Technical field

The present disclosure relates to a method of reducing current harmonic distortion for a Vienna or neutral boost PFC rectifier using adaptive correction of a feed-forward signal in a reference tracking control system and a control system. More specifically, the disclosure relates to a method of reducing current harmonic distortion for a Vienna or neutral boost PFC rectifier using adaptive correction of a feed-forward signal in a reference tracking control system and a control system as defined in the introductory parts of claim 1 and claim 9.

Background art

Current harmonic distortion has commonly been reduced using a Vienna rectifier or a NPFC (Neutral boost Power Factor Correction) rectifier, since using a Vienna or NPFC rectifier is a relatively inexpensive way to reduce harmonic content in a power grid.

In normal control systems with nonlinear transfer functions a PI controller must have high gain and bandwidth in order to precisely track a reference. A feed forward signal can lower the burden on the PI controller, but only as long as a calculated feed-forward signal matches the transfer function. The calculation of the feed-forward signal may be very difficult if the transfer function is non linear and if the feed-forward signal is changing depending on external conditions.

Such difficulties often appears in Power Factor Correction (PFC) circuits. The current controller must track a sinusoidal reference and a Pulse Width Modulation (PWM) signal must be created, which through an inductor- capacitor (LC) network creates the desired sinusoidal current. The inductor in the LC network behaves differently depending on the conduction mode. The conduction mode can be either Discontinuous Conducting Mode (DCM) or Continuous Conducting Mode (CCM).

In DCM (indicated in figure 2) the inductor current increases when the active switch in the PFC is on and decreases to zero during the switch’s off-state. When the inductor current has reached zero it stays at zero until the next on- state of the PFC switch.

In CCM (indicated in figure 3) the inductor current increases when the PFC switch is on and decreases, but does not reach zero, during the switch’s off- state. At the next on-state of the PFC switch, the inductor current increases from the level it reached in off-state of the previous switch cycle.

In DCM the maximum current is proportional to the duty-cycle, where in CCM the duty-cycle controls the currents rate of change. This difference changes the transfer function significantly and as it is hard to predict the exact moment of the transitions between the modes, it is also difficult to take this into account in the feed-forward signal.

Some types of inductor have soft saturation. That means the inductor gradually saturates through the working current range. The result is that the inductance depends on the instantaneous current floating through the inductor. This will also reduce the linearity of the transfer function.

The tracking controllers are in many cases implemented in digital components like programmable logic, m-controllers or signal processors (DSP). The control algorithm is programmed into the device and the software describes the functionality of the controller. The controller has inputs and outputs. The inputs are typically connected to voltage and current sensors in the power electronics. The outputs are connected through driver circuits or directly to digital power switches like FETs or IGBTs, which control the current flow.

These systems are digitally sampled systems. The sample rate dictates when the inputs are sampled and after the necessary calculations when the outputs are updated. This adds a delay to the control loop, which together with hardware filters limits the maximum possible loop bandwidth.

A problem with the solutions of the prior art is that such a rectifier is a unidirectional converter which means uncontrollable control sequences can appear due to discontinuous conducting mode. This is more dominant at low power levels, thus making it difficult for a PI (Proportional Integral) controller using normal feed-forward to track the reference and keep current’s total harmonics distortion (THDi) levels under IEEE519 standard. There is thus a need for improved way to deal with such problems.

CN 109831094B describes a method of reducing current harmonic distortion for a Neutral Boost PFC (NPFC) rectifier using adaptive correction of a feed forward signal in a reference tracking control system comprising a feedback controller, where the reference is a repetitive signal,

The present invention deals with how to improve reduction of current harmonic distortion in a Vienna or Boost PFC rectifier.

Summary

It is an object of the present disclosure to mitigate, alleviate or eliminate one or more of the above-identified deficiencies and disadvantages in the prior art and solve at least the above-mentioned problem. According to a first aspect there is provided a method of reducing current harmonic distortion for a Vienna or Neutral Boost PFC rectifier using adaptive correction of a feed-forward signal in a reference tracking control system comprising a feedback controller, where the reference is a repetitive signal, the method comprising the steps of:

- adding a corrective signal from the feedback controller to a memory;

- aligning a number of memory cells in the memory, the memory being a buffer table memory, with a number of PWM cycles during one cycle of the repetitive signal reference; and

- reading data stored in the buffer table memory during the previous reference cycle, and adding the data to the feed-forward signal.

