Transient overvoltage suppression method for safe and stable AC transmission system of new energy stations

Abstract

A method for suppressing transient overvoltage of a safe and stable AC transmission system of a new energy station, the steps of which include: establishing a power supply model of the new energy transmission system, and configuring a dual current loop control strategy; calculating transient overvoltage based on the power supply model and the dual current loop control strategy; establishing a relationship between negative-sequence reactive current and transient overvoltage, and calculating the permissible range of negative-sequence reactive current. When the transient overvoltage exceeds the voltage threshold, the system adjusts the negative-sequence reactive current within the permissible range to reduce the transient overvoltage. The present invention realizes quantitative analysis of the transient overvoltage problem of the non-fault phase of the AC transmission system of the new energy station under an asymmetric short circuit fault by building a calculation model, and obtains the associated variables and corresponding relationships of the transient overvoltage, thereby deriving the corresponding specific suppression method, which can accurately and safely complete the suppression of transient overvoltage, so as to ensure that the new energy AC transmission system can operate safely and stably under an asymmetric short circuit fault.

Description

Technical Field

[0001] The present invention belongs to the technical field of renewable energy power supply, and in particular relates to a method for suppressing transient overvoltage in a safe and stable AC transmission system of a renewable energy station. Background Art

[0002] At present, with the development and popularization of new energy technologies, traditional power generation methods such as thermal power generation are gradually being replaced by new energy power generation such as wind power and photovoltaic power generation. The large-scale access of new energy and other equipment to the power system has gradually evolved some power systems into a new generation of power systems with high penetration of renewable energy and high proportion of power electronic equipment.

[0003] In specific work, when a common asymmetric short-circuit fault occurs in the above system, the non-fault phase of the new energy power system often has a more serious transient overvoltage problem, and the excessively high voltage in the system will affect the safe and stable operation of the equipment in the power system, posing a serious threat to the new energy transmission capacity and the safe and stable operation of the power system. Therefore, it is very important to analyze the generation mechanism and influencing factors of transient overvoltage in the new energy transmission system, and propose a transient overvoltage calculation method and suppression strategy that meets engineering needs.

[0004] At present, the calculation method or theoretical derivation for the transient overvoltage level of the AC transmission system of renewable energy under asymmetric faults is still not perfect, and the existing research still lacks relevant theoretical derivation and calculation methods. In the prior art, there are conclusions on the non-full-phase operation analysis of the renewable energy transmission system, and the transient overvoltage of the system during the fault is analyzed by the symmetrical component method, but the conclusion does not propose a specific renewable energy control strategy, that is, a method for suppressing transient overvoltage. Furthermore, it is known that there is a transient overvoltage calculation and analysis method only for the renewable energy transmission system when a single-phase grounding fault occurs, while the transient overvoltage of the system when two-phase grounding occurs is basically obtained by simulation calculation, and it is not explained by theoretical calculation, and no method for suppressing transient overvoltage is obtained.

[0005] Therefore, when an asymmetric short circuit fault occurs in the renewable energy power system, a more serious transient overvoltage problem may occur in the non-fault phase of the fault point in the system. However, the current research methods still lack a method to accurately suppress the system transient overvoltage by optimizing the control parameters of renewable energy. It is also necessary to study the principle of transient overvoltage and propose a suppression method that can be implemented to effectively suppress the system transient overvoltage problem under asymmetric faults, thereby ensuring that the renewable energy transmission system can still operate safely and stably under asymmetric short circuit faults. Summary of the invention

[0006] The present invention provides a method for suppressing transient overvoltage of a safe and stable AC transmission system of a new energy station to solve the above-mentioned problem.

[0007] The present invention adopts the following technical solutions:

[0008] A method for suppressing transient overvoltage of a safe and stable AC transmission system of a new energy station is provided, and its main steps include:

[0009] Establish a power supply model for the new energy transmission system and configure a dual current loop control strategy on the grid-side converter;

[0010] Based on the power supply model and the dual current loop control strategy, the transient overvoltage phasor of the non-fault phase under the asymmetric short circuit fault caused by two-phase grounding is calculated. The transient overvoltage phasor formula is:

[0011]

[0012] In the formula, is the transient overvoltage phasor, is the positive sequence Thevenin open circuit voltage phasor at the fault point, is the negative sequence Thevenin open circuit voltage phasor at the fault point, is the first proportionality coefficient;

[0013] The functional relationship between the negative-sequence reactive current phasor output by the unit in the new energy transmission system and the negative-sequence Thevenin open-circuit voltage phasor at the fault point is established, and the formula is:

[0014]

[0015] In the formula, It is the negative sequence reactive current phasor output by the generator in the renewable energy transmission system. is the negative sequence grid-side line reactance, is the imaginary part;

[0016] Calculate the permissible range of the effective value of the negative-sequence reactive current phasor based on the voltage threshold of the renewable energy transmission system and the overload current of the grid-side converter;

[0017] When the effective value of the transient overvoltage phasor exceeds the voltage threshold, based on the dual current loop control strategy and according to the functional relationship and the transient overvoltage phasor formula, the effective value of the negative-sequence reactive current phasor is adjusted within the allowable range to reduce the effective value of the transient overvoltage phasor.

