# Patent

## Title
Method and apparatus for improved burst mode during power conversion

## Abstract

A method and apparatus for converting DC input power to AC output power. The apparatus comprises an input capacitor, a DC-AC inverter, a burst mode controller for causing energy to be stored in the input capacitor during at least one storage period and the energy to be drawn from the input capacitor during at least one burst period, wherein the AC output power is greater than the DC input power during the at least one burst period; a first feedback loop for determining a maximum power point (MPP) and operating the DC-AC inverter proximate the MPP; and a second feedback loop for determining a difference in a first power measurement and a second power measurement, producing an error signal indicative of the difference, and coupling the error signal to the first feedback loop to adjust at least one operating parameter of the DC-AC inverter to drive toward the MPP.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
Embodiments of the present disclosure generally relate to power conversion and, more particularly, to a method and apparatus for improved burst mode operation.
2. Description of the Related Art
Solar panels have historically been deployed in mostly remote applications, such as remote cabins in the wilderness or satellites, where commercial power was not available. Due to the high cost of installation, solar panels were not an economical choice for generating power unless no other power options were available. However, the worldwide growth of energy demand is leading to a durable increase in energy cost. In addition, it is now well established that the fossil energy reserves currently being used to generate electricity are rapidly being depleted. These growing impediments to conventional commercial power generation make solar panels a more attractive option to pursue.
Solar panels, or photovoltaic (PV) modules, convert energy from sunlight received into direct current (DC). The PV modules cannot store the electrical energy they produce, so the energy must either be dispersed to an energy storage system, such as a battery or pumped hydroelectricity storage, or dispersed by a load. One option to use the energy produced is to employ one or more inverters to convert the DC current into an alternating current (AC) and couple the AC current to the commercial power grid. The power produced by such a distributed generation (DG) system can then be sold to the commercial power company.
PV modules have a nonlinear relationship between the current (I) and voltage (V) that they produce. A maximum power point (MPP) on an I-V curve for a PV module identifies the optimal operating point of the PV module; when operating at this point, the PV module generates the maximum possible output power for a given temperature and solar irradiance. Therefore, in order to optimize power drawn from a PV module, a power conversion device coupled to the PV module, such as an inverter or a micro-inverter, generally employs a maximum power point tracking (MPPT) technique to ensure that the PV module is operated at the current and voltage levels corresponding to its MPP. The MPPT acts to rapidly adjust the PV module operating current and voltage levels in response to changes in solar irradiance and/or temperature such that the PV module can continue to operate at the MPP.
During the time period required for an MPPT technique to bias a PV module to its MPP, for example, when the solar irradiance on a PV module changes from no irradiance to increasing irradiance or at a PV module/inverter initial activation, a power conversion device coupled to the PV module will suffer from a lower efficiency until the MPP is achieved. Additionally, a power conversion device coupled to a PV module generally will suffer from a lower efficiency when the PV module is operating at a low power, e.g., low irradiance. During low irradiance, a PV module and an associated inverter may operate so inefficiently that is it best for overall system efficiency to deactivate the PV module and/or its inverter until solar irradiance increases.
Therefore, there is a need in the art for a method and apparatus for improving operation of a PV module and inverter in achieving and tracking the maximum power point.
SUMMARY OF THE INVENTION
Embodiments of the present invention generally relate to a method and apparatus for converting DC input power to AC output power. The apparatus comprises an input capacitor, a DC-AC inverter, a burst mode controller, a first feedback loop, and a second feedback loop. The burst mode controller causes energy to be stored in the input capacitor during at least one storage period and the energy to be drawn from the input capacitor during at least one burst period, wherein the AC output power is greater than the DC input power during the at least one burst period. The first feedback loop determines a maximum power point (MPP) and operates the DC-AC inverter proximate the MPP. The second feedback loop determines a difference in a first power measurement and a second power measurement, produces an error signal indicative of the difference, and couples the error signal to the first feedback loop to adjust at least one operating parameter of the DC-AC inverter to drive toward the MPP.



BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
FIG. 1 is a block diagram of a system for distributed generation (DG) in accordance with one or more embodiments of the present invention;
FIG. 2 is a block diagram of an inverter in accordance with one or more embodiments of the present invention;
FIG. 3 is a block diagram of a burst mode controller in accordance with one or more embodiments of the present invention;
FIG. 4 is a plurality of waveforms depicting collection periods of PV module power and voltage measurements during burst mode in accordance with one or more embodiments of the present invention;
FIG. 5 is a block diagram of a DC voltage controller in accordance with one or more embodiments of the present invention;
FIG. 6 is a block diagram of an MPPT controller in accordance with one or more embodiments of the present invention; and
FIG. 7 is a flow diagram of a method for operating an inverter in burst mode in accordance with one or more embodiments of the present invention.



