A Hybrid Grid-Commutated Converter and Its Main Circuit Calculation Method

Abstract

The present invention relates to a hybrid line-commutated converter device and a main circuit calculation method thereof, including: determining equipment selection parameters according to the control parameters of the hybrid line-commutated converter device and the parameters of the converter transformer; calculating the reactive power consumption of the hybrid line-commutated converter device when the reactive power output of the converter valve in the chain-type STATCOM structure is 0; based on the determined equipment selection parameters of the hybrid converter valve and the reactive power consumption of the hybrid line-commutated converter device, respectively calculating the reactive power consumption of the six-pulse converter valve and the reactive power consumption of the converter transformer under the rated condition when the chain-type STATCOM structure provides corresponding reactive power. The present invention can realize the feeding of new energy islands, does not require additional configuration of passive filters, can realize the functions of reactive power and harmonic self-compensation, and greatly reduces the floor area of the converter station. Therefore, the present invention can be widely applied to the field of high-voltage direct current transmission technology.

Description

Technical Field

[0001] The present invention belongs to the technical field of power transmission systems, and particularly relates to a hybrid line-commutated converter device and a main circuit calculation method thereof, especially a hybrid line-commutated converter device combining semi-controlled devices and fully-controlled devices and a main circuit calculation method thereof. Background Art

[0002] High-voltage direct current (HVDC) transmission technology plays an irreplaceable role in long-distance and large-capacity power transmission in China. It is a model of innovation in China's basic industries. More than thirty HVDC transmission projects have been built and put into operation in China, leading comprehensively in aspects such as high-voltage technology, equipment, standards, and projects. With the proposal of the national dual-carbon goal, as one of the main ways to consume clean energy, HVDC transmission technology will effectively promote the green transformation of the economy.

[0003] However, there are still many problems in the commutation process of traditional thyristor converters (such as LCC, line-commutated converter), for example:

[0004] (1) Thyristor converters must rely on the AC system voltage for commutation during commutation, which is not conducive to the large-scale feeding of new energy islands.

[0005] (2) While providing active power, thyristor converters also consume a large amount of reactive power (accounting for 40%-60% of the DC transmission power), and a large number of reactive power compensation devices need to be installed in the converter station.

[0006] (3) For the AC system, thyristor converters are harmonic current sources. In order to reduce the harmonic current injected into the AC system, AC filters must be installed. Therefore, in the typical configuration of converter stations in HVDC transmission projects, the number of passive filters and parallel SCs is large, and the floor area is large. Summary of the Invention

[0007] Aiming at the above problems, the purpose of the present invention is to provide a hybrid line-commutated converter device and a main circuit calculation method thereof, which combine semi-controlled devices and fully-controlled devices to replace the existing thyristor converters, enabling the line-commutated converter valve to be independent of the AC system, realizing the feeding of new energy islands, not requiring additional passive filters, and being able to achieve the functions of reactive power and harmonic self-compensation, greatly reducing the floor area of the converter station.

[0008] To achieve the above purpose, the present invention adopts the following technical solutions:

[0009] The first aspect of the present invention is to provide a hybrid line-commutated converter device, which includes: a converter transformer and a hybrid converter valve; the hybrid converter valve includes a fully controlled converter valve and a semi-controlled converter valve connected in parallel, and the phase connection terminals of the semi-controlled converter valve and the phase connection terminals of the fully controlled converter valve are respectively connected to the three phases of the converter transformer, and the converter transformer is connected to the AC system.

[0010] Preferably, the semi-controlled converter valve adopts a six-pulse converter valve, the six-pulse converter valve adopts a three-phase bridge converter structure, and each phase converter bridge includes two half-bridges, upper and lower, with a total of six bridge arms; a valve string is provided on each of the bridge arms.

[0011] Preferably, each of the valve strings adopts a semi-controlled thyristor valve string.

[0012] Preferably, the fully controlled converter valve adopts a VSC converter valve, and the VSC converter valve adopts at least one of a two-level, three-level, and modular multilevel converter valve or a chain-type STATCOM structure converter valve.

[0013] The second aspect of the present invention is to provide a method for calculating the main circuit parameters of a hybrid line-commutated converter device, including the following steps:

[0014] Determine the equipment selection parameters according to the control parameters of the hybrid line-commutated converter device and the parameters of the converter transformer;

[0015] Calculate the reactive power consumption of the hybrid line-commutated converter device when the reactive power output of the chain-type STATCOM structure converter valve is 0;

[0016] Based on the determined equipment selection parameters of the hybrid converter valve and the reactive power consumption of the hybrid line-commutated converter device, calculate the reactive power consumption of the six-pulse converter valve and the reactive power consumption of the converter transformer under rated conditions when the chain-type STATCOM structure provides the corresponding reactive power.

[0017] Preferably, the equipment selection parameters include the parallel reactance resistance value of the chain-type STATCOM structure converter valve in the hybrid converter valve, the number of sub-modules of the chain-type STATCOM structure converter valve, and the capacitance value of the sub-modules.