[0018] Optionally, in the power supply model, the grid side is equivalent to an infinite voltage source.

[0019] Optionally, the method for calculating the transient overvoltage phasor includes:

[0020] Establishing a positive-sequence equivalent circuit, a negative-sequence equivalent circuit and a zero-sequence equivalent circuit of the new energy transmission system under the asymmetric short-circuit fault, and treating the power generation side of the new energy transmission system as equivalent to a current source;

[0021] Obtaining a system equation of the new energy transmission system based on the positive-sequence equivalent circuit, the negative-sequence equivalent circuit and the zero-sequence equivalent circuit;

[0022] The transient overvoltage phasor is solved by the system equation.

[0023] Optionally, the system equation is solved by a symmetrical component method, and the system equation includes a positive sequence phasor equation, a negative sequence phasor equation, and a zero sequence phasor equation of the new energy transmission system;

[0024] The positive sequence phasor equation of the new energy transmission system is:

[0025]

[0026] In the formula, is the positive sequence phasor of the infinite voltage source, is the positive sequence grid-side line reactance, It is the positive sequence current phasor of the renewable energy transmission system. is the positive-sequence Thevenin equivalent impedance, is the positive sequence voltage phasor from the fault point to the ground, is the total reactance of the positive sequence renewable energy transmission line and transformer, is the positive sequence current phasor of the fault point to ground, is the positive sequence voltage phasor at the collection line;

[0027] The negative sequence phasor equation of the new energy transmission system is:

[0028]

[0029] In the formula, is the negative sequence voltage phasor from the fault point to the ground, is the negative-sequence Thevenin equivalent impedance, is the negative sequence current phasor from the fault point to the ground, is the negative sequence voltage phasor at the collection line, is the total reactance of the transmission line and transformer on the negative-sequence renewable energy side;

[0030] The zero-sequence phasor equation of the new energy transmission system is:

[0031]

[0032] In the formula, is the zero-sequence Thevenin open-circuit voltage phasor at the fault point, is the zero-sequence Thevenin equivalent impedance, is the zero-sequence terminal voltage phasor, is the total reactance of the zero-sequence renewable energy side transmission line and transformer, is the zero-sequence current phasor from the fault point to ground.

[0033] Optionally, the method for solving the transient overvoltage phasor equation further includes:

[0034] Create boundary conditions:

[0035]

[0036] Substituting the boundary conditions into the system equations, we obtain the formula:

[0037]

[0038] And set , the transient overvoltage phase equation is obtained as:

[0039]

[0040] In the formula, , is the second proportionality coefficient, and .

[0041] Optionally, according to the positive-sequence current and negative-sequence current states in the new energy transmission system, the negative-sequence reactive current in the dual current loop control strategy is set to be capacitive or inductive current.

[0042] Optionally, when there is a positive-sequence reactive current and a negative-sequence reactive current in the output of the renewable energy transmission system, and the positive-sequence active current and the negative-sequence active current are both zero, the negative-sequence reactive current in the dual current loop control strategy is set to be a capacitive current;

[0043] When the output of the new energy transmission system has positive-sequence reactive current, negative-sequence reactive current, and positive-sequence active current, and the negative-sequence active current is zero, the negative-sequence reactive current in the dual current loop control strategy is set to be capacitive or inductive current.

[0044] Optionally, the effective value of the transient overvoltage phasor is negatively correlated with the effective value of the negative-sequence reactive current phasor.

[0045] Optionally, the corresponding condition for an endpoint of the allowable range is that the effective value of the transient overvoltage phasor is equal to the voltage threshold.

[0046] Optionally, the corresponding condition of an endpoint of the allowable range is that the maximum single-phase current of the new energy transmission system is equal to the overload current of the grid-side converter.

[0047] Compared with the prior art, the present invention adopts the above technical solution and has the following technical effects:

[0048] The present invention combines a dual current loop control strategy and builds a calculation model to achieve quantitative analysis of the transient overvoltage problem in the non-fault phase of the AC transmission system of a new energy station under an asymmetric short circuit fault, and obtains the associated variables and corresponding relationships of the transient overvoltage, thereby deriving the corresponding specific suppression method; further, the allowable range of the associated variables is also calculated, so that the transient overvoltage can be accurately and safely suppressed in combination with the suppression method, so as to ensure that the new energy AC transmission system can operate safely and stably under an asymmetric short circuit fault. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0050] Figure 1 is a flow chart of the method of the present invention;

[0051] Figure 2 It is a schematic diagram of the corresponding system of the present invention;

[0052] FIG3 (a) is a positive sequence equivalent circuit in a non-fault equivalent circuit of the present invention;

[0053] FIG3( b ) is a negative sequence equivalent circuit in a non-fault phase equivalent circuit of the present invention;

[0054] FIG3 (c) is a zero-sequence equivalent circuit in a non-faulty phase equivalent circuit of the present invention;

[0055] Figure 4 It is the transient overvoltage simulation waveform of the non-fault phase of the system fault point in the simulation verification of the present invention;

[0056] Figure 5 It is a comparison diagram of transient overvoltages under different negative-sequence reactive current reference values ​​in Example 1 of the present invention. DETAILED DESCRIPTION

[0057] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0058] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0059] It should also be noted that the methods used in the present invention are all conventional methods unless otherwise specified; the raw materials and devices used are all conventional commercially available products unless otherwise specified.

