Power Output Control Method of CLCC Converter Station Based on Static Synchronous Compensator

Abstract

The present invention relates to a method for controlling the power output of a CLCC converter station based on a static synchronous compensator, including: switching the working mode of the static synchronous compensator to a steady-state control mode or a transient control mode according to the voltage at the point of common coupling and the voltage fluctuation situation within a period; in the steady-state control mode, a constant reactive power control strategy based on an outer-loop voltage and an inner-loop current is adopted and a filtering link is added for controlling the power output of the CLCC converter station, wherein the output of the inner-loop current link is filtered through the filtering link; in the transient control mode, a constant voltage control strategy based on an outer-loop voltage and an inner-loop current is adopted and a DC quantity compensation link is added for controlling the power output of the CLCC converter station, wherein the reference voltage input to the outer-loop voltage link is compensated through the DC quantity compensation link. Compared with the prior art, the present invention can effectively improve the power quality of the output of the CLCC converter station.

Description

Technical Field

[0001] The present invention relates to the technical field of DC power transmission, and particularly to a method and system for controlling the power output of a CLCC converter station. Background Art

[0002] Converter station designs often only consider filtering out characteristic harmonics, resulting in a relatively high content of non-characteristic harmonics flowing into the AC system, which affects the power quality of the AC system. According to actual measurements, in a high-voltage DC power transmission system with a 12-pulse converter, there are relatively large contents of 5th and 7th harmonics, and their sources mainly include: 1) Asymmetry of the AC system voltage; 2) Unequal trigger pulse intervals; 3) Unequal converter transformer impedances. Therefore, during normal operation, it is necessary to filter out the relatively large contents of 5th and 7th harmonics in the AC-side current of the high-voltage DC power transmission system.

[0003] On the other hand, the reactive power compensation of the converter station is designed according to the normal working mode, generally 40% - 60% of the active power. For a strong receiving-end system, capacitor compensation is used; a synchronous condenser or a static synchronous compensator STATCOM is used to compensate for the dynamic reactive power demand caused by faults. In a DC power transmission system with an LCC converter on the rectifier side and a CLCC converter on the inverter side (abbreviated as the LCC-CLCC hybrid HVDC system), its rectifier side still maintains the conventional LCC structure, while the inverter side adopts the CLCC structure. The CLCC converter structure can fundamentally solve the commutation failure problem and reduce the impact on the AC power grids on both sides. At the initial stage of a fault, the voltage drop of the commutation bus caused by the AC-side fault will still lead to a transient reactive power deficit in the system. At this time, STATCOM is still required to quickly provide reactive power support. Therefore, during an AC-side fault, STATCOM is required to quickly provide reactive power support, and it is also necessary to suppress oscillations as soon as possible after the fault disappears. Summary of the Invention

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

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

[0006] A method for controlling the power output of a CLCC converter station based on a static synchronous compensator includes:

[0007] Switching the working mode of the static synchronous compensator to a steady-state control mode or a transient control mode according to the voltage at the point of common coupling and the voltage fluctuation situation within a cycle;

[0008] Under 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. The output of the inner-loop current link is filtered through the filtering link.

[0009] Under 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 component compensation link is added to control the power output of the CLCC converter station. The reference voltage input to the outer-loop voltage link is compensated through the DC component 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, a connecting cable, and a converter transformer.

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

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

[0014] When the voltage U at the point of common coupling PCC satisfies U PCC ∈(0.95, 1.05) and the voltage fluctuation within a half-period satisfies , the working mode of the static synchronous compensator is switched to the steady-state control mode; otherwise, it is switched to the transient control mode. Here, T is the period, ΔU PCC is the voltage deviation, and pu is the per-unit value.

[0015] Preferably, under 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. The output of the inner-loop current link is filtered through the filtering link, specifically:

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

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

[0018] The d-axis voltage command U convd,ref and the q-axis voltage command U convq,ref of the static synchronous compensator are subjected to C dq-αβ transformation to obtain the phase voltage commands U convα,ref and U convβ,ref in the αβ coordinate system. The phase voltage commands U convα,ref and U convβ,ref are respectively superimposed with the selective filtering terms, and after the result of the superimposed filtering terms is subjected to C αβ-abc transformation, it is sent to the SPWM module.

