A DC harmonic control method and system for cascaded hybrid DC systems

Abstract

This invention proposes a DC harmonic control method for cascaded hybrid DC systems, comprising: detecting the DC side of the cascaded hybrid DC system and transforming the detected data to obtain harmonic-containing DC current data of the DC side of the cascaded hybrid DC system; performing frequency conversion switching processing on the harmonic current signal based on the DC current data to output a DC harmonic suppression reference voltage; superimposing the DC harmonic suppression reference voltage and a pre-output AC modulation reference voltage to output reference voltages for each arm of the inverter MMC of the cascaded hybrid DC system; performing nearest-approximation point-level modulation and voltage equalization control on the reference voltages of each arm of the inverter MMC based on a preset modulation strategy to output a trigger pulse; and performing DC harmonic control on the DC side of the cascaded hybrid DC system based on the DC current data and the trigger pulse. This invention achieves the suppression of harmonic currents on the DC side of the cascaded hybrid DC transmission system.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of direct current transmission, in particular to a direct current harmonic control method and system for a cascaded hybrid direct current system. BACKGROUND

[0002] With the development of high voltage direct current transmission (HVDC) technology and the application of more and more direct current transmission projects in the world, high voltage direct current transmission has become an important part of the current power transmission field. At present, the direct current transmission based on grid commutated converter (LCC-HVDC) technology has been widely used, which has the advantages of large transmission capacity, good economy and high stability. With the development of the power grid towards large-scale cross interconnection, the shortcomings of LCC-HVDC technology are increasingly apparent: power control is not flexible, commutation failure and high reactive power demand. With the development of all-controlled power electronic devices, the voltage source converter-based direct current transmission technology (VSC-HVDC) has developed rapidly in recent years, among which the most representative is the high voltage direct current transmission based on modular multilevel converter (MMC-HVDC) technology. VSC-HVDC technology makes up for the shortcomings of traditional LCC-HVDC technology such as power control inflexibility, commutation failure and reactive power demand, but it has the limitations of high cost for large-capacity transmission projects and poor response to direct current side faults. Combining the advantages and disadvantages of the two transmission methods, hybrid direct current transmission technology emerges as the times require, which uses traditional current source converters on the rectifier side and MMC converters on the inverter side, improving the flexibility of transmission power while reducing construction costs and the requirement for the capacity support capability of the inverter side AC system.

[0003] At the same time, the LCC-MMC hybrid direct current transmission technology makes the harmonic problem of the direct current system more complex due to the interaction of multiple converters. First, the rectifier side LCC converter will bring a series of harmonic problems to the AC / DC system; second, the MMC converter on the inverter side will also bring a certain amount of direct current harmonics; finally, the harmonics generated by different converters in the hybrid direct current transmission system on the direct current side may superimpose and affect each other, and the types and harmonic content of the harmonics on the direct current transmission line will increase; at the same time, the harmonics generated on both sides may enter the opposite side AC system through the transmission of the converter, causing harmonic instability problems.

[0004] The harmonic problem of LCC-HVDC has been relatively mature, and the main means for suppressing the ripple on the direct current side of this type of system is to install a filter device and a harmonic elimination reactor on the direct current side:

[0005] 1) Direct current filter

[0006] The direct current filter is the most direct and most commonly used filtering device in the direct current filtering system, and is divided into a passive filter and an active filter. The passive filter is only composed of a resistor, a capacitor and an inductor, is connected to a direct current transmission line through different combination modes, and is used for filtering out a direct current harmonic current. The passive filter has the advantages of simple parameter design and convenient operation and maintenance. However, the disadvantages are also obvious, and the specific performance is that: the parameters of the filter are easily affected by the outside world, so that the harmonic parameters cannot be matched with the filter parameters, and the detuning is prone to occur; the parameters of the filter can be matched with the parameters of the direct current transmission line, so that the harmonic is amplified, and there is a risk of parallel resonance, which seriously endangers the safe operation of the power grid. The active filter can realize dynamic tracking compensation, more harmonic frequencies can be filtered out, there is no risk of detuning, and the possibility of series and parallel resonance is small. However, due to the high cost, the active filter is still in the initial stage of research at present, and the engineering experience is less, so the active filter has not been widely applied.

