Active power decoupling method introducing three-phase voltage unbalance degree

Abstract

The invention relates to a new energy power generation technology, and aims to provide an active power decoupling method for introducing a three-phase voltage unbalance degree. Comprising the steps that voltage and current double-loop control is adopted for a three-phase energy storage converter of the T-type three-level topology; detecting a three-phase instantaneous voltage of the output side of the converter to obtain a three-phase voltage amplitude unbalance degree; pre-judging the unbalance degree of the voltage amplitude of the power grid, and continuing to execute if a preset condition is met; the three-phase voltage amplitude unbalance degree is introduced into three-phase power grid voltage dq conversion and calculation of a three-phase current feedback loop, and finally a PWM control signal is generated; and each switching tube in the converter executes actions based on the PWM control signal, so that the three-phase power is kept equal, and the frequency doubling power ripple is suppressed. According to the invention, frequency doubling ripple power oscillation under the working condition of unbalanced three-phase power grid voltage amplitude can be effectively eliminated, and the suppression target of DC side frequency doubling power ripples is realized; no additional hardware topology is added, an original control ring is reserved, and detection and control of positive and negative sequence components are omitted.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy power generation, and particularly relates to an active power decoupling strategy introducing three-phase voltage unbalance degree. Background Art

[0002] As an important part of modern power systems, energy storage systems have been widely applied in multiple fields. In particular, they play an irreplaceable role in supporting power supply-demand balance, power grid frequency regulation, power market optimization, improving the reliability of energy supply, and promoting the green energy transformation. As an important part of the energy storage system, the energy storage converter can select different topologies according to the power level. The three-level topology is usually adopted in the medium-power application scenarios of 10kW - 250kW. Among them, the T-type three-level topology has the advantages of simple control, low switching loss, small AC current harmonics, and small filter volume, and has been widely used in energy storage converters.

[0003] The suppression technology of the double-frequency ripple on the DC bus of the energy storage converter is one of the hotspots in the research of the control strategy of bidirectional energy storage converters. The double-frequency ripple suppression technology is mainly divided into two categories: passive power decoupling and active power decoupling. The traditional passive decoupling method reduces the ripple by increasing the DC bus capacitance, but this will lead to a decrease in power density, and at the same time, the electrolytic capacitor has become one of the key factors limiting the service life of the energy storage system. Different from this, active power decoupling suppresses or eliminates the double-frequency ripple on the DC bus through the active control of power electronic switches, thereby reducing the volume of electrolytic capacitors; at the same time, film capacitors are used to replace electrolytic capacitors to extend the overall life of the system. The introduction of this technology needs to ensure the bidirectional function of the charging and discharging device, that is, after the decoupling technology is introduced, the system can still operate normally in the forward rectification and reverse inversion modes, and can ensure the stability of the decoupling system under abnormal conditions such as voltage dips or surges in the grid voltage, without affecting the total harmonic distortion (THD) of the grid-connected current of the system.

[0004] For a three-phase energy storage converter, under normal operating conditions, the double-frequency ripples on the bus can cancel each other out. However, in the event of grid faults such as unbalanced three-phase grid voltages, the three-phase converter may operate abnormally, and in severe cases, it may even burn out the converter device. In addition, when the energy storage system supports the grid, the double-frequency ripples generated under unbalanced conditions will cause the temperature of the energy storage battery to rise and energy loss, significantly shortening the battery life. Therefore, the active power decoupling technology plays a particularly important role in suppressing the double-frequency ripples under unbalanced conditions. Existing active power decoupling strategies usually introduce additional decoupling topologies or separately control the positive and negative sequence components. However, introducing additional decoupling topologies will reduce the power density of the converter, increase the construction cost, and even reduce the operating efficiency of the converter and affect the system stability; without introducing a decoupling topology, by controlling the positive and negative sequence components, the double-frequency ripple of the DC-side voltage can be suppressed, but there will still be an asymmetry in the AC-side current.

[0005] Therefore, the present invention proposes a new solution to solve the above problems, which is of great significance for improving the efficiency and stability of the energy storage system in practical applications. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide an active power decoupling method that introduces the three-phase voltage unbalance degree. This method automatically introduces double-frequency ripples into the phase current using a digital control method, and finally can effectively eliminate the double-frequency ripple power oscillation under unbalanced conditions.