[0018] Preferably, when the reactive power output of the chain-type STATCOM structure converter valve is 0, the calculation formula for the reactive power consumption of the hybrid line-commutated converter device is:

[0019]

[0020] In the formula, Q conv is the reactive power consumption of the hybrid line-commutated converter device, U di0is the ideal no-load current; I d is the DC current; α is the firing angle, and μ is the commutation angle.

[0021] Preferably, the method for calculating the reactive power consumption of the six-pulse converter valve and the reactive power consumption of the converter transformer under rated conditions when the chain-type STATCOM structure provides corresponding reactive power based on the determined equipment selection parameters of the hybrid converter valve and the reactive power consumption of the hybrid line-commutated converter device includes: calculating the reactive power consumption of the hybrid line-commutated converter device based on the commutation angle when the chain-type STATCOM structure converter valve provides corresponding reactive power; performing vector analysis on the AC system to obtain the relationship between the valve-side phase voltage and the valve-side phase current and the power factor of the AC system; calculating the reactive power consumption of the converter transformer under rated conditions based on the relationship between the valve-side phase voltage and the valve-side phase current and the power factor of the AC system; calculating the reactive power consumption of the six-pulse converter valve based on the total reactive power consumption of the converter device, the reactive power consumption of the converter transformer, and the reactive power provided by the chain-type STATCOM structure converter valve.

[0022] Preferably, the calculation formula for the commutation angle μ1 is:

[0023]

[0024] where μ1 is the commutation angle when the reactive power output of the chain-type STATCOM structure converter valve is not 0, α is the firing angle, U di0N is the rated ideal no-load DC voltage, L apf is the shunt reactance of the chain-type STATCOM structure converter valve, w is the angular frequency, I s is the line current on the network side.

[0025] Preferably, the method for calculating the reactive power consumption of the converter transformer under rated conditions based on the relationship between the valve-side phase voltage and the valve-side phase current and the power factor of the AC system includes: iteratively calculating the AC system phase current I sa , and calculating the reactive power consumption of the converter transformer according to the AC system phase current I sa .

[0026] Due to the adoption of the above technical solutions, the present invention has the following advantages:

[0027] By connecting a fully-controlled commutation device in parallel with a semi-controlled commutation device, the present invention solves the problems of traditional thyristor converters that require a large number of passive filters and rely on the AC system voltage. It can replace the existing thyristor converters, eliminate the need for additional passive filters, achieve large-scale feeding of new energy into the island, and realize self-compensation of reactive power and harmonics, greatly reducing the floor area of the converter station. The present invention can be widely applied in the field of high-voltage DC transmission technology. Description of the Drawings

[0028] Figure 1 is a schematic diagram of the topology structure of the hybrid line-commutated converter valve provided by an embodiment of the present invention;

[0029] Figure 2 is a schematic diagram of the equivalent circuit of the hybrid line-commutated converter valve provided by an embodiment of the present invention;

[0030] Figure 3 is the vector relationship between the system voltage and the valve-side current provided by an embodiment of the present invention;

[0031] Figure 4 is the fundamental power factor angle of the commutation device provided by an embodiment of the present invention;

[0032] Figure 5 is the valve-side commutation process provided by an embodiment of the present invention;

[0033] Figure 6 are the waveforms of various parameters provided by an embodiment of the present invention;

[0034] Figure 7 is the valve-side current provided by an embodiment of the present invention;

[0035] Figure 8 are the waveforms of various parameters provided by an embodiment of the present invention. Detailed Embodiments

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings of 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 described embodiments of the present invention fall within the scope of protection of the present invention.

[0037] In the description of the present invention, it should be understood that the orientation or positional relationships indicated by the terms "upper", "lower", "inner", "outer", etc. are based on the orientation or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation of the present invention.

[0038] Embodiment 1

[0039] This embodiment provides a hybrid line-commutated converter device, which includes: a converter transformer and a hybrid converter valve. Among them, the hybrid converter valve includes a fully controlled converter valve and a semi-controlled converter valve connected in parallel, and the phase connection terminals of the semi-controlled converter valve and the phase connection terminals of the fully controlled converter valve are respectively connected to the three phases of the converter transformer, and the converter transformer is connected to the AC system.

[0040] Preferably, the semi-controlled converter valve adopts a six-pulse converter valve, and the six-pulse converter valve adopts a three-phase bridge converter bridge structure. Each phase converter bridge includes two half-bridges, the upper and the lower, with a total of six bridge arms; a valve string is arranged on each bridge arm.

[0041] Preferably, the valve string adopts a semi-controlled thyristor valve string.

[0042] Preferably, the fully controlled converter valve adopts a VSC converter valve. The VSC converter valve can adopt a two-level, three-level, and modular multilevel (MMC) converter valve, or can also adopt a converter valve with a chain-type STATCOM (Static Synchronous Compensator) structure.

