Energy balance control method for high-voltage high-capacity half-wave frequency conversion alternating current converter

Abstract

The invention provides an energy balance control method for a high-voltage high-capacity half-wave frequency conversion alternating current converter, which comprises the following steps of: establishing a low-frequency side switch flip angle control strategy, changing the switching time point of a low-frequency side switch from a zero crossing point to an arbitrary angle theta after the zero crossing point, calculating the periodic energy accumulation of a half bridge and a full bridge, and calculating the energy balance of the high-voltage high-capacity half-wave frequency conversion alternating current converter. The method comprises the following steps: verifying the adjustment of the flip angle, feeding back the difference value of the capacitor voltage average values of the full-bridge sub-module and the half-bridge sub-module in real time, when the difference value exceeds a threshold value, realizing the adjustment of the flip angle through PI control, and controlling the difference of the capacitor voltage of the full-bridge sub-module and the half-bridge sub-module within a reasonable range all the time. According to the invention, full-bridge switch flip angle control is provided for the problem of phase-to-phase energy imbalance of the half-wave frequency conversion alternating current converter, so that full-bridge and half-bridge energy balance in a phase unit is realized, the capacitor voltage difference between the full-bridge and the half-bridge is maintained in a reasonable range, and normal operation of the converter is realized.

Description

Technical Field

[0001] The invention relates to the technical field of half-wave frequency conversion AC converters, and in particular to an energy balancing control method for a high-voltage and large-capacity half-wave frequency conversion AC converter. Background Art

[0002] Vigorously developing new energy and accelerating the decarbonization of electricity are the only way to build a new power system and promote clean energy transformation. However, as the proportion of new energy installed capacity increases, the problem of lack of grid regulation capacity becomes prominent. The development of advanced transmission and energy storage technologies is an important measure to enhance the flexibility of the grid and power supply sides. Among them, flexible low-frequency transmission, as a new type of transmission method, greatly increases the transmission capacity of the line and reduces the electrical distance of the AC line by adopting a transmission frequency lower than the industrial frequency. It uses power electronic conversion to achieve frequency conversion and flow control of electric energy, has high operational flexibility, and has unique advantages in scenarios such as medium and long-distance offshore wind power collection and transmission.

[0003] High-voltage and large-capacity AC converters are the core equipment of flexible low-frequency AC transmission, variable-speed pumped storage and other systems. Their technical performance and economy directly determine the operational safety and economy of the system in which they are located. However, in the existing Modular Multilevel Matrix Converter (M3C) topology, each bridge arm needs to bear the sum of the peak values ​​of the AC full-wave voltage of two frequencies, and there are many power modules with low utilization and large losses. Therefore, it is urgent to innovate the main circuit topology of high-voltage and large-capacity AC converters. In order to achieve the sparseness of AC switching topology devices / modules, a literature proposed a half-wave commutated modular multilevel converter, which uses the half-wave frequency conversion idea to perform frequency conversion on the positive half-wave voltage of the sine wave, and uses the fully controlled device to reverse the polarity of the generated positive voltage according to the cycle to generate a positive and negative alternating sine wave. However, HWC-MMC has energy balance problems, especially the energy imbalance problem generated by the full-bridge and half-bridge sub-modules in the phase unit during operation. This problem will cause the capacitor voltage of these two types of submodules to deviate from the rated voltage and generate circulating current in the phase unit, which will affect the operation of the HWC-MMC. Therefore, it is urgent to develop an energy balancing control method for high-voltage and large-capacity half-wave variable frequency AC converters. Summary of the invention

[0004] In order to overcome the deficiencies of the prior art, an object of the present invention is to provide an energy balancing control method for a high voltage and large capacity half-wave variable frequency AC converter.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] The present invention provides an energy balancing control method for a high-voltage and large-capacity half-wave variable frequency AC converter, comprising:

[0007] Establish a low-frequency side switch flip angle control strategy to change the switching time point of the low-frequency side switch from the zero-crossing point to any angle θ after the zero-crossing point;

[0008] Calculate the periodic energy accumulation of the half-bridge and full-bridge to verify the adjustment of the flip angle;

[0009] The difference between the average value of the capacitor voltage of the full-bridge and half-bridge sub-modules is fed back in real time. When the difference exceeds the threshold, the flip angle is adjusted through PI control, and the difference between the capacitor voltage of the full-bridge and half-bridge sub-modules is always controlled within a reasonable range.

