HVDC Transformer DC Bias Compensation via Converter Control
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Solution Overview
Problem
In high voltage direct current (HVDC) systems, asymmetrical DC bias currents in transformers cause unwanted saturation, leading to vibration, noise, and potential transformer burn-out due to asymmetries in AC voltages generated by voltage source converters, which are more challenging to control than line commutated converters.
Innovation Solution
A method and device that detect the maxima and minima in the neutral current or calculated magnetizing current of a star-grounded transformer, generate control signals to adjust the switching of converter valves, reducing asymmetries between phases, thereby reducing transformer saturation and associated noise and vibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If voltage source converters with IGBTs and free-wheeling diodes are used in HVDC systems, then power conversion capability is improved, but asymmetrical DC bias currents are generated causing transformer saturation
Solution Approach 1:
The control device continuously monitors the AC voltages on the AC side of the converter and uses this feedback information to dynamically adjust the switching control of the converter valves. This closed-loop control ensures that asymmetries in AC voltages are detected and corrected in real-time, preventing the generation of asymmetrical DC bias currents that would otherwise saturate the transformer
Solution Approach 2:
The invention changes the control parameters of the converter valves by adjusting their switching characteristics. By modifying the switching timing and duration of the converter valves based on detected voltage asymmetries, the system compensates for inherent imbalances in voltage source converters, thereby eliminating the root cause of asymmetrical DC bias currents
2Productivity
If DC bias currents flow through the transformer, then power transmission is maintained, but transformer saturation occurs causing vibration and audible noise
Solution Approach 1:
The control device takes preliminary action by detecting and correcting asymmetries in AC voltages before they can generate harmful DC bias currents. By preemptively adjusting the converter valve switching control, the system prevents the formation of conditions that would lead to transformer saturation, vibration, and audible noise, thereby protecting the transformer while maintaining power transmission
3Duration of action of stationary object
If DC magnetization occurs in the transformer, then operational continuity is maintained, but transformer burn-out risk increases
Solution Approach 1:
Through continuous monitoring of AC voltages and dynamic adjustment of converter valve switching, the feedback control system prevents DC magnetization from developing to dangerous levels. This real-time correction mechanism maintains operational continuity while eliminating the progressive saturation that would otherwise lead to transformer burn-out
Solution Approach 2:
The control system provides beforehand cushioning by detecting voltage asymmetries and correcting them before they can cause harmful DC magnetization to accumulate. This preventive approach cushions the transformer against the gradual buildup of DC bias currents that would otherwise lead to saturation and potential burn-out
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces transformer saturation, mechanical stress, noise, and telephone interferences by eliminating asymmetrical DC magnetization in the transformer, improving operational reliability and reducing the risk of transformer burn-out.
Implementation Method 1
A transformer coupled between an AC power system and a AC/DC or DC/AC converter
Implementation Method 2
DC bias currents which means that a certain DC magnetization in the transformer occurs
Data Source
AI summary
A method and a device to compensate for an asymmetrical DC bias current in a multi-phase transformer. The transformer is connected between an AC power system and an AC/DC or DC/AC high voltage converter. For each phase of the AC side of the transformer a current quantity is determined. The current quantity reflects the time dependent behavior of the magnetizing current in the phase. Time intervals in the current quantity are determined during which the current quantity reaches a positive or a negative maximum, respectively. A DC magnetizing quantity is determined from a difference between the amplitude of the positive maximum and the amplitude of the negative maximum. An asymmetrical quantity is determined from a difference between the amplitudes of the positive and/or negative maxima of at least two of the phases and a control signal is generated from the asymmetrical quantity and provided to a control device of the converter in order to adjust the generation of the AC or DC voltage in the particular phase of the converter which corresponds to the phase of the AC side of the transformer.