Extra-high voltage direct current transmission system sending end transient overvoltage calculation and suppression method considering influence of multiple control links

Abstract

The invention discloses an extra-high voltage direct current transmission system sending end transient overvoltage calculation and suppression method considering the influence of multiple control links, and the method specifically comprises the steps: analyzing the operation characteristics and transient voltage influence factors of an LCC-HVDC system, and revealing that direct current is a key factor affecting the transient voltage change of a bus; the method comprises the following steps: decomposing the whole process of transient direct current under commutation failure according to direct current change characteristics in an operation curve of a direct current control system, and deducing an approximate analytical expression of the direct current after the fault occurs; respectively calculating voltage drops on equivalent resistance and reactance on the line, substituting the voltage drops into a direct current analytic expression, and deducing a quantitative calculation expression of transient overvoltage of the converter bus at the sending end; according to the transient overvoltage upper limit value, the allowable direct current minimum value is obtained, and the transient overvoltage suppression method under the commutation failure for improving the VDCOL link minimum current instruction value is provided. According to the invention, the quantitative analysis of the transient voltage is realized, and the transient overvoltage level of the converter bus at the sending end is effectively reduced.

Description

Technical Field

[0001] The present invention belongs to high voltage direct current transmission system technology, and in particular to a method for calculating and suppressing transient overvoltage at a sending end of an ultra-high voltage direct current transmission system taking into account the influence of multiple control links. Background Art

[0002] With the large-scale construction of new energy power generation equipment, the high-voltage direct current (LCC-HVDC) transmission project based on grid-commutated converters plays an important role in long-distance power transmission due to its advantages such as large DC transmission power, strong power regulation capability and relatively low cost. It has effectively solved the problem of reverse distribution of energy and load centers in my country [1-5]. Commutation failure is a fault unique to LCC-HVDC transmission systems [6]. Compared with DC blocking faults, commutation failures have a shorter duration and the equipment in the grid will not be cut off during the fault [7]. With the increase in the penetration rate of new energy, more and more power electronic equipment are connected to the sending-end power grid, resulting in the weaker sending-end grid structure and more serious transient overvoltage problems caused by faults [8,9]. When the transient voltage exceeds a certain threshold, it will cause the sending-end new energy equipment to be disconnected from the grid, and in severe cases, it will also cause damage to the equipment in the DC system. Therefore, it is urgent to analyze and study the transient voltage stability of the LCC-HVDC system under commutation failure.

[0003] At present, many literatures have analyzed the factors affecting transient voltage at the sending end of a high-voltage direct current transmission system under fault conditions. Literature

[10] divides the transient voltage change process into multiple stages and reveals the effects and extent of the effects of different DC control link parameters on transient voltage. Literatures

[11] and

[12] analyzed the effects of DC system parameters on transient voltage at the sending end through simulation, but did not explain or analyze the causes of these effects.

[0004] Qualitative analysis of transient voltage alone cannot fully reveal the transient change process of the system. It is also necessary to establish a mathematical model for transient voltage for further quantitative analysis. A large number of literatures have conducted research on the calculation of transient overvoltage peak value. There are three common calculation methods, namely, derivation through the relationship between reactive power increment and short-circuit capacity, short-circuit ratio index, and power flow equation

[13] . Literatures

[14] and

[15] analyze the dynamic changes of the reactive power consumption characteristics of the rectifier during commutation failure, and give a method for calculating the transient overvoltage peak value. Literature

[16] uses formulas to express the reactive power transmitted by the DC system to the AC system, further solves the analytical formula of the transient overvoltage peak value of the sending-end bus, and studies the influence of the sending-end system strength and transmission power on the transient overvoltage peak value.

[0005] Research on overvoltage suppression mainly focuses on reactive power compensation devices, wind farm operation optimization, and rectifier reactive power optimization

[17] . References

[18] and

[19] analyze the suppressive effect of reactive power compensation devices on transient overvoltage at the sending end during commutation failure. References

[20] and

[21] study the suppressive effect of improving the high voltage ride-through capability of wind farms on transient overvoltage. References

[22] and

[23] study methods for suppressing overvoltage by optimizing DC control system parameters. However, most of the research on VDCOL control is based on how to suppress commutation failure, and lacks a mechanism analysis of the impact of VDCOL control on AC overvoltage at the sending end. Since VDCOL control provides a DC current reference value to the DC control system, it affects the reactive power characteristics of the rectifier. Transient overvoltage at the sending end can also be suppressed by improving the VDCOL link.

[0006] In summary, in order to overcome the sending-end overvoltage problem caused by commutation failure at the receiving end in UHV DC, it is urgent to propose a more detailed calculation method and design a transient overvoltage suppression strategy based on it. Summary of the invention

[0007] Aiming at the problem of transient overvoltage in the sending-end system under commutation failure of the HVDC system, the present invention provides a method for calculating and suppressing transient overvoltage in the sending-end of an ultra-high voltage direct current transmission system taking into account the influence of multiple control links.

[0008] A method for calculating and suppressing transient overvoltage at the sending end of a UHV DC transmission system considering the influence of multiple control links of the present invention comprises the following steps:

[0009] Step 1: Analysis of the operating characteristics of the LCC-HVDC system and the factors affecting transient voltage reveals that DC current is the key factor affecting bus transient voltage changes.

[0010] Step 2: According to the characteristics of DC current changes in the DC control system operation curve, the entire process of transient DC current under commutation failure is decomposed to derive an approximate analytical expression for the DC current after the fault occurs.

[0011] Step 3: Calculate the voltage drop on the equivalent resistance and reactance on the line respectively, and then substitute it into the DC current analytical formula to derive the quantitative calculation expression of the transient overvoltage of the sending-end commutation bus.

[0012] Step 4: According to the upper limit of transient overvoltage, the minimum allowable DC current is obtained, and a transient overvoltage suppression method under commutation failure is proposed to improve the minimum current command value of the VDCOL link.

[0013] Furthermore, step 1 is specifically as follows:

[0014] S1.1: Analysis of operating characteristics of LCC-HVDC system.

[0015] Assume that in the equivalent circuit of the LCC-HVDC system, U s is the equivalent power supply voltage in the sending-end AC system; P s , Q s are respectively the active and reactive power transmitted from the sending end power source to the converter station along the line; R s and X s are the equivalent resistance and equivalent reactance on the sending-end system line, respectively, and their values ​​affect the electrical strength of the sending-end system; U p is the commutation bus voltage at the sending end; Q cp The reactive power compensated by the AC filter device at the sending end; U dr is the DC voltage on the rectifier side; U di is the DC voltage on the inverter side; I dc is a direct current.

[0016] In the control system of LCC-HVDC, both the rectifier side and the inverter side are equipped with constant current CC control and low voltage current limiting control VDCOL links. The constant current control link generates an advance trigger angle command value through a PI controller, and the low voltage current limiting control link generates a DC current command value by real-time measurement of the DC voltage. Moreover, the DC current command value on the inverter side is one margin lower than that on the rectifier side. In addition, the inverter side is also equipped with a current deviation CE controller and a fixed turn-off angle CEA control. The function of the current deviation control is to achieve smooth switching between CC and CEA on the inverter side.

[0017] S1.2: Analysis of factors affecting transient voltage.

[0018] During the commutation failure, the rectifier's ability to consume reactive power directly affects the transient overvoltage level of the sending-end commutation bus. During the transient response process, the rectifier consumes reactive power Q dc It is expressed as:

[0019]

[0020] Where P r The active power is transmitted to the rectifier, and the subscript r represents the rectifier side; is the power factor angle; U r0 is the DC no-load voltage.

