Methods, Systems, and Software for Controlling a Power Converter During Low (Zero)-Voltage Ride-Through Conditions

Abstract

A power converter control system having a phase tracker that is designed and configured to estimate the phase of the voltage on the power network that will be on the network when network recovers from a fault on the network. Such a power converter control system allows a power-network-connected power source to ride-through a fault event and continue supplying power thereto at the designed phase and frequency. In one embodiment, the phase tracker provides this estimate by having a response time slow enough that the voltage drop or sag caused by the fault substantially does not affect the control system. In another embodiment, the phase detector is designed and configured to freeze the frequency of its output upon detection of a fault event on the power network.

1 . A method of controlling the AC output of a power converter connected to an AC power network subject to a voltage fault that causes a network voltage on the AC power network to drop below a normal operating level during a fault period, the method comprising:
 estimating the phase angle of the voltage anticipated to be present on the AC power network when the network voltage recovers from the voltage fault; and   controlling a current of the AC output during the voltage fault as a function of the phase angle estimated.   
     
     
         2 . A method according to  claim 1 , wherein the power converter is required to remain connected to the AC power network during a maximum ride-through fault period having a very low voltage time, and said estimating includes tracking a phase of the network voltage with a time constant equal to about  1  times the very low voltage time to about  5  times the very low voltage time. 
     
     
         3 . A method according to  claim 1 , wherein the power converter is required to remain connected to the AC power network for a maximum ride-through fault period, and said estimating includes tracking a phase of the network voltage with a response time of about 1/4 to about 2 times the maximum ride-through fault period. 
     
     
         4 . A method according to  claim 1 , wherein said estimating includes tracking a phase of the network voltage with a response time of at least 25 ms. 
     
     
         5 . A method according to  claim 4 , wherein said tracking includes tracking the phase with a response time of at least 100 ms. 
     
     
         6 . A method according to  claim 5 , wherein said tracking includes tracking the phase with a response time of at least 1 s. 
     
     
         7 . A method according to  claim 1 , wherein the network voltage has an amplitude, said estimating further including changing the response time in inverse proportion to the amplitude of the network voltage. 
     
     
         8 . A method according to  claim 7 , wherein said changing the response time includes changing the response time using a phase-locked loop having an amplitude-sensitive phase detector. 
     
     
         9 . A method according to  claim 7 , wherein said estimating includes tracking a phase of the voltage with a phase-locked loop having operating parameters that are the same in both the fault period and while tracking when the network voltage is at the normal operating level. 
     
     
         10 . A method according to  claim 1 , wherein said estimating includes tracking a phase of the network voltage with a phase-locked loop. 
     
     
         11 . A method according to  claim 10 , wherein said tracking the phase with a phase-locked loop includes tracking the phase with a phase-locked loop having operating parameters that are substantially the same in both the fault period and while tracking when the network voltage is at the normal operating level. 
     
     
         12 . A method according to  claim 11 , wherein the operating parameters remain unchanged as between the fault period and while tracking when the network voltage is at the normal operating level. 
     
     
         13 . A method according to  claim 11 , further comprising:
 detecting the fault period; and   freezing at least one of the operating parameters in response to said detecting.   
     
     
         14 . A method according to  claim 11 , wherein the power converter is required to remain connected to the AC power network during a maximum ride-through fault period having a very low voltage time, and said estimating includes tracking the phase of the network voltage with a time constant equal to about 1 times the very low voltage time to about 5 times the very low voltage time. 
     
     
         15 . A method according to  claim 1 , wherein said estimating includes:
 determining when the network voltage drops below a preset level, wherein a controller phase reference signal has a frequency having a value when it is determined that the network voltage has dropped below the preset level; and   in response to said determining, freezing the frequency of the controller phase reference signal to substantially the value.   
     
     
         16 . A method according to  claim 15 , wherein said freezing includes freezing the controller phase reference signal when the network voltage drops below about 25% to about 50% of its normal operating level. 
     
     
         17 . A method according to  claim 15 , wherein the controller phase reference signal is controlled as a function of a phase error relative to the network voltage, said freezing including setting the phase error to zero. 
     
     
         18 . An apparatus for controlling the AC output of a power converter connected to an AC power network subject to a voltage fault that causes a network voltage on the AC power network to drop below a normal operating level during a fault period, the apparatus comprising:
 a control system designed and configured to:
 estimate the phase angle of the voltage anticipated to be present on the AC power network when the network voltage recovers from the voltage fault; and 
 control a current of the AC output during the voltage fault as a function of the phase angle estimated. 
   
     
     
         19 . An apparatus according to  claim 18 , wherein the power converter is required to remain connected to the AC power network for a maximum ride-through fault period having a very low voltage time, and said control system is designed and configured to track a phase of the network voltage with a time constant equal to about 1 times the very low voltage time to about 5 times the very low voltage time. 
     
     
         20 . An apparatus according to  claim 18 , wherein the power converter is required to remain connected to the AC power network for a maximum ride-through fault period, and said control system is designed and configured to track a phase of the network voltage with a response time of about ¼ to 2 times the maximum ride-through fault period. 
     
     
         21 . An apparatus according to  claim 18 , wherein said control system is designed and configured to track a phase of the network voltage with a response time of at least 25 ms. 
     
     
         22 . An apparatus according to  claim 21 , wherein said control system is designed and configured to track the phase with a response time of at least 100 ms. 
     
     
         23 . An apparatus according to  claim 22 , wherein said control system is designed and configured to track the phase with a response time of at least 1 s. 
     
     
         24 . An apparatus according to  claim 18 , wherein the network voltage has an amplitude, said control system being designed and configured to change the response time in inverse proportion to the amplitude of the network voltage. 
     
