CLCC converter station electric energy output control method based on static synchronous compensator

Abstract

The invention relates to a CLCC converter station electric energy output control method based on a static synchronous compensator, and the method comprises the steps: switching a working mode of the static synchronous compensator into a steady state control mode or a transient state control mode according to the voltage of a common connection point and the voltage fluctuation condition in a period; in the steady-state control mode, a constant reactive power control strategy based on outer loop voltage and inner loop current is adopted, a filtering link is added to carry out CLCC converter station electric energy output control, and filtering processing is carried out on output of an inner loop current link through the filtering link; in the transient control mode, the electric energy output control of the CLCC converter station is carried out by adopting a constant voltage control strategy based on the outer loop voltage and the inner loop current and adding a direct current compensation link, and the reference voltage input by the outer loop voltage link is compensated through the direct current compensation link. Compared with the prior art, the output electric energy quality of the CLCC converter station can be effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current transmission, and in particular to a method and system for controlling electric energy output of a CLCC converter station. Background Art

[0002] The design of converter stations often only considers filtering out characteristic harmonics, which results in a high content of non-characteristic harmonics flowing into the AC system, affecting the power quality of the AC system. According to actual measurements, in the HVDC transmission system with 12-pulse converters, there are large contents of 5th and 7th harmonics, the sources of which mainly include: 1) asymmetric AC system voltage; 2) unequal trigger pulse intervals; 3) unequal converter transformer impedances. Therefore, in normal operation, it is necessary to filter out the large contents of 5th and 7th harmonics in the AC side current of the HVDC transmission system.

[0003] On the other hand, the reactive power compensation of the converter station is designed according to the normal working mode, which is generally 40% to 60% of the active power. For the strong receiving end system, capacitor compensation is used; phase-shifting or static synchronous compensator STATCOM is used to compensate for the dynamic reactive power demand caused by the fault. The DC transmission system (referred to as LCC-CLCC hybrid HVDC system) uses LCC converters on the rectifier side and CLCC converters on the inverter side. The rectifier side still maintains the conventional LCC structure, while the inverter side adopts the CLCC structure. The CLCC converter structure can fundamentally solve the problem of commutation failure and reduce the impact on the AC power grid on both sides. In the early stage of the fault, the voltage drop of the commutation bus caused by the AC side fault will still cause the system transient reactive power shortage. At this time, STATCOM is still required to quickly provide reactive power support. Therefore, when the AC side fails, STATCOM is required to be able to quickly provide reactive power support, and it is also necessary to suppress oscillation as soon as possible after the fault disappears. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide a CLCC converter station power output control method based on a static synchronous compensator, which can effectively improve the output power quality of the CLCC converter station.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A method for controlling electric energy output of a CLCC converter station based on a static synchronous compensator, comprising:

[0007] According to the voltage at the common connection point and the voltage fluctuation within a cycle, the static synchronous compensator operating mode is switched to a steady-state control mode or a transient control mode;

[0008] In the steady-state control mode, a constant reactive power control strategy based on the outer loop voltage and the inner loop current is adopted and a filtering link is added to control the power output of the CLCC converter station, wherein the output of the inner loop current link is filtered by the filtering link;

[0009] In the transient control mode, a constant voltage control strategy based on the outer loop voltage and the inner loop current is adopted and a DC compensation link is added to control the power output of the CLCC converter station, wherein the reference voltage input to the outer loop voltage link is compensated by the DC compensation link.

[0010] Preferably, the CLCC converter station based on the static synchronous compensator is specifically:

[0011] The static synchronous compensator includes a voltage source converter VSC, a DC energy storage capacitor for providing DC voltage support, connecting cables and a converter transformer.

[0012] The inverter converts the DC voltage into AC voltage, and the static synchronous compensator is connected to the CLCC converter station bus through connecting cables and voltage source converters.

[0013] Preferably, the switching of the static synchronous compensator working mode to the steady-state control mode or the transient control mode according to the voltage of the common connection point and the voltage fluctuation within a cycle is specifically:

[0014] When the common connection point voltage U PCC Meet U PCC ∈(0.95,1.05) and the voltage fluctuation within half a cycle satisfy When , the static synchronous compensator operating mode is switched to the steady-state control mode, otherwise it is switched to the transient control mode; where T is the period, ΔU PCC is the voltage deviation and pu is the per-unit value.

[0015] Preferably, in the steady-state control mode, a constant reactive power control strategy based on the outer loop voltage and the inner loop current is adopted and a filtering link is added to control the power output of the CLCC converter station, wherein the output of the inner loop current link is filtered by the filtering link, specifically:

[0016] Outer loop control: d-axis direction, set DC voltage reference value U dcref =1.0pu, taking the DC voltage deviation as input, after PI control, the d-axis current reference I is generated dref , where pu is the per unit value; in the q-axis direction, set the reactive power reference value Q ref =0, the reactive power deviation is used as input, and the q-axis current reference I is generated after PI control qref ;

[0017] Inner loop control: d-axis direction, based on d-axis current reference I dref , taking the d-axis current deviation as input, after PI control, the output is combined with the offset to obtain the d-axis voltage command U of the static synchronous compensator convd,ref ;q-axis direction, according to the q-axis current reference I qref , taking the q-axis current deviation as input, after PI control, the output is combined with the offset to obtain the q-axis voltage command U of the static synchronous compensator convq,ref ;

[0018] The d-axis voltage command U for the static synchronous compensator convd,ref and q-axis voltage command U convq,ref Carry out C dq-αβ The phase voltage command U in the αβ coordinate system is obtained by transformation convα,ref and U convβ,ref , for the phase voltage command U convα,ref and U convβ,ref Superimpose the selective filtering items respectively, and perform C αβ-abc After transformation, it is sent to the SPWM module.

