Active power losses distribution methods for the modular multilevel converter

Modular Converters such as the MMC have become the new standard in VSC-HVDC applications. Their modularity has brought many industrial advantages but also increased the complexity of their operation. This paper looks at how a range of techniques may alter the balance of power losses between the IGBT modules. These techniques are based on circulating currents at the (i) fundamental frequency and (ii) second harmonic and (iii) DC voltage offset on the converter voltage waveform. Finally, conclusions on the effectiveness and potential drawbacks of these techniques are discussed

Zero phase sequence voltage injection for the alternate arm converter

The Alternate Arm Converter (AAC) is a voltage source converter being developed as an alternative to the Modular Multilevel Converter (MMC) for HVDC power transmission and reactive power compensation. Each Arm of the converter contains high voltage series IGBT Director Switches and full-bridge cells, which enables the VSC to ride through AC and DC network faults. This paper describes how the AAC can be optimised by modulating the converter terminal voltages with zerophase sequence triplen harmonic components. The optimisation reduces the ratio of the number of the full-bridge cells compared to the simpler Director Switches which offers a valuable improvement in footprint and efficiency

Assessment of DC/DC converters for use in DC nodes for offshore grids

With increasing offshore wind generation, there is a strong argument for implementing a multi-terminal DC grid offshore by the interconnection of individual HVDC links. The point of intersection of three or more lines can be used to interconnect projects with different voltage levels and to control power distribution. It is being proposed that these points, or nodes, be implemented using DC devices. A highvoltage, high-power DC/DC converter will therefore be an important component of a DC node. This paper reviews possible DC/DC converter topologies, looking at device requirement, different voltage conversion ratio and fault management. The suitability of the converters considered, for use in a node in a HVDC offshore grid, is discussed. A resonant DC/DC converter topology is considered in detail and is modelled at a conversion ratio of two, and demonstrates high power efficiency

Benefits of Operating a Four Terminal HVDC Network with a Normally Open Point

It is likely that any HVDC grids will evolve over time and will likely be multi-vendor and use several different technologies. This paper studies a four terminal network where the interconnecting link is operated as a normally open point. This allows the network to be reconfigured in the event of DC side fault, without the need for DC circuit breakers. The network uses the popular Modular Multi-level converter at one terminal and a fault tolerant converter at the other terminal. A simple cost benefit shows the economic advantage of the interconnecting link. The operation of this network is then verified through simulation for a normal operating scenario and a DC fault scenario

Power loss and thermal characterization of IGBT modules in the Alternate Arm converter

Power losses in high power HVDC converters are dominated by those that occur within the power electronic devices. This power loss is dissipated as heat at the junction of semiconductor devices. The cooling system ensures that the generated heat is evacuated outside the converter station but temperature management remains critical for the lifetime of the semiconductor devices. This paper presents the results of a study on the temperature profile of the different switches inside a multilevel converter. The steady state junction temperatures are observed through the simulation of a 20 MW Alternate Arm Converter using 1.2kA 3.3 kV IGBT modules. A comparison of the Alternate Arm Converter is made against the case of both the half-bridge and full-bridge Modular Multilevel Converter topologies. Furthermore, the concept of varying the duty-cycle of the two alternative zero-voltage states of the H-bridge modules is introduced. Simulation results demonstrate that it can change the balance of electrical and thermal stress between the two top switches and the two bottom switches of a full-bridge cell. © 2013 IEEE

The Alternate Arm Converter: A New Hybrid Multilevel Converter With DC-Fault Blocking Capability

This paper explains the working principles, supported by simulation results, of a new converter topology intended for HVDC applications, called the alternate arm converter (AAC). It is a hybrid between the modular multilevel converter, because of the presence of H-bridge cells, and the two-level converter, in the form of director switches in each arm. This converter is able to generate a multilevel ac voltage and since its stacks of cells consist of H-bridge cells instead of half-bridge cells, they are able to generate higher ac voltage than the dc terminal voltage. This allows the AAC to operate at an optimal point, called the “sweet spot,” where the ac and dc energy flows equal. The director switches in the AAC are responsible for alternating the conduction period of each arm, leading to a significant reduction in the number of cells in the stacks. Furthermore, the AAC can keep control of the current in the phase reactor even in case of a dc-side fault and support the ac grid, through a STATCOM mode. Simulation results and loss calculations are presented in this paper in order to support the claimed features of the AAC

High-frequency operation of a DC/AC/DC system for HVDC applications

Voltage ratings for HVdc point-to-point connections are not standardized and tend to depend on the latest available cable technology. DC/DC conversion at HV is required for interconnection of such HVdc schemes as well as to interface dc wind farms. Modular multilevel voltage source converters (VSCs), such as the modular multilevel converter (MMC) or the alternate arm converter (AAC), have been shown to incur significantly lower switching losses than previous two- or three-level VSCs. This paper presents a dc/ac/dc system using a transformer coupling two modular multilevel VSCs. In such a system, the capacitors occupy a large fraction of the volume of the cells but a significant reduction in volume can be achieved by raising the ac frequency. Using high frequency can also bring benefits to other passive components such as the transformer but also results in higher switching losses due to the higher number of waveform steps per second. This leads to a tradeoff between volume and losses which has been explored in this study and verified by simulation results with a transistor level model of 30-MW case study. The outcome of the study shows that a frequency of 350 Hz provides a significant improvement in volume but also a penalty in losses compared to 50 Hz