Theoretical evaluation method for MMC direct-current high-frequency harmonic waves caused by clock asynchronization

Abstract

The invention discloses a theoretical evaluation method for MMC direct-current high-frequency harmonic waves caused by clock asynchronization. The theoretical evaluation method comprises the following steps that valve-side alternating-current voltage is calculated; calculating a bridge arm reference wave; carrying out bridge arm output voltage discretization; calculating a bridge arm output voltage by considering additional time delay; carrying out fast Fourier transform on the obtained direct current voltage; and the high-frequency harmonic distribution condition is obtained through Fourier transform. According to the theoretical evaluation method for the MMC direct-current high-frequency harmonic waves caused by clock asynchronization, factors such as different time delays, alternating-current and direct-current voltage levels, sub-module capacitor voltage levels and operation working conditions are considered, the frequency band and content of the high-frequency harmonic waves caused by clock asynchronization of the upper bridge arm and the lower bridge arm are rapidly calculated and evaluated, operation is easy, and the time cost is greatly reduced.

Description

Technical Field

[0001] The invention belongs to the field of power transmission, and in particular relates to a theoretical evaluation method for MMC direct current high-frequency harmonics caused by clock asynchrony. Background Art

[0002] Modular multilevel converters (MMCs) have the advantages of modular design, a large number of levels, and easy realization of high voltage and large capacity. They are the core equipment of flexible DC transmission systems and have also played an important role in the fields of new energy access and flexible interconnection of multiple power grids. Compared with conventional two-level and three-level converter topologies with fewer levels, the multi-level waveform of MMC can effectively reduce the content of low-frequency harmonics.

[0003] Since MMC contains a very large number of switch devices, and the switch devices are not disconnected at the same time, compared with two-level and three-level converter topologies, the switch control pulse of MMC cannot be sent directly to the switch device. Instead, the switch pulse is calculated by the main controller and sent to the module control chip of each bridge arm, i.e., the module controller, by synchronous or asynchronous communication. Each module controller receives the switch data sent by the main controller and decodes it, and updates the switch device status at the clock edge, i.e., the rising edge or falling edge, until the next data is sent, and then updates the switch status at the clock edge. However, since each module controller is controlled based on its own clock, and the clocks of the module controllers of each bridge arm are not synchronized, all module controllers do not update the switch device status synchronously. Therefore, although theoretically, in the nearest level approach modulation mode of MMC, the upper and lower bridge arms bypass and put into operation submodules are carried out simultaneously, but due to the asynchrony of the switching device signals, from the perspective of the entire primary circuit, the switching process of the switching devices of the MMC bridge arm is not synchronized; that is, if the upper and lower bridge arms bypass and put into operation submodules are not completed at the same time, high-frequency harmonics may be caused in the DC circuit. Existing research has not yet found an analysis and evaluation method for MMC high-frequency harmonics caused by clock asynchrony, making it difficult to effectively evaluate the DC high-frequency harmonic components caused by clock asynchrony from a theoretical level. Summary of the invention

[0004] The present invention is proposed to solve the problems existing in the prior art, and its purpose is to provide a theoretical evaluation method for MMC direct current high-frequency harmonics caused by clock asynchrony.

[0005] The technical solution of the present invention is: a theoretical evaluation method for MMC direct current high-frequency harmonics caused by clock asynchrony, comprising the following steps:

[0006] A. Calculate the AC voltage on the valve side;

[0007] B. Calculate the bridge arm reference wave;

[0008] C. Discretize the output voltage of the bridge arm;

[0009] D. Considering the additional delay, calculate the output voltage of the bridge arm;

[0010] E. Calculate DC voltage;

[0011] F. Perform broadband fast Fourier transform to obtain the high-frequency harmonic distribution.

[0012] Furthermore, step A calculates the valve side AC voltage, and the specific process is as follows:

[0013] First, obtain the rated AC voltage, AC current, and AC connection reactance parameter data;

[0014] Then, the time domain expression of the valve-side AC voltage of the MMC is calculated;

[0015] Finally, the valve side AC voltage is obtained.

[0016] Furthermore, step B calculates the bridge arm reference wave, and the specific process is as follows:

[0017] First, obtain the valve side AC voltage and DC rated voltage;

[0018] Then, the bridge arm reference wave formula is established to calculate the bridge arm reference wave.

