MMC direct current energy consumption device trapezoidal modulation strategy based on half-bridge sub-module

Abstract

The invention relates to the technical field of power electronics, and discloses an MMC direct current energy consumption device trapezoidal modulation strategy based on a half-bridge sub-module. Comprising a standard trapezoidal voltage modulation mode and a trapezoidal modulation mode based on pulse frequency modulation; according to the standard trapezoidal voltage modulation mode, the defect of the reverse discharge process of the capacitor voltage of the full-bridge sub-module is overcome by prolonging the external discharge time of the capacitor of the energy-consuming valve sub-module in a modulation period, so that the charge and discharge balance of the capacitor of the sub-module is realized. Compared with a traditional sinusoidal voltage modulation method, half-bridge sub-modules are used for replacing full-bridge sub-modules, the use number of the sub-modules is greatly reduced, the control complexity is reduced, and the cost is saved; on the basis of a trapezoidal modulation mode of pulse frequency modulation, the dissipation power of an energy consumption device is matched with the surplus power of the system by changing the carrier frequency, and dissipation of the surplus power of the whole series of the power system is realized by combining the standard trapezoidal voltage modulation.

Description

Technical Field

[0001] The invention relates to the technical field of power electronics, in particular to a trapezoidal modulation strategy of an MMC (Modular Multilevel Converter) DC energy consumption device based on a half-bridge submodule. Background Art

[0002] With the continuous growth of global energy demand and the increasing prominence of environmental problems, offshore wind power, as a green and sustainable way of power generation, has become an important direction for energy transformation in countries around the world. Due to factors such as ecological environmental protection and waterway occupation, offshore wind power is gradually advancing from nearshore areas to offshore areas. As the distance from offshore wind farms increases, the economy and reliability of AC grid-connected solutions gradually decrease. In contrast, flexible DC transmission technology has become the preferred technical solution for large-capacity, long-distance offshore wind power grid connection due to its advantages such as no charging power limit, more flexible power control, and lower investment cost.

[0003] In an offshore wind power system that uses flexible DC grid connection, when an instantaneous ground fault occurs in the onshore AC grid, the voltage drop on the AC side of the receiving converter station will significantly reduce its active power transmission capacity. Due to the influence of communication delay, the output power of the wind farm cannot be reduced quickly, resulting in excess power, which will cause the system DC voltage to exceed the limit within 30-50 milliseconds, and eventually cause the wind turbine to be disconnected from the grid. In order to achieve such fault ride-through and improve system stability, a dissipation device is usually installed to dissipate the surplus power in the form of resistive heat energy.

[0004] Dissipation devices are divided into two types: AC energy dissipation devices and DC energy dissipation (DCC, DC Chopper) devices. AC energy dissipation devices are installed on the AC side of offshore wind farms, while DCC devices are installed on the DC side on land. Due to the limited space on offshore platforms, installing DCC devices on the DC side on land is the best choice. Currently, traditional DCC devices can be divided into four types according to the device topology: centralized DCC, distributed DCC, hybrid DCC and MMC DCC. These four DCC devices have their own advantages and disadvantages.

[0005] The centralized DCC device consists of multiple insulated-gate bipolar transistors (IGBT) in series with a centralized resistor R, and the power dissipation is adjusted by pulse width modulation (PWM Pulse-Width Modulation). This topology is simple to control and low in cost, but the di / dt and du / dt are large, and all IGBTs need to be turned off at the same time, which has high requirements for device production. The distributed DCC device evenly distributes the dissipation resistor r to each independent submodule (SM Submodule), which can reduce the voltage stress of the IGBT, but requires an expensive water cooling system to cool the resistor in each SM, which greatly increases the cost. The hybrid DCC device combines the advantages of centralized and distributed DCC devices, and consists of an SM with self-discharge function and a centralized resistor R in series. This device has higher control flexibility and can reduce system interference, but it still requires water cooling. MMC DCC is an improvement on the design of the hybrid DCC device. The SM no longer needs a built-in resistor to achieve the charge and discharge balance of the capacitor, so there is no need for a water cooling system, which greatly reduces the cost. However, the modulation strategy of the SM becomes a key factor affecting its performance.

