TECHNICAL FIELD

The present invention relates to a device, in particular to a device comprising at least two inverters. The inverters are respectively included in multiple coupled Hopf oscillators. The invention also relates a microgrid comprising the device.

BACKGROUND

Traditionally, distribution power systems are configured in radial structures. Power and short-circuit currents flow unidirectionally from distribution substations. Most protection, monitoring, and control devices are designed based on this configuration.

Recently, distributed generation has begun to emerge in the energy market, because of its value, for example, for peak shaving, combined heat and power, renewable portfolios, and transmission and distribution infrastructure deferral. In particular, a microgrid is an integrated energy system consisting of distributed energy resources and interconnected loads, which can operate in parallel with the grid- connecting or in an intentional island mode. Smart microgrids can generate, distribute, and regulate the flow of electricity to consumers.

In microgrids, the droop control method has dominated the autonomous control of parallel inverters in the last decade. Although this technique only needs local information to operate, it presents a number of problems that are solved along the literature.

The first one is that the droop coefficients that regulate frequency and amplitudes are basically proportional terms, so that in order to increase their range of values, derivative terms can be added.

The second one is that frequency and voltage are respectively related to active and reactive power when the output impedance of the generator is mainly inductive, however, in an inverter, the output impedance can be fixed by means of virtual impedance.

The third one is that in case of resistive lines/virtual impedances, active power is controlled by the inverter voltage amplitude, while the reactive power flow is dominated by the angle.

Based on these three improvements, a control architecture based on a virtual resistance, P-V and Q-f droops can be used to deal with the autonomous control of parallel connected inverters. The stability of autonomous microgrids is a critical issue considering the low inertia nature of such inverter dominated systems. Small signal based stability analysis has been reported in the prior art in order to study the stability of the autonomous droop-controlled microgrid system.

However, power droop control has the inherent drawback that it needs to calculate instantaneous active and reactive powers and then average through low pass filters, whose bandwidth may impact the transient response of the system. The authors in Yajuan et al. “A simple autonomous current-sharing control strategy for fast dynamic response of parallel inverters in islanded microgrids ”, Energy Conference (ENERGY CON), 2014 IEEE International, vol, no., pp. 182-188, 13-16 May 2014, try to increase the response of the system by using a virtual resistance. However, this method cannot deal independently with active and reactive power sharing.

The virtual oscillator control (VOC) is a time-domain controller, which allows digitally controlled inverters to emulate the dynamics of an oscillator. The method has been proven to be robust in front of load variations and can be used for a number of parallel- inverters inverters, requiring no communications, and presenting global asymptotic synchronization. The VOC in his original version has been extensively analyzed and implemented for islanded systems. In Johnson et al. “ Synchronization of parallel single- phase inverters with virtual oscillator control”, IEEE Transactions on Power Electronics, vol. 29, no. 11, pp. 6124-6138, 2014, a synchronization method based on limit cycle oscillator is proposed, and it offers inti-interference and fast response performance under grid voltage variations. However, the frequency-locked loop with extra reference. Moreover, nonlinear oscillators have been widely used to model various physical or engineering processes, and they have shown the importance in modelling and control. Oscillator models are interesting owing to their synchronization characteristics, either with other oscillators or with external driving signals. In most cases, it is difficult task to choose the right parameters of the oscillators to ensure that they will synchronize as desired. The reason is that oscillators lack plasticity, they have fixed intrinsic frequencies and cannot dynamically adapt their parameters.

Virtual oscillator control is a new and promising technique for MG, which provides a way to synchronize and control interfaced inverters without the need of communication, by mimicking the characteristics of nonlinear oscillators.

However, a problem is the parallel operation of DC-AC inverters for microgrid applications. Thus, there is a need for an improved device comprising inverters.

SUMMARY In view of the above-mentioned problems and disadvantages, embodiments of the present invention aim to improve the conventional devices comprising inverters. An object is thereby to provide a device allowing parallel operation of DC-AC inverters for a microgrid. The device should have a fast transient response- Further, it should be less sensitive to control parameters, and should enable a simple controller design. The device should provide robust performance even under load changes. A fast adjustment of active/reactive power rating should also be possible.

