Method for calculating power safe transmission range of series MMC under AC unbalanced conditions

Abstract

The present invention discloses a method for calculating the power safety transmission range of a series MMC under AC unbalanced conditions. This method is based on the establishment of a mathematical model of the series MMC under AC unbalanced conditions, and uses the symmetrical component method to analyze the unbalanced voltage at the PCC. It can effectively calculate the positive sequence, negative sequence, and zero sequence voltages at the PCC, and on this basis, analyze the constraint conditions affecting the power safety transmission range. At the same time, the power safety transmission range of the series MMC under AC unbalanced conditions is delineated on the PQ plane, which is helpful for the safe operation of this topology under AC unbalanced conditions. The calculation method has the characteristics of economy, reliability, simple implementation, and excellent performance, and can be applied to the actual project of a flexible DC transmission system adopting a three-phase series MMC.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and particularly relates to a method for calculating the power safe transmission range of a series-type MMC under AC unbalanced conditions. Background Art

[0002] China is a large energy country, but the distribution of energy supply and demand is extremely uneven. With the development of remote areas and the large-scale application of flexible DC transmission technology, there is a need to obtain power from DC transmission lines near the DC transmission line corridors. For example, four countries in Central Asia - South Asia need to draw 300 MW of DC power from the Middle Route in Afghanistan. Therefore, by leveraging the existing DC transmission line corridors, establishing and developing an economical and effective power branch station can promote the economic development along its transmission corridor, and has great engineering application value. To ensure the economy of power transmission, overhead lines need to be used, and such exposed lines are prone to temporary faults such as short circuits and flashovers. Therefore, there are requirements for the DC transmission system to have a way to handle DC side short circuit faults. Using the converter's own control to achieve rapid self-clearing of DC side faults is particularly suitable for DC transmission networks.

[0003] When a three-phase series-type SC-MMC operates under AC unbalanced conditions, the reduction of the power safe transmission range will affect its safe operation. Currently, the research on the three-phase series-type SC-MMC mainly focuses on its own operation mechanism, mathematical model, and harmonic model problems, while the research on the power safe transmission range of the three-phase series-type SC-MMC under AC unbalanced conditions has not been reported yet. Therefore, the proposal of a method for calculating the power safe transmission range of a series-type MMC under AC unbalanced conditions has positive significance and important practical value for the flexible DC transmission system. Summary of the Invention

[0004] In order to solve the problems in the prior art, the present invention proposes a method for calculating the power safe transmission range of a three-phase series-type SC-MMC operating under AC unbalanced conditions. This method can further improve its operation safety.

[0005] The present invention provides a method for calculating the power safe transmission range of a series-type MMC under AC unbalanced conditions. The actual engineering model is equivalent to a single-phase equivalent circuit of a single-ended SC-MMC, and its mathematical model is established; negative sequence current control is adopted to suppress the negative sequence current generated on the converter side under AC unbalanced conditions; the zero-sequence voltage injection method is used to avoid the breakdown and damage of switchgear and ensure that a certain power transmission is maintained at the converter station; the reference values of the positive sequence component and the zero-sequence component of the current dq axis are calculated according to the zero-sequence voltage injection method, and the positive sequence, negative sequence, and zero-sequence components of the unbalanced voltage at the PCC are analyzed by the symmetrical component method, and the voltage value, output voltage value, and AC current value at the PCC are calculated; according to the analysis and calculation results, the corresponding constraint conditions affecting the power safe transmission range of the SC-MMC are written, and its power safe transmission range is depicted in the PQ plane.

[0006] As a further improvement of the present invention, the three-phase series-type SC-MMC operates under AC unbalanced conditions, its mathematical model is established, and a negative sequence current and zero-sequence voltage injection control strategy is adopted. Under the unbalanced conditions, the symmetrical component method is used to analyze the voltage at the PCC, and the positive sequence, negative sequence, and zero-sequence components of the PCC voltage are written. The reference values of the positive sequence and zero-sequence of the current dq axis solved by the zero-sequence voltage injection method, and the positive sequence, negative sequence, and zero-sequence components of the voltage at the PCC solved by the symmetrical component method are used to write the voltage value, output voltage value, and AC current value at the PCC.

[0007] As a further improvement of the present invention, for the SC-MMC operating under AC unbalanced conditions, negative sequence current control is adopted to suppress the negative sequence current generated on the converter side under AC unbalanced conditions; the zero-sequence voltage injection method is used to avoid the breakdown and damage of switchgear and ensure that a certain power transmission is maintained at the converter station.

[0008] As a further improvement of the present invention, negative sequence current control is adopted under the AC unbalanced conditions. To suppress the negative sequence current, a negative sequence component can be added to the converter voltage to compensate for the negative sequence voltage that appears on the transformer converter side during an AC fault.

