A Coordination Method for DC Microgrid Against Hybrid Cyber-Attacks

Abstract

The present invention discloses a coordinated control method for a DC microgrid to resist hybrid network attacks, belonging to the field of regulation of DC microgrids. The present invention designs an elastic controller based on attack adaptive adjustment with a fusion error composed of a current error signal and a voltage error signal, realizing that the DC microgrid can still achieve the regulation goal when suffering from hybrid attacks. Compared with the existing control methods, this method can cope with both FDI and DoS attacks simultaneously, and the intensity of the FDI attack can be infinite, improving the robustness of the DC microgrid system. Since this method can still make the system operate stably without attacks, no additional attack detection mechanism is required, that is, the DC microgrid can operate normally whether it suffers from network attacks or not after deploying this control method.

Description

Technical Field

[0001] The present invention belongs to the field of control of DC microgrids, and more specifically, relates to a coordinated method for DC microgrids to resist hybrid cyberattacks. Background Art

[0002] A microgrid combines distributed generators, energy storage units, electric vehicles and loads, and plays an important role in future distributed energy systems. Due to the simple control of DC microgrids, low power loss, and more direct and effective deployment of distributed generation without considering reactive power and harmonics, DC microgrids have received extensive attention. In order to achieve voltage regulation and precise current distribution, decentralized droop control was first used to regulate DC microgrids. However, due to the inherent impedance of the line and its own droop factor, there is a static error between the bus voltage and current ratio and the expected value. With the development of communication technology, distributed secondary control has been widely applied, which has better flexibility, scalability and computing performance.

[0003] However, the communication links of distributed control methods are at risk of being cyberattacked, and the unhealthy states caused by the attacks will gradually spread to the entire DC microgrid network through the communication links, resulting in unstable control performance and even paralysis of the entire DC microgrid system. Therefore, designing a coordinated method for DC microgrids to resist hybrid cyberattacks, so that the system can still achieve the control objectives when under attack, is of great significance for the safe operation of DC microgrids. Summary of the Invention

[0004] In view of the above defects or improvement requirements of the prior art, the present invention provides a coordinated method for DC microgrids to resist hybrid cyberattacks, aiming to solve the problem that the existing control methods cannot achieve precise power supply ratio and bus voltage regulation when simultaneously under DoS and FDI attacks.

[0005] To achieve the above object, according to one aspect of the present invention, a coordinated control method for DC microgrids to resist hybrid cyberattacks is provided, including:

[0006] S1. Establish a hierarchical control model for each distributed power source and establish a communication network between the controllers; the hierarchical control model consists of double-loop PI control, droop control and secondary control;

[0007] S2. Interact the output current I i (t) of the local converter i and the bus voltage V b (t) with the neighbor current information I j (t) and the bus voltage reference value V * respectively to obtain the current error e I，i (t) and the voltage error eV (t);

[0008] S3. Linearly combine the voltage error signal and the current error signal to obtain a fused error term ψ i (t);

[0009] S4. Establish the internal state signal δ of the controller in the presence of a hybrid of FDI attacks and DoS attacks i (t);

[0010] S5. Construct an adaptive mitigation term from the integral of the fused error to compensate for the internal state signal δ of the controller under cyber attacks to mitigate FDI attacks; Integrate the compensated internal state δ' of the controller i (t) to obtain the final secondary control signal u i (t); i (t);

[0011] S6. Repeat steps S2 - S5 until the bus voltage regulation is completed and current sharing is achieved.

[0012] Furthermore, the internal state signal δ of the controller in the presence of a hybrid of FDI attacks and DoS attacks i (t) is:

[0013]

[0014] δ i (t) is the internal state of the controller, p(t) is the DoS attack signal, indicating the FDI attack suffered by the system.

[0015] Furthermore, Π D represents the set of times when the DoS attack acts, Π N represents the set of times when there is no DoS attack.

[0016] Furthermore, the internal state signal of the controller under cyber attacks after being compensated by the adaptive mitigation term is:

[0017]

[0018] is the adaptive mitigation term,

[0019] Furthermore, the convergence boundary expressions for the bus voltage and current sharing errors are:

[0020]

[0021] γ1, γ2 are parameters related to the eigenvalues of matrices P and Q, λ min$(P)$ is the minimum eigenvalue of matrix $P$. Matrices $P$ and $Q$ are positive definite matrices that satisfy the system Lyapunov equation and are related to the topological structure of the system.

