HVDC Inverter Frequency Control via Local Modulation
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Solution Overview
Problem
Conventional high voltage/ultrahigh voltage direct current (HVDC) transmission systems face challenges in precise and fast frequency control at the inverter side, particularly when interstation communication fails, leading to increased reactive power loss, limited direct voltage regulation, and potential commutation failures.
Innovation Solution
The method involves acquiring the frequency control function state, transmitting the deviation between the inverter side power grid frequency and rated frequency to the inverter side frequency controller, which outputs a modulation quantity ΔDP using self-adaptive parameters. This allows the inverter side to regulate power by converting between current and voltage control modes, forming new power/current orders through interstation communication, and superposing modulation quantities to achieve frequency control even during communication failures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If interstation communication is used to transmit frequency values from inverter side to rectifier side for frequency control, then frequency control function can be realized, but phase lag occurs and control precision is affected
Solution Approach 1:
The patent introduces a local frequency controller at the inverter side as an intermediary device. This local controller receives frequency values from the inverter side power grid and directly generates modulation quantities without requiring transmission through interstation communication to the rectifier side. The local controller acts as a mediator that enables frequency control while avoiding the phase lag and communication delays inherent in the conventional remote control architecture.
2Reliability
If direct voltage is regulated to control frequency when interstation communication fails, then frequency control capability is maintained, but reactive power loss increases and system safety is threatened
Solution Approach 1:
The patent replaces the mechanical/voltage-based frequency control method with a power-based control method. Instead of regulating direct voltage to control frequency (which causes reactive power loss), the system uses the frequency controller to generate modulation quantities that directly adjust transmission power. The power controller then converts these modulation quantities into power orders, achieving frequency control through power regulation rather than voltage regulation, thereby eliminating the associated reactive power losses.
3Device complexity
If rectifier side operates at fixed alpha angle to simplify control, then control complexity is reduced, but system direct voltage cannot be regulated and frequency control capability is insufficient
Solution Approach 1:
The patent divides the frequency control function into two independent segments: a frequency controller that generates modulation quantities based on frequency deviations, and a power controller that converts these modulation quantities into power orders. This segmentation allows the rectifier side to operate at a fixed alpha angle (maintaining simple control) while the inverter side's local frequency controller provides the necessary frequency regulation capability. The separation of concerns enables both simplicity and effectiveness.
4Reliability
If conventional frequency controller at rectifier side is used, then frequency control can be implemented, but control speed is slow when interstation communication fails
Solution Approach 1:
The patent implements a self-service frequency control mechanism at the inverter side. The local frequency controller autonomously measures the inverter side power grid frequency, calculates frequency deviations, and generates modulation quantities without requiring assistance from or communication with the rectifier side frequency controller. This self-service capability enables the system to maintain fast frequency control even when interstation communication fails, as the inverter side can independently respond to frequency disturbances.
Data Source
AI summary
A high voltage/ultrahigh voltage direct current transmission inverter side frequency control implementing method includes: transmitting a deviation between the inverter side power grid frequency and rated frequency to the inverter side frequency controller, wherein the frequency controller regulates and outputs a modulation quantity by adopting self-adaptive parameters according to different operation conditions; when the interstation communication is normal, the modulation quantity output of the inverter side frequency controller causes the rectifier side and the inverter side to form a new power/current order through the interstation communication; when the interstation communication is abnormal, converting the inverter side to current control from voltage control and converting the rectifier side to voltage control from current control; superposing the modulation quantity output of the inverter side frequency controller to the power/current order of the inverter side, changing the size of the transmission power to realizing the inverter side frequency control.