[0060] like Figure 1 As shown, the present invention provides a method for suppressing transient overvoltage of a safe and stable AC transmission system of a new energy station, and its main steps include:

[0061] First, it is necessary to explain the application scenario of the present invention and establish a power supply model of a new energy delivery system. Specifically, the new energy delivery system includes a new energy wind farm, a transmission line and supporting power equipment. Figure 2 As shown, a box-type substation with a Y / Δ connection method is configured at the power output end of the unit of the new energy transmission system (the power generation unit in the new energy system, i.e., the wind turbine in this embodiment, hereinafter referred to as: the unit), and is connected through a collection bus to collect electric energy, and then connected to the main power grid (the main power grid is equivalent to an infinite voltage source) through a Yg / Yg connection method. Among them, a unit converter and a grid-side converter are set at one end of the new energy unit. The irregular alternating current generated by the unit is converted into direct current through the unit converter, and then reversely converted into alternating current that meets the transmission requirements through the grid-side converter (also known as: grid-connected inverter) . Furthermore, a dual current loop control strategy is configured on the grid-side converter, namely, positive sequence current loop control and negative sequence current loop control.

[0062] Furthermore, the technical solution of the present invention will mainly discuss the asymmetric fault of two-phase grounding in the system. In order to effectively suppress the transient overvoltage problem under the asymmetric fault of new energy, the dual current loop control strategy adopted, the positive and negative sequence dual current loop control converts the positive and negative sequence electrical quantities to the d and q coordinate systems of forward rotation and reverse rotation respectively, thereby realizing the decoupling control of the positive and negative sequence electrical quantities, and finally controls the grid-side converter of the new energy transmission system of this embodiment by outputting SPWM (sinusoidal pulse width modulation) signals. The positive and negative sequence voltage reference equations given by the current loop are:

[0063] (1)

[0064] In the formula, is the d-axis component of the positive sequence voltage reference value, is the q-axis component of the positive sequence voltage reference value, is the d-axis component of the negative sequence voltage reference value, is the q-axis component of the negative sequence voltage reference value, is the d-axis component of the positive sequence current reference value, is the q-axis component of the positive sequence current reference value, is the d-axis component of the negative sequence current reference value, is the q-axis component of the negative sequence current reference value, is the d-axis component of the actual positive sequence current, is the q-axis component of the actual positive sequence current, is the d-axis component of the actual negative sequence current, is the q-axis component of the actual negative sequence current, and is the proportional gain coefficient of PI control, and is the integral gain coefficient of PI control, is the angular frequency, is the AC side inductance of the grid-side converter, is the d-axis component of the positive sequence voltage at the renewable energy grid connection point, It is the d-axis component of the negative-sequence voltage at the renewable energy grid-connected point.

[0065] Furthermore, when the wind turbine is operating normally or in a symmetrical system, the positive-sequence active current reference value of the new energy is obtained according to the DC side voltage reference value, while the positive-sequence reactive current reference value of the new energy and the negative-sequence active and reactive current reference values ​​are all set to zero. When the wind turbine is under an asymmetric fault, the dual current loop control strategy is specifically as follows: output the positive-sequence reactive current reference value according to the voltage deviation; the positive-sequence active current reference value is controlled according to the constant current; during the asymmetric grounding fault, the new energy compensates for a certain negative-sequence reactive power to suppress overvoltage, so the negative-sequence reactive current reference value is not 0, and the negative-sequence reactive current of the new energy is also controlled according to the constant current method; and the negative-sequence active current reference value of the new energy is generally set to 0.

[0066] Based on the above power supply model and dual current loop control strategy, the transient overvoltage of the non-fault phase under the asymmetric short-circuit fault caused by two-phase grounding is calculated. Since the short-circuit current phase and effective value provided by the renewable energy transmission system during the asymmetric fault are affected by the dual current loop control strategy of the renewable energy transmission system, the control influence of renewable energy should be taken into account when conducting quantitative analysis of system transient overvoltage. In calculation and analysis, one side of the renewable energy unit can be equivalent to a current source. The output current of renewable energy is determined by its control strategy and the current value of renewable energy output is equal to its reference value.

[0067] It should be pointed out that, due to the small influence of the negative sequence reactive current of new energy on single-phase grounding fault, and the change law is not obvious, the present invention does not consider the influence of the negative sequence reactive current on overvoltage during single-phase grounding fault; therefore, the transient overvoltage of the non-fault phase when the system has a two-phase grounding short circuit fault is analyzed and calculated. For the new energy transmission system through AC in the power supply model, the fault location is located on the transmission line, and the system is explained by taking the B and C two-phase grounding fault as an example. Based on this, each sequence equivalent circuit of the new energy transmission system is established, that is, the positive sequence equivalent circuit, negative sequence equivalent circuit and zero sequence equivalent circuit of the non-fault phase A in this embodiment, wherein the power generation side of the new energy transmission system is equivalent to a current source, and its output current can be determined according to the control strategy.

