Method, controller and system for subsynchronous damping control using a statcom

Abstract

A method (100) for a subsynchronous damping control, SSDC, of a STATCOM (20) connected to an alternating current, AC, power grid (2) is disclosed herein. The method comprises measuring (110) a voltage representing a voltage of the AC power grid and applying (120) a low pass filter and a high pass filter to the voltage. The method further comprises cross-coupling (130) an active power command of the voltage and a reactive power decoupler parameter of the voltage to receive a voltage phase angle parameter and cross-coupling a reactive power command of the voltage and an active power decoupler parameter of the voltage to receive a voltage magnitude parameter and combining (140) the voltage phase angle parameter and the voltage magnitude parameter to provide a voltage reference to the STATCOM. A converter for a STATCOM and a system comprising a STATCOM and said controller are also disclosed herein.

Description

Technical Field

[0001] This disclosure relates to a method for subsynchronous damping control (SSDC) of a STATCOM (connected to an AC power grid). This disclosure further relates to a controller for a STATCOM and a system including said controller. Background Technology

[0002] As electrification continues in various sectors such as transportation and heating, electricity demand is increasing. With the increasing prevalence of electric vehicles, electric heating systems, and other electrification technologies, existing power distribution infrastructure may face challenges in meeting this growing demand. Series compensation can be provided to transmission lines by adding capacitors and reactors in series. Series compensation improves the overall performance and capacity of the power system.

[0003] The challenge of series compensation lies in its potential to alter the dynamic characteristics of the system, making it more susceptible to certain oscillations, such as subsynchronous resonance (SSR) oscillations. SSRs can cause mechanical vibrations, thereby inflicting mechanical stress on grid components. Therefore, SSRs pose a threat to the power system, especially with the increasing integration of renewable energy into the grid.

[0004] Synchronous generators typically generate subsynchronous oscillations at a specific frequency. To mitigate or dampen these oscillations, methods such as power system stabilizers or subsynchronous damped control (SSDC) combined with static synchronous compensators (STATCOMs) have been proposed. SSDC introduces additional damping into the oscillations, thereby reducing the risk of oscillation exacerbation and instability.

[0005] However, the increasing grid connection of renewable energy sources replacing traditional generators has led to more complex oscillations and resonant frequencies. Furthermore, identifying the specific causes and frequencies of SSR events in different power system scenarios is challenging, and power line switching can alter resonant frequencies. This complexity renders traditional SSDC (Secure-Side Controlled Grid) insufficiently flexible for diverse power systems, thus necessitating new control strategies. Summary of the Invention

[0006] Therefore, an object of this disclosure is to provide a method for subsynchronous damping control (SSDC) of a STATCOM connected to an alternating current (AC) power grid, which overcomes at least some of the aforementioned drawbacks. For example, an object of this disclosure is to provide an input to the STATCOM to dampen a wider range of subsynchronous resonant (SSR) frequencies. Other objects of this disclosure are to provide a controller for implementing the method and a system including said controller.

[0007] According to a first aspect of this disclosure, a method for subsynchronous damped control (SSDC) of a STATCOM connected to an alternating current (AC) grid is provided. The method includes: measuring a voltage representing a voltage of the AC grid, and subjecting the voltage to a low-pass filter and a high-pass filter. The method further includes: cross-coupling an active power command and reactive power decoupler parameters of the voltage to receive a voltage phase angle parameter, and cross-coupling the reactive power command and active power decoupler parameters of the voltage to receive a voltage magnitude parameter, and combining the voltage phase angle parameter and the voltage magnitude parameter to provide a voltage reference to the STATCOM.

[0008] According to a second aspect of this disclosure, a subsynchronous damping controller for a STATCOM is provided. The controller is configured to obtain a voltage measurement representing the voltage of an AC grid to which the STATCOM is connected, and to subject the measured voltage to a low-pass filter and a high-pass filter. The controller is further configured to cross-couple an active power command of the voltage with reactive power decoupler parameters of the voltage to receive a voltage phase angle parameter, and to cross-couple the reactive power command of the voltage with active power decoupler parameters of the voltage to receive a voltage amplitude parameter, and to combine the cross-coupled voltage phase angle parameter and the cross-coupled voltage amplitude parameter to provide a voltage reference to the STATCOM.

