US-12627242-B2 - Techniques for transitioning between control modes of an active front end unit

US12627242B2US 12627242 B2US12627242 B2US 12627242B2US-12627242-B2

Abstract

Techniques to reduce discontinuities in power, for example, by controlling an electrical power output of an active front end unit during a transition between a first operating mode and a second operating mode. In some examples, to reduce discontinuities, a control unit of the active front end unit can seed the integrator of proportional-integral (PI) controllers that are brought online during the operating mode change with a value that represents the proper state of the current system, such as the measured or calculated value of the component the PI controller is controlling). In other examples, to reduce discontinuities, a control unit of the active front end unit can control reference frame alignment during a transition between a first operating mode and a second operating mode.

Inventors

Seok-Hee Han
Jesse R. Gerdes
Jackson Wai

Assignees

CATERPILLAR INC.

Dates

Publication Date: 20260512
Application Date: 20220901

Claims (16)

1 . An active front end unit including an inverter having an output coupled to a capacitor that is coupled to an electrical grid, wherein an output voltage vector is offset from a capacitor voltage vector by an angle theta defining an electrical power output, the active front end unit comprising: the inverter; and a control unit coupled to the inverter and configured to perform operations comprising: defining a first reference frame having a first axis and a second axis, wherein the first axis of the first reference frame is aligned with the capacitor voltage vector during a first operating mode, and wherein the first axis of the first reference frame is aligned with the output voltage vector during a second operating mode; defining a second reference frame having a first axis and a second axis, wherein the first axis of the second reference frame is aligned with the output voltage vector during the second operating mode, wherein the first axis of the first reference frame is offset from the first axis of the second reference frame by the angle theta during the first operating mode, and wherein the first axis of the first reference frame is aligned with the first axis of the second reference frame during the second operating mode; when transitioning from the first operating mode to the second operating mode: rotating, over a first time, the first reference frame onto the second reference frame such that the first axis of the first reference frame is aligned with the first axis of the second reference frame; and when transitioning from the second operating mode to the first operating mode: rotating, over a second time, the first reference frame away from the second reference frame such that the first axis of the first reference frame is aligned with the capacitor voltage vector.
2 . The active front end unit of claim 1 , wherein the operation of rotating, over the first time, the first reference frame onto the second reference frame such that the first axis of the first reference frame is aligned with the first axis of the second reference frame includes the operations of: performing an integration of the angle theta over the first time during which the angle theta changes to generate a voltage command; and applying a representation of the voltage command to the capacitor.
3 . The active front end unit of claim 1 , wherein the control unit is further configured to perform the operation comprising: applying a gain factor to adjust a rate of the integration.
4 . The active front end unit of claim 1 , wherein the operation of rotating, over the second time, the first reference frame away from the second reference frame such that the first axis of the first reference frame is aligned with the capacitor voltage vector includes the operations of: decreasing, over the second time, a component of the capacitor voltage vector to zero with respect to the first reference frame; and applying a representation of the component to the capacitor.
5 . The active front end unit of claim 1 , wherein the first operating mode is a frequency droop control operating mode.
6 . The active front end unit of claim 1 , wherein the control unit is further configured to perform the operation comprising: applying a seed value to an integrator of a proportional integral (PI) controller of a controller of the active front end unit, wherein the seed value is based on a measured current or voltage value to reduce a discontinuity in at least one of an output voltage, an output current, or the electrical power output of the active front end unit.
7 . The active front end unit of claim 6 , wherein the control unit is further configured to perform the operation comprising: filtering the measured current or voltage value before applying the seed value to the integrator.
8 . The active front end unit of claim 6 , wherein the operation comprising applying the seed value to the integrator of the proportional integral (PI) controller of the controller of the active front end unit includes: when the controller of the active front end unit is inactive during the first operating mode or the second operating mode: applying the seed value to the integrator of the proportional integral (PI) controller of the controller of the active front end unit.
9 . A method of controlling an electrical power output of an active front end unit during a transition between a first operating mode and a second operating mode, wherein the active front end unit has an inverter, wherein an output of the active front end unit is coupled to a capacitor that is coupled to an electrical grid, and wherein an output voltage vector is offset from a capacitor voltage vector by an angle theta defining the electrical power output, the method comprising: defining a first reference frame having a first axis and a second axis, wherein the first axis of the first reference frame is aligned with the capacitor voltage vector during the first operating mode, and wherein the first axis of the first reference frame is aligned with the output voltage vector during the second operating mode; defining a second reference frame having a first axis and a second axis, wherein the first axis of the second reference frame is aligned with the output voltage vector during the second operating mode, wherein the first axis of the first reference frame is offset from the first axis of the second reference frame by the angle theta during the first operating mode, and wherein the first axis of the first reference frame is aligned with the first axis of the second reference frame during the second operating mode; when transitioning from the first operating mode to the second operating mode: rotating, over a first time, the first reference frame onto the second reference frame such that the first axis of the first reference frame is aligned with the first axis of the second reference frame; and when transitioning from the second operating mode to the first operating mode: rotating, over a second time, the first reference frame away from the second reference frame such that the first axis of the first reference frame is aligned with the capacitor voltage vector.
10 . The method of claim 9 , wherein rotating, over the first time, the first reference frame onto the second reference frame such that the first axis of the first reference frame is aligned with the first axis of the second reference frame comprises: performing an integration of the angle theta over the first time during which the angle theta changes to generate a voltage command; and applying a representation of the voltage command to the capacitor.
11 . The method of claim 10 , comprising: applying a gain factor to adjust a rate of the integration.
12 . The method of claim 9 , wherein rotating, over the second time, the first reference frame away from the second reference frame such that the first axis of the first reference frame is aligned with the capacitor voltage vector comprises: decreasing, over the second time, a component of the capacitor voltage vector to zero with respect to the first reference frame; and applying a representation of the component to the capacitor.
13 . The method of claim 9 , wherein the first operating mode is a frequency droop control operating mode.
14 . The method of claim 9 , comprising: applying a seed value to an integrator of a proportional integral (PI) controller of a controller of the active front end unit, wherein the seed value is based on a measured current or voltage value to reduce a discontinuity in at least one of an output voltage, an output current, or the electrical power output of the active front end unit.
15 . The method of claim 14 , comprising: filtering the measured current or voltage value before applying the seed value to the integrator.
16 . The method of claim 14 , wherein applying the seed value to the integrator of the proportional integral (PI) controller of the controller of the active front end unit comprises: when the controller of the active front end unit is inactive during the first operating mode or the second operating mode: applying the seed value to the integrator of the proportional integral (PI) controller of the controller of the active front end unit.

