Solar energy is continuing to develop as a renewable energy source, and its continued focus has promoted the price reduction and efficiency improvement of solar panels. At the same time, significant advances have been made in balancing systems (BOS) devices such as inverters, chargers and energy optimizers. This article will introduce new architectures and components that affect the performance of solar BOS.
Transformerless DC/AC inverters are widely used in Europe, but in the United States, such products have only recently been used in certain areas. There are many transformerless inverter topologies, and the HERIC topology developed by the Fraunhofer Institute shows high efficiency. The structure of a traditional full-bridge inverter is shown in Figure 1. The HERIC topology is shown in Figure 2. This figure also shows two new switch/diode pairs. This topology utilizes a unique freewheeling path to reduce switching and conduction losses, increasing efficiency to over 98%.
Figure 1 Full H-bridge for transformerless inverters
Figure 2 HERIC topology for transformerless inverters
Advantages of Transformerless Inverters There are several advantages to transformerless inverters. The transformer stage of the traditional inverter needs to provide galvanic isolation, so it is heavy, expensive and has a large loss. Even high-frequency inverters with ultra-small transformers have a large energy loss, up to 1% to 2%. Every aliquot of energy is critical in the ongoing reduction of PV installation costs. Therefore, the transition to transformerless inverters will continue.
Disadvantages of transformerless inverters Transformerless inverters also have some disadvantages. As mentioned earlier, this inverter does not contain galvanic isolation provided by the transformer, which is an important safety hazard. However, integrating a complete safety mechanism, such as isolation resistance testing and residual current sensing, makes a transformerless inverter as safe as a transformer. In addition, there is evidence that grounding problems with such inverters can cause permanent damage to thin-film panels, especially some CIGS solar panels.
Common in inverter topologies are the switches in the H-bridge. As mentioned above, inverter designs are moving in the direction of reducing the size and cost of inductors/capacitors and transformers with ever higher power. High voltage/high frequency switching is required in solar inverters. However, operating the MOSFET under high voltage/high frequency conditions can result in severe conduction losses. IGBTs are often used because their conduction losses are lower than MOSFETs. However, they generate tail currents during turn-off - increasing switching losses.
ESBT
ST's emitter-switched bipolar diode (ESBT) provides a good solution. As shown in Figure 3, the ESBT's common base amplifier structure includes a high voltage BJT and a power MOSFET, and the entire device has a very low turn-on voltage drop.
Figure 3 ESBT with MOSFET driver
When an ESBT is paired with an external MOSFET and a diode/resistor, the entire circuit looks like a 3-terminal device that can be driven to a similar IGBT or power MOSFET. ESBT's turn-off energy is much lower than that of IGBTs, enabling efficient design and is ideal for high frequency, high voltage inverter designs.
Traditionally constructed rooftop solar system installation processes are also reducing BOS costs and improving performance. In this configuration, the solar panels are connected together in a series/parallel array, very sensitive to shadows and mismatches. For example, if a panel in a serial array is affected by shadows or dust, the entire output will be severely affected. One solution to this problem is to add a DC/DC converter and an extremely powerful power point tracker at the panel or series stage.
Optimizing panel-level energy optimization is a very important energy conversion and control task. These functions optimize the energy collected by the solar panel and then convert it to a continuous voltage or current while sending the operating status to the central controller. This requires a microcontroller or state machine, analog sensing circuitry, DC/DC current conversion, and wired or wireless communication.
These specific features are easy to understand and suitable for integration in a single module. This provides cost, reliability, and performance benefits. Optimized MPPT output increases system performance and leads to increased efficiency, helping to reduce system cost.
A typical MPPT integration solution is ST's SPV1020. It includes an integrated boost converter, an MPPT wired state machine, analog sensing circuitry and a PLM. The converter uses a high frequency interleaving structure that accepts smaller inductors and capacitors. This highly integrated solution will be launched later in 2010.
Solar energy is suitable for most industrial applications, such as off-grid solar powered circuit lights, signs, collision lights, security systems, data acquisition and telecommunications. Typically, solar energy is used where the grid is not accessible. However, in these places, the use of solar energy is limited by cost factors. However, like solar energy on the roof, off-grid industrial solar power systems will increase in cost and efficiency.
Off-grid power generation systems require large energy harvesters, especially batteries. These circuits require safe and efficient charging to continually improve completeness and integration. For example, Cypress Semiconductor has introduced an integrated solar charger reference design using the PowerPSoC processor. It is powered by a 12V solar panel to slowly charge a 12V lead acid battery. This reference design includes MPPT optimization and a lead acid battery charger.
The product's architecture uses a current-controlled buck rectifier for MPPT and battery charging (see Figure 4). The MPPT and battery charger embedded in the PowerPSoC use voltage and current feedback to operate the panel at peak power and control the buck controller's switches to operate the panel in peak power.
Figure 4 Block diagram of the MPPT/charger controller
In another example, ST Microelectronics developed a highly integrated HBLED solar MPPT charger/driver. This fully integrated solution features an MPPT-optimized battery charger and an integrated HBLED driver. This product will be released in late 2010 and is ideal for HBLED street lighting applications.
Displacement sensor, also known as linear sensor, is a linear device belonging to metal induction. The function of the sensor is to convert various measured physical quantities into electricity. In the production process, the measurement of displacement is generally divided into measuring the physical size and mechanical displacement. According to the different forms of the measured variable, the displacement sensor can be divided into two types: analog and digital. The analog type can be divided into two types: physical property type and structural type. Commonly used displacement sensors are mostly analog structures, including potentiometer-type displacement sensors, inductive displacement sensors, self-aligning machines, capacitive displacement sensors, eddy current displacement sensors, Hall-type displacement sensors, etc. An important advantage of the digital displacement sensor is that it is convenient to send the signal directly into the computer system. This kind of sensor is developing rapidly, and its application is increasingly widespread.
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