Reactive power support control method and system for CLCC converter under AC fault

Abstract

The invention relates to the technical field of high-voltage direct-current transmission, and discloses a reactive power support control method and system for a CLCC converter under an alternating-current fault, in order to fully explore the power support potential of the CLCC, the method comprises the following steps: firstly, starting from the forced commutation process of the CLCC during the fault period, analyzing that the trigger angle and the power have the multi-quadrant regulation and control capability; secondly, a direct current operable range and a trigger angle regulation and control range are determined; a power operation domain in the range is established; then, determining a trigger angle compensation amount and a control mode by taking no influence on active power transmitted to a receiving end alternating current system as a target; according to the method provided by the invention, the direct-current voltage and the direct-current transmission power of the inverter side during the alternating-current fault period can be kept unchanged, the voltage drop of the converter bus during the alternating-current fault period can be reduced, and the quick and stable recovery of the alternating-current voltage is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of high voltage direct current transmission, and in particular to a reactive power support control method and system for a CLCC converter under an alternating current fault. Background Art

[0002] The high-voltage direct current transmission system based on line commutated converter (LCC) has the advantages of large transmission capacity, high reliability, low loss and low cost. It has been widely used in long-distance overhead line transmission, submarine cable transmission and asynchronous networking. However, when the AC power grid fails, such as voltage drop, phase shift or three-phase imbalance, the converter may not be able to commutate normally, resulting in commutation failure. Commutation failure will reduce the DC voltage and increase the DC current, seriously affecting the stable operation of the DC transmission system. In order to completely solve the problem of commutation failure, the research and development team of the State Grid Smart Grid Research Institute proposed a converter topology with controllable shutdown capability, namely the controllable line-commutated converter (CLCC) topology. This topology realizes forced commutation based on the hybrid connection of fully controlled IGBT and half-controlled thyristor, avoiding the problem of commutation failure.

[0003] Since the commutation bus voltage drops too much during AC faults, affecting the stability of the AC system and being detrimental to the recovery process after fault removal, how to make full use of the advantage of CLCC that it will not have commutation failure and tap the power support potential of CLCC is an urgent problem to be solved in engineering. Summary of the invention

[0004] The purpose of the present invention is to provide a reactive power support control method and system for CLCC converters under AC faults, so as to solve the problem in the prior art that the voltage drop of the commutation bus during AC faults is too large, which affects the stability of the AC system and is not conducive to the recovery process after the fault is removed. The present invention takes into account the forced commutation process of CLCC during AC faults, analyzes the multi-quadrant control capability of its trigger angle and power, and establishes a power operation domain within this range in combination with the operating range of the DC current of the high-voltage DC system and the influence of the trigger angle control range on the reactive power emitted by the CLCC; with the goal of not affecting the active power transmitted to the receiving AC system, the trigger angle compensation amount and control method are determined.

[0005] In order to achieve the above purpose, the technical solutions adopted are as follows:

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

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

[0008] Determine the DC current operating range and the trigger angle control range;

[0009] Based on the DC current operable range and the trigger angle control range, a power operation domain is established under the constraints of the DC current operable range and the trigger angle control range;

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

[0011] In a second aspect, the present invention provides a reactive power support control system for a CLCC converter under an AC fault, the system comprising:

[0012] A capability analysis module configured to analyze the firing angle and multi-quadrant control capability of the CLCC converter;

[0013] A range determination module configured to determine an operable range of a DC current and a trigger angle control range;

[0014] An operation domain determination module is configured to establish a power operation domain under the constraints of the DC current operable range and the trigger angle control range based on the DC current operable range and the trigger angle control range;

[0015] The control module is configured to determine the trigger angle compensation amount based on the power operation domain under the constraints of the DC current operating range and the trigger angle control range, and smoothly put the control into operation.

[0016] The beneficial effects of the present invention are:

[0017] The present invention completely solves the commutation failure problem from the topological structure level. In order to fully explore the power support potential of CLCC, first of all, starting from the forced commutation process of CLCC during the fault, its trigger angle and power have multi-quadrant regulation capabilities. Secondly, the DC current operating range and trigger angle regulation range are determined; the power operation domain within this range is established; then, with the goal of not affecting the active power transmitted to the receiving AC system, the trigger angle compensation amount and control method are determined; the present invention can not only maintain the DC voltage and DC transmission power of the inverter side unchanged during the AC fault, but also reduce the drop of the commutation bus voltage during the AC fault, which is conducive to the rapid and stable recovery of the AC voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 An overall flow chart of a reactive power support control method for a CLCC converter under an AC fault 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 of the operation 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 a CLCC forced commutation process according to an embodiment of the present invention is shown.

