CN109638880B - Converter grid-connected system stability improving method based on admittance shaping range - Google Patents
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- H—ELECTRICITY
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Abstract
The invention discloses a converter grid-connected system stability improving method based on an admittance shaping range. Establishing an equivalent primary-dual complex circuit of a converter grid-connected system to obtain modal power of the generalized admittance; calculating the sensitivity of the real part and the imaginary part of the modal power and the characteristic value of the converter grid-connected system to the change multiple of the generalized admittance, and reflecting the influence degree of the generalized admittance on the oscillation of the converter grid-connected system by using the sensitivity; and calculating the phase angle range and the amplitude range of the change amount required by the generalized admittance of the current transformer, determining the change amount according to the phase angle range and the amplitude range of the change amount under the condition of determining the stability margin of the system, and adjusting the generalized admittance by using the change amount, thereby improving the stability of the system and achieving the effect of oscillation suppression. The impedance shaping analysis method for the converter grid-connected system can effectively improve the stability or stability margin of the system, inhibit the system oscillation and ensure the safe and stable operation of the system.
Description
Technical Field
The invention relates to a method for improving the stability of a converter grid-connected system, in particular to a method for determining a generalized admittance or a converter admittance change amount range for improving the system stability by calculating modal oscillation under an equivalent primary-dual circuit of the system, and further adjusting system parameter optimization or adding a compensation link to inhibit the system oscillation, thereby realizing the improvement of the stability.
Background
With the large-scale access of renewable clean energy such as wind, light and the like to an alternating current power grid, the power electronization degree of the alternating current power grid is continuously improved, the system is easy to generate a complex oscillation problem due to the coupling effect between the converter and the power grid, and the grid connection of the renewable energy provides a new challenge for the safety and stability of the system. The unstable problem caused by the grid connection of the converter seriously restricts the development and large-scale investment of renewable energy sources, so that the reliable and efficient oscillation suppression strategy is proposed to become an important problem to be solved urgently.
An impedance model of the converter grid-connected system is established by utilizing a generalized impedance theory, and the small interference stability of the system can be analyzed by calculating the modal power of the equivalent primary-dual complex circuit of the system. However, the method does not describe the relation between the modal power and the system oscillation in detail, and gives the admittance change direction and the shaping range for suppressing the system oscillation, so that a system oscillation suppression strategy based on complex circuit modal power analysis cannot be specifically given.
Disclosure of Invention
In order to solve the problems, the invention provides a converter grid-connected system stability improving method based on an admittance shaping range, which has a definite physical mechanism and aims to identify a part with an obvious system oscillation effect in a converter grid-connected system, optimize corresponding parameters or introduce a compensation link to change the admittance of the converter, so that the useful sum of modal power of all generalized admittances of a system equivalent primary-dual complex circuit is positive, and the system is stable with small interference.
The technical scheme of the invention comprises the following steps:
1) establishing an equivalent primary-dual complex circuit of a converter grid-connected system to obtain a generalized admittance YeiModal power O ofei；
2) Calculating real part and imaginary part of modal power and characteristic value s of converter grid-connected system to generalized admittance YeiCoefficient of variation keiThe sensitivity can reflect the influence degree of the generalized admittance on the oscillation of the converter grid-connected system by utilizing the sensitivity;
and the characteristic value s of the converter grid-connected system is a characteristic root of a state equation of the converter grid-connected system.
3) Calculating the change delta Y needed by the generalized admittance of the current transformere1_VSC、ΔYe2_VSCAnd Δ Ye3_VSCUnder the condition that the system stability margin is determined, the change amount is determined according to the phase angle range and the amplitude range of the change amount, and the generalized admittance is adjusted by the change amount, so that the stability of the system is improved, and the effect of oscillation suppression is achieved.
In the present invention, the imaginary part of the modal power, i.e. the modal power reactive Im (O)ei) If the voltage is positive, the oscillation energy storage element is regarded as an oscillation energy storage element and is not controlled; the moldReal part of the state power, i.e. modal power active Re (O)ei) When negative, it is regarded as an oscillation energy source element and needs to be controlled.
