CN112421672A

CN112421672A - Fault ride-through control method for wind power plant through VSC-HVDC grid connection

Info

Publication number: CN112421672A
Application number: CN201910767038.4A
Authority: CN
Inventors: 鲁裕婷; 都洪基
Original assignee: Nanjing University of Science and Technology
Current assignee: Nanjing University of Science and Technology
Priority date: 2019-08-20
Filing date: 2019-08-20
Publication date: 2021-02-26

Abstract

The invention discloses a fault ride-through control method for a wind farm connected to the grid via VSC-HVDC. First, a mathematical model of the VSC is established to analyze the operating characteristics of the system when the grid fails, including the transmission of active power by the converters on both sides when the fault occurs. Then, the topology structure of modular energy dissipation resistors is designed in the VSC‑HVDC system. Finally, based on the modular energy dissipation resistors, a coordinated control method for wind turbines is designed to realize fault ride-through. . Aiming at the shortcomings of the traditional fault ride-through method based on DC energy dissipation resistance, the present invention designs an improved modular energy dissipation circuit. Based on the modular energy dissipation circuit design combined with the coordinated control strategy of the wind turbine, the wind farm can be connected to the grid via VSC-HVDC. fault ride-through control.

Description

Fault ride-through control method for wind power plant through VSC-HVDC grid connection

Technical Field

The invention belongs to the field of intelligent power grids, and particularly relates to a fault ride-through control method for a wind power plant through VSC-HVDC grid connection.

Background

With the development of wind power technology in recent years, the single-machine capacity of a wind turbine generator is gradually improved, the scale of a wind power plant is gradually enlarged, and the interaction effect between the wind power plant and a traditional power grid is more and more obvious after the wind power is connected. For a power system with low wind power permeability, a wind turbine generator can be immediately disconnected when the voltage of a power grid drops, the system cannot be seriously influenced under the condition, but for the power system with high wind power permeability, the automatic disconnection of a large-scale fan can cause great impact on the power grid. The voltage stability and the frequency stability of the system are seriously threatened, and the negative benefit can hinder the large-scale development of the wind power industry. Therefore, on the background that the capacity of the wind turbine generator currently participating in grid connection accounts for a continuously increased proportion in the power grid, both new grid safety operation rules and wind power grid connection specifications require that the grid-connected wind turbine generator has certain low-voltage ride-through capability, namely, when the voltage of the power grid drops, the wind turbine generator can continuously run for a certain time in a grid connection mode and can provide reactive power support for the power grid.

The low voltage ride through means that when the voltage of the wind turbine generator connected to the power grid drops, the wind turbine generator can be connected with a power system, certain reactive power support can be provided for the power grid according to the voltage drop degree of the power grid, the power grid can be smoothly restored to a normal operation state, and therefore the wind turbine generator completes ride through operation within the low voltage time of the power grid. For grid-connected fans, low voltage ride-through is a specific operational functional requirement when a voltage drop occurs in the grid. At present, power related departments at home and abroad set detailed wind turbine generator grid-connected specifications and put different requirements on low-voltage ride-through operation according to various national conditions.

For a traditional fan operation mode, when a power grid fails, related protection measures are immediately implemented to disconnect the wind turbine generator from the power grid, the severity and duration of the power grid failure are not considered, the processing mode can guarantee the safety of the wind turbine generator to the maximum extent, and the method is feasible in a power system with low wind penetration rate. However, for an electric power system with a high proportion of wind power, if the off-grid operation mode of the wind power generator set is still adopted when the voltage of the power grid drops, the recovery difficulty of the whole power grid is increased, even the power grid fault is aggravated, the voltage of the power grid is finally collapsed, and larger-scale off-grid accidents of the generator set occur. Therefore, as the percentage of the installed capacity of the wind power to the installed capacity of the power system is continuously increased, the influence of the off-grid disconnection of the wind turbine generator on the safe and stable operation of the power grid is increased. In addition, the large-scale off-line of the wind turbine generator caused by the voltage drop of the power grid threatens the safety and stability of the power grid and causes great damage to the wind turbine generator. When the voltage of a power grid drops, the wind turbine generator set can generate unbalance of mechanical power and electromagnetic power, and the damage of related parts of the wind turbine generator set can be seriously influenced by overcurrent and overvoltage problems caused by the unbalance. The voltage drop is a common power grid disturbance condition in the actual operation of a power grid, so that the fault ride-through capability is very important for the safe and stable operation of the power grid or the wind turbine generator.

