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HVDC Links and Configurations

The HVDC link configurations can be classified into the following categories:

1. Monopole HVDC link

Symmetric monopole
Asymmetric monopole with metallic return
Asymmetric monopole with ground return

2. Bipole HVDC link

Bipole with ground return (symmetric bipole)
Bipole with metallic return (Asymmetric bipole)

1. Monopole HVDC Link

The monopole HVDC link can be configured as symmetric monopole, asymmetric monopole with metallic return and asymmetric monopole with ground return.

1.1. Symmetric Monopole HVDC Link

The symmetric monopole HVDC link is shown in Fig. 1. In this configuration, the converters are connected to the two high-voltage conductors at opposite polarity. In symmetric monopole, the ground connection can be given at multiple points. The preferred ground connection is at the middle point of the DC link capacitors. So, the steady state voltage for each pole is half of the nominal DC voltage of the converter such as +Vdc/2, -Vdc/2, where Vdc is the nominal pole-to-pole DC voltage.

Fig. 1. Symmetric monopole HVDC link.

The operating voltage of each pole in a symmetric monopole configuration is half of the nominal DC voltage (Vdc/2).
Low impedance and high impedance grounding are possible at the neutral point. The symmetric monopole is effective for high-impedance grounding via capacitors or other large impedances.
The fault clearance time constraints can be less stringent when compared to asymmetric monopole configuration.
Due to this symmetric connection, the converter transformer may not experience DC voltage stress.
In a symmetric monopole, the steady state fault current is zero unless the converter transformer is Y-g connected at the converter side.
In the case of pole-to-ground faults, the DC side is not fed by AC side currents.
It requires two fully insulated conductors which will increase the cost.

1.2. Asymmetric Monopole HVDC Link with Metallic Return

Fig. 2 shows the asymmetric monopole HVDC link with metallic return configuration. In normal operation, the voltage on the positive pole is the nominal DC voltage of the converter (Vdc) and the voltage on the metallic return conductor is zero. Normally only one converter is grounded because the earth current will flow when both converters are solidly grounded.

Fig. 2. Asymmetric monopole HVDC link with metallic return.

The asymmetric monopole can be low impedance grounded to control the voltage rise on the metallic return conductor.
In this configuration, the fault current may be uncontrolled after the fault event and therefore a fast protection scheme is required.
Increasing the grounding impedance will reduce the steady state fault current, but that may increase the voltage on the metallic return conductor.

1.3. Asymmetric Monopole HVDC Link with Ground Return

Fig. 3 shows the asymmetric monopole HVDC link with ground return configuration. Since the return path is a ground return, there is only one fully insulated conductor that can reduce the cost. Also, there is a possibility of expansion to the bipole HVDC link. However, there are some environmental concerns like permission for installing the electrodes and continuous ground current.

Fig. 3. Asymmetric monopole HVDC link with ground return.

2. Bipole HVDC Link

The bipole HVDC link can configured into bipole with ground return and bipole with metallic return HVDC links. The bipole HVDC link can provide high power rating when compared to Monopole HVDC link. Each converter is connected to the AC grid via different transformer groups. One pole uses Yg-Δ and the other pole uses Yg-Y transformer configurations to reduce the harmonics on the AC side. The bipole HVDC link can be operated as monopole HVDC when there is a fault in one pole/converter.

2.1. Bipole HVDC Link with Ground Return (Symmetric Bipole)

Fig. 4. Shows the bipole HVDC link with ground return configuration. The bipole HVDC link with metallic return can be refereed as symmetric bipole configuration. This configuration uses ground return and so the ground current can be increased when both the poles are not balanced during pole outages or maintenance periods.

Fig. 4. Bipole HVDC link with ground return.

2.2. Bipole HVDC Link with Metallic Return (Asymmetric Bipole)

Fig. 5. Shows the bipole HVDC link with metallic return configuration. This configuration can be referred as asymmetric bipole configuration. In this configuration, the extra insulated neutral conductor is required for the metallic return when compared to the bipole with ground return.

Fig. 5. Bipole HVDC link with metallic return.

3. Comparison of HVDC Links

Table 1 presents the comparison of different HVDC link configurations.

References

Kontos, Epameinondas, et al. "Impact of HVDC transmission system topology on multiterminal DC network faults." IEEE Transactions on Power Delivery, 2014.
Chaudhuri, Nilanjan, et al. Multi-terminal direct-current grids: Modeling, analysis, and control. John Wiley & Sons, 2014.
Van Hertem, Dirk, Oriol Gomis-Bellmunt, and Jun Liang. HVDC grids: for offshore and supergrid of the future. John Wiley & Sons, 2016.
Leterme, Willem, et al. "Overview of grounding and configuration options for meshed HVDC grids." IEEE Transactions on Power Delivery, 2014.
Stan, Andrei, Sorina Costinaș, and Georgiana Ion. "Overview and assessment of HVDC current applications and future trends." Energies, 2022.
ENTSO-E, HVDC Links in System Operations, 2019.
EPRI, HVDC Ground Electrode Overview 1020116, 2010.