CN112054537B - Control method of active filter for simultaneously compensating reactive power and controlling harmonic wave - Google Patents
Control method of active filter for simultaneously compensating reactive power and controlling harmonic wave Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
- H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
- H02J3/1835—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control
- H02J3/1842—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters
- H02J3/1857—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters wherein such bridge converter is a multilevel converter
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/01—Arrangements for reducing harmonics or ripples
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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- Y02E40/40—Arrangements for reducing harmonics
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Abstract
The invention relates to an active filter for simultaneously compensating reactive power and controlling harmonic wave, and a control and design method thereof, wherein a primary system of the active filter comprises: the device converter valve can be turned off and is connected with the alternating current bus through the connecting equipment; the converter valve with the turn-off device comprises 3 phases of converter arms, each phase of converter arm consists of a direct current support capacitor and the turn-off device, and the output voltage of the converter valve with the turn-off device is controlled by controlling the turn-off of the turn-off device; the connecting device comprises a connecting transformer and a connecting passive filter which are connected in parallel, wherein the connecting transformer is used for flowing fundamental frequency reactive current in a low-frequency band; the passive filter is connected to pass high frequency harmonic currents in the high frequency band. The invention can be widely applied to the field of power transmission system design.
Description
Technical Field
The invention relates to an active filter for simultaneously compensating reactive power and controlling harmonic waves, and a control and design method, and belongs to the field of power transmission system design.
Background
When the LCC direct current converter engineering operates, a large amount of reactive power is consumed, a large amount of harmonic waves are generated, mainly characteristic harmonic waves of 12 th order, 24 th order and the like and non-characteristic harmonic waves of 3 rd order and 5 th order, so that a large amount of alternating current filters and parallel capacitors are required to be arranged for reactive compensation and harmonic compensation, an alternating current filter field consisting of the alternating current filters and the parallel capacitors occupies nearly half of the occupied area of the LCC direct current converter station, and a large amount of investment is consumed.
The alternating current filter belongs to passive filter equipment, the filter effect of the alternating current filter is influenced by the impedance of an alternating current system, and the alternating current filter needs to be designed in detail according to the conditions of the alternating current system in the design process of a converter station. At present, the AC system is constructed and developed quickly, new equipment of a new technology is continuously connected into the AC system, the characteristics of the AC system are changed, and meanwhile, the filtering effect of an AC filter is also changed. Moreover, with the construction of extra-high voltage direct current engineering, converter stations are more and more dense, the impedance of an alternating current filter and an alternating current power grid has the possibility of resonance, and a harmonic source which is originally in the alternating current power grid can be amplified, so that the low-frequency harmonic of the alternating current power grid is greatly overproof. The harmonic exceeding will affect the safe operation of the equipment in the AC power network, and the network loss caused by the harmonic tide will also waste a large amount of electric energy.
Meanwhile, the alternating current filter and the parallel capacitor are controlled by the circuit breaker, and mechanical equipment has slow response and cannot adapt to rapid power change or converter locking under a fault, so that the alternating current filter and the parallel capacitor cannot cut off the alternating current overvoltage in time. In addition, the harmonic problem and the reactive compensation problem also exist in an alternating current power grid, a large number of harmonics exist in the alternating current power grid, the harmonics are mainly caused by the increasing nonlinear loads at present, and the fluctuation of alternating current voltage is also mainly controlled and stabilized by reactive compensation equipment. Therefore, if the active reactive and harmonic compensation equipment which is quickly and continuously adjusted can be used for replacing the passive compensation equipment of the traditional LCC converter station, the power response flexibility and the filtering performance of the LCC converter station can be greatly improved, the area of an alternating current filter field is reduced, the occupied land is reduced, and meanwhile, benefits are brought to an alternating current power grid.
At present, active filter equipment is applied to the direct current side of a current converter of the direct current engineering in China, replaces a direct current filter and is only applied to the alternating current side abroad. The harmonic waves generated by the converter are mostly higher-order characteristic harmonic waves of 12 th order, 24 th order and the like, and are easy to pass through capacitive equipment such as a capacitor and the like, while the reactive compensation equipment is essentially used for compensating fundamental frequency current which easily flows through inductive equipment such as an inductor and the like, and the active filter equipment cannot simultaneously give consideration to fundamental frequency reactive compensation and high-frequency filtering, so that the active filter equipment cannot be used as a passive filter of the reactive compensation equipment and the filter equipment at the same time on the alternating current side.
Disclosure of Invention
In view of the above problems, an object of the present invention is to provide an active filter for simultaneously compensating reactive power and controlling harmonic, and a control and design method thereof, which provides an effective topology structure of the active filter and designs main device parameters of the active filter.