A double-fed wind driven generator (DFIG) is the type of a current geodetic fan, and the double-fed wind driven generator set is popular in the market due to the mature technology and the relatively low power generation cost. For the condition that a double-fed wind power plant is connected with a grid through a flexible direct current transmission system, when a voltage drop fault occurs at a PCC (point of common coupling) position of a receiving-end power grid, the main problem is that the safe and stable operation of a direct current system is influenced by direct current overvoltage caused by active power imbalance transmitted by the system during the fault. The existing method is to put in an unloading circuit to consume energy and a resistor to absorb work, but the method needs large resistor capacity, and has prominent problems of land occupation and heat dissipation brought to the system and lacks practicability.

Disclosure of Invention

The invention aims to provide a fault ride-through control method for a wind power plant through VSC-HVDC grid connection.

The technical solution for realizing the purpose of the invention is as follows: a fault ride-through control method for a wind power plant through VSC-HVDC grid connection comprises the following steps:

step one, establishing a mathematical model of VSC, and analyzing the operating characteristics of a system when a power grid fails, wherein the operating characteristics include the change condition of active power transmitted by converters on two sides and the change condition of direct-current bus voltage when the power grid fails;

designing a topological structure of the modularized energy consumption resistor in the VSC-HVDC system;

and thirdly, combining a wind turbine generator design coordination control method on the basis of the modularized energy consumption resistor to realize fault ride through.

Compared with the prior art, the invention has the remarkable advantages that: (1) the invention overcomes the problems of large occupied area and difficult heat dissipation of the traditional energy consumption resistor, has small occupied area and is more convenient to dissipate heat; (2) each submodule can be mutually standby, the energy dissipation resistor is gradually put into use according to the conducting voltage of the parallel IGBT, and the bypass switch can cut off a fault module when the submodule has an internal fault so as to repair and replace the submodule; (3) the active output power of the wind power plant is coordinated, so that the whole system can ride through faults more stably.

Drawings

FIG. 1 is a flow chart of a fault ride-through control method of a wind power plant through VSC-HVDC grid connection.

Fig. 2 is an equivalent circuit diagram of d-axis and q-axis alternating current systems and direct current links of the grid-side converter.

FIG. 3 is a schematic diagram of the energy consuming resistors incorporated into the system of the present invention.

Fig. 4 is a topological structure diagram of the energy dissipation resistor of the present invention.

Fig. 5 is a schematic diagram of a portion of the RSC improvement of the present invention.

Fig. 6 is a schematic diagram of a voltage sag at the receiving PCC.

Fig. 7 is a diagram showing a change in dc bus voltage.

Fig. 8 is a schematic diagram of the real and reactive power transmitted by the grid side converter station.

Fig. 9 is a schematic diagram of the real and reactive power transmitted by the wind farm side converter station.

FIG. 10 is a graph of DC bus voltage change with a coordinated control strategy.

Fig. 11 is a graph of the real and reactive power transmitted on the grid side.

Fig. 12 is a graph of the real and reactive power transmitted at the wind farm side.

FIG. 13 is a schematic view of fan output.

Detailed Description

The grid requires that a wind power plant which is operated in a networking mode must have fault ride-through capability, and when the voltage drops to 20% of rated voltage due to grid side fault, the wind power plant must be operated in a non-networking mode within 625ms, and normal operation can be rapidly recovered after transient fault is eliminated. The invention provides a fault ride-through control method for a wind power plant through VSC-HVDC grid connection, a voltage source converter-based flexible direct current transmission system (VSC-HVDC) is widely applied by the advantages of independent, flexible and controllable active power and reactive power, no need of phase-change voltage of a power grid and the like, the permeability of the flexible direct current transmission system in the power grid is continuously improved, and the wind power plant through high-voltage direct current transmission grid connection has the fault ride-through capability for ensuring safe operation of the power grid. Aiming at the defects of the traditional fault ride-through method based on direct-current energy consumption resistance, an improved modularized energy consumption circuit is designed, and a coordination control strategy of a wind turbine generator is designed based on the modularized energy consumption circuit to realize the fault ride-through control method of a wind power plant through VSC-HVDC grid connection.