[0009] According to a third aspect of this disclosure, a system for subsynchronous damping is provided. The control system includes a subsynchronous damping controller according to a second aspect, and a STATCOM configured to receive a voltage reference from the subsynchronous damping controller. The voltage reference is used to control the injection of reactive power and / or active power (in an AC power grid) to dampen subsynchronous oscillations.

[0010] Traditional power grid systems are typically designed with synchronous machines (such as generators) for power generation. The mechanical oscillation of the generator's rotor mass introduces oscillations into the grid. In some cases, the varying mechanical masses of the generator can introduce subsynchronous resonance (SSR) into the grid, leading to instability. Oscillations from generators have a narrow frequency range, and the traditional solution for damping oscillations is to provide a power system stabilizer with reactive power compensators. Power system stabilizers dampen oscillations by regulating reactive power. However, power system stabilizers require a narrow control bandwidth, selecting specific frequencies to achieve adequate damping of the target frequency without significantly amplifying adjacent frequencies.

[0011] As more renewable energy sources are introduced into the grid, grid strength typically decreases. For example, intermittent energy sources connected to the grid via long transmission lines or cable systems are known to weaken the grid. Weak grid conditions and high power levels (i.e., from renewable energy sources) induce oscillations in the grid that are similar to those from synchronous machines. However, as the inventors have recognized, these frequencies are over a wider range than those from synchronous machine oscillations, and conventional power system stabilizers are only capable of handling a narrow frequency range.

[0012] Using the proposed method and controller, the STATCOM provides oscillation damping over a wider frequency range. The method generates a voltage reference used to control the injection of reactive and / or active power from the STATCOM into the grid, thereby strengthening the grid by reducing voltage amplitude variations and voltage phase angle variations. More specifically, reactive power injection reduces voltage amplitude variations, and active power injection reduces voltage phase angle variations. In systems requiring reactive power injection, the STATCOM can use power electronic devices to act as a source or sink of reactive power from the grid. Furthermore, the STATCOM can also absorb reactive power from the grid. In systems requiring active power injection, the STATCOM is equipped with an energy storage device for active power. Furthermore, the STATCOM can also absorb active power from the grid. Combinations of these can also be used, such as absorbing both reactive and active power, or such as injecting reactive power while absorbing active power.

[0013] Furthermore, this method is more robust than conventional power system stabilizers used in SSDCs because it has a wider frequency range that it can operate over. Conventional methods require careful tuning for specific frequency oscillations, while the method disclosed herein allows for a wider frequency range and is less sensitive to variations in the grid that may affect the oscillation frequency. In conventional methods, SSDCs are specifically designed for motor-dominated systems to dampen mechanical oscillations and inter-regional oscillations within a narrow bandwidth, thus avoiding interaction with the low-damped dynamics of the motor. STATCOMs do not have mechanical oscillations (torque control) and low-damping issues, and offer good damped power control. By changing the controller architecture while maintaining the system dynamics and without interacting with the master controller (power control or torsional vibration interaction), a wide frequency range can be allowed for SSDCs.

[0014] By measuring the voltage representing the AC grid and passing it through a low-pass filter and a high-pass filter, oscillations within a defined range can be filtered out from the voltage. This range can be defined as corresponding to a wider range of subsynchronous oscillations caused by the introduction of renewable energy sources discussed above. The low-pass filter can have a cutoff frequency between 20 Hz and 50 Hz. The high-pass filter can have a cutoff frequency between 0.01 Hz and 5 Hz. Frequencies less than 0.01 Hz are generally associated with frequency control, while frequencies greater than 50 Hz are generally associated with direct current (DC) components and harmonics. The low-pass and high-pass filters can together provide a bandpass range of 0.01 Hz to 50 Hz. As a further example, the low-pass and high-pass filters together provide a bandpass range of 5 Hz to 50 Hz or 5 Hz to 40 Hz.