[0019] Furthermore, step C discretizes the output voltage of the bridge arm, and the specific process is as follows:

[0020] Firstly, based on the bridge arm reference wave, the continuous bridge arm voltage is obtained;

[0021] The continuous bridge arm voltage is then converted to a discrete voltage based on the nearest level approximation.

[0022] Furthermore, step D takes into account the additional delay and calculates the output voltage of the bridge arm. The specific process is as follows:

[0023] First, assume that the rising edge of the lower bridge arm clock of phase x is delayed by Δt compared with that of the upper bridge arm;

[0024] Then, the expressions of the actual output voltage of the upper bridge arm and the actual output voltage of the lower bridge arm are obtained.

[0025] Furthermore, step E calculates the DC voltage, and the specific process is as follows:

[0026] First, the upper bridge arm voltage and the lower bridge arm voltage are obtained;

[0027] Then, the DC voltage is equal to the sum of the upper bridge arm voltage and the lower bridge arm voltage.

[0028] Furthermore, step F performs broadband fast Fourier transform to obtain the high-frequency harmonic distribution. The specific process is as follows:

[0029] First, the obtained DC voltage is subjected to a fast Fourier transform;

[0030] Then, the high-frequency harmonic distribution is obtained through Fourier transform.

[0031] The beneficial effects of the present invention are as follows:

[0032] The present invention takes into account factors such as different time delays, AC and DC voltage levels, submodule capacitor voltage levels, operating conditions, etc., and quickly calculates and evaluates the high-frequency harmonic frequency bands and contents caused by the asynchrony of the upper and lower bridge arm clocks. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a flow chart of the evaluation method of the present invention. DETAILED DESCRIPTION

[0034] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings and embodiments:

[0035] like Figure 1 As shown, a theoretical evaluation method for MMC DC high-frequency harmonics caused by clock asynchrony includes the following steps:

[0036] A. Calculate the AC voltage on the valve side;

[0037] B. Calculate the bridge arm reference wave;

[0038] C. Discretize the output voltage of the bridge arm;

[0039] D. Considering the additional delay, calculate the output voltage of the bridge arm;

[0040] E. Calculate DC voltage;

[0041] F. Perform broadband fast Fourier transform to obtain the high-frequency harmonic distribution.

[0042] Step A calculates the valve side AC voltage. The specific process is as follows:

[0043] First, obtain the rated AC voltage, AC current, and AC connection reactance parameter data;

[0044] Then, the time domain expression of the valve-side AC voltage of the MMC is calculated;

[0045] Finally, the valve side AC voltage is obtained.

[0046] Step B calculates the bridge arm reference wave, and the specific process is as follows:

[0047] First, obtain the valve side AC voltage and DC rated voltage;

[0048] Then, the bridge arm reference wave formula is established to calculate the bridge arm reference wave.

[0049] Step C discretizes the output voltage of the bridge arm, and the specific process is as follows:

[0050] Firstly, based on the bridge arm reference wave, the continuous bridge arm voltage is obtained;

[0051] The continuous bridge arm voltage is then converted to a discrete voltage based on the nearest level approximation.

[0052] Step D considers the additional delay and calculates the bridge arm output voltage. The specific process is as follows:

[0053] First, assume that the rising edge of the lower bridge arm clock of phase x is delayed by Δt compared with that of the upper bridge arm;

[0054] Then, the expressions of the actual output voltage of the upper bridge arm and the actual output voltage of the lower bridge arm are obtained.

[0055] Step E calculates the DC voltage, and the specific process is as follows:

[0056] First, the upper bridge arm voltage and the lower bridge arm voltage are obtained;

[0057] Then, the DC voltage is equal to the sum of the upper bridge arm voltage and the lower bridge arm voltage.

[0058] Step F performs broadband fast Fourier transform to obtain the high-frequency harmonic distribution. The specific process is as follows:

[0059] First, the obtained DC voltage is subjected to a fast Fourier transform;

[0060] Then, the high-frequency harmonic distribution is obtained through Fourier transform.

[0061] Embodiment 1

[0062] A theoretical evaluation method for MMC DC high-frequency harmonics caused by clock asynchrony comprises the following steps:

[0063] A. Calculate the AC voltage on the valve side

[0064] According to the rated AC voltage, AC current, AC connection reactor and other parameters, the time domain expression of the valve side AC voltage of the MMC is calculated as follows:

[0065]

[0066] Among them, u x-valv Indicates the valve side AC voltage of phase x, u x represents the AC voltage on the grid side of phase x, where x = a, b, c;

[0067] L acIndicates the equivalent connection inductance on the AC side;

[0068] i x It represents the alternating current of phase x, t is the time, and d is the differential sign.

