Control method and system for improving power grid stability

Abstract

The present invention provides a control method and system for power converters electrically interconnected through a power grid. The control mechanism uses a digital model of the machine shaft to emulate the electrical coupling between each pair of power converters. The power transfer calculated using this multi-shaft model is applied at the corresponding power converters, resulting in a stable, fault-tolerant power signal across the power grid.

Description

[Technical Field]

[0001] The present invention relates to the field of controlling power converters in an electric grid, and more particularly to a system and method for dynamically controlling active power exchanged between coupled grid-connected power converters electrically interconnected through a power grid. [Background technology]

[0002] Interconnection of electrical areas in an electric power system is an important issue for economic and reliability-related reasons because such interconnection provides flexibility for optimizing power flow and enables the sharing of reserves to respond to potential contingencies in the system. Interconnection also ensures multiple supply paths for loads, thereby improving reliability of supply. Electrical areas are typically interconnected through passive alternating current (AC) tie-lines because this is a cost-effective solution when the interconnected areas share the same nominal frequency and nominal voltage and are geographically close to each other. However, when areas with different nominal frequencies or nominal voltages are to be interconnected, or when the geographic distance between the areas is significant, power conversion stations are often required to actively interconnect such areas and regulate power flow using AC-DC-AC power conversion configurations. Due to ever-increasing demand, the rapid growth of renewable energy sources, and advances in power conversion technology, power system interconnection using power electronics-based active systems such as multi-terminal high-voltage direct current (MT-HVDC) systems has become an increasingly popular and effective interconnection solution in modern power systems.

[0003] As the heterogeneity and capacity of power systems increase, the complexity of power system operation also increases, raising concerns about system stability, specifically the dynamic stability of interconnected systems. One typical concern regarding the stability of large-scale interconnected systems is the occurrence of sub-synchronous oscillations in ties. Such power oscillations traditionally arise from the effects of long, high-impedance ties and cannot be naturally damped by electromechanical generators, necessitating the use of dedicated control systems. While such sub-synchronous oscillations have been studied for decades, they have received increasing attention in recent years because they may be the result of negative damping introduced by the control systems of large renewable power plants and power stations when they interact with either the electric grid or other plants and stations.

[0004] The most widely used method for dealing with the damping of subsynchronous oscillations in interconnected power systems is the power grid stabilizer (PSS), as described, for example, in P. Kundur, "Power System Stability and Control," McGraw-Hill Education, 1994, and G. Gurrala and I. Sen, "Power System Stabilizers Design for Interconnected Power Systems," IEEE Trans. Power Syst., Vol. 25, No. 2, pp. 1042-1051, May 2010. A PSS typically takes generator frequency and / or power as input signals and calculates a reference signal for the generator's exciter. The PSS adjusts the generator's instantaneous power injection to be perfectly synchronized with the given subsynchronous grid oscillation to be damped. However, the use of PSS is not a panacea because power oscillations generated by interactions between power plant / station controllers, usually driven by power converters, are more likely to occur at weaker links where PSSs have little presence and contribute less to damping. Furthermore, due to the added complexity of interconnections, selecting optimal tuning parameters for the PSS gain and compensation blocks is a complex task. In addition, tuning of PSS parameters often ignores the effects of network reconfiguration, making the PSS less robust against potential system contingencies.

[0005] Interconnected electrical areas of a power system benefit from each other in terms of load sharing and capacity reserves, but they also share challenges, as a fault in one area can lead to voltage / frequency instability in other interconnected areas. Another concern is widespread power outages in large-scale interconnected systems. In fact, interconnections can facilitate the spread of severe transient faults from one area to adjacent areas, leading to cascading failures. Thus, the stability of interconnected power systems with a high proportion of renewable energy sources has become one of the most challenging requirements to be guaranteed by system operators. Ensuring the stability of traditional interconnected power systems has been the focus of considerable effort by academia and industry since the installation of the first transmission systems. As an example, islanded systems, such as those described by N. Senroy and G.T. Heydt, "A Conceptual Framework for the Controlled Islanding of Interconnected Power Systems," IEEE Trans. Power Syst., Vol. 21, No. 2, pp. 1005-1006, May 2006, can be used to electrically separate interconnected networks into healthy and faulty areas to prevent widespread propagation of serious faults. However, because interconnected areas often support each other during faults due to their inertial properties, forcing the system to separate during a fault can further worsen the stability condition of the problematic area. A potential solution to this problem has been proposed by improving the overall system damping, thereby reducing the probability of instability during system disturbances. Another known solution to avoid oscillations between interconnected systems is to reduce the impedance of long interconnection lines. For this purpose, series capacitors have also traditionally been used to control the apparent line impedance, thereby essentially increasing the system damping.

[0006] The development of power electronics in recent decades has brought about a gradual paradigm shift in the solutions used to address stability problems in interconnected power systems. Today, system damping is not only provided by PSSs of synchronous generators or by passive solutions, but also by active units based on power electronics-based power converters. The main reasons for this paradigm shift are twofold: first, the increasing replacement of traditional synchronous generators has resulted in fewer suitable installation locations for PSSs; and second, power electronics has become a ubiquitous technology in power systems. In fact, power electronics is not only used on the load side, but is also employed in generation and transmission systems. Considering the high flexibility of power electronics, it is clear that power electronics-based power converters should also contribute to improving the stability of interconnected power systems.

[0007] In fact, similar control strategies to PSSs have been proposed for flexible AC transmission systems (FACTS) based on power converters to provide additional damping of oscillation modes; see, for example, G. Cao, ZY Dong, Y. Wang, P. Zhang, and Y. T. Oh, "VSC-based STATCOM controller for damping multi-mode oscillations," 2008 IEEE Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century, 2008, pp. 1-8, or F. H. Gundoman et al., "Review of FACTS technologies and applications for power quality in smart grids with renewable energy systems," Renew.Sustain.Energy Rev., Vol. 82, pp. 502-514, February 2018. More advanced control techniques have also been proposed in the literature to optimize the performance of power converters in addressing dynamic stability. For example, a robust controller based on a linear-quadratic Gaussian controller that ensures effective damping over a wider range than that provided by a thyristor-controlled series compensator was presented in KM Son and JK Park, "On the robust LQG control of TCSC for damping power system oscillations," IEEE Trans. Power Syst., Vol. 15, No. 4, pp. 1306-1312, 2000.Similarly, linear parameter variation methods to improve the transient stability of AC-DC interconnected power systems were considered in Q. Hui, J. Yang, X. Yang, Z. Chen, Y. Li, and Y. Teng, "A robust control strategy to improve transient stability for AC-DC interconnected power system with wind farms," ​​CSEE J. Power Energy Syst., June 2019. The fact that these methods often require a complete system model and high-level mathematical manipulation to obtain optimal gains hinders their practical deployment.

