Cascade H-bridge energy storage device control method under non-ideal power grid working condition

Abstract

The invention relates to a cascade H-bridge energy storage device control method under a non-ideal power grid working condition. The method comprises the following steps of: firstly, performing positive and negative sequence decomposition on power grid voltage by adopting DSOGI-PLL to obtain frequency and phase information of a positive sequence component; then, calculating a grid-connected current reference value under the three-phase static coordinate system according to an instantaneous power theory and Park inverse transformation; the current controller adopts dead-beat prediction control, in order to eliminate control delay in digital control, a Luenberger observer is used for predicting grid-connected current at the moment K + 1, and then a Lagrange interpolation method is used for predicting power grid voltage at the moment K + 1 and a current reference value at the moment K + 2; and finally, calculating the number of input modules at the K + 1 moment through dead-beat prediction control, and generating a switching signal according to the number of input modules through nearest level modulation. According to the invention, the negative-sequence component and the harmonic component in the grid-connected current can be suppressed under the non-ideal power grid condition, the quality of the grid-connected electric energy is improved, and the cascaded H-bridge energy storage device has the operation capability under the non-ideal power grid condition.

Description

Technical Field

[0001] The present invention belongs to the field of electric energy storage conversion, and specifically refers to a control method for a cascaded H-bridge energy storage device under non-ideal grid conditions. Background Art

[0002] With the development of renewable energy and power electronics technology, the power system's demand for flexible regulation capabilities is increasing day by day. Battery energy storage systems play an increasingly important role in frequency modulation and peak shaving, load leveling, new energy accommodation, power quality improvement, etc. due to their advantages such as fast response speed, high power density, and no requirements for installation location, and have become a key supporting technology for building a new power system. The battery energy storage device based on the cascaded H-bridge converter has the advantages of modularity, high-voltage direct connection, large single-machine capacity, multi-level output, etc., and has been widely used in high-voltage and large-capacity occasions.

[0003] Grid-connected power control is the core function of the cascaded H-bridge energy storage device. Most traditional power control strategies are PI control strategies. The Park transformation is used to transform the grid voltage and grid-connected current into direct current quantities in the synchronous coordinate system, and the PI controller is used to achieve the static error-free tracking of the grid-connected current. The PI control strategy can achieve good control effects under ideal grid conditions. However, the actual grid also includes non-ideal conditions such as grid imbalance and harmonic distortion. Under non-ideal grid conditions, the negative sequence component and harmonic component are manifested as high-frequency alternating current quantities in the synchronous coordinate system. Since the gain of the PI controller is small at high frequencies and cannot achieve static error-free tracking of the alternating current component, it is impossible to suppress the negative sequence and harmonic components in the grid-connected current under non-ideal grid conditions, resulting in distortion of the grid-connected current and reducing the quality of the grid-connected electric energy. Summary of the Invention

[0004] In view of the above problems, the present invention proposes a control method for a cascaded H-bridge energy storage device under non-ideal grid conditions, which can effectively suppress the negative sequence component and harmonic component in the grid-connected current, improve the quality of the grid-connected electric energy, and enable the cascaded H-bridge energy storage device to have the operating ability under non-ideal grid conditions.

[0005] The control method for the cascaded H-bridge energy storage device under non-ideal grid conditions of the present invention is mainly characterized in that it specifically includes the following steps:

[0006] Step 1: The controller samples the three-phase grid voltage e a (K), e b (K), e c (K), the three-phase grid-connected current i a (K), i b (K), i c (K) and the average value V of the DC voltage of the cascaded H-bridge energy storage device dc (K);

[0007] Step 2: Use DSOGI-PLL to process e a (K), e b (K), e c (K) to separate the positive and negative sequences, and obtain the phase θ(K) of the positive sequence component and the Park transformation results e d+ (K) and e q+ (K);

[0008] Step 3: The controller calculates the active current reference value i d * (K) and the reactive current reference value i q * (K);

