Review of HVDC technologies for weak grid interconnectors

Two commercially available converter technologies are line-commutated current-sourced converter (LCC) and voltage source converter (VSC). Additionally, there is growing interest in connecting multiple nodes via converters to create multiterminal DC (MTDC) systems. This subsection outlines the development and suitability of LCC, VSC, and MTDC for weak-to-weak grid connections.

As reported in [27], most HVDC subsea cables are currently installed in shallow waters, i.e., less than 500 m depth, with only three projects deeper than 1000 m: Italy-Greece interconnection (1000 m) [28], Cometa HVDC (1485 m) [29], and SAPEI (1650 m) [30]. Water depth is a crucial factor in cable selection, as the hydrostatic pressure exerted by water (approximately an increase of 1 atm for every 10 m depth increase) may deteriorate or even break the cable sheath. Three types of cables are currently used in HVDC applications: oil-filled, mass-impregnated, and extruded, each better suited for specific installations [31]. This subsection focuses on the development and ratings of these three cable types for land and subsea use.

The AC circuit breaker (CB) operation varies depending on the grid strength and location. The transient response is more pronounced in weak AC grids, leading to longer oscillations due to lower damping. These extended oscillations affected fault detection in older relays. However, modern numerical relays can accurately measure the magnitudes of symmetrical components at the power frequency [42]. In contrast to AC faults, DC faults lack a zero-crossing point, necessitating that DC circuit breakers (DC CBs) create an artificial zero-crossing point, which requires significant energy dissipation [43]. In point-to-point HVDC connections, a DC CB is not needed, as the operator can disconnect the point of connection (POC) at both AC sides of the HVDC links in the event of a fault. This principle continues to be applied in current MTDC systems due to the limitations of DC CB, although these breakers are expected to become available on the market soon. As of now, three different DC CB topologies exist [44, 45]:Placing AC CBs on the AC side of the VSC is the most economical way to protect DC systems. However, AC CBs result in longer interruption times due to mechanical movement and arc extinguishing occurring at zero-crossing [46], requiring the disconnection of the entire MTDC grid. A comprehensive comparison of fault-clearing strategies for DC grids is provided in [47]. Research proposing systematic methods for HVDC circuit breaker sizing is presented in [48], while a review of DC CBs as HVDC component ratings is discussed in [49]. In terms of operation, DC CBs require faster switching times than AC CBs, especially in multiterminal topologies, due to the limited ability of VSC or half-bridge-based MMC converters to control DC fault current during a fault to avoid complete shutdown of the multiterminal DC grid [50]. One promising research area in DC protection is the development of fast fault detection systems and operation strategies.

Weak-to-weak and weak-to-very weak AC grids are characterized by low-to-very-low AC-side power levels relative to the DC power. The concept of a weak grid does not apply to HVDC grids. However, in the context of the HVAC grid, the concept of a weak grid is associated with low grid strength, typically due to high impedance or low inertia. This condition is more prevalent in a grid with low meshed and/or long radial connections coupled with low inertia from the synchronous generators. In terms of quantification of weak grid connections, short-circuit ratio (SCR) is often employed to measure the grid strength. The SCR is an index measuring the ratio of the short-circuit level at the grid side of the converter to the converter’s rating [51]. The formula to calculate SCR, based on [3, 52], is provided in Eq. (1), with the classification based on SCR for the LCC-HVDC provided in Table 1.Where S represents the AC system’s three-phase symmetrical short-circuit level (MVA) at the converter terminal with a 1.0 p.u. AC terminal voltage and \(P_{\textrm{DC}}\) is the rated DC terminal power (MW). It is important to note that when assessing the impact of short-circuit currents on equipment, only the maximum value needs to be considered. Conversely, when evaluating limiting operating conditions, the minimum value of S at which the rated power \(P_{DC}\) can be transmitted must be used.

To the best of the authors’ knowledge, there is no consensus on the classification of SCR for VSC-HVDC. While [53] suggests an SCR range between 1.3 and 1.6 as the borderline between weak and strong systems, a more recent study by [54] indicates that the borderline SCR varies according to converter rating and power transfer.

Various indices for grid strength classification exist, such as the effective short-circuit ratio (ESCR), which accounts for the impact of filters on the fundamental frequency [51], and the equivalent ESCR (EESCR), which considers the participation of the i-th converter [55]. There are also propositions of multi-infeed-based indices such as multi-infeed interactive short-circuit ratio (MISCR) [56] and multi-infeed short-circuit ratio (MSCR) [57]. Despite the variation, the core concept of these indices remains similar, focusing on classifying the strength of an AC system, with differences primarily in their suitability for specific topologies and/or conditions.