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

[0020] The outlet voltage signal e abc of the CLCC converter station is input to the phase-locked loop unit to extract the fundamental angular frequency ω 1 , which is used to construct the C αβ-dq(n) transformation matrix;

[0021] The outlet current signal i abc of the CLCC converter station is subjected to C abc-αβ transformation to be transformed into two-phase currents i α , i β in the αβ coordinate system; the two-phase currents i α , i β are subjected to C αβ-dq(n) transformation to be transformed into currents i 1 in the rotating coordinate system dq(n) with a rotating speed of nω d(n) , i q(n) ;

[0022] The nth harmonic components are respectively obtained by low-pass filtering the currents i d(n) , i q(n) . After subtracting from the target currents with corresponding axes set to 0 and passing through the PI link, the voltage compensation expectations are respectively generated and then the nth harmonic voltage compensation expectations dq-αβ(n) are generated by C transformation to be respectively added to the phase voltage commands.

[0023] Preferably, the output voltage signal e of the CLCC converter station abc is input into the phase-locked loop unit to extract the fundamental angular frequency ω 1 , specifically:

[0024] Sample the output voltage signal e of the CLCC converter station abc , and after C abc-αβ transformation, the sampled phase voltage in the αβ coordinate system is obtained, where k is the sampling time;

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

[0026] Perform phase tracking on the positive sequence component to extract the fundamental angular frequency ω 1 .

[0027] Preferably, the input quantity of the DC quantity compensation link is the DC power change quantity or the 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 quantity compensation link is added to control the electric energy output of the CLCC converter station. Specifically, through the DC quantity compensation link, the reference voltage input to the outer loop voltage link is compensated, specifically:

[0029] Select the DC power change quantity ΔP dc as the input quantity in the DC quantity compensation link and output the DC quantity compensation term ΔU ref , and superimpose the DC quantity compensation term ΔU ref onto the reference voltage U ref of the static synchronous compensator to obtain the corrected reference voltage as U r ′ ef ;

[0030] Outer loop control: In the d-axis direction, set the DC voltage reference value U dcref = 1.0 pu, use the DC voltage deviation as the input, and generate the d-axis current reference I dref after PI control, where pu is the per-unit value; in the q-axis direction, subtract the corrected reference voltage U r ′ ef from the common connection point voltage, and generate the q-axis current reference I qref,U after PI control;

[0031] Inner loop control: In the d-axis direction, according to the d-axis current reference I dref , use the d-axis current deviation as the input, and after PI control, combine the output with the bias quantity to obtain the d-axis voltage command U convd,ref of the static synchronous compensator; in the q-axis direction, according to the q-axis current reference Iqref Taking the q-axis current deviation as the input, after PI control, the output is combined with the bias to obtain the q-axis voltage command U of the static synchronous compensator. convq,ref ;

[0032] For the d-axis voltage command U of the static synchronous compensator convd,ref and the q-axis voltage command U convq,ref perform a C dq-αβ transformation to obtain the phase voltage commands U convα,ref and U convβ,ref in the αβ coordinate system. For the phase voltage commands U convα,ref and U convβ,ref perform a C αβ-abc transformation to the abc coordinate system and then send them to the SPWM module.

[0033] Preferably, the DC quantity compensation term ΔU ref has the following calculation expression:

[0034] ΔU ref = k dc ΔP dc

[0035] where: k dc is the correction coefficient.

[0036] Preferably, the DC power change quantity ΔP dc is obtained by calculating the difference between the per-unit value of the measured DC power and the maximum value in the first half cycle.

