CN115800357A

CN115800357A - Low-cost flexible-direct networking system receiving end alternating current fault ride-through method

Info

Publication number: CN115800357A
Application number: CN202211599327.6A
Authority: CN
Inventors: 王泽一; 庞丹; 王欣铎; 于洪涛; 王志鹏; 王振浩; 赵昌鹏; 张�浩; 王尉; 刘佳佳; 刘畅
Original assignee: Changchun Power Supply Co Of State Grid Jilinsheng Electric Power Supply Co; Northeast Dianli University
Current assignee: Changchun Power Supply Co Of State Grid Jilinsheng Electric Power Supply Co; Northeast Electric Power University
Priority date: 2022-12-14
Filing date: 2022-12-14
Publication date: 2023-03-14

Abstract

A low-cost flexible direct-current networking system receiving end alternating current fault ride-through method belongs to the technical field of flexible direct current transmission. The invention aims to provide the low-cost flexible-direct networking system receiving end alternating current fault ride-through method which can fully utilize the self energy storage potential of the converter station and the self response of a new energy unit, realize flexible-direct networking system receiving end alternating current fault ride-through without depending on a communication system or additionally arranging additional equipment, and reduce the cost required by fault ride-through. The method comprises the following steps: the method comprises an overvoltage suppression method for fully utilizing energy storage potential of a converter station, a wind power plant power speed reduction control method and a receiving end alternating current fault ride-through method for cooperatively matching the converter station and the wind power plant. The method provided by the invention can meet the requirement of receiving end alternating current fault ride-through under most working conditions, does not depend on remote communication, has reliability depending on a control protection system of an original flexible-direct networking system, fully utilizes the energy storage potential of the converter station and the self response of a new energy source unit, and can reduce the cost of fault ride-through.

Description

Low-cost flexible-direct networking system receiving end alternating current fault ride-through method

Technical Field

The invention belongs to the technical field of flexible direct current transmission.

Background

In recent years, a large-scale new energy source is paid extensive attention by a flexible direct networking delivery scheme due to high power supply reliability and low unit power transmission cost, but local faults are more easily diffused to the whole system through lines due to the interconnection characteristic of the network frame.

Once a receiving end alternating current fault occurs to the system, the energy input and the energy output of the flexible direct current power grid are not balanced any more, so that the direct current voltage crosses a safety limit within tens of milliseconds, overvoltage protection is triggered, a converter station is locked, and large impact is caused to the whole system. For temporary jump or instability of the system direct-current voltage caused by temporary imbalance of input and output energy of the flexible direct-current power grid, an existing solution is to put an energy consumption device into the system during a fault period to dissipate the imbalance energy in the form of heat energy. The energy consumption devices adopted in the engineering comprise 2 types of alternating current energy consumption devices and direct current energy consumption devices. The alternating current energy consumption device is generally formed by connecting an anti-parallel thyristor and an energy consumption resistor in series, the energy consumption resistor is arranged on the alternating current side of the sending end converter station through voltage reduction and transformation groups, the smaller the single group capacity is, the higher the power control precision is, the smaller the impact of resistor switching on direct current voltage is, but the engineering cost is also correspondingly improved. The direct current energy consumption device is generally formed by connecting a plurality of Insulated Gate Bipolar Transistors (IGBTs) and energy consumption resistors in series, is arranged on a direct current circuit, and controls the switching of the direct current circuit in a mode of setting upper and lower thresholds of direct current voltage so as to dissipate unbalanced energy caused by faults. Both ac and dc energy consuming devices require sufficient space, require complete cooling equipment, and require periodic maintenance. By combining the above analysis, the existing receiving-end ac fault ride-through method (equipped with energy consumption device) has the problems of high construction cost, large floor area, additional heat dissipation cost, and the like. Therefore, there is a need in the art for a low-cost termination fault ride-through technique for a flexible-straight system.

Disclosure of Invention

The invention aims to provide the low-cost flexible-direct networking system receiving end alternating current fault ride-through method which can fully utilize the self energy storage potential of the converter station and the self response of a new energy unit, realize flexible-direct networking system receiving end alternating current fault ride-through without depending on a communication system or additionally arranging additional equipment, and reduce the cost required by fault ride-through.

The method comprises the following steps:

s1, overvoltage suppression method for fully utilizing energy storage potential of converter station

Introducing a self-adaptive pressure limiting coefficient alpha, multiplying the number of bridge arm unit input sub-modules output by a valve level controller of the converter station by the coefficient alpha, wherein the value range of the alpha is given by the pressure limiting controller and is [0,1];

a. voltage limiting controller design

If n-end converter stations in the m-end flexible direct-current power grid adopt an ANLM strategy, the equivalent capacitance value of the flexible direct-current power grid is calculated as follows:

let t =0 time fault occur, t = t _A At the moment, the number of submodules starts to be adaptively reduced to limit the DC voltage of the flexible DC power grid, t = t _B At the moment, the fault recovery or unbalanced power is cut off, the number of sub-modules starts to recover gradually, and t = t _C The number of submodules is restored to the nominal value, 0-t _C The unbalanced power of the flexible direct current power grid in the time period is delta P;

at 0-t _C And the direct-current voltages all satisfy the following expression:

at 0-t _C And the sub-module capacitor voltage satisfies the following expression:

to make the DC voltage at t _A -t _C The duration being limited to AU _d (0) Nearby, C _eq (t) the following expression needs to be satisfied:

in the formula: a is the ratio of the DC voltage target value to the steady state value;

a combination of formula (9) and formula (10), wherein formula (11) is:

order to

Further, if the DC voltage t is required _A -t _C The duration being limited to AU _d (0) In the vicinity, the voltage limiting coefficient α needs to satisfy:

in the formula: b (t) = U _C (t)/U _C (0)；

From equations (9) and (14), the relationship between B (t) and α can be obtained as follows:

in particular, when m = n, k ₁ ＝1，k ₂ And =0. At this time, the expressions of B (t) and α are as follows:

b. sub-module number recovery procedure

At t _B -t _C And in addition, the energy change of the whole flexible direct current power grid meets the following formula:

in the formula: e _N0i Storing energy for the converter station i in steady state operation;

s2, a wind power plant power speed reduction control method comprises the following steps:

the wind power plant can be rapidly reduced by reducing the d-axis component of the voltage of the bus bar of the wind power plant. The d-axis component of the voltage of the wind power plant bus bar can be actively controlled by a transmitting end converter station of the flexible direct current power grid.

