Elimination of commutation failure in LCC HVDC power converter

University of Birmingham Background
Line commuted converter (LCC) high-voltage direct current (HVDC) electrical power transmission systems are widely deployed around the world for high power applications. LCC systems are susceptible to commutation failures during faults on the AC side. Commutation failure can result in the system having to be shut down and restarted even after the fault has been cleared, potentially leading to a blackout. Also, the current in the converter lags the voltage, consuming reactive power.
Technology Overview
Researchers at the University have developed alternative solutions for elimination of commutation failures in LCC systems. LCC Systems currently reduce the probability of commutation failure without eliminating it. Our converter topology – Flexible LCC HVDC, provides firing angle control that provides:

Fast independent reactive power control
Completely eliminating commutation failure
Removing AC filters which represent at least 50% of the footprint of a typical HVDC converter station
Significantly reducing the footprint of HVDC converter station and hence the reducing the component cost and overall investment cost

The proposed method, shown in , utilizes the insertion of capacitors during commutation to help the commutation process, even in the absence of AC commutation voltage. The insertion of the capacitors is realized by using full-bridge sub-modules made up from power electronic switches (for instance, IGBTs) and anti-parallel diodes.Commutation failure under zero-impedance single phase and three-phase to ground faults can be eliminated by inserting pre-charged capacitors or series connected capacitors, which increases the commutation voltage. Each single modular capacitor can have a full bridge power electronic switch modular structure. Although the full bridge configuration is preferred there are alternative configurations that can bring new functionalities for control and protection.
Fast reactive power control and tracking at inverter side is achieved by controlling the firing angle with the proposed converter topology and positive and negative reactive power exchange with the AC network becomes possible.
Significant cost savings result from the reduction in reactive power support at the inverter side due to filter bank reduction, the smaller converter transformer rating and lower number of thyristor levels in each valve.
The new Flexible LCC HVDC proposed, is cheaper, more reliable, flexible and much smaller than the conventional LCC HVDC, see . With:

Complete elimination of commutation failure under zero impedance single-phase and three-phase-to-ground faults
Fast reactive power control at inverter side by controlling firing angle directly
Reactive control approach can be used to control reactive power at rectifier side(s)
Significant reduction in cost and complexity of filter banks and new HVDC power grids
Reduces footprint for filter bank and HVDC controllers
Large over-current on thyristor valves during commutation failure can be eliminated
Power supply maintained under the most serious fault conditions
Reduced converter power losses

The technology has the potential to be applied to existing and new LCC HVDC power grids to improve grid performance and reduce capital cost of new installations.
Extensive system modelling carried out using state-of-the-art tools. Looking for collaboration and licensing opportunities.

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