Flexible direct-current power transmission system for new energy
By combining V-F control and PQ control with P-F sag control in a flexible DC transmission system, the problem of inaccurate power balance adjustment in the event of system failure is solved, and more accurate power balance and lower probability of mutual interference is achieved.
Patent Information
- Application Number
- CN202411401482.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-10-09
- Publication Date
- 2025-02-11
AI Technical Summary
In the event of a failure, the power balance adjustment is not accurate enough due to the mutual influence between the converter stations.
A flexible DC transmission system for new energy is adopted, combined with V-F control method and PQ control based on P-F sag control, some transmitting terminal converter stations adopt V-F control, and some use PQ control, and power is achieved by adjusting the reference frequency and system frequency. balance.
It improves the accuracy of power balance adjustment in the event of a failure, reduces the probability of mutual interference between converter stations, and realizes independent regulation and rapid response of each converter station.
Smart Images
Figure CN119419894A_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the technical field related to the safe operation of power systems, and more specifically, relates to a flexible direct current transmission system for renewable energy. Background Art
[0002] The typical configuration of a flexible DC transmission system is to use a modular multilevel converter (MMC). When a system fault occurs, such as a single-pole blocking fault at the sending end or a converter station blocking fault at the receiving end, the MMC will transmit excessive power, resulting in power imbalance, which will in turn trigger a chain reaction of DC grid failures.
[0003] In order to solve the problem of power imbalance, the current converter station generally adopts the VF control method based on PF droop control. When the power flowing into the converter station increases, the power flowing in is adjusted by adjusting the frequency, that is, the phase angle, thereby solving the problem of power imbalance caused by sudden faults. However, if all converter stations adopt the VF control method based on PF droop control, affected by the low inertia characteristics of the converter station, when the electrical distance between the two converter stations is small, a small change in the power angle between the converter stations will cause a large disturbance in power, that is, the interaction between the converter stations will have a greater impact, thereby making the power balance adjustment of the system in the event of a fault not accurate enough. Summary of the invention
[0004] In view of the above defects or improvement needs of the prior art, the present invention provides a flexible direct current transmission system for renewable energy, which aims to reduce the mutual influence between converter stations, thereby improving the accuracy of power balance adjustment of the system when a fault occurs.
[0005] To achieve the above-mentioned purpose, according to one aspect of the present invention, a flexible direct current transmission system for renewable energy is provided, characterized in that it includes a sending-end structure, wherein the sending-end structure includes a plurality of sending-end converter stations for converting the alternating current on the renewable energy side into direct current, each of the sending-end converter stations includes an MMC and its corresponding MMC controller, wherein some of the sending-end converter stations adopt a VF control method based on PF droop control, and some of the sending-end converter stations adopt PQ control, the reference frequency of the phase-locked loop part in the VF control method based on PF droop control is generated by multiplying the corresponding MMC AC side active power by a first droop coefficient, the active power command value of the outer loop control part of the PQ control is generated by multiplying the system frequency by a second droop coefficient, the first droop coefficient is the ratio of the frequency interval length of the normal operation of the renewable energy to the power per unit value, the second droop coefficient is the ratio of the power per unit value to the frequency interval length of the normal operation of the renewable energy, and the power per unit value ranges from 0 p.u. to 1.1 pu.
[0006] In an optional embodiment, two of the sending-end converter stations adopt a VF control method based on PF droop control, and the remaining sending-end converter stations adopt PQ control.
[0007] In an optional embodiment, the two sending-end converter stations with the longest distance adopt a VF control method based on PF droop control, and the remaining sending-end converter stations adopt PQ control.
[0008] In an optional embodiment, the rated power per unit value is 1 p.u.
[0009] In an optional embodiment, the total rated capacity of all sending-end converter stations exceeds the wind power rated power, and the excess value is greater than or equal to the maximum rated power of the sending-end converter stations.
[0010] In an optional embodiment, a receiving-end structure is further included, wherein the receiving-end structure includes a plurality of receiving-end converter stations for converting direct current into alternating current, and each of the receiving-end converter stations includes an MMC and its corresponding MMC controller.
[0011] In an optional embodiment, the new energy includes one or more of wind power and solar energy.
[0012] In an optional embodiment, the new energy includes wind power, and the new energy side has a direct-drive wind turbine composed of a wind turbine, a synchronous generator, a back-to-back converter and its controller. The wind turbine drives the rotor of the synchronous generator to rotate after capturing wind energy, and the stator of the synchronous generator is connected to the sending end structure of the flexible direct current transmission system through the back-to-back converter.