[0021] The DC voltage on the rectifier side satisfies the following relationship:

[0022]

[0023] Where N r is the number of 6-pulse converters per pole; T r is the transformer ratio; α is the trigger angle on the rectifier side, γ is the turn-off angle on the inverter side; X cr is the commutation reactance.

[0024] Combining equation (1) and equation (2), we get Q dc The expression is:

[0025] It can be seen from formula (3) that the dynamic reactive power characteristics of the rectifier are related to the trigger angle α and the DC current I dc The reactive power consumed by the rectifier is closely related to the DC current, and the size of the DC current directly determines the size of the reactive power consumed by the rectifier.

[0026] The relationship between the surplus reactive power of the sending-end system and the transient overvoltage of the commutation bus at the sending end is expressed as:

[0027]

[0028] The expression of ΔQ is:

[0029] ΔQ=Q dc -Q cp (5)

[0030] Where ΔU is the transient voltage change of the commutation bus at the sending end; ΔQ is the reactive power exchange between the rectifier and the sending end AC system; S d It is the short-circuit capacity at the commutation bus at the sending end.

[0031] It can be seen from formula (4) that when the short-circuit capacity of the sending-end commutation bus is constant, the transient voltage change of the commutation bus directly depends on the reactive power exchange ΔQ between the rectifier and the sending-end AC system. A positive value means reactive power is absorbed from the system, and a negative value means reactive power is sent to the system. Therefore, the dynamic reactive characteristics of the rectifier in the transient process directly affect the reactive power exchange between the DC and the sending-end AC system, and further affect the transient voltage of the sending-end commutation bus.

[0032] It can be seen that DC current is the key factor affecting the transient voltage change of the bus.

[0033] Furthermore, step 2 is specifically as follows:

[0034] According to the characteristics of DC current changes in the DC control system operation curve, the entire process of transient DC current under commutation failure is decomposed into the following 6 processes:

[0035] (1) The DC current rising process.

[0036] After a three-phase short circuit occurs on the receiving-end AC bus, the receiving-end AC voltage drops due to the system short circuit fault, causing the inverter-side converter to fail in commutation. Due to the short circuit on the inverter side, the inverter-side DC voltage drops rapidly to 0, and the DC current rises rapidly in a short period of time. At this time, the CC control on the rectifier side has not responded, and the DC current is affected by the inverter-side shutdown angle control.

[0037] The cut-off angle γ and the minimum cut-off angle limit γ of the control system in steady state min The deviation value passes through the proportional integral link G PI (s) The inverter side leading trigger angle β is obtained, and the leading trigger angle is further used as a control signal to adjust and control the shutdown angle.

[0038] G γ (s) is the function for calculating the inverter side turn-off angle, which is obtained through the relationship between the turn-off angle and the leading trigger angle in steady state, as shown in formula (6):

[0039]

[0040] Where U Li is the transient voltage of the commutation bus at the receiving end, and the subscript i represents the inverter side; T i is the transformer ratio; X ci is the commutation reactance.

[0041] G meas (s) is the valve group signal measurement link, which indicates the delay effect in the signal processing process:

[0042]

[0043] In the formula, K m is the gain coefficient of the measurement link; T m To measure the link delay coefficient.

[0044] When the LCC-HVDC system operates in steady state, the DC current is expressed as:

[0045]

[0046] The above calculations give the expression of the leading trigger angle β. Substituting it into the transfer function gives the expression of the turn-off angle γ. Substituting γ into equation (8), we get the DC current expression I dc1 As shown below:

[0047]

[0048] Among them, the expression of variable K1 is:

[0049] K1=m1cosh(a1t-b1)+n1sinh(a1t-b1) (10)

[0050] In the formula, a i 、b i 、m i 、n i 、p i ,q i ,θ i , ki 、h i , i = 1 to 6 are the calculation parameters of the intermediate process; I dN is the steady-state value of DC current before the fault.

[0051] (2) The DC current decreasing process.

[0052] When the DC voltage drops to the upper limit of the VDCOL voltage threshold, VDCOL starts and adjusts the DC current command value to limit the DC current and suppress the continuous increase of the DC current.

[0053] As the DC voltage on the inverter side continues to decrease, the current command I order The actual value of the DC current is still higher than the command value, which causes the CC control on the inverter side to fail to respond. The CC control on the rectifier side works and reduces U by increasing the trigger angle α. dr , so that the DC current decreases and gradually approaches the current DC current command value; G PI (s) is the PI link transfer function, G α (s), G R (s) and G meas (s) are the transfer functions of the converter link, DC link and measurement link respectively.

[0054] The transfer functions in the control link are as follows:

[0055]

[0056]

[0057] Where Kcα is the proportional gain coefficient of the converter link; Tcα is the delay coefficient.

[0058] The relationship between the rectifier side trigger angle α and the leading trigger angle β is:

[0059] α=180°-β (13)

[0060] Substitute the obtained leading trigger angle β into formula (13) to obtain the expression of trigger angle α, and then substitute it into formula (8) to obtain the DC current expression I dc2 As shown below:

[0061]

[0062] Among them, the expressions of K2 and K3 are:

[0063] K2=m2cos(a2t-b2)+n2sin(a2t-b2) (15)

[0064] K3=p2t-q2 (16)

[0065] In the formula, I dcmax It is the maximum value that the DC current can reach during its rising process.

[0066] (3) DC current oscillation process.

[0067] At this time, the DC voltage on the inverter side has decreased to 0. From the VDCOL characteristic, it can be seen that after the DC voltage on the inverter side is lower than its voltage threshold lower limit, the output DC current command value I order Drop to the minimum current command value, and then for a period of time I order The DC current remains unchanged and oscillates within a small range around the minimum current command value.

[0068] The DC current expression of this process is I dc3 As shown below:

[0069]

[0070] (4) DC current fluctuation process.

[0071] In the DC system recovery phase after the fault is cleared, due to the overshoot of CEA control on the inverter side, the turn-off angle γ will quickly jump to above 90°. At this time, the DC current will rise slightly. The CC link on the rectifier side increases the trigger angle command value through PI control. At the same time, the DC voltage U dr The continued decline has also made I dc The DC current expression of this process is I dc4 As shown below:

[0072]

[0073] (5) DC current recovery process, i.e. constant current control.

[0074] Since the inverter has resumed normal commutation at this time, the AC bus voltage recovers quickly, and the DC voltage U di Rapidly increases, the current command value output by VDCOL gradually increases. dc When the current is less than the CC control current command value on the inverter side, the DC control system on the inverter side will switch from CEA control to CC control. During this process, the DC short-circuit current will gradually increase and approach the current command value. The regulating effect of CC control on the rectifier side can be ignored at this time, and the inverter side will take the lead in control and regulation. The DC current expression of this process is dc5 As shown below:

[0075]

[0076] (6) DC current recovery process, i.e. fixed turn-off angle control.

[0077] Under the CC control link on the inverter side, I dc Gradually approaching the current command value, when the DC current is greater than the CC control reference value on the inverter side, the CC control effect weakens. At the same time, the fixed shutdown angle is still greater than the steady-state value. The DC control system on the inverter side begins to enter the CE control for transition to ensure the stability of each transient control quantity in the process, so that the transition process of the control link can be smooth, and then switch to CEA control.

