POWER FLOW CONTROLLER WITH A FRACTIONALLY RATED BACK-TO-BACK CONVERTER

• US 20140009980A1
• Filed: 11/09/2012
• Published: 01/09/2014
• Est. Priority Date: 11/11/2011
• Status: Active Grant

• Current Assignee: Georgia Tech Research Corporation (University System of Georgia)
• Original Assignee: Varentec Incorporated (KOCH Industries Incorporated)
• Inventors: Divan, Deepakraj M., Prasai, Anish, Kandula, Rajendra Prasad
• Granted Patent: US 9,281,756 B2
• Time in Patent Office: Days
• US Class Current: 363/37
• CPC Class Codes: H02J 3/06 Controlling transfer of power between connected networks; Controlling sharing of load between connected networks
• CPC Class Codes: H02M 1/0093 wherein the output is created by adding a regulated voltage to or subtracting it from an unregulated input
• CPC Class Codes: H02M 5/458 using semiconductor devices only
• CPC Class Codes: H02M 5/4585 having a rectifier with controlled elements

Abstract

A power flow controller with a fractionally rated back-to-back (BTB) converter is provided. The power flow controller provide dynamic control of both active and reactive power of a power system. The power flow controller inserts a voltage with controllable magnitude and phase between two AC sources at the same frequency; thereby effecting control of active and reactive power flows between the two AC sources. A transformer may be augmented with a fractionally rated bi-directional Back to Back (BTB) converter. The fractionally rated BTB converter comprises a transformer side converter (TSC), a direct-current (DC) link, and a line side converter (LSC). By controlling the switches of the BTB converter, the effective phase angle between the two AC source voltages may be regulated, and the amplitude of the voltage inserted by the power flow controller may be adjusted with respect to the AC source voltages.

Claims

1. An apparatus for controlling real and reactive power flow between a first AC source and a second AC source, comprising:

a back-to-back converter comprising

an AC-DC converter coupled between a first tap and a second tap of a transformer, the AC-DC converter comprising a first set of switches,

a DC-AC converter coupled to the second AC source, the DC-AC converter comprising a second set of switches, and

a DC link coupled to the AC-DC converter and the DC-AC converter, wherein a third tap of the transformer is coupled to the first AC source, and an input voltage to the back-to-back converter is less than a voltage at the third tap of the transformer.

2. The apparatus of claim 1, wherein an input voltage to the back-to-back converter is a fraction of the voltage at the third tap of the transformer.

3. The apparatus of claim 1, wherein the transformer further comprises a set of taps floating around the voltage at the third tap of the transformer.

4. The apparatus of claim 1, wherein the transformer is an auto transformer or an isolated step-down transformer.

5. The apparatus of claim 1, further comprising the transformer.

6. The apparatus of claim 1, wherein the third tap is located between the first tap and the second tap.

7. The apparatus of claim 1, wherein the AC-DC converter comprises a first leg and a second leg, the first leg coupled to the first tap of the transformer and the second leg coupled to the second tap of the transformer.

8. The apparatus of claim 1 having an output, wherein the DC-AC converter comprises a first leg and a second leg, the first leg and the second leg coupled to the output of the apparatus.

9. The apparatus of claim 1, further comprising a control module, wherein the control module generates a common mode control signal such that the common mode current is within a first predetermined value and generates a differential mode control signal such that a differential current is within a second predetermined value.

10. The apparatus of claim 9, wherein the control module generates a first set of switching pulses to the first set of switches according to the differential mode control signal, and

a second set of switching pulse to the second set of switches according to the common mode control signal.

11. The apparatus of claim 1, wherein the first and the second set of switches are two-quadrant switches.

12. The apparatus of claim 1, wherein the DC-AC inverter is a multiple level inverter.

13. The apparatus of claim 1 having an output, further comprising:

an input filter coupled to the transformer and the AC-DC converter;

and an output filter coupled to the DC-AC converter and the output of the apparatus.

14. The apparatus of claim 1, wherein the DC link comprises a capacitor.

15. The apparatus of claim 1, further comprising a fail-normal switch, wherein the fail-normal switch is coupled across the back-to-back converter.

17. The method of claim 14, further comprising providing a reference active power, a reference series reactive power, and a reference bus voltage, wherein the step of calculating the reference common mode current uses the reference active power, the reference series reactive power, and the reference bus voltage;

18. The method of claim 15, further comprising providing a reference DC link voltage and a reference shunt reactive power, wherein the step of calculating the reference differential current uses the reference DC link voltage and the reference shunt reactive power.

20. The method of claim 17, wherein the step of generating a first set of switching pulses for the DC-AC converter and a second set of switching pulses for the AC-DC converter compares the common mode back-to-back converter output voltage reference with a carrier wave and the common mode back-to-back converter output voltage reference with the carrier wave.

16. A method of controlling a back-to-back converter comprised in a power flow controller for controlling power flow between a first AC source and a second AC source, wherein the back-to-back converter comprises an AC-DC converter, a DC link, and a DC-AC converter, comprising:

detecting an actual common mode current and an actual differential mode current;

calculating a reference common mode current and a reference differential mode current;

generating a common mode back-to-back converter output voltage reference and a differential mode back-to-back converter output voltage reference; and

generating a first set of switching pulses for the DC-AC converter and a second set of switching pulses for the AC-DC converter.

19. The method of claim 16, wherein the step of generating the common mode back-to-back converter output voltage reference and the differential mode back-to-back converter output voltage reference compares the actual common mode current to the reference common mode current and the actual differential mode current to the reference differential mode current.