A reactive power support control method and system for CLCC converters under AC fault conditions

Abstract

This invention relates to the field of high-voltage direct current (HVDC) transmission technology, and discloses a reactive power support control method and system for CLCC converters under AC faults. To fully explore the power support potential of CLCCs, this method first analyzes the forced commutation process of CLCCs during faults, demonstrating its multi-quadrant control capability for firing angle and power. Secondly, it determines the operating range of DC current and the firing angle control range, and establishes the power operating domain within this range. Then, with the goal of not affecting the active power transmitted to the receiving-end AC system, it determines the firing angle compensation amount and control method. The method provided by this invention not only maintains the inverter-side DC voltage and DC transmission power unchanged during AC faults, but also reduces the voltage drop of the converter bus during AC faults, which is beneficial for the rapid and stable recovery of AC voltage.

Description

Technical Field

[0001] This invention relates to the field of high-voltage direct current transmission technology, and in particular to a reactive power support control method and system for CLCC converters under AC fault conditions. Background Technology

[0002] High-voltage direct current (HVDC) transmission systems based on line-commutated converters (LCCs) have advantages such as large transmission capacity, high reliability, low loss, and low cost, and are currently widely used in long-distance overhead line transmission, submarine cable transmission, and asynchronous grid interconnection. However, when AC grid faults occur, such as voltage drops, phase shifts, or three-phase imbalances, the converter may fail to commutate normally, leading to commutation failure. Commutation failure causes a decrease in DC voltage and an increase in DC current, seriously affecting the stable operation of the HVDC transmission system. To completely solve the commutation failure problem, the research team of the State Grid Smart Grid Research Institute proposed a converter topology with controllable turn-off capability, namely the controllable line-commutated converter (CLCC) topology. This topology achieves forced commutation based on a hybrid connection of fully controlled IGBTs and semi-controlled thyristors, thus avoiding the commutation failure problem.

[0003] Because the voltage drop of the converter bus is too large during AC faults, it affects the stability of the AC system and is not conducive to the recovery process after the fault is cleared. Therefore, how to make full use of the advantage of CLCC not experiencing commutation failure and explore the power support potential of CLCC is an urgent problem to be solved in engineering. Summary of the Invention

[0004] The purpose of this invention is to provide a reactive power support control method and system for CLCC converters under AC faults, to solve the problem of excessive voltage drop on the converter bus during AC faults, which affects the stability of the AC system and is detrimental to the recovery process after fault clearance. This invention considers the forced commutation process of the CLCC during AC faults, analyzes its firing angle and power multi-quadrant control capability, and, combined with the influence of the DC current operating range and firing angle control range of the high-voltage DC system on the reactive power output of the CLCC, establishes a power operating domain within this range. With the goal of not affecting the active power transmitted to the receiving-end AC system, the firing angle compensation amount and control method are determined.

[0005] To achieve the above objectives, the following technical solution is adopted:

[0006] In a first aspect, the present invention provides a reactive power support control method for a CLCC converter under AC fault conditions, the method comprising:

[0007] Analyze the multi-quadrant control capability of the firing angle and power of the CLCC converter;

[0008] Determine the operating range of DC current and the range of firing angle adjustment;

[0009] Based on the operating range of DC current and the controllable range of firing angle, a power operating domain is established under the constraints of the operating range of DC current and the controllable range of firing angle.

[0010] Based on the power operating range constrained by the DC current operating range and the firing angle control range, the firing angle compensation amount is determined and smoothly put into control.

[0011] Secondly, the present invention provides a reactive power support control system for a CLCC converter under AC fault conditions, the system comprising:

[0012] The capability analysis module is configured to analyze the multi-quadrant control capability of the CLCC converter in terms of firing angle and power.

[0013] The range determination module is configured to determine the operating range of DC current and the firing angle control range;

[0014] The operating domain determination module is configured to establish a power operating domain based on the DC current operating range and the firing angle control range, constrained by the DC current operating range and the firing angle control range.

[0015] The control module is configured to determine the firing angle compensation amount and smoothly engage control based on the power operating range constrained by the DC current operating range and the firing angle adjustment range.

[0016] The beneficial effects of this invention are:

[0017] This invention completely solves the commutation failure problem at the topology level. To fully explore the power support potential of the CLCC, it first analyzes the multi-quadrant control capability of its firing angle and power, starting from the forced commutation process of the CLCC during a fault. Second, it determines the operating range of DC current and the control range of firing angle, and establishes the power operating domain within this range. Then, with the goal of not affecting the active power transmitted to the receiving-end AC system, it determines the firing angle compensation amount and control method. This invention not only maintains the inverter-side DC voltage and DC transmission power unchanged during AC faults, but also reduces the voltage drop of the converter bus during AC faults, which is conducive to the rapid and stable recovery of AC voltage. Attached Figure Description

[0018] Figure 1 A flowchart illustrating the reactive power support control method for a CLCC converter under AC fault conditions according to an embodiment of the present invention is shown.

[0019] Figure 2 A CLCC topology diagram according to an embodiment of the present invention is shown.

[0020] Figure 3 A schematic diagram illustrating the operating principle of the CLCC forced commutation mode according to an embodiment of the present invention is shown.

[0021] Figure 4 An equivalent circuit diagram of the CLCC forced commutation process according to an embodiment of the present invention is shown.

