US-12620890-B2 - Two-stage current-limiting control strategy for direct-droop-controlled grid-forming inverters

US12620890B2US 12620890 B2US12620890 B2US 12620890B2US-12620890-B2

Abstract

This document describes systems and techniques for a control system for a grid-forming inverter. In aspects, a reactive power current-limiting subsystem, based on a measured reactive power, is configured to generate a first signal representing a magnitude of a modulation waveform presentable to a pulse-width modulator configured to direct transistors in the inverter of the grid-forming inverter. An active power current-limiting subsystem, based at least in part on a measured active power, is configured to generate a second signal representing a rotating phase angle of the modulation waveform presentable to the pulse-width modulator. The reactive power current-limiting subsystem and active power current-limiting subsystem are configured to cause an alternating current (AC) output of the grid-forming inverter to resume a nominal form subsequent to current being directed around one or more transistors in an inverter to prevent an excessive output current from flowing through the one or more transistors.

Inventors

Wei Du
Sheik Mohammad Mohiuddin

Assignees

BATTELLE MEMORIAL INSTITUTE

Dates

Publication Date: 20260505
Application Date: 20240418

Claims (20)

1 . A control system for a grid-forming inverter including an instantaneous current-limiting subsystem configured to redirect one or more currents in an inverter in response to a current disturbance, comprising: feedback inputs receiving one or more outputs of the grid-forming inverter including a reactive power and an active power; a reactive power current-limiting subsystem based at least in part on the reactive power, the reactive power current-limiting subsystem being configured to generate a first signal representing a magnitude of a modulation waveform presentable to a pulse-width modulator configured to direct transistors in the inverter of the grid-forming inverter; and an active power current-limiting subsystem based at least in part on the active power, the active power current-limiting subsystem being configured to generate a second signal representing a rotating phase angle of the modulation waveform presentable to the pulse-width modulator, the reactive power current-limiting subsystem and active power current-limiting subsystem being configured to cause an alternating current (AC) output of the grid-forming inverter to resume a nominal form subsequent to current being directed around one or more transistors in an inverter of the grid-forming inverter to prevent an output current exceeding an inverter-maximum transient output current from flowing through the one or more transistors.
2 . The control system of claim 1 , wherein the control system is further configured to convert signals included in the one or more outputs from an original frame of reference into direct and quadrature components in a rotating frame of reference.
3 . The control system of claim 2 , wherein the reactive power current-limiting subsystem is further configured to generate the first signal representing the magnitude of the modulation waveform based on a quadrature component of a current output of the grid-forming inverter.
4 . The control system of claim 3 , wherein the reactive power current-limiting subsystem includes first proportional and integral controllers configured to respond to the quadrature component of the current output of the grid-forming inverter relative to selected minimum and maximum values of the quadrature component of the current output of the grid-forming inverter.
5 . The control system of claim 4 , wherein the reactive power current-limiting subsystem is further configured to adjust the first signal relative to a voltage setpoint based on a direct component of a voltage output of the grid-forming inverter.
6 . The control system of claim 2 , wherein the active power current-limiting subsystem is further configured to generate the second signal representing the phase angle of the modulation waveform based on a direct component of a current output of the grid-forming inverter.
7 . The control system of claim 6 , wherein the active power current-limiting subsystem includes second proportional and integral controllers configured to respond to the direct component of the current output of the grid-forming inverter relative to selected minimum and maximum values of the direct component of the current output of the grid-forming inverter.
8 . The control system of claim 1 , further comprising one or more low-pass filters configured to receive the one or more outputs of the grid-forming inverter and to generate one or more filtered outputs of the grid-forming inverter.
9 . The control system of claim 1 , further comprising an active and reactive power priority selection system configured to receive one or more priority inputs selectable to determine current outputs that are presented as inputs to the active current-limiting subsystem and the reactive power current-limiting subsystem.
