A power balancing control method for MMC at the receiving end of a hybrid cascaded high-voltage direct current transmission system

Abstract

The present invention relates to the technical field of protection of direct current transmission systems, and discloses a power balancing control method for MMC at the receiving end of a hybrid cascaded high-voltage direct current transmission system, comprising: S1, collecting data of LCC and MMC at the receiving end of the hybrid cascaded high-voltage direct current transmission system and calculating the real-time direct current power of LCC and MMC; S2, establishing a balancing control model for the MMC in a constant direct current power mode according to the data collected in step S1, and performing balancing control on the MMC in the constant direct current power mode according to the balancing control model. The present invention realizes automatic balancing control of the power of the constant direct current voltage MMC and the constant direct current power MMC when the direct current transmission power is reduced due to a fault at the sending end of the hybrid cascaded high-voltage direct current transmission system, thereby avoiding the phenomenon of large power fluctuations caused by the constant direct current voltage MMC bearing the direct current power shortage alone, and contributing to the safe and stable operation of the high-voltage direct current transmission system and its receiving end AC system.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current transmission system protection, and in particular to a receiving-end MMC power balancing control method of a hybrid cascade high-voltage direct current transmission system. Background Art

[0002] The high-voltage direct current transmission system has a large transmission capacity, a long transmission distance, and low transmission loss. It can effectively solve the urgent needs of new energy transmission and load center power supply, and is the only way for the development of my country's power industry. The basic principle of high-voltage direct current transmission is: at the transmission end of the high-voltage direct current transmission system, the converter is used for rectification to convert the three-phase alternating current into direct current, and the electric energy is transmitted through the high-voltage direct current transmission line. Then, at the receiving end of the high-voltage direct current transmission system, the converter is used for inversion to convert the direct current into three-phase alternating current, and the electric energy is sent to the AC system at the receiving end.

[0003] The current high-voltage direct current transmission technology can be divided into two categories: one is conventional direct current transmission using line commutated converter (LCC); the other is flexible direct current transmission using modular multilevel converter (MMC). Conventional direct current transmission has the inherent problem of commutation failure, while flexible direct current transmission is more expensive, and both have their own advantages and disadvantages. In order to maximize the technical advantages of conventional direct current transmission and flexible direct current transmission, it has become an important development trend to use both direct current transmission technologies in the same project to form a hybrid direct current transmission system. Hybrid cascade is one of the mainstream topological forms of hybrid direct current transmission. The sending end still uses LCC, the high-voltage end of the receiving end uses LCC, and the low-voltage end uses three MMCs in parallel. The high and low ends each evacuate 50% of the power. In addition, the LCC and three MMCs at the receiving end are distributed and connected to different AC systems to achieve power transmission.

[0004] In a hybrid cascade HVDC transmission system, although the topological positions of the three parallel MMCs at the receiving end are equal, their control methods are different. Usually, one MMC adopts a constant DC voltage control mode, and the other two MMCs adopt a constant DC power control mode. When a fault occurs at the sending end of the DC transmission system, resulting in a DC power shortage, since the DC power demand of the constant DC power MMC can be regarded as unchanged, the constant DC voltage MMC must significantly reduce the DC power, or even reverse the power direction. This phenomenon will cause the AC system connected to the constant DC voltage MMC to suffer significant power fluctuations, causing a significant shift in the interconnection line power flow, threatening the safe and stable operation of the power grid. Summary of the invention

[0005] The object of the present invention is to provide a power balancing control method for MMC at the receiving end of a hybrid cascaded high-voltage direct current transmission system. The method can achieve power balancing of MMCs with constant DC voltage and constant DC power when the DC transmission power is reduced due to a fault at the sending end of the hybrid cascaded high-voltage direct current transmission system, effectively avoid significant power fluctuations of the MMC with constant DC voltage, and ensure the safe operation of the AC system to which it is connected.

[0006] The present invention is achieved through the following technical solutions:

[0007] A hybrid cascade high-voltage direct current transmission system receiving-end MMC power balancing control method, comprising:

[0008] S1, collecting data of LCC and MMC at the receiving end of the hybrid cascade HVDC transmission system and calculating the real-time DC power of LCC and MMC;

[0009] S2. Establish a balanced control model for the MMC in constant DC power mode according to the data collected in step S1, and perform balanced control on the MMC in constant DC power mode according to the balanced control model.

[0010] As an optimization, the data of the receiving end LCC and MMC of the hybrid cascade HVDC transmission system include the DC current I LCC 、LCC DC voltage V LCC 、MMC DC current I MMC and the DC voltage V of the MMC MMC .

[0011] As an optimization, the real-time DC power of the LCC and the MMC includes the real-time DC power P of the LCC. LCC , Real-time DC power P of MMC in constant DC voltage mode MMC_V , the real-time DC power P of any fixed DC power mode MMC MMC_P .