The step 1) is specifically as follows:
establishing an equivalent primary-dual complex circuit of the converter grid-connected system by adopting a generalized impedance modeling method, and obtaining each generalized admittance Y according to a voltage equation at a node in the equivalent primary-dual complex circuit of the converter grid-connected systemeiVoltage difference U between both endseiAnd flow through generalized admittance YeiCurrent of (I)eiThen, the voltage difference U is appliedeiThe conjugate sum current I ofeiConjugate of (d) and modal power factor oeMultiplying to obtain generalized admittance YeiModal power O ofei(ii) a Specifically, the following formula is adopted to calculate and obtain each generalized admittance YeiModal power O at an externally forced oscillation frequencyei(s1)：
Y(s)＝Ye_VSC(s)+Ye_L(s)+Ye_C(s)
Wherein, UeiFor the component of the circuit voltage vector U at the ith generalized admittance YeiVoltage at both ends, circuit voltage vector U ═ UP,UD]TT denotes the matrix transposition, UPAnd UDRepresenting a primary voltage and a dual voltage of an equivalent primary-dual complex circuit; y iseiDenotes the ith generalized admittance, IeiTo pass through the ith generalized admittance YeiThe current of (a); arg (·) indicates the phase angle, the superscript "·" indicates the conjugation; the superscript "'" indicates the partial derivative,
means that Y(s) is obtained when s ═ s1Partial derivatives of (A), Y(s) represents the sum of the admittance matrices of the converter grid-connected system, s represents the system characteristic value, s1Representing the mode of oscillation, j representing an imaginary number, oeIs the modal power factor; y ise_VSC(s)、Ye_L(s) and Ye_CAnd(s) are respectively the admittance matrixes of a current transformer, a power grid inductor and a filter capacitor in the equivalent primary-dual complex circuit, and can be directly obtained from the equivalent primary-dual circuit of the system.
The equivalent primary-dual complex circuit is mainly composed of three generalized admittances Y of a current transformereiTwo generalized admittances Y of the grid inductanceeiAnd two generalized admittances Y of the filter capacitoreiThe structure is shown in FIG. 3. The circuit is divided into a primary circuit and a dual circuit, wherein the primary circuit comprises a second generalized admittance Y of the grid inductore2_LSecond generalized admittance Y of the filter capacitore2_CFirst and second current transformer generalized admittancese2_VSCFour generalized admittances, the dual circuit comprising the grid inductance and a third generalized admittance Ye3_LFilter capacitor third generalized admittance Ye3_CA secondary generalized admittance of the current transformer and a third generalized admittance Y of the current transformere3_VSCFour generalized admittances; second generalized admittance Y of grid inductore2_LSecond generalized admittance Y of the filter capacitore2_CAnd a second generalized admittance Y of the current transformere2_VSCThe third generalized admittance Y of the grid inductor is connected between one end of the second generalized admittance of the current transformer and the ground in parallele3_LFilter capacitor third generalized admittance Ye3_CAnd third generalized admittance Y of the current transformere3_VSCThe other end of the first generalized admittance of the first converter is connected with the other end of the first generalized admittance of the second converter in parallel connection, and then the first generalized admittance Y of the converters is formed by connecting the other ends of the first generalized admittance of the first converter and the first generalized admittance of the second converter in series connectione1_VSC。
Under an ideal condition, a converter grid-connected system which normally works is not interfered (namely, no external forced oscillation exists); in practical application, the grid-connected system of the converter which normally works is only subjected to small loadInterference (no external forced oscillation) can be avoided, so that the equivalent primary-dual complex circuit can be simplified, and the second generalized admittance Y of the grid inductance can be ignorede2_LThe original component Delta E of the external disturbance source between the earthPAnd third generalized admittance Y of grid inductancee3_LDual component Δ E of external disturbance source with groundD。
Identifying generalized admittance Y with obvious system oscillation action according to real part and imaginary part of modal powerei。
In the step 1):
1.1) determining the key oscillation energy storage element of the system. Calculating generalized admittance YeiThe imaginary part of the modal power of (1), i.e. the modal power reactive Im (O)ei) If positive, it means YeiPlay a role in increasing s1The imaginary part of (c) acts as the energy storage element for system oscillation, and | Im (O)ei) The system oscillation energy storage element with the maximum | is a key oscillation energy storage element which has the maximum influence on the oscillation frequency of the system oscillation and has the generalized admittance YeiTuning is required.
