CN110797895A - SOC balance control method under unbalanced network pressure of energy storage type MMC - Google Patents

Abstract

The invention discloses a SOC equalization control method of an energy storage type MMC under unbalanced network voltage, which enables the energy storage type MMC to perform stable SOC equalization control under the unbalanced network voltage. Firstly, the circulating current component and the sub-module instantaneous power expression in the three-phase circulating current expression under unbalanced grid voltage are analyzed, and it is found that the difference of three-phase AC power is the cause of SOC imbalance, and the double frequency component in the circulating current will affect the SOC balance control. , so the circulating current reference value is set to suppress the double-frequency circulating current, and the double-frequency component is suppressed to 0 by the quasi-PR controller. Using the improved droop control to balance the SOC values of the sub-modules can shorten the balancing time, avoid wasting energy distribution due to too long balancing time, and finally achieve SOC balance between phases and within phases.

Description

A SOC equalization control method under unbalanced grid voltage of energy storage MMC

technical field

The patent of the invention relates to a SOC equalization control method under the unbalanced grid voltage of an energy storage type MMC in flexible direct current transmission, and belongs to the field of electric power technology.

Background technique

The HVDC flexible transmission technology based on Modular Multilevel Converter (MMC), as a new type of DC transmission technology of contemporary voltage source converters, is the research focus of today's flexible DC transmission technology. Compared with the traditional DC transmission technology, the MMC-based flexible DC transmission technology does not have the problem of reactive power compensation and commutation failure. At the same time, it can supply power for passive systems and has the ability to independently adjust active and reactive power. However, the modular multilevel converter also has shortcomings. In its control, the problem of equalization of the capacitor voltage of the sub-modules and the problem of circulating current between the bridge arms has always been the focus of research.

The addition of the energy storage system has played a significant role in increasing the reliability of the power grid and giving power support to the grid. Modular Multilevel Energy Storage Converter (MM-ESC) has attracted the attention of researchers at home and abroad, and related research has been applied to medium and low voltage power grids, new energy power generation and other fields.

Since the voltage of the energy storage battery has little influence with the change of the state of charge, the voltage of the energy storage sub-module can be regarded as constant in the normal working state of the battery, and the capacitor voltage of the energy storage sub-module is equal to the battery voltage. Under steady-state conditions, the sub-module capacitor voltage is less affected by charging and discharging, the sub-module voltage is stable, and there is no voltage component of the second harmonic, so the corresponding second harmonic component will not be generated in the circulating current. Therefore, for the modular multi-level energy storage converter, the balanced control of the sub-module SOC to avoid the waste of energy storage capacity allocation has become the focus of research.

Many existing literatures have proposed SOC equalization control of sub-modules, such as realizing SOC equalization between phases and bridge arms by controlling the circulating current, adjusting the output power frequency components of each sub-module, and realizing SOC equalization in the bridge arms; The three-level SOC strategy is studied from the perspective of sub-module power, and the balanced control of SOC can also be realized.

However, under the condition of unbalanced grid voltage, the three-phase grid-connected power is unbalanced, and the inconsistency of the three grid powers will lead to inconsistent rate of change of the SOC of the sub-modules. At the same time, the double-frequency positive and sequence components in the circulating current will affect the balance control effect. Therefore, under the condition of unbalanced network voltage, the second harmonic suppression is particularly important for the SOC balance control of the sub-module.

SUMMARY OF THE INVENTION

In order to realize the stable operation of the modular multi-level energy storage converter under the condition of unbalanced grid voltage, the present invention combines the double-frequency circulating current suppression control with the improved droop control. Bridge arm circulation expression under unbalanced grid pressure. In order to avoid the influence of the second harmonic circulating current on the SOC equalization control, the reference value is set to 0, compared with the actual circulating current, and the double frequency component in the circulating current is suppressed by the quasi-proportional resonance controller. The droop control based on SOC balance regulates the charge and discharge rate of the sub-module battery by superimposing the modulating waves of circulating DC and AC components, thereby controlling the balance of SOC. The traditional droop control is to compare the current SOC value of the sub-module with the average SOC value to obtain the DC component and AC component modulation wave that should be superimposed, so as to control the battery power to achieve the balance of the SOC, but considering that under the condition of grid imbalance, the SOC The change of the average value of the value is slow, which makes the SOC equalization time too long and causes waste in energy distribution. Therefore, the present invention selects the improved droop control. Select the appropriate droop coefficient value range, and use the SOC power index function droop coefficient to achieve faster and more accurate SOC balance control.

