DRU-MMC sending end AC fault ride-through method and system

Abstract

The invention provides a DRU-MMC sending end AC fault ride-through method and system, and the method comprises the steps: enabling a receiving end MMC to maintain the DC voltage to be constant in an outer loop fixed DC bus voltage / reactive power control mode during normal operation; sampling a sending-end AC power grid voltage effective value in real time, and calculating a deviation between the effective value and a nominal value; if the deviation shows that the voltage is lower than 90% of the nominal value (determining a fault), calculating a receiving end MMC direct current bus preset voltage instruction value according to the relation between the alternating current voltage and the direct current voltage; and the outer loop voltage control module is switched to a direct current bus preset voltage / reactive power control mode, and the direct current voltage is adjusted to a preset value by adjusting the input number of the sub-modules, so that closed-loop control is formed. According to the invention, continuous power transmission during a fault period can be realized, and the problem of off-network of new energy equipment caused by power congestion in a traditional scheme is avoided.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible direct current power transmission, in particular to a DRU-MMC sending end AC fault ride-through method and system. BACKGROUND

[0002] The large-scale development of new energy bases such as wind power, photovoltaic power and deep-sea wind power is increasingly prominent. However, such new energy bases are usually geographically far away from load centers, and the problem of economically and efficiently sending out large-scale power over a long distance needs to be solved. Among them, the high-voltage direct current transmission scheme based on diode-rectifier unit (DRU) has become the preferred technical scheme in the current large-scale new energy sending out scene, because the sending end converter station has the advantages of low cost and small device size.

[0003] Since the power characteristics of the DRU-MMC transmission scheme are affected by the sending end AC grid voltage and the DC voltage, when a fault occurs at the sending end, the traditional receiving end converter station adopts a fixed DC bus voltage control strategy, and the sending end voltage drop will cause the DC voltage of the diode rectifier station to be lower than the DC voltage of the receiving end MMC, thereby causing the diode to be reverse-biased and the power transmission to be blocked. If the fault is not cleared in time or the control strategy is improper, the converter may lose stability, power transmission may be blocked, and even the system may be split.

[0004] When a three-phase short-circuit fault occurs in the sending end AC grid, the three-phase voltage decreases, causing power blockage and preventing normal transmission. By adjusting the DC bus voltage of the receiving end converter, some power can still be transmitted to the receiving end during the sending end fault, ensuring that the system power is not completely lost. During the fault, the sending end and receiving end converters can operate normally, the sub-module capacitor voltage remains normal, and the system does not lose stability or split. SUMMARY

[0005] To solve the problem of power blockage not being transmitted to the receiving end grid when a three-phase short-circuit fault occurs in the sending end AC grid of the existing DRU-MMC transmission technology, the present application discloses a DRU-MMC sending end AC fault ride-through method and system.

[0006] A DRU-MMC sending end AC fault ride-through method, comprising the following steps:

[0007] An outer loop voltage control module controls the receiving end MMC to adopt an outer loop fixed DC bus voltage / reactive power control mode during normal system operation to control the DC bus voltage to be constant;

[0008] The effective value of the sending end AC grid voltage is sampled;

[0009] The deviation of the AC voltage effective value from the system nominal voltage value is calculated;

[0010] detecting a fault based on a deviation of the effective value of the alternating voltage from a system nominal voltage value, and returning to execute the outer loop constant DC bus voltage / reactive power control mode if no fault occurs;

[0011] if a fault is detected, calculating a preset DC bus voltage instruction value of the receiving end MMC according to a relationship between the effective value of the alternating voltage and the DC voltage;

[0012] the outer loop voltage control module switches the constant DC bus voltage / reactive power control mode to a DC bus preset voltage / reactive power control mode according to the preset DC bus voltage instruction value of the receiving end MMC, so as to realize fault ride-through.

[0013] Further, the sampling of the effective value of the alternating voltage of the sending end alternating power grid comprises: synchronously sampling three-phase voltages of a point of common coupling (PCC) of the sending end alternating power grid by using a voltage sensor, and calculating the effective value of the alternating voltage.

