Method, system and storage medium for controlling transient overvoltage of a sending system

Abstract

The application discloses a control method for transient overvoltage of a sending end system, and comprises the following steps: obtaining the transient overvoltage of the sending end system; obtaining the relationship between the transient overvoltage of the sending end system and the equivalent impedance of the sending end system; predicting the equivalent impedance of the sending end system in a steady state when the transient overvoltage of the sending end system occurs through a pre-trained convolutional neural network; and adjusting the transient overvoltage of the sending end system according to the relationship between the transient overvoltage of the sending end system and the equivalent impedance of the sending end system and the equivalent impedance of the sending end system in the steady state when the transient overvoltage occurs. The application can achieve a good inhibitory effect on the transient overvoltage of the sending end system.

Description

Technical Field

[0001] This invention relates to the field of wireless communication encryption technology, and particularly to a method, system, and storage medium for controlling transient overvoltage in a transmitting system. Background Technology

[0002] While rapid economic development has brought a better life to people, it has also led to the depletion of fossil fuels and increasingly serious environmental pollution problems. Various countries and regions are advocating for energy transition and energy structure reform. Among various new energy power generation systems, wind power has received widespread attention and vigorous promotion globally due to its abundant resources, clean and efficient operation, and good environmental benefits.

[0003] my country's wind power is mainly concentrated in the southeastern coastal areas, northeastern and northwestern regions. However, the overall local electricity consumption in these areas is low, resulting in the inability to absorb the power resources locally. Excess wind power is generally transmitted to distant load centers via high-voltage direct current (HVDC) transmission systems. However, when HVDC transmission experiences commutation failures or blocking faults, the combined effect of wind turbines and HVDC transmission exacerbates the transient overvoltage problem in the sending-end system. As the penetration rate of wind farms within the sending-end system increases, the transient overvoltage level in the sending-end system will also increase under the action of wind turbines after a fault. When the voltage exceeds a certain limit, it will cause the wind turbines within the sending-end system to disconnect from the grid, resulting in transient voltage instability in the sending-end system. Summary of the Invention

[0004] To address the problems existing in the prior art, the present invention provides a method, system, and storage medium for controlling transient overvoltages in a sending-end system, which can suppress transient overvoltages in the sending-end system.

[0005] The technical problem to be solved by the present invention is achieved through the following technical solution:

[0006] In a first aspect, the present invention aims to provide a method for controlling transient overvoltage in a sending-end system, comprising:

[0007] Acquire transient overvoltages in the sending-end system;

[0008] Obtain the relationship between transient overvoltage of the sending-end system and equivalent impedance of the sending-end system;

[0009] Predict the equivalent impedance of the sending-end system in steady-state operation when a transient overvoltage occurs using a pre-trained convolutional neural network.

[0010] The transient overvoltage of the sending system is adjusted based on the relationship between the transient overvoltage of the sending system and the equivalent impedance of the sending system in steady-state operation when the transient overvoltage occurs.

[0011] In conjunction with the first aspect, further, obtaining the relationship between the transient overvoltage of the sending-end system and the equivalent impedance of the sending-end system includes:

[0012] The relationship between the transient voltage of the sending-end system and the equivalent impedance of the sending-end system can be obtained according to equation (1).

[0013]

[0014] Among them, Q cp X is the single-phase reactive power compensation value of the AC filter. sw For equivalent reactance, U s For the equivalent potential, Q r 'U' represents the minimum reactive power consumed by the rectifier-side converter station during a commutation failure. p For the transient overvoltage of the sending-end bus, U p This refers to the voltage supplied to the busbar during steady-state operation.

[0015] In conjunction with the first aspect, further, the prediction of the equivalent impedance of the sending-end system when a transient overvoltage occurs, assuming the system is operating in steady state, using a pre-trained convolutional neural network includes:

[0016] The feature quantities of electrical data during steady-state operation of the power grid node are input into a pre-trained convolutional neural network to obtain the equivalent impedance of the sending-end system during steady-state operation.

