A coordinated control strategy for LCC and MMC applicable to DC distribution networks

Abstract

This invention discloses a coordinated control strategy for LCC and MMC in DC distribution networks. The strategy employs parallel operation of the LCC and MMC, with their DC sides connected to the same DC bus and simultaneously supplying power to the DC distribution network. When the transmission power is low or power flows in reverse, the LCC is deactivated, and the MMC assumes all the transmission power. When the transmission power is high, the LCC is activated and assumes most of the power. Considering the characteristics of LCC and MMC converter stations, the MMC uses constant DC voltage control and constant reactive power control, while the LCC uses constant DC current or constant DC power control. This invention utilizes the LCC to replace the MMC in assuming part of the transmission power, reducing the number and capacity of MMC devices in the DC distribution network. The coordinated control strategy for steady-state operation of LCC and MMC in the DC distribution network employs parallel operation of the LCC and MMC, with their DC sides connected to the same DC bus and simultaneously supplying power to the DC distribution network.

Description

Technical Field

[0001] This invention relates to the field of DC power distribution technology, specifically a coordinated control strategy for LCC (Line Commutated Converter) and MMC (Modular Multilevel Converter) applicable to DC power distribution networks. Background Technology

[0002] To meet the grid connection needs of renewable energy and the power supply demands of DC loads, DC distribution networks have received widespread attention and application. However, DC distribution network systems have low inertia, and DC voltage is prone to fluctuations. In addition, the switching of loads in the DC distribution network and the random fluctuations in the output of distributed power sources such as photovoltaics and wind power can have a significant impact on DC voltage, and may even endanger the safe operation of the DC distribution network.

[0003] As the connecting hub of AC and DC power grids, grid-connected converters play a crucial role in maintaining DC power balance and suppressing DC voltage fluctuations. Existing DC distribution network projects widely use multi-mode converters (MMCs) for DC voltage control, but their high cost and maintenance restrict the further development of DC distribution systems. Compared to MMCs, multi-mode converters (LCCs) have advantages such as large capacity, low cost, and low operating losses, but also disadvantages such as limitations on minimum transmission current, slow response speed, and the need to change the DC voltage polarity when power flows in the reverse direction. Introducing LCCs into DC distribution networks to replace MMCs in handling some of the transmission power can reduce costs and maintenance, but the inherent limitations of LCCs render existing DC distribution network coordination control strategies inapplicable. Therefore, how to coordinate the operation of MMCs and LCCs in steady-state conditions in DC distribution networks is a key issue in LCC / MMC coordination control.

[0004] Existing coordinated control strategies for LCC and MMC have been widely applied in hybrid DC transmission. However, these strategies are based on high-voltage DC transmission and are unsuitable for DC distribution networks with rapidly changing power flows. Therefore, a coordinated control strategy for LCC and MMC in DC distribution networks is urgently needed. This invention, combining the characteristics of LCC and MMC, designs a power allocation, capacity ratio, and LCC activation and deactivation logic condition for LCC and MMC. When the transmission power is low and power flows in the reverse direction, the LCC is deactivated, and the MMC assumes all the transmission power. When the transmission power is high, the LCC is activated and assumes most of the power. Summary of the Invention

[0005] The purpose of this invention is to provide a coordinated control strategy for LCC and MMC suitable for DC distribution networks. The strategy utilizes LCC to replace MMC in undertaking part of the transmission power, thereby reducing the number and capacity of MMC devices in the DC distribution network. The coordinated control strategy for steady-state operation of LCC and MMC in the DC distribution network adopts the parallel operation of LCC and MMC. The DC side of LCC and MMC are connected to the same DC bus and simultaneously supply power to the DC distribution network.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A coordinated control strategy for LCC and MMC suitable for DC distribution networks is proposed. The control strategy adopts the parallel operation of LCC and MMC, with the DC side of LCC and MMC connected to the same DC bus and simultaneously supplying power to the DC distribution network.

