Method and apparatus for controlling three-phase SVPWM constant current grid-connected rectifier

Abstract

The invention relates to a power supply device (1) for a load (2), having a DC link converter with a controllable rectifier (4), which is configured to be connected to a single-phase or multi-phase electrical network (3) and to draw energy from the electrical network (3) at a constant load current (grid) in order to charge the DC link energy storage device (6), and having a DC link energy storage device (6), the control device is configured to monitor the actual intermediate circuit voltage (VDCact) and to actively or automatically allow the load (2) to draw energy from the intermediate circuit energy storage device (6) as a function of the intermediate circuit voltage (VDCact).

Description

Technical Field

[0001] The present invention relates to a power supply device for a load, the power supply device having a DC link converter with a controllable rectifier and a DC link energy storage device, in particular for an SVPWM (Space Vector Pulse Width Modulation) constant current grid-connected rectifier. The present invention also relates to an ozone generator having such a power supply device, and a method for operating such a power supply device or an ozone generator. Background Art

[0002] The power supply devices relevant here are widely used in industry, such as, but not limited to, high-power ozone generators. Such high-power ozone generators are used for water disinfection and oxidation in industry, laboratories and pilot plants. Standard applications include, for example, drinking water treatment, industrial water circulation, recycling processes, the food and beverage industry, cooling water systems, fish farming and swimming pools.

[0003] In water treatment, such systems generate highly reactive ozone (O3) and add it to the water to be treated. Due to its strong oxidizing action, ozone reacts with impurities, bacteria, fats, algae and other unwanted water components, breaking them down in the process.

[0004] Ozone is produced in an ozone generator by intermittent, repetitive high-energy pulses that break down the input oxygen O2 molecules into atomic oxygen (O), which then react with other oxygen O2 molecules to form ozone (O3). To provide the necessary energy pulses, a grid-tied inverter with a DC link capacitor is used between the load and the supply grid.

[0005] For example, a grid-connected inverter for transferring power between a power source and a fluctuating load is known from US 8796884 B2.

[0006] A control strategy for charging a pulse battery system having an inverter and a DC link capacitor is known from US 2014 / 0028249 A1.

[0007] The normal operating mode of a grid-tied inverter is to generate a constant voltage in its internal DC link capacitor. Such inverters are also called VSIs (Voltage Source Inverters). This operating mode is well known and is well suited for loads with very low or non-repetitive load fluctuations.

[0008] However, when using conventional VSIs, there is essentially a problem that fluctuations on the load side become directly apparent in the relative fluctuations in the grid power consumption. This is particularly problematic when these load fluctuations occur periodically and repeatedly. If the load fluctuations occur at frequencies within the grid fundamental frequency or in its harmonic range, this can lead to undesirable so-called total harmonic distortion (THD).

[0009] The pulsed operation of the ozone system usually generates a periodic train of energy pulses on the load side. Although the energy consumed by each energy pulse is the same, the average value of the total power is determined by the pulse frequency (number of pulses per unit time). As mentioned above, when the VSI is loaded in this way, the load-side energy pulses are directly visible on the grid-side line. This can lead to undesirable and severe distortion of the current fed by the grid, especially in cases where the current peaks occurring overlap completely, because the VSI compensates for the energy losses of the DC link immediately after each load pulse.

[0010] A complicating factor in known ozone generators is that the time constant of each pulse in normal operation is only a small fraction of the mains frequency period. Furthermore, the energy pulses may occur at a frequency that is equal to the mains operating frequency or an exact multiple thereof. Since the total power of the ozone system is regulated by the pulse frequency, the user is generally unable to predict the occurrence of the resulting distortion. The pulse frequency used therefore depends on the required total power and has a direct influence on the supply current course on the mains side. Summary of the invention

[0011] The object of the present invention is to provide a power supply device which does not have the above-mentioned problems.

[0012] This object is achieved by a power supply device having the features of claim 1, wherein the controlled rectifier is configured to be connected to a single-phase or multi-phase grid and to draw energy from the grid at a constant load current to charge a DC link energy storage device, and the power supply device comprises a control device configured to monitor the actual intermediate circuit voltage and to actively or automatically allow the load to draw energy from the intermediate circuit energy storage device in dependence on the intermediate circuit voltage. Preferred embodiments are the subject of the dependent claims.

