Transient overvoltage suppression method and system

Abstract

The invention discloses a transient overvoltage suppression method and system, and relates to the field of power system overvoltage protection, and the method comprises the steps: constructing a state space expression of a high-voltage DC power transmission system; inputting the direct current and the receiving end direct current voltage into the state space expression, and determining an equivalent control quantity; determining a tracking error determined according to the direct current and the reference current as a sliding modulus; determining a switching control quantity based on a second-order sliding mode superhelix algorithm; determining a super-spiral sliding mode constant current control rate according to the equivalent control quantity and the switching control quantity; the super-spiral sliding mode constant current control rate meets the constraint condition of Lyapunov stability; substituting the sliding mode quantity into a super-spiral sliding mode constant current control rate to determine a rectification side trigger angle signal; according to the high-voltage direct-current power transmission system, the transient overvoltage suppression of the sending end is easy to realize, the high-voltage direct-current power transmission system is convenient and simple, and the dynamic performance and the steady-state performance of the high-voltage direct-current power transmission system can be improved.

Description

Technical Field

[0001] The present application relates to the field of overvoltage protection of power systems, and in particular to a transient overvoltage suppression method and system. Background Art

[0002] At present, the construction of the "Shagohuang" new energy base is progressing steadily. New energy bases are usually far away from the load center, and high-voltage direct current transmission technology is currently the key technology for realizing long-distance power transmission. The converter station of the high-voltage direct current transmission system consumes a large amount of reactive power during normal operation, and this part of reactive power is usually compensated on-site by the filter placed in the converter station. When a fault occurs in the receiving-end AC system, a commutation failure may occur in the receiving-end converter station. During the commutation failure and the recovery process after the fault is cleared, the reactive balance in the sending-end converter station is broken, causing transient overvoltage at the sending end. With the continuous increase in the penetration rate of new energy, the strength of the sending-end AC system gradually decreases, and the transient overvoltage at the sending end caused by the commutation failure will be further aggravated. There is a risk that the new energy units may be disconnected from the grid on a large scale, which is not conducive to the stable operation of the power system. Therefore, how to suppress the transient overvoltage of the sending-end AC system after the commutation failure is an urgent problem to be solved.

[0003] The analysis of the mechanism of transient overvoltage generation at the sending end after commutation failure is relatively mature. It is generally agreed that the reactive power consumed by the rectifier station during commutation failure increases first and then decreases, causing the voltage of the AC system at the sending end to show a "first low and then high" change trend. On this basis, the current research on the transient overvoltage suppression strategy after commutation failure mainly includes two aspects: adding reactive equipment and optimizing the control link.

[0004] For the installation of reactive equipment, there are mainly two ways to dissipate surplus reactive power by installing reactive compensation and to release excess energy during faults by installing lightning arresters and energy dissipation devices. Studies have shown that the configuration of static synchronous compensator (STATCOM) can improve the strength of the power grid during faults, accelerate the system to restore stability, and reduce transient overvoltages. However, STATCOM has the problem of "reverse regulation" of voltage in the process of suppressing overvoltage, which may worsen transient overvoltages. For the optimization of the control link, studies have analyzed the transient overvoltage suppression effect under different control parameters and the impact on the dynamic and steady-state performance of the system. At present, some scholars have predicted the changes in DC current, changed the trigger angle signal in advance, and suppressed the commutation failure, but this method has high requirements on the accuracy of the measurement link and is not easy to implement. Summary of the invention

[0005] The purpose of the present application is to provide a transient overvoltage suppression method and system, which is easy to achieve the suppression of transient overvoltage at the sending end, is convenient and simple, and can improve the dynamic performance and steady-state performance of the high-voltage direct current transmission system.

[0006] To achieve the above objectives, this application provides the following solutions:

[0007] In a first aspect, the present application provides a transient overvoltage suppression method, the transient overvoltage suppression method is used in a high-voltage direct current transmission system, the equivalent circuit of the high-voltage direct current transmission system comprises: a sending-end AC system, a rectifier station, a DC transmission line, an inverter station, and a receiving-end AC system connected in sequence;

[0008] The transient overvoltage suppression method comprises:

[0009] Obtaining the DC current in the DC transmission line and the receiving-end DC voltage in the receiving-end AC system;

[0010] Construct state-space representations of HVDC transmission systems;

[0011] Inputting the DC current and the receiving end DC voltage into the state space expression to determine an equivalent control quantity;

[0012] Determine a tracking error according to the DC current and the reference current, and determine the tracking error as a sliding mode amount; the sliding mode amount satisfies the requirement that the relative order in the super-helical sliding mode control is 1;

[0013] Determine the switching control quantity based on the second-order sliding mode super-helical algorithm;

[0014] Determine a super helical sliding mode constant current control rate according to the equivalent control amount and the switching control amount; the super helical sliding mode constant current control rate satisfies the constraint condition of Lyapunov stability;

[0015] The sliding mode quantity is substituted into the super-helical sliding mode constant current control rate to determine the rectifier-side trigger angle signal; the rectifier-side trigger angle signal is used to input the rectifier station to suppress transient overvoltage of the sending-end AC system.