[0009] Step 4: Use the inverse Park transformation to transform the current reference values obtained in Step 3 into the current reference values in the three-phase stationary coordinate system i a * (K), i b * (K), i c * (K);

[0010] Step 5: Predict the grid-connected current at time K+1 through the Luenberger observer

[0011] Step 6: Use the Lagrange interpolation method to predict the grid-connected current reference values i a * (K+2), i b * (K+2), i c * (K+2) and the grid voltage e a (K+1), e b (K+1), e c (K+1);

[0012] Step 7: The deadbeat predictive control calculates the number of input modules at time K+1 according to the prediction results in Step 5 and Step 6, and the nearest level modulation generates the switching signal according to the calculated number of input modules;

[0013] Furthermore, in Step 3, the active current reference value and the reactive current reference value are calculated according to the instantaneous power theory, and the specific calculation method is:

[0014]

[0015] Among them, P * and Q* are the active power command and the reactive power command respectively; e d+ is the d-axis component after the positive sequence component of the grid voltage undergoes Park transformation.

[0016] Furthermore, in Step 4, Park inverse transformation is adopted to obtain the grid-connected current reference value in the three-phase stationary coordinate system. The specific calculation method is as follows:

[0017]

[0018] where, i a * and i b * and i c * are the grid-connected current reference values in the three-phase stationary coordinate system; θ is the phase of the positive sequence component of the grid voltage.

[0019] Furthermore, in Step 5, a Luenberger observer is used to perform lead prediction on the grid-connected current. The establishment of the Luenberger observer includes the following steps:

[0020] First, write the circuit equations according to the circuit topology of the cascaded H-bridge converter;

[0021]

[0022] where, the subscript k = a, b, c, represents the k-th phase; i k represents the grid-connected current of the k-th phase; e k represents the grid voltage of the k-th phase; v k represents the output voltage of the k-th phase of the converter; L is the grid-connected filter inductor; R is the additional loss resistance of the filter inductor.

[0023] Considering that the number of input modules in each phase in the nearest level modulation method is an integer and the cascaded H-bridge converter adopts an equalization strategy during operation to keep the DC voltage of all modules consistent, therefore, the output voltage v k of the k-th phase of the converter can be expressed as the product of the number of input modules and the average DC voltage:

[0024]

[0025] where, N k is the number of input modules of the k-th phase. When N k takes a positive value, it means that there are N k modules in the k-th phase outputting positive level. When it takes a negative value, it means that there are |N k | modules in the k-th phase outputting negative level; V dc is the average value of the DC voltages of all modules.

[0026] The current equation is discretized using the forward Euler method to obtain a discrete mathematical model of the grid-connected current:

[0027] i k (K + 1) = Ai k (K) + B[e k (K) - N k (K)V dc (K)]

[0028]

[0029] where T s is the control period; A is the state transition matrix; B is the input matrix.

[0030] The Luenberger observer is established based on the discrete current equation, and the observer equation is as follows:

[0031]

[0032] where is the predicted value of the current at time K + 1; is the predicted value of the current at time K; i k (K) is the actual sampling result of the current at time K; h is the feedback coefficient.

[0033] Furthermore, in step six, the Lagrange interpolation method is used to predict the grid voltage and current reference values, and the prediction formula is:

[0034] e k (K + 1) = 3e k (K) - 3e k (K - 1) + e k (K - 2)

[0035]

[0036] where the subscript k = a, b, c, represents the k-th phase; is the grid-connected current reference value of the k-th phase.

