Bipole HVDC Current Balancing Under Return Conduit Failure
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Solution Overview
Problem
In high voltage direct current (HVDC) power transmission networks, maintaining current balance and optimizing power transfer are challenging, especially when a return conduit failure occurs, leading to imbalances and potential over-modulation in power converters.
Innovation Solution
A bipole power transmission network with a controller that sets DC and AC voltage references, as well as reactive power exchange references, to balance current flow and optimize active power transfer between transmission conduits, preventing over-modulation by adjusting references based on current imbalance and modulation indices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If the DC voltage of the lower power transmission conduit is reduced to balance current flow, then current balance between transmission conduits is improved, but the risk of over-modulation in the power converter increases
Solution Approach 1:
The controller dynamically adjusts the DC voltage reference of the lower power transmission conduit based on the degree of current imbalance. By changing the voltage parameter in response to detected imbalance conditions, the system achieves current balance while maintaining safe operating margins to prevent over-modulation in the power converter.
2Stability of the object's composition
If the DC voltage reference is set to balance current flow, then current imbalance between transmission conduits is reduced, but the active power transfer capability is limited
Solution Approach 1:
The system dynamically adjusts the DC voltage reference based on real-time detection of current imbalance conditions. Rather than using a fixed voltage setting, the controller continuously adapts the voltage reference to achieve current balance while maximizing active power transfer within safe operating limits, preventing over-modulation while optimizing power delivery.
3Reliability
If the AC voltage reference is adjusted to prevent over-modulation, then the risk of over-modulation is reduced, but the current balancing capability is affected
Solution Approach 1:
The controller adjusts the AC voltage reference in coordination with DC voltage reference changes. When the DC voltage is reduced to balance current flow, the AC voltage reference is simultaneously adjusted to maintain an appropriate voltage ratio, preventing over-modulation while preserving the current balancing effect.
Data Source
Figure 1
AI summary
In the field of bipole power transmission networks there is a need to optimise the active power transferred downstream in the event of an issue preventing current from flowing in an associated return conduit. A bipole power transmission network (10) comprises a first upstream power converter (12) which has a first DC terminal (14) that is connected with a first transmission conduit (16) which extends, in-use, to a first downstream power converter (18). The first upstream power converter (12) also has a second DC terminal (20) that is connected with a return conduit (22) which extends, in-use, to the first downstream power converter (18) and a second downstream power converter (24). Also, the first upstream power converter (12) has at least one first AC terminal (26) which is electrically connected with a first AC power source (28). In this manner the first upstream power converter (12) is configured to transfer power between the first AC power source (28) and the first transmission conduit (16). The bipole power transmission network (10) additionally includes a second upstream power converter (32) which has a third DC terminal (34) that is connected with the return conduit (22), as well as a fourth DC terminal (36) which is connected with a second transmission conduit (38) that extends, in-use, to the second downstream power converter (24), and at least one second AC terminal (40) which is electrically connected with a second AC power source (42). In this manner the second upstream power converter (32) is configured to transfer power between the second AC power source (42) and the second transmission conduit (38). In addition to the foregoing, the bipole power transmission network (10) also includes a controller (52) that is programmed to balance the current (Idc1, Idc2) flowing in each of the first and second transmission conduits (16, 38) in the event of an issue preventing current (IdcN) from flowing in the return conduit (22), and to optimise the active power transferred downstream by the transmission conduit (16, 38) having a higher power output at the time of the issue by: setting a DC voltage (Vdc1, Vdc2) reference that the upstream power converter (12, 32) connected with the transmission conduit (16, 38) having the lower power output at the time of the issue is required to operate the said lower power transmission conduit at (16, 38); and setting an AC voltage reference (Vc1_A, Vc2_A) that the said upstream power converter (12, 32) connected with the lower power transmission conduit (16, 38) is required to maintain at the or each corresponding AC terminal (26, 40) of the said upstream power converter (12, 32) connected with the lower power transmission conduit (16, 38).