a. Design of sending end converter station controller with additional AC voltage reduction link

u _WF,abc 、i _WF,abc Respectively the voltage and the current of a bus bar of the wind power plant; u. of _WF,d 、u _WF,q Respectively a d-axis component and a q-axis component of the wind power plant busbar voltage; i.e. i _WF,d,ref 、i _WF,q,ref 、i _WF,d 、i _WF,q Respectively obtaining a d-axis target value, a q-axis target value, a d-axis component and a q-axis component of the wind power plant busbar current; omega is angular frequency of wind power plant busbar voltageRate; theta is the phase of the voltage of the wind power plant busbar; l is inductance at the outlet of the sending end converter station; m, M _d 、M _q The modulation ratio of the converter station, the d-axis component and the q-axis component of the modulation ratio, respectively, T being the time constant of the first order low pass filter.

When a hysteresis comparator in the designed controller detects that the direct current voltage of the flexible direct current power grid is rapidly increased, the occurrence of the alternating current fault of the receiving end is judged, and the target value of the d-axis component of the voltage of the bus bar of the wind power plant is actively adjusted to be low.

b. In the process of bus voltage reduction, | i _WF,d |＝i _SE,max Wherein i _SE,max For the current limiting amplitude of the sending end converter station, the change of the voltage of the wind power plant busbar approximately meets the following formula:

in the formula: c _WF Aggregating equivalent capacitance for wind farms on the wind farm bus bar, i _WF,d0 D-axis component of wind power plant outlet current in steady state;

and further obtaining the voltage change of a bus bar of the wind power plant of 0.8p.u.the required time is calculated as follows:

in the formula: u. of _b And i _b The reference values of the voltage and the current of the bus bar of the wind power plant are obtained;

the reference values of the voltage and the current are taken as follows:

in the formula: s _b Is a power reference value of the wind power plant;

during the whole voltage reduction process, the reactive current target value of the GSVSC is given as follows:

in the formula: u. of _WF,N A rated value of the voltage of the wind power plant bus bar; i.e. i _max The maximum current which can be borne by the GSVSC;

meanwhile, the active current target value of the GSVSC is given as follows:

in order to ensure that the wind Power plant has the capability of low voltage ride through, a chopper resistor R is required to be arranged between the positive electrode and the negative electrode of a direct current circuit of a Full Power Converter (FPC) in the wind turbine generator. Along with the decline of wind-powered electricity generation field busbar voltage, there can be a large amount of unbalance energy accumulation on FPC's direct current bus, need install the chopper resistance on FPC positive negative pole generating line this moment and put into use, absorb the inside unbalance energy of wind turbine generator system, guarantee wind turbine generator system steady operation.

The switching of the chopper resistor is controlled by setting the upper and lower thresholds of the voltage of the FPC DC bus, and the upper threshold is set as U _d,WT,max Then the size of the chopper resistor is designed as follows:

in the formula: p _WT,max The maximum power which can be generated by the wind turbine generator is obtained;

s3, receiving end alternating current fault ride-through method with converter station and wind power plant cooperatively matched

Setting a receiving end alternating current fault t = t of a system ₀ Causing the direct current voltage of the flexible direct current power grid to continuously rise until the sending end converter station detects the receiving end alternating current fault t = t ₁ Then, the target value of the voltage of the bus bar of the sending end converter station is set to be 0.2p.u., after a short control delay, the sending end converter station starts to realize load reduction of the wind power plant by t = t in cooperation with a chopper resistor in the wind turbine generator ₂ And finishing load shedding t = t in the wind power plant ₄ Previously, wind farms continuously inject imbalance energy into the gridThe ANLM strategy suppresses the DC overvoltage caused by unbalanced energy; ANLM strategy takes over DC voltage control period t ₃ -t ₅ Should be greater than 1.05p.u.; if the receiving end AC fault is recovered within the specified time, the number of the submodules and the DC voltage are gradually recovered to the rated value t ₄ -t ₆ In order to ensure that the sub-module capacitor energy storage is released quickly, a hysteresis comparator in a sending end converter station controller is required to detect that the direct-current voltage is recovered to 1p.u, then, the target value of the voltage of a bus bar of the wind power plant is gradually increased, so that the wind power plant is recovered to normal operation, and then the whole system is gradually recovered to steady-state operation; if the receiving end alternating current fault is not recovered within the specified time, the transmitting end converter station disconnects the alternating current side circuit breaker, and then the wind power plant is disconnected and stops running.

The method provided by the invention can meet the requirement of receiving end alternating current fault ride-through under most working conditions, meanwhile, the method does not depend on remote communication, the reliability of the method depends on a control and protection system of an original flexible-direct networking system, no additional unreliable factors are introduced, additional equipment does not need to be additionally arranged, the energy storage potential of the converter station and the self response of a new energy source unit are fully utilized, and the cost of fault ride-through can be reduced.

Drawings

FIG. 1 is a structural diagram of a large-scale wind power flexible-straight networking system;

FIG. 2 is a graph comparing theoretical calculation with simulated DC voltage;

FIG. 3 is a working schematic diagram of an adaptive NLM strategy;

FIG. 4 is a transmitting end converter station controller with an additional step down AC voltage link;

FIG. 5 is a structure diagram of a permanent magnet direct drive wind turbine generator;

FIG. 6 is a fault-crossing timing logic diagram;

FIG. 7a is a plot of the direct voltage of the results of an ANLM strategy simulation;

FIG. 7b is a graph of a single sub-module voltage plot of the results of an ANLM strategy simulation;

FIG. 7c is a graph of the number of sub-modules of the simulation result of the ANLM strategy;

FIG. 7d is a graph of energy storage in the converter station as a result of ANLM strategy simulation;

FIG. 7e is a plot of the ANLM strategy simulation results for the converter station modulation ratio;