1.2) determining the key oscillation energy source element of the system. Calculating generalized admittance YeiReal part of modal power of, i.e. modal power active Re (O)ei) If the index is negative, it means YeiPlays a role of enhancing system oscillation, is an oscillation energy source element of the system, and isei) The oscillation energy source element with the maximum | is a key oscillation source element with the maximum influence on the system stability, and the generalized admittance Y of the oscillation energy source elementeiTuning is required.
1.3) according to the generalized admittance YeiAnd selecting the most effective setting scheme for the sensitivity of the system stability. When the generalized admittance is given by YeiTo (1+ k)ei)YeiIn the process, the system characteristic value s and the generalized admittance Y are calculated againeiCoefficient of variation keiThe sensitivity of (2).
The step 2) is specifically as follows:
real part and imaginary part of modal power and system eigenvalue s versus generalized admittance YeiCoefficient of variation keiSensitivity of (2)
The expression of (a) is:
ko＝|UTY′(s1)U|-1
wherein U ═ UP,UD]T，s1Which is indicative of the mode of oscillation,
means that Y(s) is obtained when s ═ s1Partial derivatives of (A), Y(s) represents the sum of admittance matrixes of the converter grid-connected system, j represents an imaginary number, oeIs the modal power factor; y ise_VSC(s)、Ye_L(s) and Ye_CAnd(s) are respectively the admittance matrixes of a current transformer, a power grid inductor and a filter capacitor in the equivalent primary-dual complex circuit.
The sensitivity reflects the relation between the change of the characteristic value s and the change of the generalized admittance, and the larger the value is, the larger the generalized admittance Y iseiEach time a fixed ratio k is addedeiThe larger the system characteristic value variation, that is, the more obvious the influence of the generalized admittance on the system stability, the setting should be considered preferentially.
By calculating the generalized admittance Y of the key oscillation energy storage element and the key oscillation energy source elementeiAnd sequencing the elements to be set according to the sequence of the sensitivity from large to small for the sensitivity of the system stability. The element with the highest sensitivity is the weakest link under the working condition, parameter optimization or compensation should be considered firstly, if the adjustment is not enough to meet the stability margin requirement, the generalized admittance Y of the next element is consideredeiAnd setting until the system meets the requirements.
Comparing the respective generalized admittances YeiThe larger the sensitivity is, the larger the influence on the oscillation of the converter grid-connected system is, and the maximum sensitivity is taken for admittance setting.
In the invention, the sensitivity and the change amount are both directed at the generalized admittances of the three current transformers.
The change quantity delta Y of the generalized admittance of the three current transformers in the equivalent primary-dual complex circuite1_VSC、ΔYe2_VSCAnd Δ Ye3_VSCThe phase angle ranges of (a) are specifically calculated as:
wherein o iseFor the modal power factor, α represents the oscillation mode s1The phase angle of (d);
the change quantity delta Y of three generalized admittances of three inversion elements in the equivalent source-dual complex circuite1_VSC、ΔYe2_VSCAnd Δ Ye3_VSCThe range of amplitudes of (d) is calculated as:
wherein s' is a system characteristic value considering the stability margin, and is the vibration in the critical stateA swing mode; o issumAnd the sum of modal power of all generalized admittances in the converter grid-connected system is represented.
In the step 3), the variation is set according to the phase angle range and the amplitude range of the variation, the generalized admittance parameters are optimally adjusted by the variation or a compensation link is added in a converter (inverter element) controller for adjustment, so that the useful sum of the modal power of all the generalized admittances of the equivalent primary-dual complex circuit is positive and meets a certain margin, and the oscillation suppression effect is achieved.
In the present invention, the generalized admittance Y of the inverter element is changede1_VSC、Ye2_VSC、Ye3_VSCThe purpose of suppressing the system oscillation is achieved, and parameter optimization or compensation link design of a converter grid-connected system can be guided according to the change, so that the stability of the system is improved, and the effect of suppressing the oscillation is achieved.