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

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

[0024] Figure 7 A schematic diagram of reactive power exchange of 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 the DC system under different AC voltages with respect to the DC current and the trigger angle according to the embodiment of the present invention are shown; wherein, (a) is the DC current; and (b) is the trigger angle. .

[0026] Fig. 9 The partial derivatives of reactive exchange amount with respect to DC current and trigger angle under different AC voltages according to an embodiment of the present invention are shown; wherein, (a) is DC current; and (b) is trigger angle.

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

[0028] Fig.11 A flowchart of trigger angle compensation control calculation and input according to an embodiment of the present invention is shown.

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

[0030] Fig.13 A diagram showing the effect of suppressing AC bus voltage drop during a three-phase grounding fault according to an embodiment of the present invention is shown.

[0031] Fig.14 A diagram showing the effect of suppressing AC bus voltage drop during a single-phase grounding fault according to an embodiment of the present invention is shown.

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

[0033] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict.

[0034] The specific implementation of the present invention is further described in detail below in conjunction with the drawings and examples.

[0035] The embodiment of the present invention provides a reactive power support control method for a CLCC converter under an AC fault. In order to fully explore the power support potential of CLCC, firstly, starting from the forced commutation process of CLCC during the fault, its trigger angle and power have multi-quadrant control capabilities; secondly, the operating range of the DC current and the trigger angle control range of the high-voltage DC system are determined; the power operating domain within this range is established; with the goal of not affecting the active power transmitted to the receiving AC system, the trigger angle compensation amount and control method of this method are determined. This method can not only maintain the DC voltage and DC transmission power on the inverter side unchanged during the AC fault, but also reduce the drop of the commutation bus voltage during the AC fault, which is conducive to the rapid and stable recovery of the AC voltage.

[0036] Specifically, if 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 trigger angle and power of the CLCC converter.

[0038] Step 2: Determine the DC current operating range and the trigger angle control range.

[0039] Step 3: Based on the DC current operable range and the trigger angle control range, a power operation domain is established under the constraints of the DC current operable range and the trigger angle control range.

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

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

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

[0043] The topology of 6-pulse CLCC is as follows Figure 2 As shown in the figure, each bridge arm is composed of two branches in parallel, and each branch is composed of a thyristor valve and an IGBT full-control valve in series. The main branch thyristor valve adopts the series thyristor valve used in conventional LCC converters, which has high voltage and high current bearing capacity. The main branch IGBT valve transfers the current to the auxiliary branch by actively shutting down. As long as its shut-off voltage is higher than the on-state voltage drop of the auxiliary branch, the current can be successfully transferred. The IGBT full-control valve and the auxiliary branch thyristor valve are connected in parallel with the RCD damping circuit and the arrester in turn, which play the role of dynamic voltage balancing and voltage limiting respectively.

[0044] During normal operation, CLCC operates in natural commutation mode. When the two-phase bridge arm enters the commutation stage, when the main branch current decreases to the set value, V12 is turned off, V13 and V14 are turned on, and the current is transferred from the main branch to the auxiliary branch. After the main branch current passes through zero, the main thyristor V11 is subjected to reverse pressure and enters the blocking recovery period. After the specified delay ΔT, V13 is turned off with zero current, and the entire bridge arm restores its blocking capacity, completing the natural commutation process between the bridge arms. When an AC fault occurs in the receiving power grid, CLCC operates in forced commutation mode. Its working principle is as follows: Figure 3 As shown. When the two-phase bridge arm is commutating, the commutation current increases and cannot be reduced to the set value. When the main branch current flow time exceeds the allowed time, V12 is turned off, and the main branch current is transferred to the auxiliary branch; after the main branch thyristor valve is restored, the V13 sub-valve is turned off. At this time, due to the AC fault, the current does not pass zero, and the current is transferred to the capacitor C31 to charge it. When the arrester action voltage of the V13 valve is reached, the current begins to transfer to the arrester, relying on the arrester voltage to enhance the commutation voltage, and realize the forced commutation process between the bridge arms.

[0045] For the CLCC converter, taking the commutation from VT5 valve to VT1 valve as an example, when an AC fault occurs, the lightning arrester connected in parallel to the auxiliary branch will be activated to perform forced commutation. The forced commutation process is as follows: Figure 4 shown.

