CN114204586A - Second harmonic injection method for inhibiting voltage fluctuation of MMC capacitor and suitable for multiple working conditions - Google Patents
Second harmonic injection method for inhibiting voltage fluctuation of MMC capacitor and suitable for multiple working conditions Download PDFInfo
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- H—ELECTRICITY
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- H—ELECTRICITY
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Abstract
A second harmonic injection method for suppressing voltage fluctuation of an MMC capacitor suitable for multiple working conditions comprises the following steps: step 1: obtaining an expression of the instantaneous power of the three-phase bridge arm after the double-frequency circulation injection, and establishing an optimization objective function on the basis of the fundamental frequency and the double-frequency fluctuation component of the instantaneous power of the three-phase bridge arm; step 2: determining the amplitude and phase of the double frequency circulation and injection according to the optimization calculation result; and step 3: and generating a reference signal required by the controller according to the calculated amplitude and phase.
Description
Technical Field
The invention belongs to the technical field of direct current transmission, particularly relates to a flexible direct current transmission technology, and particularly relates to a second harmonic injection method for inhibiting voltage fluctuation of an MMC capacitor, which is suitable for multiple working conditions.
Background
Flexible direct current transmission (VSC-HVDC) is a new generation of direct current transmission technology following alternating current transmission, conventional direct current transmission. The flexible direct current transmission technology has the characteristics of independent active and reactive power adjustment, strong weak power grid access and low voltage ride through capability, low alternating current filtering, reactive power compensation requirements and the like. The Modular Multilevel Converter (MMC) has the characteristics of flexible control, module expansibility, low switching frequency, low harmonic content and the like. At present, MMC structures are adopted in Shanghai south-Virginia flexible direct current engineering, Nanao three-terminal flexible direct current engineering, Zhoushan five-terminal flexible direct current engineering, Xiamen +/-320 kV flexible direct current demonstration engineering and Zhang Bei direct current power grid engineering which are built in China.
However, compared with a conventional current source converter (LCC), the MMC has a problem of large volume and weight under the same capacity. Under the era background of the state of vigorously developing the open-sea wind power, the sending of the open-sea wind power through the MMC is one of the solutions generally accepted at present. The MMC sub-module capacitor occupies more than 1/2 of the sub-module volume and is about 1/3 of the cost, and the reduction of the size and the weight of the MMC is beneficial to reduction of the construction cost of an offshore platform and a converter station. The offshore wind power scene working conditions are variable, so that the capacitor voltage fluctuation suppression strategy research suitable for the multiple working conditions is developed to reduce the design requirement of the capacitor capacitance value, and the method has great theoretical and engineering significance.
Disclosure of Invention
The invention aims to effectively inhibit the fluctuation of the bridge arm power of an MMC converter valve, so as to reduce the fluctuation of the sub-module capacitor voltage and further reduce the design requirement of the sub-module capacitor capacitance value, thereby reducing the sub-module capacitor volume and finally reducing the MMC volume.
A second harmonic injection method for suppressing voltage fluctuation of an MMC capacitor suitable for multiple working conditions comprises the following steps:
step 1: obtaining a three-phase bridge arm instantaneous power expression considering double-frequency circulating current injection, and establishing an optimized objective function on the basis of the fundamental frequency and double-frequency fluctuation components of the bridge arm instantaneous power;
step 2: determining the amplitude and phase of double frequency circulation injection according to the optimization result;
and step 3: and generating a reference signal required by the controller according to the calculated amplitude and phase.
In step 1, when obtaining the three-phase bridge arm instantaneous power expression after considering double frequency circulation injection, the method includes:
1) obtaining three-phase bridge arm current expressions considering secondary harmonic current injection under different working conditions;
2) obtaining three-phase bridge arm voltage expressions under different working conditions;
3) and obtaining the instantaneous power of the three-phase bridge arm under different working conditions.