[0006] In the future, the development trend of DCC devices is low cost and lightweight design. MMC DCC devices have significant advantages in both performance and cost, so they have become the current research focus. In order to achieve the charge and discharge balance of the capacitor in the DCC device SM, a sinusoidal modulation strategy is usually used to generate a modulation voltage for the SM. However, when the surplus power in the system is large, the sinusoidal modulation voltage may become negative. Since the half-bridge SM cannot generate a negative voltage, a partial full-bridge SM must be used instead. This increases the cost and control complexity of the DCC device, and seriously affects the application prospects of the MMC DCC device. Summary of the invention

[0007] In view of the above problems, the purpose of the present invention is to provide a trapezoidal modulation strategy for an MMC DC energy consumption device based on a half-bridge submodule, which realizes the SM capacitor charge and discharge balance and the dynamic dissipation of the system's full range of surplus power under the premise that the modulation voltage is non-negative, thereby realizing the capacitor charge and discharge balance of the MMC DCC device based on the half-bridge SM at different surplus power levels. The technical solution is as follows:

[0008] A trapezoidal modulation strategy for an MMC DC energy dissipation device based on a half-bridge submodule, wherein the MMC DC energy dissipation device based on the half-bridge submodule comprises an energy dissipation valve and a centralized energy dissipation resistor R connected in series; the energy dissipation valve comprises a plurality of submodules, each of which is composed of a half-bridge; when a system fails and generates surplus power, the modulation voltage U of the energy dissipation valve is tracked by adjusting the switching instructions of each submodule in the energy dissipation valve sm, so that the voltage U of the centralized energy dissipation resistor R R The corresponding changes occur to achieve real-time control of the dissipated power and maintain the DC voltage U dc stability;

[0009] The trapezoidal modulation strategy includes a standard trapezoidal voltage modulation mode and a PFM-based trapezoidal voltage modulation mode;

[0010] The standard trapezoidal voltage modulation mode achieves submodule capacitor charge and discharge balance by extending the time for the energy-consuming valve submodule capacitor to discharge externally within a modulation cycle;

[0011] The PFM-based trapezoidal voltage modulation mode matches the dissipated power of the MMC DC energy consumption device with the surplus power of the system by changing the carrier frequency;

[0012] According to the real-time surplus power, the modulation mode is switched to make the device dissipated power equal to the system surplus power. The two modulation modes are combined to realize the dissipation of the whole series of surplus power in the power system.

[0013] Furthermore, it is characterized in that the process of determining the trapezoidal modulation voltage in the standard trapezoidal voltage modulation mode is:

[0014] Step 1a: Setting the modulation voltage of the energy consumption valve to be the sum of the DC component and the trapezoidal AC component;

[0015] Step 1b: Calculate the energy dissipated by the DC and AC components of the energy consumption valve in one cycle according to the DC component and the trapezoidal AC component of the energy consumption valve modulation voltage and the DC component and the AC component of the energy consumption branch current;

[0016] Step 1c: Calculate the average power of the DC and AC components of the energy-consuming valve according to the energy dissipated by the DC and AC components of the energy-consuming valve in one cycle;

[0017] Step 1d: Make the sum of the average DC power and the average AC power of the energy consumption valve zero;

[0018] Step 1e: Make the sum of the DC and AC average powers of the centralized energy dissipation resistor R in one modulation cycle equal to the surplus power of the system;

[0019] Step 1f: Based on the conditions of steps 1d and 1e, the trapezoidal modulation voltage expression is obtained.

[0020] Furthermore, it is characterized in that the energy dissipated by the DC and AC components of the energy-consuming valve in one cycle in step 1b is calculated as follows:

[0021] DC component of the energy consumption valve modulation voltage With trapezoidal AC components It is expressed as follows:

[0022]

[0023] Where a is the modulation coefficient, U dc is the voltage on the DC side of the system; b is the peak value of the trapezoidal AC component, Δt1 and Δt2 are the durations of the two states of the trapezoidal AC component, and T is one cycle of the modulation voltage;

[0024] Then the DC component of the energy consumption branch current is and AC component They are:

[0025]

[0026] Then the energy dissipated by the DC and AC components of the energy-consuming valve in one cycle is:

[0027]

[0028] In the formula, ΔE dc is the energy dissipated by the DC component in one cycle, ΔE ac is the energy dissipated in one cycle of the AC component;

[0029] The specific calculation of the DC component average power and AC component average power of the energy-consuming valve in step 1c is:

