A method for calculating the DC current reference value to suppress subsequent commutation failures in DC power transmission

Abstract

The present invention relates to a method for calculating a DC current reference value for suppressing subsequent commutation failures in DC power transmission, which involves the following steps: (1) By simplifying the equivalent model of the receiving-end AC system, obtaining the calculation formula for the reactive power consumed by the inverter and the reactive power balance formula for the commutation bus, and obtaining the commutation voltage expression on the inverter side; (2) Obtaining the function I of the DC current and the commutation voltage on the inverter side through elimination and substitution d = f(U L ); (3) Drawing the curve graph of I d = f(U L ) and the curve graph of the low-voltage current limiting control characteristic I dord = f(U L ) to facilitate comparing the magnitudes of I d and I dord , and selecting an appropriate DC current as the DC current reference value. The present invention studies the functional relationship between the DC current and commutation failures, selects an appropriate DC current as the DC current reference value according to the degree of commutation voltage drop, thereby limiting the DC current. The present invention can not only successfully suppress commutation failures, but also improve the fault recovery performance of the LCC-HVDC system, and does not introduce other electrical quantities except detecting the effective value of the commutation voltage

Description

Technical Field

[0001] The present invention belongs to the technical field of power system stability analysis and control. Based on the reactive power balance of the inverter commutation bus, a method for calculating the reference value of DC current to suppress subsequent commutation failures in DC transmission is proposed. Background Art

[0002] In China, the energy resources and loads are characterized by reverse distribution. Among them, the hydropower resources are concentrated in the southwestern region, the thermal power resources are concentrated in the northwestern region, while the load centers are mainly concentrated in the central and eastern coastal areas. This characteristic of "source-load separation" requires China's power grid to achieve long-distance and large-scale electric energy transmission, continuously sending the western electric energy to the central and eastern load areas.

[0003] The line commutated converter based high voltage direct current (LCC-HVDC) technology has been widely used in long-distance and large-capacity power transmission due to its characteristics such as low line loss rate, rapid power regulation, and the ability to interconnect asynchronous AC systems. High voltage direct current transmission can not only achieve the long-distance transmission of conventional energy sources such as thermal power and wind power, but also is one of the ways to achieve the long-distance transmission of large-scale new energy sources such as wind power and photovoltaic power under the new situation.

[0004] However, since thyristors are used as commutation valves in high voltage direct current transmission, the thyristors themselves do not have the ability of self-turn-off, and commutation failures of the inverter are likely to occur when the receiving-end AC system fails. Commutation failures will lead to an increase in DC current and an interruption of the DC transmitted active power. Moreover, the increase in DC current will also increase the risk of subsequent commutation failures, and long-term overcurrent will also shorten the service life of the commutation valve. In order to prevent the sudden increase in DC current after commutation failures, low voltage current limiting control is generally adopted in the DC transmission control system. Its control principle is to automatically adjust the DC current command after detecting a drop in DC voltage, thereby reducing the DC current and suppressing subsequent commutation failures. However, the design of low voltage current limiting control is relatively simple, and the DC current and DC voltage show a linear variation relationship. When the receiving-end AC system fails and causes a voltage drop, the DC current cannot be immediately limited, which results in its limited ability to suppress subsequent commutation failures. To solve the problem of subsequent commutation failures, the present invention proposes a method for calculating the reference value of DC current based on the reactive power balance of the inverter commutation bus.

[0005] By simplifying the equivalent model of the receiving-end AC system, the present invention studies the functional relationship between the DC current and the commutation voltage. On this basis, the present invention also combines the advantages of the low-voltage current-limiting control characteristics, and finally selects an appropriate DC current as the DC current reference value according to the degree of commutation voltage drop. The present invention can adjust the DC current reference value in a timely manner according to the commutation voltage, thereby limiting the DC current. In addition, the present invention can not only successfully suppress commutation failures, but also improve the fault recovery performance of the LCC-HVDC system, without introducing other electrical quantities except detecting the effective value of the commutation voltage. Summary of the Invention

[0006] The purpose of the present invention is to fully consider the influence of the reactive power consumed by the inverter and the reactive power balance of the commutation bus, simplify the equivalent model of the receiving-end AC system, study the functional relationship between the DC current and commutation failures, and accordingly propose a calculation method for the DC current reference value to suppress subsequent commutation failures in DC power transmission.