[0022] Figure 5 A voltage diagram of the CLCC-side converter bus according to an embodiment of the present invention is shown.

[0023] Figure 6 A diagram illustrating the CLCC power operating range according to an embodiment of the present invention is shown.

[0024] Figure 7 A schematic diagram of reactive power exchange in an inverter station according to an embodiment of the present invention is shown.

[0025] Figure 8 The partial derivatives of the active power transmitted by a DC system under different AC voltages according to embodiments of the present invention are shown with respect to the DC current and the firing angle; wherein, (a) is the DC current; and (b) is the firing angle.

[0026] Figure 9 The partial derivatives of reactive power exchange with respect to DC current and firing angle under different AC voltages according to embodiments of the present invention are shown; where (a) is DC current and (b) is firing angle.

[0027] Figure 10 The CLCC power operating domain according to an embodiment of the present invention is shown.

[0028] Figure 11 A flowchart illustrating the calculation and implementation process of the trigger angle compensation control according to an embodiment of the present invention is shown.

[0029] Figure 12 A block diagram of a reactive power support control strategy based on trigger angle compensation according to an embodiment of the present invention is shown.

[0030] Figure 13 A diagram illustrating the effect of suppressing AC bus voltage dips under a three-phase ground fault according to an embodiment of the present invention is shown.

[0031] Figure 14 A diagram illustrating the effect of suppressing AC bus voltage dips under a single-phase ground fault according to an embodiment of the present invention is shown.

[0032] Figure 15 A structural diagram of a reactive power support control system for a CLCC converter under AC fault conditions is shown according to an embodiment of the present invention. Detailed Implementation

[0033] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features described therein can be combined with each other.

[0034] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples.

[0035] This invention provides a reactive power support control method for CLCC converters under AC faults. To fully explore the power support potential of CLCCs, the method first analyzes the forced commutation process of CLCCs during faults, demonstrating its multi-quadrant control capability for firing angle and power. Secondly, it determines the operating range of DC current and the firing angle control range of the high-voltage DC system, establishing a power operating domain within this range. With the goal of not affecting the active power transmitted to the receiving-end AC system, the firing angle compensation amount and control method of this method are determined. This method not only maintains the inverter-side DC voltage and DC transmission power unchanged during AC faults but also reduces the voltage drop of the converter bus during AC faults, facilitating the rapid and stable recovery of AC voltage.

[0036] Specifically, such as Figure 1 As shown, the reactive power support control method of the CLCC converter under AC fault includes the following steps 1 to 4.

[0037] Step 1: Analyze the multi-quadrant control capability of the CLCC converter in terms of firing angle and power.

[0038] Step 2: Determine the operating range of DC current and the range of firing angle adjustment.

[0039] Step 3: Based on the operating range of DC current and the firing angle control range, establish the power operating domain under the constraints of the operating range of DC current and the firing angle control range.

[0040] Step 4: Based on the power operating range constrained by the DC current operating range and the firing angle control range, determine the firing angle compensation amount and smoothly put it into control.

[0041] The following embodiment will elaborate on the specific implementation steps, working principles, and effects of the above four steps.

[0042] Step 1 is to perform a multi-quadrant operation capability analysis of the CLCC. This embodiment illustrates a basic structure of a CLCC.

[0043] The topology of a 6-pulse CLCC is as follows Figure 2 As shown, each bridge arm consists of two parallel branches, and each branch is composed of a thyristor valve and an IGBT fully controlled valve connected in series. The thyristor valve in the main branch adopts the series thyristor valve used in conventional LCC converters, which has the capacity to withstand high voltage and high current. The IGBT valve in the main branch transfers the current to the auxiliary branch through active turn-off. As long as its turn-off voltage is higher than the on-state voltage drop of the auxiliary branch, the current transfer can be guaranteed. The IGBT fully controlled valve and the thyristor valve in the auxiliary branch are connected in parallel with an RCD damping circuit and a surge arrester, which play the roles of dynamic voltage equalization and voltage limiting, respectively.

[0044] During normal operation, the CLCC operates in natural commutation mode. When two phase arms enter the commutation stage, V12 turns off when the main branch current decreases to a set value, and V13 and V14 turn on, transferring current from the main branch to the auxiliary branch. After the main branch current crosses zero, the main thyristor V11 experiences reverse voltage and enters the blocking recovery period. After a specified delay ΔT, V13 turns off with zero current, and the entire bridge arm regains its blocking capability, completing the natural commutation process between bridge arms. When an AC fault occurs in the receiving-end power grid, the CLCC operates in forced commutation mode, and its working principle is as follows: Figure 3 As shown. When two-phase bridge arms are commutating, the commutation current increases and cannot be reduced to the set value. When the current carrying time of the main branch exceeds the allowable time, V12 is turned off, and the current of the main branch is transferred to the auxiliary branch. After the thyristor valve of the main branch is restored, the V13 sub-valve is turned off. At this time, due to the AC fault, the current does not cross zero. The current is transferred to capacitor C31 to charge it until it reaches the operating voltage of the surge arrester of valve V13. Then the current begins to transfer to the surge arrester. The commutation voltage is enhanced by the surge arrester voltage, and the forced commutation process between the bridge arms is realized.

[0045] For a CLCC converter, taking the commutation from valve VT5 to valve VT1 as an example, in the event of an AC fault, the surge arrester connected in parallel to the auxiliary branch trips, forcing commutation. The forced commutation process is as follows: Figure 4 As shown.