[0007] 2) smoothing reactor

[0008] In the traditional high-voltage direct current transmission project, the smoothing reactor is generally arranged at the direct current outlet of each pole of the rectifier station and the inverter station. The main functions are: ① reducing the current pulsation component (i.e. current ripple) in the direct current transmission line; ② filtering part of the harmonic; ③ when the direct current transmission line encounters a fault, it can suppress the excessively high direct current; ④ when the current is small or intermittent, it can maintain the continuity of the direct current, thereby reducing the commutation failure rate of the converter.

[0009] However, the current harmonic research of the MMC-HVDC is mostly concentrated in the double-frequency circulating current and the suppression measures, and the harmonic problem of the direct current side is less researched. Since the waveform distortion rate of the MMC alternating current voltage generally meets the requirements, the direct current filtering device is generally not arranged on the direct current side, and therefore the control method of the MMC direct current system can be improved to realize the suppression of the harmonic of the MMC-HVDC direct current side.

[0010] For the LCC-MMC hybrid direct current transmission system, the direct current harmonic problems caused by the LCC converter and the MMC converter will interact and overlap, so that the direct current harmonic problem of the hybrid direct current transmission system is more complex. At present, there are few engineering examples or experiences related to the harmonic suppression of the hybrid direct current transmission project. A joint direct current harmonic suppression method is adopted in the literature, that is, the direct current filtering device is arranged on the LCC side, and at the same time, the control strategy considering the direct current side harmonic suppression is adopted on the MMC side, which provides a new idea for solving the direct current harmonic problem of the LCC-MMC cascaded hybrid direct current transmission system. However, since the above method needs to arrange the direct current filtering device, the equipment cost is increased, the land occupation problem is brought, and even under certain operating conditions, the problems of detuning, resonance and the like affecting the safe and stable operation may occur. SUMMARY

[0011] In order to solve the above problems, the application provides a DC harmonic control method for a cascade hybrid DC system, which comprises the following steps:

[0012] The DC side of the cascade hybrid DC system is detected, and the detection data is transformed to obtain DC current data containing harmonics of the DC side of the cascade hybrid DC system;

[0013] The frequency conversion and exchange processing of the harmonic current signal is carried out based on the DC current data, and a DC harmonic suppression reference voltage is outputted;

[0014] The DC harmonic suppression reference voltage and the previously outputted AC modulation reference voltage are superimposed, the reference voltage of each bridge arm of the inverter MMC of the cascade hybrid DC system is outputted, the nearest approximation point flat modulation and voltage equalization control are carried out on the reference voltage of each bridge arm of the inverter MMC based on a preset modulation strategy, and the trigger pulse is outputted;

[0015] The DC harmonic control is carried out on the DC side of the cascade hybrid DC system based on the DC current data and the trigger pulse;

[0016] The DC harmonic control comprises suppressing the harmonic current in the DC current of the DC side of the cascade hybrid DC system, so that the harmonic current content in the DC current is less than or equal to a preset threshold value.

[0017] Optionally, the DC current data comprises a DC current phasor, a DC current component, a harmonic current and an order corresponding to the harmonic current.

[0018] Optionally, the DC harmonic control module is used for carrying out the frequency conversion and exchange processing of the harmonic current signal based on the DC current data, and outputting the DC harmonic suppression reference voltage, which comprises the following steps:

[0019] For the harmonic current of a preset order in the DC current data, the positive and negative sequence suppression voltage phasors of the corresponding order are added to the harmonic current of the preset order, the DC harmonic signal frequency conversion and exchange processing and the bridge arm AC suppression voltage amplitude calculation are sequentially carried out on the harmonic current of the preset order of the added positive and negative sequence suppression voltage phasors of the corresponding order, the three-phase AC suppression voltage reference value is obtained, and the DC harmonic suppression reference voltage is outputted based on the three-phase AC suppression voltage reference value.

[0020] Optionally, the method further comprises obtaining the harmonic current content in the DC current based on the DC current data, and carrying out the DC harmonic control on the DC side of the cascade hybrid DC system if the harmonic current content is greater than a preset threshold value.

[0021] Optionally, after the DC side of the cascaded hybrid DC system is controlled by the DC harmonic control, the feedback control result is fed back, and based on the feedback control result, it is checked whether the preset modulation strategy meets the harmonic current suppression standard under each operating condition of the cascaded hybrid DC system.

[0022] The application further provides a DC harmonic control system for a cascaded hybrid DC system, comprising a harmonic detection module, a DC harmonic control module, a power transmission control and circulating current suppression module, and a modulation strategy and capacitor voltage equalization control module.

