Patent Publication Number: US-2013234523-A1

Title: Method and apparatus providing point of interconnection control for power plants

Description:
FIELD OF THE INVENTION 
     The disclosed embodiments relate to control systems for power plants, and methods of using the same. 
     BACKGROUND OF THE INVENTION 
     Energy can be derived from many different sources, including, but not limited to, photovoltaic (“PV”) devices, wind turbines and geothermal sources. Derived energy can be collected from power plants, conditioned and then coupled to an electrical network such as a utility grid. An example of a power plant is a photovoltaic (PV) power plant containing an array of electrovoltaic devices, such as photovoltaic modules and associated interconnected electrical wires and devices such as inverters. 
     PV devices can be networked together to form a power plant such as a PV power plant. A large PV power plant may include hundreds of thousands of square feet of PV devices covering many acres. The PV devices are dispersed so as to maximize the power plant&#39;s energy-collecting capability. Energy collected by each PV device is generally pooled to one or more power converters through a number of collector cables or buses. The power converters typically include DC/AC inverters which convert direct current to alternating current for use on a coupled utility grid. The utility grid is coupled to the PV power plant via one or more power lines. The point at which the PV power plant is connected to a utility grid is referred to as a point of interconnection, or POI. Transmission lines or buses are on the utility grid side of the POI while collector lines or buses are on the PV power plant side of the POI. 
     Because the utility grid requires that supply and demand of provided electricity be carefully balanced, there is a need for a robust control of PV power plant output into the utility grid. When demand is high from the utility grid, the PV power plant may be required to increase its active power output capacity to the available generation capacity. At times of low demand, the PV power plant active power output capacity may be required to decrease. Control for the necessary increase or decrease in active power output may be facilitated by a POI control module. 
     Unlike some traditional power plants (e.g., coal, nuclear) where electricity generation at the power plant is generally constant over an extended period of time, a PV power plant is subject to significant variations in electricity output levels due to frequent disturbances in the solar resource. A passing cloud, for example, can temporarily reduce the generating power of the PV power plant by a significant amount. The POI control module is used to ensure that the active power output delivered to the utility grid does not exceed an operator provided limit when required. To the extent compensation for the fluctuations in power generation is possible, such compensation is preferably also under the control of the POI control module. 
     Therefore, the POI control module plays a vital role in a PV power plant&#39;s ability to limit active power output that meets the demands of the coupled utility grid. In addition, the POI control module also regulates voltage, power factor, or reactive power at the POI to meet the demand of the grid. The POI control module achieves this by manipulating reactive power production of the plant and controlling capacitors and inductors if the plant is equipped with them. 
     However, there may be conflicting requirements for power control for the POI control module which can be difficult to manage. For example, the voltage control must be managed within the limits of the reactive power capability of the PV plant. If the grid voltage is too high or too low, the reactive power provided by the plant will reach its limit. In addition, the POI control module must ensure that the voltage levels within the plant (e.g., at an inverter terminal) do not exceed their allowable limits. Also, the POI control module must manage the tradeoff between active and reactive power production if required. Accordingly, a POI control module which manages a PV power plant output (both active and reactive), while handling sometimes conflicting control requirements, is needed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an embodiment of a power plant control system. 
         FIG. 2  illustrates an embodiment of a point-of-interconnection control module. 
         FIG. 3A  illustrates an embodiment of a voltage controller. 
         FIG. 3B  illustrates an embodiment of a voltage droop profile used by an embodiment of a control module. 
         FIG. 4  illustrates an embodiment of a reactive power controller. 
         FIG. 5  illustrates an embodiment of a power factor controller. 
         FIG. 6  illustrates an embodiment of a method of switching controllers in a power plant control system. 
         FIG. 7  illustrates an embodiment of a point-of-interconnection control module at a power plant. 
         FIG. 8  illustrates an embodiment of a method of adding and removing static devices in a power plant control system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. It should be understood that like reference numbers represent like elements throughout the drawings. Embodiments are described with sufficient detail to enable those skilled in the art to practice them. It is to be understood that other embodiments may be employed, and that various structural, logical, and electrical changes may be made without departing from the spirit or scope of the invention. 
