Does an AAC conductor have a higher resistance compared to other conductors, even though it's made of pure aluminum

AAC (All Aluminum Conductor) is widely used in electrical power transmission and distribution systems due to its lightweight nature and conductivity. However, one of the most commonly observed characteristics of AAC conductor  is their relatively higher electrical resistance when compared to other types of conductors, such as ACSR (Aluminum Conductor Steel Reinforced) or AAAC (All Aluminum Alloy Conductor). This characteristic may seem counterintuitive because aluminum is a well-known conductor of electricity. To understand why this happens, we need to analyze various factors that influence electrical resistance in conductors.

Understanding Electrical Resistance in Conductors

Electrical resistance is a property of a material that determines how much it opposes the flow of electric current. It is governed by Ohm’s Law, which states:

$$R = \rho \frac{L}{A}$$

where:

• $R$ = Resistance (Ohms, Ω)
• $\rho$ = Resistivity of the material (Ohm-meter, Ω·m)
• $L$ = Length of the conductor (meters, m)
• $A$ = Cross-sectional area of the conductor (square meters, m²)

From this formula, we can see that resistance depends on three factors:

1. Material Resistivity
2. Length of the Conductor
3. Cross-sectional Area

Now, let’s explore why the resistance of AAC conductors is higher compared to other conductors.

Factors Affecting the Resistance of AAC Conductors

1. Pure Aluminum vs. Alloyed or Composite Conductors

AAC conductors are made entirely of pure aluminum (EC Grade Aluminum, which has a purity of 99% or more). While pure aluminum is a good conductor, it has a higher electrical resistivity than aluminum alloys or other conductor materials such as copper.

For reference:

• Resistivity of Pure Aluminum (AAC): ~2.82 × 10⁻⁸ Ω·m
• Resistivity of Aluminum Alloys (AAAC): ~2.65 × 10⁻⁸ Ω·m
• Resistivity of Copper: ~1.68 × 10⁻⁸ Ω·m

Since resistance is directly proportional to resistivity, a higher resistivity means a higher resistance. This explains why AAC conductors exhibit higher resistance than AAAC conductors, which are made of aluminum alloys with lower resistivity.

2. Stranding Effect and Contact Resistance

AAC conductors are stranded rather than solid. A stranded conductor consists of multiple aluminum wires twisted together to form a single conductor. While stranding improves flexibility, it also introduces small air gaps between the strands. These air gaps increase the effective resistance of the conductor due to the following reasons:

• Reduced Effective Cross-Sectional Area
  The presence of gaps means that the actual conducting area is slightly lower than the apparent area. This results in an increase in resistance.

• Contact Resistance Between Strands
  Even though the aluminum strands are in contact, there is still some resistance at the points of contact. This adds to the overall resistance of the conductor.

3. Increased Skin Effect in AAC Conductors

Skin effect is a phenomenon in AC (Alternating Current) systems where the current tends to concentrate near the surface of the conductor. This occurs because alternating current induces eddy currents within the conductor, causing a redistribution of current density.

• In AAC conductors, the absence of a steel core (as found in ACSR conductors) means that more current flows through the outer aluminum strands.
• Since pure aluminum has a higher resistivity, this concentration of current increases the effective resistance at high frequencies.

In contrast, in ACSR conductors, the steel core does not carry much current, so the aluminum strands remain more efficient at conducting electricity, leading to a lower overall resistance.

4. Thermal Expansion and Its Effect on Resistance

Metals expand when they are heated, and aluminum has a relatively high coefficient of thermal expansion compared to other conductive materials.

• When an AAC conductor carries current, it heats up due to electrical resistance.
• As the conductor expands, the gaps between the aluminum strands may increase, reducing effective conductivity.
• This leads to an increase in resistance over time.

In contrast, ACSR conductors, which have a steel core, experience lower thermal expansion and maintain their shape more effectively under high temperatures.

5. Absence of Reinforcing Material (Compared to ACSR Conductors)

ACSR conductors consist of a steel-reinforced core surrounded by aluminum strands. The presence of the steel core provides two advantages:

1. Improved Mechanical Strength – The steel core enables ACSR conductors to be used over longer spans without sagging.
2. Lower Electrical Resistance – Since the aluminum strands are used more efficiently in ACSR, the overall electrical resistance is lower compared to AAC.

Since AAC conductors lack this steel reinforcement, they have to be designed with a larger cross-sectional area to compensate for the increased resistance, making them less efficient in long-distance power transmission.

Comparing AAC Conductors with Other Types

From this table, we can see that AAC has a higher resistance compared to AAAC and ACSR, mainly due to its use of pure aluminum and lack of reinforcement.

Practical Implications of High Resistance in AAC Conductors

Due to its relatively high resistance, AAC conductors have specific applications where this property is acceptable. Some important implications include:

1. Shorter Transmission Distances

Since higher resistance leads to more power loss due to I²R losses, AAC conductors are not suitable for long-distance transmission lines. They are primarily used in short-distance power distribution.

2. Heat Generation and Efficiency Losses

Higher resistance means that AAC conductors tend to generate more heat when carrying current. This can lead to:

• Energy wastage due to resistive losses.
• Potential overheating if the current exceeds the rated capacity.

3. Use in Urban Areas

AAC conductors are commonly used in cities and densely populated areas where short-span distribution lines are required. The high conductivity-to-weight ratio of aluminum makes it easier to install on utility poles.

4. Suitability for Low Voltage Applications

AAC is often used in low voltage power networks where resistance-related power losses are not as critical as in high voltage transmission lines.

Conclusion

The reason why an AAC conductor has a higher resistance compared to other conductors, despite being made of pure aluminum, lies in multiple interconnected factors:

1. The intrinsic resistivity of pure aluminum is higher than that of aluminum alloys or copper.
2. The stranded construction introduces air gaps and contact resistance.
3. The absence of a steel core (as in ACSR) means that aluminum strands bear the entire electrical load, increasing resistance.
4. The thermal expansion of aluminum affects the contact between strands, further influencing resistance.
5. The skin effect causes resistance to rise in AC applications, particularly at high frequencies.

Although AAC conductors have higher resistance, they are still widely used in electrical distribution systems due to their lightweight nature, corrosion resistance, and cost-effectiveness. However, for long-distance power transmission, other conductors such as ACSR and AAAC are preferred due to their lower resistance and higher mechanical strength.

By understanding these principles, electrical engineers can make informed decisions when selecting conductors for different applications, ensuring optimal efficiency and reliability in power systems.