CN116914816A

CN116914816A - Alternating current side fault ride-through device and method for direct current series connection offshore wind farm

Info

Publication number: CN116914816A
Application number: CN202310641961.XA
Authority: CN
Inventors: 谢魏琨; 查鲲鹏; 郭高朋; 李兰芳; 张帆; 厍冬瑾
Original assignee: NARI Group Corp; China EPRI Electric Power Engineering Co Ltd
Current assignee: NARI Group Corp; China EPRI Electric Power Engineering Co Ltd
Priority date: 2023-06-01
Filing date: 2023-06-01
Publication date: 2023-10-20

Abstract

The invention provides an alternating current side fault ride-through device and method for a direct current series-connection offshore wind farm, comprising a series-connection type energy consumption device and a controller; the series energy dissipation device is connected in series on a direct current circuit between the direct current side of the offshore wind farm converter and the direct current side of the onshore converter station; the controller is respectively in communication connection with the serial energy consumption device and the onshore converter station; the controller is used for determining whether the state of the fault crossing device is a normal mode or a fault mode according to the output power of the alternating current side of the onshore converter station and the output power of the offshore wind farm converter, and inputting the series-connection type energy consumption device in the fault mode; the series-connected energy consumption device is used for consuming the power difference between the offshore wind farm converter and the onshore converter station when the state is in the fault mode. The invention inputs the series-connection type energy consumption device when the fault occurs, and consumes redundant power between the offshore wind farm converter and the onshore converter station, so that the fault can be stably traversed after the fault occurs.

Description

Alternating current side fault ride-through device and method for direct current series connection offshore wind farm

Technical Field

The invention belongs to the technical field of offshore wind power, and particularly relates to an alternating current side fault ride-through device and method for a direct current series-connection offshore wind power plant.

Background

The offshore wind power has the advantages of stable resource conditions, relatively close distance from a load center and the like, and has become an important direction for the development of the wind power in countries around the world in recent years. The alternating current power grid has inherent defects in the process of collecting offshore wind power and remotely transmitting the offshore wind power, and the application of high-voltage direct current power transmission in offshore wind power is promoted. Conventional direct current transmission has no practical application of offshore wind power transmission up to now because of the problems of large amount of filtering, reactive power compensation equipment, commutation failure and the like, and flexible direct current transmission becomes the most suitable scheme.

In the current practical engineering that offshore wind power is sent out through a flexible direct current system, wind driven generators are all assembled by parallel alternating current, and huge offshore platforms are needed to accommodate a step-up transformer and an inverter. In the direct current series connection offshore wind power plant, if the medium-voltage direct current output by the fan is directly sent out after being connected in series to obtain high-voltage direct current, an offshore platform, a transformer and corresponding equipment are not needed, and the installation and maintenance cost can be greatly reduced. However, because of the coupling characteristic between the fans connected in series, one control strategy that is more suitable for the onshore converter station is to control the direct-current side current by adjusting the direct-current side voltage, and the direct-current port voltage output by the fan converter can be adjusted according to the output power of the fan.

When the offshore wind farm adopts flexible direct current power transmission grid connection, if the receiving end alternating current power grid fails, the alternating current power output by the onshore converter station is reduced, the direct current power transmitted by the wind farm is basically not affected, the unbalanced power of the transmitting end and the receiving end can raise the capacitor voltage in the onshore converter station, and the safe and stable operation of the system is threatened.

At present, in actual engineering of grid connection of offshore wind power through a flexible direct current system, an alternating current side fault ride through (fault ride through, FRT) strategy is adopted to install a parallel energy consumption device (energy diverting circuit, EDC) on a direct current side, but the scheme may have some problems in a direct current series offshore wind farm. For example, when the dc side voltage is too low, the parallel energy consuming device may not continue to operate normally or may not meet the energy consumption requirements required for FRT; when the voltage at the direct current side changes, some parallel energy consumption devices with energy storage capacitors may have charge and discharge related problems. Therefore, in the direct current series connection offshore wind power plant, when the FRT is in fault, the fault cannot be traversed normally and stably by adopting the parallel connection type energy consumption device.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides an alternating current side fault ride-through device and method for a direct current series connection offshore wind farm.

The technical scheme provided by the invention is as follows:

the invention provides an alternating current side fault ride-through device for a direct current series-connection offshore wind farm, which comprises a series-connection energy consumption device and a controller, wherein the series-connection energy consumption device is connected with the controller;

the series energy consumption device is connected in series on a direct current circuit between the direct current side of the offshore wind farm converter and the direct current side of the onshore converter station;

the controller is respectively in communication connection with the serial energy consumption device and the onshore converter station;

the controller is used for determining whether the state of the fault crossing device is a normal mode or a fault mode according to the output power of the alternating-current side of the onshore converter station and the output power of the offshore wind farm converter, and inputting the series-connection type energy consumption device in the fault mode;

the series-connected energy consumption device is used for consuming the power difference between the offshore wind farm converter and the onshore converter station when the state is in a fault mode.

Preferably, the series energy consumption device comprises a through flow branch and an energy consumption branch which are connected in parallel;

the through-flow branch comprises a mechanical switch and a diode which are connected in series;

the energy consumption branch comprises a plurality of energy consumption submodules connected in series and a line stray inductor;

when no fault occurs on the alternating current side of the onshore converter station, the mechanical switch is in a closed state, and the energy consumption branch circuit is short-circuited;

When the alternating current side of the onshore converter station fails, the mechanical switch is in an off state, the through-flow branch is bypassed, and the energy consumption branch generates power difference between the power consumption offshore wind farm converter and the onshore converter station through a plurality of energy consumption submodules.

Preferably, the energy consumption submodule comprises a full-bridge energy consumption submodule and a half-bridge energy consumption submodule; and the full-bridge energy consumption submodule and the half-bridge energy consumption submodule are connected in series and then connected between the direct-current side of the offshore wind farm converter and the line stray inductance.

Preferably, the number of the half-bridge energy consumption sub-modules is greater than the number of the full-bridge energy consumption sub-modules.

Preferably, the rated voltage of the series-connected energy consumption device is a set percentage of the rated voltage of the direct current side of the onshore converter station.

