Patent Publication Number: US-2010117621-A1

Title: Method of load shedding to reduce the total power consumption of a load control system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional of U.S. patent application Ser. No. 11/870,889 filed Oct. 11, 2007 entitled METHOD OF LOAD SHEDDING TO REDUCE THE TOTAL POWER CONSUMPTION OF A LOAD CONTROL SYSTEM, which application claims priority from commonly-assigned U.S. Provisional Application Ser. No. 60/851,383, filed Oct. 13, 2006, and U.S. Provisional Application Ser. No. 60/858,844, filed Nov. 14, 2006, both entitled LIGHTING CONTROL SYSTEM. The entire disclosures of both applications are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a load control system comprising a plurality of load control devices for controlling the amount of power delivered to a plurality of electrical loads from an AC power source, and more particularly, to a method of shedding loads of a lighting control system in response to an estimation of the amount of power presently being consumed by the lighting control system. 
     2. Description of the Related Art 
     Reducing the total cost of electrical energy is an important goal for many electricity consumers. Most electricity customers are charged for the total amount of energy consumed during a billing period. However, since the electrical utility companies must spend money to ensure that their equipment is able to provide energy in all situations, including peak demand periods, many electrical utility companies charge their electricity consumers at rates that are based on the peak power consumption during the billing period, rather than the average power consumption during the billing period. Thus, even if an electricity consumer consumes power at a very high rate for only a short period of time, the electricity consumer will face a significant increase in its total power costs. 
     Therefore, many electricity consumers use a “load shedding” technique. This technique involves closely monitoring the amount of power presently being consumed by the electrical system. Additionally, the electricity consumers “shed loads”, i.e., turn off some electrical loads, if the total power consumption nears a peak power billing threshold set by the electrical utility. Prior art electrical systems of electricity consumers have included power meters that measure the instantaneous total power being consumed by the system. Accordingly, a building manager of such an electrical system is operable to visually monitor the total power being consumed and to turn off electrical loads to reduce the total power consumption of the electrical system if the power nears a billing threshold. 
     Many electrical utility companies offer a demand response program, in which the electricity consumers agree to shed loads during peak demand periods in exchange for incentives, such as reduced billing rates or other means of compensation. For example, the electricity utility company may request that a participant in the demand response program shed loads during the afternoon hours of the summer months when demand for power is great. Some prior art lighting control systems have offered a load shedding capability in which the intensities of all lighting loads are reduced by a fixed percentage, e.g., by 25%, in response to an input provided to the system. Such a lighting control system is described in commonly-assigned U.S. Pat. No. 6,225,760, issued May 1, 2001, entitled FLUORESCENT LAMP DIMMER SYSTEM, the entire disclosure of which is hereby incorporated by reference. 
     Since power meters tend to be rather expensive, most prior art electrical systems have included only one power meter monitoring the total power being consumed by the electrical system. Alternatively, some prior art lighting control systems, such as the Digital microWATT fluorescent lighting control system manufactured by the assignee of the present invention, have include lighting controllers that are operable to measure the power being consumed by a connected lighting load. Specifically, the lighting controllers included current transformers to measure the current flowing into the lighting controller and thus the power consumed by the lighting controller and the lighting load. However, lighting controllers including current transformers are also expensive. 
     Thus, there exists a need for a load control system that is operable to determine the power consumed by each individual electrical load in order to determine the total power being consumed by the load control system without using expensive power meters or current transformers. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a method of controlling plurality of electrical loads comprises the steps of estimating a present amount of power being consumed by each of the plurality of electrical loads, and determining the total amount of power presently being consumed by all of the plurality of electrical loads in response to the step of determining a present amount of power being consumed by each of the plurality of electrical loads. Further, the method is operable to provide a load shedding technique by additionally comparing the total amount of power to a threshold amount of power, and controlling the amount of power delivered to the plurality of electrical loads in response to the step of comparing if the total amount of power exceeds the threshold amount of power, such that the plurality of electrical loads consume a second amount of power less than the threshold amount of power. 