[0016] In a second aspect, the present application provides a transient overvoltage suppression system, the transient overvoltage suppression system is used to suppress transient overvoltages in a high-voltage direct current transmission system, the equivalent circuit of the high-voltage direct current transmission system comprising: a sending-end AC system, a rectifier station, a DC transmission line, an inverter station, and a receiving-end AC system connected in sequence;

[0017] The transient overvoltage suppression system comprises: a super-helical sliding mode constant current control link; the super-helical sliding mode constant current control link is respectively connected to the rectifier station and the DC transmission line;

[0018] The super spiral sliding mode constant current control link includes:

[0019] A data sampling module is used to obtain the DC current in the DC transmission line and the receiving-end DC voltage in the receiving-end AC system;

[0020] State space expression building module, used to build the state space expression of the high voltage direct current transmission system;

[0021] An equivalent control quantity determination module, used for inputting the DC current and the receiving end DC voltage into the state space expression to determine the equivalent control quantity;

[0022] A sliding mode amount determination module, used to determine a tracking error according to the DC current and a reference current, and determine the tracking error as a sliding mode amount; the sliding mode amount meets the requirement that the relative order in the super-helical sliding mode control is 1;

[0023] A switching control amount determination module, used for determining the switching control amount based on a second-order sliding mode super-helical algorithm;

[0024] A super-helical sliding mode constant current control rate determination module is used to determine the super-helical sliding mode constant current control rate according to the equivalent control amount and the switching control amount; the super-helical sliding mode constant current control rate satisfies the constraint condition of Lyapunov stability;

[0025] The transient overvoltage suppression module is used to substitute the sliding mode quantity into the super-helical sliding mode constant current control rate to determine the rectifier side trigger angle signal; the rectifier side trigger angle signal is used to input the rectifier station to suppress the transient overvoltage of the sending end AC system.

[0026] According to the specific embodiments provided in this application, this application discloses the following technical effects:

[0027] The present application provides a transient overvoltage suppression method and system, which determines the tracking error determined according to the DC current and the reference current as a sliding mode quantity, and determines the switching control quantity based on the second-order sliding mode super-helical algorithm, determines the super-helical sliding mode constant current control rate according to the equivalent control quantity and the switching control quantity, and substitutes the sliding mode quantity into the super-helical sliding mode constant current control rate to determine the rectifier side trigger angle signal, thereby suppressing the transient overvoltage of the sending-end AC system. The present application realizes the suppression of transient overvoltage based on the sliding mode super-helical algorithm. Since the sliding mode super-helical algorithm can avoid the jitter problem caused by the switching control, and has the advantages of simple design and easy implementation, the present application is easy to realize the suppression of transient overvoltage at the sending end, is convenient and simple, and can improve the dynamic performance and steady-state performance of the high-voltage DC transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 A schematic diagram of a transient overvoltage suppression method provided in one embodiment of the present application;

[0030] Figure 2 A circuit schematic diagram of an equivalent circuit of a high voltage direct current transmission system provided in one embodiment of the present application;

[0031] Figure 3 A schematic diagram of a second-order sliding mode motion trajectory provided in an embodiment of the present application;

[0032] Figure 4 This is a rectifier side overall control block diagram provided by an embodiment of the present application;

[0033] Figure 5 A schematic diagram of a design process of a super spiral sliding mode constant current control provided in an embodiment of the present application;

[0034] Figure 6 A schematic diagram of a design process of a super spiral sliding mode constant current control provided in another embodiment of the present application;

[0035] Figure 7 A curve diagram of the DC current response on the rectifier side under a small disturbance provided by another embodiment of the present application;

[0036] Figure 8 A curve diagram of the DC voltage response on the rectifier side under a small disturbance provided by another embodiment of the present application;

[0037] Fig. 9 A voltage curve diagram of an AC system at the sending end under a slight fault provided by another embodiment of the present application;

[0038] Fig.10 A DC current response curve diagram under a slight fault provided by another embodiment of the present application;

[0039] Fig.11 A curve diagram of active power of DC transmission under a minor fault provided by another embodiment of the present application;

[0040] Fig.12 A DC voltage curve diagram under a slight fault provided by another embodiment of the present application;

[0041] Fig.13 A reactive power curve diagram of a rectifier-side filter under a minor fault provided by another embodiment of the present application;

[0042] Fig.14 A curve diagram of the trigger angle change on the rectifier side under a slight fault provided by another embodiment of the present application;

[0043] Fig.15 A voltage curve diagram of an AC system at the sending end under a severe fault provided by another embodiment of the present application;

[0044] Fig.16 A DC current response curve diagram under severe fault conditions provided by another embodiment of the present application;

[0045] Fig.17 A curve diagram of active power of DC transmission under severe fault provided by another embodiment of the present application;

[0046] Fig.18 A DC voltage curve diagram under severe fault conditions provided by another embodiment of the present application;

[0047] Fig.19 A reactive power curve diagram of a rectifier-side filter under a severe fault provided by another embodiment of the present application;

[0048] Fig. 20 A curve diagram of the trigger angle change on the rectifier side under severe fault conditions provided in another embodiment of the present application.

[0049] Reference numerals: sending-end AC system-1, rectifier station-2, DC transmission line-3, inverter station-4, receiving-end AC system-5. DETAILED DESCRIPTION

[0050] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0051] Sliding mode control is a nonlinear control method for linear or nonlinear objects with imprecise modeling. By designing a sliding surface and a discontinuous control rate, the system state variables move along the designed sliding surface. The sliding surface is independent of disturbances and has strong robustness. At present, sliding mode control has been widely used in many fields such as spacecraft attitude tracking control, permanent magnet synchronous motor control, and flexible DC transmission control.

[0052] Traditional sliding mode control structures are relatively complex and have chattering problems, which may cause serious failures in actual engineering. Super-helical sliding mode is a second-order sliding mode. By placing discontinuous terms in sliding mode control into the high-order derivatives of the sliding surface, the chattering problem caused by switching control is avoided. At the same time, the super-helical algorithm has the advantages of simple design and easy implementation, and has been applied in energy storage converters and DC-DC converter controls.

