CN114285311B - Inverter digital control method based on COT and CFT mixing - Google Patents

Abstract

The invention discloses an inverter digital control method based on a mixture of COT and CFT. This control method uses hybrid hysteresis current control of DCFT and DCOT in the positive and negative half-cycles of the power frequency of the inverter output to avoid the impact of the extremely high digital sampling frequency when the inductor current switching period change rate is large. Dependence, thereby improving the current tracking accuracy under high voltage modulation ratio conditions. At the same time, by adjusting the timing time in real time in the positive and negative half cycles, the control capability of the switching frequency can be greatly improved under high voltage modulation ratio conditions.

Description

An inverter digital control method based on hybrid COT and CFT

Technical field

The invention belongs to the field of power electronics, and specifically relates to an inverter digital control method based on a mixture of COT and CFT.

Background technique

In the context of the global vigorous development and popularization of renewable energy in recent years, new energy photovoltaic systems have developed rapidly. The core component of the photovoltaic system is the inverter, which converts the direct current output from the photovoltaic array into the power frequency alternating current commonly used in daily life. Currently, photovoltaic systems are constantly seeking greater informatization and digitization. Inverter control methods are also constantly seeking to replace analog control with digital control to simplify the control system, enhance system reliability, reduce hardware costs and maintenance costs, etc.

Hysteresis current control has become one of the most commonly used control methods for inverters due to its simple and easy implementation, excellent stability, and fast dynamic response. However, traditional digital hysteresis current control cannot ensure that the inverter operates at high modulation. The tracking accuracy of the current and the constraints on the switching frequency under certain conditions. This is because at a certain sampling frequency, the greater the switching cycle change rate of the inductor current, the greater the sampling error. At the same time, limited by the sampling frequency, the upper limit of the switching frequency at the maximum amplitude of the inverter output will also be limited, thereby affecting the quality of the inverter output voltage. To address this problem, commonly used methods are to increase the sampling frequency, reduce the algorithm execution time, and increase the filter inductor and capacitor values. However, these methods all come at the expense of increased software and hardware costs.

Contents of the invention

In view of the shortcomings of the existing technology, the present invention proposes an inverter digital control method based on a mixture of COT and CFT. This method is based on a single-phase inverter digital hysteresis control method based on a mixture of COT and CFT to improve high modulation engineering. The tracking accuracy of current and the control of switching frequency under certain conditions.

An inverter digital control method based on a mixture of COT and CFT, which can be used for both grid-connected inverters and off-grid inverters. The inverter is a full-bridge inverter, including bridge arms Q ₁ to Q ₄ and an LC filter circuit. One end of the DC power supply on the input side is connected between bridge arms Q ₁ and Q ₂ , and the other end is connected to bridge arm Q ₃ and Q _4. After the output voltage is drawn from between the bridge arms Q ₁ and Q ₃ and between the bridge arms Q ₂ and Q ₄ , it is connected to the load through the LC filter circuit. The control system used includes voltage sensors, current sensors, analog-to-digital converters, digital PI controllers, DCOT/DCFT modules and drive circuits. The analog-to-digital converter, digital PI controller, and DCOT/DCFT module are implemented by a digital processor DSP;

The method specifically includes the following steps:

Step 1. Use current sensors and voltage sensors to collect the current flowing through the inductor in the LC filter circuit and the output voltage u _o across the load.

Step 2: Input the inductor current and output voltage obtained in Step 1 into the analog-to-digital converter for analog-to-digital conversion and sampling.

Step 3. Calculate the error value between the sampled output voltage value u _s and the output voltage reference value u _ref , and input the error value into the digital PI controller to obtain the inductor current reference value i _Lref .

Step 4. Set the inductor current deviation value ΔI, and calculate the inductor current reference upper limit value i _Lref+ and the inductor current reference lower limit value i _Lref- based on the inductor current reference value i _Lref obtained in step 3.

Preferably, the inductor current difference ΔI is a fixed value, or is calculated through a hysteresis current control constant frequency algorithm:

△I＝(U _d ² -u _o ² )/4L _N f _s ^* U _d

Where U _d is the input voltage, u _o is the inverter output voltage, f _s ^* is the given switching frequency, and L _N is the standard inductance value.