[0008] To address the shortcomings of the aforementioned methods, the concept of a virtual synchronous machine (VSM) that controls power converters to provide grid-supporting functionality has been proposed by H.P. Beck and R. Hesse, "Virtual Synchronous Machine," in 2007 9th International Conference on Electrical Power Quality and Utilization, EPQU, 2007, pp. 1–6. Due to its simplicity and effectiveness, VSM has attracted considerable attention from the power system community. VSM control schemes are often employed to realize grid-supporting and grid-forming power converters. Essentially, most VSM control schemes are based on the emulation of electromechanical synchronous generators. Therefore, most of the useful functionality of synchronous generators can be provided by VSM. Because VSMs are virtually implemented in digital controllers, all of the VSM parameters can be tuned and adapted to the operating conditions, avoiding some of the drawbacks often faced by conventional synchronous generators, such as the inherent low damping. Furthermore, advanced functionality can be easily incorporated into the VSM framework. For example, multiple rotors can be emulated simultaneously in a VSM to achieve selective damping for some vibration modes of interest.

[0009] From a physical perspective, VSM allows a power converter to have a virtual rotational frequency and inertia. Therefore, this type of VSM-controlled power converter is sometimes called a virtual rotating converter (VRC). Similar to synchronous generators, VRCs can also provide frequency support for the electrical grid in the event of a fault, as described, for example, in A. Tayyebi, D. Gross, A. Anta, F. Kupzog, and F. Dorfler, "Frequency stability of synchronous machines and grid-forming power converters," arXiv, Vol. 8, No. 2, pp. 1004-1018, 2020, and W. Zhang, D. Remon, J. Rocabert, A. Luna, J.Candela, and P. Rodriguez, "Frequency support properties of the synchronous power control for grid-connected converters," ECCE 2016-IEEE Energy Convers.Congr.Expo.Proc., 2016. Recent studies have also confirmed that the inertial response created by VRCs can support adjacent areas, see K.S. Skinder, T. Kerdphol, Y. Mitani, and D. Turschner, "Frequency Stability Assessment of Multiple Virtual Synchronous Generators for Interconnected Power Systems," IEEE Trans. Ind. Appl., Vol. PP, No. c, pp. 1-1, 2021.

[0010] However, while the above control techniques enable VRCs to provide frequency and voltage control support to interconnected AC networks, each VRC in the interconnected network still operates independently according to its own tuning parameters. That is, the controller of each VRC is controlled by considering only its own electrical area, more specifically, its own connection point to the grid, rather than the interconnected system to which it belongs. In fact, currently, there is no method other than the well-known droop method for dynamically adjusting the control of multiple VRCs to improve the dynamics and power flow of interconnected power systems. The current state of the art has yet to report an effective and widely accepted solution for controlling transient active power flows between interconnected VRCs or other power converters in the face of grid events. Furthermore, if VRC parameters are not properly set, instability may occur even between interconnected VRCs in an electrical system due to control interactions. Therefore, to improve consistent dynamic response and robustness in the event of faults, it is necessary to coordinate the control behavior of all interconnected power converters in a power grid. [Prior art documents] [Non-patent literature]

[0011] [Non-Patent Document 1] P. Kundur, "Power System Stability and Control," McGraw-Hill Education, 1994. [Non-patent document 2] G. Gurrala and I. Sen, "Power System Stabilizers Design for Interconnected Power Systems," IEEE Trans. Power Syst., Vol. 25, No. 2, pp. 1042-1051, May 2010. [Non-patent document 3] N. Senroy and G.T. Heydt, "A Conceptual Framework for the Controlled Islanding of Interconnected Power Systems," IEEE Trans. Power Syst., Vol. 21, No. 2, pp. 1005-1006, May 2006. [Non-patent document 4] G. Cao, ZYDong, Y.Wang, P. Zhang, and YTOh, "VSC based STATCOM controller for damping multi-mode oscillations," 2008 IEEE Power and Energy Society General Meeting-Conversion and Delivery of Electrical Energy in the 21st Century, 2008, pp. 1-8. [Non-patent document 5] FH Gundoman et al., "Review of FACTS technologies and applications for power quality in smart grids with renewable energy systems," Renew.Sustain.Energy Rev., Vol. 82, pp. 502-514, February 2018. [Non-patent document 6] KM Son and JK Park, "On the robust LQG control of TCSC for damping power system oscillations," IEEE Trans. Power Syst., Vol. 15, No. 4, pp. 1306-1312, 2000. [Non-Patent Document 7] Q. Hui, J. Yang, X. Yang, Z. Chen, Y. Li, and Y. Teng, “A robust control strategy to improve transient stability for AC-DC interconnected power system with wind farms,” CSEE J. Power Energy Syst., June 2019. [Non-patent document 8] H. P. Beck and R. Hesse, "Virtual synchronous machine," in 2007 9th International Conference on Electrical Power Quality and Utilisation, EPQU, 2007, pp. 1-6. [Non-Patent Document 9] A. Tayyebi, D. Gross, A. Anta, F. Kupzog, and F. Dorfler, "Frequency stability of synchronous machines and grid-forming power converters," arXiv, Volume 8, Issue 2, Pages 1004-1018, 2020. [Non-Patent Document 10] W. Zhang, D. Remon, J. Rocabert, A. Luna, JICandela, and P. Rodriguez, "Frequency support properties of the synchronous power control for grid-connected converters", ECCE 2016-IEEE Energy Convers.Congr.Expo.Proc., 2016. [Non-Patent Document 11] K.S.Skinder, T.Kerdphol, Y.Mitani, and D.Turschner, "Frequency Stability Assessment of Multiple Virtual Synchronous Generators for Interconnected Power Systems," IEEE Trans.Ind.Appl., Vol. PP, Issue c, pp. 1-1, 2021 Summary of the Invention [Problem to be solved by the invention]

[0012] It is an object of the present invention to provide a method and system that overcomes at least some of the drawbacks of the prior art. [Means for solving the problem]

[0013] According to a first aspect of the present invention, there is provided a control method for improving security of supply in a power network, the power network electrically connecting a first power converter to at least one second power converter through a power grid, the method comprising the steps of: i) for each pair of power converters consisting of a first power converter and one of a second power converter, providing a digital shaft model of a mechanical shaft to a memory element, said digital shaft model including a damping coefficient, inertia and stiffness of the mechanical shaft and emulating an electrical coupling between said pair of power converters through said power grid; ii) obtaining, in a control unit, a pair of input signals from each of said pair of power converters, each pair of input signals indicative of a rotational speed to be applied to an end of a corresponding digital shaft model; iii) determining the active power deviation in the control unit based on the calculated power transfer for each pair of power converters by applying an equation of rotation to a corresponding digital shaft model using the corresponding pair of input signals; iv) in the control unit, communicating to the first power converter a reference signal that enables the first power converter to supply the active power deviation to the power grid or absorb the active power deviation from the power grid depending on the positive value of the active power deviation.