[0014] Further, the system nominal voltage value is 1.0 pu of the rated voltage.

[0015] Further, the fault detection based on the deviation of the effective value of the alternating voltage from the system nominal voltage value comprises: if the effective value of the alternating voltage of the sending end alternating power grid is lower than 90% of the system nominal voltage value, it is determined that a fault occurs.

[0016] Further, the DC bus preset voltage / reactive power control mode specifically comprises: delivering the DC bus preset voltage instruction value of the outer loop voltage to the outer loop voltage control module through a communication network, obtaining an inner loop current instruction value of the outer loop voltage control, inputting the inner loop current instruction value into a modulation module after inner loop current control to obtain a modulation wave, adjusting the DC voltage to the DC bus preset voltage instruction value by adjusting the number of inputs and the switching state of the half-bridge / full-bridge sub-module of the modulation wave, forming a closed-loop control, maintaining power transmission during the fault, so as to realize fault ride-through.

[0017] A DRU-MMC sending end alternating fault ride-through system, comprising:

[0018] an outer loop voltage control module configured to control the receiving end MMC to adopt an outer loop constant DC bus voltage / reactive power control mode during normal operation of the system, and control the DC bus voltage to be constant;

[0019] a sampling module configured to sample an effective value of an alternating voltage of a sending end alternating power grid;

[0020] a deviation calculation module configured to calculate a deviation of the effective value of the alternating voltage from a system nominal voltage value;

[0021] a fault detection module, configured to perform fault detection based on a deviation of the calculated AC voltage effective value from a system nominal voltage value, and return the receiving end MMC to perform outer loop direct current bus voltage / reactive power control mode if no fault occurs;

[0022] a direct current bus preset voltage instruction value calculation module, configured to calculate a direct current bus preset voltage instruction value of the receiving end MMC according to a relationship between the AC voltage effective value and the direct current voltage when the fault detection module detects a fault;

[0023] an outer loop voltage control module, configured to switch the direct current bus voltage / reactive power control mode to a direct current bus preset voltage / reactive power control mode according to the direct current bus preset voltage instruction value of the receiving end MMC, and realize fault ride-through.

[0024] Further, the sampling module samples the AC voltage effective value of the sending end AC power grid, including: synchronously sampling three-phase voltage of a point of common coupling (PCC) of the sending end AC power grid by using a voltage sensor, and calculating the AC voltage effective value.

[0025] Further, the system nominal voltage value is 1.0 pu of the rated voltage.

[0026] Further, the fault detection module performs fault detection based on a deviation of the calculated AC voltage effective value from the system nominal voltage value, including: if the AC voltage effective value of the sending end AC power grid is lower than 90% of the system nominal voltage value, it is determined that a fault occurs.

[0027] Further, the direct current bus preset voltage / reactive power control mode specifically includes: delivering the direct current bus preset voltage instruction value of the outer loop voltage to the outer loop voltage control module through a communication network, the direct current bus preset voltage instruction value obtains the inner loop current instruction value through the outer loop voltage control, and then the inner loop current instruction value is input into a modulation module after inner loop current control to obtain a modulation wave, the modulation wave adjusts the direct current voltage to the direct current bus preset voltage instruction value by adjusting the number of input and the switching state of the half-bridge / full-bridge submodule, forms a closed loop control, maintains power transmission during the fault, and thus realizes fault ride-through.

[0028] Compared with the prior art, the present application has the following characteristics:

[0029] 1) The fault ride-through capability is significantly improved: after the three-phase voltage of the sending end occurs a fault, the outer loop voltage control module is switched through the fault detection module to realize direct current bus preset voltage control and adjust the direct current bus voltage of the receiving end MMC, so that power transmission can still be performed during the fault, and problems such as power blockage during the fault and large-scale disconnection of new energy power plant equipment in the traditional sending scheme are avoided.

[0030] 2) The stability of the sending system is improved: when the three-phase fault occurs in the sending end AC power grid, the fault detection module is used to determine the fault, the outer ring voltage control module is switched, the preset voltage control of the DC bus is realized, the voltage of the receiving end MMC DC bus is regulated, the power transmission is ensured during the fault, and the capacitor voltage balancing control of the sub-module is realized, thereby improving the stability of the sending system. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 Fig. 1 is a topology diagram of a new energy DRU-MMC parallel DC sending system.

