Predictive Current Control of Voltage Source Inverters Using a Discrete-Time Model

The paper [9] work presents an inclusive analysis of the OSG approach depending on most recent reference. This structure is used to study the DQ axis separation control, which is normally ignored in single-phase systems but is frequently explored for three-phase systems. We implement and analyze two separation techniques: quasi-compound proportional integral control and feed-forward control of reference current. The proposed theories and control measures were put to the test using the experimental findings.

In the study [10], the integrated (IMPPC) method proposed as a means of reducing steady-state errors in the current control system and enhancing performance of closed-loop. Additionally, compared to the conventional MPCC, Changes in the system's parameters have less of an impact on upgraded IMPCC. The successful development and use of the traditional MPCC and the enhancement of the IMPCC were shown through simulations and experimental findings. According to the experimental findings, the enhanced IMPCC offers the advantages of quick dynamic performance, no steady-state faults, and insensitivity to changes in system parameters.

The paper [11] suggested a new synchronous reference frame (DQ) current controller, for a three-phase grid-connected inverter with a Y/out transformer. The DQ derives a dynamic model of the inverter, filter, and transformer. There is also a description of the present controller's structure. The theoretical study was supported by experimental findings on a 20kW prototype, which also showed that the suggested current controller has high current control performance [11].

In our current study, DQ frame current control based current control of VSI inverter will be used.

Ramp comparison, hysteresis control, and predictive current control are the three primary methods used to regulate current in hard switched inverters. Predictive current control has the ability to provide the most accurate current control with the least amount of distortion and harmonic noise, but it is also the most complicated to install and is often matched to a particular load. In addition, there are additional issues in a real system due to mistakes induced by processing delays [12, 13]. For loads with both known and unknown back-EMF, this work proposes strategies for directly constructing a predictive current controller in a microprocessor. Because of their simplicity, these algorithms may be easily integrated into a working system, mitigating the effects of sampling delays and discretization mistakes [14]. The method given is confirmed by simulation. The majority of inverters used in industry, especially three-phase devices with medium and high power. when attaching single-phase inverter by a third leg, the power circuit of a three-phase VSI is created, shown in Figure 2.

Three switching variables, A, B, and C, will be given to the inverter if previously assumed, just one of the two power switches of each leg of the inverter is constantly run, disregarding the brief time intervals when both switches are off (dead time) [15, 16].

$\left(\begin{array}{l}V_{A B} \\ V_{B C} \\ V_{C A}\end{array}\right)=V_i\left(\begin{array}{ccc}0 & -1 & 0 \\ 0 & 1 & -1 \\ -1 & 0 & 1\end{array}\right)\left(\begin{array}{l}a \\ b \\ c\end{array}\right)$                                    (1)

There are eight states in an inverter, where all output terminals are clamped to the negative DC bus, state 0. And where they are clamped to the positive bus in state 7. Each state of an inverter is denoted by the letters ABC, forming eight states overall [17]. It is simple to demonstrate that VAB, VBC, and VCA are the instantaneous line-to-line output voltages.

2.png

Figure 2. A three-phase voltage source inverter power circuit

Hysteresis regulators is one of the primary classes of regulators. In addition to, linear PI regulators, and predictive dead-beat regulators. That have been created over the past few decades. A succinct overview of the three-phase systems' present control methods. one of the several PWM approaches, hysteresis band current control is often used Due to its clarity of use. Additionally, the rapid response current loop is the only load parameter that the method requires knowledge of. As a result of the need to manage peak-to-peak current ripple throughout the whole fundamental frequency wave. The PWM frequency swings within a band when using a current control with a fixed hysteresis band. In PWM technique with adaptive hysteresis-band current regulation, the band can be adjusted to the load to maximize performance.

The switching signals for Hysteresis current control are obtained by comparing the current error with a preset tolerance band. Based on a comparison between the actual phase current and the tolerance zone surrounding the reference current connected to that phase, this control is implemented [18].

On other hand, the phase current interactions that are frequent in three-phase systems have a negative impact on this form of band management. Since each phase current relies on its corresponding phase voltage as well as the voltage of the other two phases, this is mostly caused by interference between the commutations of the three phases. During the fundamental period, switching frequency may change depending on the load conditions, causing irregular inverter functioning.

3.png

Figure 3. Hysteresis current control schema

4.png

Figure 4. The hysteresis current control scheme in space vector

5.png

Figure 5. Diagram of a proportional and integral controller used to manage current in a three-phase voltage source converter

4.1 Hysteresis regulators

An inverters output currents (iA, iB, and iC) are measured and contrasted with the corresponding reference current waveforms (iA, iB, and iC). Current controllers provide switching variables, A, B, and C, for the inverter by applying current errors, ΔiA, ΔiB, and ΔiC. Figure 3 shows the space vector scheme [19].