[0046] According to Kirchhoff's voltage law, we can obtain...

[0047]

[0048] Organized

[0049]

[0050] In the formula, U MOV3 U represents the voltage across the surge arrester. V1 This indicates the turn-on voltage of valve VT1, which is usually very small; L μ

[0051] i represents the commutation inductance of each phase. a i c U a U c U represents the current and voltage of the corresponding phase. ac This represents the instantaneous value of the AC line voltage, where t represents time.

[0052] As can be seen from equation (2), whether commutation can proceed normally depends not only on the AC line voltage U ac It also depends on the surge arrester voltage U. MOV3 and the on-state voltage U of valve VT1 V1 Referring to the concept of LCC commutation voltage, the commutation voltage U of CLCC can be considered as... cv =U ac +U MOV3 -U V1 .

[0053] Depend on Figure 5 The difference between CLCC and LCC in the commutation process can be analyzed from the relationship between AC line voltage and firing angle. For LCC converters, when the firing angle α > 180°, due to U a c AC line voltage U ac <0, normal commutation cannot be completed. However, for CLCC converters, since the commutation voltage between bridge arms is provided by the superposition of the auxiliary branch fully controlled valve arrester voltage and the AC line voltage, the commutation voltage when the entire valve arm is turned off is increased, that is, the commutation voltage time area is increased, making the entire valve arm easier to turn off during AC faults. When the firing angle α>180°, U a c AC line voltage U ac <0, but when U MOV3 >-U ac +U V1 At that time, U cv Even with a firing angle greater than 0, normal commutation can still be achieved. Therefore, compared to LCC, CLCC has a wider adjustable range of firing angle and multi-quadrant control capability.

[0054] Therefore, the adjustment range of the firing angle α can be obtained to achieve normal commutation.

[0055] U cv =U ac +U MOV3 -U V1 >0 (3)

[0056] in

[0057] U ac =U H sin(α+30°)-U H ​​sin(α-210°) (4)

[0058] In the formula, U H The peak value of the phase voltage can be obtained from the effective value of the converter bus voltage, U. aci Seek

[0059]

[0060] Substituting equations (4) and (5) into equation (3) yields the range of values ​​for the firing angle α.

[0061]

[0062] Neglecting converter losses, the active power on the AC side of the converter at the fundamental frequency is equal to its DC power.

[0063]

[0064] In the formula, P1 is the active power at the fundamental frequency on the AC side of the converter, P d U is the DC power, U is the RMS value of the converter bus voltage, and I1 is the fundamental component of the bridge AC side line current. The power factor angle is the angle at the fundamental frequency.

[0065] Similarly, the reactive power under the fundamental frequency is

[0066]

[0067] Power factor angle The relationship with the trigger angle α is as follows

[0068]

[0069] The commutation angle μ of the 12-pulse converter is less than 30°. From equation (9), it can be seen that the power factor angle... The adjustment range also has the capability for four-quadrant operation. The power operating range of the CLCC is as follows: Figure 6 As shown; with absorption as positive, when the CLCC acts as a rectifier to absorb active power, it can absorb reactive power and generate reactive power at the same time. When the CLCC acts as an inverter to generate active power, it can absorb reactive power and generate reactive power at the same time.

[0070] The purpose of step 2 is to determine the operating range of the CLCC's DC current and the firing angle control range. While adjusting reactive power can reduce the voltage drop on the converter bus during AC faults, the multiple electrical quantities are coupled during AC faults, necessitating the determination of the CLCC's operating power range during AC faults, and then implementing reactive power control accordingly. To establish the CLCC's operating power range, the operating range of the DC current and the firing angle control range are considered, thereby establishing the feasible power domain.

[0071] In one specific embodiment, step 2 is performed by the following steps 21 and 22.

[0072] Step 21: Determine the DC current I of CLCC d Operating range.

[0073] To determine the operating range of DC current, the inverter-side turn-off angle limit, the inverter-side commutation overlap angle limit, and the converter valve overload capacity limit are considered.

[0074] (1) Inverter side turn-off angle limitation

[0075] The inverter-side turn-off angle limits the time the thyristor needs to withstand reverse voltage after the current drops to zero to ensure the thyristor recovers its blocking capability. The electrical angle corresponding to the minimum time required for the thyristor to recover its blocking capability is the minimum turn-off angle. If the turn-off angle is less than the minimum turn-off angle γ... min If the commutation angle γ is not found, the system is considered to have failed. In the inverter operation of a traditional grid-commutated converter, the minimum turn-off angle γ at 50Hz is... min The angle is 7.2°. Since the CLCC converter includes fully controlled devices such as IGBTs and surge arresters, the turn-off angle of the CLCC can be 0. Therefore, the 0 turn-off angle (γ0) is used as the extreme condition to limit the DC current operating range.

[0076]

[0077] In the formula, U di U is the DC voltage of the inverter station; N is the number of pole pairs; U aci I is the effective value of the AC bus line voltage; k is the converter transformer voltage ratio; I d X is the DC current on the inverter side; γ is the turn-off angle on the inverter side; r This refers to the equivalent commutation reactance on the secondary side of the converter transformer.

[0078] The DC current range limited by γ0 is

[0079]

[0080] In the formula, I dγ The DC current range is limited by the inverter side turn-off angle.