[0015] Cross-coupling of active and reactive power is performed due to the phenomenon that changes in active power affect reactive power and vice versa, because of the inherent interaction between these two power components in the power grid. Because of this phenomenon, cross-coupling is used in modern power systems to provide more efficient operation, improved voltage stability, and / or more efficient grid integration of renewable energy.

[0016] At and below the subsynchronous frequency range, grid voltage may be affected by disturbances such as deviations in phase angle or voltage amplitude in all three phases from their nominal values. Sources of disturbances can be varied, including equipment that causes active power oscillations, leading to grid voltage phase angle deviations (direct impact) and / or grid voltage amplitude deviations (indirect impact due to transmission systems). Reactive power equipment (SVCs, capacitor banks, etc.) may cause reactive power oscillations, resulting in variations in grid voltage amplitude. Disturbance compensation can be achieved by injecting reactive power into the grid (by minimizing phase angle and amplitude deviations). The reactive power command determines the reactive power (level) that the STATCOM converter should produce, which primarily affects grid voltage amplitude and indirectly affects grid phase angle. Similarly, the active power command determines the active power that the STATCOM converter should produce by injecting active power into the grid, which primarily affects grid phase angle. Based on the measured grid phase angle and amplitude, reactive power commands and active power commands are generated to achieve "optimal damping", that is, minimizing voltage deviation and minimizing phase angle deviation.

[0017] Cross-coupling is achieved by combining active power commands and reactive power decoupler parameters, or vice versa. Active and reactive power decoupler parameters are designed for each system to achieve the desired interaction between the STATCOM and the grid. This allows for flexible parameterization of the SSDC to achieve optimal subsynchronous oscillation damping. The cross-coupling step allows the active and reactive power commands to incorporate the inherent interaction between these two power components in the grid, thus taking into account the effects of reactive / active power (on each other).

[0018] The parameters of reactive power decouplers and active power decouplers can also be viewed as reactive power suppressors and active power suppressors, or as reactive power compensators and active power compensators. The choice of terminology may vary depending on the context, but the purpose is to cross-couple reactive and active power, and to cross-couple active and reactive power. The parameters of active and reactive power decouplers can be obtained from the power decoupler.

[0019] The reactive power decoupler parameters can be the reactive power command multiplied by the reactive power cross-coupling gain. The active power decoupler parameters can be the active power command multiplied by the active power cross-coupling gain. The corresponding gains can be designed based on the desired degree of cross-coupling influence on the other parameter (i.e., the degree to which the reactive power decoupler parameters are allowed to influence the active power command). The active power decoupler parameters and reactive power decoupler parameters are used to control the degree to which the inherent interaction between these two power components is allowed to influence the active power command and reactive power command.

[0020] The voltage reference defines the operating point of the STATCOM. During normal operation, when no subsynchronous oscillation is detected, the voltage reference can be determined by an automatic voltage control device. When a subsynchronous oscillation is detected, the subsynchronous damping control can control or influence the voltage reference using the methods provided in this disclosure.

[0021] The method may further include converting the measured voltage into a voltage amplitude and a voltage phase angle. Each of the voltage amplitude and voltage phase angle may be subjected to a low-pass filter and a high-pass filter, respectively. In other words, the voltage amplitude is subjected to a first low-pass filter and a first high-pass filter, and the voltage phase angle is subjected to a second low-pass filter and a second high-pass filter.

[0022] To further improve the efficiency of this method, the voltage can be converted into voltage amplitude and voltage phase angle. By converting the voltage into voltage phase angle and voltage amplitude, the low-pass filter and high-pass filter can be adjusted to the corresponding values.

[0023] The active power command and active power decoupler parameters can be derived from the voltage amplitude, and the reactive power command and reactive power decoupler parameters can be derived from the voltage phase angle.