Based on the same inventive concept, the invention also provides an alternating current side fault ride-through method for the direct current series connection offshore wind farm, which comprises the following steps:

determining that the state of the fault ride-through device is a normal mode or a fault mode based on the output power of the alternating-current side of the onshore converter station and the output power of the offshore wind farm converter;

When the state of the fault crossing device is in a fault mode, inputting the series energy consumption device and adjusting the voltage of the series energy consumption device and the on-shore converter station so as to consume the power difference between the offshore wind farm converter and the on-shore converter station;

when the state of the fault crossing device is in a normal mode, the series energy consumption device is cut off;

the fault ride-through device is an alternating current side fault ride-through device for the direct current series-connection offshore wind power plant.

Preferably, the adjusting the voltage of the series-connected energy consumption device and the onshore converter station includes:

obtaining a voltage reference value of the series-connected energy consumption device by using direct current control in a fault mode according to an actual value of current on a direct current line in a normal mode, and adjusting the series-connected energy consumption device according to the voltage reference value;

and obtaining a voltage reference value of the onshore converter station by using capacitor voltage stabilizing control under a fault mode according to the average capacitor voltage value in the onshore converter station, and adjusting the onshore converter station according to the voltage reference value.

Preferably, the obtaining the voltage reference value of the series energy consumption device by using the dc current control in the fault mode according to the actual value of the current on the dc line in the normal mode includes:

Obtaining a reference value of current on a direct current line, and obtaining a first voltage value through a proportional integral controller according to an actual value and the reference value of the current on the direct current line in a normal mode;

calculating to obtain the voltage feedforward quantity of the series-connected energy consumption device according to the voltage and current of the alternating current side of the onshore converter station, the current reference value of the direct current side and the output voltage of the offshore wind farm;

and superposing the first voltage value and the voltage feedforward quantity of the serial-type energy consumption device, and then limiting amplitude to obtain the voltage reference value of the serial-type energy consumption device.

Preferably, the obtaining a reference value of the current on the direct current line includes:

acquiring a power reference value output by an offshore wind farm;

and calculating a reference value of the current on the direct current line according to the power reference value output by the offshore wind farm and the output voltage of the offshore wind farm.

Preferably, the voltage feedforward amount of the series energy consumption device is calculated as follows:

in the method, in the process of the invention,is the voltage feedforward quantity of the series-type energy consumption device, < >>For the d-axis component of the voltage on the ac side of an onshore converter station, < >>For limiting the d-axis component of the current on the ac side of the onshore converter station,/i>For the current reference value u on the DC side of an onshore converter station _WF And outputting voltage for the offshore wind farm.

Preferably, the step of obtaining the voltage reference value of the onshore converter station by using the capacitor voltage stabilizing control under the fault mode according to the average capacitor voltage value in the onshore converter station includes:

obtaining a second voltage value through a proportional integral controller according to the value of the average capacitance voltage value in the onshore converter station and the set multiple of the rated capacitance voltage value in the onshore converter station;

calculating to obtain the voltage feedforward quantity of the onshore converter station according to the current reference value of the onshore converter station on the alternating current side and the current reference value of the onshore converter station on the direct current side and the output voltage of the offshore wind farm;

and superposing the second voltage value and the voltage feedforward quantity of the onshore converter station, and then limiting amplitude to obtain the voltage reference value of the onshore converter station.

Preferably, the calculation formula of the voltage feedforward quantity of the onshore converter station is as follows:

in the method, in the process of the invention,for voltage feed-forward of an onshore converter station,/->For the d-axis component of the voltage on the ac side of the onshore converter station,for limiting the d-axis component of the current on the ac side of the onshore converter station,/i>Is the current reference value of the direct current side of the onshore converter station.

Preferably, the rated voltage of the series-connected energy consumption device is determined by the rated voltage of the direct current side of the onshore converter station and the standard percentage of fault ride through.

Preferably, the rated voltage of the series-type energy consumption device is calculated as follows:

in the method, in the process of the invention,is the rated voltage of the series-connected energy consumption device, alpha is the standard percentage of fault ride-through, and +.>Is the rated voltage of the DC side of the onshore converter station.

Preferably, the operation of the series-connected energy consumption device is divided into the following modes: a normal through-flow mode, a fault commutation mode, a fault energy consumption mode and a fault recovery commutation mode;

the normal through-flow mode is used when the state of the fault crossing device is in a normal mode;

the fault commutation mode is used when the state of the fault crossing device is in transition from a normal mode to a fault mode;

the fault energy consumption mode is used when the state of the fault crossing device is a fault mode;

the fault recovery commutation mode is used when the state of the fault crossing device is that the fault mode is transited to the normal mode.

Preferably, the normal through-flow mode includes: putting into a circulation branch circuit and shorting out an energy consumption branch circuit;

the fault commutation modality includes: regulating the energy consumption sub-module on the energy consumption branch to output negative pressure, and switching off the mechanical switch on the circulation branch;

the fault energy consumption mode comprises: a circuit breaking circulation branch circuit, and an energy consumption branch circuit is input;

The fault recovery commutation modality includes: closing a mechanical switch on the flow path;

Compared with the closest prior art, the invention has the following beneficial effects:

the invention provides an alternating current side fault ride-through device and method for a direct current series-connection offshore wind farm, comprising a series-connection type energy consumption device and a controller; the series energy consumption device is connected in series on a direct current circuit between the direct current side of the offshore wind farm converter and the direct current side of the onshore converter station; the controller is respectively in communication connection with the serial energy consumption device and the onshore converter station; the controller is used for determining whether the state of the fault crossing device is a normal mode or a fault mode according to the output power of the alternating-current side of the onshore converter station and the output power of the offshore wind farm converter, and inputting the series-connection type energy consumption device in the fault mode; the series-type energy consumption device is used for consuming the power difference between the offshore wind farm converter and the onshore converter station when the state is in a fault mode.