[0068] The calculation steps include: as shown in Figure 3 (a), Figure 3 (b) and Figure 3 (c), the above-mentioned equivalent circuit is established for the non-fault phase A in the transmission line under the asymmetric short circuit fault, and then the corresponding system equation can be written out according to the symmetrical component method.

[0069] Based on the equivalent circuit, the system equation of phase A is obtained;

[0070] As shown in Figure 3 (a), the positive sequence phasor equation of phase A can be listed as:

[0071] (2)

[0072] In the formula, is the positive sequence Thevenin open circuit voltage phasor at the fault point, is the positive sequence phasor of the infinite voltage source, is the imaginary part, is the positive sequence grid-side line reactance, It is the positive sequence current phasor of the renewable energy transmission system. is the positive-sequence Thevenin equivalent impedance, is the positive sequence voltage phasor from the fault point to the ground, is the total reactance of the positive sequence renewable energy side transmission line and transformer (when the fault point is located on the transmission line close to the high voltage side of the transformer, represents the positive sequence reactance of the transformer on the new energy side). is the positive sequence current phasor of the fault point to ground, is the positive sequence voltage phasor at the collection line.

[0073] As shown in Figure 3(b), the negative sequence phasor equation of phase A can be listed as:

[0074] (3)

[0075] In the formula, is the negative sequence Thevenin open circuit voltage phasor at the fault point, is the negative sequence grid-side line reactance, is the negative-sequence reactive current phasor in this new energy system, that is, the negative-sequence reactive current phasor output by the unit in the new energy transmission system, is the negative sequence voltage phasor from the fault point to the ground, is the negative-sequence Thevenin equivalent impedance, is the negative sequence current phasor from the fault point to the ground, is the negative sequence voltage phasor at the collection line, is the total reactance of the negative sequence renewable energy side transmission line and transformer (when the fault point is located on the transmission line close to the high voltage side of the transformer, represents the negative-sequence reactance of the transformer on the new energy side).

[0076] As shown in Figure 3 (c), the zero-sequence phasor equation of phase A can be listed as:

[0077] (4)

[0078] In the formula, is the zero-sequence Thevenin open-circuit voltage phasor at the fault point, is the zero-sequence Thevenin equivalent impedance, is the zero-sequence terminal voltage phasor, is the total reactance of the zero-sequence renewable energy side transmission line and transformer, is the zero-sequence current phasor from the fault point to ground.

[0079] The transient overvoltage phasor equation is solved by the system equation. The transient overvoltage phasor equation solution method also includes:

[0080] Create boundary conditions:

[0081] (5)

[0082] Substituting the above boundary conditions into the system equation, we obtain the following formula:

[0083] (6)

[0084] Therefore, the voltage phasor formula of the non-fault phase A at the fault point can be obtained as follows:

[0085] (7)

[0086] In the formula, is the voltage phasor of the non-fault phase A at the fault point, which is the transient overvoltage phasor to be solved , and when it is approximately considered When the transient overvoltage phase The formula is:

[0087]

[0088] In the formula, is the proportionality factor, equal to ; is also the proportionality coefficient, , bring in the specific:

[0089] (8)

[0090] According to formula (7), it can be seen that the transient overvoltage of the renewable energy transmission system during the asymmetric short circuit fault is related to the positive and negative sequence control parameters of the renewable energy and the system parameters.

[0091] Optionally, according to the positive-sequence current and negative-sequence current states in the renewable energy transmission system, the negative-sequence reactive current in the dual current loop control strategy is set to be capacitive and / or inductive current. Based on this, the impact of the negative-sequence reactive current of the renewable energy transmission system on the transient overvoltage of the non-fault phase at the fault point during an asymmetric fault is mainly analyzed. Specifically, when there are positive-sequence reactive current and negative-sequence reactive current in the output of the renewable energy transmission system, and the positive-sequence active current and the negative-sequence active current are both zero, it can be inferred from the above equations, i.e., equations (2) to (5), that and Collinear, and and Then, when the renewable energy absorbs negative sequence reactive power and emits positive sequence reactive power, and Therefore, during an asymmetric short-circuit fault, when the negative sequence reactive current of the renewable energy output is increased to a capacitive current, it increases The amplitude of The overall amplitude is reduced, which can reduce the transient overvoltage problem of the non-fault phase of the fault point; if the new energy outputs inductive negative sequence reactive power, it will aggravate the overvoltage problem of the non-fault phase of the fault point. The amplitude will increase accordingly, which will also aggravate the overvoltage problem of the non-fault phase at the fault point. From the above analysis, it can be seen that the negative sequence reactive current output of new energy has a more obvious inhibitory effect on the transient overvoltage of the non-fault phase at the fault point.

