Field of the disclosure

The disclosure relates to an electric power system that can be, for example but not necessarily, an electric power system of a ship or another marine vessel.

Background

In many cases, an electric power system comprises two or more direct voltage busses configured to supply electric energy to loads and required to maintain operability independently of each other. The electric power system can be for example an electric power system of a ship or another marine vessel in which case the loads of the electric power system may comprise for example one or more propulsion motors, an alternating voltage network of the marine vessel, and other loads such as e.g. one or more bow thruster motors. The motors are advantageously alternating current “AC” motors and corresponding load-converters are inverters for converting direct voltage of a direct voltage bus into alternating voltages suitable for the AC-motors.

Cost effective designs of electric power systems of the kind mentioned above often end up with high dependency on power flow via interconnected direct voltage busses for normal operation while failure operation is still possible with disconnected direct voltage busses. In a typical design, the direct voltage busses are interconnected with a bus-tie link during normal operation, wherein the bus-tie link is provided with overcurrent protection devices, such as fuses or overcurrent relays, for disconnecting the direct voltage busses from each other in a failure situation. In conjunction with electric power systems of the kind mentioned above, it can be however challenging to maintain zonal selectivity with overcurrent protection because e.g. in a case of a fault on the bus-tie link, fault current may flow to the fault from both of the direct voltage busses and thus each of overcurrent protection devices which connect the bus-tie link to the direct voltage busses carries only a part, e.g. about a half, of the fault current. Thus, in direct voltage electric systems where availability of short-circuit current is limited and/or provided by direct voltage capacitors of the system, a high power rated bus-tie link requires high amount of capacitance on the direct voltage busses in order to provide energy required to activate overcurrent protection devices, e.g. to blow fuses, located on bus-tie link.

Summary

The following presents a simplified summary in order to provide basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.

In accordance with the invention, there is provided a new electric power system that can be, for example but not necessarily, an electric power system of a ship or another marine vessel.

An electric power system according to the invention comprises first and second direct voltage buses and at least two bus-tie links each being connected between the first and second direct voltage buses, wherein:

- each of the bus-tie links comprises at least one capacitor between first and second poles of the bus-tie link, and

- at least the first pole of each of the bus-tie links is connected via overcurrent protection devices, e.g. fuses or overcurrent relays, to first poles of the first and second direct voltage buses, each of the overcurrent protection devices being configured to break current in response to an overcurrent situation in the overcurrent protection device.

The capacitors of the above-mentioned bus-tie links form a distributed capacitor storage that is capable of providing fault currents to the electric power system to operate overcurrent protection devices so that zonal selectivity is achieved. The above-mentioned first poles of the bus-tie links and the first poles of the first and second direct voltage buses can be for example positive poles of the bus-tie links and positive poles of the first and second direct voltage buses. It however is also possible that the first poles are negative poles.

In accordance with the invention, there is provided also a new marine vessel that comprises an electric power system according to the invention. The loads of the electric power system of the marine vessel may comprise for example one or more propulsion motors, an alternating voltage network of the marine vessel, and/or other loads such as e.g. one or more bow thruster motors. The motors are advantageously alternating current “AC” motors and corresponding load-converters are inverters for converting direct voltage into alternating voltages suitable for the AC-motors.

Exemplifying and non-limiting embodiments are described in accompanied dependent claims.

Various exemplifying and non-limiting embodiments both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and nonlimiting embodiments when read in connection with the accompanying drawings.

The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require existence of unrecited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does not exclude a plurality.

Brief description of the figures

Exemplifying and non-limiting embodiments and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which: figure 1 shows a schematic illustration of an electric power system according to an exemplifying and non-limiting embodiment, and figure 2 shows a schematic illustration of an electric power system according to an exemplifying and non-limiting embodiment. Description of the exemplifying embodiments

The specific examples provided in the description given below should not be construed as limiting the scope and/or the applicability of the appended claims. Lists and groups of examples provided in the description given below are not exhaustive unless otherwise explicitly stated.

Figure 1 shows a schematic illustration of an electric power system according to an exemplifying and non-limiting embodiment. In this exemplifying case, the electric power system is an electric power system of a marine vessel 132 that can be e.g. a ship. The electric power system comprises first and second direct voltage buses 101 and 102. In the exemplifying electric power system illustrated in figure 1 , a power supply arrangement connected to the first direct voltage bus 101 comprises a battery device 131 that is configured to supply electric energy to the first direct voltage bus 101 via a direct voltage-direct voltage “DC-DC” converter 132 and via an overcurrent protection device 133. It is however also possible that an electric power system according to an exemplifying and non-limiting embodiment comprises for example a generator and/or a fuel-cell in addition to or instead of the battery device. In the exemplifying case illustrated in figure 1 , the overcurrent protection device 133 is a fuse but it is also possible that the overcurrent protection device 133 is an overcurrent relay.

The first direct voltage bus 101 comprises power outlets each of which is configured to supply electric power to a load of the electric power system and is provided with an overcurrent protection device. In figure 1 , one of the power outlets is denoted with a reference 128 and the overcurrent protection device, e.g. a fuse or an overcurrent relay, of the power outlet 128 is denoted with a reference 130. The load supplied by the power outlet 128 is denoted with a reference 129. In this exemplifying case, the load 129 comprises an alternating current “AC” propulsion motor 135 and an inverter 134 for converting the direct voltage of the first direct voltage bus 101 into alternating voltages suitable for the propulsion motor 135. Furthermore, the first direct voltage bus 101 comprises another power outlet for supplying electric energy to a load that comprises at least a part of an alternating current “AC” network 136 of the marine vessel 132. In the exemplifying electric power system illustrated in figure 1 , a power supply arrangement and loads connected to the second direct voltage bus 102 are like the power supply arrangement and the loads connected to the first direct voltage bus 101. It is however also possible that the first and second direct voltage buses are connected to different power supply arrangements and/or to different loads.