[0008] When the transmission power is low and the power flows in the opposite direction, the LCC stops operating and the MMC takes over all the transmission power; when the transmission power is high, the LCC starts operating and takes over most of the power.

[0009] Furthermore, the LCC uses constant DC current control or constant DC power control to perform the rectification function, while the MMC uses constant DC voltage control to perform the function of balancing DC side power.

[0010] Furthermore, the MMC is always operational during normal system operation.

[0011] Furthermore, when β load ≤β0, or β lcc ≤β L At that time, LCC will exit operation.

[0012] The MMC capacity value satisfies S MMC >(S LCC +S MMC )β0, where S MMC S LCC The rated DC capacity of MMC and LCC are respectively, β load β lcc The load rates of the DC bus and LCC are respectively, β0 and β L These are the critical values ​​for DC bus load rate and LCC load rate when the LCC is shut down.

[0013] Furthermore, when the power P flows from the AC grid to the DC distribution network... load When P is large, the LCC is put into operation and undertakes a large transmission power; when P load As the power gradually decreases from a large value, the LCC synchronously reduces its transmission power.

[0014] When the LCC begins to exit runtime, the system should satisfy the following:

[0015]

[0016] P LCC =S LCC *β L ,

[0017] P MMC =P load -P LCC =S LCC *(β0-β L )+S MMC *β0,

[0018] Among them, P LCC P MMC These represent the transmission power of LCC and MMC, respectively;

[0019] At this time, the load sharing ratio between the LCC and MMC is:

[0020]

[0021] When the LCC is put into operation, the power distribution relationship between the LCC and MMC before full load satisfies the above formula.

[0022] Furthermore, when the LCC is exactly saturated, that is, P LCC Gradually increase to S LCC At that time, the transmission power P undertaken by the MMC MMC =mS MMC That is, when LCC and MMC are operating in parallel in a DC distribution network, the capacity of LCC and MMC should satisfy the following equation:

[0023]

[0024] Furthermore, when P load <(S) LCC +S MMC When β0, as P load As the capacity increases, the transmission power handled by the MMC also increases.

[0025] P before LCC was put into operation MMC Less than mS MMC That is, when P MMC ≥mS MMC At that time, the LCC is immediately put into operation, and after the LCC is put into operation, the load distribution between the LCC and MMC meets the following conditions:

[0026]

[0027] When β loadWhen ≤β0, then β LCC ≤β L If LCC has not exited operation, it will exit operation immediately, while in β... MMC When ≥m, at this time If the LCC is not started, it will be started and put into operation immediately. Let s = 0 indicate that the LCC is disconnected, and s = 1 indicate that the LCC is started and put into operation. Then, the following conditions must be met:

[0028]

[0029] Immediately after the LCC is removed, the MMC takes over all the system's transmission power, that is:

[0030] P MMC =(S LCC +S MMC )β0,

[0031] At the same time, (S) LCC +S MMC )β0 and mS MMC There is a difference between them.

[0032] The beneficial effects of this invention are:

[0033] 1. The control strategy of this invention uses LCC to replace MMC to undertake part of the transmission power, thereby reducing the number and capacity of MMC devices in the DC distribution network. The coordinated control strategy for steady-state operation of LCC and MMC in the DC distribution network adopts the parallel operation of LCC and MMC. The DC side of LCC and MMC are connected to the same DC bus and supply power to the DC distribution network at the same time.

[0034] 2. The control strategy of this invention combines the characteristics of LCC and MMC, and designs the power allocation ratio, capacity ratio and state switching logic conditions for LCC to be put into and taken out of operation in DC distribution network. Compared with MMC, LCC has advantages such as large capacity, low equipment cost and low operating loss. This coordinated control strategy introduces LCC into DC distribution network to replace MMC in undertaking part of the transmission power, thereby reducing the number and capacity of MMC devices in DC distribution network. Therefore, it reduces the cost of DC distribution network equipment and maintenance costs. Attached Figure Description

[0035] The invention will now be further described with reference to the accompanying drawings.