     Disclosed herein is an apparatus providing an improved POI control module for a PV power plant and methods of using the same. The POI control module (“PCM”) is configured to manage the handling of all POI conditions that may be imposed, for example, by the connected utility grid. To do this, and as further explained below, the PCM is configured to provide automatic voltage regulation, reactive power regulation and power factor regulation as well as limiting active power when required. In addition, if the PV power plant includes capacitor banks or inductor banks for reactive power support, the PCM is configured to manage these static devices in order to add or take away reactive power generating capacity, if necessary, to augment the required reactive power. The PCM interfaces with a PV plant control module in order to coordinate control of the various components of the PV power plant. 
     The basic functions of the PCM are described below with reference to a control system  100  illustrated in  FIG. 1 . The control system  100  in  FIG. 1  illustrates control interconnections made between various control components located both at and away from a PV power plant. In control system  100 , a PCM  110  is located at or near a POI  50  between a utility grid  52  and a PV power plant  54 . PCM  110  receives various set points from, for example, an energy management system (“EMS”)  122 . The set points can include specific output requirements for the PV power plant  54 , such as commands for specific power output, and/or voltage level, or power factor set point. Alternatively, the set points can be provided to the PCM  110  via a power plant side local supervisory control module  160  such as a local supervisory control and data acquisition (“SCADA”) system with a human-machine interface (“HMI”). As explained below, the local supervisory control module  160  can be used to override the set points received from the EMS  122 . 
     The PCM  110  also receives information regarding the up-to-date voltage and current levels associated with the POI  50  and other areas of the power plant. The voltage and current levels may be measured, for example, from several locations including potential transformers and current transformers located at either transmission buses  131  on the utility grid  52  side, at collector buses  132  on the power plant  51  side, or at an interconnecting circuit breaker  740  located at the POI  50 . Transmission buses  131  deliver power to the utility grid  52  from the POI  50 . Collector buses  132  deliver power from the PV power plant  54  to the POI  50 . In addition to voltage and current, other parameters may be measured from the buses  131 ,  132  or circuit breaker  740 , such as AC frequency and amounts of active and reactive power delivered to the transmission buses  131 , or the collector buses  132  or passing through circuit breaker  740 . 
     The PCM  110  uses the set point inputs from the EMS  120  or the local supervisory control module  160  and also receives the present-value voltage and other parameter measurements from the POI buses  131 ,  132  or circuit breaker  740  to determine the output requested of the PV power plant  54 . Once determined, the PCM  110  sends an output command to a PV plant control module  140 , which functions to enforce the received command by regulating the output of a plurality of inverters  150  which are connected to the PV devices  152  in the PV power plant  54 . Each inverter  150  may connect with a plurality of individual PV devices  152 . Inverters  150  may connect directly with the plurality of individual PV devices  152 , or may alternatively connect via one or more DC/DC collectors  154 . The PV plant control module  140  can regulate both the output of the plurality of inverters  150  and the one or more DC/DC converters  154 . 
     In another representation,  FIG. 2  illustrates the PCM  110  and the entities to which the PCM  110  interconnects. Unlike  FIG. 1 , which primarily illustrates control interconnections between components of the POI control system  100 ,  FIG. 2  illustrates both control and power interconnections in greater detail. In  FIG. 2 , the PCM  110  is coupled to at least one measuring device  710 , which in turn is coupled to transmission buses  131 , collector buses  132  and the circuit breaker  740 . The PCM  110  is also coupled to a PV plant control module  140 . Additionally, the PCM  110  is in communication with either the PV plant&#39;s local supervisory control module  160  such as a SCADA system, or an external power substation with an energy management system or EMS  122 . The power interconnections in  FIG. 2  are represented by the transmission buses  131 , which are coupled to the collector buses  132  via the circuit breaker  740 . The collector buses  132  receive power from the inverters  150 . 
     The inverters  150  are each connected to a plurality of arrays of PV devices  152 , often via one or more DC/DC converters  154 . Energy collected at each PV device array  152  is directed and channeled through a series of cables to the one or more converters  154  and inverters  150 . At the DC/DC converters  154 , the generated power is collected into higher-voltage cables. At the inverters  150 , the generated power is boosted or decreased and regulated so as to be at a stable known amount of power. A PV power plant may include a hierarchy of converters  154  and inverters  150 , with higher-level DC/DC converters outputting higher voltages than lower-level DC/DC converters. At the highest level of the hierarchy are one or more central power converters that are generally in the form of DC/AC inverters  150 . These central power converters convert the direct current delivered by the lower-level converters  154  to alternating current for use on the coupled utility grid  52 . Because each converter  154  and inverter  150  in the PV power plant can be controlled to boost or decrease the output power, the power output of the PV power plant is determined by the control signals received by the converters  154  and inverters  150  (collectively, power regulators). In  FIG. 2 , the converters  154  and inverters  150  represent all of the power regulators in a PV power plant that can receive a control signal. 