[0010] Preferably, a low-frequency side switch flip angle control strategy is established, specifically:

[0011] The voltage and current variables of the half-wave variable frequency AC converter are defined as:

[0012]

[0013] In the formula, u o and i o They are the output AC voltage and current of the half-bridge side, u i and i i They are the input AC voltage and current on the full-bridge side;

[0014] Definition of full-bridge side switching signal S i And the half-bridge side switching signal S o for:

[0015]

[0016] After adding the low-frequency side switch flip angle control, S i Modify and simplify the half-wave variable frequency AC converter to:

[0017]

[0018] Among them, the half-bridge side switching signal S o unchanged, the full-bridge side switch signal S i Add the flip angle to get:

[0019]

[0020] Preferably, the periodic energy accumulation of the half bridge and the full bridge is calculated to verify the adjustment of the flip angle, specifically:

[0021] The voltage and current of the full-bridge and half-bridge modules after adding the flip angle are expressed as:

[0022]

[0023] In the formula, u f 、i f 、i h All affected by S i The influence of the flip angle in the middle will add a new control degree θ to its energy balance and calculate the half-bridge accumulated energy E H And the full bridge accumulated energy E F ,for:

[0024]

[0025] The piecewise integration method is used to derive the periodic cumulative energy of the full bridge and half bridge respectively to verify the influence of the flip angle. The full bridge cumulative energy is calculated as:

[0026]

[0027] The half-bridge accumulated energy is calculated as:

[0028]

[0029] Among them, the full-bridge accumulated energy and the half-bridge accumulated energy are both affected by the flip angle θ. When the frequency and phase angle change, the flip angle still participates in the integration of the periodic energy and affects it.

[0030] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0031] The present invention provides an energy balance control method for a high-voltage, large-capacity, half-wave variable frequency AC converter, the method comprising establishing a low-frequency side switch flip angle control strategy, changing the time point of the low-frequency side switch switching from the zero-crossing point to an arbitrary angle θ after the zero-crossing point, calculating the periodic energy accumulation of the half-bridge and the full-bridge, verifying the adjustment of the flip angle, and providing real-time feedback on the average value difference of the capacitor voltage of the full-bridge and half-bridge submodules. When the difference exceeds a threshold, the adjustment of the flip angle is achieved through PI control, and the difference between the capacitor voltage of the full-bridge and half-bridge submodules is always controlled within a reasonable range. The present invention proposes a full-bridge switch flip angle control for the inter-phase energy imbalance problem of the half-wave variable frequency AC converter, thereby achieving energy balance between the full-bridge and half-bridge in the phase unit, so that the capacitor voltage difference between the two is maintained within a reasonable range, and the normal operation of the converter is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0033] Figure 1 A schematic flow chart of an energy balancing control method for a high-voltage, large-capacity, half-wave variable frequency AC converter provided by an embodiment of the present invention;

[0034] Figure 2 It is a topological diagram of a half-wave variable frequency AC converter;

[0035] Figure 3 It is a schematic diagram of the single-phase frequency conversion link of a half-wave frequency conversion AC converter;

[0036] Figure 4 Simplify the circuit diagram for the topology;

[0037] Figure 5 It is a waveform diagram under full-bridge flip angle control;

[0038] Figure 6 Schematic diagram of full-bridge switch flip angle control. DETAILED DESCRIPTION

[0039] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0040] The present invention proposes an additional control of the flip angle of the full-bridge switch to solve the problem of phase-to-phase energy imbalance in a half-wave variable frequency AC converter. The control operation is simple, the links are few and the effect is obvious. The energy balance between the full-bridge and half-bridge phases can be achieved without affecting the original control links, so that the capacitor voltage difference between the two is maintained within a reasonable range, and the normal operation of the topology is achieved. The control method is single-phase control, that is, the three-phase full-bridge switch can adopt this strategy to achieve intra-phase energy balance, and the phases do not affect each other.

[0041] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0042] Figure 1 A flow chart of a method provided by an embodiment of the present invention, such as Figure 1As shown, the present invention provides an energy balancing control method for a high-voltage and large-capacity half-wave variable frequency AC converter, comprising:

[0043] Step 100: Establish a low-frequency side switch flip angle control strategy to change the switching time point of the low-frequency side switch from the zero-crossing point to any angle θ after the zero-crossing point;

[0044] Step 200: Calculate the periodic energy accumulation of the half bridge and the full bridge, and verify the adjustment of the flip angle;

[0045] Step 300: Feedback the average value difference of the capacitor voltage of the full-bridge and half-bridge submodules in real time. When the difference exceeds the threshold, the flip angle is adjusted through PI control, and the difference of the capacitor voltage of the full-bridge and half-bridge submodules is always controlled within a reasonable range.

