HVDC transmission and interconnectors
There is an increasing demand to interconnect power systems in order to improve both flexibility and security of supply. High-voltage DC (HVDC) transmission is a significant enabler of such interconnections and there has been an increasing market demand for such interconnections in recent years.
Author(s): Carl Barker
In this paper, the basic topologies of an HVDC transmission interconnection are introduced, the paper then goes on to introduce the differing technologies that maybe employed. The life cycle studies of an HVDC scheme are introduced including those associated with ‘owning’ an HVDC scheme and what kind of maintenance maybe expected.
High-voltage DC (HVDC) transmission provides a means of connecting two or more AC systems together whilst maintaining control of the power that flows between the AC systems. It is also a more economical way of transferring electrical power over long distances on overhead lines or even relatively short distances in underground or submarine cable . The inherent controllability makes the power flow between AC systems independent of both frequency and phase.
There are, today, two fundamental variants of HVDC; line commutated converter (LCC) technology based on thyristors as the semiconductor building block and voltage source converter (VSC) technology which typically utilises insulated gate bipolar transistors (IGBTs) as the semiconductor building block.
The fundamental concept of HVDC transmission is to convert an AC voltage into a DC voltage and then to use an opposing DC voltage to regulate the DC current that flows in the HVDC transmission circuit . In this way active power (Vdc × Idc) is controlled. The opposing DC voltage is created from a second converter and can either be derived from the second converter’s AC voltage in the case of LCC or could be generated by the converter itself in the case of VSC. With VSC it is possible for this second converter to use the energy provided by the first converter to generate its own AC voltage, hence it can supply a passive load.
The basis of an HVDC interconnection is two (or more) AC/DC converters connected in parallel via a DC transmission line or cable. Several configurations can be utilised dependent on the particular project needs and constraints as well as the conversion technology to be utilised.
The simplest arrangement of a two AC/DC converters for HVDC transmission is the ‘monopole’ arrangement, as shown in Fig 1 a. Here, there is only one high-voltage conductor connecting the two high-voltage converter terminals and this conductor could be overhead line underground/submarine cable or some combination of both. The other terminal of each converter is connected to the earth or to the sea, with the earth and/or sea providing a return conduction path. In a practical case, the actual connection to the earth or to the sea is made with ‘electrodes’  and these are typically connected to the low-voltage terminal of the converter via short (10–20 km) electrode lines which, again, can be either overhead line or underground cable.
Fig 1: Monopole HVDC interconnection
a) Monopole with earth return
b) Monopole with metallic return
A disadvantage of the use of electrodes is that the electrode material at the anode will be depleted with ampere-hours of usage, requiring replacement after some time. Also, current flow in the earth or sea will find the lowest path of resistance and this maybe a third party’s buried infrastructure, potentially resulting in erosion. It should also be noted that in some countries the use of ground or sea return is not permitted. In the case of the UK the amount of compass deviation induced by submarine cables is limited. To control this, the go and return currents need to follow a similar path such that the induced magnetic field of one cable cancels that of the other leaving little residual magnetic field at the surface. Hence, an alternative arrangement is to use a second overhead line or cable to connect the low-voltage converter terminals together, earthing only one end, Fig 1 b. Note that, as one end is earthed, this second conductor, referred to as a ‘metallic return’, is at a much lower voltage and hence lower cost, when compared with the high-voltage connection.
A ‘Symmetrical monopole’ is similar to the monopole with metallic return as shown in Fig 1 b, except that in this arrangement both conductors are operating at a DC voltage of equal magnitude but opposite polarity. Compared with the monopole, for the same DC current rating, the high-voltage conductor’s voltage rating with respect to ground is halved; however, there are now two. A typical arrangement for a symmetrical monopole is shown in Fig 2.
Fig 2: Symmetrical monopole HVDC interconnection
When comparing Figs 2–1 b, it can be seen that the mid-point voltage of the converter in the monopole arrangement is at 50% of the DC voltage whilst in the symmetrical monopole arrangement the nominal DC voltage at the mid-point is zero. This impacts on the design of the AC side of the converter, where the converter consists of only one bridge a symmetrical monopole arrangement will mean that the AC side of the converter will be exposed to approximately zero DC voltage and this will, in turn, reduce the cost of the transformer connecting the converter to the AC power grid. As discussed in Sections 4 and 5 below the single bridge arrangement is more common with a VSC converter than with an LCC converter.