[0014] The step of obtaining a pair of input signals may preferably include obtaining at least one signal indicative of rotational speed from at least one second power converter, said second power converter preferably emulating a mass rotating at said rotational speed.

[0015] The step of obtaining the pair of input signals includes obtaining at least one measurement of the AC grid frequency at a node of the at least one second power converter.

[0016] Preferably, the step of obtaining a pair of input signals may include obtaining at least one measurement of a DC voltage of a DC bus of at least one second power converter.

[0017] The reference signal may preferably include an indication of the current strength to be injected into the electrical grid by the first power converter.

[0018] Preferably, the first power converter may be controlled as a rotating mass and said reference signal may comprise an indication of the load angle of the power converter.

[0019] It may be preferable that either the inertia or the stiffness, or both the inertia and the stiffness, of at least one digital shaft model may be set to zero.

[0020] According to another aspect of the invention, a control unit is proposed, the control unit comprising: - a memory element in which digital shaft models are stored, each digital shaft model including a damping coefficient, an inertia, and a stiffness of a machine shaft configured to emulate an electrical coupling between a pair of power converters electrically connected through a power grid, each pair of power converters consisting of the same first power converter and one second power converter; - receiving means configured to receive a pair of input signals from each of said pair of power converters, each pair of input signals indicative of a rotational speed to be applied to an end of a corresponding digital shaft model; a computing means, determining an active power deviation based on the calculated power transfer for each pair of power converters by applying an equation of rotation to the corresponding digital shaft model using the corresponding acquired pair of input signals; generating a reference signal that enables the first power converter to supply the determined active power deviation to the power grid or absorb the determined active power deviation from the power grid depending on the positivity of the determined active power deviation; a computing means configured to - transmitting means configured to communicate the reference signal to a first power converter; Equipped with.

[0021] According to a further aspect of the present invention, a control system for a power grid is proposed, the control system comprising a plurality of control units according to an aspect of the present invention, each control unit configured to determine an active power deviation of one power converter of a plurality of power converters electrically interconnected through a power grid.

[0022] According to yet another aspect of the present invention, there is provided a power network in which a plurality of power converters are interconnected through a power grid, the power network comprising at least one control unit according to an aspect of the present invention for controlling at least one power converter.

[0023] The power grid may preferably be equipped with a control system according to an aspect of the present invention.

[0024] According to another aspect of the present invention there is provided a computer program comprising computer readable code means which, when executed on a computer system, causes the computer system to carry out a method according to an aspect of the present invention.

[0025] According to a final aspect of the present invention there is provided a computer program product comprising a computer readable medium having stored thereon a computer program according to an aspect of the present invention.

[0026] The proposed invention enables the use of a controller that digitally emulates additional mechanical coupling between electrically interconnected power converters through virtual mechanical shafts. The proposed virtual multi-shaft (VMS) controller and associated method for dynamically controlling the exchange of active power between multiple physically interconnected power conversion stations in an electrical grid establishes additional virtual links that emulate the effect of mechanical shafts coupling the converter stations to increase system damping in the event of a fault and reduce instability risk. The method schedules dynamic active power flows between the power conversion stations and enables real-time adjustment of system parameters to ensure appropriate system damping levels under prevailing operating conditions. The basic principle of the proposed controller is to regulate the dynamic power exchange between two or more coupled power conversion stations connected in an electrical grid so that the power conversion stations follow the natural power exchange that occurs between two equivalent masses coupled by a mechanical shaft. This basic operating principle is extended to multiple interconnected power conversion stations in the entire power system by emulating a set of virtual mechanical shafts that connect all power stations in the grid. The proposed method contributes to improving the dynamic stability of the electric grid in case of faults and contingencies. The proposed controller and method are able to dynamically control the active power exchanged between interconnected power conversion stations in the electric grid, allowing all stations to support each other during disturbances that may affect the stability of the system.

[0027] Some embodiments of the present invention are illustrated by the following figures, which do not limit the scope of the invention: [Brief explanation of the drawings]

[0028] [Figure 1] 1 is a schematic diagram of a power system according to a preferred embodiment of the present invention, including a control unit according to a preferred embodiment of the present invention; [Figure 2] 1 is a schematic diagram of a power system according to a preferred embodiment of the present invention, including a control unit according to a preferred embodiment of the present invention; [Figure 3] 1 is a schematic diagram of a power system according to a preferred embodiment of the present invention, including a control unit according to a preferred embodiment of the present invention; [Figure 4] FIG. 2 is a diagram of a digital shaft model used in a preferred embodiment of the method according to the invention. [Figure 5] 1 is a schematic diagram of a power system according to a preferred embodiment of the present invention, including a control system according to a preferred embodiment of the present invention; [Figure 6] FIG. 1 is a conceptual control diagram of a control system according to a preferred embodiment of the present invention. [Figure 7] FIG. 2 is a block diagram illustrating the major steps performed by a control system in accordance with a preferred embodiment of the present invention. [Figure 8] FIG. 1 is a diagram of a power network with three AC areas interconnected through both AC ties and a multi-terminal HVCD network. [Figure 9] FIG. 9 is a diagram of the power grid of FIG. 8, including a digital shaft model. [Figure 10] 1 is a schematic control diagram of an active power control loop of a power converter including a control unit according to a preferred embodiment of the present invention; [Figure 11A] 9 is a plot of dynamic frequency deviation showing simulation results obtained using known methods and methods according to preferred embodiments of the present invention for area 1 of the power grid shown in FIG. 8; [Figure 11B] 9 is a plot of dynamic frequency deviation showing simulation results obtained using known methods and methods according to preferred embodiments of the present invention for area 2 of the power grid shown in FIG. 8; [Figure 11C] 9 is a plot of dynamic frequency deviation showing simulation results obtained using known methods and methods according to preferred embodiments of the present invention for area 3 of the power grid shown in FIG. 8; [Figure 12A] 9 is a plot of power flow deviations for different study cases of the power grid shown in FIG. 8. [Figure 12B] 9 is a plot of results obtained using a method according to a preferred embodiment of the present invention of power flow deviations in different study cases of the power grid shown in FIG. 8; [Figure 12C] 9 is a plot of results obtained using a method according to a preferred embodiment of the present invention of power flow deviations in different study cases of the power grid shown in FIG. 8; DETAILED DESCRIPTION OF THE INVENTION

[0029] In this section, the present invention will be described in more detail based on preferred embodiments and figures. Similar reference numerals will be used throughout the various embodiments of the present invention to describe similar or identical concepts. For example, reference numerals 100, 200, 300, and 400 each refer to a control unit according to the present invention, but in different embodiments.

