阅读说明：本技术 一种储能型mmc不平衡网压下的soc均衡控制方法 (SOC balance control method under unbalanced network pressure of energy storage type MMC ) 是由 夏向阳 徐雷 刘奕玹 于 2019-11-27 设计创作，主要内容包括：本发明公开了一种储能型MMC不平衡网压下的SOC均衡控制方法,能使储能型MMC在不平衡网压下进行稳定的SOC均衡控制。首先分析不平衡网压下三相环流表达式中环流成分和子模块瞬时功率表达式,发现三相交流功率的差异是引起SOC不均衡的原因,同时环流中的2倍频分量会影响SOC均衡控制,故以抑制2倍频环流为目标设定环流参考值,通过准PR控制器将2倍频分量抑制为0。使用改进后下垂控制来均衡子模块SOC值,可以缩短均衡的时间,避免因为均衡时间过长而导致能源分配上的浪费,最终达到相间和相内的SOC均衡。(The invention discloses a SOC balance control method under unbalanced network pressure of an energy storage type MMC, which can enable the energy storage type MMC to carry out stable SOC balance control under the unbalanced network pressure. Firstly, analyzing a circulation component and a submodule instantaneous power expression in a three-phase circulation expression under unbalanced network pressure, finding that the difference of three-phase alternating current power is the cause of SOC unbalance, and simultaneously, 2-frequency multiplication components in circulation can influence SOC balance control, so that a circulation reference value is set by taking 2-frequency multiplication circulation inhibition as a target, and the 2-frequency multiplication components are inhibited to be 0 through a quasi-PR controller. The improved droop control is used for balancing the SOC values of the submodules, so that the balancing time can be shortened, the waste of energy distribution caused by overlong balancing time is avoided, and the inter-phase and intra-phase SOC balancing is finally achieved.)

1. The utility model provides a SOC balanced control that energy storage type MMC unbalanced network pushed down which characterized in that:

step S1: and in the step of calculating the ring current of the bridge arm, deriving an expression of the ring current of the bridge arm under different working conditions according to a mathematical model and a topological structure of the MMC.

Step S2: and in the three-phase power calculation step, the three-phase power under the unbalanced network is calculated, and the imbalance of the three-phase power is the cause of the SOC imbalance.

Step S3: and 2, in the step 2 frequency doubling circulation restraining step, according to the conclusion obtained in the step S1, after the circulation reference value is set and compared with the circulation, the 2 frequency doubling circulation restraining is realized through a quasi PR controller.

Step S4: and a droop coefficient selection step, namely modifying the droop coefficient into a droop coefficient with an exponential function in consideration of energy distribution waste caused by too slow change of the SOC reference value under the unbalanced network.

Step S5: in the improved droop control link, the corrected droop coefficient is selected, so that the faster SOC balance speed is realized.

2. The method for controlling SOC equalization under unbalanced network voltage of energy-storage MMC as claimed in claim 1, wherein the loop current expression in step S1 is:

in the formula i_jp，i_jpRespectively upper and lower bridge arm currents, i_{diffj_dc}Is the direct component of the circulating current, i_{diffj_m}Is the ac component amplitude.

Under the condition of unbalanced network voltage, the expressions of bridge arm voltage and bridge arm current are as follows:

3. the method for controlling SOC equalization under unbalanced network voltage of energy-storing MMC of claim 1, characterized by the instantaneous power expression of a single sub-module in step S2:

4. the method for controlling SOC equalization under unbalanced network voltage of energy-storing MMC as claimed in claim 1, wherein the transfer function of the quasi-PR controller in step S3 is:

in the formula K_p，K_rRespectively a proportionality coefficient and a resonance coefficient.

5. The SOC balancing control method under unbalanced network voltage of energy-storing MMC as claimed in claim 1, wherein the conventional droop control expression in step S4 is:

the improved sag factor is:

in the formula S₁Current SOC value of the submodule; k is a radical of_d1，k_d2Droop control coefficients of the inter-phase SOC and the intra-phase SOC are respectively; u shape₁Is the direct current component in the superposed control wave; u. of₁Is an alternating component in the superposed control wave; s₀Is the reference value of the sub-module SOC.

6. The method for controlling SOC equalization under unbalanced network voltage of energy-storage MMC as claimed in claim 1, wherein the modified droop control expression in step S5 is:

in the formula, the parameter k₁，k₂，ρ₁，ρ₂The design is adjusted according to the actual condition of the system.

Technical Field

The invention relates to an SOC balance control method under unbalanced network voltage of an energy storage type MMC in flexible direct current transmission, and belongs to the technical field of electric power.