Description

TECHNICAL FIELD This document relates generally to electrical power equipment and more specifically to an energy storage system inverter apparatus. BACKGROUND Using various operating modes, a control unit of an active front end unit can service a grid, e.g., electrical grid or microgrid, which is equipped with gas or diesel gensets. The operating modes include a current control operating mode, a voltage control operating mode, and a frequency control operating mode. Transitioning between different operating modes can be needed to meet changing requirements of a system connected to an electrical grid, for example, gensets can be on or off. U.S. Pat. No. 9,893,523 discloses systems, methods and apparatus for controlling an energy delivery system including providing an energy management system (EMS) having an automatic generation control (AGC) system including a load frequency control (LFC) module and an economic dispatch (ED) module; determining a regulation requirement based upon a predefined set of nested system control zones and a current area control error (ACE); determining regulation allocation based on a pre-defined set of gain factors associated with the nested system control zones; and implementing corrections to the operation of the energy delivery system based upon solution results of the determined regulation allocation. SUMMARY This disclosure describes techniques to reduce discontinuities in power, for example, by controlling an electrical power output of an active front end unit during a transition between a first operating mode and a second operating mode. In some examples, to reduce discontinuities, a control unit of the active front end unit can seed the integrator of proportional-integral (PI) controllers that are brought online during the operating mode change with a value that represents the proper state of the current system, such as the measured or calculated value of the component the PI controller is controlling). In other examples, to reduce discontinuities, a control unit of the active front end unit can control reference frame alignment during a transition between a first operating mode and a second operating mode. In an aspect, this disclosure is directed to an active front end unit including an inverter having an output coupled to a capacitor that is coupled to an electrical grid, wherein an output voltage vector is offset from a capacitor voltage vector by an angle theta defining an electrical power output, the active front end unit comprising: the inverter; and a control unit coupled to the inverter and configured to perform operations comprising: defining a first reference frame having a first axis and a second axis, wherein the first axis of the first reference frame is aligned with the capacitor voltage vector during a first operating mode, and wherein the first axis of the first reference frame is aligned with an output voltage vector during a second operating mode; defining a second reference frame having a first axis and a second axis, wherein the first axis of the second reference frame is aligned with the output voltage vector during the second operating mode, wherein the first axis of the first reference frame is offset from the first axis of the second reference frame by the angle theta during the first operating mode, and wherein the first axis of the first reference frame is aligned with the first axis of the second reference frame during the second operating mode; when transitioning from the first operating mode to the second operating mode: rotating, over a first time, the first reference frame onto the second reference frame such that the first axis of the first reference frame is aligned with the first axis of the second reference frame; and when transitioning from the second operating mode to the first operating mode: rotating, over a second time, the first reference frame away from the second reference frame such that the first axis of the first reference frame is aligned with the capacitor voltage vector. In another aspect, this disclosure is directed to an active front end unit including an inverter having an output coupled to a capacitor that is coupled to an electrical grid, wherein an output voltage vector is offset from a capacitor voltage vector by an angle theta defining an electrical power output, the active front end unit comprising: the inverter; and a control unit coupled to the inverter and configured to perform operations comprising: when transitioning from a first operating mode to a second operating mode: commanding a proportional integral (PI) controller of a control unit of the active front end unit to be in an inactive mode; and applying a seed value to an integrator of the PI controller, wherein the seed value is based on a measured current or voltage value to reduce a discontinuity in at least one of an output voltage, an output current, or the electrical power output of the active front end unit. In yet another aspect, this disclosure is