Control of meshed HVDC transmission grid
The need for power transmission from remote generation sources far from major load centres is growing in tandem with global energy consumption. Many isolated remote areas benefit from favourable geographic conditions for renewable energy generation. However, the challenge lies in connecting these remote renewable energy sources to the national grid to fulfil rising demand. Constructing a separate transmission corridor to link them to major load centres is not economically feasible. Hence, the most viable option is to connect them to the nearby power transmission corridor. In the case of a high-voltage AC transmission corridor, it is easy to tap the power to feed a local AC network or to integrate local renewable generation into high-voltage AC lines using transformers. However, AC transmission lines have limitations in transferring power over long distances and in pushing power from one area to another. High-voltage direct current (HVDC) transmission is always preferred over conventional high-voltage AC transmission for long-distance point-to-point bulk power transmission.
The expansion of High Voltage Direct Current (HVDC) transmission systems is creating opportunities to connect multiple HVDC systems, forming DC transmission grids where power is pushed from one area to another. These DC grids could establish interconnected systems that link various AC power systems, large-scale offshore wind farms, solar parks, MVDC grids, etc., as illustrated in Fig. 1. There has been significant interest in academia as well as the industry in DC transmission grids due to recent advancements in power electronics. To ensure the flexibility and controllability of interconnected DC grids, it is important to develop suitable AC-DC converters as well as DC-DC converters. Converters with modular structures are preferred for high-voltage applications due to scalability and ease of repair. HVDC converters could also be utilised to provide multiple services to the AC grid, such as supplying grid inertia and damping oscillations.
Fig. 1 Illustration of a meshed HVDC grid
Challenges
One of the primary advantages of DC power transmission is that power flow magnitude and direction can be controlled. However, the control system must be fast to respond. The mismatch in DC voltages between two nodes will result in a large current flow. Hence, any attempt to connect an intermediary point in the DC transmission line should be done carefully.
Another important fact is that remotely located AC networks of isolated rural villages and remote renewable energy sources, which need to be integrated with the HVDC transmission line, will be very weak. Integrating a weak AC network at a low voltage level with an HVDC transmission line has several challenges, such as commutation failures, voltage fluctuations, etc. If a weak AC network is to be integrated, self-commutated converters/grid-forming converters need to be used.
The fault-tolerant aspect of the scheme is also important. During DC faults, there will be a sudden dip in DC voltage, which can cause a sharp increase in current. To control and limit the current from tapping stations during such scenarios is a concern. The converter used for power tapping should have fault-tolerant feature.
The DC-DC converter should also have high voltage gain, as the tapping station and the HVDC line have large voltage differences. An isolated converter may be necessary if the configurations of the two DC systems to be interconnected are different. Modular converters are preferred for ease of repair, maintenance, and transportation. Bi-directional converters may be required in some applications.
Fig 2. Hybrid HVDC with LCC and VSC converters
The major research aspects in this domain are summarised below.
| [1] |
Fault tolerant converters for HVDC grid applications |
| [2] |
Hybrid HVDC with VSC and LCC systems (Combining the bulk power capability of LCC HVDC system and the ability of VSC to get connected to weak grids) |
| [3] |
Integration of wind/solar farms with HVDC system |
| [4] |
Grid forming control of HVDC systems: HVDC connected to a passive load/ 100% IBR-based grid |
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