[0013] In general, compared with the prior art, the flexible direct current power transmission system for new energy provided by the present invention has the following beneficial effects:
[0014] The present invention combines the VF control method based on PF droop control and PQ control, wherein the converter station using the VF control method based on PF droop control is called a VF converter station. When the power flowing into the VF converter station increases, the phase angle can be adjusted by adjusting the reference frequency, and then the power flowing in can be adjusted to increase the output of active power and achieve power balance regulation. Moreover, the reference frequency of the VF control method based on PF droop control is obtained by multiplying the active power on the AC side of the MMC by the droop coefficient, and the autonomous regulation of the reference frequency can be achieved. The converter station using PQ control is called a PQ converter station. The active power command value of the outer loop control part is generated by multiplying the system frequency by the droop coefficient, and is affected by the control of the VF converter station. Under the control of the VF control mode, when the power rises, the system frequency rises accordingly, so that the active power command value of the PQ converter station also increases accordingly, increasing the output of the active power of the PQ converter station, thereby jointly bearing the power output caused by the fault with the VF converter station to achieve power balance regulation, and because the active power command value of the outer loop control part of the PQ control is generated by multiplying the system frequency by the droop coefficient, the active power command value can be autonomously regulated. Therefore, the flexible direct current transmission system for new energy mentioned in the present invention is adopted, combined with VF control and PQ control, the two are interrelated and influence each other, which can solve the power imbalance problem caused by system faults, and each converter station can achieve autonomous regulation, without the need for communication and coordination, and can respond quickly. Moreover, the present invention combines the VF control method and the PQ control, which can reduce the probability of mutual interference between converter stations and improve the accuracy of power balance adjustment of the system when a fault occurs, compared with the VF control method that only adopts the PF droop control. BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Figure 1 is a topological structure diagram of a new energy transmission grid in one embodiment;
[0016] Figure 2 is a control block diagram of a VF control method based on PF droop control in one embodiment of the present invention;
[0017] Figure 3 is a control block diagram of PQ control in one embodiment of the present invention;
[0018] Figure 4 is a calculation method of the droop coefficient in one embodiment of the present invention, wherein (a) is a calculation method of the first droop coefficient, and (b) is a calculation method of the second droop coefficient;
[0019] Figure 5 1 is a simulation curve of electrical parameters after a VF converter station fault in an embodiment of the present invention, wherein (a) is a power change curve, (b) is a frequency change curve, and (c) is a voltage change curve;
[0020] Figure 6 1 is a simulation curve of electrical parameters after a PQ converter station fault in an embodiment of the present invention, wherein (a) is a power change curve, (b) is a frequency change curve, and (c) is a voltage change curve. DETAILED DESCRIPTION
[0021] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.
[0022] like Figure 1 The figure shows the topological structure of the renewable energy transmission grid, which includes a renewable energy power generation structure, a sending-end structure, a DC power grid, a receiving-end structure and a receiving-end AC power grid. The renewable energy power generation structure converts wind energy, solar energy and other renewable energy into AC power, and the sending-end structure then converts the AC power into DC power, which is then transmitted over long distances through the DC power grid. The receiving-end structure receives the transmitted DC power and converts it into AC power, and connects it to the AC power grid to supply power to the load.
[0023] The present invention mainly improves the control mode of the receiving end structure, so that the converter station of the receiving end structure can quickly and accurately adjust the power balance when a system failure occurs.
[0024] Specifically, the present invention discloses a flexible direct current transmission system for renewable energy, characterized in that it includes a sending-end structure, wherein the sending-end structure includes a plurality of sending-end converter stations for converting alternating current on the renewable energy side into direct current, each of the sending-end converter stations includes an MMC and its corresponding MMC controller, wherein some of the sending-end converter stations adopt a VF control method based on PF droop control, and some of the sending-end converter stations adopt PQ control, the reference frequency of the phase-locked loop part in the VF control method based on PF droop control is generated by multiplying the corresponding MMC AC side active power by a first droop coefficient, the active power command value of the outer loop control part of the PQ control is generated by multiplying the system frequency by a second droop coefficient, the first droop coefficient is the ratio of the frequency interval length of normal operation of renewable energy to the power per unit value, the second droop coefficient is the ratio of the power per unit value to the frequency interval length of normal operation of renewable energy, and the power per unit value ranges from 0p.u. to 1.1pu.
[0025] Among them, the VF control method based on PF droop control and PQ control are both improved on the basis of traditional control.