       FIG. 2  also illustrates additional detail with respect to the PCM  110 . In  FIG. 2 , the PCM  110  includes a master control module  750  and a plurality of controllers  220 ,  300 ,  400  and  500 . As is explained in greater detail below, the master control module  750  functions to enable one of the controllers  300 ,  400  and  500  to output a command to the PV plant control module  140 . Controller  220  within PCM  110  also outputs a command to the PV plant control module  140 , but is continually enabled, whereas only one of controller  300 ,  400  and  500  is enabled at any given moment. Commands are output by the controllers  220 ,  300 ,  400  and  500  at the direction of the master control module  750  and in response to inputs received by each controller. Controllers  220 ,  300 ,  400  and  500  receive as inputs measurements from the at least one measuring device  710 . Controllers  220 ,  300 ,  400  and  500  and master control module  750  also receive as inputs set points received via local control from the PV power plant&#39;s SCADA system  160  or via remote control from a utility grid&#39;s EMS  122 . For example, when responding to remote control, the controllers  220 ,  300 ,  400  and  500  in the PCM  110  respond to external set points issued from an EMS  122 . The EMS  122  is used by a utility grid  52  (or the generation company) to determine the energy needs of the grid. Those needs are then translated to set points that are sent from the EMS  122  to the controllers  220 ,  300 ,  400  and  500  and the master control module  750  in PCM  110 . The PCM  110  also relays the received set points via, for example, the master control module  750 , to the local PV power plant SCADA system  160  so that the PV power plant can monitor both the set points received by the PCM  110  and the response of the PV power plant. 
     When the PCM  110  is responding to local control by the PV power plant&#39;s SCADA system  160 , an operator at or remotely operating through the SCADA system  160  switches the PCM  110  from external control to local control. Local control allows a local PV power plant operator to override external set points from EMS  122 , ensuring that demands made of the PV power plant are consistent with the PV power plant&#39;s goals and safe operating criteria. The PCM  110  operates under local control until an operator at or remotely operating through the PV power plant&#39;s SCADA system  160  switches the control back to EMS  122 . Control can also be switched automatically in response to pre-defined conditions. 
     The controllers  220 ,  300 ,  400  and  500  in PCM  110  receive measured parameters from measuring device  710  as inputs. The measured parameters received by a controller may include voltage measurements, reactive power measurements, power factor measurements as well as active power measurements. The controllers  220 ,  300 ,  400  and  500  use the received parameters to determine commands to output to the PV plant control module  140 . At any given moment, controller  220  is active but only one of controllers  300 ,  400  and  500  is enabled to output a command, as dictated by the master control module  750 . 
     Controller  220  in PCM  110  is an active power limit controller. The active power limit controller  220  outputs a command to limit the amount of active power produced by the PV power plant. The active power limit controller&#39;s output is in response to an input maximum active power set point set by the PV power plant&#39;s SCADA system  160 , for example. Thus, using the active power limit controller  220 , the PCM  110  is enabled to receive a maximum active power set point and output to the PV plant control module  140  an active power command that will result in limiting the active power output from the PV power plant to no more than the input maximum active power set point. 
     In addition to commands output by the active power limit controller  220 , commands are also output by one of controllers  300 ,  400  and  500 . Although the controllers  300 ,  400  and  500  respond to various set points and parameters, each controller outputs similar types of commands. That is, each controller outputs a command for a desired amount of reactive power from PV power plant  54 . For example, and as explained in greater detail below, controller  300  is a voltage controller. Voltage controller  300  receives a set point requiring a specific voltage at either a transmission bus  131 , a collector bus  132  or a circuit breaker  740 . The voltage controller  300  also receives a measured parameter indicating to the controller  300  the voltage at the transmission bus  131 , collector bus  132  or circuit breaker  740 . The voltage controller  300  uses this information to output a command to the PV plant control module  140  for a required amount of reactive power to be output from the PV power plant. The required amount of reactive power will result in the desired voltage at the measured transmission bus  131 , collector bus  132  or circuit breaker  740 . Thus, controllers  300 ,  400  and  500  may be required to compute a necessary amount of reactive power based on the received set point. 
