the ship, (which corresponds to 14 to 26Hz).
The generator output is rectified and converted to the ship's frequency, 60Hz, by the FKI Synchrosil converter, which is rated at a nominal power of 3,500kW.
The Synchrosil converter is based upon a standard forced air-cooled unit used in industrial drive systems, but for this application is configured as a 24-pulse synchro-converter.
In contrast to more standard applications, the input bridge of the converter is subject to low frequency and, hence, this is the limiting device as regards variable frequency derating, rather than the output bridge.
A further feature of the converter set-up is that the input rectifier is fired with minimum delay angle under steady state conditions to present the shaft generator with the highest possible power factor.
The power circuit of the converter comprises four power loops in two pairs, each pair feeding a double wound transformer whose windings are phase shifted to obtain 24 pulse operation.
To minimise the possibilities of harmonics, the dc voltages and currents of the four loops are balanced under closed loop control.
A frequency control outer loop provides a common current demand to four inner current controls with an accuracy of the order of 1%.
Because of the low operating speeds of the generator, each Synchrosil unit is used in conjunction with a static excitation system.
The latter obtains its power supply from the generator output and provides a constant terminal voltage to the input bridge of the Synchrosil converter over the full shaft speed range.
Linked to the Synchrosil converters, as the driven element in each system, are synchronous condensers.
Configured as 4-pole salient brushless machines, the synchronous condensers supply the total volt-ampere requirements (var) for the ships.
They are fitted with brushless exciters - 6 pole brushless "Pony" motors - and conventional automatic voltage regulators (AVR), identical to those used on the diesel generators.
As a result, the output characteristics of the system are identical to those of the diesel sets, enabling parallel operation of the generators in any combination, using quadrature current droop control Smooth and Precise System Starting When the shaft generator system is required to come on-line the synchronous condenser is accelerated, by means of the low-voltage pony motor, to a speed where natural commutation is possible.
The full speed of this motor corresponds to about 2/3 of the 60Hz synchronous speed of the 4-pole synchronous condenser.
When the former reaches a low slip speed it is tripped off and the synchronous condenser is accelerated by the converter system to achieve synchronism with the diesel generators.
Overall, this method of starting has proved to be both efficient and effective and offers the added advantage that it eliminates the need for rotor position feedback.
System Safeguards Ensure Smooth Transfer Between Generators.
The safeguards built into the shaft generator system mean that, if at any time during its operation the main engine is instructed to operate below the minimum speed of 60 rev / min, an automatic transfer system cuts in and transfers the load from the shaft generator to the diesel generator and disconnects and de-energises the former unit.
The main engine governor is then allowed to reduce the shaft speed.
All these operations are fully automatic and require no manual input from the ship's personnel on the bridge.
The proof of the efficacy of any system of this type rests on its performance in actual use.
In this respect the shaft generator system has been an unqualified success.
The ships have now been earning revenue for over a year and the experience gained has proved the soundness of the design of the complete system and it's components in this application.
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