[0032] Figure 2 Fig. 4 is a control module block diagram of an embodiment of the present application.

[0033] Figure 3 Fig. 5 is an outer ring DC bus voltage / reactive power mode control strategy of an embodiment of the present application.

[0034] Figure 4 Fig. 6 is a fault ride-through control diagram based on a preset DC voltage of an embodiment of the present application.

[0035] Figure 5 Fig. 7 is a relationship diagram between the DRU DC voltage and the sending end AC voltage of an embodiment of the present application.

[0036] Figure 6 Fig. 8 is an outer ring DC bus preset voltage / reactive power mode control strategy of an embodiment of the present application.

[0037] Figure 7 Fig. 9 is a flowchart of a DRU-MMC sending end AC fault ride-through method of an embodiment of the present application.

[0038] Figure 8 Fig. 10 is a DRU-MMC sending end three-phase short-circuit fault simulation diagram of an embodiment of the present application.

[0039] Figure 9 Fig. 11 is a DRU-MMC fault ride-through simulation diagram based on a preset DC voltage of an embodiment of the present application. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme of the embodiments of the present application will be described clearly and completely below with reference to the drawings of the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0041] Reference Figure 1The application provides a new energy DRU-MMC parallel direct current transmission system, which comprises a sending end new energy unit, a sending end DRU converter, a direct current transmission line, a receiving end MMC, a receiving end converter transformer and a communication network.

[0042] The sending end new energy unit: the new energy generating equipment is collected to a point on common coupling (PCC) through an alternating current line or a booster transformer.

[0043] The sending end DRU converter: the sending end DRU converter can adopt a 6-pulse rectifier topology or a 12-pulse rectifier topology or a double 12-pulse rectifier topology and is connected to the PCC point through the sending end converter transformer.

[0044] The sending end DRU converter comprises a converter transformer and a DRU unit, and the DRU unit can adopt a 6-pulse rectifier topology or a 12-pulse rectifier topology or a double 12-pulse rectifier topology.

[0045] The direct current transmission line: the direct current transmission line connects the sending end DRU converter and the receiving end MMC and can adopt a direct current cable or an overhead line.

[0046] The receiving end MMC: the receiving end MMC adopts a three-phase half-bridge full-bridge hybrid MMC and is connected to the sending end DRU converter through the direct current transmission line.

[0047] The receiving end MMC is a three-phase half-bridge full-bridge hybrid MMC, each phase of the receiving end MMC comprises N full-bridge sub-modules connected in series, N≥1, and M half-bridge sub-modules connected in series, M≥1.

[0048] The receiving end converter transformer: the three phases of the receiving end MMC are connected to the receiving end converter transformer and then connected to a receiving end land power grid.

[0049] The communication network: the voltage state of the alternating current side power grid of the sending end DRU converter is collected in real time, and the collected information is transmitted to an outer ring voltage control module through the communication network.

[0050] Preferably, referring to Figure 2 The control module block diagram provided by the embodiment of the application comprises an outer ring voltage control module, a sending end DRU converter control module, a receiving end MMC control module and a communication network. dcref The outer ring voltage control module comprises a parameter direct current voltage reference value V dcFRTref , a direct current voltage preset value V dc , a direct current voltage actual value V ref. The submodule capacitor voltage control module includes j-phase upper (denoted by subscript p) and lower (denoted by subscript n) bridge arm (denoted by subscript z) submodule capacitor voltage v pj_cz , v nj_cz , capacitor voltage reference value v C_ref . The inner loop current control module includes DC current reference value I dcref , reactive current reference value I qjref , AC current reference value i ac_jref , each bridge arm DC current reference value i leg_dcjref , each bridge arm AC current reference value i leg_njref , each upper and lower bridge arm current reference value i arm_p(n)jref , zero sequence voltage reference value v 0ref , actual value of upper and lower bridge arm current i pj , i nj . The modulation module includes each phase upper and lower bridge arm modulation wave v pj_ref , v nj_ref , phase-locked loop module and communication module.