     Thus, using the voltage controller  300 , the PCM  110  is enabled to receive a voltage set point and output to the PV plant control module  140  a reactive power command that will result in a corresponding reactive power output from the PV power plant which will achieve the input voltage set point as long as the active power limit monitored by controller  220  is not exceeded. The reactive power output from the PV power plant results in a stable voltage at the POI that is at or within a predefined range of the required voltage set point. 
       FIG. 3A  illustrates the voltage controller  300 . The voltage controller  300  receives as input a voltage set point Vsp. The voltage set point Vsp is summed by summer  310  with a droop voltage signal Vdr from droop voltage source  340 . The droop voltage signal Vdr is provided in order to enable the PV power plant output to be more stable. In essence, the PV power plant has additional resistance built-in to the plant that is used to compensate for sudden changes in the PV power plant&#39;s load. In an uncompensated circuit, a sudden change in load will cause the output voltage to temporarily droop or sag. A circuit that is compensated with additional resistance is less susceptible to voltage droop. However, this means that the voltage to be output by the inverters of the PV power plant is actually different than the received set point voltage. Thus, in voltage controller  300 , a droop voltage signal Vdr provided by droop voltage source  340  is added to the voltage set point Vsp at summer  310  in order to derive a target voltage Vtg. 
     At comparator  320 , the target voltage Vtg is compared with the voltage V measured at a bus at the POI. If the measured voltage V is within a predefined range or limit LIM of the target voltage Vtg, then the comparator  320  outputs an OFF signal to PID controller  330 , and the voltage controller  300  does not output a command. In other words, because the measured voltage V and the target voltage Vtg are close to each other, no change in output from the PV power plant is necessary. However, if comparator  320  determines that the measured voltage V is not within a predefined range or limit LIM of the target voltage Vtg, then the comparator  320  outputs an ON signal to PID controller  330 , ultimately resulting in the PID controller  330  outputting a reactive power command VARcmd to plant control module  140 . 
     PID controller  330  includes a proportional, integral and derivative (“PID”) control loop, as is known in the art. PID controller  330  accepts as inputs the target voltage Vtg and the measured voltage V. Modeling a closed loop feedback system, the PID controller  330  outputs a VARcmd command to the plant control module  140  in order to control the PV power plant inverters and/or converters to provide an output which will produce a reactive power voltage within the limit specified by comparator  320 . This means that, in addition to providing a closed loop feedback system, PID box  330  also converts the received voltages into a desired reactive power. 
     The output reactive power command VARcmd is output from the voltage controller  300  and may be output to the PV plant control module  140  of  FIGS. 1 and 2 , under the direction of the master control module  750 . The reactive power command VARcmd is also returned as feedback to a droop voltage source  340  in controller  300  to provide an appropriate droop voltage Vdr to be summed with the received voltage set point Vsp. 
     The droop voltage Vdr is generated by the source  340  in accordance with a droop voltage profile  200 , illustrated in  FIG. 3B . In the profile  200 , a voltage droop relationship between voltage V (on the y-axis) and reactive power VAR (on the x-axis) is shown. For a known reactive power VAR, the profile  200  shows the corresponding voltage V that compensates for possible voltage droop. In the profile  200 , a reactive power VAR with a high magnitude is capped at a droop voltage V so as to ensure that voltages in the PV power plant are not driven beyond rated limits. The droop voltage profile  200  is an example of a possible droop voltage profile; other profiles may be used depending on the design of the PV power plant. 
     The power command VARcmd, which is a request or command sent to the PV plant control module  140  to change the amount of reactive power being generated. The reactive power command VARcmd indicates the amount of reactive power that is required. The reactive power command VARcmd is communicated to the PV plant control module  140  so that the required amount of reactive power is output from the inverters  150 . Thus, the reactive power command VARcmd results in an update to the output voltage V at the POI that is within a predefined limit LIM of the voltage set point Vsp. 
     The PCM  110  also includes a reactive power controller  400 . As with the voltage controller  300 , the reactive power controller  400  is responsive to a received set point by determining a necessary reactive power output from the PV power plant. The received set point is a reactive power set point, and the reactive power controller  400  outputs a command to the PV plant control module  140  that results in a stable reactive power at the POI that is at or near the required reactive power set point. 
