I would like to thank all the people and institutions who have contributed directly and indirectly to this work. Tasmia Baten, tutor of this thesis: it would not have been possible to complete this work without his invaluable guidance, advice and support. Grid-connected photovoltaic (PV) systems supply electricity directly to the electrical grid, operating in parallel with the conventional source.
This paper deals with the design and simulation of a three-phase inverter in the MATLAB SIMULINK environment, which can be part of photovoltaic grid-connected systems.
Abbreviations
Introduction
Literature Review
D-Q transformation
- Abc_to_dq0 Transformation
- Dq0_to_abc Transformation
- Phase lock loop (PLL)
- Advantage of D-Q transformation
This allows you to eliminate time-varying inductances by referencing the stator and rotor quantities to a fixed or rotating reference frame. Id and Iq represent the two DC currents flowing in the two equivalent rotor windings (d winding directly on the same axis as the field winding, and q winding on the quadrature axis), producing the same flux as the stator Ia, Ib and Ic currents. The dq0_to_abc Transformation block performs the inverse of the so-called Park transformation, which is commonly used in three-phase electric machine models.
It transforms three quantities (direct axis, quadratic axis, and zero-sequence components) expressed in a two-axis reference frame back into phase quantities. The dq0_to_abc transformation block is used in the synchronous machine block model where the stator quantities refer to the rotor. The Park transformation then eliminates the time-varying inductances by referencing the stator and rotor quantities to a fixed or rotating reference frame.
The Id and Iq currents represent the two DC currents flowing in the two equivalent rotor windings (d winding on the same axis as the field winding, and q winding in quadratic) and produce the same flux as the stator Ia, Ib and Ic -flow. The DC component of the mixed signal (proportional to the phase difference between these two signals) is extracted with a variable frequency average value.
Control unit
Design of alternative current controller
P-I controller
However, in terms of the speed of the response and overall stability of the system, it has a negative impact. This controller is mostly used in areas where the speed of the system is not an issue. The combination of proportional and integral terms is important to increase the speed of the reaction and also to eliminate the steady state error.
A PID controller works by correcting the error between a measured process variable and a desired set point by calculating and then executing a corrective action that can adjust the process accordingly - and quickly - to keep the error to a minimum .
Negative feedback
Negative feedback is widely used in mechanical and electronic engineering, but it also occurs naturally in living organisms and can be observed in many other fields from chemistry and economics to physical systems such as climate.
Inverter
Working principle of inverter
- Active switching state
- Zero switching state
In a three-phase inverter, we have three lags (lag a, lag b, lag c) and we have three switches. So when the switches are on or off, we have different voltages at terminal a, b, c with respect to neutral point. With different switching conditions and also look at the voltage waveform when the switching frequency is low.
In switch state 111, all of the above switches are on and interconnected.
Instrument
When the input signal is within the range defined by the lower limit and upper limit parameters, the input signal is passed through unchanged. When the input signal is outside these limits, the block clamps the signal to the upper or lower limit. ABC_to_DQ0 transform: Perform the Park transform from the three-phase reference frame (abc) to the dq0 reference frame.
The abc_to_dq0 transform block calculates the direct axis, quadratic axis, and zero-sequence quantities in a two-axis rotating reference frame for a three-phase sinusoidal signal. The output of the PLL is coupled to the input port of abc to dq0 and dq0 to the abc block. The PI voltage controller is used to generate the Id reference and reduce the steady state error.
Figure 4.3 shows the input voltage waveform of the PI voltage regulator and Figure 4.4 shows the output voltage of the PI voltage regulator. Then the actual Id current, which is a negative feedback generated by the abc_to_dq0 block, is compared with the Id reference and then put into the PI current controller. The PI current controller reduces the steady state error and this is the reason to use the PI controller.
Current Id and current Iq are transformed into three references abc by the dq0_to_abc transformation block. Fig-4.9 shows the waveform before the multiplexer and Fig-4.10 shows the waveform of the signal after the multiplexer. We connected a three-phase V-I meter block after the inverter for measuring three-phase voltage and current.
The Abc_to_dq0 transformation block is used after the converter to perform the Park transformation from the three-phase (abc) reference frame to the dq0 reference frame. The output of this Id and Iq block is connected to two PI current controllers. A phase loop is used to generate the angular frequency, which is connected to two abc_to_dq0 blocks and a dq0_to_abc block.
A DC/AC converter is used to convert the DC voltage of a PV system into AC voltage. The output voltage of the inverter is pure sine wave and the output current of the inverter should be continuous.
