GridSweep Simulation: Measuring Subsynchronous Impedance Spectra of Distribution Feeder

TL;DR

The paper addresses the challenge of detecting oscillation risk in distribution feeders with inverter-based resources by estimating the subsynchronous apparent impedance using GridSweep, a small 1-kW, 120-V probing device. By combining active subsynchronous probing with GPS-synchronized continuous point-on-wave recordings and processing the measurements in the $dq$ frame, the method retrieves the impedance components $Z_{dd}$ and $Z_{qd}$ and identifies oscillation modes as spectral peaks. Validation is conducted via EMT simulations, comparing GridSweep results to ground truth in simple RLC topologies and to current-injection benchmarks in complex feeder/IBR scenarios, with attention to the effects of low-pass filtering and measurement location. The findings demonstrate that GridSweep can reveal dominant subsynchronous modes and the influence of IBR controls, offering a practical approach for assessing oscillation risk using only 120-V outlet connections on distribution grids.

Abstract

Peaks and troughs in the subsynchronous impedance spectrum of a distribution feeder may be a useful indication of oscillation risk, or more importantly lack of oscillation risk, if inverter-based resource (IBR) deployments are increased on that feeder. GridSweep is a new instrument for measuring the subsynchronous impedance spectra of distribution feeders. It combines an active probing device that modulates a 120-volt 1-kW load sinusoidally at a user-selected GPS-phase locked frequency from 1.0 to 40.0 Hz, and with a recorder that takes ultra-high-precision continuous point-on-wave (CPOW) 120-volt synchrowaveforms at 4 kHz. This paper presents a computer simulation of GridSweep's probing and measurement capability. We construct an electromagnetic transient (EMT) simulation of a single-phase distribution feeder equipped with multiple inverter-based resources (IBRs). We include a model of the GridSweep probing device, then demonstrate the model's capability to measure the subsynchronous apparent impedance spectrum of the feeder. Peaks in that spectrum align with the system's dominant oscillation modes caused by IBRs.

Figures (18)

• Figure 1: Illustration of the GridSweep, represented by $R$, interacting with the rest of the system, represented by $Y$.
• Figure 2: Pre-processing CPOW voltage measurement to extract magnitude and angle in the $dq$ frame.
• Figure 3: Block diagram of the perturbation and voltage relationship.
• Figure 4: Testbed 1. $R=0.2$ p.u., $X_L =0.25$ p.u., $X_C = 0.1$ p.u. $R_0 = 10$ p.u.
• Figure 5: (a) The input chirp signal perturbation and output measurement data of voltage magnitude and angle. (b) Comparison of the estimated model output and the measured output.