Current Source Converter Gate-Assisted Commutation
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Solution Overview
Problem
Current high voltage direct current (HVDC) transmission systems face limitations in power conversion efficiency, availability, and fault current management due to the requirements of reversible HVDC transmission link voltage and susceptibility to inverter commutation failures, especially in line commutated converters (LCCs) and voltage source converters (VSCs).
Innovation Solution
A current source converter (CSC) design that incorporates gate-assisted natural commutation processes to manage DC link voltage reversal, improve AC power factor, and rapidly limit fault currents, while maintaining efficiency and availability, similar to conventional LCCs, and enabling operation in point-to-point DC transmission links and multi-terminal DC grids.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If line commutated converters (LCCs) are used for HVDC transmission, then power transmission efficiency is improved, but reversible power flow capability deteriorates due to severe limitations on HVDC cable insulation and multi-terminal grid adoption
Solution Approach 1:
The patent inverts the conventional LCC approach by using voltage source converters (VSCs) with forced commutation instead of natural commutation. This inversion allows the HVDC transmission link voltage to remain unipolar while achieving reversible power flow capability, thereby resolving the contradiction between transmission efficiency and adaptability.
Solution Approach 2:
The patent changes the commutation method parameter from natural commutation in LCCs to forced commutation in VSCs. This parameter change enables the system to maintain unipolar HVDC voltage while achieving reversible power flow, thus resolving the technical contradiction.
2Ease of operation
If phase control is used to regulate power flow in LCCs, then power flow control is improved, but harmonic distortion and power factor deterioration worsen
Solution Approach 1:
The patent replaces the mechanical phase control method in LCCs with electronic pulse width modulation (PWM) control in VSCs. This substitution eliminates the generation of harmonic distortion and power factor issues while maintaining power flow control capability, thereby resolving the contradiction.
Solution Approach 2:
The patent uses periodic PWM switching actions in VSCs to control power flow, replacing the continuous phase control in LCCs. This periodic switching enables precise power flow regulation without generating harmful harmonics, resolving the technical contradiction.
3Adaptability or versatility
If VSCs are used to overcome LCC limitations, then reversible power flow and harmonic mitigation are improved, but power conversion efficiency deteriorates
Solution Approach 1:
The patent employs dynamic control strategies in VSCs that adapt switching frequencies and modulation indices to operating conditions. This dynamic operation minimizes switching losses while maintaining reversible power flow capability, thereby resolving the contradiction between adaptability and efficiency.
Solution Approach 2:
The patent optimizes VSC operating parameters such as switching frequency and modulation depth to minimize power losses. By dynamically adjusting these parameters based on load conditions, the system achieves both reversible power flow capability and high conversion efficiency, resolving the technical contradiction.
4Loss of energy
If LCCs are used for HVDC transmission, then power transmission efficiency is improved, but fault current limitation capability deteriorates
Solution Approach 1:
The patent implements feedback control in VSCs that detects fault conditions and rapidly responds by adjusting switching states to limit fault currents. This feedback mechanism provides inherent fault current limitation capability while maintaining high transmission efficiency, thereby resolving the contradiction.
Solution Approach 2:
The patent incorporates preliminary protection mechanisms in VSCs, including pre-configured switching patterns and control algorithms that prepare the system for fault conditions. This preliminary action enables rapid fault current limitation while maintaining efficient normal operation, resolving the technical contradiction.
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 CSC design enhances power conversion efficiency, reduces harmonic distortion, and limits fault currents without the need for DC circuit breakers, improving overall system reliability and flexibility in HVDC transmission systems.
Implementation Method 1
a naturally commutated inverting mode where, during each commutation event, an incoming first power semiconductor switching device is turned 'on' by gate control at a point in time that is in advance of a reference time such that anode current in the incoming first power semiconductor switching device increases at a determined rate and anode current in an outgoing first power semiconductor switching device decreases at a determined rate
Implementation Method 2
the available circuit commutated turn-off time is greater than the recovery time that is applicable with an open circuit gate terminal bias applied
Data Source
AI summary
A converter, and in particular a current source converter, including a bridge having an AC terminal for each of one or more AC lines, and first and second DC terminals. A converter arm is connected between each respective AC terminal and the first DC terminal, and between each respective AC terminal and the second DC terminal. Each converter arm includes a first power semiconductor switching device capable of being turned ‘on’ and ‘off’ by gate control and having a recovery time. The converter is adapted to be operated in one or more inverting modes.