Drawings

FIG. 1 is a schematic diagram of an AC side fault ride-through device for a DC serial offshore wind farm according to the present invention;

FIG. 2 is a schematic diagram of the overall topology of a DC serial offshore wind farm of the present invention for delivering power to shore;

fig. 3 is a schematic diagram of the topology of an onshore converter station according to the present invention;

FIG. 4 is a schematic diagram of the topology of a serial-type energy dissipation device according to the present invention;

FIG. 5 is a schematic diagram of the output voltage of the energy dissipating sub-module according to the present invention in different states;

FIG. 6 is a schematic flow chart of an AC side fault ride-through method for a DC serial offshore wind farm according to the present invention;

FIG. 7 is a schematic diagram of a fault ride-through strategy according to the present invention;

fig. 8 is a schematic diagram of the control strategy of an onshore converter station according to the invention;

FIG. 9 is a schematic diagram of a control strategy of a serial-type energy consumption device according to the present invention;

FIG. 10 is a schematic diagram showing the voltage, current and power variations of the fault ride-through device during the fault ride-through process according to the present invention;

FIG. 11 is a schematic diagram of a period of the output voltage of the energy consuming submodule according to the present invention;

FIG. 12 is a schematic diagram of a hysteresis waveform of the output voltage of the energy consuming submodule according to the present invention;

FIG. 13 is a schematic diagram of a hysteresis comparator according to the present invention;

FIG. 14 is a schematic diagram of the overall fault ride-through operation of the present invention;

FIG. 15 is a schematic circuit diagram corresponding to the fault-ride-through action of the present invention;

FIG. 16 is a schematic diagram of a simulation waveform of the proposed method under condition 1;

fig. 17 is a schematic diagram of a simulation waveform of the proposed method in case of operating condition 2 of the present invention.

Detailed Description

The following describes the embodiments of the present invention in further detail with reference to the drawings.

Example 1:

the invention provides an alternating current side fault ride-through device for a direct current series-connection offshore wind farm, which is shown in figure 1 and comprises a series-connection energy consumption device and a controller;

Fig. 2 is a schematic diagram of the overall topology of a dc series offshore wind farm for delivering power to shore, where a wind generator outputs dc voltage via a wind energy conversion system, and after being connected in series, the dc voltage is sent to shore via a dc link, and is connected in series with an energy consumption device and an onshore converter station, where the ac side of the onshore converter station is connected to an ac grid.

Wherein, the maximum power point tracking control of each fan is givenRespectively emitted power reference valueTransmitted to an onshore converter station through an optical fiber, and a direct current reference value +.>Each fan outputs a direct current voltage u through a converter (wind energy conversion) _WTj (1. Ltoreq.j.ltoreq.M), all u _WTj The direct current voltage u output by the wind power plant is obtained by adding _WF Transmitted to an onshore converter station via optical fibers. The wind power plant outlet is connected with the inductor and then connected with the direct current circuit, so that the wind power plant outlet is connected with the series-connected energy consumption device on the shore, and is connected with the onshore converter station after passing through the inductor. The direct current flowing in the direct current loop is i _dc The output voltage of the energy consumption device is u _EDC The direct-current side voltage of the onshore converter station is u _dc 。

Fig. 3 is a schematic diagram of the topology of an onshore converter station employing a full-bridge modular multilevel converter (modular multilevel converter, MMC). The full-bridge MMC has 3 phases and 6 bridge arms, and each bridge arm is formed by cascading n full-bridge Submodules (SM) and is connected in series with a bridge arm inductor L. Wherein, the liquid crystal display device comprises a liquid crystal display device, Representing ac three-phase output voltages (3 quantities); />Representing three-phase output currents (3 quantities); />Representing the output voltages (6 quantities) of the upper and lower bridge arms of the three phases; />Representing the upper and lower leg currents (6 quantities) of the three phases; u (u) _dc Represents a direct-current side voltage (1 quantity); i.e _dc Shows the DC side current (1 quantity)。

Explanation: the subscript ac represents alternating current (alternative current), and the subscripts a, b, c represent abc three phases, respectively;subscripts p and n denote positive and negative (positive and negative), respectively, of the DC side, i.e. upper and lower legs, e.g +.>Representing the upper bridge arm voltage of phase a; u (u) _dc The subscript dc in (a) indicates direct current (direct current).

The MMC has 6n total full-bridge submodules, each full-bridge submodule contains a capacitor, and the voltage of the capacitor is u _capj Summing all capacitor voltages and calculating the average u _cap U is generally required to be controlled _cap For its rated voltage valueTo ensure safe and stable operation of MMC

The bridge arm voltage of the full-bridge MMC consists of three parts, namely 0.5u direct current component _dc Ac componentCirculation componentThe voltage calculation method of the three-phase six bridge arms is as follows:

the onshore converter station is a full-bridge MMC, wherein the total number of the full-bridge MMC is 6 bridge arms, the total number of the full-bridge submodules is 6n, and the capacitor voltage in each module is u _capj All that is required is to performMeasuring and averaging to obtain MMC average capacitance voltage

The alternating-current side output three-phase alternating-current voltage of the onshore converter station isThree-phase alternating current is->Through a transformation ratio of k _T 1, a transformer connected with an AC power grid, wherein the three-phase AC voltage of the AC power grid is +.>When AC power grid has AC fault, AC voltage +.>The amplitude of (c) will decrease.

FIG. 4 is a schematic diagram of a topology of a series-connected energy dissipating device, as shown in FIG. 4, including a through-flow branch and an energy dissipating branch connected in parallel;

The energy consumption submodule comprises a half-bridge energy consumption submodule which is connected in series with the direct current side of the offshore wind farm converter. The energy consumption submodule comprises a full-bridge energy consumption submodule and a half-bridge energy consumption submodule; and the full-bridge energy consumption submodule and the half-bridge energy consumption submodule are connected in series and then connected between the direct-current side of the offshore wind farm converter and the line stray inductance. The number of the half-bridge energy consumption sub-modules is larger than that of the full-bridge energy consumption sub-modules.

In addition, the rated voltage of the series-type energy consumption device is a set percentage of the rated voltage of the direct current side of the onshore converter station.