The operating difficulties of the HVDC interconnector depend on the strength of the AC grid. In the event of strong-to-strong or weak-to-strong interconnection, no significant operating difficulties are reported. On the other hand, in the weak-to-weak and weak-to-very weak grid interconnection, various adverse phenomena are expected, such as oscillation, interference, voltage drop, harmonic distortion, and frequency deviation [3]. These are often attributed to: It has been known that LCC-HVDC systems require a sufficiently strong AC network to commutate current from one valve to another during normal operation. In networks with low short-circuit capacity, the AC voltage may dip significantly during switching transients or disturbances, leading to commutation failures. This means the thyristors in the LCC cannot turn off as required, causing a disruption in the HVDC transmission [60, 61]. Furthermore, LCC-HVDC systems require reactive power support from the AC network. In strong networks, this is typically not a problem. However, in weak networks, the additional reactive power demand can lead to further voltage drops and exacerbate voltage instability [62]. Therefore, it is not common to use LCC-HVDC in a weak AC grid condition [60‐62].

There is no significant difference in terms of challenges faced by onshore or offshore grids. All issues shown above can still occur in either onshore or offshore grids. The difference between those is in terms of conductor selection (offshore cable, onshore overhead line) and, thus, its unique challenge underlying the conductor selection. In terms of actual HVDC as weak grid interconnection, the reported impact of weak grid parts in Zhangbei MTDC revolves around unstable grid connection, restriction on renewable generation, frequency fluctuation, and system disconnection [4]. Whereas in the Caprivi HVDC link, it is reported to encounter difficulties in complex control settings and instability issues [63]. It is important to note that although one project is offshore and the other is onshore, both face challenges primarily due to low grid strength.

In terms of reinforcement to meet the minimum requirement for HVDC connection, several aspects can be considered. The AC side can be reinforced by increasing its voltage level, boosting its line rating (through a larger cross section), and/or installing new transmission lines. Existing AC lines can also be converted into DC lines, as it is extensively discussed in [64, 65]. These reinforcements not only aim to meet the minimum requirement for a reliable operation of the HVDC link but also to increase the short-circuit capacity of the system, which permits higher power transfer by the HVDC. It might also be necessary to reset the existing protection scheme to incorporate the HVDC link. A fast and reliable substation communication is also helpful when connecting to weak AC grids. PMUs can be used for better monitoring and control of HVDC systems, especially if such system requires timestamp [66], while fiber optics can be used for communication between substations. However, several methods have been developed to establish communication-free control for HVDC systems [67‐69].

In terms of converter technology, offshore LCC-HVDC requires additional commutation voltage support in the form of a synchronous condenser [70] or STATCOM [71]. A STATCOM typically has lower power losses but also a lower power rating than a synchronous condenser [16]. There are no practical synchronous condenser installations in an offshore LCC-HVDC platform. This could be attributed to the cost and the footprint requirement. However, from a research perspective, STATCOM and other FACTS technologies have been introduced to extend the operating range of LCC-HVDC connected to weak AC grids [72‐75]. LCC-HVDC is also prone to commutation failure if connected to a weak AC grid, a risk that can be reduced by using a synchronous condenser [76], STATCOM[77], or a controllable capacitor [78, 79]. Comparison between these methods is documented in [71, 80]. There are also several investigations on the feasibility of transforming a decommissioned power plant into a synchronous condenser to support a weak AC grid [81‐83].

On the other hand, VSC-HVDC, with its ability to independently control both active and reactive power, can eliminate the need for synchronous condenser and reactive power devices. Reference [70] concludes that a 2000–2500 MW VSC-HVDC link at the Bipole III of Manitoba Hydro could potentially eliminate the need for 1000 Mvar from new synchronous condensers. However, reactive power support devices can be integrated into the VSC-HVDC system if a connection to an SCR \(< 1\) is desired. In this context, VSC is preferable to LCC for weak grid connections, as AC-side reinforcement becomes either too complex or costly when the SCR is too low. Consequently, for an Archipelagic country like Indonesia or an energy island-type connection, VSC-HVDC is more suitable than LCC-HVDC.

An AC-side network reinforcement is limited to point-to-point HVDC grids, as mentioned in the early part of this section. The idea of adding the previously mentioned AC-side network reinforcement could be implemented to help increase the short-circuit capacity and regulate voltage or power fluctuation of the weak part of MTDC, as can be seen in Zhangbei MTDC. Zhangbei is the latest MTDC (commissioned in 2018), where two of its four terminals have low short-circuit capacities and high voltage fluctuations due to their location at the end of the Jibei power grid and significant renewable energy transfer [4]. A pumped storage facility has been built in Zhangbei to help reduce frequency fluctuation and improve the short-circuit capacity of the system.

In summary, while most of the proposed solutions in mitigating the adverse impact of weak grid interconnection found in the literature are in the form of advance control approaches (elaborated in the next chapter), there is another solution in the form of AC grid strength enhancement as mentioned in this section. Even though this enhancement may prove to be costly, it permits commercially available and industrially tested control approaches and electrical apparatus to be deployed with slight to no alterations. A combination of AC-side network reinforcement with the appropriate control could also be exercised to either widen transmission capacity or eliminate operating difficulties due to a weak network. This is a potential research topic, given the increasing prevalence of energy island-type networks, weak grid island interconnections, and planned MTDC grids.