FIG. 7f is a graph of the ANLM strategy simulation results for the converter station AC side power;

FIG. 8a is a direct current voltage graph of a simulation result of a wind power plant load shedding control strategy;

FIG. 8b is a graph of the converter station alternating current side power curve of the wind farm load shedding control strategy simulation result;

FIG. 8c is a voltage amplitude graph of a bus of the wind farm 2 as a result of the wind farm load shedding control strategy simulation;

FIG. 8d is a graph of the output power of the wind farm 2 as a result of the wind farm load shedding control strategy simulation;

FIG. 8e is a chopper resistor switching control curve diagram of a wind power plant load shedding control strategy simulation result;

FIG. 8f is a chopper resistor absorbed energy curve diagram of a wind power plant load shedding control strategy simulation result;

FIG. 9a is a simulation result DC voltage plot of the proposed fault ride-through strategy;

FIG. 9b is a graph of a single sub-module voltage plot of the simulation results of the proposed fault-ride-through strategy;

FIG. 9c is a graph of the voltage amplitude of the busbar of the wind farm 2 as a result of a simulation of the proposed fault ride-through strategy;

FIG. 9d is a graph of the simulation result wind farm 2 output power for the proposed fault ride-through strategy;

FIG. 9e is a graph of chopper resistance absorbed energy from a simulation of the proposed fault ride-through strategy;

FIG. 9f is a simulation result converter station energy storage graph of the proposed fault ride-through strategy;

FIG. 9g is a graph of the number of sub-modules of the simulation result for the proposed fault ride-through strategy;

FIG. 10a is a graph comparing simulation results versus mid-wind field 2 bus voltage;

fig. 10b is a graph of simulation results versus stroke field output power.

Detailed Description

The following detailed description is made in conjunction with the accompanying drawings:

1. large-scale wind power flexible-straight networking system structure and MMC basic working principle

1.1 Large-scale wind power warp-soft-straight networking system structure

A large-scale wind farm based on a Permanent Magnet Synchronous Generator (PMSG) adopts four-terminal true bipolar ring wiring with a flexible operation mode through a flexible direct networking system, and the structure of the large-scale wind farm is shown in fig. 1. The respective converter station information is shown in table 1.

Table 1 basic information of the converter stations

Tab.1Basic information for each converter station

1.2 basic working principle of half-bridge type MMC

In a Half-bridge MMC structure commonly used in the flexible and straight engineering, each phase of an upper bridge arm and a lower bridge arm is formed by connecting N Half-bridge sub-modules (HBSM), a bridge arm resistor and a bridge arm reactor in series. The half-bridge MMC mostly adopts an NLM strategy as a modulation mode. Firstly, the number of submodules put into an MMC single-phase bridge arm is controlled to be N all the time by an NLM strategy, and the voltage U of the MMC direct-current side is ensured _d The size is constant. And secondly, the NLM strategy generates a corresponding three-phase voltage waveform on the alternating current side by controlling the switching of the bridge arm sub-modules. The alternating current-direct current side voltage of the MMC meets the following relational expression:

in the formula: m is the modulation ratio of MMC output, u _sm The MMC AC side can output ideal AC voltage only when M is within the range of 0-1, which is the target value of the output voltage of the MMC AC side.

2. Wind power receiving end alternating current fault transient overvoltage mechanism analysis through flexible-direct networking system

2.1 System Steady State operation analysis

A flexible direct current power grid consisting of m (m is more than or equal to 3) end converter stations is arranged, eachAll stations adopt vector current control, and each station can realize independent control of active power and reactive power of an alternating current side. For a scene that large-scale wind power is networked through a flexible direct-connection network, when the wind power station runs in a steady state, the sending end converter station needs to provide reliable voltage for wind power station grid connection, namely d-axis and q-axis components of alternating-current side voltage are kept unchanged under the control of the sending end station, and the collected wind power is only related to wind power station outlet current as shown in a formula (2); the alternating-current side voltage of the receiving end converter station is given by the alternating-current power grid connected with the receiving end converter station, and the receiving end converter station can control i _d,RE And i _q,RE Realizing active control on the power of the alternating current side of the converter, as shown in formula (3)

In the formula: subscript SE represents the transmitting end converter station, subscript RE represents the receiving end converter station, subscript WF represents the wind farm, and subscript GS represents the receiving end ac grid.

When the system operates in a steady state, the stored energy of each converter station keeps dynamic balance, the AC and DC sides of each converter station and the submodules are periodically stored and released, only a place for energy interchange is provided for the AC and DC sides, and the whole flexible DC power grid has the following energy balance expression within an AC voltage cycle:

in the formula: p _loss For the grid loss, p, of the flexible grid _i For the AC side power, P, of each converter station _i For each converter station dc side power i =1,2,3, ·, m.

2.2 transient overvoltage mechanism

When the system has a receiving end alternating current fault, the output active power of the system continuously decreases, and as can be known from the formula (4), if the active power continuously collected at the alternating current side of the transmitting end converter station is larger than the residual power transmission limit of the receiving end converter station in the period, unbalanced energy can be continuously accumulated in the flexible direct current power grid, and the converter station serves as a main energy storage device in the flexible direct current power grid and takes the task of temporarily storing the unbalanced energy.

During the receiving end alternating current fault period, compared with the energy storage change of the sub-module capacitor, the energy storage change of the bridge arm inductor in the converter station is extremely small, and for simplifying analysis, the energy storage of the converter station can be approximate to the energy storage of the sub-module capacitor.

During the receiving end alternating current fault period, under the combined action of capacitor voltage sequencing and a voltage-sharing algorithm, 6N sub-modules in each station are switched alternately, the sub-module voltages rise in turn, and finally unbalanced energy is absorbed.

If the fault occurs at the moment t =0, the expression of the continuous rising process of the sub-module capacitor voltage in the flexible direct current power grid during the fault is as follows:

in the formula: delta P _∑ For unbalanced power in the entire flexible DC network, C _0i Is the sub-module capacitance value of each converter station.

