Conversion of AC transmission lines to HVDC using current modulation

Modeling and control of extended multiterminal high voltage direct current systems with three-wire bipole structure

“…The rated DC voltage is ±400 kV, and station B is the sending end and station A and C are the receiving ends. The voltage and current characteristics of this system are similar to that described in Figure . For power transmission from station B to A, the amplitudes of I AB1 and I AB2 change between 1 and 2.5 kA, and the amplitude of I AB3 is 1.5 kA; As for power transmission from B to C, the amplitudes of I BC1 and I BC2 change between 0.73 and 2.73 kA, and the amplitude of I BC3 is 2 kA, which satisfies the line thermal limitation (2 kA).…”

“…In this paper, in order to keep the capacitor voltage balanced and accelerate the regulation, the feed-forward control and cascaded PI controls are applied. According to Equations (7) and (9), the reference voltage can be calculated preliminarily, and U xref is achieved by adding the output of constant current control to the reference voltage. U xref is used to control the DC voltage of the CRC through the second PI controller and is also fed into the calculation formula of the upper arm voltage u jp and the lower arm voltage u jn in phase j, which contribute to the capacitor voltage balancing.…”

“…T p and T n mainly depend on the thermal characteristics of the line conductors. For the tripole HVDC structure, if T p is 4 or 5 min, the conductor temperature fluctuation will be several degrees centigrade, the same order of magnitude caused by sudden cloud cover or puffs of wind . Because the power expansion mechanism of the TWBS is similar to the tripole HVDC structure, T p and T n in the TWBS are also in the order of several minutes.…”

“…It is readily comprehensible that a line with single circuit could be converted into a monopole or bipole HVDC link. In order to increase the transmission capacity as much as possible after conversion, the tripole HVDC structure was proposed in , which makes full use of the three‐phase wires of an existing AC line and expands the transmission capacity to 1.37 times of the bipole HVDC structure.…”

“…If the tripole HVDC structure is adopted for converting AC lines into MTDC systems, the converters should act coordinately and synchronously in the transition process, due to the periodical reversal of DC voltages and DC currents on the third pole , which demands accurate and fast communication. So compared with the monopole and bipole structure, the tripole HVDC structure is more complex for converting AC lines into MTDC systems with many terminals, and its extensibility is limited, although its transmission capacity is larger than that of the monopole or bipole HVDC structure .…”