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

[0047]

[0048] Arranged

[0049]

[0050] Where U MOV3 Indicates the voltage across the arrester; U V1 Indicates the opening voltage of VT1 valve, which is usually very small; L μ

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

[0052] From formula (2), we can see that whether the commutation can be normal at this time depends not only on the AC line voltage U ac , also depends on the arrester voltage U MOV3 and the opening voltage U of VT1 valve V1 , referring to the concept of LCC commutation voltage, it can be considered that the CLCC commutation voltage U 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 trigger angle. For LCC converter, when the trigger angle α>180°, due to U a c , AC line voltage U ac <0, the normal commutation process cannot be completed. However, for CLCC converters, since the commutation voltage between the bridge arms is provided by the auxiliary branch full-control valve arrester voltage and the AC line voltage, the commutation voltage when the entire valve arm is shut down is increased, that is, the commutation voltage time area is increased, making the entire valve arm easier to shut down during AC faults. When the trigger angle α>180°, U a c , AC line voltage U ac <0, but when U MOV3 >-U ac +U V1 When U cv >0, normal commutation can still be achieved. Therefore, compared with LCC, CLCC has a wider adjustable range of trigger angle and has multi-quadrant control capability.

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

[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] Where U H is the phase voltage peak value, which can be obtained from the effective value of the commutation bus voltage U aci Obtain

[0059]

[0060] Substituting equations (4) and (5) into equation (3), we can obtain the range of the trigger angle α:

[0061]

[0062] When the converter loss is ignored, the fundamental active power on the AC side of the converter is equal to its DC power.

[0063]

[0064] Where P 1 is the fundamental active power on the AC side of the converter, P d is the DC power, U is the effective value of the commutation bus voltage, I 1 is the fundamental component of the line current on the bridge AC side, is the power factor angle under the fundamental wave.

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

[0066]

[0067] Power factor angle The relationship between the trigger angle α is:

[0068]

[0069] The commutation angle μ of the 12-pulse converter is less than 30°. From formula (9), we can see that the power factor angle The adjustment range of CLCC also has the ability to operate in four quadrants. Figure 6 As shown; taking absorption as positive, when CLCC absorbs active power as a rectifier, it can both absorb reactive power and emit reactive power. When CLCC emits active power as an inverter, it can both absorb reactive power and emit reactive power.

[0070] The purpose of step 2 is to determine the CLCC DC current operating range and trigger angle control range. The voltage drop of the commutation bus during AC faults can be reduced by regulating reactive power, but multiple electrical quantities during AC faults are coupled with each other. It is necessary to determine the CLCC power operating range during AC faults and perform reactive power regulation on this basis. In order to establish the CLCC power operating range, the DC current operating range and trigger angle control range are considered, and then the power feasible domain is established.

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

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

[0073] In order to determine the operable range of DC current, the inverter side shut-off angle limit, inverter side commutation overlap angle limit and converter valve overload capacity limit are considered.

[0074] (1) Inverter side shutdown angle limit

[0075] The inverter side turns off angle limit thyristor needs to withstand a period of reverse voltage after the current drops to 0 to ensure that the thyristor recovers its blocking ability. The electrical angle corresponding to the minimum time required for the thyristor to recover its blocking ability is the minimum turn-off angle. If the turn-off angle is less than the minimum turn-off angle γ min , then it is judged that the system has commutation failure. In the inverter operation process of the traditional grid commutation converter, the minimum turn-off angle γ corresponding to 50Hz min =7.2°. Since the CLCC converter contains fully controlled IGBT and lightning arrester, the turn-off angle of CLCC can be 0. Therefore, the 0 turn-off angle (γ 0 ) as an extreme operating condition to limit the DC current operating range.

[0076]

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

[0078] γ 0 The limited DC current range is

[0079]

[0080] In the formula, I dγThis is the DC current range considering the inverter side turn-off angle limitation.

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

[0082] The commutation overlap angle of the 12-pulse converter needs to be controlled within a certain range, usually 15° to 30°. Too large an overlap angle will lead to increased current fluctuations and harmonics, while too small an overlap angle may lead to unstable system response and excessive stress on switching devices. Reasonable adjustment of the overlap angle is the key to ensuring converter performance and power quality, so the commutation overlap angle limit in this paper is μ<30°.

[0083] The inverter side commutation angle formula is:

[0084]

[0085] When the commutation overlap angle μ is 30° at most, the DC current range limited by the inverter commutation angle is

[0086]

[0087] In the formula, I dμ This is the DC current range that takes into account the inverter side commutation overlap angle limitation.

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

[0089] The overload capacity of the converter valve refers to the maximum current that the converter valve can withstand outside the normal operating range. This capacity is usually a short-term overload, and can withstand current values ​​exceeding the rated current within a few cycles to dozens of cycles. The overload capacity of the converter valve is to cope with some short-term overload conditions, such as grid failures, transient responses during the commutation process, etc. For a 12-pulse converter station, the thyristor's second-level overload capacity can reach 1.4 times the rated current, that is, I dh ≤1.4pu, the specific value depends on the design standard of the converter valve.

