Patent Publication Number: US-2011058398-A1

Title: Power converter system and method

Description:
TECHNICAL FIELD 
     The present invention relates to an electrical power conversion system and method and, more particularly, to an electrical power conversion system and method for converting electrical power generated from various types of power sources and for transferring a converted electrical power into an electrical grid. 
     BACKGROUND 
     In many industrialized countries a growing need for energy is being sensed and decision makers are looking towards supplying energy made from renewable sources such as renewable electrical power sources. There are several types of renewable electrical power source that have been perfected over the years, such as photovoltaic arrays that transform solar energy into electrical power and wind turbines that transform wind energy into electrical power. For transferring electrical power to an end user device, these electrical power sources are connected to an electrical grid which transports electrical power to various power consumption sites. 
     The electrical power already present in the electrical grid follows a certain waveform, and for transferring electrical power into the grid a compatible waveform must be produced by the electrical power source. However, the waveform within the electrical grid can vary depending on various conditions such as the amount of power generated by the connected electrical power sources and the amount of power drawn from the loads that are connected to the electrical grid. Moreover, these renewable electrical power sources do not generate a constant amount of electrical power and the amount of electrical power generated depends on the type of electrical power source. 
     The electrical power generated from each electrical power source is in the form of a DC (direct current). The electrical grid however only accepts an AC (alternating current) waveform having a given voltage and given frequency. For being accepted into the electrical grid, the DC must be converted into a compatible AC power signal. For doing so, it is common practice to convert the voltage of the DC into a desired voltage using a DC/DC converter. Once converted into the desired voltage, the converted DC is then inverted into a desired AC power signal using a DC/AC inverter. 
     In U.S. Pat. No. 5,077,652 there is disclosed a DC to AC converter that is connected to a load. This converter uses a DC/DC converter to boost an input DC from a low voltage to a higher voltage. The output of the DC/DC converter is connected to a DC/AC inverter, the inverter inverts the generated higher voltage DC into an AC power signal having a desired frequency. The DC/DC converted is connected to a controller module that controls the converter based on a voltage feedback from the output of the DC/DC converter, the controller module regulates the output voltage of the converter based on a predetermined voltage. 
     However the voltage at the output of the DC/DC converter cannot be adjusted to the waveform variations in the electrical grid. Also, the available power cannot efficiently be converted into an AC power signal when the DC input power fluctuates. 
     Moreover, in the disclosed DC to AC converter there is no way to verify if the electrical grid is operational and if the converter should feed the electrical grid with electrical power. In the case of an un-operational electrical grid such as during a power blackout, the electrical grid operators have no control over the various power sources that are connected to the electrical grid. If the power sources keep on feeding an un-operational electrical grid, an electrical grid islanding situation will occur and this can result into a hazardous situation, affecting the security of maintenance personal and damaging electrical network devices or even damaging end user devices that are connected to the electrical grid. 
     Consequently an efficient way of adjusting the generated AC power signal to the varying waveform of the electrical grid and the varying DC input power would be advantageous to minimize energy losses. Moreover, a safer way of feeding electrical power into the electrical grid by various distributed power sources is required to prevent hazardous situations. 
     SUMMARY 
     According to one aspect of the invention, there is provided an electrical power conversion system for connecting an electrical power source to an electrical grid that can draw more electrical power than the electrical power source can provide comprising an input module for generating a high voltage DC power signal from a variable low DC power signal of the electrical power source based on a voltage command, an output module connected to the high voltage DC power signal for generating an AC power signal with a peak voltage based on the voltage command according to a frequency command and a phase command, an electrical grid interface for selectively connecting the AC power signal to the electrical grid and to measure an electrical grid waveform for generating an electrical grid measurement including voltage, phase and frequency, and a controller for determining an available power at the low DC power signal to allow the input module to supply the high voltage DC power signal, for setting the phase command with respect to grid phase measured by the electrical grid interface in accordance with the available power, for setting the voltage command based on grid voltage measured by the electrical grid interface, for setting the frequency command based on grid frequency measured by said electrical grid interface and for detecting loss of the electrical grid to control the grid interface to disconnect the AC power signal from said electrical grid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by way of the following detailed description of embodiments of the invention with reference to the appended drawings, in which: 
         FIG. 1A  is a block diagram of an electrical power conversion system for connecting a power source to an electrical grid where a high DC power signal voltage measurement is used to determine an input power information, according to an embodiment; 
         FIG. 1B  is a block diagram of an electrical power conversion system for connecting a power source to an electrical grid where a voltage command is used to determine an input power information, according to an embodiment; 