[0007] To solve the technical problem, the solution of the present invention is:

[0008] Provide an active power decoupling method that introduces the three-phase voltage unbalance degree, including:

[0009] (1) Adopt voltage-current double-loop control for the three-phase energy storage converter with a T-type three-level topology;

[0010] (2) Detect the three-phase instantaneous voltages on the output side of the converter to obtain the three-phase voltage amplitude unbalance degree; pre-judge the grid voltage amplitude unbalance degree, and if the preset conditions are met, continue to perform the subsequent operations;

[0011] (3) Introduce the three-phase voltage amplitude unbalance degree into the calculation of the three-phase grid voltage dq transformation and the three-phase current feedback loop, and finally generate a PWM control signal;

[0012] (4) Each switching tube in the converter performs actions based on the PWM control signal to make the three-phase power equal, thereby suppressing the double-frequency power ripple.

[0013] As a preferred embodiment of the present invention, in step (2), the unbalance degree of the grid voltage amplitude is pre-judged according to formula (1):

[0014]

[0015] In the formula, F a 、F b 、F c are the unbalance degrees of the three-phase voltage amplitudes respectively; F a,b,c is any one of F a 、F b 、F c .

[0016] As a preferred embodiment of the present invention, in step (2), if the unbalance degree of the grid voltage amplitude does not meet the preset condition, it indicates that the grid fault is serious, and the converter should be disconnected from the grid.

[0017] As a preferred embodiment of the present invention, in step (3), when introducing the unbalance degree of the three-phase voltage amplitudes into the dq transformation of the three-phase grid voltage, the calculation is performed according to the following formulas (2)-(3):

[0018]

[0019]

[0020] In the above formulas, is the d-axis component of the grid instantaneous voltage under the three-phase unbalance condition; v d refers to the d-axis grid voltage obtained under three-phase balance; ω is the grid angular frequency; t is time; is the initial phase angle of the double-frequency component.

[0021] As a preferred embodiment of the present invention, when introducing the unbalance degree of the three-phase voltage amplitudes into the three-phase current feedback loop, the current values obtained after the abc-dq transformation are i d and i q , and the corresponding command values i d,cmd and i q,cmd should be ensured to remain unchanged.

[0022] As a preferred embodiment of the present invention, in step (4), each switching tube in the converter performs actions based on the PWM control signal, so that the magnitudes of the three-phase currents are adjusted in the ratio of 1 / F a 、1 / F b 、1 / F c , and the actual output current is 1 / F (a,b,c) times, so as to keep the current error zero; that is, when the voltage of the fault phase drops, the current of the fault phase will increase correspondingly, so that the three-phase power remains equal to suppress the double-frequency power ripple.

[0023] As a preferred embodiment of the present invention, in step (4), after introducing the regulation of the three-phase voltage unbalance degree, the three-phase grid currents on the output side of the converter are as shown in Equation (4):

[0024]

[0025] wherein, i a1 , i b1 , i b1 respectively refer to the three-phase grid currents after introducing the regulation of the three-phase voltage unbalance degree; i La , i Lb , i Lc respectively refer to the sampling values of the three-phase currents; i g is the peak-to-peak value of the three-phase currents when the power grid operates normally.

[0026] As a preferred embodiment of the present invention, a neutral point potential balance control is adopted for the three-phase energy storage converter with a T-type three-level topology, and by adjusting the average value of the three-phase currents, the neutral point current is made to approach zero.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] 1. The present invention obtains the unbalance degree of the three-phase voltage amplitudes by instantaneously sampling the three-phase voltages, and automatically introduces double-frequency ripples into the phase currents by digital control, which can effectively eliminate the double-frequency ripple power oscillation under the condition of unbalanced three-phase grid voltage amplitudes, and achieve the goal of suppressing the double-frequency power ripple on the DC side.

[0029] 2. When performing decoupling operation, the present invention only needs to consider the unbalance of the voltage amplitudes, and the double-frequency ripple can be calculated by using a simplified form without the need for conventional positive and negative sequence voltage separation processing; additional hardware topologies can be not added, the original control loop can be retained, and the detection and control of positive and negative sequence components can be omitted. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is the overall control block diagram of the present invention for introducing the three-phase voltage unbalance degree.

[0031] Figure 2 is the quantitative drawing of the ripple power component of the present invention.

[0032] Figure 3 is the schematic diagram of the active power after introducing the three-phase voltage unbalance degree of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] The technical solutions in the embodiments of the present invention will be further described below in conjunction with the drawings, but the protection scope of the present invention is not limited thereto.

[0034] The three-phase energy storage converter based on the T-type three-level topology of the present invention proposes an active power decoupling method introducing the three-phase voltage unbalance degree, and its control block diagram is as shown in Figure 1 shown. In Figure 1 , for the T-type three-level circuit, a voltage-current double-loop control is adopted. The input of the voltage outer loop is the DC-side voltage v dc , and the inputs of the current inner loop are i d and i q obtained after the abc-dq transformation of the three-phase sampled currents, and the phase angle θ is calculated by the phase-locked loop from the three-phase sampled voltages.