In order to achieve the purpose, the invention adopts the following technical scheme:
in a first aspect of the present invention, there is provided an active filter for simultaneously compensating reactive power and controlling harmonics, the primary system of the active filter comprising: a turn-off device converter valve and a connection device; the converter valve of the turn-off device is connected into an alternating current bus through the connecting equipment, and the alternating current bus is respectively connected with an alternating current system and a harmonic source; the converter valve of the turn-off device consists of 3 phases of converter arms, each phase of converter arm consists of a direct current support capacitor and a turn-off device, and the output voltage of the converter valve of the turn-off device is controlled by controlling the turn-on and turn-off of the turn-off device; the connecting device comprises a connecting transformer and a connecting passive filter which are connected in parallel, wherein the connecting transformer is used as an inductive device and is used for flowing fundamental frequency reactive current in a low frequency band; the connected passive filter functions as a capacitive device for passing high frequency harmonic currents in a high frequency band.
In a second aspect of the present invention, there is provided a method for controlling an active filter for simultaneously compensating for reactive power and controlling harmonics, comprising the steps of: (1) Measuring harmonic current of a harmonic source and harmonic current connected with a passive filter in an active filter, calculating harmonic modulation voltage of a converter valve of a turn-off device in the active filter based on measured current data, and performing harmonic control; (2) Measuring fundamental wave current connected with the passive filter and the transformer and fundamental frequency voltage of an alternating current bus, and calculating fundamental wave modulation voltage of a converter valve of a turn-off device in the active filter based on current and voltage data obtained by measurement to perform reactive compensation; (3) And (3) adding the harmonic modulation voltage of the converter valve of the turn-off device obtained in the step (1) and the fundamental modulation voltage of the converter valve of the turn-off device obtained in the step (2) to obtain the total modulation voltage of the converter valves of the turn-off device in the active filter.
Further, in the step (1), the method for calculating the harmonic modulation voltage of the converter valve in the active filter comprises the following steps: (1.1) measuring the harmonic current of the harmonic source and the low-voltage side harmonic current connected with the passive filter in the active filter to obtain the harmonic current i of the harmonic source s And low-voltage side harmonic current i connected with passive filter F (ii) a (1.2) converting the harmonic current i of the harmonic source s And low-voltage side harmonic current i connected with passive filter F Subtracting to obtain the high-voltage side target harmonic current i of the connected transformer T1 (ii) a (1.3) high-voltage side target harmonic current i of the connecting transformer T1 Multiplying the transformation ratio k of the connecting transformer to obtain the low-voltage side target harmonic current i of the connecting transformer T2 (ii) a (1.4) connecting the low-voltage side target harmonic current i of the transformer T2 And low-voltage side harmonic current i connected with passive filter F Adding to obtain a control target value i of the converter valve outlet current of the turn-off device C (ii) a (1.5) inputting the obtained control target value of the converter valve outlet current of the turn-off device into an active filter harmonic current control inner ring, and outputting corresponding converter valve harmonic modulation voltage through a preset controller; and (1.6) repeating the steps (1.1) to (1.5) until the output of the active filter reaches a stable state.
Further, in the step (2), the method for calculating the fundamental wave modulation voltage of the converter valve of the turn-off device in the active filter comprises the following steps: (2.1) measuring the low-voltage side currents of the connecting transformer and the connecting passive filter respectively to obtain the low-voltage side currents of the connecting transformer and the connecting passive filter; (2.2) multiplying the low-voltage side current of the connecting transformer by 1/k times to obtain the high-voltage side current of the connecting transformer, and adding the high-voltage side current of the connecting transformer and the low-voltage side current of the connecting passive filter to obtain the total current of the high-voltage side of the active filter; (2.3) carrying out fundamental wave extraction on the total current of the high-voltage side of the active filter to obtain a fundamental wave component of the total current of the high-voltage side of the active filter; (2.4) measuring the fundamental frequency voltage of the alternating current bus, and calculating according to the obtained fundamental frequency voltage of the alternating current bus and the fundamental component of the total current on the high-voltage side of the active filter obtained in the step (2.3) to obtain the reactive power injected into the alternating current bus by the active filter; and (2.5) subtracting the reactive power injected into the alternating current bus by the active filter obtained in the step (2.4) from a reactive set value of the active filter, calculating the obtained difference value through a controller to obtain the amplitude of the fundamental frequency modulation voltage of the converter valve of the active filter, and multiplying the amplitude of the fundamental frequency modulation voltage of the active filter by the same-phase unit voltage of the fundamental wave voltage of the alternating current bus extracted in the step (2.3) to obtain the fundamental frequency modulation voltage of the active filter.
In a third aspect of the present invention, there is provided a method for designing an active filter for simultaneously compensating reactive power and controlling harmonics, comprising the steps of: (1) Performing equivalent analysis on the active filter to obtain an equivalent circuit diagram of the active filter, and performing harmonic analysis and fundamental frequency analysis on the active filter based on the equivalent circuit diagram; (2) And designing equipment elements of the active filter based on an equivalent circuit diagram of the active filter and the results of harmonic analysis and fundamental frequency analysis.