[0019] Preferably, the selective filtering item is the desired item for the nth harmonic voltage compensation, and the acquisition process is specifically as follows:

[0020] The output voltage signal of the CLCC converter station is abc Input to the phase-locked loop unit to extract the fundamental angular frequency ω1, which is used to construct C αβ-dq(n) Transformation matrix;

[0021] The output current signal i of the CLCC converter station abc Carry out C abc-αβ Transformation, transformed into the two-phase current i in the αβ coordinate system α ,i β ; For two-phase current i α ,i β Carry out C αβ-dq(n) Transformation, transformed into the current i in the rotating coordinate system dq(n) with a rotation speed of nω1 d(n) ,i q(n) ;

[0022] For the current i d(n) ,i q(n) The nth harmonic component is obtained by low-pass filtering, and the voltage compensation expectation is generated by the PI link after the difference is made with the target current set to 0 for the corresponding axis. Then by C dq-αβ(n) Transformation generates nth harmonic voltage compensation expectation To be added to the phase voltage instructions respectively.

[0023] Preferably, the outlet voltage signal e of the CLCC converter station is abc Input to the phase-locked loop unit to extract the fundamental angular frequency ω1, specifically:

[0024] The output voltage signal of CLCC converter station abc Sampling is performed through C abc-αβ After transformation, the sampled phase voltage in the αβ coordinate system is obtained, where k is the sampling time;

[0025] By using the time delay T4 method, the sampled phase voltage is expressed as the sum of the positive sequence component and the negative sequence component;

[0026] The phase of the positive sequence component is tracked and the fundamental angular frequency ω1 is extracted.

[0027] Preferably, the input quantity of the DC quantity compensation link is a DC power change or a DC voltage.

[0028] Preferably, in the transient control mode, a constant voltage control strategy based on the outer loop voltage and the inner loop current is adopted and a DC compensation link is added to control the power output of the CLCC converter station, wherein the reference voltage input to the outer loop voltage link is compensated by the DC compensation link, specifically:

[0029] Select the DC power change ΔP in the DC compensation link dc As input quantity and output DC compensation term ΔU ref , the DC compensation term ΔU ref The reference voltage U added to the static synchronous compensator ref The corrected reference voltage is U r ' ef ;

[0030] Outer loop control: d-axis direction, set DC voltage reference value U dcref =1.0pu, taking the DC voltage deviation as input, after PI control, the d-axis current reference I is generated dref , where pu is the per-unit value; in the q-axis direction, the corrected reference voltage U r ' ef The voltage difference with the common connection point is used to generate the q-axis current reference I after PI control. qref,U ;

[0031] Inner loop control: d-axis direction, based on d-axis current reference I dref , taking the d-axis current deviation as input, after PI control, the output is combined with the offset to obtain the d-axis voltage command U of the static synchronous compensator convd,ref ;q-axis direction, according to the q-axis current reference I qref, taking the q-axis current deviation as input, after PI control, the output is combined with the offset to obtain the q-axis voltage command U of the static synchronous compensator convq,ref ;

[0032] The d-axis voltage command U for the static synchronous compensator convd,ref and q-axis voltage command U convq,ref Carry out C dq-αβ The phase voltage command U in the αβ coordinate system is obtained by transformation convα,ref and U convβ,ref , for the phase voltage command U convα,ref and U convβ,ref Carry out C αβ-abc After transformation to the abc coordinate system, it is sent to the SPWM module.

[0033] Preferably, the DC compensation term ΔU ref The calculation expression is:

[0034] ΔU ref =k dc ΔP dc

[0035] Where: k dc is the correction factor.

[0036] Preferably, the DC power variation ΔP dc It is obtained by subtracting the DC power measurement per unit value from the maximum value of the first half cycle.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] 1) The present invention designs a control link of a static synchronous compensator STATCOM, and switches the working mode of the static synchronous compensator to a steady-state control mode or a transient control mode according to the voltage of the common connection point and the voltage fluctuation within a cycle: in the steady-state control mode, a constant reactive power control strategy based on the outer loop voltage and the inner loop current is adopted, and a filtering link is added to control the power output of the CLCC converter station, wherein the output of the inner loop current link is filtered by the filtering link, and the maximum available capacity of the STATCOM can be maintained during normal operation, and the STATCOM eliminates non-characteristic harmonics generated by the converter station, thereby improving the power quality of the receiving-end system during normal operation; in the transient control mode, a constant voltage control strategy based on the outer loop voltage and the inner loop current is adopted, and a DC compensation link is added to control the power output of the CLCC converter station, wherein the reference voltage input to the outer loop voltage link is compensated by the DC compensation link, and by coordinating with the DC system, the STATCOM can quickly respond to the voltage drop of the receiving-end AC power grid during a fault, and provide reactive support for the receiving-end power grid.