     The reactive power controller  400  is illustrated in  FIG. 4 . The reactive power controller  400  receives as an input a reactive power set point VARsp. The reactive power set point VARsp is received either locally from the PV power plant&#39;s SCADA system or from an external EMS  122  at, for example, power substation  120 . At summer  410 , the reactive power set point VARsp is summed with a reactive power droop signal VARdr. As with the voltage controller  300 , a droop signal from droop signal source  440  is added to the set point signal in order to compensate for any expected voltage droop that could occur as a result of changes in PV power plant load. In reactive power controller  400 , the reactive power droop signal VARdr is determined using the droop voltage profile  200  of  FIG. 3B . The voltage V at a transmission bus  131 , collector bus  132  or circuit breaker  740  at the POI is input to droop signal source  440 , and, in accordance with the droop profile  200 , box  440  outputs a reactive power droop signal VARdr. 
     At summer  410 , the reactive power set point VARsp and the reactive power droop signal VARdr are summed to yield a reactive power target signal VARtg. At comparator  420 , the reactive power target signal VARtg is compared with the PV power plant&#39;s output reactive power VAR, as measured at the transmission bus  131 , collector bus  132  or circuit breaker  740  at the POI. If the measured reactive power VAR is within a predefined range or limit LIM of the reactive power target signal VARtg, an OFF signal is output to the PID controller  430 , and the controller  400  outputs no command. If, however, the measured reactive power VAR differs from the reactive power target signal VARtg by more than the predefined range or limit LIM, then the comparator  420  outputs an ON signal to the PID controller  430 . 
     The PID controller  430  accepts as inputs the measured reactive power VAR and the reactive power target signal VARtg and applies them to a PID closed loop feedback system to output a reactive power command VARcmd. The reactive power command VARcmd indicates the total amount of reactive power that is required from the PV power plant to obtain the reactive power set point VARsp within the limit LIM set by comparator  420 . The reactive power command VARcmd is communicated to the PV plant control module  140  so that the PV plant control module  140  can order the determined amount of reactive power from the inverters  150  and/or converters  154 . 
       FIG. 5  illustrates the power factor controller  500 , located within PCM  110 . As with the other controllers, the power factor controller  500  is responsive to a received power factor set point and determines a necessary reactive power output command VARcmd from the PV power plant. The output command VARcmd results in a stable power factor at the POI that is at or near the required power factor set point. Power factor, or PF, is a ratio between active power and apparent power. In an electric power system, a system with a low power factor draws more current than a system with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will often charge a higher cost to industrial or commercial customers where there is a low power factor. Thus, both utilities and the PV power plant may have a motivation to set a power factor set point. 
     The power factor controller  500  receives as input a power factor set point PFsp, which is set locally from the PV power plant&#39;s SCADA system  160  or remotely from an external EMS  122  at, for example, power substation  120 . At comparator  510 , the received power factor set point PFsp is compared with the PV power plant&#39;s output power factor PF, as determined through measurements at either a transmission bus  131 , a collector bus  132  or a circuit breaker  740  at the POI. If the measured power factor PF is within a predefined range or limit LIM of the power factor set point PFsp, the comparator  510  outputs an OFF command to the PID controller  520 , and no command is output by the power factor controller  500 . If, however, the measured power factor PF is not within the predefined range or limit LIM of the power factor set point PFsp, the comparator  510  outputs an ON command to the PID controller  520 . The PID controller  520  uses the measured power factor PF and the power factor set point PFsp as inputs and applies a closed loop feedback system to determine an output command VARcmd. The PID controller  520  converts the power factor inputs into a reactive power command output VARcmd. The reactive power command VARcmd indicates the total amount of reactive power that is required from the PV power plant in order to maintain the required power factor at the POI. The reactive power command VARcmd is communicated to the PV plant control module  140  so that the PV plant control module  140  can order the determined amount of reactive power from the inverters  150  and/or the converters  152 . 
     The plurality of active power limit controllers  220 , voltage controllers  300 , the reactive power controllers  400  and the power factor controllers  500  are implemented by the PCM  110 . The PCM  110  may include various numbers of each controller in order to correspond to the numbers of transmission and collector buses, as well as for circuit breaker  740 . 
