WO2022074278A1

WO2022074278A1 - Synchronous control method for a power conversion unit

Info

Publication number: WO2022074278A1
Application number: PCT/ES2021/070735
Authority: WO
Inventors: Pedro RODRÍGUEZ CORTÉS; Andrés TARRASÓ MARTÍNEZ; Ngoc Bao LAI; José Ignacio CANDELA GARCÍA
Original assignee: Universitat Politecnica De Catalunya
Priority date: 2020-10-09
Filing date: 2021-10-08
Publication date: 2022-04-14
Also published as: ES2905693A1

Abstract

Proposed is a synchronous control method for a power conversion unit. The method allows the synchronous characteristic of a conventional generator to be emulated, the implementation of said method being based on a traditional current controller and on the control voltage generated. By defining a virtual electromotive force and the system control voltage, the method implements a synchronisation structure to maintain the stability of the system and generate a synchronous compensation current, thereby promoting the stability of the frequency and the voltage at the point of common coupling of the power conversion unit. The method also enables control and adjustment of the basic parameters of the system, inertia, the damping factor and the virtual impedance value, allowing the mains voltage and frequency to be improved and supported dynamically.

Description

SYNCHRONOUS CONTROL METHOD FOR A POWER CONVERSION UNIT

TECHNICAL SECTOR

The present invention belongs to the field of electrical system controllers and presents a control structure capable of emulating the electromechanical response of a synchronous generator, allowing to regulate the active and reactive power that is generated, while providing inherent support for maintenance. of the level of voltage and frequency in the electrical network by using the control voltage generated through the current controller. This invention makes it possible to avoid the use of an external network voltage measurement, as well as a synchronization system based on this measurement, allowing direct synchronization embedded in the current control loop, maintaining the functionalities of active and reactive power control. generated as well as the voltage and frequency regulation characteristics of a synchronous generator, which allows a dynamic response to disturbances in the electrical network, interconnected to a larger or isolated network, just as synchronous generators operate, without sacrificing a fast response in power control.

BACKGROUND OF THE INVENTION

In recent years, electrical power generation systems have undergone a major change, giving rise to a more heterogeneous and distributed generation system. This change has been largely a consequence of the strong push for renewable energy in this sector, which has led to the replacement of traditional generation systems by generation plants based on renewable energy. Photovoltaic and wind energy have turned out to be options with great potential for progress in the integration of renewable sources into the electricity grid, accumulating a large part of the energy injected into the electricity grid worldwide. These renewable sources are connected to the electrical grid through power electronic converters, also known as power converters or inverters. These conversion elements commonly work as a voltage source (VSC, Voltage Source Converter), and are responsible for regulating the flow of active and reactive power injected into the electrical network by controlling the output voltage of the conversion unit.

Renewable energies, such as photovoltaic and wind energy, present a great intermittence in the injection of energy as they depend largely on weather conditions. Likewise, its dynamic behavior presents clear differences with the way in which the rest of the generation units are controlled and operated, not only due to its response speed but also due to its lack of inertia and network support functionalities. For this reason, as the proportion of renewables in the energy mix increases, the new power injection systems to the grid have begun to emulate the more conventional power injection systems to the electricity grid. For this, they take synchronous generators as a reference, which are capable of contributing to the stability of both frequency and voltage of the electrical network thanks to their ability to store energy in their rotating axis as mechanical energy. This energy can be delivered to the electrical grid as additional electrical power during grid disturbances. Similarly, they support the level of tension.

The new renewable energy plants connected to the grid through power converters, despite having a much faster and more effective behavior than traditional generation systems, lack mechanical elements in which to store additional mechanical energy to deliver to the plant. power grid in the event of sharp transitions or grid disturbances. This translates into a great reduction in the stability of the systems connected to the electrical network, which increases their vulnerability to imbalances and disturbances in the electrical network.

This has prompted research and development of new solutions to be able to integrate the lack of inertia internally in renewable energy systems, thus expanding their functionality and allowing a high level of integration of this type of energy while maintaining the levels of system stability provided by renewable energy systems. traditional generators. As a result of this work, different control strategies have been proposed to modify the references of active and reactive power, achieving an improvement in the stability of the system without giving up the generic features of active and reactive power control. These strategies focus on emulating the electromechanical and electromagnetic characteristics of synchronous generators, which provide the ability to store energy in the form of kinetic energy in a virtual rotor.