Hereby is achieved a method for correcting errors in signals when the reference is a repetitive signal hereby making it possible to use more simple and not expensive equipment for reduction of current harmonic distortion.

According to some embodiments, the number of memory cells in the buffer table memory is changed to equal the number of memory cells with the number of PWM cycles during one cycle of the repetitive signal reference.

Hereby is achieved that that the data stored in the buffer represents exactly one complete cycle of the reference.

According to some embodiments, the number of PWM cycles during one cycle of the repetitive signal reference is changed to equal the number of PWM cycles with the number of memory cells in the buffer table memory.

Hereby is achieved that the method adapts the number of PWM cycles to the number of cells in the buffer table memory.

According to some embodiments, the data stored in the buffer table memory represents exactly one complete cycle of the repetitive signal reference. Hereby is achieved that the method is adapted to the situation where the data stored in the buffer table memory is aligned with one complete cycle of the repetitive signal reference.

According to some embodiments, a delay in the buffer table memory is calculated as one cycle minus time used for measuring and calculating the data.

Hereby is achieved that the timing is optimized and that the error is corrected at a proper point in time

According to some embodiments, the method is carried out in discontinuous conducting mode (DCM).

An advantage with the present disclosure is that using discontinuous conducting mode at lower power levels where a PI controller is not able to track it completely, using this new adaptive feed-forward method helps to overcome the limitation of a Vienna or NPFC Rectifier, keeps THDi levels low at low power and also keeping production cost low.

According to some embodiments, the method is carried out in continuous conducting mode (CCM).

An advantage with the present disclosure is that using continuous conducting mode at high power levels, using this new adaptive feed-forward method helps to overcome the limitation of a Vienna or NPFC Rectifier, keeps THDi levels low at high power and also keeping production cost low.

According to some embodiments of the method, there is a switching between DCM and CCM due to the currents varying with the mains frequency, Hereby is achieved that DCM is dominating at low power and CCM is dominating at high power, causing the effect of the correction of the feed forward signal gradually decreasing with increasing power.

According to some embodiments, the method is carried out at low power levels.

Hereby is achieved that even though low power levels provides great harmonics distortion and difficulties to control with PI controllers, the present method makes it possible to reduce harmonic distortion significantly.

According to a second aspect there is provided a control system comprising a rectifier of a Vienna or neutral boost PFC type, and a reference tracking control system further comprising: - a PI controller with an adaptive correction of a feed forward signal and a feedback controller, where an adaptive correction circuit comprises a buffer table memory, where a number of memory cells in the buffer table memory equals a number of PWM cycles during one cycle of the reference; - at least one tracking controller having an input and an output, which tracking controller is configured for controlling a current flow in a sample system where a sample rate dictates when inputs are sampled and when outputs are updated following calculations performed in the controller.

Hereby is achieved an adaptive system for correcting errors in signals when the reference is a repetitive signal hereby making it possible to use more simple and not expensive equipment for reduction of current harmonic distortion.

According to some embodiments of the second aspect, the buffer table memory is adapted to equal the number of memory cells with the number of PWM cycles during one cycle of the repetitive signal reference. Hereby is achieved that the system is adapted to the situation where the data stored in the buffer table memory is aligned with one complete cycle of the repetitive signal reference.

Effects and features of the second aspect are to a large extent analogous to those described above in connection with the first aspect. Embodiments mentioned in relation to the first aspect are largely compatible with the second aspect.

The present disclosure will become apparent from the detailed description given below. The detailed description and specific examples disclose preferred embodiments of the disclosure by way of illustration only. Those skilled in the art understand from guidance in the detailed description that changes and modifications may be made within the scope of the disclosure.