[0090] Considering the inverter side shut-off angle limit, inverter side commutation overlap angle limit and converter valve overload capacity limit, the minimum DC current is taken as the upper limit value, that is,

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

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

[0093] Usually I dmin Take 10% of the rated DC current, that is, I dmin=0.1pu. For high-voltage DC transmission systems, low-voltage current limiting (VDCOL) is generally configured. To prevent the DC current from being discontinuous, the minimum current limit value I of the low-voltage current limiting output can be taken. dvmin As the lower limit of the DC system, the DC current operating range of the high-voltage DC system can be determined.

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

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

[0096] In the quasi-steady state process of the traditional 12-pulse DC engineering, the minimum value of the trigger angle α min is about 110°, the turn-off angle γ is usually 15°~20°, and the commutation overlap angle μ≤30°. At this time, the power factor angle The converter can only consume reactive power. For CLCC converters, the upper limit of the trigger angle α is greater than 180°. The CLCC converter generates reactive power.

[0097] The upper limit of the trigger angle α is determined by formula (6):

[0098]

[0099] In the formula, α max is the upper limit of the trigger angle α; U V1 is the opening voltage of VT1 valve.

[0100] This gives the maximum operating range of the firing angle

[0101] α min <α<α max (17)

[0102] The trigger angle α in the quasi-steady state is

[0103]

[0104] In the formula, α 0 is the value of the trigger angle α in the quasi-steady state, U aciN is the effective value of the AC bus voltage on the inverter side in quasi-steady state,

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

[0106] Therefore, the actual trigger angle α adjustment range in quasi-steady state is

[0107] α0 <α<α max (19)

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

[0109]

[0110]

[0111] In the formula, α qmin It is the lower limit of the trigger angle adjustment when CLCC generates reactive power.

[0112] Combined with the upper limit of the CLCC trigger angle α, the adjustment range of the trigger angle α under CLCC reactive power can be obtained:

[0113]

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

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

[0116] Since CLCC will not have commutation failure, it is usually used as an inverter station in the high-voltage DC transmission system. The schematic diagram of reactive power exchange between it and the AC system is as follows: Figure 7 shown.

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

[0118]

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

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

[0121]

[0122] Where U aci is the effective value of the AC bus line voltage, I d is the DC current on the inverter side; U di is the DC voltage of the inverter station; N is the number of pole pairs; α, β and μ are the trigger angle, advance trigger angle and commutation overlap angle of the inverter station respectively; is the power factor angle of the converter; k is the voltage ratio of the converter transformer; Xr is the equivalent commutation reactance on the secondary side of the converter transformer; B c It is the equivalent accommodation of reactive power compensation device; P d is the active power transmitted by the DC system, Q I Reactive power generated by the inverter station;

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

[0124] In formula (24), P d For DC current I d And the partial derivative of the trigger angle α can be obtained

[0125]

[0126] In formula (28), Q ac For DC current I d And the partial derivative of the trigger angle α can be obtained

[0127]

[0128]

[0129] By incorporating the control parameters of the CIGRE standard model into equations (29) to (32) and considering the normalization of the AC / DC system parameters, we can obtain the active power P transmitted by the DC system under different AC voltage drops caused by AC faults: d , reactive exchange quantity Q ac About I d The partial derivatives of and α are Figure 8 and Fig. 9 shown.

[0130] Depend on Figure 8 It can be seen that the active power P transmitted by the DC system under different AC voltages is d About DC current I d The partial derivatives of and the trigger angle α are both positive numbers. This indicates that as the DC current and the trigger angle increase, the active power generated by the converter station increases. Fig. 9 It can be seen that the reactive exchange quantity Q under different AC voltages is ac About DC current I d The partial derivatives of are all negative, and the reactive exchange quantity Q under different AC voltages acThe partial derivatives of the trigger angle α are all positive numbers. This means that as the DC current increases, the reactive power generated by the converter station decreases, that is, the reactive power consumed increases, more reactive power is absorbed from the AC side, and a greater reactive burden is imposed on the AC system in the fault. As the trigger angle increases, the reactive power generated by the converter station increases, that is, the reactive power consumed decreases, and less reactive power is absorbed from the AC side, reducing the reactive burden of the AC system in the fault.

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

[0132] In the above DC current I d Under the common constraints of the operating range and the trigger angle α adjustment range, the DC transmission power P can be determined d , CLCC converter generates reactive power Q I , reactive power Q absorbed by the AC system ac operating range.

