Patent Application
20160009354
Embodiments of the invention relate generally to a direct current (DC) power system and more specifically to a DC power system for marine applications.
In the last few decades, marine traffic has increased substantially across the world due to tremendous rise in cargo transport vessels, warships, offshore oil ships, passenger ships etc. These vessels or ships have many electrical loads on board. Variable speed electric drives for pumps, fans, electric propulsion installations, lighting and air conditioning are some examples of the electrical loads on board of a ship. The electrical loads operate at various different voltages and frequencies, and thus need separate electric power supplies.
Most of the ships use alternating current (AC) power system architecture. However, more recently some ships are using DC power system architecture including energy storage devices or electrical generators to meet the demands of the plurality of electrical loads. If a load needs alternating current (AC) power supply, the DC power may be converted into AC power with help of power electronic converters. Similarly, if the electrical generator is AC then the AC power may be converted into DC power via power electronic converters. Generally, a plurality of generators supply power to a plurality of DC buses which in turn supply power to a particular electrical load. There are examples where a single DC bus can also provide power to the entire ship electrical load.
Challenges with conventional DC power system include integration of multiple DC buses with different voltage levels, isolating the faults in the DC power system in a very short time and integrating the various energy storage devices and generators. For these and other reasons, there is a need for the present DC power system for marine applications.
In accordance with an embodiment of the present technique, a power system for a marine ship is provided. The power system includes a plurality of protection zones wherein each protection zone comprising a plurality of direct current (DC) buses. The DC buses which do not have same DC voltage are coupled to each other via at least one DC to DC converter and at least one energy source is coupled to at least one DC bus via a power electronic converter. Furthermore, at least two protection zones are coupled to each other via at least one bus-tie converter.
In accordance with another embodiment of the present technique, a method of providing power to a marine ship is provided. The method includes providing power to at least one DC bus in the marine ship via an energy source connected to a power electronic converter and connecting at least two protection zones of the marine ship via at least one bus-tie converter. The method also includes coupling DC buses which do not have same DC voltage in one protection zone with each other via at least one DC to DC converter.
Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The term “or” is meant to be inclusive and mean one, some, or all of the listed items. The use of “including,” “comprising” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “connected” and “coupled” are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings, whether direct or indirect. Furthermore, the terms “circuit” and “circuitry” and “controller” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together to provide the described function.
Turning now to the drawings, by way of example in
Furthermore, in protection zone 102, AC generators 108, 110 feed power to a DC bus 120 via power electronic converters 122 and 124 respectively, and energy storage device 112 feeds power to a DC bus 126 via a power electronic converter 128. Power electronic converters 122, 124 are AC to DC converters as they have to convert power from AC generators to the DC bus whereas power electronic converter 128 is a DC to DC converter as it couples a DC energy storages device to a DC bus. The two DC buses 120 and 126 do not have same DC voltage and hence are coupled to each other via a DC to DC converter 130. DC to DC converter 130 may be a bidirectional DC to DC converter or an unidirectional DC to DC converter. Further, loads 132 and 134 are connected to DC bus 120 via power electronic converters 136 and 138 respectively and loads 140 and 142 are connected to DC bus 122 via power electronic converters 144 and 146 respectively. Depending on whether the load is an AC load or a DC load, power electronic converter 136, 138, 144 and 146 may be AC to DC converters or DC to DC converters.
In similar manner, in protection zone 104, AC generators 114, 116 feed power to a DC bus 150 via power electronic converters 152 and 154 respectively, and energy storage device 118 feeds power to a DC bus 156 via a power electronic converter 158. The two DC buses 150 and 156 are coupled to each other via a DC to DC converter 160. DC Furthermore, loads 162 and 164 are connected to DC bus 150 via power electronic converters 166 and 168 respectively and loads 170 and 172 are connected to DC bus 152 via power electronic converters 174 and 176 respectively. Based on the type of load i.e., an AC load or a DC load, power electronic converter 166, 168, 174 and 176 may be AC to DC converters or DC to DC converters. It should be noted that in the embodiment shown, all converters have been shown to have only one input terminal and one output terminal for ease of explanation. However, a negative terminal or a reference terminal is always there at the input and the output of each of the converter.
Power System 100 further includes a controller 180 to control the operation of bus-tie converters 106, 188 and other converters in protection zones 102 and 104. It should be noted that even though a single controller 180 is shown in the embodiment of
For example, if there is a fault on DC bus 126 or the subsequent branches connected to it (e.g., branches 182, 184, 186 in
In one embodiment of the present technique, a second bus-tie converter 188 may be used to connect low voltage buses 126 and 156. In other words, protection zones 102 and 104 may be coupled to each other via two bus-tie converters 106 and 188. Bus-tie converter 106 couples medium voltage buses 120 and 150 and bus-tie converter 188 couples low voltage buses 126 and 156. It should be noted that low voltage buses 126 and 156 operate at a low voltage with respect to medium voltage buses 120 and 150 respectively. This embodiment allows low voltage buses 126 and 156 and their corresponding branches to operate normally even if one of the medium voltage buses 120 or 150 is faulty and unable to supply any power.
In another embodiment, the power flow between two zones, zone 1 and zone 2 may need to be only unidirectional i.e., either from zone 1 to zone 2 or from zone 2 to zone 1. In such an embodiment, first DC to DC converter 302 and second DC to DC converter 304 both can be unidirectional DC to DC converters. Examples of unidirectional DC to DC converters include half bridge IGBT/diode converters, i.e., asymmetrical H bridge with a diode and an IGBT in each leg, or diode bridge converters.
In operation a controller (e.g., controller 180 of
For example, if the controller finds a fault in zone 1 or its branches, the controller will stop the operation of first converter 302 and thus, stops the energy flow between zone 1 and zone 2. During this state, the controller 180 would enable second converter 304 to either charge energy storage device 306 or supply power to the loads. In other words, even though there is a fault in zone 1, zone 2 would continue to operate normally. The controller would further operate one or more converters in zone 1 in current control mode. If the fault is transient then once the fault is cleared, the controller 180 will resume the operation of first converter 302 and thus, the system will resume normal operation. However, if the fault is not cleared within a certain time i.e., if it is a permanent fault then converter 302 will be disabled and a mechanical switch between zone 1 and zone 2 will be open.
In an embodiment, where there are two bus-tie converters (e.g., 106, 188 of
In operation, if the power needs to be transferred from bus A to bus B then bus A converter 402 will act as a DC to AC converter and bus B converter 404 will act as an AC to DC converter. In this case, bus A converter 402 converts input DC voltage into medium/high frequency AC voltage and bus B converter 404 converts medium/high frequency AC voltage into DC voltage. Similarly, if the power needs to be transferred from bus B to bus A then bus B converter 404 will act as a DC to AC converter and bus A converter 402 will act as an AC to DC converter. Transformer 406 provides a coupling between AC connections of bus A converter 402 and AC connections of bus B converter 404. As discussed earlier if power is being supplied to bus B from bus A and if there is a fault on bus B then converter 400 will operate in current control mode. Furthermore, if power transfer needs to be unidirectional, i.e., only from bus A to bus B or vice versa then the full bridge IGBT converters 410, 412 may be replaced with full bridge diode converters.
Advantages of the present technique include high power density, high efficiency and excellent fault handling capability because of use of high or medium frequency DC to DC converters. Furthermore, since the energy storage elements are integrated in a bus-tie converter, the power system at different zones can share the same energy storage. Another advantage of the present technique is that the bus tie converter can disconnect the two zones extremely fast during a fault scenario.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