[0035] The three-phase unbalance degree generally refers to the amplitude and phase differences of the three-phase voltages, which affect the stability and efficiency of the power system. In the case of unbalanced three-phase grid voltages, there will be a double-frequency power ripple on the DC side. The present invention introduces the three-phase voltage unbalance degree, specifically referring to introducing the grid amplitude variations F a , F b , F c obtained by sampling the grid voltages into the calculations of the three-phase grid voltage dq transformation and the three-phase current feedback loop. There are various calculation methods for the three-phase unbalance degree, such as the national standard, the simplified national standard, IEEE, NEMA, and GIGRE methods, etc. Any existing calculation method can be applicable, and the present invention does not make any restrictions.

[0036] The active power decoupling method introducing the three-phase voltage unbalance degree of the present invention is specifically as follows:

[0037] 1. In the process of active power decoupling, for the three-phase energy storage converter with the T-type three-level topology, a voltage-current double-loop control is adopted; at the same time, a neutral-point potential balance control is adopted, and the neutral-point current is made to approach zero by adjusting the average value of the three-phase currents.

[0038] 2. Detect the three-phase instantaneous voltages on the output side of the converter to obtain the three-phase voltage amplitude unbalance degree; pre-judge the grid voltage amplitude unbalance degree.

[0039] Specifically, pre-judge the grid voltage amplitude unbalance degree according to formula (1):

[0040]

[0041] In the formula, F a , F b , F c are respectively the three-phase voltage amplitude unbalance degrees; F a,b,c is any one of F a , F b , F c . The effect of this decoupling strategy decreases with the increase of the voltage unbalance degree, and the selection of the judgment threshold is based on the condition that the suppression ratio of the double-frequency power ripple exceeds 70%.

[0042] If the unbalance degree of the grid voltage amplitude does not meet the preset condition, it indicates that the grid fault is serious, and the converter should be disconnected from the grid. If the preset condition described in formula (1) is met, the subsequent operations are continued.

[0043] 3. Introduce the unbalance degree of the three-phase voltage amplitude into the calculation of the three-phase grid voltage dq transformation and the three-phase current feedback loop, and finally generate the PWM control signal.

[0044] (1) Considering that the three-phase voltage unbalance caused by the grid fault is the voltage amplitude unbalance, the present invention introduces the unbalance degree of the voltage amplitude for relevant calculations, and the processing steps of positive and negative sequence separation can be omitted.

[0045] Therefore, when only considering the case of grid voltage amplitude unbalance, the expression of the three-phase grid voltage after dq transformation is as shown in formula (2):

[0046]

[0047] In the above formulas, is the d-axis component of the grid instantaneous voltage under three-phase unbalanced conditions; v d refers to the d-axis grid voltage obtained under three-phase balanced conditions; ω is the grid angular frequency; t is the time; is the initial phase angle of the double-frequency component.

[0048] It can be seen from the above formula that v d1 obtained under three-phase voltage amplitude unbalance contains a DC component and a double-frequency AC component. At this time, the active power p d multiplied by i o will also contain a double-frequency AC component.

[0049] (2) If the unbalance degrees F a , F b , F c of the three-phase voltage amplitude meet the judgment in (2), the unbalance degrees F a , F b , F c of the three-phase voltage are introduced into the three-phase current feedback loop of the three-phase energy storage converter control loop, so that the power of each phase is kept equal, thereby suppressing the double-frequency power ripple. In this process, the current values obtained after the abc-dq transformation are i d and i q , and it should be ensured that the corresponding command values i d,cmd and i q,cmd remain unchanged.

[0050] 4. Each switching tube in the converter performs actions based on the PWM control signal to make the three-phase power equal and thus suppress the double-frequency power ripple.

[0051] Each switching device in the converter performs actions based on the PWM control signal. Due to the negative feedback characteristic of the current loop, the magnitudes of the three-phase currents will be adjusted according to the ratio of 1 / F a 、1 / F b 、1 / F c so that the actual output current is 1 / F (a,b,c) times to keep the current error zero; that is, when the voltage of the faulty phase drops, the current of the faulty phase will increase accordingly, making the three-phase power equal to suppress the second-harmonic power ripple.

[0052] After introducing the regulation of the three-phase voltage unbalance degree, the three-phase grid currents on the output side of the converter are as shown in Equation (4):

[0053]

[0054] In the formula, i a1 、i b1 、i b1 respectively refer to the three-phase grid currents after introducing the regulation of the three-phase voltage unbalance degree; i La 、i Lb 、i Lc respectively refer to the sampled values of the three-phase currents; i g is the peak-to-peak value of the three-phase currents when the power grid operates normally.