The MMC network side three-phase voltage can be expressed as follows:
in the formula, x, y and z are three-phase voltage drop coefficients, and the values are 0-1; t is the transformer transformation ratio; u shapemThe valve side alternating voltage amplitude under the steady state condition; omega is the fundamental angular frequency; gamma is the transformer shift angle;
the positive, negative and zero sequence voltages of the network side are obtained through abc-dq0 transformation and are expressed as follows:
wherein
Defined as a twiddle factor;
when the MMC operates in a steady state, x, y, z are 1; under the condition of unbalanced alternating voltage, three phases are not symmetrical any more, and negative sequence voltage and current components can occur. The unbalance degree and the unbalance mode are different, the falling coefficients of the three-phase voltage are different, and the sequence components of the three-phase voltage are different.
The a-phase leg current considering second harmonic current injection can be expressed as:
wherein iau、ialRespectively an a-phase upper bridge arm current and a lower bridge arm current;
respectively a power factor angle and a second harmonic current phase angle; i isdc_aThe direct current quantity of the a-phase bridge arm current is obtained; i ismMeasuring the current amplitude for alternating current; k is a radical of2Is the second harmonic injection coefficient; lambda is an alternating current correction coefficient;
the a-phase bridge arm voltage can be expressed as:
wherein u isau、ualRespectively an a-phase upper bridge arm voltage and a lower bridge arm voltage; m isa +、ma -Respectively a phase positive sequence voltage modulation ratio and a phase negative sequence voltage modulation ratio; alpha is alphaa +、αa -Respectively is a phase positive and negative sequence alternating voltage phase angle; u shapedcIs a direct current side voltage value;
due to the symmetry of the bridge arms, the method only analyzes the upper bridge arm, and the power fluctuation power characteristic of the lower bridge arm is the same as that of the upper bridge arm. The instantaneous power fluctuation P of the bridge arm on the a phase can be known by the expressions of the current and the voltage of the bridge armauCan be expressed as:
the specific expression of each secondary component in the above formula is as follows:
since the energy cannot accumulate indefinitely on the submodules, the dc component should be 0. The direct current component flowing through the a-phase bridge arm can be obtained as follows:
in the step 1, the fundamental frequency and the double-frequency fluctuation component of the instantaneous power of the three-phase bridge arm are used as an inhibition target, and when an optimization target function is established, the fundamental frequency and the double-frequency fluctuation of the instantaneous power of the three-phase upper bridge arm are obtained according to the instantaneous power of the three-phase upper bridge arm containing double-frequency circulation injection, so that a fluctuation amplitude is obtained;
establishing an objective function according to the obtained fundamental frequency of the instantaneous power of the three-phase bridge arm and the fluctuation amplitude of the double frequency fluctuation as follows:
wherein, | Pxu_1I and I Pxu_2I is the fundamental frequency of the instantaneous power of the three-phase upper bridge arm and the fluctuation amplitude of the double frequency fluctuation respectively, and x is a, b and c; k represents a weight coefficient of the secondary fluctuation of the capacitor voltage.
The fundamental frequency and double frequency fluctuation of the instantaneous power of the bridge arm on the phase a are as follows:
wherein, Pau_1And Pau_2Fundamental frequency and double frequency fluctuation of instantaneous power of an upper bridge arm of the phase a are respectively;
to Pau_1And Pau_2Performing an identity transform to obtain:
Pau_1＝Aau_1sinωt+Bau_1cosωt
Pau_2＝Aau_2sin2ωt+Bau_2cos2ωt
obtaining P according to the identity transformation resultau_1And Pau_2The fluctuation amplitude is:
the above expressions take the phase a as an example, but if the voltage drop degrees of the three-phase power grid are different, the fluctuation characteristics of the three-phase bridge arm are asymmetric, but the calculation modes of the phases are the same, and the expressions of the phase b and the phase c can be obtained only by correcting the positive-negative sequence voltage modulation ratio and the positive-negative sequence voltage phase angle in the above formulas. The derivation calculation hereinafter is also performed by taking the a-phase as an example. From the above analysis, the fundamental frequency of the instantaneous power of the b-phase bridge arm and the c-phase bridge arm and the amplitude P of the double frequency fluctuation can be obtained in the same waybu_1、Pbu_2And Pcu_1、Pcu_2Comprises the following steps:
generating a reference signal according to the amplitude and the phase of the double frequency circulation injection, and realizing the injection of the double frequency circulation by combining a modulation algorithm comprises the following steps:
amplitude phase k based on double frequency circulating current injection2、
And generating a reference signal in combination with a harmonics generator;
the reference signal is injected into the controller through the double frequency circulation to obtain a double frequency circulation modulation voltage signal u2Finally, the injection of the double frequency circulation is realized by changing the modulation wave of the modulation algorithm.