The electric power system comprises bus-tie links each being connected between the first and second direct voltage buses 101 and 102. In the exemplifying case illustrated in figure 1 , the electric power system comprises three bus-tie links 103, 104, and 105. It is however also possible that an electric power system according to an exemplifying and non-limiting embodiment comprises only two bus-tie links or more than three bus-tie links between two direct voltage buses. Each of the bus-tie links comprises capacitors between first and second poles of the bus-tie link. The bus-tie link 103 comprises capacitors 106 and 107, the bus-tie link 104 comprises capacitors 108 and 109, and the bus-tie link 105 comprises capacitors 110 and 111. On each of the bus-tie links, the two capacitors are advantageously in a vicinity of the ends of the bus-tie link under consideration so that most of the physical length of the bus-tie link is between the two capacitors. Each capacitor may comprise for example one or more electric double layer capacitor “EDLC” elements.

The first pole of each of the bus-tie links 103-105 is connected via overcurrent protection devices, e.g. fuses or overcurrent relays, to first poles 118 and 119 of the first and second direct voltage buses 101 and 102. The first poles can be e.g. positive voltage poles. Each of the overcurrent protection devices is configured to break current in response to an overcurrent situation in the overcurrent protection device under consideration. The bus-tie link 103 is connected via overcurrent protection devices 112 and 113 to the first poles 118 and 119 of the first and second direct voltage buses 101 and 102. Correspondingly, the bus-tie link 104 is connected via overcurrent protection devices 114 and 115 to the first poles of the first and second direct voltage buses and the bus-tie link 105 is connected via overcurrent protection devices 116 and 117 to the first poles of the first and second direct voltage buses. For the sake of illustration, we consider an exemplifying fault situation in which there is a fault, e.g. a short circuit or a line-to-earth short circuit, on one of the bus-tie links. Without limiting the generality, we can consider a fault situation in which there is a fault 137 on the bus-tie link 105. In this exemplifying case, current is supplied to the fault 137 by the capacitors 110 and 111. Furthermore, current is supplied to the fault 137 by a capacitor 126 of the first direct voltage bus 101 via the overcurrent protection device 116 and by a capacitor 127 of the second direct voltage bus 102 via the overcurrent protection device 117. Yet furthermore, current is supplied to the fault 137 by capacitors 106 and 107 of the bus-tie link 103 via the overcurrent protection devices 112, 113, 116, and 117. Yet furthermore, current is supplied to the fault 137 by capacitors 108 and 109 of the bus-tie link 104 via the overcurrent protection devices 114, 115, 116, and 117. Thus, the total currents of the overcurrent protection devices 116 and 117 comprise currents from many sources and therefore the overcurrent protection devices 116 and 117 break currents and thereby separate the bus-tie link 105 and the fault 137 from other parts of the electric power system prior to the overcurrent protection devices 112-115 have conducted so much currents for a so long time that overcurrent protection devices 112-115 would react.

Next, we consider an exemplifying fault situation in which there is a fault on one of the direct voltage buses. Without limiting the generality, we can consider a situation in which there is a fault 138 on the first direct voltage bus 101. In this exemplifying case, current is supplied to the fault 138 by the capacitor 126. Furthermore, current is supplied to the fault 138 by capacitors 106-111 via the overcurrent protection devices 112, 114, and 116. As a corollary, the overcurrent protection devices 112, 114, and 116 break currents and thereby the first direct voltage bus 101 is separated from the second direct voltage bus 102.

As illustrated by the above-presented exemplifying fault situations, the capacitors 106-111 of the bus-tie links 103-105 form a distributed capacitor storage that is capable of providing fault currents to the electric power system to operate overcurrent protection devices so that zonal selectivity is achieved. Figure 2 shows a schematic illustration of an electric power system according to an exemplifying and non-limiting embodiment. The electric power system comprises first and second direct voltage buses 201 and 202. The electric power system comprises three bus-tie links 203, 204, and 205 each being connected between the first and second direct voltage buses 201 and 202. Each of the bus-tie links comprises a capacitor between first and second poles of the bus-tie link. The bustie link 203 comprises a capacitor 206, the bus-tie link 204 comprises a capacitor 208, and the bus-tie link 205 comprises a capacitor 210. The first pole of each of the bus-tie links 203-205 is connected via overcurrent protection devices 212, 213, 214, 215, 216, and 217, e.g. fuses or overcurrent relays, to first poles 218 and 219 of the first and second direct voltage buses 201 and 202. Correspondingly, the second pole of each of the bus-tie links 203-205 is connected via overcurrent protection devices 220, 221 , 222, 223, 224, and 225 to second poles 226 and 227 of the first and second direct voltage buses 201 and 202. In the exemplifying electric power system illustrated in figure 2, the above-mentioned first poles are positive poles and the above-mentioned second poles are negative poles. In the exemplifying electric power system illustrated in figure 2, the bus-tie links comprise switch disconnectors 231 for disconnecting an electric connection implemented with the bus-tie links between the first and second direct voltage buses 201 and 202.

The specific examples provided in the description given above should not be construed as limiting the applicability and/or the interpretation of the appended claims. Lists and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.