An HVDC power transmission network (also called an HVDC power distribution system or meshed HVDC grid) uses direct current (DC) for the transmission of electrical power, in contrast to the more common alternating current (AC) systems. For long-distance transmission, HVDC systems may be less expensive and may suffer lower electrical losses. In general, an HVDC power transmission system comprises at least one long-distance HVDC link or cable for carrying direct current a long distance, e.g. under sea, and converter stations for converting alternating current to direct current for input to the HVDC power transmission system and converter stations for converting direct current back to alternating current.
In an meshed or, highly meshed HVDC grid, the DC node voltage might not be sufficient to control load flow congestion. A series connected converter may alleviate this problem by adding a fictitious resistance in the line. If the injected series voltage is VAB, to ensure that the cable current is Idc, a fictitious series resistance Rinj=VAB/Idc is injected. Insertion of the series converter in a congested HVDC network is shown in FIG. 1 where a series converter is connected between A and B.
In a voltage source converter (VSC) based DC grid, the current direction changes based on the direction of power flow. Hence, for complete control of load flow, the series converter works in all the four quadrants as illustrated in FIG. 2.
The AC-DC converters in FIG. 1 represent DC Grid converter stations typically with asymmetrical monopoles with separate converter for positive and negative polarity, or they can be balanced bipolar converters. Both of the converter topologies are shown in FIG. 3. In FIG. 3, for both of the converter types the series converter location is shown. For symmetric voltage profile in the positive and negative lines a series converter (shown as SC in FIG. 3) is positioned in both positive and negative lines as shown in FIG. 3. The aim is to change the load sharing between the two cables x and y (both are positive lines) or, between the two cables z and q (both are negative lines) which is achieved by the series converter.
The series converter can be a voltage source converter (VSC). However, the harmonic content of the series injected voltage must be within permissible limits. In case the series converter is powered from an AC source (local AC grid, transformer tertiary winding) the power quality also has to be maintained at the AC side, such that the AC source is not polluted due to the series converter. These conditions may necessitate filters both in the DC and the AC side of the series converter.
Since the converter is connected in series with the HVDC line, rated current can flow through it, although the voltage produced by the converter is relatively small. Multi-level voltage source converters can reduce the voltage across each of the switching device which forms the converter, but rated line current still flows through them.
The protection of the converter is also an important criterion. VSCs are naturally vulnerable to DC side faults (faults on the HVDC cable). This necessitates requirement of fast and costly protection equipment so as to bypass the series converter efficiently at the event of a fault. Additional reactors might also be required in the main HVDC line to limit cable discharge current through the series converter at the time of the fault (pole-ground/pole-pole). These reactors need to be designed for rated current, thereby significantly adding to the cost of the overall solution.
Series converters of different known designs are described in WO 2012/037966, WO 2012/037964 and DE 1513827.