[0012] As an optimization, the balanced control model of the fixed DC power mode MMC performs balanced control on the fixed DC power mode MMC in the following specific method: the DC power command value of the fixed DC power mode MMC is corrected to a variable value, and the real-time DC power P of the LCC is calculated based on the DC power command value of the fixed DC power mode MMC. LCC Correct the DC power control command value P of the constant DC power mode MMC in real time ref .

[0013] As an optimization, the DC power control command value P of the fixed DC power mode MMC ref for:

[0014]

[0015] Among them, Pref_N P is the DC power control command value (manually set) of the MMC in the DC power mode under normal operating conditions; LCC_N is the DC power of LCC under normal operating conditions; ΔP is the preset power drop threshold value; ΔP ref It is the DC power control instruction adjustment amount calculated in real time.

[0016] As an optimization, the DC power control instruction adjustment amount ΔP calculated in real time ref The calculation formula is as follows:

[0017] ΔP ref =k p (P MMC_P -P MMC_V )+k i ∫(P MMC_P -P MMC_V )dt

[0018] Among them, k p is the proportional coefficient of the PI regulator; k i is the integral coefficient of the PI regulator, P MMC_P is the real-time DC power of the fixed DC power MMC, P MMC_V is the real-time DC power of the MMC with a constant DC voltage.

[0019] As an optimization, the power drop threshold ΔP is 0.1*P LCC_N .

[0020] As an optimization, the proportional coefficient k of the PI regulator p is 4.

[0021] As an optimization, the integral coefficient k of the PI regulator i Take 3.

[0022] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0023] 1. The present invention realizes automatic power balancing control of the constant DC voltage MMC and the constant DC power MMC when the DC transmission power is reduced due to a fault at the sending end of the hybrid cascaded high-voltage DC transmission system, thereby avoiding the large power fluctuation caused by the constant DC voltage MMC bearing the DC power shortage alone, and facilitating the safe and stable operation of the high-voltage DC transmission system and its receiving-end AC system;

[0024] 2. The present invention has simple calculation and clear physical meaning. It does not need to change the structure and hardware of the high-voltage direct current transmission system. It can achieve power balancing effect by performing simple addition, subtraction, multiplication and division operations based on the existing electrical parameters and control parameters of the system. It has low requirements on hardware and software, fast speed and is suitable for engineering applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative work. In the drawings:

[0026] Figure 1 The figure is a simulation diagram of active power curves of the fixed DC power MMC and the fixed DC voltage MMC at the receiving end after the hybrid cascaded high-voltage DC transmission system receiving end MMC power balancing control method of the present invention is adopted when a DC power shortage is caused by a fault at the sending end of the DC transmission system (the black line is the line of the fixed DC power MMC, and the gray line is the line of the fixed DC voltage MMC);

[0027] Figure 2 This is a simulation diagram of the active power curves of the fixed DC power MMC and the fixed DC voltage MMC at the receiving end when a fault occurs at the sending end of the DC transmission system resulting in a DC power shortage, without adopting the hybrid cascaded high-voltage DC transmission system receiving end MMC power balancing control method described in the present invention (the black line is the line of the fixed DC power MMC, and the gray line is the line of the fixed DC voltage MMC). DETAILED DESCRIPTION

[0028] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0029] Example

[0030] A hybrid cascade high voltage direct current transmission system receiving end MMC power balancing control method, characterized by comprising:

[0031] S1, collecting data of LCC and MMC at the receiving end of the hybrid cascade HVDC transmission system and calculating the real-time DC power of LCC and MMC;

[0032] S2. Establish a balanced control model for the MMC in constant DC power mode according to the data collected in step S1, and perform balanced control on the MMC in constant DC power mode according to the balanced control model.

[0033] In this embodiment, the data of the receiving end LCC and MMC of the hybrid cascade high voltage direct current transmission system include the DC current I LCC 、LCC DC voltage V LCC 、MMC DC current IMMC and the DC voltage V of the MMC MMC .

[0034] In this embodiment, the real-time DC power of the LCC and the MMC includes the real-time DC power P of the LCC. LCC , Real-time DC power P of MMC in constant DC voltage mode MMC_V , the real-time DC power P of any fixed DC power mode MMC MMC_P Of course, the changes of the two parallel constant DC power mode MMCs are the same, so here we only look at one of the constant DC power mode MMCs.

[0035] In this embodiment, the balanced control model of the constant DC power mode MMC performs balanced control on the constant DC power mode MMC in a specific method as follows: the DC power command value of the constant DC power mode MMC is corrected to a variable value, and the real-time DC power P of the LCC is calculated based on the DC power command value of the constant DC power mode MMC. LCC Correct the DC power control command value P of the constant DC power mode MMC in real time ref .