[0053] However, there is currently a lack of relevant literature discussing the application of sliding mode control in line commutated converter based high voltage direct current (LCC-HVDC) systems. How to implement LCC-HVDC system modeling suitable for sliding mode control design, and the performance of applying sliding mode control to the constant current control link for transient overvoltage suppression remain to be studied. Therefore, this application utilizes the advantages of super-spiral sliding mode control and proposes a transient overvoltage suppression method and system, which is based on the LCC-HVDC system transient overvoltage suppression strategy of super-spiral sliding mode constant current control, while suppressing transient overvoltages at the sending end, improving the dynamic performance and steady-state performance of the high-voltage direct current transmission system.

[0054] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0055] In an exemplary embodiment, Figure 1 As shown, a transient overvoltage suppression method is provided, and the transient overvoltage suppression method is used in a high-voltage direct current transmission system.

[0056] See also Figure 2 The equivalent circuit of the high-voltage direct current transmission system includes: a sending-end AC system 1, a rectifier station 2, a DC transmission line 3, an inverter station 4 and a receiving-end AC system 5 connected in sequence. Figure 2 Where S1 is the equivalent power supply of the AC system at the sending end, u s1 is the equivalent power supply voltage of the AC system at the sending end, R s1 and L s1 are the equivalent resistance and inductance of the AC system at the sending end, i s1 and u r are the current and voltage at the junction of the sending-end AC system 1 and the sending-end AC bus, AC filter1 is the sending-end filter and reactive power compensation equipment, U1 is the sending-end AC bus voltage in the sending-end AC system 1, PCC represents the interface point between the power grid and the independent power source, k r is the transformer ratio at the sending end, U d1 is the DC voltage at the sending end of the AC system 1 (i.e., the DC voltage at the converter station at the sending end), Rd and L d are the equivalent resistance and inductance of the DC transmission line, I d is the DC current, U d2 is the receiving-end DC voltage in the receiving-end AC system 5 (i.e., the receiving-end converter station DC voltage), k i is the transformer ratio of the receiving end, U2 is the AC bus voltage of the receiving end, i s2 and u i are the current and voltage at the junction of the receiving end AC system 5 and the receiving end AC busbar, R s2 and L s2 are the equivalent resistance and inductance of the receiving end AC system, S2 is the equivalent power supply of the receiving end AC system, u s2 is the equivalent power supply voltage of the receiving-end AC system, and AC filter2 is the receiving-end filter and reactive power compensation equipment.

[0057] The following first analyzes the mechanism of transient overvoltage at the sending end caused by commutation failure at the receiving-end converter station.

[0058] like Figure 2 As shown, when a fault occurs in the receiving-end AC system 5, it may induce a commutation failure in the receiving-end converter. During the commutation failure, the upper and lower valve groups of a phase of the receiving-end converter will be turned on at the same time, causing the DC pole voltage of the receiving-end converter to drop rapidly to zero. Among them, the fault can be divided into the following stages:

[0059] 1) DC current increasing stage.

[0060] The valves of the upper and lower bridge arms of the inverter are turned on at the same time, which is equivalent to the DC side of the valve being short-circuited and the AC side being open-circuited. The DC current I d In the rising stage. In addition, after the converter valve is turned on, the arc extinction angle will be instantly reduced to 0, and the commutation failure prediction function (CFPED) link will respond to increase the arc extinction angle, thereby improving the commutation margin and preventing continuous commutation failure.

[0061] At the same time, the AC voltage at the receiving end drops rapidly at the moment the disturbance occurs. From formula (1), we can see that the DC voltage U d2 Then it decreased.

[0062] U d2 =1.35U2 cosβ+(3 / π)X C2 I d (1)

[0063] Where: β is the leading trigger angle of the inverter side; I d is the DC current; X C2 is the commutation reactance of inverter station 4; U2 is the receiving end AC voltage; U d2 is the receiving end DC voltage.

[0064] Since the DC current I d has reached a higher level, and the DC voltage U d1 It has not been affected by the fault. From formula (2), we can know that the reactive power Q consumed by rectifier station 2 is d Increase. When the system is in steady state, the reactive power required by the rectifier station 2 is mainly provided by the AC filter group and the shunt capacitor group. However, these two types of equipment cannot provide sufficient reactive power instantaneously due to insufficient speed. Therefore, the sending-end AC system 1 needs to supply reactive power to the rectifier station 2, causing the sending-end commutation bus voltage U1 to drop and a transient low voltage to appear.

[0065]

[0066] Where: is the power factor angle at the sending end; Q d It is the reactive power consumed by rectifier station 2.

[0067] Therefore, from formula (3), we can know that the DC voltage U d1 Then it decreased.

[0068] U d1 =1.35U1 cosα-(3 / π)X C1 I d (3)

[0069] Where: α is the trigger angle on the rectifier side; X C1 is the commutation reactance of rectifier station 2; U1 is the AC voltage at the sending end; U d1 is the DC voltage at the sending end.

[0070] 2) DC current reduction stage.

[0071] In order to suppress the rise in the first stage, the constant current control link (CCA) on the rectifier side is started to increase the sending end trigger angle α to reduce the DC current I d In addition, due to the drop of the DC voltage value in the first stage and the increase of , the low voltage current limiting link (VDCOL) will also respond to reduce the command value. The reduction of the command value will increase the input of the CCA link during the fault, causing excessive overshoot of the current, so the DC current I d After the reduction, the DC current I d At the end of the action delay, I d It reaches a peak value and then begins to decrease, and the fault enters the DC current reduction stage.

