In a power or communication cable, how does the charging reactive power (MVAR) change as the operating frequency is increased?

Introduction / Context:
Cables behave predominantly like capacitors. The reactive power drawn due to their capacitance is commonly referred to as charging MVAR. Understanding how this charging requirement varies with frequency is essential in power system planning and in high-frequency communications links using long coaxial or underground cables.

Given Data / Assumptions:

• Cable acts as a distributed capacitance with negligible series resistance for this conceptual analysis.
• System voltage magnitude is considered fixed while frequency is varied.
• Charging MVAR is the reactive power associated with the cable's shunt capacitance.

Concept / Approach:

The fundamental relation for capacitive reactive current is I_c = V * ω * C, where ω = 2 * π * f. The corresponding reactive power is Q_c = V^2 * ω * C when line-to-neutral quantities are used. Therefore, Q_c is directly proportional to frequency f for a given voltage and capacitance.

Step-by-Step Solution:

Verification / Alternative check:

Utility practice shows that raising system frequency (or long EHV cable lengths) noticeably increases reactive charging demand, often requiring shunt reactors for compensation.

Why Other Options Are Wrong:

• 'decreases' and 'remain the same': contradict Q_c ∝ f.
• 'decreases or remains the same': not supported by the capacitive model.
• 'first increases then decreases': no such behavior without additional frequency-dependent mechanisms (e.g., resonance), which are not implied here.

Common Pitfalls:

Confusing copper losses (which depend on current and skin effect) with reactive charging; assuming inductive line behavior dominates in short overhead lines—cables are different due to higher capacitance.

Final Answer:

increases