Since the regulation speed of the fixed dc voltage station is slower than the charging speed of the sub-module capacitor, it can be assumed that the unbalanced power of the entire flexible dc grid during the fault is a fixed value Δ P, and equation (5) can be further:

under the action of the traditional NLM strategy, the direct current voltage is supported by the voltage of the N sub-modules, so that the process of further obtaining unbalanced energy to cause the direct current voltage to rise is as follows:

in the formula: c _eq Is the equivalent capacitance of the whole flexible direct current power grid.

Under the extreme condition that alternating current short-circuit faults occur at the receiving end of the system, the most serious fault scenario is that a near-zone three-phase alternating current short-circuit fault occurs at a receiving end converter station MMC4 with a large capacity, and the unbalanced power in a single-pole loop of the system shown in the figure 1 is about 1300MW under the consideration of the regulation effect of a fixed direct current voltage station. Fig. 2 shows the calculated dc voltage of the flexible dc network in comparison with a simulated value in this fault scenario.

The parameters of table 2 were substituted into formula (7) to obtain: at 75ms after the fault occurs, the direct current voltage can cross 650kv (1.3 p.u.), so that the flexible direct current power grid is locked, which is very close to the simulation result (72 ms) shown in fig. 2, and the reasonability of formula derivation is verified.

3. According to the ANLM strategy study, as shown in the formula (7), if the capacitance of the sub-modules of the converter station can be increased or the number of the sub-modules put into the converter station can be reduced, the direct-current overvoltage caused by the temporary imbalance of the input energy and the output energy can be suppressed. In practical engineering, increasing the sub-module capacitance value during a fault is difficult to achieve. Therefore, the present section proposes an ANLM strategy, introduces a self-adaptive voltage limiting coefficient α based on the conventional NLM strategy, and multiplies the number of bridge arm unit input sub-modules output by a valve stage controller of a converter station by the coefficient α, where α is given by the voltage limiting controller and has a value range of [0,1]. When the receiving end station has a fault and the direct-current voltage rapidly rises, the additional voltage limiting controller is adopted to give a voltage limiting coefficient alpha to reduce the number of sub-modules put into the converter station at the same time, increase the equivalent capacitance value of the converter station, limit the direct-current voltage and fully excavate the energy storage potential of the converter station.

3.1 Voltage limiting controller design

let t =0 time instant fault occur, t = t _A At the moment, the number of submodules starts to be adaptively reduced to limit the DC voltage of the flexible DC power grid, t = t _B At the moment, fault recovery or unbalanced power is cut offThe number of modules starts to recover gradually, t = t _C The number of time submodules is restored to the nominal value.

At 0-t _C And the direct-current voltages all satisfy the following expression:

in the formula: a is the ratio of the DC voltage target value to the steady state value.

Conjunctive formula (6) and formula (7), the above formula can be:

order to

Further, if the DC voltage t is required _A -t _C The duration being limited to AU _d (0) Nearby, the voltage limiting coefficient α needs to satisfy:

in the formula: b (t) = U _C (t)/U _C (0) Can be obtained by combining the formula (6) and the formula (9)

In particular, when m = n, k ₁ ＝1，k ₂ ＝0

As can be seen from equation (15), when each converter station in the flexible direct current power grid adopts the ANLM strategy, the dc voltage is limited to be near the target value only by requiring that the voltage limiting coefficient α be adaptive to the variation of the average value of the capacitor voltage of the submodule in the flexible direct current power grid. And at this time, the sub-module capacitor voltage of the flexible direct current power grid during the fault period can be considered to be approximately equal to U _d (t)/α N, which makes the design of the voltage limiting controller more convenient.

Based on the above analysis, when each converter station in the flexible direct current power grid adopts the ANLM strategy, the implementation manner of the ANLM strategy is shown in fig. 3.

In FIG. 3, the coefficient K _G The magnitude of the direct current voltage is determined by a direct current voltage target value during the self-adaptive change of the number of the submodules, and during the alternating current fault of a receiving end, the flexible direct current power grid needs to improve the power sending capacity by increasing the active current component of a receiving end station, so that the direct current voltage target value is selected to be larger than a direct current voltage steady-state value and lower than a direct current voltage safety limit value.

As can be seen from equation (1), during the period of limiting the dc voltage by the ANLM strategy, as long as the ac voltage target value of the health station is within the interval of 0 to a, the modulation ratio output by the converter station will not exceed the limit, and still within the modulation range, the operation safety of the ac side of the health station will not be affected.

3.2 sub-Module number recovery Process analysis

t _B At the moment, unbalanced power is removed, the sub-module capacitor energy storage begins to be gradually released, and the input number of the sub-modules of each converter station is gradually recovered. At t _B -t _C And in addition, the energy change of the whole flexible direct current power grid meets the following formula:

in the formula: e _N0i The energy storage is the energy storage of the converter station i in steady operation.

In steady state, the generation and consumption of energy in the converter station is considered to be a dynamically balanced processThe energy storage of the time-varying converter station i is of a size E _N0i The constant value of (2).

As can be seen from equation (16), the length of time required for the number of submodules to recover depends on the difference between the output and input powers of the flexible dc power grid.

3.3 comparative analysis of ANLM strategy and traditional NLM strategy

If α is 1 and remains constant during the fault, the rising trends of the dc voltage and the submodule capacitor voltage remain the same, in order to ensure that the dc voltage increase does not exceed the maximum permissible value Δ U _dmax The energy storage increment of the capacitor of a single sub-module has the following limitation:

at the same time, it is also limited by the voltage withstand constraints of the sub-module capacitors:

in the formula: delta U _Cmax And increasing the maximum allowable value for the sub-module capacitor voltage.

Therefore, during the power imbalance of the flexible direct current power grid, under the traditional NLM strategy, the energy storage increment of the capacitor of a single sub-module is constrained as follows:

ΔE _C ≤min(ΔE _Cmax1 ,ΔE _Cmax2 ) (19)

however, if during the fault, α can change adaptively according to the rise of the mean value of the sub-module capacitor voltage, the dc voltage is limited and stabilized, and the sub-module capacitor energy storage only needs to satisfy the constraint of equation (18).