DETAILED DESCRIPTION
FIG. 1 is a block diagram of a system 100 for distributed generation (DG) in accordance with one or more embodiments of the present invention. This diagram only portrays one variation of the myriad of possible system configurations. The present invention can function in a variety of distributed power generation environments and systems.
The system 100 comprises a plurality of inverters 1021, 1022 . . . 102n, collectively referred to as inverters 102, a plurality of PV modules 1041, 1042 . . . 104n, collectively referred to as PV modules 104, an AC bus 106, and a load center 108.
Each inverter 1021, 1022 . . . 102n is coupled to a PV module 1041, 1042 . . . 104n, respectively. In some embodiments, a DC-DC converter may be coupled between each PV module 104 and each inverter 102 (e.g., one converter per PV module 104). Alternatively, multiple PV modules 104 may be coupled to a single inverter 102 (i.e., a centralized inverter); in some embodiments, a DC-DC converter may be coupled between the PV modules 104 and the centralized inverter.
Each inverter 102 employs an MPPT technique to operate the subtending PV module 104 at its MPP such that the PV module 104 generates an optimal power output for a given temperature and solar irradiation. The inverters 102 are coupled to the AC bus 106, which in turn is coupled to the load center 108. The load center 108 houses connections between incoming power lines from a commercial power grid distribution system and the AC bus 106. The inverters 102 convert DC power generated by the PV modules 104 into AC power, and meter out AC current that is in-phase with the AC commercial power grid voltage. The system 100 couples the generated AC power to the commercial power grid via the load center 108.
In accordance with one or more embodiments of the present invention, the inverters 102 employ a “burst mode” during initial operation. In burst mode, the inverters 102 store energy over one or more AC grid voltage cycles (“energy storage periods”) and subsequently “burst” the stored energy to the commercial power grid (“burst periods”). The length of the energy storage periods is determined such that a ripple voltage overriding the PV module output voltage remains below a desired ripple voltage threshold.
In addition to improving the efficiency of the inverters 102, the burst mode facilitates a rapid convergence to the MPP utilizing an MPPT technique described below. Upon operating proximate the MPP, and if the PV modules 104 are supplying sufficient output power, the inverters 102 deactivate the burst mode and operate in a continuous mode, utilizing the MPPT technique to remain proximate to the MPP. In the event that the solar irradiance and/or temperature changes to a level such that the output power from the PV modules 104 drops below a burst mode threshold, one or more of the inverters 102 switch back to burst mode.
FIG. 2 is a block diagram of an inverter 102 in accordance with one or more embodiments of the present invention. The inverter 102 comprises an I-V monitoring circuit 204, an input capacitor 220, a DC-AC inverter 208, a DC voltage controller 210, an MPPT controller 212, a burst mode controller 224, and a conversion control module 214. The inverter 102 is coupled to the PV module 104 and to the commercial power grid.
The I-V monitoring circuit 204 is coupled to the PV module 104, the input capacitor 220, and the burst mode controller 224. The burst mode controller 224 is further coupled to the DC voltage controller 210, the MPPT controller 212, and the conversion control module 214. Additionally, the MPPT controller 212 is coupled to the DC voltage controller 210, and the DC voltage controller 210 is coupled to the conversion control module 214. The DC voltage controller 210 functions to bias the PV module 104 at a DC voltage set point (i.e., a desired PV module operating voltage), while the MPPT controller 212 drives such DC voltage set point to correspond to the MPP voltage. The burst mode controller 224 functions to switch the inverter 102 between continuous mode and burst mode, and additionally measures input power from the PV module 104 that is utilized by the MPPT controller 212 in determining the DC voltage setpoint.
The I-V monitoring circuit 204 monitors the instantaneous input voltage and current, Vin and Iin, respectively, from the PV module 104. The input capacitor 220, in addition to being coupled to the I-V monitoring circuit 204, is coupled to the DC-AC inverter 208, and the DC-AC inverter 208 is further coupled to the DC voltage controller 210, the conversion control module 214, and the commercial power grid. A current Icap flows through the input capacitor 220, and a current Iinv flows to the DC-AC inverter 208.
The conversion control module 214 receives a reference signal from the commercial power grid, and provides the control signals for the DC-AC inverter 208 to convert the received DC current, Iinv, to a required AC output current, Ireq. One example of such power conversion is commonly assigned U.S. Patent Application Publication Number 2007/0221267 entitled “Method and Apparatus for Converting Direct Current to Alternating Current” and filed Sep. 27, 2007, which is herein incorporated in its entirety by reference. The AC output current Ireq from the DC-AC inverter 208 is coupled to the commercial power grid such that it is in-phase with the commercial AC current.
