Controlling an energy storage device

1 . A method of controlling an energy storage device arranged to provide frequency support for an AC power system, the method comprising:

setting an amount of energy stored in the energy storage device depending on a power system frequency of the AC power in the AC power system,

wherein the energy storage device comprises an energy storage capacitor,

wherein setting the amount of energy stored in the energy storage capacitor depending on the power system frequency comprises setting a voltage of the energy storage capacitor depending on the power system frequency,

wherein an energy supply device comprises an AC-DC converter connected to the AC power system and a DC-link,

wherein the energy supply device further comprises a DC-DC converter connected to the DC-link, the DC-DC converter being connected to the energy storage capacitor, and wherein the AC-DC converter and/or the DC-DC converter is controlled by a control signal, and wherein setting the amount of energy stored in the energy storage device comprises:

deriving a charging or discharging action depending on the power system frequency and a charging state indicating signal of the energy storage device, wherein deriving the charging or discharging action comprises:

deriving the control signal depending on the power system frequency and the charging state indicating signal; and

controlling the energy supply device, connected to exchange energy with the energy storage device, based on the control signal.

2 . The method according to claim 1 , wherein the energy storage capacitor comprises a supercapacitor and/or an electrolytic capacitor.

3 . The method according to claim 1 ,

wherein the control signal is for controlling the voltage at the energy storage capacitor.

4 . The method according to claim 1 ,

wherein the energy supply device comprises a tap transformer connected to the AC power system, the AC-DC converter being connected to the tap transformer, and

wherein the AC-DC converter is controlled by the control signal for controlling the voltage at the DC side and at the energy storage device.

5 . The method according to claim 1 ,

wherein the energy storage device comprises an energy storage flywheel,

wherein the AC-DC converter being connected to a flywheel driver for the flywheel, and

wherein the AC-DC converter and/or the flywheel driver is controlled by the control signal for controlling a rotational speed of the flywheel.

6 . The method according to claim 1 , wherein a frequency signal is obtained by measuring the power system frequency and low pass filtering the measured power system frequency.

7 . The method according to claim 1 ,

wherein the charging state or amount of energy of the energy storage device or voltage at the energy storage device is set such that the higher the power system frequency above a nominal power system frequency, the higher the charging state or amount of energy of the energy storage device or voltage at the energy storage device, and/or

wherein the charging state or amount of energy of the energy storage device or voltage at the energy storage device is set such that the lower the power system frequency below a nominal power system frequency, the lower the charging state or amount of energy of the energy storage device or voltage at the energy storage device.

8 . The method according to claim 1 ,

wherein the charging state or amount of energy of the energy storage device is set such that the closer the power system frequency to an upper power system frequency limit, the higher the charging state or amount of energy of the energy storage device or voltage at the energy storage device, and/or

wherein the charging state or amount of energy of the energy storage device is set such that the closer the power system frequency to a lower power system frequency limit, the lower the charging state or amount of energy of the energy storage device or voltage at the energy storage device.

9 . The method according to claim 1 ,

wherein the energy storage device and the energy supply device are arranged to supply active energy to the AC power system, if the power system frequency falls below the nominal power system frequency and if further the rate of change of the frequency is negative, and/or

wherein the energy storage device and the energy supply device are arranged to receive active energy from the AC power system, if the power system frequency exceeds above the nominal power system frequency and if further the rate of change of the frequency is zero or positive.

10 . An arrangement for controlling an energy storage device arranged to provide frequency support for an AC power system, the arrangement comprising:

a processing module adapted to set an amount of energy stored in the energy storage device depending on a power system frequency of the AC power in the AC power system,

wherein the energy storage device comprises an energy storage capacitor,

wherein setting the amount of energy stored in the energy storage capacitor depending on the power system frequency comprises setting a voltage of the energy storage capacitor depending on the power system frequency,

wherein an energy supply device comprises an AC-DC converter connected to the AC power system and a DC-link,

wherein the energy supply device further comprises a DC-DC converter connected to the DC-link, the DC-DC converter being connected to the energy storage capacitor, and wherein the AC-DC converter and/or the DC-DC converter is controlled by a control signal, and wherein the amount of energy stored in the energy storage device comprises:

a charging or discharging action derived from the power system frequency and a charging state indicating signal of the energy storage device,

wherein the charging or discharging action comprises:

the control signal derived from the power system frequency and the charging state indicating signal; and

wherein the energy supply device is controlled, connected to exchange energy with the energy storage device, based on the control signal.

11 . An energy storage system, comprising:

the energy storage device comprising the energy storage capacitor according to claim 10 .

12 . A wind turbine, comprising:

a rotation shaft to which plural rotor blades are connected;

a generator driven by the rotation shaft;

a generator converter connected to the generator and connectable to the AC power system; and

the energy storage system according to claim 11 .