[0030] It should be noted that features described with respect to a particular embodiment described herein may be combined with features of other embodiments unless expressly stated to the contrary. In order to focus on features unique to the present invention, features that are commonly known in the art will not be explicitly mentioned.

[0031] FIG. 1 illustrates a power network 1000 according to a preferred embodiment, in which a first power converter 01 is electrically interconnected with a second power converter 02 through a power grid 10. The power network 1000 comprises a control unit 100 configured to control the first power converter 01. The control unit uses a method that relies on a digital shaft model 12, also referred to as a virtual shaft, that emulates a rotating mechanical shaft used to transfer power between the first power converter 01 and the second power converter 02 to improve security of supply in the power network. The digital shaft model 12 is stored in a memory element 110, from which the control unit 100 has read access, at least, to the digital shaft model's parameter values ​​representing inertia H, damping coefficient D, and stiffness K, as well as the corresponding equations of motion. These parameters are used to calculate the rotational frequency ω, ω, and ωt, respectively, as shown in FIG. i and ω j The model parameters are selected depending on the desired target behavior that the control method is to achieve.

[0032] The digital shaft model 12 uses a pair of input signals received through suitable receiving means 120 in the control unit 100, where a first input 101 is obtained from the first power converter 01, for example through a dedicated communication channel or by measurements in the power grid, and a second input 102 is obtained from the second power converter 02. The input signals 101, 102 indicate the respective rotational speeds applied to both ends of the digital shaft model 12, which end mimic the electrical connection between the two power converters 01 and 02. The input rotational frequencies 101, 102 may be obtained through various measurements depending on the type of power converter. As non-limiting examples, the following cases may be considered:

[0033] If the power converter providing the input signal is an AC node, the rotational speed estimate applied to the corresponding end of the digital shaft model is the grid frequency ω grid is given by

[0034] If the power converter providing the input signal is a DC bus, the rotational speed estimation applied to the corresponding end of the digital shaft model is calculated by measuring the DC bus voltage and using the following formula:

number

[0035] If the power converter is internally controlled by a rotating shaft model, the rotational frequency of that model is provided as an input signal to the corresponding end of the digital shaft model used by the control unit 100 .

[0036] The control unit 100 comprises a processor 130 configured to apply, e.g., by means of appropriately formulated software code instructions, an equation of rotation to the digital shaft model 12 using the pair of input signals 101, 102 as input. The equation of rotation of the machine shaft is given, for example, in this specification by equations (3) and (4) below. As a result, an active power deviation 140 is obtained. If the obtained active power deviation is positive, this means that the first power converter 01 should inject correspondingly more power into the power grid 10 to stabilize the power signal at the link to the second power converter 02. If the obtained active power deviation is negative, the first power converter should absorb corresponding power. Therefore, by means of suitable transmission means 150, a reference signal 160 for controlling the first power converter 01 is communicated to the first power converter 01. The first power converter 01 realizes the corresponding determined active power deviation 140 upon receiving the reference signal 160.

[0037] The reference signal 160 may be, by way of non-limiting example, a determined active power deviation 140p shaft The corresponding current may be determined, for example, by the following equation:

number

[0038] The reference signal 160 is calculated based on the load angle:

number

[0039] FIG. 2 illustrates a power network 2000 according to a preferred embodiment, in which a first power converter 01 is electrically interconnected with a plurality of second power converters 02, 03, ..., 0N through a power grid 10, where N is an integer. The overall functionality is similar to that of the power network described in connection with FIG. 1. The power network 2000 includes a control unit 200 configured to control the first power converter 01. The control unit uses a method that relies on a digital shaft model 12, 13, ..., 1N, also referred to as a virtual shaft, to improve supply stability in the power network. The digital shaft model emulates a rotating mechanical shaft used to transfer power between the first power converter 01 and each of the second power converters 02, 03, ..., 0N, respectively. The digital shaft models 12, 13, ..., 1N are stored in a memory element 210, which the control unit 200 can read and access at least using the parameter values ​​of the digital shaft models representing the inertia H, the damping coefficient D, and the stiffness K, as well as the corresponding equations of motion for each digital shaft model. These parameters are respectively related to the rotational frequency ω, as shown in FIG. i and ω j The model parameters are selected depending on the desired target behavior that the control method is to achieve.

[0040] Each digital shaft model 12, 13, ..., 1N uses a pair of input signals received through suitable receiving means 220 in the control unit 200, the first input signal 101 of each pair being obtained from a first power converter 01 and the second input signal 102, 103, ..., 10N of a given pair being obtained from each of the second power converters 02, 03, ..., 0N. The input signals 101, 102, 103, ..., 10N indicate the respective rotational speeds applied to the ends of the digital shaft model 12, 13, ..., 1N, which respectively mimic the electrical connections between the two power converters 01 and 02, between the two power converters 01 and 03, and between the two power converters 01 and 0N.

[0041] The control unit 200 comprises a processor 230 configured to apply, e.g., by means of appropriately formulated software code instructions, an equation of rotation to each digital shaft model 12, 13, ..., 1N by using as input a corresponding pair of input signals (101, 102), (101, 103), ..., (101, 10N). The equation of rotation of the machine shaft is given, for example, herein by equations (3) and (4) below. The partial power deviations obtained for each digital shaft model are preferably summed to obtain an active power deviation 240. Then, using suitable transmitting means 250, a reference signal 260 for controlling the first power converter 01 is communicated to the first power converter 01. The first power converter 01 realizes the corresponding determined active power deviation 240 upon receiving the reference signal 260.