[0030]

[0031]

[0032] Where P dc is the average power of the DC component of the energy-consuming valve, P ac is the average power of the AC component of the energy-consuming valve;

[0033] The calculation formula in step 1d is as follows:

[0034]

[0035] The calculation formula in step 1e is as follows:

[0036]

[0037] Where P R is the average power dissipated in R; k is the surplus power factor; P max It is the maximum surplus power of the system, which is equivalent to the DC voltage U across the resistor R when the energy-consuming valve is not put into SM. dc Power at time; I dc and I ac They are direct current and alternating current respectively;

[0038] In step 1f, the trapezoidal modulation voltage expression is obtained by equation (9) and equation (10):

[0039]

[0040] Select the trapezoidal modulation voltage when Δt1 = 1 / 2·Δt2. The maximum value of the modulation voltage is 1.256U. dc , the non-negative interval is (0~0.438), that is, using 1.256 times the half-bridge SM can dissipate (0~0.438)P max of surplus power.

[0041] When the ratio of Δt1 to Δt2 is different, the maximum value of the trapezoidal modulation voltage is different, and the non-negative interval of the modulation voltage is also different. Taking into account the equipment cost and control effect, the present invention selects the trapezoidal modulation voltage when Δt1 = 1 / 2·Δt2. At this time, the maximum value of the modulation voltage is 1.256U dc , the non-negative range is 0 to 0.438. Therefore, only 1.256 times of the half-bridge SM is needed to dissipate 0 to 0.438P max That is, compared with the traditional modulation method, the standard trapezoidal modulation voltage prolongs the time for the submodule capacitor to discharge externally, thereby achieving capacitor charge and discharge balance.

[0042] Furthermore, the process of determining the trapezoidal modulation voltage in the trapezoidal voltage modulation mode based on PFM is as follows:

[0043] Step 2a: Determine the trapezoidal modulation voltage in the PFM-based trapezoidal voltage modulation mode. The maximum value is U m , the minimum value is 0, and the expression is as follows:

[0044]

[0045] Where, t1, t2 and t3 are the trapezoidal modulation voltages respectively. The end time points of the first, second and third states;

[0046] The current I flowing through the energy consuming device sm for:

[0047]

[0048] Step 2b: According to the law of conservation of energy, the sum of the energy absorbed and released by the energy-consuming valve in one modulation cycle should be zero to achieve the charge and discharge balance of the SM capacitor in the energy-consuming valve; by performing piecewise integration of equations (12) and (13), it is calculated:

[0049]

[0050] Substituting t1=t3-t2 into equation (14), we obtain:

[0051]

[0052] Step 2c: The power dissipated by the energy-consuming device is matched with the surplus power by adjusting the carrier frequency of the trapezoidal wave; assuming that the carrier frequency D is:

[0053]

[0054] When D=0, the modulation voltage of the energy-consuming valve is 0, and the voltage across R is U dc , dissipated power P R Equal to the maximum surplus power P of the system max When D = 1, the power dissipation is minimum. Using the segmented integration method, the minimum average power of the energy dissipation resistor R in each modulation cycle is calculated as follows:

[0055]

[0056] When t3 = T, the power dissipation of the resistor is the smallest, so the minimum power dissipation of the resistor is:

[0057]

[0058] The maximum value of the trapezoidal modulation voltage under the condition of Δt1=1 / 2·Δt2 is 1.256U dc Substitute U m , get the minimum power dissipation of the resistor That is, the trapezoidal voltage modulation mode based on PFM can dissipate (0.23~1)P max Surplus power within range.

[0059] Therefore, the surplus power generated during the system failure process is 0~P max , the power dissipation of the first mode is (0~0.438)P max , the second mode is (0.23~1)P max , the combination of the two can satisfy (0~P max ) power dissipated, only 1.256 times of the half-bridge sub-module is needed to dissipate the surplus power of the system under all operating conditions. Therefore, compared with the traditional modulation method, the trapezoidal voltage modulation strategy proposed in the present invention can significantly save the use of power devices, thereby reducing costs.

