The Latest Trends in Solar Inverter Design

Solar modules generate a DC voltage, and solar inverters convert this DC power into AC power, which is then fed into the grid . This article will explore the latest trends in solar inverter design.

One important trend is the adoption of higher powers. Today, solar power plants with peak power generation exceeding 100kW are becoming more common, and this trend also exists in smaller-scale power generation systems: the average power increases from 5kWp to 10kWp.

Figure 1 Common solar inverter topology: boost + H-bridge

The boost + H-bridge topology is one of the most commonly used topologies for solar inverters. It is a two-stage non-isolated topology. The first stage is the boost stage, which is used to increase the variable output voltage of the module (for example, 100V – 500V) to a larger intermediate voltage, which must be greater than the actual peak mains voltage (such as 230V x sqrt(2), or >325V). The boost stage also has an important role, that is, in order to maximize efficiency, the solar module must operate to produce as much power as possible, and the power curve of the solar module can be obtained by multiplying the output current by the output voltage value. There is a maximum point in the power characteristic, which is called the "maximum power point" or MPP, and this exact location varies with factors such as module type, temperature and sunlight shadow.

Using a software technique called maximum power point tracking, or MPPT, along with a customized algorithm, the inverter’s input stage tracks this maximum power point.

The second stage of the inverter converts the constant intermediate voltage into an AC voltage of 50Hz, which is then fed into the mains. This output is synchronized with the phase and frequency of the mains. Since this stage is connected to the mains, it must meet certain safety standards even in fault conditions. In addition, there is a new draft of VDE 0126-1-1 related to the Low Voltage Directive, which requires solar inverters to actively support the main power grid even in the case of power quality degradation to minimize the risk of more general power outages. Under the existing regulatory restrictions, it is possible to design an inverter that can shut down in real time in the event of a power outage to achieve self-protection. However, when solar inverters become popular and account for a considerable share of total power generation, if the connected solar inverters are directly shut down when a power outage occurs, it is possible to cause a larger-scale main grid power outage, because the inverters will be shut down one by one and quickly reduce the power in the grid. The new draft directive therefore aims to improve the stability and power quality of the mains distribution network at the expense of only making the inverter output stage slightly more complex.

Solar inverters must be reliable to minimize maintenance and downtime costs. They must also be efficient to maximize power generation. Solar inverter designers also put considerable effort into maximizing efficiency.

There are many ways to improve the efficiency of a boost inverter. Since the boost inverter can operate in continuous conduction mode or boundary conduction mode (CCM or BCM), different optimization schemes are derived. In CCM mode, a major cause of losses is the reverse recovery current of the boost diode; in this case, a silicon carbide diode or Fairchild Semiconductor's Stealth diode is generally used. Solar inverters are more commonly operated in BCM mode, and although CCM mode is usually recommended for this power level, the reason for using BCM mode is that the forward voltage of the diode is much lower in BCM mode. In addition, BCM mode also has much higher EMI filter and boost inductor ripple current. Here, good high-frequency inductor design is a solution.

A new approach is to use two interleaved boost stages instead of one. This way, the current through each inductor and each switch can be halved. In addition, with interleaving, the ripple current on one stage can compensate for the ripple current on another stage, thus removing the input ripple current over a wide operating input range. Controls such as the FAN9612 interleaved BCM PFC can easily meet the requirements of the solar boost stage.

There are two options for the boost switch in the inverter: IGBT or MOSFET. For input stages that require a rated switching voltage of more than 600V, 1200V IGBT fast switches such as FGL40N120AND are often used. For input stages that only require a rated voltage of 600V/650V, MOSFETs are selected.

Designers of output H-bridge stages have traditionally used 600V/650V MOSFETs, but new draft specifications requiring four-quadrant operation of the output stage have rekindled interest in IGBTs in this area. MOSFETs have built-in body diodes, but their switching performance is poor compared to the combined package diodes used in IGBTs. New field-stop IGBTs can switch voltages at speeds of 10V/ns, which greatly improves conduction losses over older products. The integrated diode has excellent soft recovery performance, which helps reduce EMI caused by high di/dt above 500A/us. For 16kHz-25kHz switching, IGBTs such as Fairchild Semiconductor's FGH60N60UFD are recommended.

Three-level