Hence, it is to be understood that the herein disclosed disclosure is not limited to the particular component parts of the device described or steps of the methods described since such device and method may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It should be noted that, as used in the specification and the appended claim, the articles "a", "an", "the", and "said" are intended to mean that there are one or more of the elements unless the context explicitly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps.

Terminology -The term “buffer table memory” is to be interpreted as a memory acting as an adaptive table where a number of memory cells in the buffer table memory is adjusted during each cycle to match the number of values to be stored in the table at each cycle. The term " low power level" is to be interpreted as being 30% or less in relation to maximum power level and the term “high power level” is to be interpreted as being 70% or more in relation to maximum power level.

Brief descriptions of the drawings

The above objects, as well as additional objects, features and advantages of the present disclosure, will be more fully appreciated by reference to the following illustrative and non-limiting detailed description of example embodiments of the present disclosure, when taken in conjunction with the accompanying drawings.

Figure 1 shows schematically a block diagram of a control part of a PFC;

Figure 2 shows an example of behaviour of an inductor current in discontinuous conducting mode (DCM);

Figure 3 shows an example of behaviour of an inductor current in continuous conducting mode (CCM);

Figure 4 shows a feed-forward reference tracking control system;

Figure 5 shows a feed-forward reference tracking control system with adaptive correction according to an embodiment of the present disclosure;

Figure 6 shows an example of an output with low power and without correction;

Figure 7 shows an example of an output with low power and with feed-forward correction;

Figure 8 shows an example of an output with high power and without correction; and Figure 9 shows an example of an output with high power and with feed-forward correction.

Detailed description

The present disclosure will now be described with reference to the accompanying drawings, in which preferred example embodiments of the disclosure are shown. The disclosure may, however, be embodied in other forms and should not be construed as limited to the herein disclosed embodiments. The disclosed embodiments are provided to fully convey the scope of the disclosure to the skilled person.

Figure 1 shows a block diagram showing a control part of a PFC, here in relation to a Vienna or Neutral Boost PFC (NPFC) Rectifier which uses adaptive correction of a feed-forward signal in a reference tracking control system.

An aspect discloses an Adaptive Feed-Forward Correction for Vienna or NPFC Rectifier to reduce current harmonic distortion at low power levels.

This method makes it possible to work-around well-known limitations of a Vienna or Neutral Boost PFC (NPFC) Rectifier using adaptive correction of a feed-forward signal in a reference tracking control system 2.

In a control system 2 shown in figure 4, an actual measured value is subtracted from a reference forming an error signal. In sampled systems, this error signal is basis for correction of a following cycle. The error signal is an input to the feedback controller 3. An output of the feedback controller 3 is added to the feed-forward signal, forming an input to a Pulse Width Modulation (PWM) and power stage 9. In case the reference input is a repetitive signal e.g. a sinusoidal grid voltage, an amplitude level is changing all the time. As long as a transfer function for a load (PFC-choke) P _r (0) has a linear transfer function, a transfer function k _ff (0) for the feed-forward signal is easily calculated and the error at the input of the feedback controller will be almost zero. When P _r (0) is non linear, it is almost impossible to calculate the feed-forward transfer function k _ff (0), as the nonlinearities depend on conditions like temperature, voltage, current and used inductors actual parameters. This causes that the output does not track the reference and an error is feed into the feedback controller. The feedback controller 3 will partly correct the error in the following PWM cycle, but only partly due to lack of gain and bandwidth. Furthermore, the correction will be delayed at least one PWM cycle.

By the new adaptive system as shown in figure 5, the controller is improved, in relation to the reference being a repetitive signal. When errors like the above described occur, the corrective signal from the feedback controller 3 is added to a memory 4. The memory 4 is a buffer table memory, where the number of memory cells in the buffer table memory equals the number of PWM cycles during one cycle of the reference.

This means that if the reference frequency changes, either the number of memory cells in the buffer memory 4 must be changed or the PWM frequency must be changed. Important is that the data stored in the buffer memory 4 represents exactly one complete cycle of the reference.