As shown in fig. 1, a method for controlling a wind farm to ride through a VSC-HVDC grid connection fault includes the following steps:

the method comprises the following steps of firstly, describing a topological structure and a working principle of the VSC, establishing a mathematical model of the VSC, and analyzing the operating characteristics of a system when a power grid fails;

Furthermore, in the first step, because the double-fed wind turbine generator is connected with the power grid through the flexible direct-current transmission system, and the wind turbine realizes decoupling control with the power grid through the direct-current transmission line, when the power grid side fails, a corresponding control strategy can be adopted on a converter part between the direct-current line and the power grid side. The mathematical model of the grid-side converter under the dq synchronous rotation coordinate system is as follows:

in the formula (1), the grid side current is used as the current of the converterThe flow direction is positive direction, U_sdAnd U_sqD-axis and q-axis components of the grid voltage connected to the grid side respectively; u shape_gdAnd U_gqOutputting d-axis and q-axis components of the alternating voltage for the grid-side converter; i.e. i_sdAnd i_sqD-axis and q-axis components of the net side current, respectively; omega is the fundamental angular frequency of the power grid; r and L are respectively an alternating current side circuit resistor and a filter inductor.

Therefore, according to the mathematical model of the grid-side converter under the dq two-phase synchronous rotating coordinate system, equivalent circuits of a d-axis and q-axis alternating current system and a direct current link of the grid-side converter can be obtained, as shown in fig. 2(a), (b) and (c).

If losses in the network side converter and the reactor are neglected, it can be seen from fig. 2 that active power P injected into the grid through the network side converter_invCan be expressed as:

with grid voltage oriented on the d-axis, there is U_sqWhen 0, formula (2) can be rewritten as:

and the output active power P of the wind field side converter_recComprises the following steps:

P_rec＝U_dci_rec (4)

during normal operation, direct current voltage keeps invariable, and according to the power balance principle, the power of direct current bus both sides equals, promptly:

the energy Δ P flowing through the dc-side capacitor can then be expressed as:

in a steady state, the power of the inverters on both sides is equal, so that Δ P is equal to 0, the dc voltage is constant at the reference value, and no current passes through the capacitor.

When the power grid is in fault, under the condition that the current limitation of the converter is not considered in an ideal state, the power grid voltage is limited by U_sdInstantly falls to U'_sdBecause the wind turbine generator and the power grid are basically isolated through the grid-connected converter system, the active power output by the wind field side converter is basically kept constant, and the d-axis current of the grid side converter also needs to be i in order to maintain the active power balance of the two side converters_sdIs increased to i'_sdNamely, the following steps are provided:

in actual operation, the grid-side converter has certain current limiting measures, the passing current is not allowed to increase infinitely, the current limiting effect of a limiter is considered, and the actual instantaneous d-axis current at the moment is set as i_″sdAnd i ″)_sd＜i′_sdThen, there are:

P_rec＝P′_inv+ΔP (9)

p 'in the formula (8)'_invThe actual active power of the network side is obtained when the voltage of the power grid drops.

From the above analysis, it can be known that the grid voltage drop causes P_invThe direct-current bus voltage U is greatly reduced, the active power output by the wind turbine generator is basically kept stable, the active deviation delta P of two sides of a direct-current link is larger than zero, namely, the energy generated by the double-fed wind turbine generator cannot be completely input into a power grid, redundant unbalanced energy is injected into a middle direct-current bus, and the direct-current bus voltage U is directly caused by charging a capacitor_dcRises rapidly. If effective measures are not taken in time, the large fluctuation of the direct current voltage can cause the direct current bus capacitor and the full capacitorDamage to the power converter device. Therefore, the doubly-fed wind power system and the whole power grid cannot be guaranteed to operate safely and stably based on the conventional control strategies of the power grid side and the grid side under the ideal steady-state working condition of the power grid, and the control strategy of the current conversion system needs to be further optimally designed.