[0023] The harmonic detection module is used for detecting the DC side of the cascaded hybrid DC system and transforming the detection data to obtain the DC current data containing harmonics of the DC side of the cascaded hybrid DC system.

[0024] The DC harmonic control module is used for performing frequency conversion and exchange processing of the harmonic current signal based on the DC current data and outputting a DC harmonic suppression reference voltage.

[0025] The DC harmonic control module comprises a plurality of control loops, and based on the DC current data and the trigger pulse, the control loops are started to control the DC side of the cascaded hybrid DC system, the DC harmonic control comprises suppressing the harmonic current in the DC current of the DC side of the cascaded hybrid DC system, so that the harmonic current content in the DC current is less than or equal to the threshold value of the control loop.

[0026] The power transmission control and circulating current suppression module is used for outputting an AC modulation reference voltage.

[0027] The modulation strategy and capacitor voltage equalization control module is used for superimposing the DC harmonic suppression reference voltage and the AC modulation reference voltage to output the reference voltage of each bridge arm of the inverter MMC of the cascaded hybrid DC system, performing nearest approximation point flat modulation and voltage equalization control on the reference voltage of each bridge arm of the inverter MMC based on the modulation strategy of the capacitor voltage equalization control module, and outputting the trigger pulse.

[0028] Optionally, the DC current data comprises any one or more of the following: DC current phasor, DC current component, harmonic current, and order corresponding to the harmonic current.

[0029] Optionally, the DC harmonic control module performs frequency conversion and exchange processing of the signal based on the DC current data to output the DC harmonic suppression reference voltage, comprising:

[0030] The preset order harmonic current in the direct current data is added with positive and negative sequence suppression voltage phasors of corresponding orders, the preset order harmonic current of the added positive and negative sequence suppression voltage phasors of corresponding orders is subjected to direct current harmonic signal frequency conversion exchange processing and bridge arm alternating current suppression voltage amplitude calculation in turn, three-phase alternating current suppression voltage reference values are obtained, and a direct current harmonic suppression reference voltage is output based on the three-phase alternating current suppression voltage reference values.

[0031] Optionally, the direct current harmonic suppression module is further configured to obtain harmonic current content in the direct current based on the direct current data, and determine that the control loop is put into operation if the harmonic current content is greater than a threshold value of the control loop.

[0032] Optionally, the direct current harmonic control module feeds back a control result to the modulation strategy and capacitor voltage sharing control module after performing direct current harmonic control on the direct current side of the cascade hybrid direct current system, and the modulation strategy and capacitor voltage sharing control module checks whether the modulation strategy meets the suppression standard of harmonic currents under each operating condition of the cascade hybrid direct current system based on the feedback control result.

[0033] Compared with the prior art, the method has the following beneficial effects:

[0034] The application provides a direct current harmonic control method for a cascade hybrid direct current system, which comprises the following steps: detecting a direct current side of the cascade hybrid direct current system to obtain direct current data containing harmonic currents of the direct current side of the cascade hybrid direct current system; performing frequency conversion exchange processing on the harmonic current signals based on the direct current data to output a direct current harmonic suppression reference voltage; superimposing the direct current harmonic suppression reference voltage and a previously output alternating current modulation reference voltage to output reference voltages of each bridge arm of a cascade hybrid direct current system inverter MMC, performing nearest approximation point flat modulation and voltage sharing control on the reference voltages of each bridge arm of the inverter MMC based on a preset modulation strategy to output trigger pulses; and performing direct current harmonic control on the direct current side of the cascade hybrid direct current system based on the direct current data and the trigger pulses.

[0035] The application further provides a DC harmonic control system for a cascade hybrid DC system, comprising: a harmonic detection module, a DC harmonic control module, a power transmission control and circulating current suppression module, and a modulation strategy and capacitor voltage equalization control module; the harmonic detection module is used for detecting a DC side of the cascade hybrid DC system to obtain DC current data containing harmonics of the DC side of the cascade hybrid DC system; the DC harmonic control module is used for frequency conversion and exchange processing of harmonic current signals based on the DC current data to output a DC harmonic suppression reference voltage; the DC harmonic control module comprises a plurality of control loops, and the control loops are started based on the DC current data and a trigger pulse to perform DC harmonic control on the DC side of the cascade hybrid DC system, the DC harmonic control comprising suppression of the harmonic current in the DC current of the DC side of the cascade hybrid DC system, so that the harmonic current content in the DC current is less than or equal to a threshold value of the control loop; and the power transmission control and circulating current suppression module is used for outputting an AC modulation reference voltage. The system of the application suppresses the harmonic current of the DC side of the hybrid DC power transmission system through the DC harmonic control module, avoids the problems of material cost and maintenance manpower cost caused by installation of DC filter hardware devices in the cascade hybrid DC power transmission system, and has economy and operation reliability; the plurality of control loops of the DC harmonic control module can suppress harmonic currents of multiple orders, and have flexibility. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 Flow chart of the method of the application;