Further, in the step (1), when performing an equivalent analysis on the active filter, the high-voltage side of the connection transformer is equivalent to a parallel capacitor C p The low voltage side is equivalent to a parallel inductor L p The connection between the high-voltage side and the low-voltage side is equivalent to a connecting inductor L s And satisfies the following conditions:
C p ＝(k-1)/ω 2 L
L s ＝L/k
L p ＝L/k(k-1)
wherein k is the transformation ratio of the connecting transformer; l is an equivalent value of the short-circuit inductor of the connecting transformer at the high-voltage side; ω is the frequency.
Further, in the step (2), the method for designing the device element of the active filter based on the equivalent circuit diagram of the active filter and the results of the harmonic analysis and the fundamental frequency analysis includes the following steps: (2.1) designing the tuning frequency of the passive filter according to the harmonic frequency to be filtered based on the equivalent circuit diagram of the active filter, and designing the connection equipment of the active filter according to the harmonic filtering requirement and the reactive output requirement; and (2.2) calculating the final voltage output and current output of the active filter according to the harmonic compensation requirement and the reactive compensation power requirement of the active filter design requirement, and designing the capacity of a converter valve of a turn-off device in the active filter according to the final voltage output and current output.
Further, in the step (2.1), when each parameter in the connection device is designed, the equivalent connection inductance L of the connection transformer is enabled to be equal s And connecting the passive filter to resonate at a frequency located about midway between the target harmonic order and the fundamental frequency.
Further, in the step (2.1), specific parameters of the passive filter are determined according to the single-time switching reactive power allowed by the system and the target filtering frequency, and specific parameters of the transformer are determined according to the parallel resonance frequency and the reactive power output capacity of the passive filter.
Further, in the step (2.2), the method for designing the converter valve capacity of the turn-off device of the active filter includes the following steps: (2.2.1) calculating the harmonic voltage output and the harmonic current output of the active filter required under the subharmonic according to the maximum harmonic compensation requirement of the design requirement of the active filter; (2.2.2) calculating the minimum fundamental voltage output and the fundamental current output of the converter valve of the active filter according to the maximum reactive compensation power requirement of the design requirement of the active filter; (2.2.3) calculating to obtain the final voltage output and current output of the active filter according to the minimum fundamental voltage output and fundamental current output of the converter valve of the active filter, the harmonic voltage output and the harmonic current output which are obtained through calculation; and (2.2.4) designing the capacity and the composition equipment of the converter valve of the active filter according to the final voltage output and current output of the active filter.
Due to the adoption of the technical scheme, the invention has the following advantages: 1. the converter valve adopts a grid-connected connection mode that the converter valve of the turn-off device of the active filter is directly connected into an alternating current system through a parallel structure of the passive filter and the transformer, the passive filter is used as a harmonic branch for harmonic compensation, the transformer branch is used as a fundamental frequency current branch for reactive compensation, and the converter valve capacity of the turn-off device with smaller turn-off capacity is used for simultaneously compensating reactive power and harmonic; 2. the invention provides a basic active filter control strategy, and has engineering practicability; 3. the invention provides an equivalent analysis circuit of an active filter, which is convenient for analyzing harmonic waves passing through the active filter and designing a connection passive filter and a connection transformer of the active filter.
Drawings
FIG. 1 is a schematic diagram of the topology of an active filter designed by the present invention;
FIG. 2 is a schematic diagram of an active filter control strategy designed by the present invention;
FIG. 3 is an equivalent circuit diagram of an active filter designed according to the present invention;
FIG. 4 is an active filter harmonic approximation equivalent circuit designed by the present invention;
fig. 5 is an active filter fundamental frequency approximation equivalent circuit of an embodiment of the present invention.
Detailed Description
The invention is described in detail below with reference to the figures and examples. It is to be noted, however, that the following drawings are provided only for the purpose of better understanding of the present invention and the following description of the embodiments is merely illustrative and is in no way intended to limit the invention or its use. The numerical expressions and numerical values of the steps set forth in the embodiments do not limit the scope of the present invention, and this embodiment and other embodiments may have different values.
Techniques, methods known to those of ordinary skill in the relevant art may not be discussed in detail, but are intended to be part of the specification where appropriate.
It should be noted that once a certain reference number and letter are defined in a certain figure or expression, further discussion thereof is not required in subsequent figures.
