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Abstract
side, and thus the AC side voltages are decoupled. This topology has the ability to control power flow between its AC ports and can supply a radial network section with a controllable voltage. By having the ability to control the power flow it is possible to distribute redundancy over different feeders when needed. In the current practice, every feeder must be able to supply the full load of another feeder, and can therefore only be loaded up to around fifty percent of its power rating. Sharing the redundancy over more feeders allows the connection of loads and generation units beyond this limit. Since the AC voltages on the different IN ports are unrelated, the IN can connect networks with different voltage amplitudes, phase angles and/or frequencies, which makes it possible to also share redundancy in such situations. Controlling the power flow in a meshed network can also be used to optimize voltage profiles, and thus maximize the penetration level of distributed generation units in the network. Alternatively, the power flow can be optimized to reduce losses in the network. During a network disturbance, the IN can prevent spreading of this disturbance, support the disturbed network, temporarily supply part of the network as a radial network, and restore meshed operation after the disturbance. To allow the Intelligent Node (IN) to perform the described tasks, the IN converters need to be able to respond quickly to planned and unplanned events in the power system, such as load changes, shortcircuits and the opening and closing of load-break switches. The ability of the converters to do so, depends, besides on their ratings, mainly on the controls that drive them. Furthermore, the protection system of the IN needs to prevent damage to the IN components due to over-currents and over-voltages. At the converter level two basic operating modes exist: power flow control and voltage control. The first operating mode is used in meshed network operation, and called P Q control mode. The converter controls its power exchange with the network by controlling its output current. In the second operating mode, called V control mode, the converter defines the amplitude, frequency and phase angle of the voltage on its AC port. The converter behaves as a voltage source with a fixed frequency and supplies or consumes the active and reactive power as required by the connected loads and generators of a radial network section. In the proposed IN concept, at least one of the converters of the IN is galvanically connected to the ’central grid’, and operates in P Q control mode, in order to supply the connected sections and to control the DC bus voltage. To fully utilize the capabilities of the interconnected converters, the IN control concept also includes specific detection schemes and addi-