Online Parametric Estimation of Grid Impedance Under Unbalanced Grid Conditions

Abstract

Whereas power-electronics-based power systems are expected to enable more integration of renewable energy sources, they could pose crucial challenges including stability issues due to the Thévenin impedance seen by inverters. Such problems could be mitigated by enabling the inverters to estimate the grid impedance by including a grid impedance estimation technique into their control loop. However, one aspect which has been overlooked thus far is that the accuracy of such grid impedance estimation techniques is significantly affected by various grid conditions. For instance, the unbalance in three-phase power systems causes unwanted oscillations at double the fundamental frequency in the inverters control loops. Therefore, this paper proposes a simple and reliable online estimation of the grid impedance under unbalanced conditions. The technique is based on wide-band impedance estimation incorporated into the control loop of the positive sequence of a three-phase grid-connected inverter equipped with a positive-and negative-sequence control (PNSC) strategy. Additionally, complex curve fitting is utilized to obtain parametric models of the grid impedance. To demonstrate the efficacy of the proposed grid impedance estimation technique, extensive case studies are performed. These include: (1) unbalanced operations of both resistive-inductive (RL) and resistive-inductive-capacitive (RLC) models of the grid, (2) background harmonics, and (3) asymmetrical impedances of the network.

Review of Local Network Impedance Estimation Techniques

“…Introduce P or Q step change The techniques described in [25], [53], [87], [88] require two sets of measurements, where P and Q deviations can be either positive or negative. The impedance is subsequently calculated algebraically as per (19) and (20), or similar. The Park transformation values for PCC voltage and grid current are used, such that U = U q − jU d and I n = I nq − jI nd .…”

Considerations on Current and Voltage Unbalance of Nonlinear Loads in Residential and Educational Sectors

“…The power losses that occur in AC power systems depended on frequency, waveform distortion, and unbalance [7,12,23,50]. In the presence of phase displacement, unbalance and waveform distortion it is difficult, but very important, to correctly define the apparent power (S) and power factor (PF), which represent a measure of the system's power delivery capability [5,12,14].…”

“…A good power theory should explain and describe power-related phenomena in electrical systems for all possible situations and should be able to be used for filter design and reactive compensation to improve the power factor [5,8]. Among the numerous approaches to the power theory, the Current Physical Components (CPC) theory, developed by Czarnecki [8,15,23], the Conservative Power Theory (CPT), developed by Tenti [6] and the p-q instantaneous power theory, developed by Akagi [12], are the most distinguished.…”

“…One of the main differences of these approaches consists in the description of the power properties of the systems in the time domain, and in the frequency domain respectively. Another difference is the use of instantaneous values of power, and the averaged values (over the period T) respectively [5,6,8,12,15,23].…”

“…With the availability of these power quality analyzers, much more precise control of the processes has been developed, which makes the processes even more susceptible to the effects of power system disturbances. The power quality degradation may result in other adverse effects, such as overheating of the transformer, errors in measurement, voltage unbalances, and reductions in efficiency [10,[21][22][23].…”