Cable sizing is a critical aspect of solar projects, especially in large installations where long cable lengths are involved. Cables play a crucial role in transmitting electrical energy from the solar panels to the inverter, and from there to the grid or battery bank. Inadequate cable sizing can result in significant power losses, voltage drop, and even system failure. In this blog post, we will discuss the importance of cable sizing in solar projects, how much AC and DC cables are typically used on a per MW basis, voltage drop criteria for cable sizing calculations, and provide a step-by-step example of cable sizing calculations.
Importance of Cable Sizing in Solar Projects
Cable sizing is critical in solar projects as it determines the amount of electrical energy that can be transmitted from the solar panels to the inverter. The size of the cable is determined by several factors, including the current carrying capacity, cable length, ambient temperature, and voltage drop. If the cable is undersized, it can lead to significant power losses, voltage drop, and even system failure. On the other hand, oversizing the cable can increase the cost of the project unnecessarily.
AC and DC Cable Sizing in Solar Projects
In solar projects, both AC and DC cables are used. AC cables are used to transmit power from the inverter to the grid, while DC cables are used to connect the solar panels to the inverter. The amount of cable used in a solar project varies depending on the size of the installation. On a per MW basis, the typical amount of AC and DC cables used are:
- AC Cable: 1.5 to 2 km
- DC Cable: 15 to 20 km
Voltage Drop Criteria for Cable Sizing Calculations
Voltage drop is the reduction in voltage that occurs as electrical energy is transmitted through the cable. Voltage drop is affected by several factors, including the cable length, current, and cable resistance. To ensure that the voltage drop is within acceptable limits, industry standards specify maximum voltage drop percentages for various applications. For instance, for solar projects, the maximum voltage drop is typically around 2% for AC cables and 1% for DC cables.
Step-by-Step Cable Sizing Calculation Example
To demonstrate cable sizing calculations, we will use the following data for a 500 kWp solar power plant:
- Selected Cable Specification: 3.5C X 50 Sq. mm Armoured Aluminum Cable
- Cable Resistance: 0.82 Ohm/km
- Cable Reactance: 0.078 Ohm/km
- Current Carrying Capacity: 140 A
- Ambient Temperature: 35 deg. C
- Inverter Voltage: 415 V
- Max AC Output Current: 96 A
- Approx Cable Length: 75 m
Step 1: Calculate Full Load Current
Full Load Current is calculated as follows:
Full Load Current = Inverter Max AC Output Current / Power Factor
In this case, Power Factor (cos ϕ) is assumed to be 1.
Full Load Current = 96 A / 1 = 96 A
Step 2: Calculate Voltage Drop
Voltage Drop is calculated as follows:
Voltage Drop = Full Load Current x Cable Resistance x Cable Length / 1000
In this case, Cable Length is 75 m.
Voltage Drop = 96 A x 0.82 Ohm/km x 75 m / 1000 = 5.9024 V
Step 3: Calculate % Voltage Drop
% Voltage Drop is calculated as follows:
% Voltage Drop = Voltage Drop / Inverter Voltage x 100
% Voltage Drop = 5.9024 V / 415 V x 100 = 1.42%
The % Voltage Drop for this installation is within
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