[0043] Embodiment 2

[0044] As Figure 1 shown, it is a structural schematic diagram of a hybrid line-commutated converter device provided by this embodiment. The converter device includes a six-pulse converter valve and a VSC converter valve. Among them, the VSC converter valve adopts a converter valve with a chain-type STATCOM structure. The six-pulse converter valve is a three-phase bridge converter bridge. Each phase converter bridge includes two half-bridges, the upper and the lower, which are composed of a total of six bridge arms. A semi-controlled thyristor valve string is arranged on each bridge arm. Among them, the valve strings T1 and T4, T3 and T6, and T2 and T5 respectively form three arm pairs. The center terminals of each arm pair are correspondingly connected to the A, B, and C phases of the converter transformer; the VSC converter valve, that is, the converter valve with a chain-type STATCOM structure, is connected to the AC system in a directly hung manner, that is, the phases of the converter valve with a chain-type STATCOM structure are directly connected to the center terminals of each arm pair of the six-pulse converter valve. In the figure, n is the grounding point on the grid side; a, b, and c are the connection points of the grid-side A, B, and C phases with the three phases at the outlet of the chain-type STATCOM converter valve; A', B', and C' are the connection points of the grid-side A, B, and C phases with the center terminals of each arm pair of the six-pulse converter valve.

[0045] As Figure 2 shown, it is Figure 1 the equivalent circuit diagram of the hybrid line-commutated converter device in Figure 2 In sa U sb Usc are the voltages of phases A, B, and C of the commutation transformer; I sa , I sb , I sc are the line currents at the outlets of phases A, B, and C of the commutation transformer; I va , I vb , I vc are the line currents on the valve side at the outlets of phases A, B, and C of the commutation transformer; L d is the reactance value of the smoothing reactor; P is the DC power; I d is the DC current; U d is the DC voltage; I apfa , I apfb , I apfc are respectively the currents of phases A, B, and C of the commutation transformer at the outlet of the commutation valve of the chain - type STATCOM structure; U apfa , U apfb , U apfc are the voltages of phases A, B, and C of the commutation transformer at the outlet of the chain - type STATCOM structure.

[0046] The working process of the hybrid commutation valve is analyzed below. Its working process includes commutation and non - commutation processes. Taking phases A and B as examples, it is introduced as follows:

[0047] (1) Non - commutation process

[0048] In the non - commutation process, only the fundamental wave is considered. When phases A and B are operating, there are:

[0049]

[0050] In the formula, L r is the equivalent reactance of the commutation transformer, and L apf is the shunt reactance of the commutation valve of the chain - type STATCOM structure.

[0051] (2) Commutation process

[0052] In the commutation process, only the fundamental wave is considered. When phase A commutes to phase B, if the harmonic compensation function of the STATCOM structure is not considered, there are:

[0053]

[0054]

[0055]

[0056] Based on formula (4), when the reactive power output of the commutation valve of the chain - type STATCOM structure is 0, the above formulas (2) - (4) can be converted to:

[0057]

[0058] (U sa -U sb )dwt = 2L(I d -0) (6)

[0059]

[0060] Wherein, I d is the direct current, w is the angular frequency, t represents time, α is the firing angle, and μ is the commutation angle.

[0061] When the reactive power output of the cascaded STATCOM structure is not 0, there is:

[0062]

[0063] Wherein, I apfa0 and I apfa are respectively the currents of phase A of the converter transformer at the outlet of the converter valve of the cascaded STATCOM structure before and after commutation; I apfb0 and I apfb are respectively the currents of phase B of the converter transformer at the outlet of the converter valve of the cascaded STATCOM structure before and after commutation.

[0064] When phase A commutes to phase B, the voltage drop generated by phase A due to the commutation reactance causes the voltage of phase A at the connection point of the converter valve of the cascaded STATCOM structure to decrease. The cascaded STATCOM structure is equivalent to a voltage source, and the current output I apfa will increase, that is, I apfa -I apfa0 > 0; Due to the voltage drop generated by phase B due to the commutation reactance, the voltage of phase B at the connection point of the cascaded STATCOM structure increases. The cascaded STATCOM structure is equivalent to a three-phase voltage source, and the current output I apfb will decrease, that is, I apfb -I apfb0 < 0; Therefore, there is:

[0065] ΔU = L(I apfa -I apfa0 ) + L(I apfb -I apfb0 ) > 0 (9)

[0066]

[0067] Wherein, ΔU is the voltage drop caused by the commutation reactance.

[0068] Since the value on the right side decreases, the corresponding commutation angle μ decreases, that is, after the converter valve of the parallel cascaded STATCOM structure, the commutation angle during the commutation process will be reduced.

[0069] In summary, after the commutation valve of the shunt-connected chain STATCOM structure, the commutation angle will be reduced during the commutation process, thereby reducing the reactive power consumption of the LCC thyristor converter. At the same time, the shunt-connected chain STATCOM structure can provide reactive power compensation, and there is no need to set up passive filters.

[0070] When the VSC converter is an MMC converter, the commutation of the MMC converter can be directly carried out, and the commutation of the MMC converter does not depend on the AC system.