[0026] like Figure 2 The control block diagram of the VF control method based on PF droop control is shown, which includes a phase-locked loop PPL, a coordinate transformation module, an outer loop controller and an inner loop controller, where f0 is the rated frequency of the power system, P s is the active power on the AC side of the MMC, k f1 is the first droop coefficient, P s With k f1 After multiplication, the reference frequency f is generated ref , frequency is added and then integrated to obtain the phase angle θ, u s_abc is the three-phase voltage on the AC side of the MMC at the sending end, i s_abc is the three-phase current of the MMC AC side at the sending end, u sd 、u sq are the dq components of the AC voltage at the sending end MMC, i sd 、i sq are the dq components of the AC side current of the MMC at the sending end, u sref is the voltage d component command value of the MMC AC side at the sending end, u cdref is the d-axis component of the MMC modulation voltage command value at the sending end, u cqref is the q-axis component of the MMC modulation voltage command value at the sending end, ω is the phase-locked frequency, L is the decoupling inductance coefficient, u ca 、u cb 、u cc are the three-phase voltages of the DC side of the MMC at the sending end. In this control block diagram, the reference frequency f ref According to the active power P on the AC side of the MMC s Directly calculated, when the active power P s When the reference frequency f ref The phase angle θ increases accordingly, which in turn increases the active power input to the MMC and realizes autonomous adjustment of power balance.
[0027] like Figure 3 The control block diagram of PQ control is shown, which includes an outer loop controller, an inner loop controller and an active power command value calculation module, where f is the system frequency, k f2 is the second droop coefficient, f and k f2 After multiplication, the active power command value P is generated ref The outer loop controller and the inner loop controller are traditional PQ control, and the active power command value calculation module is a new module of the present invention. s When it increases, the system sending frequency f decreases, P ref Increase and realize autonomous adjustment of power balance.
[0028] like Figure 4The figure shows the calculation method of the droop coefficient, wherein (a) is the calculation method of the first droop coefficient, and (b) is the calculation method of the second droop coefficient. The first droop coefficient is the ratio of the frequency interval length of the normal operation of the new energy to the power per unit value, and the second droop coefficient is the ratio of the power per unit value to the frequency interval length of the normal operation of the new energy. Usually, the frequency interval of the normal operation of the new energy is 48Hz~50.2Hz, the frequency interval length is 2.2Hz, and the power per unit value is 1p.u. at the rated frequency. The present invention selects the power per unit value within the range of 0p.u.~1.1pu, preferably greater than the per unit value at the rated frequency. For example, 1p.u. can be selected.
[0029] In one embodiment, two sending-end converter stations adopt the VF control method based on PF droop control, and the remaining sending-end converter stations adopt PQ control. The probability of simultaneous failure of the two converter stations using the VF control method based on PF droop control is very small, so the voltage support of the system can be guaranteed. Furthermore, it is preferred that the two sending-end converter stations with the farthest distance adopt the VF control method based on PF droop control, and the remaining sending-end converter stations adopt PQ control. By increasing the distance, mutual interference between the two converter stations can also be prevented.
[0030] In one embodiment, in order to meet the restriction condition that when a converter station with the largest capacity fails, its power can be transferred to other converter stations and will not exceed the rated power of other converter stations, the system margin should be designed as follows:
[0031] P SM =P S∑ -P REN ≥P Smax
[0032] Where P SM is the operating margin of the sending-end converter station, P S∑ is the total rated capacity of the sending-end converter station, P REN is the rated power of new energy, P Smax is the maximum rated power of the converter station.
[0033] The present invention combines the VF control method based on PF droop control and PQ control, wherein the converter station using the VF control method based on PF droop control is called a VF converter station. When the power flowing into the VF converter station increases, the phase angle can be adjusted by adjusting the reference frequency, and then the power flowing in can be adjusted to increase the output of active power and achieve power balance regulation. Moreover, the reference frequency of the VF control method based on PF droop control is obtained by multiplying the active power on the AC side of the MMC by the droop coefficient, and the autonomous regulation of the reference frequency can be achieved. The converter station using PQ control is called a PQ converter station. The active power command value of the outer loop control part is generated by multiplying the system frequency by the droop coefficient, and is affected by the control of the VF converter station. Under the control of the VF control mode, when the power rises, the system frequency rises accordingly, so that the active power command value of the PQ converter station also increases accordingly, increasing the output of the active power of the PQ converter station, thereby jointly bearing the power output caused by the fault with the VF converter station to achieve power balance regulation, and because the active power command value of the outer loop control part of the PQ control is generated by multiplying the system frequency by the droop coefficient, the active power command value can be autonomously regulated. Therefore, the flexible direct current transmission system for new energy mentioned in the present invention is adopted, combined with VF control and PQ control, the two are interrelated and influence each other, which can solve the power imbalance problem caused by system faults, and each converter station can achieve autonomous regulation, without the need for communication and coordination, and can respond quickly. Moreover, the present invention combines the VF control method and the PQ control, which can reduce the probability of mutual interference between converter stations and improve the accuracy of power balance adjustment of the system when a fault occurs, compared with the VF control method that only adopts the PF droop control.
[0034] The effect of the above scheme is verified as follows.