The invention provides an SOC equalization control method under unbalanced network pressure of an energy storage MMC, comprising:

Step S1: in the calculation link of the bridge arm circulation current, the expressions of the bridge arm circulation current under different working conditions are derived according to the mathematical model and topology structure of the MMC.

Step S2: in the three-phase power calculation link, the three-phase power under the unbalanced grid voltage is calculated and obtained, and the unbalance of the three-phase power is the cause of the unbalanced SOC.

Step S3: 2-fold frequency circulating current suppression link, according to the conclusion drawn in step S1, set the circulating current reference value and compare with the circulating current and then pass a quasi-PR controller to realize the 2-fold frequency circulating current suppression.

Step S4: in the selection of the droop coefficient, considering that the SOC reference value changes too slowly under the unbalanced grid pressure, resulting in wasted energy distribution, the droop coefficient is modified to a droop coefficient with a power index function.

Step S5: In the improved droop control link, the modified droop coefficient is selected, and boundary conditions are set to achieve a faster SOC equalization speed.

beneficial effect

The invention analyzes the circulation components of the MMC-ESC under unbalanced network pressure, and designs a double-frequency circulation component suppression link to suppress the double-frequency component in the circulation, so as to avoid it affecting the control circulation of the subsequent SOC equalization. A new method for selecting the droop control coefficient is proposed. Compared with the traditional droop control, the SOC balance between phases and within phases can be achieved more quickly, and the waste of energy distribution can be avoided.

Description of drawings

Fig. 1 is the SOC equalization control flow chart;

Fig. 2 is MMC topology structure diagram;

Fig. 3 is the topological structure diagram of MMC-ESC sub-module;

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

FIG. 5 is a droop graph of droop control.

Specific implementation method

The topology of MMC is shown in Figure 2. According to its topology and Kirchhoff's current-voltage law, the bridge arm current and bridge arm voltage expressions can be obtained:

where L _a is the bridge arm inductance; j=a, b, c; u _j is the phase output voltage; i _j is the AC measurement output current; u _jp and u _jn are the equivalent voltages of the upper and lower bridge arms respectively; i _jp and i _jn are the upper and lower bridge arm currents respectively; U _dc is the DC bus voltage.

i _diffj is the j-phase circulation, and its expression is:

where i _{diffj_dc} is the DC component in the j-phase circulating current; i _{diff_m} is the amplitude of the j-phase i-second circulating current.

Analysis of the internal circulation mechanism of the MMC shows that under symmetrical conditions, the MMC circulation mainly contains secondary current harmonic components with negative sequence properties. The circulating current of the second harmonic will affect the capacitor voltage fluctuation and the output voltage of the bridge arm. Therefore, the work of suppressing the AC circulating current under the symmetrical condition will take the second harmonic component as the main goal.

However, in the case of unbalanced power grid, if the circulating current is not suppressed, positive and negative sequence components will appear in the bridge arm voltage and bridge arm current:

At the same time, the modulation function of the upper and lower bridge arms in the unbalanced condition is:

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

At this time, the harmonics on the bridge arm will cause the fluctuation of the capacitor voltage. Since the switching action couples the voltage fluctuation to the output side, the entire bridge arm produces voltage fluctuations, thereby forming a circulating current. It is worth noting that the second harmonic components at this time also include positive sequence components, which will affect the balance control of SOC and need to be suppressed.

The sub-module topology of MM-ESC is shown in Figure 3. The energy storage sub-module adopts the connection method of the battery directly connected in parallel with the two ends of the capacitor. The switching functions of the upper and lower bridge arms can be expressed as:

where m _i is the modulation ratio of the phase.

The output voltage of a single submodule can be obtained:

The instantaneous power expression of a single submodule can then be obtained:

In the formula, P _dc is the DC power, and P _aci is the AC power of the i-phase.

According to the result of formula (9), we can get

In the formula, s ₀ is the initial charge state of the sub-module, and Q represents the total power of the energy storage battery.

It can be seen from equation (10) that the state of charge of the sub-module is affected by the DC power and the AC power. Under the unbalanced grid voltage, the three-phase AC power is inconsistent, and the SOC cannot be balanced.

