High Frequency Bi-directional AC Power Transmission

Abstract

A method may involve transmitting power between a tethered aerial vehicle equipped with wind turbines for generating power and a ground station configured to interconnect the generated power to an electrical distribution network. The power may be transmitted using high voltage, high frequency AC electrical signals, and transformers at the ground station and the aerial vehicle can scale the AC voltage for use at the respective locations. Converters at the ground station and the aerial vehicle can then convert the transformed voltage to DC. The AC voltage may be transmitted through the tether at a resonant frequency of the tether based in part on an internal capacitance between multiple conductive paths on the tether.

What is claimed is: 
     
         1 . An aerial vehicle comprising:
 a generator disposed in the aerial vehicle to generate power;   a converter disposed in the aerial vehicle to convert the generated power to a high frequency AC voltage;   a transformer disposed in the aerial vehicle to transform the high frequency AC voltage to a high voltage transmission level for transmission through a tether to a ground station; and   at least one inductor electrically coupled to the transformer, wherein the high frequency AC voltage oscillates at a resonant frequency of the tether, and wherein the resonant frequency is based on an internal tether capacitance of the tether and on the at least one inductor.   
     
     
         2 . The aerial vehicle of  claim 1 , further comprising:
 DC leads coupled to the converter to provide a DC voltage associated with the generated power; and   AC leads coupled to the converter to output the high frequency AC voltage with respect to the DC leads.   
     
     
         3 . The aerial vehicle of  claim 2 , further comprising a plurality of switches included in the converter to reverse polarity of the AC leads with respect to the DC leads while voltage across the AC leads is zero or within a predefined threshold to zero. 
     
     
         4 . The aerial vehicle of  claim 1 , wherein the transformer electrically couples with the tether to provide the high frequency AC voltage such that a first conductive path of the tether conducts the high frequency AC voltage with respect to a second conductive path of the tether. 
     
     
         5 . The aerial vehicle of  claim 1 , further comprising a wind turbine situated on the aerial vehicle, wherein the generator is electrically coupled to the wind turbine to generate the power. 
     
     
         6 . The aerial vehicle of  claim 1 , further comprising a propeller to generate thrust on the aerial vehicle. 
     
     
         7 . The aerial vehicle of  claim 6 , further comprising a motor to drive the propeller based on power drawn from the ground station via the tether. 
     
     
         8 . The aerial vehicle of  claim 1 , further comprising a control system that receives power drawn from the ground station via the tether to operate the aerial vehicle. 
     
     
         9 . The aerial vehicle of  claim 1 , wherein the ground station transforms and converts the high frequency AC voltage to a voltage configured to be distributed over a power distribution grid. 
     
     
         10 . The aerial vehicle of  claim 1 , wherein the ground station receives power from a power distribution grid, wherein the ground station converts and transforms the received power to another high frequency AC voltage at the high voltage transmission level for transmission through the tether to the aerial vehicle, and wherein the transformer of the aerial vehicle transforms the other high frequency AC voltage transmitted by the ground station from the high voltage transmission level to a lower voltage level. 
     
     
         11 . The aerial vehicle of  claim 10 , wherein the converter of the aerial vehicle converts the other high frequency AC voltage transformed by the transformer of the aerial vehicle to a DC voltage. 
     
     
         12 . The aerial vehicle of  claim 11 , wherein the aerial vehicle draws power from the converted DC voltage to power the aerial vehicle. 
     
     
         13 . The aerial vehicle of  claim 1 , further comprising a DC bus electrically coupled to the generator, wherein the generator is configured as a motor-generator that alternately: (i) uses energy from the DC bus to generate thrust on the aerial vehicle, (ii) generates electrical energy from motion of the aerial vehicle, and (iii) using the generated electrical energy, supplies the DC bus. 
     
     
         14 . The aerial vehicle of  claim 13 , further comprising:
 a propeller coupled to the motor-generator; and   a motor drive coupled between the DC bus and the motor-generator, wherein the motor drive: (i) draws energy from the DC bus for the motor-generator to use for generating the thrust via the propeller, and (ii) supplies the DC bus with electrical energy generated via the motor-generator from movement of the propeller.   
     
     
         15 . The aerial vehicle of  claim 1 , further comprising a current source connected to the converter, wherein the internal tether capacitance, wherein the internal tether capacitance of the tether charges upon reversing polarity of the current source, via the converter, such that a voltage across first and second conductive paths in the tether changes between positive and negative values in response to a given polarity reversal. 
     
     
         16 . The aerial vehicle of  claim 1 , wherein the high voltage transmission level from the transformer exceeds 2 kilovolts. 
     
     
         17 . The aerial vehicle of  claim 1 , wherein the resonant frequency is greater than 2 kilohertz. 
     
     
         18 . A tether for coupling an aerial vehicle at a first end of the tether to a ground station at a second end of the tether, the tether comprising:
 a first conductive path extending between the first end and the second end;   a second conductive path extending between the first end and the second end, wherein the tether receives a high frequency AC voltage at the first end, wherein the tether transmits the high frequency AC voltage from the first end to the second end by the first conductive path conducting the high frequency AC voltage with respect to the second conductive path, and wherein the first conductive path and the second conductive path define an internal tether capacitance of the tether; and   at least one inductor disposed at the first end of the tether, wherein the high frequency AC voltage oscillates at a resonant frequency of the tether, and wherein the resonant frequency is based on the internal tether capacitance and on the at least one inductor.   
     
     
         19 . A ground station that couples with an aerial vehicle via a tether, the ground station comprising:
 a transformer for receiving a high frequency AC voltage transmitted through the tether at a high voltage transmission level, wherein the transformer transforms the high frequency AC voltage from the high voltage transmission level to a lower voltage level;   a converter that converts the transformed high frequency AC voltage to a voltage to be distributed over a power distribution grid, wherein the ground station provides power to the power distribution grid using the converted voltage from the converter; and   at least one inductor for electrically coupling the transformer with the tether, wherein the high frequency AC voltage transmitted through the tether oscillates at a resonant frequency of the tether, and wherein the resonant frequency is based on an internal tether capacitance of the tether and on the at least one inductor.   
     
     
         20 . The ground station of  claim 19 , wherein the tether transmits the high frequency AC voltage such that a first conductive path of the tether conducts the high frequency AC voltage with respect to a second conductive path of the tether.