HVDC Inverter Control via Differential Voltage and Current Minimization
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Solution Overview
Problem
Existing methods for controlling high-voltage direct current (HVDC) transmission systems with voltage source converters (VSCs) are limited, as they require non-zero operating currents to avoid gap currents and cannot specify zero setpoint currents, making them inefficient and inflexible.
Innovation Solution
The method involves self-commutated converters with power semiconductors that can be switched off, where the rectifier control minimizes the sum of differential DC voltage and absolute setpoint direct current, and the inverter control minimizes the sum of differential direct voltage and current, allowing for zero setpoint currents and flexible regulation of VSCs connected via a DC voltage connection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If non-zero operating currents are used to avoid gap currents, then reliability is improved, but flexibility and efficiency deteriorate due to inability to specify zero setpoint currents
Solution Approach 1:
The invention changes the control parameter from current magnitude to current direction (polarity). By allowing the current direction to reverse while maintaining non-zero magnitude, the system achieves both gap current avoidance and zero power transfer capability. This is implemented through independent control of current magnitude and polarity, enabling the converters to operate with minimal current to prevent gap currents while still achieving effective power flow reversal.
2Productivity
If voltage polarity reversal is used to reverse power flow in classic HVDC systems, then power flow direction is improved, but device complexity increases due to additional switching requirements
Solution Approach 1:
The invention inverts the conventional approach to power flow reversal. Instead of reversing voltage polarity as in classic HVDC systems, the invention reverses current direction while maintaining constant voltage polarity. This inversion simplifies the switching mechanism because the voltage source converters can naturally reverse current flow by changing the switching state of their power semiconductors, eliminating the need for complex voltage polarity reversal mechanisms.
3Loss of energy
If minimal operating current is used to reduce losses, then efficiency is improved, but reliability deteriorates due to gap current occurrence
Solution Approach 1:
The invention implements dynamic control of current magnitude and polarity separately. The system continuously adjusts the current level to maintain it above the gap current threshold while minimizing power losses. This dynamic adjustment allows the converters to operate at the lowest possible current that still prevents gap currents, optimizing the trade-off between efficiency and reliability. The control system monitors current levels and automatically adjusts to maintain optimal operating conditions.
Data Source
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AI summary
The invention relates to a method for regulating at least two inverters (1), which may be regulated either as rectifier or inverter, connected to each other by a DC link (4), for energy transmission and/or distribution, wherein a measured DC voltage (Udc_r1... Udc_rr; Udc_il... Udc_ii) and a measured DC current ( Idc_r1... Idc_rr; Idc_il... Idc_ii ) is measured at each inverter (1) and transmitted to a rectifier regulator (7_rr) for regulating the corresponding rectifier or to an inverter regulator (8_ii) for regulating the corresponding inverter, wherein each rectifier regulator (7_rr) and each inverter regulator (8_ii) give the difference between a given set DC voltage (Udco) and the relevant received measured DC voltage (Udc_r1... Udc_rr, Udc_il... Udc_ii) to give a differential DC voltage (du) and the difference between a set DC current (Idco_rl Idco_rr, Id-co_il Idco_n) and the corresponding received measured DC current (Idc_r1... Idc_rr, Idc_il... Idc_ii) to give a differential DC current (di), wherein the measured DC current, the measured DC voltage, the set DC current and the set DC voltage are normalised with which a regulation of inverters comprising switchable power semiconductors connected by a DC link can be carried out, wherein the proviso of set currents being zero is possible. According to the invention, each inverter is a self-commutated inverter with switchable power semiconductors and the rectifier regulation (7_rr) of the provided inverter (1) is regulated such that the sum of the product of the differential voltages (du) and the value of given set DC current ( |Idco_r|) at th corresponding rectifier and the differential current (di) is a minimum (du* | Idco_r | +di -->Min) and the inverter regulation (8_ii) regulates the corresponding inverter (1) such that the sum between the differential voltage (du) and the differential current (di) is a minimum (du+di -->Min).