[0017] In conjunction with the first aspect, the characteristic quantities of the electrical data during the steady-state operation of the power grid node are further shown in the following formula:

[0018]

[0019]

[0020] e tb,m =[U tb,m ,θ tb,m ,P tb,m Q tb,m (4)

[0021] Where E represents the characteristic quantity of electrical data, E n Let e ​​represent the nth selected sample. tb,m This indicates at time point t b The eigenvalues ​​of the next node m; U tb,m θ tb,m P tb,m Q tb,m Representing time point t respectively b Voltage, phase angle, active power, and reactive power of the load at node m; w ta Representing time point t a The magnitude of the wind speed injected downwind.

[0022] In conjunction with the first aspect, further, the adjustment of the transient overvoltage of the sending-end system includes:

[0023] The equivalent impedance of the sending system when the transient overvoltage occurs is obtained based on the relationship between the transient overvoltage of the sending system and the equivalent impedance of the sending system. Then, the equivalent impedance is adjusted by a controllable series compensation device to the equivalent impedance of the sending system in steady-state operation when the transient overvoltage occurs, so that the transient overvoltage is suppressed and the sending system is restored to steady-state operation.

[0024] Secondly, a control system for transient overvoltage in a sending-end system is provided, characterized in that it includes:

[0025] Transient overvoltage acquisition module, used to acquire transient overvoltages in the sending-end system;

[0026] The relationship acquisition module is used to acquire the relationship between the transient overvoltage of the sending-end system and the equivalent impedance of the sending-end system;

[0027] The steady-state impedance acquisition module is used to predict the equivalent impedance of the sending-end system in steady-state operation when a transient overvoltage occurs, using a pre-trained convolutional neural network.

[0028] The transient overvoltage adjustment module is used to adjust the transient overvoltage of the sending system based on the relationship between the transient overvoltage of the sending system and the equivalent impedance of the sending system, as well as the equivalent impedance of the sending system in steady-state operation when the transient overvoltage occurs.

[0029] In conjunction with the second aspect, the operations performed by the relationship acquisition module further include:

[0030] The relationship between the transient voltage of the sending-end system and the equivalent impedance of the sending-end system can be obtained according to equation (1).

[0031]

[0032] Among them, Q cp X is the single-phase reactive power compensation value of the AC filter. sw For equivalent reactance, U s For the equivalent potential, Q r 'U' represents the minimum reactive power consumed by the rectifier-side converter station during a commutation failure. p For the transient overvoltage of the sending-end bus, U p This refers to the voltage supplied to the busbar during steady-state operation.

[0033] In conjunction with the second aspect, further, the operation performed by the steady-state impedance acquisition module includes: inputting the feature quantities of electrical data of the power grid node during steady-state operation into a pre-trained convolutional neural network to obtain the equivalent impedance of the sending-end system during steady-state operation.

[0034] In conjunction with the second aspect, the operations performed by the transient overvoltage adjustment module further include:

[0035] The equivalent impedance of the sending system when the transient overvoltage occurs is obtained based on the relationship between the transient overvoltage of the sending system and the equivalent impedance of the sending system. Then, the equivalent impedance is adjusted by a controllable series compensation device to the equivalent impedance of the sending system in steady-state operation when the transient overvoltage occurs, so that the transient overvoltage is suppressed and the sending system is restored to steady-state operation.

[0036] Thirdly, a computer-readable storage medium is provided having a computer program stored thereon that, when executed by a processor, implements the steps of the method described in any of the first aspects.

[0037] The beneficial effects of this invention are as follows: Based on the equivalent circuit of the sending-end system, this invention finds the relationship between the transient overvoltage and the equivalent impedance of the sending-end system under the fault of commutation failure. Then, through the neural network, it finds the equivalent impedance that should be corresponding to the system operating in steady state when the fault occurs. Finally, by adjusting the equivalent impedance through a controllable series compensation device, the transient overvoltage of the system is indirectly adjusted so that the system can return to steady state, and the transient overvoltage of the system is well suppressed. Attached Figure Description

[0038] Figure 1 This is a flowchart of the present invention;

[0039] Figure 2 This is the simplified equivalent circuit diagram of the sending-end system in this invention;

[0040] Figure 3 This is a schematic diagram illustrating the control principle of impedance control via a controllable series compensation device in this invention.

