Flexible DC additional control method for suppressing subsynchronous oscillation

A sub-synchronous oscillation and additional control technology, applied in the direction of reducing/preventing power oscillation, power transmission AC network, etc., can solve the problem of limited sub-synchronous oscillation suppression effect, achieve fast response, strong versatility, and easy access

Inactive Publication Date: 2017-03-15

BEIJING SIFANG JIBAO AUTOMATION

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AI-Extracted Technical Summary

Problems solved by technology

[0005] At present, the additional excitation damping control has a good suppression effect in suppressing small disturb...

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Abstract

The invention discloses a flexible DC additional control method for suppressing subsynchronous oscillation. The flexible DC additional control method comprises the following steps of (1) detecting AC bus voltage of a rectifier side and extracting subsynchronous information; (2) selecting a broadband-pass single-channel DC additional damping controller; (3) superposing an output current signal of the controller and inner-loop d-axis control current in a flexible DC controller to obtain a control signal for controlling a flexible DC transmission converter valve; (4) adding a low-frequency filter to a DC voltage feedback link of a DC outer-loop voltage controller and filtering out a low-frequency component, caused by subsynchronous oscillation, in DC voltage; and (5) generating a trigger pulse from the control signal of the flexible DC converter valve through a modulation link and finally providing additional electromagnetic torque to suppress the subsynchronous oscillation. According to the method, damping of oscillation frequencies of all subsynchronous oscillation bands can be achieved, the method is not limited by a rectifier side control strategy and can be easily realized in a project, and the subsynchronous stability of a system can be improved.

Application Domain

Electric power transfer ac networkPower oscillations reduction/prevention

Technology Topic

Inner loopVIT signals +13

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Examples

Experimental program(1)

Example Embodiment

[0038] In order to make it easy to understand the technical means, creative features, objectives and effects achieved by the present invention, the technical solutions of the present invention will be further described in conjunction with specific implementations below.

[0039] figure 1 Shown is the schematic diagram of the flexible DC topology and the DC damping additional control wiring diagram that suppresses subsynchronization. The DC rectifier station includes N units of thermal power generators from G1 to GN that may produce subsynchronous oscillations. The distance between each unit and the converter bus on the rectifier side is represented by LineN. The AC voltage u of the converter bus measured by the DC rectifier station side voltage transformer abc It is sent to the flexible DC sub-synchronous damping controller, and the generated control output signal is attached to the flexible straight controller.

[0040] The flexible DC additional control method for suppressing subsynchronous oscillation disclosed in the present invention, as attached figure 2 As shown, including the following steps:

[0041] Step 1: Measure the AC bus voltage of the rectifier station of the flexible DC transmission system, and obtain the frequency deviation signal of the AC bus voltage relative to the rated power frequency of the grid through the phase-locked link PLL; considering that it is difficult to obtain the speed deviation signal of the high-pressure cylinder of the actual engineering steam turbine, Using rectifier station AC bus voltage u abc As an input signal. During subsynchronous oscillation, the three-phase instantaneous voltage of the converter bus contains a subsynchronous component that is complementary to the oscillation frequency of the turbine generator shaft system. The AC bus frequency deviation signal can be extracted through the PLL phase lock link, which contains the required subsynchronous information. ;

[0042] Step 2: Take the frequency deviation signal obtained in step 1 as the input signal, and then pass through the DC additional damping controller composed of four links: band-pass filtering, phase compensation, gain amplification, and amplitude limiting. The signal obtained is the suppression second. Synchronous oscillation DC damping control signal; the types of flexible direct additional damping controllers mainly include narrowband multi-channel and wideband single-channel. Here we choose the wideband single-channel control type with strong engineering versatility and relatively simple parameter setting. The control method pays attention to ensuring the suppression of the main oscillation modes without causing inappropriate adjustment of other modes. Such as image 3 As shown, the parameter setting principles are as follows:

[0043] The broadband pass single-channel filter link, after the input frequency deviation signal is passed through the filter link, all the subsynchronous frequency (usually between 10 and 40 Hz) components of the torsional vibration mode of the system can pass, and the power frequency, Super-synchronous oscillation signal and noise signal; adopts a second-order Butterworth band-pass filter, the passband frequency is 7Hz~50Hz, that is, the cutoff frequency is ω c1 = 7Hz, ω c2 =50Hz;

[0044] The phase compensation link is the key to parameter tuning. The compensation goal is to provide positive damping for the oscillation frequency in the subsynchronous frequency band, that is, the phase difference between Δf and the additional electromagnetic torque generated by it should be within the range of -90° to +90°. Considering the safety margin, the range is usually -45° to +45°. Use 2~4 shapes like (1+sT 1 )/(1+sT 2 ) To perform phase compensation in the lead-lag link; in engineering design, the phase angle φ that needs to be compensated can be determined by time-domain simulation or field test; after that, the time constant is determined by the following formula:

[0045]

[0046] Among them, ω x Is the frequency of the selected phase compensation; φ is ω x Corresponding phase angle to be compensated; T 1 , T 2 Is the time constant of the lead-lag link; s is the Lagrangian operator.

