Abstract
Due to the possibility of putting a large number of modules consisting of switches and capacitors connected in series, the modular multilevel converter (MMC) can easily be scaled to high power and high voltage power conversion, which is an attractive feature for filter-less and transformer-less design and helpful to achieve high efficiency. However, a significantly increased amount of sub-modules in a MMC may increase the requirements for sensors and also increase the risk of failures. As a result, fault detection and diagnosis of MMC sub-modules are of great importance for continuous operation and post-fault maintenance. Therefore, in this paper, an effective fault diagnosis technique for real-time diagnosis of the switching device faults covering both the open-circuit faults and the short-circuit faults in MMC sub-modules is proposed, in which the faulty phase and the fault type is detected by analyzing the difference among the three output load currents, while the localization of the faulty switches is achieved by comparing the estimation results by the adaptive observer. In contrast to other methods that use additional sensors or devices, the presented technique uses the measured phase currents only, which are already available for MMC control. In additional, its operation, effectiveness and robustness are confirmed by simulation results under different operating conditions and load conditions.
Highlights
Modular Multilevel Converter-High Voltage Direct Current (MMC-HVDC) systems [1,2,3,4,5,6] are gradually becoming a possible solution compared to the traditional power transmission technology for connecting offshore energy plants located at longer distances to the transmission grids due to a series of merits compared to other existing power transmission systems such as higher output voltage levels, modular construction, larger maintenance intervals, improved reliability and reduced costs
There are two different types of the switching device fault in a sub-module: open-circuit fault which appears due to lifting of the bonding wires in a switch module caused by over-temperature or aging and usually does not cause additional serious damage to the system if the protection system functions well; short-circuit faults are caused by wrong gating signals, overvoltage, or high temperature and could cause additional damage to other components in the circuit, so the short circuit fault should be treated fast and carefully
(2) Open-circuit fault in T1 (Figure 3a): as shown in Table 2, the sub-module operates as normal when the arm current ikm > 0, the arm current still goes through D1 and C to charge the capacitor when the gating signal Sp(n),i,k = 1 and the arm current flows through T2 to bypass the capacitor when the gating signal Sp(n),i,k = 0; when ip(n),k is negative, the module is in normal operation when
Summary
Renewable energies, especially the off-shore wind energy and marine energies, have been drawing a lot of interest due to their features of low pollution, sustainable development, few disturbances and large capacity, which have made the expansion of energy plants from onshore to offshore the trend. A direct approach to detect faults is to add additional sensors to each semiconductor switching device, to each cell, or to use a gate drive module capable of detecting faults and providing feedback [7] This method combines the monitoring process and the identification process and the time consumed in the detection is quite short, it is a quite inefficient way to detect faults as the faults are detected manually which has low efficiency, especially when a large number of switching devices and cells are involved. These additional sensors and signals increase the cost, and the implementation complexity.
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