The object is achieved by the embodiments provided in the enclosed independent claims. Advantageous implementations of the embodiments are further defined in the dependent claims. According to a first aspect, the invention relates to device, comprising a first inverter, wherein the first inverter is configured to invert a DC input voltage into an AC output voltage, a first controller, wherein the first controller is configured to provide a reference voltage for the first inverter on the basis of an AC output current of the first inverter, a second inverter, wherein the second inverter is configured to invert a DC input voltage into an AC output voltage, a second controller, wherein the second controller is configured to provide a reference voltage for the second inverter on the basis of an AC output current of the second inverter, wherein the first inverter and the first controller form a first Hopf oscillator, wherein the second inverter and second controller form a second Hopf oscillator, and wherein the first Hopf oscillator is coupled to the second Hopf oscillator.

The device is in particular related to parallel inverters, which means that the inverters (number ³ 2) connect to a common load in parallel. Each inverter may be configured to invert a DC input voltage into an AC output voltage. The controller of each inverter may be the same, and it may be configured to provide a voltage reference for a corresponding inverter on the basis of an AC output current of the corresponding inverter.

This allows the interconnection of a number of renewable generators and energy storage system at the same distribution grid, providing the following advantages: fast active and reactive current sharing among parallel connected inverters, novel application with Hopf oscillator synchronization, simple controller design, fast adjustment of active/reactive power rating, robust performance with loads changes, plug and play functionalities. Thus, and improved device is provided.

Moreover, the invention solves the following major problems, slow transient response, coupling active/reactive power, sensitive control parameters, slow changes need when adjusting active/reactive power rating and poor performances with different loads.

Furthermore, embodiments of the invention relate to the parallel operation of DC-AC inverters in a microgrid. The inverters utilize local measurement signal to realize paralleling, without any communication to exchange information between each other.

Synchronization of coupled oscillation is a natural property which can be utilized in many areas, such as neural processes, coherency in plasma physics, communications, and electric power systems. In particular, the oscillator based paralleling is a fast and simple control method, which can offer a way to integrate renewable resources on the community level and allow for customer participation in the electricity enterprise.

In an implementation form of the device of the first aspect, the first controller and the second controller are identical. In an implementation form of the device of the first aspect, the first inverter and/or the second inverter is a three-phase inverter, in particular comprising three half-bridges and three filters, such as L, LCL, and LC filters.

Each inverter has thus a three-phase configuration, in particular comprising the three half-bridges and the three filters.

In an implementation form of the device of the first aspect, the first controller and/or the second controller is an oscillator controller.

In particular, each controller is mainly an oscillator controller.

In an implementation form of the device of the first aspect, the first inverter and the second inverter are not in mutual communication.

In particular, none of the inverters of the device are in mutual communication.

In an implementation form of the device of the first aspect, the first controller is configured to detect the output current of the first inverter and/or the second controller is configured to detect the output current of the second inverter. In particular, the controller of each inverter is configured to detect the output current of the local inverter.

In an implementation form of the device of the first aspect, the device is configured to perform a Clark transformation on the output current of the first inverter and/or second inverter, before the output current is detected by the first controller and/or second controller, respectively.

In particular the device is configured to perform a Clark transformation on the output current of each single inverter.

In an implementation form of the device of the first aspect, the first controller and the second controller are configured to generate the reference voltage according to: wherein i _ab is the output current of the each inverter, after Clarke transformation, V _ab is the controller output reference voltage, r is a steady state amplitude of oscillation of the each Hopf oscillator, w is the each Hopf oscillator’s frequency, respectively, and k is a current gain. In an implementation form of the device of the first aspect, the device is further configured to perform the following steps in order to calculate a value of k:

1) run a model for k without load;

2) calculate a voltage Vpk;

3) if Vpk < Vmin, wherein Vmin corresponds to a voltage of the first inverter without rated load, then decrease k and go to step 1);

4) if Vpk > Vmax, wherein Vmax corresponds to a voltage of the first inverter with full-rated load, then increase k and go to step 1); and

5) if Vmin<Vpk<Vmax, then stop.

