Flexible distribution systems through the application of multi back-to-back converters : concept, implementation and experimental verification

In recent years the planning and operation of electrical power systems has changed significantly. The unbundling of the formerly integrated generation, transmission, distribution and delivery companies and the growing penetration of DG is increasing the complexity and uncertainty in distribution network planning and operation. Due to the uncertainty, network investments that are done to anticipate growth load or the connection of expected new generators may turn out to be uneconomic. The complexity leads to a higher risk of failures. This stimulates the distribution network operator to consider flexible alternatives to traditional network reinforcements and flexible operation measures. This thesis concerns a power flow control device based on power electronics, called Intelligent Node (IN), which can provide such needs. The idea to control power flow by the application of power electronics is not new in itself. The existing applications are mainly aimed at transmission systems, with its high voltage and power levels and existence of advanced measurement and control systems. In this thesis, an overview is given of existing applications. Successful experiences in the transmission network cannot be applied directly to the distribution system due to its different network topology and operation. For example, the low-inductive character of underground cables, as opposed to the mostly inductive impedance of overhead lines, requires adaptations on the methods to control voltages and power flow in the distribution system. Also the phase-byphase operation of load-break switches is only found in distribution networks, and the availability of measurement data for all network nodes cannot be taken for granted. The IN consists of multiple converters interconnected on their DC 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, short-circuits 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 PQ 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 PQ 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 additional control and protections, which can change the operating mode and set-points of the converters or shut down the IN in response to power system events. When the power system is in normal operation conditions and the converters are in PQ control mode, the IN controls the power flow in the meshed network according to centrally determined P and Q set-points. During a short-circuit, and the resulting voltage dip, the IN should no longer follow these setpoints, but inject reactive power to mitigate the voltage dip. To do so, a control scheme was developed, which is only active when the network voltage is outside of a certain voltage band. Although it is assumed that meshed operation is the normal situation, it might be necessary to operate some parts of the network radially for some time. In order to perform maintenance or repair work on a certain network section, it can be necessary for instance to isolate it by opening the switches on each of its sides. In such a situation, the IN can supply a resulting radial part of the network, with the applicable converter in V control mode. To do so, the applicable IN converter must stop controlling the power flow and start controlling the voltage level instead, after detecting the change in the network. A control and detection scheme was developed to implement this functionality. After the maintenance or repair work has been finished, the radial part of the grid is to be reconnected to the rest of grid. To maintain IN operation and minimize voltage discontinuities after restoring meshed operation, it is necessary that the voltage of the radial network section is synchronized with the voltage of the rest of the network. Therefore, the voltage amplitude, frequency and phase angle are periodically measured at a remote location, and transmitted to the IN with a random but limited time delay. To determine the maximum remote measurement interval, the statistics of frequency variations in the public electricity network have been gathered through measurements. The maximum interval is determined as a function of acceptable phase angle difference between the networks. After detecting that the meshed network has been restored, the applicable converter must be able to change from V control mode to PQ control mode, without disconnecting from the grid or stopping operation. The operation of circuit breakers is in many networks performed simultaneously on all three phases. In other medium voltage networks, for example in the Netherlands, however, the phase-by-phase operation of load-break switches is common, given the wide-spread application of manually operated, compact epoxy resin insulated, single-phase switchgear. Phase-by-phase connection and disconnection of grid areas requires a different IN behavior. The control and detection schemes were developed both for three-phase and for phase-byphase switchgear operation. Existing back-to-back applications cannot make the described transitions without supply interruption, neither for three-phase nor for phase-by-phase switchgear operation. The developed control and detection schemes are implemented in a laboratory-scale set-up. The main components of this set-up are two 400V, three-phase converters, connected on their DC sides, with the possibility to connect the AC sides to a radial network with a resistive load or to the public low voltage network. With this set-up, experiments are performed, focusing on the connection and disconnection of network areas and on voltage sag and swell mitigation. The experimental verification of the connection and disconnection control and detection schemes, as well as the voltage dip and swell mitigation implementation, shows a successful implementation of the concept