[0037] Furthermore, the solution steps of the deadbeat predictive control in step seven are as follows:

[0038] First, establish the current equation at time K + 1:

[0039] i k (K + 2) = Ai k (K + 1) + B[e k (K + 1) - N k (K + 1)V dc (K + 1)]

[0040] To ensure that the grid-connected current at time K+2 tracks the reference value, use the current reference value i k * (K+2) to replace i k (K+2) in the above formula, and obtain the theoretical optimal number of modules N k * (K+1) at time K+1:

[0041]

[0042] The grid-connected current i k (K+1) at time K+1 is obtained by the lead prediction of the Luenberger observer; considering that the battery voltage is relatively stable, within adjacent control cycles, it can be considered that the DC voltage of the H-bridge converter remains unchanged, that is, V dc (K) = V dc (K+1), so the solution formula of the deadbeat prediction can be rewritten as:

[0043]

[0044] Considering that the solved N k * (K+1) is not necessarily an integer, and only an integer number of modules can be put into the nearest level modulation. To ensure that the finally put-in number of modules is optimal, select the two integers N k * adjacent to N 1 and N 2 , respectively predict the grid-connected currents i 1 and i 2 corresponding to N 1 and N 2 (K+2), and calculate the absolute deviations J k * between the grid-connected current i 1 and J 2 , and determine the optimal number of modules by comparing the magnitudes of J 1 and J 2 .

[0045]

[0046] Finally, the nearest level modulation generates a switching signal to drive the cascaded H-bridge converter according to the calculated optimal number of input modules.

[0047] The control method of the cascaded H-bridge energy storage device under the non-ideal grid condition proposed by the present invention has the following advantages compared with the existing methods:

[0048] (1) It can effectively suppress the negative sequence component and harmonic component in the grid-connected current, and improve the power quality of the cascaded H-bridge energy storage device when connected to the grid under non-ideal grid conditions;

[0049] (2) The deadbeat predictive control directly solves the control quantity based on the three-phase stationary coordinate system model, without the need to separate the positive and negative sequences of the current and extract harmonics, greatly reducing the complexity of the controller;

[0050] (3) The Luenberger observer and Lagrange interpolation method are introduced to predict the system state and control commands, overcoming the control delay in digital control, and at the same time, the sampling feedback also further ensures the accuracy of the prediction results; Brief Description of the Drawings

[0051] Figure 1 It is the topology diagram of the cascaded H-bridge energy storage device of the present invention.

[0052] Figure 2 It is the structure diagram of the Luenberger observer of the present invention.

[0053] Figure 3 It is the control block diagram of the control method of the cascaded H-bridge energy storage device under non-ideal grid conditions of the present invention.

[0054] Figure 4 It is the structure diagram of the DSOGI-PLL of the present invention.

[0055] Figure 5 It is the schematic diagram of the simulation results of the present invention under unbalanced grid conditions.

[0056] Figure 6 It is the schematic diagram of the simulation results of the present invention under grid harmonic distortion conditions.

[0057] Figure 7 It is the schematic diagram of the FFT analysis results of the grid-connected current of the present invention under grid harmonic distortion conditions. Detailed Embodiment

[0058] In order to more clearly describe the technical content of the present invention, the following will be further described in combination with specific embodiments.

[0059] Before detailing the embodiments according to the present invention, it should be noted that in the following text, the term "comprising", "including" or any other variant is intended to cover non-exclusive inclusion, such that a process, method, article or device comprising a series of elements not only includes those elements but also includes other elements not explicitly listed, or elements inherent to such process, method, article or device.

[0060] Please refer to Figure 1As shown in the figure, it is the circuit topology diagram of the cascaded H-bridge energy storage device of the present invention. Its three-phase as a whole adopts a Y-type connection method. Each phase is composed of a cascade of multiple H-bridge power units. The DC side of the H-bridge power module is connected to the energy storage battery; the AC side is directly connected to the medium and high voltage power grid through a filter inductor, and there is no need for a power frequency transformer between the power grid and the energy storage converter.