[0035] The AC sides of Zhangbei Converter Station and Kangbao Converter Station are connected through a line with an equivalent inductance of 0.02H. The positive and negative poles of Zhangbei Station adopt VF control based on PF droop control, and Kangbao Station adopts PQ droop control. According to the rated capacity of Zhangbei and Kangbao Stations, the operating margin at this time should be 1500MW, and the simulation is carried out under the condition of 3000MW of new energy power.
[0036] (1) Voltage Control Converter Station Lockout Fault Simulation
[0037] The simulation is set to be 0.3s when the positive converter station of Zhangbei Station is locked due to a fault. The simulation curve is shown in Figure 5. Figure 5As can be seen from Figure (a) in the figure, after the positive pole fault of Zhangbei Station is locked, the power is quickly transferred to the negative pole VF control converter of Zhangbei, and the increase of the power flowing into the negative pole VF control converter of Zhangbei passive control leads to an increase in the system frequency due to the existence of the VF frequency droop control logic, as shown in Figure (b). Under the influence of the VF frequency droop control, the system frequency increases, which in turn leads to an increase in the power transmission of PQ Station, realizing the adjustment of power balance, so that the system voltage can be kept stable, as shown in Figure (b). Figure 6 As shown in Figure (c).
[0038] (2) PQ control converter station locking fault simulation
[0039] The simulation is set to lock the positive pole of Kangbao due to a fault at 0.3s. The power curves of Zhangbei Station and Kangbao Station are as follows: Figure 6 As shown in Figure (a), it can be seen that the power of the negative pole of Kangbao is quickly transferred to the Zhangbei converter. Because of the existence of the VF frequency droop control logic, the system frequency increases, as shown in Figure (b) of 6. The increase in system frequency leads to an increase in the negative pole transmission power of Kangbao Station, achieving power balance regulation. After the fault, the system voltage is stable, as shown in Figure 6. Figure 6 As shown in Figure (c).
[0040] The above verifies the effectiveness of the control strategy.
[0041] The technical features of the above-mentioned embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification. It should be noted that "in one embodiment", "for example", "for example", etc. of the present invention are intended to illustrate the present invention, rather than to limit the present invention.
[0042] The above-mentioned embodiments only express several implementation methods of the present invention, and the description is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent application. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.
Claims
1. A flexible direct current transmission system for renewable energy, characterized in that: It includes a sending-end structure, which includes multiple sending-end converter stations that convert AC power on the new energy side into DC power. Each of the sending-end converter stations includes an MMC and its corresponding MMC controller, wherein some of the sending-end converter stations adopt a VF control method based on PF droop control, and some of the sending-end converter stations adopt PQ control. The reference frequency of the phase-locked loop part in the VF control method based on PF droop control is generated by multiplying the corresponding MMC AC side active power with a first droop coefficient, and the active power command value of the outer loop control part of the PQ control is generated by multiplying the system frequency with a second droop coefficient. The first droop coefficient is the ratio of the frequency interval length of normal operation of the new energy to the power per unit value, and the second droop coefficient is the ratio of the power per unit value to the frequency interval length of normal operation of the new energy. The power per unit value ranges from 0p.u. to 1.1pu.
2. The flexible direct current transmission system for renewable energy as claimed in claim 1, characterized in that: The two sending-end converter stations adopt a VF control method based on PF droop control, and the remaining sending-end converter stations adopt PQ control.
3. The flexible direct current transmission system for renewable energy as claimed in claim 2, characterized in that: The two sending-end converter stations with the longest distance adopt VF control based on PF droop control, and the remaining sending-end converter stations adopt PQ control.
4. The flexible direct current transmission system for renewable energy as claimed in claim 1, characterized in that: The rated power per unit value is 1 p.u.
5. The flexible direct current transmission system for renewable energy as claimed in claim 1, characterized in that: The total rated capacity of all sending-end converter stations exceeds the rated power of wind power and the excess value is greater than or equal to the maximum rated power of the sending-end converter stations.
6. The flexible direct current transmission system for renewable energy as claimed in claim 1, characterized in that: It also includes a receiving-end structure, which includes a plurality of receiving-end converter stations for converting direct current into alternating current, and each of the receiving-end converter stations includes an MMC and its corresponding MMC controller.
7. The flexible direct current transmission system for renewable energy as claimed in claim 1, wherein the renewable energy comprises one or more of wind power and solar energy.
8. The flexible direct current transmission system for renewable energy as claimed in claim 1, wherein the renewable energy includes wind power, and the renewable energy side has a direct-drive wind turbine composed of a wind turbine, a synchronous generator, a back-to-back converter and a controller thereof, wherein the wind turbine drives the rotor of the synchronous generator to rotate after capturing wind energy, and the stator of the synchronous generator is connected to the sending end structure of the flexible direct current transmission system through the back-to-back converter.