Abstract:
The subject matter of the invention is the provision of a power-management system and method for photovoltaic installations connecting to the grid, which incorporates monitoring active and/or reactive power, account being taken at all times of grid requirements, in order to allow temporary demand arising in the grid to be addressed, thereby contributing to regulation of the grid and to the stability and quality thereof.

Description:
[0001]    This application is a continuation of International Application PCT/ES/2008/000560, filed Aug. 12, 2008, which is hereby incorporated by reference in its entirety. 
     
    
     OBJECT OF THE INVENTION 
       [0002]    The present invention is applicable to the field of power generation systems based on renewable energy and, more specifically, photovoltaic solar energy. 
         [0003]    The object of the invention is to provide a power management system for grid-connected photovoltaic installations, incorporating an active and reactive power control that takes grid requirements into account at a given time. The management system provides the photovoltaic facility with an active and reactive power reserve which allows it to address temporary grid requirements. By means of said management, the photovoltaic facility can behave as a conventional energy source (thermal, nuclear . . . ), thereby participating in power grid regulation and contributing to the stability and quality thereof. 
       BACKGROUND OF THE INVENTION 
       [0004]    At present, grid-connected photovoltaic installations are widely acknowledged in our society. They are installations formed by a, plurality of photovoltaic generators (photovoltaic field) and at least one electronic converter that transforms the energy produced by the photovoltaic field (direct current) for injection in the power grid (alternating current). 
         [0005]    In this regard, U.S. Patent Publication No. 2008122449 A1 discloses a control scheme for a PhotoVoltaic unit to suitably drive the injection of active and reactive power into a radial distribution grid. An active and reactive power control from preestablished setpoints is disclosed but it is not disclosed how the preestablished setpoints are established or determined. 
         [0006]    There is another documents, related with the prior art, as for example IEEE pp. 1463-1468, 15-19 June 2008 of DELFINO, F et al. titled “An integrated active and reactive power control scheme for grid-connected photovoltaic production systems” where is disclosed a system that tries to overcome those problems derived from the sunlight fluctuations, but without taking into account how to manage reactive and active power. 
         [0007]    Photovoltaic generator is understood to be any device capable of transforming solar energy into electricity. 
         [0008]    All photovoltaic generators have a characteristic voltage-intensity (V-I) curve. This curve varies according to irradiance and the temperature of the photovoltaic generator. There is a voltage-power (V-P) curve associated to this V-I curve that relates the energy produced by the photovoltaic generator to its operating voltage. In order to maximize the energy produced by the photovoltaic field, the photovoltaic converters are equipped with at least one maximum power point (MPP) tracker. Said MPP tracker (generally DC/DC conversion structures with input voltage control) determines the operating voltage that will maximize the energy produced by association of the generators connected thereto. 
         [0009]    On the other hand, the irradiance incident upon the generator is a function of the angle formed by the plane of the panel that is perpendicular to the sun. Solar trackers are often used to increase the energy produced by photovoltaic installations, whereon the photovoltaic generators are disposed. Solar trackers are mobile structures that pivot around one or two axles (azimuthal and zenithal) controlled by a group of actuators that orient the photovoltaic generators towards the sun, maximizing the irradiance incident thereupon. 
         [0010]    Grid-connected photovoltaic systems inject the energy produced by the installation into the power grid. In order to guarantee power grid stability and maintain the frequency and voltage within the established limits, there must be a balance between the power generated and the power consumed. In the case of imbalances between generated and consumed power, the different generation sources must readjust the power generated, thereby re-establishing the balance. To this end, conventional sources are obliged to supply extra power or limit active power (P) and supply or consume reactive power (Q) when required by the power grid. Specifically, the following relationship is fulfilled: 
         [0000]    
       
