HVDC Frequency Set-Point Control Without Station Communication Links
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Solution Overview
Problem
HVDC power transmission systems face challenges in regulating active power flow when a dedicated communication link between HVDC stations is unavailable, particularly in offshore installations where weather conditions can damage wired links and impact wireless communications, leading to potential over-frequency issues in AC grids.
Innovation Solution
A control apparatus that determines a frequency set-point for an AC network connected to an HVDC station based on measured DC voltage, using a disturbance detector to differentiate between deliberate modulation and disturbances, allowing for active power control even without a communication link. This apparatus operates in two modes: one where it sets a default frequency if no communication link is available, and another where it receives control data if a link is present.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a dedicated communication link is used between HVDC stations for frequency control, then active power regulation precision is improved, but system reliability deteriorates due to vulnerability to weather conditions and cable damage
Solution Approach 1:
The system uses the DC voltage signal itself to carry control information from the onshore station to the offshore station, eliminating the need for separate communication infrastructure. The DC voltage serves dual purposes: power transmission and control signal carrier, making the system self-sufficient and immune to external communication failures
Solution Approach 2:
The DC voltage signal performs multiple functions simultaneously: it transmits active power and carries frequency control information. By encoding control commands in DC voltage variations, the system uses an existing signal for dual purposes, removing dependency on dedicated communication links
2Reliability
If DC voltage modulation is used for control signaling, then communication reliability is improved by eliminating dedicated links, but measurement precision deteriorates due to difficulty in distinguishing deliberate modulation from disturbances
Solution Approach 1:
The system implements a feedback mechanism where the onshore station monitors DC voltage and sends corrective control signals when disturbances are detected. The offshore station feeds back its frequency status through DC voltage variations, enabling the onshore station to adjust and maintain accurate frequency control despite noise
Solution Approach 2:
The system applies preliminary filtering and validation to DC voltage signals before interpreting them as control commands. By anticipating potential disturbances and applying pre-processing filters, the system prevents false interpretation of noise as deliberate modulation
3Device complexity
If the system operates without communication links, then system complexity is reduced, but active power control capability deteriorates due to inability to regulate power flow
Solution Approach 1:
The offshore HVDC station autonomously monitors DC voltage and automatically adjusts its active power output in response to detected variations, without requiring complex external control systems. This self-service capability maintains full control functionality while simplifying the overall system architecture
Solution Approach 2:
DC voltage acts as an intermediary carrier that transmits control information from the onshore station to the offshore station. By using DC voltage as the mediator for control signaling, the system maintains sophisticated control capability without requiring separate communication infrastructure
Data Source
AI summary
A control apparatus is described for controlling a frequency set-point for a first AC network electrically connected to a first HVDC station to regulate active power. The controller has a frequency controller operable in a first mode of operation to determine a frequency set-point for the first AC network based on a measured DC voltage at the first HVDC station. A disturbance detector is configured to monitor the measured value of DC voltage at the first HVDC station for a predetermined characteristic indicative that a variation in measured DC voltage does correspond to a known modulation applied to the DC voltage by a second HVDC station. The frequency controller is configured to determine the frequency set-point for the first AC network based on a measured value of DC voltage if said predetermined characteristic is detected, and to control the frequency set-point to a predetermined default frequency if not detected.