#voltagedropcalculation

Designing electrical systems for solar farms requires a methodical approach that balances efficiency, safety, and regulatory compliance. With a properly designed electrical system, it ensures that the energy harvested from photovoltaic (PV) arrays is transmitted with minimal losses and delivered reliably to the grid or storage systems.

Key factors in this process include voltage drop calculation, cable resistance, soil resistivity, and the application of derating factors. Each helps maintain the performance and integrity of the entire electrical infrastructure. In this guide, we'll discuss how to integrate these factors to prepare effective solar farm electrical system designs.

Key Components of Solar Farm Electrical System Design

A solar farm's electrical layout typically includes PV modules, combiner boxes, inverters, step-up transformers, and grid connection points. Power flow involves transferring generated direct current (DC) from solar panels, converted to alternating current (AC), then delivered to the final export point.

The system should accommodate steady-state and fluctuating currents caused by environmental factors like irradiation variability. Furthermore, the infrastructure should handle extended cable runs, often over hundreds of metres, which introduces specific design challenges related to voltage drop and thermal effects.

Importance of Voltage Drop Calculation

Voltage drop refers to the reduction in voltage as electric current flows through a conductor. This is an important design consideration in solar farms, where cables often span long distances. With excessive voltage drop, it can result in significant energy losses and non-compliance with industry standards. The Australian standard AS/NZS 3008 recommends a maximum voltage drop of 5% between the source and the load.

The voltage drop (Vd) can be calculated using the following formula. However, note that the voltage drop calculation is not limited to the formula provided. Other formulas may also be used, considering factors such as cross-sectional area, conductor resistivity, and more:

Vd = I · (Rcos + Xsin) · L

Where:

I = current (A)

R = resistance of the conductor (Ω/km)

X = reactance of the conductor (Ω/km)

L = one-way length of the conductor (km)

θ = phase angle (based on power factor)

Both AC and DC systems require accurate voltage drop calculations. In AC systems, reactance is considered alongside resistance. In DC systems, only resistance is used. That said, utilising voltage drop calculators or engineering software ensures precision, especially when working with parallel cables and large-scale layouts.

Role of Cable Resistance in Design

Cable resistance is a fundamental property that directly affects the efficiency of a solar farm. It's determined by the conductor material, cross-sectional area, and temperature. Copper and aluminium are the most commonly used materials. Copper has lower resistance but is more expensive, while aluminium is cost-effective but has higher resistance.

Moreover, resistance increases with the length of the cable and the temperature. When designing an electrical system, engineers refer to the resistance per kilometre from the cable datasheet. They then use this value to calculate the equivalent resistance for the actual cable length, adjusting for expected operating temperatures.

By choosing cables with low resistance, you can reduce power losses and contribute to the overall efficiency of the installation. However, oversizing cables to reduce resistance should be justified economically, as it can significantly increase material and installation costs.

Moreover, reducing power loss is not limited to cable size selection. Designing the most effective cable route using the shortest possible distance also helps minimise voltage drop. Thus, selecting the correct cable size along the shortest route results in the most effective design.

Accounting for Soil Resistivity

Soil resistivity affects the thermal performance of buried cables and the design of the earthing system. High soil resistivity can lead to poor heat dissipation, resulting in elevated cable temperatures and derating of the current-carrying capacity. It also impacts the effectiveness of grounding systems, which are needed for safety and fault protection.

That said, resistivity is influenced by soil composition, moisture content, and temperature. Therefore, it's highly recommended that site-specific soil resistivity testing be conducted rather than relying on generic values. Measurements typically use the Wenner or Schlumberger methods to obtain accurate data.

Furthermore, designers should refer to Table 11 in AS/NZS 3008 for adjusting cable ratings based on measured soil thermal resistivity. For example, thermal resistivity can exceed 2.5 K.m/W in dry or rocky conditions. To maintain safe operating conditions, either cable derating or installation techniques such as thermal backfill may be necessary.

Applying Derating Factors

Derating factors adjust the current-carrying capacity of cables based on environmental and installation conditions. Failure to apply these factors can lead to overheating, insulation degradation, and cable failure.

Common factors include ambient temperature, cable grouping, soil thermal resistivity, and depth of laying. High ambient temperatures reduce a cable’s ability to dissipate heat, while closely grouped cables generate mutual heating. Soil thermal resistivity impacts underground heat dissipation, and the depth of laying affects cooling, especially with seasonal temperature changes.

For example, cables rated for 90°C may require derating if installed in environments consistently above 40°C. Similarly, multiple cables in a common trench should be derated to prevent overheating.

Integrating All Factors into a Practical Design Process

A structured approach to solar farm electrical design should incorporate the following steps:

Define system voltage and load profiles based on PV output and inverter ratings.

Map cable routes and calculate estimated distances between components.

Select preliminary cable sizes based on current requirements.

Calculate voltage drop to ensure compliance with performance standards.

Evaluate cable resistance for chosen materials and adjust the cable size if necessary to achieve the appropriate resistance.

Measure or estimate soil resistivity and apply thermal resistivity values to underground cable calculations.

Apply derating factors for temperature, grouping, and installation conditions.

Refine cable selection based on adjusted current-carrying capacities and cost-benefit analysis.

Use simulation tools or professional software to verify the design under worst-case scenarios. 

This iterative process ensures the design's technical and economic factors are considered, resulting in a reliable and cost-effective power distribution network.

Ensure Compliant Solar Farm Electrical System Design with CableHero

Effective electrical design helps maximise the return on investment (ROI) in solar farm projects. By rigorously applying voltage drop calculations, understanding cable resistance, and accounting for soil resistivity, engineers can ensure an efficient, cost-effective and compliant system.