      
       S=P+iQ  
      
     
         [0011]    S=apparent or complex power 
         [0012]    P=active power 
         [0013]    Q=reactive power 
         [0014]    In photovoltaic systems, the energy generated is closely related to weather conditions, resulting in a lack of control thereupon. The difficulty of estimating the power that can be supplied by the photovoltaic system at a given time, in view of irradiance uncertainty, prevents the photovoltaic systems of the state of the art from having a power reserve that can be supplied in the event of increased demand. 
         [0015]    On the other hand, in the photovoltaic systems of the state of the art, current is injected into the grid at a preset power factor (generally cos φ=1), due to which it does not participate in reactive power regulation. 
         [0016]    As the number of grid-connected photovoltaic installations grows, so do their negative effects on power grid stability. Therefore, there is a need for developing a management system that will allow control over active and reactive power, providing the system with a power reserve (active and reactive) that will enable it to address temporary power grid requirements. 
       DESCRIPTION OF THE INVENTION 
       [0017]    In order to achieve the objectives and avoid the aforementioned drawbacks, the present invention consists of a method and system for power management in grid-connected photovoltaic installations, which incorporates a control for active and reactive power, taking into account grid requirements at a given time. The invention provides an active and reactive power reserve that allows it to address temporary grid requirements. 
         [0018]    In accordance with the present invention, a power management system for a photovoltaic installation formed by:
       at least one control unit (UC) for managing the active and/or reactive power of the photovoltaic installation,   at least one photovoltaic generator,   at least one electronic converter (CE) for transforming direct current into alternating current.       
 
         [0022]    In the case of active power management, the system has at least one array formed by the foregoing photovoltaic generators, the energy production of which is individually controlled. Said array is called power control minimum unit (UMCP). 
         [0023]    In order to control the active power of the photovoltaic installation, the present invention comprises the following stages:
       Establish an active power reserve setpoint for the installation.   Determine a first array of UMCPs that will operate without limiting its active power.   Estimate the power producible by the photovoltaic installation.   Determine a second array of UMCPs that will operate limiting its active power at a certain value.       
 
         [0028]    Limitation of the active power of the elements of the second UMCP array may be established through the active power reserve setpoint and estimation of producible power. 
         [0029]    In turn, the aforementioned estimation of producible power may be obtained through one or several of the following methods:
       Through the power generated by the first UMCP array.   Through at least one balanced photovoltaic cell.   Taking into account the orientation of any of the UMCP arrays with respect to the sun.   Based on a sample of the V-I curve of the UMCP, which is obtained from the periodic variation in operating voltage of the UMCP.
 
In a preferred embodiment of the invention, the active power reserve setpoint is determined using an economic optimization algorithm and/or grid frequency control loop and/or any other external requirement and/or using an installation power output variation rate control loop.
       
 
         [0034]    In a preferred embodiment, the number of units that form the second UMCP array is selected based on the limitation of active power required from said array, taking into account that the number of units that form the first UMCP array must constitute a representative sample under the operating conditions of the photovoltaic installation. 
         [0035]    In another preferred embodiment, a percentage of the active power reserve is obtained by:
       Modifying the orientation of at least one of the units of the second UMCP array.   Controlling the operating voltage of at least one of the units of the second UMCP array.   Connecting and disconnecting at least one UMCP.       
 
         [0039]    Another preferred embodiment includes, in the photovoltaic installation, at least one controlled load that allows consumption of the active power reserve of the installation, being a percentage of the active power reserve of the installation. 
         [0040]    The controlled load may be an electric load or an energy storage unit, such as an electrolyzer, a battery, a flywheel or supercapacitor. Depending on its nature, it can be connected to the power grid or at the entrance of at least one CE. The controlled load is subordinated to grid active power demand. If the grid were to require extra power, consumption of the controlled load would decrease or even be switched off. This will ensure that the power reserve is adequately used. 
         [0041]    The energy stored in each energy storage unit can be used to control the variations in the power output of the installation. For example, in the event of a variation in irradiance that produces as a consequence a proportional variation in the power output, the storage units will be used to supply energy, controlling said variation in power in accordance with preset criteria. 
         [0042]    The method also makes reference to reactive power control on establishing a reactive power setpoint for the photovoltaic installation and, subsequently, determining the reactive power generated or consumed by each electronic converter of the installation. 
         [0043]    The reactive power setpoint can be determined using an economic optimization algorithm, a grid voltage control loop or external requirements. 
         [0044]    In a preferred embodiment of the invention, the reactive power generated or consumed by each electronic converter of the photovoltaic installation, is determined based on its reactive power generation or consumption capacity. 
         [0045]    In another preferred embodiment, the reactive power setpoint is modified in at least one of the electronic converters using an internal voltage regulation loop to maintain its output voltage within the established limits. 
         [0046]    Reactive power can be controlled independently in each of the electronic converters. 
         [0047]    The system can also comprise:
       at least one local control unit (UCL) associated to each UMCP;   at least one communications network that interconnects at least one UCL with at least the UC and at least one CE.       
 