Document WO2017044922 presents a control method for frequency variations by means of a power conversion unit, represented as a voltage source in series with an impedance, which is modified to control the network frequency. In CN104377697A and US201 101531 13A1, two control strategies are presented that emulate the dynamics of the rotor of a synchronous machine, allowing the power conversion unit to provide the dynamic support functionalities in both frequency and voltage of the synchronous generator.

A large part of the methods to integrate synchronous systems are based on the calculation of the voltage to be generated by the power conversion unit, and use the virtual impedance to produce the necessary voltage drop to control the system. In this way the system behaves in an equivalent way to a controlled voltage source. These strategies present stability problems due to the derivative component of the virtual impedance, which presents an unstable behavior, giving rise to an inappropriate dynamic response to the appearance of disturbances and transient phenomena in the electrical network.

In ES2402465B1 a control is presented that modifies the virtual impedance control and performs a control based on the virtual admittance. The difference between the voltage at the inverter terminals and the voltage generated by the virtual rotor generates a reference current to be followed by a current control that acts on an internal control loop. In this way, the converter acts as a controlled current source against the electrical network, thus avoiding the problem of the derivative component of the virtual impedance. These methods require knowing the voltage of the coupling point of the power converter, since thanks to this value the nested control loops are made to emulate the electromechanical and electromagnetic behavior of the synchronous machine.

EXPLANATION OF THE INVENTION

According to the above, the present invention proposes a new synchronization method for a current controller, capable of emulating the electromechanical characteristic of a synchronous generator and providing a synchronous compensation current to maintain a power conversion unit, or power converter, stable and provide support in frequency and voltage to the electrical network. Thus avoiding the use of an exclusive synchronization system and the use of external voltage measurements of the power converter. The synchronous control method for a power conversion unit object of this invention, which is particularly based on synchronization by means of the current controller, is a control system that allows emulating the dynamic behavior of a synchronous generator, without the need to use of a specific synchronization system or an external voltage measurement. Thus providing an improvement in the stability of the system both in frequency and voltage. Likewise, it is capable of acting in interconnected networks, as well as in isolated systems, providing network-forming features.

This control method operates the power conversion system using the classic current control strategies for a power converter connected to the electrical network or in an isolated system acting as a former, being also capable of emulating the characteristics and behavior of a synchronous machine. Classical control systems are composed of a current controller in synchronous coordinates, dq , or a controller in stationary coordinates, a/3 . Commonly, control systems in synchronous coordinates establish synchronization systems to be able to rotate at the same network speed, which usually measure the voltage of the electrical network and use it to synchronize the entire control system. These synchronization loops can be made up of different algorithms, where the most common are the so-called PLLs (Phase Lock Loop), which allow synchronization to the grid voltage by means of a grid voltage monitoring algorithm.

The synchronous control method object of this invention avoids the use of traditional electrical grid synchronization systems and uses the control voltage reference itself generated by the current controller as a synchronization element, which has the ability to emulate the electromechanical and electromagnetic behavior of a synchronous machine, thus allowing the control of active and reactive power against voltage and frequency variations in the electrical network. Synchronization based on the use of the current controller is performed by the control voltage generated by the current controller. This generated voltage subtracted from the internal electromotive force (EMF) of the virtual synchronous machine is capable of providing the difference in voltage between them. Thus defining a difference in magnitude and phase, which through a virtual impedance is translated into a synchronous compensation current. To generate the internal EMF of the machine virtual synchronous, the phase angle obtained by the electromechanical loop is used. The input of this control loop is linked to the power generated by the synchronization system, which is defined by the synchronous compensation current and the inverter control voltage. In this way, the system is able to determine the necessary phase angle for the internal EMF of the control system, so that the virtual impedance generates the synchronous compensation current. In this way improving the stability of the control system, and generating compensation currents during transients of the electrical network, both in frequency and voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

To complement the description of the invention and in order to clarify the operation of the control method, the figures are described where the following is presented by way of illustration and not limitation:

Figure 1 shows the general scheme of a power converter connected to the electrical network.

Figure 2 shows the internal control structure, where the current controller and the synchronization system to determine the synchronous compensation current are presented.

Figure 3 shows the control structure of the synchronization system in detail.

Figure 4 shows the internal control structure where the synchronization system generates a synchronous compensation power.

Fig. 5 shows the internal control structure of compensation synchronous power generation in detail.