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

[0038] 1) By designing the control link of the static synchronous compensator STATCOM, according to the common connection point voltage and the voltage fluctuation situation within the period, the working mode of the static synchronous compensator is switched to the steady-state control mode or the 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. Among them, the output of the inner-loop current link is filtered through the filtering link, which can utilize the maximum available capacity of the STATCOM during normal operation, and the STATCOM eliminates the non-characteristic harmonics generated by the converter station, 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 quantity compensation link is added to control the power output of the CLCC converter station. Among them, the reference voltage input to the outer-loop voltage link is compensated through the DC quantity compensation link. 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 power support for the receiving-end power grid.

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

[0040] 3) The T / 4 method is used to extract the positive and negative sequence components, and the negative sequence component interface is reserved to facilitate subsequent function expansion. 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 It is a schematic diagram of the bridge arm of a CLCC converter station;

[0043] Figure 3 It is a control block diagram of a static synchronous compensator (STATCOM);

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

[0045] Figure 5 It is a block diagram of an additional filtering link;

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

[0047] Figure 7 It is a block diagram of the structure of the measurement and sampling part;

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

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

[0050] Figure 10 It is a DC reference value selection link;

[0051] Figure 11 They are the 5th and 7th harmonics of the current fed into the power grid when the trigger angle difference is 2 degrees;

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

[0053] Figure 13 It is the reactive power fed into the AC system during a single-phase fault;

[0054] Figure 14 It is the reactive power generated by a single-phase static synchronous compensator (STATCOM);

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

[0056] Figure 16 is the reactive power fed into the AC system during a three-phase fault;

[0057] Figure 17 is the reactive power generated by the three-phase static synchronous compensator STATCOM. Specific implementation manners

[0058] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0059] Embodiment 1

[0060] This embodiment provides a method for controlling the power output of a CLCC converter station based on a static synchronous compensator. The method includes:

[0061] According to the voltage at the point of common coupling and the voltage fluctuation within a period, switch the working mode of the static synchronous compensator to the steady-state control mode or the transient control mode;

[0062] 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 through the filtering link;

[0063] 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 quantity 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 through the DC quantity compensation link.

[0064] Embodiment 2

[0065] This embodiment provides a method for controlling the power output of a CLCC converter station based on a static synchronous compensator. The method includes:

[0066] According to the voltage at the point of common coupling and the voltage fluctuation within a period, switch the working mode of the static synchronous compensator to the steady-state control mode or the transient control mode;

[0067] Under the steady-state control mode, a constant reactive power control strategy based on the outer-loop voltage and inner-loop current is adopted, and a filtering link is added to control the power output of the CLCC converter station. The output of the inner-loop current link is filtered through the filtering link.

[0068] Under the transient control mode, a constant voltage control strategy based on the outer-loop voltage and inner-loop current is adopted, and a DC component compensation link is added to control the power output of the CLCC converter station. The reference voltage input to the outer-loop voltage link is compensated through the DC component compensation link.

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

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

[0071] Figure 2 It is the basic structure of the CLCC converter. Each arm consists of a main branch and an auxiliary branch. The main branch consists of a main thyristor valve V11 and a 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 large current. The V12 sub-valve is composed of a small number of IGBTs in series with diodes, and can effectively transfer the current to the auxiliary branch through the active turn-off characteristic 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 commutation. After a predetermined delay, V14 performs active turn-off, ensuring the integrity and reliability of commutation. This topology gives full play to the high current-carrying capacity and low loss characteristics of thyristors, and at the same time combines the large current turn-off ability of IGBTs, enabling the controllable recovery of the main-branch thyristor valve and the controllable turn-off of the arm. Using this converter topology can fundamentally solve the commutation failure problem.

