JNTU B.Tech IV Year HVDC TRANSMISSION Previous Question Paper (July/August - 2022 - R18)

Surge Impedance Loading (SIL) is the power delivered by a lossless transmission line to a purely resistive load equal to its surge impedance. It is defined by the formula:

where V is the line-to-line voltage and Z_c is the surge impedance (sqrt(L/C)).

The physical significance is that at SIL, the reactive power generated by the line capacitance exactly equals the reactive power absorbed by the line inductance. Consequently, the voltage profile remains flat along the line, and the power factor is unity at both ends.

The voltage profile is governed by the load relative to SIL:

• If the load is less than SIL, the line acts as a capacitor, causing a voltage rise at the receiving end (Ferranti effect).
• If the load is greater than SIL, the line acts as an inductor, causing a voltage drop at the receiving end.
• As the line length increases, these effects are amplified, requiring compensation (like shunt reactors or series capacitors) to maintain the voltage profile within acceptable limits.

An HVDC converter station acts as the interface between AC and DC systems. The essential apparatus includes:

• Converter Transformers: Step up or down the AC voltage and provide the necessary phase shift for multi-pulse operation.
• Converter Valves: Thyristor-based bridges that perform the rectification (AC to DC) or inversion (DC to AC) process.
• Smoothing Reactors: Large inductors placed in series with the DC line to reduce ripple current, prevent commutation failure, and limit fault currents.
• Harmonic Filters: AC and DC filters (tuned or high-pass) used to suppress harmonics generated by the converter process to prevent interference and heating.
• Reactive Power Compensation: Shunt capacitor banks or synchronous condensers to supply the reactive power consumed by the converters.
• DC Switchgear: Includes DC circuit breakers, disconnectors, and surge arresters to protect the station against lightning and switching surges.

The schematic typically shows the AC bus connected to the converter transformer, followed by the valve bridge, with smoothing reactors on the DC side and filters connected to the AC bus.