As shown in fig. 4, the series-connected energy consumption device (energy diverting circuit, EDC) is divided into two branches: a normal through-flow branch and an energy consumption branch. The normal through-flow branch consists of a diode D and a quick mechanical switch S _BRK The diode is used for limiting the unidirectional flow of direct current, and the mechanical switch is used for controlling the on-off of the branch. The energy consumption branch is formed by connecting n energy consumption submodules in series, wherein m are based on half-bridge, L are based on full-bridge submodules, and L is the stray inductance of the circuit. The energy consuming device sub-modules (SM) are composed of a capacitor, an energy consuming resistor R, IGBT (insulated gate bipolar transistor) and a diode, and the energy consuming device is a device for consuming energy by using the resistor, and the resistor of the device is distributed in each energy consuming sub-module. The energy consumption submodule is divided into a half-bridge submodule and a full-bridge submodule, the half-bridge can only output positive voltage, and the full-bridge can also output negative voltage. Both sub-modules have the same effect when consuming energy.

D _1H 、D _1F 、D _2F And D _f Is a diode S _1Hj 、S _1Fj 、S _2Fj And S is _Rj Is IGBT, R is energy dissipation resistor, C is capacitor. i.e ₁ Indicating the current through the current branch, i ₂ Indicating the current flowing through the energy-consuming branch, u _SMj (1. Ltoreq.j. Ltoreq.n) is the output voltage of n sub-modules, u _L Is the voltage across the stray inductance.

As shown in fig. 5, in the half-bridge sub-module, D _1H For limiting the direction of the branch current, D _f Stray inductance freewheel for resistor S _R For controlling the capacitor to discharge the resistor to maintain voltage balance,S _1H for controlling the output voltage of the submodule to be 0-u _C (capacitor voltage). Quan Qiaozi module increases S compared to half bridge submodule _2F And D _2F By controlling S _1F And S is _2F To make the output voltage of the module be-u _C ～u _C The output negative voltage may be used to consume the power plant replacement stream. It should be noted that the anti-parallel diode at two ends of the IGBT in the sub-module is used to prevent the IGBT from being damaged due to back pressure, and the circuit does not pass through when it is operating normally. In addition, the parameters of capacitance and resistance in the half bridge module and the Quan Qiaozi module are the same, so that when the two sub-modules are input in the fault, the output voltage and the consumed energy are the same. Only a very small number of full-bridge sub-modules need to be designed for circuit commutation.

The energy consumption device submodule is divided into 3 parts: the current is conducted unidirectionally through the half bridge or full bridge circuit, the capacitor and the chopper energy dissipation circuit. The half-bridge or full-bridge circuit is used for controlling the output voltage of the submodule; the full bridge circuit also has the function of outputting negative voltage, and is used for transferring current from the through current branch to the energy consumption branch. The chopper energy consumption circuit is used for controlling the capacitor voltage.

IGBTs are a type of controllable unidirectional-conduction switching device. When the signal s=0 it receives, the IGBT turns off and current cannot flow; when it receives a signal s=1, the IGBT turns on, and a current flows in the direction of the arrow.

The meaning of the input of the submodule is that if current flows in the forward direction, the voltage can be output, the half bridge can only output positive voltage, and the full bridge can also output negative voltage; but if no current flows, the output voltage is 0.

The alternating-current side fault traversing device for the direct-current series-connection offshore wind power plant provided by the invention has the advantages that the reaction is quicker and more reliable in engineering practice. Mainly aims at a conventional flexible direct current transmission system adopting constant direct current voltage control, and two ends of a converter station are connected with energy consumption devices in parallel. Compared with the parallel energy dissipation devices of the same type, the serial energy dissipation device can adapt to the direct current voltage and current which are changed by the serial direct current offshore wind power system; the series energy consumption device can be designed according to 80% of rated voltage of a direct current transmission system, 20% of submodules are saved, and the volume and cost of the device are reduced. According to the invention, the direct-current side voltage of the converter station is reduced in the fault period, and direct-current side current control can be continuously carried out through the series-type energy consumption device, so that the direct-current stability and the MMC capacitor voltage stability are always kept before and after the fault.

In summary, the ac side fault ride through device for the dc tandem offshore wind farm provided by the present invention, in the dc tandem offshore wind farm, the tandem energy dissipation device can adapt to the varying dc voltage under the dc control, and the problem that the parallel energy dissipation device may exist under the circumstance is not encountered. In addition, if the series-connection type energy consumption device is designed according to the low voltage crossing standard (minimum 20%) of the current offshore wind farm, the series-connection type energy consumption device can be designed according to 80% of the rated voltage of the direct current side, and compared with the parallel-connection type energy consumption device of the same type, the number of energy consumption sub-modules can be reduced by 20%.

Example 2:

based on the same inventive concept, the invention provides an alternating current side fault ride-through method for a direct current series connection offshore wind farm, as shown in fig. 6, based on the fault ride-through device, comprising the following steps:

step 1: determining that the state of the fault ride-through device is a normal mode or a fault mode based on the output power of the alternating-current side of the onshore converter station and the output power of the offshore wind farm converter;

step 2: when the state of the fault crossing device is in a fault mode, inputting the series energy consumption device and adjusting the voltage of the series energy consumption device and the on-shore converter station so as to consume the power difference between the offshore wind farm converter and the on-shore converter station;

Step 3: when the state of the fault crossing device is in a normal mode, the series energy consumption device is cut off;

As shown in fig. 7, after an ac fault occurs, the wind field side is not affected, and the dc power p is outputted _WF Is unchanged. For the maximum alternating current power which can be output by MMC after faultShould be compared with the wind field power p _WF And the magnitude relation of the fault crossing is used for judging a specific fault crossing method.

Defining an ac side residual capacity p of an onshore converter station _res The method comprises the following steps:

(1)p _res when the power consumption is more than or equal to 0, the shore converter station can maintain the power balance, and the energy consumption device inputs the signal S _fault =0. The converter station can complete fault ride-through by means of normal mode control;

(2)p _res when the power consumption is less than 0, the power balance of the onshore converter station cannot be maintained, and the energy consumption device inputs the signal S _fault =1. The fault crossing can be completed by controlling the fault mode and putting in the energy consumption device.

In conclusion, from p _res If it is greater than 0, a fault ride-through policy flow diagram as shown in FIG. 7 may be obtained.