[0009] As a further improvement of the present invention, the zero-sequence voltage injection method is adopted under the AC unbalanced conditions. To balance the active power of each phase of the SC-MMC, a zero-sequence voltage is injected on the converter side.

[0010] As a further improvement of the present invention, when a single-phase ground fault occurs in phase c at the PCC, the symmetrical component method is used to analyze the PCC voltage under unbalanced conditions, and its positive sequence, negative sequence, and zero-sequence components are respectively:

[0011]

[0012]

[0013]

[0014] Among them, U + , U - and U 0 represent the amplitudes of the positive-sequence component, negative-sequence component, and zero-sequence component of the voltage at the PCC, respectively; and represent the phase angles of the positive-sequence component, negative-sequence component, and zero-sequence component of the voltage, respectively. k and represent the voltage ratio of phase c at the PCC under unbalance and the angle of the position of its unbalance caused by the voltage of phase c, respectively.

[0015] Therefore, the equivalent PCC voltage on the converter side is:

[0016]

[0017] As a further improvement of the present invention, the actual running unbalance degree k and are:

[0018]

[0019] Among them, Z s represents the grid impedance.

[0020] As a further improvement of the present invention, the AC current on the grid side of the transformer is:

[0021]

[0022] Among them, P t and Q t represent the active power and reactive power injected into the SC-MMC, respectively.

[0023] As a further improvement of the present invention, the three-phase expression of the equivalent output voltage of the converter is:

[0024]

[0025] As a further improvement of the present invention, the zero-sequence component of the output voltage can be calculated by the zero-sequence component of the current obtained by the zero-sequence voltage injection method.

[0026] As a further improvement of the present invention, the power safe transmission range of the three-phase series-connected SC-MMC under the AC unbalance condition helps the safe operation of this topology under the AC unbalance condition.

[0027] The beneficial effects of the present invention are as follows: (1) The present invention is directed to a three-phase series-connected SC-MMC operating under unbalanced AC conditions. Based on the establishment of a mathematical model of the series-connected MMC under unbalanced AC conditions, the method uses the symmetrical component method to analyze the unbalanced voltage at the PCC, can effectively calculate the positive-sequence, negative-sequence, and zero-sequence voltages at the PCC, and on this basis, analyzes the constraint conditions affecting the power safe transmission range. At the same time, the power safe transmission range of the series-connected MMC under unbalanced AC conditions is delineated on the PQ plane, which helps the safe operation of this topology under unbalanced AC conditions. (2) The method of the present invention has the characteristics of economy, reliability, simple implementation, and excellent performance, and can be applied to the actual project of a flexible DC transmission system using a three-phase series-connected MMC. Description of the Drawings

[0028] Figure 1 is the topological structure diagram of the three-phase series-connected SC-MMC;

[0029] Figure 2 is the topological structure diagram of the SC-MMC of phase a;

[0030] Figure 3 is the single-phase equivalent circuit diagram of the single-ended SC-MMC under unbalanced AC conditions;

[0031] Figure 4 is the equivalent circuit diagram when a single-phase fault (SPF) occurs at phase c of the PCC point;

[0032] Figures 5(a) to 5(c) is the phasor diagram of the asymmetrical components of the PCC voltage. Among them, Fig. 5(a) is the positive-sequence component diagram of the voltage at the PCC, Fig. 5(b) is the negative-sequence component diagram of the voltage at the PCC, and Fig. 5(c) is the zero-sequence component diagram of the voltage at the PCC;

[0033] Figure 6 is the flowchart of the method of the present invention. Detailed Embodiment

[0034] As Figure 6 shown, the present invention discloses a method for calculating the power safe transmission range of a series-connected MMC under unbalanced AC conditions, including the following steps:

[0035] Step 1: Equivalent the actual engineering model to the single-phase equivalent circuit of the single-ended SC-MMC, and establish a mathematical model of the series-connected MMC;

[0036] Step 2: Adopt negative-sequence current control to suppress the negative-sequence current generated on the converter side under unbalanced AC conditions, and adopt the zero-sequence voltage injection method to avoid the breakdown and damage of the switching equipment and ensure that the converter station maintains a certain power transmission at the same time;

[0037] Step 3: Use the symmetrical component method to analyze the positive-sequence, negative-sequence, and zero-sequence components of the unbalanced voltage at the PCC;

[0038] Step 4: Write the constraint conditions corresponding to the power safe transmission range of SC-MMC according to the analysis and calculation results;

[0039] Step 5: Depict the power safe transmission range of the series MMC under AC unbalanced conditions in the PQ plane.

[0040] In step 1, SC-MMC operates under AC unbalanced conditions, and its mathematical model is established.

[0041] In step 3, according to the zero-sequence voltage injection method, the reference values of the positive sequence and zero sequence of the current in the dq axis are calculated. Under unbalanced conditions, the voltage at the PCC is analyzed by the symmetrical component method, and the positive sequence, negative sequence, and zero sequence components of the PCC voltage can be written out, and the voltage value, output voltage value, and AC current value at the PCC are calculated.

