The present invention relates to systems and methods of distributing DC power such as, for example, from solar panels to electrical devices. Traditional solar panel installations use an AC inverter in order to convert the high voltage DC electricity supplied from the solar panels into 60 Hz AC voltage that can then be directly tied to the utility grid. The AC voltage is then distributed to the loads within the building or back to the utility grid.
Many electronic devices used today do not directly use the 60 Hz AC voltage that is distributed throughout buildings and households through the power grid. Instead, the devices first convert the 60 Hz AC voltage into high voltage DC. The DC voltage is then used to power the device. However, every time electrical power is transformed from DC to AC or from AC to DC there are losses in efficiency that are given off in the form of heat. These losses from DC to AC and from AC to DC conversions can add up to between 8-12% of the power that could have been delivered from a solar array.
The systems described herein improve the efficiency of such power systems by supplying the high voltage DC power generated from a solar panel array directly to DC-powered devices. This type of DC microgrid installation does not require an inverter as the DC voltage is distributed directly to the loads (i.e., the DC-powered devices), which reduces overall system cost and complexity while also eliminating a notable source of power loss. However, the downside of not using an inverter is that modern inverters typically have functionality built into it them provide Maximum Power Point Tracking (“MPPT”) for the solar array. MPPT is used in order to maximize the power supplied by the solar array by adjusting the output voltage of the solar array in order to deliver the maximum output power. This functionality could be restored by adding individual MPPT modules on the solar arrays, but this would add to system cost, lowers efficiency due to a DC-to-DC conversion, and reduces reliability by adding components.
Instead, various embodiments described herein provide a method of control for a lighting ballast to allow it to be used in a system with variable DC power distribution. The solar array MPPT functionality is realized by varying the loads in order to reach the optimum operating voltage of the solar array. A supplemental DC power supply is added to the system in order to accomplish the MPPT function. This same power supply is used to power the loads when the solar array can't produce the necessary power requirements demanded by the loads. Thus, MPPT functionality is provided without requiring a DC-to-AC inverted or a separate MPPT module for each solar array. Furthermore, system provides for consistent load performance that is not affected by the varying DC voltage supplied to them. In this way, for example, the occupants of the building do not notice any change in behavior of their surrounding environment.
In one embodiment, the invention provides a microgrid system comprising a DC power bus, a solar array, a controllable DC power supply, and a lighting ballast. The solar array is coupled to the DC power bus and configured to provide DC power to the DC power bus. The controllable DC power supply is configured to apply a variable voltage to the DC power bus. In some embodiments, the variable voltage of the controllable DC power supply is controlled to achieve a maximum power point. The lighting ballast is configured to control an amount of power applied to a DC load (such as, for example, a light source). The lighting ballast monitors the variable voltage of the DC power bus and defines a pulse width modulated power control signal based on the voltage of the DC power bus. The voltage of the DC power bus is controllable applied to the DC load based on the pulse-width modulated power control signal.
In some embodiments, the amount of power applied from the DC power bus to the DC load is adjusted by varying a frequency at which the DC power is applied to the DC load. In other embodiments, the amount of power applied from the DC power bus to the DC load is adjusted by varying a DC power duty cycle.
In another embodiment, the invention provides a ballast for selectively applying DC power from a variable voltage DC power bus to a DC load. The ballast includes at least one switch coupled between the DC power bus and the DC load. A processor is coupled to the at least one switch and controls the operation of the at least one switch. A non-transient computer-readable memory stores instructions that are executed by the processor to control the operation of the processor. The processor determines a voltage on the variable voltage DC power bus and defines a pulse-width modulate power control signal based on the determined voltage. The at least one switch is then operated based on the pulse-width modulated power control signal to apply DC power from the DC power bus to the DC load at a first frequency.
Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.
Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.
One or more DC loads 101 draw power from the DC power supply 103 and the solar array 105 through the DC bus. The DC load(s) 101 include an electronic ballast circuit to provide signal conditioning for an applicable light source such as, for example, induction, fluorescent, and LED light sources. In other constructions, other types of DC loads, such as, for example, HVAC equipment, pumps, fans, compressors, and inverters, are included in addition to or instead of the light source.