Background

A high-voltage flexible direct-current power transmission technology based on a Modular Multilevel Converter (MMC) is a novel direct-current power transmission technology of a current voltage source Converter, and is a research focus of the current flexible direct-current power transmission technology. Compared with the traditional direct current transmission technology, the flexible direct current transmission technology based on the MMC can not have the problems of reactive compensation and commutation failure, and meanwhile, the flexible direct current transmission technology can supply power for a passive system and has the capability of independently adjusting active power and reactive power. However, the modular multilevel converter also has disadvantages, and in the aspect of control of the modular multilevel converter, the problem of equalization of capacitance and voltage of sub-modules and the problem of circulation among bridge arms are always important for research.

The energy storage system plays an obvious role in increasing the reliability of power supply of a power grid and providing power support for the power grid. At present, Modular Multilevel Energy Storage Converter (MM-ESC) attracts the attention of researchers at home and abroad, and related researches are applied to the fields of medium and low voltage power grids, new Energy power generation and the like.

Because the voltage of the energy storage battery has little influence along with the change of the charge state, the voltage of the energy storage sub-module is constant under the normal working state of the battery, and the capacitor voltage of the energy storage sub-module is equal to the voltage of the battery. Under the steady state working condition, the sub-module capacitor voltage is less influenced by charging and discharging, the sub-module voltage is stable, and the voltage component of the second harmonic wave cannot exist, so that the corresponding second harmonic wave component cannot be generated in the circulating current. Therefore, for the modular multilevel energy storage converter, the balancing control of the sub-module SOC which avoids the waste of energy storage capacity configuration becomes the key point of research.

In many existing documents, SOC balance control of submodules is proposed, for example, interphase and inter-bridge arm SOC balance is realized by controlling circulation currents, output power frequency components of each submodule are adjusted, and SOC balance in a bridge arm is realized; a three-level SOC strategy is researched from the angle of controlling phase, bridge arm and sub-module power, and balance control of SOC can be achieved.

However, under the condition of unbalanced grid voltage, three-phase grid-connected power is unbalanced, and the inconsistency of the three grid powers can cause inconsistency of the SOC change rates of the submodules. Meanwhile, 2 frequency multiplication positive and sequence components in the circulating current can influence the balance control effect, so that under the working condition of unbalanced network voltage, the second harmonic suppression is particularly important for the SOC balance control of the submodule.

Disclosure of Invention

In order to realize the stable operation of the modular multilevel energy storage converter under the network voltage unbalance working condition, the invention combines the double-frequency circulating current inhibition control and the improved droop control, and firstly, a bridge arm circulating current expression under the unbalanced network voltage is obtained according to a topological structure diagram of the MMC-ESC. In order to avoid the influence of the second harmonic circulating current on the SOC balance control, a reference value is set to be 0, and compared with the actual circulating current, the second harmonic component in the circulating current is restrained by the quasi-proportional resonant controller. And the droop control based on SOC balance regulates and controls the charge-discharge rate of the sub-module battery by superposing the circulating current direct current and alternating current component modulation waves, so that the balance of the SOC is controlled. In the traditional droop control, the direct current component and alternating current component modulation waves which are to be superposed are obtained by comparing the current SOC value of the submodule with the SOC average value, so that the battery power is controlled to reach the balance of the SOC, but the invention selects the improved droop control in consideration of the problem that the balance time of the SOC is too long and the waste of energy distribution is caused under the condition of unbalanced power grid, because the change of the SOC average value is slow. And selecting a proper droop coefficient value range, and adopting the droop coefficient of the SOC power indicator function, thereby achieving faster and more accurate SOC balance control.

The invention provides a SOC balance control method under the unbalanced network voltage of an energy storage type MMC, which comprises the following steps:

step S1: and in the step of calculating the ring current of the bridge arm, deriving an expression of the ring current of the bridge arm under different working conditions according to a mathematical model and a topological structure of the MMC.

Step S2: and in the three-phase power calculation step, the three-phase power under the unbalanced network is calculated, and the imbalance of the three-phase power is the cause of the SOC imbalance.

Step S3: and 2, in the step 2 frequency doubling circulation restraining step, according to the conclusion obtained in the step S1, after the circulation reference value is set and compared with the circulation, the 2 frequency doubling circulation restraining is realized through a quasi PR controller.

Step S4: and a droop coefficient selection step, namely modifying the droop coefficient into a droop coefficient with an exponential function in consideration of energy distribution waste caused by too slow change of the SOC reference value under the unbalanced network.

Step S5: in the improved droop control link, the corrected droop coefficient is selected, the boundary condition is set, and the faster SOC balance speed is realized.

Advantageous effects

The invention analyzes the circulating current composition of the MMC-ESC under the unbalanced network pressure, and inhibits the 2-frequency component in the circulating current by designing a 2-frequency circulating current component inhibition link, thereby avoiding influencing the control circulating current of the subsequent SOC balance. Compared with the traditional droop control, the method has the advantages that the inter-phase and intra-phase SOC balance is achieved more quickly, and the waste caused by energy distribution is avoided.