[0024] The method may further include subjecting each of the voltage magnitude and voltage phase angle to a gain. These gains provide a means of adjusting and fine-tuning the behavior of the method and the controller to achieve desired performance objectives, such as stability, responsiveness, accuracy, and robustness.

[0025] A STATCOM can include a converter and an energy storage device, wherein a controller is used to control the converter. The energy storage device can store active power, thus enabling the STATCOM to inject active power into the grid. A STATCOM can use power electronic devices to act as a source or sink of reactive power in the grid.

[0026] The effects and features of the second and third aspects are generally similar to those described above in conjunction with the first aspect. The embodiments mentioned in the first aspect are at least generally compatible with the second and third aspects. It should further be noted that, unless explicitly stated otherwise, this disclosure relates to all possible combinations of features.

[0027] The further scope of this disclosure will become apparent from the detailed description given below. However, it should be understood that while the detailed description and specific examples indicate preferred embodiments of this disclosure, they are given by way of illustration only, as various variations and modifications within the scope of this disclosure will become apparent to those skilled in the art from this detailed description.

[0028] Therefore, it should be understood that this disclosure is not limited to the specific components of the methods and systems described herein. It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. Attached Figure Description

[0029] The above and other aspects of this disclosure will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 A system including STATCOM is illustrated schematically.

[0030] Figure 2 The method according to the first embodiment is illustrated schematically.

[0031] Figure 3 The method according to the second embodiment is illustrated schematically. Detailed Implementation

[0032] The present disclosure is described below by way of several illustrative examples. It should be understood that these examples are provided for illustration and explanation only and are not intended to limit the scope of the present disclosure.

[0033] Figure 1 The diagram illustrates system 1, which includes a static synchronous compensator (STATCOM) 20 and a subsynchronous damping controller 40 (further referred to as controller 40). STATCOM 20 includes a converter 22 and an energy storage device 30. STATCOM 20 is connected to the power lines of grid 2. The connection to the grid can be achieved via transformer 10.

[0034] STATCOM 20 is a power electronic device used in a power system to provide precise voltage control and reactive power compensation through power electronics technology. In this disclosure, STATCOM can further provide active power compensation by adding energy storage devices for active power. Converter 40 is capable of generating or absorbing reactive power to regulate the voltage of the AC grid (or the connected electrical network). STATCOM 20 operates by injecting or absorbing reactive / active power in response to changes in system conditions, such as load changes or disturbances (e.g., subsynchronous resonance (SSR)), thereby helping, for example, to maintain voltage levels within acceptable limits. Controller 40 is used to control the timing and manner of STATCOM 20's injection of active and reactive power. Reactive power injection can reduce voltage amplitude variations, and active power injection can reduce voltage phase angle variations. Controller 40 provides a voltage reference to STATCOM 20, which is used to control the injection of active and reactive power to dampen subsynchronous oscillations.

[0035] To obtain a voltage reference, controller 40 executes method 100, which combines... Figure 2 and Figure 3 Further discussion.

[0036] exist Figure 2 The diagram illustrates a method 100 for subsynchronous damped control (SSDC) of a STATCOM 20 connected to an AC power grid 2. Method 100 includes measuring 110 a voltage representing the voltage of the AC power grid 2. Measurement step 110 can be implemented, for example, by a potential transformer, a voltage transformer, or a voltage sensor.

[0037] Method 100 further includes passing the measured voltage through a 120° low-pass filter and a high-pass filter. By passing the voltage through the 120° low-pass and high-pass filters, oscillations within a defined frequency range can be filtered out from the voltage. In the low-pass filter, a cutoff frequency is used to provide an upper limit allowing a signal to pass through with minimal attenuation, thereby effectively allowing frequencies below this cutoff frequency to propagate while suppressing frequencies above it. In the high-pass filter, a cutoff frequency is used to provide a lower limit allowing a signal to pass through with minimal attenuation, thereby effectively allowing frequencies above this cutoff frequency to propagate while suppressing frequencies below it. In other words, the low-pass filter provides an upper limit, and the high-pass filter provides a lower limit, thus specifying a range.

