The instant application relates to islanded or weakly-connected DC or mixed DC-AC power systems, and more particularly to electric power generation and distribution systems for islanded or weakly-connected DC or mixed DC-AC power systems.
Conventional electric power generation and distribution systems for islanded or weakly-connected DC or mixed DC-AC power systems such as shipboard and off-shore power systems typically use single-voltage generation systems having synchronous or induction generators driven by prime movers. The windings of each generator are electrically connected to one another to form a single voltage output for each generation system. The generator winding connections are typically realized by transformers, DC/DC converters or AC/DC converters to form the single voltage output. Such systems have rigid prime mover speed requirements and limited voltage flexibility, high cost and lower efficiency.
According to an embodiment of a dual-voltage power generation system, the power generation system comprises a prime mover configured for adjustable speed operation and a doubly-fed induction generator driven by the prime mover and comprising a multi-phase stator winding and a multi-phase rotor winding. A first output terminal of the dual-voltage power generation system is electrically connected to the multi-phase stator winding, and a second output terminal is electrically connected to the multi-phase rotor winding. The dual-voltage power generation system further comprises a first converter having an AC side connected to one of the multi-phase windings and an AC or DC side connected to one of the output terminals. The multi-phase stator winding has a different turns ratio than the multi-phase rotor winding and the first output terminal is electrically isolated from the second output terminal so that the generator has two isolated power supply outputs at different voltage levels in a first configuration.
According to an embodiment of a method of configuring a dual-voltage power generation system for operation, the method comprises: configuring a prime mover for driving a doubly-fed induction generator at variable speed, the generator comprising a multi-phase stator winding and a multi-phase rotor winding having different turns ratios; electrically connecting a first output terminal of the dual-voltage power generation system to the multi-phase stator winding; electrically connecting a second output terminal of the dual-voltage power generation system to the multi-phase rotor winding; connecting an AC side of a first converter to one of the multi-phase windings and an AC or DC side of the first converter to one of the output terminals; and electrically isolating the first output terminal from the second output terminal so that the dual-voltage power generation system has two isolated power supply outputs at different voltage levels in a first configuration.
According to an embodiment of a power generation and distribution system, the system comprises a higher-voltage DC bus for supplying power to large drive-fed motors, a lower-voltage DC bus for supplying power to small drive-fed motors and a first plurality of dual-voltage power generation systems. Each of the dual-voltage power generation systems comprises a prime mover configured for adjustable speed operation, a doubly-fed induction generator driven by the prime mover and comprising a multi-phase stator winding and a multi-phase rotor winding having different turns ratios, a first DC output terminal electrically connected to the higher-voltage DC bus, and a second DC output terminal electrically connected to the lower-voltage DC bus and electrically isolated from the first DC output terminal. Each of the dual-voltage power generation systems further comprises a first converter having an AC side connected to the multi-phase stator winding and a DC side connected to the first DC output terminal and a second converter having an AC side connected to the multi-phase rotor winding and a DC side connected to the second DC output terminal.
According to another embodiment of a power generation and distribution system, the system comprises a higher-voltage DC bus for supplying power to drive-fed motors, a lower-voltage AC bus for supplying power to at least one of direct-on-line AC motors and auxiliary AC loads and a plurality of dual-voltage power generation systems. Each of the dual-voltage power generation systems comprises a prime mover configured for adjustable speed operation, a doubly-fed induction generator driven by the prime mover and comprising a multi-phase stator winding and a multi-phase rotor winding having different turns ratios, a DC output terminal electrically connected to the higher-voltage DC bus, an AC output terminal directly connected to the multi-phase rotor winding and electrically connected to the lower-voltage AC bus, the AC output terminal being electrically isolated from the DC output terminal, and a converter having an AC side connected to the multi-phase stator winding and a DC side connected to the DC output terminal.
Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.
