Abstract:
Power inverters are controlled in response to reactive power support commands received from a power grid such that at least one power inverter provides reactive power to the power grid. The reactive power provided is based on each power inverter&#39;s reactive power capacity only while the total power capacity of each power inverter providing the reactive power is not exhausted in generating real power.

Description:
BACKGROUND 
       [0001]    This invention generally relates to renewable energy sources, and more particularly, to systems and methods for controlling power in renewable energy sources. 
         [0002]    Solar, wind and hydro power generation systems are generally categorized as renewable variable power generation systems because the energy is harvested from naturally occurring and plentiful sources of energy. However, the amount of power produced by such systems may be unstable. The power generated by a solar farm for example, may vary as a function of cloud coverage and the position of the sun in the sky. Such a solar farm may have a plurality of energy harvesting panels with associated photovoltaic cells and inverters that may require power monitoring and control for coordinating and providing power to an electrical grid. For example, a utility may monitor the grid power demand and may need to communicate with the solar farm to determine if the solar farm has the capacity to meet some or all of the power demand. 
         [0003]    It is common to connect many small solar inverters to the electrical grid, making the collection of inverters appear as one power plant. Electrical power grids generally require both a source of active power and a source of reactive power. Active power (Watts) is generally provided to the electrical grid based upon the capacity of each power generation system inverter. Reactive power (VARs) is generally proportioned equally among the inverters based upon the number of inverters in the power generation system. Such proportioned reactive power generation disadvantageously results in a loss of expensive active power from highly productive inverters due to equal sharing. 
         [0004]    In view of the foregoing, there is a need for a power inverter system and method of operation that distributes reactive power support from among multiple power inverters, favoring those inverters that have remaining capacity, rather than dividing the total VAR support equally among all the inverters, especially those inverters operating at the maximum rated output power. 
       BRIEF DESCRIPTION 
       [0005]    One embodiment of the present disclosure is directed to a power inverter system, comprising: 
         [0006]    one or more power inverters configured to receive power from at least one corresponding energy source; and 
         [0007]    at least one controller configured to control each power inverter via algorithmic software reactive power support commands such that at least one power inverter provides reactive power to a power grid based on its reactive power capacity and only while its total power capacity is not exhausted in generating real power 
         [0008]    Another embodiment of the present disclosure is directed to a method of operating a power inverter system, the power inverter system comprising one or more power inverters receiving power from at least one energy source and further comprising at least one power inverter controller, the method comprising: 
         [0009]    polling each power inverter to determine its reactive power (VAR) capacity; and 
         [0010]    controlling each power inverter in response to power inverter controller VAR support commands such that each power inverter having reactive power capacity provides reactive power to a power grid based on its reactive power capacity and only while its total power capacity is not exhausted in generating real power. 
     
    
     
       DRAWINGS 
         [0011]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawing, wherein: 
           [0012]      FIG. 1  illustrates the capacity of a power inverter to support reactive power and active power according to one embodiment; and 
           [0013]      FIG. 2  illustrates a solar farm configured to support reactive power and active power according to one embodiment. 
       
    
    