[0133]

[0134] Where P dmax , P dmin , Q Imax , Q Imin and Q acmax , Q acmin For a certain U aci in I d The maximum and minimum values ​​of the DC transmission power, the reactive power emitted by the CLCC, and the reactive power absorbed by the AC system under the constraints of and α.

[0135] According to the above relationship between active power and reactive power with respect to the partial derivative of the firing angle and DC current, the above extreme values ​​can be expressed as

[0136]

[0137] Reactive power Q generated by CLCC I About DC transmission power P d The power operation domain under the DC current operating range and the trigger angle α adjustable range is as follows: Fig.10 shown.

[0138] Depend on Fig.10 It can be seen that (I dN ,α 0 ) is the steady-state operating point, that is, the corresponding trigger angle and rated DC current in steady state. At this time, the corresponding 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 CLCC trigger angle during the fault period so that the DC current is within the operable range.

[0139] In a certain embodiment, the trigger angle compensation amount is determined in step 4 and smoothly put into control, specifically: in order to reduce the drop degree of the commutation bus voltage during the AC fault, the reactive power is controlled by compensating the trigger angle on the inverter side, so as to achieve the purpose of suppressing the AC voltage drop. In order not to affect the active power transmitted by the receiving AC system, the trigger angle compensation amount is determined with the goal of ensuring that the DC voltage of the inverter station remains unchanged. In order to maintain the DC voltage of the inverter station unchanged, the main control on the inverter side adopts constant DC voltage control, and switches to predictive fixed shutdown angle control during the recovery period after the fault is removed; the reactive support control strategy is mainly composed of four parts: constant DC voltage control link, predictive fixed shutdown angle control, trigger angle compensation amount calculation link and fault detection link. In steady state, it runs in constant DC voltage control. If a fault is detected and U aci ac,γ＝0 The trigger angle compensation strategy is started on the basis of constant DC voltage control; the actual trigger angle compensation is compared with the expected trigger angle compensation, and the minimum of the two is used for compensation; in order to balance the problem of drastic fluctuation of commutation bus voltage caused by excessive harmonics during single-phase faults, the lowest point of the effective value of the commutation bus voltage is taken within a cycle, and the trigger angle compensation is performed through a smooth curve. When a fault is detected, the compensation link is exited, and the control system switches from constant DC voltage control to predictive fixed turn-off angle control until it returns to constant DC voltage control after full recovery.

[0140] Specifically, in order to reduce the voltage drop of the commutation bus during AC faults, this method proposes to control the reactive power by compensating the trigger angle on the inverter side, so as to achieve the purpose of suppressing the AC voltage drop. In order not to affect the active power transmitted by the receiving AC system, the trigger angle compensation amount is determined with the goal of ensuring that the DC voltage of the inverter station remains unchanged.

[0141]

[0142] Where U diN is the rated DC voltage on the inverter side, α 1 In order to maintain the DC voltage of the inverter station unchanged, the reference value of the trigger angle after compensation is required, I di is the DC current on the inverter side during the fault. Using Taylor's formula, ignoring the second-order differential terms and remainder terms, the DC current expansion is:

[0143]

[0144] In the formula, Δt 1 is the predicted time constant of the first-order differential term, Δt 2 is the prediction time constant of the second-order differential term, t 0 is the initial time; Δt 1 =10ms, Δt​2 =1ms;

[0145] From equations (35) and (36), we can get the reference value of the trigger angle after compensation:

[0146]

[0147] Let α 1 =α 0 +Δα ref, Substituting into equation (37), we can get the trigger angle that needs to be compensated:

[0148]

[0149] In the formula, α 0 is the trigger angle of the inverter station in quasi-steady state, which can be known from formula (18); Δα ref It is the reference value of the trigger angle compensation.

[0150] In order to maintain the DC voltage of the inverter station unchanged, the inverter side main control in this paper adopts constant DC voltage control, and switches to predictive fixed shutdown angle control during the recovery period after fault removal;

[0151] The principle of predictive fixed shut-off angle control is:

[0152]

[0153] In the formula, γ ref is the cut-off angle reference value, I dcref is the current reference value;

[0154] Based on the constant DC voltage control, the reactive power control strategy of compensating the trigger angle is adopted. The calculation and input process of the trigger angle to be compensated are as follows: Fig.11 As shown, U ac,γ＝0 is the effective value of the commutation bus voltage at 0 turn-off angle in quasi-steady state, which can be obtained from formula (10):

[0155]

[0156] First, the actual values ​​of electrical quantities such as commutation bus voltage, DC power, trigger angle and DC current are collected, and whether a fault occurs in the inverter-side AC system is determined by whether the fault detection link is activated; when the system detects that the effective value of the commutation bus voltage is lower than the effective value of the commutation bus voltage when the shutdown angle reference value is 0 during the fault period, the trigger angle compensation strategy is started.

