Bipole HVDC Converter Control for Neutral Current Balancing
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Solution Overview
Problem
Existing HVDC grid systems face challenges in controlling neutral currents due to asymmetries and contingencies, particularly when fast centralized control adjustments are not feasible, leading to instability and inefficiencies.
Innovation Solution
A bipole power exchange arrangement with converters controlled by dual voltage references, including a residual voltage reference based on droop control for neutral current balancing and symmetrical voltage references for power exchange, decoupling control goals to ensure stability without centralized intervention.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If local setpoint controls with integral part are used to control neutral current to zero, then current balancing between poles is achieved under symmetrical conditions, but the control cannot handle asymmetrical operations and requires deactivation
Solution Approach 1:
The control system dynamically adapts its behavior based on operating conditions. The converter control switches between different control modes: using local setpoint controls with integral part for symmetrical operations, and transitioning to a different control strategy when asymmetry is detected. This dynamic adaptation allows the system to maintain reliable neutral current control while handling both symmetrical and asymmetrical operations effectively.
2Reliability
If centralized control regulations are used to react to contingencies, then coordination between HVDC stations is achieved, but fast changes to control behavior are not possible and intervention time is delayed
Solution Approach 1:
The converter control system performs self-service by autonomously detecting contingencies and implementing appropriate control actions without requiring external centralized intervention. The control system continuously monitors operating conditions and can independently adjust converter voltage references and power dispatch to maintain grid stability, thereby eliminating the time delay associated with centralized control signaling and decision-making.
Solution Approach 2:
The control system employs continuous feedback mechanisms where the converter control monitors neutral pole current and other system parameters in real-time. When deviations from normal operation are detected, the feedback loop triggers immediate local control adjustments. This closed-loop feedback enables rapid response to contingencies by continuously comparing actual system state with desired operating conditions and making corrective actions without waiting for centralized control instructions.
3Adaptability or versatility
If individual power dispatch of each pole is used during asymmetrical operation, then different voltages at both DC poles are achieved, but currents are forced into the neutral system
Solution Approach 1:
The control system dynamically adjusts voltage and power parameters based on the degree and nature of asymmetry detected. By modifying converter voltage references and power dispatch parameters in response to changing operating conditions, the system can accommodate different voltage levels at DC poles while simultaneously controlling neutral current through coordinated parameter adjustments, thus achieving asymmetrical operation without forcing excessive currents into the neutral system.
Data Source
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AI summary
The present invention relates to a bipole power exchange arrangement (20) designed to exchange power between a power source and a DC network, comprising a first converter (21) arranged between a positive DC pole and a neutral DC pole, a second converter (25) arranged between the neutral DC pole and a negative DC pole, and a converter control (26, 27) providing a first converter voltage reference (Ucvr1) for the first converter and a second converter voltage reference (Ucvr2) for the second converter. According to the invention the converter voltage references each comprise at least two additive components, wherein one of the components is a residual voltage reference (Uresref) calculated using a droop control function based on a neutral pole current. The present invention further relates to a method for operating such arrangement.