In the event of failure, p _res When < 0, the onshore converter station cannot maintain power balance, and fault mode control must be designed.

If p after failure occurs _res When < 0, the normal mode capacitor voltage stabilizing control fails, and the MMC capacitor voltage will continue to rise, so that the fault mode capacitor voltage stabilizing control needs to be designed: average value u of capacitance voltage of all submodules of MMC _cap For controlled quantity, 1.05 times the capacitance voltage ratingFor controlling the reference value; the invention uses MMC DC voltage limiting value +.>Output of the DC current control in the normal mode as output +.>The DC power flowing into the MMC can be reduced by limiting amplitude, and the rise of the capacitor voltage is restrained.

Explanation: the core of the invention is a series-connected energy consumption device and two fault mode controls (fault mode capacitor voltage stabilizing control and fault mode direct current control) designed for putting into the energy consumption device. Output quantity of normal mode capacitor voltage stabilizing control and normal mode DC current controlAnd->Are all of upper limit (+)>And->) Normal mode control is achieved by adjusting within limits.

When an ac fault occurs, the normal mode capacitive voltage regulation control is to be active (continue to control u _cap Controlled as) Output +.>Must be increased if the fault is severe +>Eventually will be greater than the upper limit +.>(human given), but this is not allowed for device safety, so the normal mode capacitive voltage regulation control fails.

At this time, the energy consumption device must be put into to consume the redundant power to ensure u _cap And the control is not out of control. In the dc loop, the wind farm voltage and the onshore converter station dc voltage are equal (u _WF ＝u _dc ) The method comprises the steps of carrying out a first treatment on the surface of the Now if a series-connected energy consumption device is to be put into operation, u _WF ＝u _dc +u _EDC I.e. wind farm voltage u _WF Unchanged, required DC voltage u of onshore converter station _dc Descending, the schematic diagram is shown in figure 3. Therefore, a fault mode capacitor voltage stabilizing control is designed for actively reducing the DC voltage u of the onshore converter station _dc In this way, the normal mode dc control controlled by the onshore converter station fails and we wish to take over the dc control by the consumer.

However, fault mode capacitive voltage regulation control enables DC voltage u _dc Reduce%As an upper output limit for the normal mode DC current control, such that the reference value +.>Reducing the actual DC voltage u _dc Naturally will decrease), will cause the control of the normal mode DC current to be out of control, the wind field voltage u _WF And u is equal to _dc Will cause the direct current side current i to be _dc Raised. At this time, if the series-connected energy consumption device is put into charge of voltage difference, the direct current can be controlled, which is the fault mode direct current control: direct current is used as control quantity, and direct voltage born by energy consumption device is +. >As output.

In the fault crossing strategy of the flow submitted by the invention, the fault mode control is started or not by the output signal S judged in the first step _fault And (5) determining. If the AC side low voltage fault occurs, signal S _fault =0, blocking the fault mode control, and resetting the Proportional Integral (PI) links of the fault mode capacitor voltage stabilizing control and the direct current control all the time, so that the energy consumption device is not put into; if signal S _fault And =1, soBarrier mode control must be started, fault mode capacitance voltage stabilizing control and feedforward quantity of direct current controlChanging, unlocking the PI link and adjusting on the basis of feedforward quantity, and obtaining the MMC average capacitance voltage u _cap And direct current i _dc Controlled to a reference value.

In the fault mode capacitor voltage stabilizing control and the direct current control provided by the invention, the proper feedforward quantityThe method is not only beneficial for the controlled quantity to quickly reach a steady-state point, but also determines the matching mode of normal mode and fault mode control, and the feedforward quantity calculation method is described as follows:

wherein: will beThe upper limit is set to +.>(1.05 times the DC voltage rating, to cope with transient processes), and (2)>The lower limit is set to 0.

Letter meaning in the formula: in the same manner as described above,for the d-axis component of the voltage on the ac side of the onshore converter station, is- >Is the limiting value of the d-axis component of the alternating current. />Is a reference value for DC current control, u _WF And outputting voltage for the wind farm.

Thus, the regulation of the voltage of the series-connected consumer and the onshore converter station comprises:

using direct current control under a fault mode to obtain a voltage reference value of the serial energy consumption device according to the actual value of the current on the direct current line under the normal mode and adjusting the serial energy consumption device according to the voltage reference value;

and the voltage reference value of the on-shore converter station is obtained by using capacitance voltage stabilizing control under a fault mode according to the average capacitance voltage value in the on-shore converter station, and the on-shore converter station is regulated according to the voltage reference value.

Firstly, the step of obtaining the voltage reference value of the series energy consumption device by using the direct current control in the fault mode according to the actual value of the current on the direct current line in the normal mode comprises the following steps:

the method comprises the steps of obtaining a reference value of current on a direct current line in a target-to-target-period mode, and obtaining a first voltage value through a proportional-integral controller according to an actual value and the reference value of the current on the direct current line in a normal mode;

the voltage feedforward quantity of the series-type energy consumption device is calculated according to the voltage and current of the alternating current side and the current reference value of the direct current side of the onshore converter station and the output voltage of the offshore wind farm;

Wherein the obtaining the reference value of the current on the direct current line includes:

(1) acquiring a power reference value output by an offshore wind farm;

(2) and calculating a reference value of the current on the direct current line according to the power reference value output by the offshore wind farm and the output voltage of the offshore wind farm.

The voltage feedforward quantity of the series energy consumption device is calculated as follows:

And obtaining a voltage reference value of the onshore converter station by using capacitor voltage stabilizing control under a fault mode according to the average capacitor voltage value in the onshore converter station, wherein the voltage reference value comprises the following components:

the method comprises the steps that a second voltage value is obtained through a proportional integral controller according to the value of the average capacitance voltage value in the onshore converter station and the value of the set multiple of the rated capacitance voltage value in the onshore converter station;

The method comprises the steps that a voltage feedforward quantity of an onshore converter station is calculated according to current reference values of an alternating current side and a direct current side of the onshore converter station and output voltage of an offshore wind farm;

The calculation formula of the voltage feedforward quantity of the onshore converter station is as follows:

When the direct-current series-connection offshore wind farm is connected with the grid through the flexible direct-current transmission system, an alternating-current side fault-ride-through (fault ride-through) strategy is considered, and the control of a normal mode and an alternating-current fault mode is included. If there is no ac fault, the full-bridge MMC control strategy only requires the capacitor voltage regulation control (normal mode), ac current control, dc current control (normal mode) and MMC switch signal modulation for these 4 blocks. The structure and control strategy of the full-bridge MMC itself is already in existence.