[0060] The beneficial effects of the present invention are:

[0061] 1) The trapezoidal modulation strategy of the present invention includes two modes, wherein the standard trapezoidal voltage modulation mode compensates for the lack of the reverse discharge process of the full-bridge sub-module capacitor voltage by extending the time for the energy-consuming valve sub-module capacitor to discharge externally within a modulation cycle, thereby achieving the sub-module capacitor charge and discharge balance. Compared with the traditional sinusoidal voltage modulation method, the use of a half-bridge sub-module replaces the full-bridge sub-module and greatly reduces the number of sub-modules used, reduces the complexity of control and saves costs.

[0062] 2) The present invention adopts a trapezoidal modulation mode based on pulse frequency modulation, changes the carrier frequency, makes the power dissipated by the energy-consuming device match the surplus power of the system, and combines it with the standard trapezoidal voltage modulation to achieve the dissipation of the whole series of surplus power of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 It is the topological structure of the MMC DCC device based on the half-bridge submodule described in the method of the present invention.

[0064] Figure 2 It is the DC / AC component of the standard trapezoidal modulation mode described in the method of the present invention.

[0065] Figure 3 The standard trapezoidal modulation mode in the method of the present invention is different under Δt1:Δt1 The curve of extreme value changing with k.

[0066] Figure 4 It is the modulation voltage of the trapezoidal voltage modulation mode based on pulse frequency modulation described in the method of the present invention.

[0067] Figure 5 It is the parameter variation curve of the energy consumption valve and the concentrated resistor R in one modulation cycle in the present invention. Figure 6 This is the trapezoidal voltage modulation mode process based on pulse frequency modulation described in the present invention.

[0068] Figure 7 It is the offshore wind power VSC-HVDC system simulation model described in the method of the present invention.

[0069] Figure 8 It is the waveform of the simulation result of the trapezoidal voltage modulation strategy described in the method of the present invention. DETAILED DESCRIPTION

[0070] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0071] The present invention provides a trapezoidal modulation strategy for an MMC DCC device based on a half-bridge submodule, wherein the trapezoidal modulation strategy includes two modes, wherein the standard trapezoidal voltage modulation mode in the trapezoidal modulation strategy compensates for the lack of a reverse discharge process of a full-bridge submodule capacitor voltage by extending the time for the capacitor of the energy-consuming valve submodule to discharge externally within a modulation cycle, thereby achieving a submodule capacitor charge and discharge balance. Compared with the traditional sinusoidal voltage modulation method, the use of a half-bridge submodule replaces the full-bridge submodule and greatly reduces the number of submodules used, reduces the complexity of control and saves costs. In addition, the trapezoidal modulation mode based on pulse frequency modulation (PFM) in the trapezoidal modulation strategy, by changing the carrier frequency, makes the energy-consuming device dissipated power match the surplus power of the system, and is combined with the standard trapezoidal voltage modulation strategy to achieve the dissipation of a full range of surplus power in the power system.

[0072] To achieve the above object, the present invention provides the following technical solution: a trapezoidal modulation strategy of an MMC DCC device based on a half-bridge submodule, comprising the following contents:

[0073] A1. Typical MMC DCC device based on half-bridge submodule;

[0074] A2. Basic principles of standard trapezoidal voltage modulation mode;

[0075] A3. The basic principle of trapezoidal voltage modulation mode based on PFM.

[0076] 1. The typical MMC DCC device structure based on half-bridge submodule is as follows: Figure 1 As shown, it consists of an energy-consuming valve and a centralized energy-consuming resistor R in series. The energy-consuming valve contains multiple SMs, and the SMs are composed of half bridges. When a system failure occurs and surplus power is generated, the modulation voltage U of the energy-consuming valve can be tracked by adjusting the switching instructions of each SM in the energy-consuming valve. sm , so that the voltage U of the centralized energy dissipation resistor R R The corresponding changes occur, thereby achieving real-time control of the dissipated power and maintaining the DC voltage U dc stability.

[0077] 2. The basic principle of the standard trapezoidal voltage modulation mode is:

[0078] The modulation voltage of the energy consumption valve is set to the sum of the DC component and the trapezoidal AC component, such as Figure 2 As shown. Figure 2 It can be seen that the DC component and trapezoidal AC component of the energy-consuming valve modulation voltage can be expressed by the following formula:

[0080]

[0081] In the formula, is the DC component of the trapezoidal modulation voltage, a is the modulation coefficient (0 <a<1),U dc is the DC side voltage of the system.