Station controls represent the highest level in the HVDC control hierarchy, where the system operator or coordinator establishes the reference values for both upper and lower-level controls. This includes AC and DC voltage references (\(V_{\textrm{AC}\_\textrm{ref}}\) and \(V_{\textrm{DC}\_\textrm{ref}}\), respectively), as well as active and reactive power references (\(P_{\textrm{AC}\_\textrm{ref}}\) and \(Q_{\textrm{AC}\_\textrm{ref}}\), respectively), and AC frequency. The dispatch must be coordinated with the DC grid operator to avoid violating the set reference, which could lead to system instability. The station controls are commonly integrated into the energy management systems, and the time constant is usually minutes or hours. The strength of connected AC systems is considered when constructing the algebraic models and the related constraints. Therefore, the droop-related controls are still the main part, for example, voltage [84, 85] and active power [86] droop control strategies.

The upper-level control’s task is to transform the dispatch command into a voltage reference for IGBT in the lower-level control. The non-island control is used when the VSC-HVDC converter is connected to an AC system with active synchronous generation [18]. Vector control is the most common control strategy.

The upper-level island control can be defined as the scenario in which the VSC-HVDC converter is connected to either (1) an AC system with a disconnected load or asynchronous generation, or (2) a very weak AC system [18]. The difference between island and non-island upper-level control is the island control does not require PLL for reference angle but rather an independent oscillator that controls the frequency of the system.

Synchronverters extend the idea of PSC, where it replicates all of the synchronous generator’s concepts in a converter [107]. The synchronverters divide their block into power and electronic parts, similar to synchronous generators and they are able to perform frequency droop real power regulation, voltage droop, and reactive power regulation. However, implementation of synchronverters in an MTDC grid is difficult as it can only act as a master–slave configuration [108].

In weak grid scenarios—characterized by low short-circuit ratios (SCR) and significant resistive components—the control strategies must adapt to ensure reliable operation. Firstly, traditional synchronverters lack inherent current control, which can lead to issues during voltage sags. To address this, a current-controlled variation of the synchronverter has been proposed [109], incorporating an internal current controller. This approach helps prevent damage to the power converter and maintains system stability by managing the reference current derived from the voltage equation in the dq reference frame. Secondly, in weak grids, the line impedance includes considerable resistance, leading to cross-coupling between active power and voltage, as well as reactive power and phase angle.

The original synchronverter control mechanisms, which assume purely inductive line impedance, need modifications to account for these resistive effects. This necessitates the development of advanced control strategies that can effectively decouple these interactions to maintain stable operation [110]. Moreover, the synchronverter control can dynamically adjust the system’s inertia through a derived inertia function. This feature is particularly beneficial in weak grid conditions, where maintaining voltage stability is crucial. The ability to vary inertia helps the system respond more effectively to disturbances, although the impact on output may be minimal [111].

Virtual oscillator control (VOC) is an attractive technology for VSC-HVDC working under an island or grid-connected mode. The principle of VOC differs from PSC and synchronverters. The PSC and synchronverters control are all linear controls, while VOC control is a nonlinear control method. VOC introduces a nonlinear oscillation block into the control loop. Therefore, the inverter can generate AC voltage with controllable amplitude and frequency. The VOC is attractive because of its robustness and power balancing capabilities compared to PSC and synchronverters [112].

The time-domain oscillator is the first variant of VOC, introduced as a controller for single-phase inverters by [113‐115]. The studies show the advantage of VOC control, and in [116] extended it to three-phase VOC inverters. Van der Pol oscillator is the second variant of VOC, which has droop control functionality while also providing an enhanced dynamic response speed [112, 117]. However, it cannot follow dispatch commands and can only work independently, because the oscillator itself is an independent system whose structure and parameters cannot be changed during operation. The third variant, and also the latest one, is the dispatchable VOC [118‐120], which can provide an interface to grid operators to control its output power and frequency while also maintaining the advantage of other VOC variants.

A unified virtual synchronous control is proposed for multiterminal VSC-HVDC systems [108]. The control is based on the emulation of the synchronous generator’s rotor (inertia emulation and damping) for grid synchronization with the dynamics within the DC capacitor and DC grid taken into account. Each VSC-HVDC converter performs an equal role in the voltage droop control scheme.

The emergence of energy islands, MTDC with distant offshore locations, and island-to-island connections increases the possibility of having a weak-to-weak or even weak-to-very weak grid interconnection using HVDC. Thus, suitable control methods will be a very challenging issue in maintaining the stable operation of the HVDC link. Based on the discussion in the previous subsection, a table presenting the suitability of a specific control method for weak-to-weak AC-side connection is provided in Table 2, where it shows that modified feedforward control might be the most suitable control approach for the Indonesian and other weak grids.

In summary, as elaborated in Table 2, available control techniques designed for strong grids cannot be directly used for weak grids and thus require further adjustment for weak grid interconnection. Future work in this domain may involve the development of novel control techniques or the modification of existing controls to accommodate weak grid interconnections.