LTP Modeling and Analysis of Frequency Coupling in PLL-Synchronized Converters for Harmonic Power Flow Studies

Abstract

As known, nonlinear loads in power systems originate harmonic distortion and power quality issues. Converterinterfaced loads exhibit a nonlinear behaviour as well and may largely contribute to increase the harmonic pollution. The nonlinearities introduced by the PLL-synchronization and power control originate, indeed, a coupling mechanism between fundamental and harmonic frequencies. These harmonic coupling effects are not captured by traditional Norton/Thevenin equivalent converter models, leading to inaccurate harmonic power flow analyses. This paper proposes a Linear Time Periodic model of a PLL-synchronized converter to be used in Harmonic Power Flow analyses. A realistic 18-bus distribution grid hosting substantial amount of power-electronic interfaced resources is used as a case study. It is revealed that, in high grid loadability condition and with distorted grid supply voltage, the harmonics are significantly amplified by the converters and the fundamental components of the buses voltages are reduced, representing a risk for the voltage stability. This phenomenon is also influenced by the tuning of the current control loop and the PLL. The accuracy of the presented analyses is validated by comparing the harmonic power flow results with time-domain simulations.

Impedance-Based Stability Analysis of Grid-Connected Inverters under the Unbalanced Grid Condition

“…The frequencies of the oscillation harmonic components are mirror-coupled about the fundamental frequency [20]. In addition to the PLL, the grid impedance and other dq-axis control asymmetries have a great influence on frequency-coupling oscillations [21,22]. The traditional impedance models consider the inverter as a single-input single-output (SISO) system with positive and negative sequence impedance decoupling.…”