The DC voltage controller 210 employs a first feedback loop (an “inner” loop) 216 to bias the PV module 104 at the DC voltage setpoint by modulating the current Iin drawn from the PV module 104. The first feedback loop 216 comprises the I-V monitoring circuit 204, the burst mode controller 224, the MPPT controller 212, the DC voltage controller 210, and the DC-AC inverter 208. The DC voltage controller 210 receives a signal indicative of the PV module DC (i.e., average) input voltage VDC from the burst mode controller 224, and receives the DC voltage setpoint from the MPPT controller 212. Based on a difference between the VDC and the DC voltage setpoint, the first feedback loop 216 drives the DC-AC inverter 208 to generate Ireq such that the appropriate current Iin is drawn from the PV module 104 to bias the PV module 104 at the DC voltage setpoint. Thus, the first feedback loop 216 iteratively computes a difference between the average PV module operating voltage and a DC voltage setpoint for the PV module 104, and accordingly adjusts the current Iin drawn from the PV module 104 such that the PV module 104 is biased at the DC voltage setpoint (i.e., at an operating current and voltage that approximately corresponds to the MPP).
The MPPT controller 212 employs a second feedback loop 218 (an “outer” loop) to adjust the DC voltage setpoint such that it corresponds to the MPP voltage. The second feedback loop 218 comprises the I-V monitoring circuit 204, the burst mode controller 224, the MPPT controller 212, and the DC voltage controller 210. The burst mode controller 224 receives signals indicative of the instantaneous PV module input current and voltage, Iin and Vin, respectively, through the I-V monitoring circuit 204, and computes the instantaneous input power, Pin, from the PV module 104. The burst mode controller 224 further processes the input power Pin, as described in greater detail below, to obtain a first and a second power measurement and provides such power measurements to the MPPT controller 212. The MPPT controller 212 determines a power difference between the first and second power measurements; based on the difference, the MPPT controller 212 determines whether the PV module operating voltage must be increased or decreased to reach the MPP, modifies the DC voltage setpoint accordingly, and supplies the new DC voltage setpoint to the DC voltage controller 210. Additionally, a power difference of zero indicates that the PV module 104 is currently biased at the MPP, and the MPPT controller 212 supplies the corresponding DC voltage setpoint to the DC voltage controller 210. The second feedback loop 218 thus iteratively determines whether the PV module 104 is operating proximate the MPP and, in the case where the PV module 104 is not operating proximate the MPP, modifies at least one operating parameter within the first feedback loop 216 to achieve the MPP (i.e., the outer loop “fine tunes” the setting established by the inner loop).
Upon initial operation or sufficiently low output power from the PV module 104, the burst mode controller 224 operates the inverter 102 in burst mode, wherein during energy storage periods (e.g., one or more AC grid voltage cycles of 16.67 msec), the input capacitor 220 stores energy that is subsequently supplied to the DC-AC inverter 208 during a burst period. During burst mode, the burst mode controller 224 drives the DC voltage controller 210 such that the output current generated by the inverter 102, Ireq, is a “burst mode current”, IB. Once the PV module 104 is operating proximate the MPP voltage, and if the PV module 104 is generating sufficient output power (i.e., if the PV module output power is greater than a pre-determined burst mode threshold), the burst mode controller 224 operates the inverter 102 in continuous mode. If the PV module output power subsequently drops below the burst mode threshold, the burst mode controller 224 switches the inverter 102 back to burst mode operation.
During energy storage periods of burst mode operation, the DC-AC inverter 208 is driven such that no output current is generated by the inverter 102 (i.e., IB is zero). During such periods, current is precluded from flowing to the DC-AC inverter 208, thereby causing current generated by the PV module 104 to charge the input capacitor 220. The inverter 102 remains in an energy storage period for a number of AC grid voltage cycles, Noff, as determined by the burst mode controller 224, before beginning a burst period of one AC grid voltage cycle. During the burst period, the burst mode controller 224 causes the DC voltage controller 210 to drive the DC-AC inverter 208 such that the burst current IB is generated in accordance with the energy that has been stored in the input capacitor 220 during the previous energy storage period. As a result, the inverter output current during a burst period, IB, is greater than the inverter output current during continuous mode, Ireq, for a given level of solar irradiance and temperature. In some embodiments, IB=Ireq*(Noff+1).
Due to the charging and discharging of the input capacitor 220 during burst mode, a ripple voltage (“burst mode ripple voltage”, or Vr) overrides the average voltage across the input capacitor 220, and results in a corresponding burst mode ripple voltage overriding the PV module DC voltage, VDC. As such, the PV module operating voltage fluctuates in accordance with the magnitude of the burst mode ripple voltage Vr; the greater the burst mode ripple voltage Vr, the greater the PV module operating voltage excursion. Such a fluctuation in the PV module operating voltage results in reduced efficiency of the inverter 102. For example, when operating proximate the MPP voltage, a larger voltage fluctuation around the MPP voltage results in a greater period of time that the PV module 104 is operating off of the MPP voltage, resulting in less than optimal power being drawn from the PV module 104.
The magnitude of the burst mode ripple voltage Vr varies in accordance with the length of time the input capacitor 220 charges and discharges; i.e., with the energy storage and burst periods. The burst mode controller 224 controls the energy storage period, i.e., the number of AC grid voltage cycles of the energy storage period, Noff, such that the burst mode ripple voltage Vr remains below a pre-determined threshold, thereby improving the efficiency of the inverter 102 during burst mode operation.
