In the sub-projects DG DemoNet Concept and BAVIS, the integration of DER is addressed. Voltage control concepts were developed in numerical simulation environments and based on real network data (three case studies typical of Austrian medium voltage networks). Moreover, both the economic and technical efficiency have been evaluated and compared to a reference scenario. Based on this experience, DG DemoNet Validation analysed whether the results obtained from the simulations were also valid and effective under real network conditions.
As a result, two different solutions for voltage control in MV networks have been validated and successfully demonstrated (coordinated voltage control and distributed voltage control) on a test platform in two different medium voltage networks (Lungau in Salzburg and Großes Walsertal in Vorarlberg). Based on actual voltage measurements at critical grid nodes together with the measurement of the actual active and reactive power contributions of all controllable DGs, voltage set values for the transformer’s AVC (Automatic Voltage Control) and reactive power set values for the DGs are periodically calculated and set. This keeps all voltages (i.e. the voltage range, the spread between the highest and lowest voltage in the grid) within the allowed voltage limits, thereby avoiding active power curtailment. The influence of the individual contribution of the reactive power is determined by the contribution matrix, representing the actual network topology.
Distributed Voltage Control (‘level control’): In this control mode, an optimal set point for the on load tap changer’s automatic voltage controller (AVC), based on network measurement of critical nodes, is calculated. Four different strategies have been implemented and can be chosen online to keep the voltages on the ‘upper limit’, ‘centred’, ‘lower limit’ or ‘minimize’ the number of taps.
Coordinated Voltage Control (‘range control’): Here the objective is to optimize the reactive power of distributed generators together with set point of the transformer to keep the voltages within the limits. In this controller mode the range controller keeps the spreading of the maximum and minimum voltage within the accepted voltage band, regardless of the absolute voltage values and once the spreading is small enough the level controller shifts the voltages into the allowed voltage band between the upper and lower voltage limits. This is illustrated in Fig. 1, where the green line represents the adjustment in the voltage range compared to the yellow line.
The independence of the two control modes makes it possible to separate the two control objectives. Thus, it ensures a continuous change of the AVC’s voltage set point (‘level control’). This guarantees that the well-established transformer’s AVC works as intended as a primary control loop, responsible for coping with the short-term variations of the high voltage side and of sudden changes in topology. With just the information about the transformer’s dead-band and the actual voltage measurements and the contribution of the DG’s reactive power on the actual network switching state, the CVCU safeguards the operation of the network between the allowed voltage limits.
Figure 1: Schematic functional diagram of the central voltage control unit
Beside the technical evaluation, the developed voltage control concepts have been economically validated in the investigated grid sections. In the development phase a cost benefit analysis has been performed based on the simulations. These results were validated during the field test (demonstration) phase with actual data on related cost. A specific work package was dedicated to the cost benefit analysis . The economic analyses were performed with the net present value method for a 20 years period.
Figure 2 shows the costs and cost reductions of the developed new solutions for three case studies in order to increase the hosting capacity of the medium voltage network. These costs are compared to the cost of reinforcement options and network extension hosting the same amount of distributed generation with current network planning and operation approaches (DG share in the 3 networks between 60% and 90%). Depending on the different grid structures and network topologies the cost reduction in the three networks ranges from 5% to 80%. The two networks in which the field test and demonstration took place are Case Study 1 and 2
Figure 2 – costs and cost reduction compared to the reference scenario network reinforcement
The next step is to investigate the replicability and scalability of the developed solution in Austria as well as in Europe to identify networks where similar problems may occur and whether the solutions will be suitable. Therefore the project chain DG DemoNet, in particular the demonstration site in Salzburg (Upper Austria) and the respective distribution system operator Salzburg AG, as well as the Austrian Institute, are partners in the European Project iGREENGrid. The project focuses on investigating replicability and scalability of the specific solutions by establishing a family of relevant national projects focused on the effective integration of variable distributed generation in power distribution grids.
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