A short length of cable between the dead end tower and the power transformer

Question

Examoogle · Accepted Answer

Option A: Always reduces the steepness of the incident wave

Explanation & Concepts:
A short length of cable inserted between an overhead line (high surge impedance) and a transformer (low surge impedance) acts as a surge modifier. It effectively reduces the steepness of the incident surge voltage wave, thereby protecting the transformer insulation from high-frequency transients.

Working Principle: The principle relies on the concept of surge impedance (characteristic impedance $Z$). When an incident wave travels from a high-impedance medium ($Z_1$) to a lower-impedance cable section ($Z_2$), and then to the transformer, reflections and refractions occur at the junctions. The capacitance of the short cable section slows down the wavefront rise time (steepness), effectively filtering the high-frequency components of the lightning surge.

Key Formulas: $\alpha = \frac{2Z_2}{Z_1 + Z_2}$ — Refraction coefficient of voltage; $Z = \sqrt{\frac{L}{C}}$ — Surge impedance of a transmission line or cable

Key Points: The surge impedance of a cable is much lower than that of an overhead transmission line.; Reducing the steepness ($dV/dt$) reduces the stress on the transformer's end-turn insulation.; This technique is a passive protective measure against switching and lightning surges.; The length of the cable is typically short, serving as a buffer.

Advantages: Low cost and maintenance-free protection; Effective reduction in voltage stress at the transformer terminals

Disadvantages: Limited effectiveness for very long surges; Requires impedance matching for optimal performance

Applications: HV Substations connected via overhead lines; Transformer protection in power distribution networks

Additional Info: The steepness of a wave is defined by the rate of rise of the voltage ($dV/dt$).. Option B is incorrect because the effect is universal for surges entering from a higher impedance line to a lower impedance cable.

Answer: A is correct — A short length of cable reduces the steepness of the incident wave by utilizing its inherent surge impedance characteristics.

Exam Tip: Remember that surge impedance $Z$ is inversely proportional to the square root of capacitance $C$; cables have higher capacitance than overhead lines, leading to lower surge impedance.

Examoogle · Answer

Option B: Reduces the steepness of the wave under certain conditions only

Examoogle · Answer

Option C: It does not change the steepness

Examoogle · Answer

Option D: None of above

Correct Answer

Discussion (0)

Correct Answer

Related Questions

Discussion (0)