Search

The large‐scale integration of remote renewable energy resources into existing AC grids is one of the main drivers for the construction of multi‐terminal direct current (MTDC) systems. Today, a few MTDC systems are already in execution stage or even in operation. However, with a growing number of extended MTDC systems, the probability of high‐voltage DC (HVDC) insulation faults is likely to increase and therefore, reliable solutions for handling of such disturbances are crucial in order to maintain high availability. Since most HVDC converters are realised by modular multilevel converters (MMC) converters are based on half‐bridge (HB) modules, they lack an inherent capability to control DC fault currents. Therefore, additional equipment like HVDC breakers needs to be installed as well. Among the various breaker types, the so‐called 'hybrid' DC circuit breaker (DCCB) is considered most popular. This contribution provides a more holistic view on the overall system behaviour, represented AC grid, MMC HVDC converter, DC reactor and DCCB altogether. It discusses the physical interactions of these main components in MTDC systems. In particular, the influence of the electrical parameters of the hybrid DCCB and its required DC reactors on the MMC converter's internal current and voltage conditions is outlined. [ABSTRACT FROM AUTHOR]

The large-scale integration of remote renewable energy resources into existing AC grids is one of the main drivers for the construction of multi-terminal direct current (MTDC) systems. Today, a few MTDC systems are already in execution stage or even in operation. However, with a growing number of extended MTDC systems, the probability of high-voltage DC (HVDC) insulation faults is likely to increase and therefore, reliable solutions for handling of such disturbances are crucial in order to maintain high availability. Since most HVDC converters are realised by modular multilevel converters (MMC) converters are based on half-bridge (HB) modules, they lack an inherent capability to control DC fault currents. Therefore, additional equipment like HVDC breakers needs to be installed as well. Among the various breaker types, the so-called 'hybrid' DC circuit breaker (DCCB) is considered most popular. This contribution provides a more holistic view on the overall system behaviour, represented AC grid, MMC HVDC converter, DC reactor and DCCB altogether. It discusses the physical interactions of these main components in MTDC systems. In particular, the influence of the electrical parameters of the hybrid DCCB and its required DC reactors on the MMC converter's internal current and voltage conditions is outlined. [ABSTRACT FROM AUTHOR]

In this article, a modified hybrid dc breaker and a half-bridge modular multilevel converter (MMC) are employed to interrupt the dc fault current in a high-voltage direct current transmission system. In the hybrid dc breaker, parallel to the main branch, a series connection of an energy-absorbing capacitor and a Thyristor stack is employed. When a dc fault current is recognized, the Thyristor stack is triggered and the load commutation switch in the main branch is turned off. In addition, in the fault clearing state, the MMC is programmed to generate zero voltages at all arms. Using this mechanism, a second-order RLC circuit with zero input is formed, and the fault current is cleared in a very short time. A freewheeling path is provided to bypass the dc line when the equivalent inductance seen from the dc line is large. In contrast to conventional approaches, the peak of dc fault current is limited to a specific limit and the necessity to ultrafast mechanical switches is avoided. In addition, the amplitude of arms currents and corresponding insulated-gate bipolar transistors are kept in the safe operating area. A detailed circuit analysis of the new method is given and design formulas are extracted. Simulation results are provided in PSCAD/EMTDC environment, and experimental results are presented for a scaled-down prototype. [ABSTRACT FROM AUTHOR]

A major challenge in a multiterminal high-voltage direct current (MT-HVdc) system is the detection and subsequent removal of dc faults, i.e., line–line or line–ground faults. This technology is crucial to make emerging MT-HVdc systems reliable and stable. Current detection solutions are either too sensitive to load variations causing false positives or are too slow to detect faults before the equipment is damaged. This paper proposes a framework and case examples for fault detection based on features derived from the wavelet transform of the grid current. The algorithm is intended to control new high-speed dc breakers (such as assembly HVdc breaker) for improved fault protection. This paper customizes the detection strategy to specific fault frequencies using multiresolution analysis, whose magnitudes are significant in the vicinity of the fault, and are attenuated at remote grid terminals. A model of a three-terminal MT-HVdc system is developed in power systems computer aided design/electromagnetic transients including dc. The results have been post processed in matrix laboratory and simulation results verify the effectiveness of the proposed solution for preventing dc faults on an HVdc system. [ABSTRACT FROM AUTHOR]

High di/dt during the close-open operation process of hybrid dc breaker will cause differential mode interference (DMI) in the energy supply system of the breaker's driver units due to their inductive coupling. The generated DMI may lead to false action of the breaker, which is an important issue that should be carefully considered in design stage. A method to analyze DMI based on time-domain wideband circuit simulations is proposed in this article. First, a time-domain waveform expression of DMI voltage source is derived, and then wideband equivalent circuit models of major components in the energy supply system are established. Furthermore, a standardized design methodology to reduce DMI is suggested. Applying the method to a high-frequency energy supply system of a ±200 kV hybrid dc breaker, simulation results show that the DMI level for two DMI mitigation measures is reduced by about 90% and 80% of the initial value, respectively. [ABSTRACT FROM AUTHOR]

Voltage source converters (VSCs)-based high-voltage direct current grid (HVDC grid) is increasingly being used in high-level and long-distance power transmission. DC breakers could be applied to isolate fault on long overhead DC links, whereas most of these faults would be temporary and can be recovered after some deionisation time. The objective of this article lies in proposing and implementing DC line soft reclosing sequence based on the controllability of hybrid HVDC breaker (HHB). The proposed method has been evaluated in a four-terminal VSC-based HVDC grid with HHBs at each DC lines' end. The analysis and simulations in PSCAD/EMTDC show that the proposed sequential operation of HHB can softly recover the faulty line and identify whether the fault is temporary or permanent, with little impact on the whole DC system. [ABSTRACT FROM AUTHOR]

Fast and reliable DC fault detection is one of the main challenges for modular multilevel converter (MMC) based DC grid with DC circuit breakers (DCCBs). This paper extracts the high-frequency components in transient voltages by wavelet transform and proposes a fault identification method based on the difference of square of transient voltages to identify the faulted lines for DC grids using overhead lines. Meanwhile, a faulted pole discrimination method based on the difference between the change of positive and negative pole voltages is presented. A line protection scheme including detection activation, fault identification, faulted pole discrimination, and post-fault re-closing is designed. Using only the local measurements, the scheme can realize the protection of the whole line without communication and has the capability of fault resistance endurance and anti-disturbance. The proposed method is tested with a four-terminal MMC-based DC grid in PSCAD/EMTDC. The selection methods of threshold values are presented and the impact of DCCB operation on the reliability of DC fault protection is analyzed. Simulation results verify the fast detection and reliability of the designed DC line protection scheme. [ABSTRACT FROM AUTHOR]

Different HVDC grid types and the respective protection options are discussed. An earthed bipole HVDC grid was modeled in PSCAD, and using simulation results, the necessity of di/dt limiting inductors to contain the rise of fault currents within the capacity of current hybrid dc breakers is demonstrated. The impact of different inductor sizes on current rise was studied. A fault detection and localization scheme using the rate of change of voltage measured at the line side of the di/dt limiting reactors is proposed. The protection system was modeled and tested under different fault types and locations. The results show that the proposed method of HVDC grid protection is feasible using the current hybrid dc breaker technology. A systematic procedure for setting the necessary protection threshold values is also demonstrated. [ABSTRACT FROM AUTHOR]