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Which type of HVDC link can provide the more than half the rated power transfer capacity under the fault in any one conductor condition?
Homopolar
Bipolar
Monopolar
Unipolar
Homopolar
A homopolar HVDC link consists of two conductors having the same polarity (usually negative) and utilizes the ground as a return path. During a fault on one conductor, the other conductor remains operational and continues to transmit power, effectively allowing the system to maintain a significant portion of its capacity using the healthy line and ground return.
A homopolar HVDC link consists of two conductors having the same polarity (usually negative) and utilizes the ground as a return path. During a fault on one conductor, the other conductor remains operational and continues to transmit power, effectively allowing the system to maintain a significant portion of its capacity using the healthy line and ground return.
PtotalтАЛ=VdтАЛ├Ч(Id1тАЛ+Id2тАЛ) тАФ Total power transfer in healthy state
PfaultтАЛ=VdтАЛ├ЧIdтАЛ тАФ Power transfer with one pole isolated
In a homopolar system, both lines are operated with the same polarity and the ground acts as the neutral path. Since each conductor is connected to its own converter station capable of independent operation, isolating one faulty pole still leaves the healthy pole functioning with ground return, enabling more than 50% power transfer capability compared to a bipolar system which loses exactly 50% capacity if one pole fails and the metallic return is unavailable.
Homopolar links use the ground as a return path under normal conditions.
Bipolar links use two poles of opposite polarity (positive and negative).
In bipolar systems, a fault on one pole reduces capacity to 50% if the return path is not metallic.
Homopolar configuration is rarely used compared to bipolar due to earth current issues.
Reduced insulation costs as both poles are at same polarity
Enhanced reliability as healthy pole can operate independently with ground return
Continuous ground current leads to electrolysis and interference issues
Requires high-quality grounding electrodes at both ends
Earlier HVDC installations
Systems where ground return is permissible and cost-saving is prioritized over environmental concerns
| Feature | Homopolar | Bipolar |
|---|---|---|
Pole Fault Capacity |
| Exactly 50% Capacity |
Homopolar links are less common than bipolar links in modern transmission due to the environmental impact of continuous earth return currents.
Option B (Bipolar) is incorrect because while it is the most popular type, a fault on one pole typically reduces power transfer exactly to 50% unless a metallic return is available.
A is correct тАФ A homopolar link allows the healthy conductor to continue carrying power while using the ground as a return, maintaining more than half the rated capacity.
Always remember that in HVDC systems, the 'metallic return' is the key differentiator for capacity maintenance during a single-pole fault.