[0071] Embodiment 3

[0072] Based on the hybrid grid-commutated converter device provided in the above Embodiment 1, this embodiment provides a main circuit calculation method for the hybrid grid-commutated converter device, which specifically includes the following steps:

[0073] Step 1: Determine the equipment selection parameters according to the control parameters of the hybrid grid-commutated converter device and the parameters of the commutation transformer;

[0074] Step 2: Calculate the reactive power consumption of the hybrid grid-commutated converter device when the reactive power output of the commutation valve in the chain STATCOM structure is 0;

[0075] Step 3: Based on the equipment selection parameters of the hybrid commutation valve determined in Step 1 and the reactive power consumption of the hybrid grid-commutated converter device in Step 2, calculate the reactive power consumption of the six-pulse commutation valve and the reactive power consumption of the commutation transformer under rated conditions when the chain STATCOM structure provides corresponding reactive power.

[0076] Preferably, in the above Step 1, the equipment selection parameters to be determined include the shunt reactance value of the commutation valve in the chain STATCOM structure in the hybrid commutation valve, the number of sub-modules of the commutation valve in the chain STATCOM structure, and the capacitance value of the sub-modules.

[0077] Preferably, the calculation formula for the shunt reactance of the commutation valve in the chain STATCOM structure is:

[0078] L apf =(100*1.414*1000*t) / (2*I IGBT -I apf1 *k*1.414) (11)

[0079] In the formula, I IGBT is the rated current of the IGBT, k is the safety margin of the chain STATCOM structure, t is the trigger time of the valve board card, and I apf1 is the maximum fundamental wave current output of the commutation valve in the chain STATCOM structure.

[0080] The connection reactance impedance value Z of the commutation valve in the chain STATCOM structureapf The calculation formula is:

[0081] Z apf = ωL apf (12)

[0082] Preferably, the selection of the converter valve sub-module of the chain-type STATCOM structure needs to be based on the maximum peak voltage and maximum peak current that the converter valve of the chain-type STATCOM structure may generate. The maximum peak current on the valve side should not exceed the current capacity of the IGBT device, and the maximum peak voltage on the valve side is formed by superimposing the number of sub-modules.

[0083] Specifically, the calculation formula for the capacitance value of the sub-module is:

[0084]

[0085] In the formula, C0 is the capacitance value of the sub-module, V peak is the maximum phase peak voltage of the converter valve of the chain-type STATCOM structure, I peak is the maximum peak current of the converter valve of the chain-type STATCOM structure, N is the number of sub-modules, U c is the rated voltage, ω N is the rated angular frequency; ε is half of the allowable peak-to-peak fluctuation of the sub-module capacitor voltage.

[0086] Among them, the calculation formulas for V peak and I peak are respectively:

[0087] V peak = U1 + U h1 + U h2 (14)

[0088] I peak = I1 + I h1 (15)

[0089] Among them, U1 is the fundamental voltage peak; U h1 is the harmonic voltage peak corresponding to the harmonic compensated by the converter valve of the chain-type STATCOM structure; U h2 is the harmonic voltage peak corresponding to the harmonic blocked by the converter valve of the chain-type STATCOM structure; I1 is the fundamental current peak; I h1 is the harmonic current peak of the harmonic compensated by the converter valve of the chain-type STATCOM structure.

[0090] Preferably, the calculation formula for the equivalent reactance of the converter transformer is:

[0091]

[0092] The impedance calculation formula of the converter transformer is:

[0093] Z x = d x / 2 * U di0N / I dN (17)

[0094] Wherein, L r is the equivalent reactance of the commutation transformer, d x is the short - circuit impedance of the commutation transformer, U s is the line voltage on the network side, S N is the rated capacity of the three - phase transformer; Z x is the impedance of the commutation transformer, U di0N is the rated ideal no - load DC voltage, I dN is the rated DC current.

[0095] Preferably, in the above step 2, according to the control parameters of the hybrid line - commutated converter device, when the reactive power output of the converter valve in the STATCOM structure is 0, the calculation formula for the reactive power consumption of the hybrid line - commutated converter device is:

[0096]

[0097]

[0098] Wherein, U di0 is the ideal no - load current, U di0N is the rated ideal no - load DC voltage, Q conv is the reactive power consumption of the hybrid line - commutated converter device; U d is the DC voltage, I d is the DC current; α is the firing angle, μ is the commutation angle; d x is the short - circuit impedance of the commutation transformer, d r is the percentage of the commutation transformer resistance, I dN is the rated DC current, U T is the conduction voltage drop of the converter valve.

[0099] Preferably, in the above step 3, it specifically includes the following steps:

[0100] Step 3.1, Calculate the reactive power consumption of the hybrid line - commutated converter device based on the commutation angle when the STATCOM structure provides corresponding reactive power.

[0101] Among them, the calculation formula for the commutation angle during the commutation process is:

[0102]

[0103] Wherein, U ab is the line voltage between phases a and b.