The components in the figures are not necessarily to scale, instead emphasis being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts. In the drawings:
According to the embodiments described herein, electric power generation and distribution are provided for islanded or weakly-connected DC or mixed DC-AC power systems such as shipboard and off-shore power systems. The electric power generation and distribution systems include a combination of double-fed induction generators (DFIGs) and power electronic converters configured to output at least two isolated voltage levels without using transformers, DC/DC converters or AC/DC converters to electrically connect the windings of each DFIG. For example, a typical configuration can include a medium voltage output and a low voltage output. The output voltages may be all in DC or a mix of DC and AC. The overall generation and distribution system has reduced weight, volume and capital cost compared to conventional systems.
The dual-voltage power generation system 100 further comprises at least a first converter 114 having an AC side 116 connected to one of the multi-phase windings 106, 108 and an AC or DC side 118 connected to one of the output terminals 110, 112. The first converter 114 can be any standard converter such as an AC/DC converter or an AC/DC/AC converter. While the first converter 114 is shown as an AC(˜)/DC(=) converter in
According to the embodiment of
Owing to the different turns ratio between the multi-phase stator and rotor windings 106, 108 of the DFIG 104, the first output terminal 110 of the dual-voltage power generation system 100 i.e. the terminal connected to the DC side 118 of the first AC/DC converter 114 is at a higher DC voltage level (e.g. a relatively medium voltage or MVDC in
In some embodiments, at least one of the AC/DC converters 114, 120 is a self-commutated AC/DC converter i.e. both turn-on and turn-off of the converter can be controlled. Each self-commutated AC/DC converter can control the frequency of voltage and current at the AC side 116, 122 of the self-commutated AC/DC converter. The dual-voltage power generation system 100 can also include an optional crowbar circuit 126 connected to the multi-phase winding 106, 108 at the AC side 116, 122 of the first and/or second converter 114, 120. Each crowbar circuit 126 is operable to bypass the converter 114, 120 to which it is connected and short-circuit the corresponding multi-phase winding 106, 108 of the DFIG 104 at the AC side 116, 122 of that converter 114, 120. The construction and operation of crowbar circuits is well known in the electric power generation and distribution arts, and therefore no further explanation is given in this regard.
Operation of the dual-voltage power generation system 100 of
The dual-voltage power generation system 100 is set in a first configuration when ωm>ωs and ωm=ωs+ωr, where ωm is the equivalent electrical frequency of rotation of the prime mover 102, ωs is the electrical frequency of the multi-phase stator winding 106 and ωr is the electrical frequency of the multi-phase rotor winding 108. In the first configuration (the diagram labeled “Normal Generation” in
P
MVDC=(ωs/ωm)Pem−LOSSMV (1)
where LOSSMV is power loss along the MVDC path. Power generation into the LVDC bus is the remaining fraction of the electromechanical power as given by:
P
LVDC=(ωr/ωm)Pem−LOSSLV (2)
where LOSSLV is power loss along the LVDC path. Power generation to either the LVDC or MVDC bus may be independently reduced to zero. The shaft speed of the prime mover 102 is variable, which allows optimal efficiency of the prime mover 102.
The dual-voltage power generation system 100 is set in a second configuration when ωs>ωm and ωs=ωm+ωr. To enable the second configuration (the diagram labeled “LVDC Back Feeding Generation” in
The dual-voltage power generation system 100 is set in a third configuration when ωs<ωm and ωr=ωm+ωs. To enable the third configuration (the diagram labeled “MVDC Back Feeding Generation” in
In case of stator (or rotor) side AC/DC converter failure, the dual-voltage power generation system 100 is set in a fourth configuration. In the fourth configuration, the stator (or rotor) side crowbar circuit 126 bypasses the faulty converter 114/120 and short-circuits the stator (or rotor) terminals. The generator 100 continues to operate in induction mode and generates power into the rotor (or stator) side circuit.
