Electrical transmission or interconnection systems – Plural supply circuits or sources – Substitute or emergency source
Reexamination Certificate
1999-03-11
2001-01-23
Ballato, Josie (Department: 2836)
Electrical transmission or interconnection systems
Plural supply circuits or sources
Substitute or emergency source
C307S064000, C322S039000
Reexamination Certificate
active
06177738
ABSTRACT:
BACKGROUND OF INVENTION
The invention relates to an electrical system for use in isolation, the system comprising a main engine, such as a gas turbine or a diesel engine, a generator driven by the main engine for generating a first alternating voltage, a first rectifier unit for converting the first alternating voltage generated by the generator into a direct voltage on a DC bus, and at least one inverter unit for converting the direct voltage on the DC bus into a second alternating voltage.
In accordance with the above, the invention relates to an electrical system for use in isolation, which means, for example, an electrical system of a ship or some other station, for example one located in a desert, isolated from other electric networks. An above type of electrical system for a ship is known, for example, from European Patent 97,185. The publication teaches the use of a synchronous generator as a shaft generator driven by the shaft of the main engine, the synchronous generator supplying power to two electric networks. In one network, the voltage and frequency are determined by the voltage and frequency of the shaft generator, i.e. they are directly dependent on the rotation speed of the ship's main engine. To the other electric network, power is supplied by an AC inverter arrangement, which comprises a rectifier circuit and an inverter circuit, by which is generated an alternating voltage with a constant amplitude and frequency. The two separate electric networks are used so that the ship's primary actuators, such as motors, pumps, compressors, ventilating and cooling systems, etc. obtain the power supply directly from the network that is being fed by the synchronous generator, whereas the electric power required by radio equipment, navigation systems and lighting is obtained via the AC inverter arrangement.
The above-described known system has some defects. For example, a separate energizing circuit and a separate electric motor have to be arranged so as to be able to start the main engine. Another major problem may arise when the electricity produced by the inverter system is not necessarily of high quality, i.e. it contains harmonic waves, which are harmful in many applications. Also, in case of main engine failure, the system is also too vulnerable, if there are no auxiliary engines or arrangements by which the ship's propeller can be driven even in case of such main engine failure.
U.S. Pat. Nos. 4,883,973 and 4,330,743, respectively, teach an electrical system for a vehicle and an aircraft. In both systems, the same electric machine is used both as a motor when the main engine, i.e. the motor of the vehicle or aircraft, is started, and as a generator generating electric power to the electrical system after the motor has started. In U.S. Pat. No. 4,883,973, the motor is an asynchronous machine that is coupled to the vehicle's motor through a bimodal gear set. The gear set operates as a reduction drive during the starting mode and as a direct drive during the generating mode. When the vehicle's motor is being started, the rotary speed of the asynchronous machine has to be registered by a tachometer so as to effect feedback control in the inverter arrangement controlled by a computer-based controller feeding the asynchronous machine, so that the arrangement can maintain the desired voltage/frequency ratio in the output voltage. The motor used in the electrical aircraft system disclosed in U.S. Pat. No. 4,330,743 is a synchronous machine coupled to an aircraft engine via a drive unit containing a hydraulic torque converter. When the aircraft engine is being started, the synchronous machine is used as a motor, whereby the inverter unit feeding the synchronous machine needs data on the position of the rotor. When the aircraft engine is being started, the synchronous machine operates as a motor, and its speed is at first raised to the operating speed. After this, the hydraulic torque converter is used to accelerate the aircraft engine gradually to a desired speed so as to start the engine.
Although U.S. Pat. Nos. 4,883,973 and 4,330,743 teach the use of one and the same electric machine both as a motor when the main engine is being started and as a generator when the main engine is running, expensive and clumsy electrotechnical control arrangements are needed particularly for the use as a motor in the arrangement of both publications, the control arrangements comprising a tachometer or a rotor position transmitter, and torque converter and gear arrangements. The torque converter and gear arrangements used are such that they cannot be applied reasonably to transmission of high power, such as megawatts. Also, the operation of the electrical systems known from the two publications is based on direct voltage supply obtained, for example, from a battery. This sets certain limitations to application of these systems.
BRIEF DESCRIPTION OF INVENTION
The object of the present invention is to provide an electrical system which is designed for use in isolation and in which the defects and problems of the above-described known systems are overcome. This is achieved with an electrical system according to the invention, which is characterized in that the generator driven by the main engine for generating the first alternating voltage is an asynchronous machine coupled directly to the shaft of the main engine, and that the first rectifier unit also comprises an inverter part for converting the direct voltage of the DC bus into an alternating voltage with an adjustable frequency to said asynchronous machine so as to use the machine as a motor. Depending on the structure of the rest of the system, there are several partly alternative reasons for the use of said asynchronous machine as a motor. For example, when the machine is used as a motor, the gas turbine used as the main engine can be started, or the ship's propeller shaft can be driven as a “take-me-home” motor in case of main engine failure, irrespective of whether the main engine is a gas turbine or a diesel engine.
When the asynchronous machine is used as a shaft generator coupled to the shaft of the main engine, a situation arises in which one and the same machine is used both as a motor starting the gas turbine which is used as a main engine, and as a generator generating electric power from the shaft of the main engine. This simplifies the system, and the number of components needed is reduced. Further, the asynchronous machine, such as a squirrelcage induction motor, is much cheaper than a synchronous motor, and needs substantially less servicing.
The electrical system of the invention can be designed to further comprise a diesel engine/synchronous generator combination for generating a third alternating voltage on an AC bus, and a second rectifier unit for converting the third alternating voltage into a direct voltage on said DC bus so that it can be further converted in the inverter part of the first rectifier unit into an alternating voltage to said asynchronous machine when the machine is used as a motor. In the system of the invention, any auxiliary electricity needed, for example, for starting the main engine can be generated by means of the diesel engine/synchronous generator combination. Further, when necessary, for example in case of main engine failure, the system allows generation of electric power that can supply both the ship's other electric network and the electric motor coupled to the shaft of the main engine by the above-described arrangement, and so the ship's propeller can also be driven in this way, although only at a low rotary speed. The result, however, is a “take me home” arrangement.
The electrical system of the invention can be designed to further comprise an electric motor mechanically arranged to run with the shaft of the synchronous generator, the electric motor being arranged to receive the second alternating voltage generated by the inverter unit and to drive the synchronous generator so as to make it generate said third alternating voltage on the AC bus; and
Hentunen Erkki
Multanen Tero
ABB Industry OY
Ballato Josie
DeBeradinis Robert L
Dykema Gossett PLLC
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