[0078] The expression of the leading trigger angle β is obtained by calculation. Substituting β into the transfer function, the expression of the turn-off angle γ is obtained. Substituting γ into equation (8), the expression of the DC current I is obtained. dc6 As shown below:

[0079]

[0080] K4=m6cos(a6t+b6)+n6sin(a6t+b6) (21)

[0081] Furthermore, step 3 is specifically as follows:

[0082] From the power flow calculation relationship from the power source to the commutation bus in the equivalent circuit of the LCC-HVDC sending-end system, the longitudinal component and transverse component of the voltage drop on the transmission line of the sending-end system can be obtained respectively, so the voltage at the commutation bus at the sending end is expressed as:

[0083]

[0084] In the formula, ΔU s is the longitudinal component of the voltage drop in the AC line; δU s It is the transverse component of the voltage drop in the AC line.

[0085] In steady state, the active power and reactive power transmitted on the transmission line of the sending end system are the output power P of the sending end power supply. s and Q s , that is, it satisfies the following relationship:

[0086]

[0087] The reactive power generated by the reactive compensation device is expressed as:

[0088]

[0089] In the formula, B c It is the equivalent susceptance of the reactive power compensation device at the sending end.

[0090] When the commutation bus voltage increases, the reactive power compensation of the AC filter will increase with the voltage increase. At the same time, when the commutation failure fault is restored, the reactive power consumed by the converter station will also change. Therefore, the actual reactive power transmitted by the converter station to the AC system during the transient process is expressed as:

[0091]

[0092] Where U p0 is the voltage of the AC busbar at the sending end before the fault, Q cp0 is the reactive power output by the AC filter device before the fault; ignoring the influence of the commutation reactance, the active and reactive power consumed by the rectifier is expressed as:

[0093]

[0094] During the transient process, the active power and reactive power exchanged between the DC system and the sending-end AC system are:

[0095]

[0096] By simplifying the analysis of the sending-end system, it is considered that the voltage drop between the sending-end power supply and the sending-end commutation bus only exists in the equivalent impedance of the line. The voltage drop on the equivalent resistance and equivalent reactance is calculated respectively, and the calculation expression of the transient voltage of the sending-end commutation bus is obtained as follows:

[0097]

[0098] In the formula, R s is the equivalent resistance of the AC power grid at the sending end, X s is the equivalent reactance of the AC power grid at the sending end.

[0099] Combining equations (27) and (28), we can derive U p The expression is:

[0100]

[0101] In the formula, the expression of algebraic b is:

[0102]

[0103] By substituting the DC current analytical expression into the above formula, the quantitative calculation formula for the transient voltage at the sending-end commutation bus can be obtained.

[0104] Furthermore, step 4 is specifically as follows:

[0105] VDCOL is an important control link in the DC control system. It can detect the DC voltage in real time and output the corresponding DC current command value according to the characteristic curve to adjust the current and promote the commutation process. The VDCOL characteristic curve mainly consists of two parts: one is the VDCOL inflection point parameter, that is, the low-voltage starting threshold U dl and high voltage start threshold U dh , current command value lower limit I dl And the upper limit of current command value I dh ; The second is the VDCOL linear type. The traditional VDCOL voltage and current are linearly related with a fixed slope.

[0106] The DC current command value output by the VDCOL link is expressed as:

[0107]

[0108] Where U d * is the converted voltage per unit value at the midpoint of the DC line, which is used as the input value of VDCOL; k and b are the coefficients of the operating characteristic curve, where k = (I dh -I dl ) / (U dh -U dl ), b=-U dl (I dh -I dl ) / (U dh -U dl ).

[0109] The DC current starts to recover after reaching the minimum value in its fluctuation stage. Solving the DC current expression in this stage, the minimum DC current is 0.23pu. When the transient overvoltage peak is limited to below 1.1pu, according to formula (32), it can be obtained that the DC current must be higher than 0.4pu.

[0110]

[0111] Since there is a measurement link G in the transfer function of the DC current drop, oscillation and fluctuation process meas (s) to measure the delay T m The influence of this causes the constant current control link on the rectifier side to increase the trigger angle command during the DC current fluctuation stage, causing the DC current to overshoot, and the DC current will drop to a smaller value below the lower limit of the current command value of 0.55pu.

[0112] After the improved VDCOL, the DC current overshoot effect caused by the constant current control on the rectifier side remains basically unchanged in this stage, so the lower limit of the current command value needs to be increased by ΔI dl=0.4-0.23=0.17pu. By adding this value to the original control strategy, it can be obtained that the lower limit of the current command value output by the VDCOL link needs to be greater than 0.72pu.

[0113] The beneficial technical effects of the present invention are:

[0114] 1) By analyzing the influence of the DC control system on the transient voltage during the commutation failure process, the present invention obtains the conclusion that the DC current is the key influencing factor affecting the transient voltage at the sending end, and derives the approximate analytical expression of the DC current. The voltage drop between the commutation bus and the equivalent power supply at the sending end caused by power transmission is further considered, and the quantitative calculation formula of the transient voltage at the commutation bus at the sending end under commutation failure is derived, thereby realizing the quantitative analysis of the transient voltage.

[0115] 2) Compared with the conventional VDCOL control link, the present invention improves the VDCOL control link based on the transient overvoltage limit constraint condition. The current command value output by the improved VDCOL control link can suppress excessive DC current drop, effectively reduce the transient overvoltage level of the sending-end commutation bus, and contribute to the safe and stable operation of equipment in the high-voltage DC sending-end system. BRIEF DESCRIPTION OF THE DRAWINGS

[0116] Figure 1 It is the equivalent circuit of LCC-HVDC system.

[0117] Figure 2 It is the DC control system in LCC-HVDC.

[0118] Figure 3 This is the transient voltage curve of the commutation bus at the sending end under commutation failure.

[0119] Figure 4 This is the DC system operation curve during fault recovery.

[0120] Figure 5 is the inverter side fixed turn-off angle control transfer function.

[0121] Figure 6 This is the block diagram of the constant current control transfer function on the rectifier side.

[0122] Figure 7 It is the constant current control transfer function of the rectifier side after the low-voltage current limit reaches the threshold.

[0123] Figure 8 is the inverter side fixed turn-off angle control transfer function.

[0124] Fig. 9 It is the VDCOL characteristic curve.

[0125] Fig.10The relationship between the low voltage current limiting link parameters and overvoltage (a) VDCOL link current command value lower limit I dl ; (b) VDCOL link high voltage start threshold U dh (c) VDCOL link low voltage start threshold U dl ).

[0126] Fig.11 To improve the VDCOL curve.

[0127] Fig.12 Comparison diagram between theoretical calculation and actual simulation (a) DC current I dc Transient response process; (b) Transient response process of trigger angle α on the rectifier side; (c) Bus voltage U at the sending end p transient response process).

[0128] Fig.13 Figure 2 shows the transient voltage and DC current of the commutation bus at the sending end under a three-phase short circuit fault ((a) transient voltage of the commutation bus at the sending end; (b) DC current).

[0129] Fig.14 Figure 2 shows the transient voltage and DC current of the commutation bus at the sending end under a single-phase short circuit fault ((a) transient voltage of the commutation bus at the sending end; (b) DC current). DETAILED DESCRIPTION

[0130] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0131] A method for calculating and suppressing transient overvoltage at the sending end of a UHV DC transmission system considering the influence of multiple control links of the present invention comprises the following steps:

[0132] Step 1: Analysis of the operating characteristics of the LCC-HVDC system and the factors affecting transient voltage reveals that DC current is the key factor affecting bus transient voltage changes.