[0072] I d After the reduction, the DC voltage U d1 Before it can change, the reactive power demand Q of the rectifier dHowever, the safety control device has limited reaction capacity due to the delay, and the fixed reactive power compensation device at the sending end cannot be withdrawn in time, and continues to send a large amount of reactive power that is 40% to 60% of the DC active power. As a result, there is excess reactive power at the sending end converter station, and a large amount of reactive power is even reversed and poured into the AC system, causing transient overvoltage on the sending end converter bus.

[0073] 3) DC recovery stage.

[0074] The voltage, current and other indicators gradually returned to the stable operating level, the reactive power demand of rectifier station 2 increased, reached a new balance with the AC system, and the transient voltage returned to the steady-state value before the fault. Therefore, the transient overvoltage phenomenon caused by commutation failure has the typical characteristics of "first decreasing and then increasing".

[0075] From the above analysis, it can be seen that the change of system state after commutation failure is related to the DC current I d The low current formed by the overshoot of the DC current in the constant current control link is the key factor leading to transient overvoltage at the sending end. Optimizing the constant current control and improving the DC current response characteristics are a solution to suppress transient overvoltage at the sending end after commutation failure.

[0076] Next, the super-helical sliding mode control principle is analyzed.

[0077] Sliding mode variable structure control can switch the control function according to the real-time state of the system so that the system state point always remains on the sliding surface. The sliding surface is obtained through artificial design according to the control objective. It is less affected by external interference and less sensitive to parameter changes. Therefore, it has strong robustness.

[0078] Consider a nonlinear system of the form

[0079]

[0080] Where x is the state variable of the system; u is the control signal, which is used as the control input; f(x) and g(x) are smooth unknown functions; and s(x) is the sliding mode with input x.

[0081] Let s(x) = 0 be the sliding surface. The control objective is to make the system state reach the sliding surface s(x) = 0 within a finite time and have a second-order sliding mode. In other words, the system state reaches the following sliding manifold within a finite time:

[0082]

[0083] Assume that the relative order of the sliding mode s(x) with respect to the control input u is 1, that is, At this time there are:

[0084]

[0085] The derivative of the control input u As a new control variable, by designing discontinuous control Make the sliding mode s(x) approach zero and maintain the second-order sliding mode, that is, The control input u is obtained by The second-order sliding mode motion trajectory is obtained by integrating , so it is continuous, thus suppressing the system chattering. Figure 3 shown.

[0086] In order to ensure the strictness of the second-order sliding mode, the smooth unknown functions f(x) and g(x) are globally bounded:

[0087]

[0088] Where: C, K m and K M All are normal numbers.

[0089] The super spiral algorithm refers to the state trajectory spirally converging to the origin in a finite time. This algorithm is the only one among the existing second-order sliding mode algorithms that does not require the derivative information of the sliding mode quantity. The algorithm can be directly applied when the relative order of the sliding mode s with respect to the control input u is 1 without introducing a new control variable. Can simplify the design of control systems.

[0090] The switching control rate based on the second-order sliding mode super-helical algorithm, that is, the expression of the switching control amount is as follows:

[0091]

[0092] Where: u sw represents the switching control amount; u1 represents the first control amount; u2 represents the second control amount; represents the first-order derivative of u2; s represents the sliding modulus; λ1 and λ2 are both constants; sign represents the sign function.

[0093] At this time, the discontinuous high-frequency switching quantity λ2sign(s) no longer directly affects the control law u sw , but appears in the switching control law in the form of time integral, thereby obtaining a continuous control signal, thus eliminating the chattering defect in traditional sliding mode control.

[0094] The above analysis of the robustness of super-helical sliding mode control and the design ideas of the control rate can effectively suppress the common chattering problems in traditional sliding mode control. Based on the design ideas of the control rate given in step 2 and the second-order sliding mode super-helical algorithm, it will be applied to the constant current control design of the high-voltage direct current transmission system. By designing the super-helical sliding mode constant current control law, the transient overvoltage suppression method is realized.

[0095] Still see Figure 1 , the transient overvoltage suppression method comprises:

[0096] Step 101: obtaining a DC current in a DC transmission line and a receiving-end DC voltage in a receiving-end AC system.

[0097] Step 102: Construct a state space expression of the high voltage direct current transmission system.

[0098] Step 103: input the DC current and the receiving-end DC voltage into the state space expression to determine an equivalent control variable.

[0099] Step 104: determining a tracking error according to the DC current and the reference current, and determining the tracking error as a sliding mode amount; the sliding mode amount meets the requirement that the relative order in the super-helical sliding mode control is 1.

[0100] Step 105: Determine the switching control amount based on the second-order sliding mode super-helical algorithm.

[0101] Step 106: Determine a super-helical sliding mode constant current control rate according to the equivalent control amount and the switching control amount; the super-helical sliding mode constant current control rate satisfies the constraint conditions of Lyapunov stability.

[0102] Step 107: Substitute the sliding mode quantity into the super-helical sliding mode constant current control rate to determine the rectifier-side trigger angle signal; the rectifier-side trigger angle signal is used to input the rectifier station to suppress transient overvoltage of the sending-end AC system.

[0103] In another exemplary embodiment of the present application, step 102 specifically includes:

[0104] 1) Establish an equivalent mathematical model of the high voltage direct current transmission system.

[0105] Specifically, according to Figure 2 , the dynamic equation of the high voltage direct current transmission system can be written out, and the dynamic equation can be used as an equivalent mathematical model. The expression of the equivalent mathematical model is:

[0106]

[0107] Among them, I d Indicates direct current; Indicates I d The first derivative of d Indicates the equivalent inductance of the DC transmission line; U d1 represents the DC voltage at the sending end of the AC system 1; R d Indicates the equivalent resistance of the DC transmission line; U d2 Represents the receiving-end DC voltage in the receiving-end AC system 5.