In order to realize fault ride-through, the scheme provided by the invention is to increase capacitance voltage stabilizing control (fault mode) of the MMC, install a series-connected energy consumption device in a direct current line, and provide a corresponding energy consumption device control strategy.

Fig. 8 is a schematic diagram of a control strategy for an onshore converter station, capacitive voltage regulation control (normal mode). It is desirable to average MMC capacitance voltage u _cap Controlled to the rated voltage of the capacitorThe difference between the two is sent to a Proportional Integral (PI) controller, and the feedforward quantity is added after the output>Through a clipping link (clipping value is +.>) Obtaining output +.>(a d-axis component reference value of alternating current).

An alternating current control. D-axis component reference value of AC currentAnd q-axis component reference value->Obtaining a three-phase alternating current reference value +.>To make the actual alternating current +.>Control to->The two are input into a Proportional Resonance (PR) controller by difference, and the feedforward quantity is overlapped after the output>Obtaining an MMC bridge arm voltage alternating current component reference value +.>

Capacitive voltage regulation control (failure mode). It is desirable to average MMC capacitance voltage u _cap Controlled to be 1.05 times of rated voltage of capacitor(the capacitor voltage rises at the time of failure and is therefore slightly higher than the nominal value). The difference between the two is sent to a Proportional Integral (PI) controller, and the feedforward quantity is added after the output>Obtaining output through a limiting link>

Dc current control (normal mode). It is desirable to apply a DC current i _dc Control to a reference valueThe two are subtracted to obtain an error, the error is input into a proportional integral controller (PI), and the output quantity is superimposed with a feedforward quantity u _WF Then go through a limiting link (the limiting value is the output of fault mode capacitor voltage stabilizing control +.>) Obtaining the direct-current voltage reference value +.>

Explanation: the proportional-integral (PI) controller is a common controller and comprises two parts of proportional and integral terms of input quantity, and the mathematical model is

y(t)＝K _p x(t)+K _i ∫x(t)dt

Wherein: y (t) is the output of PI, x (t) is the input of PI, K _p Is a proportionality coefficient, K _i Is an integral coefficient.

From the equation, the bridge arm voltage of the MMC is decoupled into dc, ac and loop components, which can be controlled separately. The direct current control, the circulation control and the alternating current control inner loop respectively output 3 reference values of bridge arm voltages: direct current componentCirculation component->And AC component->Wherein the reference value of the alternating current control +.>Can be decomposed into d-axis current reference values by inverse park transformation (a common coordinate transformation method for transforming 3 variables of an abc coordinate system and a dq0 coordinate system into each other)>And q-axis current reference value->q-axis current reference value->The reactive outer ring is given or directly set manually according to actual conditions, and is usually 0; d-axis current reference value->Given by the normal mode capacitive voltage regulation control of MMC, here for the purpose of making +.>Can respond quickly in the event of AC failure, a feed-forward quantity is added >The calculation formula is as follows:

wherein: p is p _WF Power is delivered to the wind farm;a three-phase ac voltage (right side of the transformer in fig. 2) that is an onshore ac grid, with a phase angle θ; />Is->Converted to a three-phase ac voltage at the outlet of the onshore converter station (left side of the transformer in fig. 2), -a converter station for converting the ac voltage to a dc voltage> For the amplitude of the three-phase ac voltage at the outlet of the converter station (d-axis component), by +.>Obtained by park forward conversion, k _T Is the voltage variation value of the secondary side voltage and the primary side voltage of the transformer.

Explanation: "park transformation" (a common coordinate transformation method, transforming abc coordinate system into dq0 coordinate system), its transformation formula is as follows

Park forward transform:

inverse park transformation:

wind farm delivery Power (p) _WF ) The energy consuming device absorbs power (p _EDC ) And the onshore converter station DC side power (p _dc ) Can be expressed as:

wherein: u (u) _WF 、u _EDC And u _dc Respectively outputting voltage of wind power plant and energy consumption device and onshore current conversionStation dc side voltage.

The alternating current power output by the onshore converter station is

Wherein:and->Respectively by->And->And performing park forward conversion.

To protect the device from damage due to over-current, the ac current control must limit the current amplitude,the upper limit is(e.g., set to 1.1 times the rated current value). After the fault occurs, the maximum alternating current power which can be transmitted by the system is

Wherein:for alternating current +.>Clipping of the d-axis component of +.>Is the d-axis component of the ac voltage. (voltages and currents on alternating current side of onshore converter station)

Neglecting loss of direct current transmission line, wind farm output power p _WF With the DC active p of the input MMC _dc Equal, wind farm output voltage u _WF With MMC DC voltage u _dc Equal. In steady state, the DC power p of the input MMC _dc And output AC active power p _ac Equal, i.e.

Fig. 9 is a schematic diagram of a control strategy for a series-connected energy consumer, dc current control (failure mode). It is desirable to apply a DC current i _dc Control to a reference valueThe two are subtracted to obtain an error, the error is input into a proportional-integral controller (PI), and the output quantity is superimposed with the feedforward quantity +.>Then the output voltage reference value of the energy consumption device is obtained through a limiting link>Input signal S of energy consumption device _fault And a mechanical switch status signal S _BRK Will control how the energy consuming device acts.

And controlling the capacitance voltage of the submodule. The capacitance voltage of each sub-module of the energy consumption device is u _Cj (j is not less than 1 and not more than n), which are required to be controlled within a certain range, and are usually taken as rated values in engineering0.9 to 1.1 times of the total weight of the composition. Will u _Cj Input respective hysteresis control, output switch S of respective chopper energy consumption circuit _Rj Is a signal of (a).