     
         25 . An apparatus according to  claim 24 , wherein said control system includes a phase-locked loop having an amplitude-sensitive phase detector designed and configured to change the response time as a function of the amplitude of the network voltage. 
     
     
         26 . An apparatus according to  claim 25 , wherein said phase-locked loop has operating parameters that are the same in both the fault period and while tracking when the network voltage is at the normal operating level. 
     
     
         27 . An apparatus according to  claim 18 , wherein said control system includes a phase-locked loop designed and configured to track a phase of the network voltage. 
     
     
         28 . An apparatus according to  claim 27 , wherein said phase-locked loop has operating parameters that are substantially the same in both the fault period and while tracking when the network voltage is at the normal operating level. 
     
     
         29 . An apparatus according to  claim 28 , wherein the operating parameters remain unchanged as between the fault period and while tracking when the network voltage is at the normal operating level. 
     
     
         30 . An apparatus according to  claim 28 , wherein said control system is designed and configured to:
 detect the fault period; and   freeze at least one of the operating parameters in response to said detecting.   
     
     
         31 . An apparatus according to  claim 28 , wherein the power converter is required to remain connected to the AC power network during a maximum ride-through fault period having a very low voltage time, and said phase-locked loop is designed and configured to track the phase of the network voltage with a time constant equal to about 1 times the very low voltage time to about 5 times the very low voltage time. 
     
     
         32 . An apparatus according to  claim 18 , wherein said control system is designed and configured to:
 determine when the network voltage drops below a preset level, wherein a controller phase reference signal has a frequency having a value when it is determined that the network voltage has dropped below the preset level; and   in response to determining when the network voltage drops below a preset level, freeze the frequency of the controller phase reference signal to substantially the value.   
     
     
         33 . An apparatus according to  claim 32 , wherein said control system is designed and configured to freeze the controller phase reference signal when the network voltage drops below about 25% to about 50% of its normal operating level. 
     
     
         34 . An apparatus according to  claim 32 , wherein said control system includes a phase-locked loop controlling the controller phase reference signal as a function of a phase error relative to the network voltage, said control system designed and configured to set the phase error to zero in response to determining when the network voltage drops below a preset level. 
     
     
         35 . A machine-readable storage medium containing machine-executable instructions for performing a method of controlling the AC output of a power converter connected to an AC power network subject to a voltage fault that causes a network voltage on the AC power network to drop below a normal operating level during a fault period, said machine-executable instructions comprising:
 a first set of machine-executable instructions for estimating the phase angle of the voltage anticipated to be present on the AC power network when the network voltage recovers from the voltage fault; and   a second set of machine-executable instructions for controlling a current of the AC output during the voltage fault as a function of the phase angle estimated.   
     
     
         36 . A machine-readable storage medium according to  claim 35 , wherein said first set of machine-executable instructions includes machine-executable instructions for tracking a phase of the network voltage with a time constant equal to about  1  times a very low voltage time period to about  5  times the very low voltage time. 
     
     
         37 . A machine-readable storage medium according to  claim 35 , wherein said first set of machine-executable instruction includes machine-executable instructions for tracking a phase of the network voltage with a response time of about ⅓ to 2 times a maximum ride-through fault period. 
     
     
         38 . A machine-readable storage medium according to  claim 35 , wherein said first set of machine-executable instruction includes machine-executable instructions for tracking a phase of the network voltage with a response time of at least 25 ms. 
     
     
         39 . A machine-readable storage medium according to  claim 38 , wherein said first set of machine-executable instruction includes machine-executable instructions for tracking the phase with a response time of at least 100 ms. 
     
     
         40 . A machine-readable storage medium according to  claim 39 , wherein said first set of machine-executable instruction includes machine-executable instructions for tracking the phase with a response time of at least 1 s. 
     
     
         41 . A machine-readable storage medium according to  claim 35 , wherein the network voltage has an amplitude, said first set of machine-executable instruction including machine-executable instructions for changing the response time in inverse proportion to the amplitude of the network voltage. 
     
     
         42 . A machine-readable storage medium according to  claim 41 , wherein said machine-executable instructions for changing the response time include machine-executable instructions for changing the response time using a phase-locked loop having an amplitude sensitive phase detector. 
     
     
         43 . A machine-readable storage medium according to  claim 41 , wherein said machine-executable instructions for tracking the phase with a phase-locked loop include machine-executable instructions for tracking the phase with a phase-locked loop having operating parameters that are the same in both the fault period and while tracking when the network voltage is at the normal operating level. 
     
     
         44 . A machine-readable storage medium according to  claim 35 , wherein said machine-executable instructions for tracking include machine-executable instructions for tracking the phase with a phase-locked loop. 
     
     
         45 . A machine-readable storage medium according to  claim 35 , wherein said machine-executable instructions for tracking the phase with a phase-locked loop include machine-executable instructions for tracking the phase with a phase-locked loop having operating parameters that are the same in both the fault period and while tracking when the network voltage is at the normal operating level. 
     
     
         46 . A machine-readable storage medium according to  claim 27 , wherein said first set of machine-executable instructions includes machine-executable instructions for:
 determining when the network voltage drops below a preset level, wherein a controller phase reference signal has a frequency having a value when it is determined that the network voltage has dropped below the preset level; and   in response to said determining, freezing the frequency of the controller phase reference signal to substantially the value.   
     
     
         47 . A machine-readable storage medium according to  claim 37 , wherein said machine-executable instructions for freezing includes machine-executable instructions for freezing the controller phase reference signal when the network voltage drops below about 25% to about 50% of its normal operating level. 
     
     
         48 . A machine-readable storage medium according to  claim 37 , wherein the controller phase reference signal is controlled as a function of a phase error relative to the network voltage, said machine-executable instructions for freezing including machine-executable instructions for setting the phase error to zero.