The invention has the beneficial effects that:
the method is based on the modal power analysis of the system equivalent source-dual complex circuit, identifies the part with high stability sensitivity in the converter grid-connected system by calculating the modal oscillation power of the system equivalent source-dual complex circuit, optimizes the adjustment parameters or introduces a compensation link to change the converter admittance, can effectively improve the system stability or stability margin, inhibits the system oscillation, and ensures the safe and stable operation of the system.
Drawings
Fig. 1 is an equivalent circuit diagram of converter grid connection in simulation verification according to an embodiment of the present invention.
Fig. 2 is a block diagram of vector control of a converter in simulation verification according to an embodiment of the present invention.
FIG. 3 is an equivalent source-pair complex circuit of the system in simulation verification according to an embodiment of the present invention.
Fig. 4 is an oscillation curve in the electromagnetic transient simulation of the original system in the simulation verification according to the embodiment of the present invention.
Fig. 5 is a schematic diagram of a system characteristic value moving range in simulation verification according to an embodiment of the present invention.
Fig. 6 is a block diagram of a control of a system compensation link in simulation verification according to an embodiment of the present invention.
Fig. 7 is an oscillation curve in the electromagnetic transient simulation of the compensated system in the simulation verification according to the embodiment of the present invention.
Detailed Description
The invention is described in further detail below with reference to the figures and the embodiments.
The specific embodiment of the method according to the invention is implemented completely as follows:
and (3) constructing a converter grid-connected model in Matlab/Simulink software, as shown in figure 1. The converter adopts a double-loop vector control strategy based on a phase-locked loop, as shown in fig. 2, and the power factor of the converter output power is 1. The physical meanings of the variables in FIGS. 1 and 2 are shown in Table 1 below:
table 1 converter grid-connected system variable corresponding table
The parameter values of the variables in the converter grid-connected system are shown in the following table 2:
table 2 example simulation verification of parameter values of converter variables
The system primary-dual complex circuit formed by generalized impedance theory modeling is shown in fig. 3. The generalized admittances and the current transformer admittances in the system and the modal powers thereof are obtained by calculation according to the data in table 2 and are shown in table 3. The line inductance is further increased, the inductance Lg on the ac power grid side in the embodiment 1 is increased from 0.55p.u. to 0.56p.u., the inductance Lg on the ac power grid side in the embodiment 2 is increased from 0.4p.u. to 0.41p.u., each generalized admittance and converter admittance and modal power thereof in the system are shown in table 4, and since the modal power of the system changes from positive to negative, the system changes from stable to unstable, the oscillation waveform of the system in the electromagnetic transient simulation is shown in fig. 4, and it can be seen from the graph that the system oscillation changes from attenuation to divergence, and an unstable phenomenon occurs.
TABLE 3 generalized admittance and modal power at oscillation frequency
TABLE 4 generalized admittance and modal power after varying line inductance
Changing the admittance Y of a converter according to said tuning principle of the inventiong11And Yg22Wherein Y isg11And Yg22The diagonal elements of the transformer admittance matrix are in relation to the transformer generalized admittance:
therefore, the setting principle is consistent with the generalized admittance setting principle of the converter, and the setting principle and the generalized admittance setting principle are equivalent and can be mutually converted.
The system characteristic value moving range obtained according to step 2.1) is shown in fig. 5.
Are respectively aligned with Yg11And Yg22Setting to obtain a critical state satisfying a margin, wherein the oscillation mode change amount delta s is1The compensation link shown in fig. 6 is added to the converter control shown in fig. 1, and the input signal, the application position of the output signal and the parameters of the compensation link are shown in table 5.
TABLE 5 Compensation Link parameters
Before and after the compensation step is added, the admittance and modal power of the system converter are shown in table 6.
TABLE 6 Compensation front and rear system converter admittance and modal power thereof
As can be seen from table 6:
1) for example 1, setting Yg11Then, Og11When the value is changed from-0.92-1.25 i to-0.86-1.25 i, the system characteristic value is shifted from the horizontal to the left, and the system stability is enhanced; setting Yg22Then, Og22After the system characteristic value is changed from-0.95 to 1.31i to 0.89 to 1.31i, the system characteristic value is horizontally shifted from-0.3230 +89.1128i to-1.5891 +89.1203i, and the system stability is enhanced.
