Energy supporting device
An energy supporting device for a high voltage direct current, HVDC, transmission system is provided. The HVDC transmission system includes a first HVDC converter and a second HVDC converter connected to each other via an HVDC link. The energy supporting device comprises a plurality of cells configured to be connected in series to the HVDC link, a resistor electrically connected between the plurality of cells and an electrical reference potential, and a bypass switch configured to allow the resistor to be bypassed. A cell includes a full-bridge arrangement of power switches and an energy storage device electrically connected to the full-bridge arrangement. The energy supporting device is configured to be operated in: a charging mode, in which the bypass switch is in a closed state for allowing the resistor to be bypassed, and in which the energy storage device of at least one cell of the plurality of cells is receiving electrical energy from the HVDC link, and in a dissipation mode, in which the bypass switch is in an open state for allowing the resistor to dissipate electrical energy from the HVDC link, and in which the energy storage devices of the plurality of cells are bypassed, and in an energy release mode, in which the energy storage device of at least one cell of the plurality of cells is discharging electrical energy to the HVDC link.
1. An energy supporting device for providing grid forming and/or virtual synchronous machine, VSM, capabilities to a high voltage direct current, HVDC, transmission system including a first HVDC converter and a second HVDC converter connected to each other via an HVDC link, wherein the energy supporting device comprises:
a plurality of cells configured to be connected in series to the HVDC link, wherein a cell includes:
a full-bridge arrangement of power switches; and
an energy storage device electrically connected to the full-bridge arrangement;
a resistor electrically connected between the plurality of cells and an electrical reference potential; and
a bypass switch configured to allow the resistor to be bypassed;
wherein the energy supporting device is configured to be operated in:
a charging mode, in which the bypass switch is in a closed state for allowing the resistor to be bypassed, and in which the energy storage device of at least one cell of the plurality of cells is receiving electrical energy from the HVDC link;
a dissipation mode, in which the bypass switch is in an open state for allowing the resistor to dissipate electrical energy from the HVDC link, and in which the energy storage devices of the plurality of cells are bypassed; and
an energy release mode, in which the energy storage device of at least one cell of the plurality of cells is discharging electrical energy to the HVDC link to thereby provide the grid forming and/or VSM capabilities.
2. An energy supporting device according to claim 1 , wherein the energy supporting device is further configured to operate in:
an idle mode, in which the full-bridge arrangement of at least one of the plurality of cells is configured to block electrical energy from the HVDC link from passing through the energy supporting device.
3. An energy supporting device according to claim 1 , wherein the full-bridge arrangement of a cell of the plurality of cells is configured to:
in the charging mode, allow the energy storage device of the cell to receive electrical energy;
in the dissipation mode, allow the energy storage device of the cell to be bypassed; and
in the discharging mode, allow the energy storage device of the cell to discharge electrical energy.
4. An energy supporting device according to claim 1 , wherein a cell of the plurality of cells further comprises a DC/DC converter, wherein the energy storage device of the cell is connected to the full-bridge arrangement via the DC/DC converter.
5. An energy supporting device according to claim 1 ,
wherein the energy storage device of a cell of the plurality of cells comprises a super-capacitor and/or a battery.
6. An energy supporting device according to claim 1 , further comprising a control unit configured to control the power switches of the full-bridge arrangements of the plurality of cells and the bypass switch.
7. A high voltage direct current, HVDC, transmission system including a first HVDC converter and a second HVDC converter connected to each other via an HVDC link, and comprising:
at least one energy supporting device according to claim 1 , wherein the energy supporting device is connected to the HVDC link.
8. An HVDC transmission system according to claim 7 , configured as a symmetrical monopole HVDC transmission system or a bipolar HVDC transmission system.
9. An HVDC transmission system according to claim 8 , comprising at least two of the energy supporting device, wherein a first energy supporting devices is connected to a first transmission line of the HVDC link, and a second energy supporting devices is connected to a second transmission line of the HVDC link.
10. An HVDC transmission system according to claim 8 , further comprising:
a control system configured to control the power switches of the full-bridge arrangement of the plurality of cells and the bypass switch; and
at least one sensor configured to sense a current and/or a voltage of the HVDC link and communicatively coupled to the control system;
wherein the control system is further configured to cause the energy supporting device to assume one of the modes based on the received current and/or voltage of the HVDC link.
11. A method for providing grid forming and/or virtual synchronous machine, VSM, capabilities to a high voltage direct current, HVDC, transmission system including a first HVDC converter and a second HVDC converter connected to each other via an HVDC link, wherein the grid forming and/or virtual synchronous machine, VSM, capabilities is provided by means of an energy supporting device comprising:
a plurality of cells connected in series to the HVDC link, wherein a cell includes:
a full-bridge arrangement of power switches; and
an energy storage device electrically connected to the full-bridge arrangement;
a resistor electrically connected between the plurality of cells and an electrical reference potential; and
a bypass switch configured to allow the resistor to be bypassed;
the method comprising the steps of:
dissipating a surplus of electrical energy from the HVDC link by opening the bypass switch of the energy supporting device and transmitting the surplus of electrical energy to the resistor of the energy supporting device; and
releasing electrical energy to the HVDC link by discharging electrical energy from the energy storage device of at least one cell of the plurality of cells to thereby provide the grid forming and/or VSM capabilities.
12. A method according to claim 11 , further comprising the step of: absorbing electrical energy from the HVDC link by closing the bypass switch of the energy supporting device and transmitting the electrical into an energy storage device of at least one cell of the plurality of cells of the energy supporting device.
13. A method according to claim 12 , wherein the step of absorbing electrical energy further comprises:
transmitting the electrical energy into a number of electrical storage devices of the plurality cells, wherein the number is based on a sensed level of charge of the electrical storage devices.
14. A method according to claim 11 , wherein the step of dissipating a surplus of electrical energy further comprises:
dissipating the surplus of electrical energy from the HVDC link if
a level of the surplus electrical energy of the HVDC link is above a predetermined threshold.
15. A method according to claim 11 , comprising:
performing the step of dissipating a surplus of electrical energy in response to a first support request; and
performing the step of discharging a surplus of electrical energy in response to a second support request.