     According to another embodiment of the present invention the present invention, a load control system for controlling the amount of power delivered to a plurality of electrical loads from an AC power source comprises a plurality of load control devices and a central controller operable to determine the total amount of power being delivered to all of the electrical loads. The load control devices are coupled to the electrical loads for control of the amount of power delivered to the electrical loads. Each load control device is characterized by a first value corresponding to the present amount of power being delivered to a corresponding at least one of the electrical loads. A central controller is operatively coupled to the load control devices, such that the load control devices are operable to control the amount of power delivered to the electrical loads in response to the controller. Each of the load control devices is operable to transmit the first value to the controller, and the controller is operable to determine the total amount of power being delivered to all of the electrical loads in response to the first value of each of the plurality of load control devices. 
     The present invention further provides a method for using a computing device to reduce power usage for a plurality of load devices without using power meters that measure actual power usage. The method comprises the steps of: (1) defining a power usage goal value that represents a preferred amount of power to be used for at least one of the plurality of load devices; (2) estimating a power usage value representing actual power usage for the at least one load device at a particular time; and (3) automatically reducing power to the at least one load device when the power usage value exceeds the power usage goal value until the power usage value is equal to or lower than the power usage goal value. 
     In addition, the present invention provides a system for reducing power usage for a plurality of load devices by using a load shedding techniques without using power meters that meter actual power usage. The system comprises: (1) means for electronically defining a power usage goal value that represents a preferred amount of power to be used by at least one load device; (2) means for estimating an amount of power usage for the at least one load device at a particular time to calculate a power usage value; and (3) means for automatically reducing power to the at least one load device when the power usage value exceeds the power usage goal value until the power usage value is equal to or lower than the power usage goal value. 
     According to another aspect of the present invention, a method of automatically reducing power consumption in a load control system is presented. The load control system includes a controller and a plurality of load control devices controlling the amount of power delivered to a plurality of electrical loads. The method comprises the steps of: (1) configuring a load shedding tier defining a load shed parameter for each of the electrical loads; (2) the controller determining the total amount of power presently being consumed by all of the plurality of electrical loads; (3) the controller comparing the total amount of power to a threshold amount of power; (4) the controller automatically transmitting a digital message to the plurality of load control devices if the total amount of power exceeds a threshold amount of power; and (5) the load control devices controlling the amount of power delivered to the electrical loads in accordance with the load shed parameters of the load shedding tier in response to the digital message transmitted by the controller. 
     According to another embodiment of the present invention, a load control system for automatically controlling the amount of power delivered from an AC power source to a plurality of electrical loads comprises a plurality of load control devices coupled to each of the electrical loads for controlling the amount of power delivered to the electrical loads. They system further comprises a central controller operable to determine the total amount of power presently being consumed by all of the plurality of electrical loads, compare the total amount of power to a threshold amount of power, and automatically transmit a digital message to the plurality of load control devices if the total amount of power exceeds a threshold amount of power. The load control devices are each operable to control the amount of power delivered to the connected electrical load in accordance with a load shed parameter of a load shedding tier in response to the digital message transmitted by the controller. 
     The present invention further provides a central controller for a load control system having a plurality of load control devices for controlling the amount of power delivered from an AC power source to a plurality of electrical loads. The central controller comprises: (1) means for configuring a load shedding tier defining a load shed parameter for each of the electrical loads; (2) means for determining the total amount of power presently being consumed by all of the plurality of electrical loads; (3) means for comparing the total amount of power to a threshold amount of power; and (4) means for automatically transmitting a digital message to the plurality of load control devices if the total amount of power exceeds a threshold amount of power, such that the load control devices control the amount of power delivered to the electrical loads in accordance with the load shed parameters of the load shedding tier in response to the digital message. 
     In addition, the present invention provides a load control device of a load control system for controlling the amount of power delivered from an AC power source to an electrical load. The load control device comprises a load control circuit, a control circuit, a memory, and a communication circuit. The load control circuit is adapted to be coupled to the AC power source and the electrical load to control the amount of power delivered to the electrical load. The control circuit is coupled to the load control circuit for controlling the amount of power delivered to the electrical load. The memory is coupled to the control circuit and is operable to store a first load shed parameter for a first load shedding tier. The communication circuit is coupled to the control circuit and is operable to receive a digital message representative of the total power of the load control system exceeding a threshold amount of power. The control circuit is operable to control the amount of power delivered to the electrical load in accordance with the first load shed parameter of the first load shedding tier in response to receiving the digital message a first time. 