[0051] The outer loop voltage control module selects the constant DC bus voltage / reactive power control mode and the DC bus preset voltage / reactive power control mode according to the operating characteristics of the receiving end MMC. The DC current command value and the reactive current command value can be obtained by switching the operating mode;

[0052] The submodule capacitor voltage control module controls the submodule capacitor voltage of the MMC to be at the command value in the steady state or fault ride-through working condition, and the submodule capacitor voltage control current command value is obtained by calculation;

[0053] The inner loop current control module controls the bridge arm current to track the bridge arm current reference value generated by the outer loop voltage control module and the submodule capacitor voltage control module;

[0054] The modulation module generates the driving pulse of each submodule inside the MMC;

[0055] The phase-locked loop module tracks the positive sequence voltage phase signal of the MMC grid connection point;

[0056] The fault detection module detects whether a fault occurs by detecting the sending end AC power grid voltage.

[0057] The communication module detects the sending end power grid voltage in real time, and transmits the sampling value to the receiving end MMC through communication and is used to generate the DC bus voltage preset command value.

[0058] Based on the above control block diagram, referring to Figure 3 , when the receiving end MMC works in the outer loop constant DC bus voltage / reactive power mode, the actual value of the DC bus voltage V dc , the DC bus voltage command reference value Vdcref The actual value V dc is subtracted from the actual value V dcref , and input into a PI regulator for PI control to obtain a DC bus current reference value I pdc , for the inner loop current control. Wherein K idc and K ref are proportional coefficient and integral coefficient in the outer loop DC bus voltage control, and s is Laplace operator.

[0059] (1)

[0060] The actual value Q of the AC side reactive power is collected, and the AC side reactive power command value Q ref is subtracted from the actual value Q of the AC side reactive power, and input into a PI regulator for PI control and limiting to obtain a reactive current amplitude command value I qref , and the reactive current amplitude I qref is multiplied by the phase of the three-phase reactive current respectively to obtain three-phase reactive current command values i qjref (j=a, b, c), wherein K pQ and K iQ are proportional coefficient and integral coefficient in the outer loop DC bus voltage control. For inner loop current control:

[0061] (2)

[0062] (3)

[0063] In the formula, ω + t is the positive sequence phase of the AC side.

[0064] Referring to Figure 4 , Figure 5 After the sending end power grid fails, the effective value V s of the line voltage at the PCC of the sending end AC power grid is measured in real time, and the DC side voltage V DC_DRU of the DRU is:

[0065] (4)

[0066] In the formula: n is the number of DRU units, for example, n=1 corresponds to a 6-pulse DRU converter, and n=2 represents a 12-pulse DRU converter. α is the power factor angle of the AC side of the DRU converter.

[0067] After the sending end power grid fails, the effective value V sReal-time measurements are performed, and faults are determined based on the degree of deviation of the effective value of the AC line voltage. If a three-phase short-circuit fault is determined to have occurred in the AC system at the sending end, the DC side voltage of the DRU is calculated according to formula (4), and then multiplied by a power transmission coefficient k to obtain the preset command value V of the DC voltage of the MMC at the receiving end. dcFRTref .

[0068] Reference Figure 6 Collect the actual value of DC bus voltage V dc The preset command value V of the receiving end MMC DC voltage is set. dcFRTref The actual value of DC bus voltage V dc The difference is calculated and input to the PI regulator for PI control to obtain the DC bus current reference value I. dcref， Used for inner loop current control:

[0069] (5)

[0070] Based on the above, the outer loop voltage control module includes a constant DC bus voltage / reactive power control mode and a DC bus preset voltage / reactive power control mode.

[0071] Preferably, see Figure 7 This invention provides a method for AC fault ride-through at the sending end of a DRU-MMC, the specific steps of which include:

[0072] Step 1: Outer Loop Constant DC Bus Voltage / Reactive Power Control. During normal system operation, the outer loop voltage control module controls the receiving-end MMC to adopt the outer loop constant DC bus voltage / reactive power control mode to keep the DC bus voltage constant.

