When designing a solar panel array, several key factors determine the optimal number of panels that can be connected to an inverter. The goal is to balance maximizing system performance while ensuring safety and compliance with industry standards. In this article, we’ll explore why Voc (open-circuit voltage) is prioritized over Vmp (maximum power point voltage) in system design, the rationale behind the 1.1 safety factor, and how to approach designing residential installations under the updated AS/NZS 4777.1 standards.

Why Voc is Prioritised Over Vmp in Design

Voc (Open-Circuit Voltage):
1. Voc represents the maximum voltage a solar panel can generate when it’s not under load (no current is flowing). This is a critical parameter because it indicates the highest possible voltage the panel will produce under the most extreme conditions, such as very cold temperatures.
Vmp (Maximum Power Point Voltage):
1. Vmp is the voltage at which the panel generates its maximum power under standard operating conditions. While Vmp is important for efficiency calculations, it is not as critical for ensuring safety because it represents typical, not peak, conditions.
Why Voc is More Critical:

Voc is prioritized because it represents the worst-case scenario in terms of voltage. When temperatures drop, the Voc of each panel increases, potentially pushing the system voltage beyond what the inverter can handle. This can damage the inverter and pose safety risks. Vmp, while essential for efficiency calculations, does not reflect these edge cases, which is why designers focus on Voc for determining the maximum number of panels per string.

The Importance of the 1.1 Safety Factor

The commonly applied 1.1 multiplier accounts for potential voltage increases due to cold temperatures. Solar panels are tested at 25°C (Standard Test Conditions, STC), but in colder conditions, the voltage can rise significantly.

Accounting for Temperature Effects:

The 1.1 multiplier is a safety factor applied to Voc to account for the increase in voltage that occurs in cold conditions. Each panel has a temperature coefficient for voltage, typically around -0.3%/°C. This coefficient means that for every degree Celsius the temperature drops below the Standard Test Condition (STC) of 25°C, the panel’s Voc increases by 0.3%.

Calculating the Adjusted Voc:

In colder climates, where temperatures may drop significantly, this factor becomes more critical. For example, if the temperature drops 30°C below STC, the Voc of a panel could increase by 9% or more. This is why the standard 1.1 multiplier is often used as a baseline, but in some cases, a factor of 1.15 or 1.2 might be more appropriate.

Example calculation:

Panel Voc: 42V
Temperature drop: 30°C
Adjusted Voc: 42V×(1+(0.003×30))=42V×1.09=45.78V

Using this adjusted Voc, you can safely determine the number of panels per string while ensuring the total voltage stays within the inverter’s safe operating range.

Residential Installations: 600V vs. 1000V DC Limitations

600V DC Limitation in AS/NZS 4777.1:2016:
1. Historically, residential installations in Australia and New Zealand were capped at 600V DC. This limit was imposed to reduce risks in residential environments, where the likelihood of improper handling or accidental contact is higher. This restriction led to shorter strings, more wiring, and potentially higher installation costs.
1000V DC Under AS/NZS 4777.1:2024:
1. The new 2024 standard allows residential systems to operate up to 1000V DC. This change aligns with international practices and enables installers to use longer strings, reducing wiring complexity and losses, and potentially lowering overall system costs. However, this increase also requires careful planning to ensure that safety margins are maintained, particularly when considering temperature-related voltage increases.

Commercial Installations: Higher Complexity and Larger Systems

Higher Voltage and Power Capacities:
1. In commercial installations, the maximum allowable DC voltage is typically 1000V or even 1500V. This allows for much longer strings and larger arrays, reducing overall system costs due to fewer connections and lower wiring losses.
Considerations for Large-Scale Systems:
1. Phase Balancing: Commercial systems often involve three-phase power, so balancing the load across phases is critical. The updated AS/NZS 4777.1 standard now introduces specific phase balance limits, such as capping single-phase installations at 30 kVA.
2. Grid Protection: Larger systems require more sophisticated protection, including interface protection for systems above 200 kW. In some cases, commercial systems need to integrate SCADA (Supervisory Control and Data Acquisition) systems to meet the requirements of the Distribution Network Service Provider (DNSP).
Oversizing Considerations:
1. Commercial inverters often allow some degree of DC oversizing (e.g., up to 1.3 times the inverter’s rated AC output). This strategy improves energy yield during low-light conditions but requires careful planning to avoid excessive power clipping during peak production.