     According to another aspect of the present invention, a method of determining a setpoint of a load control device for controlling the amount of power delivered to an electrical load located in a space comprises the steps of: (1) initially setting the value of the setpoint equal to a desired level; (2) limiting the value of the setpoint to an occupied high-end trim if the space is occupied; (3) limiting the value of the setpoint to a daylighting high-end trim determined by a daylighting procedure; and (4) subsequently reducing the value of the setpoint in response to a load shed parameter. 
     According to another embodiment of the present invention, a method of controlling the amount of power delivered from an AC power source to an electrical load located in a space, comprises the steps of: (1) receiving a digital message containing a command to control the amount of power delivered to the electrical load to a desired level; (2) detecting if the space is occupied; and (3) determining a daylighting high-end trim using a daylighting procedure. The improvement comprises the steps of: (4) receiving a load shed parameter; and (5) determining the amount of power to be delivered to the electrical load by limiting the amount of power to be delivered to the electrical load to the minimum of the desired level of the digital message, an occupied high-end trim, and the daylighting high-end trim, and by reducing the amount of power to be delivered to the electrical load in response to the load shed parameter. 
     The present invention further provides a load control device for controlling the amount of power delivered from an AC power source to an electrical load located in a space. The load control device comprises: (1) means for initially setting the value of the setpoint equal to a desired level; (2) means for limiting the value of the setpoint to an occupied high-end trim if the space is occupied; (3) means for limiting the value of the setpoint to a daylighting high-end trim determined by a daylighting procedure; and (4) means for subsequently reducing the value of the setpoint in response to a load shed parameter. 
     In addition, the present invention provides a load control device of a load control system for controlling the amount of power delivered from an AC power source to an electrical load located in a space. The load control device comprises a load control circuit, a control circuit, a memory, a communication circuit, an occupancy sensor input, and a daylight sensor input. The load control circuit is adapted to be coupled to the AC power source and the electrical load to control the amount of power delivered to the electrical load. The control circuit is coupled to the load control circuit for controlling the amount of power delivered to the electrical load, to the memory for storing a load shed parameter, and to the communication circuit for receiving a digital message representative of a desired amount of power to deliver to the electrical load. The occupancy sensor input receives an occupancy sensor signal representative of whether the space is occupied, such that the control circuit is operable to determine an occupied high-end trim in response to the occupancy sensor signal. The daylight sensor input receives a daylight sensor signal representative of the total illumination in the space, such that the control circuit is operable to determine a daylighting high-end trim in response to the daylighting sensor signal. The control circuit is operable to determine the amount of power to be delivered to the electrical load by limiting the amount of power to be delivered to the electrical load to the minimum of the desired level of the digital message, the occupied high-end trim, and the daylighting high-end trim, and by reducing the amount of power to be delivered to the electrical load in response to the load shed parameter. 
     Other features and advantages of the present invention will become apparent from the following description of the invention that refers to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram of a lighting control system according to the present invention; 
         FIG. 2  is a simplified block diagram of a digital electronic dimming ballast of the lighting control system of  FIG. 1 ; 
         FIG. 3  is an example of a format of a ballast power consumption table of the personal computer of the lighting control system of  FIG. 1 ; 
         FIG. 4  is a flowchart of the load shedding procedure executed by the PC according to the present invention; 
         FIG. 5  is a flowchart of a load shed parameter update procedure executed by a control circuit of the ballast of  FIG. 2 ; and 
         FIG. 6  is a flowchart of a setpoint procedure executed periodically by the control circuit of the ballasts of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The foregoing summary, as well as the following detailed description of the preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purposes of illustrating the invention, there is shown in the drawings an embodiment that is presently preferred, in which like numerals represent similar parts throughout the several views of the drawings, it being understood, however, that the invention is not limited to the specific methods and instrumentalities disclosed. 
       FIG. 1  is a simplified block diagram of a lighting control system  100  according to the present invention. Preferably, the lighting control system  100  is operable to control the level of illumination in a space by controlling the intensity level of the electrical lights in the space and the daylight entering the space. As shown in  FIG. 1 , the lighting control system  100  is operable to control the amount of power delivered to (and thus the intensity of) a plurality of lighting load, e.g., a plurality of fluorescent lamps  102 , using a plurality of digital electronic dimming ballast  110 . Further, the lighting control system  100  may additionally include a plurality of other load control devices (not shown), such as dimmers or motor speed control modules, which include appropriate load control circuits that are well known to one having ordinary skill in the art. The lighting control system  100  is further operable to control the position of a plurality of motorized window treatments, e.g., motorized roller shades  104 , to control the amount of daylight entering the space. 
