LCC HVDC Thyristor Bridge Capacitor Module Insertion
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Solution Overview
Problem
Line commutated converters (LCC) in high-voltage DC systems are susceptible to commutation failure during faults, leading to potential blackouts and inefficient reactive power management, as they can only switch on thyristors and consume reactive power, unlike voltage source converters (VSC) which can switch and produce/reactive power on demand.
Innovation Solution
Incorporating capacitor modules into the bridge circuits of LCCs to increase effective commutation voltage, reduce current through thyristors, and enable balanced capacitor voltage, allowing for independent reactive power control and eliminating commutation failures by using methods like 'push', 'pull', and 'push & pull' capacitor insertion strategies during commutation periods.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If LCC systems use thyristors that can only be switched on, then the system can transmit larger power, but the system becomes susceptible to commutation failure during faults
Solution Approach 1:
The patent introduces capacitor modules as intermediary elements inserted into the bridge circuit arms. These capacitors act as mediators that provide additional commutation voltage during fault conditions, enabling successful commutation of thyristors even when AC voltage is compromised. The capacitor modules bridge the gap between the limited thyristor switching capability and the reliability requirement during faults.
Solution Approach 2:
The patent dynamically changes the commutation voltage parameter by inserting capacitor modules into the bridge circuit during fault conditions. This parameter change provides the additional voltage boost needed to overcome the reduced AC voltage during faults, enabling reliable thyristor commutation. The capacitor insertion transforms the commutation voltage from insufficient to sufficient levels.
2Device complexity
If LCC systems operate with conventional thyristor switching, then the system structure is simpler, but commutation time is extended and reactive power consumption increases
Solution Approach 1:
The capacitor modules are pre-charged before insertion into the bridge circuit. This preliminary charging action ensures that when the capacitors are inserted during commutation, they immediately provide the required voltage boost without delay. The pre-charged capacitors are ready to act instantaneously, reducing overall commutation time.
Solution Approach 2:
The capacitor modules maintain continuous support during the commutation process by remaining inserted throughout the commutation period. This continuous presence ensures uninterrupted commutation voltage support, preventing any gaps or interruptions that would extend the commutation time. The useful action of voltage support is maintained continuously throughout the critical commutation interval.
3Use of energy by moving object
If LCC systems use conventional operation mode, then the system consumes reactive power, but the system cannot produce or control reactive power on demand
Solution Approach 1:
The patent transforms the static reactive power consumption characteristic into a dynamic controllable feature. By controlling the insertion and extraction timing of capacitor modules, the system can dynamically adjust reactive power exchange with the AC network. This dynamic control enables the converter to operate in both consuming and generating modes, providing adaptability to different grid conditions.
Solution Approach 2:
The capacitor modules serve multiple functions: they provide commutation voltage during faults, enable reactive power consumption during normal operation, and can be controlled to provide reactive power generation when needed. This multi-functionality makes the LCC system universal, capable of performing various reactive power management tasks similar to VSC systems.
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 completely eliminates commutation failures under single-phase-to-ground and three-phase-to-ground faults, reduces commutation time, and enables independent fast reactive power control, leading to cost savings, smaller converter transformer ratings, lower losses, and reduced thyristor levels.
Implementation Method 1
Each arm comprises an upper and lower thyristor connected in series, an associated branch extending from between the upper and lower thyristors, and at least one capacitor module for each phase. The, or each capacitor module is operable to insert a capacitor into the respective arm of the bridge circuit.
Data Source
Figure 1
Figure 2a~2b
Figure 3a~3b
AI summary
A line commutated converter, LCC, for a high-voltage, direct current, HVDC, power converter comprises at least one bridge circuit for connection to at least one terminal of a DC system. Each bridge circuit comprises a plurality of arms, and each arm is associated with a respective phase of an AC system. Each arm comprises an upper and lower thyristor connected in series, an associated branch extending from between the upper and lower thyristors, and at least one capacitor module for each phase. The, or each capacitor module is operable to insert a capacitor into the respective arm of the bridge circuit.