[0042] In step 5, according to the constraint conditions, the power safe transmission range is depicted in the PQ plane by the scanning method and the analytical method. For SC-MMC operating under AC unbalanced conditions, negative sequence current control is adopted to suppress the negative sequence current generated on the converter side under AC unbalanced conditions; the zero-sequence voltage injection method is adopted to avoid the breakdown and damage of the switching equipment and ensure that the converter station maintains a certain power transmission at the same time.

[0043] Negative sequence current control is adopted under AC unbalanced conditions. To suppress the negative sequence current, the negative sequence component can be added to the converter voltage to compensate for the negative sequence voltage that appears on the transformer converter side during AC faults.

[0044] The zero-sequence voltage injection method is adopted under AC unbalanced conditions. To balance the active power of each phase of SC-MMC, the zero-sequence voltage is injected on the converter side.

[0045] When a single-phase grounding fault occurs in phase c at the PCC, the symmetrical component method is used to analyze the PCC voltage under unbalanced conditions, and its positive sequence, negative sequence, and zero sequence components are respectively:

[0046]

[0047]

[0048]

[0049] Among them, U + , U - and U 0 respectively represent the amplitudes of the positive sequence component, negative sequence component, and zero sequence component of the voltage at the PCC; and respectively represent the phase angles of the positive sequence component, negative sequence component, and zero sequence component of the voltage, k and They respectively represent the voltage ratio of phase c at the PCC during imbalance and the angle of the imbalance position caused by the voltage of phase c.

[0050] Therefore, the equivalent PCC voltage on the converter side is:

[0051]

[0052] During actual operation, the unbalance degree k and are:

[0053]

[0054] Among them, Z s represents the grid impedance.

[0055] The AC current on the grid side of the transformer is:

[0056]

[0057] Among them, P t and Q t respectively represent the active power and reactive power injected into the SC-MMC.

[0058] The three-phase expression of the equivalent output voltage on the converter side is:

[0059]

[0060] Among them, and respectively represent the amplitudes of the positive-sequence, negative-sequence, and zero-sequence components of the output voltage, and respectively represent the phase angles of the positive-sequence, negative-sequence, and zero-sequence components of the output voltage.

[0061] The zero-sequence component of the output voltage can be calculated by the zero-sequence component of the current obtained through the zero-sequence voltage injection method.

[0062] Characterizing the power safe transmission range of the SC-MMC under AC unbalanced conditions helps the safe operation of this topology under AC unbalanced conditions.

[0063] The present invention is directed to a three-phase series MMC operating under AC unbalanced conditions, as Figure 1 shown. This three-phase series SC-MMC is composed of three-phase MMCs connected in series. Each phase of the MMC has four arms, and all four arms are composed of half-bridge sub-modules (HBSM). Among them, VT n (n = 1, 2) are fully controlled switching devices (usually IGBTs), VD n (n = 1, 2) are anti-parallel diodes of the switching devices, C is the capacitance value of the sub-module, U c is the sub-module voltage, USM is the output voltage of the sub-module. The output voltage U of the HBSM SM is 0 or U c , L 0 is the inductance value of the arm reactor, U dc is the DC-side voltage value of the three-phase series MMC.

[0064] Figure 2 is the topology of phase a of the SC-MMC system, where u ai (i = 1, 2, 3, 4) is the voltage generated by the switching of the HBSMs in the four arms of phase a, U d is one-third of the DC-side bus voltage of the converter, u a and i a are the voltage and current on the secondary side of the transformer respectively, u MN is the output voltage.

[0065] Figure 3 is the single-phase equivalent circuit diagram of the single-ended SC-MMC under AC unbalanced conditions, where u s is the grid voltage, where Z s represents the grid impedance, Z g represents the grounding impedance to simulate the voltage imbalance at the PCC, where X T is the fundamental leakage reactance of the transformer, X L is the arm reactance.

[0066] Figure 4 is the equivalent circuit diagram of the SPF occurring at phase c of the PCC point. When the SPF occurs at phase c of the PCC, the voltage imbalance is simulated by connecting the grounding impedance Z g at phase c of the PCC.

[0067] Figure 5 is the phasor diagram of the asymmetric components of the PCC voltage. Under unbalanced conditions, the positive-sequence, negative-sequence, and zero-sequence components of the voltage at the PCC are respectively:

[0068]

[0069]

[0070]

[0071] Among them, U + , U - and U 0 represent the amplitudes of the positive-sequence, negative-sequence, and zero-sequence components of the voltage at the PCC respectively; and represent the phase angles of the positive-sequence, negative-sequence, and zero-sequence components of the voltage respectively, and k and represent the voltage ratio of phase c at the PCC during imbalance and the angle of the position where the voltage of phase c causes its imbalance respectively.

