Secure operation of high-renewable power systems : facilitating network flexibility by power flow routers

Abstract

Ensuring secure and economic operations is an important task for power system operators. Traditionally, the operators mostly rely on node power flexibility (e.g., generation dispatch, energy storages, demand response) to handle the impact caused by renewable uncertainties. On the other hand, network flexibility has been paid with little attention. However, this is no longer the situation nowadays. Power networks are becoming more flexible thanks to the development of power electronic techniques. In this thesis, we study how power flow routers (PFRs) as a representative controller, facilitate network flexibility in power systems. One potential hardware implementation of PFRs is a pair of power electronic transformers installed at two terminals of a line, which implies that PFRs are applicable to both high-voltage (HV) and low-voltage (LV) systems. This is different from the traditional network controllers (e.g., thyristor controlled series compensators) which are limited to HV systems. This thesis exploits the potential of PFRs to address some important security issues in system operation, including transient stability problems in HV systems and voltage regulation problems in LV systems. We make contributions in five aspects. First, based on the AC power flow model with PFRs, we develop a new linear approximation model for power flow equations with PFRs. Numerical tests show that this linear model has high accuracy in voltage magnitude and provides an alternative in power system optimization problems. Second, we propose a robust transient stability-constrained optimal power flow (OPF) problem that addresses the impact of renewable uncertainties by coordinating PFR tuning and generation re-dispatch. We reveal that PFRs unlock the dispatchability of cost-effective generators so that the system can achieve high robustness of transient stability in a more economic way. Third, we investigate the value of PFRs in helping battery energy storage systems (BESSs) for renewable power accommodation. A multi-period optimization model is designed to minimize the BESS capacity while satisfying the voltage constraints. Results show the effectiveness of PFRs in replacing BESSs and hint that PFRs may also effectively mitigate voltage volatility. Fourth, we introduce PFRs into droop-controlled microgrids which have highly volatile voltage profiles under high renewables. We propose a chance-constrained OPF problem and design an iterative algorithm to determine the set points of distributed generator outputs, voltage magnitudes, system frequency and the parameters of PFRs. Case studies show that PFRs perform well in voltage volatility mitigation especially under high droop gains. Fifth, a chance-constrained optimization model is designed to estimate the flexibility area at the transmission-distribution interface where PFRs are installed in the distribution system. Compared to the traditional model with only node power flexibility, the flexibility area is much enlarged with the presence of PFRs. Overall, the thesis offers a new viewpoint that network flexibility is a significant supplement to node power flexibility to achieve a more secure and economic operation.