Static exciter of a field winding and method for operating the same

1. A static exciter of a field winding of an electric generator comprising:

a rectifier connected to an electric grid and to the field winding;

a unidirectional electronic switch connected in series with the rectifier;

a capacitor bank;

a further switch disposed electrically between the unidirectional electronic switch and the capacitor bank;

a control unit connected to the further switch and configured to close the further switch when a voltage indicative of a grid voltage drops below a first prefixed voltage value so as to connect the capacitor bank to the field winding so as to supply energy to the field winding from the capacitor bank and configured to interrupt current flowing between the capacitor bank and the field winding by opening the further switch when the voltage indicative of the grid voltage exceeds a second prefixed voltage value so as to disconnect the capacitor bank from the field winding, such that the capacitor bank supplies energy to the field winding only for the time needed,

wherein the control unit is configured to open or control the further switch with a prefixed delay, and

wherein the control unit includes at least one voltage drop band associated with a prefixed closing delay time for the further switch, wherein the closing delay time is zero for a voltage lower than the first prefixed voltage value, and wherein the control unit includes at least one voltage rise band associated with a prefixed opening delay time for the further switch, wherein the opening delay time is zero for a voltage greater than the second prefixed voltage value.

2. The static exciter as recited in claim 1 , wherein the unidirectional electronic switch includes a forward biased diode.

3. The static exciter as recited in claim 1 , wherein the second prefixed voltage value is greater than or equal to the first prefixed voltage value.

4. The static exciter as recited in claim 2 , wherein the diode is connected in parallel with the capacitor bank.

5. The static exciter as recited in claim 1 , further comprising a voltage transformer having a low voltage side and configured to feed an automatic voltage regulator of the field winding, wherein a grid voltage drop is measurable at the low voltage side.

6. The static exciter as recited in claim 1 , wherein the rectifier includes a thyristor bridge.

7. The static exciter as recited in claim 1 , wherein the control unit is configured to open the further switch when a voltage of the capacitor bank drops below a third prefixed voltage value.

8. A method for operating a static exciter of a field winding of an electric generator, wherein the static exciter includes a rectifier connected to an electric grid and to the field winding, a unidirectional electronic switch connected in series with the rectifier, a capacitor bank, a further switch disposed between the unidirectional electronic switch and the capacitor bank, and a control unit, the method comprising:

connecting the capacitor bank to the field winding by closing the further switch when a grid voltage drops below a first prefixed voltage value; and

interrupting current flowing from the capacitor bank to the field winding by opening the further switch when the grid voltage exceeds a prefixed voltage value such that the capacitor bank supplies energy to the field winding only for the time needed,

wherein the opening and the closing of the further switch is performed with a prefixed delay,

wherein the control unit includes a plurality of voltage drop bands each associated with a prefixed closing delay time for the further switch, wherein the closing delay time is zero for a voltage lower than the first prefixed voltage value, and wherein the control unit includes a plurality of voltage rise bands each associated with a prefixed opening delay time for the further switch, wherein the opening delay time is zero for a voltage greater than the second prefixed voltage value, and further comprising:

measuring an actual voltage indicative of the grid voltage to determine to which of the plurality of voltage drop bands the grid voltage corresponds when the grid voltage drops, wherein the closing the further switch is performed according to the prefixed closing delay time after the actual voltage enters the voltage drop band; and

measuring the actual voltage indicative of the grid voltage to determine to which of the plurality of voltage rise bands the grid voltage corresponds when the grid voltage rises, wherein the opening the further switch is performed according to the prefixed opening delay time after the actual voltage enters the voltage rise band.

9. The method as recited in claim 8 , wherein the unidirectional electronic switch includes a forward biased diode.

10. The method as recited in claim 8 , wherein the second prefixed voltage value is greater than or equal to the first prefixed voltage value.

11. The method as recited in claim 8 , further comprising opening the further switch when a voltage of the capacitor bank drops below a third prefixed voltage value.