Step 5. Use the zero-crossing comparator to detect the zero-crossing point of the voltage reference value u _ref and switch the inverter control mode:

s5.1. When the voltage reference value u _ref is negative, the COT digital control mode is used, specifically: when the inductor current value i _L sampled in step 2 is less than the inductor current reference lower limit value i _Lref- , control the bridge arm Q _1. Q ₄ is turned on, Q ₂ and Q ₃ are turned off, and the timing T _on time starts; when the timing T _on ends, the control bridge arms Q ₁ and Q ₄ are turned off, and Q ₂ and Q ₃ are turned on;

s5.2. When the voltage reference value u _ref is positive, the CFT digital control mode is used, specifically: when the inductor current value i _L sampled in step 2 is greater than the inductor current reference upper limit value i _Lref+ , control the bridge arm Q ₁ , Q ₄ is turned off, Q ₂ and Q ₃ are turned on, and the timing T _off time starts; when the timing T _off ends, the control bridge arms Q ₁ and Q ₄ are turned on, and Q ₂ and Q ₃ are turned off.

The present invention has the following beneficial effects

This method can greatly reduce the dependence on the digital sampling frequency under high modulation conditions, thereby improving the current tracking accuracy under high modulation conditions. At the same time, by adjusting the timing time of the DSP timer in real time, the quasi-constant frequency function of hysteresis current control can be realized. By controlling the timing time near the peak-to-peak value of the inverter output voltage to be very small, the switching frequency here can be increased. is very high, which increases the freedom of switching frequency control.

Description of the drawings

Figure 1 is the schematic diagram of constant on-time modulation;

Figure 2 is the schematic diagram of constant off-time modulation;

Figure 3 is a schematic diagram of a single-phase full-bridge inverter circuit based on hybrid digital hysteresis current control of COT and CFT;

Figure 4 is a structural block diagram of hybrid digital hysteresis current control based on COT and CFT;

Figure 5 shows the inductor current waveform based on hybrid digital hysteresis current control of COT and CFT;

Figure 6 is a schematic diagram of traditional digital hysteresis current control.

Detailed ways

The present invention will be further explained below in conjunction with the accompanying drawings; however, the protection scope of the present invention is not limited thereto. It should be pointed out that if there are processes or symbols that are not specifically described in detail, those skilled in the art can understand and implement them by referring to the existing technology.

This embodiment takes bipolar SPWM control technology as an example, that is, among the four bridge arms, Q ₁ and Q ₄ act synchronously, Q ₂ and Q ₃ act synchronously and are complementary to Q ₁ and Q ₄ .

The control principle diagrams of the COT (Constant On-Time, constant switch on-time) and CFT (Constant Off-Time, constant switch off-time) are shown in Figures 1 and 2. In COT mode, the current reference lower limit value is used as the comparison signal. When the sampling current is less than the current reference lower limit value, the flip-flop Q outputs a high level to turn on Q ₁ and Q ₄ , and the timing t _on time is started at the same time. At the end of the t _on time, the flip-flop Q output is low, and the inductor current decreases. In CFT mode, the current reference upper limit value is used as the comparison signal. When the sampling current is greater than the current reference upper limit, the flip-flop Q outputs a low level to turn off Q ₁ and Q _4. At the same time, the timing t _off time starts, and the timing t _off At the end of the time, the flip-flop Q outputs a high level, and the inductor current rises.

The basic circuit diagram of the control system used in this embodiment is shown in Figure 3. The midpoint of the upper and lower bridge arms of the inverter is connected to the LC filter. Loads are connected in parallel at both ends of the filter capacitor. The current sensor samples the current and voltage of the filter inductor. The sensor samples the voltage of the output load. After the signal is conditioned, it is input to the ADC module, PI controller and DCOT/DCFT module implemented by DSP. The control signal obtained by the internal calculation of DSP is connected to the drive circuit through the GPIO port of DSP, and then drives the switch. Q ₁ , Q ₂ , Q ₃ and Q ₄ actions. The control structure block diagram is shown in Figure 4. For the convenience of analysis, the devices in the circuit structure are all ideal devices.