[0036] Figure 1 This is a schematic diagram of the parallel operation structure of LCC and MMC in the DC distribution network of the present invention;

[0037] Figure 2 This is the LCC state switching logic diagram in the DC distribution network of the present invention;

[0038] Figure 3 This is the wiring diagram of the LCC and MMC simulation system in the DC distribution network of this invention;

[0039] Figure 4 These are simulation waveforms of the steady-state power allocation strategy between LCC and MMC in the DC distribution network of this invention:

[0040] Wherein, (a) is the DC bus voltage waveform; (b) is the LCC current command value waveform; (c) is the LCC output current waveform; and (d) is the LCC firing angle waveform.

[0041] Figure 5 This is a simulation waveform of the control strategy when the LCC is activated in the DC distribution network of the present invention;

[0042] Wherein, (a) is the DC bus voltage waveform; (b) is the LCC current command value waveform; (c) is the LCC output current waveform; and (d) is the LCC firing angle waveform.

[0043] Figure 6 This is a simulation waveform of the control strategy when LCC is disconnected in the DC distribution network of the present invention;

[0044] Among them, (a) is the DC bus voltage waveform; (b) is the LCC current command value waveform; (c) is the LCC output current waveform; and (d) is the LCC firing angle waveform. Detailed Implementation

[0045] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0046] A coordinated control strategy for LCC and MMC suitable for DC distribution networks, wherein the control strategy adopts a parallel operation mode of LCC and MMC, such as... Figure 1 As shown, the DC sides of the LCC and MMC are connected to the same DC bus and supply power to the DC distribution network at the same time.

[0047] When the transmission power is low and the power flows in the opposite direction, the LCC stops operating and the MMC takes over all the transmission power; when the transmission power is high, the LCC starts operating and takes over most of the power.

[0048] Among them, the LCC adopts constant DC current control or constant DC power control and undertakes the rectification function, while the MMC adopts constant DC voltage control and undertakes the function of balancing DC side power.

[0049] Assume the rated capacity of the MMC is SMMC The rated DC capacity of the LCC is S LCC The load factors of the DC bus, LCC, and MMC are β. load β lcc β mmc When the LCC is out of operation, the critical values ​​for the DC bus load rate and the LCC load rate are β0 and β, respectively. L .

[0050] Let P be the power transmitted from the AC grid to the DC distribution network. load And P load >0 indicates that the transmission power is positive. To prevent discontinuous DC current, the DC current of an LCC is generally not less than 10% of its rated current during operation. Since the DC voltage of a DC distribution network is basically maintained at its rated value during normal operation, it can be approximated that the transmission power of an LCC is generally not less than 10% of its rated capacity during operation, i.e., β. lcc ≥10%.

[0051] To allow for some margin, let's take β. lcc ≤β L When LCC exits operation, β L >10%, meaning the minimum forward transmission power of the LCC is S. LCC β L Furthermore, the power of an LCC cannot be reversed when the polarity of the DC side voltage remains unchanged, while an MMC does not have this limitation.

[0052] Obviously, in order to leverage the advantages of LCC—large capacity, low cost, and low operating losses—compared to S... MMC S LCC The value of should be as large as possible within the allowable range in order to handle more transmission power, thereby reducing the transmission power and capacity of MMC.

[0053] Meanwhile, to leverage the MMC's fast dynamic response and achieve better voltage control, the MMC should always be operational during normal system operation. load ≤S LCC β L When this happens, the LCC should exit operation; otherwise, the MMC will enter inverter mode.

[0054] To allow for some margin, we might as well set P load ≤(S LCC +S MMC When β0, the LCC exits operation. In order for the MMC to be able to handle all the transmission power after the LCC exits operation, its capacity value should meet the following condition: MMC >(S LCC +S MMC )β0.