[0038] Compared to traditional power system stabilizers, Method 100 allows a wider range of frequencies to pass through low-pass and high-pass filters. The introduction of renewable energy sources (such as wind farms) that are typically located far from where the electricity is generated results in longer transmission lines in the grid. Renewable energy weakens the grid. Weak grid conditions and high power levels (i.e., from renewable energy sources) induce oscillations in the grid, similar to those from synchronous machines such as hydropower. However, in weaker grids with highly interconnected intermittent energy sources, oscillations typically result in a wider range of subsynchronous frequencies. Therefore, the low-pass filter can have a cutoff frequency between 20 Hz and 50 Hz, and the high-pass filter can have a cutoff frequency between 0.01 Hz and 5 Hz. Frequencies less than 0.01 Hz are typically associated with frequency control, while frequencies greater than 50 Hz are typically associated with direct current (DC) components and harmonics. The low-pass and high-pass filters can together provide a bandpass range of 0.01 Hz to 50 Hz. As a further example, the low-pass filter and the high-pass filter together provide a bandpass range of 5 Hz to 50 Hz or 5 Hz to 40 Hz.

[0039] Method 100 further includes cross-coupling the active power command of voltage 130 with the reactive power decoupler parameters of voltage to receive voltage phase angle parameters, and cross-coupling the reactive power command of voltage 130 with the active power decoupler parameters of voltage to receive voltage amplitude parameters.

[0040] Cross-coupling typically refers to the interaction or interdependence between two or more variables or systems, where a change in one variable affects the behavior of another, and vice versa. The cross-coupling of active and reactive power130 is performed due to the inherent interaction between these two power components in the power grid, where changes in active power affect reactive power, and vice versa.

[0041] Cross-coupling 130 in Method 100 is achieved by combining active power commands with reactive power decoupler parameters. Active power decoupler and reactive power decoupler parameters are designed for each system to achieve the desired interaction between STATCOM 20 and the grid 2. This allows for flexible parameterization of the SSDC to achieve optimal subsynchronous oscillation damping. The cross-coupling step 130 allows the active power command and reactive power command to incorporate the inherent interaction between these two power components in the grid.

[0042] The active power command, together with the reactive power decoupler parameters, generates the voltage phase angle parameter, and the reactive power command, together with the active power decoupler parameters, generates the voltage magnitude parameter. Combining the voltage phase angle and voltage magnitude parameters, a voltage reference is provided to converter 22 of the STATCOM 20. The voltage reference defines the operating point of the STATCOM 20.

[0043] The controller 40 for the STATCOM 20 is configured to perform the above-described method. Specifically, the controller 40 is configured to obtain a voltage measurement representing the voltage of the AC grid, and to subject the measured voltage to a low-pass filter and a high-pass filter. The controller 40 is further configured to cross-couple the active power command of the voltage with the reactive power decoupler parameters of the voltage to receive voltage phase angle parameters, and to cross-couple the reactive power command of the voltage with the active power decoupler parameters of the voltage to receive voltage amplitude parameters, and to combine the cross-coupled voltage phase angle parameters and the cross-coupled voltage amplitude parameters to provide a voltage reference to the STATCOM 20.

[0044] exist Figure 3 Alternative embodiments of method 100 are schematically illustrated in the diagram. Figure 3 In the middle, for Figure 2 The method provides some additional steps, such as combining... Figure 2 As described, method 100 includes measuring a voltage 110 representing the voltage of AC power grid 2. Method 100 further includes converting the voltage 115 into a voltage magnitude and a voltage phase angle. Each of the voltage magnitude and voltage phase angle may be subjected to a low-pass filter 120A and a high-pass filter 120B, respectively.

[0045] Method 100 further includes subjecting each of the voltage magnitude and voltage phase angle to a gain of 125A and 125B, respectively. These gains provide a means for adjusting and fine-tuning the behavior of the method and controller to achieve desired performance objectives, such as stability, responsiveness, accuracy, and robustness.