When operating in continuous mode, the inverter 102 continually generates output power in-phase with the AC grid power (i.e., during each AC grid voltage cycle). As such, the inverter output power oscillates between zero output power at the AC grid voltage zero-crossings and peak output power at the AC grid voltage peak positive and negative amplitudes. When the inverter output power is set at zero, current from the PV module 104 is precluded from flowing to the DC-AC inverter 208 and therefore charges the input capacitor 220. When the inverter output power is set at peak, energy stored in the input capacitor 220 is utilized, in addition to the instantaneous power from the PV module 104, to generate a peak inverter output power of twice the average PV module output power. Thus, the charging and discharging of the input capacitor 220 during continuous mode provides an AC component overriding the average power provided by the PV module 104.
During both the continuous and burst modes of operation, the ripple voltage caused by the charging and discharging of the input capacitor 220 provides an opportunity for maximum power point tracking. As the ripple voltage across the PV module 104 varies above and below an average (i.e., DC) PV module voltage, the PV module output power varies in accordance with the ripple voltage. If the PV module 104 produces more power when operating above its DC voltage than when operating below its DC voltage, then the PV module DC voltage is below the MPP and must be increased to reach the MPP. If the PV module 104 produces more power when operating below its DC voltage than when operating above its DC voltage, then the PV module DC voltage is above the MPP and must be decreased to reach the MPP. Thus, the difference between the average power produced by the PV module 104 when it is operating above its DC voltage and when it is operating below its DC voltage indicates in which direction the PV module DC voltage must be shifted to achieve the MPP. Additionally, if the difference is zero, the PV module 104 is biased at the MPP. In some embodiments, such a power difference may be determined based on the PV module output power during certain phases of the AC grid voltage, as further described below.
FIG. 3 is a block diagram of a burst mode controller 224 in accordance with one or more embodiments of the present invention. The burst mode controller 224 comprises a multiplier 302, an operating mode module 304, and a power measurement module 306.
The multiplier 302 receives signals indicative of the instantaneous input current and voltage from the PV module 104, Iin and Vin, respectively, through the I-V monitoring circuit 204, and generates an output signal indicative of the instantaneous input power from PV module 104, Pin. The output of the multiplier 302 is coupled to the power measurement module 306 and to the operating mode module 304; additionally, the power measurement module 306 receives inputs of Iin and Vin. The operating mode module 304 is further coupled to the DC voltage controller 210, and receives a signal indicative of an estimated PV module input current, Iest, from the DC voltage controller 210. The Iest is an estimated input current to be drawn from the PV module 104 that will result in biasing the PV module 104 at a desired DC voltage setpoint. The operating mode module 304 is also coupled to the power measurement module 306, and additionally receives an input signal of a burst mode power threshold, PB.
The operating mode module 304 compares the PV module input power Pin to the burst mode threshold PB. If the inverter 102 is operating in continuous mode and Pin is less that PB, the operating mode module 304 switches the inverter 102 to burst mode; once Pin is greater than PB, the operating mode module 304 switches the inverter 102 back to continuous mode.
The power measurement module 306 receives an input indicative of the AC grid voltage phase from the conversion control module 214, for example, from a phase lock loop of the conversion control module 214. The power measurement module 306 integrates the PV module input power Pin during certain portions of the AC grid voltage cycle to obtain a first power “bin”, PB1, and a second power “bin”, PB2. Additionally, the PV module input voltage Vin is integrated over a portion of the AC grid voltage cycle to obtain a DC voltage bin, VDCB, where VDCB is utilized to determine a DC (i.e., average) PV module input voltage, VDC. In some embodiments, during each cycle of the AC grid voltage when operating in continuous mode, the power measurement module 306 integrates Pin during a 90°-180° phase of the AC grid voltage cycle to obtain PB1, and integrates Pin during a 180°-270° phase of the same AC grid voltage cycle to obtain PB2. The PV module input voltage Vin is integrated over the entire 90°-270° phase to obtain VDCB. During burst mode, the power bins and the average PV module input voltage are determined as described further below.
The power measurement module 306 determines a first and a second power measurement, P1 and P2, respectively, based on PB1 and PB2. During continuous mode, P1=PB1, and P2=PB2; during burst mode, the power measurements are determined as described further below. The first and second power measurements are supplied to the MPPT controller 212 for determining whether the PV module 104 is operating at, above, or below the MPP, and any required shift in the DC voltage setpoint to achieve the MPP. Additionally, the power measurement module 306 determines a PV module DC voltage, VDC, based on VDCB. The power measurement module 306 supplies VDC to the DC voltage controller 210 for determining Ireq. The new DC voltage setpoint and the required output current Ireq are applied during the next AC grid voltage cycle.
When switching from continuous to burst mode, the operating mode module 304 determines a maximum number of AC grid voltage cycles, Noff, for an energy storage period such that the burst mode ripple voltage Vr will remain below a pre-determined threshold. In some embodiments, a threshold of 10% of the PV module DC voltage VDC is utilized. In some embodiments, Noff is computed as follows:

NoffC*Vdc*Vr/Pin*T 

In the above equation, C is the capacitance of the input capacitor 220, VDC is the PV module DC voltage, Vr is the burst mode ripple voltage, Pin is the input power from the PV module 104, and T is the AC grid voltage cycle period.
During burst mode, the power measurement module 306 integrates the PV module input power Pin over certain portions of the AC grid voltage cycle during an energy storage period to obtain PB1 and PB2. To determine the portions of the AC grid voltage cycle over which to integrate Pin, the Noff AC grid cycles of the energy storage period are partitioned into two equal portions—a “first half” of the energy storage period and a “second half” of the energy storage period, where the first half occurs prior to the second half. For example, for an energy storage period of one AC grid voltage cycle, the first half comprises the AC grid voltage cycle from 0°-180°, and the second half comprises the same AC grid voltage cycle from 180°-360°. For an energy storage period of two AC grid voltage cycles, the first half comprises the entire first AC grid voltage cycle, and the second half comprises the entire second AC grid voltage cycle. For energy storage periods greater than two AC grid voltage cycles, the first and second halves are analogously defined.
To obtain PB1, the power measurement module 306 integrates the PV module input power Pin during any of the 90°-270° AC grid voltage phase periods occurring within the first half of the energy storage period. To obtain PB2, the power measurement module 306 integrates the PV module input power Pin during any of the 90°-270° AC grid voltage phase periods occurring within the second half of the energy storage period. Additionally, the power measurement module 306 integrates the PV module input voltage Vin over each 90°-270° phase during the entire energy storage period, along with the 90°-270° phase during the subsequent burst period, to obtain VDCB.
The measured power bins PB1 and PB2 are then used by the power measurement module 306 to determine the first and second power measurements, P1 and P2, respectively. In some embodiments, P1=PB1/Noff and P2=PB2/Noff during burst mode. Additionally, the power measurement module 306 determines VDC based on VDCB. The first and second power measurements P1 and P2 are supplied to the MPPT controller 212 and VDC is supplied to the DC voltage controller 210 for the appropriate MPPT and inverter output current processing. The output of such processing, i.e., the new DC voltage setpoint and the required output current from the inverter 102, are applied to the burst period following the next energy storage period.
In some instances, the input capacitor 220 may degrade over time and degrade the performance of the inverter 102. In order to identify such a condition, the operation of the input capacitor 220 may be evaluated during burst mode when the number of AC grid voltage cycles in the energy storage period (i.e., Noff) is two or more. In some embodiments, the power measurement module 306 determines a difference between the average input voltage Vin from the PV module 104 during the first and second AC grid voltage cycles of the energy storage period (i.e., ΔV). The capacitance C of the input capacitor 220 can then be estimated as follows:

C=Iin/Fgrid*ΔV 

Where Iin is the input current from the PV module 104, and Fgrid is the frequency of the grid voltage. If the capacitance is below a capacitance threshold, the power measurement module 306 may provide an output alarm to indicate the condition.
FIG. 4 is a plurality of waveforms 400 depicting collection periods of PV module power and voltage measurements during burst mode in accordance with one or more embodiments of the present invention.
The waveform 402 corresponds to a computed energy storage period of one AC grid voltage cycle. During a first AC grid voltage cycle T1, an inverter is operating in continuous mode. The power bins PB1 and PB2 are obtained by integrating the PV module input power Pin during the 90°-180° and 180°-270° phases of the AC grid voltage, respectively, and the PV module input voltage Vin is integrated during the entire 90°-270° period to obtain the DC voltage bin VDCB. P1 is set to PB1, P2 is set to PB2, and the average DC voltage VDC is computed from VDCB. The power and voltage measurements PB1, PB2, and VDC are utilized to determine the inverter operating parameters for the next AC grid voltage cycle in which the inverter generates output power.
At the end of T1, the inverter switches to burst mode and enters an energy storage period. The inverter generates no output power during such a period. The power bin PB1 is obtained by integrating the PV module input power Pin during the 90°-270° phase within the first half of the energy storage period (i.e., the 90°-180° phase of T2), and the power bin PB2 is obtained by integrating the PV module input power Pin during the 90°-270° phase within the second half of the energy storage period (i.e., the 180°-270° phase of T2). Additionally, the PV module input voltage Vin is integrated during the entire 90°-270° period of T2 and to obtain the voltage bin VDCB.
At T3, the inverter begins a burst period and begins generating output power. The operating parameters determined during T1 are adjusted accordingly to drive the inverter to generate the burst mode current, IB. During T3, the PV module input voltage Vin is integrated during the entire 90°-270° period of T3 and added to the voltage bin VDCB. P1 is set to PB1, and P2 is set to PB2, and VDC is computed from VDCB. The power and voltage measurements P1, P2, and VDC are utilized to determine the inverter operating parameters for the next AC grid voltage cycle in which the inverter generates output power.
At T4, the inverter begins an energy storage period. Analogous to the operation during T2, PB1, PB2, and VDCB are obtained. At time T5, the inverter begins a burst period. The operating parameters determined during T3 are adjusted accordingly to drive the inverter to generate the burst mode current, IB. Analogous to the operation during T3, the PV module input voltage Vin is integrated during the entire 90°-270° period of T5 and added to the voltage bin VDCB. P1 is set to PB1, and P2 is set to PB2, and VDC is computed from VDCB. The power and voltage measurements P1, P2, and VDC are utilized to determine the inverter operating parameters for the next AC grid voltage cycle in which the inverter generates output power.
The waveform 404 corresponds to a computed energy storage period of two AC grid voltage cycles. During a first AC grid voltage cycle T1, an inverter is operating in continuous mode. The power bins PB1 and PB2 are obtained by integrating the PV module input power Pin during the 90°-180° and 180°-270° phases of the AC grid voltage, respectively, and the PV module input voltage Vin is integrated during the entire 90°-270° period to obtain the DC voltage bin VDCB. P1 is set to PB1, P2 is set to PB2, and the average DC voltage VDC is computed from VDCB. The power and voltage measurements P1, P2, and VDC are utilized to determine the inverter operating parameters for the next AC grid voltage cycle in which the inverter generates output power.
At T2, the inverter switches to burst mode and enters an energy storage period of two AC grid voltage cycles. The inverter generates no output power during such a period. The power bin PB1 is obtained by integrating Pin during the portions of the 90°-270° phases occurring within the first half of the energy storage period (i.e., the 90°-270° phase of T2), and the power bin PB2 is obtained by integrating Pin during portions of the 90°-270° phases occurring within the second half of the energy storage period (i.e., the 90°-270° phase of T3). Additionally, the PV module input voltage Vin is integrated during each 90°-270° period of the energy storage period, i.e., the 90°-270° phase periods of T2 and T3 to obtain the voltage bin VDCB.
At T4, the inverter begins a burst period and begins generating output power. The operating parameters determined during T1 are adjusted accordingly to drive the inverter to generate the burst mode current, IB. During T4, the PV module input voltage Vin is integrated during the entire 90°-270° period of T4 and added to the voltage bin VDCB. P1 is set to PB1/2, P2 is set to PB2/2, and VDC is computed from VDCB. The power and voltage measurements P1, P2, and VDC are utilized to determine the inverter operating parameters for the next AC grid voltage cycle in which the inverter generates output power.
At T5, the inverter begins an energy storage period. Analogous to the operation during T2 and T3, PB1, PB2, and VDCB are obtained. At time T7, the inverter begins a burst period. The operating parameters determined during T4 are adjusted accordingly to drive the inverter to generate the burst mode current, IB. Analogous to the operation during T2 and T3, the PV module input voltage Vin is integrated during the entire 90°-270° period of T7 and added to the voltage bin VDCB. P1 is set to PB1/2, P2 is set to PB2/2, and VDC is computed from VDCB. The power and voltage measurements P1, P2, and VDC are utilized to determine the inverter operating parameters for the next AC grid voltage cycle in which the inverter generates output power.
The waveform 406 corresponds to a computed energy storage period of three AC grid voltage cycles. During a first AC grid voltage cycle T1, an inverter is operating in continuous mode. The power bins PB1 and PB2 are obtained by integrating the PV module input power Pin during the 90°-180° and 180°-270° phases of the AC grid voltage, respectively, and the PV module input voltage Vin is integrated during the entire 90°-270° period to obtain the DC voltage bin VDCB. P1 is set to PB1, P2 is set to PB2, and the average DC voltage VDC is computed from VDCB. The power and voltage measurements P1, P2, and VDC are utilized to determine the inverter operating parameters for the next AC grid voltage cycle in which the inverter generates output power.
At T2, the inverter switches to burst mode and enters an energy storage period of three AC grid voltage cycles. The inverter generates no output power during such a period. The power bin PB1 is obtained by integrating Pin during the portions of the 90°-270° phases occurring within the first half of the energy storage period (i.e., the 90°-270° phase of T2 and the 90°-180° phase of T3), and the power bin PB2 is obtained by integrating Pin during portions of the 90°-270° phases occurring within the second half of the energy storage period (i.e., the 180°-270° phase of T3 and the 90°-270° phase of T4). Additionally, the PV module input voltage Vin is integrated during each 90°-270° period of the energy storage period, i.e., the 90°-270° phase periods of T2, T3, and T4 to obtain the voltage bin VDCB.
At T5, the inverter begins a burst period and begins generating output power. The operating parameters determined during T1 are adjusted accordingly to drive the inverter to generate the burst mode current, IB. During T5, the PV module input voltage Vin is integrated during the entire 90°-270° phase and added to the voltage bin VDCB. P1 is set to PB1/3, P2 is set to PB2/3, and VDC is computed from VDCB. The power and voltage measurements P1, P2, and VDC are utilized to determine the inverter operating parameters for the next AC grid voltage cycle in which the inverter generates output power.
FIG. 5 is a block diagram 500 of a DC voltage controller 210 in accordance with one or more embodiments of the present invention. The DC voltage controller 210 comprises an adder/subtractor 502, a proportional-integral (PI) controller 504, a gain module 506, and a burst mode adjustment module 508. The DC voltage controller 210 utilizes the first feedback loop 216 to control the current supplied to the DC-AC inverter 208 such that the PV module 104 is biased at the DC voltage setpoint.
The adder/subtractor 502 receives a signal indicative of the PV module DC voltage, VDC, from the burst mode controller 224, and a signal indicative of the DC voltage setpoint from the MPPT controller 212. The output of the adder/subtractor 502 couples a difference between VDC and the DC voltage setpoint to the PI controller 504. The PI controller 504 acts to correct the difference by estimating an input current, Iest, to be drawn from the PV module 104 that will result in biasing the PV module 104 at the DC voltage setpoint.
The output of the PI controller 504 is coupled to the gain module 506 and provides a signal indicative of Iest to the gain module 506. The gain module 506 further receives an input of the AC grid voltage amplitude, VAC, from the conversion control module 214, and VDC from the burst mode controller 224, and determines the required output current from the inverter, Ireq, to draw Iest from the PV module 104. In some embodiments, Ireq is calculated as follows:

Ireq=VDC*Iest/VAC*eff

In the above equation, eff is an efficiency rating of the inverter.
The output of the gain module 506 is coupled to the burst mode adjustment module 508. The burst mode adjustment module 508 adjusts the required inverter output current Ireq during burst mode such that it corresponds to the burst mode output current, IB, in accordance with a control signal received from the burst mode controller 224. The output of the burst mode adjustment module 508 is coupled to the DC-AC inverter 208 and drives the DC-AC to generate an output current Ireq during continuous mode and an output current IB during burst mode.
FIG. 6 is a block diagram of an MPPT controller 212 in accordance with one or more embodiments of the present invention. The MPPT controller 212 comprises a power difference module 602, an integrator 604, and an adder 608. The MPPT controller 212 utilizes the second feedback loop 218 to determine a DC voltage setpoint for the PV module 104 corresponding to the MPP voltage.
The power difference module 602 receives signals indicative of the first and second power measurements, P1 and P2, from the burst mode controller 224. The power difference module 602 computes a power difference between P1 and P2 and utilizes the power difference to determine an error signal, ε. In some embodiments, the power difference is computed as (P2−P1)/(P2+P1).
The error signal ε from the power difference module 602 is coupled to the integrator 604. The integrator 604 integrates the error signal ε and couples the integrated error signal to the adder 608. The adder 608 additionally receives an input of a nominal voltage, Vnom, where Vnom is an initial estimate of the MPP voltage. The integrated error signal acts to “fine tune” the nominal voltage such that the DC voltage setpoint (i.e., the sum of the integrated error and the nominal voltage) corresponds to the MPP voltage. The DC voltage setpoint is then supplied to the DC voltage controller 210 in order to drive the PV module 104 to operate at the DC voltage setpoint.
FIG. 7 is a flow diagram of a method 700 for operating an inverter in burst mode in accordance with one or more embodiments of the present invention. In some embodiments, such as the embodiment described below, a power conversion device, such as a DC-AC inverter, is coupled to the PV module and converts DC power from the PV module to AC power, where such AC power is coupled to a commercial power grid. In some embodiments, multiple PV modules may be coupled to a single centralized DC-AC inverter; alternatively, individual PV modules may be coupled to individual DC-AC inverters (e.g., one PV module per DC-AC inverter). In some embodiments, a DC-DC converter may be coupled between the PV module or PV modules and the DC-AC inverter.
The method 700 begins at step 702 and proceeds to step 704. At step 704, the inverter operates in continuous mode and generates an output current, Ireq, such that the PV module is biased proximate the MPP voltage. At step 708, a first and a second power measurement of the input power from the PV module, P1 and P2, respectively, are obtained. In some embodiments, the first power measurement comprises integrating the input power Pin from the PV module during a 90°-180° phase of an AC grid waveform cycle to obtain a first “power bin”, PB1, where P1=PB1, and the second power measurement comprises integrating Pin during the 180°-270° phase of the same AC grid waveform cycle to obtain a second “power bin”, PB2, where P2=PB2. In some embodiments, Pin may be sampled during such phases to obtain the first and second power measurements; for example, Pin may be sampled at a rate of 256 times the commercial power grid frequency. In alternative embodiments, the first and second power measurements may be obtained during different phases of an AC grid waveform cycle.
Additionally, an average, or DC, voltage from the PV module, VDC, is computed. In some embodiments, the voltage Vin from the PV module is integrated during the 90°-270° phase of the AC grid voltage to obtain a DC voltage “bin”, VDCB. VDC is then computed based on VDCB.
The method 700 proceeds to step 710, where P1, P2, and VDC are utilized to determine a DC voltage setpoint for the PV module such that the DC voltage setpoint corresponds to the MPP voltage. At step 712, a required inverter output current, Ireq, that will result in the PV module being biased at the desired DC voltage setpoint is determined. Steps 704 through 712 of the method 700 comprise an outer feedback loop that determines whether the PV module is currently biased at the MPP voltage, and, if necessary, adjusts the DC voltage setpoint to achieve the MPP.
The method 700 proceeds to step 714, where a determination is made whether the PV module input power exceeds a burst mode threshold. If the condition at step 714 is met, the method 700 returns to step 704 and the inverter continues to operate in continuous mode. If the condition at step 714 is not met, the method 700 proceeds to step 716.
At step 716, a maximum number of AC grid voltage cycles, Noff, for energy storage periods is determined based on a burst mode ripple voltage threshold. In some embodiments, a burst mode ripple voltage threshold of 10% of the PV module DC voltage VDC is utilized. At step 718, the inverter switches to burst mode, and begins an energy storage period of Noff AC voltage grid cycles. During the energy storage period, the inverter does not produce any output power, and power generated by the PV module is stored in the inverter.
At step 720, the first and second power bins, PB1 and PB2, are collected, along with the DC voltage bin, VDCB. In some embodiments, the Noff AC grid cycles are partitioned into two equal portions—a “first half” of the Noff AC voltage grid cycles and a “second half” of the Noff AC voltage grid cycles, where the first half occurs prior to the second half. PB1 is obtained by integrating the PV module input power Pin during any of the 90°-270° AC grid voltage phase periods occurring within the first half, and PB2 is obtained by integrating the PV module input power Pin during any of the 90°-270° AC grid voltage phase periods occurring within the second half. Additionally, the PV module input voltage Vin over each 90°-270° phase during the energy storage period to obtain VDCB.
The method 700 proceeds to step 724. At step 724, following the energy storage period, a burst period is activated and the inverter begins to generate output power. In some embodiments, the burst period comprises a single AC grid voltage cycle. During the burst period, the required output current Ireq determined in step 712 is adjusted such that the inverter generates a burst current, IB, in accordance with the amount of energy stored during the energy storage period. In some embodiments, IB=Ireq*(Noff+1).
At step 728, an input voltage from the PV module is measured and added to the DC voltage bin VDCB; in some embodiments, the PV module input voltage Vin is integrated over the 90°-270° phase of the AC grid voltage cycle and added to VDCB. The method proceeds to step 730, where P1, P2, and VDC are determined. In some embodiments, P1=PB1/Noff, P2=PB2/Noff, and VDC is the average of VDCB. At step 732, the DC voltage setpoint for the PV module is determined based on P1, P2, and VDC. At step 734, the required inverter output current, Ireq, that will result in the PV module being biased at the desired DC voltage setpoint is determined.
The method 700 proceeds to step 736, where a determination is made whether operation of the inverter should continue. If the condition at step 736 is met, the method 700 returns to step 714; if the conditions at step 736 is not met, the method 700 proceeds to step 738 where it ends.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