10 . A single-loop droop controller for a grid-forming inverter comprising: an instantaneous current-limiting subsystem configured to redirect one or more currents around one or more transistors in an inverter when the one or more currents exceed an inverter-maximum transient output current; an active and reactive current-limiting subsystem including feedback inputs receiving one or more outputs of the grid-forming inverter including a reactive power and an active power, including: a reactive power current-limiting subsystem based at least in part on the reactive power, the reactive power current-limiting subsystem being configured to generate a first signal representing a magnitude of a modulation waveform presentable to a pulse-width modulator configured to direct transistors in the inverter of the grid-forming inverter; and an active power current-limiting subsystem based at least in part on the active power, the active power current-limiting subsystem being configured to generate a second signal representing a rotating phase angle of the modulation waveform presentable to the pulse-width modulator, the reactive power current-limiting subsystem and active power current-limiting subsystem being configured to cause an alternating current output of the grid-forming inverter to resume a nominal form subsequent to current being directed around one or more transistors in an inverter of the grid-forming inverter to prevent an output current exceeding the inverter-maximum transient output current from flowing through the one or more transistors.
11 . The single-loop droop controller of claim 10 , wherein the instantaneous current-limiting subsystem includes: a hysteresis module configured to compare an output current detected across one or more transistors in an inverter controlled by the single-loop droop controller with the inverter-maximum transient output current and to generate an overcurrent signal, the overcurrent signal presenting a fault signal responsive to the output current exceeding the inverter-maximum transient output current; and a logic array configured to logically combine a plurality of gate signals generated by a pulse-width modulator with the overcurrent signal to present modified gate signals to the one or more transistors, the logic array being configured to replace one or more of the gate signals in the modified gate signals with a gate disable signal responsive to the overcurrent signal presenting the fault signal to prevent the output current from flowing through the one or more transistors.
12 . The single-loop droop controller of claim 10 , wherein the active and reactive current-limiting subsystem is further configured to convert signals included in the one or more the outputs from an original frame of reference into direct and quadrature components in a rotating frame of reference.
13 . The single-loop droop controller of claim 12 , wherein the reactive power current-limiting subsystem is further configured to generate the first signal representing the magnitude of the modulation waveform based on a quadrature component of a current output of the grid-forming inverter.
14 . The single-loop droop controller of claim 13 , wherein the reactive power current-limiting subsystem includes first proportional and integral controllers configured to respond to the quadrature component of the current output of the grid-forming inverter relative to selected minimum and maximum values of the quadrature component of the current output of the grid-forming inverter.
15 . The single-loop droop controller of claim 14 , wherein the reactive power current-limiting subsystem is further configured to adjust the first signal relative to a voltage setpoint based on a direct component of a voltage output of the grid-forming inverter.
16 . The single-loop droop controller of claim 12 , wherein the active power current-limiting subsystem is further configured to generate the second signal representing the phase angle of the modulation waveform based on a direct component of a current output of the grid-forming inverter.
17 . A method of adjusting control signals in a grid-forming inverter including an instantaneous current-limiting subsystem configured to redirect one or more currents in an inverter in response to a current disturbance, comprising: monitoring one or more outputs of the grid-forming inverter including a reactive power and an active power; based at least in part on the reactive power, generating a first signal representing a magnitude of a modulation waveform presentable to a pulse-width modulator configured to direct transistors in the inverter of the grid-forming inverter; based at least in part on the active power, generating a second signal representing a rotating phase angle of the modulation waveform presentable to the pulse-width modulator; and adjusting the first signal and the second signal to cause an alternating current (AC) output of the grid-forming inverter to resume a nominal form subsequent to current being directed around one or more transistors in an inverter of the grid-forming inverter to prevent an output current exceeding an inverter-maximum transient output current from flowing through the one or more transistors.
18 . The method of claim 17 , further comprising converting signals that include the one or more outputs from an original frame of reference into direct and quadrature components in a rotating frame of reference.
19 . The method of claim 18 , further comprising generating the first signal representing the magnitude of the modulation waveform based on a direct component of a voltage output of the grid-forming inverter and a quadrature component of the current output of the grid-forming inverter.
20 . The method of claim 18 , further comprising generating the second signal representing the phase angle of the modulation waveform based on a direct component of the current output of the grid-forming inverter.