         [0050]    In a preferred embodiment, the system includes, in the UC, active power management means for monitoring the active power of the different UMCPs, establishing the operating mode of each UMCP and sending active power setpoints to each UMCP. 
         [0051]    In other preferred embodiments, the photovoltaic installation includes a device capable of controlling the position of at least one UMCP (for example, a tracker) with respect to the sun. Alternatively, a device capable of controlling the operating voltage of at least one UMCP may be included. 
         [0052]    In a preferred embodiment, the UC and UCL form part of a single device. 
         [0053]    By way of example, the UC, UCL and CE may be housed in the same casing or in different casings (discrete elements). 
         [0054]    Below, in order to help to better understand this specification and forming an integral part thereof, it is accompanied by figures wherein the object of the invention has been represented in an illustrative and non-limiting manner, in addition to some figures belonging to the state of the art which have been previously described. 
         [0055]    In preferred embodiment, the system for power management in a photovoltaic installation which comprises at least one control unit (UC), at least one photovoltaic generator, at least one electronic converter (CE) for transforming direct current into alternating current. The UC comprises means for managing power in the photovoltaic installation, selected from among active power management means, reactive power management means and a combination thereof. The system for power management in a photovoltaic installation further comprises at least one array of photovoltaic generators, the energy production of which is individually controlled (power control minimum unit—UMCP —), one local control unit (UCL) associated to each photovoltaic generator, a communications network that interconnects at least one local control unit (UCL) with at least the control unit (UC) and with at least one electronic converter (CE). The system further includes at least one controlled load that consumes the active energy generated by the photovoltaic installation, being a percentage of the active power reserve of the installation. Said controlled load is connected in parallel to the entrance of the electronic converters (CE) or in parallel to the exit of the electronic converters (CE). 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0056]      FIG. 1  shows a preferred embodiment of the elements that integrate a photovoltaic installation, according to the invention. 
           [0057]      FIG. 2  shows a preferred embodiment for controlling the active power of the installation. 
           [0058]      FIG. 3  shows a preferred embodiment for controlling the reactive power of the installation. 
           [0059]      FIG. 4  represents a practical example of the voltage-power curve of a conventional photovoltaic generator. 
       
    
    
     DESCRIPTION OF ONE OR SEVERAL EXAMPLES OF EMBODIMENT OF THE INVENTION 
       [0060]    Examples of the invention are described below, quoting references from the figures. 
         [0061]      FIG. 1  shows a schematic view of a photovoltaic system wherein the proposed power management system is implemented. The photovoltaic system is formed by:
       a control unit (UC) ( 104 );   a plurality of power control minimum units (UMCP_ 1 -UMCP_n) (102 1  . . .  102   n ), each of which is connected to a local control unit (UCL_ 1 -UCL_n) ( 103   1  . . .  103   n );   a plurality of electronic converters (CE_ 1 -CE_m) ( 101   1  . . .  101   m ) whereto at least one UMCP is connected. Said CE transform the energy produced by the different UMCP connected thereto for injection into the power grid ( 106 );   a communications network ( 105 ) that interconnects the different local control units (UCL_ 1 -UCL_n) ( 103   1  . . .  103   n ) with the control unit (UC) ( 104 ) and the electronic converters (CE 1 -CE m ) ( 101   1  . . .  1014         
 
         [0066]    Each previously described UMCP (UMCP_ 1 -UMCP_n) ( 102   1  . . .  102   n ) will depend on the characteristics of the installation and which may be:
       An array of photovoltaic generators disposed on at least one solar tracker. In installations with solar trackers, the position of the tracker can be controlled, which provides the possibility of controlling the energy produced by said generators.   An array of photovoltaic generators connected to at least one conversion structure (DC/DC or DC/AC) with input voltage control. Each conversion structure provides independent control over the operating voltage of the associated generators, providing the possibility of controlling the energy produced by said generators.   An array of photovoltaic generators with sectioning elements that allow connection and disconnection thereof from the electronic converters (CE 1 -CE m ) ( 101   1 - 101   m ).   Any combination of the foregoing.       
 