In an implementation form of the device of the first aspect, the device is further configured to perform an inverse Clark transformation on the reference voltages.

In an implementation form of the device of the first aspect, the device further comprises a pulse-width-modulation, PWM, unit and wherein the output reference voltage is provided as input to the PWM unit after the inverse Clark transformation.

According to a second aspect, the invention relates to a microgrid comprising the device of any one of the first aspect or any one of the implementation forms thereof.

It has to be noted that all devices, elements, units and means described in the present application could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof.

BRIEF DESCRIPTION OF DRAWINGS The above described aspects and implementation forms of the present invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which:

FIG. 1 shows a schematic representation of a device comprising a first and a second inverter, according to an embodiment; FIG. 2 shows a direction field of a Hopf oscillator of a device according to an embodiment;

FIG. 3 shows a 3-D plot of a steady state amplitude of oscillation of a Hopf oscillator of a device according to an embodiment;

FIG. 4 shows a 3-D plot of a Hopf oscillator of a device according to an embodiment;

FIG. 5 shows a dynamic response of a power of an inverter of a device according to an embodiment;

FIG. 6 shows a diagram of a process for computing a control parameter of a device according to an embodiment; FIG. 7 shows a schematic diagram of a droop controller and of a Hopf controller according to an embodiment;

FIG. 8 shows a schematic diagram of a power response of a droop controller and of a Hopf controller according to an embodiment;

FIG. 9 shows a schematic diagram of a phase and of state solutions of a Van der Pol oscillator and of a Hopf oscillator according to an embodiment; FIG. 10 shows a voltage amplitude and a frequency of a controller and of an inverter of a device according to an embodiment;

FIG. 11 shows an active power (a) and a reactive power (b) sharing waveform under inverter connection according to an embodiment; FIG. 12 shows two inverters current sharing waveform under inverter connection according to an embodiment;

FIG. 13 shows a current and voltage of loads waveform under inverter connection according to an embodiment;

FIG. 14 shows an active power (a) and a reactive power (b) sharing waveform under inverter removal according to an embodiment;

FIG. 15 shows two inverters current sharing waveform under inverter removal according to an embodiment;

FIG. 16 shows a current and voltage of loads waveform under inverter removal according to an embodiment; FIG. 17 shows an active power (a) and a reactive power (b) sharing waveform under non-linear load connections according to an embodiment;

FIG. 18 shows two inverters current sharing waveform under non-linear load connections according to an embodiment;

FIG. 19 shows current and voltage of loads waveform under nonlinear connection according to an embodiment; and

FIG. 20 shows experimental result of transient responses according to an embodiment. DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a schematic representation of a device 100 according to an embodiment, the device 100 comprising a first inverter 101 and a second inverter 102.

The first inverter 101 is configured to invert a DC input voltage into an AC output voltage. Moreover, the device 100 comprises a first controller 103, wherein the first controller 103 is configured to provide a reference voltage for the first inverter 101 on the basis of an AC output current of the first inverter 101.

The second inverter 102 is configured to invert a DC input voltage into an AC output voltage. Moreover, the device 100 comprises a second controller (not shown in the figure), wherein the second controller is configured to provide a reference voltage for the second inverter 102 on the basis of an AC output current of the second inverter 102.

The first inverter 101 and the first controller 103 form a first Hopf oscillator, and the second inverter 102 and second controller form a second Hopf oscillator, and the first Hopf oscillator is coupled to the second Hopf oscillator. The device 100 can be comprised in a microgrid. The synchronization and current/power sharing process are essential for the operation of microgrid in islanded mode. Collections of oscillators are observed to synchronize in a diverse variety of systems, despite the inevitable differences between the oscillators. Therefore, the coupled Hopf oscillator concept can be applied in a microgrid to achieve paralleling operation. In the following, for the sake of completeness, a summary of the theoretical background about Hopf oscillators is given.