[0061] In this embodiment, the power grid voltage level is 35 kV, the rated power is 10 MW, the filter inductor is 20 mH, the number of H-bridge modules per phase is 46, and the control period is 50 μs. The specific setting method is as follows:

[0062] 1. Mathematical modeling of the energy storage device

[0063] According to the circuit topology of the cascaded H-bridge energy storage device, the grid-connected current equation is listed as shown in Equation (1):

[0064]

[0065] Among them, the subscript k = a, b, c, indicating the k-th phase; i k represents the grid-connected current of the k-th phase; e k represents the grid voltage of the k-th phase; v k represents the output voltage of the k-th phase of the converter; L is the grid-connected filter inductor; R is the additional loss resistance of the filter inductor.

[0066] The nearest level modulation is adopted in the embodiment, so Equation (1) can be further rewritten as:

[0067]

[0068] Among them, N k represents the number of input modules of the k-th phase; V dc is the average value of the DC voltage.

[0069] 2. Construction of the Luenberger observer

[0070] The forward Euler method is used to discretize Equation (2), and the discretized current equation is shown in Equation (3):

[0071] i k (K + 1) = Ai k (K) + B[e k (K) - N k (K)V dc (K)]

[0072]

[0073] Among them, T s represents the execution period of the controller.

[0074] The Luenberger observer is constructed based on Equation (3), and its structure is as Figure 2 shown. According to Figure 2 the following, the equation of the Luenberger observer can be obtained as shown in Equation (4):

[0075]

[0076] where is the predicted current value at the (K + 1)th moment; is the predicted current value at the Kth moment; i k (K) is the actual sampling result of the current at the Kth moment; h is the feedback coefficient.

[0077] 3. Solving formula for deadbeat predictive control

[0078] By recursively advancing Equation (3) by one beat, the current equation at the (K + 1)th moment is obtained:

[0079] i k (K + 2) = Ai k (K + 1) + B[e k (K + 1) - N k (K + 1)V dc (K + 1)] (5)

[0080] After eliminating the control delay, the control quantity takes effect at the (K + 1)th moment, and at the same time, the grid-connected current should track the current reference value at the (K + 2)th moment. Substituting the current reference value i k * (K + 2) into Equation (5) gives the solving equation for the control quantity of deadbeat predictive control:

[0081]

[0082] where N k * (K + 1) is the theoretically optimal control quantity that meets the requirements. For this embodiment, the control quantity is the number of input modules.

[0083] 4. Grid-connected current control

[0084] The overall control block diagram of the control method described in the present invention is as Figure 3 shown. In the Kth control cycle, the controller first samples the energy storage device to obtain the three-phase grid voltages e a (K), e b (K), e c (K), the three-phase grid-connected currents i a (K), i b (K), i c (K), and the average DC voltage V dc(K). After that, DSOGI-PLL is used to separate the positive and negative sequences of the three-phase grid voltage, and the phase θ(K) of the positive-sequence component of the grid voltage and the Park transformation results e d+ (K), e q+ (K), Figure 4 is the schematic diagram of DSOGI-PLL.

[0085] After that, the controller receives the active power command P * (K) and the reactive power command Q * (K), and calculates the reference values of the active current and the reactive current according to the instantaneous power theory. The calculation method is shown in Equation (7):

[0086]

[0087] Then, the Park inverse transformation shown in Equation (8) is used to transform the reference values of the active and reactive currents in the synchronous coordinate system into the reference values of the currents in the three-phase stationary coordinate system.

[0088]

[0089] The Lagrange interpolation method is used to predict the reference value of the grid-connected current and the grid voltage in advance, and the reference value of the current at time K+2 and the grid voltage at time K+1 are obtained.

[0090] e k (K+1) = 3e k (K) - 3e k (K-1) + e k (K-2)

[0091]

[0092] The Kalman observer equation shown in Equation (4) is used to predict the grid-connected current at time K+1, and the predicted value is denoted as Considering that the DC-side voltage is relatively stable, the average value V of the DC voltage collected in this period dc (K) is used as the DC voltage V in the next period dc (K+1). Substitute i k * (K+2) and V dc (K) into Equation (6) to solve the theoretically optimal number of modules N at time K+1 k * (K+1).