[0133] S1.1: Analysis of operating characteristics of LCC-HVDC system.

[0134] The equivalent circuit of LCC-HVDC system is as follows Figure 1 Shown: U s is the equivalent power supply voltage in the sending-end AC system; P s , Q s are respectively the active and reactive power transmitted from the sending end power source to the converter station along the line; R s and X s are the equivalent resistance and equivalent reactance on the sending-end system line, respectively, and their values ​​affect the electrical strength of the sending-end system; U p is the commutation bus voltage at the sending end; Q cp The reactive power compensated by the AC filter device at the sending end; Udr is the DC voltage on the rectifier side; U di is the DC voltage on the inverter side; I dc is a direct current.

[0135] Control system in LCC-HVDC Figure 2 As shown, both the rectifier side and the inverter side are provided with constant current (CC) control and low voltage current limiting control (VDCOL) links. The constant current control link generates an advance trigger angle command value through a PI controller, and the low voltage current limiting control link generates a DC current command value by measuring the DC voltage in real time. Moreover, the DC current command value on the inverter side is one margin (0.1pu) lower than that on the rectifier side. In addition, the inverter side is also provided with a current deviation (CE) controller and a constant turn-off angle (CEA) control, wherein the function of the current deviation control is to achieve smooth switching between CC and CEA on the inverter side.

[0136] S1.2: Analysis of factors affecting transient voltage.

[0137] Commutation failure only occurs on the inverter side of the system, usually caused by an AC fault on the inverter side. At the initial moment of the fault, the DC current suddenly rises, causing the rectifier's ability to consume reactive power to increase. The rectifier absorbs a large amount of reactive power from the sending-end system, causing the transient voltage of the sending-end commutation bus to drop; during the commutation failure recovery process, the DC current is small, and the sending-end converter station consumes less reactive power, but the sending-end AC filter device cannot be removed in time, resulting in a large amount of surplus reactive power flowing to the sending-end system, causing transient overvoltage at the bus. The transient voltage curve is as follows: Figure 3 shown.

[0138] During the commutation failure, the rectifier's ability to consume reactive power directly affects the transient overvoltage level of the sending-end commutation bus. During the transient response process, the rectifier consumes reactive power Q dc It is expressed as:

[0139]

[0140] Where P r Transmit active power to the rectifier (subscript r represents the rectifier side, the same below); is the power factor angle; U r0 is the DC no-load voltage.

[0141] The DC voltage on the rectifier side satisfies the following relationship:

[0142]

[0143] Where N r is the number of 6-pulse converters per pole; T r is the transformer ratio; α is the trigger angle on the rectifier side, γ is the turn-off angle on the inverter side; X cris the commutation reactance (which can be ignored for simplified analysis).

[0144] Combining equation (1) and equation (2), we get Q dc The expression is:

[0145]

[0146] It can be seen from formula (3) that the dynamic reactive power characteristics of the rectifier are related to the trigger angle α and the DC current I dc The reactive power consumed by the rectifier is closely related to the DC current, and the size of the DC current directly determines the size of the reactive power consumed by the rectifier.

[0147] The relationship between the surplus reactive power of the sending-end system and the transient overvoltage of the commutation bus at the sending end is expressed as:

[0148]

[0149] The expression of ΔQ is:

[0150] ΔQ=Q dc -Q cp (5)

[0151] Where ΔU is the transient voltage change of the commutation bus at the sending end; ΔQ is the reactive power exchange between the rectifier and the sending end AC system; S d It is the short-circuit capacity at the commutation bus at the sending end.

[0152] It can be seen from formula (4) that when the short-circuit capacity of the sending-end commutation bus is constant, the transient voltage change of the commutation bus directly depends on the reactive power exchange ΔQ between the rectifier and the sending-end AC system. A positive value means reactive power is absorbed from the system, and a negative value means reactive power is sent to the system. Therefore, the dynamic reactive characteristics of the rectifier in the transient process directly affect the reactive power exchange between the DC and the sending-end AC system, and further affect the transient voltage of the sending-end commutation bus.

[0153] In summary, the main reason for transient overvoltage in the sending-end commutation bus is that during commutation failure, the overshoot of the current control link on the rectifier side reduces the DC current to a very small value, even close to zero, causing the trigger angle command value to change, thereby reducing the rectifier's ability to consume reactive power, and the AC filter device injects a large amount of reactive power into the sending-end AC system, causing overvoltage. It can be seen that DC current is a key factor affecting the transient voltage change of the bus. In order to quantitatively analyze the transient voltage change characteristics of the sending-end commutation bus under commutation failure, it is necessary to clarify the transient response process of the DC control system and DC current.

[0154] Step 2: According to the characteristics of DC current changes in the DC control system operation curve, the entire process of transient DC current under commutation failure is decomposed to derive an approximate analytical expression for the DC current after the fault occurs.

[0155] The detailed calculation method of DC current needs to consider the role of control link switching and establish the transfer function of the relevant control link. Due to the nonlinear characteristics of the thyristor devices in the commutation device, the action response of the commutation device during the fault is very complex, which makes it difficult to accurately describe the dynamic changes during the transient period after the fault, and involves complex differential calculations. Only a linearization method can be used to approximate the DC system within a period of time after the fault to reduce the complexity of the problem. The DC system operation curve during the LCC-HVDC system fault recovery period is as follows: Figure 4 shown.

[0156] The DC control system in the LCC-HVDC system contains multiple control links. In the transient process caused by the fault, different control links will switch and multiple control links will act simultaneously. Therefore, according to the action switching logic of the control link, the entire transient process is divided into three stages, namely the initial stage of the fault, the control system response stage after the fault causes the commutation failure, and the fault clearing and recovery stage. Further, according to the characteristics of the DC current change in the DC control system operation curve, the entire process of transient DC current under commutation failure is decomposed and can be divided into the following 6 processes:

[0157] (1) The DC current rising process.

[0158] After a three-phase short circuit occurs on the AC busbar at the receiving end, the AC voltage at the receiving end drops due to the system short circuit fault, causing the inverter-side converter to fail to commutate. Due to the short circuit on the inverter side, the DC voltage on the inverter side drops rapidly to 0, and the DC current rises rapidly in a short period of time. At this time, the CC control on the rectifier side has not responded yet, and the DC current is affected by the inverter-side turn-off angle control. The inverter-side fixed turn-off angle control transfer function block diagram is shown in the figure. Figure 5 shown.

[0159] The cut-off angle γ and the minimum cut-off angle limit γ of the control system in steady state min The deviation value passes through the proportional integral link G PI (s) The inverter side leading trigger angle β is obtained, and the leading trigger angle is further used as a control signal to adjust and control the shutdown angle.

[0160] G γ (s) is the function for calculating the inverter side turn-off angle, which is obtained through the relationship between the turn-off angle and the leading trigger angle in steady state, as shown in formula (6):

[0161]

[0162] Where U Liis the transient voltage of the commutation bus at the receiving end, and the subscript i represents the inverter side; T i is the transformer ratio; X ci is the commutation reactance.

[0163] G meas (s) is the valve group signal measurement link, which indicates the delay effect in the signal processing process:

[0164]

[0165] In the formula, K m is the gain coefficient of the measurement link; T m To measure the link delay coefficient.