[0039] 2) In transient control mode, the DC power variation is used as the input of additional control. The DC power variation is calculated by subtracting the current value from the maximum value in the past half cycle to reduce DC second harmonic interference under asymmetric fault conditions.

[0040] 3) The T / 4 method is used to extract the positive-sequence and negative-sequence components, and the negative-sequence component interface is retained to facilitate subsequent function expansion. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a schematic diagram of the structure of a DC inverter station containing a static synchronous compensator STATCOM;

[0042] Figure 2 This is a schematic diagram of the bridge arm of the CLCC converter station;

[0043] Figure 3 This is the control block diagram of the static synchronous compensator STATCOM;

[0044] Figure 4 It is the control block diagram of static synchronous compensator STATCOM in steady-state control mode;

[0045] Figure 5 This is the block diagram of the additional filtering link;

[0046] Figure 6 It is the control block diagram of static synchronous compensator STATCOM in transient control mode;

[0047] Figure 7 This is the structural block diagram of the measurement sampling part;

[0048] Figure 8 It is the block diagram of SPLL;

[0049] Fig. 9 It is a schematic block diagram of the power output control of the CLCC converter station containing a static synchronous compensator STATCOM;

[0050] Fig.10 It is the DC reference value selection link;

[0051] Fig.11 The 5th and 7th harmonics of the current sent to the grid when the trigger angle differs by 2 degrees;

[0052] Fig.12 It is the effective value of the AC bus voltage of the converter transformer during a single-phase fault;

[0053] Fig.13 Send reactive power into the AC system for single-phase faults;

[0054] Fig.14 It is the reactive power generated by the single-phase static synchronous compensator STATCOM;

[0055] Fig.15 It is the effective value of the AC bus voltage of the converter transformer during a three-phase fault;

[0056] Fig.16 Send reactive power into the AC system for three-phase faults;

[0057] Fig.17 It is the reactive power generated by the three-phase static synchronous compensator STATCOM. DETAILED DESCRIPTION

[0058] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0059] Example 1

[0060] This embodiment provides a CLCC converter station power output control method based on a static synchronous compensator, the method comprising:

[0061] According to the voltage at the common connection point and the voltage fluctuation within a cycle, the static synchronous compensator operating mode is switched to a steady-state control mode or a transient control mode;

[0062] In the steady-state control mode, a constant reactive power control strategy based on the outer loop voltage and inner loop current is adopted with an additional filtering link to control the power output of the CLCC converter station, wherein the output of the inner loop current link is filtered through the filtering link;

[0063] In transient control mode, a constant voltage control strategy based on outer loop voltage and inner loop current is adopted with an additional DC compensation link to control the power output of the CLCC converter station, in which the reference voltage input to the outer loop voltage link is compensated by the DC compensation link.

[0064] Example 2

[0065] This embodiment provides a CLCC converter station power output control method based on a static synchronous compensator, the method comprising:

[0066] According to the voltage at the common connection point and the voltage fluctuation within a cycle, the static synchronous compensator operating mode is switched to a steady-state control mode or a transient control mode;

[0067] In the steady-state control mode, a constant reactive power control strategy based on the outer loop voltage and inner loop current is adopted with an additional filtering link to control the power output of the CLCC converter station, wherein the output of the inner loop current link is filtered through the filtering link;

[0068] In transient control mode, a constant voltage control strategy based on outer loop voltage and inner loop current is adopted with an additional DC compensation link to control the power output of the CLCC converter station, in which the reference voltage input to the outer loop voltage link is compensated by the DC compensation link.

[0069] Next, the method of this embodiment is introduced in detail.

[0070] Figure 1 The schematic diagram of the DC inverter station structure with a static synchronous compensator STATCOM. The static synchronous compensator STATCOM consists of a voltage source converter (VSC), a DC energy storage capacitor, a connecting cable and a converter transformer. The DC side capacitor provides DC voltage support for it, and the inverter converts the DC voltage into AC voltage. The STATCOM is connected to the grid-side bus of the CLCC converter station through connecting cables and converter transformers.

[0071] Figure 2 It is the basic structure of the CLCC converter. Each bridge arm consists of a main branch and an auxiliary branch. The main branch consists of the main thyristor valve V11 and the low-voltage IGBT valve V12. The V11 sub-valve continues the design of the traditional thyristor converter valve and has the ability to withstand high voltage and high current; the V12 sub-valve is composed of a small number of IGBTs and diodes in series, which can effectively transfer the current to the auxiliary branch through the active shutdown characteristics of the IGBT. The auxiliary branch consists of a high-voltage IGBT valve V13 and a small-current thyristor valve V14. It mainly temporarily accepts the current from the main branch during the commutation period. After a predetermined delay, V14 performs active shutdown to ensure the integrity and reliability of the commutation. This topology gives full play to the high current capacity and low loss characteristics of the thyristor, and combines the high current shutdown capability of the IGBT to achieve controllable recovery of the main branch thyristor valve and controllable shutdown of the bridge arm. The use of this converter topology can fundamentally solve the problem of commutation failure.