[0055] Performing the Park transformation on Equation (4), the expression of i d1 with respect to F a,b,c is as shown in Equation (5):

[0056]

[0057] Among them,

[0058]

[0059] Among them, i d is the d-axis voltage obtained when the normal grid voltage is balanced.

[0060] For simplicity of analysis, only the case of phase A fault is considered here (i.e., F b = 1, F c = 1). Combining Equations (1) and (5), the output active power p o can be deduced as shown in Equation (7):

[0061]

[0062] Equation (7) represents the active power p o after introducing the three-phase voltage unbalance degree, which consists of a DC component and an AC component is composed of, where is the active power p o of the ripple power component.

[0063] In order to further analyze the magnitude of, a quantitative plot is made of it, and the result is as Figure 2 shown.

[0064] The schematic diagram of the active power after introducing the three-phase voltage unbalance degree is as Figure 3 shown.

[0065] Through the above steps in this embodiment, the three-phase voltage unbalance degree is introduced into the control system of the T-type three-level circuit, so that the power of each phase remains the same, thereby eliminating the double-frequency ripple and making the three-phase output power reach balance.

[0066] Different from the common active decoupling strategies in the prior art, the decoupling method proposed by the present invention does not require additional hardware costs, does not need to modify the original control loop, does not need to detect and control the positive and negative sequence components under the three-phase unbalance condition, and can operate in both rectification and inversion modes.

[0067] It should be understood that the above-described embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. All deformations that can be directly derived or associated by those of ordinary skill in the art from the content disclosed in the present invention should be considered as the protection scope of the present invention.

Claims

1. An active power decoupling method for introducing three-phase voltage unbalance degree, characterized in that, Including: (1) Adopting voltage-current double-loop control for a three-phase energy storage converter with a T-type three-level topology; (2) Detecting the three-phase instantaneous voltage on the output side of the converter to obtain the unbalance degree of the three-phase voltage amplitudes; pre-judging the unbalance degree of the grid voltage amplitude, and if the preset conditions are met, continue to perform subsequent operations; (3) Introducing the unbalance degree of the three-phase voltage amplitudes into the calculation of the three-phase grid voltage dq transformation and the three-phase current feedback loop, and finally generating a PWM control signal; (4) Each switch tube in the converter performs actions based on the PWM control signal to keep the three-phase power equal, thereby suppressing the double-frequency power ripple.

2. The method according to claim 1, wherein In the step (2), the unbalance degree of the grid voltage amplitude is pre-judged according to formula (1): Where, F a , F b , F c are respectively the unbalance degrees of the three-phase voltage amplitudes; F a,b,c is any one of F a , F b , F c .

3. The method according to claim 1, wherein In the step (2), if the unbalance degree of the grid voltage amplitude does not meet the preset conditions, it indicates that the grid fault is serious, and the converter should be disconnected from the grid.

4. The method according to claim 1, wherein In the step (3), when introducing the unbalance degree of the three-phase voltage amplitudes into the three-phase grid voltage dq transformation, the calculation is carried out according to the following formulas (2)-(3): In the above formulas, is the d-axis component of the grid instantaneous voltage under unbalanced three-phase conditions; v d refers to the d-axis grid voltage obtained under balanced three-phase conditions; ω is the grid angular frequency; t is time; is the initial phase angle of the double-frequency component.

5. The method according to claim 1, characterized in that, In the said step (3), when introducing the three-phase voltage amplitude unbalance degree into the three-phase current feedback loop, the current values obtained after abc-dq transformation are i d and i q . It shall be ensured that the corresponding command values i d,cmd and i q,cmd remain unchanged.

6. The method according to claim 1, wherein In the step (4), each switching tube in the converter performs actions based on the PWM control signal, so that the magnitudes of the three-phase currents are adjusted according to the ratio of 1 / F a , 1 / F b , 1 / F c , and the actual output current is 1 / F (a,b,c) times to keep the current error zero; that is, when the voltage of the faulty phase drops, the current of the faulty phase will increase accordingly, so that the three-phase power remains equal to suppress the double-frequency power ripple.

7. The method according to claim 1, characterized in that, In the step (4), after introducing the adjustment of the three-phase voltage unbalance degree, the three-phase grid currents on the output side of the converter are as shown in formula (4): where, i a1 , i b1 , i b1 respectively refer to the three-phase grid currents after introducing the three-phase voltage unbalance adjustment; i La , i Lb , i Lc respectively refer to the sampled values of the three-phase currents; i g is the peak-to-peak value of the three-phase currents during normal grid operation.

8. The method according to claim 1, characterized in that Adopting neutral point potential balance control for a three-phase energy storage converter with a T-type three-level topology, and making the neutral point current approach zero by adjusting the average value of the three-phase currents.

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