[0104] That is, the calculation formula for the commutation angle μ1 is:

[0105]

[0106] In the formula, μ1 is the commutation angle when the reactive power output of the chain - type STATCOM structure is not 0, α is the triggering angle, U di0N is the rated ideal no - load DC voltage, L r is the equivalent reactance of the commutation transformer, and w is the angular frequency.

[0107] Based on the commutation angle μ1 and formula (19), the reactive power consumption of the hybrid network commutation converter at this time is calculated.

[0108] Step 3.2: Conduct a vector analysis on the AC system to obtain the relationship between the valve - side phase voltage, valve - side phase current and the power factor of the AC system.

[0109] As Figure 3 shown, when the reactive power output of the chain - type STATCOM structure is not 0, after the commutation valve of the chain - type STATCOM structure is connected in parallel on the valve side, it is considered that the valve - side power factor remains unchanged, and it is considered that the parallel connection of the STATCOM on the valve side does not affect the power factor of the original LCC converter. At this time, the included angle between the AC system phase voltage and the valve - side phase current is (μ1 is the commutation angle when the reactive power output of the commutation valve of the chain - type STATCOM structure connected in parallel on the valve side is not 0). Among them,

[0110]

[0111]

[0112] In the formula, is the included angle between the AC system phase voltage U sa and the AC system valve - side phase current I va ; θ1 is the angle by which the valve - side phase voltage U va lags behind the AC system phase voltage U sa ; θ2 is the angle by which the valve - side phase current I va leads the AC system phase current I sa ; the included angle between the valve - side phase voltage U va and the valve - side phase current I va is U wL is the voltage drop of the commutation reactance.

[0113] Step 3.3: Based on the relationship between the valve - side phase voltage, valve - side phase current and the power factor of the AC system, calculate the reactive power consumption of the commutation transformer under rated conditions.

[0114] Specifically, the calculation method for the reactive power consumption of a converter transformer includes: iteratively calculating the AC system phase current I sa , and calculating the reactive power consumption of the converter transformer based on the AC system phase current I sa .

[0115] First, calculate the voltage drop U wL = I apf * wL apf ;

[0116] Second, given I sa , calculate U sa , and calculate U wL , U sa , α, μ1 to obtain U va :

[0117]

[0118] Third, calculate θ1 based on U va :

[0119] |U wL | 2 = |U va | 2 + |U sa | 2 - 2 * |U va | * |U sa | * cos(θ1) (25)

[0120] Then, calculate I s ' a .

[0121] |I va | 2 = |I sa | 2 + |I apfa | 2 + 2 * |I sa | * |I apfa | * sin(α + μ1 / 2 - θ1) (26)

[0122] Finally, compare the calculated I s ' a with the given I sa . If the difference is less than a preset threshold (e.g., 0.001), it is considered that the given initial value I sa is correct and no further iteration is required. At this time, all steady-state parameters of the main circuit can be obtained based on the parameters of I sa .

[0123] Step 3.4: Calculate the reactive power consumption of the six-pulse converter valve based on the total reactive power consumption of the commutation device, the reactive power consumption of the commutation transformer, and the reactive power provided by the converter valve of the cascaded STATCOM structure.

[0124] Embodiment 4

[0125] The following is the calculation method for the primary main equipment parameters of the STATCOM structure connected in parallel on the valve side. These parameters reflect the ability of the STATCOM structure connected in parallel on the valve side to suppress harmonic currents in the converter station and the reactive power compensation ability of the converter station. At the same time, the harmonic capabilities of each order can be flexibly allocated and controlled. The technical parameters of the hybrid converter valve are as follows in the table.

[0126] 1. Design parameter calculation

[0127] Taking a six-pulse converter valve of a ±800 kV project as an example, the relevant control parameters of the commutation device are shown in Tables 1 and 2 below:

[0128] Table 1 Control parameters of the hybrid network-commutated converter device

[0129] <![CDATA[U d (DC voltage)]]> 200 kV <![CDATA[I d (Direct current)]]> 5 kA <![CDATA[U di0N (Rated ideal no-load DC voltage)]]> 232 <![CDATA[α N (Rated firing angle)]]> 15° <![CDATA[U va (Valve-side phase voltage)]]> 99.2 kV <![CDATA[U s (line voltage on the grid side)]]> 525 kV

[0130] Table 2 Control parameters of the commutation transformer

[0131] <![CDATA[S N (Rated capacity of three-phase transformer)]]> 1250 MVA <![CDATA[d x (Short-circuit impedance of the converter transformer)]]> 20％ <![CDATA[U va (Valve-side phase voltage)]]> 99.2 kV Converter transformer step size 0.0125

[0132] (1) Parallel impedance of the converter valve of the cascaded STATCOM structure

[0133] Take the safety margin k = 1.5 of the STATCOM structure connected in parallel on the valve side, and select the rated current of the IGBT as I IGBT = 3000 A, the rated voltage is 4.5 kV. Considering that after the valve overcurrent, the valve board card is triggered to act after t = 70 us, its connection reactance is:

[0134] L apf =(100 * 1.414 * 1000 * 70 * 10 -6 ) / (2 * 3000 - 1690 * 1.5 * 1.414) = 4 mH

[0135] The impedance value of the connection reactance Z apf = 1.29 Ω

[0136] (2) Sub-module of the converter valve of the cascaded STATCOM structure

[0137] The calculation of the commutation chain needs to consider the control function of the shunt STATCOM. In the embodiments of the present invention, the function of the shunt cascaded STATCOM structure is mainly to filter out the characteristic harmonic waves of 12k±1 times at the converter station, such as the 11th, 13th, 23rd, 25th harmonic waves, etc., and block, that is, not to compensate for the 6k±1 times harmonic waves. Therefore, it is necessary to calculate the harmonic voltage that needs to be increased by the converter valve of the cascaded STATCOM structure according to the harmonic current of the converter station.

[0138] Based on the control parameters in Table 1, taking the main filtering of the characteristic harmonic current of the converter station as an example, the parameters of the connecting reactor are designed, and the harmonic current of the converter station is shown in Table 3 below.

[0139] Table 3 Harmonic current of the converter station

[0140]

[0141]

[0142] Table 4 Calculation of the output of the blocked harmonic voltage of the shunt STATCOM

[0143] h I / kA Upeak / kV 5 0.72053 23.63684 7 0.4632 21.27326 17 0.07564 8.436607 19 0.05931 7.39348 Total 33.72

[0144] Based on the above principles, the voltage of the corresponding harmonics needs to be increased in the valve-side shunt cascaded STATCOM structure as shown in Table 4. At this time, the harmonic voltage of the valve-side shunt STATCOM Among them, U 11 、U 13 、U 23 、U 25 、U 35 、U 37 respectively represent the peak values of the harmonic voltages at frequencies such as 11, 13, 23, 25, 35, 37, etc.

[0145] Except for the harmonic waves of 5, 7, 17, 19 and other times flowing out, the harmonic waves of the remaining times are all compensated. The harmonic voltage of the valve-side shunt STATCOM at this time is shown in Table 5. At this time, the harmonic voltage of the valve-side shunt STATCOM Among them, U5, U7, U 17 、U 19 respectively represent the harmonic voltages at frequencies such as 5, 7, 17, 19, etc.

[0146] Table 5 Calculation of the output of the compensated harmonic voltage of the shunt cascaded STATCOM structure

[0147]

[0148]

[0149] If the reactive power output of the converter valve in the valve-side parallel chain-type STATCOM structure is considered as Qapf = 200 Mvar, then the current I of phase A of the converter transformer at the outlet of the chain-type STATCOM structure converter valve apfa is:

[0150] I apfa = Qapf / 1.732 / Uva = 200 / 1.732 / 99.2 = 1 kA

[0151] The fundamental voltage of the converter valve in the valve-side chain-type STATCOM structure is:

[0152] Uva + Zr * I apfa == 101.3 kV

[0153] The maximum peak voltage of the converter valve in the valve-side chain-type STATCOM structure is:

[0154] V peak = 101.3 * 1.414 + 6.04 + 33.72 = 183 kV

[0155] The maximum peak current of the converter valve in the valve-side chain-type STATCOM structure is the peak value of the linear superposition of the fundamental current and the harmonic current:

[0156] I peak = (1 + 0.65) * 1.414 = 2.3 kA

[0157] Select high-reliability press-pack type IGBT devices with 4500V / 3000A, the short-time overcurrent tolerance is 6000A (DC) / 1ms, and the rated voltage Uc can generally be taken as about 2.4 kV. After calculation, the number of sub-modules is 77 at this time, and considering redundancy N = 84.

[0158] Capacitance value of the sub-module:

[0159]

[0160] Take ε = 0.1, then C0 = 3.8 mF.

[0161] (3) Converter transformer

[0162] Inductance of the converter transformer

[0163] Impedance Z of the converter transformer x = d x / 2 * U di0N / I dN = 0.1 * 232 / 5 = 4.64 Ω

[0164] 2. Reactive power consumption calculation of the hybrid line-commutated converter device

[0165] The difference between the hybrid converter valve and the conventional LCC converter valve lies in that a cascaded STATCOM structure converter valve is connected in parallel on the valve side. When the cascaded STATCOM structure outputs power and when it does not output power, the reactive power consumption of the LCC converter valve will be different. First, calculate the reactive power consumption of the converter device when the reactive power output of the cascaded STATCOM is 0.

[0166] According to the control parameters in Table 1 and Table 2, the calculation results are shown in Table 6 below.

[0167] Table 6 Calculation results of the reactive power consumption of the converter device when the output of the STATCOM connected in parallel on the valve side is 0

[0168]

[0169]

[0170] When the STATCOM output is 0, the reactive power consumption of the converter device is 563 Mvar.