[0072] Figure 3 It is the design of the STATCOM system-level control logic. The STATCOM system-level control is divided into two modes: steady-state control mode and transient control mode. When the voltage U at the point of common coupling PCC satisfies U PCC ∈(0.95, 1.05) and the voltage fluctuation within half a cycle satisfies If so, switch the operating mode of the static synchronous compensator to the steady-state control mode; otherwise, switch to the transient control mode. Here, 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 at 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 the STATCOM enters the transient control mode. If so, go to Step 3; otherwise, go to Step 4;

[0075] Step 3: The STATCOM switches to the transient control mode. The d-axis control switches the DC voltage reference value to U dcref1 , and the reference current limit is taken as min{1.5, 1.5 / U d}, max{-1.5, -1.5 / U d}; the q-axis control switches to the AC voltage control, and the 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, and ΔU dc,ref is the DC system coordinated control link. The reference current limit is taken as min{1.5, 1.5 / U q}, max{-1.5, -1.5 / U q}. Specifically, as shown in Figure 6 .

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

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

[0078] I. Steady-state control mode

[0079] The steady-state control mode is the control strategy of the STATCOM during the normal operation of the system, and it has two main goals:

[0080] Goal 1: During the steady-state operation, the reactive power generated by the STATCOM is as close to zero as possible, and the purpose is to reserve as much available capacity as possible for the dynamic response of the STATCOM;

[0081] Objective 2: Filter out the non-characteristic harmonic components fed from the DC system into the receiving-end AC power grid.

[0082] The control strategy is as shown in the block Figure 4 as follows:

[0083] 1. Outer-loop control

[0084] In the d-axis direction, set the DC voltage reference value U dcref = 1.0 pu, take the DC voltage deviation as the input, and generate the d-axis current reference I after passing through a PI control ( dref regulator). Among them, the d-axis current reference I dref takes the current limit of the power transistor IGBT, ±1.5 pu, and pu is the per-unit value;

[0085] In the q-axis direction, to meet Control Objective 1, considering that capacitors are used for reactive power compensation during normal system operation, set the reactive power reference value Q ref = 0, take the reactive power deviation as the input, and generate the q-axis current reference I qref after passing through PI control;

[0086] 2. Inner-loop control

[0087] In the d-axis direction, based on the d-axis current reference I dref , take the d-axis current deviation as the input, and after passing through PI control (the PI regulator with slow adjustment Q ), combine the output with the bias to obtain the d-axis voltage command U convd,ref of the static synchronous compensator;

[0088] In the q-axis direction, based on the q-axis current reference I qref , take the q-axis current deviation as the input, and after passing through PI control, combine the output with the bias to obtain the q-axis voltage command U convq,ref of the static synchronous compensator;

[0089] 3. Additional filtering link

[0090] Perform a C convd,ref transformation on the d-axis voltage command U convq,ref and the q-axis voltage command U dq-αβ of the static synchronous compensator to obtain the phase voltage commands U convα,ref and U convβ,ref in the αβ coordinate system. Add selective filtering terms to the phase voltage commands U convα,ref and U convβ,ref respectively. After performing a C αβ-abc transformation on the result after adding the filtering terms, send it to the SPWM module.

[0091] The additional filtering link adopts an n - harmonic filtering link based on the instantaneous reactive power theory. The specific scheme is as follows Figure 5 shown, where ω 1 is the fundamental angular frequency, and the subscript (n) represents the n - th harmonic value of the electrical quantity. SPLL is a phase - locked loop, and C abc-αβ is the transformation matrix for converting the electrical quantity from the abc coordinate system to the αβ coordinate system, and C αβ-abc is the transformation matrix for converting the electrical quantity from the αβ coordinate system to the abc coordinate system, and C αβ-dq(n) is the transformation matrix for converting the electrical quantity from the αβ coordinate system to the dq0 coordinate system with nω 1 as the reference value:

[0092]

[0093]

[0094]

[0095] The specific implementation of the additional filtering link is as follows:

[0096] 1) Input the output voltage signal e abc of the CLCC converter station into the phase - locked loop unit to extract the fundamental angular frequency ω 1 for constructing the C αβ-dq(n) transformation matrix;

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

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

[0099]

[0100]

[0101] Perform phase tracking on the positive - sequence component to extract the fundamental angular frequency ω 1 .