In practical engineering, the withstand voltage multiple of the sub-module capacitor and the IGBT is larger than the safety limit value of the system direct-current voltage, namely delta U _dmax <NΔU _Cmax Therefore, under the ANLM strategy, the energy storage upper limit of the capacitor of a single sub-module is higher. The difference value of the energy storage upper limit of the flexible direct current power grid under the ANLM strategy and the traditional NLM strategy is calculated as follows:

further, the expression of the minimum time t' that the ANLM strategy can strive for system processing faults is as follows:

in the formula: delta P _max The maximum unbalanced power caused by the fault.

Combining the above analysis, the ANLM strategy has the following advantages compared to the conventional NLM strategy:

1) From the viewpoint of energy utilization efficiency. The ANLM strategy properly improves the voltage of the sub-modules of each station by reducing the number of the sub-modules of each station under the condition that the voltage resistance of the sub-modules is not exceeded, and the unbalanced energy caused by alternating current faults of receiving ends is absorbed by fully utilizing the voltage margin of the sub-modules. After the receiving end is recovered to be normal, unbalanced energy absorbed by the sub-module capacitor is stably released into a receiving end power grid, and the utilization efficiency of the energy is improved.

2) From the point of view of fault handling time. The ANLM strategy strives for more sufficient time for accurately switching the energy consumption devices and reducing the load of the wind power plant, and the fault ride-through capability of the system is improved.

The safety limit for the dc voltage is 1.3p.u., while the submodule usually has a withstand voltage limit of 2p.u ^[20] However, since the sub-module voltage fluctuates greatly at the end of the fault, it is more appropriate to set the safety limit to 1.5p.u. The parameters of Table 2 were substituted into the formulae (20) and (21): compared with the traditional NLM strategy, the ANLM strategy enables the energy storage upper limit of the flexible direct current power grid to be improved by about 128MJ, and meanwhile, the fault handling time of at least about 98ms can be strived for the system. Namely, under the ANLM strategy, the upper limit of the energy storage of the flexible direct current power grid is about 270MJ, and the ride-through of any type of receiving end alternating current fault with the duration not exceeding 170ms can be realized.

However, if the receiving-end ac fault lasts for a long time, the sub-module may be damaged and the converter station may be threatened to operate safely because the sub-module absorbs excessive unbalanced energy and the voltage of the sub-module rises to a safe boundary value, and the proposed ANLM strategy cannot implement fault ride-through of the dc overvoltage caused by the unbalanced energy alone, and can be completed only by cooperating with other control strategies.

4. Receiving end alternating current fault ride-through strategy based on cooperative matching of ANLM strategy and chopper resistor inside wind turbine generator set 4.1 wind power plant rapid load shedding control strategy

From the equation (2), it can be seen that the wind farm can be rapidly derated by reducing the d-axis component of the wind farm bus voltage. In order to avoid the locking of the sending end converter station caused by the over-high voltage reduction rate, an alternating voltage-alternating current double closed-loop control structure is mostly adopted in the engineering, and a sending end converter station controller with an additional alternating voltage reduction link is designed as shown in fig. 4.

In fig. 4: u. of _WF,abc 、i _WF,abc Respectively the voltage and the current of a bus bar of the wind power plant; u. of _WF,d 、u _WF,q Respectively representing a d-axis component and a q-axis component of the voltage of a bus of the wind power plant; i.e. i _WF,d,ref 、i _WF,q,ref 、i _WF,d 、i _WF,q Respectively obtaining a d-axis target value, a q-axis target value, a d-axis component and a q-axis component of the current of the wind power plant bus; omega is the angular frequency of the wind power plant busbar voltage; theta is the phase of the voltage of the wind power plant busbar; l is an inductor at an outlet of the sending end converter station; m, M _d 、M _q The modulation ratio of the converter station, the d-axis component and the q-axis component of the modulation ratio, respectively, T being the time constant of the first order low pass filter.

When the hysteresis comparator detects that the direct-current voltage is rapidly increased, the controller judges that the alternating-current fault of the receiving end occurs, and the unbalanced energy fed into the flexible direct-current power grid by the wind power plant is reduced by reducing the voltage target value of the wind power plant bus bar.

As shown in FIG. 4, u _WF,q Under the control of the sending end converter station, u is approximately 0 and can be approximately considered _WF,d Namely, the voltage of the bus bar of the wind power plant is considered according to the most serious fault situation, the voltage of the bus bar needs to be gradually reduced to 0.2p.u.nearby, and the wind power plant can not feed unbalanced energy into the flexible direct current power grid any more. In order to make the voltage track the target value quickly during the voltage drop of the bus bar i _WF,d Will increase all the time, but because ofThe existence of an inner loop current amplitude limiting link, finally i _WF,d |＝i _SE,max Wherein i _SE,max For the current limiting amplitude of the sending end converter station, the change of the voltage of the wind power plant busbar approximately meets the following formula:

in the formula: c _WF Aggregating equivalent capacitance for wind farms on the wind farm bus bar, i _WF,d0 Is the d-axis component of the wind farm outlet current at steady state.

When the wind farm is operating at full load, i _WF,d0 Is 1p.u. The converter station usually has 1.1 times the overload operation capability, i.e. i _SE,max Is 1.1p.u.

Therefore, the voltage change of the bus bar of the wind power plant is 0.8p.u. the required time is calculated as follows:

in the formula: u. u _b And i _b The reference values of the voltage and the current of the wind power plant bus bar are provided.

The reference values of the voltage and the current are taken as follows:

in the formula: s _b Is the power reference value of the wind power plant.

The time required by the voltage change of the bus bar of the wind power plant in full-load operation to be 0.8p.u.can be calculated by the combined type (23) and the formula (24), and the time required by the voltage of the bus bar to be reduced to 0.2p.u.considering the controller delay of 30ms is about 180ms.

4.2 Low Voltage ride through control of wind turbines

The common structure of the large-scale wind power plant internal unit based on the PMSG is shown in figure 5.