[0082]

[0083] In the formula, is the AC component of the trapezoidal modulation voltage, b is the peak value of the trapezoidal AC component, Δt1 and Δt2 are the durations of the two states of the trapezoidal AC component, and T is one cycle of the modulation voltage.

[0084] Then the DC component of the energy consumption branch current is and AC component They are:

[0085]

[0086] According to formulas (1), (2), (3) and (4), the energy dissipated by the DC and AC components of the energy-consuming valve in one cycle can be obtained as follows:

[0087]

[0088] In the formula, ΔE dc is the energy dissipated by the DC component in one cycle, ΔE ac It is the energy dissipated in one cycle of the AC component.

[0089] The average power of the DC and AC components of the energy-consuming valve can be calculated according to the following formula.

[0090]

[0091] Where P dc is the average power of the DC component of the energy-consuming valve, P ac is the average power of the AC component of the energy-consuming valve.

[0092] In order to keep the capacitor voltage in the energy-consuming valve SM stable, the modulation voltage U sm The charge and discharge of the SM capacitor should be balanced, so the average power P of the DC component of the energy-consuming valve dc The average power of the AC component P ac The sum should be zero. The calculation formula is as follows:

[0093]

[0094] Meanwhile, to maintain the stability of the DC voltage, within one modulation period, the energy dissipated in the centralized dissipating resistor R in the energy dissipating device should be equal to the energy fed into the system. That is to say, the sum of the DC and AC average powers of the centralized dissipating resistor R within one modulation period is equal to the surplus power of the system. The calculation formula is as follows:

[0095]

[0096] In the formula, P R is the average power dissipated in R; k(0 < k < 1) is the surplus power coefficient; P max is the maximum surplus power of the system, which is equivalent to the power when the DC voltage across the resistor R is U dc when the energy dissipating valve does not input SM.

[0097] From equations (9) and (10), the trapezoidal modulation voltage expression can be obtained as:

[0098]

[0099] It can be seen from equation (11) that the peak value of the trapezoidal modulation voltage is related to Δt1 and Δt2. When Δt1 = 0, the trapezoidal wave becomes a rectangular wave; when Δt2 = 0, the trapezoidal wave becomes a triangular wave. These correspond to two extreme cases. As Figure 3 shown, the curves of the extreme values of the modulation voltage varying with the surplus power coefficient k under different ratios of Δt1 to Δt2 are plotted.

[0100] From Figure 3 it can be seen that when the ratio of Δt1 to Δt2 is different, the maximum value of the trapezoidal modulation voltage is different, and the non - negative interval of the modulation voltage is also different. The maximum values and non - negative intervals of the modulation voltage under different ratios are shown in Table 1.

[0101] Table 1

[0102]

[0103] Taking into account both the equipment cost and the control effect, the present invention selects the trapezoidal modulation voltage under the condition of Δt1 = 1 / 2·Δt2. It can be seen from Table 1 that at this time, the maximum value of the modulation voltage is 1.256U dc , and the non - negative interval is 0 - 0.438. Therefore, only 1.256 times the half - bridge SM is needed to dissipate the surplus power of 0 - 0.438P max .

[0104] 3. The basic principle of the trapezoidal voltage modulation mode based on PFM is specifically as follows:

[0105] As known from the above, the trapezoidal voltage modulation mode proposed by the present invention can only dissipate 0 - 0.438Pmax The surplus power cannot dissipate 0~P max Therefore, the present invention proposes a trapezoidal voltage modulation mode based on PFM, and its basic principle is as follows.

[0106] It is a trapezoidal modulation voltage based on the trapezoidal voltage modulation mode of PFM, such as Figure 4 As shown. The maximum value of the trapezoidal modulation voltage is U m , the minimum value is 0, and the expression is as follows:

[0107]

[0108] Ideally, the voltage across the MMC DCC device is the rated DC voltage U dc , then the current I flowing through the energy consuming device sm for

[0109]

[0110] According to the law of conservation of energy, in order to achieve the charge and discharge balance of the SM capacitor in the energy-consuming valve, the sum of the energy absorbed and released by the energy-consuming valve in one modulation cycle should be zero. This can be calculated by performing piecewise integration on equations (12) and (13).

