US-12627156-B2 - Method and controller for controlling a power converter connected to a grid

US12627156B2US 12627156 B2US12627156 B2US 12627156B2US-12627156-B2

Abstract

A method for controlling a power converter connected to a grid, wherein the power converter is initially controlled in a grid-forming mode to output current at a nominal voltage, based on control data provided by a controller. The method includes determining that a first parameter exceeds a first threshold. Then: controlling the power converter in a first limiting mode, by limiting the power converter output current based on a present value and/or a reference value; and determining the output current of the power converter. Responsive to determining that the output current is reaching an equilibrium condition: determining a virtual impedance for the power converter; modifying the control system based on the virtual impedance; and controlling the power converter in a constrained grid-forming mode, including the modified control system determining the control data based on measured signals indicative of currents and/or voltages downstream from the power converter, and a reference signal.

Inventors

Philip Joseph HART
Hanchao LIU
Atinuke ADEMOLA-IDOWU
Matias Uolevi BERG
Krishnakumar RAMAN VASUDEVAN

Assignees

GE INFRASTRUCTURE TECHNOLOGY LLC

Dates

Publication Date: 20260512
Application Date: 20241114
Priority Date: 20231124

Claims (15)

1 . A method for controlling a power converter connected to a grid, wherein the power converter is initially controlled in a grid-forming mode to output current at a nominal voltage, based on control data provided by a controller, the controller comprising a control system which determines the control data based on measured signals indicative of currents and/or voltages downstream from the power converter, and a reference signal; the method comprising: determining, by the controller, that a first parameter related to the controller and/or the power converter exceeds a first threshold; responsive to determining that the first parameter exceeds the first threshold: controlling, by the controller, the power converter in a first limiting mode, by limiting the power converter output current based on a present value and/or a reference value; determining, by the controller, the output current of the power converter; responsive to determining that the output current is at or is close to an equilibrium condition: determining, by the controller, a virtual impedance for the power converter based on the output current of the power converter; modifying, by the controller, the control system based on the virtual impedance; and controlling, by the controller, the power converter in a first constrained grid-forming mode, comprising the modified control system determining the control data based on measured signals indicative of currents and/or voltages downstream from the power converter, and a reference signal.
2 . The method of claim 1 , further comprising: determining, by the controller, that a second parameter related to the controller and/or the power converter exceeds a second threshold; responsive to determining that the second parameter exceeds the second threshold: controlling, by the controller, the power converter in a second limiting mode, by limiting the power converter output current based on an updated present value and/or an updated reference value; determining, by the controller, an updated output current of the power converter; responsive to determining that the updated output current is at or is close to an equilibrium condition: determining, by the controller, an updated virtual impedance for the power converter based on the updated output current of the power converter; further modifying, by the controller, the control system based on the updated virtual impedance; and controlling, by the controller, the power converter in a second constrained grid-forming mode, comprising the further modified control system determining the control data based on the measured signals indicative of currents and/or voltages downstream from the power converter, and the reference signal.
3 . The method of claim 1 , wherein the control system is a cascaded control system including a voltage controller and a current controller; wherein the voltage controller uses a voltage control system to produce first data, based on the reference signal and the measured signals, and outputs the first data to the current controller; the current controller uses a current control system to produce second data, based on the first data and the measured signals; the controller determines the control data based on the second data; and the controller controls the power converter in the first limiting mode by setting the first data to be equal to the present value and/or the reference value, thereby limiting the power converter output current.
4 . The method of claim 1 , wherein the control system is a direct voltage controller; the controller comprises a limiting controller; and the controller controls the power converter in the first limiting mode by the limiting controller restricting the output current of the power converter from increasing past the present value or the reference value, thereby limiting the power converter output current.
5 . The method of claim 1 , wherein the determining, by the controller, the virtual impedance for the power converter further comprises determining, by the controller, the virtual impedance and a fixed virtual voltage offset for the power converter based on the output current of the power converter; and the modifying, by the controller, the control system based on the virtual impedance further comprises modifying, by the controller, the control system based on the virtual impedance and the fixed virtual voltage offset.
6 . The method of claim 1 , wherein the modifying of the control system based on the virtual impedance comprises: determining, by the controller, an adjustment term for the control system by calculating a virtual voltage or a virtual current as a function of the virtual impedance and the output current; and implementing, by the controller, the adjustment term into the control system.