       [0014]    While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
       DETAILED DESCRIPTION 
       [0015]      FIG. 1  illustrates the capacity of a power inverter ( 10 ) to support reactive power (VARS) and active power (Watts) according to one embodiment. Power inverter  10  may be part of a solar, wind, battery or hydro power generation system. Assuming power inverter  10  provides power to an electrical grid based on solar power, it can be seen that power inverter  10  has a total power capacity that comprises a reactive power capacity  12  and an active power capacity  14 . The active power capacity  14  is less than the total power capacity of the inverter  10  since one or more solar panels, described in further detail herein, from which the power inverter  10  draws its power, is generating less than its optimum power due to solar panel damage, inclimate weather, and/or solar panel debris build-up, and so on. 
         [0016]    Although the embodied power inverter system  100  is described herein with reference to a solar farm  102 , solar power inverters  108 ,  110 ,  112 ,  114  and solar panels  104 , other power inverter system embodiments using wind, battery and/or hydro power elements can just as easily be implemented in accordance with the principles described herein. Solar farm  102  for example, may comprise a wind farm, a hydro farm, and/or a battery farm. Solar power inverters  108 ,  110 ,  112 ,  114  may just as easily comprise wind power inverters, battery power inverters, and/or hydro power inverters. Solar panels  104  may just as easily comprise wind turbines, batteries, and/or hydro plants/turbines. 
         [0017]      FIG. 2  illustrates power inverter system  100  comprising a solar farm  102  configured to support reactive power and active power according to one embodiment. Solar farm  102  comprises a plurality of solar power inverters  108 ,  110 ,  112 ,  114 . Power inverters  108 ,  110  and  114  are each providing active power to an electrical power grid  120  based upon available power generated via corresponding power sources (solar panels)  104 . Solar panels  104  are generating less than optimal solar energy that may be a result of panel damage, inclimate weather, and/or solar panel debris build-up, and so on, as stated herein. Power inverters  108 ,  110  and  114  therefore are each generating active output power that is less than its respective optimal power generation capacity. Each power inverter thus has reserve capacity to also provide VAR power to the electrical power grid  120 . 
         [0018]    Power inverter  112  however, is providing its optimal active power to the electrical power grid  120  since its corresponding solar panel  106  is generating its maximum solar energy from which power inverter  112  draws its power. Since the optimal active power of power inverter  112  usurps its entire power generation capacity, power inverter  112  has no power capacity reserve to generate VAR power to the electrical grid  120 . 
         [0019]    Power inverter system  100  can also be seen to comprise a central controller  130 . Controller  130  continuously polls the plurality of power inverters  108 ,  110 ,  112 ,  114  to determine the present real power capacity and present reactive power capacity of each power inverter. Upon receipt of a VAR command via a power grid feedback link  132 , the controller  130  proceeds to request VAR support from power inverters  108 ,  110 ,  114  with sufficient capacity to support generation of VAR power. Since power inverter  112  is already generating power at its maximum capacity, power inverter  112  is not requested to provide VAR power. VAR power is instead requested only from power inverters  108 ,  110  and  114 . According to one embodiment, VAR power is requested from power inverters  108 ,  110  and  114  in proportion to the amount of VAR power capacity available from each power inverter. Thus, for example, if power inverter  110  has twice the VAR capacity available as that available from power inverter  108 , then power inverter  110  is request to supply twice the VAR power as that requested from power inverter  108 . This method of operation ensures that power inverters with higher real power production do not become curtailed due to the assumption that VAR power requests should be shared equally among the plurality of power inverters  108 ,  110 ,  112  and  114  operating in response to the central controller  130 . 
         [0020]    Although power inverter system  100  comprises a central controller  130 , other embodiments may comprise a distributed control architecture using the principles described herein. A distributed control architecture may, for example, employ a control structure that is distributed about each inverter such that communications are accomplished via a mesh or neural type network. A distributed control structure provides advantages over a centralized control structure in that failure of a central controller results in overall system failure; whereas failure of a single inverter controller in a distributed control structure does not adversely impact the operation of the inverters still operating under localized control in the distributed control structure. 
         [0021]    In summary explanation, a power inverter that operates together with a set of solar panels that have become damaged or covered by debris would have a difficult time extracting rated power from those solar panels, and therefore will have remaining power capacity. Such power inverters would then be favored during a VAR support request. According to one embodiment, a central controller responsive to algorithmic software would be configured to request VAR power in proportion to the VAR power capacity available from each power inverter or in inverse proportion to the real power being generated from each power inverter. A power inverter system configured to operate in response to such VAR requests thus maximizes the respective farm real power output while still supporting VAR requests. According to one aspect, a power inverter is configured to adjust the level at which VAR demand, e.g. from the plant control, takes priority over kW demand. By default, kW will always take priority, and VAR demand will not be fulfilled unless kW available is less than 100%. The level adjustment is implemented according to one embodiment as a power factor at which kW has priority, i.e. if this adjustment were PF=0.9, then kW will take priority until the point at which the VAR demand causes the inverter power factor to fall below 0.9. 
         [0022]    According to another embodiment, a plurality of VAR banks  140  that may comprise, for example, a plurality of shunt capacitors and/or a plurality of series reactors are integrated with the solar farm  102  and in electrical communication with one or more power grid  120  substations. Upon receipt of a VAR command via the power grid feedback link  132 , the controller  130  proceeds to request VAR support back to the power grid substations for controlling the VAR banks  140  by switching shunt capacitors and series in or out of the system in a fashion familiar to those skilled in the art. Controller then  130  proceeds to request additional VAR support from power inverters  108 ,  110 ,  114  with sufficient capacity to further refine the generation of requested VAR power. Fine tuning VAR power in this manner avoids wasted VAR power while more closely meeting the desired VAR requirements. Fine tuning VAR power in this manner further allows the power inverter system  100  to support VAR power requests in the event requested VAR power is deficient solely through use of inverters  108 - 114 , e.g. when all inverters  104 ,  106  are running at full capacity. VAR power control in the absence of such inverter control techniques is well known in the VAR control system art, and so further details regarding techniques for implementing capacitor and reactor switching to support VAR requests will not be discussed in further detail herein. 
         [0023]    According to another embodiment, a central controller  130  or a plurality of distributed inverter controllers are configured to control each power inverter  108 - 114  such that reactive power is provided during periods of darkness to support sun free reactive power. Each inverter  108 - 114  thus comprises a dedicated inverter controller that may operate in combination with or independently of a central controller  130 . Providing sun free reactive power according to one aspect is implemented by ensuring an inverter is fully loaded prior to requesting sun free reactive power from another inverter having VAR capacity. This technique promotes increased inverter efficiencies since the inverters  108 - 114  are generally designed to function at or near a designed operating point. 
         [0024]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.