[0092] Optionally, when the output of the renewable energy transmission system has positive-sequence reactive current, negative-sequence reactive current, positive-sequence active current, and the negative-sequence active current is zero, it can be inferred from the above equations that and Collinear, but at this time and When the new energy sources emit capacitive negative sequence reactive power and inductive positive sequence reactive power, and Therefore, when the negative sequence reactive current output by renewable energy is increased to a capacitive current, the overvoltage problem of the non-fault phase at the fault point can also be reduced.

[0093] Therefore, the transient overvoltage can be effectively suppressed by selecting appropriate new energy negative sequence reactive current.

[0094] In addition, according to formula (8), it can be seen that the transient overvoltage of the non-fault phase at the fault point is also related to k The value is related. When two phases are short-circuited and grounded, k ( k >0) increases, the fault point is not the fault phase U A The voltage amplitude will also increase.

[0095] From the above calculation conclusions, it can be seen that the effective value of the transient overvoltage phasor is negatively correlated with the effective value of the negative-sequence reactive current phasor of the renewable energy transmission system. That is, by adjusting the capacitive negative-sequence reactive current in the double current loop, the effective value of the negative-sequence reactive current phasor of the renewable energy transmission system is increased, thereby reducing the effective value of the transient overvoltage phasor and suppressing the transient overvoltage.

[0096] On this basis, the allowable range of negative-sequence reactive current of the renewable energy transmission system is calculated based on the voltage threshold of the renewable energy transmission system and the overload current of the grid-side converter. Optionally, the corresponding condition of one of the endpoints of the allowable range is that the effective value of the transient overvoltage phasor is equal to the voltage threshold, which can also be understood as the effective value of the transient overvoltage is equal to the voltage threshold; wherein, the effective values ​​of voltage and current are different from the effective values ​​of voltage and current phasors, but there is a specific relationship in the AC system, such as: effective value of voltage phasor = effective value of voltage, effective value of current phasor = effective value of current. Specifically, the situation when renewable energy only outputs positive-sequence reactive current and negative-sequence reactive current, and both positive and negative-sequence active currents are zero, is analyzed. During an asymmetric short-circuit fault, the negative-sequence reactive power of renewable energy can only emit capacitive reactive power (i.e., absorb negative-sequence reactive power to reduce transient overvoltage), and the renewable energy emits positive-sequence inductive reactive power due to entering low voltage ride-through control. When assuming that the negative sequence reactive current of renewable energy is 0, the voltage of the non-fault phase at the fault point will exceed the voltage limit value after an asymmetric ground fault occurs in the system, then consider using the method of making renewable energy output negative sequence reactive current to suppress overvoltage. , and when the negative sequence reactive current of the renewable energy source is zero, when the effective value of the transient overvoltage phasor exceeds the voltage threshold, then:

[0097] (9)

[0098] In the formula, is the effective value of the voltage phasor at the fault point of phase A when no negative sequence reactive current is output, that is, the effective value of the transient overvoltage phasor existing at this time; is the effective value of the positive-sequence Thevenin open-circuit voltage phasor at the fault point; is the voltage threshold, that is, the voltage limit value of the line.

[0099] When the new energy starts to output negative sequence reactive current to suppress transient overvoltage, the effective value of the non-fault phase voltage phasor at the fault point is:

[0100] (10)

[0101] In the formula, is the effective value of the voltage phasor at the fault point of phase A, that is, the effective value of the transient overvoltage phasor; is the effective value of the negative sequence Thevenin open circuit voltage phasor at the fault point, and at this time and The phase is reversed.

[0102] make , the reference value range of the effective value of the negative sequence reactive current phasor in the renewable energy transmission system can be calculated:

[0103] (11)

[0104] (12)

[0105] (13)

[0106] It should be noted that the negative-sequence reactive current phasor in the formula has a non-positive effective value due to the current direction problem, so the modulus operation is performed on both sides of the inequality and the direction of the inequality sign is swapped; when the equation (13) takes the equal sign, the endpoint value of an allowable range of the negative-sequence reactive current can be obtained.

[0107] When the positive sequence active current of renewable energy is not zero, it is not convenient to give an analytical expression for the negative sequence reactive current of renewable energy. However, the boundary value of the negative sequence reactive circuit can be obtained by solving the equation group. The boundary calculation equation group of the reference value of the negative sequence reactive current of renewable energy is as follows. A constraint equation is added on the basis of the simultaneous equations (2), (3), (4) and (5):

[0108] (14)

[0109] By solving the above equations, the reference boundary value of the negative-sequence reactive current of renewable energy can be obtained.

[0110] Therefore, when the renewable energy has no positive-sequence active current output during the fault period, the range of negative-sequence reactive current can be calculated using equation (13); and when the renewable energy has positive-sequence active current output during the fault period, the range of negative-sequence reactive current can be solved by solving the simultaneous equations (2), (3), (4), (5) and (14).