[0013] Using the power supply device according to the present invention, the controlled rectifier can be controlled to obtain energy from the power grid at a constant charging current and charge the DC link energy storage device until a preset DC link voltage is reached, wherein the control device monitors the actual DC link voltage and allows the load to extract energy from the DC link energy storage device only when the actual DC link voltage is equal to a preset trigger voltage.

[0014] This power supply is able to achieve such an operation mode that the constant current of the grid-side line takes precedence over the constant voltage of the DC link. This is achieved by logical coupling between the grid-side controlled rectifier and the load-side inverter, and by accepting significant ripple or voltage drop of the DC link energy storage device, such as in the form of capacitors, which is not accepted by traditional VSI systems.

[0015] During operation of the power supply device according to the invention, the DC link converter first transmits energy pulses to the load on the load side. After each energy pulse, the voltage of the DC link energy storage device drops significantly. This energy loss is then compensated by a constant current from the grid unit until the energy losses of the DC link are fully compensated. Only then is the next load-side energy pulse delivered.

[0016] This operating mode ensures that a constant current is always drawn from the grid, which recharges the DC link storage device after each discharge, i.e. after each delivered energy pulse. In this case, the current is a function of the power setpoint desired and set by the user. The pulse frequency on the load side depends on the current with which the DC link storage device is recharged. Higher currents lead to faster charging and thus to higher pulse frequencies on the load side.

[0017] Since the charging current of the DC link energy storage device on the grid side is controlled by the control device or the user to be a constant value, the pulse frequency of the energy pulse on the load side no longer has any effect on the shape of the grid side current. Therefore, using the power supply device according to the present invention, undesirable total harmonic distortion (THDi) in the current no longer occurs.

[0018] The next load-side energy pulse is only triggered when the voltage of the DC link capacitor reaches a value to be defined, the so-called trigger voltage, which is selected to be below the nominal DC link voltage. To this end, the control constantly monitors the actual DC link voltage. Above this trigger voltage level, the control continuously reduces the current until the nominal DC link voltage is reached. This is a cascaded voltage and current control (CC / CV) with constant current or voltage limitation. This ensures that the power supply is operated with a constant input current (CC, in production mode) or a constant DC link voltage (CV, in no-load operation).

[0019] In particular, the load can be a conventional completely passive ozone generator, since the power supply device can be said to release the energy pulse independently. The load does not need to actively request it. In this way, conventional loads, in particular ozone generators, can be equipped with the power supply device according to the invention.

[0020] In a preferred embodiment, the control device is further configured to control a controlled rectifier.

[0021] In a particularly preferred embodiment, the control device is configured to regulate and / or set the load current in order to charge or recharge the DC link energy storage device. Thus, the control device can be used to regulate the duration between two energy pulses on the load side, thereby regulating the effective total power output of the system.

[0022] In another embodiment, the power supply device comprises an input device for the controller or the load, the input device being configured to monitor whether power is drawn from the DC link energy storage device, and the control device is further configured to terminate the charging process of the DC link energy storage device. By this embodiment, once the control device receives a signal from the input device, i.e., from the controller or the load, respectively, that energy is no longer to be extracted from the DC link energy storage device, the control device can terminate the charging process of the DC link energy storage device. Thus, the control device can switch to an idle mode in which the DC link is no longer charged.

[0023] The invention also relates to an ozone generator having the power supply device.

[0024] The invention also relates to a method for operating a power supply device as described above or for an ozone generator having such a power supply device.

[0025] According to the present invention, the rectifier is controlled in such a way that it draws energy from the grid at a constant load current and charges the DC link energy storage device until a preset intermediate circuit voltage is reached, wherein the control device monitors the actual intermediate circuit voltage, and wherein the control device allows the load to draw energy from the intermediate circuit energy storage device only when the actual intermediate circuit voltage is equal to or greater than a preset trigger voltage.

[0026] In one embodiment, the control device regulates and / or sets the load current so as to recharge the DC link energy storage device to a preset DC link voltage within a preset duration.

[0027] In another embodiment, once the control device receives a signal from the input device (ie, from the controller or the load) that power is no longer to be drawn from the DC link energy storage device, the control device terminates the charging process.

[0028] The advantages of the method according to the invention result from the above-mentioned advantages of the power supply unit according to the invention.