Two current controllers can be used in place of three controllers, one controller for each phase of the inverter. the space vector Components (id and iq), and $\vec{i} *$ of output currents are calculated and compared with corresponding components, id and iq, of the reference current vector, $\vec{i} *$ based on the park transformation provided by (2) and applied to the output currents of>an inverter. Figure 4 for the d-component controller shows the three-level outputs of the present controllers, zd and zq. The Table 1 illustrates the Hysteresis current control switching in the below.

Table 1. DQ hysteresis current control switching

For (zd, zq)=(0, 0), state 0 or state 7, “state 1 for (0, 1) and (1, 1), state 2 for (1, 1), state 3 for (1, 0), state 4 for (1, 0), state 5 for (1, 1), and state 6 for (1, 1) and (0, 1). These states produce the best control effects.

$\vec{\imath}=\left[\begin{array}{l}i_d \\ i_q\end{array}\right]=\left[\begin{array}{rrr}1 & -\frac{1}{2} & -\frac{1}{2} \\ 0 & \frac{\sqrt{3}}{2} & -\frac{\sqrt{3}}{2}\end{array}\right]\left[\begin{array}{l}i_A \\ i_B \\ i_C\end{array}\right]$                           (2)

A switching table receives the signals zd and zq and applies them to a particular state of the inverter.

4.2 Linear proportional-integral regulators

Linear control is used by the PI regulator. The input of PI controller is the variation between the reference value and the feedback value. The output is the linear combination of the PI and the deviation. The PI controller is the most frequently used in power electronics due to its simplicity and dependability. The following is an expression for the transfer function of a traditional PI controller in the continuous domain:

$G_{p i}(s)=K_p+\frac{k_i}{s}$                       (3)

where, the proportional and integral benefits, respectively, are denoted by K_pand K_i.

In a power electronic system, a the $G_{p i}(s)=K_p+\frac{k_i}{s}$ reel-phase, two-level VSC is frequently employed and is regarded as the controlled plant. Figure 5 displays the control diagram for the PI controller used to control the current in VSC, where Vcabc is the converter voltage, vdc is the DC voltage, and C is the DC capacitance, and Ugabc is the grid voltage of the point of common coupling, Igabc is the grid current, Zf is the impedance of the filter, which could be a simple L filter or an LCL filter, Zg is the impedance of the weak grid, and Zf represents the impedance of the weak grid.

Three-phase alternate currents in the stationary frame must be converted to DC signals in the synchronous frame since the PI controller can only accomplish a zero steady-state control of the DC signal. As a result, the coordinate transformation and phase-locked loop are essential components of this control system [20].

4.3 Predictive dead-beat regulators

Dead predictive control is a method used for the determination of the necessary control action during each sample period through calculations depends on the circuit model of the controlled system, in contrast to PI controllers that collect integrated errors. For system output values to remain comparable to reference signals, equivalent circuit fidelity is crucial.

When a quick dynamic response is demanded, the deadbeat predictive current control technique is utilized. It is quite simple to program. This control does, however, have certain shortcomings, including model-assumption errors and unintended delays brought on by calculation and modulation time [21].

To achieve good current regulation with little harmonic distortion, algorithms for predictive current regulation have been described that are amenable to practical application in a digital controller. The algorithms account for errors introduced by digital sampling delays and can be used in setups where the back emf is either directly measurable, as in PWM rectifier systems, or indirectly estimated from the measured current change in the previous switching cycles, as in a variable speed motor drive setup.

Can used in either single or three phase inverter systems, are very reliable, and provide accurate current regulation, even when overmodulated or out of tune. The effectiveness of the new algorithms has been shown via both simulation and experiment. The new VSI's predictive current control algorithm is first tested for its efficacy. As discussed in results section, that the higher thyristor is activated when the current flips from negative to positive, whereas the lower thyristor generates its triggering pulse when the current flips from positive to negative. The phase current exhibits a THD of 15% due to the mistake of switching signals computation brought on by the phase delay, constrains in this work its effecting of delaying phase currents have much less distortion at the zero-crossing point to solve this issue should utilizing a new method for predication for reaching to real value for current and voltage has a zero error rate. The main benefits are that the new concept can be used across the board for power conversion, the Z-source inverter is a buck boost inverter (something the more conventional V-source and I source inverters don't offer), it can be used in situations where the input voltage varies widely, all conventional PWM control schemes can be used, and the theoretical input-output relationship remains the same, and the cost is decreased and the efficiency is increased. The average switching device power of a Z-source inverter is lower in the low boost ratio band (1/2). The dc boosted PWM inverter is the optimal setup when operating at a low voltage and requiring a boost ratio greater than 2. The recognized RHP zero in the Z-source impedance network cannot be removed by modifying the Z-source values, which is a drawback. There is a need for more research on compensation strategies. In future directions we will try to predicate the value of current control by using different neural network algorithms and comparison the results with current study.