[0022] On the other hand, the present invention provides a coordinated controller for a DC microgrid against hybrid network attacks, including:

[0023] A hierarchical control model construction module that constructs a hierarchical control model for each distributed power source and establishes a communication network between the controllers; the hierarchical control model consists of double-loop PI control, droop control, and secondary control;

[0024] An error acquisition module that takes the output current $I$ i (t) and the bus voltage $V$ b (t) of the local converter $i$, and respectively interacts with the neighbor current information $I$ j (t) and the bus voltage reference value $V$ * to obtain the current error $e$ I，i (t) and the voltage error $e$ V (t);

[0025] An error fusion module that linearly combines the voltage error signal and the current error signal to obtain a fused error term $\psi$ i (t);

[0026] A network attack signal establishment module that establishes the internal state signal $\delta$ i (t) of the controller in the presence of both FDI attacks and DoS attacks;

[0027] An error compensation module that forms an adaptive mitigation term from the integral of the fused error to compensate for the internal state signal $\delta$ i (t) of the controller under network attack to mitigate the FDI attack; integrates the compensated internal state $\delta'$ i (t) of the controller to obtain the final secondary control signal $u$ i (t);

[0028] An iterative control module that is used to repeatedly execute the functions of the error acquisition module, the error fusion module, the network attack signal establishment module, and the error compensation module until the bus voltage regulation is completed and current sharing is achieved.

[0029] Furthermore, the internal state signal of the controller under network attack after being compensated by the adaptive mitigation term is:

[0030]

[0031] is the adaptive mitigation term, The fused error term $\psi$ i (t); $p(t)$ is the DoS attack signal, Indicates the FDI attack suffered by the system.

[0032] Generally speaking, compared with the prior art, the above technical solutions conceived by the present invention can achieve the following beneficial effects.

[0033] The existing methods are only effective for single FDI attacks and DoS attacks, and cannot resist the hybrid attacks composed of the two attacks. The method of the present invention can effectively alleviate the attacks of the hybrid network on the DC microgrid, so that the DC microgrid can still achieve the regulation of the bus voltage and the accurate power supply ratio even when it is jointly attacked by an infinite FDI attack and a DoS attack of a certain duration; and it can operate normally without network attacks, without additional attack detection technologies, the control process is simple and does not require any global information, and has the advantages of plug and play. Description of the Drawings

[0034] Figure 1 Is the structural diagram of the i-th DC / DC converter and its local hierarchical controller;

[0035] Figure 2 Is the control block diagram of the method proposed by the present invention. Detailed Embodiments

[0036] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0037] Glossary of Terms in the Present Invention

[0038] Network attack: An attack on a communication network;

[0039] Hybrid network attack: The hybrid network attack discussed in this patent includes false data injection (FDI) attacks and denial of service attacks (DoS);

[0040] False data injection attack (FDI): Maliciously tampering with the transmitted data in a communication network;

[0041] Denial of service attack (DoS): Maliciously occupying a communication network and unable to transmit information normally;

[0042] DC microgrid: A small power system that integrates renewable energy, energy storage, and loads through a DC bus and power electronic devices (converters);

[0043] Coordination control: The controller interacts with information in the communication network. By coordinating multiple power electronic devices, two control objectives of voltage regulation and power distribution of the entire microgrid system are achieved;

[0044] Voltage regulation: Adjust the DC bus voltage to be stable at the rated value;

[0045] Current distribution: Adjust the distributed source end in the DC microgrid;

[0046] The method steps of the present invention will be described in detail below in conjunction with the accompanying drawings, including:

[0047] (1) For each distributed power source, a hierarchical control model is designed. The hierarchical control model consists of double-loop PI control, droop control, and secondary control. The secondary controller obtains the secondary control signal through the voltage-current feedback loop after collecting the neighbor information and bus voltage information, and participates in the droop control to obtain the voltage reference signal. The double PI closed-loop control acts on the DC / DC converter through the output PWM signal, so that the output voltage of the DC / DC converter tracks the reference signal on a fast time scale;

[0048] Specifically, as Figure 1 shown. I i and V i respectively represent the output current and output voltage of the converter i, R i and X i are line impedances, and the bus voltage is represented by V b . The secondary control signal consists of a feedback loop formed by the bus voltage information and the current information given by the neighbor. As a compensation signal for the droop control, it forms a reference voltage together with the specified voltage: where, V * is the rated voltage of the DC microgrid, m i is the droop control coefficient, and u i (t) is the secondary control signal. Subsequently, it is provided to the double-loop PI controller to generate a PWM signal to act on the controller to complete the control.