[0007] The present invention solves its technical problems through the following technical solutions:

[0008] A calculation method for the DC current reference value to suppress subsequent commutation failures in DC power transmission specifically includes the following steps:

[0009] (1) Equivalent the receiving-end AC system to a voltage source with internal impedance. From the commutation bus to the receiving-end AC voltage source, according to the direction of voltage drop, the commutation voltage on the inverter side satisfies the following expression:

[0010]

[0011] Where: E s is the line voltage effective value of the receiving-end AC system voltage source;

[0012] U L is the effective value of the commutation voltage on the inverter side;

[0013] P d is the active power transmitted by DC;

[0014] Q c is the reactive power absorbed by the AC system from the DC system;

[0015] R s and X s are respectively the resistance and reactance of the internal impedance Z s of the AC system voltage source, satisfying Z s =R s +X s ;

[0016] At the commutation bus of the inverter, the reactive power balance formula is:

[0017] Q c =Q f -Q d =B c U L 2 -Q d (2)

[0018] Among them: Q d is the reactive power consumed by the inverter;

[0019] Q f is the reactive power provided by reactive power compensation devices such as filters, satisfying Q f =B c U L 2 ;

[0020] Bc is the equivalent susceptance of the reactive power compensation device;

[0021] In formula (2), the reactive power consumed by the inverter can be further expressed as:

[0022]

[0023] Among them: is the power factor angle;

[0024] I d is the DC current;

[0025] k is the turns ratio of the converter transformer;

[0026] N is the number of 6-pulse converters;

[0027] U d0 is the ideal no-load DC voltage, and satisfies

[0028] Through the above calculation process, the reactive power consumed by the inverter and the reactive power balance formula of the inverter commutation bus can be obtained, preparing for the calculation of the DC current reference value;

[0029] (2) From formula (1), the function Q c =f1(U L , P d ) can be obtained. From formulas (2) and (3), the function Q c =f2(U L , P d , I d ) can be obtained. Considering that Q c takes the same value in the above two functions, that is, f1(U L , P d ) = f2(U L , P d,I d ), By solving this equation, the variable Q can be eliminated c , and then I can be obtained d and U L , P d 's function, the function I d = f(U L , P d ) is expressed as follows:

[0030]

[0031] Where: Δ = (U L X s ) 2 +Z s 2 (E s 2 -U L 2 -P d 2 Z s 2 / U L 2 +2P d R s );

[0032] The function I d = f(U L , P d ) has two variables, namely U L and P d . In fact, the active power of the DC control system can only be obtained by measuring the product of the DC voltage and the DC current;

[0033] In equation (4), if the calculation of P d is related to I d , this will make the function I d = f(U L , P d ) more complex. For the above reasons, it is necessary to establish the relationship between P d and U L , and then eliminate P d in the function I L = f(U d , P d ). According to the steady-state model of the DC transmission system, the expressions of the DC voltage and the DC current can be represented as equation (5). Assuming that the extinction angle γ after commutation failure is the minimum extinction angle γ min , and the lead angle β will not be adjusted in a short time, that is, β = β N , then the active power P d can be expressed as equation (6):

[0034]

[0035]

[0036] It can be seen from Equation (6) that when X C , γ min and β N are all constants, the active power P d is only determined by the commutation voltage U L . Substituting (6) into (4), the DC current can be written as a function only related to the commutation voltage, that is, I d = f(U L ); According to the function I d = f(U L ), it can be known that by measuring U L , I d can be obtained;

[0037] (3) Substitute U L into the function I d = f(U L ) to obtain the curve of I d = f(U L ). In addition, changing the independent variable of the low-voltage current limiting control from the DC voltage to the commutation voltage, the curve of I dord = f(U L ) is obtained. To ensure that the DC current can be immediately limited after commutation failure occurs, a smaller value should be selected as the DC current reference value. It can be seen from the figure that when the commutation voltage drops to a small extent, that is, U L > 160 kV, at this time I d is less than I ord , so I d needs to be used as the DC current reference value I dref . When the commutation voltage drops significantly, that is, U L < 95 kV, at this time I d is greater than I ord , so I ord needs to be used as the DC current reference value I dref . When the drop degree of the commutation voltage is between 95 - 160 kV, in order to ensure the continuity of the current change, the change of the DC current reference value in this stage is linearly processed in the first order; Finally, combining the above three cases, the characteristic curve of the DC current reference value I dref changing with the commutation voltage is obtained. The DC current reference value I dref can be expressed as:

[0038]

[0039] Where: ULN is the rated commutation voltage. Taking the DC transmission standard test model as an example, U LN = 230 kV;