[0055] Based on the above analysis, when P load When the voltage is large, the LCC should be put into operation and bear a large transmission power; while when P load As P gradually decreases, the LCC should also decrease its transmission power synchronously; when P... load Gradually decrease to (S) LCC +S MMC When β0, if P LCC This represents the transmission power of the LCC at this time, so P can be obtained. LCC =S LCC β lcc In order to make S LCC Take the largest possible value for β; obviously, then β... lcc The minimum value β should be taken. L That is, when the LCC begins to exit runtime, the system should satisfy the following equation:

[0056]

[0057] That is, when the total load P of the DC bus load =(S LCC +S MMC When β = 0, the load on the LCC is:

[0058] P LCC =S LCC *β L (2)

[0059] The load on the MMC at this time is:

[0060] P MMC =P load -P LCC =S LCC *(β0-β L )+S MMC *β0 (3)

[0061] By combining equations (2) and (3), the load distribution ratio between LCC and MMC at this time can be obtained as follows:

[0062]

[0063] That is, when the LCC is put into operation, the power distribution relationship between the LCC and MMC before saturation should satisfy equation (4), so as to ensure that the system should satisfy equation (1) when the LCC starts to exit operation.

[0064] In a DC distribution network, the capacity ratio of LCC to MMC is crucial. To ensure the MMC can effectively suppress voltage fluctuations, the MMC should have a certain amount of spare capacity before the LCC reaches saturation. Let's assume that as P... loadWhen increasing, when the LCC is exactly saturated, that is, P LCC gradually increases to S LCC When, the transmission power borne by the MMC is:

[0065] P MMC = mS MMC (5)

[0066] Among them, 0 < m < 1, then by combining formula (4) and formula (5), we can get:

[0067]

[0068] That is, when the LCC and the MMC operate in parallel in the DC distribution network, the capacities of the LCC and the MMC should satisfy formula (6).

[0069] The state switching logic condition for the LCC to be put into and withdrawn from operation in the DC distribution network. When the system's P at a certain moment load < (S LCC + S MMC )β0, according to the above analysis, at this time the LCC has been withdrawn from operation, and the MMC bears all the transmission power. As P load increases, the transmission power borne by the MMC also increases accordingly.

[0070] Similarly, in order to enable the MMC to effectively suppress voltage fluctuations, before the LCC is put into operation, P MMC should not exceed mS MMC , that is, when P MMC ≥ mS MMC , the LCC should be immediately put into operation, and after the LCC is put into operation, the LCC and the MMC perform load distribution according to the formula.

[0071] From the above analysis, when β load ≤ β0, at this time β LCC ≤ β L , if the LCC has not been withdrawn from operation, it should be immediately withdrawn from operation, and when β MMC ≥ m, at this time if the LCC has not been started, it should be immediately started and put into operation. If s = 0 represents that the LCC is cut off and s = 1 represents that the LCC is started and put into operation, then we can get

[0072]

[0073] That is, the relationship between s and β load is as Figure 2 shown.

[0074] At the same time, it should be considered that at the moment after the LCC is cut off, the MMC bears all the transmission power of the system, that is, P MMC = (S LCC+S MMC )β0, in order to prevent frequent investment and withdrawal of LCC, (S LCC +S MMC )β0 and mS MMC There should be a reasonable difference between them.

[0075] To verify the effectiveness of the proposed control strategy, a simulation was conducted to verify the steady-state coordinated control strategy of LCC and MMC in a DC distribution network. The simulation system for LCC and MMC in a DC distribution network is as follows: Figure 3 As shown in Table 1, the simulation parameters are as follows: the LCC converter station is a 12-pulse converter station composed of two LCC converters connected in series.

[0076] Table 1. A set of simulation parameters for the steady-state coordinated control strategy of LCC and MMC in DC distribution networks.

[0077]

[0078]

[0079] Based on the parameters in Table 1, the steady-state coordinated control strategy of LCC and MMC in DC distribution network was simulated and verified. The simulation results are as follows: Figure 4 , Figure 5 and Figure 6 As shown.