Compared with the prior art, the invention has the following technical effects:
the invention takes the fundamental frequency and the double-frequency fluctuation component of the instantaneous power as the suppression target, carries out the optimization design of the amplitude and the phase of the injected double-frequency circulating current, and finally injects the calculated double-frequency circulating current into the three phases by changing the modulation wave signal. The method can effectively inhibit the fluctuation of the converter valve bridge arm power, further reduce the fluctuation of the sub-module capacitor voltage, further reduce the design requirement of the sub-module capacitor capacitance value, reduce the sub-module capacitor volume, and finally reduce the volume of the MMC, and has important significance for reducing the volume and weight of the MMC and further reducing the construction cost of an offshore platform and a converter station.
Drawings
FIG. 1 is a flow chart of the present invention;
fig. 2 is a system control block diagram of the present invention.
Detailed Description
As shown in fig. 1 and fig. 2, a second harmonic injection method for suppressing voltage fluctuation of an MMC capacitor suitable for multiple operating conditions mainly includes the following steps:
step 1: and establishing an optimization objective function by taking the fundamental frequency and the double-frequency fluctuation component of the instantaneous power of the three-phase bridge arm as an inhibition objective.
In the embodiment of the invention, firstly, a three-phase bridge arm instantaneous power expression after double frequency circulation injection is deduced and considered, and then an optimization objective function is established.
1) And considering the instantaneous power expression of the three-phase bridge arm after the injection of the double-frequency circulating current.
The MMC network side three-phase voltage can be expressed as follows:
in the formula, x, y and z are three-phase voltage drop coefficients, and the values are 0-1; u shapemIs the amplitude of the alternating voltage under steady state conditions; omega is the fundamental angular frequency; gamma is the transformer shift angle;
the positive, negative and zero sequence voltages of the network side are obtained through abc-dq0 transformation and are expressed as follows:
wherein
Defined as a twiddle factor;
when the MMC operates in a steady state, x, y, z are 1; under the condition of unbalanced alternating voltage, three phases are not symmetrical any more, and negative sequence voltage and current components can occur. The unbalance degree and the unbalance mode are different, the falling coefficients of the three-phase voltage are different, and the sequence components of the three-phase voltage are different.
