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New Interface for Control and Coordination of Solar Energy



Although significant progress has been made in harvesting solar energy for a single input power system, there has been little focus on the control and coordination of solar energy conversion with other actively controlled sources and storage devices. The operation and control of solar power conversion using a multiport power electronic interface (MPEI) is a new possibility explored here, with special emphasis on load sharing and battery recovery modes.

Solar power conversion usually takes place in two forms: solar-to-chemical and solar-to-electrical. The typical applications are a solar-based battery charger and a solar-source inverter. Single-source two-stage structures, and multi-converter front-end systems are commonly used for solar power processing but they have problems controlling the output voltage or balancing the power between harvesting and consumption.

The control structure for solar maximum power point tracking (MPPT) in a load sharing mode can provide optimal harvesting from a solar panel and provide immunity to load dynamics. The current-mode maximum power transfer (CMMPT) method is used to transfer maximum power to the battery during charge recovery. The control system is implemented in a Texas Instruments TMS320F2812 DSP and experimental results show the effectiveness of the control system.

MPEI background

The MPEI provides connections for multiple sources and storage as well as an integrated control system structure for optimal power management. The control aspect of solar power processing includes load sharing and a battery recovery mode. It is a self-sustaining multiple input/output static power electronic converter that interfaces with different sources, storages and loads. The integrated control system of the MPEI enables excellent system dynamic and steady state performance. It provides optimal renewable energy harvesting, energy management and economical utility grid interactions.

A six-phase-leg structure for the MPEI can interact with fuel cell, solar panel and wind turbine renewable sources, as well as one battery for energy storage, and a single-phase ac-load. As indicated in Figure 1, the upper switch for unidirectional converters is disabled, using only paralleled fast diodes. Additionally, the ports that interface with the battery storage and ac-load are bidirectional. The objects under investigation are circled in Figure 1, including the phase legs for solar, battery and inverter stages.


Figure 1: Multi-port power electronic interface: five-port implementation.

Solar-battery load-sharing mode

In a load sharing scenario (Figure 2), the output voltage can be controlled by a battery switching cell. For the solar switching cell, the current reference is directly fed from the MPPT routine and is under current control. The battery switching cell is under average current mode (ACM) control (Figure 3), and the dc-link voltage is regulated by the battery voltage controller, whose output is used as a current reference for the inner battery current loop. This control system offers a controlled power source as well as a controlled voltage source. Given limited power harvested from the solar panel, the voltage source (the battery switching cell) will dynamically provide the rest of the power in demand.


Figure 2: Maximum power transfer by means of load sharing.

The generic ACM control block diagram involves an outside voltage loop and an inner current loop. In the case of a direct solar current control, the design target only resides inside the blue block. The control-to-current transfer function can be obtained from a small signal average model of boost converter.


Figure 3: Average current mode control diagram.

Since the dc-link voltage is controlled by the battery switching cell, the current loop crossover frequency can be set high. A PI (proportional-integral) controller is used in the solar current loop design. The battery switching cell design follows the ACM control structure.

Maximum power transfer in battery recovery mode

In the battery recovery mode operation, there is no demand from the ac side and the harvested maximum power from the solar panel is directly transferred to battery storage. Since the solar panel voltage varies with sun irradiation intensity, buck or boost operation will be needed at different irradiation levels. Independent control of the solar power and battery charging current might cause power imbalance and lead to dc link-voltage fluctuation. A current-mode maximum power transfer (CMMPT) control structure is proposed for this particular operation mode (Figure 4). Shown here, the control system structure for CMMPT is presented in transfer function blocks. GiL is the equivalent control-to-charging current plant with inner current loop under PI controller compensation. GiL1d is the control-to-solar current transfer function, and GiL2d is the control-to-charging current transfer function. The charging current reference is generated by the MPPT routine. The charging current controller generates the inner current reference for solar output current; the resulting PWM signal will gate two active switches synchronously.

Figure 4: Control system for current-mode maximum power transfer (CMMPT).

The proposed controller offers several advantages:
  • A flexible input/output voltage level.
  • A dynamic link between input current and charging current, hence the input power and output power.
  • Solves the instability problem of a boost switching cell and reduces the number of control loops.
  • Indirectly stabilizes the dc-link capacitor voltage.
Experimental results

The start-up phase of the MPPT in load sharing mode took about 10 s to reach maximum power point. The solar switching cell is under direct current control for maximum power tracking while the battery switching cell is under ACM control to stabilize the dc-link voltage. During a pulse load of 300 W, the tracking of the solar power was hardly affected by the load dynamics, and the battery switching cell handled the load dynamics by providing a pulse current. This new interface successfully showed how to control and process solar energy using load sharing and battery recovery modes.

References
  1. Maximum Solar Power Transfer in Multi-port Power Electronic Interface, APEC 2010 paper by Wei Jiang and Babak Fahimi
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