[0171] 3. Research on reactive power consumption calculation

[0172] (1) Total reactive power consumption of the converter device

[0173] The calculation of the total reactive power consumption of the converter device includes the hybrid converter valve and the converter transformer. Under the condition of the same trigger angle, due to the cascaded STATCOM structure converter valve, the actual commutation angle is reduced, and the total reactive power loss of the converter device is reduced.

[0174] At this time, the current flowing through both the cascaded STATCOM structure converter valve and the AC system is DC current.

[0175] When the reactive power output of the cascaded STATCOM structure converter valve connected in parallel is 200 Mvar, there is:

[0176] I apfa = 1 kA

[0177] I s = 4 kA

[0178] α = 15 degrees

[0179] At this time, the commutation angle is:

[0180]

[0181] At this time, the reactive power consumption of the converter device is:

[0182]

[0183] In the formula, Q conv is the reactive power consumption of the converter device.

[0184] (2) Reactive power consumption of the converter transformer

[0185] The following calculates the reactive power consumption of the converter transformer under the rated condition of 1.0 pu:

[0186] The current I of the six-pulse converter valve v and the DC current I d have the following relationship: I v = 0.816I d . Through Fourier decomposition, the fundamental wave I v1 = 0.78I d . When the DC power is 1.0 pu, the DC current I d = 5 kA. The STATCOM is connected in parallel on the valve side of the converter transformer. The fundamental wave current I v1 at the outlet of the valve side of the converter valve = 0.78 * 5 = 3.9 kA.

[0187] When the SVG provides 200 Mvar, the iterative result obtained through the iterative method is: |I sa | = 3.48 kA

[0188] The impedance Z x of the converter transformer is approximately 4.64 Ω. Then the maximum reactive power consumption of the converter transformer is:

[0189] Q transformer = Zx * I sa * I sa * 3 = 4.64 * 3.48 * 3.48 * 3 = 169 Mvar

[0190] (3) Reactive power consumption of the six-pulse converter valve

[0191] When the reactive power output of the chain-type STATCOM is 200 Mvar, the total reactive power consumption of the converter device is 524 Mvar, and the reactive power consumption of the converter transformer is 169 Mvar. At this time, the reactive power consumption of the six-pulse converter valve is: 524 - 169 = 355 Mvar. Therefore, after the chain-type STATCOM structure provides 200 Mvar of reactive power, the AC system still needs to provide 155 Mvar of reactive power.

[0192] Example 5

[0193] This example further introduces the present invention through simulation research.

[0194] (1) System conditions

[0195] The system conditions for the research on the hybrid converter valve with STATCOM connected in parallel on the valve side are:

[0196] The DC system is a 200 kV / 5000 A six-pulse rectifier; the smoothing reactor is 150 mH; the converter transformer is of the YY type, with a turns ratio of 525 kV / 173.2 kV and an impedance of 20%; the inverter is replaced by a controlled DC voltage source.

[0197] The AC system is an infinite power source, the bus voltage of the converter station is constantly 525 kV, and there is no AC filter.

[0198] The structure of the cascaded STATCOM is a three-phase star connection structure with the neutral point ungrounded, and the high-voltage end is connected between the converter transformer and the converter valve.

[0199] (2) Research conditions

[0200] Before the cascaded STATCOM structure is put into the reactive power compensation function, the DC voltage is 200 kV, the DC current is 5 kA, the firing angle alpha = 15 degrees, and the reactive power output Qc of the cascaded STATCOM structure is 0 MVar; the converter consumes 348 Mvar of reactive power, the AC system provides 559 Mvar of reactive power, and the converter transformer absorbs 210 Mvar. After measurement, the commutation angle is 25.2 degrees.

[0201] When the cascaded STATCOM structure is put into operation, the output of the cascaded STATCOM structure is 200 Mvar. At t = 0.75 s, the cascaded STATCOM structure is put into the reactive power compensation function, the DC voltage is 200 kV, the DC current is 5 kA, the number of taps is reduced by 4, and the firing angle is 17 degrees. The reactive power output Qc of the cascaded STATCOM structure is 200 MVar, the converter consumes 200 Mvar, the converter transformer consumes 200 Mvar, and the AC system provides 200 Mvar. After measurement, the commutation angle is 21.6 degrees.

[0202] According to the calculation, when the cascaded STATCOM structure provides 200 Mvar, the total reactive power consumption of the converter device is 524 Mvar, and the total reactive power consumption of the converter transformer is 158 Mvar. At this time, the consumption of the converter valve is: 524 - 158 = 366 Mvar. When the cascaded STATCOM structure provides 200 Mvar of reactive power, 166 Mvar of reactive power still needs to be provided by the system. If the reactive power consumption of the converter transformer is considered, the AC system should provide 324 Mvar.

[0203] According to the simulation, when the cascaded STATCOM structure provides 200 Mvar, the converter transformer consumes 200 Mvar of reactive power, the converter valve consumes 340 Mvar, and the AC system provides 340 Mvar. The AC system provides 340 Mvar.

[0204] Conclusion: The specific comparison between the theoretical calculation and the actual simulation is shown in Table 7 below, and they are basically consistent.