The a-phase leg current considering second harmonic current injection can be expressed as:
wherein iau、ialRespectively an a-phase upper bridge arm current and a lower bridge arm current;
respectively a power factor angle and a second harmonic current phase angle; i isdc_aIs the direct current component of the a-phase bridge arm current; i ismMeasuring the current amplitude for alternating current; k is a radical of2Is the second harmonic injection coefficient; lambda is an alternating current correction coefficient;
the a-phase bridge arm voltage can be expressed as:
wherein u isau、ualRespectively an a-phase upper bridge arm voltage and a lower bridge arm voltage; m isa +、ma -Respectively a phase positive sequence voltage modulation ratio and a phase negative sequence voltage modulation ratio; alpha is alphaa +、αa -Respectively is a phase positive and negative sequence alternating voltage phase angle; u shapedcIs a direct current side voltage value;
due to the symmetry of the bridge arms, the method only analyzes the upper bridge arm, and the power fluctuation power characteristic of the lower bridge arm is the same as that of the upper bridge arm. The instantaneous power fluctuation P of the bridge arm on the a phase can be known by the expressions of the current and the voltage of the bridge armauCan be expressed as:
the specific expression of each sub-component in the formula (5) is shown in the formula (6)
Since the energy cannot accumulate indefinitely on the submodules, the dc component should be 0. The direct current component flowing through the a-phase bridge arm can be obtained as follows:
and establishing an optimization objective function by taking the fundamental frequency and the double-frequency fluctuation component of the instantaneous power of the three-phase bridge arm as an inhibition objective. The main process is as follows:
the fundamental frequency and the double frequency fluctuation of the instantaneous power of the bridge arm on the phase a can be obtained by the formula (6), as shown in the formula (8):
wherein, Pau_1And Pau_2Fundamental frequency and double frequency fluctuation of instantaneous power of an upper bridge arm of the phase a are respectively;
to Pau_1And Pau_2Performing an identity transform to obtain:
obtaining P according to the identity transformation resultau_1And Pau_2The fluctuation amplitude is:
the above expressions take the phase a as an example, but if the voltage drop degrees of the three-phase power grid are different, the fluctuation characteristics of the three-phase bridge arm are asymmetric, but the calculation modes of the phases are the same, and the expressions of the phase b and the phase c can be obtained only by correcting the positive-negative sequence voltage modulation ratio and the positive-negative sequence voltage phase angle in the above formulas. The derivation calculation hereinafter is also performed by taking the a-phase as an example. From the above analysis, P can be obtainedbu_1、Pbu_2And Pcu_1、Pcu_2The fluctuation amplitude is:
the difficulty with the second harmonic injection strategy is how to choose the amplitude and phase of the injected harmonics. For a particular operating regime (m and
fixed), it requires determining the amplitude I of the second harmonic current2And phase
There are two variables in total. To maximally suppress the capacitor voltage fluctuation, it is required to reduce the fundamental frequency and the second harmonic component in the bridge arm power as much as possible, and therefore the objective function is defined herein as
Wherein, | Pxu_1I and I Pxu_2RespectivelyThe fluctuation amplitude of the instantaneous power fundamental frequency and double frequency fluctuation of the three-phase upper bridge arm is x, a, b and c; k represents a weight coefficient of the secondary fluctuation of the capacitor voltage. Since the influence of the double frequency power fluctuation on the overall power fluctuation amplitude is smaller, the k value is set to 0.5 here. It should be noted that the value of k is not fixed, and can be adjusted appropriately according to actual conditions.
Step 2: and performing global optimization by taking the objective function minimization as an objective to obtain the optimal amplitude and phase of the double frequency circulation injection.
In the embodiment of the invention, global optimization is carried out by taking objective function minimization as a target, and different steady-state modulation ratios m and power factor angles are used
To determine k2、
Optimal values for both variables. It should be noted that the present invention does not limit the optimization method, and the user can select the optimization method according to the actual situation.
It will be appreciated by those skilled in the art that since the objective function is based on the capacitor voltage fluctuation, its minimized solution can achieve a minimization of the capacitor voltage fluctuation, i.e. if the amplitude and phase of the optimal solution are injected, the objective can be achieved, in particular, by designing the parameter k2、
Can realize suppression of the fluctuation of the capacitor voltage.
And step 3: and generating a reference signal according to the amplitude and the phase of the injection, and combining a modulation algorithm to realize the injection of the double frequency circulation.
In the embodiment of the invention, the optimal amplitude and phase of double frequency circulating current injection are obtained, namely k is obtained2、
Optimal values, after which harmonic generation can be combinedGenerating a reference signal i2d *And i2q *Wherein i is2d *Reference signal, i, for d-axis double frequency circulating injection controller2q *A reference signal for the q-axis double frequency circulating current injection controller. The reference signal is injected into the controller through the double frequency circulation to obtain a double frequency circulation modulation voltage signal u2Finally, the injection of the double frequency circulation is realized by changing the modulation wave of the modulation algorithm.