[0072] Figure 3 It is the design of STATCOM system-level control logic. STATCOM system-level control is divided into two types: steady-state control mode and transient control mode: when the common connection point voltage U PCC Meet U PCC ∈(0.95,1.05) and the voltage fluctuation within half a cycle satisfy When , the static synchronous compensator operating mode is switched to the steady-state control mode, otherwise it is switched to the transient control mode; where T is the period, ΔU PCC is the voltage deviation, and pu is the per-unit value. The specific steps are described as follows:

[0073] Step 1: Collect the voltage signal of the PCC point, the current signal sent by the CLCC converter station, the current signal sent by the STATCOM, and the voltage and current signals at the receiving end of the system;

[0074] Step 2: Determine whether STATCOM enters transient control mode, if yes, go to step 3; otherwise go to step 4;

[0075] Step 3: STATCOM switches to transient control mode, and the d-axis control switches the DC voltage reference value to U dcref1 , the reference current limit is min{1.5,1.5 / U d},max{-1.5,-1.5 / U d}; q-axis control switches to AC voltage control, reference quantity U PCC,ref1 =U PCC,ref +ΔU dc,ref , where U PCC,ref is the reference value obtained through the reference voltage selection link, ΔU dc,ref For the DC system coordination control link, the reference current limit is min{1.5,1.5 / U q},max{-1.5,-1.5 / U q}. Specific examples: Figure 6 shown.

[0076] Step 4: STATCOM switches to steady-state control mode, and the d-axis control switches the DC voltage reference value to U dcref , the reference current limit is 1.5; the q-axis control is switched to reactive power control, the goal is to minimize the reactive power output of STATCOM; and the additional filtering link is started at the same time. Figure 4 shown.

[0077] The control logic design in each mode is as follows:

[0078] 1. Steady-state control mode

[0079] The steady-state control mode is the control strategy of STATCOM when the system is operating normally, and it has two main goals:

[0080] Objective 1: During steady-state operation, the reactive power generated by the STATCOM is as close to zero as possible, with the aim of retaining as much available capacity as possible for the dynamic response of the STATCOM;

[0081] Objective 2: Filter out the non-characteristic harmonic components sent by the DC system to the receiving AC power grid.

[0082] Control strategy as box Figure 4 As shown, specifically:

[0083] 1. Outer loop control

[0084] In the d-axis direction, set the DC voltage reference value U dcref =1.0pu, taking the DC voltage deviation as input, through PI control ( Regulator) to generate the d-axis current reference I dref , where the d-axis current reference I dref Take the current limit of the power tube IGBT as ±1.5pu, where pu is the per-unit value;

[0085] In the q-axis direction, in order to meet the control target 1, considering that the capacitor is used for reactive power compensation when the system is working normally, the reactive power reference value Q is set ref =0, the reactive power deviation is used as input, and the q-axis current reference I is generated after PI control qref ;

[0086] 2. Inner loop control

[0087] In the d-axis direction, according to the d-axis current reference I dref , taking the d-axis current deviation as input, and controlling it through PI control (slow-speed regulator PI Q ), the output is combined with the offset to obtain the d-axis voltage command U of the static synchronous compensator. convd,ref ;

[0088] In the q-axis direction, according to the q-axis current reference I qref , taking the q-axis current deviation as input, after PI control, the output is combined with the offset to obtain the q-axis voltage command U of the static synchronous compensator convq,ref ;

[0089] 3. Additional filtering

[0090] The d-axis voltage command U for the static synchronous compensator convd,ref and q-axis voltage command U convq,ref Carry out C dq-αβ The phase voltage command U in the αβ coordinate system is obtained by transformation convα,ref and U convβ,ref , for the phase voltage command U convα,ref and U convβ,ref Superimpose the selective filtering items respectively, and perform C αβ-abc After transformation, it is sent to the SPWM module.

[0091] The additional filtering link adopts the nth harmonic filtering link based on the instantaneous reactive power theory. The specific scheme is as follows Figure 5 As shown in the figure, ω1 is the fundamental angular frequency, the subscript (n) represents the nth harmonic value of the electrical quantity, SPLL is a phase-locked loop, C abc-αβ is the transformation matrix of electrical quantity from abc coordinate system to αβ coordinate system, C αβ-abcis the transformation matrix of electrical quantity from αβ coordinate system to abc coordinate system, C αβ-dq(n) It is the transformation matrix of the electrical quantity from the αβ coordinate system to the dq0 coordinate system with nω1 as the reference value:

[0092]

[0093]

[0094]

[0095] The additional filtering link is specifically implemented as follows:

[0096] 1) The output voltage signal of the CLCC converter station is abc Input to the phase-locked loop unit to extract the fundamental angular frequency ω1, which is used to construct C αβ-dq(n) Transformation matrix;

[0097] Specifically, the output voltage signal e of the CLCC converter station abc Sampling is performed through C abc-αβ After transformation, the sampled phase voltage in the αβ coordinate system is obtained, where k is the sampling time;

[0098] By using the time delay T / 4 method, the sampled phase voltage is expressed as the sum of the positive sequence component and the negative sequence component, and the expression is:

[0099]

[0100]

[0101] The phase of the positive sequence component is tracked and the fundamental angular frequency ω1 is extracted.