## Claims

1. An apparatus for converting DC input power to AC output power, comprising:
an input capacitor;
a DC-AC inverter;
a burst mode controller for causing energy to be stored in the input capacitor during at least one storage period and the energy to be drawn from the input capacitor during at least one burst period, wherein the AC output power is greater than the DC input power during the at least one burst period;
a first feedback loop for determining a maximum power point (MPP) and operating the DC-AC inverter proximate the MPP; and
a second feedback loop for determining a difference in a first power measurement and a second power measurement, producing an error signal indicative of the difference, and coupling the error signal to the first feedback loop to adjust at least one operating parameter of the DC-AC inverter to drive toward the MPP.

2. The apparatus of claim 1, wherein the first power measurement measures the DC input power during a first phase range of at least one cycle of a commercial power grid and the second power measurement measures the DC input power during a second phase range of the at least one cycle.

3. The apparatus of claim 2, wherein the first phase range and the second phase range are of equal length.

4. The apparatus of claim 1, wherein the at least one storage period is determined such that a ripple voltage overriding a voltage of the input capacitor satisfies a ripple voltage threshold.

5. The apparatus of claim 1, wherein the second feedback loop comprises an integrator for integrating the error signal.

6. The apparatus of claim 1, wherein the burst mode controller causes the AC output power to be greater than the DC input power.

7. The apparatus of claim 1, wherein the at least one storage period and the at least one burst period occur when the DC input power satisfies a burst mode power threshold.

8. The apparatus of claim 7, wherein the power burst threshold is 30% of a power rating of a device providing the DC input power.

9. The apparatus of claim 1, wherein the AC output power is zero during the at least one storage period.

10. The apparatus of claim 1, wherein the burst mode controller determines a capacitance of the input capacitor.

11. The apparatus of claim 10, wherein an alarm is raised when the capacitance satisfies a capacitance threshold.

12. A method for converting DC input power to AC output power, comprising:
storing energy in an input capacitor during at least one storage period;
drawing energy from the input capacitor during at least one burst period, wherein the AC output power is greater than the DC input power during the at least one burst period; and
operating a DC-AC inverter proximate a maximum power point (MPP) while employing a maximum power point tracking (MPPT) technique.

13. The method of claim 12, wherein the MPPT technique comprises:
determining the MPP;
operating the DC-AC inverter proximate the MPP;
determining a difference in a first power measurement and a second power measurement;
producing an error signal indicative of the difference; and
adjusting using the error signal at least one operating parameter of the DC-AC inverter to drive toward the MPP.

14. The method of claim 13, wherein the first power measurement measures the DC input power during a first phase range of at least one cycle of a commercial power grid and the second power measurement measures the DC input power during a second phase range of the at least one cycle.

15. The method of claim 14, wherein the first phase range and the second phase range are of equal length.

16. The method of claim 13, further comprising integrating the error signal.

17. The method of claim 12, wherein the at least one storage period is determined such that a ripple voltage overriding a voltage of the input capacitor satisfies a ripple voltage threshold.

18. The method of claim 12, wherein the at least one storage period and the at least one burst period occur when the DC input power satisfies a burst mode power threshold.

19. The method of claim 18, wherein the power burst threshold is 30% of a power rating of a device providing the DC input power.

20. The system of claim 19 wherein, in each of the at least one inverters, the first power measurement measures the DC input power during a first phase range of at least one cycle of a commercial power grid and the second power measurement measures the DC input power during a second phase range of the at least one cycle.

21. The system of claim 20 wherein, in each of the at least one inverters, an alarm is raised when the capacitance satisfies a capacitance threshold.

22. The system of claim 19 wherein, in each of the at least one inverters, the at least one storage period is determined such that a ripple voltage overriding a voltage of the input capacitor satisfies a ripple voltage threshold.

23. The method of claim 12, wherein the AC output power is zero during the at least one storage period.

24. The method of claim 12, further comprising determining a capacitance of the input capacitor.

25. The method of claim 24, further comprising raising an alarm when the capacitance satisfies a capacitance threshold.

26. A system for power conversion, comprising:
at least one photovoltaic (PV) module;
at least one inverter, coupled to the at least one PV module, comprising:
a DC-AC inverter, wherein the DC-AC inverter converts DC input power to AC output power;
an input capacitor;
a burst mode controller for causing energy to be stored in the input capacitor during at least one storage period and the energy to be drawn from the input capacitor during at least one burst period, wherein the AC output power of the DC-AC inverter is greater than the DC input power of the DC-AC inverter during the at least one burst period;
a first feedback loop for determining a maximum power point (MPP) and operating the DC-AC inverter proximate the MPP; and
a second feedback loop for determining a difference in a first power measurement and a second power measurement, producing an error signal indicative of the difference, and coupling the error signal to the first feedback loop to adjust at least one operating parameter of the DC-AC inverter to drive toward the MPP.

27. The system of claim 26 wherein, in each of the at least one inverters, the at least one burst mode controller determines a capacitance of the at least one input capacitor.