[0102] The structure block diagram of the sampling part is as shown in Figure 7 , and the SPLL block diagram is as shown in Figure 8 .

[0103] 2) Perform C abc transformation on the output current signal i abc-αβ of the CLCC converter station to transform it into two - phase currents i α , i β in the αβ coordinate system.; For the two-phase current i α , i β perform a C αβ-dq(n) transformation to transform it into the current i 1 in the rotating coordinate system dq(n) with a rotational speed of nω d(n) , i q(n) . At this time, the nth harmonic component in the original current is transformed into the direct current i d(n) , i q(n) .

[0104] 3) Respectively perform low-pass filtering LPF on the currents i d(n) , i q(n) to obtain the nth harmonic components. After subtracting from the target current with the corresponding axis set to 0, the voltage compensation expectations are respectively generated through the PI link and then the nth harmonic voltage compensation expectations are generated through the C dq-αβ(n) transformation to be respectively added to the phase voltage commands.

[0105] II. Transient control mode

[0106] In the transient control mode, STATCOM still adopts PQ decoupled control. Considering the coordination with the DC link, an additional DC voltage change rate compensation module is added to the outer loop control. This mode corresponds to a large fluctuation in the voltage at the PCC point, and its control objective is to quickly provide reactive power support when the voltage at the PCC point drops and restore the voltage level at the PCC point as soon as possible. The control block diagram is as Figure 6 shown.

[0107] The objective of the d-axis control in the transient control mode is to keep the STATCOM system operating stably and ensure the safety of the STATCOM device itself. When the AC side voltage drops and causes the DC voltage to drop, if the reference value of U dref = 1 is still maintained, it may lead to DC current oscillation. Therefore, a DC reference value selection link is designed. When the DC voltage drops, the DC voltage reference value is synchronously lowered. The design of the DC reference value selection link is as Figure 10 shown.

[0108] 1. DC quantity compensation link

[0109] In the CLCC converter station, the selectable state variables generally include the extinction angle, DC voltage, DC current, and DC active power. Since CLCC forces commutation during a fault, the extinction angle can be specified and thus 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 candidates for DC side coordination variables.

[0110] The electrical characteristics comparison of DC voltage and DC power during a fault is shown in Table 1. Considering that it is expected that the STATCOM can provide reactive power support for the AC system as much as possible during a fault and adapt to various fault conditions, the change in DC power 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] In the formula: ΔU ref is the additional reference value of the q-axis outer loop voltage control of the STATCOM considering the coordinated influence of the DC side; U ref is the reference value of the q-axis outer loop voltage control of the original system of the STATCOM, and k dc_p is the correction coefficient.

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

[0114]

[0115] In practical applications, it is found that when the STATCOM voltage recovers to 0.95 pu after the fault disappears, the control strategy switches to the steady-state mode at this time, but there is still power oscillation in the STATCOM. To accelerate the STATCOM's recovery to the steady state, this 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 disturbance conditions in normal steady state, the DC power hardly changes and the steady-state control is not affected.

[0118] In this embodiment, considering that the STATCOM aims to quickly restore the voltage at the PCC point and increase the DC power output. 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. It can be seen from the analysis that at the beginning of the AC side voltage dip, 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 will continue to oscillate to the steady state subsequently.

[0119] To quickly increase the reactive power output of the STATCOM in the initial stage of the fault and suppress the oscillation after the fault disappears, the change in DC power ΔP is selected in the DC quantity compensation linkdc Take the input quantity and output the DC compensation term ΔU ref = k dc ΔP dc , where k dc is the correction coefficient. Add the DC compensation term ΔU ref to the reference voltage U ref of the static synchronous compensator to obtain the corrected reference voltage U r ′ ef . The change in DC power ΔP dc is obtained by calculating the difference between the per-unit value of the measured DC power and the maximum value in the first half cycle.