[0041] Figure 4 This is a schematic diagram of a 4-machine, 11-node model in an embodiment of the present invention;

[0042] Figure 5 This refers to the transient voltage of the sending-end bus when the wind power penetration rate is 30% in this invention.

[0043] Figure 6 This is a comparison diagram of the transient voltage of the sending-end bus before and after using the control method of the present invention. Detailed Implementation

[0044] To further describe the technical features and effects of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0045] Example 1

[0046] like Figure 1 As shown, a method for controlling transient overvoltage in a sending-end system includes the following steps:

[0047] Step 1: Obtain the transient overvoltage of the sending-end system;

[0048] Step 2: Obtain the relationship between the transient overvoltage of the sending-end system and the equivalent impedance of the sending-end system;

[0049] To simplify the calculation and analysis process, the sending-end system has been partially simplified, such as... Figure 2 As shown.

[0050] Among them, U s X is the equivalent potential. sw For the equivalent impedance, U p Q is the voltage of the sending-end bus during steady-state operation. cp P is the single-phase reactive power compensation amount for the AC filter. ac Q represents the active power of a single-phase line. ac This refers to the reactive power of a single-phase line.

[0051] Generally, during system operation, the reactive power compensated by the rectifier-side converter is approximately equal to the reactive power consumed by the converter, and the voltage and equivalent potential of the sending-end bus are equal. However, in actual systems, when the transmitted power is too large, the reactive power consumed by the system's equivalent reactance will also be significant. This reactive power loss cannot be ignored, as it will also cause a substantial voltage drop.

[0052] The voltage of the sending-end bus during steady-state operation is:

[0053]

[0054] The reactive power consumed by the equivalent impedance is:

[0055] By applying a forward-backward substitution method to equation (2), the reactive power output of the equivalent power source can be obtained as follows:

[0056]

[0057] During the commutation failure process, the transient voltage of the sending-end system is "low first and then high." Therefore, the reactive power compensation in the converter station is no longer a constant, but a quantity that changes with the voltage. In other words, the transient characteristics of reactive power compensation must be considered. Based on the above characteristics, the reactive power exchanged between the AC and DC systems can be derived as shown in equation (4):

[0058]

[0059] Among them, Q r ' is the minimum reactive power consumed by the rectifier-side converter station during commutation failure; Q' is the reactive power exchanged between the AC and DC systems (DC to AC transmission is positive); U' pThis refers to the transient overvoltage of the sending-end bus. In actual systems, the reactive power consumed by the converter during commutation failure is constantly changing. Here, to simplify the calculation, only the minimum reactive power consumed is considered.

[0060] The transient voltage of the converter bus in the sending-end system is:

[0061]

[0062] in,

[0063] In actual fault processes, the active power exchange between AC and DC systems is very small and can be ignored. Therefore, in the calculation, the influence of the voltage drop transverse component can be ignored, and only the longitudinal component is considered. Further solving the equation yields:

[0064]

[0065] Equation (6) shows that the transient overvoltage is related to the equivalent reactance X. sw Based on equation (6), it was found that the amplitude of transient overvoltage is positively correlated with the equivalent impedance of the system. This is because when the equivalent impedance of the system decreases, the short-circuit ratio of the system increases. Therefore, with the same reactive power surplus, the larger the short-circuit ratio, the lower the overvoltage amplitude of the system. Thus, the transient overvoltage of the sending-end system can be further suppressed by reducing the system impedance.

[0066] Step 3: Predict the equivalent impedance of the sending-end system in steady-state operation when a transient overvoltage occurs using a pre-trained convolutional neural network.