7 . The method of claim 1 , wherein the controller determines the virtual impedance by: calculating, whilst the power converter is in the first limiting mode, a virtual voltage defined as a voltage difference between an internal voltage phasor reference for the power converter and a node voltage at a node between an output of the power converter and the grid; and based on a Thevenin equivalent circuit of the power converter connected to the grid, including the virtual voltage, the output current of the power converter in the equilibrium condition, and the virtual impedance, using circuit analysis methods to resolve the Thevenin equivalent circuit for the virtual impedance.
8 . The method of claim 7 , further comprising: reducing, by the controller, the virtual impedance, as a result of the voltage difference between the internal voltage phasor reference for the power converter and the node voltage at the node reducing in magnitude, and thereby returning to control the power converter in the initial grid-forming mode.
9 . The method of claim 1 , wherein a time period from determining that the first parameter related to the controller and/or the power converter exceeds the first threshold, to the controller completing the step of controlling the power converter in the first liming mode, is less than or equal to 50 ms.
10 . The method of claim 1 , wherein the determining that the first parameter related to the controller and/or power converter exceeds the first threshold comprises one or more of: determining that a current reference of a dq-frame current regulator using commanded angle or Phase Locked Loop, PLL, angle has saturated; and/or determining that a d or q current reference inputted into a Proportional Integral, PI, controller in a current controller has saturated; and/or determining that a d or q current reference to be inputted, following an inverse reference frame transformation, into a Proportional Resonant, PR, controller in a current controller has saturated; and/or determining that a d or q current reference to be inputted, following an inverse reference frame transformation, into a deadbeat controller in a current controller has saturated; and/or determining that a phasor current limit has been reached; and/or determining that an active power limit has been reached; and/or determining that an energy limit has been reached; and/or determining that a component is pulse dropping or blocking; and/or determining that a voltage reference has saturated; and/or determining that a voltage output has reached a limit; and/or determining that a modulation index has saturated.
11 . A controller for controlling a power converter connected to a grid, the controller comprising: a control system arranged to determine control data based on measured signals indicative of currents and/or voltages downstream from the power converter, and a reference signal; wherein the controller is arranged to control the power converter in a grid-forming mode to output current at a nominal voltage, based on the control data; and wherein the controller is configured to: determine that a first parameter related to the controller and/or the power converter exceeds a first threshold; responsive to determining that the first parameter exceeds the first threshold: control the power converter in a first limiting mode, wherein the controller is configured to limit the power converter output current based on a present value and/or a reference value; determine the output current of the power converter; responsive to determining that the output current is at or is close to an equilibrium condition: determine a virtual impedance for the power converter based on the output current of the power converter; modify the control system based on the virtual impedance; and control the power converter in a first constrained grid-forming mode, wherein the modified control system is configured to determine the control data based on measured signals of currents and/or voltages downstream from the power converter, and a reference signal.
12 . The controller of claim 11 , wherein the controller is further configured to: determine that a second parameter related to the controller and/or the power converter exceeds a second threshold; responsive to determining that the second parameter exceeds the second threshold: control the power converter in a second limiting mode, wherein the controller is configured limit the power converter output current based on an updated present value and/or an updated reference value; determine an updated output current of the power converter; responsive to determining that the updated output current is at or is close to an equilibrium condition: determine an updated virtual impedance for the power converter based on the updated output current of the power converter; further modify the control system based on the updated virtual impedance; and control the power converter in a second constrained grid-forming mode, wherein the further modified control system is configured to determine the control data based on the measured signals and the reference signal.
13 . The controller of claim 11 , wherein the control system is a cascaded control system comprising a voltage controller and a current controller; wherein the voltage controller is configured to use a voltage control system to produce first data, based on the reference signal and the measured signals, and is further configured to output the first data to the current controller; the current controller is configured to use a current control system to produce second data, based on the first data and the measured signals; the controller is further configured to determine the control data based on the second data; and the controller is further configured to control the power converter in the first limiting mode by setting the first data to be equal to the present value or the reference value, thereby limiting the power converter output current.
14 . The controller of claim 11 , wherein the controller is configured to modify the control system based on the virtual impedance comprises that the controller is configured to determine an adjustment term for the control system by calculating a virtual voltage or a virtual current as a function of the virtual impedance and the output current, and implement the adjustment term into the control system.
15 . A power converter comprising: a DC side for connection to a DC source; an AC side for connection to a grid; and the controller of claim 11 .