     Each of the fluorescent lamps  102  is coupled to one of the digital electronic dimming ballasts  110  for control of the intensities of the lamps. The ballasts  110  are operable to communicate with each other via digital ballast communication links  112 . A common communication protocol used for digital ballast communication links is the digital addressable lighting interface (DALI) protocol. However, the present invention is not limited to ballasts  110  and digital ballast communication links  112  using the DALI protocol. 
     The digital ballast communication links  112  are also coupled to digital ballast controllers (DBCs)  114 , which provide the necessary direct-current (DC) voltage to power the communication links  112 , as well as assisting in the programming of the lighting control system  100 . Each of the ballasts  110  is operable to receive inputs from a plurality of sources, for example, an occupancy sensor (not shown), a daylight sensor (not shown), an infrared (IR) receiver  116 , or a wallstation  118 . The ballasts  110  are operable to transmit digital messages to the other ballasts  110  in response to the inputs received from the various sources. Preferably, up to 64 ballasts  110  are operable to be coupled to a single digital ballast communication link  112 . 
     The ballasts  110  may receive IR signals  120  from a handheld remote control  122 , e.g., a personal digital assistant (PDA), via the IR receiver  116 . The remote control  122  is operable to configure the ballast  110  by transmitting configuration information to the ballasts via the IR signals  120 . Accordingly, a user of the remote control  122  is operable to configure the operation of the ballasts  110 . For example, the user may group a plurality of ballasts into a single group, which may be responsive to a command from the occupancy sensor. Preferably, a portion of the programming information (i.e., a portion of a programming database) is stored in memory of each of the ballasts  110 . An example of the method of using a handheld remote control to configure the ballasts  110  is described in greater detail in co-pending commonly-assigned U.S. patent application Ser. No. 11/375,462, filed Mar. 13, 2006, entitled HANDHELD PROGRAMMER FOR LIGHTING CONTROL SYSTEM, the entire disclosure of which is hereby incorporated by reference. 
     Referring back to  FIG. 1 , each of the motorized roller shades  104  comprises an electronic drive unit (EDU)  130 . Each electronic drive unit  130  is preferably located inside the roller tube of the associated roller shade  104 . The electronic drive units  130  are responsive to digital messages received from a wallstation  134  via a shade communication link  132 . The user is operable to open or close the motorized roller shades  104 , adjust the position of the shade fabric of the roller shades, or set the roller shades to preset shade positions using the wallstation  134 . The user is also operable to configure the operation of the motorized roller shades  104  using the wallstations  134 . Preferably, up to 96 electronic drive units  130  and wallstations  134  are operable to be coupled to the shade communication link  132 . A shade controller (SC)  136  is coupled to the shade communication link  132 . An example of a motorized window treatment control system is described in greater detail in commonly-assigned U.S. Pat. No. 6,983,783, issued Jan. 10, 2006, entitled MOTORIZED SHADE CONTROL SYSTEM, the entire disclosure of which is hereby incorporated by reference. 
     A plurality of processors  140  allow for communication between a personal computer (PC)  150  and the load control devices, i.e., the ballasts  110  and the electronic drive units  130 . Each processor  140  is operable to be coupled to one of the digital ballast controllers  114 , which is coupled to the ballasts  110  on one of the digital ballast communication links  112 . Each processor  140  is further operable to be coupled to the shade controller  136 , which is coupled to the motorized roller shades  114  on one of the shade communication links  114 . The processors  140  and the PC  150  are coupled to an inter-processor link  152 , e.g., an Ethernet link, such that the PC  150  is operable to transmit digital messages to the processors  140  via a standard Ethernet switch  154 . 
     The PC  150  operates as a central controller for the lighting control system  100  and executes a graphical user interface (GUI) software, which is displayed on a display screen  156  of the PC. The GUI allows the user to configure and monitor the operation of the lighting control system  100 . During configuration of the lighting control system  100 , the user is operable to determine how many ballasts  110 , digital ballast controllers  114 , electronic drive units  130 , shade controllers  136 , and processors  140  that are connected and active using the GUI software. Further, the user may also assign one or more of the ballasts  110  to a zone or a group, such that the ballasts  110  in the group respond together to, for example, an actuation of the wallstation  118 . The PC  150  includes a memory for storing the programming data of the lighting control system  100 . The PC  150  is operable to transmit an alert to the user in response to a fault condition, such a fluorescent lamp that is burnt out. Specifically, the PC  150  sends an email, prints an alert page on a printer, or displays an alert screen on the screen  156 . 