[0067] Based on the relationship between transient overvoltage and system equivalent impedance obtained in step two, input variables are formed using each node of the power grid as a unit. The electrical data of the power grid nodes in steady state are concatenated in chronological order to obtain a matrix that can characterize the state of the power grid. During the concatenation process of the power grid nodes, the CNN (Convolutional Neural Network) can learn the operating state at different time points, and the concatenation of each node can also characterize the topological relationship of the power grid nodes to a certain extent.

[0068] The characteristic quantities of electrical data are shown in the following formula:

[0069]

[0070]

[0071] e tb,m =[U tb,m ,θ tb,m ,P tb,m Q tb,m (9)

[0072] Where E represents the characteristic quantity of electrical data, En Let e ​​represent the nth selected sample. tb,m This indicates at time point t b The eigenvalues ​​of the next node m; U tb,m θ tb,m P tb,m Q tb,m Representing time point t respectively b Voltage, phase angle, active power, and reactive power of the load at node m; w ta Representing time point t a The magnitude of the wind speed injected downwind.

[0073] This invention analyzes power system data obtained from a wide-area measurement system and constructs an impedance reference value X for a controllable series compensation system by training a CNN. ref The input matrix E is formed by the variables of each node in the system; therefore, the input to the training set is e. tb,m The set E is formed. Transient overvoltage estimation is essentially finding the mapping E→U. After calculating the required impedance reference value during the fault, a data-driven online calculation method is further introduced into the control block diagram of the original controllable series compensation. The controllable series compensation control principle diagram after introducing online calculation with a convolutional neural network is as follows. Figure 3 As shown. Figure 3 As can be seen, the control method proposed in this invention introduces a convolutional neural network to calculate and modify the impedance reference value online, based on the original controllable series compensation impedance control principle diagram. When a system fault occurs, the convolutional neural network will first calculate the required impedance reference value, so that the controllable series compensation capacitor can modify the system impedance according to the reference value calculated by the convolutional neural network, thereby suppressing the transient overvoltage of the sending end system.

[0074] Step 4: Adjust the transient overvoltage of the sending-end system;

[0075] Based on the equivalent impedance of the sending-end system when the transient overvoltage occurs, obtained in step three, if the sending-end system is in steady-state operation, and the relationship between the transient overvoltage and the equivalent impedance obtained in step two, the equivalent impedance is adjusted to the equivalent impedance of the sending-end system in steady-state operation when the transient overvoltage occurs, through a controllable series compensation device (generally a variable capacitor), so that the transient overvoltage is suppressed and the sending-end system is restored to steady-state operation.

[0076] This invention uses the electromagnetic transient simulation software EMTP-RV to modify the IEEE 4-machine 11-node model as follows: Figure 4As shown. The accuracy of the proposed calculation method for transient overvoltage in the wind-fire bundled sending-end system and the feasibility of the proposed control method are verified by simulation. The wind power penetration rate inside the sending-end system is set to 30%, and the transient overvoltage amplitude of the sending-end system is observed after a three-phase ground fault occurs in the receiving-end AC system. The fault is set to occur at 30s in the inverter side of the conventional DC transmission. The fault is cleared after 0.2s. 0.1s after the fault occurs, a single-pole blocking fault occurs in the DC transmission line, and the reactive power compensation device in the converter station is cleared 200ms after the blocking fault. First, the transient overvoltage amplitude of the sending-end system when the wind power penetration rate is 30% is calculated to be 1.19pu according to Equation (6). The transient overvoltage amplitude of the sending-end system at fault is observed by fault simulation on the simulation software. Figure 5 As shown, by comparing the magnitudes of transient overvoltages obtained from mechanism calculations and simulation examples, it can be seen that the error can be controlled within 5%. Therefore, the proposed mechanism calculation formula can be applied to the estimation of transient overvoltages in the wind-fire bundled power supply system.