[0040] I dref Combines the advantages of low-voltage current limiting control, can quickly limit the DC current at the initial stage of the fault, and slowly increase the DC current at the initial stage of fault recovery. Since the reactive power demand of the inverter is considered when calculating the DC current reference value I d This method can also prevent the commutation voltage at the inverter side from dropping sharply while adjusting the DC current reference value. Taking this DC current reference value as the new DC current command value and sending it into the DC control system can suppress the occurrence of subsequent commutation failures. It should be noted that Equation (1) is applicable to faults occurring at the inverter commutation bus. When faults occur at other positions of the AC line, Equation (1) needs to be corrected; however, considering that when the severity of the fault is the same, for the inverter, the commutation bus fault is more serious than the AC line fault; therefore, under the same fault severity, if the proposed DC current reference value calculation method can suppress the commutation failure caused by the commutation bus fault, it can also suppress the commutation failure caused by the AC line fault.

[0041] Compared with the prior art, the positive effects that the present invention can produce include the following points:

[0042] 1. When calculating the DC current reference value, the present invention considers the reactive power demand of the inverter. By ensuring the reactive power balance of the commutation bus, it can prevent the commutation voltage from dropping sharply while adjusting the DC current reference value.

[0043] 2. The DC current reference value calculation method described in the present invention combines the advantages of low-voltage current limiting control, and can not only quickly limit the DC current at the initial stage of the fault, but also make the DC current slowly recover at the initial stage of fault recovery.

[0044] 3. The DC current reference value calculation method described in the present invention only requires one measurement data of the effective value of the commutation voltage and does not need to introduce other electrical quantities. Brief Description of the Drawings

[0045] Figure 1 is the equivalent model diagram of the AC system on the inverter side of the present invention;

[0046] Figure 2 is the characteristic curve diagram of the DC current changing with the commutation voltage of the present invention; Figure 2 (a) is the curve diagram of the function I d = f(U L ); Figure 2 (b) is the curve diagram of the low-voltage current limiting control characteristic I dord = f(U L );

[0047] Figure 3 This is the flowchart of the present invention. Specific embodiments

[0048] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will further elaborate in detail on a method for calculating the DC current reference value to suppress subsequent commutation failures in DC transmission in combination with the drawings in the embodiments of the present invention. Among them, the described embodiments are some embodiments of the present invention, rather than all embodiments. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed present invention, but merely represents the selected embodiments of the present invention.

[0049] A method for calculating the DC current reference value to suppress subsequent commutation failures in DC transmission includes the following steps:

[0050] (1) By simplifying the equivalent model of the receiving-end AC system, the calculation formula for the reactive power consumed by the inverter and the reactive power balance formula of the commutation bus are obtained. Combining with the direction of voltage drop, the expression satisfied by the commutation voltage on the inverter side is obtained, preparing for the next calculation of the DC current;

[0051] (2) Based on the formulas obtained in step 1, the function I of the DC current and the commutation voltage on the inverter side is obtained through methods such as elimination and substitution. d =f(U L );

[0052] (3) The curve graphs of the above function I d =f(U L ) and the low-voltage current limiting control characteristic I dord =f(U L ) are respectively drawn to facilitate comparing the magnitudes of I d and I dord . Furthermore, a suitable DC current can be selected as the DC current reference value according to the degree of drop of the commutation voltage.

[0053] The following will elaborate in detail on the technical solutions of the present invention in combination with the drawings and embodiments.

[0054] 1) The receiving-end AC system is equivalent to a voltage source with internal impedance. From the commutation bus to the receiving-end AC voltage source, according to the direction of voltage drop, the commutation voltage on the inverter side satisfies the following expression:

[0055]

[0056] Where: E s is the line voltage effective value of the receiving-end AC system voltage source;

[0057] U Lis the effective value of the commutation voltage on the inverter side;

[0058] P d is the active power transmitted by DC;

[0059] Q c is the reactive power absorbed by the AC system from the DC system;

[0060] R s and X s are respectively the resistance and reactance of the internal impedance Z of the AC system voltage source, that is, Z s = R s + X s . s

[0061] At the inverter commutation busbar, the reactive power balance formula is:

[0062] Q c = Q f - Q d = B c U L 2 - Q d (2)

[0063] Where: Q d is the reactive power consumed by the inverter, Q f is the reactive power provided by reactive power compensation devices such as filters, and it satisfies Q f = B c U L 2 , where Bc is the equivalent susceptance of the reactive power compensation device.

[0064] In formula (2), the reactive power consumed by the inverter can be further expressed as:

[0065]

[0066] In the formula: is the power factor angle, I d is the DC current, k is the turn ratio of the commutation transformer, N is the number of 6-pulse converters, U d0 is the ideal no-load DC voltage, and it satisfies

[0067] Through the above calculation process, the reactive power consumed by the inverter and the reactive power balance formula of the inverter commutation busbar can be obtained, preparing for the calculation of the DC current reference value later.