[0037] Figure 2 Structure diagram of the system of the application;

[0038] Figure 3 LCC-MMC hybrid DC power transmission example system structure diagram of the system example of the application;

[0039] Figure 4 Three-time DC harmonic content comparison diagram of the system example of the application before and after three-time DC harmonic control loops are put into operation. DETAILED DESCRIPTION

[0040] Reference will now be made to the drawings to describe the exemplary embodiments of the present application in detail. The application may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and fully convey the scope of the application to those skilled in the art. Like reference numerals refer to like elements throughout the specification. It will be understood that when an element or layer is referred to as being "on" another element or substrate, it can be directly on the element or substrate or intervening layers can also be present. In addition, it will also be understood that when an element is referred to as being "coupled" to another element, it can be directly coupled to the other element or intervening layers can also be present. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0041] The terms used herein, including technical terms, have meanings commonly understood by those skilled in the art, unless otherwise specified. In addition, it is understood that terms defined in commonly used dictionaries should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and should not be interpreted in an idealized or overly formal sense.

[0042] In order to solve the problems of the prior art, such as the need to install a direct current filtering device, the increase of equipment cost, the problem of occupying land, and even the problem of affecting safe and stable operation under certain operating conditions, such as detuning and resonance, the present application provides a direct current harmonic control method and system for a cascaded hybrid direct current system. The method of the present application obtains direct current data through detection of the direct current side of the cascaded hybrid direct current system, outputs a trigger pulse, and realizes suppression of the harmonic current on the direct current side of the cascaded hybrid direct current transmission system according to the trigger pulse. The system of the present application suppresses the harmonic current on the direct current side of the hybrid direct current transmission system through a direct current harmonic control module, avoids the problems of material cost and maintenance manpower cost caused by the installation of a direct current filtering hardware device in the cascaded hybrid direct current transmission system, and has economic efficiency and operation reliability. The multiple control loops of the direct current harmonic control module can suppress harmonic currents of multiple orders, and have flexibility.

[0043] In order to better understand the present application, the content of the present application will be further described below in combination with the drawings and examples of the specification.

[0044] Example 1:

[0045] The present application provides a direct current harmonic control method for a cascaded hybrid direct current system, as shown in Figure 1 , which includes:

[0046] Step 1: detecting the direct current side of the cascaded hybrid direct current system and transforming the detection data to obtain the harmonic-containing direct current data of the direct current side of the cascaded hybrid direct current system. The direct current data includes any one or more of the following: direct current phasor, direct current component, harmonic current, and order corresponding to the harmonic current.

[0047] Step 2: performing frequency conversion and exchange processing on the harmonic current signal based on the direct current data, and outputting a direct current harmonic suppression reference voltage;

[0048] Step 3: superimposing the direct current harmonic suppression reference voltage and the pre-output alternating current modulation reference voltage to output a reference voltage for each bridge arm of the cascaded hybrid direct current system inverter MMC, performing nearest approximation point flat modulation and voltage equalization control on the reference voltage for each bridge arm of the inverter MMC based on a pre-set modulation strategy, and outputting a trigger pulse;

[0049] Step 4: Perform DC harmonic control on the DC side of the cascaded hybrid DC system based on DC current data and trigger pulses; the DC harmonic control includes suppressing the harmonic current in the DC current on the DC side of the cascaded hybrid DC system, so that the harmonic current content in the DC current is less than or equal to a preset threshold value.

[0050] The specific implementation steps are as follows:

[0051] Step 2 involves frequency conversion processing of the harmonic current signal based on the DC current data, outputting a DC harmonic suppression reference voltage, including:

[0052] For the harmonic current of a preset order in the DC current data, positive and negative sequence suppression voltage phasors of the corresponding order are added to the harmonic current of the preset order. The harmonic current of the preset order with the added positive and negative sequence suppression voltage phasors of the corresponding order is subjected to DC harmonic signal frequency conversion switching processing and bridge arm AC suppression voltage amplitude calculation in sequence to obtain the three-phase AC suppression voltage reference value. Based on the three-phase AC suppression voltage reference value, the DC harmonic suppression reference voltage is output.