[0077] To verify the feasibility of the proposed control method, the above-mentioned method of changing the equivalent reactance was applied to the improved IEEE 4-machine 11-node system, such as... Figure 4 The controllable series compensation device (TCSC) is connected to the AC tie bus between nodes 6 and 7, thereby changing the equivalent impedance of the sending-end system during a fault and suppressing transient overvoltages. Simultaneously, a convolutional neural network is applied to calculate the impedance values ​​of each bus within the power grid online, and the compensation amount of the series compensation device is adjusted based on the calculation results. This allows the system to suppress transient overvoltages by changing the equivalent impedance of the system during a fault. The fault setting is a three-phase ground fault occurring on the inverter side of the conventional DC transmission line at 30 seconds. The fault is cleared after 0.2 seconds. A single-pole blocking fault occurs in the DC transmission line 0.1 seconds after the fault occurs, and the reactive power compensation device in the converter station is disconnected 200 ms after the blocking fault. By comparing the transient overvoltage amplitude of the wind-fire bundled sending-end system before and after the addition of the control system, it is found that... Figure 6 As shown, the proposed control method can effectively suppress transient overvoltages in the sending-end system.

[0078] Example 2

[0079] The present invention also provides a control system for transient overvoltage in a sending-end system, comprising:

[0080] Transient overvoltage acquisition module, used to acquire transient overvoltages in the sending-end system;

[0081] The relationship acquisition module is used to acquire the relationship between the transient overvoltage of the sending-end system and the equivalent impedance of the sending-end system;

[0082] The steady-state impedance acquisition module is used to predict the equivalent impedance of the sending-end system in steady-state operation when a transient overvoltage occurs, using a pre-trained convolutional neural network.

[0083] The transient overvoltage adjustment module is used to adjust the transient overvoltage of the sending system based on the relationship between the transient overvoltage of the sending system and the equivalent impedance of the sending system, as well as the equivalent impedance of the sending system in steady-state operation when the transient overvoltage occurs.

[0084] The operations performed by the relationship acquisition module include:

[0085] The relationship between the transient voltage of the sending-end system and the equivalent impedance of the sending-end system can be obtained according to equation (1).

[0086]

[0087] Among them, Q cp X is the single-phase reactive power compensation value of the AC filter. sw For equivalent reactance, U s For the equivalent potential, Q r 'U' represents the minimum reactive power consumed by the rectifier-side converter station during a commutation failure. p For the transient overvoltage of the sending-end bus, U p This refers to the voltage of the sending-end bus during steady-state operation.

[0088] The steady-state impedance acquisition module performs the following operations: inputting the feature quantities of electrical data of the grid node during steady-state operation into a pre-trained convolutional neural network to obtain the equivalent impedance of the sending-end system during steady-state operation.

[0089] The operations performed by the transient overvoltage adjustment module include:

[0090] The equivalent impedance of the sending system when the transient overvoltage occurs is obtained based on the relationship between the transient overvoltage of the sending system and the equivalent impedance of the sending system. Then, the equivalent impedance is adjusted to the equivalent impedance of the sending system when the transient overvoltage occurs, which is the same as the equivalent impedance of the sending system in steady-state operation when the transient overvoltage occurs, so that the transient overvoltage is suppressed and the sending system can be restored to steady-state operation.

[0091] Example 3

[0092] The present invention also provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps described in the method for controlling transient overvoltage in a sending-end system.

[0093] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0094] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0095] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0096] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0097] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for controlling transient overvoltage in a sending-end system, characterized in that, include: Acquire transient overvoltages in the sending-end system; Obtain the relationship between transient overvoltage of the sending-end system and equivalent impedance of the sending-end system; Predict the equivalent impedance of the sending-end system in steady-state operation when a transient overvoltage occurs using a pre-trained convolutional neural network. The transient overvoltage of the sending system is adjusted based on the relationship between the transient overvoltage of the sending system and the equivalent impedance of the sending system in steady-state operation when the transient overvoltage occurs. The relationship between the transient overvoltage of the sending-end system and the equivalent impedance of the sending-end system includes: The relationship between the transient overvoltage of the sending-end system and the equivalent impedance of the sending-end system can be obtained according to equation (1). （1）； in, This is the single-phase reactive power compensation amount for the AC filter. For equivalent impedance, For equivalent potential, This represents the minimum reactive power consumed by the rectifier-side converter station during a commutation failure. This refers to the transient overvoltage of the sending-end bus. This refers to the voltage of the sending-end bus during steady-state operation.