Figure 6 shows the internal control structure, where the current controller and the synchronization system are integrated, in addition to the droop regulation loops.

PREFERRED EMBODIMENT OF THE INVENTION

Next, a preferred embodiment of the invention is presented with reference to the previously mentioned figures.

In a possible preferred embodiment of the synchronous control system and method for a power conversion unit based on synchronization by means of the current controller, is presented for a classic configuration of the aforementioned power conversion unit (or power converter) that is connected to the network in Figure 1. In particular, the power converter consists of an external DC source (1) which can be generated by any type of renewable energy, a power inverter (2) with the corresponding controller (3) and an output filter (4) to reduce the noise level at the converter output.

As shown in figure 2, the internal control of the power converter is made up of a current control loop (8), which through the error between the current measurement (7) and the current references, the external reference (5) and the compensation current (6), is capable of generating a control voltage (9), which defines the voltage level at the output of the power inverter (2). This voltage is used as input for the synchronization system (10) which is capable of determining its phase (1 1) and generating a synchronous compensation current (6). As shown in Figure 3, the synchronization system uses an FEM (12) called E _v . By means of the voltage difference between the internal voltage of the FEM virtual machine (12) and the control voltage (9) a compensation current (6) is generated, which depends directly on the parameters of the virtual impedance (13). This element consists of a resistance and inductance value that generate the synchronous compensation current (6) depending on the error between the control and internal voltages. By means of this compensation current and the power converter control voltage, the output power of the power inverter (2) is calculated, which enters an active power control block (14). This block (14) has the capacity to generate a variation in angular speed Aro (15) depending on the variation in active power previously calculated by means of the compensation current and the control voltage. This speed added to the nominal angular speed of the system (16) determines the angular speed of the electrical network. The integration of this variable allows to generate the phase angle of the EMF, thus allowing to generate variables in phase with the control voltage.

In one embodiment, the system may be implemented in a synchronous coordinate system, dq, which defines the variables at stationary values. In this way the virtual impedance (13) can be defined in the synchronous coordinate system with the necessary inductive and resistive values. Thus enabling the system to be able to generate the compensation current in synchronous coordinates, also defining the local phase of the system by means of the active power control loop (14) to carry out the transformations necessary for internal control.

In another embodiment, the system can be implemented in a stationary coordinate system, cr/3, which defines the control variables as rotating vectors in time. In this way, the virtual impedance (13) can work with the traditional impedance transfer function. Thus defining the level of synchronous compensation current in the system. Forcing the system to define the phase angle (11 ) to generate the internal EMF of the virtual machine.

In another embodiment, the virtual admittance can be represented only as a matrix of values, which do not depend on a transfer function and only present the average value of the virtual impedance. In this way, the transfer function presented by the virtual impedance for different harmonic frequencies is avoided.

In another embodiment, the compensation system can be defined by a compensation synchronous power (19), which is defined by the synchronization system, represented in Figure 4. This reference added to the possible external active power reference (18 ) and reactive (17), are sent to the power to current transformation equations (20), which through the control voltage (9) is capable of defining the reference current of the current controller, as shown in Fig. figure 5.

In another embodiment, droop type control loops can be integrated into the current control system. By means of the power to reference current transformation block, the system is capable of defining the droop -f curve characteristic for active power (22) and Q-\/ for reactive power (21).

A person skilled in the art could make changes and modifications to the embodiments described above, without departing from the scope of the invention, as defined in the attached claims.

Claims

8

CLAIMS Synchronous control method for a power conversion unit, which is based on synchronization by means of a current controller, characterized in that it comprises defining a synchronous compensation current and a phase angle of the voltage using the control voltage itself the power conversion unit. Method according to claim 1, further comprising generating a synchronous compensation current using a virtual impedance, wherein the synchronous compensation current depends on the control voltage and an internal electromotive force, EMF, of a virtual synchronous machine. Method according to claim 2, wherein the virtual impedance is defined by a transfer function or by averaged parameters. Method according to claim 2, further comprising using an active power control block to calculate a phase angle of the EMF, wherein the active power control block has as input a power difference generated by the synchronous compensation current . The method according to claim 2, further comprising defining a synchronous compensation power using the synchronous compensation current and the control voltage of the power conversion unit, thereby defining a power control in the power conversion unit. Method according to claim 2, further comprising integrating droop control loops or external power or current setpoints within a current control loop of the power conversion unit.