Description

TECHNICAL FIELD The present invention relates to a method and a controller for controlling a power converter connected to a grid, and more particularly for controlling the power converter by using a virtual impedance. BACKGROUND In power transmission networks, DC power is converted to AC power where it is necessary to interconnect DC and AC networks. In any such power transmission network, power conversion means, also known as converters, power converters, inverters, power inverters, or power-electronics based resources, are required at each interface between AC and DC power to effect the required conversion from AC to DC or from DC to AC. When converting DC power to AC power, for example, at an interface between a DC transmission line and a grid, a power converter may be operated in a grid-following mode (GFL) or a grid-forming mode (GFM). In the GFL mode, the power converter utilizes fast current-regulation loops to control active and reactive power exchanged with the grid, achieving relatively constant active and reactive power exchange in a sub-transient to transient timescale (e.g., between approximately 10 ms and 150 ms). The power converter uses a current reference for the active component of current to achieve a desired active power output. A power converter operating in a GFL mode includes functions that manage the voltage and/or reactive power in a manner that results in a command for the reactive component of current. Wide-bandwidth current regulators then develop commands for voltage to be applied by the power converter to the grid, such that the actual currents closely track the commands. A power converter operating in a GFL mode therefore provides a current-source characteristic in the sub-transient to transient timescale. Alternatively, a power converter operating in a GFM mode provides a voltage-source characteristic in the sub-transient to transient timescale, where the phase angle and magnitude of the voltage are controlled to remain mostly static in this timescale to achieve the regulation functions demanded by the grid. With this structure, current will flow according to the demands of the grid while the converter contributes to establishing a voltage and frequency for the grid. A power converter operating in a GFM mode is thus distinct from a power converter operated in a GFL mode, because the GFM mode regulates, in the sub-transient timescale (e.g., <150 ms), the AC voltage and frequency of the power converter instead of AC current. However, if a fault event occurs in the grid, or other severe grid disturbance, herein referred to as a “grid event”, it is possible that the power converter will experience a current, power, or energy overload, because the power converter is regulating voltage and not current, power or energy. This overload can cause the power converter operating in a GFM mode to trip offline, or, in a worst-case scenario, damage the semiconductor devices within the power converter and indefinitely render the power converter inoperable. During such a grid event, self-protection of the power converter is to be ensured, and current limiting is typically achieved via switching the control to a GFL mode. However, under some conditions, switching the control mode in such a manner has the potential to jeopardize the stability of the power converter and the stability of the overall power system. SUMMARY OF THE INVENTION In light of these considerations and emerging grid requirements, it is desired to develop methods that allow the power converter to retain some semblance of GFM behaviour while operating close to (but not exceeding) hardware limits. According to a first aspect, there is provided a method for controlling a power converter connected to a grid, wherein the power converter is initially controlled in a grid-forming mode to output current at a nominal voltage, based on control data provided by a controller, the controller comprising a control system which determines the control data based on measured signals indicative of currents and/or voltages downstream from the power converter, and a reference signal. The method comprises determining, by the controller, that a first parameter related to the controller and/or the power converter exceeds a first threshold. Responsive to determining that the first parameter exceeds the first threshold: controlling, by the controller, the power converter in a first limiting mode, by limiting the power converter output current based on a present value and/or a reference value; and determining, by the controller, the output current of the power converter. Responsive to determining that the output current is at or is close to an equilibrium condition: determining, by the controller, a virtual impedance for the power converter based on the output current of the power converter; modifying, by the controller, the control system based on the virtual impedance; and controlling, by the controller, the power converter in a first constrained grid