[0036] In this embodiment, the DC power control command value P of the fixed DC power mode MMC ref for:

[0037]

[0038] Among them, P ref_N P is the DC power control command value of the MMC in DC power mode under normal operating conditions (this command value is set manually and can be set according to actual conditions); LCC_N is the DC power of LCC under normal operating conditions; ΔP is the preset power drop threshold value; ΔP ref It is the DC power control instruction adjustment amount calculated in real time.

[0039] In this embodiment, the DC power control instruction adjustment amount ΔP obtained by real-time calculation ref The calculation formula is as follows:

[0040] ΔP ref =k p (P MMC_P -P MMC_V )+k i ∫(P MMC_P -P MMC_V )dt

[0041] Among them, k p is the proportional coefficient of the PI regulator; k i is the integral coefficient of the PI regulator, P MMC_Pis the real-time DC power of the fixed DC power MMC, P MMC_V is the real-time DC power of the MMC with a constant DC voltage.

[0042] In this embodiment, the power drop threshold value ΔP is 0.1*PLCC_N.

[0043] In this embodiment, the proportional coefficient kp of the PI regulator is 4.

[0044] In this embodiment, the integral coefficient ki of the PI regulator is 3.

[0045] In actual operation, the power drop threshold value ΔP, the proportional coefficient kp of the PI regulator, and the integral coefficient ki of the PI regulator can be fine-tuned according to actual conditions, and do not necessarily have to be the above values.

[0046] The principle of the present invention is as follows:

[0047] The hybrid cascade HVDC transmission system uses LCC at the sending end, LCC at the high-voltage end of the receiving end, and three MMCs in parallel at the low-voltage end. The high and low voltage ends of the receiving end each evacuate 50% of the power. In order to ensure the stability of the DC voltage at the low-voltage end and the reasonable distribution of power, one of the three parallel MMCs adopts a constant DC voltage control mode, and the other two adopt a constant DC power mode. The basic principles of the two control modes are similar. The DC side voltage / power of the MMC is collected in real time and subtracted from the corresponding command values ​​to generate a DC voltage / power deviation, which is then input into the MMC inner loop current controller to form a negative feedback control. When a fault occurs at the sending end of the DC transmission system and causes a DC power shortage, the total DC power flowing into the three MMCs decreases rapidly. At this time, since the constant DC power MMC has the ability to maintain its own DC power, the DC power shortage will be borne solely by the constant DC voltage MMC, resulting in a large fluctuation in the power of the connected AC system, threatening the safety of the power grid.

[0048] In order to solve the above problems, the present invention provides a power balancing control method for the MMC at the receiving end of a hybrid cascaded high-voltage direct current transmission system. When it is detected that the DC power of the high-end converter LLC drops to a certain extent, the power command value of the fixed DC power MMC is corrected to a variable, and the power command value of the fixed power MMC is closed-loop adjusted according to the actual power difference between the fixed DC power MMC and the fixed DC voltage MMC. The larger the difference between the two, the greater the downward adjustment of the power command value of the fixed DC power MMC, thereby achieving power balancing of the three MMCs.

[0049] First, the control and protection device of the HVDC transmission system collects the DC voltage and DC current of the receiving end LCC and MMC in real time, and calculates the DC power P of the LCC in real time. LCC , DC power P of MMC in constant DC voltage mode MMC_V, the DC power P of any fixed DC power mode MMC MMC_P .

[0050] According to the real-time DC power P of the LCC valve group LCC The DC power control command value P of the MMC in the fixed DC power mode ref Real-time adjustment is performed. Once it is determined that the DC power drops seriously, the command value of the fixed DC power mode MMC is changed from a fixed value to a variable value. ref The value of is as follows:

[0051]

[0052] Among them, P ref_N P is the DC power control command value of the MMC in DC power mode under normal operating conditions; LCC_N is the DC power of the LCC valve group under normal operating conditions; ΔP is the preset power drop threshold value; ΔP ref The DC power control command adjustment amount is calculated in real time, and its calculation formula is as follows:

[0053] ΔP ref =k p (P MMC_P -P MMC_V )+k i ∫(P MMC_P -P MMC_V )dt

[0054] Among them, k p is the proportional coefficient of the PI regulator; k i is the integral coefficient of the PI regulator. Using the above formula, the power command value of the constant power MMC can be adjusted in a closed loop according to the actual power difference between the constant power MMC and the constant DC voltage MMC.

[0055] Simulation experiment

[0056] To verify the effectiveness of the proposed method, the CIGRE DC transmission standard test system under PSCAD / EMTDC was used as a model to carry out simulation experiments. The AC busbar at the sending end of the hybrid cascade high-voltage cascade system was set to have an ABC three-phase grounding fault at 2s, and the grounding resistance was set to 5Ω. The power drop of the receiving-end MMC was compared when the method of the present invention was put into use after the fault.