The dynamics of a Hopf oscillator can be described by the following equations: wherein x, y are the states of the oscillator, w refers to the oscillator frequency, and r is an amplitude parameter, which determines the steady state amplitude of oscillation Fig.2 shows that the directions of the vector field of the Hopf oscillator and oscillator states are able to stabilize in a limit-cycle with desired frequency and phase (the initial state is (0, 0)). Therefore, the solutions of the system are x = r sin(wt + q ₀ ) , y = r cos(wt + q ₀ )

, where is the initial phase of the state. From Fig. 2, a limit cycle can be seen which is a closed orbit in the phase spate, that is, a non-constant trajectory, but returning to its starting point, r ² will maintain the oscillations in the system. When r has a ramping change, the 3-D plot are shown in Fig. 3. It can be seen that the system follows r rapidly. It implies that the Hopf oscillator has a robust performance under different reference. The control structure of a Hopf oscillator control for parallel three-phase LC-filter inverter is depicted in Fig. 1. The controller 103 is shaped with the i _ab current as follow: where i _ab is the output current of the inverter after Clarke transformation, V _ab is the controller output, which is utilized to generate the reference voltage with the inverse Clarke transform.

In the single-loop of Hopf oscillator, with regarding of the external periodic perturbation i _ab , the oscillator phase dynamics are presented by the following differential equations: wherein V is the amplitude of output, k is the current gains, q is the instantaneous phase angle of inverter output. The values of the parameters are tuned to depend on the power ratings and dynamical speed. Then, the voltage references under ab axis are generated, which will divided by DC voltage. Finally, after ab - abc transformation, a conventional sine-triangle PWM scheme is used to generate a switching signal.

FIG. 4 shows a 3-D plot of a current gain of a Hopf oscillator of a device 100 according to an embodiment. The current gain k is an important parameter which will affect the performance of system or device 100. Fig. 4 illustrates the phase orbit under different k.

From Fig. 4, it can be seen that if k changes ramping up from 0 to 7000, the output amplitude of voltage will not change. If k is overlarge, the voltage and current has a linear droop relationship. Therefore, k is designed to have small values to make the voltage follow the references.

FIG. 5 shows a dynamic response of the power of the inverter 101 of the device 100 according to an embodiment. In particular, Fig.5 illustrates the dynamics response of the system or device 100 under different k. As it can be seen, if k is smaller, then the system is faster. If k is less than 100, the overshoot of transfer dynamics is higher which will make the system unstable. In an embodiment, k=100.

Therefore, in order to standardize the design, k can be chosen such that, when oscillator output current is 1A, the inverter 101 is loaded to full rated capacity The values of k that ensure this are:

A system of inverters 101, 102 with different power ratings connected in parallel shares the load power in proportion to the ratings, if the current gains are chosen as suggested by equation (4). This directly follows as a consequence of real-root of output voltage. can be set to be constant to make sure the system operation in rated power.

FIG. 6 shows a flow diagram of a process for computing the control parameter of the device 100 according to an embodiment.

The iterative design process is shown in Fig. 6. The Hopf oscillator has a stable oscillation at the rated system frequency at initial state. The maximum and minimum load voltages correspond to the no load and full-rated load cases, respectively. Consequently, the V _min £ V _pk £ V _max will be satisfied across the entire rated load range. In particular, the device 100 can further configured to perform the following steps in order to calculate a value of k:

1) run a model for k without load;

2) calculate a voltage Vpk;

3) if Vpk < Vmin, wherein Vmin corresponds to a voltage of the first inverter 101 without rated load, then decrease k and go to step 1);

4) if Vpk > Vmax, wherein Vmax corresponds to a voltage of the first inverter 101 with full-rated load, then increase k and go to step 1); and

5) if Vmin<Vpk<Vmax, then stop.

FIG. 7 shows a schematic diagram of a droop controller and of a Hopf controller according to an embodiment.