[0042] FIG. 3 shows details of a power network 4000 according to a preferred embodiment, in which multiple power converters (not shown) are electrically interconnected through a power grid. The overall functionality is similar to that of the power network described in connection with FIGS. 1 and 2. The power network 2000 includes a control unit 300 configured to control multiple N power converters. The power network may therefore be considered a control system with multiple control units, each similar to the control units previously described in connection with FIGS. 1 and 2, respectively. The control system may also be implemented in a distributed manner within the power network without departing from the scope of the present invention. The control unit 300 uses a previously described method for determining an active power deviation 340(1) for controlling the power converters 01, which relies on a digital shaft model 12, 13, ... 1N, also referred to as a virtual shaft, and communicates the determined active power deviation 340(1) to the power converters 01 via a corresponding reference signal 360(1) via a transmission means 350. The control unit 300 uses the same method to control the power converters 02: the memory element 310 also includes, for each electrical connection linking the power converter 02 to a power converter 01...0N, a digital shaft model 21, 23,..., 2N, and so on. The control unit 300 determines active power deviations 340(2)..., 340(N) to be applied at the corresponding power converters 02..., 0N by applying the corresponding rotation equations and corresponding input signals 101, 102,..., 10N to improve the power signal stability of the power grid 3000. The determined corresponding active power deviations are communicated to the power converters 02..., 0N by reference signals 360(2)..., 360(N), respectively, similar to the active power deviations previously described in connection with FIG. 2 dealing with the control signal of the power converter 01.

[0043] In all embodiments, the acquisition of the input signals 101, 102, ..., 10N in the control units 100, 200, 300 may include a pre-filtering step, as previously discussed, in which measurements acquired from the corresponding power converters are converted to the rotational frequencies of the corresponding digital shaft models. The control method may further be applied solely based on a filter selecting a predetermined rotational frequency range and damping coefficient. Optionally, a discriminator may be used to select the time period during which the input signals are actually used to control the power converters.

[0044] The control unit may also be configured to select a frequency range and a damping factor for the active power deviation 140, 260, 360 to be delivered by the corresponding power converter before transmitting the reference signal 160, 260, 360. If the determined active power deviations are outside the predetermined range, they are discarded. Further optionally, a discriminator may be used to select a time period when the reference signal should be executed at the corresponding power converter.

[0045] In the following, further preferred embodiments of the present invention will be described to highlight specific features and provide further explanation, but without limiting the scope of the present invention to these embodiments.

[0046] FIG. 5 shows a conceptual representation of a set of electrical areas / sections of a power grid 4000 according to a preferred embodiment of the present invention, which may be interconnected with each other by passive ties if they have compatible voltage and frequency levels. 10 If the nominal voltages and frequencies of the electrical areas are different from each other, the interconnection of the electrical areas should be achieved using other active methods than simple ties. Several grid-connected power conversion stations 01, 02, 03, 04 are connected to such an electrical area. Such conversion stations may have several power conversion ports, which may allow interconnection through a dedicated active interconnection network, similar to that in a high-voltage direct current (HVDC) system. In such cases, the power conversion stations convert area-specific magnitudes, such as voltage and frequency, into the common electrical magnitudes of the active interconnection network to enable the exchange of active power between the areas. As previously explained, the main purpose of the control system 4500 using the control unit 400, also referred to as a VMS controller, according to the present invention is to dynamically control the exchange of real power between power conversion stations of the grid 10 by emulating the physical effect of a mechanical shaft coupling such power conversion stations, as exemplified by the link 14. The VMS controller 4500 generates a reference signal of real power deviation for each of the coupled power conversion stations using a set of state inputs 101, 102, 103, 104. It should be noted that the dynamic power exchange managed by the VMS controller to emulate the effect of a virtual mechanical shaft is possible even when there is no dedicated network interconnecting the power conversion stations. However, to achieve the real power exchange set by the VMS controller, each station must have a power system connected to one of its ports.

[0047] The power conversion stations are assumed to be power electronics-based power converters in this embodiment. The low-level controllers at each power conversion can be based on any existing control technique. The power converters used in the stations are called virtual rotating converters (VRCs) in the following if they are based on the virtual synchronous machine (VSM) technique. This is because power converters controlled by the VSM technique emulate a virtual mass rotating at a synchronous frequency. Under this technique, a conventional grid-following converter can be understood as a zero-inertia VRC.

[0048] The purpose of the VMS controller 4500, 400 is to enable dynamic control of the power exchanged between the interconnected VRCs, or equivalently, between the power converters 01, 02, 03, 04. That is, the VMS controller reinforces the inherent electrical link provided by the electrical grid to contribute to the attenuation of instantaneous angular frequency deviations from the nominal value of the coupled VRCs, despite the fact that the nominal frequency values ​​of the interconnected areas may differ from each other, as occurs when asynchronous AC areas are connected through a DC link. Thus, in the event of transient deviations in the system frequency as a result of disturbances, transient power exchanges between the virtually coupled power conversion stations will occur according to the mechanical characteristics of the virtual shaft connecting such stations, minimizing the impact of such disturbances on the dynamic stability of the system.

[0049] The VMS controller sees each power conversion station of the interconnected electrical system as a virtual rotating mass. The normalized virtual rotational frequency of each VRC coupling is used as an input signal for the VMS controller to calculate the instantaneous active power reference of such VRC. At the system level, the VMS controller thus emulates a complex virtual mechanical system in which virtual rotating masses are coupled through multiple virtual mechanical shafts. These virtual mechanical shafts would have no effect if all VRCs remained synchronized. However, as soon as a deviation occurs between the rotational frequencies of the virtual mechanical shafts, the effects of the virtual shafts emulated by the VMS controller, which depend on the mechanical parameters of such virtual shafts, will result in dynamic power exchange between the power conversion stations.

[0050] The basic operating principle of a VMS controller comes from the mechanical connection between two rotating masses. Figure 4 shows a conceptual representation of such a mechanical system when connecting the i-th rotating mass with the j-th rotating mass. In this figure, the masses are rotated at an angular frequency ω i and ω j These two masses are assumed to rotate at a constant speed. These two masses are connected by a virtual mechanical shaft, which has mechanical parameters, i.e., damping D ij , stiffness K ij , and inertia H ij These three mechanical parameters of the shaft determine the dynamic power exchange between the two virtually interconnected rotating masses, which will correspond to the instantaneous active power exchanged between the two virtually coupled power conversion stations. The dynamic behavior of this mechanical system can be described in terms of the transmitted power through the well-known equations of rotational motion, mathematically expressed in equations (3) and (4), presented further below:

[0051] 6 shows a conceptual control diagram of a virtual shaft emulator or control unit used in this embodiment interconnecting two grid-tied power conversion stations. The virtual shaft emulator receives as input signals the virtual rotational frequencies (ω i and ω j However, as will be discussed later, the input signal to this emulator does not represent the virtual rotation frequency, but rather the virtual rotation frequency (ω i ' and ω j The signal pre-processing stage may be any other state signal representing a magnitude that can be converted into a signal of the magnitude '). In fact, this signal pre-processing stage properly prepares and conditions the signals received from the virtually coupled power conversion station so that they are in a suitable form to be processed in the emulation of the rotational equations of motion block. Furthermore, the post-processing block prepares the output actuation signal of the virtual shaft emulator according to the specifications set by the power conversion station. As an example of the functionality of the post-processing block in a particular implementation of the virtual shaft emulator, the actuation signal may be generated only in case of certain transient events or only in a certain frequency range.