[0057] The simulation results of MMC DC power are shown in Figure 1-2As shown in the figure, 1s-1.5s is the system startup process, during which the power of the constant DC voltage MMC and the constant DC power MMC both rise to 650MW. The system operates stably between 1.5s and 2s, and the power of the MMC remains constant. At 2s, a fault disturbance occurs at the sending end, and the overall power transmission capacity of the DC system decreases. Figure 2 It can be seen that when the patented method is not used, only the power of the constant DC voltage MMC drops, the lowest drop is 0MW, while the power of the constant DC power MMC is still controlled at the command value; Figure 1 It can be seen that when the method of this patent is adopted, the power of the constant DC power MMC follows the power of the constant DC voltage MMC and is appropriately reduced. The decline of the three is basically the same, with the lowest drop to 400MW. In summary, the present invention realizes the automatic power balance control of the constant DC voltage and constant DC power MMC when the DC transmission power is reduced due to a fault at the sending end of the hybrid cascaded high-voltage DC transmission system, avoids the large power fluctuation caused by the constant DC voltage MMC bearing the DC power shortage alone, and contributes to the safe and stable operation of the high-voltage DC transmission system and its receiving-end AC system.

[0058] It should also be noted that, in this specification, relational terms such as first and second, etc. are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations.

[0059] At the same time, the devices used in this specification are all existing devices, and those skilled in the art can obtain them through purchase, reference, etc.

[0060] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0061] In summary, although the present invention has been disclosed in the above embodiments, it is not intended to limit the present invention. A person with ordinary knowledge in the technical field to which the present invention belongs can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be determined by the scope of the attached patent application.

Claims

1. A method for controlling MMC power balancing at the receiving end of a hybrid cascaded high-voltage direct current transmission system, characterized in that: include: S1, collecting data of LCC and MMC at the receiving end of the hybrid cascade HVDC transmission system and calculating the real-time DC power of LCC and MMC; S2. Establishing a balanced control model for the MMC in constant DC power mode according to the data collected in step S1, and performing balanced control on the MMC in constant DC power mode according to the balanced control model; The specific method of the balanced control model of the fixed DC power mode MMC for balanced control of the fixed DC power mode MMC is: the DC power command value of the fixed DC power mode MMC is corrected to a variable value, and the real-time DC power of the LCC is used to control the DC power command value of the fixed DC power mode MMC. P LCC Correct the DC power control command value of the fixed DC power mode MMC in real time P ref； The DC power control command value of the MMC in the fixed DC power mode P ref for: ； in, P ref_N The DC power control command value of the MMC in DC power mode is determined under normal operating conditions; P LCC_N is the DC power of LCC under normal operating conditions; Δ P is the preset power drop threshold; Δ P ref It is the DC power control instruction adjustment amount calculated in real time.

2. The method for controlling MMC power balancing at the receiving end of a hybrid cascaded high-voltage direct current transmission system according to claim 1 is characterized in that: The data of the receiving-end LCC and MMC of the hybrid cascaded high-voltage direct current transmission system include the direct current of the LCC, the direct current voltage of the LCC, the direct current of the MMC and the direct current voltage of the MMC.

3. The method for controlling MMC power balancing at the receiving end of a hybrid cascaded high-voltage direct current transmission system according to claim 1, characterized in that: The real-time DC power of the LCC and MMC includes the real-time DC power of the LCC P LCC , Real-time DC power of MMC in constant DC voltage mode P MMC_V , the real-time DC power of any fixed DC power mode MMC P MMC_P .

4. The method for controlling MMC power balancing at the receiving end of a hybrid cascaded high-voltage direct current transmission system according to claim 1 is characterized in that: The DC power control instruction adjustment amount Δ calculated in real time P ref The calculation formula is as follows: ； in, k p is the proportional coefficient of the PI regulator; k i is the integral coefficient of the PI regulator, P MMC_P is the real-time DC power of the fixed DC power MMC, P MMC_V is the real-time DC power of the MMC with a constant DC voltage.

5. The method for controlling MMC power balancing at the receiving end of a hybrid cascaded high-voltage direct current transmission system according to claim 1, characterized in that: The power drop threshold Δ P 0.1* P LCC_N .

6. The method for controlling MMC power balancing at the receiving end of a hybrid cascaded high-voltage direct current transmission system according to claim 4, characterized in that: The proportional coefficient of the PI regulator k p is 4.

7. The method for controlling MMC power balancing at the receiving end of a hybrid cascaded high-voltage direct current transmission system according to claim 4, characterized in that: The integral coefficient of the PI regulator k i Take 3.

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