       FIG. 2  is a simplified block diagram of the digital electronic dimming ballast  110 , which is driving three fluorescent lamps L 1 , L 2 , L 3  in parallel. The load control circuit of the ballast  110  comprises a front end  210  and a back end  220 . The front end  210  includes a rectifier  230  for generating a rectified voltage from an alternating-current (AC) mains line voltage, and a filter circuit, for example, a valley-fill circuit  240 , for filtering the rectified voltage to produce a direct-current (DC) bus voltage. The valley-fill circuit  240  is coupled to the rectifier  230  through a diode  242  and includes one or more energy storage devices that selectively charge and discharge so as to fill the valleys between successive rectified voltage peaks to produce a DC bus voltage. The DC bus voltage is the greater of either the rectified voltage or the voltage across the energy storage devices in the valley-fill circuit  240 . 
     The back end  220  includes an inverter  250  for converting the DC bus voltage to a high-frequency AC voltage and an output circuit  260  comprising a resonant tank circuit for coupling the high-frequency AC voltage to the lamp electrodes. A balancing circuit  270  is provided in series with the three lamps L 1 , L 2 , L 3  to balance the currents through the lamps and to prevent any lamp from shining brighter or dimmer than the other lamps. The front end  210  and back end  220  of the ballast  110  are described in greater detail in commonly-assigned U.S. Pat. No. 6,674,248, issued Jan. 6, 2004, entitled ELECTRONIC BALLAST, the entire disclosure of which is hereby incorporated by reference. 
     A control circuit  280  generates drive signals to control the operation of the inverter  250  so as to provide a desired load current to the lamps L 1 , L 2 , L 3 . The control circuit  280  is operable to control the intensity of the lamps L 1 , L 2 , L 3  from a low-end trim (i.e., a minimum intensity) to a high-end trim (i.e., a maximum intensity). A power supply  282  is connected across the outputs of the rectifier  230  to provide a DC supply voltage, V cc , which is used to power the control circuit  280 . A communication circuit  284  is coupled to the control circuit  280  and allows the control circuit  280  to communicate with the other ballast  110  on the digital ballast communication link  112 . The ballast  110  further comprises a plurality of inputs  290  having an occupancy sensor input  292 , a daylight sensor  294 , an IR input  296 , and a wallstation  298  input. The control circuit  280  is coupled to the plurality of inputs  290  such that the control circuit  280  is responsive to the occupancy sensor, the daylight sensor, the IR receiver  116 , and the wallstation  118  of the lighting control system  100 . The control circuit  280  is operable to determine a setpoint, i.e., the desired intensity of the connected lamp  102 , in response to the communication circuit  284  and the plurality of inputs  290 . The control circuit  280  is also coupled to a memory  286  for storage of the operational information of the ballast  110 , e.g., the setpoint, the high-end trim, the low-end trim, a serial number, etc. 
     An example of a digital electronic dimming ballast operable to be coupled to a communication link and a plurality of other input sources is described in greater detail in co-pending commonly-assigned U.S. patent application Ser. No. 10/824,248, filed Apr. 14, 2004, entitled MULTIPLE-INPUT ELECTRONIC BALLAST WITH PROCESSOR, and U.S. patent application Ser. No. 11/011,933, filed Dec. 14, 2004, entitled DISTRIBUTED INTELLIGENCE BALLAST SYSTEM AND EXTENDED LIGHTING CONTROL PROTOCOL. The entire disclosures of both applications are hereby incorporated by reference. 
     During normal operation of the lighting control system  100 , the PC  150  communicates with the ballasts  110  and the electronic drive units  130  using a polling technique. The PC  150  polls the load control devices by transmitting a polling message to each of the ballasts  110  and electronic drive units  130  in turn. To send a polling message to a specific ballast  110 , the PC  150  transmits the polling message to the processors  140 . If a processor  140  that receives the polling message is coupled to the digital ballast controller  114  that is connected to the specific ballast  110 , the processor  140  re-transmits the polling message to the digital ballast controller  114 . Upon receipt of the polling message, the digital ballast controller  114  simply re-transmits the polling message to the specific ballast  110 . 