[0111]

[0112] Substituting t1=t3-t2 into equation (14), we can obtain

[0113]

[0114] The implicit constraint in equation (15) is t3 ≥ t1, which leads to

[0115] U m ≥U dc (17)

[0116] From formulas (15) and (16), it can be inferred that the time intervals t1, t2, and t3 in the trapezoidal voltage modulation waveform need to satisfy a certain proportional relationship to achieve the charge and discharge balance of the SM capacitor in the energy-consuming valve. Using the derived formula and basic principles, the modulation voltage within a modulation cycle can be plotted: Energy consumption branch current I sm , Instantaneous power of energy-consuming valve P sm , the instantaneous power P of the centralized energy dissipation resistor R R and energy consumption valve energy E sm Schematic diagram, such as Figure 5 shown.

[0117] from Figure 5It can be seen that the energy consumption device exists in four different stages within one modulation cycle.

[0118] The first stage (0~t1): the modulation voltage of the energy consumption valve changes from 0 to U m Linear increase. When U sm The current I flowing into the energy consumption device increases. sm The instantaneous power P of the energy-consuming valve sm First it increases and then decreases, while the instantaneous power P of the centralized energy dissipation resistor R R The SM capacitor of the energy-consuming valve absorbs energy. When When sm The instantaneous power P of the energy-consuming valve sm becomes negative, as U sm At this time, the SM capacitor of the energy-consuming valve begins to discharge and the energy begins to decrease.

[0119] The second stage: (t1~t2): modulation voltage of energy consumption valve Keep in U m , and the energy of the capacitor is released linearly. At this time, I sm , P sm and P R Remain unchanged.

[0120] The third stage (t2-t3): energy consumption valve modulation voltage The linear drop to 0 is the reverse process of the first stage. The capacitor is discharged first and then charged. At time t3, the capacitor energy is the same as at time 0, achieving charge and discharge balance.

[0121] The fourth stage: (t3~T): modulation voltage of energy consumption valve is 0, and the voltage on the centralized energy dissipation resistor R is U dc The instantaneous power P of R R Maximum, that is, the maximum remaining power P of the system max .

[0122] According to formula (10), the system surplus power range is 0 to kP max In order to maintain the DC voltage U dc In order to ensure the stability of the system, the power dissipated by the centralized energy dissipation resistor R in the energy dissipation device needs to be equal to the surplus power of the system. The present invention achieves matching between the power dissipated by the energy dissipation device and the surplus power by adjusting the carrier frequency of the trapezoidal wave.

[0123] The specific principle is as follows

[0124] Assume the carrier frequency D is

[0125]

[0126] According to formula (18), t3 varies between 0 and T, so D varies between 0 and 1. Since all SMs of the energy-consuming valve need to run once in one cycle, there is a minimum time t3, which cannot be close to zero. When D = 0, the modulation voltage of the energy-consuming valve is 0, and the voltage across R is U dc , dissipated power P R Equal to the maximum surplus power P of the system max When D = 1, the power dissipation is minimum. Using the segmented integration method, the minimum average power of the energy dissipation resistor R in each modulation cycle is calculated as follows:

[0127]

[0128] When t3 = T, the power dissipation of the resistor is minimum, which is equal to

[0129]

[0130] According to formula (20), as U m The decrease of P R The maximum value of the trapezoidal modulation voltage under the condition of Δt1 = 1 / 2·Δt2 is 1.256U dc Substitute U m , can be obtained

[0131] Therefore, the trapezoidal voltage modulation mode based on PFM can dissipate (0.23~1)P max The surplus power in the range of 0~0.438P is dissipated by the above standard trapezoidal voltage modulation mode. max The surplus power in the range of 1.256 half-bridge submodules can be used to dissipate the surplus power of the whole system. Therefore, compared with the classic sinusoidal voltage modulation strategy, the trapezoidal voltage modulation strategy proposed in this paper can significantly save the use of power devices, thereby reducing costs. The process of the trapezoidal voltage modulation mode based on pulse frequency modulation is as follows: Figure 6 shown.

[0132] The present embodiment is further described in detail below through a specific case.

[0133] A ±100kV / 200MW offshore wind power grid-connected power system simulation model is built in the MATLAB / Simulink environment, and an MMC DCC device is configured between DC poles. Figure 7The offshore sending-end converter adopts constant power control, and the onshore receiving-end converter adopts constant DC voltage control. The key parameters of the model are shown in Table 2. The simulation considers the most serious three-phase short-circuit fault. According to the fault ride-through standard, the fault duration is 2s.

