Enhancing PV Safety, Performance With Parallel Microinverter Tech

The global shift toward renewables is leading to a surge in solar energy adoption. According to the IEA, global solar photovoltaic (PV) installations stood at about 135 GW in 2020, a 23% increase from 2019. As solar installations continue to grow, there will be an increased focus on improving the safety, reliability, and performance of PV systems. Microinverter technology is a viable solution for addressing the limitations of conventional string inverters and optimizing solar energy systems for maximum efficiency and safety.

Smart micro inverters are set to optimize solar energy systems in the near future. Image used courtesy of Hoymiles

Challenges With Conventional String Inverters

In a typical string inverter system, PV panels are connected in series, with voltages up to 1000 VDC or greater. Such high voltages increase the risk of fire hazards, as any fault or short circuit can lead to DC arc faults. These fires are difficult to put out and reach temperatures over 3000 °C. The risk of fires is particularly concerning in residential and industrial settings where PV systems are installed on rooftops, often close to flammable materials.

In addition to the fire hazard, the higher-voltage DC wiring in string inverters also increases electric shock risk during installation and maintenance. DC voltages greater than 120 V can be lethal, and accidental contact with live wires can cause serious injury or even death. To mitigate these safety risks, working with high-voltage components requires specialized safety equipment, such as insulated tools, personal protective equipment (PPE), and proper grounding and bonding techniques.

Installation and maintenance procedures also become more complex and time-consuming, as technicians must exercise extreme caution. This complexity not only increases installation and maintenance costs but also requires highly trained and experienced personnel to ensure safe and proper handling of the high-voltage components.

In a string inverter system, all panels in that string depend on a single inverter, creating a single point of failure. If that inverter fails (e.g., due to a faulty component, software malfunction, or overheating), the entire array goes offline until it is repaired or replaced. This centralized architecture reduces the overall system reliability. It also increases maintenance costs as the cost of repairing or replacing the inverter can be substantial, especially if the product is no longer under warranty.

The Barrel Effect and Sub-Optimal Energy Production

String inverters are prone to the “Barrel Effect,” which can significantly reduce the energy output of the PV system. The Barrel Effect occurs when the least-performing panel in a series string limits the current and output of the entire string. Where one panel in the string is shaded, dirty, or underperforming due to manufacturing defects or degradation, it acts as a bottleneck, restricting current flow and reducing the output of the entire string.

Cloud shading is one cause of inconsistent solar panel performance. Image used courtesy of Adobe Stock

The Barrel Effect arises from inconsistencies in the output characteristics of PV panels, as panels from the same manufacturing batch tend to have variations in manufacturing tolerances. Over time, factors like uneven soiling, partial shading from nearby objects like trees and buildings, or aging can further widen the disparities in panel performance.

Benefits of Parallel Microinverter Technology

With microinverters, panels can be installed in various orientations and angles, including on multiple roof faces. This flexibility allows optimal site utilization and can be particularly advantageous in complex or space-constrained installations.

Microinverters overcome string inverters' safety and performance challenges through their parallel design. Rather than connecting the panels in series, microinverters allow each panel to operate independently, with its own inverter converting DC into AC at the module level. This distributed architecture eliminates the need for high-voltage DC connections between panels.

When a single panel fails or underperforms, a microinverter system (left) provides 4.67 kW power compared to only 4.30 kW for a traditional string inverter single (right). Image used courtesy of Hoymiles

Unlike string inverters, microinverter systems typically operate at DC voltages below 60 V, which falls within the low voltage category. The lower-voltage operation reduces the risk of fire during installation and maintenance, as the energy available for arc faults is much lower. Even if a fault occurs, the low voltage makes the system touch-safe.

Microinverters also provide built-in compliance with rapid shutdown requirements, a safety feature mandated by the U.S. National Electrical Code (NEC). Rapid shutdown requires PV systems to quickly reduce DC voltages to safe levels when an emergency occurs, ensuring the safety of first responders. With microinverters, the AC output can be easily isolated, and the low DC voltages at the panel level meet rapid shutdown requirements without additional equipment.