Fig. 10 illustrates the voltage, current and power variations of a fault ride through device during a fault ride through. The rated voltage of the series-type energy consumption device is determined by the rated voltage of the direct current side of the onshore converter station and the standard percentage of fault ride-through.

The rated voltage of the series energy consumption device is calculated as follows:

When the voltage of the shore alternating current power grid drops to 0.2pu, the shore converter station can still output 0.2pu alternating current power, and the maximum power emitted by the offshore wind farm is rated power (1.0 pu), so that the maximum unbalanced power required to be consumed by the energy consumption device is 0.8pu. Rated current of energy consumption deviceEqual to the rated value of the direct current of the offshore wind power system +.>The consumer outputs a voltage setpoint value->0.8 times of the rated DC voltage of the offshore wind power system>

In the high-voltage large-capacity MMC engineering, a crimping IGBT with rated voltage of 4.5kV is generally selected as a power device, and certain consideration is given toAfter voltage margin, corresponding module capacitance voltage ratingTypically designed to be 2.1kV.

When all n energy consumption sub-modules of the energy consumption device are required to be input, the rated value of 80% direct-current voltage is required to bear at mostThe number of series sub-modules should be

The rated power of the n sub-modules is the rated power of the wind power plant, and is combined with the rated power of the DC side of the onshore converter stationSo the submodule resistance is

Selecting a smaller capacitor C reduces device size and cost, but the smaller capacitor will produce a larger voltage ripple, or higher S, when facing the same input power _R Switching frequency. S is S ₁ The switching frequency of (2) can be selected according to engineering common values, such as 100Hz. For a single sub-module, if the output voltage is 1pu when the DC current is constant, the absorbed power is maximum, so S should be determined under this condition _R Maximum switching frequency. S is S _R Is about the switching frequency of

Wherein: k is the ratio of the transmission power to the rated power,and C is the capacitance value of the submodule, and h is the difference between the threshold value and the rated value of the hysteresis-controlled capacitance voltage. When k=0.5, the switching frequency takes a maximum value.

For the working state of the fault ride-through device, the operation of the series energy consumption device is divided into the following 4 modes: a normal through-flow mode, a fault commutation mode, a fault energy consumption mode and a fault recovery commutation mode;

the excellent normal through-flow mode is used when the state of the fault crossing device is in a normal mode;

the excellent fault current conversion mode is used when the state of the fault crossing device is in transition from a normal mode to a fault mode;

the excellent fault energy consumption mode is used when the state of the fault crossing device is a fault mode;

And the excellent fault recovery commutation mode is used when the state of the fault crossing device is that the fault mode is transited to the normal mode.

Wherein:

the normal through-flow mode includes: putting into a circulation branch circuit and shorting out an energy consumption branch circuit;

the fault current conversion mode comprises the following steps: regulating the energy consumption sub-module on the energy consumption branch to output negative pressure, and switching off the mechanical switch on the circulation branch;

the fault energy consumption mode comprises the following steps: a circuit breaking circulation branch circuit, and an energy consumption branch circuit is input;

the fault recovery commutation mode includes: closing a mechanical switch on the flow path.

Specifically, 4 modes: normal through-flow, fault commutation, fault energy consumption and fault recovery commutation modes. Wherein S is _fault Input signal for energy consumption device (0 does not input, and the current branch is passed; 1 inputs, and the energy consumption branch is passed), S _BRK Is a status signal (0 closed, 1 open) of the fast mechanical switch.

1) Normal through-flow mode

When the system is operated without failure, the energy consumption device is not put into operation, and the mechanical switch is closed (S _fault ＝0，S _BRK =0), all the submodules are in the put-in state (S _{1_H} ＝S _{1_F} ＝S _{2_F} No input of energy dissipation resistor (S) _R =0), the direct current completely passes through the normal current branch (i ₁ ＝i _dc ) The energy consuming device consumes no power.

2) Failure commutation mode

When the power balance of the onshore converter station cannot be maintained, an energy consumption device needs to be put into, and a signal S is triggered _fault From 0 to 1, (S) _fault ＝1，S _BRK =0) the energy consuming device starts the commutation after receiving the signal. In order to transfer current from the current-carrying branch to the energy-consuming branch, the energy-consuming device output voltage must be made smaller than zero (u _EDC <0). The half-bridge submodule is completely cut off/bypassed (S _{1_H} =1), the output voltage is 0; full-bridge submodule outputs a negative voltage (S) _{1_F} ＝S _{2_F} =1). Because the stray inductance L is smaller, the commutation time is very short, and i is the time after the commutation is completed ₁ =0, at which time the fast mechanical switch S can be opened _BRK And (5) safely disconnecting.

3) Failure energy consumption mode

After the current conversion is finished, the mechanical switch S _BRK After disconnection (S) _fault ＝1，S _BRK =1), the energy consuming device enters energy consuming mode, i ₂ ＝i _dc . To make the energy consumption device output the reference voltageThe invention adopts a carrier phase-shifting based pulse width modulation (CPS-PWM) method. Voltage reference value +.>The duty cycle d can be calculated as

For the voltage reference value of the energy-consuming device, < >>Is the voltage reference value of the energy consumption device. The duty ratio d is the time duty ratio that the output voltage of the submodule is positive, the energy consumption submodule inputs and outputs the positive voltage for a duration T', and then cuts off and outputs the 0 voltage for a duration T ₁ -T', as shown in fig. 11. The carrier phase-shifting pulse width modulation requires that the output voltage of each sub-module is d with a duty ratio, the number of sub-modules is n, but the time lags behind delta T=T in sequence ₁ Rectangular wave of/n, T as shown in FIG. 12 ₁ Representing the period of the output voltage of the sub-module.

Because the current flowing through each sub-module in the series energy consumption device is the same, when CPS-PWM is adopted, the high precision of output voltage can be ensured, and the dissipation power is distributed evenly among all the sub-modules. Furthermore, the submodule capacitor voltage is balanced only by the respective switch S _R Control to maintain capacitor voltage at nominal valueIn the vicinity, a hysteresis comparator can be used for obtaining a switching signal, and the control is simple, real-time and quick.