[0111] From the above-mentioned influence of negative sequence reactive current on transient overvoltage, it can be known that when the amplitude of negative sequence reactive current output by renewable energy is increased, the amplitude of transient overvoltage of non-fault phase at fault point can be reduced. Therefore, according to the boundary of negative sequence current reference value of renewable energy obtained by the above calculation, a part of the selection range of negative sequence reactive current reference value of renewable energy can be obtained. In order to prevent overcurrent of grid-side converter of renewable energy transmission system, the negative sequence reactive current The modulus value should not be too large. Therefore, the corresponding condition for the other end point of the allowable range is that the maximum single-phase current of the new energy transmission system is equal to the overload current of the grid-side converter. Specifically, the current of each phase needs to be calculated by the following formula, which is classified and discussed according to the common transformer wiring method. The formula is:

[0112] After passing through the Yy0 transformer, the current of each phase can be calculated by the following formula:

[0113] (15)

[0114] The current after passing through the Yd11 transformer can be calculated by the following formula:

[0115] (16)

[0116] In the formula, is the current phasor of phase A, is the B phase current phasor, is the C phase current phasor, is 1 unit phase shift, expressed as e j120° or e -j120° .

[0117] In this way, the current phasor of each phase can be solved and the corresponding effective value can be obtained for comparison with the overload current (limiting current of the grid-side converter) to determine whether the effective value of the negative-sequence reactive current phasor mentioned above is appropriate. It is also possible to reversely select the largest single-phase current in the calculation result and take it equal to the overload current to obtain the corresponding endpoint value of the allowable range, and combine it with the aforementioned endpoint value to form a specific allowable range. The above formula can also be used to determine whether the current adjusted new energy is overcurrent. If the calculated negative-sequence reactive current reference value can make the three-phase currents all within an acceptable range, then the current reference value can be used; if the calculated negative-sequence reactive current cannot make the three-phase currents all within an acceptable range, then the current reference value cannot be used.

[0118] When the effective value of the transient overvoltage phasor exceeds the voltage threshold, based on the dual current loop control strategy and according to the functional relationship and the transient overvoltage phasor formula, the effective value of the negative-sequence reactive current phasor of the renewable energy transmission system is adjusted within the allowable range to reduce the effective value of the transient overvoltage phasor, thereby suppressing the transient overvoltage problem.

[0119] Simulation verification, based on PSCAD simulation software, build the following in the simulation Figure 2 The power supply model of the renewable energy transmission system shown in the figure uses an aggregate model of 5,000 direct-drive wind turbines to simulate renewable energy stations. During normal operation, the renewable energy output active power is 8,000MW, and the renewable energy is connected to the 220kV infinite power grid via AC lines. All electrical quantities in the calculation are expressed in per-unit values, with a base capacity of 8,000MW and a base voltage of 220kV.

[0120] 1) Comparison between simulation and calculation of transient overvoltage

[0121] Simulation conditions: This part mainly verifies the effect of changing the negative sequence reactive current of renewable energy on the transient overvoltage of the non-fault phase of the fault point in the system. During fault crossing, the reference value of the positive sequence active and reactive current of renewable energy is set to a fixed value and remains unchanged, and the reference value of the negative sequence reactive current of renewable energy is changed to observe the change trend of overvoltage. The transformer reactance is 0.05pu, and the parameters of the unit length of the AC line are shown in Table 1 below:

[0122] Unit length parameter (pu / km) Line positive sequence reactance parameters 1.3×10-3 Line negative sequence reactance parameters 1.3×10-3 Line 0-sequence reactance parameters 2.6×10-3

[0123] Table 1

[0124] The length of the AC line is 150km. The negative-sequence reactive current of the new energy is 0, 0.1, and 0.2pu respectively, the positive-sequence active current of the new energy is fixed at 0.2pu, and the positive-sequence reactive current of the new energy is fixed at 0.4pu. The simulation value and calculated value of the transient overvoltage peak of the new energy are shown in Table 2. A two-phase ground short circuit occurs on the transmission line at 1.2s, and the duration is 0.4s. The transient overvoltage simulation results of the non-fault phase of the system fault point are shown in Figure 4 As shown, the waveform shown is the voltage fundamental amplitude.

[0125] The transient overvoltage simulation values ​​and calculated values ​​are recorded as shown in Table 2 below:

[0126] New energy negative sequence reactive current (pu) Transient overvoltage simulation value (pu) Transient overvoltage calculation value (pu) 0 1.325 1.314 0.1 1.296 1.284 0.2 1.267 1.255

[0127] Table 2

[0128] It can be seen from Table 2 that with the increase of the amplitude of the negative-sequence reactive current of renewable energy, under the same short-circuit fault setting, the transient overvoltage peak value gradually decreases, which is consistent with the above-mentioned characteristics of the influence of negative-sequence reactive current on overvoltage. And the theoretical calculated value of overvoltage is basically consistent with the change trend of the simulation value. From the simulation results, in order to reduce transient overvoltage, the amplitude of the negative-sequence reactive current of renewable energy can be appropriately increased.