Example one
As shown in fig. 1, the active filter for simultaneously compensating reactive power and controlling harmonic according to the present invention includes a primary system including a converter valve and a connection device, wherein the converter valve is connected to an ac bus via the connection device, and the ac bus is connected to an ac system and a harmonic source, respectively. The converter valve of the turn-off device consists of 3 phases of converter arms, each phase of converter arm consists of a direct current support capacitor and a turn-off device, and the output voltage of the converter valve of the turn-off device is controlled by controlling the turn-on and turn-off of the turn-off device. As zero sequence harmonic is filtered, the 3-phase converter bridge arms are connected in a mode that a Y-connection neutral point is grounded. The connecting device comprises a transformer and a passive filter which are mutually connected in parallel, wherein the transformer is used as inductive equipment, has very low impedance at a low frequency band and mainly passes through fundamental frequency reactive current; the passive filter is a capacitive device, has low impedance in a high frequency band, and mainly passes high frequency harmonic current.
Example two
As shown in fig. 2, the present invention further provides a method for controlling an active filter for simultaneously compensating reactive power and controlling harmonic, including the following steps:
(1) And measuring the harmonic current of a harmonic source and the low-voltage side harmonic current connected with a passive filter in the active filter, calculating the harmonic modulation voltage of a converter valve in the active filter based on the measured current data, and performing harmonic control.
The active filter takes the harmonic current as a control quantity, and controls the harmonic current flowing into the alternating current bus at the high-voltage side of the active filter to be the same as the harmonic current flowing out of the harmonic source in amplitude and opposite in phase, and the harmonic currents are mutually offset. In the topological structure of the active filter designed by the invention, the connecting transformer and the connecting passive filter as connecting equipment are in a parallel structure, the low-voltage side current and the high-voltage side current of the connecting passive filter are the same, and the low-voltage side current of the connecting transformer is k times of the high-voltage side current (k is the transformer transformation ratio), so that the control target of the high-voltage side current of the active filter cannot be directly converted into the harmonic current control target of the converter valve outlet of the active filter. Therefore, the invention adopts the following method to carry out harmonic control:
(1.1) measuring the harmonic current of the harmonic source and the low-voltage side harmonic current (equal to the high-voltage side harmonic current of the passive filter) connected with the passive filter in the active filter to obtain the harmonic current i of the harmonic source s And low-voltage side harmonic current i connected with passive filter F ；
(1.2) converting the harmonic current i of the harmonic source s And low-voltage side harmonic current i connected with passive filter F Subtracting to obtain the high-voltage side target harmonic current i of the connecting transformer T1 ；
(1.3) high-voltage side target harmonic current i of the connecting transformer T1 Multiplying by k to obtain the low-voltage side target harmonic current i of the connecting transformer T2 ；
(1.4) Low-voltage side target harmonic current i of the connecting transformer T2 And low-voltage side harmonic current i connected with passive filter F Adding to obtain a control target value i of the converter valve outlet current C ；
(1.5) inputting the obtained control target value of the converter valve outlet current into an active filter harmonic current control inner ring, and outputting corresponding converter valve harmonic modulation voltage through a certain controller;
(1.6) repeating the step (1.1) to the step (1.5) until the output of the active filter reaches a stable state.
(2) And measuring the fundamental wave current of the low-voltage side of the passive filter and the transformer and the fundamental frequency voltage of the alternating-current bus, and calculating the fundamental wave modulation voltage of the converter valve in the active filter based on the measured current voltage data to perform reactive compensation.
The transformer presents pure inductance at fundamental frequency, the passive filter presents pure capacitance at fundamental frequency, and the two devices still present pure capacitance or pure inductance after being connected in parallel. Therefore, the connection structure of the active filter designed by the invention basically does not present resistance at fundamental frequency, and if the converter valve of the active filter generates voltage in the same direction as the alternating current bus, reactive current can be injected into or absorbed by the alternating current bus. Specifically, the reactive compensation method comprises the following steps:
(2.1) measuring the low-voltage side currents of the connecting transformer and the connecting passive filter respectively to obtain the low-voltage side currents of the connecting transformer and the connecting passive filter;
(2.2) multiplying the low-voltage side current of the connecting transformer by 1/k times to obtain the high-voltage side current of the connecting transformer, and adding the high-voltage side current of the connecting transformer and the low-voltage side current of the connecting passive filter to obtain the total current of the high-voltage side of the active filter;
(2.3) carrying out fundamental wave extraction on the total current of the high-voltage side of the active filter to obtain a fundamental wave component of the total current of the high-voltage side of the active filter;
(2.4) measuring the fundamental frequency voltage of the alternating current bus, and calculating the obtained fundamental frequency voltage of the alternating current bus and the fundamental component of the total current on the high-voltage side of the active filter obtained in the step (2.3) to obtain the reactive power injected into the alternating current bus by the active filter;
and (2.5) subtracting the reactive power injected into the alternating current bus by the active filter obtained in the step (2.4) from a reactive set value of the active filter, calculating the amplitude of the fundamental frequency modulation voltage of the converter valve of the active filter by a proper controller, and multiplying the amplitude of the fundamental frequency modulation voltage of the active filter by the same-phase unit voltage of the fundamental wave voltage of the alternating current bus extracted in the step (2.3) as the fundamental wave of the fundamental frequency modulation voltage of the converter valve of the active filter to obtain the fundamental frequency modulation voltage of the active filter.