Drawings

FIG. 1 is a SOC equalization control flow diagram;

FIG. 2 is a diagram of an MMC topology;

FIG. 3 is a topological structure diagram of a MMC-ESC submodule;

FIG. 4 is a control block diagram of second harmonic circulating current suppression based on a quasi-PR controller;

fig. 5 is a droop graph for droop control.

Detailed description of the invention

The topological structure of the MMC is shown in fig. 2, and according to the topological structure and kirchhoff current-voltage law, bridge arm current and bridge arm voltage expressions can be obtained:

in the formula L_aBridge arm inductance; j ═ a, b, c; u. of_jOutputting the voltage for the phase; i.e. i_jMeasuring output current for alternating current; u. of_jpAnd u_jnRespectively equivalent voltages of an upper bridge arm and a lower bridge arm; i.e. i_jpAnd i_jnRespectively the upper and lower bridge arm currents; u shape_dcIs the dc bus voltage.

i_diffjIs j-phase circulation, and the expression is as follows:

in the formula i_{diffj_dc}As a direct component of the j-phase circulating current；i_{diff_m}The amplitude of the j-phase i-times circulating current.

Analyzing the mechanism of the MMC internal circulation, under the symmetrical working condition, the MMC circulation mainly comprises 2 times of current harmonic components and is in a negative sequence property. And the 2 nd harmonic circulation can affect the capacitance voltage fluctuation and the bridge arm output voltage, so that the work of alternating current circulation suppression under the symmetrical working condition takes 2 nd harmonic component as a main target.

However, under the condition of unbalanced power grid, if the ring current is not suppressed, a positive sequence component and a negative sequence component appear in the bridge arm voltage and the bridge arm current:

meanwhile, the modulation functions of the upper and lower bridge arms under the unbalanced condition are as follows:

at this time, the average current of the upper and lower arms is:

at the moment, the harmonic waves on the bridge arm can cause the fluctuation of the capacitor voltage, the voltage fluctuation is coupled to the output side due to the switching action, and the whole bridge arm generates the voltage fluctuation, so that a circulating current is formed. Note that the 2 nd harmonic component at this time also includes a positive sequence component, which affects the balance control of the SOC and needs to be suppressed.

The submodule topological structure of the MM-ESC is shown in figure 3, and the energy storage submodule adopts a connection mode that a battery is directly connected with two ends of a capacitor in parallel. The switching function of the upper and lower arms can be expressed as:

in the formula m_iIs the modulation ratio of that phase.

The output voltage of a single submodule can be obtained:

the instantaneous power expression for a single sub-module can then be derived:

in the formula P_dcFor direct current power, P_aciI-phase ac power.

According to the result of formula (9), a

In the formula s₀For the initial charge state of the submodule, Q represents the total charge of the energy storage battery.

From the equation (10), the state of charge of the sub-modules is affected by the direct current power and the alternating current power, and under the unbalanced network voltage, the three-phase alternating current power is inconsistent, and the SOC cannot be balanced.

In order to eliminate the 2-frequency multiplication positive sequence and negative sequence components in the circulation, the circulation reference value is set to be 0, the circulation is compared with the circulation reference value, and then the 2-frequency multiplication circulation is restrained through a quasi-PR controller, as shown in FIG. 4. Wherein the transfer function of the quasi-PR controller is:

the SOC estimation method adopts an ampere-hour integration method, which is defined as:

the balance control of the SOC is divided into SOC interphase control and SOC in-phase control, wherein the SOC interphase balance is controlled by superposing a direct current component by a modulating wave, the SOC in-phase balance is controlled by superposing an alternating current component by the modulating wave, and for the traditional SOC balance droop control, an expression is shown as a formula (5), and a droop control curve is shown as a graph in FIG. 5:

in the formula S₁The current SOC value of the submodule is obtained; k is a radical of_d1，k_d2Droop control coefficients of the inter-phase SOC and the intra-phase SOC are respectively; u shape₁Is the direct current component in the superposed control wave; u. of₁Is an alternating component in the superposed control wave; s₀Is the reference value of the sub-module SOC.

Under the unbalanced network voltage, the change of the SOC value average value is slow, so that the equalization time of the SOC is too long, and the waste on energy distribution is caused, therefore, the improved droop coefficient of the power exponential function is adopted:

wherein the parameter k₁，k₂，ρ₁，ρ₂The design needs to be adjusted according to the actual condition of the system, and after the improved droop coefficient of the power exponent function is used, the time for SOC equalization can be shortened, and the waste caused by energy distribution in the equalization process is reduced.

Under the unbalanced network pressure, in order to enable the submodule battery to be in a normal working range, the SOC value of the submodule battery is controlled to be in the following range:

10≤S≤90 (15)

and finally, obtaining a corrected droop control expression:

through the links of the above concepts, the influence of the 2 nd harmonic circulating current on the SOC balance can be eliminated, so that the SOC balance can be achieved more quickly through the improved droop control.

The above description is only exemplary of the present invention and should not be taken as limiting the invention, as any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.