Patent Description:
In many cases, an electric power system comprises a direct voltage rail, one or more battery elements for supplying electric energy to the direct voltage rail, and one or more load-converters for converting the direct voltage of the direct voltage rail into voltages suitable for one or more loads of the electric power system. The electric power system can be for example an electric power system of a ship in which case the loads of the electric power system may comprise one or more propulsion motors, an alternating voltage network of the ship, and other loads such as e.g. one or more bow thruster motors. The motors are advantageously alternating current "AC" motors and the corresponding load-converters are inverters for converting the direct voltage of the direct voltage rail into alternating voltages suitable for the AC-motors.

In many cases it is advantageous that the direct voltage of the direct voltage rail is higher than the direct voltages of the battery elements. In these cases, each of the battery elements is typically connected with a voltage-increasing supply-converter to the direct voltage rail. The supply-converter comprises typically an inductor coil whose first pole is connected to the respective battery element, a controllable switch between the ground and the second pole of the inductor coil, and an unidirectionally conductive component, e.g. a diode, for providing a path for electric current from the inductor coil towards the direct voltage rail in response to a situation in which the controllable switch is in a non-conductive state.

In an electric power system of the kind described above, there is typically a need for a selective protection so that, in a case of a fault, a portion of the electric power system which is functionally separated from the rest of the electric power system is as small as possible. For example, in a case of fault in one of the load-converters, only the faulty load-converter is functionally separated from the rest of the electric power system. In order to implement the selective protection, each of the above-mentioned supply-converters is typically connected via a fuse or another over-current protector to the direct voltage rail. Correspondingly, each of the above-mentioned load-converters is connected via a fuse or another over-current protector to the direct voltage rail.

An inherent challenge related to the above-described approach is that, in many fault situations, the supply-converters of the kind described above may be incapable of supplying sufficient fault current within a sufficiently short time after the beginning of a fault situation. Therefore, the fault current through a fuse or another over-current protector may be insufficient to burn the fuse or to switch the other over-current protector into a non-conductive state sufficiently fast. Therefore, there is a risk that a faulty portion of the electric power system is not correctly separated from the rest of the electric power system. It is naturally possible to provide the supply-converters with additional means for supplying sufficient fault current from the battery elements within a sufficiently short time but this would make the supply-converters significantly more complex and less cost effective.

Publication <CIT> describes a method for protecting a direct-current "DC" electric power distribution system that includes one or more alternating current/direct current "AC/DC" converters and/or one more DC/DC converters, and one or more loads, connected by DC-buses. The method, which is carried out in response to a detection of a fault somewhere in the system, begins with limiting an output current of each of one or more of the converters so that each of these converters outputs a limited DC-current. After the current limitation of the one or more converters has taken place, one or more protection devices in the system are activated, where the activating at least partly depends on the limited DC-currents. The activation may comprise an automatic opening of one or more protection devices, wherein the opening of each protection device is based on a respective device current exceeding a respective threshold for a respective period of time. In this method, a correct operation of the protection devices is achieved so that the limited DC-currents are controlled so that the activation of the protection devices is successful. This approach, however, complicates the control of the converters.

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. An electric power system according to the invention comprises:.

The capacitor system may comprise for example one or more electric double layer capacitors "EDLC" which may be also called "super capacitors". The electric energy stored by the capacitor system mitigates a voltage drop in the direct voltage rail during a fault situation, and the fault current available from the capacitor system enables a selective protection. The capacitor system can be either a centralized capacitor system which is connected to one point of the direct voltage rail or the capacitor system can be a distributed capacitor system comprising many capacitor elements connected to many points of the direct voltage rail. Each over-current protector can be for example a fuse or a relay switch responsive to current exceeding a pre-determined limit.

It is to be noted that an electric power system according to an exemplifying and non-limiting embodiment of the invention may comprise more than one direct voltage rail and more than one capacitor system connected to the direct voltage rails so that the direct voltage rails are interconnected with one or more over-current protectors and each direct voltage rail is connected to one of the capacitor systems.

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

A number of exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims.

Exemplifying and non-limiting embodiments of the invention and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which:.