[0052] As indicated above, the active power deviation to be transmitted by an elastic mechanical shaft connecting two rotating masses can be calculated through the equations of rotational motion, given here as a non-limiting example in equations (3) and (4) below. The values ​​of the mechanical parameters of the virtual shaft can be changed at any time depending on the requirements of the application. However, in certain implementations of the virtual shaft emulator, the block responsible for calculating the equations of rotational motion linking two coupled power conversion stations can implement more complex equations than those shown in (3) and (4), because this block can, for example, emulate the behavior of several coupled shafts operating in parallel, whose individual parameters can be adjusted depending on the operating conditions or used only for certain frequency ranges.

[0053] The output signal of the rotary motion emulation block in Figure 6 represents the instantaneous active power deviation, Δp, to be exchanged between the two coupled power conversion stations. i and Δp j This output signal is post-processed in a dedicated post-processing block to determine the reference signal, Δp * i and Δp * j Through this block, the output signal of the virtual shaft emulator can be adapted to the requirements imposed by the internal controller of the coupled power conversion station, as well as to the limitations imposed by the technology of the power converters and by the interconnection network.

[0054] The output signal of the virtual shaft emulation block shown in FIG. 6 is provided as an additional input to the system responsible for setting up operable power flows between two coupled power conversion stations.

[0055] In a typical power grid where multiple power conversion stations can be virtually cross-coupled, a VMS controller can be realized by setting up several virtual machine shaft emulators that virtually cross-couple all the converter stations of the grid. A simplified diagram of a VMS system or control unit is shown in Figure 7, where it can be seen how the virtual rotational frequencies of the coupled power conversion stations 101, 102, -10N are all used as input signals, and how output signals representing the instantaneous active power deviation to be exchanged by each pair of coupled stations are generated in the corresponding virtual shaft emulation blocks. Such power references dynamically adjust the active power to be exchanged between the power conversion units of the grid, improving the dynamic stability of the entire grid due to the damping effect provided by the set of virtual shafts.

[0056] When the coupled power conversion station is driven by a virtual synchronous machine controller, the VMS controller simply inputs the virtual rotational frequency (ω) of the power conversion station as an input signal to the virtual shaft emulation block shown in Figure 6. i , ω j ) can easily regulate the active power exchanged between such power conversion stations. However, the VMS controller also works well when the coupled power conversion stations are controlled by any other type of conventional controller, such as a grid-following controller, which does not emulate a rotating mass using a virtual rotational frequency but estimates the frequency and / or phase angle of the AC voltage at the connection point to the grid. In such cases, the VMS controller preprocesses the estimated grid frequency at the connection point of the power conversion stations and converts it into a corresponding normalized virtual frequency. From the normalized virtual frequency, the VMS controller determines the active power to be transmitted through the virtual shaft. If the area to be coupled to the interconnection network through the power conversion station operates on direct current, the VMS controller will use a preprocessing block to estimate the equivalent virtual rotational frequency from another suitable energy state signal sent by the power conversion station, such as the DC bus voltage.

[0057] In this way, the VMS controller can regulate the dynamic active power exchanged between several areas linked to the interconnection network through various types of power conversion stations, minimizing the impact of faults and contingencies on the stability of the system.

[0058] Further preferred embodiments of the present invention are described using the exemplary drawings of FIGS.

[0059] The power system used to present the following preferred embodiment of the VMS controller (control unit) and method is shown in FIG. 8, where three AC areas are interconnected through both passive ties and a multi-terminal HVDC (MT-HVDC) network coupled through power conversion stations 01, 02, and 03, also referred to as VRC1, VRC2, and VRC3, respectively. 10 Each AC area of ​​the power system may consist of multiple synchronous generators. However, for simplicity of explanation, an aggregated model is used to simulate the AC areas of the system. Thus, Area 1 and Area 2 consist of two generation companies (GENCOs) with synchronous generators, while Area 3 has only one GENCO with synchronous generators. Parameters for the three AC areas are provided in Table I. Similarly, the aggregated load in each area is represented as being managed by a distribution company (DISCO). The AC-DC power conversion stations of the interconnected MT-HVDC network are controlled as virtual rotating converters (VRCs) by using a virtual synchronous machine (VSM) controller. Specifically, in this embodiment, a synchronous power controller (SPC) is used to control the VRC. The virtual multi-shaft (VMS) controller sends active power deviation references to the three AC-DC power conversion stations of the MT-HVDC network to damp the system frequency oscillations.

[0060] [Table 1]

[0061] Frequency control in each area of ​​an interconnected system is a crucial issue in ensuring dynamic stability during load / generation fluctuations and contingencies. System operators use different frequency control strategies with different time scales to keep area frequencies near their nominal values. Frequency regulation for a given area is generally performed by a power-frequency control strategy, typically implemented by a primary and a secondary controller. Such a control strategy provides a set of primary reference values ​​to generating stations within the area based on local frequency measurements, while a central supervisory controller provides a set of secondary reference values ​​to all generating stations within the area based on an area control error (ACE) calculation.

[0062] To simulate the dynamic frequency response of the power system shown in Figure 8, the frequency deviation of the ith area (i = 1, 2, 3) of the system is assumed to be given by the simplified equation shown in (1).

number

[0063] To further illustrate the behavior of this preferred embodiment of the VMS controller, a comparison of the dynamic frequency response in the electrical areas of the power system when a load step occurs in Area 1 is shown in Figure 8, where three interconnection cases between the electrical areas are considered: i) the electrical areas are interconnected using only passive AC ties; ii) the electrical areas are interconnected using both passive AC ties and a MT-HVDC network controlled with a conventional proportional control scheme; and iii) the electrical areas are interconnected using both passive AC ties and a MT-HVDC network controlled by a VMS controller.