[0120] The change in DC power ΔP dc is obtained by calculating the difference between the per-unit value of the measured DC power and the maximum value in the first half cycle

[0121] 2. Outer loop control

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

[0123] In the q-axis control, subtract the corrected reference voltage U′ ref from the common connection point voltage, and generate the q-axis current reference I qref,U after PI control. The limit value of this DC current is taken as the current limit ±1.5 pu of the power transistor IGBT and 1.5 pu of the STATCOM capacity limit, calculated as the upper limit min{1.5, 1.5 / U d} and the lower limit max{-1.5, -1.5 / U d .

[0124] 3. Inner loop control

[0125] In the d-axis direction, based on the d-axis current reference I dref , take the d-axis current deviation as the input, and after PI control, combine the output with the bias quantity to obtain the d-axis voltage command U convd,ref of the static synchronous compensator;

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

[0127] For the d-axis voltage command U of the static synchronous compensator convd,ref and the q-axis voltage command U convq,ref perform C dq-αβ transformation to obtain the phase voltage commands U convα,ref and U convβ,ref in the αβ coordinate system. For the phase voltage commands U convα,ref and U convβ,ref perform C αβ-abc transformation to the abc coordinate system and then send them to the SPWM module.

[0128] Next, taking an actual LCC-CLC CHVDC system as a prototype, build an HVDC model and its control system in PSCAD, as Figure 9 shown, where the non-characteristic harmonic compensation link considers the 5th and 7th harmonics, and the STATCOM capacity is 300 MVar. Conduct simulation analysis on the normal operation and short-circuit conditions under rated conditions.

[0129] Case 1: Normal operation condition, the STATCOM is controlled in the steady-state control mode. Considering that the trigger angles of YY and YD transformers differ by 2°, and the additional selective filtering link considers the 5th and 7th harmonics. The DC is put into operation at 0.5 s, the STATCOM is put into operation at 1.0 s, and the additional selective filtering link is put into operation at 1.3 s.

[0130] The effective values of the 5th and 7th harmonics in the current fed into the sending-end power grid are as shown in the appendix Figure 11 shown. Before the STATCOM is put into operation, there are certain fluctuations in the harmonic current. After the STATCOM is put into operation, it has an inhibitory effect on the current fluctuations. Before the additional selective filtering link, the content of the 7th harmonic is relatively large, while the content of the 5th harmonic is relatively small. This is because the AC side system is symmetrical and there is no negative sequence component, and the 6n + 1th harmonic is generated in the positive sequence component. After the additional selective filtering link is put into operation, the effective value of the 7th harmonic drops from 0.1 kA to 0.05 kA, and the decline rate reaches 50%. Since the content of the 5th harmonic is small itself, there is no obvious change.

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

[0132] Case 2: A single-phase grounding fault occurs on the AC bus of the HVDC inverter side

[0133] A single-phase fault with a grounding inductance of 0.001 H occurs on the commutation bus of the inverter station. The fault occurs at 0.5 s and lasts for 0.1 s. Figures 12 to 14The AC bus voltages of the converter station, the active and reactive powers received by the receiving-end AC system, and the reactive power output of the STATCOM are respectively shown under the conditions without STATCOM (Scheme 1), with STATCOM (Scheme 2) + STATCOM additional DC active power variation control (Scheme 3).

[0134] Figure 12 The variation of the effective value of the AC voltage of the converter bus under three schemes is shown. It can be seen from the figure that the voltage of the AC bus of the converter transformer changes little under the three conditions. Only in the recovery period, Scheme 3 considering STATCOM recovers faster. Because CLCC will not have commutation failures, the DC system still maintains a certain power transmission during the fault period, with less reactive power demand. The bus voltage of the converter station is mainly restricted by the recovery characteristics of the strong AC system, so there is basically no change, and only shows a slightly faster recovery than the case without STATCOM compensation during the recovery period.