[0108] 2) Based on the equivalent mathematical model, the state space expression of the high voltage direct current transmission system is constructed with the direct current as the state variable and the rectifier-side trigger angle signal input to the rectifier station 2 as the control signal.

[0109] Specifically, by substituting formula (3) into formula (9), we can obtain:

[0110]

[0111] Since the trigger angle α of the constant current control signal is contained in the nonlinear term, it does not meet the general form of controller design. Therefore, the first-order Taylor expansion linearization is performed at the system stable operating point α0, and the processed system dynamic equation is obtained as follows:

[0112]

[0113] It can be further expressed as a state space expression:

[0114]

[0115] Among them, x represents the state variable; represents the first-order derivative of x; u represents the control signal; I d represents the DC current; α represents the trigger angle signal on the rectifier side; a(x) represents the state intermediate variable; b represents the control coefficient; L d represents the equivalent inductance of the DC transmission line; U1 represents the sending-end AC bus voltage in the sending-end AC system 1; α0 represents the trigger angle signal corresponding to the stable operation point in the high-voltage DC transmission system; X c1 Represents the commutation reactance of rectifier station 2; R d Indicates the equivalent resistance of the DC transmission line; U d2 Represents the receiving-end DC voltage in the receiving-end AC system 5.

[0116] In another exemplary embodiment of the present application, the process of determining the sliding modulus in step 104 is:

[0117] The design goal of constant current control is to control the DC current I d Can strictly track its reference signal I dref . Select the tracking error according to the design goal:

[0118] e=x * -x=I dref -I d (13)

[0119] The sliding mode is defined using tracking error:

[0120] s=e (14)

[0121] Where s represents the sliding mode; e represents the tracking error; I dref Represents the reference current; I d Indicates direct current.

[0122] The derivative of the sliding modulus is:

[0123]

[0124] because Therefore, the selected sliding mode quantity s satisfies the relative order of 1 with respect to the control input u, and super-helical sliding mode control can be used.

[0125] In another exemplary embodiment of the present application, the control method adopted in this embodiment is essentially equivalent variable structure control, and the control signal u is composed of the equivalent control quantity u eq and the switching control quantity u sw They are responsible for bringing the state variable x into the sliding surface and ensuring that the state variable x does not leave the sliding surface. The expression of the control signal u is:

[0126] u=u eq +u sw (16)

[0127] Will and u=u eq Substituting into equation (15), we can get the system equivalent control quantity u eq The specific expression is:

[0128]

[0129] Among them, u eq represents the equivalent control quantity; a(x) represents the state intermediate variable; b represents the control coefficient.

[0130] Therefore, step 103 specifically includes: 1) inputting the DC current and the receiving end DC voltage into the state space expression to calculate the state intermediate variable a(x) and the control coefficient b. 2) determining the equivalent control quantity according to the state intermediate variable and the control coefficient.

[0131] Among them, the switching control quantity u sw The super-helical sliding mode control is adopted, and the expression of the switching control quantity is shown in equation (8), which will not be repeated here.

[0132] Substituting equations (8) and (17) into equation (16), we can obtain the expression of the super-helical sliding mode constant current control rate:

[0133]

[0134] Among them, u represents the control signal; a(x) represents the state intermediate variable; b represents the control coefficient; s represents the sliding mode; λ1 and λ2 are both constants.

[0135] In another exemplary embodiment of the present application, verification is required, and the process is as follows:

[0136] The stability of the control law is analyzed by substituting equation (28) into equation (15), and the first-order derivative expression of the sliding mode s is obtained as follows:

[0137]

[0138] In order to verify the stability of the designed super-helical sliding mode constant current control, the stability of equation (19) is proved by using the Lyapunov function. For the convenience of analysis, the above equation is changed into the following form:

[0139]

[0140] In the formula, m = -λ1b, n = -λ2b.

[0141] Introduce new state variables:

[0142]

[0143] Derivative of the above formula:

[0144]

[0145] The derivative of Z is:

[0146]

[0147] Define the Lyapunov function:

[0148] V(Z)=Z T PZ (24)

[0149] Where P is a symmetric positive definite matrix, and λ1 and λ2 are both constants.

[0150]

[0151] It can be obtained that Lyapunov function V(Z)≥0.

[0152] Taking the derivative of V(Z) we get:

[0153]

[0154] The matrix Q is:

[0155]

[0156] Therefore, when the matrix Q is a positive definite matrix, the function V(Z) < 0. According to Lyapunov's second law, equation (19) is stable.

[0157] set up

[0158]

[0159] The matrix Q can be simplified as:

[0160]

[0161] Its characteristic values ​​are:

[0162]

[0163] To make the matrix Q a positive definite matrix, we only need p1>0, then:

[0164]

[0165] Substituting m and n, we can obtain the constraint that the matrix Q is a positive definite matrix:

[0166]

[0167] When the above formula (12) is satisfied, it can be guaranteed Therefore, as long as the control parameters satisfy the constraint condition that the matrix Q is a positive definite matrix, the control system is Lyapunov stable, that is, the constraint condition of the positive definite matrix mentioned above is used as the constraint condition of Lyapunov stability.

[0168] Combine the following Figure 4 and Figure 5 An overall process of the above transient overvoltage suppression method is described.

[0169] The control process of the rectifier side of the HVDC system under super-helical sliding mode constant current control is as follows: Figure 4 As shown, Figure 4 In the figure, ST-SMC is the proposed super-helical sliding mode constant current control link, I dref is the DC current reference value, K mr / 1+sT mr is the transfer function of the DC current measurement link, where K mr is the proportional coefficient of the measuring link, T mr The transfer function is used to measure the time constant of the DC current I d After filtering, the DC current I d With reference current I dref The difference is input into ST-SMC as the sliding modulus s.