And further, in the second step, energy consumption resistors are put between the direct current side converter station and the power grid side converter station, the modular energy consumption resistors are designed according to the idea of the modular converter, and the topological structure of the energy consumption resistors is designed.

From the above analysis, it can be seen that when a fault occurs at the PCC of the receiving-end power grid and causes a voltage drop, the fault ride-through operation of the system can be ensured by eliminating the difference power between the dc side and the power grid side, so that a controllable energy consumption resistor R can be put between the dc side and the power grid side converter station_chTo absorb the differential power during the fault, as shown in fig. 3, to maintain the stability of the dc voltage during the fault, and ensure the fault-ride-through operation of the system. The resistor capacity and the occupied area required by adopting a single energy consumption resistor to absorb the differential power are large, and higher requirements are put on the heat dissipation of the system.

Aiming at the defects, a dissipative resistor R is provided_chSplit into n resistors R with same resistance_nAre connected in parallel, each individual resistor R_nCan be independently put in and cut off, the topological structure is shown in figure 4, and a series of sub-modules (SM) are connected in parallel to form a modular energy consumption circuit. Each sub-module consumes power when independently turned on

Wherein, U_dcAnd the rated voltage of the direct current side is represented, and even if the alternating current network side has serious faults to drop the voltage of a PCC point to 0, the GSVSC cannot transmit power, and the wind power plant still normally transmits power.

If n submodules are in total, when all the submodules are opened, the power difference when the most serious fault occurs on the power grid side, namely the sum of the direct current system, is completely consumedConstant active power P_dcThen, then

P_dc＝nP_n (11)

The resistance element value corresponding to each submodule can be obtained as follows:

each submodule is internally provided with a separate resistor R_nThe energy dissipation resistor is connected with two parallel IGBTs in series, the conduction voltages of the IGBTs are set to be different, and therefore when the direct current voltage rises, the IGBTs are gradually conducted according to the rise of the voltage, and the energy dissipation resistor is gradually put into use. Setting when direct current voltage exceeds a design threshold value U'_dcAt that time, the modular energy consuming circuit begins to be triggered. Faults with different severity degrees can generate different power difference delta P, and the modularized energy consumption circuit can obtain the number n of the sub-modules which need to be put into the modularized energy consumption circuit at present according to the power difference delta P at the moment_a. Through the design, the capacity of the energy consumption resistor of each submodule is only 1/n of the original direct current energy consumption resistor, and the defect that the original direct current energy consumption resistor is the largest is overcome. The more the number of the designed submodules is, the smaller the resistance capacity corresponding to each submodule is, and the heat dissipation of the resistor is more facilitated.

Further, in the third step, on the basis of the modular energy consumption resistor designed in the second step, the output power of the wind turbine generator is adjusted by combining a fault ride-through coordination control strategy designed by the wind turbine generator.

In order to realize the balance of active power during the fault period, the active output of the wind power plant is simultaneously adjusted on the basis of the modularized energy consumption resistor, so that the wind turbine generator can smoothly pass through the fault. Therefore, the active power outer ring of the rotor side frequency converter of the doubly-fed wind turbine generator is improved.

For a doubly-fed wind generator, the following relationships apply:

wherein, P_sRepresenting the active power, P, output by the stator of a doubly-fed wind generator_eRepresenting the electromagnetic power of the generator, P_cus、P_fesDenotes the stator copper and iron losses, P_m、P′_mRepresenting input mechanical power and mechanical power losses, P, of doubly-fed machines_mecRepresenting the net mechanical power absorbed by the doubly fed machine and s representing the slip.

In order to realize the maximum power tracking of the wind turbine, the optimal power curve of the wind turbine and the wind turbine rotating speed omega are used_rTo calculate the active power command P output by the generator in real time^*To output mechanical power P by fan_wInstead of P in the formula (13)_mCan obtain

Wherein, C_pRepresenting the wind energy utilization factor, p representing the air density, S representing the area swept by the fan blades, and v representing the wind speed.

