Analyzing Statements about EHV Lines

The question asks to identify the statement that is INCORRECT regarding Extra High Voltage (EHV) transmission lines. Let's examine each option provided.

Evaluating EHV Line Characteristics Statements

• Statement 1: The line can be easily tapped and or extended as the power need not be converted.

  EHV lines carry power over long distances at very high voltages to minimize losses. Tapping an EHV line directly for distribution or lower voltage transmission is not easy. It requires substations to step down the voltage significantly using transformers. Therefore, the power voltage *does* need to be converted (stepped down) to make it usable or manageable for distribution or industrial purposes. This statement is likely incorrect because tapping isn't "easy" without voltage conversion.

• Statement 2: The surface-voltage gradient on conductors becomes lower as the voltage increases.

  The electric field strength at the surface of a conductor, known as the surface voltage gradient, is directly related to the voltage applied to the conductor. For a simple cylindrical conductor in free space, the electric field ($E$) at the surface is given by:

  \(E = \frac{V}{r \ln(D/r)}\)

  Where \(V\) is the voltage, \(r\) is the conductor radius, and \(D\) is the distance to a reference point (or distance between conductors in a simplified line model). This formula shows that increasing the voltage \(V\) will generally increase the electric field \(E\), and thus the surface voltage gradient, if the conductor geometry (\(r\) and \(D\)) remains constant. High surface voltage gradients can lead to corona discharge. To manage the gradient at EHV levels, engineers use larger conductors or, more commonly, bundled conductors. However, the fundamental principle is that increasing voltage *increases* the gradient, not lowers it, unless specific design measures are taken to counteract this effect. The statement claims the gradient becomes *lower* as voltage increases, which contradicts this principle.

• Statement 3: Current density increases as voltage increases with charging current.

  EHV lines have significant capacitance to ground and between conductors. The charging current (\(I_c\)) is proportional to the line voltage (\(V\)) and the line capacitance (\(C\)) and frequency (\(f\)): \(I_c = 2\pi f C V\). As voltage \(V\) increases, the charging current \(I_c\) also increases. Current density is the current per unit cross-sectional area of the conductor. If the conductor size doesn't increase proportionally much more than the charging current increases, the current density associated with the charging current will tend to increase as voltage increases. This statement seems plausible in the context of charging current effects at EHV.

• Statement 4: Voltage can be steeped up or stepped down to the required level using transformers.

  Transformers are essential components of AC power systems, including EHV transmission. They are used at generating stations to step up voltage for transmission and at substations to step down voltage for distribution and utilization. This statement is a fundamental fact about power transmission and is correct.

Identifying the Incorrect Statement

Based on the analysis:

• Statement 4 is correct.
• Statement 3 is plausible regarding the effect of increasing charging current with voltage.
• Statement 1 is incorrect about easy tapping without voltage conversion.
• Statement 2 is fundamentally incorrect regarding the relationship between voltage and surface voltage gradient on a given conductor geometry. Increasing voltage tends to *increase*, not lower, the gradient, requiring design considerations to mitigate this.

The statement that is most fundamentally and unequivocally incorrect based on the physics of electric fields around conductors is the second one. While EHV line designs use techniques (like bundling) to *limit* the gradient at high voltages compared to a single conductor of the same cross-section, the statement claims it becomes *lower* simply as voltage increases, which is not true without those specific design considerations.

Therefore, the INCORRECT statement is that the surface-voltage gradient on conductors becomes lower as the voltage increases.