[0081] (2) Inverter-side commutation overlap angle limitation

[0082] The commutation overlap angle of a 12-pulse converter needs to be controlled within a certain range, typically 15° to 30°. An excessively large overlap angle can lead to increased current fluctuations and harmonics, while an excessively small overlap angle can result in unstable system response and excessive stress on switching devices. Properly adjusting the overlap angle is crucial for ensuring converter performance and power quality; therefore, this paper limits the commutation overlap angle to μ < 30°.

[0083] The inverter-side commutation phase angle formula is:

[0084]

[0085] When the commutation overlap angle μ is at its maximum of 30°, the DC current range limited by the inverter-side commutation angle is:

[0086]

[0087] In the formula, I dμ To account for the DC current range limited by the commutation overlap angle on the inverter side.

[0088] (3) Overload capacity limitation of converter valve

[0089] The overload capacity of a converter valve refers to the maximum current it can withstand outside its normal operating range. This capacity is typically for short-term overloads, lasting from a few cycles to tens of cycles, allowing it to withstand current values ​​exceeding its rated current. The overload capacity of a converter valve is designed to handle short-term overload conditions, such as grid faults or transient responses during the converter process. For a 12-pulse converter station, the thyristor's second-level overload capacity can reach 1.4 times its rated current, i.e., I0.05. dh ≤1.4pu, the specific value depends on the design standard of the converter valve.

[0090] Considering the minimum DC current under the limitations of inverter-side turn-off angle, inverter-side commutation overlap angle, and converter valve overload capacity, we have:

[0091] I dmax =min(I dγ ,I dμ ,I dh (14)

[0092] In the formula, I dmax This is the upper limit of the DC current.

[0093] Normally I dmin Take 10% of the rated DC current, i.e., I dmin =0.1 pu. For high-voltage direct current transmission systems, low-voltage current limiting (VDCOL) is generally configured. To prevent discontinuous DC current, the minimum current limit value I of the low-voltage current limiting output can be taken. dvminThis serves as the lower limit for the DC system. Therefore, the operating range of the DC current for a high-voltage DC system can be determined.

[0094] I dvmin <I d <I dmax (15)

[0095] Step 22: Determine the firing angle α control range of CLCC.

[0096] In the traditional quasi-steady-state process of a 12-pulse DC engineering project, the minimum value of the firing angle α is αmin. min The angle is approximately 110°, the turn-off angle γ is typically 15°–20°, the commutation overlap angle μ ≤ 30°, and the power factor angle is at this point. The converter can only consume reactive power. For CLCC converters, the upper limit of the firing angle α is greater than 180°, at which point the power factor angle... The CLCC converter generates reactive power.

[0097] The upper limit of the firing angle α is determined by equation (6).

[0098]

[0099] In the formula, α max U represents the upper limit of the firing angle α; V1 This is the turn-on voltage for valve VT1.

[0100] This allows us to determine the maximum operating range of the firing angle.

[0101] α min <α<α max (17)

[0102] The firing angle α in quasi-steady state takes the value of

[0103]

[0104] In the formula, α0 is the value of the firing angle α under quasi-steady state, U aciN This represents the effective value of the inverter-side AC bus voltage under quasi-steady-state conditions.

[0105] I dN U is the rated DC current on the inverter side. diN This is the rated DC voltage on the inverter side.

[0106] Therefore, the actual firing angle α adjustment range under quasi-steady state is:

[0107] α0<α<α max (19)

[0108] Furthermore, the adjustment range of the firing angle α when the CLCC converter outputs reactive power can be obtained. The lower limit of the firing angle α can be determined by the relationship between the power factor angle, the firing angle, and the commutation overlap angle.

[0109]

[0110]

[0111] In the formula, α qmin Adjust the lower limit of the firing angle under reactive power output for CLCC.

[0112] Based on the upper limit of the CLCC firing angle α, the adjustment range of the firing angle α under the reactive power output of the CLCC can be obtained.

[0113]

[0114] Step 3 is used to determine the power feasible region of CLCC, which is carried out by steps 31 to 32 below.

[0115] Step 31: Analyze the power characteristics of CLCC under different AC voltage drops.

[0116] Since CLCCs do not experience commutation failure, they are typically used as inverter stations in high-voltage direct current transmission systems. A schematic diagram of reactive power exchange between the CLCC and the AC system is shown below. Figure 7 As shown.

[0117] Depend on Figure 7 The mathematical model of the inverter side of the system can be obtained as follows:

[0118]

[0119] Q ac =Q I +Q c (27)

[0120] Substituting equations (25) and (26) into equation (27), we get:

[0121]

[0122] In the formula, U aci I is the effective value of the AC bus line voltage. d U is the DC current on the inverter side. di α represents the DC voltage of the inverter station; N represents the number of pole pairs; α, β, and μ represent the firing angle, lead firing angle, and commutation overlap angle of the inverter station, respectively. X is the power factor angle of the converter; k is the voltage ratio of the converter transformer; r B is the equivalent commutation reactance on the secondary side of the converter transformer; c For equivalent capacity of reactive power compensation devices; Pd Q represents the active power transmitted by a DC system. I The reactive power generated by the inverter station;

[0123] Q c Q represents the reactive power generated by the reactive power compensation device. ac This represents the reactive power exchange between the converter station and the AC system. A positive value indicates that the AC system absorbs reactive power from the inverter station, while a negative value indicates that the AC system provides reactive power to the inverter station.