2) For example 2, Y is setg11Then, Og11After the system characteristic value is changed from-0.12 +0.55i to-0.05 +0.55i, the system characteristic value is horizontally shifted from-0.07 +68.94i to-1.43 +68.79i, so that the system stability is enhanced; setting Yg22Then, Og22After the value is changed from-0.41 +0.82i to-0.35 +0.82i, the system characteristic value is horizontally shifted from-0.07 +68.94i to-1.36 +68.80i, and the system stability is enhanced.
In summary, it can be seen that, for the two systems of embodiment 1 and embodiment 2, the tuning method based on modal power changes Yg11Or Yg22And then, the system oscillation mode can be horizontally shifted to the left, and the stability of the system is enhanced. In addition, the electromagnetic transient simulation result can also illustrate the above conclusion:
1) for example 1, when t is 0.5s, the line inductance in the ac power grid becomes 0.55p.u. from 0.54p.u., and when t is 4.5s, the line inductance becomes 0.56p.u., and Y is setg11And Yg22Then, terminal voltage oscillation curves of the system are shown in fig. 7(a) and 7(b), respectively. As can be seen from a comparison of FIG. 5(a), setting Yg11Or Yg22Then, the line inductance is 0.55p.u., the oscillation in the systemGradually decays, the decay rate is obviously faster than that of the original system, and when the line inductance is increased to 0.56p.u., the oscillation in the system is still gradually decayed, and the system is stable with small interference. This indicates setting Yg11Or Yg22The stability of the system of example 1 is improved.
2) For example 2, when t is 0.5s, the line inductance in the ac power grid becomes 0.39p.u. to 0.4p.u., and when t is 4.5s, the line inductance becomes 0.41p.u., and Y is setg11And Yg22Then, terminal voltage oscillation curves of the system are shown in fig. 7(c) and 7 (d). As can be seen by comparing FIG. 5(b), setting Yg11Or Yg22Then, when the line inductance is 0.40p.u., the oscillation in the system is gradually attenuated, the attenuation speed is obviously faster than that of the original system, and when the line inductance is increased to 0.41p.u., the oscillation in the system is still gradually attenuated, and the system is small in interference and stable. This indicates setting Yg11Or Yg22The stability of the system of example 1 is improved.
Therefore, the modal power-based admittance shaping range calculation method provided by the invention can identify weak links seriously influenced by the system on stability damage by analyzing the modal power of the equivalent primary-dual complex circuit of the system, provides a calculation method of the admittance shaping range of the converter for improving the system stability, and further guides the design of parameter optimization or compensation links of the system, thereby achieving the effect of inhibiting the system oscillation.
The foregoing detailed description is intended to illustrate and not limit the invention, which is intended to be within the spirit and scope of the appended claims, and any changes and modifications that fall within the true spirit and scope of the invention are intended to be covered by the following claims.