[0072] Therefore, the equivalent PCC voltage on the converter side is:

[0073]

[0074] The actual operating unbalance degree k and are:

[0075]

[0076] where Z s represents the grid impedance.

[0077] The AC current on the grid side of the transformer is:

[0078]

[0079] where P t and Q t respectively represent the active power and reactive power injected into the SC-MMC.

[0080] The three-phase expression of the equivalent output voltage of the converter is:

[0081]

[0082] A method for calculating the power safe transmission range of a three-phase series-connected SC-MMC under AC unbalanced conditions proposed by the present invention helps the safe operation of this topology under AC unbalanced conditions and can be applied to the actual project of flexible DC transmission systems.

[0083] The above content is a further detailed description of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention belongs, without departing from the concept of the present invention, several simple deductions or substitutions can still be made, and all should be regarded as belonging to the protection scope of the present invention.

Claims

1. A method for calculating the power safe transmission range of a series MMC under AC unbalanced conditions, characterized in that, it includes the following steps: Step 1: Equivalent the actual engineering model to a single-phase equivalent circuit of a single-ended SC-MMC, and establish a mathematical model of the series MMC; Step 2: Adopt negative sequence current control to suppress the negative sequence current generated on the converter side under AC unbalanced conditions. To avoid the breakdown and damage of switching equipment and ensure that the converter station maintains a certain power transmission at the same time, adopt the zero-sequence voltage injection method; Step 3: Use the symmetrical component method to analyze the positive sequence, negative sequence and zero sequence components of the unbalanced voltage at the PCC; Step 4: Write down the corresponding constraint conditions affecting the power safe transmission range of the SC-MMC according to the analysis and calculation results; Step 5: Depict the power safe transmission range of the series MMC under AC unbalanced conditions in the PQ plane; In the said Step 1, the SC-MMC operates under AC unbalanced conditions, and establish its mathematical model; In the said Step 3, according to the zero-sequence voltage injection method, calculate the positive sequence and zero sequence reference values of the current in the dq axis. Under unbalanced conditions, analyze the voltage at the PCC by the symmetrical component method, and the positive sequence, negative sequence and zero sequence components of the PCC voltage can be written down, and calculate the voltage value, output voltage value and AC current value at the PCC; In the said Step 5, according to the constraint conditions, use the scanning method and the analytical method to depict its power safe transmission range in the PQ plane; When a single-phase ground fault occurs in phase c at the PCC, use the symmetrical component method to analyze the PCC voltage under unbalanced conditions, and its positive sequence, negative sequence and zero sequence components are respectively: where U represents the voltage at the PCC, U + , U - and U 0 represent the magnitudes of the positive-sequence, negative-sequence, and zero-sequence components of the voltage at the PCC, respectively; and represent the phase angles of the positive-sequence, negative-sequence, and zero-sequence components of the voltage, respectively. k and represent the voltage ratio of phase c at the PCC during unbalance and the angle of the location of its unbalance caused by the voltage of phase c, respectively; Therefore, the equivalent PCC voltage on the converter side is: ω is the AC angular frequency, and t is the time.

2. The method according to claim 1, characterized in that, For the SC-MMC operating under AC unbalanced conditions, adopt negative sequence current control to suppress the negative sequence current generated on the converter side; To avoid the breakdown and damage of switching equipment and ensure that the converter station maintains a certain power transmission at the same time, adopt the zero-sequence voltage injection method.

3. The method according to claim 1, characterized in that, Under AC unbalanced conditions, adopt negative sequence current control. To suppress the negative sequence current, the negative sequence component can be added to the converter voltage to compensate for the negative sequence voltage appearing on the transformer converter side during AC faults.

4. The method according to claim 1, characterized in that, Under AC unbalanced conditions, adopt the zero-sequence voltage injection method. To balance the active power of each phase of the SC-MMC, inject zero-sequence voltage on the converter side.

5. The method according to claim 1, characterized in that, In actual operation, the unbalance degree k and is: Among them, Z s represents the grid impedance, and Z g is the ground short-circuit impedance.

6. The method according to claim 1, characterized in that, The AC current on the transformer grid side is: Among them, P t and Q t respectively represent the active power and reactive power injected into the SC-MMC, is the positive sequence component of the d-axis voltage of the PCC.

7. The method according to claim 1, characterized in that, The three-phase expression of the equivalent output voltage on the converter side is: Among them, and represent the amplitudes of the positive, negative, and zero-sequence components of the output voltage respectively, and represent the phase angles of the positive, negative, and zero-sequence components of the output voltage respectively.

8. The method according to claim 7, characterized in that, The zero-sequence component of the output voltage can be calculated by the zero-sequence component of the current obtained by the zero-sequence voltage injection method.

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