Each PMSG in the wind farm is connected to a flexible direct current Power Grid through a Full Power Converter (FPC), each FPC is composed of a Machine side voltage source type Converter (MSVSC) and a Grid side voltage source type Converter (GSVSC), the MSVSC is controlled by Maximum Power Point Tracking (MPPT), the control target is that it is expected to be a PV node, and the control target is matched with pitch control to ensure that the output Power of the PMSG is constant, the GSVSC is controlled by constant direct current voltage, and reactive Power is controlled while stabilizing the direct current voltage of the FPC. In order to ensure that the wind power plant has the low voltage ride through capability, a chopper resistor R is required to be arranged between the positive electrode and the negative electrode of the direct current circuit of each FPC.

When the voltage of a bus bar of the wind power plant is reduced to be below 0.9p.u, the wind generation set enters a low voltage ride through mode, the GSVSC needs to operate in a current-limiting control mode with reactive current priority, and in the whole voltage reduction process, the reactive current target value of the GSVSC is given as follows ^[21] ：

In the formula: u. of _WF,N A rated value of the voltage of the wind power plant bus bar; i.e. i _max The maximum current that the GSVSC can withstand.

Meanwhile, the active current target value of the GSVSC is given as follows:

as can be seen from the formula (26), as the bus bar voltage of the wind power plant decreases, the output active power of the GSVSC will be greatly reduced, and the MSVSC continuously collects the electric energy in the period, so that a large amount of unbalanced energy is accumulated on the DC bus bar of the FPC, and the DC bus bar voltage U is enabled to be higher than the output active power of the GSVSC _d,WT And rapidly rises. At the moment, the chopper resistors which need to be installed on the positive and negative electrode buses of the FPC are put into use, unbalanced energy inside the wind turbine generator is absorbed, the FPC direct-current overvoltage is restrained, and the stable operation of the wind turbine generator is guaranteed.

Switching of the chopper resistor is controlled by setting the upper and lower thresholds of the voltage of the FPC direct current bus in engineeringSetting its upper limit threshold value as U _d,WT,max (1.15 times the rated dc voltage is selected here), the size of the chopper resistor is designed as follows:

in the formula: p _WT,max The power can be the maximum power which can be generated by the wind turbine generator.

4.3 sequential logic for Fault ride-through

The system can be ensured to safely and stably continuously run within the specified time of fault ride-through the cooperative coordination of the ANLM strategy and the wind power plant load reduction control strategy, and the sequential logic is shown in FIG. 6.

Receiving end alternating current fault of the system (t = t) ₀ ) Causing the direct-current voltage of the flexible direct-current power grid to continuously rise until the sending end converter station detects the receiving end alternating-current fault (t = t) ₁ ) Then, the target value of the voltage of the bus bar of the sending end converter station is set to be 0.2p.u., and after a short control delay, the sending end converter station starts to realize load reduction of the wind power plant in cooperation with a chopper resistor in the wind turbine generator (t = t) ₂ ) And load shedding is finished in the wind power plant (t = t) ₄ ) Previously, wind farms continuously inject unbalanced energy into the grid, which may cause dc overvoltages if no measures are taken, and the ANLM strategy proposed herein suppresses dc overvoltages caused by unbalanced energy. To avoid the converter station at the transmitting end from mistaking that the fault is recovered, the ANLM strategy takes over the direct-current voltage control period (t) ₃ -t ₅ ) Should be greater than 1.05p.u. If the receiving end AC fault is recovered within the specified time, the sub-module number and the DC voltage are gradually recovered to the rated value (t) ₄ -t ₆ ) In order to ensure that the sub-module capacitor stored energy is quickly released, a hysteresis comparator in the sending end converter station controller is required to detect that the direct-current voltage is recovered to 1p.u. If the receiving end alternating current fault is not recovered within the specified time, the transmitting end converter station disconnects the alternating current side circuit breaker, and then the wind power plant is disconnected and stops running.

In the action process of the strategy, unbalanced energy caused by the fault is stored by the converter station and is gradually absorbed by chopper resistors inside the wind turbine generators. In order to enable the wind turbine generator set to have the capacity of coping with the fault risk, a chopper resistor is arranged in each generator set, and the ANLM strategy only utilizes the voltage margin of the sub-modules to enable the converter station to temporarily absorb unbalanced energy, so that extra equipment does not need to be additionally arranged, and extra cost is not increased. The chopper resistors are respectively arranged in each wind turbine generator and are usually 2-5 omega ^[22] The volume is very small, compared with the direct configuration of an energy consumption device, the occupied area cost can be reduced, and meanwhile, the heat dissipation pressure can be effectively relieved due to the small resistance value.

According to the current development progress of flexible direct current primary equipment, the switching time response scale of a control submodule of a converter station valve control system is about 180-315 mu s ^[23] The transient response of the direct-current fault overvoltage is matched sufficiently, the switching of the chopper resistor only depends on the local signal of the wind turbine generator, remote communication is not needed, and the reliability and the rapidity of the provided strategy meet the requirements.

5. Simulation analysis

A simulation model of the wind power grid-connected system with four flexible ends and a straight end as shown in the figure 1 is built in the PSCAD. The detailed simulation parameters of each station are shown in table 2, the

wind power plants

1 and 2 are respectively formed by 750 equivalent wind power generation units and 1500 equivalent wind power generation units with the rated power of 2MW through single-machine multiplication, and the wind power generation units are structurally shown in fig. 5. To ensure the simulation accuracy, the simulation step size and the control step size are set to 25us and 200us, respectively.

TABLE 2 converter station simulation parameters Tab.2Parameters of MMC

In the section, by taking the near-zone three-phase alternating-current short-circuit fault with different durations of the occurrence of the MMC4 as an example, 3 different receiving-end alternating-current fault ride-through strategies are simulated and contrastively analyzed.

5.1 use of ANLM strategy only

In the simulation working condition, the valve level control strategies of all converter stations in the flexible direct current power grid all adopt ANLM strategies, and the target value of direct current voltage during the self-adaptive change period of the sub-module is selected to be 1.12pu. The failure start time is set to t =2.0s and the duration is set to 170ms. The simulation results are shown in fig. 7.