[0166] When the LCC-HVDC system operates in steady state, the DC current is expressed as:

[0167]

[0168] The above calculations give the expression of the leading trigger angle β. Substituting it into the transfer function gives the expression of the turn-off angle γ. Substituting γ into equation (8), we get the DC current expression I dc1 As shown below:

[0169]

[0170] Among them, the expression of variable K1 is:

[0171] K1=m1cosh(a1t-b1)+n1sinh(a1t-b1) (10)

[0172] In the formula, a i 、b i 、m i 、n i 、p i ,q i ,θ i , k i 、h i , i = 1 to 6 are the calculation parameters of the intermediate process; I dN is the steady-state value of DC current before the fault.

[0173] (2) The DC current decreasing process.

[0174] When the DC voltage drops to the upper limit of the VDCOL voltage threshold, VDCOL starts and adjusts the DC current command value to limit the DC current and suppress the continuous increase of the DC current.

[0175] As the DC voltage on the inverter side continues to decrease, the current command I orderThe actual value of the DC current is still higher than the command value, which causes the CC control on the inverter side to fail to respond. The CC control on the rectifier side works and reduces U by increasing the trigger angle α. dr , so that the DC current decreases and gradually approaches the current DC current command value. The transfer function block diagram is as follows Figure 6 As shown, G PI (s) is the PI link transfer function, G α (s), G R (s) and G meas (s) are the transfer functions of the converter link, DC link and measurement link respectively.

[0176] The transfer functions in the control link are as follows:

[0177]

[0178] Where Kcα is the proportional gain coefficient of the converter link; Tcα is the delay coefficient.

[0179] The relationship between the rectifier side trigger angle α and the leading trigger angle β is:

[0180] α=180°-β (13)

[0181] Substitute the obtained leading trigger angle β into formula (13) to obtain the expression of trigger angle α, and then substitute it into formula (8) to obtain the DC current expression I dc2 As shown below:

[0182]

[0183] Among them, the expressions of K2 and K3 are:

[0184] K2=m2cos(a2t-b2)+n2sin(a2t-b2) (15)

[0185] K3=p2t-q2 (16)

[0186] In the formula, I dcmax It is the maximum value that the DC current can reach during its rising process.

[0187] (3) DC current oscillation process.

[0188] At this time, the DC voltage on the inverter side has decreased to 0. From the VDCOL characteristic, it can be seen that after the DC voltage on the inverter side is lower than its voltage threshold lower limit, the output DC current command value I order Drop to the minimum current command value, and then for a period of time I order remains unchanged, and the DC current oscillates in a small range around the minimum current command value. The transfer function diagram is as follows: Figure 7 shown.

[0189] The DC current expression of this process is I dc3 As shown below:

[0190]

[0191] (4) DC current fluctuation process.

[0192] In the DC system recovery phase after the fault is cleared, due to the overshoot of CEA control on the inverter side, the turn-off angle γ will quickly jump to above 90°. At this time, the DC current will rise slightly. The CC link on the rectifier side increases the trigger angle command value through PI control. At the same time, the DC voltage U dr The continued decline has also made I dc The DC current expression of this process is I dc4 As shown below:

[0193]

[0194] (5) DC current recovery process, i.e. constant current control.

[0195] Since the inverter has resumed normal commutation at this time, the AC bus voltage recovers quickly, and the DC voltage U di Rapidly increases, the current command value output by VDCOL gradually increases. dc When the current is less than the CC control current command value on the inverter side, the DC control system on the inverter side will switch from CEA control to CC control. During this process, the DC short-circuit current will gradually increase and approach the current command value. The regulating effect of CC control on the rectifier side can be ignored at this time, and the inverter side will take the lead in control and regulation. The DC current expression of this process is dc5 As shown below:

[0196]

[0197] (6) DC current recovery process, i.e. fixed turn-off angle control.

[0198] Under the CC control link on the inverter side, I dc Gradually approaching the current command value, when the DC current is greater than the CC control reference value on the inverter side, the CC control effect weakens, and the fixed turn-off angle is still greater than the steady-state value. The DC control system on the inverter side begins to enter the CE control for transition to ensure the stability of each transient control quantity in the process, so that the transition process of the control link can be smooth, and then switch to CEA control. The transfer function block diagram is as follows: Figure 8 As shown, the cut-off angle calculation transfer function G r (s), PI link transfer function G PI(s) and the transfer function of the measurement link are consistent with the transfer function expression in the previous process.

[0199] The expression of the leading trigger angle β is obtained by calculation. Substituting β into the transfer function, the expression of the turn-off angle γ is obtained. Substituting γ into equation (8), the expression of the DC current I is obtained. dc6 As shown below:

[0200]

[0201] K4=m6cos(a6t+b6)+n6sin(a6t+b6) (21)

[0202] Step 3: Calculate the voltage drop on the equivalent resistance and reactance on the line respectively, and then substitute it into the DC current analytical formula to derive the quantitative calculation expression of the transient overvoltage of the sending-end commutation bus.

[0203] In actual DC transmission projects, the voltage drop caused by the equivalent impedance on the transmission line at the sending end needs to be considered, so current research generally uses the power flow equation to calculate the transient voltage at the sending end bus. The present invention performs a simplified analysis, calculates the voltage drop on the equivalent resistance and reactance on the line respectively, and then substitutes it into the DC current analytical formula to derive the quantitative calculation expression of the transient voltage at the commutation bus.

[0204] From the power flow calculation relationship from the power source to the commutation bus in the equivalent circuit of the LCC-HVDC sending-end system, the longitudinal component and transverse component of the voltage drop on the transmission line of the sending-end system can be obtained respectively, so the voltage at the commutation bus at the sending end is expressed as:

[0205]

[0206] In the formula, ΔU s is the longitudinal component of the voltage drop in the AC line; δU s It is the transverse component of the voltage drop in the AC line.

[0207] In steady state, the active power and reactive power transmitted on the transmission line of the sending end system are the output power P of the sending end power supply. s and Q s , that is, it satisfies the following relationship:

[0208]

[0209] The reactive power generated by the reactive compensation device is expressed as:

[0210]

[0211] In the formula, B c It is the equivalent susceptance of the reactive power compensation device at the sending end.

[0212] When the commutation bus voltage increases, the reactive power compensation of the AC filter will increase with the voltage increase. At the same time, when the commutation failure fault is restored, the reactive power consumed by the converter station will also change. Therefore, the actual reactive power transmitted by the converter station to the AC system during the transient process is expressed as:

[0213]

[0214] Where U p0 is the voltage of the AC busbar at the sending end before the fault, Q cp0 is the reactive power output by the AC filter device before the fault; ignoring the influence of the commutation reactance, the active and reactive power consumed by the rectifier is expressed as:

[0215]

[0216] During the transient process, the active power and reactive power exchanged between the DC system and the sending-end AC system are:

[0217]

[0218] By simplifying the analysis of the sending-end system, it is considered that the voltage drop between the sending-end power supply and the sending-end commutation bus only exists in the equivalent impedance of the line. The voltage drop on the equivalent resistance and equivalent reactance is calculated respectively, and the calculation expression of the transient voltage of the sending-end commutation bus is obtained as follows:

[0219]

[0220] In the formula, R s is the equivalent resistance of the AC power grid at the sending end, X s is the equivalent reactance of the AC power grid at the sending end.

