Short Answer:

Power transfer capability of a transmission line is inversely related to the line’s impedance. This means that as the impedance of the line increases, the ability to transfer power decreases. Impedance includes both resistance and reactance, and it limits how much current can flow for a given voltage.

Lower impedance allows more power to be transmitted efficiently over the line with less voltage drop and energy loss. On the other hand, high impedance restricts current flow, increases losses, and reduces system stability. Therefore, managing line impedance is important for safe and effective power transmission.

Detailed Explanation:

Power Transfer Capability and Impedance

Power transfer capability refers to the maximum amount of electrical power that can be transferred from the source to the load through a transmission line without exceeding safe limits. It depends on several factors, but one of the most critical among them is the impedance of the transmission path.

Impedance (Z) is the combination of resistance (R) and reactance (X) in an AC circuit. It determines how much the circuit opposes the flow of current. The relationship between power transfer and impedance is based on Ohm’s Law and the power equations used in electrical engineering.

Mathematical Relation

The power transferred in an AC transmission line can be approximated using the following equation (for a simplified system):

P = (V₁ × V₂ / X) × sin(δ)

Where:

• P = Active power transferred
• V₁ and V₂ = Sending and receiving end voltages
• X = Line reactance (dominant part of impedance in high-voltage systems)
• δ = Power angle (difference in phase between V₁ and V₂)

From this formula, we can clearly see that power (P) is inversely proportional to reactance (X). That means:

• If X increases → P decreases
• If X decreases → P increases

This inverse relationship shows that lower impedance results in higher power transfer capability.

Practical Impact of High Impedance

1. Reduced Current Flow
   High impedance restricts current flow, which means less power reaches the load. This results in inefficient transmission, especially over long distances.
2. Voltage Drop
   Increased impedance causes a significant voltage drop along the line, reducing the voltage available at the receiving end. This affects the performance of electrical equipment.
3. Power Losses
   High impedance, especially due to resistance, leads to greater I²R losses, where energy is lost as heat. This reduces the overall efficiency of the power system.
4. System Stability Issues
   High impedance can make the power system less stable. A small change in load or angle difference can cause large changes in current and voltage, leading to voltage collapse or blackout in worst cases.

How to Improve Power Transfer Capability

1. Reducing Line Impedance
   Using larger diameter conductors, shorter line routes, and better conductor materials (like copper) can reduce resistance and reactance, lowering total impedance.
2. Series Compensation
   Installing series capacitors reduces the effective reactance of the line. This increases power transfer capability without changing the physical layout of the line.
3. Increasing Transmission Voltage
   Higher voltage reduces current for the same power, and therefore, reduces the effect of impedance. This is why high-voltage transmission is used for long distances.
4. Using Parallel Transmission Paths
   Adding parallel lines reduces the overall impedance of the transmission path, increasing the current-carrying capacity and improving reliability.
5. Advanced Technologies (FACTS Devices)
   Devices like STATCOM, SVC, and TCSC dynamically control the line impedance and reactive power, allowing more power to be transferred safely and efficiently.

Real-World Examples

• In long rural transmission lines, high impedance can make power delivery inefficient and unstable. Engineers reduce impedance by using wider conductor spacing and high-voltage levels.
• In urban networks, short distances and thick conductors reduce impedance, allowing high power flow even at moderate voltages.

Conclusion

Power transfer capability in a transmission line is strongly affected by its impedance. As impedance increases, the ability to transfer power decreases due to higher voltage drops, losses, and reduced current flow. To ensure efficient and stable power transmission, impedance must be minimized or compensated using technical solutions such as series capacitors, high-voltage lines, and FACTS devices. Proper management of impedance ensures that electrical energy is delivered safely, economically, and with high reliability.