Power flow controller with a fractionally rated back-to-back converter

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 (B TB) 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.

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.