[0080] Figure 4 The simulation waveforms for power distribution between the LCC and MMC are shown. At initial system startup, the equivalent load power is 10MW. A 2MW load power is suddenly added at t=2.5s, and the load is suddenly removed at t=8.5s. The initial command value for the LCC is 350A. Starting from t=0s, the system updates the command value of the LCC's DC current every two seconds to ensure that the output power ratio of the LCC to the MMC is 7:3.

[0081] in Figure 4 (a) shows the DC bus voltage waveform. Except for fluctuations caused by load switching, the DC voltage can be stabilized at 20kV.

[0082] Figure 4 (b) shows the waveform of the LCC current command value. At t = 4s, the LCC triggers an update to the current command value, which increases linearly at a rate of 30A / s to 420A to maintain the load power ratio of the LCC and MMC at 7:3. Similarly, after the load is suddenly removed at t = 8.5s, the LCC command current value remains unchanged at 420A, and the removed load power is borne by the MMC. At t = 10s, the LCC triggers an update to the current command value, which decreases linearly at a rate of 30A / s to 350A to maintain the load power ratio of the LCC and MMC at 7:3.

[0083] Figure 4 (c) shows the output current waveform of the LCC.

[0084] Figure 4 (d) shows the LCC firing angle change waveform. During the current command value adjustment process, the LCC can adjust its firing angle in a timely manner so that its output current can track its current command value, and the LCC firing angle is always kept within a reasonable range of 5-20° throughout the entire operation. It operates stably under the designed control strategy.

[0085] Figure 5 The simulation waveforms represent the control strategy when the LCC is activated. Initially, the system is unloaded and the LCC is not in operation. To simulate a gradual increase in load power, 1MW of load is activated every second starting from 2.5s, eventually reaching an equivalent load of 10MW.

[0086] in Figure 5 (a) shows the DC bus voltage waveform. Except for fluctuations caused by load switching, the DC voltage can be stabilized at 20kV. Figure 5 (b) shows the waveform of the LCC current command value. As the load power increases, the LCC can be unlocked and engaged at t=6s, and the current command value increases linearly to 350A to maintain the output power ratio of LCC to MMC at 7:3.

[0087] Figure 5 (c) shows the output current waveform of the LCC. Figure 5 (d) shows the LCC trigger angle change waveform. At t=6s, the LCC unlock trigger pulse is put into operation, and the trigger angle drops rapidly to below 40°. At the same time, the LCC output current increases linearly with the current command value, and finally the current stabilizes at 350A. The DC bus voltage fluctuation is very small throughout the entire process, realizing the smooth input of the LCC and verifying the feasibility of the LCC input control strategy.

[0088] Figure 6 The simulation waveform is for the control strategy when the LCC is disconnected. When the system is initially running, the LCC is put into operation with an equivalent load of 10MW. To simulate the gradual decrease of load power, the load power is disconnected by 1MW per second starting from 4.5s, and the final load power is 1MW.

[0089] in Figure 6 (a) shows the DC bus voltage waveform. Except for fluctuations caused by load switching, the DC voltage can be stabilized at 20kV.

[0090] Figure 6 (b) shows the waveform of the LCC current command value. As the load decreases, the LCC is triggered to enter the cut-off procedure at t=6s. First, the LCC current command value decreases linearly to 0.

[0091] Figure 6 (c) shows the output current waveform of the LCC.

[0092] Figure 6 (d) shows the LCC trigger angle change waveform. At t=6s, the LCC output current decreases linearly following the command value. When the output current approaches zero, the trigger angle quickly moves to 150°, and the LCC trigger pulse is blocked. The DC bus voltage fluctuation is very small throughout the entire disconnection process, realizing the smooth disconnection of the LCC and verifying the feasibility of the LCC disconnection control strategy.

[0093] According to the simulation waveform of the steady-state coordinated control strategy of LCC and MMC in DC distribution network, the coordinated control strategy of LCC and MMC in steady-state operation proposed in this invention can utilize LCC to replace MMC to undertake part of the transmission power, thus achieving the control purpose and expected effect, and verifying the effectiveness and feasibility of the proposed coordinated control strategy of LCC and MMC in steady-state operation in distribution network.