By reading the data which was stored in the previous reference cycle and adding it to the feed-forward signal, the error in the control system 2 is reduced. The delay in the buffer memory 4 must be one cycle minus time used for measuring and calculating the data. Flereby it is secured that the error correction signal will be inserted where the error is expected to appear, and thereby eliminating or at least reducing the error. If the error is not completely eliminated, the residual error is measured and will be added to the data in the buffer memory 4. The new and more precise data, which now is in the buffer memory 4, will be used in the next cycle of the reference.

As each cell of table data is updated by adding the corrective signal from the feedback controller 3, the data in the table 4 improves after each cycle of the reference signal. As long as the repetitive reference signal is unchanged, cycle after cycle, the errors will decrease towards zero.

When the reference signal changes, new data is added to the table 4, so that the content of the table 4 is continuously updated to decrease the error level.

The figures below show improvements using adaptive feed-forward correction. The measurements are made using clean power supply. The more sinusoidal the current waves, the lesser THDi levels.

Figures 6 and 7 shows the output signals before and after adaptive feed forward correction at 0.5 kW of power, respectively. From figure 6 it can be observed that low power provides great harmonics distortion and difficult to control with PI controllers.

Figures 8 and 9 shows the output signals before and after adaptive feed forward correction at 4.0 kW of power, respectively.

The first aspect of this disclosure shows a method of reducing current harmonic distortion for a Vienna or Neutral Boost PFC (NPFC) rectifier 1 using adaptive correction of a feed-forward signal in a reference tracking control system 2 comprising a feedback controller 3, where the reference is a repetitive signal, the method comprising the steps of: - adding a corrective signal from the feedback controller 3 to a memory 4; - aligning or equalling a number of memory cells in the memory, being a buffer table memory 4, with a number of PWM cycles during one cycle of the repetitive signal reference; and - reading data stored in the buffer table memory 4 during the previous reference cycle, and adding the data to the feed-forward signal.

In an embodiment of the aspect, the number of cells in the buffer table 4 is changed to equal the number of memory cells with the number of PWM cycles during one cycle of the repetitive signal reference.

In an embodiment of the aspect, the number of PWM cycles during one cycle of the repetitive signal reference is changed to equal the number of PWM cycles with the number of memory cells in the buffer table memory 4.

In an embodiment of the aspect, the data stored in the buffer table memory 4 represents exactly one complete cycle of the repetitive signal reference.

In an embodiment of the aspect, a delay in the buffer table memory 4 is calculated as one cycle minus time used for measuring and calculating the data.

In an embodiment of the aspect, the method is carried out in discontinuous conducting mode DCM.

In an embodiment of the aspect, the method is carried out in continuous conducting mode CCM.

In an embodiment of the aspect, the method is carried out at low power levels.

The second aspect of this disclosure shows a control system comprising a rectifier of a Vienna or neutral boost PFC (NPFC) type 1 , and a reference tracking control system further comprising: - a PI controller with an adaptive correction of a feed forward signal and a feedback controller, where an adaptive correction circuit 5 comprises a buffer table memory 4, where a number of memory cells in the buffer table memory equals a number of PWM cycles during one cycle of the reference; - at least one tracking controller 6 having an input and an output, which tracking controller 6 is configured for controlling a current flow in a sample system where a sample rate dictates when inputs are sampled and when outputs are updated following calculations performed in the controller 6.

In an embodiment of the second aspect, the buffer table memory 4 is adapted to equal the number of memory cells with the number of PWM cycles during one cycle of the repetitive signal reference.

As an example, the PWM frequency can be 40 kHz and the repetitive signal reference is the mains frequency of the voltage supply (50 Hz in Europe and 60 Hz in USA).

Dividing the mains frequency up in the PWM frequency, you will get the number of memory cells required in the buffer table memory per cycle of the repetitive signal reference and each value in the table is adjusted 50 times per second (once for each cycle of the repetitive signal reference), which in this example will be 800 for Europe (50 Hz).

The period times can also be specified as 20 ms (50 Hz) and 25 ps (40 kHz) respectively.

The person skilled in the art realizes that the present disclosure is not limited to the preferred embodiments described above. The person skilled in the art further realizes that modifications and variations are possible within the scope of the appended claims. Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed disclosure, from a study of the drawings, the disclosure, and the appended claims.