[0053] After step 3 is implemented, the following steps are also included: obtaining the harmonic current content in the DC current based on the DC current data; if the harmonic current content is greater than a preset threshold value, then DC harmonic control is performed on the DC side of the cascaded hybrid DC system.

[0054] After step 4, the process further includes: performing DC harmonic control on the DC side of the cascaded hybrid DC system, feeding back the control results, and verifying, based on the feedback control results, whether the preset modulation strategy meets the harmonic current suppression standard under each operating condition of the current cascaded hybrid DC system.

[0055] Example 2:

[0056] Based on the same inventive concept, a DC harmonic control system for a cascaded hybrid DC system implements the function of a "virtual DC filter" through internal control of the MMC (Multi-Channel Controller). Multiple control loops are designed for DC harmonic suppression to address the 3k, 6k, and 12k (k = 1, 2, ...) order DC harmonics that may arise from the coexistence of the MMC and LCC (Limited-Channel Controller) converters. Ultimately, through the multi-loop additional DC harmonic suppression control strategy of the MMC, the DC-side harmonics generated by the LCC and MMC devices are suppressed, avoiding the problems associated with adding a DC filter. Figure 2 As shown, the system of the present invention includes: a harmonic detection module, a DC harmonic control module, a power transmission control and circulating current suppression module, and a modulation strategy and capacitor voltage equalization control module;

[0057] The harmonic detection module is used to detect the DC side of the cascaded hybrid DC system and transform the detection data to obtain the harmonic-containing DC current data of the DC side of the cascaded hybrid DC system; wherein, the DC current data includes: DC current phasor, DC current components, harmonic current, and the order corresponding to the harmonic current.

[0058] The DC harmonic control module is used to perform frequency conversion switching processing of harmonic current signals based on DC current data and output a DC harmonic suppression reference voltage.

[0059] The DC harmonic control module includes multiple control loops. Based on DC current data and trigger pulses, the control loops are started to perform DC harmonic control on the DC side of the cascaded hybrid DC system. The DC harmonic control includes suppressing the harmonic current in the DC current on the DC side of the cascaded hybrid DC system, so that the harmonic current content in the DC current is less than or equal to the threshold value of the control loop.

[0060] The power transmission control and circulating current suppression module is used to output an AC modulation reference voltage;

[0061] The modulation strategy and capacitor voltage equalization control module are used to superimpose the DC harmonic suppression reference voltage and the AC modulation reference voltage to output the reference voltage of each bridge arm of the cascaded hybrid DC system inverter MMC. Based on the modulation strategy of the capacitor voltage equalization control module, the reference voltage of each bridge arm of the inverter MMC is subjected to nearest approximation point leveling modulation and voltage equalization control to output trigger pulses.

[0062] The specific implementation operations of each module in this invention are as follows:

[0063] The harmonic detection module detects the DC side of the cascaded hybrid DC system, using the system's synchronous angular frequency as a reference. It then performs a Fourier transform on the detected data to obtain DC current data, including: DC current phasor, DC current components, harmonic currents, and the corresponding orders of the harmonic currents.

[0064] The harmonic current components satisfy the following relationship:

[0065] I d =I d0 +I dH1 +…+I dHm

[0066] Among them, I d I is the DC current phasor containing harmonics. d0 For the DC current component, I dHi (i=1,…,m) represents the main harmonic currents on the DC side;

[0067] The DC harmonic control module is located in the MMC bridge arm and is used for frequency conversion processing of harmonic current signals based on DC current data, outputting a DC harmonic suppression reference voltage, including:

[0068] For the harmonic current of a preset order in the DC current data, positive and negative sequence suppression voltage phasors of the corresponding order are added to the harmonic current of the preset order. The harmonic current of the preset order with the added positive and negative sequence suppression voltage phasors of the corresponding order is subjected to DC harmonic signal frequency conversion switching processing and bridge arm AC suppression voltage amplitude calculation in sequence to obtain the three-phase AC suppression voltage reference value. Based on the three-phase AC suppression voltage reference value, the DC harmonic suppression reference voltage is output.

[0069] Specifically, the preset orders are explained below as 3k, 6k, and 12k:

[0070] For the 3k, 6k, and 12k harmonic currents on the DC side, each control loop in the DC harmonic control module adds corresponding positive and negative sequence suppression voltage phasors of order 3k±1, 6k±1, and 12k±1 to the 3k, 6k, and 12k harmonic currents on the DC side. After transformation, the DC harmonic suppression reference voltage is determined according to the DC harmonic suppression reference voltage calculation formula.