When a coordinated fault ride-through strategy is adopted, a fault occurs at the power grid side, and the direct-current voltage exceeds a set threshold value U'_dcCalculating the number n of submodules to be put in_aAnd because the number of the modules is an integer, the residual power can not be consumed, a new power shortage can be generated at the moment, the part is borne by the load shedding operation of the wind turbine generator, and the new power shortage delta P^*＝ΔP-n_aP_nAt this time, a load shedding instruction and a new instruction value P are sent to the wind power plant₁ ^*＝P^*-ΔP^*The wind turbine generator system does not operate according to a maximum power curve any more, but performs load shedding operation, the electromagnetic torque of the wind turbine generator system is reduced, the rotating speed of the fan can be temporarily increased, but most of power shortage is consumed by the energy dissipation resistor when the wind turbine generator system is matched with the energy dissipation resistor, the load shedding instruction value obtained by the wind turbine generator system is small, the rotating speed increasing value of the fan is limited, and the wind turbine generator system can stably pass through faults. A schematic diagram of a modified part of the rotor-side frequency converter is shown in fig. 5.

The present invention will be described in detail with reference to the following examples and drawings.

Examples

At 4s, when a three-phase ground fault occurs at the PCC in fig. 3, the voltage drops from 1p.u. to 0.2p.u., as shown in fig. 6, the duration is 625ms, comparing the case of using the coordinated control strategy with the case of not using the coordinated control strategy. When the coordination control strategy is not adopted, the voltage change situation of the direct current bus is shown in fig. 7, the active power and the reactive power transmitted by the power grid side converter station are shown in fig. 8, and the active power and the reactive power transmitted by the wind farm side converter station are shown in fig. 9. Fig. 7 shows a change situation of the dc bus voltage after the voltage at the receiving PCC drops, where the dc bus voltage rapidly rises due to the active power imbalance because the active transmission capability of the GSVSC drops. Fig. 8 shows the active power and reactive power changes transmitted by the GSVSC, the active power transmitted during the fault decreases, and after the fault, the energy stored by the dc capacitor and the active power absorbed by the WFVSC are transmitted to the grid through the GSVSC, so that the active power transmitted to the grid at the moment of fault recovery increases instantaneously, and the bus voltage does not return to normal until the bus voltage drops to the rated value. The WFVSC transmits active and reactive power, which is almost constant because the WFVSC still receives wind farm transmission power and transfers power to the GSVSC during a fault, as shown in fig. 9. According to the simulation result, if no measures are taken during the fault period, the voltage of the direct current bus is increased, great impact is generated on the power grid, and the stable operation of the power grid is not facilitated. After the coordination control strategy is adopted, the change situation of the direct current bus voltage is shown in fig. 10, the change situations of active power and reactive power transmitted by the converter stations on the grid side and the wind farm side are shown in fig. 11 and 12, and the output situation of the wind turbine is shown in fig. 13. Fig. 10 shows the variation of the dc bus voltage, which has a small fluctuation value, and achieves a very smooth fault ride-through effect. At this time, most of the differential power is consumed by the energy consumption resistor, so that the active load shedding of the DFIG is not obvious, and therefore, as can be seen from fig. 13, the power output by the wind turbine generator is gradually reduced during the fault period, the output is gradually increased after the fault is recovered, and the instantaneous increase and decrease are not caused. Fig. 12 shows the active power and the reactive power transmitted by the GSVSC, and as can be seen by comparing with fig. 9, the fluctuation of the power is reduced, and the active power output by the GSVSC is relatively smooth. Fig. 11 shows the real and reactive power transmitted by the WFVSC, and during a fault, the transmitted real power is reduced correspondingly, while the reactive power is not changed much.

Claims

1. a fault ride-through control method of wind farm through VSC-HVDC grid connection, is characterized in that, comprises the following steps:

Step 1: Establish a mathematical model of the VSC, and analyze the operating characteristics of the system when the power grid fails, including the change of the active power transmitted by the converters on both sides and the change of the DC bus voltage when the fault occurs;

Step 2, designing the topology of the modular energy dissipation resistor in the VSC-HVDC system;

Step 3: On the basis of the modular energy dissipation resistor, the coordinated control method of the wind turbine is designed to realize the fault ride-through.