Description

INCORPORATION BY REFERENCE This application claims the benefit of U.S. Provisional Patent Application No. 63/404,060 (the “First Provisional Application”), filed Sep. 6, 2022, U.S. Provisional Patent Application No. 63/460,780, filed Apr. 20, 2023 (the “Second Provisional Application”), and U.S. patent application Ser. No. 18/241,739, filed Sep. 1, 2023 (the “Non-Provisional Application”). This application incorporates by reference the entirety of the First Provisional Application, the Second Provisional Application, and the Non-Provisional Application. STATEMENT AS TO RIGHTS TO DISCLOSURES MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT This disclosure was made with U.S. Government support under Contract DE-AC0576RL01830 awarded by the U.S. Department of Energy. The U.S. Government has certain rights in the invention. BACKGROUND Grid-forming inverters are increasingly used to improve the stability of a power grid by enabling renewable power sources to supply power to the power grid to supplement conventional generating sources. Most renewable sources, such as solar power systems and wind power systems, typically provide sources of direct current (DC) power, either directly or via storage batteries in which the DC power generated is stored for later use. Although power grids operate using alternating current (AC) power, grid-forming inverters convert DC power sources to AC power for supply to the power grid. To a power grid, a grid-forming inverter fundamentally behaves as a voltage source behind an impedance. A potential disadvantage of the grid-forming inverter is that transistors or other components of grid-forming inverters may be damaged during overcurrent episodes, such as occurrence of short circuits, if protective measures are not used. Droop control is a widely used technique for managing current flow in a grid-forming inverter to one of most mature grid-forming control strategies. Two common forms of droop-control systems are multi-loop droop-controlled systems and single-loop droop-controlled systems. A multi-loop droop-controlled system typically controls the magnitude and frequency of an inverter filter capacitor voltage according to the droop laws using cascaded inner voltage and current loops to achieve fast control of the filter capacitor voltage. A single-loop droop-controlled system directly controls magnitude and frequency of a modulation waveform according to the droop laws without using a cascaded inner current loop. Multi-loop droop-controlled systems often focus on modifying a current reference of the cascaded inner current loop of multi-loop droop-controlled systems to limit overcurrent episodes, but these methods are not applicable to single-loop droop-controlled structures that do not use a cascaded inner current loop. A single-loop droop-controlled system—as the name may imply—is simpler in structure than a multi-loop droop-controlled system, but a single-loop droop-controlled system may be more vulnerable to overcurrent episodes than multi-loop droop-controlled systems. Unfortunately, if protective measures are used to redirect excessive DC currents to protect components in the grid-forming inverter, such protective measures may disrupt the AC current that is generated by the grid-forming inverter. The grid-forming inverter combines the DC currents to generate a sinusoidal AC current. Thus, making changes to the DC currents could interfere with the ability of the grid-forming inverter to generate the sinusoidal AC current. SUMMARY This document describes systems and techniques for a control system for a grid-forming inverter. In aspects, a reactive power current-limiting subsystem, based on a measured reactive power, is configured to generate a first signal representing a magnitude of a modulation waveform presentable to a pulse-width modulator configured to direct transistors in the inverter of the grid-forming inverter. An active power current-limiting subsystem, based at least in part on a measured active power, is configured to generate a second signal representing a rotating phase angle of the modulation waveform presentable to the pulse-width modulator. The reactive power current-limiting subsystem and active power current-limiting subsystem are configured to cause an alternating current (AC) output of the grid-forming inverter to resume a nominal form subsequent to current being directed around one or more transistors in an inverter to prevent an excessive output current from flowing through the one or more transistors. This Summary introduces simplified concepts related to systems and techniques for a control strategy for direct-droop-controlled grid-forming inverters, including instantaneous current-limiting as well as reactive and active current-limiting, as further described in the Detailed Description and Drawings. This Summary is not intended to identify essential features of the claimed subject matter, nor is it intended for use in determining the scope of t