By taking two monopoles, Fig 1, and using a common return path for both, the ‘bipole’ arrangement shown in Fig 3 can be created. By balancing the currents in Pole 1 and Pole 2 in Fig 3 the DC current will flow in the two high-voltage conductors only; thereby, eliminating any losses associated with the metallic return conductor (I 2R) or erosion of the electrodes, if used. However, in the event of the failure of either Pole 1 or Pole 2, the remaining, healthy, Pole retains a DC current path and hence can continue to transmit power.
Fig 3: Bipole HVDC interconnection
The bipole arrangement can be further enhanced, in terms of its flexibility, with the addition of DC commutating switches which permit the high-voltage conductor of the Pole out of service to be used as a metallic return path of the healthy pole in order to either parallel the existing metallic return path and hence reduce the transmission losses or to minimise the ampere-hour depletion of the electrodes by commutating the current into the new ‘temporary’ metallic return path.
All of the previously discussed HVDC configurations can have additional converters added in order to make the scheme ‘multi-terminal’. The additional terminal(s) could be added in series or in parallel. However, a series connection requires the continuous flow of DC current between the other converters in order to be able to transmit power, whilst the parallel arrangement requires a common voltage connection. The flexibility of the parallel converter arrangement, not requiring a DC current flow through all converters, means that this arrangement is the preferred one for HVDC .
Comparison of LCC and VSC for HVDC transmission
LCC HVDC has been commercially available since the mid-1950s, initially utilising mercury-arc valves as the switching elements (hence the term ‘valve’ is still used for a phase arm within a converter) and since the 1970s the technology has used series strings of thyristors as the switching devices. VSC, on the other hand, was only introduced in the late 1990s and the present modular multi-level converter (MMC) topology, which is now being universally adopted, was only introduced in the late 2000s. Whilst the fundamental objective of transferring DC power between converters remains the same, irrespective of the technology, the operational characteristics and associated equipment differ in characteristics and capability. Table 1 summarises some of the main differences between LCC and VSC.
inherently absorbs reactive power irrespective of active power flow direction
can absorb or supply reactive power independent of the active power being transferred so long as the total current remains within the converter valve rating
LCC necessitates the addition of 3-phase AC shunt capacitor banks (usually configured as harmonic filters) to meet reactive power exchange limits with the AC system
converter switching operation produces non-sinusoidal AC currents plus DC side voltage ripple
the MMC synthetic sine wave generation means that the converter produced waveforms have a very low harmonic content
LCC will always require AC side harmonic filters (see also reactive power), whereas VSC may not require filtering
large converter station footprint required, mainly due to AC harmonic filters and associated AC switchyard
typically, no need for harmonic filters; therefore, smaller station footprint
converters buildings are larger for VSC but the overall site area is smaller than for LCC
LCC typically utilises 125 mm thyristors able to operate at 8 kV/3500 A. With 150 mm thyristors, the current rating can be increased to 5000 A
present VSC is based on IGBT semiconductors with a typical rating of 3.3 kV/1500 A
the relatively low power rating of the IGBT compared with the thyristor means that LCC has lower capital cost and lower losses at higher power
the converter requires an existing AC source (e.g. a synchronous machine) in order to commutate
the converter can generate an AC supply to a passive load
the VSC can contribute to an AC grid restoration scheme at an earlier part of the process
power flow reversed by reversing the polarity of the DC voltage
power flow reversed by reversing the direction of the DC current flow
power reversal in an LCC scheme is a global change compared with VSC where the change may only be at one station in a multi-terminal scheme, i.e. large multi-terminal networks are easier to plan and operate using VSC converters
DC line fault blocking capability
the LCC can inherently bring the fault current down to zero through control action and then restore operation once the fault is cleared
the basic VSC circuit cannot interrupt a DC side fault and hence must trip the converter AC circuit breakers to interrupt the current
to clear a DC side fault in a VSC scheme necessitates the complete separation of the HVDC scheme from the grid by opening the converter AC circuit breakers. This can be avoided but only by the addition of equipment, either HVDC circuit breakers or additional semiconductors within the converter
Table 1: Comparison of LCC and VSC
Main characteristics of an LCC HVDC converter
The basic building block of an LCC converter is the thyristor which is a semiconductor switch that can conduct current, in one direction only, when both positive voltages are placed across its terminals and a ‘gate pulse’ is applied. By controlling the time when the gate pulse is applied within each AC system cycle, the DC voltage produced by the converter can be controlled and hence the DC power flow can be controlled.