(3) And (3) adding the harmonic modulation voltage of the converter valve obtained in the step (1) and the fundamental modulation voltage of the converter valve obtained in the step (2) to obtain the total modulation voltage of the converter valve in the active filter.
EXAMPLE III
As shown in fig. 3 to 5, the present invention further provides a design method of an active filter for simultaneously compensating reactive power and controlling harmonic, including the following steps:
(1) And performing equivalent analysis on the active filter to obtain an equivalent circuit diagram of the active filter, and performing harmonic analysis and fundamental frequency analysis on the active filter based on the equivalent circuit diagram.
a. Equivalent circuit analysis
The connecting device of the active filter designed by the invention is formed by connecting the connecting transformer and the connecting passive filter in parallel, and the connecting transformer belongs to the magnetic coupling device, and the primary side and the secondary side are not directly connected through a circuit, so the problems of complicated analysis and invisibility are solved. And circuit equivalence is carried out on the connecting transformer, which is beneficial to design of each device.
Fig. 3 shows an equivalent circuit diagram of the active filter. Wherein, the connection transformer is equivalent to a PI type circuit, and the high-voltage side of the connection transformer is equivalent to a parallel capacitor C p The connection between the high-voltage side and the low-voltage side is equivalent to a connecting inductor L s The low-voltage side of the connecting transformer is equivalent to a parallel inductor L p And satisfies the following formula:
C p ＝(k-1)/ω 2 L (1)
L s ＝L/k (2)
L p ＝L/k(k-1) (3)
wherein k is the transformer transformation ratio; l is an equivalent value of the transformer short-circuit inductor on the high-voltage side; ω is the frequency.
C in FIG. 3 1 For connecting component capacitors of passive filters, L 1 For connecting the component inductors of the passive filter, APF stands for the converter valve of the active filter, U s Representing the equivalent voltage source of the AC system, i s Representing harmonic source harmonic currents.
It can be seen that the parallel capacitor C p The capacitance value of (C) varies with the frequency, the parallel capacitance C p May be incorporated into the system impedance; parallel inductance L p The filtering effect is not influenced, but the current stress flowing through the converter valve is influenced; connection ofThe inductor Ls and the connecting passive filter need to be designed so that the required capacity of the converter valve reaches an optimal value.
b. Harmonic analysis
When calculating the path of the harmonic current and the harmonic capacity of the converter valves of the active filter, the approximate equivalent circuit diagram of the active filter is shown in fig. 4. In the figure, i s-h Is a counter-cancellation current, U, of the harmonic current of the harmonic source to be generated by the active filter h Is the harmonic voltage output of the converter valve of the active filter, and can be seen that U h Is actually i s-h In equivalent connection with the inductor L of the transformer s And the opposite value of the voltage drop over the parallel arrangement connecting the passive filters. i.e. i T3-h Is U h Equivalent parallel inductance L of transformer in connection p The reverse current of the current generated, it can be seen that the harmonic current of the active filter contributes i APF-h Is i T3-h And i s-h And (4) summing. i.e. i T2-h Is an equivalent connection inductor L of a current-connected transformer s Current of (i) F-h Is the current flowing through the connection filter.
c. Fundamental frequency analysis
When calculating the magnitude of the fundamental reactive power and the fundamental frequency capacity of the converter valve of the active filter, the approximate equivalent circuit diagram of the active filter is shown in fig. 5. Because the ac system impedance is generally small, it is negligible. In the figure, i Q The designed total reactive current of the active filter flows into an alternating current system to generate target output reactive power. It is noted that the total reactive capacity generated by the active filter is capacitively biased by the presence of the connected passive filter. As can be seen from FIG. 5, i T1-1 The voltage of the alternating current system is equivalent to a parallel capacitor C in a transformer p Up generated base frequency current according to i Q And i T1-1 The equivalent connection inductance L of the inflow transformer can be calculated s At a base frequency current i T2-1 And a base frequency current i flowing through the passive filter F-1 And the corresponding voltage drop is subtracted from the alternating current system voltage to obtain the fundamental frequency voltage output of the converter valve of the active filter. Equivalent parallel inductance L of converter valve with fundamental frequency voltage output connected with transformer p High birthGenerating a current i T3-1 ，i T3-1 ， i T2-1 And i F-1 And adding to obtain the fundamental current output of the active filter.
(2) And designing equipment elements of the active filter based on an equivalent circuit diagram of the active filter and the results of harmonic analysis and fundamental frequency analysis.
