Q5Power System - I
Question
Q.5. Draw a simple scheme of HVDC converter station and describe briefly components of the converter station.
Answer
An HVDC converter station consists of converter transformers, thyristor/IGBT valve groups (rectifier/inverter), AC and DC harmonic filters, smoothing reactors, and reactive power compensation equipment, all working together to convert AC to DC (or vice versa) for transmission.
A typical HVDC converter station comprises the following key components, arranged between the AC system connection and the DC transmission line:
Converter transformers step the AC system voltage to the level required by the valve bridge and provide the necessary phase-shifted (typically star-star and star-delta) secondary windings for 12-pulse operation, as well as galvanic isolation between the AC system and the DC valve group. Valve groups (thyristor-based for LCC, or IGBT-based for VSC) perform the actual AC-DC (rectifier) or DC-AC (inverter) power conversion, controlled by firing-angle (LCC) or pulse-width-modulation (VSC) control systems. Smoothing reactors, connected in series on the DC side, reduce DC current ripple and limit the rate of rise of fault current in the event of a DC-side fault, protecting the valves. AC harmonic filters, tuned to the characteristic harmonics generated by the converter (5th, 7th, 11th, 13th for a 12-pulse bridge), prevent these harmonics from propagating into the connected AC system, and often also supply part of the converter's reactive power requirement. DC filters similarly suppress the characteristic DC-side voltage ripple harmonics from propagating along the DC line and causing telephone/communication interference. Reactive power compensation equipment (shunt capacitor banks, synchronous condensers, or SVC/STATCOM) supplies the substantial reactive power that LCC-type converters inherently consume (VSC-based converters, by contrast, can independently control their own reactive power exchange and generally need much less or no additional external compensation).
Two such converter stations, one at each end of the DC line, together with the DC transmission line/cable itself, form the complete HVDC link; the station operating as rectifier controls DC current (or power) via its firing angle, while the station operating as inverter controls DC voltage (or extinction-angle margin), with an overall master control system coordinating the current-order and power-order between the two ends to maintain stable, adjustable power transfer, including scheduled ramping and, if required, complete reversal of power flow direction.