[0102] The structural block diagram of the measurement sampling part is as follows Figure 7 As shown, the SPLL block diagram is as follows Figure 8 shown.

[0103] 2) The output current signal i of the CLCC converter station abc Carry out C abc-αβ Transformation, transformed into the two-phase current i in the αβ coordinate system α ,i β ; For two-phase current i α ,i β Carry out C αβ-dq(n) Transformation, transformed into the current i in the rotating coordinate system dq(n) with a rotation speed of nω1 d(n) ,i q(n) At this time, the nth harmonic component in the original current is transformed into DC i d(n) ,i q(n) .

[0104] 3) For the current i d(n) ,i q(n) The nth harmonic component is obtained by low-pass filtering LPF, and the voltage compensation expectation is generated through the PI link after the difference is made with the target current set to 0 for the corresponding axis. Then by C dq-αβ(n) Transformation generates nth harmonic voltage compensation expectation To be added to the phase voltage instructions respectively.

[0105] 2. Transient control mode

[0106] In transient control mode, STATCOM still uses PQ decoupling control, taking into account the coordination with the DC link, that is, adding an additional DC voltage change rate compensation module in the outer loop control. This mode corresponds to a large fluctuation in the PCC point voltage, and its control goal is to quickly provide reactive support when the PCC point voltage drops and restore the PCC point voltage level as soon as possible. The control block diagram is shown in the figure. Figure 6 shown.

[0107] The goal of d-axis control in transient control mode is to keep the STATCOM system working stably and ensure the safety of the STATCOM equipment itself. dref =1 may cause DC current oscillation, so the DC reference value selection link is designed. When the DC voltage drops, the DC voltage reference value is lowered synchronously. The DC reference value selection link is designed as follows: Fig.10 shown.

[0108] 1. DC compensation link

[0109] In the CLCC converter station, the selectable state variables generally include the arc extinction angle, DC voltage, DC current and DC active power. Since CLCC forces commutation during a fault, the arc extinction angle can be specified and therefore cannot be used as a coordination variable. The DC current first rises and then falls, and its change cannot intuitively reflect the reactive power demand of the AC system. The changes in DC voltage and DC active power are consistent with the reactive power demand of the AC system and can be used as alternatives to the DC side coordination variables.

[0110] The comparison of electrical characteristics of DC voltage and DC power during fault is shown in Table 1. Considering that STATCOM is expected to provide reactive support to the AC system as much as possible during fault and adapt to various fault conditions, the DC power change is used as the input of the additional input link, that is:

[0111] ΔU ref =k dc_p ΔP dc , U r ' ef =U ref -ΔU ref(6)

[0112] Where: ΔU ref It is the additional reference value for the q-axis outer loop voltage control of STATCOM considering the influence of the coordinated DC side; U ref is the reference value of the q-axis outer loop voltage control of the original STATCOM system, k dc_p is the correction factor.

[0113] Table 1 Comparison of DC voltage and DC power characteristics during fault period

[0114]

[0115] In actual applications, it is found that when the STATCOM voltage recovers to 0.95pu after the fault disappears, the control strategy switches to the steady-state mode, but the STATCOM still has power oscillation. In order to speed up the recovery of STATCOM to steady state, the additional strategy is considered to be applied to the steady-state strategy.

[0116] ΔQ ref = k′ dc_p ΔP dc , Q′ ref =Q ref -ΔQ ref (7)

[0117] Under small disturbances in normal steady state, the DC power basically does not change and the steady-state control is not affected.

[0118] In this embodiment, the goal of STATCOM is to quickly restore the PCC point voltage and increase the DC transmission power. The DC voltage of the converter station is affected by the AC system voltage, but changes faster than the AC voltage, which directly affects the DC current and DC power. Analysis shows that when the AC side voltage drops, the DC active power drops rapidly, and then the change slows down. After the fault disappears, the DC active power quickly recovers to the initial value, and then continues to oscillate to a steady state.

[0119] In order to quickly increase the reactive power output of STATCOM in the early stage of fault and suppress oscillation after the fault disappears, the DC power change ΔP is selected in the DC power compensation link. dc As input quantity and output DC compensation term ΔU ref =k dc ΔP dc , k dc As the correction coefficient, the DC compensation term ΔU ref The reference voltage U added to the static synchronous compensator ref The corrected reference voltage is U r ' ef . DC power change ΔP dcIt is obtained by subtracting the DC power measurement per unit value from the maximum value of the first half cycle.

[0120] DC power change ΔP dc The DC power measurement standard value is calculated by subtracting the maximum value of the first half cycle.