In order to eliminate the double-frequency positive and negative sequence components in the circulating current, set the circulating current reference value to 0, compare the circulating current with the circulating current reference value, and then use a quasi-PR controller to suppress the double-frequency circulating current, as shown in Figure 4 shown. The transfer function of the quasi-PR controller is:

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

The SOC balance control is divided into SOC interphase control and SOC intraphase control. The SOC interphase balance is controlled by superimposing the DC component of the modulated wave, and the SOC intraphase balance is controlled by the modulation wave superimposing the AC component. For the traditional SOC balance droop control, the expression is as follows Equation (5) is shown, and its droop control curve is shown in Figure 5:

where S ₁ is the current SOC value of the sub-module; k _d1 , k _d2 are the droop control coefficients of inter-phase SOC and intra-phase SOC respectively; U ₁ is the DC component in the superimposed control wave; u ₁ is the AC in the superimposed control wave component; S ₀ is the reference value of the sub-module SOC.

Under the unbalanced grid pressure, the average SOC value changes slowly, which makes the SOC equalization time too long, resulting in waste of energy distribution. Therefore, the improved power index function droop coefficient is adopted:

The parameters k ₁ , k ₂ , ρ ₁ , ρ ₂ need to be adjusted and designed according to the actual situation of the own system. After using the improved droop factor of the power index function, it can speed up the SOC equalization time and reduce the energy distribution during the equalization process. of waste.

Under the unbalanced grid voltage, in order to keep the sub-module battery in the normal working range, its SOC value should be controlled within the following range:

10≤S≤90 (15)

Finally, the revised droop control expression is obtained:

Through the above several concepts, we can eliminate the influence of the second harmonic circulation on the SOC balance, so as to achieve the SOC balance more quickly through the improved droop control.

The above description is only an embodiment of the present invention, and is not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention. .

Claims (6)

1. a kind of SOC equalization control under unbalanced network pressure of energy storage type MMC, it is characterized in that: Step S1: in the calculation link of the bridge arm circulation current, the expressions of the bridge arm circulation current under different working conditions are derived according to the mathematical model and topology structure of the MMC. Step S2: in the three-phase power calculation link, the three-phase power under the unbalanced grid voltage is calculated and obtained, and the unbalance of the three-phase power is the cause of the unbalanced SOC. Step S3: 2-fold frequency circulating current suppression link, according to the conclusion drawn in step S1, set the circulating current reference value and compare with the circulating current and then pass a quasi-PR controller to realize the 2-fold frequency circulating current suppression. Step S4: in the selection of the droop coefficient, considering that the SOC reference value changes too slowly under the unbalanced grid pressure, resulting in wasted energy distribution, the droop coefficient is modified to a droop coefficient with a power index function. Step S5: In the improved droop control link, the revised droop coefficient is selected to achieve a faster SOC equalization speed. 2. the SOC equalization control method under the unbalanced grid pressure of energy storage type MMC according to claim 1, is characterized in that the circulating current expression in step S1 is: where i _jp , i _jp are the currents of the upper bridge arm and the lower bridge arm respectively, i _{diffj_dc} is the DC component in the circulating current, and i _{diffj_m} is the amplitude of the AC component. The expressions of bridge arm voltage and bridge arm current under unbalanced grid voltage:

. 3. the SOC equalization control method under the unbalanced grid pressure of energy storage type MMC according to claim 1, is characterized in that the instantaneous power expression of the single submodule in step S2:

. 4. the SOC equalization control method under the unbalanced grid pressure of energy storage type MMC according to claim 1, is characterized in that the transfer function of the quasi-PR controller in step S3 is:

where K _p and K _r are the proportional coefficient and the resonance coefficient, respectively. 5. the SOC equalization control method under the unbalanced grid pressure of energy storage type MMC according to claim 1, it is characterized in that the traditional droop control expression in step S4 is:

The improved sag factor is:

where S ₁ is the current SOC value of the sub-module; k _d1 , k _d2 are the droop control coefficients of inter-phase SOC and intra-phase SOC, respectively; U ₁ is the DC component in the superimposed control wave; u ₁ is the AC component in the superimposed control wave ; S ₀ is the reference value of the sub-module SOC. 6. The SOC equalization control method under the unbalanced grid pressure of energy storage type MMC according to claim 1, is characterized in that the improved droop control expression in step S5 is:

In the formula, the parameters k ₁ , k ₂ , ρ ₁ , ρ ₂ need to be adjusted and designed according to the actual situation of the own system.

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