Operating a wind turbine being connected to a utility grid both via a HVDC power connection and via an umbilical AC cable with a network bridge controller performing a power and a voltage control

1. A method for controlling the operation of a wind turbine,

wherein the wind turbine includes (i) a mechanical drive train, (ii) an electric generator being mechanically connected to the drive train, and (iii) a power converter being electrically connected to the generator,

wherein the wind turbine forms a part of a wind park which is connectable to a utility grid (i) via a HVDC power transmission system and/or (ii) via an AC auxiliary power transmission system,

and wherein the wind park is in a third operational mode in which the wind park is connected to the utility grid both via the HVDC power transmission system and via the AC auxiliary power transmission system, the method comprising

receiving, by a power controller,

an active power reference signal being indicative for the active power which is supposed to be generated by the wind turbine and

an active power feedback signal being indicative for the active power which is actually generated by the wind turbine;

determining, by the power controller and based on the active power reference signal and on the active power feedback signal, a first voltage control signal and a power controller frequency signal;

receiving, by an umbilical power controller

a power reference signal being indicative for a desired power flow through the AC auxiliary power transmission system and

a power feedback signal being indicative for the actual power flowing via the AC auxiliary power transmission system;

determining, by the umbilical power controller and based on the power reference signal and on the power feedback signal, a power offset frequency signal being indicative for an actual power offset within the AC auxiliary power transmission system;

receiving, by a HVDC power controller, the power controller frequency signal;

determining, by the HVDC power controller and based on the power controller frequency signal, a second voltage control signal;

receiving, by a theta integrator unit, the power offset frequency signal;

determining, by the theta integrator unit and based on the power offset frequency signal, an actual angle signal being indicative for an actual angle between a rotating dq reference frame and a stationary abc reference frame; and

controlling the operation of a network bridge of the power converter based on the first voltage control signal, the second voltage control signal, and the actual angle signal.

2. The method as set forth in claim 1 , further comprising

determining, by the umbilical power controller and based on the power reference signal and on the power feedback signal, an umbilical power angle signal;

wherein the actual angle signal is given by the combination of (a) the direct output of the theta integrator unit and (b) the umbilical power angle signal.

3. The method as set forth in claim 1 , further comprising

receiving, by the theta integrator unit, the power controller frequency signal;

wherein the actual angle signal is determined further based on the power controller frequency signal.

4. The method as set forth claim 3 , wherein

the actual angle signal further depends on a nominal frequency signal being indicative for the nominal frequency of an AC system which collects AC power generated by the wind turbine and by other wind turbines of the wind park.

5. The method as set forth in claim 1 , further comprising

receiving, by a power feedforward unit, the active power reference signal; and

determining, by the power feedforward unit and based on the active power reference signal, a power feedforward voltage signal;

wherein the first voltage control signal is further based on the determined power feedforward voltage signal.

6. The method as set forth in claim 5 , wherein the first voltage control signal is given by the sum of (i) the power feedforward voltage signal and

(ii) a power controller voltage signal; wherein the power controller voltage signal is determined

by the power controller in response

to the active power reference signal and

to the active power feedback signal.

7. The method as set forth in claim 1 , further comprising

receiving a signal being indicative for the nominal voltage the AC system which collects AC power generated by the wind turbine and by other wind turbines of the wind park;

wherein the second voltage control signal further depends on the signal.

8. A network bridge controller for controlling the operation of a network bridge of a power converter of a wind turbine, wherein the network bridge controller is capable of adopting a configuration having in an activated state

a power controller;

an HVDC power controller; and

a theta integrator unit;

wherein the network bridge controller is configured for performing an operational control of the wind turbine by carrying out the method as set forth in claim 1 .

9. A wind turbine comprising

a mechanical drive train having a wind rotor with at least two rotor blades;

an electric generator being mechanically connected to the drive train;

a power converter being electrically connected to the generator, wherein the power converter comprises

(i) an AC-DC generator bridge for rectifying an AC power input provided by the generator,

(ii) a direct current link receiving the rectified an AC power input, and

(iii) a DC-AC network bridge which converts a DC power of the DC link into an AC power output; and

a network bridge controller as set forth in claim 8 .

10. A power generating and forwarding system comprising

a wind park comprising a plurality of wind turbines;

an AC system for collecting the AC power being generated by the plurality of wind turbines

an HVDC power transmission system being connectable to the AC system and/or to a utility grid for transferring DC power from the AC system to the utility grid, wherein the HVDC power transmission system comprises a HVDC diode rectifier, a HVDC power transmission cable, and a DC-AC converter; and

an AC auxiliary power transmission system being connectable to the AC system and/or to the utility grid for transferring AC power between the utility grid and the AC system and in particular from the utility grid to the AC system;

wherein at least some of the wind turbines are a wind turbine as set forth in claim 9 .

11. A computer program for controlling the operation of a wind turbine, the computer program, when being executed by a data processor and in particular by a data processor of a wind turbine controller and/or of a network bridge controller, is adapted for controlling and/or for carrying out the method as set forth in claim 1 .