[0093] Finally, select the two integers adjacent to N k * (K+1), denoted as N1 With N 2 Substitute N 1 With N 2 into Equation (5) respectively to calculate i 1 (K + 2) and i 2 (K + 2), calculate i 1 (K + 2) and i 2 (K + 2) and the reference value i k * (K + 2) to obtain the deviation J 1 between J 2 , compare J 1 and J 2 . The number of modules to be finally invested can be determined by comparing the magnitudes of J

[0094]

[0095] 5. Simulation Verification

[0096] Build a simulation model of this example using Matlab / Simulink Figure 5 The simulation results of the cascaded H-bridge energy storage system under unbalanced grid conditions are given, where Figure 5 (a) is the unbalanced grid voltage, and the amplitudes of the three-phase voltages are 1.2 times, 1.0 times, and 0.8 times the per-unit value respectively Figure 5 (b) is the grid-connected current. It can be seen from Figure 5 that under unbalanced grid conditions, the grid-connected current can still remain three-phase symmetrical

[0097] Inject 5th and 7th harmonics of 10% per-unit value into the ideal grid Figure 6 are the simulation results of the cascaded H-bridge energy storage system under grid harmonic distortion conditions Figure 6 (a) is the distorted grid voltage Figure 6 (b) is the grid-connected current Figure 7 is the FFT analysis result of the grid-connected current. Combining Figure 6 and Figure 7 it can be seen that when the grid is harmonically distorted, the waveform of the grid-connected current is still a three-phase symmetrical sine wave, and the total distortion rate is only 0.43%

[0098] It should be understood that each part of the present invention can be implemented by hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution device

[0099] In the description of this specification, the descriptions referring to terms such as "one embodiment", "some embodiments", "example", "specific example", or "embodiment", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0100] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

[0101] In summary, a control method for a cascaded H-bridge energy storage device under non-ideal grid conditions proposed by the present invention can suppress the negative sequence and harmonic components in the grid-connected current under non-ideal grid conditions, and the three-phase grid-connected currents are symmetric sine waves. Ideal control effects are demonstrated in the implementation examples.

[0102] In this specification, the present invention has been described with reference to its specific embodiments. However, it is obvious that various modifications and transformations can still be made without departing from the spirit and scope of the present invention. Therefore, the specification and the drawings should be regarded as illustrative rather than restrictive.

Claims

1. A control method for a cascaded H-bridge energy storage device under non-ideal power grid conditions, characterized in that: The method comprises the following steps: Step 1: The controller samples and obtains the three-phase grid voltage e a (K), e b (K), e c (K), three-phase grid-connected current i a (K) i b (K) i c (K) and the average DC voltage V of the cascaded H-bridge energy storage device dc (K); Step 2: Use the three-phase phase-locked loop DSOGI-PLL to measure the three-phase grid voltage e a (K), e b (K), e c (K) to separate the positive and negative sequences, and obtain the phase θ(K) of the positive sequence component and the Park transformation result e of the positive sequence component d+ (K) and e q+ (K); Step 3: According to the instantaneous power theory, the active current reference value i is calculated from the active power command and reactive power command of the current cycle. d * (K) and reactive current reference value i q * (K); Step 4: Use Park inverse transformation to transform the obtained current reference value into the current reference value i in the three-phase stationary coordinate system a * (K) i b * (K) i c * (K); Step 5: Predict the grid-connected current at time K+1 using the Lumberg observer Step 6: Use Lagrange interpolation method to predict the grid-connected current reference value i at time K+2 a * (K+2), i b * (K+2), i c * The grid voltage e at time (K+2) and time K+1 a (K+1), e b (K+1), e c (K+1); Step 7: The prediction results in step 5 and step 6 are used to calculate the number of modules put into operation at the K+1th moment according to the deadbeat prediction control, and the nearest level modulation generates a switching signal according to the calculated number of modules put into operation.