[0073] Step 2: Real-time sampling of the effective value of AC voltage in the sending-end AC grid. A voltage sensor is used to synchronously sample the three-phase voltage at the point of common coupling (PCC) of the sending-end AC grid, and the effective value of the AC voltage is calculated.

[0074] Step 3: Calculate the deviation between the effective voltage value and the nominal voltage value. Dynamically compare the effective AC voltage value collected in Step 2 with the system's nominal voltage value (e.g., 1.0 pu of the rated voltage).

[0075] Step 4: Determine if a fault has occurred. Based on the deviation between the effective value of the AC voltage calculated in Step 3 and the nominal system voltage, determine if a fault has occurred. If the three-phase grid voltage at the sending end is lower than 90% of the nominal system voltage, a fault is determined to have occurred. If no fault has occurred, return to Step 1; if a fault has occurred, proceed to Step 5.

[0076] Step 5: Calculate the preset voltage command value of the receiving-end MMC DC bus at this time. Specifically, calculate the preset voltage command value of the DC bus based on the relationship between the effective value of the AC voltage and the DC voltage.

[0077] Step six: the communication network delivers the DC bus preset voltage instruction value to the outer loop voltage control module. Specifically, the instruction value of the DC bus preset voltage calculated in step five is delivered to the outer loop voltage control module through the communication network.

[0078] Step seven: the outer loop voltage control module switches the control strategy after receiving the DC bus preset voltage instruction value. Specifically, after receiving the DC bus preset voltage instruction value delivered by the communication network, the DC bus voltage / reactive power control mode is switched to the DC bus preset voltage / reactive power control mode.

[0079] Step eight: the receiving end MMC DC bus preset voltage / reactive power control. The DC bus preset voltage of the outer loop voltage is delivered to the outer loop voltage control module through the communication network. The DC bus preset voltage instruction value is obtained through the inner loop current control after the outer loop voltage control, and then input into the modulation module to obtain the modulation wave. By adjusting the number of input and the switching state of the half-bridge / full-bridge sub-module, the DC voltage is adjusted to the DC bus preset voltage instruction value in real time, forming a closed loop control, maintaining power transmission during the fault, and thus realizing fault ride-through. In this embodiment, refer to Figure 8 The simulation diagram of the DRU-MMC sending end three-phase short-circuit fault is shown in the figure. As shown in the simulation, at 0.7s, a three-phase ground fault occurs, the sending end grid three-phase voltage drops, the receiving end MMC adopts the constant DC bus voltage control, the DC voltage remains unchanged, according to the DRU AC / DC voltage transmission power characteristic, the sending end three-phase voltage after conversion is lower than the DC voltage, the power transmission stops, the three-phase voltage decreases to zero, the active power and the DC current are both zero.

[0080] Refer to Figure 9 After the DC voltage preset control, the DC bus voltage of the receiving end MMC is adjusted to be lower than the converted sending end AC three-phase voltage value. After the voltage adjustment, the active power transmission is restored, that is, a part of the power transmission is maintained during the fault, and the fault ride-through control is realized.

[0081] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto. Any changes or replacements within the technical scope disclosed by the present application can be easily thought by any person skilled in the art, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for DRU-MMC sending end AC fault ride through, characterized in that, The method comprises the following steps: The outer ring voltage control module controls the receiving end MMC to adopt the outer ring constant DC bus voltage / reactive power control mode during normal operation of the system to control the DC bus voltage to be constant; The effective value of the AC voltage of the sending end AC power grid is sampled; The deviation between the effective value of the AC voltage and the system nominal voltage value is calculated; Fault detection is performed based on the calculated deviation between the effective value of the AC voltage and the system nominal voltage value, and if no fault occurs, the outer ring constant DC bus voltage / reactive power control mode is returned to be executed; If a fault is detected, the preset DC bus voltage instruction value of the receiving end MMC is calculated according to the relationship between the effective value of the AC voltage and the DC voltage; The outer ring voltage control module switches the constant DC bus voltage / reactive power control mode to the DC bus preset voltage / reactive power control mode according to the preset DC bus voltage instruction value of the receiving end MMC, and realizes fault ride-through.