Claims (1)
1. A method for improving stability of a converter grid-connected system based on an admittance shaping range is characterized by comprising the following steps:
1) establishing an equivalent primary-dual complex circuit of a converter grid-connected system to obtain a generalized admittance YeiModal power O ofei；
2) Calculating real part and imaginary part of modal power and characteristic value s of converter grid-connected system to generalized admittance YeiCoefficient of variation keiThe sensitivity can reflect the influence degree of the generalized admittance on the oscillation of the converter grid-connected system by utilizing the sensitivity;
3) calculating the change delta Y needed by the generalized admittance of the current transformere1_VSC、ΔYe2_VSCAnd Δ Ye3_VSCUnder the condition that the system stability margin is determined, the change amount is determined according to the phase angle range and the amplitude range of the change amount, and the generalized admittance is adjusted by the change amount, so that the stability of the system is improved, and the effect of oscillation suppression is achieved;
the step 1) is specifically as follows:
establishing an equivalent primary-dual complex circuit of the converter grid-connected system by adopting a generalized impedance modeling method, and obtaining each generalized admittance Y according to a voltage equation at a node in the equivalent primary-dual complex circuit of the converter grid-connected systemeiVoltage difference U between both endseiAnd flow through generalized admittance YeiCurrent of (I)eiThen, the voltage difference U is appliedeiThe conjugate sum current I ofeiConjugate of (d) and modal power factor oeMultiplying to obtain generalized admittance YeiModal power O ofei(ii) a Specifically, the following formula is adopted to calculate and obtain each generalized admittance YeiModal power O at an externally forced oscillation frequencyei(s1)：
Y(s)＝Ye_VSC(s)+Ye_L(s)+Ye_C(s)
Wherein, UeiFor the component of the circuit voltage vector U at the ith generalized admittance YeiVoltage at both ends, circuit voltage vector U ═ UP,UD]TT denotes the matrix transposition, UPAnd UDRepresenting a primary voltage and a dual voltage of an equivalent primary-dual complex circuit; y iseiDenotes the ith generalized admittance, IeiTo pass through the ith generalized admittance YeiThe current of (a); arg (·) indicates the phase angle, the superscript "·" indicates the conjugation; the superscript "'" indicates the partial derivative,
means that Y(s) is obtained when s ═ s1Partial derivatives of (A), Y(s) represents the sum of the admittance matrices of the converter grid-connected system, s represents the system characteristic value, s1Representing the mode of oscillation, j representing an imaginary number, oeIs the modal power factor; y ise_VSC(s)、Ye_L(s) and Ye_C(s) admittance matrixes of a current transformer, a power grid inductor and a filter capacitor in the equivalent primary-dual complex circuit respectively;
the step 2) is specifically as follows:
real part and imaginary part of modal power and system eigenvalue s versus generalized admittance YeiCoefficient of variation keiSensitivity of (2)
The expression of (a) is:
ko＝|UTY′(s1)U|-1
wherein U ═ UP,UD]T，s1Which is indicative of the mode of oscillation,
is expressed by the formula Y(s)s＝s1Partial derivatives of (A), Y(s) represents the sum of admittance matrixes of the converter grid-connected system, j represents an imaginary number, oeIs the modal power factor; y ise_VSC(s)、Ye_L(s) and Ye_C(s) admittance matrixes of a current transformer, a power grid inductor and a filter capacitor in the equivalent primary-dual complex circuit respectively;
the change quantity delta Y of the generalized admittance of the three current transformers in the equivalent primary-dual complex circuite1_VSC、ΔYe2_VSCAnd Δ Ye3_VSCThe phase angle ranges of (a) are specifically calculated as:
wherein o iseFor the modal power factor, α represents the oscillation mode s1The phase angle of (d);
the change quantity delta Y of three generalized admittances of three inversion elements in the equivalent source-dual complex circuite1_VSC、ΔYe2_VSCAnd Δ Ye3_VSCThe range of amplitudes of (d) is calculated as:
wherein s' is a system characteristic value considering the stability margin and is an oscillation mode in a critical state; o issumAnd the sum of modal power of all generalized admittances in the converter grid-connected system is represented.
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| CN106532685A (en) * | 2016-10-26 | 2017-03-22 | 浙江大学 | Generalized impedance criterion calculation method for stability analysis of grid-connected inverter and application |
| CN106786776A (en) * | 2017-02-15 | 2017-05-31 | 云南电网有限责任公司 | A kind of method using generalized impedance method analysis grid-connected inverter system stability is corrected |
| CN108509697A (en) * | 2018-03-19 | 2018-09-07 | 浙江大学 | A kind of converter system original-dual circuit modeling method considering disturbance |
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| CN106532685A (en) * | 2016-10-26 | 2017-03-22 | 浙江大学 | Generalized impedance criterion calculation method for stability analysis of grid-connected inverter and application |
| CN106786776A (en) * | 2017-02-15 | 2017-05-31 | 云南电网有限责任公司 | A kind of method using generalized impedance method analysis grid-connected inverter system stability is corrected |
| CN108509697A (en) * | 2018-03-19 | 2018-09-07 | 浙江大学 | A kind of converter system original-dual circuit modeling method considering disturbance |
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