[0170] Specifically, first, the DC current I dand receiving end DC voltage U d2 Sampling is performed to calculate a(x) and b in equation (12). DC current I d The difference between the reference current and the current after the measurement link is used as the sliding mode quantity s input. Substituting it into formula (18) can obtain the rectifier side trigger angle signal α output by the super spiral sliding mode constant current control. The trigger angle signal α will be transmitted to the rectifier side converter as a control signal to change the output DC voltage on the rectifier side, and the DC current will also change accordingly until the DC current reference value is reached. By outputting the trigger angle signal α, the DC current is controlled so that it has better dynamic response characteristics during the fault period. By improving the low current level of the DC current during the fault period, the reactive power consumption of the converter station is increased, thereby suppressing transient overvoltage.

[0171] In summary, the design process of super spiral sliding mode constant current control is as follows: Figure 5 First, the equivalent mathematical model of the high-voltage direct current transmission system is established, as shown in formula (9). Based on the mathematical model, the DC current I d is the state variable, and the rectifier side trigger angle signal α is the control signal to obtain the DC system state space expression, as shown in equation (12), and written as the general form of controller design

[0172] Taking advantage of the fact that the sliding mode in the sliding mode control is insensitive to disturbances, according to the design goal of the constant current controller, the DC current deviation is defined as the sliding mode quantity s, and it is judged whether the sliding mode quantity s meets the requirement of the relative order of 1 in the super-helical sliding mode control. If it does not meet the requirement, the sliding mode quantity s needs to be redefined. The equivalent variable structure control design idea is adopted, and the control input u is determined by the equivalent control quantity u eq and the switching control quantity u sw Finally, the stability check is carried out based on the Lyapunov function, and the conditions that the control parameters need to meet to make the controller stable are given.

[0173] An experimental process is provided below to verify the effectiveness of the above transient overvoltage suppression method.

[0174] In order to verify the effectiveness of super-spiral sliding mode constant current control in suppressing transient overvoltage at the sending end after commutation failure, based on the PSCAD / EMTDC simulation platform and the conventional DC standard model of the International Conference on Large Electric Systems (CIGRE), the rectifier-side PI constant current control is replaced by the super-spiral sliding mode constant current control mentioned above. Figure 6 As shown, Figure 6 Medium dc Represents the equivalent inductance of the DC transmission line, R dc Indicates the equivalent resistance of the DC transmission line, U dc Indicates the DC voltage at the midpoint of the transmission line, I dcrIndicates the DC current at the sending end, I dic Represents the receiving end DC current, C dc Represents the capacitance of the DC transmission line to ground. The system related parameters are shown in Table 1.

[0175] Table 1 CIGRE HVDC standard model system parameters

[0176]

[0177]

[0178] Example 1: Small disturbance simulation comparison.

[0179] In order to evaluate the performance of super-helical sliding mode constant current control, a small disturbance simulation test is carried out based on PSCAD / EMTDC. At the same time, since the super-helical sliding mode control equation (8) contains a nonlinear function sign, it is difficult to solve the eigenvalue, so the performance evaluation is only carried out through time domain simulation.

[0180] After the DC system runs stably for a period of time, the reference voltage on the inverter side drops at 3s. Figure 7 and Figure 8 They are the response curves of DC current and DC voltage on the rectifier side under small disturbances.

[0181] Depend on Figure 7 and Figure 8 It can be seen that the PI control has a large DC current fluctuation amplitude under small disturbance conditions, while the super-helical sliding mode constant current control designed in this paper has a smaller DC current fluctuation amplitude and a faster recovery speed, and its control performance is better than that of PI control. In terms of the DC voltage change amplitude and recovery speed, the super-helical sliding mode constant current control designed in this paper has a slightly better effect, but is significantly faster than PI control in terms of recovery speed.

[0182] Example 2: Comparison of minor fault simulation.

[0183] In this section, a grounding inductor is connected at the receiving end AC bus to simulate a single-phase grounding fault in the AC transmission line. A single-phase grounding fault occurs at the receiving end AC bus at 1 second, and the fault lasts for 0.1 second.

[0184] The system response curves of different control strategies under minor faults are as follows: Figure 9-14 shown. Fig. 9 The voltage curve of AC system 1 at the sending end under a slight fault is shown. When a fault occurs, the transient overvoltage of super-helical sliding mode constant current control is significantly reduced compared with the traditional PI constant current control. Fig.10 The DC current response curve under a minor fault is shown. Under the action of super-helical sliding mode constant current control, the overcurrent and undercurrent levels during the fault are significantly improved. Fig.11 and Fig.12The curves of DC transmission active power and DC voltage under minor faults are shown respectively. The recovery speed is significantly accelerated under the action of super-helical sliding mode constant current control. Fig.13 The reactive power curve of the filter on the rectifier side under a minor fault is shown, and the reactive redundancy is reduced during the fault; Fig.14 The trigger angle change curve of the rectifier side under a minor fault is shown. It can be seen that the change speed of the trigger angle of the super-spiral sliding mode constant current control is significantly improved compared with the traditional PI constant current control. After the single-phase fault of the receiving-end AC busbar, the system fails to commutate, and the sending-end AC system 1 generally shows a trend of first low and then high. Since the super-spiral sliding mode constant current control can respond more to current changes than the PI constant current control, it can increase or decrease the trigger angle of the rectifier side more quickly, thereby reducing the overcurrent level and improving the low current level caused by overshoot. The decrease in the overcurrent level leads to a decrease in the reactive power absorbed by the rectifier from the sending-end AC system 1, and the transient low voltage is improved; the increase in the low current level causes the reactive power consumed by the rectifier to increase, and the reactive surplus on the rectifier side is reduced, so that the transient overvoltage can be more significantly improved. The transient overvoltage at 1.07s dropped from 1.155pu to 1.101pu. While improving the transient overvoltage, the recovery rate of the DC voltage and active power is accelerated, and the dynamic and steady-state performance of the DC system is also enhanced to a certain extent.