[0046] The active power command and active power decoupler parameters are derived from the voltage amplitude, while the reactive power command and reactive power decoupler parameters are derived from the voltage phase angle.

[0047] The reactive power decoupler parameters can be the reactive power command multiplied by 150A of reactive power cross-coupling gain. The active power decoupler parameters can be the active power command multiplied by 150B of active power cross-coupling gain. The corresponding gains can be designed based on the desired degree of cross-coupling influence on the other parameter (i.e., the degree to which the reactive power decoupler parameters are allowed to influence the active power command). The active power and reactive power decoupler parameters are used to control the extent to which the inherent interaction between these two power components is allowed to influence the active power command and reactive power command.

[0048] Method 100 further includes combining voltage phase angle parameters and voltage magnitude parameters 140 to provide a voltage reference to converter 22 of STATCOM 20.

[0049] While this disclosure is readily adaptable to various modifications and alternatives, specific examples have been shown and described with reference to the accompanying drawings in order to clearly explain the various advantageous aspects of this disclosure. However, it should be understood that the specific embodiments and drawings herein are not intended to limit this disclosure to the particular forms disclosed. Rather, it is intended to cover all modifications, equivalents, and alternatives falling within the scope of the appended claims, including possible combinations of the various elements of these specific examples.

[0050] Item list of examples Project 1. A method (100) for subsynchronous damped control SSDC of a STATCOM (20) connected to an AC power grid (2), the method comprising: The measurement (110) represents the voltage of the AC power grid. The voltage is subjected to a (120) low-pass filter and a high-pass filter. Cross-couple (130) the active power command of the voltage and the reactive power decoupler of the voltage to receive voltage phase angle parameters, and cross-couple the reactive power command of the voltage and the active power decoupler of the voltage to receive voltage amplitude parameters, and The voltage phase angle parameter and the voltage amplitude parameter are combined (140) to provide a voltage reference to the STATCOM.

[0051] Project 2. The method according to Project 1 further includes... The measured voltage is converted (115) into voltage amplitude phasor and voltage phase angle phasor. In this process, each of the voltage amplitude phasor and the voltage phase angle phasor is subjected to a low-pass filter and a high-pass filter, respectively.

[0052] Project 3. According to the method described in Project 2, wherein the active power command and the active power decoupler originate from the voltage amplitude phasor, and the reactive power command and the reactive power decoupler originate from the voltage phase angle phasor.

[0053] Item 4. The method according to Item 2 or 3 further includes subjecting each of the voltage magnitude phasor and the voltage phase angle phasor to a gain of (150A, 150B), respectively.

[0054] Project 5. The method according to any one of the preceding projects, wherein the low-pass filter has a cutoff frequency between 20 Hz and 50 Hz.

[0055] Project 6. The method according to any one of the preceding projects, wherein the high-pass filter has a cutoff frequency between 0.01 Hz and 5 Hz.

[0056] Item 7. The method according to any one of the preceding items, wherein the low-pass filter and the high-pass filter together provide a bandpass range of 0.01 Hz to 50 Hz.

[0057] Project 8. A subsynchronous damping controller (40) for a STATCOM (20), wherein the controller is configured to: Obtain a voltage measurement value representing the voltage of the AC power grid to which the STATCOM is connected. The measured voltage is subjected to a low-pass filter and a high-pass filter. The active power command and the reactive power decoupler of the voltage are cross-coupled to receive voltage phase angle parameters, and the reactive power command and the active power decoupler of the voltage are cross-coupled to receive voltage amplitude parameters. The voltage phase angle parameter and the voltage amplitude parameter after cross-coupling are combined to provide a voltage reference to the STATCOM.

[0058] Item 9. The controller described in Item 8 is further configured as follows: The voltage is converted into a voltage amplitude phasor and a voltage phase angle phasor, wherein each of the voltage amplitude phasor and the voltage phase angle phasor is subjected to a low-pass filter and a high-pass filter, respectively.