<FIG> shows a schematic illustration of an electric power system according to an embodiment of the invention. In this exemplifying case, the electric power system is an electric power system of a ship. The electric power system comprises a direct voltage rail <NUM>, battery elements, and supply-converters for transferring electric energy from the battery elements to the direct voltage rail. In <FIG>, three of the battery elements are denoted with figure references <NUM>, <NUM>, and <NUM>, and three of the supply-converters are denoted with figure references <NUM>, <NUM>, and <NUM>. The electric power system comprises load-converters for converting the direct voltage of the direct voltage rail <NUM> into voltages suitable for loads of the electric power system. In <FIG>, three of the load-converters are denoted with figure references <NUM>, <NUM>, and <NUM>. In the exemplifying case illustrated in <FIG>, the loads of the electric power system comprise a propulsion system, bow thruster motors, and an alternating voltage network of the ship. In <FIG>, one of the electric motors of the propulsion system is denoted with a figure reference <NUM>, one of the bow thruster motors is denoted with a figure reference <NUM>, and a part of the alternating voltage network of the ship is denoted with a figure reference <NUM>. In the exemplifying case illustrated in <FIG>, the load-converters comprise inverters for converting the direct voltage of the direct voltage rail into alternating voltages suitable for the loads of the electric power system.

The electric power system comprises over-current protectors connected between the direct voltage rail <NUM> and each of the one or more supply-converters and other over-current protectors connected between the direct voltage rail <NUM> and each of the one or more load-converters. In <FIG>, three of the over-current protectors connected between the direct voltage rail <NUM> and the supply-converters are denoted with figure references <NUM>, <NUM>, and <NUM>, and three of the over-current protectors connected between the direct voltage rail <NUM> and the load-converters are denoted with figure references <NUM>, <NUM>, and <NUM>. Each over-current protector can be for example a fuse or a relay switch responsive to current exceeding a pre-determined limit.

The electric power system further comprises a capacitor system <NUM> connected to the direct voltage rail <NUM>. The capacitor system <NUM> may comprise for example one or more high-capacitance electric double layer capacitors "EDLC" which are also called "super capacitors". The capacitance of the capacitor system <NUM> is advantageously at least <NUM> F, more advantageously at least <NUM> F, and yet more advantageously at least <NUM> F. The capacitor system <NUM> is capable of supplying fault current that is sufficient to switch an appropriate one of the above-mentioned over-current protectors into a non-conductive state in response to a fault causing a voltage drop at an electrical node connected to the direct voltage rail via the one of the over-current protectors. For example, if there is a fault in the load-converter <NUM> and/or in the electric motor <NUM> such that the voltage of an electric node <NUM> drops, the capacitor system <NUM> supplies fault current through the over-current protector <NUM> so that the over-current protector <NUM> is switched into the non-conductive state and, as a corollary, a part <NUM> of the electric power system is separated from the rest of the electric power system. In the exemplifying electric power system illustrated in <FIG>, the capacitor system <NUM> is connected to the direct voltage rail <NUM> via an over-current protector <NUM>. The current limit of the over-current protector <NUM> is sufficiently higher than the current limit of each of the over-current protectors <NUM>-<NUM> and <NUM>-<NUM> in order to achieve a selective protection.

The exemplifying electric power system illustrated in <FIG> comprises another direct voltage rail <NUM> so that the direct voltage rails <NUM> and <NUM> of the electric power system are connected to each other via over-current protectors <NUM> and <NUM>. The portions of the electric power system connected to different ones of the direct voltage rails are advantageously operable independently of each other in order to improve the operational reliability of the electric power system. The electric power system comprises another capacitor system <NUM> connected to the direct voltage rail <NUM> and capable of supplying fault current in the same way as the above-mentioned capacitor system <NUM>.

The exemplifying electric power system illustrated in <FIG> comprises charging converters <NUM> and <NUM> for receiving electric energy from an external power network <NUM> and for supplying the electric energy to the direct voltage rails <NUM> and <NUM> via over-current protectors <NUM> and <NUM>. The electric power system comprises means for charging the battery elements with the electric energy received from the power network <NUM>. The supply-converters can be provided with components and control systems for enabling the supply-converters to transfer electric energy from the direct voltage rails to the battery elements. It is, however, also possible that the electric power system comprises separate converters for charging the battery elements.

