Method of starting a wind park

1. A method of starting a wind park comprising a plurality of wind turbines connectable in a collector system connectable to a utility grid, the method comprising:

starting at least one first wind turbine, each being equipped with a utility grid independent energy supply and a grid forming function, to produce electrical energy from wind energy, utilizing the respective grid independent energy supply for starting;

performing the grid forming function by the at least one first wind turbine to achieve a reference voltage in the collector system;

starting at least one second wind turbine and/or at least one third wind turbine to produce energy by conversion of wind energy, utilizing energy provided in the collector system for starting,

wherein performing the grid forming function by the first wind turbine includes:

ramping up a converter output voltage of a converter of the first wind turbine in a controlled fashion until a current limit and/or power limit of the converter is reached,

wherein ramping up the converter output voltage of the converter of the first wind turbine is performed such as to balance the system load by:

monitoring an actual frequency of the collector system voltage;

deriving a limited reference voltage based on a frequency difference between the actual frequency and a nominal frequency of the collector system voltage; and

controlling the converter of the first wind turbine based on the limited reference voltage.

2. The method according to claim 1 , wherein the at least one second wind turbine is equipped with a grid forming function, the method further comprising after starting the at least one second wind turbine:

performing the grid forming function by the second wind turbine to achieve the reference voltage in the collector system.

3. The method according to claim 1 , further comprising:

connecting the first wind turbine to the collector system after having started the first wind turbine;

connecting the second wind turbine to the collector system before starting the second wind turbine;

connecting the third wind turbine to the collector system before starting the third wind turbine;

automatically synchronizing the electrical output of the second wind turbine and/or the third wind turbine with that of the first wind turbine, wherein each of the third wind turbine performs a current control at a converter output terminal of the third wind turbine.

4. The method according to claim 1 , wherein at least one of the second wind turbine and the third wind turbine is started, if a collector system voltage is between predefined voltage limits.

5. The method according to claim 1 , wherein deriving the limited reference voltage comprises:

deriving a maximum, a minimum and an offset of the reference voltage based on a measured collector system voltage and the frequency difference;

deriving the limited reference voltage based on the maximum, the minimum and the offset of the reference voltage and the reference voltage.

6. The method according to claim 1 ,

wherein at least one of the reference voltage and the limited reference voltage is lower than a nominal voltage of the collector system until the power produced by conversion of wind of collector system connected wind turbines essentially matches at least one of active and reactive power demand of the collector system and

wherein at least one of the reference voltage and the limited reference voltage is greater than an actual voltage of the collector system.

7. The method according to claim 1 , wherein performing the grid forming function by the first wind turbine supports active and/or reactive power requirements of the collector system including the wind turbines connected thereto.

8. The method according to claim 1 , wherein a power reference of each of the at least one first and the at least one second wind turbine is derived, based on a preliminary power reference and a power reference offset derived based on the frequency difference, and based on a set fraction of maximum available power of the respective first or second wind turbine.

9. The method according to claim 1 , wherein a wind turbine power reference of each of the first and second wind turbines is limited by a power limit derived from a park master maximum power reference and a power offset derived, in dependence of the frequency difference.

10. The method according to claim 1 , further comprising:

starting at least one other first wind turbine to produce electrical energy from wind energy before starting any second wind turbine, utilizing the respective grid independent energy supply for starting.

11. The method according to claim 1 , further comprising:

running synchronisation control regarding adjusting magnitude and phase of the collector system voltage to respective values of the utility grid; and

connecting the collector system to the utility grid, if a magnitude and/or a phase of the collector system voltage essentially match those of the utility grid in a predefined margin.

12. The method according to claim 1 , wherein the respective grid independent energy supply of each first wind turbine is disconnected from the collector system and connectable to the collector system,

wherein at least one of the grid independent energy supply of any of the at least one first wind turbine comprises at least one of:

a Diesel and/or hydrogen powered generator;

a solar cell system;

an electric energy storage, in particular battery, accumulator, capacitor bank.

13. A wind park comprising a plurality of wind turbines connectable in a collector system connectable to a utility grid, the wind park being adapted to perform the method according to claim 1 .