[0029] The basic idea of ​​all the above embodiments is that the pulse sequence / pulse frequency at which the load or ozone generator operates is no longer constant but is generated as a function of the rate of voltage rise in the DC link which in turn is a function of the constant current drawn from the grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The invention is explained in more detail below with reference to the accompanying drawings, which are only used for a better understanding of the principles of the invention and are not limited to the embodiments shown.

[0031] They appear as: Figure 1 : A schematic diagram of an embodiment of a power supply device according to the present invention; Figure 2 : Overview of the control operations of the control device of the power supply unit; Figure 3a: U / I characteristics of conventional prior art VSI; and Figure 3b : Exemplary U / I characteristics of a power supply device according to the present invention.

[0032] Description of Reference Numerals 1 Power supply device 2 loads / ozone generator 3. Power Grid 4 Grid-side rectifier 5. Load-side inverter 6 DC link storage device I_grid Charging current VDC_nom Nominal or specified DC link voltage VDC_act Actual DC link voltage VDC_trig Trigger voltage dT_min The time period before the DC link energy storage device is charged dT_pulse Time interval between two load-side energy pulses DETAILED DESCRIPTION Figure 1 A schematic diagram of an embodiment of the invention is shown. A power supply 1 has a DC link converter between a three-phase grid 3 and a load 2 in the form of an ozone generator. On the grid side, the DC link converter has a controlled rectifier 4 and on the load side, an inverter 5. The DC link converter also has a DC link energy storage device 6 in the form of a capacitor.

[0033] According to the invention, the rectifier 4 is controlled in such a way that it draws an adjustable constant charging current I_grid from the grid 3, thereby charging the DC link energy storage device 6 until it reaches a preset DC link voltage VDC_nom. During the charging process, the control device continuously monitors the actual DC link voltage VDC_act. As soon as the actual DC link voltage reaches the value of the preset trigger voltage VDC_trig, the control device allows the load 2 to extract energy from the DC link energy storage device 6, thereby delivering a load-side energy pulse. Subsequently, new energy is only possible to output when the DC link energy storage device 6 is charged again, or when the actual voltage VDC_act reaches the value of the specified trigger voltage VDC_trig again.

[0034] In this way, the intermediate energy storage device 6 is continuously charged, resulting in a constant charging current I_grid. This charging current remains constant even if energy is drawn from the DC link energy storage device 6, because the trigger voltage VDC_trig is less than the specified DC link voltage VDC_nom, and energy is already drawn from the DC link storage 6 before the controlled rectifier 4 reduces the charging current by reaching VDC_nom.

[0035] For better understanding, Figure 2 The specific sequence of control operations of the control device of the power supply unit 1 is shown. On the left side of the figure, the control operation of the controlled rectifier 4 is shown. On the right side of the figure, the control operation of the load-side inverter 5 is shown. The two sides are logically coupled by the functions shown in the figure, because the arrangement of the grid side and the load side directly depends on the monitored actual DC link voltage VDC_act of the DC link energy storage device 6.

[0036] Figure 2 The control operation sequence of the controlled rectifier 4 shown on the left starts with checking whether the operating point of the charging current I_grid set by the user is not zero. In other words, whether the user has put the power supply device 1 into operation. If this is the case, the controlled rectifier draws a constant charging current I_grid equal to the set operating point from the power supply system, thereby charging the DC link energy storage device 6, and continuously monitoring the actual DC link voltage VDC_act. If the voltage level of the actual DC link voltage VDC_act reaches or exceeds the value of the specified DC link voltage VDC_nom, the charging current I_grid is reduced to keep the specified DC link voltage VDC_nom at the specified value. However, due to the above-mentioned logical coupling, this does not happen during normal operation.

[0037] Figure 2The control operation sequence of the load-side inverter shown on the right also checks the actual DC link voltage VDC_act. If the actual DC link voltage VDC_act exceeds the value of the specified trigger voltage VDC_trig, which is lower than the specified DC link voltage VDC_nom, an energy pulse is delivered to the load. If the actual DC link voltage VDC_act is lower than the specified trigger voltage VDC_trig, the inverter waits and the DC link energy storage device 6 continues to charge.

[0038] 3 a and 3 b compare, by way of example, the U / I (voltage and current) characteristics of a conventional VSI and the power supply device 1 according to the present invention.