Specifically, the method comprises the following steps:
(2.1) designing the tuning frequency of the connection passive filter according to the harmonic times needing to be filtered based on the equivalent circuit diagram of the active filter, and designing parameters such as the transformation ratio, the short-circuit impedance and the like of the connection transformer by combining the harmonic filtering requirement and the reactive power output requirement, so that the equivalent connection inductance L of the connection transformer s And connecting the passive filter to resonate at a frequency located about midway between the target harmonic order and the fundamental frequency.
According to the equivalent circuit diagram shown in fig. 3, the connection device of the active filter designed by the invention can be equivalent to the parallel connection of the passive filter and the inductor. If the passive filter is designed as the simplest single-tuned filter, as shown in fig. 3, the impedance characteristic of the passive filter is analyzed, and it can be known that the connection device has inductance as the main characteristic at low frequency and presents low impedance; with increasing frequency, the shunt inductance L p The passive filter generates parallel resonance with the passive filter, and the connecting equipment presents extremely high impedance; as the frequency increases to approach the resonant frequency of the passive filter, the impedance of the connection device takes the passive filter as a main characteristic and finally reaches a minimum value; the frequency continues to increase and the impedance of the connected device appears inductive and increases continuously.
It can be seen that the connection device will present a low impedance mainly at the fundamental frequency and the tuning frequency of the passive filter, so the tuning frequency of the passive filter should be designed according to the number of harmonics to be filtered, and the parameters of transformation ratio and short-circuit impedance of the connection transformer should be designed in combination with the requirements of harmonic filtering and reactive output.
As can be seen from the harmonic analysis circuit fig. 4, in order to filter out the target harmonic current using as little converter valve capacity as possible, the passive filter should be tuned to the target harmonic order. The capacity of a typical passive filter is limited by acThe system can accept single-switching reactive change, and the capacity of the passive filter is close to the maximum capacity accepted by the alternating current system as much as possible. The short-circuit impedance of the connection transformer should be as small as possible in order to output reactive power, but for harmonic filtering, it can be seen from fig. 4 that if the short-circuit impedance of the connection transformer is too small or the transformation ratio is too large, the total parallel impedance will increase around the tuning frequency of the passive filter, if the equivalent connection inductance L is used s And passive filters resonate near the tuning frequency of the passive filter, making filtering at the tuning frequency very difficult. In addition, if the short-circuit impedance of the connection transformer is too small or the transformation ratio is too large, the equivalent parallel inductance L will be caused p The current flowing upwards is overlarge, and the current output of the converter valve of the active filter is increased. Taken together, the equivalent connection inductance L should be made s And the passive filter resonates at a frequency located in the middle vicinity of the target harmonic order and the fundamental frequency.
And (2.2) calculating the final voltage output and current output of the active filter according to the harmonic compensation requirement and the reactive compensation power requirement of the active filter design requirement, and designing the capacity of a converter valve in the active filter according to the final voltage output and current output.
Specifically, the method comprises the following steps:
and (2.2.1) calculating the harmonic voltage output and the harmonic current output of the active filter required under the subharmonic according to the maximum harmonic compensation requirement of the design requirement of the active filter.
As can be seen from the approximate equivalent circuit diagram 4 of the harmonic of the active filter, the maximum value of the harmonic source current to be filtered is known to be i s0-h The active filter needs to generate a reverse current i s-h Counteracting it, the harmonic voltage output U of converter valve of active filter h Can be calculated according to equation (4)
Wherein
To connect the impedance of the passive filter at the target harmonic order,
equivalent connection of inductor L for connecting transformer s Impedance at the target harmonic order.
Converter valve harmonic current output i of active filter APF-h Can be calculated according to equation (5) and is the target harmonic current i s-h The current i generated on the equivalent parallel inductance of the transformer by the converter valve T3-h And (4) summing.
And (2.2.2) calculating the minimum fundamental voltage output and the fundamental current output of the converter valve of the active filter according to the maximum reactive compensation power requirement of the design requirement of the active filter.
As can be known from the active filter fundamental frequency approximate equivalent circuit diagram 5, when the fundamental frequency voltage of the converter valve of the active filter and the fundamental frequency voltage of the ac bus are in the same phase, the current output of the converter valve of the active filter and the fundamental frequency current input to the ac system by the active filter both change linearly as the amplitude of the fundamental frequency voltage of the converter valve of the active filter changes linearly.
As can be seen from FIG. 5, the current i in the equivalent parallel capacitance of the transformer T1-h Calculation is as in formula (6)
Active filter design injects reactive current i into AC system Q The fundamental voltage output of the active filter converter valve is U APF-1 Calculated as (7)
In the formula (I), the compound is shown in the specification,
in order to connect the fundamental impedance of the passive filter,
the fundamental wave impedance of the inductor is equivalently connected with the connecting transformer.