     In response to receiving the polling message, the specific ballast  110  transmits a status message to the PC  150 . The status message is transmitted in a relaying fashion back to the PC  150 , i.e., in a reverse order than how the polling message is transmitted from the PC  150  to the ballast  110 . Preferably, the status message includes the present intensity of the fluorescent lamp. For example, the ballast  110  may transmit the present intensity as a number between 0 and 127 corresponding to the percentage between off (i.e., a number of 0) and the high-end value (i.e., a number of 127). 
     According to the present invention, the PC  150  estimates a total power consumption of the lighting control system  100  (i.e., a power usage value) using one or more operational characteristics of the ballasts  110  rather than using power meters or current transformers to measure the actual input current of the ballasts. Preferably, the PC  150  simply determines the total amount of power presently being consumed by the lighting control system  100  in response to the number, wattage, and type of lamps  102  connected to the ballasts  110  and the present intensities of the ballasts. Alternatively, a single ballast  110  could be operable to estimate the power consumption of the ballast rather than the PC  150  performing the computation. 
     The PC  150  is operable to determine the power presently being consumed by each of the ballasts  110  by using the present intensity of each ballast and one of a plurality of ballast power consumption tables  300 . A unique ballast power consumption table  300  (i.e., a look-up table) for each type of ballast is stored in the memory of the PC  150 . An example of the format of the ballast power consumption tables  300  is shown in  FIG. 3 . The table  300  comprises a first column  310  of intensity levels (i.e., index values), which correspond to the lighting intensity levels received by the PC  150  from the ballasts  110 , i.e., numbers from 0 to 127. The table  300  also comprises a second column of corresponding power consumption amounts for each of the intensity levels of the first column  310 , i.e., P 0  through P 127  as shown in  FIG. 3 . The values of the power consumption of the ballast  110  may range, for example, from 14.8 W at low-end to 65 W at high-end for a 277V 10% ballast operating two T5 HE fluorescent lamps in parallel. Preferably, the plurality of ballast power consumption tables  300  are determined by actual measurements of the current drawn by the different types of ballasts at different operating voltages under different operating conditions. The data for the plurality of ballast power consumption tables  300  is then stored in the memory of the PC  150 . 
     The PC  150  determines the power consumption of each ballast by locating the power consumption amount in the second column  320  of the table  300  adjacent the intensity value (that was received from the ballast  110 ) in the first column  310 . For example, if the PC  150  receives an intensity level of three (3) from the ballast  110 , the PC  150  assumes that the ballast is presently consuming an amount of power of P 3 . Once the PC  150  has determined the power consumption of each of the ballast  110  in the lighting control system  100 , the PC can sum the power consumption values to determine the total power consumption of the lighting control system  100 . Preferably, the PC  150  is operable to display (i.e., graphically represent) the total estimated power consumption of the lighting control system  100  on the screen  156  of the PC. Alternatively, each ballast  110  could store the appropriate power consumption table  300  in the memory  286 . Each ballast  110  could then determine the power consumption using the present intensity, and simply transmit the present power consumption to the PC  150 . 
     The PC  150  is operable to use the estimated total power consumption as part of a load shedding procedure  400  (shown in  FIG. 4 ). The PC  150  is operable to compare the total power consumption to a load shedding power threshold (i.e., a power usage goal value), which may be set, for example, by a billing threshold of an electrical utility company. If the total power consumption exceeds the threshold, the PC  150  is operable to cause the ballasts  110  to shed loads, i.e., to dim the lamps to a lower intensity, using either a manual load shedding mode or an automatic load shedding mode. When executing the manual load shedding mode, the PC  150  is operable to display on the screen  156  or transmit (e.g., via email) a warning message that the load shedding power threshold has been exceeded. In response to such a warning message, a building manager may manually control the lamps  102  to lower levels, for example, by selecting a lighting preset via the PC  150 . The PC  150  is also operable to display on the screen  156  the load shedding power threshold and an estimate of the power savings (i.e., the amount of power that would be consumed without load shedding minus the estimated amount of power presently being consumed using load shedding). 