[0124] In equation (24), P d For DC current I d Taking the partial derivative with the firing angle α, we can obtain

[0125]

[0126] In equation (28), Q will be... ac For DC current I d Taking the partial derivative with the firing angle α, we can obtain

[0127]

[0128]

[0129] By incorporating the control parameters of the CIGRE standard model into equations (29) to (32), and considering the per-unit scaling of AC / DC system parameters, the active power P transmitted by the DC system under different AC voltage drops caused by AC faults can be obtained. d Reactive power exchange quantity Q ac About I d The partial derivatives of α and α are as follows: Figure 8 and Figure 9 As shown.

[0130] Depend on Figure 8 It can be seen that the active power P transmitted by the DC system under different AC voltages d Regarding DC current I d The partial derivatives with respect to the firing angle α are both positive. This indicates that the active power generated by the converter station increases with the increase of both the DC current and the firing angle. Figure 9 It can be seen that the reactive power exchange quantity Q under different AC voltages ac Regarding DC current I d The partial derivatives are all negative, and the reactive power exchange quantity Q under different AC voltages is... acThe partial derivatives with respect to the firing angle α are all positive. This indicates that as the DC current increases, the reactive power output of the converter station decreases, meaning the reactive power consumed increases, and more reactive power is absorbed from the AC side, placing a greater reactive power burden on the AC system during a fault. Conversely, as the firing angle increases, the reactive power output of the converter station increases, meaning the reactive power consumed decreases, and less reactive power is absorbed from the AC side, reducing the reactive power burden on the AC system during a fault.

[0131] Step 32: Determine the power feasible region of CLCC.

[0132] In the aforementioned DC current I d Under the combined constraints of the operating range and the firing angle α adjustment range, the DC transmission power P can be determined. d The CLCC converter outputs reactive power Q I The reactive power Q absorbed by the AC system ac The operating range.

[0133]

[0134] In the formula, P dmax P dmin Q Imax Q Imin and Q acmax Q acmin For a given U aci in I d Under the constraints of α, the maximum and minimum values ​​of DC transmission power, reactive power generated by CLCC, and reactive power absorbed by AC system.

[0135] Based on the partial derivatives of active and reactive power with respect to firing angle and DC current, the above extreme values ​​can be expressed as follows:

[0136]

[0137] The reactive power Q generated by CLCC I Regarding DC transmission power P d The power operating range within the DC current operating range and the adjustable firing angle α is as follows: Figure 10 As shown.

[0138] Depend on Figure 10 It can be seen that (I) dN α0) represents the steady-state operating point, i.e., the firing angle and rated DC current corresponding to steady state. At this point, the CLCC consumes reactive power, the AC system absorbs a small amount of reactive power, and the AC system is in an overcompensated state. The shaded area represents the adjustable range of DC transmission power and reactive power obtained by adjusting the firing angle of the CLCC during a fault, keeping the DC current within the operating range.

[0139] In one embodiment, step 4 determines the firing angle compensation amount and smoothly engages control. Specifically, to mitigate the voltage drop of the converter bus during AC faults, reactive power is controlled on the inverter side by compensating the firing angle, thereby suppressing AC voltage drops. To avoid affecting the active power transmitted by the receiving-end AC system and to ensure the DC voltage of the inverter station remains constant, the firing angle compensation amount is determined. To maintain the DC voltage of the inverter station constant, the main control on the inverter side adopts constant DC voltage control, switching to predictive constant turn-off angle control during the recovery period after fault clearance. This reactive power support control strategy mainly consists of four parts: constant DC voltage control, predictive constant turn-off angle control, firing angle compensation amount calculation, and fault detection. In steady state, it operates under constant DC voltage control; when a fault is detected and U... aci ac,γ＝0 The firing angle compensation strategy is initiated based on constant DC voltage control. The actual firing angle compensation is compared with the desired firing angle compensation, and the minimum of the two is used for compensation. To balance the severe fluctuations in converter bus voltage caused by excessive harmonics during single-phase faults, the lowest point of the effective value of the converter bus voltage within one cycle is selected, and firing angle compensation is performed using a smoothing curve. Upon fault detection, the compensation phase is terminated, and the control system switches from constant DC voltage control to predictive constant turn-off angle control until full recovery, after which it reverts to constant DC voltage control.

[0140] Specifically, to mitigate the voltage drop on the converter bus during AC faults, this method proposes controlling reactive power on the inverter side through a compensated firing angle, thereby suppressing AC voltage drops. To ensure the active power transmitted by the receiving-end AC system remains constant and to maintain the DC voltage at the inverter station, the firing angle compensation amount is determined.

[0141]

[0142] In the formula, U diN The rated DC voltage on the inverter side is α1, which is the compensated firing angle reference value required to maintain the DC voltage of the inverter station constant. di For the inverter-side DC current during a fault, using Taylor's formula, neglecting second-order and higher derivative terms and remainder terms, we obtain the DC current expansion as follows:

[0143]

[0144] In the formula, Δt1 is the prediction time constant of the first-order differential term, Δt2 is the prediction time constant of the second-order differential term, and t0 is the initial time; Δt1 = 10 ms, Δt2 = 1 ms;

[0145] From equations (35) and (36), the reference value of the compensated firing angle can be obtained as follows: ​

[0146]

[0147] Let α1 = α0 + Δα ref, Substituting into equation (37), we obtain the firing angle that needs to be compensated as follows:

[0148]

[0149] In the formula, α0 is the firing angle of the inverter station in quasi-steady state, which can be obtained from equation (18); Δα ref This is a reference value for the firing angle compensation.