[0046] First, the topology of the half-wave variable frequency AC converter is introduced. Figure 2 As shown, the voltage matching part is composed of a cascade of half-bridge submodules (HBSM) or full-bridge submodules (FBSM), and the number of cascades is determined by system requirements; the frequency conversion part is composed of a cascade of full-bridge submodules (FBSM), and the number of cascades is determined by system requirements. By switching on / off a specific number of submodules (SM) in the voltage matching part, an AC voltage with a specific amplitude and frequency can be generated on the power frequency side of the converter through a half-wave commutation part in which controllable switching devices are connected in series; based on the number of submodules (SM) switched on in the voltage matching part, a specific number of submodules (SM) are switched on / off in the frequency conversion part, and an AC voltage u with a specific amplitude and frequency can be generated on the A phase side of the converter through the half-wave commutation part. sa .

[0047] The frequency conversion process of the half-wave frequency conversion AC converter mainly includes four links, such as Figure 3 The AC voltage u x It is determined by the number of submodules put into the voltage matching part, and is a positive voltage of a specific amplitude and frequency. The positive voltage is composed of the positive half cycle of the sine wave of the specific amplitude and frequency, such as Figure 3 As shown in step 1, it has nothing to do with the number of submodules put into the frequency conversion part. a is a positive voltage of a specific amplitude and frequency, which is composed of the positive half cycle of a sine wave of the specific amplitude and frequency, such as Figure 3 As shown in link 1, its amplitude and frequency are jointly determined by the sub-modules of the voltage matching part and the frequency conversion part. The half-wave commutation part is composed of cascaded controllable switching devices. The number of cascades is determined by the system requirements. x , the polarity is reversed at a certain frequency and interval to generate a positive and negative alternating sine wave as the one-phase power frequency output of the converter, such as Figure 3As shown in step 3. Similarly, the low-frequency voltage generation part is composed of cascaded controllable switch devices. The number of cascades is determined by the system requirements. The positive voltage u generated by the voltage matching part and the frequency conversion part is a , the polarity is reversed at a certain frequency and interval to generate a positive and negative alternating sine wave as a single-phase low-frequency output of the converter, such as Figure 3 As shown in link 4.

[0048] The half-wave variable frequency AC converter topology will produce full-bridge and half-bridge energy offset during operation, and the capacitor voltages of the two will become increasingly different during operation. This requires a control strategy to adjust the energy balance between the two and maintain the relative stability of the capacitor voltage. Therefore, the present invention establishes a low-frequency side switch flip angle control strategy, and its specific principle is: Figure 3 In link 4, the low-frequency side switch needs to be switched at the sine zero crossing point, and the sine waveform is converted into a half-wave waveform through the on-off of the switch. The low-frequency side switch flip angle control strategy is to change the time point of the low-frequency side switch switching from the zero crossing point to any angle θ after the zero crossing point. The sub-module energy accumulation model is established. The above steps are introduced in detail to explain the principle:

[0049] In step 100, a low-frequency side switch flip angle control strategy is established, specifically:

[0050] The half-wave variable frequency AC converter topology is simplified, and the simplified structure is as follows: Figure 4 As shown, according to Figure 4 The voltage and current variables of the half-wave variable frequency AC converter are defined as:

[0051]

[0052] In the formula, u o and i o They are the output AC voltage and current of the half-bridge side, u i and i i They are the input AC voltage and current on the full-bridge side;

[0053] Definition of full-bridge side switching signal S i And the half-bridge side switching signal S o for:

[0054]

[0055] After adding the low-frequency side switch flip angle control, S i Modifications are made, and the simplified half-wave variable frequency AC converter is simplified again. The period energy is derived using the double frequency condition as an example to illustrate the influence of the switch flip angle on the energy of the two. The simplified related parameters are:

[0056]

[0057] Among them, the half-bridge side switching signal S o unchanged, the full-bridge side switch signal S i Add the flip angle to get:

[0058]

[0059] In step 200, the periodic energy accumulation of the half bridge and the full bridge is calculated to verify the adjustment of the flip angle, specifically:

[0060] The voltage and current of the full-bridge and half-bridge modules after adding the flip angle are expressed as:

[0061]

[0062] In the formula, u f 、i f 、i h All affected by S i The influence of the flip angle in the middle will add a new control degree θ to its energy balance and calculate the half-bridge accumulated energy E H And the full bridge accumulated energy E F ,for:

[0063]

[0064] Among them, the integral cannot be realized due to the influence of the irregular waveform. The segmented integration method is adopted to derive the periodic cumulative energy of the full bridge and half bridge respectively, and verify the influence of the flip angle. Among them, the full bridge cumulative energy is calculated as:

[0065]

[0066] The half-bridge accumulated energy is calculated as:

[0067]

[0068]

[0069] From the above, we can see that the accumulated energy of the full bridge and the accumulated energy of the half bridge are both affected by the flip angle θ. When the frequency and phase angle change, the flip angle still participates in the integration of the periodic energy and affects it. Therefore, the energy distribution between the full bridge and the half bridge can be achieved by controlling the flip angle.