         FIG. 2A  is a flow chart of an electrical power conversion method for connecting a power source to an electrical grid where a high DC power signal voltage measurement is used to determine an input power information, according to an embodiment; 
         FIG. 2B  is a flow chart of an electrical power conversion method for connecting a power source to an electrical grid where a voltage command is used to determine an input power information, according to an embodiment; 
         FIG. 3A  is a block diagram of an input module of the system having a low DC power signal as input and where a voltage command is received by an AC generator, according to an embodiment; 
         FIG. 3B  is a block diagram of an input module of the system having a high DC power signal as input and where a voltage command is received by an AC generator, according to an embodiment; 
         FIG. 3C  is a block diagram of an input module of the system where a voltage command is received by a voltage regulator of a low AC power signal, according to an embodiment; 
         FIG. 3D  is a block diagram of an input module of the system where a voltage command is received by a transformer, according to an embodiment; 
         FIG. 4  is a block diagram of an output module of the system where a voltage command, phase offset command and a frequency command are received by a sine wave generator, according to an embodiment; 
         FIG. 5A  is a block diagram of an interface module according to an embodiment; 
         FIG. 5B  is a graph representing a phase displacement between an AC output waveform and a grid waveform; 
         FIG. 5C  is a graph representing a stretched zero crossing of the AC output waveform for detecting an islanding situation of the electrical grid; 
         FIG. 5D  is a graph representing a truncated voltage of the AC output waveform for detecting an islanding situation of the electrical grid; 
         FIG. 6  is a block diagram of three converters being synchronized for generating a three phase AC output; and 
         FIG. 7  is a block diagram of the system connectable to a configuration manager, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Presented in  FIG. 1A  is an electrical power conversion system  100  that is depicted as being adapted to connect to a power source  102  such as a wind turbine or a photovoltaic array and to connect to an electrical grid  104 . Once connected to the power source  102  and the electrical grid  104 , this system  100  is able to transfer power from the power source  102  into the electrical grid  104 . In general, the electrical grid  104  can retrieve more power than the power source  102  can provide and the amount of power that the power source  102  can generate is normally variable. Depending on various environmental conditions such as wind speed in the case of a wind turbine or light units in the case of a photovoltaic array, the amount of power the power source  102  is able to generate varies. For this reason, the system  100  is able to adapt to the power generated by the power source  102  and efficiently convert this power into a waveform that is compatible with the electrical grid  104 . Although in this embodiment the system  100  is adapted to connect to a power source  102  that retrieves energy from a wind turbine or a photovoltaic array, it will be understood by a skilled person that the system  100  is also adapted to connect to a power source  102  of any other type that retrieves energy from either a renewable or a non-renewable power source. 
     According to an embodiment, the system  100  is adapted to dynamically generate an AC power signal having a waveform that is compatible with the waveform of the electrical grid  104 . The system  100  draws a variable low DC power signal directly from the power source  102  or from a battery that has been charged by the power source  102 . The system  100  then converts this low DC power signal into an AC power signal having a waveform that is compatible with the waveform of the electrical grid  104 . Before generating the AC power signal, the waveform of the electrical grid  104  is first analyzed for detecting variations. The waveform of the electrical grid  104  is variable as the amount of power transferred into the electrical grid  104  from the connected power sources  102  is variable and as the amount of power drawn by the loads from the electrical grid  104  is also variable. Although the system  100  described herein is adapted to connect to a power source that generates a variable low DC power signal, it will be understood by a skilled person that it is possible for this system  100  to connect to a power source that generates a power signal that has a higher voltage than that of the waveform of the electrical grid  104 . In such a case, the system  100  converts a high DC power signal into an AC power signal having a waveform that is compatible with the waveform of the electrical grid  104  by voltage down conversion. 
     Moreover, as electrical power transportation standards can differ from one country or region to another, the acceptable waveform range on the electrical grid can also differ. According to an embodiment, the system  100  is adapted to dynamically generate a waveform that is compatible to either one of the various electrical standards such as: 120V at 50 Hz/60 Hz, 240V at 50 Hz/60 Hz, 550V at 50 Hz/60 Hz, etc. 
     To do this, as further presented in  FIG. 1A , the system  100  comprises an electrical grid interface  106 , an output controller  108 , an input controller  110 , an input module  112  and an output module  114 . The interface  106  is a point of connection between the system  100  and the electrical grid  104 , it allows analyzing the grid&#39;s waveform and providing grid waveform information to the other components of the system  100 . The output controller  108  receives the grid waveform information as an electrical grid measurement. From the grid measurement, it is possible for the output controller  108  to determine a voltage set point, a frequency set point and a phase set point each set point being based on a corresponding parameter of the grid waveform. For example, if the waveform of the grid is of 120V at 60 Hz with a 10 degree phase, the voltage set point, the frequency set point and the phase set point would be fixed accordingly. These set points are used as a guideline for the system  100  for generating an AC power signal that can be transferred into the electrical grid  104  while minimizing power loss. 