[0150] To maintain a constant DC voltage at the inverter station, this paper adopts constant DC voltage control for the main control on the inverter side, and switches to predictive constant turn-off angle control during the recovery period after fault clearance.

[0151] The predictive shut-off angle control principle is as follows:

[0152]

[0153] In the formula, γ ref I is the reference value for the shut-off angle. dcref This is the current reference value;

[0154] A reactive power control strategy with compensated firing angle is adopted based on constant DC voltage control. The calculation of the firing angle to be compensated and the activation process are as follows: Figure 11 As shown, where U ac,γ＝0 The effective value of the converter bus voltage at 0 turn-off angle under quasi-steady-state conditions can be obtained from equation (10).

[0155]

[0156] First, the actual values ​​of electrical quantities such as converter bus voltage, DC power, firing angle, and DC current are collected. The fault detection circuit is used to determine whether a fault has occurred in the inverter-side AC system. When the system detects that the effective value of the converter bus voltage is lower than the effective value of the converter bus voltage when the cut-off angle reference value is 0 during the fault period, the firing angle compensation strategy is activated.

[0157] The block diagram of the reactive power support control strategy is as follows: Figure 12 As shown, it mainly consists of four parts: a constant DC voltage control loop, a predictive constant turn-off angle control loop, a trigger angle compensation calculation loop, and a fault detection loop. In steady state, it operates under constant DC voltage control; upon detecting a fault, and U... aci ac,γ＝0 ​The firing angle compensation strategy is initiated based on constant DC voltage control. The actual firing angle compensation is compared with the desired firing angle compensation, and the minimum of the two is used for compensation. To balance the severe fluctuations in converter bus voltage caused by excessive harmonics during single-phase faults, the lowest point of the effective value of the converter bus voltage within one cycle is selected, and firing angle compensation is performed using a smoothing curve. Upon fault detection, the compensation phase is terminated, and the control system switches from constant DC voltage control to predictive constant turn-off angle control until full recovery, after which it reverts to constant DC voltage control.

[0158] To implement and verify the effectiveness of the proposed method in suppressing converter bus voltage dips during AC faults, a 12-pulse 800kV / 5kA LCC-CLCC DC transmission system simulation model was built in PSCAD / EMTDC based on the CIGRE HVDC standard test model. The proposed reactive power support control strategy was simulated and verified for AC faults of varying severity on the inverter side, with two scenarios set up: Scenario 1 without control strategy; Scenario 2 with control strategy in effect. Simulation analyses were performed for three-phase ground faults and single-phase ground faults on the inverter side of the LCC-CLCC system.

[0159] See Figure 13 As shown, when a severe three-phase ground fault occurs on the inverter side (through a 0.08H inductor), the converter bus voltage in Scenario 2 drops significantly, severely impacting system frequency and voltage stability. In Scenario 1, the voltage drop on the converter bus is less severe; the converter bus voltage in Scenario 1 increases by 55kV compared to Scenario 2. Compared to the unimplemented strategy, the voltage support capability of the converter bus is improved by 10.5% when the strategy is implemented, which is beneficial for improving voltage stability during faults and rapid recovery after fault clearance.

[0160] See Figure 14 As shown, when a severe single-phase ground fault occurs on the inverter side (through a 0.01H inductor), the CLCC converter in Scenario 2 continues to consume reactive power, causing a significant voltage drop in the converter bus and severely impacting system voltage stability. After implementing strategies in Scenario 1, the voltage drop in the converter bus is significantly reduced. At this point, the converter bus voltage in Scenario 1 is 60kV higher than in Scenario 2, and the voltage support capability of the converter bus is improved by 11.5%, enhancing the rapid and stable recovery capability of the AC / DC system.

[0161] This invention also provides a reactive power support control system for a CLCC converter under AC fault conditions. Please refer to [link / reference]. Figure 15 The system includes:

[0162] Capability analysis module 151 is configured to analyze the multi-quadrant control capability of the CLCC converter in terms of firing angle and power.

[0163] The range determination module 152 is configured to determine the operating range of DC current and the firing angle control range;

[0164] The operating domain determination module 153 is configured to establish a power operating domain under the constraints of the DC current operating range and the firing angle control range, based on the DC current operating range and the firing angle control range.

[0165] The control module 154 is configured to determine the firing angle compensation amount and smoothly engage control based on the power operating range constrained by the DC current operating range and the firing angle adjustment range.

[0166] It should be noted that the reactive power support control system of the CLCC converter under this AC fault belongs to the same technical concept as the previously described method, and has the same technical principle and beneficial effect, so it will not be repeated here.

[0167] The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions also fall within the scope of the present invention, and the patent protection scope of the present invention should be defined by the claims.