[0039] FIG. 3 a shows the U / I characteristic of a conventional VSI according to the prior art. The lower line shows the curve of the supply current I_grid on the grid side, while the upper line shows the curve of the load current I_load delivered to the load. The actual DC link voltage corresponds to the nominal voltage VDC_nom of the DC link and always remains constant, since the charging current I_grid on the supply side immediately replenishes the energy delivered to the load. Therefore, for each energy pulse in the two characteristic curves I_grid, I_load, the current peak occurs at the same point marked by the vertical dashed line, the spacing of which corresponds to the time interval dT_pulse between two load-side energy pulses. It can be said that the energy pulses on the load side are mapped into the supply current on the grid side. As mentioned above, this is particularly disadvantageous if the interval dT_pulse between two energy pulses corresponds to the interval of the grid voltage, or if the tapping occurs at a frequency corresponding to an integer multiple of the grid operating frequency.

[0040] In contrast, Figure 3bThe U / I characteristic of the device according to the invention is shown. In this case, the charging current I_grid on the grid side is constant in segments I, II and III, while the actual DC link voltage shows a drop at the point of the energy pulse on the load side. In segment I, the device is turned off, so the charging current I_grid is always zero. If the user increases the operating point of the charging current I_grid to a non-zero value and the first energy pulse is delivered on the load side (segment II), the actual DC link voltage VDC_act drops below the value of the DC link voltage VDC_nom and below the value of the trigger voltage VDC_trig. The DC link energy storage device is immediately charged by the constant charging current I_grid. As soon as the actual DC link voltage reaches the value of the set trigger voltage VDC_trig, another load-side energy pulse is issued, causing the actual DC link voltage VDC_act to drop again. This process is repeated until the user turns off the device (segment IV) or changes the operating point of the charging current I_grid (segment III). By adjusting the operating point of the charging current I_grid, the time period dT_min until the DC link energy storage device is charged is changed, thereby changing the interval between two load-side energy pulses. Therefore, ultimately, the total energy output can be set by the charging current I_grid, so that the charging current I_grid is set constant in each case, without the resulting pulse frequency having any influence on the course of the grid current.

Claims

1. A power supply device (1) for a load (2), the power supply device having a DC link converter with a controllable rectifier (4) and a DC link energy storage device (6), the controllable rectifier (4) being configured to be connected to a single-phase or multi-phase power grid (3) and to draw energy from the power grid (3) at a constant load current (I_grid) to charge the DC link energy storage device (6), and the power supply device being provided with a control device, the control device being configured to monitor an actual intermediate circuit voltage (VDC_act) and, based on the intermediate circuit voltage (VDC_act), actively or automatically allow the load (2) to draw energy from the intermediate circuit energy storage device (6).

2. The power supply device (1) according to claim 1, characterized in that: The control device is also configured to control the controllable rectifier (4).

3. The power supply device (1) according to claim 1 or 2, characterized in that: The control device is configured to regulate and / or set the load current (I_grid) in order to charge or recharge the DC link energy storage device (6).

4. The power supply device (1) according to any one of claims 1 to 3, characterized in that: The power supply device comprises an input device for a controller or a load, wherein the input device is configured to monitor whether energy is drawn from the DC link energy storage device (6), and the control device is also configured to terminate the charging process of the DC link energy storage device (6).

5. An ozone generator having a power supply device (1) according to any one of the preceding claims.

6. A method for operating a power supply device (1) according to any one of claims 1 to 4 or an ozone generator according to claim 5, wherein - controlling the rectifier (4) to draw energy from the grid (3) at a constant load current (I_grid) and to charge the DC link energy storage device (6) until a preset intermediate circuit voltage (VDC_nom) is reached, -in, The control device monitors the actual intermediate circuit voltage (VDC_act), and -in, The control device allows the load (2) to extract energy from the intermediate circuit energy storage device (6) only when the actual intermediate circuit voltage (VDC_act) is equal to or greater than a preset trigger voltage (VDC_trig).

7. The method according to claim 6, characterized in that The control device regulates and / or sets the load current (I_grid) so that the DC link energy storage device (6) is recharged to a preset DC link voltage (VDC_trig) within a preset duration (dT_min).

8. The method according to claim 6 or 7, characterized in that: Once the control device receives a signal from an input device (i.e. a controller or a load) indicating that energy is no longer to be drawn from the DC link energy storage device (6), the charging process is terminated immediately.

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