As shown in Figure 5, when Q ₁ and Q ₄ are turned on, the inductor current i _L increases, and when Q ₂ and Q ₃ are turned on, the inductor current i _L decreases. After sampling the inverter output voltage u _o , ADC conversion is performed to obtain u _s , and an error signal is generated with the output voltage reference value u _ref and sent to the digital PI controller. The inductor current reference value i _Lref is obtained and sent to the DCOT/DCFT module. The DCOT/DCFT module is internally divided into a CFT module and a COT module. The CFT module and the COT module are in an interlocking relationship. The direction of the inverter output voltage is determined through the zero-crossing detector.

When the inverter output voltage is in the positive half cycle, the CFT module logic is executed, and the inductor current reference upper limit i _Lref+ in the positive half cycle is i _Lref +ΔI. When the inductor current sampling value i _L ≥ i _Lref+ , Q ₁ and Q ₄ are turned off and Q ₂ and Q ₃ are turned on. At the same time, the timing module starts timing T _off . At this time, it is in a switching state with a large change rate of the inductor current. The inductor current needs to be sampled. When the timing of the digital controller reaches T _off , Q ₁ and Q ₄ are turned on and Q ₂ and Q ₃ are turned off. At this time, it enters the switching state with a small change rate of the inductor current. It is necessary to start sampling the inductor current and judge whether to turn off again. Cut off Q ₁ and Q ₄ . Until the state of the zero-crossing comparator flips and enters the negative half cycle of the inverter output.

When in the negative half cycle of the inverter output, the COT module logic is executed. The lower reference limit of the inductor current in the negative half cycle i _Lref- is i _Lref -ΔI. When the inductor current sampling value i _L ≤ i _Lref- , Q ₁ and Q ₄ are turned on and Q ₂ and Q ₃ are turned off. At the same time, the timing module starts timing T _on . At this time, it is a switching state with a large change rate of the inductor current. There is no need to sample the inductor current. When the timing of the digital controller reaches T _on , Q ₁ and Q ₄ are turned off and Q ₂ and Q ₃ are turned on. At this time, it enters the switching state with a small change rate of the inductor current. It is necessary to start sampling the inductor current and judge again whether it is conductive. Pass Q ₁ , Q ₄ . Until the state of the zero-crossing comparator flips and enters the positive half cycle of the inverter output. The inverter circuit continuously operates in COT mode and CFT mode.

When using this method, the duty cycle of the inverter output is greater than 0.5 when the inverter output is in the positive half cycle, and the duty cycle is less than 0.5 in the negative half cycle. When the inverter output voltage is at the maximum value of the positive half cycle under high regulation conditions, the duty cycle value approaches 1. At this time, the rising slope of the inductor current is relatively gentle when the switch is turned on, while the inductor current decreases when it is turned off. The slope is very steep. Similarly, when the inverter output voltage is at the maximum value of the negative half cycle under high regulation conditions, the duty cycle value approaches 0. At this time, the slope change of the inductor current is exactly opposite to that of the positive half cycle. The traditional digital hysteresis current sampling control principle diagram is shown in Figure 6. It can be seen that the sampling error increases when the inductor current change rate is large. Obviously, the sampling error in the switching state with a small change rate of the inductor current is smaller than that in the switching state with a large change rate of the inductor current. Therefore, when the inverter output is in the positive half cycle and the duty cycle is greater than 0.5, the change rate of the inductor current in the switch on state is small but the change rate in the off state is large. CFT technology can be used to sample only the current in the on state. The shutdown state is controlled by timing. On the contrary, the negative half-cycle duty cycle of the inverter output is less than 0.5. The change rate of the inductor current in the off-state of the switch is small but the change rate in the on-state is large. COT technology can be used to sample only the current in the off-state and the on-state. Timing control is also used.

It should be noted that the above embodiments are only used to illustrate the present invention and do not limit the technical solutions described in the present invention. At the same time, although this specification has described the present invention in detail with reference to the above embodiments, those of ordinary skill in the art It should be understood that the present invention can still be modified or equivalently substituted; therefore, all technical solutions and improvements that do not depart from the spirit and scope of the present invention should be covered by the protection scope of the appended claims of the present invention.