Claims

1. A reactive power support control method for a CLCC converter under AC fault conditions, characterized in that, The method includes: Analyze the multi-quadrant control capability of the firing angle and power of the CLCC converter; Determine the operating range of DC current and the range of firing angle adjustment; Based on the operating range of DC current and the controllable range of firing angle, a power operating domain is established under the constraints of the operating range of DC current and the controllable range of firing angle. Based on the power operating range constrained by the DC current operating range and the firing angle control range, the firing angle compensation amount is determined and smoothly put into control. Analysis of the multi-quadrant control capability of the firing angle and power of the CLCC converter, including: For CLCC converters, in the event of an AC fault, the surge arresters connected in parallel to the auxiliary branch trip, forcing commutation. According to Kirchhoff's voltage law, the following equations are derived: According to equation (1), we get: In the formula, U MOV3 U represents the voltage across the surge arrester. V1 Indicates the on-state voltage of valve VT1; L μ i represents the commutation inductance of each phase. a i c U a U c U represents the current and voltage of the corresponding phase. ac The value represents the instantaneous value of the AC line voltage, and t represents time. Referring to the concept of LCC commutation voltage, the commutation voltage U of CLCC can be considered as... cv =Uac+U MOV3 -U V1 ; For CLCC, normal commutation can be achieved when the adjustment range of the firing angle α is as shown in equation (3): IN cv =U ac +U MOV3 -IN V1 >0 (3) in U ac =U H sin(α+30°)-U H sin(α-210°) (4) In the formula, U H The peak phase voltage is calculated using the following formula: In the formula, U aci This represents the effective value of the converter bus voltage. Substituting equations (4) and (5) into equation (3), we obtain the range of values ​​for the trigger angle α as follows: Neglecting converter losses, the active power on the AC side of the converter under the fundamental frequency is equal to its DC power, calculated using the following formula: In the formula, P1 is the active power at the fundamental frequency on the AC side of the converter, P d U is the DC power, U is the RMS value of the converter bus voltage, and I1 is the fundamental component of the bridge AC side line current. The power factor angle at the fundamental frequency; The reactive power under the fundamental frequency is calculated using the following formula: Determine the power factor angle The relationship with the firing angle α is as follows: In the formula, μ is the commutation angle of the pulsating converter.

2. The method as described in claim 1, characterized in that, Determine the operating range of DC current and the adjustable range of firing angle, including: The operating range of DC current is determined based on the inverter-side turn-off angle limit, the inverter-side commutation overlap angle limit, and / or the converter valve overload capacity limit. Based on the range of values ​​for the firing angle α, the firing angle adjustment range is determined.

3. The method as described in claim 2, characterized in that, Based on inverter-side turn-off angle limitations, inverter-side commutation overlap angle limitations, and / or converter valve overload capacity limitations, the operating range of DC current is determined, including: When considering the inverter-side turn-off angle limit, the DC current operating range is limited by the 0 turn-off angle γ0 as the extreme operating condition. U is calculated using the following formula. di : In the formula, U di U is the DC voltage of the inverter station; N is the number of pole pairs; U aci I is the effective value of the AC bus line voltage; k is the converter transformer voltage ratio; I d X is the DC current on the inverter side; γ is the turn-off angle on the inverter side; r This refers to the equivalent commutation reactance on the secondary side of the converter transformer. The DC current range limited by γ0 is: In the formula, I dγ To account for the DC current range limited by the inverter side turn-off angle; When considering the commutation overlap angle limit on the inverter side, the commutation overlap angle limit is taken as μ<30°; The inverter-side commutation angle is calculated using the following formula: When the commutation overlap angle μ is at its maximum of 30°, the DC current range limited by the inverter-side commutation angle is: In the formula, I dμ To account for the DC current range limited by the commutation overlap angle on the inverter side; When considering the overload capacity limit of the converter valve, the DC current range for the overload capacity limit of the converter valve is determined based on the second-level overload capacity of the thyristor. Considering the minimum DC current under the limitations of inverter-side turn-off angle, inverter-side commutation overlap angle, and converter valve overload capacity, as the upper limit, we obtain: IN dmax =min(I dγ ,IN dμ ,IN dh ) (14) In the formula, I dmax This is the upper limit of the DC current. Take the minimum current limit value I of the low-voltage current-limited output. dvmin As the lower limit of the DC system, the operating range of DC current for the high-voltage DC system is defined as follows: I dvmin <I d <I dmax (15) In the formula, I d This represents the DC current in a high-voltage DC system.

4. The method as described in claim 1, characterized in that, Based on the range of values ​​for the firing angle α, the firing angle adjustment range is determined, including: Based on the range of values ​​for the firing angle α, the upper limit of the firing angle α is determined as follows: In the formula, α max U represents the upper limit of the firing angle α; V1 This is the turn-on voltage for valve VT1; Based on the upper limit of the trigger angle α, the maximum operating range of the trigger angle is obtained: α min ＜α＜α max (17) In the formula, α min This is the lower limit of the value of the trigger angle α; Calculate the firing angle α under quasi-steady state: In the formula, α0 is the value of the firing angle α under quasi-steady state, U aciN I is the effective value of the inverter-side AC bus voltage under quasi-steady-state conditions. dN U is the rated DC current on the inverter side. diN This is the rated DC voltage on the inverter side; Based on the value of the firing angle α under quasi-steady state, the actual firing angle α adjustment range under quasi-steady state is determined as follows: α0＜α＜α max (19) According to the power factor angle The lower limit of the firing angle α is determined by the relationship between the firing angle α and the commutation overlap angle μ: In the formula, α qmin Provides the lower limit for firing angle adjustment under reactive power for CLCC; Based on the upper limit of the CLCC firing angle α, the adjustment range of the firing angle α under reactive power output by the CLCC is:

5. The method as described in claim 4, characterized in that, Based on the DC current operating range and the firing angle control range, a power operating domain is established under the constraints of the DC current operating range and the firing angle control range, including: Analyze the power characteristics of CLCC under different AC voltage drops; Based on the power characteristics of CLCC under different AC voltage drops, as well as the operating range of DC current and the range of firing angle adjustment, a power operating domain is established under the constraints of the operating range of DC current and the range of firing angle adjustment.