[0129] 2) Optimization of negative sequence reactive current parameters to suppress transient overvoltage

[0130] This section uses the above calculation formula to derive the boundary value of negative sequence reactive current, so that the transient overvoltage during asymmetric faults is within the acceptable limit. And it is generally required that the transient overvoltage peak does not exceed 1.3pu. According to the previous simulation analysis, the negative sequence reactive current of renewable energy has a significant impact on transient overvoltage, so the transient overvoltage of the system can be adjusted by optimizing the negative sequence reactive current value.

[0131] Example 1: Analyze the situation when the new energy does not output positive sequence active current. Let the system have a two-phase grounding fault between phases B and C. Take the fault occurring on the transmission line close to the high-voltage side of the transformer as an example for analysis. The line parameters are shown in Table 3:

[0132] Reactance per unit length (pu / km) Line positive sequence parameters 1.3×10-3 Line 0 sequence parameters 3×10-3

[0133] Table 3

[0134] The line length is 150km. kThe value is larger than the amplitude system above. During the fault period, the positive sequence active current output by the renewable energy is 0pu, and the positive sequence reactive current is 0.5. According to calculations, when the negative sequence reactive current of the renewable energy is 0, the voltage of the non-fault phase at the fault point will exceed the voltage limit value after an asymmetric ground fault occurs. Therefore, consider using the method of making the renewable energy output negative sequence reactive current to suppress overvoltage. According to formula (13), let the voltage threshold Equal to 1.3pu, infinite system voltage U is 1pu, and the system parameters are brought into it, then the negative sequence reactive current boundary value can be calculated when the renewable energy has no positive sequence active current component. The calculated boundary of the renewable energy negative sequence reactive current is 0.28pu, which is approximated to 0.3pu, and it can be judged that the renewable energy will not overcurrent. And if the negative sequence reactive current amplitude is greater than the boundary value, the transient overvoltage can be lower than 1.3pu. The simulation results are as follows Figure 5 The statistical results are shown in Table 4:

[0135] New energy negative sequence reactive current reference value (pu) Voltage simulation (pu) 0 1.395 0.3 1.303 0.4 1.273

[0136] Table 4

[0137] Example 2, analyzes the situation where new energy has a positive-sequence active current component output. The system has a two-phase grounding fault in phases B and C at the same location, and uses the same line parameters as Example 1, with a line length of 150km. During the fault, the positive-sequence active current output by the new energy is fixed at 0.2pu; the positive-sequence reactive current is taken as 0.3pu to 0.6pu respectively. The benchmark capacity is still 8000MVA, and the benchmark voltage is 220kV. The leakage reactance of the transformer is taken as 0.05pu.

[0138] According to the calculation, when the negative-sequence reactive current of renewable energy is 0, the voltage of the non-fault phase at the fault point will exceed the voltage limit value after an asymmetric ground fault occurs. Then, the negative-sequence reactive current boundary values ​​are calculated, and simulations are performed for the cases where renewable energy has no negative-sequence reactive current output (corresponding to the voltage value in Table 5 - no negative-sequence current) and renewable energy outputs according to the calculated negative-sequence reactive current boundary value (corresponding to the voltage value in Table 5 - negative-sequence current). The simulation and calculation results are listed in Table 5, as follows (it can be judged that renewable energy will not over-current based on the set negative-sequence reactive current reference value):

[0139] New energy positive sequence reactive current (pu) Negative sequence reactive current boundary value (pu) Voltage value - no negative sequence current (pu) Voltage value - with negative sequence current (pu) Voltage control target (pu) 0.3 0.08 1.331 1.308 1.3 0.4 0.17 1.362 1.311 1.3 0.5 0.27 1.393 1.310 1.3 0.6 0.36 1.423 1.310 1.3

[0140] Table 5

[0141] It can be seen from the simulation results that the transient overvoltage peak increases with the increase of positive sequence reactive current; and when the transient overvoltage of the non-fault phase increases, the boundary value of the negative sequence reactive current increases accordingly to achieve the suppression of overvoltage. And when the new energy sets its negative sequence reactive current to the calculated boundary value, the transient overvoltage of the non-fault phase can be basically kept near the voltage limit value. The effectiveness of the proposed suppression method is proved. Since the calculation result of the negative sequence reactive current boundary value has a certain error, it can be appropriately increased on the basis of the calculated negative sequence reactive current boundary value so that the overvoltage can be less than 1.3pu.

[0142] With the help of the above equations, the reasonable values ​​of the control parameters of renewable energy can be roughly calculated to optimize the control parameters, thereby effectively suppressing transient overvoltage. Under certain working conditions, by optimizing the reactive current of renewable energy, the transient overvoltage peak value of the non-fault phase of the fault point can be made not to exceed the overvoltage limit value. However, in the case of overcurrent in the renewable energy converter, it will not be possible to optimize and adjust according to the calculated reactive current boundary of renewable energy, so other methods need to be considered to limit transient overvoltage.