[0185] Example 3: Severe fault simulation comparison.

[0186] In this embodiment, a grounding inductor is connected to the receiving-end AC bus to simulate a three-phase grounding fault in the AC transmission line. A three-phase grounding fault occurs in the receiving-end AC bus at 1 second, and the fault lasts for 0.1 second.

[0187] The system response curves of different control strategies under severe faults are as follows: Figure 15-Figure 20 As shown, Fig.15 The voltage curve of AC system 1 at the sending end under severe fault is shown. When the fault occurs, the system fails to commutate for two cycles. After the second commutation failure occurs, the transient overvoltage of super-helical sliding mode constant current control is significantly reduced compared with the traditional PI constant current control. Fig.16 The DC current response curve under severe fault is shown. Under the action of super-helical sliding mode constant current control, the over-current and under-current levels during the fault are significantly improved. Fig.17 and Fig.18 The curves of DC transmission active power and DC voltage under severe faults are shown respectively. The recovery speed is significantly accelerated under the action of super-helical sliding mode constant current control. Fig.19 The reactive power curve of the filter on the rectifier side under severe fault is shown, and the reactive redundancy is reduced during the fault; Fig. 20The trigger angle change curve of the rectifier side under severe faults is shown. It can be seen that the change speed of the trigger angle of the super spiral sliding mode constant current control is significantly improved compared with the traditional PI constant current control. After the three-phase fault of the AC bus at the receiving end, the system has two cycles of commutation failure, and the effective value of the overvoltage of the AC bus at the sending end has two peaks. In the first commutation failure cycle, since the low current level under the two control strategies is roughly the same, the first peak of the overvoltage of the AC bus at the sending end is also roughly the same, reaching the first overvoltage peak of 1.175pu at 1.15s; during the system recovery process, after the second commutation failure occurs, the DC current under the PI constant current control rapidly decreases from the maximum value at the time of commutation failure, and even drops to 0, so the second overvoltage peak of 1.245pu appears on the AC bus at the sending end. The super-spiral sliding mode constant current control increases or decreases the trigger angle on the rectifier side more quickly, thereby reducing the overcurrent level after the second commutation failure and improving the low current level caused by overshoot, significantly improving the transient low voltage and reducing the transient overvoltage of the AC bus at the sending end. The second peak is only 1.121pu. While improving the transient overvoltage, the recovery rate of the DC voltage and active power is accelerated, the trigger angle on the rectifier side does not reach the limit, and the dynamic and steady-state performance of the DC system is also enhanced to a certain extent.

[0188] Based on the same inventive concept, the embodiment of the present application also provides a transient overvoltage suppression system for implementing the transient overvoltage suppression method involved above. The implementation scheme for solving the problem provided by the system is similar to the implementation scheme recorded in the above method, so the specific limitations of one or more transient overvoltage suppression system embodiments provided below can refer to the limitations of the transient overvoltage suppression method above, and will not be repeated here.

[0189] In an exemplary embodiment, a transient overvoltage suppression system is provided, which is used to suppress transient overvoltages in a high-voltage direct current transmission system. The equivalent circuit of the high-voltage direct current transmission system includes: a sending-end AC system 1, a rectifier station 2, a DC transmission line 3, an inverter station 4, and a receiving-end AC system 5 connected in sequence.

[0190] The transient overvoltage suppression system includes: a super-helical sliding mode constant current control link; the super-helical sliding mode constant current control link is connected to the rectifier station 2 and the DC transmission line 3 respectively.

[0191] The super spiral sliding mode constant current control link includes:

[0192] The data sampling module is used to obtain the DC current in the DC transmission line 3 and the receiving-end DC voltage in the receiving-end AC system 5 .

[0193] The state space expression building module is used to build the state space expression of the high voltage direct current transmission system.

[0194] The equivalent control quantity determination module is used to input the DC current and the receiving end DC voltage into the state space expression to determine the equivalent control quantity.

[0195] The sliding mode amount determination module is used to determine the tracking error according to the DC current and the reference current, and determine the tracking error as the sliding mode amount; the sliding mode amount meets the requirement that the relative order is 1 in the super-helical sliding mode control.

[0196] The switching control amount determination module is used to determine the switching control amount based on the second-order sliding mode super-helical algorithm.

[0197] The super-helical sliding mode constant current control rate determination module is used to determine the super-helical sliding mode constant current control rate according to the equivalent control amount and the switching control amount; the super-helical sliding mode constant current control rate satisfies the constraint conditions of Lyapunov stability.

[0198] The transient overvoltage suppression module is used to substitute the sliding mode quantity into the super-helical sliding mode constant current control rate to determine the rectifier side trigger angle signal; the rectifier side trigger angle signal is used to input the rectifier station 2 to suppress the transient overvoltage of the sending end AC system 1.