2. the fault ride-through control method of grid-connected wind farm through VSC-HVDC according to claim 1, is characterized in that, in step 1, establishes the mathematical model of VSC, analyzes the operating characteristic when the power grid breaks down, is specifically:

The mathematical model of the grid-side converter in the dq synchronous rotating coordinate system is:

In formula (1), the grid-side current flows from the converter to the grid as the positive direction, U _sd and U _sq are the d-axis and q-axis components of the grid voltage connected to the grid side, respectively; U _gd and U _gq are the grid-side switching are the d-axis and q-axis components of the output AC voltage of the current transformer; i _sd and i _sq are the d-axis and q-axis components of the grid-side current, respectively; ω is the angular frequency of the grid fundamental wave; R and L are the AC-side line resistance and filter, respectively inductance;

The energy ΔP flowing through the DC side capacitor can be expressed as:

Among them, U _dc is the DC capacitor voltage, P _rec is the active power output by the wind farm-side converter, P _inv is the active power injected into the grid by the grid-side converter, and at steady state, the powers of the converters on both sides are equal, ΔP = 0;

When the power grid fails, in the ideal state without considering the current limit of the converter, the grid voltage drops from U _{sd to U' sd} _{instantaneously} , the d-axis current of the grid-side converter increases from is _sd to i' _sd , and the grid The active power output by the side converter becomes:

In actual operation, considering the current limiting effect of the limiter, the actual instantaneous d-axis current at this time is set as i″ _sd , and i″ _sd <i′ _sd , then:

_{Prec =P'inv} ₊ ΔP (5)

In formula (8), _P'inv is the actual active power on the grid side when the grid voltage drops.

3. The fault ride-through control method for grid-connected wind farms via VSC-HVDC according to claim 1, characterized in that, in step 2, an energy dissipation resistor R _ch is put in between the DC side and the grid side converter station to reduce the power consumption The energy resistor R _ch is divided into n resistors R _n with the same resistance value in parallel. The power consumed by each sub-module when it is independently turned on is

Among them, U _dc represents the rated voltage of the DC side; if there are n sub-modules in total, when all the sub-modules are turned on, all the power difference when the most serious fault occurs on the grid side is consumed, that is, the rated active power P _dc of the DC system, then

P _dc =nP _n (7)

The value of the resistance element corresponding to each sub-module is:

The interior of each sub-module is composed of a separate resistor _Rn connected in series with two parallel IGBTs, and the on-voltage of the IGBTs is set to be different. When the DC voltage exceeds the design threshold U' _dc , the modular energy-consuming circuit starts to be triggered; Faults of different severity will generate different power balance ΔP, and the modular energy-consuming circuit obtains the number na of sub-modules that need to be put into operation according to the power balance _ΔP at this time.

4. The fault ride-through control method for grid-connected wind farms via VSC-HVDC according to claim 1, wherein in step 3, on the basis of the modular energy dissipation resistors designed in step 2, the fault ride-through coordination is designed in conjunction with wind turbines The control strategy is to adjust the output power of the wind turbine, according to the power balance formula of the double-fed wind turbine:

Among them, P _s represents the active power output by the stator of the DFIG, P _e represents the electromagnetic power of the generator, P _cus and P _fes represent the stator copper loss and stator iron loss, and P _m and P′ _m represent the DFIG Input mechanical power and mechanical power loss, P _mec represents the net mechanical power absorbed by the DFIG, and s represents the slip;

According to the optimal power curve of the wind turbine and the rotational speed ω _r of the wind turbine, the active power command P ^* output by the generator is calculated in real time, and the output mechanical power P _w of the wind turbine is used to replace P _m in formula (9) to obtain:

Among them, C _p represents the wind energy utilization coefficient, ρ represents the air density, S represents the area swept by the fan blades, and v represents the wind speed.

5. The fault ride-through control method for wind farms connected to the grid via VSC-HVDC according to claim 4, characterized in that when a coordinated fault ride-through strategy is adopted, a fault occurs on the grid side, and the DC voltage exceeds the set threshold U' _dc , Calculate the number of sub-modules na that should be put into use at this time. Since the number of modules is _an integer, there will be surplus power that cannot be consumed, resulting in a new power shortage. This part is borne by the wind turbine load shedding operation, and the new power shortage ΔP ^* = ΔP- n _a P _n , then a load shedding command will be issued to the wind farm, and the new command value P ₁ ^* =P ^* -ΔP ^* .