[0094] The coordinated control strategy for steady-state operation of LCC and MMC in DC distribution networks proposed in this invention can utilize LCC to replace MMC in undertaking part of the transmission power, thereby reducing the number and capacity of MMC devices in DC distribution networks.

[0095] The coordinated control strategy for steady-state operation of LCC and MMC in DC distribution networks adopts a parallel operation mode for LCC and MMC, with the DC side of both LCC and MMC connected to the same DC bus and simultaneously supplying power to the DC distribution network. Based on the characteristics of LCC and MMC, the power allocation ratio, capacity matching ratio, and state switching logic conditions for LCC activation and deactivation in the DC distribution network were designed.

[0096] Compared to MMC, LCC has advantages such as larger capacity, lower equipment cost, and lower operating losses. This coordinated control strategy introduces LCC into the DC distribution network to replace MMC in handling part of the transmission power, thereby reducing the number and capacity of MMC devices in the DC distribution network. This, in turn, reduces the cost of DC distribution network equipment and maintenance.

[0097] The technical means disclosed in this invention are not limited to those disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention.

Claims

1. A LCC and MMC coordinated control strategy suitable for a direct current power distribution network, characterized in that, The control strategy adopts a parallel operation mode of the LCC and the MMC, the DC sides of the LCC and the MMC are connected to the same DC bus, and the LCC and the MMC supply power to the DC distribution network; When the transmission power is small and the power flows reversely, the LCC is out of operation, and the MMC bears all the transmission power; when the transmission power is large, the LCC is put into operation and bears most of the power; When the transmission power from the alternating current grid to the direct current distribution grid is large When the transmission power is large, the LCC is put into operation and bears large transmission power When the transmission power gradually decreases from large, the LCC synchronously decreases transmission power When the LCC starts to be out of operation, the system should satisfy: wherein, , respectively represent the transmission power of LCC, MMC; , respectively represent the rated DC capacity of MMC, LCC, , respectively represent the load rate of DC bus, LCC, and respectively represent the critical value of DC bus load rate and the critical value of LCC load rate when LCC exits operation. At this time, the load distribution ratio of the LCC and the MMC is: When the LCC is put into operation, the power distribution relationship of the LCC and the MMC before full load satisfies the above formula. 2.The LCC and MMC coordinated control strategy for a DC power distribution network according to claim 1, wherein, The LCC adopts fixed DC current control or fixed DC power control, bears the rectification role, the MMC adopts fixed DC voltage control, and bears the role of balancing the DC side power.

3. The LCC and MMC coordinated control strategy suitable for a direct current power distribution network according to claim 2, characterized in that, The MMC is always put into operation during the normal operation of the system.

4. The LCC and MMC coordinated control strategy suitable for a direct current power distribution network according to claim 3, characterized in that, When or LCC exits operation; The MMC capacity value meets .

5. The LCC and MMC coordinated control strategy suitable for DC power distribution network according to claim 4, characterized in that, When the LCC is just saturated, that is gradually increases to the transmission power borne by the MMC That is, when the LCC and the MMC are parallelly operated in the DC power distribution network, the capacity of the LCC and the MMC should satisfy the formula: 。 6. The LCC and MMC coordinated control strategy suitable for a direct current power distribution network according to claim 5, characterized in that, When the transmission power of the MMC increases with the increase of . Before LCC is put into operation less than that is, when LCC is immediately put into operation, and after LCC is put into operation, and the load distribution of LCC and MMC satisfies: When , at this time , the LCC is not exited from operation, then it is immediately exited from operation, and in , at this time , the LCC is not started, then it is immediately started and put into operation, in order to = 0 indicates that the LCC is cut off, = 1 indicates that the LCC is started and put into operation, then it is satisfied: In the moment after the LCC is cut off, the MMC bears all the transmission power of the system, that is: At the same time, There is a difference between There is a difference between

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