[0134] Table 2 Key parameters of the model

[0135]

[0136] The offshore sending power is set to 0.1 times the rated power (i.e. 20WM). The waveform of the power system during the three-phase short circuit is as follows: Figure 8 As shown in the figure, U ac is the voltage on the AC side of the receiving end on shore, U dc is the DC voltage, Number is the energy consumption valve sub-model input quantity instruction, U c is the submodule capacitor voltage, I sm is the bridge arm current of the energy-consuming branch. Figure 8 It can be seen that U ac It dropped to 0 at 0.5s and recovered to the rated value at 2.5s. At the beginning of the fault, the output capacity of the onshore receiving converter station dropped to 0, while the wind farm continued to transmit power to the power system, resulting in U dc Rising, when U dc When it rises to the preset threshold of 1.05pu, the MMC DCC device is put into operation to dissipate the surplus power transmitted by the power system and maintain the DC voltage U dc stability.

[0137] Depend on Figure 8 It can be seen that at the beginning of the fault, since the energy-consuming device had just been put into use, its dissipated power did not fully match the surplus power of the system, resulting in an increase in the DC voltage, but the peak value did not exceed 1.1pu, which was within the safe range. During the entire fault ride-through process, the DC voltage was basically maintained at around 1p.u., which could ensure that the wind turbine would not be disconnected from the grid for 2s and meet the requirements of the power system.

[0138] During the fault ride-through stage, the energy-consuming valve puts the sub-module into operation according to the trapezoidal modulation strategy. The number of sub-modules put into operation is step-shaped, and the variation is ±1, so it will not cause large voltage fluctuations and electromagnetic interference.

[0139] Depend on Figure 8 It can be seen that the submodule capacitor voltage is basically maintained at 1 p.u., which proves that the trapezoidal modulation strategy proposed in the present invention can maintain the charge and discharge balance of the submodule capacitor voltage.

[0140] It can be seen from the bridge arm current that when the bridge arm current is negative, the submodule capacitor discharges externally; when it is positive, the submodule capacitor charges externally. The trapezoidal modulation strategy proposed in the present invention prolongs the time for the submodule capacitor to discharge externally to make up for the lack of reverse discharge, thereby achieving the submodule capacitor voltage charge and discharge balance.

[0141] Therefore, the trapezoidal modulation strategy of the MMC DCC device based on the half-bridge submodule proposed in the present invention can dissipate the surplus power of the power system when the power system fault is crossed, while maintaining the charge and discharge balance of the submodule capacitor voltage. Compared with the sinusoidal modulation strategy, fewer power devices are used, thereby saving costs.

Claims

1. A trapezoidal modulation strategy for an MMC DC energy consumption device based on a half-bridge submodule, characterized in that: The MMC DC energy dissipation device based on the half-bridge submodule includes an energy dissipation valve and a centralized energy dissipation resistor R connected in series; the energy dissipation valve includes multiple submodules, each of which is composed of a half-bridge; when a system fails and generates surplus power, the modulation voltage U of the energy dissipation valve is tracked by adjusting the switching instructions of each submodule in the energy dissipation valve. sm , so that the voltage U of the centralized energy dissipation resistor R R The corresponding changes occur to achieve real-time control of the dissipated power and maintain the DC voltage U dc stability; The trapezoidal modulation strategy includes a standard trapezoidal voltage modulation mode and a PFM-based trapezoidal voltage modulation mode; The standard trapezoidal voltage modulation mode achieves submodule capacitor charge and discharge balance by extending the time for the energy-consuming valve submodule capacitor to discharge externally within a modulation cycle; The trapezoidal voltage modulation mode based on PFM changes the carrier frequency so that the dissipated power of the MMC DC energy consumption device matches the surplus power of the system; According to the real-time surplus power, the modulation mode is switched to make the device dissipated power equal to the system surplus power. The two modulation modes are combined to realize the dissipation of the whole series of surplus power in the power system.