     According to one embodiment, the output controller  108  sends the voltage set point to the input controller  110 . Based in part on this voltage set point, the input controller  110  generates a voltage command for the input module  112 . The input module  112  is the entry point of the low DC power signal generated by the power source  102 . Based on the voltage command, the input module  112  generates a high DC power signal for sending to the output module  114 . 
     Depending in part on the amount of power available and in part on the voltage command, the voltage of the generated high DC power signal varies. For obtaining a high DC power signal having the desired voltage, the voltage command must be adjusted to the power available. According to one embodiment, the system  100  has a feedback loop of the high DC power signal voltage measurement. Based on this measurement, the input controller  110  adjusts the voltage command which is a duty cycle command for the input module  112  to maintain, increase or decrease the voltage of the high DC power signal. 
     Based on the high DC power signal voltage measurement, according to one embodiment of the system  100 , the input controller  110  is adapted to monitor the current of the high DC power signal and limits the current when the current is higher than a given threshold. 
     According to another embodiment, based on the high DC power signal voltage measurement, the input controller  110  generates an input power info for the output controller  108 . The input power info holds information concerning the available power generated by the power source  102  at the low DC power signal. It will be understood by a skilled person that the input power info can also be generated based on a low DC power signal voltage measurement. 
     According to one embodiment, the output controller  108  determines a phase offset command based in part on the input power info. In a case where the available power is too low and the system  100  is unable to generate a high DC power signal with the desired voltage, the output controller  108  determines a phase offset command to generate an AC power signal having a current that is high enough to transfer the available amount of power into the electrical grid  104 . The output controller  108  determines the phase offset command also based in part on the phase set point so that the output module  114  generates an AC power signal that is in phase with the grid&#39;s waveform. 
     Similarly, the output controller  108  determines the frequency command based on the frequency set point so that the output module  114  generates an AC power signal that has a same frequency as the grid&#39;s waveform. Once determined, both the phase offset command and the frequency command are sent to the output module  114 , the output module  114  in turn is adapted to receive the high DC power signal and to process it based on the phase offset command and the frequency command for generating the AC power signal. 
     According to another embodiment, the output controller  108  determines a rectifying voltage command based on the input power info and the voltage set point. In a case where the available power is sufficient to generate an AC power signal having the desired voltage, the rectifying voltage command can adjust the voltage of the AC power signal when the high DC power signal voltage is not at the desired level or is too high. Although not shown in  FIG. 1A , this rectifying voltage command is sent to the output module  114 , the output module being adapted to receive the rectifying voltage command and to process the high DC power signal for generating the AC power signal based on the rectifying voltage command. 
     Further presented in  FIG. 1A , the AC power signal is sent to the Interface  106  for further waveform processing and for generating an adjusted AC power signal. The interface  106  verifies if all waveform transferring conditions are met and if this is the case, the Interface  106  then transfers the adjusted AC power signal into the electrical grid  104 . 
     There is presented in  FIG. 2A  a method for generating the adjusted AC power signal from the variable low DC power signal, the adjusted AC power signal being compatible with the waveform of the electrical grid, according to the system  100  of  FIG. 1A . 
     Presented in  FIG. 1B  is the system  100  depicted according to another embodiment. In this system  100 , the voltage command is a duty cycle command. The input controller  110  generates a pulse width modulation pattern that determines a duty cycle command based on the voltage set point. In this embodiment, the input controller  110  does not need to measure the voltage of the generated high DC power signal to generate the input power info for sending to the output controller  108 . Based on the pulse width modulation pattern and the voltage of the low DC power signal, the input controller  110  is adapted to determine the input power info. 
     There is presented in  FIG. 2B  a method for generating an adjusted AC power signal from a variable low DC power signal, the adjusted AC power signal being compatible with the waveform of the electrical grid, according to the system  100  of  FIG. 1B . 
     Presented in  FIG. 3A  is the input module  112 , according to an embodiment of the system  100 . This input module  112  has an AC generator  300 , a transformer  302   a  and a rectifier  304 . The AC generator  300  is adapted to receive the low DC power signal and convert it into a low AC power signal having a voltage that is adapted to the transformer  302   a . In one embodiment, the AC generator  300  has a four transistor full-bridge circuit for inverting the low DC power signal to a low AC power signal. The voltage of the low AC power signal is adjusted by the voltage command which controls the four transistor full-bridge circuitry. 