Active filter converter valve fundamental current output i APF-1 Calculating as shown in equation (8):
the fundamental frequency current of the high-voltage side of the active filter connected transformer is shown as the formula (9)
Through analysis, when the current flowing through the active filter connecting transformer is zero,
the active filter sends out capacitive reactive power to a system, and the converter valve of the active filter sends out capacitive fundamental current; when the fundamental current output of the converter valve of the active filter is zero,
the active filter injects capacitive reactive power into the system; when the active filter injects zero reactive power into the system,
the active filter converter valve emits inductive fundamental current.
In order to fully utilize the capacity of the converter valve of the active filter, the point where the current output of the converter valve is zero is generally taken as a zero working point, and at the moment, the active filter outputs capacitive reactive power integrally. And determining the maximum inductive reactive power and the maximum capacitive reactive power of the active filter input alternating current system according to the design requirements, and calculating the fundamental voltage output and the fundamental current output of the converter valve of the active filter according to the formulas.
And (2.2.3) calculating to obtain the final voltage output and current output of the active filter according to the minimum fundamental voltage output and fundamental current output of the converter valve of the active filter, the harmonic voltage output and the harmonic current output.
And the final voltage output of the active filter is the sum of the harmonic voltage output and the fundamental voltage output. The final current output of the active filter is the sum of the harmonic current output and the fundamental current output.
And (2.2.4) designing the capacity and the composition equipment of the converter valve of the active filter according to the final voltage output and the current output of the active filter.
Example four
The following takes a specific embodiment applied to a newly-built ± 800kv,8000mw extra-high voltage dc converter station as an example to specifically describe the design method of the active filter:
given that the maximum single-switch reactive fluctuation allowed by an alternating current system accessed by the converter station is 300MVA, an active filter is required to filter 11 and 13 th harmonics generated by the converter station and generate a smoothly modulated dynamic reactive power of 200MVA, so that the fixed capacity of other passive reactive compensation equipment such as a parallel capacitor is increased. The maximum 11 and 13 currents generated at full power of the converter station are 328A and 238A respectively, and are filtered by 4 active filters.
The active filter in this embodiment adopts a basic topology as shown in fig. 1 and a basic control strategy as shown in fig. 2. In FIG. 1, MMC Valve is converter bridge arm of converter Valve, uc is converter bridge arm voltage, i F For connecting the low-side current of the passive filter, i s Harmonic current, i, being a harmonic source T1 For connecting the high-side current of the transformer, i T2 Is the low-side target harmonic current of the connecting transformer, zs is the ac system impedance. The fixed capacity of the connected passive filter is 300MVA, the form of the single-tuned filter is selected, the tuning point is set to be 12 times, and the specific parameters are shown in table 1.
Table 1 connecting passive filter device parameters
| Connecting passive filter devices | Parameters of the equipment |
| C1 | 3.466×10 -6 uF |
| L1 | 20.32mH |
| R1 | 100Ω |
The capacity of the transformer for the converter station is generally 240MVA, the transformation ratio is 525kV/35kV, and the fundamental frequency impedance | Z of the passive filter C1 I | =525 × 525/300=919 Ω, and if the equivalent connection impedance of the connection transformer and the passive filter resonate at the intermediate frequency between 12 times and the fundamental frequency, that is, at the frequency of 6 times and 300Hz, the impedance ω L of the equivalent connection inductor of the connection transformer is the impedance ω L s ＝ωL T The/k should be designed to be 25.52 omega, and the short-circuit impedance of the connecting transformer can be calculated to be 33.3%.
At the moment, the passive filter is connected with the equivalent connecting inductor in parallel, and the impedance multiple is equivalently increased
Is composed of
When the current of the converter valve of the active filter is zero, the transformer is equivalently connectedInductor L s And connecting the total current in the passive filter and the equivalent parallel inductor L of the transformer p The same current in the power supply, and the output of the active filter voltage is
The reactive current injected into the ac system at this time passes through a capacitive current calculated as 0.288 kA.
The corresponding reactive power is-260 MVA. I.e. the power zero of the active filter generates a capacitive reactive of 260MVA. The reactive output of the active filter is in a capacitive reactive power range of 160 MVA-360 MVA when the aim is to generate 200MVA dynamic reactive power.
When 160MVA capacitive reactive power is generated, the output of the converter valve is 29kV effective value of line voltage, and the output of current is 1663A effective value.
When 360MVA is generated, the output of the converter valve is 39kV effective value of line voltage, and the current output is 1663A effective value.
Therefore, 4500V/2100A IGBT devices can be selected, the rated working voltage is selected to be 2.4kV, Y is connected with a neutral point to be grounded, and each phase of the converter valve needs 13 IGBT sub-modules.