The thyristor, as a single device, does not have sufficient voltage rating for HVDC applications and hence a single valve (phase arm) of a converter consists of many devices in series in order to achieve a higher-voltage rating. For example, a 500 kVdc converter may comprise 12 valves, each valve comprising around 72 series connected thyristors. The challenge for the valve designer is therefore to ensure that the AC, DC and transient voltages are shared across the series string of thyristors and that coherent switching is achieved. This necessitates the addition of extra components within the valve making the thyristor a small, but necessary, part of the complete valve, Figs 4 and 5.
Fig 4: Typical section of an LCC thyristor valve indicating six series thyristors plus associated components
Fig 5: Interconnection France-Angleterre HVDC 3-phase thyristor valve arrangement 
The AC–DC conversion and vice versa is achieved by switching the part of the AC waveform that is connected to the DC terminals throughout the 3-phase AC cycle. The actual conversion process is discussed in many texts [4,6] and is not elaborated further here. The converter operation produces harmonics: AC current harmonics and DC voltage harmonics. The DC side harmonics do not always require filtering, particularly when the DC transmission circuit is entirely via underground or submarine cables. However, the AC side harmonics always require filtering, necessitating the inclusion of 3-phase AC filter banks to shunt the harmonic currents away from the AC system to which the converter is connected.
The basic building block of the LCC converter is the 6-pulse bridge, consisting of six valves and this produces AC harmonics of the order of 6n ± 1 (i.e. 5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th etc.). The 5th and 7th harmonics in particular are problematic for the filter designer as they are of a large magnitude [Ih=Ifundamental⋅(1/h)Ih=Ifundamental⋅(1/h)] and relatively close to the fundamental frequency making it difficult to design a filter that can remove the majority of the harmonic current without incurring any fundamental frequency losses (i.e. the active power that is being exchanged). For this reason, almost all LCC converters are constructed in a 12-pulse arrangement, comprised of two 6-pulse converters, series connected on the DC side and parallel connected on the AC side, Fig 6. By utilising different transformer side connections (one star, one delta), the harmonics of the two 6-pulse bridges are phase shifted such that 5th and 7th cancel, 11th and 13th add, 17th and 19th cancel, 23rd and 25th add etc. Hence, with the 12-pulse arrangement the number of harmonics are reduced and the lowest AC harmonic frequency to be filtered is the 11th harmonic, reducing the complexity of the filtering.
Fig 6: 12-Pulse LCC converter
Another consequence of the operation of an LCC converter is that it will always absorb reactive power and the amount of reactive power absorbed will be approximately proportional to the active power flow; irrespective of the power flow direction. To avoid having to exchange excessive amounts of reactive power with the AC system to which the converter is connected reactive power is typically supplied local to the converter in the form of 3-phase AC capacitor banks. To optimise the utilisation of equipment, the reactive power banks are typically designed to be AC harmonic filters. However, noting that the reactive power changes with active power, the reactive power supply has to be changed in proportion. This is achieved by subdividing the reactive power supply into switchable banks (utilising AC circuit breakers). The designer’s task is to optimise the number and size of the reactive power banks whilst ensuring that the reactive power exchange with the AC system is maintained and that the correct banks are connected to give the required AC harmonic performance.
Main characteristics of a VSC HVDC converter
The VSC HVDC converter has gone through a number of generations of topology since its first introduction in the late 1990s. However, today the majority of new schemes are based on the MMC topology, Fig 7 a. In this arrangement, each valve consists of multiple switchable sub-modules. Each sub-module has, associated with it, an energy storage device, typically a capacitor. By switching in or bypassing each sub-module, the valve voltage can be modified. By correctly sequencing the module switching a waveshape, with a very low harmonic content, can be synthesised.