     The transformer  302   a  has two secondary windings and increases the voltage of the low AC power signal by a predetermined ratio to generate a high AC power signal. 
     The rectifier  304  is at least one diode and is connected to the transformer  302   a  to filter a positive voltage of the high AC power signal and generate the high DC power signal. 
     Depending on the power source  102 , it is possible for the power source  102  to generate a DC having a voltage that is higher than the voltage set point. In such a case, according to yet another embodiment of this system  100 , the input module  112  such as presented in  FIG. 3B  may be used. In this input module  112 , a high DC power signal is received by the AC generator  300  a voltage command controls the AC generator  300  for generating a high AC power signal having a voltage that is adapted to the transformer  302   b . The transformer  302   b  then decreases the high AC power signal to a low AC power signal. The low AC power signal is then sent through the rectifier  304  to filter a positive voltage of the low AC power signal and generate a low DC power signal. 
     According to yet another embodiment of the input module  112 , there is presented in  FIG. 3C  another input module  112  that is adapted to down convert a voltage of a high DC power signal to a low DC power signal. In this input module  112 , there is a voltage regulator  306  that is connected to the transformer  302   c  for regulating the voltage of the low AC power signal based on the voltage command. A skilled reader will understand that this input module  112  can also be used to up convert a voltage of a low DC power signal to a high DC power signal. 
     According to yet another embodiment of the input module  112 , there is presented in  FIG. 3D  a transformer  302   d  that is adapted to receive a voltage command. The transformer  302   d  has multiple secondary windings and allows up converting a voltage of the low AC power signal by various transformation ratios. A selection of the voltage transformation ratio is done by sending the voltage command to the transformer  302   d . This can be particularly useful for systems  100  that are adapted to various electrical transportation standards. A skilled reader will understand that the transformer  302   d  can be of a type that allows down converting a voltage of a high AC power signal by various transformation ratios. 
     Presented in  FIG. 4  there is the output module  114  having an AC generator  400 , a sine wave generator  402  and a low pass filter  404 . According to an embodiment, the AC generator  400  is an H-bridge inverter and generates an AC power signal from the high DC power signal. The AC generator  400  is commanded by a duty cycle that is controlled by a pulse width modulation generated by the sine wave generator  402 . The sine wave generator  402  being controlled by at least one of a voltage command, a phase offset command or a frequency command. 
     According to yet another embodiment, the low pass filter  404  is connected to the output of the AC generator  400  and removes high frequency components such as harmonics so that only the fundamental component of the AC generator output is transferred to the electrical grid  104 . 
     Presented in  FIG. 5A , there is the interface  106  having among others an inductor  500  and an analyzer  502 . The inductor  500  protects the components of the system  100  against surcharges from the electrical grid  104 . Additionally, the inductor  500  introduces a small phase difference between the system&#39;s output and the grid&#39;s voltage, such as graphically represented in  FIG. 5B . This phase difference is necessary to allow the transferring of power into the electrical grid  104 . The analyzer  502  analyzes the waveform of the electrical grid  104  and generates the grid measurement. Based on the grid measurement, the analyzer  502  also generates a de-phase command. According to one embodiment, the inductor  500  introduces the small phase difference between the system&#39;s output and the grid&#39;s voltage based on the de-phase command. It will be understood by a skilled person in the art that the analyzer  502  can be an independent module or part of another module of the system such as the output controller  108  or the output module  114 . 
     Further presented in  FIG. 5A , the interface  106  has an islanding detector  504  and a switch  506 . The islanding detector  504  is adapted to detect an islanding situation or a loss of the electrical grid based on an analysis of the grid waveform. When an islanding situation is detected, the islanding detector  504  signals a switch  506  to disconnect the system  100  from the electrical grid  104 . 
     An islanding situation can occur when the electrical grid  104  is made un-operational. For example, when a technician wishes to do maintenance work on the electrical grid  104  he will render the electrical grid un-operational producing an electrical blackout. However, he will have no control on the power sources  102  that are connected to the electrical grid  104  and if not disconnected, power from the power sources  102  can still be transferred into the electrical grid  104  and jeopardise his safety. Therefore it is required by various safety standards to automatically disconnect all power sources  102  from the electrical grid  104  when an islanding situation occurs. 
     The islanding detector  504  is adapted to detect an islanding situation by using one or a combination of islanding detection methods. According to one embodiment the detector  504  is adapted to detect an islanding situation by monitoring the grid voltage and is adapted to signal a disconnection when the measured voltage of the grid  104  is higher or lower than an acceptable range. The acceptable range can be a predetermined range or can be a range that is set through a configuration of the system  100 . 