2. The method for controlling transient overvoltage in a sending-end system according to claim 1, characterized in that, The equivalent impedance predicted by the pre-trained convolutional neural network when the system is in steady-state operation during transient overvoltage includes: The feature quantities of electrical data during steady-state operation of the power grid node are input into a pre-trained convolutional neural network to obtain the equivalent impedance of the sending-end system during steady-state operation.

3. The method for controlling transient overvoltage in a sending-end system according to claim 2, characterized in that, The characteristic quantities of the electrical data of the power grid node during steady-state operation are shown in the following formula: （2）； （3）； （4）； in, Characteristic quantities representing electrical data Indicates the selected first One sample, Indicates a point in time Next node eigenvalues; , , , Representing time points Next node Voltage, phase angle, active power and reactive power of the load; Indicates a point in time The magnitude of the wind speed injected downwind.

4. The method for controlling transient overvoltage in a sending-end system according to claim 1, characterized in that, The adjustment of transient overvoltage in the sending-end system includes: The equivalent impedance of the sending system when the transient overvoltage occurs is obtained based on the relationship between the transient overvoltage of the sending system and the equivalent impedance of the sending system. Then, the equivalent impedance is adjusted by a controllable series compensation device to the equivalent impedance of the sending system in steady-state operation when the transient overvoltage occurs, so that the transient overvoltage is suppressed and the sending system is restored to steady-state operation.

5. A control system for transient overvoltage in a sending-end system, characterized in that, include: Transient overvoltage acquisition module, used to acquire transient overvoltages in the sending-end system; The relationship acquisition module is used to acquire the relationship between the transient overvoltage of the sending-end system and the equivalent impedance of the sending-end system; The steady-state impedance acquisition module is used to predict the equivalent impedance of the sending-end system in steady-state operation when a transient overvoltage occurs, using a pre-trained convolutional neural network. The transient overvoltage adjustment module is used to adjust the transient overvoltage of the sending system based on the relationship between the transient overvoltage of the sending system and the equivalent impedance of the sending system, as well as the equivalent impedance of the sending system in steady-state operation when the transient overvoltage occurs. The operations performed by the relationship acquisition module include: The relationship between the transient overvoltage of the sending-end system and the equivalent impedance of the sending-end system can be obtained according to equation (1). （1）； in, This is the single-phase reactive power compensation amount for the AC filter. For equivalent reactance, For equivalent potential, This represents the minimum reactive power consumed by the rectifier-side converter station during a commutation failure. This refers to the transient overvoltage of the sending-end bus. This refers to the voltage of the sending-end bus during steady-state operation.

6. A control system for transient overvoltage in a sending-end system according to claim 5, characterized in that, The operation performed by the steady-state impedance acquisition module includes: inputting the feature quantities of electrical data of the power grid node during steady-state operation into a pre-trained convolutional neural network to obtain the equivalent impedance of the sending-end system during steady-state operation.

7. A control system for transient overvoltage in a sending-end system according to claim 5, characterized in that, The operations performed by the transient overvoltage adjustment module include: The equivalent impedance of the sending system when the transient overvoltage occurs is obtained based on the relationship between the transient overvoltage of the sending system and the equivalent impedance of the sending system. Then, the equivalent impedance is adjusted by a controllable series compensation device to the equivalent impedance of the sending system in steady-state operation when the transient overvoltage occurs, so that the transient overvoltage is suppressed and the sending system is restored to steady-state operation.

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