Small-signal analysis of wind parks (WPs) connected to power systems is performed using either impedance-based analysis or state-space analysis. Studies that model wind parks using state-space equations have led to significant progress in identifying the causes of oscillations and its participation factors. However, unlike traditional synchronous generation, standardized and accurate analytical models of the dynamic behavior of WPs are not available. Furthermore, due to industrial secrecy issues, this information is typically not provided by manufacturers, who provide a black box EMT model for studies. Thus, although analysis using state-space equations brings advantages in analyzing the phenomenon, it may not be feasible when complete models are not made available by manufacturers.

In this context, the impedance-based approach has gained prominence to analyze oscillations due to wind park interactions with the grid. This approach uses the impedance profile as a function of frequency, i.e., terminal characteristics, which are obtained by frequency scanning from the model provided by the manufacturer. With the impedance model of the WP and the grid, the Nyquist criterion can be applied to verify stability or stability margin. This approach is effective for stability analysis.

To address the system small-signal analysis based on the impedance, the system is separated into the system and the inverter-based resource (IBR). The power system subsystem is modeled by its Thevenin equivalent circuit consisting of an ideal voltage source in series with an output impedance, while the grid-connected inverter is modeled as a current source in parallel with an output impedance. This is the typical analyses performed to evaluate the impact on small-signal stability of WP on power systems.

However, in power systems such as the Brazilian one, it is common for multiple WPs to share a single connection point to the transmission network. These WPs can have different technologies (type III or IV) and each wind turbine can have different nominal capacity. Thus, it is not practical to directly consider one single WP using typical aggregation techniques. Therefore, rather than evaluating the impact of a single WP, the collective impact of the WPs connected to a specific point of the system should be assessed.

This paper presents the procedure to compute the equivalent impedance representing multiple WPs based on the impedance model of each WP. The procedure is applied to case based on a point of coupling of different WPs connected to the Northeast region of the Brazilian interconnected power system. The equivalent system is modeled in EMTP, the WPs are represented by generic-EMT model of wind parks, and the impedance scan is performed to obtain the impedance model of each WP. The use of the method allows identifying critical operating conditions for that group of WPs connected to that point of connection, due to weak grid conditions.