Droop control methods have been widely used as distributed control of parallel converters, and aims to proportionally share active and reactive powers with adjusting frequency and output voltage amplitudes of each converter locally in order to emulate the behavior of a synchronous generator. In order to apply droop to some certain conditions, it can be extended or add to some loops such as: adaptive voltage droop, synchronized reactive compensation, Q-V Dot droop and virtual impedance loop etc. The conventional droop controller consists of three control loops droop loop, voltage controller, and current controller.

From Fig.7, it can be seen that the Hopf controller 103 only needs output current on the input signal side. Regarding the output signal side, the active and reactive power sharing performances of two inverters 101 and 102 are shown in Fig. 8.

In contrast, Hopf oscillator acts on instantaneous measurements and inherently provides a faster and better damped response. Moreover, the coupled Hopf oscillator can regulate frequency adaptively which is unrealizable for droop method, and more complex secondary control is unnecessary for Hopf method. Note that the faster performance is because that the Hopf does not apply low-pass fdter and double loops.

In conclusion, for any improvement and extension methods from droop, the power calculation is needed, which is able to distinguish the droop and Hopf. Furthermore, the response speed also can verify this. FIG. 9 shows a schematic diagram of a phase and of state solutions of a Van der Pol oscillator and of a Hopf oscillator according to an embodiment;

Due to the owning of global asymptotic synchronization features, the Van der Pol oscillator has been extensively analysed and implemented for islanded converter control. However, its phase evolution is difficult to describe in contrast to Hopf oscillator.

Moreover, the comparisons between the Van der Pol oscillator and Hopf oscillation are shown in Fig.9. Fig.9(a)(b)(c)(d) show the phase portrait and state solutions of the oscillators, and it can be seen that PBO is structurally stable in an unique limit-circular, and the xy can be ideal orthogonal sinusoid. The Van der Pol oscillator generates a limit shapeless cycle, and distorted outputs xy with same perturbation. Otherwise its output approximates a limit-cycle when parameter stays in a small range. Consequently, Hopf performs better power quality to compare with Van der Pol.

This provides the following advantages: no active reactive/power should be calculated, and it only includes a voltage loop. Thus, the technology provides fast response. DC link of power electronics converters can be saved due to the fast response provided.

The Hopf-oscillator based controller is independent from the loads. It means that no matter how the loads changes, the system operation is stable.

At the same time, the method can adjust active/reactive power ratting in real time, being suitable to be controlled by a superior control level coming from a local controller or an energy management system.

The aforementioned features cannot be achieved by the existing methods at the same time.

For the sake of completeness, in the following, first, the derivation of the voltage- and frequency- regulation characteristics of the Hopf-oscillator-controlled inverters will be described. The derivation is based on an averaging analysis of the Hopf oscillator dynamics.

The dynamics of one Hopf oscillator under perturbation can be described by equations (1) and (2). By differentiating V _a = Vsin(q) , V _b = Vcos(q) with respect to time, the following equations are obtained: Substituting equation (5) in equation (1), the phase dynamics equations of Hopf oscillator are obtained as follows: Subsequently, if T is a period of a periodic signal V(t), then the average value is given by definition by:

In order to simplify the averaged model, the dynamics of the inverter terminal voltage is defined as: where w and w ^* are the nominal frequency of inverter output, and the steady state frequency of inverter output, respectively. The angles q and q ^* donate the phase offset with respect to w and w ^* respectively. Therefore, in the averaged model, the single Hopf oscillator dynamical system under 2p -periodic function in time are presented as follows:

According to Newton-Leibniz formula, the first item of (9) equation can be derived as: Considering the perturbation ί _ab , which is related to the instantaneous active and reactive power P, Q, the definition and average equations during a period of active and reactive power can be expressed as:

P = V cos (w t)i _a + V sin(wt)i _b

Q = V sin (w t)i _a - V cos(wt)i _b (11) In 2p -periodic, the average and reactive power (11) can be obtained as:

Substituting (10), (11) and (12) into (9), the nonlinear equations can be rewritten as:

Therefore, the dynamical equation of amplitude and phase of one oscillator are expressed as:

Remarkably, with Hopf oscillator control, the voltage-amplitude and phase dynamics are directly linked to the average active and reactive power outputs of the inverter 101. Consequently, these averaged dynamics can be leveraged for synthesizing Hopf oscillators so that the inverter satisfies voltage- and frequency-regulation specifications in the steady state.