A control system 107 monitors one or more system voltages and one or more system currents and provides a control signal to regulate the operation of the DC power supply 103. In the example of
To prevent this undesirable effect, the DC electronics ballast in the DC load 101 compensates for this varying DC system voltage in such a way that output power provided to the lamp remains constant regardless of the input DC voltage. In some constructions, the ballast is configured to operate one or more switches (e.g. FETs) that control whether power is applied to the lamp. The frequency at which the switches are operated can be controllably adjusted such that the resonant tank circuit of the lamp shifts away from its resonant point. This can be done by increasing or decreasing the frequency depending on the resonant point set by the ballast circuit. By varying this frequency as a function of the DC input voltage, the output power can be maintained at a relatively constant level thus not changing the light output of the lamp for different DC input voltages.
In other constructions, the ballast pulse-width modulates the output with a varying duty cycle that is a function of the DC input voltage. Higher DC input voltages results in lower duty cycling of the fundamental frequency in order to achieve the same overall average light output. One advantage of this method is that it is more easily applied to LED lamp outputs than the method described above. This method also makes it easier to optimize the electronics for a given fundamental frequency.
If the DC input voltage provided to the load is within the defined range, the ballast circuit samples the lamp output (step 211) and determines whether the lamp is lit (step 213). If the lamp is not already lit, the ballast control circuit executes a lamp strike sequence (step 215) to light the lamp. If, however, the lamp is already lit, the ballast control system uses pulse width modulation to adjust the output of the lamp based on the DC input voltage (step 217). The ballast control system then again samples the DC input voltage (step 203) and continues to adjust the lamp output based on the DC input voltage until the lamp is turned off or until the DC input voltage leaves the defined range.
An auxiliary power supply module 307 draws power from the DC power bus 306 and produces a 12V auxiliary power source 309 and a 3.3V auxiliary power source 311 which are used to operate portions of the ballast control system as described in further detail below. In some constructions, a separate auxiliary power supply module 307 is connected directly to the DC power bus 306 and provides operating power to each ballast system connected to the DC power bus 306. In other constructions, the auxiliary power supply module 307 is incorporated into the same hardware (i.e., housing) as a single ballast control system 300.
The ballast control system 300 is operated by a microprocessor 315. The microprocessor 315 is powered by the 3.3V auxiliary power source 311 and executes instructions stored on a memory unit such as EEPROM 317. In this example, EEPROM 317 is also powered by the 3.3V auxiliary power source 311. In this example, the microprocessor 315 provides three pulse-width modulated control outputs—a high-side PWM signal 319, a low-side PWM signal 321, and a dimming PWM signal 323. The three PWM control outputs are provided to a pair of AND gates 325, 327. The first AND gate 325 produces a high-side FET control signal 329 based on the high-side PWM signal 319 and the dimming PWM signal 323. Similarly, the second AND gate 327 produces a low-side FET control signal 331 based on the low-side PWM signal 321 and the dimming PWM signal 323.
The high-side FET control signal 329 and the low-side FET control signal 331 are both provided to a FET driver module 333 which operates a plurality of switches within a half-bridge module 313. The FET driver 333 receives power from the 12V auxiliary power source 309 and opens the high-side switch(es) of the half bridge module 313 based on the high-side FET control signal 329. Similarly, the FET driver 333 opens the low-side switch(es) of the half bridge module 313 based on the low-side FET control signal 331.
The state of the high-side and low-side switches of the half bridge module 313 control whether (and how) power is provided from the DC power bus 306 to the ballast network 335 and, subsequently, to the lamp itself As described above in reference to
The same circuit can be used to control the power provided to the lamp by adjusting the fundamental frequency of the high-side PWM signal 319 and the low-side PWM signal 321. Instead of using the dimming PWM signal 323 to adjust the duty cycle, the dimming PWM signal 323 is held high. The microprocessor 315 then adjusts the frequency of the high-side PWM signal 319 and the frequency of the low-side PWM signal 321 to control the power applied to the lamp. This secondary control mechanism can aso be implemented by a simplified version of the circuit illustrated in
Thus, the invention provides, among other things, a power distribution system for providing DC power directly from a solar array to a load (such as, for example, a light source) and a ballast control system that operates to provide consistent power to a load despite a varying DC voltage on a DC power bus. Various features and advantages of the invention are set forth in the following claims.
This application claims the benefit of U.S. Provisional Patent Application No. 61/811,206, filed Apr. 12, 2013, the entire contents of which are incorporated herein by reference.
Number | Date | Country | |
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61811206 | Apr 2013 | US |