[0047] The gain link determines the size of the additional electromagnetic torque that the input signal can provide after the phase compensation link; the larger the amplification factor in the proportional amplification link, the better the SSO suppression effect; but too much gain will affect the high frequency stability of the system; In engineering design, use digital or dynamic simulation methods to determine the minimum K value that can suppress SSO as a gain factor;

[0048] Limiting link, the size of the limiting value reflects the maximum value of the additional electromagnetic torque that the suppression system can provide, which is determined according to the upper and lower limits of the DC damping control; it is based on the maximum acceptable value of the controller i MAX , Minimum i MIN Quantity, determine the additional output signal i SSDC.

[0049] If you need to suppress the 25.0Hz sub-synchronous frequency, the design parameters are shown in the following table.

[0050] Table 1 Design parameters

[0051]

[0052] Step 3: Measure the DC voltage of the rectifier station, then pass through a low-frequency filter with a cut-off frequency of 10Hz, filter out the low-frequency components caused by subsynchronous oscillations in the DC voltage, and then input it to the DC voltage outer loop controller, the DC voltage outer loop controller The output signal is the inner ring d-axis reference current signal;

[0053] When the flexible DC rectifier side adopts constant DC voltage control mode, when subsynchronous oscillation occurs, the DC voltage will contain f n -f s AC component of frequency; where f n Is the rated frequency of the grid, f s This is the subsynchronous oscillation frequency; if the AC component is introduced to the given inner loop current, the paired subsynchronous suppression effect and the AC side will be affected. The parameter design of the filter requires the ability to filter out low-frequency AC components.

[0054] Step 4: Add the DC damping additional control signal output in step 2 with the inner loop d-axis reference current signal obtained in step 3 (superimposed as the input signal of the current inner loop controller, and the flexible converter valve is triggered by the current inner loop controller. Control signal; for flexible DC transmission systems, the superimposition position of the subsynchronous suppression signal can be in the outer loop controller or the inner loop controller. The flexible DC outer loop control mainly consists of 4 typical modes, active power control mode and reactive power control mode , DC voltage control mode and AC voltage control mode, such as Figure 4 Shown. . When the suppression signal is superimposed on some control link of the outer loop, such as superimposed on the active outer loop P ref , Reactive outer ring Q ref , Constant DC voltage outer loop U DC_REF Or constant AC voltage outer ring U S_REF , Its inhibitory effect is affected by the operation mode and is not universal.

[0055] When the suppression signal is superimposed on the inner loop q-axis reactive current control link, what it changes is the reactive power exchanged between the converter station and the AC system. Because reactive power is generally compensated locally, it is not suitable for transmission on the line. For the generator set far from the bus of the converter station, the reactive power superposition method basically has no suppression effect; it can be seen that for the flexible DC system, the best superposition position of the subsynchronous suppression signal is the d-axis current inner loop reference value i dref , Its superposition method is like Figure 4 Shown.

[0056] Step 5: Input the control signal of the flexible and straight converter valve obtained in step 4 into the trigger pulse modulator, and the output signal of the trigger pulse modulator is the trigger pulse for opening and closing the converter valve. The trigger pulse controls the switching The opening and closing of the flow valve finally generates additional electromagnetic torque to suppress subsynchronous oscillation. The control signal of the flexible straight converter valve generates a trigger pulse after passing through the pulse modulation link, such as Figure 4 As shown, an additional electromagnetic torque is finally provided to provide a positive electrical damping torque for the system to suppress subsynchronous oscillation.

[0057] The applicant gave a detailed description and description of the embodiments of the present invention in conjunction with the accompanying drawings of the specification, but those skilled in the art should understand that the above embodiments are only preferred embodiments of the present invention, and the detailed description is only to help readers better understand The spirit of the present invention is not a limitation on the protection scope of the present invention. On the contrary, any improvement or modification made based on the inventive spirit of the present invention should fall within the protection scope of the present invention.

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