[0064] In the known conventional control method, which will be used below as a baseline, a conventional proportional controller is used to control a power conversion station of an HVDC link, which interconnects two AC areas of a power system, i.e., the i-th and k-th AC areas, which are further interconnected through a passive AC tie line. The input signal to such a proportional controller is the frequency deviation (Δω) of the i-th and k-th AC areas. i and Δω k ), and the power flow deviation (ΔP tieAC,ik ) The output signal from the proportional controller is the active power reference signal (ΔP * tieAC,ik ) is used to generate the active power reference signal (ΔP * tieAC,ik ) can be expressed as:

number

[0065] It is worth noting that the small signal dynamic response of the HVDC link is DC Furthermore, it should be emphasized that the internal controller of the power conversion station considered in this preferred use case is based on SPC and therefore naturally behaves as a VRC. This particular embodiment of the local-level power conversion controller is very convenient for presenting the preferred embodiment of the system-level VMS controller, because each VRC has a virtual rotation speed that must be perfectly synchronized with the frequency of the AC area in steady state, which allows an intuitive understanding that several such power conversion stations can be virtually coupled through mechanical shafts emulated by VMS controllers.

[0066] 9 symbolically represents a part of the virtual mechanical coupling between the power conversion stations 01, 02, and 03 (VRC1, VRC2, and VRC3) of FIG. 8. In this diagram, it can be seen how the VMS controller is a multi-input, multi-output system and calculates in real time the instantaneous active power deviation to be transmitted through each of the virtual shaft-coupled power conversion stations controlled as VRCs in case of deviations in the rotational frequency of the power conversion stations. However, it is worth emphasizing that the VMS controller can also be used to regulate the power exchanged between the power conversion stations based on classical control methods, such as those used in grid-following power converters, since the virtual rotational speed can be generated, for example, from the measurement of the AC grid frequency by using the well-known phase locked loop (PLL) or from the energy status of the converter as an input variable for signal pre-processing.

[0067] In this preferred embodiment of the VMS controller, the equation of motion that determines the power transmitted from each end of the shaft connecting the ith and jth VRCs in FIG. 9, where i, j=1, 2, 3, is, for each unit:

number

[0068] Equations (3) and (4) are typical equations of motion for an elastic shaft and are used in a VMS controller to determine the power to be transmitted between two coupled VRCs. Note that the power transmitted from the i-th end of a given shaft does not necessarily match the power transmitted from the k-th end due to power losses and elastic behavior of the emulated mechanical shaft. Specifically, proper selection of the mechanical parameters of the VMS controller's virtual axis can not only help all VRCs in the network rotate at the same frequency in steady state, but also modify the system's inherent response to improve the system's dynamic stability.

[0069] The VMS controller controls the frequencies (ω1, ω2, ... ω) of all coupled VRCs (i.e., power converters) to realize a set of cross-coupled shafts in a general interconnection network with multiple power conversion stations. N ) as input signals and calculates the dynamic equations of the different virtual axes that they combine to determine the power reference to be sent to each of the VRCs. Figure 10 shows an implementation of a VMS controller for the ith power conversion station of the system of Figure 9, where i, j, k=1, 2, 3.

[0070] As shown in Figure 10, this scheme can be extended to N stations by simply adding more equations of motion to the VMS controller corresponding to additional shafts in the system. Figure 10 shows a control unit 500 for controlling a power converter 01 of a power grid 5000, considering, for example, i=1. Inputs provided to control unit 500 include input signal 101 (from power converter 01) and input signals 102,...,10N from power converters 2-N electrically interconnected with power converter 01 through the grid. A reference signal 560 is generated based on the input signals and a corresponding digital shaft model, one of which is denoted by reference 1j.

[0071] Thus, each power conversion station in the interconnected system receives a corresponding reference signal 560 from the VMS controller 500 and modifies the active power of each power conversion station, which aggregates the individual contributions of all virtual shafts connecting such power conversion station to the rest of the system. There are several techniques that can be used to select values ​​for the system's virtual shaft parameters, such as eigenvalue positioning by small-signal analysis, frequency analysis to damp harmonic oscillations, passivity-based analysis, and data-driven analysis. In any case, these parameters are modified in real time, allowing the VMS controller to adapt to the system's operating conditions at any time. It should be mentioned that delays introduced by the communication system between the VRC's local controller and the system-level VMS controller affect the dynamic response of the system and must be considered when selecting values ​​for the virtual shaft parameters.

[0072] Since the internal controller of the power conversion station in this preferred embodiment is based on an SPC, FIG. 10 shows blocks for the power loop controller, PLC (power loop controller), and voltage-controlled oscillator (VCO) of the SPC. Similarly, blocks for the droop controller and DC voltage controller of the power converter are also shown in FIG. 10. As already indicated above, the internal controller of the power conversion stations 01, 02, 03, and 04 can also be based on any other control method of the grid-tied converter other than an SPC. In such a case, the virtual rotational frequency used as input to the VMS controller would be obtained from other relevant magnitudes of the power converter, such as the grid frequency estimated by a PLL. It should be noted here that while the VMS controller shown in FIG. 10 generates an active power reference signal 560 to the power conversion station of the interconnected system, any other signal, such as an active current reference or a virtual rotational frequency reference, could also be used to control the power flow of the converter station. It should also be mentioned that the power references generated by the VMS controller must be considered with respect to limitations inherent in the power conversion station, e.g., DC bus current or voltage limits, to ensure safe system operation.

[0073] To demonstrate the effectiveness of the VMS controller over other existing solutions, some simulation results are presented below when used in the actively interconnected system shown in Figure 8.

[0074] Figures 11A-11C present the dynamic frequency response, respectively, for each of the three areas of the interconnected system of Figure 8 under the event of a 0.03 pu load connection in Area 1. To evaluate the impact of the VMS controller on the frequency response of the three interconnected areas of the system, the following interconnection cases and controllers were considered: i) Electrical alternating current areas interconnected only through passive AC ties (AC links); ii) Electrical alternating current areas interconnected through both passive AC ties and MT-HVDC networks (conventional MTDC links) with power conversion stations controlled by the conventional system-level proportional control scheme described; iii) Electrical alternating current areas interconnected through both passive AC ties and MT-HVDC networks equipped with power conversion stations controlled by the control scheme shown in Figure 10 and equipped with a system level VMS controller or control unit (the present invention) according to a preferred embodiment of the present invention.