[0157] The block diagram of the reactive support control strategy is as follows Fig.12 As shown in the figure, it mainly consists of four parts: constant DC voltage control link, predictive fixed turn-off angle control, trigger angle compensation calculation link and fault detection link. In steady state, it runs in constant DC voltage control. If a fault is detected and Uaci ac,γ＝0 The trigger angle compensation strategy is started on the basis of constant DC voltage control; the actual trigger angle compensation is compared with the expected trigger angle compensation, and the minimum of the two is used for compensation; in order to balance the problem of drastic fluctuation of commutation bus voltage caused by excessive harmonics during single-phase faults, the lowest point of the effective value of the commutation bus voltage is taken within a cycle, and the trigger angle compensation is performed through a smooth curve. When a fault is detected, the compensation link is exited, and the control system switches from constant DC voltage control to predictive fixed turn-off angle control until it returns to constant DC voltage control after full recovery.

[0158] In order to implement and verify the method proposed in this embodiment to suppress the voltage drop of the commutation bus 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 support control strategy was simulated and verified for AC faults of different severity on the inverter side, and the following two scenarios were set at the same time: Scenario 1 does not put the control strategy into use; Scenario 2 puts the control strategy into use. Simulation analysis was performed on the three-phase grounding fault and single-phase grounding fault on the inverter side of the LCC-CLCC system.

[0159] See also Fig.13 As shown in the figure, when a serious three-phase grounding fault occurs on the inverter side (through a 0.08H inductor), the voltage of the commutation bus in scenario 2 drops sharply, seriously affecting the frequency and voltage stability of the system. In scenario 1, the voltage drop of the commutation bus is reduced; at this time, the voltage of the commutation bus in scenario 1 increases by 55kV compared with that in scenario 2. Compared with the non-investment strategy, the voltage support capacity of the commutation bus when the investment strategy is put into use is increased by 10.5%, which is conducive to improving the voltage stability during the fault and the rapid recovery capability after the fault is removed.

[0160] See also Fig.14 As shown in the figure, when a single-phase grounding fault occurs on the inverter side (through a 0.01H inductor), the CLCC converter in scenario 2 continues to consume reactive power, and the commutation bus voltage drops sharply, seriously affecting the system voltage stability. After the strategy is adopted in scenario 1, the voltage drop of the commutation bus is greatly reduced. At this time, the commutation bus voltage in scenario 1 increases by 60kV compared with scenario 2, and the voltage support capacity of the commutation bus increases by 11.5%, which improves the rapid and stable recovery capability of the AC / DC system.

[0161] The embodiment of the present invention also provides a reactive power support control system for a CLCC converter under AC fault conditions, see Fig.15 , the system comprises:

[0162] A capability analysis module 151 is configured to analyze the firing angle and the multi-quadrant control capability of the CLCC converter;​

[0163] A range determination module 152, configured to determine a DC current operable range and a trigger angle control range;

[0164] The operation domain determination module 153 is configured to establish a power operation domain under the constraints of the DC current operable range and the trigger angle control range based on the DC current operable range and the trigger angle control range;

[0165] The control module 154 is configured to determine the trigger angle compensation amount based on the power operation domain constrained by the DC current operating range and the trigger angle control range, and smoothly put it into control.

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

[0167] The above implementation modes are only used to illustrate the present invention, but not to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions also belong to the scope of the present invention. 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, characterized in that: The method comprises: Analyze the multi-quadrant control capability of the firing angle and power of CLCC converters; Determine the DC current operating range and the trigger angle control range; Based on the DC current operable range and the trigger angle control range, a power operation domain is established under the constraints of the DC current operable range and the trigger angle control range; Based on the power operation domain constrained by the DC current operating range and the trigger angle control range, the trigger angle compensation amount is determined and smoothly put into control.