[0069] u x and i x The specific expression is as follows:

[0070]

[0071] B. Calculate the bridge arm reference wave

[0072] According to the valve side AC voltage and DC rated voltage, calculate the bridge arm reference wave according to the following formula:

[0073]

[0074] C. Discretization of bridge arm output voltage

[0075] The continuous bridge arm voltage in the above formula (4) is converted into a discrete voltage based on the nearest level approximation as follows:

[0076]

[0077] D. Considering the additional delay, calculate the bridge arm output voltage

[0078] Assume that the rising edge of the lower bridge arm clock of phase x is delayed by Δt compared with that of the upper bridge arm. Therefore, the actual output voltage of the upper and lower bridge arms is specifically expressed as follows:

[0079]

[0080] E. Calculate DC voltage

[0081] According to the obtained upper bridge arm output voltage and lower bridge arm output voltage, the DC voltage is equal to the sum of the upper arm output voltage and the lower bridge arm output voltage, that is:

[0082] u dc =u' xp-NLM (t)+u' xn-NLM (t) (7).

[0083] F. Broadband Fast Fourier Transform

[0084] Perform fast Fourier transform on the DC voltage to obtain the high-frequency harmonic distribution.

[0085] The present invention takes into account factors such as different time delays, AC and DC voltage levels, submodule capacitor voltage levels, operating conditions, etc., and quickly calculates and evaluates the high-frequency harmonic frequency bands and contents caused by the asynchrony of the upper and lower bridge arm clocks.

Claims

1. A theoretical evaluation method for MMC DC high-frequency harmonics caused by clock asynchrony, characterized by: The following steps are involved: A. Calculate the AC voltage on the valve side; B. Calculate the bridge arm reference wave; C. Discretize the output voltage of the bridge arm; D. Considering the additional delay, calculate the output voltage of the bridge arm; E. Calculate DC voltage; F. Perform broadband fast Fourier transform to obtain the high-frequency harmonic distribution.

2. The method for theoretically evaluating MMC direct current high-frequency harmonics caused by clock asynchrony according to claim 1, characterized in that: Step A calculates the valve side AC voltage. The specific process is as follows: First, obtain the rated AC voltage, AC current, and AC connection reactance parameter data; Then, the time domain expression of the valve-side AC voltage of the MMC is calculated; Finally, the valve side AC voltage is obtained.

3. The method for theoretically evaluating MMC direct current high-frequency harmonics caused by clock asynchrony according to claim 1, characterized in that: Step B calculates the bridge arm reference wave, and the specific process is as follows: First, obtain the valve side AC voltage and DC rated voltage; Then, the bridge arm reference wave formula is established to calculate the bridge arm reference wave.

4. The method for theoretically evaluating MMC direct current high-frequency harmonics caused by clock asynchrony according to claim 1, characterized in that: Step C discretizes the output voltage of the bridge arm, and the specific process is as follows: Firstly, based on the bridge arm reference wave, the continuous bridge arm voltage is obtained; The continuous bridge arm voltage is then converted to a discrete voltage based on the nearest level approximation.

5. The method for theoretically evaluating MMC direct current high-frequency harmonics caused by clock asynchrony according to claim 1, characterized in that: Step D considers the additional delay and calculates the bridge arm output voltage. The specific process is as follows: First, suppose x The rising edge of the lower bridge arm clock is delayed compared to the upper bridge arm Δt ; Then, the expressions of the actual output voltage of the upper bridge arm and the actual output voltage of the lower bridge arm are obtained.

6. The method for theoretically evaluating MMC direct current high-frequency harmonics caused by clock asynchrony according to claim 1, characterized in that: Step E calculates the DC voltage, and the specific process is as follows: First, the upper bridge arm voltage and the lower bridge arm voltage are obtained; Then, the DC voltage is equal to the sum of the upper bridge arm voltage and the lower bridge arm voltage.

7. The method for theoretically evaluating MMC direct current high-frequency harmonics caused by clock asynchrony according to claim 1, characterized in that: Step F performs broadband fast Fourier transform to obtain the high-frequency harmonic distribution. The specific process is as follows: First, the obtained DC voltage is subjected to a fast Fourier transform; Then, the high-frequency harmonic distribution is obtained through Fourier transform.

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