Fig 7: MMC VSC converter
a) As an HVDC converter
b) As two STATCOM’s
Deconstructing the converter topology depicted in Fig 7 a, it can be seen that the MMC converter is constructed from two STATic COMpensator (STATCOM) circuits, Fig 7 b , parallel on the AC side and series connected on the DC side. The converter is, therefore, capable of operating as a STATCOM, providing reactive power and an active power source at the same time. However, it should be noted that the converter conducts both the AC current and the DC current simultaneously and hence there is a direct relationship between the total amount of reactive power available from a converter which is rated for a defined active power and vice versa.
Whilst the STATCOM utilises a ‘full-bridge’ arrangement , Fig 8 a in order to optimise the overall circuit design this can be simplified  when the main function is active power transfer between AC and DC to the ‘half-bridge’ arrangement as shown in Fig 8 b. This half-bridge arrangement has reduced the number of semiconductors through which the current must pass, hence both reducing the capital cost and the converter losses. However, this arrangement means that there is an uncontrolled conduction path between the AC and DC terminals via the free-wheel diode D2. Under normal operation, the peak of the AC voltage must always be lower than the DC voltage in order to ensure that D2 is reverse biased and hence does not conduct. This therefore dictates, to a large extent, the selection of the converter AC side voltage. Under DC fault conditions where the DC voltage magnitude falls below that of the AC voltage D2 will conduct necessitating the opening of the converter AC circuit breaker. Where the HVDC transmission connection is a point-to-point and comprises only cable as the transmission media, the fault is likely to be permanent, resulting in an outage until the cable is repaired. However, in the event of an overhead DC line being part of the HVDC conduction path, this may suffer from temporary faults, once cleared, power flow can be restored. However, having tripped the AC breaker the power restoration time will be in excess of the typical AC system auto-reclose time which must be considered in the scheme planning.
Fig 8: MMC sub-module types
As previously noted, the real and reactive power capabilities of the converter are interdependent. However, the total available power is not simply a function of the current rating of the converter. The peak AC valve voltage is a function of the voltage generated across the valve reactors and the connection transformer leakage reactance (which will be a function of the reactive current). This peak AC valve voltage must be maintained below the DC voltage to avoid D2 conduction hence reactive power and the circuit inductance influence the selection of the nominal valve winding voltage and hence the nominal converter current and hence the losses associated with active power transfer. The result of these interactions is that a typical real/reactive (P/Q) capability characteristic for a VSC converter is similar to that shown in Fig 9.
Fig 9: Typical real/reactive power operating range of a VSC converter
A typical ±320 kVdc converter will consist of in excess of 300 sub-modules per valve in a 6-valve arrangement as shown in Fig 10. By individually switching each of these sub-modules, the resulting synthesised waveform has minimum harmonics, Fig 11. The need for harmonic filters is therefore, in most cases, obviated from the perspective of converter-generated harmonics. However, the converter presents an inductor/capacitor circuit to the AC system which could, under some conditions act to magnify existing background harmonics. The design engineer is therefore responsible for optimising the design and deciding on whether to use the converter to damp these interactions or whether to include a passive filter to provide the necessary damping.
Fig 10: Typical ±320 kVdc VSC converter
Fig 11: Output voltage waveform from a VSC converter
Studies associated with an HVDC scheme’s life cycle
HVDC transmission schemes are large, complex, projects that require detailed studies during the project life cycle .
At the planning stage, the studies fall into two categories: (a) project validation, to demonstrate the technical/economic value of the project and (b) specification studies, to provide data to the bidders to allow them to perform their design studies (e.g. short-circuit levels, background harmonics etc.).
During the design phase, studies are performed by the equipment manufacturers, initially to generate sufficient design detail for the bid, rating main circuit plant, establishing achievable performance etc. After contract award, the contractor will perform further detailed studies finalising equipment ratings and establishing how the equipment will operate within the wider network to which it is connected. Contract control equipment will be used along with real-time simulation systems to study system interactions further and to provide benchmarking for the commissioning stage testing.
