# Dynamic Impedance Estimation: Challenges and Considerations
## 1 Introduction
Thus, it is necessary to propose some strategies to reduce the noise's influence in estimating harmonic components through low-cost computational algorithms [9; 10]. In considering possible applications of the harmonic synchrophasor, it is worth to mention the harmonic responsibilities attribution and the harmonic network estimation. The first application aims to quantify the responsibility for harmonic distortion, at a given point in the system, by the various harmonic sources [11] and the second allows, for example, performing islanding detection in systems with distributed generation [12], protection system adjustment, and improvement of the system control and operation. Estimating a network's harmonic impedance based on online measurements is not an easy task. The methods that try to solve this problem are divided into passive and active methods. Passive methods seek to estimate the impedance from the current and voltage signals available on the measurement point without causing any disturbance in the network, although they can take advantage of any disturbances that affect the system. On the other hand, active methods cause some intentional disturbance in the network [13; 14], and from the observation of current and voltage, they can estimate the impedance more accurately. Compared to the passive ones, the disadvantages of active methods are the possible deterioration in the PQ and external devices' need to cause disturbances in the network. The injection of small signals called Gaussian modulated signals (GMS), as suggested in [15], does not degrade the PQ and produces accurate impedance estimates. Besides, this injection can be implemented in the control loop of the distributed generators' power converters. In this case, there is no need for external hardware to inject the GMS pulses. However, since most of the non-linear loads connected to the power system cannot inject the GMS to estimate the impedance, it is also important to investigate methods for estimating impedance by passive approaches. In this way, estimation methods based on the independent component analysis (ICA) are highlighted [16; 17; 18]. These works partially solve the problem of estimating the impedance at the point of common coupling (PCC) on the load and utility sides. The challenge is to extend the methodology for estimating the harmonic transfers considering several harmonic sources contributing to a given point in the system. Regardless of the method used to estimate harmonic impedance of the system, synchronized measurement is required, either of the harmonic phasor or the disturbance waveform (in the case of GMS injection), seeking to mitigate the effects of the fundamental frequency variation of the noise and the errors from instrumentation systems. In this context, this work will discuss the challenges related to the dynamic impedance estimation and present some solution proposals. Figure 1 illustrates the three main points to be addressed in this work, which will be shown in Section 2, Section 3, and Section 4 of this paper. The conclusions of the work are then presented in Section 5.