The mixed DC-AC power generation system 300 can output a variable or fixed AC frequency depending on the system design. For variable AC frequency operation, the AC output (LVAC) of the mixed DC-AC power generation system 300 has a variable frequency. The prime mover 102 controls the shaft frequency and the AC/DC converter 114 controls its AC-side electrical frequency. The prime mover 102 is in variable-speed operation to achieve optimal efficiency. Power sharing between the DC and AC outputs 110, 112 is independent from the shaft speed. Depending on the relationship between the stator, rotor, and shaft electrical frequencies, the power flow scenarios between the DC and AC outputs 110, 1112 is the same as those illustrated in
For fixed AC frequency operation, the AC output 112 of the mixed DC-AC power generation system 300 has a fixed frequency. The prime mover 102 controls the shaft frequency and the AC/DC converter 114 controls its AC-side electrical frequency. Power sharing between the DC and AC outputs 110, 112 is dependent on the shaft speed. All four power configurations illustrated in
In each dual-voltage power generation system 100/200 of the first group, power sharing between the MV and LV DC outputs 110, 112 is independent from the shaft speed. Power flow into the MV or LV DC buses is reversible. Distributed energy resources (DERs) 504, including energy storage and fuel cells, can be connected to the LVDC bus, MVDC bus, or both. DERs 504 connected to either bus can be used to compensate for load consumption at both buses. The AC loads may be supplied from the LVDC bus or from the MVDC bus (not shown) through DC/AC converters 506. Optional grid (AC or DC grid) connections 508 can exist for some amount of energy exchange depending on specific applications. An AC grid connection can be connected to the MVDC bus or to the LVDC bus (not shown). Switches 510 with protection functions are connected between converters and DC buses, and between multiple DC busses.
The power generation and distribution system 500 can also include a second group of the dual-voltage power generation systems 300/400 previously described herein, configured to have a DC output terminal 110/112 electrically connected to the higher-voltage DC bus and an AC output terminal 112/110 directly connected to the multi-phase stator winding 108 of the corresponding DFIG 104. The dual-voltage power generation systems 300/400 in the second group each have a single AC/DC converter 114/120. The AC side (˜) of the converter 114/120 is connected to the multi-phase stator or rotator winding 108, 108 of the corresponding DFIG 104, and the DC side (=) of the converter 114/120 is connected to the corresponding DC output terminal 110/112. The AC output terminal 112/110 of the dual-voltage power generation systems 300/400 in the second group are also electrically connected to a lower-voltage AC bus (LVAC1, LVAC2). The lower-voltage AC buses supply power to at least one of direct-on-line AC motors and auxiliary AC loads 512.
Each mixed DC-AC dual-voltage power generation system 300/400 has one converter 114/120 for electrically connecting the multi-phase stator or rotor winding 106, 108 of the corresponding DFIG 104 to one of the MVDC buses (MVDC1, MVDC2) via the DC output terminal 110/112 of the respective mixed DC-AC dual-voltage power generation system 300/400. The AC output terminal 112/110 of each mixed DC-AC dual-voltage power generation system 300/400 is directly connected to the other multi-phase winding 106, 108 of the DFIG 104 and electrically connected to a lower-voltage AC bus (LVAC1, LVAC2). The medium voltage DC buses connect to the DC outputs 110/112 of the mixed DC-AC dual-voltage power generation systems 300/400 and the DC outputs of the single-voltage power generation systems 602, and supply energy to large drive-fed motor loads 502. The low voltage AC buses (LVAC1, LVAC2) connect to the AC outputs 112/110 of the mixed DC-AC dual-voltage power generation systems 300/400, and supply energy to direct-on-line AC motors and/or auxiliary AC loads 512. DERs 504, including energy storage and fuel cells, can be connected to the MVDC bus and/or LVAC bus and an optional grid 508 can connect to the MVDC bus or LVAC bus (not shown) as previously described herein in connection with
Terms such as “first”, “second”, and the like, are used to describe various elements, regions, sections, etc. and are not intended to be limiting. Like terms refer to like elements throughout the description.
As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.
With the above range of variations and applications in mind, it should be understood that the present invention is not limited by the foregoing description, nor is it limited by the accompanying drawings. Instead, the present invention is limited only by the following claims and their legal equivalents.