     The automatic load shedding mode provides for automatic control of the lamps  102  in response to the power consumption exceeding the load shedding power threshold, rather than requiring a building manager to intervene. During the automatic load shedding mode, the PC  150  dims the lamps in response to the load shedding condition using load shedding “tiers”. A tier is defined as a combination of predetermined load shed parameters (i.e., load shedding amounts) for each of the individual electrical loads or groups of electrical loads. For example, “Tier  1 ” may comprise shedding loads in an office space by 20%, in a hallway space by 40%, and in a lobby by 10%, while “Tier  2 ” may comprise shedding loads in the office space by 30%, in the hallway space by 50%, and in the lobby by 30%. Preferably, each successive tier reduces the amount of power being delivered to the electrical loads. Accordingly, the PC  150  is operable to consecutively step through each of the tiers to continue decreasing the total power consumption of the lighting control system  100  if the total power consumption repeatedly exceeds the load shedding threshold. 
     Preferably, the PC  150  controls each of the ballasts  110  to consume less power by transmitting the load shed parameter (which is chosen according to the next load shedding tier) to each of the ballasts. The load shed parameter represents a level of desired load shedding to be applied to the setpoint determined by the control circuit  280  of each of the ballasts  110  (i.e., the load shed parameter represents a percentage of the present setpoint). After determining the setpoint in response to the communication circuit  284  and the plurality of inputs  290 , the control circuit  280  of each ballast  110  preferably multiples the setpoint by a factor that is dependent upon the load shed parameter, as will be described in greater detail below. Since the load control system  100  does not simply reduce the high-end trim of the ballasts  110  in response to the total power consumption exceeding the load shedding power threshold (as in some prior art load control systems), the load control system always controls the lamps  102  to a lower intensity during the load shedding procedure  400  of the present invention, even if the ballasts  110  are receiving inputs from occupancy sensors and daylight sensors. 
       FIG. 4  is a flowchart of the load shedding procedure  400  executed by the PC  150  according to the present invention. First, the PC  150  transmits a polling message to the next device, i.e., the next ballast  110 , at step  410 . Preferably, the PC  150  starts with the first ballast  110  and steps through each ballast  110  as the load shedding procedure  400  loops. Next, the procedure  400  loops until the PC  150  receives a status message back from the polled ballast  110  at step  412  or a timeout expires at step  414 . If the timeout expires at step  414  before the PC  150  receives a status message at step  412 , the PC  150  transmits a polling message to the next ballast  110  at step  410 . 
     If the PC  150  receives a status message back from the polled ballast  110  at step  412 , the PC determines the present power consumption of the polled ballast  110  using the intensity level from the status message and the appropriate ballast power consumption table  300  at step  416 . To determine which of the plurality of ballast power consumption tables  300  that are stored in memory to use, the PC  150  uses the information about the ballast  110  (i.e., the type of the ballast, the wattage, number of lamps, etc.), which is part of the database stored in memory. At step  418 , the PC  150  determines the total power consumption by summing the present power consumption of the each of the individual ballasts  110 . At step  420 , the PC  150  displays the total power consumption from step  418  on the screen  156 . 
     If the load shedding threshold is exceeded at step  422 , a determination is made at step  424  if the automatic load shedding mode is enabled. If so, the PC  150  determines if there are more load shedding tiers to implement at step  426 . If there are more load shedding tiers to implement at step  426 , the PC controls the ballasts  110  to the intensity levels set by the next tier at step  428 . As previously mentioned, the PC  150  updates a load shed parameter of each of the ballasts according to the next tier. Preferably, the load shed parameter has a value that ranges between zero (0) and 100, such that a load shed parameter of zero corresponds to no load shedding, while a load shed parameter of 100 causes the lamp  102  to be turned off. For example, the PC  150  may transmit a load shed parameter of 20 to a first ballast and a load shed parameter of 40 to a second ballast. Accordingly, the first ballast will store the value 20 as its load shed parameter and the second ballast will store the value 40 as its load shed parameter using a load shed parameter update procedure  500 . 
       FIG. 5  is a flowchart of the load shed parameter update procedure  500  executed by the control circuit  280  of the ballasts  110  when a digital message is received via the communication circuit  284  at step  510 . If the received message is a load shed parameter at step  512 , the ballast  110  overwrites the load shed parameter in memory with the new load shed parameter of the received message at step  514  and the procedure  500  exits at step  516 . Otherwise, the ballast  110  processes the received message accordingly at step  518  and exits at step  516 . 