[0068] 2) From formula (1), the function Q c = f1(U L , P d), the function Q can be obtained from equations (2) and (3). c = f2(U L , P d , I d ).

[0069] Considering that Q has the same value in the above two functions, i.e., f1(U c , P L ) = f2(U d , P L , I d ), by solving this equation, the variable Q can be eliminated d , and then the function of I c and U d , P L can be obtained. The expression of the function I d = f(U d , P L ) is as follows: d )

[0070]

[0071] Where: Δ = (U L X s ) 2 + Z s 2 (E s 2 - U L 2 - P d 2 Z s 2 / U L 2 + 2P d R s ).

[0072] The function I d = f(U L , P d ) has two variables, namely U L and P d . In fact, the active power of the DC control system can only be obtained by measuring the product of the DC voltage and the DC current. In equation (4), if the calculation of P d is related to I d , this will make the function I d = f(U L , P d ) more complex. For the above reasons, it is necessary to establish the relationship between P d and U L , and then eliminate the function I d = f(U L, P d P in ( d . According to the steady-state model of the HVDC system, the expressions of DC voltage and DC current can be represented by Equation (5). Assuming that the extinction angle γ after commutation failure is the minimum extinction angle γ min , and the lead angle β will not be adjusted in a short time, that is, β = β N , then the active power P d can be expressed as Equation (6):

[0073]

[0074]

[0075] It can be seen from Equation (6) that when X C , γ min and β N are all constants, the active power P d is only determined by the commutation voltage U L . Substituting (6) into (4), the DC current can be written as a function related to the commutation voltage, that is, I d = f(U L ). According to the function I d = f(U L ), it can be known that by measuring U L , I d can be obtained.

[0076] 3) Substitute U L into the function I d = f(U L ) to obtain the curve of I d = f(U L ). In addition, by changing the independent variable of the low-voltage current limiting control from DC voltage to commutation voltage, the curve of I dord = f(U L ) is obtained. To ensure that the DC current can be immediately limited after commutation failure occurs, a smaller value should be selected as the DC current reference value. As can be seen from Figure 2 (a), when the commutation voltage drops slightly, that is, U L > 160 kV, at this time I d is less than I ord , so I d needs to be used as the DC current reference value I dref . When the commutation voltage drops significantly, that is, U L < 95 kV, at this time I d is greater than I ord , so I ord needs to be used as the DC current reference value I drefWhen the drop degree of the commutation voltage is between 95 and 160 kV, in order to ensure the continuity of the current change, the change of the DC current reference value in this stage is linearly processed by the first order. Finally, combining the above three cases, the DC current reference value I dref The characteristic curve with the change of the commutation voltage is shown in Fig. 2(b). The DC current reference value I dref can be expressed as:

[0077]

[0078] In the formula: U LN is the rated commutation voltage. Taking the DC transmission standard test model as an example, U LN = 230 kV.

[0079] I dref Combines the advantages of low-voltage current limiting control, can quickly limit the DC current at the initial stage of the fault, and slowly increase the DC current at the initial stage of the fault recovery. Since the reactive power demand of the inverter is considered when calculating I d in the DC current reference value, this method can also prevent the sharp drop of the commutation voltage on the inverter side while adjusting the DC current reference value. Taking this DC current reference value as the new DC current command value and sending it into the DC control system can suppress the occurrence of subsequent commutation failures. It should be noted that Equation (1) is applicable to the faults occurring at the inverter commutation bus. When faults occur at other positions of the AC line, Equation (1) needs to be corrected. However, considering that when the severity of the fault is the same, for the inverter, the commutation bus fault is more serious than the AC line fault. Therefore, under the same fault severity, if the proposed DC current reference value calculation method can suppress the commutation failure caused by the commutation bus fault, it can also suppress the commutation failure caused by the AC line fault.