[0143] The above are only preferred embodiments of the present invention. It should be pointed out that, for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for suppressing transient overvoltage in a safe and stable AC transmission system of a new energy station, characterized in that the steps include: Establish a power supply model for the new energy transmission system and configure a dual current loop control strategy on the grid-side converter; Based on the power supply model and the dual current loop control strategy, the transient overvoltage phasor of the non-fault phase under the asymmetric short circuit fault caused by two-phase grounding is calculated. The transient overvoltage phasor formula is: In the formula, is the transient overvoltage phasor, is the positive sequence Thevenin open circuit voltage phasor at the fault point, is the negative sequence Thevenin open circuit voltage phasor at the fault point, is the first proportionality coefficient; The functional relationship between the negative-sequence reactive current phasor output by the unit in the new energy transmission system and the negative-sequence Thevenin open-circuit voltage phasor at the fault point is established, and the formula is: In the formula, It is the negative sequence reactive current phasor output by the generator in the renewable energy transmission system. is the negative sequence grid-side line reactance, is the imaginary part; Calculate the permissible range of the effective value of the negative-sequence reactive current phasor based on the voltage threshold of the renewable energy transmission system and the overload current of the grid-side converter; When the effective value of the transient overvoltage phasor exceeds the voltage threshold, based on the dual current loop control strategy and according to the functional relationship and the transient overvoltage phasor formula, the effective value of the negative-sequence reactive current phasor is adjusted within the allowable range to reduce the effective value of the transient overvoltage phasor; In the power supply model, the grid side is equivalent to an infinite voltage source; The method for calculating the transient overvoltage phase comprises: Establishing a positive-sequence equivalent circuit, a negative-sequence equivalent circuit and a zero-sequence equivalent circuit of the new energy transmission system under the asymmetric short-circuit fault, and treating the power generation side of the new energy transmission system as equivalent to a current source; Obtaining a system equation of the new energy transmission system based on the positive-sequence equivalent circuit, the negative-sequence equivalent circuit and the zero-sequence equivalent circuit; Solving the transient overvoltage phasor by means of the system equation; The system equations are solved by a symmetrical component method, and the system equations include a positive sequence phasor equation, a negative sequence phasor equation, and a zero sequence phasor equation of a new energy transmission system; The positive sequence phasor equation of the new energy transmission system is: In the formula, is the positive sequence phasor of the infinite voltage source, is the positive sequence grid-side line reactance, It is the positive sequence current phasor of the renewable energy transmission system. is the positive-sequence Thevenin equivalent impedance, is the positive sequence voltage phasor from the fault point to the ground, is the total reactance of the positive sequence renewable energy transmission line and transformer, is the positive sequence current phasor of the fault point to ground, is the positive sequence voltage phasor at the collection line; The negative sequence phasor equation of the new energy transmission system is: In the formula, is the negative sequence voltage phasor from the fault point to the ground, is the negative-sequence Thevenin equivalent impedance, is the negative sequence current phasor from the fault point to the ground, is the negative sequence voltage phasor at the collection line, is the total reactance of the transmission line and transformer on the negative-sequence renewable energy side; The zero-sequence phasor equation of the new energy transmission system is: In the formula, is the zero-sequence Thevenin open-circuit voltage phasor at the fault point, is the zero-sequence Thevenin equivalent impedance, is the zero-sequence terminal voltage phasor, is the total reactance of the zero-sequence renewable energy side transmission line and transformer, is the zero-sequence current phasor from the fault point to the ground; The method for solving the transient overvoltage phase equation also includes: Create boundary conditions: Substituting the boundary conditions into the system equations, we obtain the formula: And set , the transient overvoltage phase equation is obtained as: In the formula, , is the second proportionality coefficient, and .

2. The transient overvoltage suppression method for a safe and stable AC transmission system of a new energy station according to claim 1 is characterized in that: According to the positive-sequence current and negative-sequence current states in the new energy transmission system, the negative-sequence reactive current in the dual current loop control strategy is set to be capacitive or inductive current.

3. The transient overvoltage suppression method for a safe and stable AC transmission system of a new energy station according to claim 2 is characterized in that: When there are positive-sequence reactive current and negative-sequence reactive current in the output of the renewable energy transmission system, and both the positive-sequence active current and the negative-sequence active current are zero, the negative-sequence reactive current in the dual current loop control strategy is set to be a capacitive current; When the output of the new energy transmission system has positive-sequence reactive current, negative-sequence reactive current, and positive-sequence active current, and the negative-sequence active current is zero, the negative-sequence reactive current in the dual current loop control strategy is set to be capacitive or inductive current.

4. The transient overvoltage suppression method for a safe and stable AC transmission system of a new energy station according to claim 3 is characterized in that: The effective value of the transient overvoltage phasor is negatively correlated with the effective value of the negative-sequence reactive current phasor.

5. The transient overvoltage suppression method for a safe and stable AC transmission system of a new energy station according to any one of claims 1 to 4, characterized in that: The corresponding condition of an endpoint of the allowable range is that the effective value of the transient overvoltage phasor is equal to the voltage threshold.

6. The transient overvoltage suppression method for a safe and stable AC transmission system of a new energy station according to claim 5 is characterized in that: The corresponding condition of an endpoint of the allowable range is that the maximum single-phase current of the new energy transmission system is equal to the overload current of the grid-side converter.

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