     Once the ballast  110  has stored the load shed parameter in memory, the ballast uses a setpoint procedure  600  to determine a lighting setpoint (which controls the intensity of the lamp  102 ) from the load shed parameter.  FIG. 6  is a flowchart of the setpoint procedure  600 , which is preferably executed periodically by the control circuit  280  of the ballasts  110 , for example, every 2.5 msec. During the setpoint procedure  600 , the control circuit  280  uses an occupancy high-end trim (OCC_HET), which represents the high-end trim of the ballast  110  when a connected occupancy sensor has detected an occupied state in the space in which the ballasts  110  and the occupancy sensor are located. 
     Further, the control circuit  280  uses a daylighting high-end trim (DAY_HET), which represents the high-end trim of the ballast  110  determined from a daylight reading of a connected daylight sensor using a daylighting algorithm. Preferably, the daylighting algorithm attempts to maintain the total illumination (from both daylight and artificial light, i.e., from the lamps  102 ) in the space in which the ballasts  110  and the daylight sensor are located substantially constant. The daylighting algorithm accomplishes this goal by decreasing the value of the daylighting high-end trim if the total illumination in the space increases, and increasing the value of the daylighting high-end trim if the total illumination decreases. Examples of daylighting algorithms are described in greater detail in commonly-assigned U.S. Pat. No. 4,236,101, issued Nov. 25, 1980, entitled LIGHT CONTROL SYSTEM, and U.S. Pat. No. 7,111,952, issued Sep. 26, 2006, entitled SYSTEM TO CONTROL DAYLIGHT AND ARTIFICIAL ILLUMINATION AND SUN GLARE IN A SPACE. The entire disclosures of both applications are hereby incorporated by reference. 
     Referring to  FIG. 6 , the setpoint procedure  600  begins at step  602 . If the ballast  110  has received a digital message via the communication circuit  284  at step  604 , a determination is made at step  606  as to whether the received message contains an intensity command, i.e., a command to change the intensity of the lamp  102 . If so, the control circuit  280  adjusts the setpoint according to the intensity command of the received message at step  608  and the procedure  600  moves on to step  612 . If a digital message has not been received at step  604  or the receive message does not contain an intensity command at step  606 , the procedure  600  simply continues to step  612 . 
     If an occupancy sensor that is connected to the ballast  110  is signaling that the space is occupied at step  612 , a determination is made at step  614  as to whether the occupancy high-end trim OCC_HET is less than the present setpoint. If so, the setpoint is set to the occupancy high-end trim OCC_HET at step  616  and the procedure  600  continues on to step  618 . If the space is not occupied at step  612  or the occupancy high-end trim OCC_HET is not less than the present setpoint at step  614 , the procedure  600  continues on to step  618 , where a determination is made as to whether a daylighting algorithm is enabled. If the daylighting algorithm is enabled at step  618  and the daylighting high-end trim DAY_HET is less than the present setpoint at step  620 , the setpoint is set to the daylighting high-end trim DAY_HET at step  622  and the setpoint is stored in memory at step  624 . If the daylighting algorithm is not enabled at step  618  or if the daylighting high-end trim DAY_HET is not less than the present setpoint at step  620 , the present setpoint is simply stored in memory at step  624 . 
     At step  626 , the setpoint is updated based on the load shed parameter that was received during the load shed parameter update procedure  800  of  FIG. 8 . Specifically, the setpoint is set using the following equation: 
       Setpoint=Setpoint·(100−Load Shed Parameter)/100.  (Equation #1) 
     For example, if no load shedding is desired, the load shed parameter is zero and the setpoint is not changed according to Equation #1. Further, if the load shed parameter is 100, the setpoint is equal to zero, and thus, the ballast  110  turns the lamp  102  off. A load shed parameter between zero and 100 causes the setpoint to be scaled accordingly. The setpoint procedure  600  exits at step  628 . 
     Therefore, the PC  150  is operable to cause a ballast  110  to begin load shedding by transmitting a load shed parameter having a value greater than zero to the ballast  110 . The control and logic in regards to determining the values of the load shed parameters and determining when to automatically shed loads (i.e., if automatic load shedding mode is enabled) is executed by the PC  150 . 
     Referring back to  FIG. 7 , if the automatic load shedding mode is not enabled at step  724  or if there are not more tiers to implement at step  726 , the PC  150  transmits an alert, i.e., sends an email, prints an alert page on a printer, or displays a warning message on the display screen  156 . If the load shedding threshold is not exceeded at step  722 , the procedure  700  simply loops to poll the next device at step  710 . 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.