[0070] Among them, the waveform under full-bridge flip angle control is as follows Figure 5 The specific control strategy is as shown in Figure 6As shown, by real-time feedback of the difference in the average value of the capacitor voltage of the full-bridge and half-bridge sub-modules, when the difference exceeds the threshold, the flip angle is adjusted through PI control, and the difference in the capacitor voltage of the full-bridge and half-bridge sub-modules is always controlled within a reasonable range.

[0071] Among them, the difference: Generally speaking, under different working conditions, the full-bridge and half-bridge submodules have corresponding rated voltages. At the same time, the system returns the actual capacitor voltage average value of the full-bridge and half-bridge submodules in real time. The difference is the absolute value obtained by subtracting the actual capacitor voltage value from the rated voltage;

[0072] Threshold: The voltage of the capacitor of the full-bridge and half-bridge submodules fluctuates during normal operation. If the system needs to operate normally and stably with small losses, the voltage of the submodule capacitor needs to be controlled within a certain fluctuation range. The fluctuation range can be adjusted according to the actual working conditions. Generally, the fluctuation rate will be controlled within ±5%. The threshold is: rated voltage of the full / half-bridge submodule capacitor * specified fluctuation rate;

[0073] Specific control method: When the difference of the capacitance of the full-bridge or half-bridge sub-modules is greater than the threshold, the flip angle is increased / decreased through the PI link to reduce the difference to within the threshold range;

[0074] Reasonable range: set according to the requirements of the working conditions. If the volatility is required to be high, then the volatility needs to be controlled within a smaller range. Generally, the volatility is controlled within the range of ±5% of the submodule capacitor voltage.

[0075] This control is an additional control, and the flip angle is an additional degree of freedom, which does not affect the normal control of the half-wave variable frequency AC converter. Therefore, the control of the flip angle is valid whether in normal conditions or fault conditions.

[0076] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other. Specific examples are used in this article to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea; at the same time, for those of ordinary skill in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and scope of application. In summary, the content of this specification should not be understood as a limitation on the present invention.

Claims

1. A method for energy balance control of a high-voltage, large-capacity, half-wave variable frequency AC converter, characterized in that: include: Establish a low-frequency side switch flip angle control strategy to change the switching time point of the low-frequency side switch from the zero-crossing point to any angle θ after the zero-crossing point; Calculate the periodic energy accumulation of the half-bridge and full-bridge to verify the adjustment of the flip angle; The difference between the average value of the capacitor voltage of the full-bridge and half-bridge sub-modules is fed back in real time. When the difference exceeds the threshold, the flip angle is adjusted through PI control, and the difference between the capacitor voltage of the full-bridge and half-bridge sub-modules is always controlled within a reasonable range.

2. The method according to claim 1, characterized in that Establish a low-frequency side switch flip angle control strategy, specifically: The voltage and current variables of the half-wave variable frequency AC converter are defined as: In the formula, u o and i o They are the output AC voltage and current of the half-bridge side, u i and i i They are the input AC voltage and current on the full-bridge side; Definition of full-bridge side switching signal S i And the half-bridge side switching signal S o for: After adding the low-frequency side switch flip angle control, S i Modify and simplify the half-wave variable frequency AC converter to: Among them, the half-bridge side switching signal S o unchanged, the full-bridge side switch signal S i Add the flip angle to get:

3. The method according to claim 2, characterized in that Calculate the periodic energy accumulation of the half-bridge and full-bridge to verify the adjustment of the flip angle, specifically: The voltage and current of the full-bridge and half-bridge modules after adding the flip angle are expressed as: In the formula, u f 、i f 、i h All affected by S i The influence of the flip angle in the middle will add a new control degree θ to its energy balance and calculate the half-bridge accumulated energy E H And the full bridge accumulated energy E F ,for: The piecewise integration method is used to derive the periodic cumulative energy of the full bridge and half bridge respectively to verify the influence of the flip angle. The full bridge cumulative energy is calculated as: The half-bridge accumulated energy is calculated as: Among them, the full-bridge accumulated energy and the half-bridge accumulated energy are both affected by the flip angle θ. When the frequency and phase angle change, the flip angle still participates in the integration of the periodic energy and affects it.

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