With the fast development of renewable energy resources, harmonics in the modern distribution system become more serious due to the wide adoption of power electronic devices. Nowadays, the distribution-level phasor measurement units (D-PMUs), which can provide synchronized harmonic data with high time resolution, are under fast development. To take advantage of the D-PMUs, a hierarchical harmonic contribution (HC) determination method is proposed in this paper for systems with multiple harmonic sources. The HCs are determined by the widely linear complex partial least squares (WL-CPLS) method which is both accurate and efficient. In the actual power system, the D-PMUs are not fully installed at all buses and the harmonics monitored by each D-PMU are actually the aggregate effects of multiple harmonic sources in the subarea. Based on the above consideration, a hierarchical HC strategy is proposed to locate the main harmonic contributors layer by layer. The proposed method can be applied to the radial distribution system with any number of D-PMU placement. The methodologies are illustrated and verified by the IEEE 37-bus system.

To address the DC-link voltage control issue in flywheel energy storage systems (FESSs), a DC-link voltage control strategy using a capacitor-energy-based super-twisting sliding mode controller (CE-STSMC), integrated with a disturbance observer, is proposed in this article. First, an exponential term is incorporated into the STSMC algorithm to enhance its convergence rate. Then, the improved STSMC is employed as the voltage-loop controller to mitigate the insufficient anti-disturbance capability of conventional control methods. To improve the system robustness, a nonlinear disturbance observer (NDOB) is developed to estimate the load power. The estimated disturbance is further feedforward-compensated into the improved STSMC controller. Finally, experiments are carried out on a 2.2 kW FESS prototype under DC-link voltage step and sudden load-change conditions, which demonstrates the effectiveness and superiority of the proposed control strategy.