2. The control method of the cascaded H-bridge energy storage device under non-ideal power grid conditions according to claim 1, characterized in that: The step 3 calculates the active current reference value and the reactive current reference value in the following manner: Among them, P * With Q * They are active power command and reactive power command respectively; e d+ is the d-axis component of the grid voltage positive sequence component after Park transformation; i d * is the active current reference value, i q * It is the reactive current reference value.

3. The control method of the cascaded H-bridge energy storage device under non-ideal power grid conditions according to claim 2, characterized in that: The fourth step calculates the current reference value in the following manner: Among them, i a * 、i b * 、i c * is the grid-connected current reference value in the three-phase stationary coordinate system; θ is the phase of the positive sequence component of the grid voltage.

4. The control method of the cascaded H-bridge energy storage device under non-ideal power grid conditions according to claim 1, characterized in that: The step 5 uses the Romberg observer to make advance predictions of the grid-connected current, wherein the establishment of the Romberg observer includes the following steps: First, write the circuit equation according to the circuit topology of the cascaded H-bridge converter: Wherein, the subscript k = a, b, c, represents the kth phase; i k represents the grid-connected current of the kth phase; e k Indicates the grid-connected current of phase k; v k represents the output voltage of the kth phase of the converter; L is the grid-connected filter inductor; R is the loss resistance added to the filter inductor; The output voltage v of the kth phase of the converter is k Expressed as the product of the number of modules put into operation and the average DC voltage, the circuit equation is: Among them, N k is the number of modules put into operation in phase k, N k A positive value indicates that the kth phase has N k The module outputs a positive level. When the value is negative, it means that the kth phase has |N k | module outputs negative level; V dc is the mean DC voltage of all modules; The forward Euler method is used to discretize the current equation and obtain the discrete mathematical model of the grid-connected current: i k (K+1)=Ouch k (K)+B[e k (K)-N k (K)V dc (K)] Among them, T s is the control cycle, A is the state transfer matrix, and B is the input matrix; The Lumberg observer is based on the discrete current equation. The observer equation is as follows: in, is the predicted current value at time K+1; is the predicted current value at time K; i k (K) is the actual sampling result of the current at time K; h is the feedback coefficient.

5. The control method of the cascaded H-bridge energy storage device under non-ideal power grid conditions according to claim 4, characterized in that: The step six uses the Lagrange interpolation method to predict the grid voltage and current reference values, and the prediction formula is: yes k (K+1)=3e k (K)-3e k (K-1)+e k (K-2) Wherein, subscript k = a, b, c, indicating the kth phase; is the grid-connected current reference value of the kth phase.

6. The control method of the cascaded H-bridge energy storage device under non-ideal power grid conditions according to claim 5, characterized in that: The solution steps of the deadbeat predictive control in step seven are as follows: First, establish the current equation at time K+1: i k (K+2)=Ouch k (K+1)+B[e k (K+1)-N k (K+1)V dc (K+1)] To ensure that the grid-connected current tracks the reference value at time K+2, the current reference value i at time K+2 is used. k * (K+2) replaces i in the above formula k (K+2), get the theoretical optimal number of input modules N at time K+1 k * (K+1): Among them, the grid-connected current i at time K+1 is k (K+1) is obtained by the advance prediction of the Lumberg observer; the solution formula of the deadbeat prediction is rewritten as: Select and N k * (K+1) two adjacent integers N1 and N2, respectively predict the grid-connected currents i1(K+2) and i2(K+2) corresponding to N1 and N2, and calculate the current reference value i k * (K+2) is the absolute deviation between J1 and J2, where: The optimal number of modules can be determined by comparing the sizes of J1 and J2; Finally, the nearest level modulation generates a switching signal to drive the cascaded H-bridge converter according to the calculated optimal number of input modules.

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