         [0071]    Each UMCP can operate under two operating modes governed by the UC through its corresponding UCL whereto it is connected: observer mode (MO) and reserve mode (MR). When a UMCP operates under MO mode it supplies the maximum active power available at a given time (depending on the weather conditions), without any restriction thereupon. When a UMCP operates under MR mode, the UC limits the active power supplied by said UMCP. 
         [0072]    When part of the installation is made to operate under MR mode, the total active power of the installation remains below the maximum power available under instantaneous radiation conditions. The maximum active power available is estimated based on the data supplied by the UMCP under MO mode. Of the total UMCP of the installation (n), there will be (u) UMCPs that will operate under MR mode and (n-u) UMCPs that will operate under MO mode. 
         [0073]      FIG. 2  represents a preferred embodiment for controlling the active power of the photovoltaic installation. The invention envisages obtaining a setpoint that consists of an active power reserve, which can be defined as a percentage of the active power produced in the installation at a given time (% P res     —   UC). In another embodiment, obtaining the setpoint can be defined as a percentage of the nominal active power of the installation or as a desired active power value for the photovoltaic system in absolute units (for example, kW). The value of said setpoint is selected in a block ( 203 ) based on several criteria, which can be:
       a value established by the power grid operator (% P res     —   RED);   a value generated in the UC ( 104 ) in accordance with several predefined criteria in an optimization algorithm (% P res     —   OPT). This algorithm takes into account parameters ( 201 ) such as production optimization, active power reserve, grid stability at a given time, bonuses on account of the active power reserve, tariff information, profit optimization, etc.;   a value generated in the UC based on grid frequency (% P res     —   FRE). In this case, grid frequency (F grid ) is subtracted from a reference frequency (F grid, ref ) in a block ( 204 ) which provides control over grid frequency stability, obtaining a frequency error. This error is applied to a controller included in the block ( 204 ) in order to obtain the percentage of active power reserve (% P res     —   FRE);   a setpoint generated in the UC ( 104 ) based on the active power output variation rate of the installation. This provides control over active power output in the event of variations in irradiance, for the purpose of mitigating the effect of said variations in irradiance on active power output. The generation of this setpoint is obtained from a predefined variation rate (TVP ref ) and from the total active power defined below) produced by the system, through a block ( 206 ).       
 
         [0078]    The UC ( 104 ) also receives the measurement of active power of each UMCP as an entry (UMCP_P 1 -UMCP_P n ). These data are used to calculate the total active power (P tot ) produced by the system, through the blocks ( 211 ,  212 ,  213 ), according to the following expression: 
         [0000]        P   tot =Σ( UMCP   —   P   1    . . . UMPC   —   P   u )+Σ( UMCP   —   P   U+1    . . . UMCP   —   P   n )
 
         [0079]    The selected active power reserve setpoint as a percentage, % P res P_UC, is converted into absolute units in the block ( 205 ), in accordance with the following equation: 
         [0000]        P   totres =%  P   res     —     UC·P   tot    
         [0080]    In other embodiments, this power value in absolute units could be given as a direct setpoint. 
         [0081]    The UC ( 104 ) calculates the average value of the active power supplied by the UMCPs under reserve mode—MR—through the block ( 213 ) (  P   u ) and under observer mode—MO—through the block ( 214 ) (  P   n-u ). These values are used to determine the estimated current active power reserve in the installation, through the blocks ( 216 ,  217 ), according to the equation: 
         [0000]        est   P   res =(   P     n-u     P     u )· u  
 
         [0082]    The figure also includes the existence of a controlled load that consumes part of the active energy generated by the photovoltaic installation (P cc ). Said controlled load allows the use of the active power reserve. The value of the in controlled load (P m ) is subtracted from the total active power reserve (P totres    1  the block ( 218 ), obtaining the active power reserve value (P res ), according to the following expression: 
         [0000]    
       
      
       P 
       res 
       =P 
       totres 
       −P 
       cc  
      
     
         [0083]    The estimated active power reserve ( est P res ) is subtracted from the setpoint P res  in the block ( 207 ), giving the error (ε_P res ), according to the following expression ( 207 ): 
         [0000]      (ε —   P   res   =P   res − est   P   res )
 