[0221] Combining equations (27) and (28), we can derive U p The expression is:

[0222]

[0223] In the formula, the expression of algebraic b is:

[0224]

[0225] By substituting the DC current analytical expression into the above formula, the quantitative calculation formula of the transient voltage at the sending end commutation bus can be obtained. Step 4: According to the upper limit of transient overvoltage, the minimum allowable DC current is obtained, and a transient overvoltage suppression method under commutation failure is proposed to improve the minimum current command value of the VDCOL link.

[0226] VDCOL is an important control link in the DC control system. It can detect the DC voltage in real time and output the corresponding DC current command value according to the characteristic curve to adjust the current and promote the commutation process. The VDCOL characteristic curve is as follows: Fig. 9 As shown in Figure 1, it mainly consists of two parts: one is the VDCOL inflection point parameter, that is, the low voltage start threshold U dl and high voltage start threshold U dh , current command value lower limit I dl And the upper limit of current command value I dh ; The second is the VDCOL linear type. The traditional VDCOL voltage and current are linearly related with a fixed slope.

[0227] The DC current command value output by the VDCOL link is expressed as:

[0228]

[0229] Where U d * is the converted voltage per unit value at the midpoint of the DC line, which is used as the input value of VDCOL; k and b are the coefficients of the operating characteristic curve, where k = (I dh -I dl ) / (U dh -U dl ), b=-U dl (I dh -I dl ) / (U dh -U dl ).

[0230] like Fig.10 The influence of relevant parameters on the overvoltage of the AC bus at the sending end after the commutation failure is given. The high and low starting thresholds of the DC voltage in the VDCOL characteristic curve determine the dynamic performance of the low-voltage current limiting control. If the high-voltage starting threshold is too large, the current command value will fluctuate violently under small disturbances. If it is too small, the system will not react sensitively to voltage fluctuations. If the low-voltage starting threshold is too low, the current will be restored prematurely. The difference between the high and low voltage starting thresholds determines the rate of change of the current command value. The larger the value, the better the effect of limiting the DC current. However, the current will rise quickly during the voltage recovery process after the fault, which has a greater impact on the system.

[0231] When the DC current follows the VDCOL link outputs the minimum current command value I dl When it decreases and continues to decrease to its minimum value due to the overshoot of the constant current control link, the reactive power consumed by the rectifier also reaches the minimum. At this time, the severity of the corresponding transient overvoltage is the greatest.

[0232] Since the inflection point parameters and slope of the traditional VDCOL characteristic curve are fixed, the system cannot obtain better control performance during the transient change of the system. Therefore, the present invention affects the DC current recovery characteristics by improving the lower limit of the DC current command value output by the VDCOL link, adjusts the DC current during the transient overvoltage of the sending-end commutation bus, increases the reactive power consumption of the rectifier, and reduces the degree of reactive power imbalance between the AC and DC systems, so as to achieve the effect of suppressing the transient overvoltage peak.

[0233] right Fig.10 Analysis shows that the lower limit of the current command value I dl The change has a greater impact on transient voltage than other inflection point parameters. Therefore, the suppression strategy proposed in the present invention takes the transient voltage of 1.1pu of the sending-end commutation bus as a constraint condition, and obtains the minimum limit of the DC current through the quantitative calculation expression of the transient voltage. The lower limit of the current command value output by VDCOL is improved according to the limit. The minimum value of the DC current in the decline process is increased by suppressing the DC current from dropping too low, thereby achieving a method of suppressing transient overvoltage.

[0234] Analysis of the DC current curve shows that the DC current begins to recover after reaching the minimum value in its fluctuation stage. Solving the DC current expression in this stage, the minimum DC current value is 0.23pu. When the transient overvoltage peak is limited to below 1.1pu, according to formula (32), it can be obtained that the DC current must be higher than 0.4pu.

[0235]

[0236] Since there is a measurement link G in the transfer function of the DC current drop, oscillation and fluctuation process meas (s) to measure the delay T m The influence of this causes the constant current control link on the rectifier side to increase the trigger angle command during the DC current fluctuation stage, causing the DC current to overshoot, and the DC current will drop to a smaller value below the lower limit of the current command value of 0.55pu.

[0237] The improved VDCOL characteristic curve is shown in the figure below: Fig.11 As shown in the figure, in this stage, the DC current overshoot effect caused by the constant current control on the rectifier side remains basically unchanged, so the lower limit of the current command value needs to be increased by the value ΔI dl =0.4-0.23=0.17pu. By adding this value to the original control strategy, we can get that the lower limit of the current command value output by the VDCOL link needs to be greater than 0.72pu. Simulation verification:

[0238] In order to verify the transient voltage calculation formula and transient overvoltage suppression strategy derived from the DC current analytical formula when the system commutation failure is caused by the receiving end fault, the present invention establishes a test system based on the CIGRE DC transmission standard model in PSCAD / EMTDC, where the specific parameters of the CIGRE model are shown in Table 1.

[0239] Table 1 Actual simulation parameters of CIGRE standard model

[0240]

[0241]

[0242] In the CIGRE model, a three-phase short-circuit fault is set at the receiving end AC busbar, causing the system to fail to commutate. The short-circuit ratio of the sending end AC system is 2.5, the rated transmission power of the DC line is 1000MW, the fault occurrence time is set to 2s, the fault duration is 0.1s, the grounding inductance is 0.01H, the grounding resistance is 0.01Ω, and the simulation time is 2.35s. At this time, the commutation failure has been completely restored and the system has returned to a steady-state operation state.

[0243] Simulation analysis of transient voltage quantitative calculation method:

[0244] Based on the previous analysis of DC current, the DC current curve can be drawn by substituting the derived expressions of each segment of DC current into the CIGRE standard model parameters.

[0245] From the beginning of the fault (t = 2s) to the recovery to the steady state (t = 2.35s), the DC current is divided into 6 stages, corresponding to Fig.12 Since the trigger angle curve and the transient voltage curve of the sending-end commutation bus are calculated based on the DC current, the trigger angle and transient voltage curve are also divided into the above 6 stages.

[0246] Fig.12 The comparison between the theoretical calculation curves of DC current, trigger angle on the rectifier side and transient voltage of AC bus at the sending end under commutation failure of LCC-HVDC system and the actual simulation curves of CIGRE standard example is given.

[0247] In the theoretical calculation process, it is necessary to consider the change of the DC current reference value and calculate in real time based on the difference between the actual DC current value and the reference value. Therefore, in the above trigger angle α and transient voltage U p The calculation is performed by further dividing the intervals (2) and (5) of the curve into sub-intervals.

[0248] It can be seen from the above simulation diagram that when the system fails to commutate, the DC current will surge in a short time. After a period of time, the trigger angle on the rectifier side begins to rise, suppressing the rise of the DC current. Then the VDCOL link on the inverter side starts to generate a DC current command value, causing the DC current to begin to decline. After a small change near the minimum command value, it begins to enter the recovery process and gradually rises to the steady-state value. The transient voltage shows a "first low and then high" change trend, proving that the simulation results of the DC current and transient voltage response process are consistent with the above theoretical analysis results. The actual simulation results have high-frequency oscillations with small amplitudes, and the theoretical calculation is the result of linear approximation processing. Therefore, this calculation method can only describe the general trend of transient changes, and cannot accurately describe the details of the changes in the system electrical quantities during the transient response period.