Claims (8)

1. An inverter digital control method based on mixing of COT and CFT, wherein COT is constant switch on time, CFT is constant switch off time, and the inverter is a full-bridge inverter comprising a bridge arm Q ₁ ～Q ₄ And an LC filter circuit, wherein one end of the DC power supply on the input side is connected with the bridge arm Q ₁ 、Q ₂ The other end is connected with the bridge arm Q ₃ 、Q ₄ Between, output voltage is from bridge arm Q ₁ 、Q ₃ Between and bridge arm Q ₂ 、Q ₄ After being led out, the power supply is connected to a load through an LC filter circuit; the method is characterized in that: the method specifically comprises the following steps:

step 1, collecting current flowing through an inductor and output voltage u at two ends of a load in an LC filter circuit _o ；

Step 2, carrying out analog-to-digital conversion on the inductance current and the output voltage obtained in the step 1 and sampling;

step 3, calculating the sampled output voltage value u _s And output voltage reference u _ref The error value is input into a digital PI controller to obtain an inductance current reference value i _Lref ；

Step 4, setting an inductance current deviation value delta I, and according to the inductance current reference value I obtained in the step 3 _Lref Calculating the upper limit value i of the inductor current reference _Lref+ With the inductor current reference lower limit i _Lref- ；

Step 5, according to the output voltage reference value u _ref Determining an inverter control mode:

s5.1 when the voltage reference value u _ref If negative, adopting a COT digital control mode, specifically: when the inductance current value i obtained by sampling in the step 2 _L Less than inductor current referenceLower limit value i _Lref- At the time, control arm Q ₁ 、Q ₄ Conduction, Q ₂ 、Q ₃ Turn off and start timing T _on Time; when timing T _on At the end, control arm Q ₁ 、Q ₄ Turn off, Q ₂ 、Q ₃ Conducting;

s5.2 when the voltage reference value u _ref If yes, adopting a CFT digital control mode, specifically: when the inductance current value i obtained by sampling in the step 2 _L Is larger than the reference upper limit value i of the inductance current _Lref+ At the time, control arm Q ₁ 、Q ₄ Turn off, Q ₂ 、Q ₃ Turned on and start timing T _off Time; when timing T _off At the end, control arm Q ₁ 、Q ₄ Conduction, Q ₂ 、Q ₃ And (5) switching off.

2. The digital control method for the inverter based on the mixture of COT and CFT according to claim 1, wherein: the inductance current deviation value is calculated delta I according to a hysteresis control constant frequency algorithm:

wherein U is _d For input voltage u _o For the inverter output voltage, f _s ^* For a given switching frequency, L _N Is the standard inductance value.

3. The digital control method for the inverter based on the mixture of COT and CFT according to claim 1, wherein: the control system used by the method comprises a voltage sensor, a current sensor, an analog-to-digital converter, a digital PI controller, a DCOT/DCFT module and a driving circuit.

4. A digital control method for an inverter based on a mixture of COT and CFT according to claim 1 or 3, wherein: the analog-to-digital converter, the digital PI controller and the DCOT/DCFT module are realized by a digital signal processor DSP;the current sensor and the voltage sensor are used for collecting current flowing through the inductor and output voltage u across the load in the LC filter circuit _o The input analog-to-digital converter performs analog-to-digital conversion and sampling; the DCOT/DCFT module detects the voltage reference value u through the zero-crossing comparator _ref The zero crossing point of the COT digital control mode and the CFT digital control mode is realized; the driving circuit is used for switching the switching state of the bridge arm according to the output signal of the DCOT/DCFT module.

5. The digital control method for the inverter based on the mixture of COT and CFT according to claim 1, wherein: at the timing T of step 5 _on Or timing T _off The corresponding switching action judgment is not needed according to the sampling value of the inductance current in the duration time period.

6. The digital control method for the inverter based on the mixture of COT and CFT according to claim 1, wherein: by dynamically adjusting the timing T _on Or timing T _off The quasi-constant frequency control of the bridge arm switching frequency is realized.

7. The digital control method for the inverter based on the mixture of COT and CFT according to claim 1, wherein: by reducing the timing T when the inverter is at a high voltage modulation ratio output _on Or timing T _off The duration of the bridge arm switch is maintained in a high-frequency switch switching state, and the accurate control of the switching frequency under the high-voltage modulation ratio is realized.

8. The digital control method for an inverter based on mixing of COT and CFT according to any one of claims 1, 2, 3, 5, 6 or 7, wherein: the method is used for controlling the off-grid inverter or the grid-connected inverter.