[0121] 2. Outer loop control

[0122] In the d-axis direction, set the DC voltage reference value U dcref =1.0pu, taking the DC voltage deviation as input, after PI control, the d-axis current reference I is generated dref The DC current limit is the 1.5pu current limit of the power tube IGBT and the 1.5pu STATCOM capacity limit, which is calculated as the upper limit min{1.5,1.5 / U q}, lower limit max{-1.5,-1.5 / U q}, where pu is the per-unit value;

[0123] In terms of q-axis control, the corrected reference voltage U′ ref The voltage difference with the common connection point is used to generate the q-axis current reference I after PI control. qref,U The DC current limit is the current limit of the power tube IGBT ±1.5pu and the 1.5pu STATCOM capacity limit, and the upper limit min{1.5,1.5 / U d}, lower limit max{-1.5,-1.5 / U d}.

[0124] 3. Inner loop control

[0125] In the d-axis direction, according to the d-axis current reference I dref , taking the d-axis current deviation as input, after PI control, the output is combined with the offset to obtain the d-axis voltage command U of the static synchronous compensator convd,ref ;

[0126] In the q-axis direction, according to the q-axis current reference I qref , taking the q-axis current deviation as input, after PI control, the output is combined with the offset to obtain the q-axis voltage command U of the static synchronous compensator convq,ref ;

[0127] The d-axis voltage command U for the static synchronous compensator convd,ref and q-axis voltage command U convq,ref Carry out C dq-αβ The phase voltage command U in the αβ coordinate system is obtained by transformation convα,ref and U convβ,ref , for the phase voltage command U convα,ref and U convβ,ref Carry out Cαβ-abc After transformation to the abc coordinate system, it is sent to the SPWM module.

[0128] Next, taking an actual LCC-CLCCHVDC system as a prototype, a HVDC model and its control system are built in PSCAD. Fig. 9 As shown in the figure, the non-characteristic harmonic compensation link considers the 5th and 7th harmonics, and the STATCOM capacity is 300MVar. The normal operation and short circuit conditions under rated conditions are simulated and analyzed.

[0129] Case 1: Under normal operating conditions, STATCOM adopts steady-state control mode. Considering that the trigger angle of YY and YD transformers differs by 2°, the 5th and 7th harmonics are considered in the additional selective filtering link. 0.5 seconds to put DC into operation, 1.0 seconds to put STATCOM into operation, and 1.3 seconds to put the additional selective filtering link into operation.

[0130] The effective values ​​of the 5th and 7th harmonics in the current sent to the power grid at the sending end are shown in the attached figure. Fig.11 As shown. Before STACOM was put into operation, the harmonic current had a certain fluctuation, and after STATCOM was put into operation, the current fluctuation was suppressed. Before the additional selective filtering link, the 7th harmonic content was large, while the 5th harmonic content was small. This is because the AC side system is symmetrical, there is no negative sequence component, and 6n+1 harmonics are generated in the positive sequence component. After the additional selective filtering link was put into operation, the effective value of the 7th harmonic decreased by 50% from 0.1kA to 0.05kA. The 5th harmonic did not change significantly because of its small content.

[0131] It can be seen that STATCOM with additional selective filtering can specifically filter out non-characteristic harmonics with large contents in itself and the system.

[0132] Case 2: Single-phase ground fault on the AC busbar on the HVDC inverter side

[0133] A single-phase fault with a grounding inductance of 0.001H occurred in the inverter station commutation bus. The fault occurred in 0.5 seconds and lasted for 0.1 seconds. Figure 12 to Figure 14 The bus voltage of the converter station, the active power and reactive power received by the receiving AC system, and the reactive power output of STATCOM are shown respectively without STATCOM (Scheme 1), with STATCOM (Scheme 2) + STATCOM additional DC active power variation control (Scheme 3).

[0134] Fig.12The changes in the effective value of the AC voltage of the converter bus under the three schemes are shown. As can be seen from the figure, the voltage of the converter AC bus does not change much in the three cases, and the scheme 3 that considers STATCOM only during the recovery period recovers faster. Because CLCC will not have commutation failure, the DC system still maintains a certain power transmission during the fault, and the reactive power demand is small. The bus voltage of the converter station is mainly subject to the recovery characteristics of the strong AC system, so there is basically no change, and it only shows a slightly faster recovery period than the case without STATCOM compensation.

[0135] Fig.13 The reactive power delivered to the AC system under three schemes is shown. As can be seen from the figure, without STATCOM compensation, the reactive power delivered by the converter station filter is reduced due to the low bus voltage during the fault period, and power needs to be reversed from the system to the DC system. After the fault disappears, the reactive demand on the DC side increases due to the rapid recovery of the bus voltage, and some reactive power needs to be absorbed from the system. After the installation of STATCOM, reactive power is provided to the system during the fault period and about 0.5 seconds after the fault disappears, which increases the reactive power delivered to the AC system, improves the reactive support capacity of the AC system, and contributes to the voltage stability of the AC system and the successful DC commutation of other LCCs in the system. Comparing the results with and without STATCOM additional DC control, it can be clearly seen that the reactive power emitted by the STATCOM additional DC control scheme (Scheme 3) during the fault period and within 0.5 seconds after the fault disappears is greater than that of the non-additional DC control scheme (Scheme 2). After the voltage is restored, the former absorbs less reactive power and can return to steady state more quickly.