In the overall system control block diagram shown in fig. 2, the upper dashed box shows the operation of the double frequency loop injection controller, k2And
indicating that the amplitude and phase of the double frequency circulation needs to be injected; t isabc/dqThe module being a dq transformation module, Tdq/abcThe module is a dq inverse transformation module; i.e. i2d *And i2q *Respectively are dq axis double frequency current reference values; the PI module is a proportional integral link; l isarmIs the bridge arm reactance value; i.e. idiffIs a phase circulation measurement; i.e. i2fdAnd i2fqD-axis double frequency circulation and q-axis double frequency circulation respectively; the difference between the dq axis measured value of the double frequency circulation and the reference value is subjected to PI regulation to obtain d-axis double frequency circulation modulation voltage and q-axis double frequency circulation modulation voltage, and the d-axis double frequency circulation modulation voltage and the q-axis double frequency circulation modulation voltage are subjected to dq inverse transformation to generate a double frequency circulation modulation voltage signal u2。
And the lower dotted line frame shows the whole work control block diagram of the system. Double closed-loop controller for generating modulated wave reference signal ej_ref(ii) a The double-frequency circulating current injection controller outputs a double-frequency circulating current modulation voltage signal u2And further influences the modulation link, thereby realizing injection of frequency doubling circulation.
Through the above description of the embodiments, it is clear to those skilled in the art that the above embodiments can be implemented by software, and can also be implemented by software plus a necessary general hardware platform. With this understanding, the technical solutions of the embodiments can be embodied in the form of a software product, which can be stored in a non-volatile storage medium (which can be a CD-ROM, a usb disk, a removable hard disk, etc.), and includes several instructions for enabling a computer device (which can be a personal computer, a server, or a network device, etc.) to execute the methods according to the embodiments of the present invention.
Claims (6)
1. A second harmonic injection method for suppressing voltage fluctuation of an MMC capacitor suitable for multiple working conditions is characterized by comprising the following steps:
step 1: obtaining a three-phase bridge arm instantaneous power expression considering double-frequency circulating current injection, and establishing an optimized objective function on the basis of the fundamental frequency and double-frequency fluctuation components of the three-phase bridge arm instantaneous power;
step 2: determining the amplitude and the phase of double frequency circulation injection according to the optimization calculation result;
and step 3: and generating a reference signal required by the controller according to the calculated amplitude and phase.
2. The method of claim 1, wherein in step 1, when obtaining the instantaneous power expression of the three-phase bridge arm after considering double frequency circulating current injection, the method comprises the following steps:
1) obtaining three-phase bridge arm current expressions considering secondary harmonic current injection under different working conditions;
2) obtaining three-phase bridge arm voltage expressions under different working conditions;
3) and obtaining the instantaneous power of the three-phase bridge arm under different working conditions.
3. The method of claim 2,
the MMC network side three-phase voltage can be expressed as follows:
in the formula, x, y and z are three-phase voltage drop coefficients, and the values are 0-1; t is the transformer transformation ratio; u shapemIs the valve side under steady state conditionsAn alternating voltage amplitude; omega is the fundamental angular frequency; gamma is the transformer shift angle;
the positive, negative and zero sequence voltages of the network side are obtained through abc-dq0 transformation and are expressed as follows:
wherein
Defined as a twiddle factor;
when the MMC operates in a steady state, x, y, z are 1; under the condition of unbalanced alternating voltage, three phases are not symmetrical any more, and negative sequence voltage and current components can occur. The unbalance degree and the unbalance mode are different, the falling coefficients of the three-phase voltage are different, and the sequence components of the three-phase voltage are different.