     The different controllers in the PCM  110  each have distinctly different goals (e.g., meeting a voltage set point, meeting a reactive power set point, or meeting a power factor set point). The different goals of each controller can potentially result in contradictory commands arising from the PCM  110  if each of controllers  300 ,  400  and  500  were to operate simultaneously and independent of each other. For example, the voltage controller  300 , which is designed to maintain a specific voltage on a transmission bus, can potentially output a reactive power command VARcmd that results in a large amount of reactive power generation by inverters connected to a collector bus, thus resulting in a collector bus overvoltage condition. If a separate voltage controller  300  was maintaining a voltage on the collector bus  132 , the two voltage controllers could be in conflict with each other. As another example, a reactive power controller  400  maintaining a specific reactive power on a transmission bus  131  could result in the transmission bus power factor shifting beyond a specified control range. Thus, in this example, the reactive power controller  400  for the transmission bus could be in conflict with a power factor controller  500  for the same transmission bus. 
     Therefore, in order to avoid such conflict, the PCM  110  includes the master control module  750  that coordinates the actions of the controllers  300 ,  400  and  500 . The master control module  750  may include software or hardware and may be a combination thereof The master control module  750  outputs overriding control signals to the controllers  300 ,  400  and  500  to enable or disable the controllers. Under the master control module  750 , the operations of each of the PCM&#39;s controllers  300 ,  400  and  500  are coordinated so that only one controller is active at any given time and is able to output a reactive power command VARcmd. The active controller  300 ,  400 ,  500  is selected by an operator providing an input to master control module  750  using either remote or local control. Alternatively, the active controller is selected by the PCM  110  in accordance with priorities established by an operator or that are predefined. The active one of controllers  300 ,  400 ,  500  remains active, while the other controllers  300 ,  400 ,  500  each monitor respective parameters. However, if a controller identifies that its monitored parameter is approaching an upper or lower limit or is shifting from a corresponding set point by more than a predetermined limit LIM, the controller sends a signal to the master control module  750  and then the master control module  750  may require that the active controller become inactive and that the controller that identified the shifting parameter becomes the new active controller. Once the parameters being monitored by the new active controller are returned to within an allowable range of the monitored set point, the master control module  750  may return control to the previous active controller. 
     In order to justify switching controllers, a controller&#39;s parameter must either exceed a set threshold or shift beyond a predefined range LIM bounding the controller&#39;s respective set point. The predefined ranges or thresholds are stored and used by comparators  320 ,  420  and  510  in controllers  300 ,  400  and  500 , respectively. Multiple limits or alarms may be configured for any given controller, if desired. 
     Time limits or deadbands can also be set that prevent frequent controller switching, if desired. For example, a minimum time limit can be set that prevents controllers from switching too quickly after a previous switch. Additionally, a range of values may be set for each controller such that a variation of the controller&#39;s monitored parameters within the deadband or range will not result in controller switching. Time limits or deadbands can be configured and individually enabled or disabled by an operator and are stored in comparators  320 ,  420  and  510 . 
     There may be circumstances when multiple set points are received that cannot all be satisfied simultaneously. For example, it is possible that the PCM  110  could receive a power factor set point and a voltage set point that cannot both be satisfied at the same time. In such a case, the set point with a highest priority is satisfied. The lower priority set point remains unsatisfied as long as the conflicting set points exist. Priorities are programmed into the master control module  750  and may be updated by an operator. Priorities may reflect efforts to maintain the safety and integrity of the PV power plant and/or may reflect contractual agreements between the PV power plant and coupled utility grids. 
       FIG. 6  illustrates a conflict resolution method  600  applied by the master control module  750 . At step  610 , the master control module determines an active controller. The determination is made based upon received set points and operator priorities. At step  620 , a conflict is identified. The conflict could be that a parameter being monitored by a non-active controller has triggered an alarm because the parameter is shifting away from its set point (step  626 ). The conflict could also be that a set point has been received that is incompatible with, but does not replace, an existing set point (step  628 ). If the competing set points are potentially compatible, the master control module switches control to the controller that triggered the alarm (step  630 ). The newly activated controller remains active until its monitored parameter is returned to its set point, and then the master control module switches control back to the previously determined active controller (step  640 ). If, at step  620 , the competing set points are not compatible, the master control module switches control to the controller with the highest priority, as defined by an operator or according to a predefined priority ranking (step  650 ). 