Therefore, the regulation characteristic can be analysed in deep based on (14). The relationship of voltage amplitude and active power, frequency and reactive power is shown in Fig. 10. The equilibria of (13) is able to be solved based on the solutions of the nonlinear equation as follow:

Where and refer to the equilibrium steady-state RMS-voltage amplitude and average active power output, respectively. Therefore, the positive roots of (15) are given by:

Therefore, the two roots are real values, if the equilibrium real power output satisfies:

Consequently, the critical value for active power is set. Therefore, the maximum of output voltage can be defined based on which is related to the voltage reference and the parameter k.

Consider the phasor-angle dynamics in (13). The equilibrium of frequency can be derived as:

Therefore, the equilibrium frequency can be designed based on voltage amplitude and reactive power.

In the following, an example of an inverter connection with resistive loads will be given.

In this example, the voltage reference is 311 V, the filter is 0.1 ohm + 3 mF, the load is 100 ohm. The control parameter k=100. Figs.11-13 illustrate the active power and reactive power, a phase of currents, voltage and current of loads when the second inverter 102 connects to the system at 1s.

In the following, an example of an inverter removal with resistive-inductive loads will be given. In this example, the resistive and inductive loads (100 ohm + 0.1 H) are connected to the system. The second inverter 102 is removed at 1 s. The simulation results are shown in Fig. 14-16.

In particular, in Fig. 14, the active power (upper) and reactive power (below) sharing waveform under inverter removal are shown. In Fig. 15, the two inverters current sharing waveforms (A phase) under inverter removal are shown, while in Fig. 16, the current and voltage of loads waveform under inverter removal are shown.

In the following, a case of support of nonlinear loads is given.

In this case, the nonlinear loads is connected at Is. The nonlinear loads is a rectifier with RC circuit (50 ohm + 60 mF). The simulation results are shown in Fig.17-19.

In particular, Fig. 17 shows the active power (upper) and reactive power (below) sharing waveform under nonlinear loads connection, Fig. 18 shows the two inverters current sharing waveform (A phase) under nonlinear loads connection, and Fig. 19 shows the current and voltage of loads waveform under nonlinear loads connection. Simulation results show that embodiments of the invention provide fast response, robustness, sharing precision and stable operation under islanded control mode.

FIG. 20 shows experimental results of transient responses according to an embodiment.

Experimental results showing the controller’s performance under different scenarios are depicted as follows. The performance of the proposed method was verified by the dSPACE 1006 real-time platform with 2.2 kW inverters. The DC voltage is 650 V and the voltage reference is 311 V. The IGBT’s switching frequency is 10 kHz, and the L-filter inductance is 3 mH. The oscillator parameters are k=52,4 , w ₀ = 100p , and the initial state is (311 0). The resistive loads are 150 ohm. Fig. 20 shows the transient responses of loads voltage and output current in different scenarios. Fig. 21. (al) (a2) illustrates the current transit response for load connection in the ac common bus. The proposed controller can achieve stable operation with 1:1 currents sharing ratio. The transient responses for sudden second inverter removal are shown in Fig. 21 (bl) (b2). Note that the remaining first inverter 101 abruptly increases the current output when the second inverter 102 is removed, which ensures the load voltage continues to meet requirement. The waveforms of Fig. 21 (cl), (c2) depict the transient response to current ratio changes in resistive load from 1:1 to 2:1. It can be observed that the increase and decrease of output current are nearly instantaneous, and load voltage is also stable. Therefore, it can be concluded that the load voltage are stable based on oscillator controller, and the proposed method can achieve synchronization and current sharing fast.

The present invention has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.