[0249] The simulation results show that the theoretical calculation results of the transient voltage of the sending-end commutation bus are roughly consistent with the actual simulation results in terms of change trend, and the calculation error at the peak of the transient overvoltage is about 0.21%. Therefore, within the allowable error range, this calculation method can be used as a theoretical calculation method for the transient overvoltage of the sending-end commutation bus, and provide a theoretical basis for the study of transient overvoltage suppression strategies in engineering design.

[0250] Analysis of the effect of improving VDCOL on transient overvoltage suppression

[0251] In order to verify the effectiveness of the control strategy proposed in this invention on transient overvoltage suppression, based on the CIGRE standard example in PSCAD, three-phase short circuit and single-phase short circuit grounding faults are set at the inverter side commutation bus to simulate different operating conditions, such as Fig.13 , 14 shown.

[0252] Under different short-circuit fault conditions, the transient voltage and DC current variation characteristics of the sending-end commutation bus under the original control strategy of the CIGRE model and the system control strategy after the improved VDCOL control link proposed in the present invention are compared.

[0253] By comparing the two control strategies under different short-circuit fault conditions, it can be seen that the transient overvoltage under the proposed control strategy is below 1.1pu, which proves that the control strategy has a significant inhibitory effect on transient overvoltage.

Claims

1. A method for calculating and suppressing transient overvoltage at the sending end of a UHV DC transmission system considering the influence of multiple control links, characterized in that: The following steps are involved: Step 1: Analysis of the operating characteristics of the LCC-HVDC system and the factors affecting transient voltage reveals that DC current is the key factor affecting bus transient voltage changes; Step 2: According to the characteristics of DC current changes in the DC control system operation curve, the entire process of transient DC current under commutation failure is decomposed to derive an approximate analytical expression of the DC current after the fault occurs; Step 3: Calculate the voltage drop on the equivalent resistance and reactance on the line respectively, and then substitute it into the DC current analytical formula to derive the quantitative calculation expression of the transient overvoltage of the sending-end commutation bus; Step 4: According to the upper limit of transient overvoltage, the minimum allowable DC current is obtained, and a transient overvoltage suppression method under commutation failure is proposed to improve the minimum current command value of the VDCOL link.

2. According to the method for calculating and suppressing transient overvoltage at the sending end of a UHV DC transmission system considering the influence of multiple control links in claim 1, it is characterized in that: The step 1 is specifically as follows: S1.1: Analysis of operating characteristics of LCC-HVDC system; Assume that in the LCC-HVDC system equivalent circuit, U s is the equivalent power supply voltage in the sending-end AC system; P s , Q s are respectively the active power and reactive power transmitted from the sending end power source to the converter station along the line; R s and X s are the equivalent resistance and equivalent reactance on the sending-end system line, respectively, and their values ​​affect the electrical strength of the sending-end system; U p is the commutation bus voltage at the sending end; Q cp The reactive power compensated by the AC filter device at the sending end; U dr is the DC voltage on the rectifier side; U di is the DC voltage on the inverter side; I dc is direct current; In the control system of LCC-HVDC, both the rectifier side and the inverter side are equipped with constant current CC control and low voltage current limiting control VDCOL links. The constant current control link generates the advanced trigger angle command value through the PI controller, and the low voltage current limiting control link generates the DC current command value by real-time measurement of the DC voltage. The DC current command value on the inverter side is one margin lower than that on the rectifier side. In addition, the inverter side is also equipped with a current deviation CE controller and a fixed turn-off angle CEA control. The function of the current deviation control is to achieve smooth switching between CC and CEA on the inverter side. S1.2: Analysis of factors affecting transient voltage; During the commutation failure, the rectifier's ability to consume reactive power directly affects the transient overvoltage level of the sending-end commutation bus. During the transient response process, the rectifier consumes reactive power Q dc It is expressed as: Where P r The active power is transmitted to the rectifier, and the subscript r represents the rectifier side; is the power factor angle; U r0 is the DC no-load voltage; The DC voltage on the rectifier side satisfies the following relationship: Where N r is the number of 6-pulse converters per pole; T r is the transformer ratio; α is the trigger angle on the rectifier side, γ is the turn-off angle on the inverter side; X cr is the commutation reactance; Combining equation (1) and equation (2), we get Q dc The expression is: It can be seen from formula (3) that the dynamic reactive power characteristics of the rectifier are related to the trigger angle α and the DC current I dc The reactive power consumed by the rectifier is closely related to the DC current, and the size of the DC current directly determines the size of the reactive power consumed by the rectifier. The relationship between the surplus reactive power of the sending-end system and the transient overvoltage of the commutation bus at the sending end is expressed as: The expression of ΔQ is: ΔQ=Q dc -Q cp (5) Where ΔU is the transient voltage change of the commutation bus at the sending end; ΔQ is the reactive power exchange between the rectifier and the sending end AC system; S d is the short-circuit capacity at the commutation busbar at the sending end; It can be seen from formula (4) that when the short-circuit capacity of the commutation bus at the sending end is constant, the transient voltage change of the commutation bus directly depends on the reactive power exchange ΔQ between the rectifier and the sending end AC system. The positive value is the reactive power absorbed from the system, and the negative value is the reactive power sent to the system. Therefore, the dynamic reactive characteristics of the rectifier in the transient process directly affect the reactive power exchange between the DC and the sending end AC system, and further affect the transient voltage of the commutation bus at the sending end. It can be seen that DC current is the key factor affecting the transient voltage change of the bus.

