The integration of large-scale offshore wind generation into modern power systems demands extensive studies to ensure reliable and secure operation. These studies must address multiple aspects, including steady-state behavior such as voltage profiles, active and reactive power requirements, and dynamic performance during transient events like faults. The complexity increases significantly when offshore wind farms are considered for black start restoration, as this abnormal operating condition introduces unique challenges in both steady state and transient conditions of each restoration step. One of the challenges in system restoration planning study is to sift through numerous possible restoration scenarios and paths to identify those that are technically feasible.
To address these requirements, the research work presents a comprehensive set of steady state power flow optimization analysis conducted using tools developed by EPRI to establish the potential system restoration plan following a system blackout. The tools which utilize the offshore wind farm model and the onshore transmission network data created in PSS/E format, are aimed at determining the optimal cranking path to a list of non-black-start units and critical loads specified by the user while satisfying the constraints such as voltage limits, active power and reactive power output of the generator units. This was tested using a case study for a 2.7 GW off shore wind farm project planned for the North eastern region of the United States.
Two stages of black start restoration have been considered in the study, the black start restoration of the offshore wind farm network and the step-by-step energization of the onshore transmission network to pick up critical loads and stabilize the restored power system until reaching the next potential non-blackstart resources.
During early restoration, as the system is lightly loaded, high voltages are of major concern. To find a transmission path for energizing a non-blackstart unit or a critical load, the charging current of each line is calculated. A path with the least charging current is selected to be energized. Alternatively, to find a path with least switching time, the switching time of the line can also be used as a weight. The shortest path algorithm for weighted undirected graph is applied to find the path with least total weights.
To satisfy the operating constraints, active and reactive outputs of generating units are adjusted and some non-critical load or dispatchable loads may have to be picked up. To achieve this objective, the secondary problem is designed as an optimal power flow problem, which checks the feasibility and the operating point of the entire system considering ramping rates of generating units and other power flow constraints. The study provides detailed information on the MW and Mvar loading of the wind turbine generators and the voltage profiles along the cranking paths for each generating scenario and the total time required to reach the proposed critical load on the system as well as recommending the required balancing loads to be included along the cranking path to satisfy the network constraints. These analyses provide the necessary prerequisites for more detailed dynamic and EMT domain studies, which are utilized concurrently to validate the network restoration steps.