The concern for power quality has been on the rise in recent years for all kinds of electric power systems. Harmonic analysis is especially interesting in the case of Offshore Wind Power Plants (OWPPs) due to the special susceptibility of these systems to harmonic issues and to the inherent differences in their behavior with respect to other electric networks. The aim of this article is to provide an overview of the basic concepts for harmonic analysis in OWPPs and to outline the typical assessment procedure used to this day. Some of the limitations of current industry practice will be outlined, as well as some possible complementary approaches for its improvement.
In this paper, the output impedance of a three-phase inverter based on a dual current control (also called double synchronous reference frame current control) is modelled, which includes: the current loop gain, the control delay, and the Phase-Locked Loop (PLL). The impact of these parameters on the impedance is then analysed by a sensitivity study. The model is derived using transfer matrices and complex transfer functions, and it results in a compact impedance formulation that can be used in harmonic small-signal stability studies and system-wide steady-state harmonic calculations.
To accurately simulate electric vehicle DC fast chargers' (DCFCs') harmonic emission, a small time step, i.e., typically smaller than 10 μs, is required owing to switching dynamics. However, in practice, harmonics should be continuously assessed with a long duration, e.g., a day. A trade-off between accuracy and time efficiency thus exists. To address this issue, a multi-time scale modeling framework of fast-charging stations (FCSs) is proposed. In the presented framework, the DCFCs' input impedance and harmonic current emission in the ideal grid condition, that is, zero grid impedance and no background harmonic voltage, are obtained based on a converter switching model with a small timescale simulation. Since a DCFC's input impedance and harmonic current source are functions of the DCFC's load, the input impedance and harmonic emission at different loads are obtained.Thereafter, they are used in the fast-charging charging station modeling, where the DCFCs are simplified as Norton equivalent circuits.In the station level simulation, a large time step, i.e., one minute, is used because the DCFCs' operating power can be assumed as a constant over a minute. With this co-simulation, the FCSs' long-term power quality performance can be assessed time-efficiently, without losing much accuracy.
A voltage imbalance at the AC terminals of a threephase inverter creates a ripple in the power signal on the DC side. In order to minimize this ripple, several techniques can be applied, in which a double Synchronous Reference Frame (SRF) current control structure is very typical. In this approach, both the positive and negative sequence currents are controlled. This technique has been shown to have an adequate response against imbalances; however, this paper shows that in the typical implementation of the double SRF control, the output AC pq instantaneous powers will have a constant error due to the phaseangle misalignment of the negative sequence with the positive sequence. Based on mathematical formulations and simulation results, this paper shows that this AC-power error exists, and that it is due to the above reason. In order to overcome this, this paper proposes to have a phase-tracking system that specifically follows the negative sequence phase-angle. The results show that this implementation is able to properly control the output AC power.
To accurately simulate the harmonic emission of EV DC fast chargers (DCFCs) and the harmonic voltage of the power grid to which the chargers are connected, a small time-step, i.e., typically smaller than 10µs, is required. However, for harmonic assessment, a long timescale, typically a day, is required. A conflict between accuracy and time efficiency exists. To address this issue, a multitimescale modeling framework of fast charging stations (FCSs) is proposed in this paper. In the presented framework, the DCFCs' input impedance and harmonic current emission in the ideal grid condition, i.e., the grid impedance is zero and there are no background harmonic voltages, is obtained firstly through a converter switch model with a small timescale. Since the DCFC's input impedance and harmonic current source change in the charging course, the input impedance and harmonic emission at different input power should be obtained. Then, the DCFCs' input impedance and harmonic emission will be used in the fast-charging station modeling, where the DCFCs are simplified as their Norton equivalent circuits. In the station level modeling, a bigger time step, i.e., 1 minute, is used, since the DCFCs' operating power can be assumed as a constant in one minute. With this framework, the FCSs' long-term power quality performance can be assessed efficiently without neglecting the DCFCs' small timescale dynamics.
This paper presents a small-signal model for powerelectronics converters that use a typical control structure in wind energy applications: the double Synchronous Reference Frame (SRF) current control. The paper considers the presence of unbalanced currents and voltages, and analyses their impact on the frequency couplings of the converter. In addition, it is revealed that, in the presence of negative-sequence voltage synchronization, the converter presents an additional coupling at −2f1 − fp.
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