2. The DRU-MMC sender side AC fault ride through method of claim 1, wherein: The effective value of the AC voltage of the sending end AC power grid is sampled, including synchronously sampling the three-phase voltage of the PCC point of the sending end AC power grid by using a voltage sensor, and calculating the effective value of the AC voltage.

3. The DRU-MMC sender side AC fault ride through method of claim 1, wherein: The system nominal voltage value is 1.0 pu of the rated voltage.

4. The DRU-MMC sender side AC fault ride through method of claim 1, wherein: The fault detection based on the calculated deviation between the effective value of the AC voltage and the system nominal voltage value includes that if the effective value of the AC voltage of the sending end AC power grid is lower than 90% of the system nominal voltage value, it is determined that a fault occurs.

5. The DRU-MMC sender side AC fault ride through method of claim 1, wherein: The DC bus preset voltage / reactive power control mode specifically includes that the DC bus preset voltage instruction value of the outer ring voltage is transmitted to the outer ring voltage control module through a communication network, the DC bus preset voltage instruction value is subjected to outer ring voltage control to obtain an inner ring current instruction value, the inner ring current instruction value is subjected to inner ring current control to obtain a modulation wave, the modulation wave adjusts the input number and switching state of the half-bridge / full-bridge submodule to adjust the DC voltage to the DC bus preset voltage instruction value in real time, forms a closed-loop control, maintains power transmission during a fault, and thus realizes fault ride-through.

6. A DRU-MMC sending end AC fault ride through system, characterized in that, The method comprises the following steps: The outer ring voltage control module controls the receiving end MMC to adopt the outer ring constant DC bus voltage / reactive power control mode during normal operation of the system to control the DC bus voltage to be constant; The effective value of the AC voltage of the sending end AC power grid is sampled by the sampling module; The deviation between the effective value of the AC voltage and the system nominal voltage value is calculated by the deviation calculation module; The fault detection module performs fault detection based on the calculated deviation between the effective value of the AC voltage and the system nominal voltage value, and if no fault occurs, the outer ring constant DC bus voltage / reactive power control mode is returned to be executed by the receiving end MMC; The DC bus preset voltage instruction value calculation module calculates the preset DC bus voltage instruction value of the receiving end MMC according to the relationship between the effective value of the AC voltage and the DC voltage when the fault detection module detects a fault; The outer ring voltage control module switches the constant DC bus voltage / reactive power control mode to the DC bus preset voltage / reactive power control mode according to the preset DC bus voltage instruction value of the receiving end MMC, and realizes fault ride-through.

7. The DRU-MMC sender side AC fault ride through system of claim 6, wherein: The sampling module samples the effective value of the alternating voltage of the sending end alternating current power grid, comprising: synchronously sampling the three-phase voltage of the PCC point of the sending end alternating current power grid by using a voltage sensor, and calculating the effective value of the alternating voltage.

8. The DRU-MMC sender side AC fault ride through system of claim 6, wherein: The system nominal voltage value is 1.0 pu of the rated voltage.

9. The DRU-MMC sender side AC fault ride through system of claim 6, wherein: The fault detection module detects faults based on the deviation of the calculated effective value of the alternating voltage from the system nominal voltage value, comprising: if the effective value of the alternating voltage of the sending end alternating current power grid is lower than 90% of the system nominal voltage value, it is determined that a fault has occurred.

10. The DRU-MMC sender side AC fault ride through system of claim 6, wherein: The DC bus preset voltage / reactive power control mode, specifically comprising: transmitting the DC bus preset voltage instruction value of the outer loop voltage to the outer loop voltage control module through the communication network, the DC bus preset voltage instruction value obtains the inner loop current instruction value through the outer loop voltage control, and then inputs the modulation module after the inner loop current control to obtain the modulation wave, the modulation wave adjusts the input quantity and the switching state of the half-bridge / full-bridge submodule, adjusts the DC voltage to the DC bus preset voltage instruction value in real time, forms a closed-loop control, maintains power transmission during the fault, and realizes fault ride-through.

Images