[0205] Table 7 Parameters of theoretical calculation and actual simulation results

[0206] Parameter Calculation Simulation △Q Qv (Converter valve absorption) 366 Mvar 340 Mvar 26 Mvar Qtran (Converter transformer absorption) 158 Mvar 200 Mvar -42 Mvar Total reactive power provided (Qs + Qc) 524 Mvar 540 Mvar 16 Mvar

[0207] The above embodiments are only used to illustrate the present invention, and the structures, connection manners, manufacturing processes, etc. of the various components can all be changed. Any equivalent transformation and improvement made on the basis of the technical solution of the present invention should not be excluded from the protection scope of the present invention.

Claims

1. A method for calculating the main circuit parameters of a hybrid line-commutated converter device, characterized in that The steps include: Determine the equipment selection parameters according to the control parameters of the hybrid line-commutated converter (LCC) device and the parameters of the converter transformer; Among them, the hybrid LCC device includes: A converter transformer and a hybrid converter valve; The hybrid converter valve includes a fully controlled converter valve and a semi-controlled converter valve connected in parallel, and the phase connection terminals of the semi-controlled converter valve and the phase connection terminals of the fully controlled converter valve are respectively connected to the three phases of the converter transformer, and the converter transformer is connected to the AC system; The fully controlled converter valve adopts a voltage source converter (VSC) valve, and the VSC valve adopts a cascaded static synchronous compensator (STATCOM) structure valve; Calculate the reactive power consumption of the hybrid LCC device when the reactive power output of the cascaded STATCOM structure valve is 0; Based on the determined equipment selection parameters of the hybrid converter valve and the reactive power consumption of the hybrid LCC device, calculate respectively under the rated condition the reactive power consumption of the six-pulse converter valve and the reactive power consumption of the converter transformer when the cascaded STATCOM structure provides the corresponding reactive power.

2. The main circuit parameter calculation method of a hybrid line-commutated converter device according to claim 1, characterized in that The semi-controlled converter valve adopts a six-pulse converter valve, and the six-pulse converter valve adopts a three-phase bridge converter structure. Each phase converter bridge includes two half-bridges, upper and lower, with a total of six arms; a valve string is provided on each of the arms.

3. The main circuit parameter calculation method of a hybrid line-commutated converter device according to claim 2, characterized in that, Each of the valve strings adopts a semi-controlled thyristor valve string.

4. The main circuit parameter calculation method of a hybrid line-commutated converter device as described in claim 1, characterized in that: The equipment selection parameters include the parallel reactance value of the cascaded STATCOM structure valve in the hybrid converter valve, the number of sub-modules of the cascaded STATCOM structure valve, and the capacitance value of the sub-modules.

5. The main circuit parameter calculation method of a hybrid line-commutated converter device according to claim 1, characterized in that: The calculation formula for the reactive power consumption of the hybrid LCC device when the reactive power output of the cascaded STATCOM structure valve is 0 is: In the formula, is the reactive power consumption of the hybrid line-commutated converter device, is the ideal no-load current; is the DC current; is the firing angle, is the commutation angle.

6. The main circuit parameter calculation method of a hybrid line-commutated converter device according to claim 1, characterized in that: The method for calculating respectively under the rated condition the reactive power consumption of the six-pulse converter valve and the reactive power consumption of the converter transformer when the cascaded STATCOM structure provides the corresponding reactive power based on the determined equipment selection parameters of the hybrid converter valve and the reactive power consumption of the hybrid LCC device includes: Commutation angle when the chain - type STATCOM - based converter valve provides corresponding reactive power , the reactive power consumption of the hybrid line - commutated converter device is calculated; Conduct a vector analysis on the AC system to obtain the relationship between the valve-side phase voltage and current and the power factor of the AC system; Based on the relationship between the valve-side phase voltage and current and the power factor of the AC system, calculate the reactive power consumption of the converter transformer under the rated condition; Based on the total reactive power consumption of the converter device, the reactive power consumption of the converter transformer, and the reactive power provided by the cascaded STATCOM structure valve, calculate the reactive power consumption of the six-pulse converter valve.

7. The main circuit parameter calculation method of a hybrid line-commutated converter device as claimed in claim 6, characterized in that: The commutation angle is calculated by the following formula: Wherein, is the commutation angle when the reactive power output of the converter valve of the chain - type STATCOM structure is not 0, is the trigger angle, is the rated ideal no - load DC voltage, is the shunt reactance of the converter valve of the chain - type STATCOM structure, is the angular frequency, is the line current on the grid side.

8. The main circuit parameter calculation method of a hybrid line-commutated converter device as claimed in claim 6, characterized in that: The method for calculating the reactive power consumption of the converter transformer under the rated condition based on the relationship between the valve-side phase voltage and current and the power factor of the AC system includes: Based on the relationship between the valve-side phase voltage and valve-side phase current and the power factor of the AC system, the AC system phase current is iteratively calculated. , and the reactive power consumption of the converter transformer is calculated according to the AC system phase current. ​

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