[0059] Project 10. The controller according to Project 9, wherein the active power command and the active power decoupler are derived from the voltage amplitude phasor, and the reactive power command and the reactive power decoupler are derived from the voltage phase angle phasor.

[0060] Item 11. The controller according to Item 9 or 10 is further configured to subject each of the voltage magnitude phasor and the voltage phase angle phasor to a gain.

[0061] Item 12. The controller according to any one of Items 8 to 11, wherein the low-pass filter has a cutoff frequency between 20 Hz and 50 Hz.

[0062] Item 13. The controller according to any one of Items 8 to 12, wherein the high-pass filter has a cutoff frequency between 0.01 Hz and 5 Hz.

[0063] Item 14. The controller according to any one of Items 8 to 13, wherein the low-pass filter and the high-pass filter together provide a bandpass range of 0.01 Hz to 50 Hz.

[0064] Project 15. A system (1) for subsynchronous damping, the control system comprising: The subsynchronous damping controller (40) according to any one of items 8 to 14. STATCOM (20) is configured to receive the voltage reference from the subsynchronous damping controller. The voltage reference is used to control the injection of reactive power and / or active power to dampen subsynchronous oscillations.

Claims

1. A method (100) for subsynchronous damped control SSDC of a STATCOM (20) connected to an AC power grid (2), the method comprising: The measurement (110) represents the voltage of the AC power grid, and the measured voltage is converted (115) into voltage amplitude and voltage phase angle. The voltage amplitude and the voltage phase angle are subjected to a (120) low-pass filter and a high-pass filter, respectively. The active power command of the voltage amplitude and the reactive power decoupler parameters of the voltage are combined (130) to receive the voltage phase angle parameters, and the reactive power command of the voltage phase angle and the active power decoupler parameters of the voltage are combined to receive the voltage amplitude parameters, and The voltage phase angle parameter and the voltage amplitude parameter are combined (140) to provide a voltage reference to the STATCOM.

2. The method of claim 1, further comprising subjecting each of the voltage amplitude and the voltage phase angle to a gain of (150A, 150B).

3. The method according to any one of the preceding claims, wherein, The low-pass filter has a cutoff frequency between 20 Hz and 50 Hz.

4. The method according to any one of the preceding claims, wherein, The high-pass filter has a cutoff frequency between 0.01 Hz and 5 Hz.

5. The method according to any one of the preceding claims, wherein, The low-pass filter and the high-pass filter together provide a bandpass range of 00.1 Hz to 50 Hz.

6. A subsynchronous damping controller (40) for a STATCOM (20), wherein, The controller is configured to: Obtain a voltage measurement representing the voltage of the AC power grid to which the STATCOM is connected, and convert the voltage into voltage amplitude and voltage phase angle. The voltage amplitude and the voltage phase angle are subjected to a low-pass filter and a high-pass filter, respectively. The active power command and reactive power decoupler parameters of the voltage are combined to receive the voltage phase angle parameter, and the reactive power command and active power decoupler parameters of the voltage are cross-coupled to receive the voltage amplitude parameter. The voltage phase angle parameter and the voltage amplitude parameter are combined to provide a voltage reference to the STATCOM.

7. The controller of claim 6 is further configured to subject each of the voltage amplitude and the voltage phase angle to a gain.

8. The controller according to any one of claims 6 to 7, wherein, The low-pass filter has a cutoff frequency between 20 Hz and 50 Hz.

9. The controller according to any one of claims 6 to 8, wherein, The high-pass filter has a cutoff frequency between 0.01 Hz and 5 Hz.

10. The controller according to any one of claims 6 to 9, wherein, The low-pass filter and the high-pass filter together provide a bandpass range of 0.01 Hz to 50 Hz.

11. A system (1) for subsynchronous damping, the control system comprising: The subsynchronous damping controller (40) according to any one of claims 6 to 10. STATCOM (20) is configured to receive the voltage reference from the subsynchronous damping controller. The voltage reference is used to control the injection of reactive power and / or active power to dampen subsynchronous oscillations.

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