         [0084]    This error is introduced into a controller ( 208 ) which can be a PI (proportional-integral) or any other kind of controller. The output of said controller is applied to a limiter block ( 209 ) which limits it in accordance to several criteria, such as for example, the characteristics of the UMCPs, the number of UMCPs operating under MR mode and the total active power produced by the installation so that it does not exceed the established limit active power. The limited output is the maximum percentage of nominal active power that must not be exceeded by the UMCPs operating under MR mode (% P n     —   MR), 
         [0085]    The number of UMCPs operating under MR and MO mode is selected at a given time based on the operating conditions of each of the UMCPs through the block ( 215 ). When the value % P n     —   MR descends from a certain threshold, the UC ( 104 ) passes at least one UMCP from MO mode to MR mode. The selection of the UMCPs operating under MO mode is carried out so that the sample is representative of the installation (UMCP_M 1  . . . UMCP_M n ). 
         [0086]    The operating mode of each UMCP and the value of % P n     —   MR are transmitted to the UCL through the communications network ( 105 ). The UCL is in charge of fulfilling the limitation established by the % P n     —   MR, governing the operation of each UMCP. 
         [0087]    In another preferred embodiment, at least one photovoltaic cell calibrated so as to estimate the active power available in the installation is incorporated in the system, which allows a reduction in the number of UMCPs operating under MO mode. 
         [0088]    In another preferred embodiment applicable to installations consisting of photovoltaic generators disposed on solar trackers, the UC ( 104 ) determines the position of the different trackers to obtain the active power reserve required based on the equations that govern the irradiance incident upon the generator in accordance with its orientation with respect to the sun. 
         [0089]      FIG. 4  shows an example of operation applicable to UMCPs wherein input voltage is controlled. Here, the UMCPs under MO mode operate at the maximum power point (MPP) ( 401 ), while the UMCPs under MR mode operate at a voltage ( 402 ) so as to enable the power limitation established by the control method (active power reserve). 
         [0090]      FIG. 3  represents a preferred embodiment for controlling the reactive power of the photovoltaic installation. In said invention, the active and reactive power of each of the CEs is monitored and the reactive power setpoint of each is determined. 
         [0091]    The reactive power setpoint (Q ref     —   UC) can be defined as a percentage of the active power of the installation (% Q ref ) or as an absolute value. Said setpoint is selected in the block ( 304 ) in accordance with several criteria, which may be:
       a value established by the power grid operator (Q ref     —   RED);   a value generated in the UC ( 104 ) (Q ref     —   OPT) in accordance with several criteria predefined in a block ( 303 ), based on an optimization algorithm. This algorithm could take into account parameters represented by a block ( 302 ), such as production optimization, grid stability at a given time, bonuses on account of the active power reserve, tariff information, profit optimizations, etc.,   a value generated in the UC ( 104 ) based on grid voltage (V red ). In this case, grid voltage (V red ) is subtracted from the reference voltage (V red ,  ref ) in a block ( 301 ), obtaining a voltage error. This error is applied to a controller included in the block ( 301 ) to obtain the reactive power setpoint (Q ref     —   VOL).       
 
         [0095]    Subsequently, the UC ( 104 ) determines the reactive power that must be supplied by each of the CE (CE_Q 1ref  . . . CE_Q mref ). This distribution is carried out in a coordinated manner between the different CEs through a block ( 305 ) that executes an optimization algorithm which takes into account:
       The active power of each CE (CE_P 1  . . . CE_P m ).   The reactive power of each CE (CEQ 1  . . . CEQ m ).   The effort required from each (proportion of reactive power produced with respect to its capacity), for the purpose of reducing the electrical stress of the CEs.       
 
         [0099]    The optimization algorithm also takes into account parameters such as capacity to respond to the different CEs, etc. 
         [0100]    The values of CE_Q 1ref  . . . CE_Q mref  are transmitted to the different CEs through the communications network ( 105 ). 
         [0101]    In a preferred embodiment, the CE may include a fast voltage regulation loop to modify the setpoint received from the UC in order to maintain the voltage at the exit of the CE within the established limits. 
         [0102]    In other embodiments, reactive power can be controlled independently in each of the CEs, following the established criteria.