Optimizing Performance Characteristics With Module-Level MPPT

Microinverters utilize maximum power point tracking (MPPT) technology at the module level. This allows each module to operate at its optimal power point. MPPT uses an algorithm that constantly adjusts the inverter's impedance to match the panels’ changing output characteristics, ensuring maximum power extraction under varying conditions.

In string inverter installations, MPPT is applied across the entire string, finding a single maximum power point that is a compromise between these connected panels. However, a centralized approach fails to account for the characteristics and conditions of each panel, leading to suboptimal power extraction.

In contrast, microinverters perform MPPT independently for each module. This allows every panel to operate at its individual peak power point throughout the day, adapting to changing sunlight conditions, temperature variations, and even partial shading.

Microinverters support independent maximum power point tracking for each module. Image used courtesy of Adobe Stock

Module-level MPPT in microinverters can also help mitigate the effects of panel mismatch and degradation over time. As panels age, their output characteristics change at different rates. With string inverters, the MPPT algorithm does not compensate for mismatches, reducing overall system performance. Microinverters, on the other hand, continuously optimize the power output of each panel independently, maintaining high efficiency throughout the system's lifetime.

Eliminating the Barrel Effect

Since the parallel architecture of microinverters allows each PV panel to operate independently, it effectively eliminates the barrel effect that plagues string inverter systems. When a panel is shaded or underperforming, it does not limit the current or output of the neighboring panels. This independent operation ensures that each panel contributes its maximum possible power to the system.

The distributed architecture of a microinverter eliminates single points of failure. If one microinverter fails, only the connected panel is affected, while the rest of the system continues to operate normally.

Hoymiles Microinverter Solutions

Hoymiles HMS-1000 series 2-in-1 microinverters are designed for high-powered panels and can connect two PV modules. The four products in the family support 700, 800, 900, and 1000 VA of peak output power. The HMS-2000 series 4-in-1 microinverters support four panels with peak output powers of 1600 to 2000 VA.

Hoymiles series microinverters can connect up to 4 panels. Image used courtesy of Hoymiles

These products achieve CEC peak efficiencies of 96.7%, minimizing conversion losses and maximizing energy harvest. Their wide MPPT voltage range of 16 to 60 V enables reliable operation across a range of environmental conditions and panel configurations.

Energy Efficiency

Hoymiles microinverters can generate power from sunrise to sunset, even under low-light conditions. With a start-up voltage of just 22 V, these microinverters can begin producing power earlier in the day and continue operating later into the evening compared to string inverters. This extended production window can result in up to 1.5% higher yield over the same time period.

Safety and Performance

The module-level MPPT in Hoymiles microinverters is finely tuned to rapidly adjust the output, enabling accurate maximum power point tracking. This algorithm can rapidly respond to changes in irradiance and temperature, ensuring optimal power extraction in changing conditions.

Protection Features

Hoymiles microinverters incorporate several advanced features to prioritize safety and optimize performance. The fully parallel circuit design maintains DC voltages below 60 V, mitigating the risk of shock and arc faults. The units are tested to 6000 V surge limits and have an IP67 enclosure rating, ensuring reliable operation in harsh environmental conditions.

Cloud Monitoring

HMS-1000 and HMS-2000 series microinverters offer cloud monitoring capabilities for remote system monitoring and performance analysis. The monitoring platform provides detailed insights at the panel level, enabling faster identification and troubleshooting of performance issues. This granular monitoring helps maintain system efficiency and reduces maintenance costs over the system's lifetime.

A Cleaner Energy Future

The transition to a cleaner energy future requires increased solar energy adoption and continuous innovation in PV system technology. Professional system designers and DIY users, like homeowners, can benefit from the many advantages of parallel microinverters.

Parallel microinverters overcome the limitations of string inverters in terms of safety, performance, and reliability:

• Their lower operating voltages can minimize the risk of fire outbreaks and electric shock.
• The parallel architecture prevents underperforming panels from limiting the energy output of the PV array.
• Their flexibility allows installation at various orientations and angles.
• With fewer system-wide failures and repairs, maintenance expenses can be reduced over the lifetime of the installation, providing peace of mind and financial protection for system owners.

To learn more about Hoymiles microinverters and how they combine performance and safety features, visit the Hoymiles website.

This article was first published on EEPower.