[0049] First, according to Kirchhoff's theorem, the power flow equation of the DC microgrid is obtained as R i I i =V i -V b . The double-loop PI control makes the output voltage V i track the reference voltage on a fast time scale that is At this time, the expression of the bus voltage with respect to the rated value can be obtained: V b (t)=V * -(m i +R i )I i (t)+ui (t).

[0050] To achieve the stable and economic operation of the DC microgrid load, it is usually necessary to meet two control objectives: 1. V b (t) = V * , 2. d i and d j represent the proportionality coefficients of the expected output currents of the i-th and j-th respectively; i and j represent the i-th and j-th DC / DC converters respectively;

[0051] (2) Establish a communication network between the controllers. The communication network is represented by graph theory G = {V, ε}, where V represents each distributed controller node and ε represents the edges connecting each node. N i represents the neighbor set of the i-th DC / DC converter. Each controller obtains the local output current I i (t) and V b (t) through sensors, and interacts with the neighbor current information I j (t) and the bus voltage reference value V * to obtain the current error e I，i (t) and the voltage error e V (t);

[0052] For the control objectives in (1), the error variables are defined as follows:

[0053] e V (t) = V b (t) - V * ,

[0054]

[0055] where e V represents the voltage error between the bus voltage and the rated voltage value, and e I，i represents the current distribution error of the output current of the i-th converter. N i represents the neighbor set of the i-th DC / DC converter.

[0056] (3) Due to the constraints of the power flow in the DC microgrid, the voltage and current are coupled. To be able to analyze the voltage and current errors simultaneously, a linear fusion error ψ i (t) = α i e V (t) + βe I，i (t) is established, where α i and β represent the weight coefficients of the current error and the voltage error respectively. Subsequently, the purpose of controlling the bus voltage and the output current can be achieved by analyzing the fusion error.

[0057] (4) As Figure 2 shown, establish the FDI attack signal, which is used to represent the FDI attack suffered by the system. Its magnitude is unknown, but the rate of change is finite. The FDI attack usually acts on the tampering of valid information. The internal state signal with the FDI attack added evolves into

[0058] (5) Establish the DoS attack signal. The DoS attack affects the performance of the DC microgrid by blocking the network channel. It occurs when each converter communicates with its neighbor nodes and can be described by a switching function:

[0059]

[0060] where Π D represents the set of times when the DoS attack acts, and Π N represents the set of times when there is no DoS attack. When p(t) is 0, it means that the DoS attack causes the communication network to be blocked, and at this time, the FDI attack cannot act on the controller either; when p(t) is 1, there is no DoS attack, the communication is normal, and the FDI attack can affect the controller at this time. Based on this, the internal state of the controller evolves into

[0061] (6) Design an adaptive mitigation term based on the internal signals in the presence of the hybrid attack. This term is composed of the integral of the fusion error, that is At this time, the internal state of the controller is updated to Due to the presence of network attacks, the local data obtained by the sensor at the next sampling moment will deviate from the true data, which will further increase the fusion error. At this time, the mitigation term

[0062] can adaptively reduce the error, and finally make the bus voltage and current equalization error reach a consistent ultimate boundedness; where γ1 and γ2 are parameters related to the eigenvalues of matrices P and Q, and λ min (P) is the minimum eigenvalue of matrix P. Matrices P and Q are positive definite matrices that satisfy the system Lyapunov equation, which is related to the topological structure of the system. By appropriately increasing the gain coefficients α i and β of the fusion error, γ1 can be reduced, thereby reducing this convergence boundary.

[0063] (7) To eliminate chattering and reduce deviation, integrate the internal state δ i (t) of the controller to obtain the final secondary control signal u i (t);

[0064] (8) Repeat steps (2)-(7) based on the above secondary control signal until the bus voltage regulation is completed and current sharing is achieved.