The above embodiments are only used for illustrating the present invention, and the structure, connection mode, manufacturing process, etc. of the components may be changed, and all equivalent changes and modifications performed on the basis of the technical solution of the present invention should not be excluded from the protection scope of the present invention.
Claims (2)
1. A method of controlling an active filter simultaneously compensating for reactive power and controlling harmonics, the primary system of the active filter simultaneously compensating for reactive power and controlling harmonics comprising: a turn-off device converter valve and a connection device; the converter valve of the turn-off device is connected into an alternating current bus through the connecting equipment, and the alternating current bus is respectively connected with an alternating current system and a harmonic source; the converter valve of the turn-off device consists of 3 phases of converter arms, each phase of converter arm consists of a direct current support capacitor and a turn-off device, and the output voltage of the converter valve of the turn-off device is controlled by controlling the turn-on and turn-off of the turn-off device; the connecting device comprises a connecting transformer and a connecting passive filter which are connected in parallel, wherein the connecting transformer is used as inductive equipment and is used for flowing fundamental frequency reactive current at a low frequency band; the passive filter is used as a capacitive device and is used for flowing high-frequency harmonic current in a high-frequency band; the method is characterized by comprising the following steps:
(1) Measuring harmonic current of a harmonic source and harmonic current connected with a passive filter in an active filter, calculating harmonic modulation voltage of a converter valve of a turn-off device in the active filter based on current data obtained by measurement, and performing harmonic control;
in the step (1), the method for calculating the harmonic modulation voltage of the converter valve of the turn-off device in the active filter comprises the following steps:
(1.1) measuring the harmonic current of the harmonic source and the low-voltage side harmonic current connected with the passive filter in the active filter to obtain the harmonic current i of the harmonic source s And low-voltage side harmonic current i connected with passive filter F ；
(1.2) converting the harmonic current i of the harmonic source s And low-voltage side harmonic current i connected with passive filter F Subtracting to obtain the high-voltage side target harmonic current i of the connecting transformer T1 ；
(1.3) high-voltage side target harmonic current i of the connecting transformer T1 Multiplying the transformation ratio k of the connecting transformer to obtain the low-voltage side target harmonic current i of the connecting transformer T2 ；
(1.4) connecting the low-voltage side target harmonic current i of the transformer T2 And low-voltage side harmonic current i connected with passive filter F Adding to obtain a control target value i of the converter valve outlet current of the turn-off device C ；
(1.5) inputting the obtained control target value of the converter valve outlet current of the turn-off device into an active filter harmonic current control inner ring, and outputting corresponding converter valve harmonic modulation voltage through a preset controller;
(1.6) repeating the step (1.1) to the step (1.5) until the output of the active filter reaches a stable state;
(2) Measuring fundamental current connected with a passive filter and a transformer and fundamental frequency voltage of an alternating current bus, and calculating fundamental modulation voltage of a converter valve of a turn-off device in the active filter based on current and voltage data obtained by measurement to perform reactive compensation;
(3) And (3) adding the harmonic modulation voltage of the converter valve of the turn-off device obtained in the step (1) and the fundamental modulation voltage of the converter valve of the turn-off device obtained in the step (2) to obtain the total modulation voltage of the converter valve of the turn-off device in the active filter.
2. A method of controlling an active filter for simultaneous reactive compensation and harmonic control as claimed in claim 1, characterized by: in the step (2), the method for calculating the fundamental wave modulation voltage of the converter valve of the turn-off device in the active filter comprises the following steps:
(2.1) measuring the low-voltage side currents of the connecting transformer and the connecting passive filter respectively to obtain the low-voltage side currents of the connecting transformer and the connecting passive filter;
(2.2) multiplying the low-voltage side current of the connecting transformer by 1/k times to obtain the high-voltage side current of the connecting transformer, and adding the high-voltage side current of the connecting transformer and the low-voltage side current of the connecting passive filter to obtain the total current of the high-voltage side of the active filter;
(2.3) performing fundamental wave extraction on the total current of the high-voltage side of the active filter to obtain a fundamental wave component of the total current of the high-voltage side of the active filter;
(2.4) measuring the fundamental frequency voltage of the alternating current bus, and calculating according to the obtained fundamental frequency voltage of the alternating current bus and the fundamental component of the total current on the high-voltage side of the active filter obtained in the step (2.3) to obtain the reactive power injected into the alternating current bus by the active filter;
and (2.5) subtracting the reactive power injected into the alternating current bus by the active filter obtained in the step (2.4) from a reactive power set value of the active filter, calculating the obtained difference value through a controller to obtain the amplitude of fundamental wave modulation voltage of a converter valve of the active filter, and multiplying the same-phase unit voltage of the fundamental frequency voltage of the alternating current bus extracted in the step (2.4) by the fundamental wave of the fundamental wave modulation voltage of the converter valve of the active filter to obtain the fundamental wave modulation voltage of the active filter.
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