[0136] Fig.14 The reactive power generated by STATCOM under three schemes is shown. It can be seen from the figure that the reactive power generated by the STATCOM additional DC control scheme (Scheme 3) during the fault period and within 0.5 seconds after the fault disappears is greater than that of the non-additional DC control scheme (Scheme 2). After the voltage is restored, the former absorbs less reactive power and can return to steady state more quickly.

[0137] Case 3: Three-phase grounding fault on the AC busbar on the HVDC inverter side

[0138] A single-phase fault with a grounding inductance of 0.02H occurred in the inverter station commutation bus. The fault occurred in 0.1 second and lasted for 0.1 second, that is, the fault disappeared in 0.15 seconds.

[0139] Figure 15 to Figure 17The bus voltage of the converter station, the active and reactive power sent by the DC system to the inverter-side converter station, and the simulation diagram of the active and reactive power sent by the converter station to the system are shown in three schemes: (without STATCOM (Scheme 1), STATCOM access scheme with conventional control (Scheme 2), and STATCOM access scheme with additional DC compensation control (Scheme 3). Compared with the results of single-phase fault, since it is a symmetrical fault, there is no negative sequence component at the receiving end during and after the fault, so the volatility of the variables is much smaller.

[0140] Fig.15 The changes in the effective value of the AC voltage of the converter bus under the three schemes are shown. As can be seen from the figure, the voltage of the converter transformer AC bus does not change much in the three cases. Scheme 3, which only considers STATCOM during the fault and recovery period, recovers faster. Because CLCC will not have commutation failure, the DC system still maintains a certain power transmission during the fault, with low reactive power demand. The bus voltage of the converter station is mainly subject to the recovery characteristics of the strong AC system, so there is basically no change, and it is only slightly faster than the case without STATCOM compensation during the fault and recovery period.

[0141] Fig.16 The reactive power delivered to the AC system under the three schemes is shown. Comparing the results with and without STATCOM additional DC control, it can be clearly seen that the reactive power generated by the STATCOM additional DC control scheme (Scheme 3) during the fault period and within 0.5 seconds after the fault disappears is greater than that of the scheme without additional DC control (Scheme 2). After the voltage is restored, the former absorbs less reactive power and can return to steady state more quickly.

[0142] Fig.17 The reactive power generated by STATCOM under three schemes is shown. It can be seen from the figure that the reactive power generated by the STATCOM additional DC control scheme (Scheme 3) during the fault period and within 0.5 seconds after the fault disappears is greater than that of the non-additional DC control scheme (Scheme 2). After the voltage is restored, the former absorbs less reactive power and can return to steady state more quickly.

[0143] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed by the present invention, and these modifications or replacements should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A method for controlling electric energy output of a CLCC converter station based on a static synchronous compensator, characterized in that: include: According to the voltage at the common connection point and the voltage fluctuation within a cycle, the static synchronous compensator operating mode is switched to a steady-state control mode or a transient control mode; In the steady-state control mode, a constant reactive power control strategy based on the outer loop voltage and the inner loop current is adopted and a filtering link is added to control the power output of the CLCC converter station, wherein the output of the inner loop current link is filtered by the filtering link; In the transient control mode, a constant voltage control strategy based on the outer loop voltage and the inner loop current is adopted and a DC compensation link is added to control the power output of the CLCC converter station, wherein the reference voltage input to the outer loop voltage link is compensated by the DC compensation link.

2. A method for controlling electric energy output of a CLCC converter station based on a static synchronous compensator according to claim 1, characterized in that: The CLCC converter station based on static synchronous compensator is specifically: The static synchronous compensator includes a voltage source converter VSC, a DC energy storage capacitor for providing DC voltage support, a connecting cable, and a converter transformer; The inverter converts the DC voltage into AC voltage, and the static synchronous compensator is connected to the CLCC converter station bus through connecting cables and voltage source converters.

3. The method for controlling electric energy output of a CLCC converter station based on a static synchronous compensator according to claim 1, characterized in that: The switching of the static synchronous compensator working mode to the steady-state control mode or the transient control mode according to the voltage of the common connection point and the voltage fluctuation within the cycle is specifically: When the common connection point voltage U PCC Meet U PCC ∈(0.95,1.05) and the voltage fluctuation within half a cycle satisfy When , the static synchronous compensator operating mode is switched to the steady-state control mode, otherwise it is switched to the transient control mode; where T is the period, ΔU PCC is the voltage deviation and pu is the per-unit value.