2. The trapezoidal modulation strategy of the MMC DC energy consumption device based on the half-bridge submodule according to claim 1 is characterized in that: The process of determining the trapezoidal modulation voltage in the standard trapezoidal voltage modulation mode is as follows: Step 1a: Setting the modulation voltage of the energy consumption valve to be the sum of the DC component and the trapezoidal AC component; Step 1b: Calculate the energy dissipated by the DC and AC components of the energy consumption valve in one cycle according to the DC component and the trapezoidal AC component of the energy consumption valve modulation voltage and the DC component and the AC component of the energy consumption branch current; Step 1c: Calculate the average power of the DC component and the average power of the AC component of the energy consumption valve according to the energy dissipated by the DC and AC components of the energy consumption valve in one cycle; Step 1d: making the sum of the average power of the DC component and the average power of the AC component of the energy consumption valve zero; Step 1e: Make the sum of the DC and AC average powers of the centralized energy dissipation resistor R in one modulation cycle equal to the surplus power of the system; Step 1f: Based on the conditions of steps 1d and 1e, the trapezoidal modulation voltage expression is obtained.

3. The trapezoidal modulation strategy of the MMC DC energy consumption device based on the half-bridge submodule according to claim 2 is characterized in that: In step 1b, the energy dissipated by the DC and AC components of the energy-consuming valve in one cycle is calculated as follows: DC component of the energy consumption valve modulation voltage With trapezoidal AC components It is expressed as follows: Where a is the modulation coefficient, U dc is the voltage on the DC side of the system; b is the peak value of the trapezoidal AC component, Δt1 and Δt2 are the durations of the two states of the trapezoidal AC component, and T is one cycle of the modulation voltage; Then the DC component of the energy consumption branch current is and AC component They are: Then the energy dissipated by the DC and AC components of the energy-consuming valve in one cycle is: In the formula, ΔE dc is the energy dissipated by the DC component in one cycle, ΔE ac is the energy dissipated in one cycle of the AC component; In step 1c, the DC component average power and AC component average power of the energy-consuming valve are calculated as follows: Where P dc is the average power of the DC component of the energy-consuming valve, P ac is the average power of the AC component of the energy-consuming valve; In step 1d, the calculation formula is as follows: In step 1e, the calculation formula is as follows: Where P R is the average power dissipated in R; k is the surplus power factor; P max It is the maximum surplus power of the system, which is equivalent to the DC voltage U across the resistor R when the energy-consuming valve is not put into SM. dc Power at time; I dc and I ac They are direct current and alternating current respectively; In step 1f, the trapezoidal modulation voltage expression is obtained by equation (9) and equation (10): Select the trapezoidal modulation voltage when Δt1 = 1 / 2·Δt2. The maximum value of the modulation voltage is 1.256U. dc , the non-negative interval is (0~0.438), that is, using 1.256 times the half-bridge SM dissipation (0~0.438)P max of surplus power.

4. The trapezoidal modulation strategy of the MMC DC energy consumption device based on the half-bridge submodule according to claim 1 is characterized in that: The process of determining the trapezoidal modulation voltage in the trapezoidal voltage modulation mode based on PFM is as follows: Step 2a: Determine the trapezoidal modulation voltage in the PFM-based trapezoidal voltage modulation mode. The maximum value is U m , the minimum value is 0, and the expression is as follows: Where, t1, t2 and t3 are the trapezoidal modulation voltages respectively. The end time points of the first, second and third states; The current I flowing through the energy consuming device sm for: Step 2b: According to the law of conservation of energy, the sum of the energy absorbed and released by the energy-consuming valve in one modulation cycle should be zero to achieve the charge and discharge balance of the SM capacitor in the energy-consuming valve; by performing piecewise integration of equations (12) and (13), it is calculated: Substituting t1=t3-t2 into equation (14), we obtain: Step 2c: The power dissipated by the energy-consuming device is matched with the surplus power by adjusting the carrier frequency of the trapezoidal wave; assuming that the carrier frequency D is: When D=0, the modulation voltage of the energy-consuming valve is 0, and the voltage across R is U dc , dissipated power P R Equal to the system's maximum surplus power P max ; When D = 1, the power dissipation is minimum; Using the piecewise integration method, the minimum average power of the energy dissipation resistor R in each modulation cycle is calculated as follows: When t3 = T, the power dissipation of the resistor is the smallest, so the minimum power dissipation of the resistor is: The maximum value of the trapezoidal modulation voltage under the condition of Δt1=1 / 2·Δt2 is 1.256U dc Substitute U m , get the minimum power dissipation of the resistor That is, the trapezoidal voltage modulation mode based on PFM can dissipate (0.23~1)P max Surplus power within range.

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