3. The method for calculating and suppressing transient overvoltage at the sending end of a UHV DC transmission system considering the influence of multiple control links according to claim 2 is characterized in that: The step 2 is specifically as follows: According to the characteristics of DC current changes in the DC control system operation curve, the entire process of transient DC current under commutation failure is decomposed into the following 6 processes: (1) DC current rising process; After a three-phase short circuit occurs on the AC busbar at the receiving end, the AC voltage at the receiving end drops due to the system short circuit fault, causing the inverter-side converter to fail in commutation. Due to the short circuit on the inverter side, the DC voltage on the inverter side drops rapidly to 0, and the DC current rises rapidly in a short period of time. At this time, the CC control on the rectifier side has not responded, and the DC current is affected by the inverter-side turn-off angle control. The cut-off angle γ and the minimum cut-off angle γ of the control system in steady state min The deviation value passes through the proportional integral link G PI (s) obtaining the inverter side leading trigger angle β, which is further used as a control signal to adjust and control the turn-off angle; G γ (s) is the function for calculating the inverter side turn-off angle, which is obtained through the relationship between the turn-off angle and the leading trigger angle in steady state, as shown in formula (6): Where U Li is the transient voltage of the commutation bus at the receiving end, and the subscript i represents the inverter side; T i is the transformer ratio; X ci is the commutation reactance; G meas (s) is the valve group signal measurement link, which indicates the delay effect in the signal processing process: In the formula, K m is the gain coefficient of the measurement link; T m To measure the link delay coefficient; When the LCC-HVDC system operates in steady state, the DC current is expressed as: The above calculations give the expression of the leading trigger angle β. Substituting it into the transfer function gives the expression of the turn-off angle γ. Substituting γ into equation (8), we get the DC current expression I dc1 As shown below: Among them, the expression of variable K1 is: K1=m1cosh(a1t-b1)+n1sinh(a1t-b1) (10) In the formula, a i , b i 、m i 、n i 、p i ,q i ,θ i , k i 、h i , i=1~6 are the calculation parameters of the intermediate process; I dN is the steady-state value of DC current before the fault; (2) DC current decreasing process; When the DC voltage drops to the upper limit of the VDCOL voltage threshold, VDCOL starts and adjusts the DC current command value to limit the DC current and suppress the continuous increase of the DC current; As the DC voltage on the inverter side continues to decrease, the current command I order The actual value of the DC current is still higher than the command value, which causes the CC control on the inverter side to fail to respond. The CC control on the rectifier side works and reduces U by increasing the trigger angle α. dr , so that the DC current decreases and gradually approaches the current DC current command value; G PI (s) is the PI link transfer function, G α (s), G R (s) and G meas (s) are the transfer functions of the converter link, DC link link and measurement link respectively; The transfer functions in the control link are as follows: Where, Kcα is the proportional gain coefficient of the converter link; Tcα is the delay coefficient; The relationship between the rectifier side trigger angle α and the leading trigger angle β is: α＝180°-β (13) Substitute the obtained leading trigger angle β into formula (13) to obtain the expression of trigger angle α, and then substitute it into formula (8) to obtain the DC current expression I dc2 As shown below: Among them, the expressions of K2 and K3 are: K2=m2cos(a2t-b2)+n2sin(a2t-b2) (15) K3=p2t-q2 (16) In the formula, I dcmax It is the maximum value that can be reached during the DC current rising process; (3) DC current oscillation process; At this time, the DC voltage on the inverter side has decreased to 0. From the VDCOL characteristic, it can be seen that after the DC voltage on the inverter side is lower than its voltage threshold lower limit, the output DC current command value I order Drop to the minimum current command value, and then for a period of time I order remains unchanged, and the DC current oscillates within a small range around the minimum current command value; The DC current expression of this process is I dc3 As shown below: (4) DC current fluctuation process; In the DC system recovery phase after the fault is cleared, due to the overshoot of CEA control on the inverter side, the turn-off angle γ will quickly jump to above 90°. At this time, the DC current will rise slightly. The CC link on the rectifier side increases the trigger angle command value through PI control. At the same time, the DC voltage U dr The continued decline has also made I dc The DC current expression of this process is I dc4 As shown below: (5) DC current recovery process, i.e. constant current control; Since the inverter has resumed normal commutation at this time, the AC bus voltage recovers quickly, and the DC voltage U di Rapidly increases, the current command value output by VDCOL gradually increases. dc When the current is less than the CC control current command value on the inverter side, the DC control system on the inverter side will switch from CEA control to CC control. During this process, the DC short-circuit current will gradually increase and approach the current command value. The regulating effect of CC control on the rectifier side can be ignored at this time, and the inverter side will take the lead in control and regulation. The DC current expression of this process is dc5 As shown below: (6) DC current recovery process, i.e. fixed turn-off angle control; Under the CC control link on the inverter side, I dc Gradually approaching the current command value, when the DC current is greater than the CC control reference value on the inverter side, the CC control effect weakens, and at the same time, the fixed shutdown angle is still greater than the steady-state value. The DC control system on the inverter side begins to enter the CE control for transition, ensuring the stability of each transient control quantity in the process, so that the transition process of the control link can be smooth, and then switch to CEA control; The expression of the leading trigger angle β is obtained by calculation. Substituting β into the transfer function, the expression of the turn-off angle γ is obtained. Substituting γ into equation (8), the expression of the DC current I is obtained. dc6 As shown below: K4=m6cos(a6t+b6)+n6sin(a6t+b6) (21).

4. The method for calculating and suppressing transient overvoltage at the sending end of a UHV DC transmission system considering the influence of multiple control links according to claim 3 is characterized in that: The step 3 is specifically as follows: From the power flow calculation relationship from the power source to the commutation bus in the equivalent circuit of the LCC-HVDC sending-end system, the longitudinal component and transverse component of the voltage drop on the transmission line of the sending-end system can be obtained respectively, so the voltage at the commutation bus at the sending end is expressed as: In the formula, ΔU s is the longitudinal component of the voltage drop in the AC line; δU s is the transverse component of the voltage drop in the AC line; In steady state, the active power and reactive power transmitted on the transmission line of the sending end system are the output power P of the sending end power supply. s and Q s , that is, it satisfies the following relationship: The reactive power generated by the reactive compensation device is expressed as: In the formula, B c is the equivalent susceptance of the reactive power compensation device at the sending end; When the commutation bus voltage increases, the reactive power compensation of the AC filter will increase with the voltage increase. At the same time, when the commutation failure fault is restored, the reactive power consumed by the converter station will also change. Therefore, the actual reactive power transmitted by the converter station to the AC system during the transient process is expressed as: Where U p0 is the voltage of the AC busbar at the sending end before the fault, Q cp0 is the reactive power output by the AC filter device before the fault; Ignoring the effect of commutation reactance, the active and reactive power consumed by the rectifier is expressed as: During the transient process, the active power and reactive power exchanged between the DC system and the sending-end AC system are: By simplifying the analysis of the sending-end system, it is considered that the voltage drop between the sending-end power supply and the sending-end commutation bus only exists in the equivalent impedance of the line. The voltage drop on the equivalent resistance and equivalent reactance is calculated respectively, and the calculation expression of the transient voltage of the sending-end commutation bus is obtained as follows: In the formula, R s is the equivalent resistance of the AC power grid at the sending end, X s is the equivalent reactance of the AC power grid at the sending end; Combining equations (27) and (28), we can derive U p The expression is: In the formula, the expression of algebraic b is: By substituting the DC current analytical expression into the above formula, the quantitative calculation formula for the transient voltage at the sending-end commutation bus can be obtained.

5. The method for calculating and suppressing transient overvoltage at the sending end of a UHV DC transmission system considering the influence of multiple control links according to claim 4 is characterized in that: The step 4 is specifically as follows: VDCOL is an important control link in the DC control system. It can detect the DC voltage in real time and output the corresponding DC current command value according to the characteristic curve to adjust the current and promote the commutation process. The VDCOL characteristic curve mainly consists of two parts: one is the VDCOL inflection point parameter, that is, the low-voltage starting threshold U dl and high voltage start threshold U dh , current command value lower limit I dl And the upper limit of current command value I dh ; The second is the VDCOL linear type. The traditional VDCOL voltage and current are linearly related with a fixed slope; The DC current command value output by the VDCOL link is expressed as: Where U d * is the converted voltage per unit value at the midpoint of the DC line, which is used as the input value of VDCOL; k and b are the coefficients of the operating characteristic curve, where k = (I dh -I dl ) / (U dh -U dl ), b=-U dl (I dh -I dl ) / (U dh -U dl ); The DC current starts to recover after reaching the minimum value in its fluctuation stage. The DC current expression in this stage is solved to obtain the minimum value of the DC current as 0.23pu. When the transient overvoltage peak is limited to less than 1.1pu, according to formula (32), it can be obtained that the DC current must be higher than 0.4pu. Since there is a measurement link G in the transfer function of the DC current drop, oscillation and fluctuation process meas (s) to measure the delay T m The influence of the current fluctuation causes the constant current control link on the rectifier side to increase the trigger angle command during the DC current fluctuation stage, causing the DC current to overshoot, and the DC current will drop to a smaller value below the lower limit of the current command value of 0.55pu; After the improved VDCOL, the DC current overshoot effect caused by the constant current control on the rectifier side remains basically unchanged in this stage, so the lower limit of the current command value needs to be increased by ΔI dl =0.4-0.23=0.17pu. By adding this value to the original control strategy, it can be obtained that the lower limit of the current command value output by the VDCOL link needs to be greater than 0.72pu.

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