As can be seen from fig. 7 (a) - (c), the ANLM strategy limits the dc voltage of the grid to around 560kV (1.12p.u.) by means of sub-module adaptive switching during fault ride-through. In addition, the voltage of each station submodule does not exceed 3.3kV (1.5p.u.), is far lower than the withstand voltage limit, and cannot be damaged. As can be seen from fig. 7 (d), the flexible dc network continues to absorb the unbalanced energy of about 268MJ by means of the converter station throughout the fault. As can be seen from fig. 7 (e) and (f), the modulation ratios of the healthy stations during fault ride-through are both within the modulatable range, the voltage on the ac side of the healthy stations is kept stable, and the ANLM strategy does not affect the power transmission on the healthy ac side. The simulation results of fig. 7 are substantially in accordance with the inference in section 3.3 herein. However, if the failure lasts longer, the sub-modules are at risk of being damaged. In conclusion, the ANLM strategy can realize the ride-through of the short-time receiving end alternating current fault through the short-time energy storage of the sub-module, but cannot independently realize the ride-through of the receiving end alternating current fault with longer duration.

5.2 Rapid load shedding control strategy only adopting wind power plant

In the simulation working condition, the design of the MMC2 controller is shown in FIG. 4, and the valve level control strategies of all the stations are the traditional NLM strategies. The failure start time is set to be t =2.0s, the duration is set to be 600ms, and the control delay of the wind farm and the transmitting end station is considered to be 30ms. The simulation results are shown in fig. 8.

As can be seen from fig. 8, due to the limitation of the current loop and the influence of the control delay, the load reduction control strategy for the wind farm proposed in sections 4.1 to 4.2 cannot reduce the load in time at the initial stage of the fault. Before the wind power plant is greatly unloaded, the unbalanced energy can be continuously accumulated by the flexible direct current power grid, so that the direct current voltage exceeds a safety threshold value of 650kV (1.3 p.u.). Therefore, the load reduction control of the wind power plant proposed in subsection 4.1-4.2 cannot independently realize the crossing of the deep receiving end alternating current fault of the system.

5.3 cooperative fault ride-through strategy as proposed herein

The simulation working condition adopts a cooperative fault ride-through strategy provided by subsection 4.3, the MMC2 controller is designed as shown in FIG. 4, and the valve level control strategies of all converter stations adopt an ANLM strategy. The control delay of the fault scenario and the wind farm and the sending end station is consistent with subsection 5.2. The simulation results are shown in fig. 9.

As can be seen from fig. 9 (a), the proposed strategy limits the dc voltage to below 650kV (1.3p.u.) throughout the fault ride-through. As can be seen from fig. 9 (d) - (g), only before the wind farm is greatly reduced in load, the ANLM strategy absorbs the unbalanced energy accumulated by the flexible direct current power grid by reducing the number of sub-modules and increasing the equivalent capacitance of the converter station, so as to strive for time for load reduction of the wind farm. After 2.20s, the unbalanced energy is basically absorbed by a chopper resistor inside the fan, and the wind power plant does not feed the unbalanced energy into the flexible direct current power grid any more. Fig. 10 shows a simulation comparison graph of fig. 9 (c) and 8 (c) and fig. 9 (d) and 8 (d), and it can be seen from fig. 10 that the ANLM strategy slightly affects the waveform quality of the wind farm bus voltage, but does not affect the amplitude thereof, so that the ANLM strategy hardly affects the 2 effects of the wind farm load shedding control proposed in the section 4.1-4.2 herein. Comparing fig. 9 (b) and fig. 7 (b), it can be seen that in the proposed cooperative coordination strategy, the peak value of the sub-module capacitor voltage of each station during the fault period is low, and after the fault is over, the sub-module release energy is jointly adjusted by the wind farm 2 and the MMC3, the sub-module voltage is quickly restored to the rated value, and compared with the sole adoption of the ANLM strategy, the proposed cooperative coordination ride-through strategy has a lower requirement on the withstand voltage of the sub-module capacitor. In summary, the cooperative ride-through strategy proposed in section 4.3 of this document can implement ride-through of the ac fault at the receiving end with long duration and deep fault degree, and can meet the ride-through requirement of the ac fault at the receiving end under most working conditions.

6. Conclusion

(1) And analyzing a direct-current overvoltage mechanism of the wind power under the alternating-current fault of the receiving end of the flexible direct-current networking system in detail from the angle of energy balance to obtain an expression of the direct-current voltage during the fault.

(2) The ANLM strategy capable of realizing the direct-current voltage emergency limitation is provided, the converter station adopting the ANLM strategy has a higher energy storage upper limit, more sufficient time can be provided for the system to process faults, and the fault ride-through capability of the system is improved.

(3) And providing a receiving end alternating current fault ride-through strategy in which an ANLM strategy is cooperatively matched with a chopper resistor inside the wind turbine generator. The proposed cooperative ride-through strategy can meet the requirement of alternating current fault ride-through of a receiving end under most working conditions, meanwhile, the strategy does not depend on remote communication, the reliability of the strategy depends on a control and protection system of an original wind power grid system through flexible and direct connection, no additional unreliable factor is introduced, additional equipment does not need to be additionally arranged, the energy storage potential of a converter station and the self response of a wind power generator set are fully utilized, and the cost of fault ride-through can be reduced.

The symbol names involved in the invention are as follows:

C _0i : sub-module capacitance values of each converter station; u shape _d : a direct current voltage signal of the flexible direct current power grid; u shape _d0 : the steady-state value of the direct-current voltage of the flexible direct-current power grid; delta U _dmax : the maximum allowable value of the direct-current voltage increment; u shape _C : sub-module capacitance voltage signals of the flexible direct current power grid; u shape _C0 : the stable voltage value of the sub-module capacitor of the flexible direct current power grid; delta U _Cmax : the maximum allowable value of the voltage increment of the sub-module capacitor; Δ P: unbalanced power; delta P _max : maximum unbalanced power due to a fault; a: the ratio of the DC voltage target value to the steady state value; p is a radical of formula _RE : the power sent by the receiving end converter station; p is a radical of formula _SE : the power collected by the sending end converter station; e _N0i : energy storage during steady-state operation of the converter station i; u. u _WF,abc 、i _WF,abc : voltage and current of a wind power plant busbar; u. of _WF,d 、u _WF,q : d-axis component and q-axis component of wind power plant busbar voltage; i.e. i _WF,d,ref 、i _WF,q,ref : d-axis target value and q-axis target value of wind power plant busbar current; i.e. i _WF,d 、i _WF,q : d-axis component and q-axis component of wind power plant busbar current; ω: angular frequency of wind farm bus voltage; θ: the phase of the voltage of the wind power plant busbar; l: an inductance at an outlet of the sending end converter station; m, M _d 、M _q : a modulation ratio of the converter station, a d-axis component and a q-axis component of the modulation ratio; t: time constant of first order low pass filter；i _SE,max : the current limiting amplitude of the sending end converter station; c _WF : the wind power plant is aggregated with equivalent capacitance on a wind power plant bus; i.e. i _WF,d0 : d-axis component of wind farm bus current at steady state; s _b : a power reference value of the wind farm; u. of _b : a wind farm voltage reference value; i all right angle _b : a current reference value of the wind power plant; i.e. i _max : maximum current that GSVSC can withstand; i.e. i _dref : an active current target value of the GSVSC; i.e. i _qref : reactive current target value of GSVSC; u. u _WF,N : rated value of wind farm bus voltage; u shape _d,WT,max : the upper limit threshold value of the chopper resistor switching; p _WT,max : the maximum power which can be generated by the wind turbine generator; t is t ₀ Time: receiving end AC fault occurs; t is t ₁ Time: a transmitting end converter station detects a receiving end alternating current fault; t is t ₂ Time: the wind power plant starts to load down; t is t ₃ Time: the self-adaptive NLM strategy starts to take over the direct-current voltage control; t is t ₄ Time: the load reduction of the wind power plant is completed; t is t ₅ Time: finishing the control of the direct current voltage by the self-adaptive NLM strategy; t is t ₆ Time: the direct-current voltage of the flexible direct-current power grid is restored to 1p.u.; t is t ₇ Time: the wind power plant recovers normal operation; t is t _A Time: the number of the sub-modules starts to be reduced in a self-adaptive manner; t is t _B Time: starting adaptive recovery of the number of sub-modules; t is t _C Time: the number of submodules is restored to the nominal value.

Claims

1. A low-cost flexible-direct networking system receiving end alternating current fault ride-through method is characterized in that: the method comprises the following steps:

a. voltage limiting controller design

let t =0 time instant fault occur, t = t _A At the moment, the number of submodules starts to be adaptively reduced to limit the DC voltage of the flexible DC power grid, t = t _B At the moment, the fault recovery or unbalanced power is cut off, the number of sub-modules starts to recover gradually, and t = t _C The number of submodules is restored to the nominal value, 0-t _C The unbalanced power of the flexible direct current power grid in the time period is delta P;

at 0-t _C And the direct-current voltages all satisfy the following expression:

a combination of formula (9) and formula (10), wherein formula (11) is:

order to

in the formula: b (t) = U _C (t)/U _C (0)；

in particular, when m = n, k ₁ ＝1，k ₂ =0. At this time, the expressions of B (t) and α are as follows:

b. sub-module number recovery procedure

the wind power plant can be rapidly reduced by reducing the d-axis component of the voltage of the bus bar of the wind power plant. The d-axis component of the wind farm bus voltage can be actively controlled by the transmitting converter station of the flexible direct current power grid.

u _WF,abc 、i _WF,abc Respectively the voltage and the current of a bus bar of the wind power plant; u. of _WF,d 、u _WF,q Respectively representing a d-axis component and a q-axis component of the voltage of a bus of the wind power plant; i.e. i _WF,d,ref 、i _WF,q,ref 、i _WF,d 、i _WF,q Respectively obtaining a d-axis target value, a q-axis target value, a d-axis component and a q-axis component of the wind power plant busbar current; omega is the angular frequency of the voltage of the bus bar of the wind power plant; theta is the phase of the voltage of the wind power plant busbar; l is inductance at the outlet of the sending end converter station; m, M _d 、M _q The modulation ratio of the converter station, the d-axis component and the q-axis component of the modulation ratio, respectively, T being the time constant of the first order low pass filter.

in the formula: u. of _b And i _b Reference values of the voltage and the current of a bus bar of the wind power plant;

the reference values of the voltage and the current are taken as follows:

in the formula: s _b The power reference value of the wind power plant;

in the formula: u. of _WF,N A rated value for the wind farm busbar voltage; i.e. i _max The maximum current bearable by the GSVSC;

meanwhile, the active current target value of the GSVSC is given as follows:

The switching of the chopper resistor is controlled by setting the upper and lower thresholds of the voltage of the FPC DC bus, and the upper threshold is set as U _d,WT,max Then, the size of the chopper resistor is designed as follows:

in the formula: p _WT,max Maximum value capable of being sent out by wind turbine generatorPower;

Setting a receiving end alternating current fault t = t of a system ₀ Causing the direct current voltage of the flexible direct current power grid to continuously rise until the sending end converter station detects the receiving end alternating current fault t = t ₁ Setting the target value of the bus bar voltage to be 0.2p.u by the transmitting-end converter station, and after a short control delay, starting to realize the load reduction t = t of the wind power plant by the transmitting-end converter station in cooperation with a chopper resistor in the wind turbine generator ₂ And completing load shedding t = t in the wind power plant ₄ Firstly, the wind power plant continuously injects unbalanced energy into the flexible direct current power grid, if no measures are taken, direct current overvoltage can be caused, and the ANLM strategy inhibits the direct current overvoltage caused by the unbalanced energy; ANLM strategy takes over DC voltage control period t ₃ -t ₅ Should be greater than 1.05p.u.; if the receiving end AC fault is recovered within the specified time, the sub-module number and the DC voltage are gradually recovered to the rated value t ₄ -t ₆ In order to ensure that the sub-module capacitor stored energy is quickly released, a hysteresis comparator in a sending end converter station controller is required to detect that the direct-current voltage is recovered to 1p.u, then, the target value of the voltage of a bus bar of the wind power plant is gradually increased, so that the wind power plant is recovered to normal operation, and then, the whole system is gradually recovered to steady-state operation; if the receiving end alternating current fault is not recovered within the specified time, the transmitting end converter station disconnects the alternating current side circuit breaker, and then the wind power plant is disconnected and stops running.