Step-by-Step Design Process

Determine the Inverter’s Maximum DC Input Voltage:
1. For example, if the inverter’s maximum voltage is 1000V, this is the upper limit for your string’s combined Voc.
Adjust the Voc Based on the Safety Factor:
Adjusted Voc=42V×1.1=46.2V
Calculate the Maximum Number of Panels Per String:
1000V / 46.2V ≈21 panels per string

Round down to ensure safety.
Verify Against MPPT Limits:
1. Check the inverter’s datasheet for the number of strings and panels that can be connected to each MPPT. Ensure your design stays within these limits.
Consider the Inverter’s Power Capacity:
1. Even if the voltage is within range, ensure that the total DC power does not exceed the inverter’s recommended input power. Oversizing the array can lead to power clipping, where the inverter cannot process all the available power, reducing system efficiency.
Check Local Regulations and Standards:
1. For residential installations, ensure compliance with the updated AS/NZS 4777.1 standard, particularly the new 1000V limit and interface protection requirements.

Useful Link

https://ausinet.com.au/max_voc_min_vmp/

Conclusion

Designing a solar panel array involves balancing performance, safety, and regulatory compliance. By focusing on Voc instead of Vmp, applying an appropriate safety factor, and considering updated standards like the 1000V limit under AS/NZS 4777.1:2024, you can design a system that maximizes efficiency while ensuring safety. The considerations differ slightly between residential and commercial installations, with commercial systems offering more flexibility but requiring more complex design and grid protection measures.

For residential systems, the transition from 600V to 1000V allows for more efficient designs with longer strings, while commercial systems benefit from larger arrays with streamlined wiring and lower losses. By following these principles, your team can design safe, compliant, and efficient solar solutions tailored to any installation scenario.

Other Considerations

When designing the number of solar panels for a system, there are several additional factors you should be aware of to ensure both safety and optimal performance. These considerations go beyond simply calculating the maximum number of panels per string and include system-specific, environmental, and regulatory aspects.

1. Irradiance and Shading Conditions

Shading Impact: Shading from trees, buildings, or other obstructions can reduce the energy output of an entire string if even one panel is shaded. This is because panels in series share the same current. To mitigate this, consider using:
- Optimizers or Microinverters: These devices allow individual panels to operate independently, reducing the effect of shading.
- String Configuration: Plan your string layout to avoid shading issues and maximize exposure to sunlight.
Irradiance Variation: Consider how panel orientation and tilt impact irradiance throughout the year. Suboptimal angles can lower output and affect string sizing if you’re attempting to maximize energy production.

2. System Voltage Drop

Voltage drops occur in long wiring runs between panels and the inverter. While this may not directly affect panel count, it does impact overall system efficiency. Be mindful of the length of your DC cables and consider upsizing cables to minimize losses, particularly for larger commercial systems.

3. Inverter Efficiency Curves and Power Clipping

Efficiency at Different Loads: Inverters have different efficiency curves, meaning they operate most efficiently within certain input voltage ranges. Oversizing your array can cause power clipping (when the inverter reaches its maximum output and cuts off excess power), especially during peak production hours.
Oversizing Ratios: Depending on your inverter, oversizing the DC capacity (up to 1.3 times the inverter’s AC rating) can increase energy yield without significant losses due to clipping.

4. Thermal Effects on Panels and Inverter Performance

Temperature Effects on Panels: Beyond voltage increases due to cold temperatures, high temperatures also affect panel performance. While Voc increases in cold, the overall efficiency of panels decreases in hot conditions due to higher resistive losses.
Inverter Cooling: Ensure that the inverter’s cooling capabilities align with the expected temperature range. High ambient temperatures can lead to thermal derating, where the inverter reduces its output to prevent overheating.

5. Regulatory Requirements

Local Building Codes and Fire Safety Regulations: In some regions, there are specific guidelines regarding setback distances, fire safety access, and roof loading. These factors may limit how many panels can be installed and where they can be placed.
Compliance with Grid Connection Standards: In regions like Australia, the updated AS/NZS 4777.1 and other standards impose grid protection measures, phase balancing requirements, and limits on export to the grid. Be sure your design aligns with both national and local standards.

6. Future Expansion and Upgradability

Consider whether the system might need to be expanded in the future. If so, leave room in the design for additional panels or an inverter upgrade. This is particularly relevant in commercial systems, where expansions are common as energy needs grow.

7. Safety and Grounding

Proper Grounding and Earthing: Ensure the system is properly grounded, especially for higher-voltage systems (like those operating at 1000V). Poor grounding can lead to dangerous electrical faults and affect the longevity of your components.
Surge Protection: Depending on the location, surge protection devices (SPDs) may be required to protect against lightning strikes or voltage surges, especially in commercial installations.

8. Cable Management and Layout Optimization

String Layout: Optimize panel placement to minimize cable lengths, reducing voltage drop and installation costs. This is particularly important in large commercial setups where cable runs can be extensive.
Conduit and Raceway Planning: Plan the layout in a way that reduces the need for long conduits and raceways, which can introduce additional costs and complexity.

9. Energy Storage and Backup Integration

If the system includes battery storage, ensure that the inverter and battery systems are compatible and that the panels are sized to provide sufficient energy for both immediate use and storage. This may involve using hybrid inverters that can manage both grid-tied and off-grid modes.