[0071] Based on the DC harmonic characteristics of MMC, the 3k+1 (or 6k+1, 12k+1)th positive sequence voltage or the 3k-1 (or 6k-1, 12k-1)th negative sequence voltage on the AC side of the MMC arm will generate the 3k (or 6k, 12k)th DC characteristic harmonic on the DC side of the MMC.

[0072] Therefore, the DC harmonic suppression reference voltage u dH_refj The calculation formula is as follows:

[0073] u dH_refj =k1u 3k+1j +k2u 3k-1j +k3u 6k+1j +k4u 6k-1j +k5u 12k+1j +k6u 12k-1j

[0074] Among them, u 3k+1j u 3k-1j u 6k+1j u 6k-1j u 12k+1j u 12k-1jThe 3k+1 (or 6k+1, 12k+1) positive sequence voltage or 3k-1 (or 6k-1, 12k-1) negative sequence voltage is added to each control loop. The voltage amplitude is linearly related to the amplitude of the 3k (or 6k, 12k) DC characteristic harmonic detected by the harmonic detection module. The proportionality coefficient is generally taken as 2 to 8. The principle for selecting the initial phase value of the voltage is: the initial phase of the a-phase voltage of the 3k±1 (or 6k±1, 12k±1) additional voltage is the same as the initial phase of the 3k (or 6k, 12k) DC characteristic harmonic detected by the harmonic detection module. The b and c phase voltages are obtained according to the abc phase sequence and the positive and negative sequence relationship. k1, k2, k3, k4, k5, and k6 represent the control coefficients of each harmonic control loop, which need to be obtained by parameter debugging in conjunction with the application system example.

[0075] For the j-th (j=a,b,c) phase bridge arm of the MMC, a DC harmonic suppression reference voltage u is added to the AC modulation reference voltage uref_1j obtained through the power control and circulating current suppression modules. dH_refj The reference voltage of the j-th (j=a,b,c) phase bridge arm of the cascaded hybrid DC system inverter MMC is output. Based on the modulation strategy of the capacitor voltage equalization control module, the reference voltage of each bridge arm of the inverter MMC is subjected to nearest approximation point leveling and voltage equalization control to output trigger pulses.

[0076] The DC harmonic suppression module is also used to obtain the harmonic current content in the DC current based on the DC current data. If the harmonic current content is greater than the threshold value of the control loop, then the control loop is determined to be in operation.

[0077] Specifically, for the control loop, the first step is to determine whether the loop is operational through comparison and logic gates, setting the control loop action threshold to |I|. dHi_ref | If the DC harmonic content I dHi_p =I dHi / I d The value is greater than the action threshold of the loop, i.e., I dHi_p >|I dHi_ref If |, then the control loop is activated.

[0078] Then, the control loop receives the amplitude and phase information of the specific order harmonic current obtained from the harmonic detection module, and suppresses it through the harmonic control loop, so that I... dHi_p ≤|I dHi_ref |

[0079] The DC harmonic suppression module is also used to obtain the harmonic current content in the DC current based on the DC current data. If the harmonic current content is greater than the threshold value of the control loop, then the control loop is determined to be in operation.

[0080] After performing DC harmonic control on the DC side of the cascaded hybrid DC system, the DC harmonic control module feeds back the control results to the modulation strategy and capacitor voltage equalization control module. Based on the feedback control results, the modulation strategy and capacitor voltage equalization control module verifies whether the modulation strategy meets the harmonic current suppression standard under various operating conditions of the current cascaded hybrid DC system.

[0081] Example 3:

[0082] by Figure 3 The ±400kV LCC-MMC hybrid DC transmission example system shown is used to verify the system in Example 2. The example system uses a single LCC converter station on the rectifier side and a parallel connection of two LCC and MMC converter stations with two landing points on the inverter side. The receiving-end system is a 500kV AC system. To address the DC current harmonic problem present on the MMC DC output side of this example system, the DC harmonic detection module first analyzes and finds that the third harmonic is the main component of the MMC DC current. A 3kth DC harmonic control loop is then implemented in the MMC, and the suppression of the third DC harmonic is achieved through software control.