[0080] The above content is only an embodiment of the present invention, and its purpose is not to limit the system and method proposed by the present invention. The protection scope of the present invention is subject to the claims. Without departing from the spirit and scope of the present invention, various obvious modifications or changes in form and details made by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A method for calculating the DC current reference value to suppress subsequent commutation failures in HVDC transmission, specifically including the following steps: Step (1): Equivalent the receiving-end AC system as a voltage source with internal impedance. From the commutation bus to the receiving-end AC voltage source, according to the direction of voltage drop, the commutation voltage on the inverter side satisfies the following expression: Where: E s is the rms value of the line voltage of the receiving-end AC system voltage source; U L is the effective value of the commutation voltage on the inverter side; P d is the active power for DC transmission; Q c Reactive power absorbed by the AC system from the DC system; R s and X s are the resistance and reactance of the internal impedance Z s of the AC system voltage source, respectively, satisfying Z s = R s + X s ; At the commutation bus of the inverter, the reactive power balance formula is: Q c = Q f -Q d = B c U L 2 -Q d (2) Where: Q d is the reactive power consumed by the inverter; Q f The reactive power provided for reactive power compensation devices such as filters satisfies Q f = B c U L 2 ; Bc is the equivalent susceptance of the reactive power compensation device; In formula (2), the reactive power consumed by the inverter can be further expressed as: Wherein: is the power factor angle; I d is a direct current; k is the turns ratio of the converter transformer; N is the number of 6-pulse converters; U d0 is the ideal no-load DC voltage and satisfies Through the above calculation process, the reactive power consumed by the inverter and the reactive power balance formula of the inverter commutation bus can be obtained, preparing for the calculation of the DC current reference value later; In step (2), the function Q can be obtained from Equation (1) c = f1(U L , P d ). From Equations (2) and (3), the function Q c = f2(U L , P d , I d ). Considering that Q c takes the same value in the above two functions, i.e., f1(U L , P d ) = f2(U L , P d , I d ), by solving this equation, the variable Q c can be eliminated, and then the function of I d with respect to U L and P d can be obtained. The expression of the function I d = f(U L , P d ) is as follows: Where: Δ = (U L X s ) 2 + Z s 2 (E s 2 - U L 2 - P d 2 Z s 2 / U L 2 + 2P d R s ); Function I d = f(U L , P d ) has two variables, namely U L and P d . In fact, the active power of the DC control system can only be obtained by measuring the product of the DC voltage and the DC current; In Equation (4), if P d is calculated in relation to I d , this will make the function I d = f(U L , P d ) more complex. For the above reasons, it is necessary to establish the relationship between P d and U L , and then eliminate P d in the function I L = f(U d , P d ). According to the steady-state model of the HVDC system, the expressions of DC voltage and DC current can be expressed as Equation (5). Assuming that the extinction angle γ after commutation failure is the minimum extinction angle γ min , and the advance trigger angle β will not be adjusted in a short time, that is, β = β N , then the active power P d can be expressed as Equation (6): As can be seen from Equation (6), when X C , γ min and β N are all constants, the active power P d is only determined by the commutation voltage U L . Substituting (6) into (4), the DC current can be written as a function only related to the commutation voltage, that is, I d = f(U L ); According to the function I d = f(U L ), it can be known that by measuring U L , I d can be obtained; Step (3) Substitute U L into the function I d = f(U L ), to obtain the curve of I d = f(U L ). Change the independent variable of the low-voltage current limiting control from the DC voltage to the commutation voltage, to obtain the curve of I dord = f(U L ). To ensure that the DC current can be immediately limited after commutation failure occurs, a smaller value should be selected as the DC current reference value. When U L > 160 kV, at this time I d is less than I ord , so it is necessary to use I d as the DC current reference value I dref . When U L < 95 kV, at this time I d is greater than I ord , so it is necessary to use I ord as the DC current reference value I dref . When the drop degree of the commutation voltage is between 95 - 160 kV, to ensure the continuity of the current change, the change of the DC current reference value is linearly processed in the first order. Finally, combining the above three cases, obtain the characteristic curve of the DC current reference value I dref changing with the commutation voltage. The DC current reference value I dref can be expressed as: Where: U LN is the rated commutation voltage; I dref Combining the advantages of low-voltage current-limiting control, it can quickly limit the DC current at the initial stage of the fault and slowly increase the DC current at the initial stage of fault recovery. Since the I in calculating the DC current reference value d takes into account the reactive power demand of the inverter, this method can also prevent the commutation voltage at the inverter side from dropping sharply while adjusting the DC current reference value; taking this DC current reference value as the new DC current command value and sending it into the DC control system can suppress the occurrence of subsequent commutation failures. Equation (1) is applicable to faults occurring at the commutation bus of the inverter. When faults occur at other positions of the AC line, Equation (1) needs to be corrected; however, considering that when the severity of the fault is the same, for the inverter, the commutation bus fault is more serious than the AC line fault; therefore, under the same fault severity, if the proposed DC current reference value calculation method can suppress the commutation failures caused by the commutation bus fault, it can also suppress the commutation failures caused by the AC line fault.

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