[0199] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0200] This article uses specific examples to illustrate the principles and implementation methods of this application. The description of the above embodiments is only used to help understand the method and core ideas of this application. At the same time, for those skilled in the art, according to the ideas of this application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. A transient overvoltage suppression method, characterized in that: The transient overvoltage suppression method is used in a high-voltage direct current transmission system, wherein the equivalent circuit of the high-voltage direct current transmission system comprises: a sending-end AC system, a rectifier station, a DC transmission line, an inverter station, and a receiving-end AC system connected in sequence; The transient overvoltage suppression method comprises: Obtaining the DC current in the DC transmission line and the receiving-end DC voltage in the receiving-end AC system; Construct state-space representations of HVDC transmission systems; Inputting the DC current and the receiving end DC voltage into the state space expression to determine an equivalent control quantity; Determine a tracking error according to the DC current and the reference current, and determine the tracking error as a sliding mode amount; the sliding mode amount satisfies the requirement that the relative order in the super-helical sliding mode control is 1; Determine the switching control quantity based on the second-order sliding mode super-helical algorithm; Determine a super helical sliding mode constant current control rate according to the equivalent control amount and the switching control amount; the super helical sliding mode constant current control rate satisfies the constraint condition of Lyapunov stability; The sliding mode quantity is substituted into the super-helical sliding mode constant current control rate to determine the rectifier-side trigger angle signal; the rectifier-side trigger angle signal is used to input the rectifier station to suppress transient overvoltage of the sending-end AC system.

2. The transient overvoltage suppression method according to claim 1, characterized in that: Construct the state space expression of the HVDC transmission system, including: Establish an equivalent mathematical model of the HVDC transmission system; Based on the equivalent mathematical model, the state space expression of the high voltage direct current transmission system is constructed by taking the direct current as the state variable and the rectifier-side trigger angle signal input to the rectifier station as the control signal.

3. The transient overvoltage suppression method according to claim 1, characterized in that: The state space expression is: Among them, x represents the state variable; represents the first-order derivative of x; u represents the control signal; I d represents the DC current; α represents the trigger angle signal on the rectifier side; a(x) represents the state intermediate variable; b represents the control coefficient; L d represents the equivalent inductance of the DC transmission line; U1 represents the sending-end AC bus voltage in the sending-end AC system; α0 represents the trigger angle signal corresponding to the stable operating point in the high-voltage DC transmission system; X c1 Represents the commutation reactance of the rectifier station; R d Indicates the equivalent resistance of the DC transmission line; U d2 Represents the receiving end DC voltage in the receiving end AC system.

4. The transient overvoltage suppression method according to claim 1, characterized in that: Inputting the DC current and the receiving end DC voltage into the state space expression to determine the equivalent control quantity specifically includes: Input the DC current and the receiving end DC voltage into the state space expression to calculate the state intermediate variable and the control coefficient; The equivalent control amount is determined according to the state intermediate variable and the control coefficient; the expression for determining the equivalent control amount is: Among them, u eq represents the equivalent control quantity; a(x) represents the state intermediate variable; x represents the state variable; b represents the control coefficient.

5. The transient overvoltage suppression method according to claim 1, characterized in that: The expression of the sliding modulus is: s=e; e=I dref -I d ; Where s represents the sliding mode; e represents the tracking error; I dref Represents the reference current; I d Indicates direct current.

6. The transient overvoltage suppression method according to claim 1, characterized in that: The expression of the switching control amount is: Among them, u sw represents the switching control quantity; u1 represents the first control quantity; u2 represents the second control quantity; u·2 represents the first-order derivative of u2; s represents the sliding mode; λ1 and λ2 are both constants; sign represents the sign function.

7. The transient overvoltage suppression method according to claim 1, characterized in that: The expression of the super helical sliding mode constant current control rate is: Among them, u represents the control signal; a(x) represents the state intermediate variable; x represents the state variable; b represents the control coefficient; s represents the sliding mode; λ1 and λ2 are both constants; sign represents the sign function.

8. The transient overvoltage suppression method according to claim 7, characterized in that: The constraints for Lyapunov stability are:

9. The transient overvoltage suppression method according to claim 2, characterized in that: The expression of the equivalent mathematical model is: Among them, I d Indicates direct current; I d The first derivative of d Indicates the equivalent inductance of the DC transmission line; U d1 Represents the DC voltage at the sending end of the AC system; R d Indicates the equivalent resistance of the DC transmission line; U d2 Represents the receiving end DC voltage in the receiving end AC system.

10. A transient overvoltage suppression system, characterized in that: The transient overvoltage suppression system is used to suppress transient overvoltage in a high-voltage direct current transmission system, the equivalent circuit of which comprises: a sending-end AC system, a rectifier station, a DC transmission line, an inverter station and a receiving-end AC system connected in sequence; The transient overvoltage suppression system comprises: a super-helical sliding mode constant current control link; the super-helical sliding mode constant current control link is respectively connected to the rectifier station and the DC transmission line; The super spiral sliding mode constant current control link includes: A data sampling module is used to obtain the DC current in the DC transmission line and the receiving-end DC voltage in the receiving-end AC system; State space expression building module, used to build the state space expression of the high voltage direct current transmission system; An equivalent control quantity determination module, used for inputting the DC current and the receiving end DC voltage into the state space expression to determine the equivalent control quantity; A sliding mode amount determination module, used to determine a tracking error according to the DC current and a reference current, and determine the tracking error as a sliding mode amount; the sliding mode amount meets the requirement that the relative order in the super-helical sliding mode control is 1; A switching control amount determination module, used for determining the switching control amount based on a second-order sliding mode super-helical algorithm; A super-helical sliding mode constant current control rate determination module is used to determine the super-helical sliding mode constant current control rate according to the equivalent control amount and the switching control amount; the super-helical sliding mode constant current control rate satisfies the constraint condition of Lyapunov stability; The transient overvoltage suppression module is used to substitute the sliding mode quantity into the super-helical sliding mode constant current control rate to determine the rectifier side trigger angle signal; the rectifier side trigger angle signal is used to input the rectifier station to suppress the transient overvoltage of the sending end AC system.

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