[0083] The implementation process is as follows:

[0084] The harmonic detection module detects the harmonic order and harmonic voltage amplitude on the DC side of the cascaded hybrid DC system. Using the system's synchronous angular frequency as a reference, a Fast Fourier Transform (FFT) is performed on the DC side current of the MMC to obtain the harmonic components on the DC side of the cascaded hybrid transmission system. Each harmonic component satisfies the following relationship:

[0085] I d =I d0 +I dH3 +I dH6 +I dH12 +…

[0086] Where Id represents the DC current phasor containing harmonics, I d0 I represents the DC current component. dH3 I dH6 I dH12 The main harmonic current components on the DC side are the 3rd, 6th, and 12th harmonics. Among them, the 3rd harmonic problem is prominent in this example system. After the system is put into operation for 3 seconds, the amplitude of the 3rd harmonic of the DC side current of the MMC can reach 25A and will continue to increase, posing a certain threat to the safe and stable operation of the system.

[0087] The three-loop control system of the DC harmonic control module is applied to the system of this example. The three loops are respectively attached with the corresponding positive and negative sequence suppression voltage phasors of the 3k±1, 6k±1, and 12k±1 orders for the characteristic harmonic currents of the DC side.

[0088] Specifically, for a DC harmonic control loop of any order harmonic, the first step is to determine whether the loop is operational through comparison and logic gates, setting the control loop's action threshold to |I|. dHi_ref | If the DC harmonic content I dHi_p =I dHi / I d The value is greater than the action threshold of the loop, i.e., I dHi_p >|I dHi_ref If the harmonic control loop is activated, the control threshold for the 3rd harmonic is set to 0.4%, and the control thresholds for the 6th and 12th harmonics are set to 0.3%. The DC current measurement value of the MMC is 5kA, so the corresponding reference values ​​for the 3rd, 6th, and 12th current harmonic control actions are 20, 15, and 15A, respectively.

[0089] Then, the control loop receives the amplitude and phase information of the 3rd, 6th, and 12th current harmonics from the harmonic detection module (higher-order harmonics are ignored). For the main 3rd current harmonic problem, it suppresses it through the 3kth harmonic control loop, making I... dH3_p ≤|I dH3_ref |=20A.

[0090] Based on the DC harmonic characteristics of the MMC, the fourth positive-sequence voltage or the second negative-sequence voltage on the AC side of the MMC arm will generate the third DC characteristic harmonic on the DC side of the MMC. Therefore, this embodiment uses a DC harmonic suppression reference voltage u. dH3_refj Designed as follows:

[0091] u dH3_refj =k1u 4j +k2u 2j

[0092] Among them, u 4j u 2j The 4th positive sequence voltage or 2nd negative sequence voltage is added to the 3k DC harmonic control circuit. The voltage amplitude is linearly related to the amplitude of the 3rd DC characteristic harmonic detected in step 1, which is 5.1578 and 7.3684 times, respectively. The principle for selecting the initial phase value of the voltage is: the initial phase of the a-phase voltage of the 3rd added voltage is the same as the initial phase of the 3rd DC characteristic harmonic detected by the harmonic detection module, and the b and c phase voltages are obtained according to the abc phase sequence and the positive and negative sequence relationship. k1 and k2 represent the control amplification coefficients of the 3rd harmonic controller. The parameters are adjusted in conjunction with the application system example, and the optimal values ​​are k1 = 0.8 and k2 = 0.7.

[0093] Comparison of the amplitude of the third harmonic of the MMC DC side current before and after the 3k DC harmonic control circuit is put into operation, for example Figure 4As shown, the MMC harmonic control strategy effectively suppressed the third harmonic, proving the effectiveness of the control strategy and avoiding the hassle of installing a corresponding DC filter.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of the claims of the present invention pending approval.

Claims

1. A DC harmonic control method for a cascaded hybrid DC system, characterized in that, The method includes: The DC side of the cascaded hybrid DC system is detected, and the detection data is transformed to obtain the DC current data containing harmonics on the DC side of the cascaded hybrid DC system. The harmonic current signal is converted and switched based on DC current data, and a DC harmonic suppression reference voltage is output. The DC harmonic suppression reference voltage and the pre-output AC modulation reference voltage are superimposed to output the reference voltage of each bridge arm of the cascaded hybrid DC system inverter MMC. Based on the preset modulation strategy, the reference voltage of each bridge arm of the inverter MMC is subjected to nearest approximation level modulation and voltage equalization control to output trigger pulses. DC harmonic control is performed on the DC side of the cascaded hybrid DC system based on DC current data and trigger pulses; The DC harmonic control includes suppressing the harmonic current in the DC current on the DC side of the cascaded hybrid DC system, so that the harmonic current content in the DC current is less than or equal to a preset threshold value.