[0135] Figure 13 The reactive power fed into the AC system under three schemes is shown. It can be seen from the figure that without STATCOM compensation, during the fault period, due to the low bus voltage, the reactive power output by the converter station filter decreases, and power needs to be inverted from the system to the DC system. After the fault disappears, due to the rapid recovery of the bus voltage, the reactive power demand on the DC side increases, and part of the reactive power needs to be absorbed from the system. After installing STATCOM, it provides reactive power for the system during the fault period and about 0.5 seconds after the fault disappears, increasing the reactive power fed into the AC system, improving the reactive power support ability of the AC system, and contributing to the voltage stability of the AC system and the successful commutation of other LCC DCs in the system. By comparing the results with and without STATCOM additional DC quantity control, it can be clearly seen that the STATCOM additional DC quantity control scheme (Scheme 3) emits more reactive power than the scheme without additional DC quantity control (Scheme 2) during the fault period and within 0.5 seconds after the fault disappears. After the voltage recovers, the former absorbs less reactive power and can return to the steady state more quickly.

[0136] Figure 14 The reactive power output by STATCOM under three schemes is shown. It can be seen from the figure that the STATCOM additional DC quantity control scheme (Scheme 3) emits more reactive power than the scheme without additional DC quantity control (Scheme 2) during the fault period and within 0.5 seconds after the fault disappears. After the voltage recovers, the former absorbs less reactive power and can return to the steady state more quickly.

[0137] Case 3: A three-phase grounding fault occurs on the AC bus at the inverter side of the HVDC

[0138] A single-phase fault with a grounding inductance of 0.02H occurs on the converter bus of the inverter station. The fault occurs at 0.1 s and lasts for 0.1 s, that is, the fault disappears at 0.15 s.

[0139] Figures 15 to 17 Three scenarios are respectively shown: the bus voltage of the converter station, the active and reactive power fed by the DC system into the inverter-side converter station, and the simulation diagrams of the active and reactive power fed by the converter station into the system under three scenarios (Scenario 1: without STATCOM; Scenario 2: STATCOM access scheme with conventional control; Scenario 3: STATCOM access scheme with additional DC current compensation control). Compared with the results of single-phase faults, since it is a symmetrical fault and there is no negative sequence component at the receiving end during and after the fault, the volatility of the variables is much smaller.

[0140] Figure 15 The variation of the effective value of the AC voltage of the converter bus under three scenarios is shown. It can be seen from the figure that the voltage of the AC bus of the converter transformer changes little in the three cases. Only in Scenario 3 considering STATCOM during and after the fault recovery is faster. Because CLCC will not occur commutation failure, the DC system still maintains a certain power transmission during the fault, with less reactive power demand. The bus voltage of the converter station is mainly restricted by the recovery characteristics of the strong AC system, so there is basically no change, and it only shows a slightly faster recovery than the case without STATCOM compensation during and after the fault.

[0141] Figure 16 The situation of reactive power fed into the AC system under three scenarios is shown. By comparing the results with and without STATCOM additional DC current control, it can be clearly seen that the STATCOM additional DC current control scheme (Scenario 3) emits more reactive power than the scheme without additional DC current control (Scenario 2) during the fault and within 0.5 seconds after the fault disappears. After the voltage recovers, the former absorbs less reactive power and can return to the steady state more quickly.

[0142] Figure 17 The situation of reactive power emitted by STATCOM under three scenarios is shown. It can be seen from the figure that the STATCOM additional DC current control scheme (Scenario 3) emits more reactive power than the scheme without additional DC current control (Scenario 2) during the fault and within 0.5 seconds after the fault disappears. After the voltage recovers, the former absorbs less reactive power and can return to the steady state more quickly.

[0143] As described above, it is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of various equivalent modifications or substitutions, and these modifications or substitutions should all be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to 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 transient control mode, a constant voltage control strategy based on outer loop voltage and 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; 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.

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 power output of a CLCC converter station based on a static synchronous compensator according to claim 1, 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.

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 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.

6. 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.

7. A method for controlling electric energy output of a CLCC converter station based on a static synchronous compensator according to claim 6, 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.

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: The DC compensation term ΔU ref The calculation expression is: ΔU ref =k dc ΔP dc Where: k dc is the correction factor.

9. 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: 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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