2. The method according to claim 1, characterized in that Analyze the multi-quadrant control capability of the firing angle and power of the CLCC converter, including: For CLCC converters, when an AC fault occurs, the lightning arrester in parallel with the auxiliary branch is activated to perform forced commutation. According to Kirchhoff's voltage law, According to formula (1), we get: Where U MOV3 Indicates the voltage across the arrester; U V1 Indicates the opening voltage of VT1 valve; L μ represents the commutation inductance of each phase, i a 、i c , U a , U c Indicates the current and voltage of the corresponding phase, U ac represents the instantaneous value of the AC line voltage, and t represents time; Referring to the concept of LCC commutation voltage, it can be considered that the CLCC commutation voltage U cv =Uac+U MOV3 -U V1 ; For CLCC, the adjustment range of the trigger angle α is as shown in formula (3), which can achieve normal commutation: IN cv =U ac +U MOV3 -IN V1 >0 (3) in U ac =U H sin(α+30°)-U H sin(α-210°) (4) Where U H is the phase voltage peak value, and the calculation formula is: Where U aci is the effective value of the commutation bus voltage; Substituting equations (4) and (5) into equation (3), the range of the trigger angle α is: When the converter loss is ignored, the fundamental active power on the AC side of the converter is equal to its DC power. The calculation formula is: Where P1 is the fundamental active power on the AC side of the converter, P d is the DC power, U is the effective value of the commutation bus voltage, I1 is the fundamental component of the bridge AC side line current, is the power factor angle under fundamental wave; The reactive power under fundamental wave is calculated by the following formula: Determining the Power Factor Angle The relationship between the trigger angle α is: Where μ is the commutation angle of the pulsating converter.

3. The method according to claim 2, characterized in that Determine the DC current operating range and firing angle control range, including: Determining the DC current operating range based on the inverter side shut-off angle limit, the inverter side commutation overlap angle limit and / or the converter valve overload capacity limit; Based on the value range of the trigger angle α, the trigger angle control range is determined.

4. The method according to claim 3, characterized in that Based on the inverter-side shut-off angle limit, the inverter-side commutation overlap angle limit and / or the converter valve overload capacity limit, the DC current operating range is determined, including: When considering the inverter side shutdown angle limit, the 0 shutdown angle γ0 is used as the extreme working condition to limit the DC current operating range; Calculate U by the following formula di : Where U di is the DC voltage of the inverter station; N is the number of pole pairs; U aci is the effective value of the AC bus voltage; k is the converter transformer voltage ratio; I d is the DC current on the inverter side; γ is the turn-off angle on the inverter side; X r is 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 The DC current range to consider the inverter side turn-off angle limit; When considering the inverter side commutation overlap angle limit, the commutation overlap angle limit is μ<30°; The inverter side commutation angle is calculated using the following formula: When the commutation overlap angle μ is 30° at most, the DC current range limited by the inverter commutation angle is In the formula, I dμ The DC current range considering the inverter side commutation overlap angle limitation; When considering the overload capacity limit of the converter valve, the DC current range of the converter valve overload capacity limit is determined according to the second-level overload capacity of the thyristor; Considering the inverter side shut-off angle limit, inverter side commutation overlap angle limit and the minimum DC current under the converter valve overload capacity limit as the upper limit, we get: IN dmax =min(I dγ ,IN dμ ,IN dh ) (14) In the formula, I dmax is the upper limit of DC current; Take the minimum current limit value I of the low voltage current limiting output dvmin As the lower limit of the DC system, the DC current operating range of the high-voltage DC system is determined as: I dvmin <I d <I dmax (15) In the formula, I d Represents the DC current of the high voltage DC system.

5. The method according to claim 3, characterized in that Based on the value range of the trigger angle α, the trigger angle control range is determined, including: Based on the value range of the trigger angle α, the upper limit of the trigger angle α is determined as: In the formula, α max is the upper limit of the trigger angle α; U V1 is the opening voltage of VT1 valve; According to 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 is the lower limit of the trigger angle α; Calculate the value of the trigger angle α in the quasi-steady state: Where α0 is the value of the trigger angle α in the quasi-steady state, U aciN is the effective value of the AC bus voltage on the inverter side in quasi-steady state, I dN is the rated DC current on the inverter side, U diN is the rated DC voltage on the inverter side; According to the value of the trigger angle α in the quasi-steady state, the actual trigger angle α adjustment range in the quasi-steady state is determined as follows: α0＜α＜α max (19) According to the power factor angle The relationship between the trigger angle α and the commutation overlap angle μ determines the lower limit of the trigger angle α: In the formula, α qmin It is the lower limit of the trigger angle adjustment when CLCC emits reactive power; According to the upper limit of the CLCC trigger angle α, the adjustment range of the trigger angle α when the CLCC emits reactive power is:

6. The method according to claim 5, characterized in that Based on the DC current operating range and the trigger angle control range, a power operation domain is established under the constraints of the DC current operating range and the trigger 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 DC current operating range and the trigger angle control range, a power operation domain is established under the constraints of the DC current operating range and the trigger angle control range.