6. The method as described in claim 5, characterized in that, Based on the power characteristic analysis of CLCC under different AC voltage dips, the active power P transmitted by the DC system under different AC voltage dips caused by AC faults is obtained. d Reactive power exchange quantity Q ac ,include: The mathematical model of the inverter side of the system is determined as follows: Q ac =Q I +Q c (27) Substituting equations (25) and (26) into equation (27), we get: In the formula, U aci I is the effective value of the AC bus line voltage. d U is the DC current on the inverter side. di α represents the DC voltage of the inverter station; N represents the number of pole pairs; α, β, and μ represent the firing angle, lead firing angle, and commutation overlap angle of the inverter station, respectively. X is the power factor angle of the converter; k is the voltage ratio of the converter transformer; r B is the equivalent commutation reactance on the secondary side of the converter transformer; c For equivalent capacity of reactive power compensation devices; P d Q represents the active power transmitted by a DC system. I The reactive power generated by the inverter station; Q c Q represents the reactive power generated by the reactive power compensation device. ac This refers to the reactive power exchange between the converter station and the AC system. In equation (24), P d For DC current I d Taking the partial derivative with the firing angle α, we get: In equation (28), Q is... ac For DC current I d Taking the partial derivative with the firing angle α, we get: Substituting the control parameters of the CIGRE standard model into equations (29) to (32), and considering the per-unit scaling of AC / DC system parameters, we obtain the active power P transmitted by the DC system under different AC voltage drops caused by AC faults. d Reactive power exchange quantity Q ac About I d The partial derivatives of α.

7. The method as described in claim 6, characterized in that, Based on the power characteristic analysis of CLCC under different AC voltage dips, and considering the power characteristics of CLCC under different AC voltage dips, as well as the DC current operating range and firing angle control range, a power operating domain is established under the constraints of the DC current operating range and firing angle control range, including: In DC current I d Determine the DC transmission power P under the combined constraints of the operating range and the firing angle α adjustment range. d The CLCC converter outputs reactive power Q I The reactive power Q absorbed by the AC system ac Its operating range is: In the formula, P dmax P dmin Q Imax Q Imin and Q acmax Q acmin For a given U aci in I d Under the constraints of α, the maximum and minimum values ​​of DC transmission power, reactive power generated by CLCC, and reactive power absorbed by AC system; Based on the partial derivatives of active power and reactive power with respect to firing angle and DC current, P dmax P dmin Q Imax Q Imin and Q acmax Q acmin Represented as 8. The method as described in claim 7, characterized in that, Based on the power operating range constrained by the DC current operating range and the firing angle control range, the firing angle compensation amount is determined and smoothly implemented, including: U is determined by the following formula. diN : In the formula, U diN The rated DC voltage on the inverter side is α1, which is the compensated firing angle reference value required to maintain the DC voltage of the inverter station constant. di This refers to the DC current on the inverter side during a fault. Using Taylor's formula, neglecting second-order and higher differential terms and remainder terms, the expansion of DC current is obtained as follows: In the formula, Δt1 is the prediction time constant of the first-order differential term, Δt2 is the prediction time constant of the second-order differential term, and t0 is the initial time. From equations (35) and (36), the reference value of the compensated firing angle is: Let α1 = α0 + Δα ref Substituting into equation (37), the reference value for the firing angle compensation is obtained as follows: In the formula, α0 is the firing angle of the inverter station in quasi-steady state; Δα ref This is a reference value for the firing angle compensation. During the recovery period after fault clearance, switch to predictive fixed shut-off angle control; The formula for predictive cut-off angle control is expressed as: In the formula, γ re f is the reference value for the shut-off angle, I dcref This is the current reference value; A reactive power control strategy with compensated firing angle is adopted based on constant DC voltage control. The reactive power control strategy with compensated firing angle includes: In the event of a fault, determine U aci Is it less than U? ac,γ＝0 ;where U ac,γ＝0 The effective value of the converter bus voltage at 0 turn-off angle under quasi-steady-state conditions is calculated using the following formula: If U aci <U ac,γ＝0 Then the trigger angle compensation amount Δα is input. re1 The reference value Δα for trigger angle compensation is calculated using equation (38). ref ; If the firing angle compensation amount Δα re1 >Δα ref Then Δα is invested ref If the trigger angle compensation amount Δα re1 ≤Δα ref Then Δα is re-inputted. re1 .

9. A reactive power support control system for a CLCC converter under AC fault conditions, used to implement the method as described in any one of claims 1 to 8, characterized in that, The system includes: The capability analysis module is configured to analyze the multi-quadrant control capability of the CLCC converter in terms of firing angle and power. The range determination module is configured to determine the operating range of DC current and the firing angle control range; The operating domain determination module is configured to establish a power operating domain based on the DC current operating range and the firing angle control range, constrained by the DC current operating range and the firing angle control range. The control module is configured to determine the firing angle compensation amount and smoothly engage control based on the power operating range constrained by the DC current operating range and the firing angle adjustment range.

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