CONVERTER AND METHOD FOR OPERATING A CONVERTER

Abstract

A converter includes an AC side to be connected to an AC network at a connecting point, a grid following controller configured to control a steady state current at the connecting point, and a grid forming controller configured to actively control frequency and voltage at the connecting point. The grid following controller and the grid forming controller share an underlying current controller. A method for operating the converter is also provided.

Claims

1. A converter, comprising: an AC side of the converter; a connecting point for connecting said AC side to an AC network; a grid following controller configured to control a steady state current at said connecting point; a grid forming controller configured to actively control at least one of a frequency or a voltage at said connecting point; and an underlying current controller shared by said grid following controller and said grid forming controller.

2. The converter according to claim 1, wherein said grid forming controller and said grid following controller have outputs, and a virtual admittance is connected between said outputs and said underlying current controller.

3. The converter according to claim 1, wherein said grid forming controller includes at least one of a voltage angle or a frequency controller for active power control.

4. The converter according to claim 3, wherein said at least one of a voltage angle or a frequency controller is based on a model of an ideal answer of an electrical machine by using a transfer function.

5. The converter according to claim 1, wherein said grid forming controller includes a voltage magnitude controller for reactive power control.

6. The converter according to claim 5, wherein said voltage magnitude controller includes a linear controller configured to change a reference amplitude of an AC voltage based on a reactive power reference and on a measured reactive power.

7. The converter according to claim 1, wherein said grid forming controller is configured for a sequence selective control, generating separate control setpoints for said underlying current controller for a positive and a negative sequence system.

8. The converter according to claim 1, wherein said grid following controller includes a current setpoint controller to control at least one of a fixed active and reactive power operation point or a converter energy balance.

9. The converter according to claim 1, which further comprises a DC side of the converter to be connected to at least one of a DC link or an electrical energy storage device.

10. The converter according to claim 1, wherein the converter is a modular multilevel converter.

11. A method for operating a converter, the method comprising steps of: providing a converter according to claim 1; performing a grid following control by using said grid following controller; performing a grid forming control by using said grid forming controller; and switching over from said grid following control to said grid forming control in an uninterrupted operation.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0024] FIG. 1 is a schematic circuit diagram of a converter according to an embodiment of the invention;

[0025] FIG. 2 is a schematic circuit diagram of the converter of FIG. 1; and

[0026] FIG. 3 is a block diagram of a control structure for the converter of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is seen a converter 1 with an AC side 2 and a DC side 3. The converter 1 is a voltage source converter, in particular a modular multilevel converter (MMC). The AC side 2 of the converter 1 is connected with an AC grid 4 at a connecting point 2a. The DC side 3 of the converter is connected with an electrical energy storage device 5 through a DC link 6. The converter 1 is configured to stabilize the AC grid 4 by exchanging active and reactive power with the AC grid 4.

[0028] An MMC 7 shown in FIG. 2 includes three phase branches 8a-c and six converter valves (also denoted as arms) 9a-f. Every converter arm 9a-f extends between one of the DC poles or terminals 10a,b, constituting a DC side of the converter 7, and one of the AC terminals 11a-c, constituting an AC side of the converter 7. Each converter valve 9a-f includes an arm inductance L and a number of switching modules 12 connected in series. The number of switching modules 12 in every converter arm 9a-f is in general arbitrary (not restricted to two per valve) and can be adapted to the given application. According to the example shown in FIG. 2, all switching modules 12 are so-called full-bridge switching modules. A proper control of the semiconductor switches of a given switching module 12 creates a positive, a negative or a zero voltage across its terminals.

[0029] FIG. 3 shows a converter control system 13 suitable for the converter shown in FIG. 1 and or FIG. 2 connected with an AC grid at a connecting point. It includes a grid following controller 14 and a grid forming controller 15 both implemented as control modules within the control system 13. As its input, the control system 13 (the respective controllers 14, 15) receives all of the information usually needed for the converter control, in particular the measured AC and DC side currents and voltages and the AC grid frequency as well as the corresponding reference values. The grid following and the grid forming controllers 14, 15 share an underlying current controller 17. The outputs of the grid forming and the grid following controllers 14, 15 are connected with the underlying current controller 17 through a virtual admittance 16.

[0030] The grid forming controller 15 is configured to actively control the frequency and voltage at a connecting point (e.g. connecting point 2a in FIG. 1). It includes a voltage angle and a frequency controller for active power control (which can be implemented as functional sub-units of the grid forming controller). The voltage angle and/or a frequency controller is based on a model of an ideal answer of an electrical machine by using a transfer function. The model is a machine model including electromechanical and electromagnetic properties of the converter. The grid forming controller 15, or the model, respectively, can be implemented e.g. as a virtual synchronous machine (VISMA), a virtual synchronous generator or a synchronous power controller. The grid forming controller 15 also includes a voltage magnitude controller for reactive power control having a linear controller (each of those preferably being implemented as functional sub-units of the grid forming controller) configured to change the reference amplitude of the AC voltage based on a reactive power reference and on a measured reactive power. Moreover, the grid forming controller 15 generates separate control setpoints to the underlying current controller for a positive and a negative sequence system.

[0031] The grid following controller 14 includes a current setpoint controller (which can be implemented as functional sub-units of the grid following controller) to control a fixed active and reactive power operation point and/or a converter energy balance.

[0032] The underlying current controller 17 can include for example a second order generalized integrator, a dq controller, a resonant controller or similar.