Hysteresis comparator: setting 1 upper hysteresis limit and 1 lower hysteresis limit, e.g.And->Switch S _R Shut down (S) _R =0), the capacitor charges, and the capacitor voltage rises. When the hysteresis upper limit is 1.1, the switch S _R Conduction (S) _R =1), the capacitance discharges through the dissipation resistor R, and the capacitance voltage drops. When the hysteresis lower limit is 0.9, the switch S is further switched _R Shut down (S) _R =0), the capacitor charges. Such a process is repeated all the time, the capacitance voltage is always +.>And->The variation between these corresponds to controlling the average value to +. >As shown in fig. 13.

4) Failure recovery commutation modality

After the mechanical switch is closed (S) _fault ＝0，S _BRK =0), all the submodules reenter the put-in state (S _{1_H} ＝S _{1_F} ＝S _{2_F} =0). The n sub-modules apply reverse voltage to the inductor, and direct current is transferred from the energy consumption branch to the normal current branch.

The overall fault-ride-through operation is shown in fig. 14, and a circuit schematic diagram corresponding to the operation is shown in fig. 15.

In order to verify the effectiveness of the method and the applicability of the energy consumption device topology, a simulation model is built in PSCAD/EMTDC software, and specific parameters are shown in table 1. Because the invention researches a fault ride-through strategy of the alternating current side of the direct current series-connected offshore wind farm, the offshore wind farm is replaced by a controlled voltage source for simplicity.

TABLE 1 Key model simulation parameters

Two typical operating conditions are presented here as simulated waveforms:

working condition 1) transmitting power 0.2pu, DC current 0.2pu, wind field voltage 1.0pu, AC voltage dropping to 0.2pu

In the working condition, the transmission power is set to be 0.2pu, the direct current is set to be 0.2pu, the wind field voltage is set to be 1.0pu, the alternating voltage drops to be 0.2pu, the onshore converter station can maintain the power balance, and the simulation waveform is shown in fig. 16.

Working condition 2) transmission power 1.0pu, DC current 1.0pu, wind field voltage 1.0pu, AC voltage drop to 0.2pu

In the working condition, the transmission power is set to be 1.0pu, the direct current is set to be 1.0pu, the wind field voltage is set to be 1.0pu, the alternating voltage drops to be 0.2pu, the onshore converter station cannot maintain power balance, the energy consumption device is required to be put into fault ride through, and the simulation waveform is shown in figure 17.

In conclusion, the method is quicker and more reliable in response in engineering practice. The current alternating current side fault ride-through method is mainly aimed at a conventional flexible direct current transmission system adopting constant direct current voltage control, and energy consumption devices are connected in parallel at two ends of a converter station. Compared with the parallel energy dissipation devices of the same type, the serial energy dissipation device can adapt to the direct current voltage and current which are changed by the serial direct current offshore wind power system; the series energy consumption device can be designed according to 80% of rated voltage of a direct current transmission system, 20% of submodules are saved, and the volume and cost of the device are reduced. According to the invention, the direct-current side voltage of the converter station is reduced in the fault period, and direct-current side current control can be continuously carried out through the series-type energy consumption device, so that the direct-current stability and the MMC capacitor voltage stability are always kept before and after the fault.

It should be noted that the above embodiments are only for illustrating the technical solution of the present invention and not for limiting the scope of protection thereof, and although the present invention has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that various changes, modifications or equivalents may be made to the specific embodiments of the application after reading the present invention, and these changes, modifications or equivalents are within the scope of protection of the claims appended hereto.

Claims

1. An alternating current side fault ride-through device for a direct current series connection offshore wind farm is characterized by comprising a series connection type energy consumption device and a controller;

2. The apparatus of claim 1, wherein the series-connected energy dissipation device comprises a through-flow branch and an energy dissipation branch in parallel;

3. The apparatus of claim 2, wherein the energy consuming sub-modules comprise a full bridge energy consuming sub-module and a half bridge energy consuming sub-module; and the full-bridge energy consumption submodule and the half-bridge energy consumption submodule are connected in series and then connected between the direct-current side of the offshore wind farm converter and the line stray inductance.

4. The apparatus of claim 3, wherein the half-bridge energy consuming sub-modules and the full-bridge energy consuming sub-modules are each a plurality, and the number of half-bridge energy consuming sub-modules is greater than the number of full-bridge energy consuming sub-modules.

5. The apparatus of claim 1, wherein the rated voltage of the series-connected energy consuming device is a set percentage of the rated voltage of the dc side of the onshore converter station.

6. An ac side fault ride-through method for a dc series offshore wind farm, comprising:

the fault ride-through device is an alternating current side fault ride-through device for a direct current series offshore wind farm according to any one of claims 1-5.

7. The method of claim 6, wherein said adjusting the voltage of the series-connected energy consuming device and the onshore converter station comprises:

8. The method of claim 7, wherein the obtaining the voltage reference value of the series-connected energy consumption device using the direct current control in the fault mode based on the actual value of the current on the direct current line in the normal mode comprises:

9. The method of claim 8, wherein the voltage feedforward amount of the series-connected energy consuming device is calculated as follows:

10. The method of claim 7, wherein the deriving the voltage reference value of the onshore converter station using capacitive voltage regulation control in failure mode based on the average capacitive voltage value in the onshore converter station comprises:

11. The method of claim 10, wherein the voltage feedforward amount of the onshore converter station is calculated as follows:

in the method, in the process of the invention,for voltage feed-forward of an onshore converter station,/->For the d-axis component of the voltage on the ac side of the onshore converter station,for limiting the d-axis component of the current on the ac side of the onshore converter station,/i>For on-shore converter stationsCurrent reference on the current side.

12. The method of claim 6, wherein the rated voltage of the series-connected energy consuming device is determined by the rated voltage and a percent of fault ride-through criteria on the dc side of the onshore converter station.

13. The method of claim 12, wherein the voltage rating of the series-connected energy consuming device is calculated as follows:

14. The method of claim 6, wherein the operation of the series-connected energy consuming device is divided into the following modes: a normal through-flow mode, a fault commutation mode, a fault energy consumption mode and a fault recovery commutation mode;

15. The method of claim 14, wherein the normal through-flow mode comprises: putting into a circulation branch circuit and shorting out an energy consumption branch circuit;