CURRENT ISSUE
Contests
Feature Article
|
|
Issue #211 February 2008
Intelligent Energy Solutions
Solar-Powering the Circuit Cellar
Part 3: Wiring & Electronics
by Steve Ciarcia
Start | Back to the Solar Panels | Wiring and Inverters | It's All About Architecture | Maximum Power Point Tracking | So, Does It Work? | Conservation | Soaking Up Some Photons | Sources & PDF
MAXIMUM POWER POINT TRACKING
The magnitude of the inverter output current is set by the MPPT control. If the PV array output power is greater than the inverter rating (which may be reduced at elevated ambient temperature), the MPPT control sets the output current to match the inverter power rating. If the PV array output power is less than the inverter power rating, the MPPT sets the inverter output current to maximize the PV array power (i.e., it tracks the array’s maximum power point).
Because the PV array voltage and current are interrelated by the I-V characteristic of the PV cells (governed by the semiconductor physics of the cells), an MPPT algorithm controls either array voltage or current to maximize power output. The general relationship between PV array voltage or current and output power (under uniform irradiance) is shown in Figure 4.
|
| Figure 4—Characteristic PV array power curve. |
Maximum power point systems typically control array voltage (by adjusting the power converter input voltage) and the task of the MPPT algorithm becomes one of finding the voltage at the array’s maximum power point (VMPP). The VMPP stays relatively constant with changes in irradiance, except at low levels, but changes substantially with cell temperature. Therefore, it is not possible to just find and set VMPP for a particular PV array. It must be tracked. Fortunately, because VMPP changes mainly with array temperature (i.e., relatively slowly), the tracking algorithm does not have to be fast to achieve acceptable accuracy.
There are several basic MPPT techniques and a multitude of variations on the theme. The algorithm used in the GT inverters is somewhat nonconventional, but it uses the characteristics of the entire PV array/inverter system to determine the maximum power point.
The system starts at the extreme right side of the array power-voltage (P-V) curve (zero power, PV array open-circuit voltage). The MPPT controller increases output power by increasing the output current magnitude command. As power is increased, the system operates along the array P-V curve, approaching the maximum power point (see Figure 5). The maximum power point is a boundary between stable and unstable regions of operation. To the right of the max power point, the system is stable. To the left of the max power point, the system is unstable and the array voltage will tend to “crash” to zero if output power is increased. The system is at the margin of stability at the maximum power point and the array voltage oscillates at this point (the magnitude of oscillation is quite small, about 1% of the array DC voltage). The MPPT controller detects this oscillation and uses it to find VMPP.
|
| Figure 5—Tracking P-V to determine the maximum power point. |
Once VMPP is found, the MPPT controller holds the system at this voltage by adjusting the output current magnitude. The controller monitors the magnitude of the array voltage oscillations. If the magnitude exceeds a limit, indicating that the operating point is moving to the left of VMPP towards instability (i.e., the actual VMPP has moved right), the array voltage is increased to bring the system back to VMPP. Every 0.7 s, the MPPT controller adjusts the VMPP setpoint to see if the actual maximum power point has moved to the left. If power can be increased without the magnitude of oscillations exceeding the limit, the setpoint is changed to the improved value.
The Xantrex GT series MPPT controller is implemented in a Freescale Semiconductor HC12 microcontroller. The microcontroller is also responsible for the user interface and data network functions. A separate Microchip Technology PIC18F series microcontroller is used for the grid interconnect protection functions specified by the IEEE Standard for Interconnecting Distributed Resources with the Electric Power Systems. These require the inverter to rapidly shut off the output connected to the grid if the grid voltage or frequency goes outside specified limits. The pulse width modulators and inner-control loops are implemented with discrete analog and digital circuits.
Finally, like most of the electronics I buy, I always like to look under the hood. Before taking apart the inverters on my wall, however, I spent some time e-mailing the kind folks at Xantrex and they were happy to answer my questions and provide me with a picture of the inside of the GT5 so I didn’t have to disassemble mine (see Photo 5). ;-)
|
| Photo 5—This is an inside look at the GT5 inverter. The toroid isolation transformer is at the upper right and the DC bus electrolytic capacitors are to the left of the transformer. There are a lot of inductors and capacitors—used both as parts of the power conversion circuits and as filters to remove ripple and EMI. The power semiconductors are not visible because they are attached to the heatsink on the other side of the main PCB. |
|