[0065] In the present invention, an elastic controller based on attack adaptive adjustment is designed through a fusion error composed of a current error signal and a voltage error signal, enabling the DC microgrid to still achieve the regulation goal when under hybrid attacks. From the perspectives of control and system, the FDI attack causes continuous changes in the state of the closed-loop system, and the DoS attack makes the closed-loop system include a switching process. The combined hybrid switched closed-loop system brings challenges and difficulties to stability analysis due to the combination of state changes and switching. Compared with existing control methods, this method can cope with both FDI and DoS attacks simultaneously, and the intensity of the FDI attack can be infinite, enhancing the robustness of the DC microgrid system. Since this method can still keep the system operating stably without attacks, no additional attack detection mechanism is required, that is, the DC microgrid can operate normally whether it is under cyber attacks after deploying this control method.

[0066] It is easy for those skilled in the art to understand that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A coordinated control method for a DC microgrid against hybrid network attacks, characterized in that, Including: S1. Establish a hierarchical control model for each distributed power source and establish a communication network between each controller; The hierarchical control model consists of double closed-loop PI control, droop control and secondary control; S2. Output current I of local converter i i (t) and bus voltage V b (t) are respectively interacted with neighbor current information I j (t) and bus voltage reference value V * to obtain current error e I,i (t) and voltage error e V (t); S3. Linearly combine the voltage error signal and the current error signal to obtain a fused error term ψ i (t); S4. Establish the internal state signal δ of the controller in the co - existence of FDI attack and DoS attack i (t); δ i (t) is the internal state of the controller, and p(t) is the DoS attack signal, indicating the FDI attack suffered by the system; S5. An adaptive relaxation term is formed by integrating the fusion error to compensate for the internal state signal δ i (t) of the controller under cyber - attack to mitigate the FDI attack. After being compensated by the adaptive relaxation term, the internal state signal of the controller under cyber - attack is as follows: is an adaptive relaxation term, Integrate the compensated internal state δ’ i (t) of the controller to obtain the final secondary control signal u i (t); S6. Repeat steps S2 - S5 until the bus voltage regulation is completed and current sharing is achieved.

2. The coordinated control method of a DC microgrid against hybrid network attacks according to claim 1, characterized in that ∏ D represents the set of times when a DoS attack takes effect, ∏ N represents the set of times when no DoS attack takes effect.

3. A coordinated control method for a DC microgrid against hybrid network attacks according to claim 1, characterized in that, The convergence boundary expressions of the bus voltage and current sharing error are: γ1 and γ2 are parameters related to the eigenvalues of matrices P and Q, λ min (P) is the minimum eigenvalue of matrix P. Matrices P and Q are positive definite matrices that satisfy the system Lyapunov equation and are related to the topological structure of the system.

4. A coordinated controller for a DC microgrid against hybrid network attacks, characterized in that, Including: A hierarchical control model construction module that establishes a hierarchical control model for each distributed power source and establishes a communication network between each controller; The hierarchical control model consists of double closed-loop PI control, droop control and secondary control; Error acquisition module, which takes the output current I of the local converter i i (t) and the bus voltage V b (t), and interacts with the neighbor current information I j (t) and the bus voltage reference value V * respectively, to obtain the current error e I,i (t) and the voltage error e V (t); The error fusion module linearly combines the voltage error signal and the current error signal to obtain a fused error term ψ i (t); Network attack signal establishment module, establishing the internal state signal δ of the controller where FDI attack and DoS attack coexist i (t); δ i (t) is the internal state of the controller, and p(t) is the DoS attack signal. represents the FDI attack suffered by the system; Error compensation module, which consists of the integral of the fused error to form an adaptive relaxation term, compensates for the internal state signal δ i (t) of the controller under cyber-attack to mitigate the FDI attack. After being compensated by the adaptive relaxation term, the internal state signal of the controller under cyber-attack is as follows: is an adaptive relaxation term, Integrate the compensated internal state δ’ i (t) of the controller to obtain the final secondary control signal u i (t); An iterative control module that is used to repeatedly execute the functions of the error acquisition module, error fusion module, network attack signal establishment module and error compensation module until the bus voltage regulation is completed and current sharing is achieved.

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