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The arm average-value model (AAVM) of MMC assumes identical SM capacitor voltages and ignores the sorting of capacitor voltages and therefore greatly reduces the simulation burden. However, the existing AAVM cannot consider the effect of anti-parallel diode and fails to correctly simulate the behavior of arm blocking. In view of this, an improved AAVM which considers the effect of anti-parallel diodes is proposed to simulate the arm blocking. Three thyristors are used to associate the arm current circuit and capacitor circuit in AAVM. In normal state, all thyristors are controlled off and the improved AAVM is performing the same with AAVM. At the instant of arm blocking, all thyristors are controlled on to allow the arm current to charge the SM capacitor, which emulates the effect of anti-parallel diodes. The improved AAVM is compared with the detailed switching model of MMC based on a benchmark system established in PSCAD/EMTDC. The simulation result shows that the improved AAVM behaves the same as the detailed model of MMC at both the steady state and the transient state and the improved AAVM can correctly reflect the behaviors of arm blocking while the simulation time is greatly reduced.

Electromagnetic loop networks exist in the process of the power grid development from low voltage levels to high voltage levels. There are some problems in many multi-terminal electromagnetic loop networks, such as the excessive short-circuit current, the uncontrollable power flow and the complex stability control measures. In order to solve these problems, a reconstruction method for the multi-terminal electromagnetic loop network is proposed, which integrates the loop-decoupling technology and the MMC-HVDC. Finally, simulations are carried out in the Urumqi power grid based on PSD-BPA, and the result shows that the method proposed in this paper is effective, the multi-terminal electromagnetic loop network can operate safely after reconstructing.

In flexible DC transmission, if the DC fault occurs, not only the 2-level voltage source converter(VSC) and the modular multilevel converter(MMC) based on the half-bridge submodule(HBSM) cannot suppress the DC fault current, but also the DC capacitor will increase the DC fault current. Furthermore, DC circuit breakers are too expensive. In order to overcome the issue that the converter can not block the DC fault current, this paper took advantage of the unidirectional conductivity of current source PWM converter(CSC) which is a dual with VSC, studied H7-CSC topology which can greatly reduce the CSC loss, and provided the modulation and control strategy of H7-CSC. In addition, the start-up control strategy and the DC fault treatment strategy of DC network consist of H7-CSC were proposed; the three terminal DC network simulation model was set up which composes a line-commutated converter(LCC) as a rectifier and two H7-CSCs as the inverters, then simulation verification was carried out in PSCAD/EMTDC. The results show that the DC network model proposed by this paper can achieve the smooth start-up, stable operation, DC fault removal and system restart. This model realizes DC fault removal through the control of each converter, without DC circuit breaker, which provides new ideas for the development of DC power grid in the future.