     An additional feature of the PCM  110  is illustrated in  FIG. 7 .  FIG. 7  is similar to the illustration of  FIG. 2 , except that in  FIG. 7 , the PCM  110  is also coupled to static devices such as capacitor banks  810  and/or inductor banks  820  that are present at the PV power plant and which can be selectively coupled to the collector bus  132 . The capacitor and inductor banks  810 ,  820  are often present in a PV power plant in order to provide additional power resources that can be used to meet the set points received by the PCM  110 . The PCM  110  is configured to directly control the use of the capacitor and inductor banks  810 ,  820 , if present in the PV power plant, as described below. 
     In general, at a PV power plant, capacitor and inductor banks  810 ,  820  are used as slow-acting devices, meaning that the capacitor and inductor banks have slower response times than the more dynamically-responsive inverters controlled by the PV plant control module. The capacitor and inductor banks  810 ,  820  in a PV power plant are used to either extend the PV power plant&#39;s ability to provide reactive power or to preserve the dynamic control range of the PV power plant&#39;s inverters  150  and converters  154  for contingencies. In other words, by using the slow-acting static devices to provide a portion of the PV power plant&#39;s reactive power, a greater proportion of the PV power plant&#39;s inverters  150  and converters  154  may be available to act quickly to meet any sudden changes in power demands. 
     When the PCM  110  uses controllers  300  and  500  to monitor and respond to voltage and power factor set points, respectively, the PCM  110  can output commands that result in the PV plant control module  140  directing inverters  150  and converters  154  to meet the command set points. However, in response to changes in voltage and power factor set points, the PCM  110  may also issue commands to add or remove static devices such as capacitor and inductor banks  810 ,  820 . This is illustrated in method  850  of  FIG. 8 . 
     In method  850 , the PCM  110  receives a set point (step  855 ). If, as a result of a set point received at the PCM  110 , an increase in reactive power is required, meaning that the PCM  110  is boosting (step  860 ), the PCM  110  can direct that either an inductor  820  be removed or a capacitor  810  be added. The PCM  110  checks to see that no inductors  820  are currently under its control (step  865 ). If one is, the inductor is removed (step  870 ). If there is not, a capacitor is added (step  875 ). Additional inductors  820  may be removed (if already under the control of the PCM  110 ) or additional capacitors  810  may be added. 
     If, as a result of a set point received at the PCM  110 , a decrease in reactive power is required, meaning that the PCM  110  is bucking (step  880 ), the PCM  110  can direct that either a capacitor  810  be removed or an inductor  820  be added. The PCM  110  checks to see that no capacitors  810  are currently under its control (step  885 ). If one is, the capacitor is removed (step  890 ). If there is not, an inductor is added (step  895 ). Additional capacitors  810  may be removed (if already under the control of the PCM  110 ) or additional inductors  820  may be added. 
     Therefore, when the PCM  110  adds or removes a static device in response to a received set point, the PCM&#39;s own reactive power command VARcmd that is output is changed based on the compensation provided by the static devices. As is shown in step  899  of method  850 , the output reactive power command VARcmd is changed by adding or subtracting a multiple of the step size (determined by the capacitors  810  or inductors  820 ), indicating the amount of reactive power added or subtracted by the addition or removal of static devices. 
     The above description relates to embodiments wherein the PCM  110  sends reactive power commands to a separate PV plant control module  140  that then controls the output of PV power plant inverters  150  and/or converters  154 . Of course, variations such as combining the PCM  110  and PV plant control module  140  are also contemplated by the above description. Additionally, while the above description relates specifically to a PV power plant, the PCM  110  and PV plant control module  140  may be used with other types of power plants, including wind and geothermal power plants. Furthermore, the PCM  110  and PV plant control module  140  may be each implemented in either hardware or as software on a processor, or in a combination of hardware and software. As specific examples, the controllers  220 ,  300 ,  400 ,  500 , the master control module  750 , the PID controllers  330 ,  430  and  520 , and PV plant control module  140  may each be implemented in hardware, software, or a combination thereof. The above description and drawings are only to be considered illustrative of specific embodiments, which achieve the features and advantages described herein. Modifications and substitutions to specific process conditions can be made. Accordingly, the embodiments of the invention are not considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.