The a-phase leg current considering second harmonic current injection can be expressed as:
wherein iau、ialRespectively an a-phase upper bridge arm current and a lower bridge arm current;
respectively a power factor angle and a second harmonic current phase angle; i isdc_aIs the direct current component of the a-phase bridge arm current; i ismMeasuring the current amplitude for alternating current; k is a radical of2Is the second harmonic injection coefficient; lambda is an alternating current correction coefficient;
the a-phase bridge arm voltage can be expressed as:
wherein u isau、ualRespectively a phase upper bridge armPressing and lower bridge arm voltage; m isa +、ma -Respectively a phase positive sequence voltage modulation ratio and a phase negative sequence voltage modulation ratio; alpha is alphaa +、αa -Respectively is a phase positive and negative sequence alternating voltage phase angle; u shapedcIs a direct current side voltage value;
due to the symmetry of the bridge arms, the method only analyzes the upper bridge arm, and the power fluctuation power characteristic of the lower bridge arm is the same as that of the upper bridge arm. The instantaneous power fluctuation P of the bridge arm on the a phase can be known by the expressions of the current and the voltage of the bridge armauCan be expressed as:
the specific expression of each secondary component in the above formula is as follows:
since the energy cannot accumulate indefinitely on the submodules, the dc component should be 0. The direct current component flowing through the a-phase bridge arm can be obtained as follows:
4. a method according to one of claims 1 to 3, characterized in that: in the step 1, the fundamental frequency and the double-frequency fluctuation component of the instantaneous power of the three-phase bridge arm are used as an inhibition target, and when an optimization target function is established, the fundamental frequency and the double-frequency fluctuation of the instantaneous power of the three-phase upper bridge arm are obtained according to the instantaneous power of the three-phase upper bridge arm containing double-frequency circulation injection, so that a fluctuation amplitude is obtained;
establishing an objective function according to the obtained fundamental frequency of the instantaneous power of the three-phase bridge arm and the fluctuation amplitude of the double frequency fluctuation as follows:
wherein, | Pxu_1I and I Pxu_2I is the fundamental frequency of the instantaneous power of the three-phase upper bridge arm and the fluctuation amplitude of the double frequency fluctuation respectively, and x is a, b and c; k represents a weight coefficient of the secondary fluctuation of the capacitor voltage.
5. The method of claim 3, wherein: the fundamental frequency and double frequency fluctuation of the instantaneous power of the bridge arm on the phase a are as follows:
wherein, Pau_1And Pau_2Fundamental frequency and double frequency fluctuation of instantaneous power of an upper bridge arm of the phase a are respectively;
to Pau_1And Pau_2Performing an identity transform to obtain:
Pau_1＝Aau_1sinωt+Bau_1cosωt
Pau_2＝Aau_2sin2ωt+Bau_2cos2ωt
obtaining P according to the identity transformation resultau_1And Pau_2The fluctuation amplitude is:
the above expressions take a-phase as an example, but if the three-phase power grid voltage drops to different degrees, the three-phase power grid voltage does not drop to the same degreeSimilarly, the fluctuation characteristics of the three-phase bridge arm are not symmetrical, but the calculation modes of all phases are the same, and the expressions of the phase b and the phase c can be obtained only by correcting the positive-negative sequence voltage modulation ratio and the positive-negative sequence voltage phase angle in the above formulas. The derivation calculation hereinafter is also performed by taking the a-phase as an example. From the above analysis, the fundamental frequency of the instantaneous power of the b-phase bridge arm and the c-phase bridge arm and the amplitude P of the double frequency fluctuation can be obtained in the same waybu_1、Pbu_2And Pcu_1、Pcu_2Comprises the following steps:
6. the method of claim 1, wherein: generating a reference signal according to the amplitude and the phase of the double frequency circulation injection, and realizing the injection of the double frequency circulation by combining a modulation algorithm comprises the following steps:
amplitude phase k based on double frequency circulating current injection2、
And generating a reference signal in combination with a harmonics generator;
the reference signal is injected into the controller through the double frequency circulation to obtain a double frequency circulation modulation voltage signal u2Finally, the injection of the double frequency circulation is realized by changing the modulation wave of the modulation algorithm.
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