[0075] The results shown in Figures 11A-11C demonstrate the beneficial impact of the proposed VMS controller in improving frequency stability and power distribution in interconnected electrical areas during fault events. Indeed, the VMS significantly improved the system's dynamic performance and damped frequency oscillations more effectively than using only a MT-HVDC system driven by a passive AC tie and a conventional system-level proportional controller, ultimately contributing to improving the system's dynamic stability. The figures show how the VMS controller can significantly damp transient frequency oscillations after the initial swing of the system, while the conventional proportional controller takes longer to damp such oscillations and has a larger amplitude compared to the VMS controller.

[0076] To evaluate the ability of the VMS controller to modulate the strength of dynamic support between coupled power stations, which allows each power conversion station to dynamically adjust the dynamic capacity it provides to the remaining virtually coupled stations when the system is affected by a fault, three study cases were analyzed for the interconnected system of Figure 8, and the results are shown in Figures 12A-12C.

[0077] Thus, Case A studies the dynamic power flow between the AC areas of Figure 8 when the MT-HVDC system is controlled by the conventional high-level proportional scheme described above. Simulation results for Case A are shown in Figure 12A. In the second and third case studies, i.e., Cases B and C, a VMS controller or control unit according to the present invention is used to dynamically control the power exchanged between the electrical AC areas through the power conversion stations of the MT-HVDC system. The dynamic active power share during the transient period between the power conversion stations is controlled through the use of different parameter sets for the virtual shaft of the VMS controller, as listed in Tables II and III for Use Cases B and C, respectively. Thus, Figure 12B shows how Areas 1 and 2 have a larger active power share than Area 3 when the parameter set for Case B is used. Similarly, Figure 12C shows how Areas 1 and 3 have a larger active power share than Area 2 when the parameter set for Case C is used. These results demonstrate the effectiveness of the VMS controller in adjusting the dynamic active power share between different interconnected AC areas simply by changing the parameter set of the virtual shaft that connects the generating stations of the system.

[0078] [Table 2]

[0079] [Table 3]

[0080] Those skilled in the art will be able, from this specification and the accompanying figures, to create computer program code to perform the described functionality without undue burden and the exercise of inventive skill.

[0081] It will be understood that the detailed description of certain preferred embodiments is given by way of example only, since various changes and modifications within the scope of the present invention will be apparent to those skilled in the art, the scope of protection being defined by the following set of claims.

Claims

1. 1. A control method for improving security of supply in a power network (1000, 2000, 3000, 4000, 5000) electrically connecting a first power converter (01) to at least one second power converter (02, 03, . . . 0N) through a power grid (10), comprising: i) for each pair of power converters consisting of a first power converter (01) and one of the second power converters (02, 03, ..., 0N), providing a digital shaft model (12, 13, ..., 1N) of a mechanical shaft in a memory element (110, 210, 310), said digital shaft model including a damping coefficient, inertia and stiffness of the mechanical shaft and emulating an electrical coupling between said pair of power converters through said power grid; ii) obtaining in the control unit (100, 200, 300, 400) pairs of input signals (101, 102; 101, 103; ...; 101, 10N) from each of said pairs of power converters, each pair of input signals being indicative of a rotational speed to be applied to an end of a corresponding digital shaft model; iii) determining the active power deviation (140, 240, 340) in the control unit based on the calculated power transfer for each pair of power converters by applying a rotational equation to a corresponding digital shaft model using a corresponding pair of input signals; iv) in the control unit (100, 200, 300, 400), communicating to the first power converter (01) a reference signal (160, 260, 360) that enables the first power converter (01) to supply the active power deviation (140, 240, 340) to the power grid or absorb the active power deviation (140, 240, 340) from the power grid depending on the positivity of the active power deviation. A control method comprising:

2. 2. The control method of claim 1, wherein the step of obtaining a pair of input signals includes obtaining at least one signal indicative of a rotational speed from at least one second power converter (02), the second power converter emulating a mass rotating at the rotational speed.

3. 3. The control method of claim 1 or 2, wherein the step of obtaining a pair of input signals includes obtaining at least one measurement of AC grid frequency at a connection point of the at least one second power converter (02).

4. 4. The control method of claim 1, wherein the step of obtaining a pair of input signals comprises obtaining at least one measurement of a DC voltage of a DC bus of at least one second power converter (802).

5. 5. The control method of claim 1, wherein the reference signal (160, 260, 360) comprises an indication of a current strength to be injected into the power grid by the first power converter.

6. 5. The control method of claim 1, wherein a first power converter (01) is controlled as a rotating mass, and the reference signal (160, 260, 360) comprises an indication of a load angle of the power converter.

7. 7. A control method according to claim 1, wherein either the inertia or the stiffness, or both the inertia and the stiffness, of at least one digital shaft model are set to zero.

8. a memory element (110, 210, 310) in which digital shaft models (11, 13, ..., 1N) are stored, each digital shaft model including a damping coefficient, an inertia, and a stiffness of a mechanical shaft configured to emulate an electrical coupling between a pair of power converters (01, 02, ..., 0N) electrically connected through a power grid (10), each pair of power converters consisting of the same first power converter and one second power converter; receiving means (120, 220, 320) configured to receive a pair of input signals from each of said pair of power converters, each pair of input signals indicative of a rotational speed to be applied to an end of a corresponding digital shaft model; A calculation means (130, 230, 330) comprising: determining an active power deviation (140, 240, 340) based on the calculated power transfer for each pair of power converters by applying an equation of rotation to the corresponding digital shaft model using the corresponding acquired pair of input signals; The first power converter generates a reference signal (160, 260, 360) that enables the first power converter to supply the determined active power deviation to the power grid or absorb the determined active power deviation from the power grid depending on the positivity of the determined active power deviation. a computing means configured to transmitting means (150, 250, 350) configured to communicate said reference signal to a first power converter (01); A control unit (100, 200, 300, 400) comprising:

9. 10. A control system (3500, 4500) comprising a plurality of control units (100, 200, 300, 400) according to claim 8, each control unit being configured to determine the active power deviation of one power converter of a plurality of power converters (01, 02, ..., 0N) electrically interconnected through a power grid (10).

10. A power network (1000, 2000, 3000, 4000, 5000) in which a plurality of power converters (01, 02, ..., 0N) are interconnected through a power grid (10), and which comprises at least one control unit (100, 200, 300, 400) according to claim 8 for controlling at least one power converter.

11. 11. The power grid of claim 10, comprising the control system of claim 9.

12. A computer program comprising computer readable code means which, when executed on a computer system, causes the computer system to carry out the method of claim 1.

13. 13. A computer program according to claim 12, further comprising computer readable code means which, when executed on a computer system, causes the computer system to carry out the method of any one of claims 2 to 7.

14. A computer program product comprising a computer readable medium having stored thereon a computer program according to claim 12 or 13.

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