     According to another embodiment the detector  504  is adapted to detect an islanding situation by monitoring the grid frequency and is adapted to signal a disconnection when the measured frequency of the grid  104  is higher or lower than an acceptable range. The acceptable range can be a predetermined range or can be a range that is set through a configuration of the system  100 . 
     According to another embodiment the detector  504  is adapted to detect an islanding situation by inducing a small perturbation near the zero-crossing of the adjusted AC power signal, such as can be seen in  FIG. 5C . A shift command is sent to the inductor  500  for inducing this small perturbation in the voltage of the adjusted AC power signal. This slightly modifies the effective frequency of the adjusted AC power signal. As this perturbation is variable and has a positive feedback, when the grid is present this perturbation cannot be detected and the system operates normally. However, if the grid is not present, the frequency of the grid will be outside of the acceptable range and the islanding detector  504  will then signal a disconnection. 
     According to yet another embodiment the detector  504  is adapted to detect and islanding situation by inducing a small perturbation of the voltage amplitude of the adjusted AC power signal, such as can be seen in  FIG. 5D . A shift command is sent to the inductor  500  for inducing this small perturbation in the voltage amplitude of the adjusted AC power signal. As this perturbation is variable and has a positive feedback, when the grid is present this perturbation cannot be detected and the system operates normally. However, if the grid is not present, the voltage of the grid will be outside of the acceptable range and the islanding detector  504  will then signal a disconnection. 
     It will be understood by a skilled reader that the shift commands of  FIG. 5C  or  FIG. 5D  can be sent to the output controller for having the small perturbation induced by the output module rather than by the inductor  500 . 
     Presented in  FIG. 6 , three systems  100  are adapted to generate an adjusted AC power signal for each phase of an electrical power line. Each system is connectable to one phase of the electrical power line and is controlled by a synchronization and diagnostic bus  602  for phase integrity purposes. According to an embodiment, the system  100  is adapted to connect to a synchronization and diagnostic bus  602  to which a total of three systems  100  are connectable. The bus  602  has a system connection detector for dynamically detecting a connection of the system  100  to the bus  602  and for dynamically assigning a master or slave function to each connected system  100 . According to an embodiment, the connection detector is adapted to assign the master function on a first come first serve basis, the first connected system  100   a  is assigned the master function and the two other systems ( 100   b  and  100   c ) that are subsequently connected are assigned the slave function. Various other conditions can be used by the connection detector for assigning the master or slave function to each system ( 100   a ,  100   b  and  100   c ) without departing from the scope of the invention. 
     According to an embodiment, each system ( 100   a ,  100   b  and  100   c ) has a synchronization manager  600  that is the connection point of the system  100  to the bus  602 . As presented in  FIGS. 5A and 7 , the manager  600  is connected to the interface  106  and more specifically to the analyzer  502 . As each system ( 100   a ,  100   b  and  100   c ) adapts to a corresponding phase of the electrical grid  104 , the manager  600  of each system ( 100   a ,  100   b  and  100   c ) sends a phase measurement of the electrical grid  104  to the bus  602 . Based on the phase measurement, a phase measurement analyzer of the bus  602  compares the phase measurement of each slave system ( 100   b  and  100   c ) to the phase measurement of the master system  100   a . If the phase measurement analyzer detects that a phase delta between the master system  100   a  and either one of the slave systems ( 100   b  and  100   c ) is outside an acceptable range—typically plus or minus one hundred twenty degrees—a phase abnormality alert is sent by the analyzer to the managers  600  of each system ( 100   a ,  100   b ,  100   c ). In response, when the manager  600  receives such a phase abnormality alert, it sends a disconnection command to the switch  506 . Consequently all three systems ( 100   a ,  100   b  and  100   c ) are then disconnected. 
     As further presented in  FIG. 7 , it will be understood by a skilled reader that the system  100  can comprise other components such as an input filter  700 , an output filter  702 , an isolation barrier  704 , etc. 
     As even further presented in  FIG. 7 , the system  100  is adapted to be configured by an electrical line operator through a configuration manager  706  that is connected to the system  100 . The configuration manager  706  can be of a type that remains connected to the system  100  while in operation or of a type that is disconnectable from the system  100  once configured. The configuration manager  706  is adapted to allow the operator to set parameters of the system  100 . According to one embodiment, the configuration manager is a computerized system that allows the operator to set at least one parameter in at least one of the components of the system  100  such as the interface  106 , the output controller  108 , the input controller  110 , the input module  112 , the output module  114 , the synchronization manager  600 , the input filter  700 , the output filter  702  and the isolation barrier  704 .