Power outages can damage severely critical infrastructures such as telecommunication networks, financial services, water supplies and hospitals and completely shutting down production at companies. While most power blackouts usually last from some minutes to few hours, some can last days or even weeks. Furthermore, it is likely that these kinds of events would become more recurrent due to the stranded expansion of aged Electrical Power System (EPS) infrastructures in Europe for coping with the increased societal and environmental pressure for massive deployment of adopting Renewable Energy Source (RES)-based generation, which is known for having a variable characteristic. It is expected that RES will become the dominant factor in the power grid and will gradually replace Conventional Synchronous Generation Units (CGU), and it is known that Offshore Wind Farms (OffWF) are more suitable for large-scale generation applications. However, RES, including Wind Farms (WF), are not well aligned to work with EPS that were designed fifty - to - sixty years ago, due to reduced inertia, reduced short-circuit power, and limited control capabilities that imply the integration of RES. Moreover, more grids will contain more renewables, and thus more risks of outages could arise as one fault can trigger another one as a domino effect, deriving in widespread disruptions, including regional blackouts.
In this context, the contemporary control systems that regulate WF inject power to the grid via Power-Electronic Interfaces (PEI), and their schemes are designed to not interfere actively with the safe and secure regulation of a large-scaled EPS. On the contrary, they are just limited to inject a predetermined power injection setpoint with a current-injection control method that assumes all the time this power has load demand to go (grid-following control). However, this is not the case when unplanned islanding or an outage arises in an EPS, as the WF currently do not possess a control system that regulates the frequency of an island.
This MSc Thesis Project presents the design, implementation and testing of a control system attached to type-4 Wind Turbines (WT) that can manage and tightly ensure the load and generation balance during any circumstance, including a massive blackout. This control method can successfully regulate voltage, reactive power and frequency, which can be adapted automatically to the real-time conditions of the grid. The scheme takes the grid-following control approach as a starting point, which was modified in order to have Grid-Forming and Black-Start capabilities. The proposed new control approach for grid-forming and black-start design was implemented in DIgSILENT PowerFactory 2018, where a total of seventeen large-scaled type-4 WF are located into a three-area EPS. The Permanent-Magnet Synchronous Generator (PMSG) WF containing the proposed grid-forming control system are accompanied by Hydro, Nuclear and Thermal Plants, accounting CGU. Additionally, the three-area EPS also contains two HVDC Transmission Systems composed each by two Voltage-Source Converters (VSC), which also have similar grid-forming and black-start capabilities, and seven Battery-Energy Storage Systems (BESS), which give auxiliary power to the Black-Start Units and frequency support to the areas with loss of generation. The WF, BESS and the HVDC stations are interfaced via modelled Modular Multi-level Converter (MMC) controlled voltage sources.
The proposed grid-forming and black-start capabilities of the three-area EPS were tested with several EMT simulations reproducing severe short-circuit faults followed by a loss-of-generation scenario, the blocking of the HVDC converter stations and a massive relay protection program, resulting in the full outage and isolation of the area responsible for the largest power supply in the three-area EPS. After the blackout, the three-area EPS performed a Restoration plan, from the generation resetting and the reconnection of lines, transformers, etc., to the final (cold) load pick-up stage. In order to evaluate the advantage of using utility-scaled WF with grid-forming controllers to execute a conjunct Black-Start and Restoration plan, two operational scenarios were performed: one with 90% wind power share and another without any participation of wind power. As a consequence of the implementation of the proposed grid-forming control systems, the simulation results endorse that an EPS with 90% wind power share can steer a Black-Start and Restoration operation when required.