2. The method according to claim 1, characterized in that, The DC current data includes: DC current phasor, DC current components, harmonic current, and any one or more of the orders corresponding to the harmonic current.

3. The method according to claim 1, characterized in that, The frequency conversion switching processing of harmonic current signals based on DC current data, outputting a DC harmonic suppression reference voltage, includes: For the harmonic current of a preset order in the DC current data, positive and negative sequence suppression voltage phasors of the corresponding order are added to the harmonic current of the preset order. The harmonic current of the preset order with the added positive and negative sequence suppression voltage phasors of the corresponding order is subjected to DC harmonic signal frequency conversion switching processing and bridge arm AC suppression voltage amplitude calculation in sequence to obtain the three-phase AC suppression voltage reference value. Based on the three-phase AC suppression voltage reference value, the DC harmonic suppression reference voltage is output.

4. The method according to claim 1, characterized in that, The method further includes: obtaining the harmonic current content in the DC current based on DC current data; if the harmonic current content is greater than a preset threshold value, then performing DC harmonic control on the DC side of the cascaded hybrid DC system.

5. The method according to claim 1, characterized in that, After performing DC harmonic control on the DC side of the cascaded hybrid DC system, the control results are fed back. Based on the feedback control results, it is verified whether the preset modulation strategy meets the harmonic current suppression standard under various operating conditions of the current cascaded hybrid DC system.

6. A DC harmonic control system for a cascaded hybrid DC system, characterized in that, The system includes: a harmonic detection module, a DC harmonic control module, a power transmission control and circulating current suppression module, and a modulation strategy and capacitor voltage equalization control module. The harmonic detection module is used to detect the DC side of the cascaded hybrid DC system and transform the detection data to obtain the harmonic-containing DC current data of the DC side of the cascaded hybrid DC system. The DC harmonic control module is used to perform frequency conversion switching processing of harmonic current signals based on DC current data and output a DC harmonic suppression reference voltage. The DC harmonic control module includes multiple control loops. Based on DC current data and trigger pulses, the control loops are started to perform DC harmonic control on the DC side of the cascaded hybrid DC system. The DC harmonic control includes suppressing the harmonic current in the DC current on the DC side of the cascaded hybrid DC system, so that the harmonic current content in the DC current is less than or equal to the threshold value of the control loop. The power transmission control and circulating current suppression module is used to output an AC modulation reference voltage; The modulation strategy and capacitor voltage equalization control module are used to superimpose the DC harmonic suppression reference voltage and the AC modulation reference voltage to output the reference voltage of each bridge arm of the cascaded hybrid DC system inverter MMC. Based on the modulation strategy of the capacitor voltage equalization control module, the reference voltage of each bridge arm of the inverter MMC is modulated and equalized to output a trigger pulse.

7. The system according to claim 6, characterized in that, The DC current data includes: DC current phasor, DC current components, harmonic current, and the order corresponding to the harmonic current.

8. The system according to claim 6, characterized in that, The DC harmonic control module is used to perform frequency conversion switching processing of harmonic current signals based on DC current data, and outputs a DC harmonic suppression reference voltage, including: For the harmonic current of a preset order in the DC current data, positive and negative sequence suppression voltage phasors of the corresponding order are added to the harmonic current of the preset order. The harmonic current of the preset order with the added positive and negative sequence suppression voltage phasors of the corresponding order is subjected to DC harmonic signal frequency conversion switching processing and bridge arm AC suppression voltage amplitude calculation in sequence to obtain the three-phase AC suppression voltage reference value. Based on the three-phase AC suppression voltage reference value, the DC harmonic suppression reference voltage is output.

9. The system according to claim 6, characterized in that, The DC harmonic control module is also used to obtain the harmonic current content in the DC current based on the DC current data. If the harmonic current content is greater than the threshold value of the control loop, then the control loop is determined to be in operation.

10. The system according to claim 6, characterized in that, After performing DC harmonic control on the DC side of the cascaded hybrid DC system, the DC harmonic control module feeds back the control results to the modulation strategy and capacitor voltage equalization control module. Based on the feedback control results, the modulation strategy and capacitor voltage equalization control module verifies whether the modulation strategy meets the harmonic current suppression standard under various operating conditions of the current cascaded hybrid DC system.

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