7. The method according to claim 6, characterized in that Based on the power characteristic analysis of CLCC under different AC voltage drops, the active power P transmitted by the DC system under different AC voltage drops caused by AC faults is obtained. d And reactive exchange Q ac ,include: Determine the mathematical model of the system inverter side, expressed as: Q ac =Q I +Q c (27) Substituting equation (25) and equation (26) into equation (27), we obtain: Where U aci is the effective value of the AC bus line voltage, I d is the DC current on the inverter side; U di is the DC voltage of the inverter station; N is the number of pole pairs; α, β and μ are the trigger angle, advance trigger angle and commutation overlap angle of the inverter station respectively; is the power factor angle of the converter; k is the voltage ratio of the converter transformer; X r is the equivalent commutation reactance on the secondary side of the converter transformer; B c It is the equivalent accommodation of reactive power compensation device; P d is the active power transmitted by the DC system, Q I Reactive power generated by the inverter station; Q c The reactive power generated by the reactive power compensation device; Q ac is the reactive power exchange between the converter station and the AC system; In formula (24), P d For DC current I d And the partial derivative of the trigger angle α is obtained: In formula (28), Q ac For DC current I d And the partial derivative of the trigger angle α is obtained: Substituting the control parameters of the CIGRE standard model into equations (29) to (32) and considering the normalization of the AC / DC system parameters, we can obtain the active power P transmitted by the DC system under different AC voltage drops caused by AC faults: d , reactive exchange quantity Q ac About I d and the partial derivatives of α.

8. The method according to claim 7, characterized in that Based on the power characteristics analysis of CLCC under different AC voltage drops, the power characteristics of CLCC under different AC voltage drops, the DC current operating range and the trigger angle control range, the power operation domain under the constraints of the DC current operating range and the trigger angle control range is established, including: The DC current I d Under the common constraints of the operating range and the trigger angle α adjustment range, the DC transmission power P is determined d , CLCC converter generates reactive power Q I , reactive power Q absorbed by the AC system ac The operating range is: Where P dmax , P dmin , Q Imax , Q Imin and Q acmax , Q acmin For a certain U aci in I d The maximum and minimum values ​​of DC transmission power, reactive power emitted by CLCC and reactive power absorbed by AC system under the constraints of and α; According to the partial derivative relationship between 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 Expressed as 9. The method according to claim 8, characterized in that Based on the power operation domain under the constraints of the DC current operating range and the trigger angle control range, the trigger angle compensation amount is determined and smoothly put into control, including: Determine U by the following formula diN : Where U diN is the rated DC voltage on the inverter side, α1 is the reference value of the trigger angle after compensation required to maintain the DC voltage of the inverter station unchanged, I di is the DC current on the inverter side during the fault period; Using Taylor's formula, ignoring the second-order differential terms and remainder terms, the DC current expansion is: Where 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; According to equations (35) and (36), the reference value of the trigger angle after compensation is: Let α1=α0+Δα ref Substituting into formula (37), the reference value of the trigger angle compensation is obtained as follows: Where α0 is the trigger angle of the inverter station in quasi-steady state; Δα ref is the reference value of the trigger angle compensation; Switching to predictive fixed turn-off angle control during the recovery period after fault removal; The formula of predictive fixed cut-off angle control is expressed as: In the formula, γ ref is the cut-off angle reference value, I dcref is the current reference value; A reactive power control strategy of compensating the trigger angle is adopted on the basis of constant DC voltage control. The reactive power control strategy of compensating the trigger angle includes: In the event of a fault, determine U aci Is it less than U ac,γ＝0 ; where U ac,γ＝0 is the effective value of the commutation bus voltage at 0 turn-off angle in quasi-steady state, and the calculation formula is: If U aci <U ac,γ＝0 , then the trigger angle compensation Δα is input re1 , and the trigger angle compensation reference value Δα is calculated by formula (38) ref ; If the trigger angle compensation value Δα re1 >Δα ref , then invest Δα ref ; If the trigger angle compensation Δα re1 ≤Δα ref , then reinvest Δα re1 .

10. A reactive power support control system for a CLCC converter under AC fault, characterized in that: The system comprises: A capability analysis module configured to analyze the firing angle and multi-quadrant control capability of the CLCC converter; A range determination module configured to determine an operable range of a DC current and a trigger angle control range; An operation domain determination module is configured to establish a power operation domain under the constraints of the DC current operable range and the trigger angle control range based on the DC current operable range and the trigger angle control range; The control module is configured to determine the trigger angle compensation amount based on the power operation domain under the constraints of the DC current operating range and the trigger angle control range, and smoothly put the control into operation.

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