Almost inevitably over the life of the equipment, there will be changes to the network to which it is connected. Studies may therefore be required to ensure that these changes do not push the existing equipment outside of its operating capability. If disturbances in the equipment operation occur, it may be necessary to perform studies to understand the nature of the equipment response and to identify, if necessary, any remedial action required.
Typical maintenance needs for an HVDC converter
Much of the equipment associated with an HVDC converter station is common with an AC substation, for example; AC circuit breakers, disconnectors, earth switches, insulators, auxiliary systems (batteries, battery chargers, diesel generators, distribution panels etc.), transformers, Sequence Control and Data Acquisition (SCADA), AC protection, communication systems etc. All of these equipment will require some form of routine maintenance, be it routine either in the form of simply visual inspection or some form of invasive testing. However, in addition to these items there is equipment specifically associated with HVDC .
The converter valves are at the heart of the converter station. As they are constructed from series connected elements, more elements than are required for the rating are always included such that a failure of an element does not necessitate the shutdown of the HVDC scheme. The number of redundant elements included is a function of the prescribed maintenance interval, typically once every 2 years for an HVDC converter. During an actual outage, two activities are normally carried out, cleaning of part of the converter (for example, one-third of the converter such that after three maintenance outages the complete valve hall has been cleaned) and replacing any failed elements. Today’s HVDC converters include very comprehensive monitoring techniques providing online fault monitoring so that equipment needing repair or replacement can be readily identified.
LCC valves are typically maintained in situ, that is, the repairs are carried out on the valve itself. Conversely, with MMC VSC converters, each sub-module is a self-contained element and maintenance is performed by replacing a complete sub-module.
The HVDC converters will inherently generate losses in the form of heat and this heat has to be conducted away from the converters. This is achieved by the use of a de-ionised water or water/glycol mix (dependent on the site environmental conditions). The glycol can be either ethylene or propylene dependent on extremes of minimum ambient temperature. The water is pumped via a cooling plant to outdoor coolers, typically air-blast coolers. Fans and pumps in particular require maintenance but redundant elements are included, so that maintenance does not necessitate an HVDC station outage.
HVDC transmission can come in many forms; considering both the means of interconnection and the technology employed. The two basic technologies, LCC and VSC, each have their own advantages and disadvantages making the selection of technology needs based decision. It can also be seen that the implementation of an HVDC transmission scheme requires many studies and engineering decisions in order to optimise the final solution.
- Mazzanti G. Marzinotto M.: ‘Extruded cables for high-voltage direct-current transmission’ (IEEE Press, Piscataway, NJ, USA, 2013), p. 18.
- ‘Introduction to HVDC’. Available at http://www.gegridsolutions.com/youtube_vdo/watch.aspx?v=int_to_hig_vol_dir_cur_hvdc, accessed 4 December 2016.
- ‘HVDC ground electrode design’ (EPRI, Palo Alto, California, EL-2020, 1981).
- Kimbark E. W.: ‘Direct current transmission: volume 1’ (Wiley-Blackwell, USA, 1971).
- D’Aubigny A. Monkhouse D. Houston B. et al.: ‘Experience from IFA 2000 France–England HVDC interconnector refurbishment project’. CIGRÉ, Paris, France, August 2016.
- Arrillaga J.: ‘High voltage direct current transmission’ (IET Press, United Kingdom, 1998, 2nd edn.).
- ‘Introduction to VSC’. Available at http://www.gegridsolutions.com/youtube_vdo/watch.aspx?v=evo_of_vol_sou_con_tec, accessed 4 December 2016.
- Knight R. C. Young D. J. Trainer D. R.: ‘Relocatable GTO-based static VAr compensator for NGC substations’. CIGRÉ, Paris, France, August 1998.
- Lesnicar A. Marquardt R.: ‘An innovative modular multilevel converter topology suitable for a wide power range’. IEEE Bologna PowerTech Conf., Bologna, Italy, June 2003.
- ‘Modelling and simulation studies to be performed during the lifecycle of HVDC systems’ (CIGRÉ, Brochure 563, 2013).
- Kirby N. M. Coullon J. L.: ‘Optimizing the availability of HVDC systems via effective asset management’. CIGRÉ Canada Conf., Winnipeg, Canada, September 2015.