<FIG> illustrates the main circuit of the supply-converter <NUM> for transferring electric energy from the battery element <NUM> to the direct voltage rail <NUM>. The other supply-converters can be similar to the supply-converter <NUM>. The main circuit of the supply-converter <NUM> comprises an inductor coil <NUM> whose first pole is connected to the battery element <NUM>. The main circuit comprises a first controllable switch <NUM> between the ground and the second pole of the inductor coil <NUM>. The first controllable switch <NUM> can be for example an insulated gate bipolar transistor "IGBT", a gate turn-off thyristor "GTO", a bipolar transistor, or a field-effect transistor "FET". The main circuit of the supply-converter <NUM> comprises a first unidirectionally conductive component <NUM> for providing a path for electric current from the inductor coil <NUM> towards the direct voltage rail <NUM> in response to a situation in which the first controllable switch <NUM> is in a non-conductive state. The first unidirectionally conductive component <NUM> can be for example a diode. The supply-converter <NUM> is a voltage-increasing converter, i.e. a boost-converter, capable of transferring electric energy from the battery element <NUM> to the direct voltage rail <NUM> in a controlled way when the voltage VDC BAT of the battery element is smaller than the voltage VDC RAIL of the direct voltage rail <NUM>.

In the exemplifying case illustrated in <FIG>, the supply-converter <NUM> further comprises components for enabling the supply-converter to transfer electric energy from the direct voltage rail <NUM> to the battery element <NUM> so as to charge the battery element <NUM>. The main circuit of the supply-converter <NUM> comprises a second controllable switch <NUM> for conducting electric current arriving from the direct voltage rail <NUM> to the second pole of the inductor coil <NUM>. The second controllable switch <NUM> can be for example an insulated gate bipolar transistor "IGBT", a gate turn-off thyristor "GTO", a bipolar transistor, or a field-effect transistor "FET". The main circuit of the supply-converter <NUM> comprises a second unidirectionally conductive component <NUM> for providing a path for electric current from the ground to the second pole of the inductor coil <NUM> in response to a situation in which the second controllable switch <NUM> is in a non-conductive state. The second unidirectionally conductive component <NUM> can be for example a diode. With the aid of the inductor coil <NUM>, the second controllable switch <NUM>, and the second unidirectionally conductive component <NUM>, the supply-converter <NUM> is capable of charging the battery element <NUM> by transferring electric energy from the direct voltage rail <NUM> to the battery element <NUM> in a controlled way when the voltage VDC BAT of the battery element is smaller than the voltage VDC RAIL of the direct voltage rail <NUM>. In an exemplifying case where the first and second controllable switches <NUM> and <NUM> are IGBTs or metal oxide semiconductor field effect transistors "MOSFET", the first and second unidirectionally conductive components <NUM> and <NUM> can be body diodes of the IGBTs or the MOSFETs.

Each of the supply-converters and each of the load-converters of the electric power system may comprise a controller for controlling the operation of the converter under consideration. It is also possible that a single controller is configured to control many converters. A controller can be implemented with one or more processor circuits each of which can be a programmable processor circuit provided with appropriate software, a dedicated hardware processor such as for example an application specific integrated circuit "ASIC", or a configurable hardware processor such as for example a field programmable gate array "FPGA". Furthermore, each controller may comprise one or more memory circuits. The controllers are not shown in <FIG> and <FIG>.

Claim 1:
An electric power system comprising:
- a direct voltage rail (<NUM>),
- one or more battery elements (<NUM>-<NUM>),
- one or more supply-converters (<NUM>-<NUM>) for transferring electric energy from the one or more battery elements to the direct voltage rail, each supply-converter being connected to one of the one or more battery elements,
- one or more load-converters (<NUM>-<NUM>) for converting direct voltage of the direct voltage rail into voltages suitable for one or more loads of the electric power system, and
- first over-current protectors (<NUM>-<NUM>, <NUM>-<NUM>) connected between the direct voltage rail and each of the one or more supply-converters and between the direct voltage rail and each of the one or more load-converters,
characterized in that the electric power system further comprises a capacitor system (<NUM>) connected to the direct voltage rail via a second over-current protector (<NUM>) and comprising one or more capacitors, the capacitor system being capable of supplying fault current for switching one of the first over-current protectors into a non-conductive state in response to a fault causing a voltage drop at an electrical node connected to the direct voltage rail via the one of the first over-current protectors, wherein a current limit of the second over-current protector is higher than a current limit of each of the first over-current protectors.