DC fault detection and location in meshed multi-terminal HVDC systems based on DC reactor voltage change rate

The change rate of the DC reactor voltage with predefined protection voltage thresholds is proposed to provide fast and accurate DC fault detection in a meshed multi-terminal HVDC system. This is equivalent to the measurement of the second derivative of the DC current but has better robustness in terms of EMI noise immunization. In addition to fast DC fault detection, the proposed scheme can also accurately discriminate the faulty branch from the healthy ones in a meshed DC network by considering the voltage polarities and amplitudes of the two DC reactors connected to the same converter DC terminal. Fast fault detection leads to lower fault current stresses on DC circuit breakers and converter equipment. The proposed method requires no telecommunication, is independent of power flow direction, and is robust to fault resistance variation. Simulation of a meshed three-terminal HVDC system demonstrates the effectiveness of the proposed DC fault detection scheme

DC/DC converters based on hybrid MMC for HVDC grid interconnection

This paper presents a multi-terminal high-power DC/DC converter configuration based on hybrid MMC topology with fault blocking capability for interconnecting HVDC systems. Its main functions include bidirectional power flow, step-up and step-down operation and fault isolation equivalent to a DC circuit breaker. By contrast to the conventional MMC based DC/DC converter, the proposed DC/DC converter with hybrid MMC configuration has the advantage of being able to block the DC/DC converter terminal connecting to faulty DC grid section, while continue operating the other terminals connected to healthy DC grid sections. The proposed DC/DC converter operation is analyzed and its control is described. Simulation results using Matlab/Simulink are presented to demonstrate the robust performance during dc fault conditions

DC fault protection strategy considering DC network partition

This paper investigates DC network partition and alternative DC fault protection strategy for Multi-terminal HVDC (MTDC) system. Fast acting DC Circuit Breakers (DCCBs) or fault blocking DC-DC converters can be configured at strategic locations to allow the entire MTDC system to be operated interconnected but partitioned into islanded DC network zones following faults. In case of any DC fault event, the DCCBs or DC-DC converters at the strategic cable connections that link the different DC network partitions are opened or blocked such that the faulty DC network zone is quickly isolated from the remaining of the MTDC system. Thus, the healthy DC network zone can remain operational or recover quickly to restore power transmission. Each DC zone can be protected using AC circuit breakers and DC switches for cost reduction. The validity of the proposed protection strategy is confirmed using MATLAB/SIMULINK simulation

Protection of large partitioned MTDC networks using DC-DC converters and circuit breakers

This paper proposes a DC fault protection strategy for large multi-terminal HVDC (MTDC) network where MMC based DC-DC converter is configured at strategic locations to allow the large MTDC network to be operated interconnected but partitioned into islanded DC network zones following faults. Each DC network zone is protected using either AC circuit breakers coordinated with DC switches or slow mechanical type DC circuit breakers to minimize the capital cost. In case of a DC fault event, DC-DC converters which have inherent DC fault isolation capability provide ‘firewall’ between the faulty and healthy zones such that the faulty DC network zone can be quickly isolated from the remaining of the MTDC network to allow the healthy DC network zones to remain operational. The validity of the proposed protection arrangement is confirmed using MATLAB/SIMULINK simulations

Active control of DC fault currents in DC solid-state transformers during ride-through operation of multi-terminal HVDC systems

When a pole-to-pole dc fault occurs in a multi-terminal HVDC system, it is desirable that the stations and dc solid-state transformers on healthy cables continue contributing to power transfer, rather than blocking. To reduce the fault current of a modular multilevel converter based dc solid-state transformer, active fault current control is proposed, where the dc and ac components of fault arm currents are regulated independently. By dynamically regulating the dc offset of the arm voltage rather than being set at half the rated dc voltage, the dc component in the fault current is reduced significantly. Additionally, reduced ac voltage operation of the dc solid-state transformer during the fault is proposed, where the ac voltage of transformer is actively limited in the controllable range of both converters in the transformer to effectively suppress the ac component of the fault current. The fault arm current peak and the energy absorbed by the surge arrester in the dc circuit breakers are reduced by 31.8% and 4.9% respectively, thereby lowering the capacities of switching devices and circuit breakers. Alternatively, with the same fault current level, the dc-link node inductance can be halved by using the proposed control, yielding lowered cost and volume. The novel active fault current control mechanism and the necessary control strategy are presented and simulation results confirm its feasibility

Control and operation of multi-terminal DC systems for integrating large offshore wind farms

This paper discusses control and operation of multi-terminal DC systems for integrating large offshore wind farms. It was presented at the 7th International Workshop on Large-Scale Integration of Wind Power and Transmission Networks for Offshore Wind Farms in 2008

Studies of coordinated zone protection strategies for DC grid

DC grid technology is one of the effective methods for collecting, transmitting and accommodating large-scale renewable energy over long-distance. Voltage source converter (VSC) based DC grid technology has become a preferred technical scheme because of its inherent advantages, such as flexible bidirectional DC power control capability. However, VSC based DC grid is also facing many operation control and protection problems to be solved, especially the problem of fault protection for the DC grid. This paper first analyzed the fault characteristics of the DC grid, and then through a typical DC grid configuration, a zone protection strategy is proposed with the objective of using a minimum number of DCCBs. The simulation results using MATLAB/Simulink platform show that the zone protection strategy is feasible and effective. The study in this paper provides a technical reference for the research and development of fault protection and isolation for DC grid in the future

Pre-charging and DC fault ride-through of hybrid MMC based HVDC systems

Compared to half-bridge based MMCs, full-bridge based systems have the advantage of blocking dc fault, but at the expense of increased power semiconductors and power losses. In view of the relationships among ac/dc voltages and currents in full-bridge based MMC with the negative voltage state, this paper provides a detailed analysis on the link between capacitor voltage variation and the maximum modulation index. A hybrid MMC, consisting of mixed half-bridge and full-bridge circuits to combine their respective advantages is investigated in terms of its pre-charging process and transient dc fault ride-through capability. Simulation and experiment results demonstrate the feasibility and validity of the proposed strategy for a full-bridge based MMC and the hybrid MMC

Protection and post-fault recovery of large HVDC networks using partitioning and fast acting DC breakers at strategic locations

DC fault protection arrangements for a large multi-terminal HVDC network are proposed where fast-acting DC circuit breakers are only used at strategic locations with the large DC network to be operated interconnected but partitioned into islanded DC zones in case of any DC fault events in one of the DC zones. Each DC network zone can be protected using low cost, slow protection devices such as AC circuit breakers coordinated with DC switches or slow mechanical type DC circuit breakers. This ensures the maximum ‘loss-of-infeed’ for any AC networks connected to the large HVDC system is kept within acceptable limits with reduced investment in protection cost as expensive fast acting DC circuit breakers are kept to a minimum. A post-fault recovery method of the faulty section is proposed including the reconnection with the healthy part of the network to ensure reliable and smooth restoration of the large multi-terminal HVDC network. A detailed pre-fault and post-fault power flow analysis is also conducted in a multi-terminal DC network with DC voltage droop control. The proposed protection arrangements and post-fault recovery method are validated by simulation of a two-zone, six-terminal DC network with respective radial and meshed configurations