4. The method for controlling electric energy output of a CLCC converter station based on a static synchronous compensator according to claim 1, characterized in that: In the steady-state control mode, a constant reactive power control strategy based on the outer loop voltage and the inner loop current is adopted and a filtering link is added to control the power output of the CLCC converter station, wherein the output of the inner loop current link is filtered by the filtering link, specifically: Outer loop control: d-axis direction, set DC voltage reference value U dcref =1.0pu, taking the DC voltage deviation as input, after PI control, the d-axis current reference I is generated dref , where pu is the per unit value; in the q-axis direction, set the reactive power reference value Q ref =0, the reactive power deviation is used as input, and the q-axis current reference I is generated after PI control qref ; Inner loop control: d-axis direction, based on d-axis current reference I dref , taking the d-axis current deviation as input, after PI control, the output is combined with the offset to obtain the d-axis voltage command U of the static synchronous compensator convd,ref ;q-axis direction, according to the q-axis current reference I qref , taking the q-axis current deviation as input, after PI control, the output is combined with the offset to obtain the q-axis voltage command U of the static synchronous compensator convq,ref ; The d-axis voltage command U for the static synchronous compensator convd,ref and q-axis voltage command U convq,ref Carry out C dq-αβ The phase voltage command U in the αβ coordinate system is obtained by transformation convα,ref and U convβ,ref , for phase voltage command U convα,ref and U convβ,ref Superimpose the selective filtering items respectively, and perform C αβ-abc After transformation, it is sent to the SPWM module.

5. The method for controlling electric energy output of a CLCC converter station based on a static synchronous compensator according to claim 4, characterized in that: The selective filtering term is the desired term for the nth harmonic voltage compensation, and the acquisition process is specifically as follows: The output voltage signal of the CLCC converter station is abc Input to the phase-locked loop unit to extract the fundamental angular frequency ω1, which is used to construct C αβ-dq(n) Transformation matrix; The output current signal i of the CLCC converter station abc Carry out C abc-αβ Transformation, transformed into the two-phase current i in the αβ coordinate system α ,i β ; For two-phase current i α ,i β Carry out C αβ-dq(n) Transformation, transformed into the current i in the rotating coordinate system dq(n) with a rotation speed of nω1 d(n) ,i q(n) ; For the current i d(n) ,i q(n) The nth harmonic component is obtained by low-pass filtering, and the voltage compensation expectation is generated by the PI link after the difference is made with the target current set to 0 for the corresponding axis. Then by C dq-αβ(n) Transformation generates nth harmonic voltage compensation expectation To be added to the phase voltage instructions respectively.

6. A method for controlling electric energy output of a CLCC converter station based on a static synchronous compensator according to claim 5, characterized in that: The outlet voltage signal of the CLCC converter station is abc Input to the phase-locked loop unit to extract the fundamental angular frequency ω1, specifically: The output voltage signal of CLCC converter station abc Sampling is performed through C abc-αβ After transformation, the sampled phase voltage in the αβ coordinate system is obtained, where k is the sampling time; By using the time delay T / 4 method, the sampled phase voltage is expressed as the sum of the positive sequence component and the negative sequence component; The phase of the positive sequence component is tracked and the fundamental angular frequency ω1 is extracted.

7. The method for controlling electric energy output of a CLCC converter station based on a static synchronous compensator according to claim 1, characterized in that: The input quantity of the DC compensation link is the DC power change or the DC voltage.

8. The method for controlling electric energy output of a CLCC converter station based on a static synchronous compensator according to claim 7, characterized in that: In the transient control mode, a constant voltage control strategy based on the outer loop voltage and the inner loop current is adopted and a DC compensation link is added to control the power output of the CLCC converter station, wherein the reference voltage input to the outer loop voltage link is compensated by the DC compensation link, specifically: Select the DC power change ΔP in the DC compensation link dc As input quantity and output DC compensation term ΔU ref , the DC compensation term ΔU ref The reference voltage U added to the static synchronous compensator ref The corrected reference voltage is U r ' ef ; Outer loop control: d-axis direction, set DC voltage reference value U dcref =1.0pu, taking the DC voltage deviation as input, after PI control, the d-axis current reference I is generated dref , where pu is the per-unit value; in the q-axis direction, the corrected reference voltage U r ' ef The voltage difference with the common connection point is used to generate the q-axis current reference I after PI control. qref,U ; Inner loop control: d-axis direction, based on d-axis current reference I dref , taking the d-axis current deviation as input, after PI control, the output is combined with the offset to obtain the d-axis voltage command U of the static synchronous compensator convd,ref ;q-axis direction, according to the q-axis current reference I qref , taking the q-axis current deviation as input, after PI control, the output is combined with the offset to obtain the q-axis voltage command U of the static synchronous compensator convq,ref ; The d-axis voltage command U for the static synchronous compensator convd,ref and q-axis voltage command U convq,ref Carry out C dq-αβ The phase voltage command U in the αβ coordinate system is obtained by transformation convα,ref and U convβ,ref , for phase voltage command U convα,ref and U convβ,ref Carry out C αβ-abc After transformation to the abc coordinate system, it is sent to the SPWM module.

9. A method for controlling electric energy output of a CLCC converter station based on a static synchronous compensator according to claim 8, characterized in that: The DC compensation term ΔU ref The calculation expression is: ΔU ref =k dc ΔP dc Where: k dc is the correction factor.

10. The method for controlling electric energy output of a CLCC converter station based on a static synchronous compensator according to claim 8, characterized in that: The DC power variation ΔP dc It is obtained by subtracting the DC power measurement per unit value from the maximum value of the first half cycle.

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