Editing Power inverter (section)

===Advanced designs===
[[File:H-bridge inverter cjc.png|thumb|upright=0.5|[[H-bridge]] inverter circuit with transistor switches and antiparallel diodes]]
There are many different power [[circuit topology (electrical)|circuit topologies]] and [[control system|control strategies]] used in inverter designs.<ref>{{cite conference |last1=Unruh |first1=Roland |title=Evaluation of MMCs for High-Power Low-Voltage DC-Applications in Combination with the Module LLC-Design |book-title=22nd European Conference on Power Electronics and Applications (EPE'20 ECCE Europe) |date=Oct 2020 |pages=1–10 |isbn=978-9-0758-1536-8 |s2cid=222223518 |doi=10.23919/EPE20ECCEEurope43536.2020.9215687 |url=https://ieeexplore.ieee.org/document/9215687 |url-access=subscription}}</ref> Different design approaches address various issues that may be more or less important depending on the way that the inverter is intended to be used. For example, an electric motor in a car that is moving can turn into a source of energy and can, with the right inverter topology (full H-bridge) charge the car battery when decelerating or braking. In a similar manner, the right topology (full H-bridge) can invert the roles of "source" and "load", that is, if for example the voltage is higher on the AC "load" side (by adding a solar inverter, similar to a gen-set, but solid state), energy can flow back into the DC "source" or battery.

Based on the basic [[H-bridge]] topology, there are two different fundamental control strategies called basic frequency-variable bridge converter and PWM control.<ref>{{cite book |title=Principles of Power Electronics |date=1991 |isbn=978-0201096897 |pages=169–193 |last1=Kassakian |first1=John G. |publisher=Addison-Wesley}}</ref> Here, in the left image of H-bridge circuit, the top left switch is named as "S1", and others are named as "S2, S3, S4" in counterclockwise order.

For the basic frequency-variable bridge converter, the switches can be operated at the same frequency as the AC in the electric grid. However, it is the rate at which the switches open and close that determines the AC frequency. When S1 and S4 are on and the other two are off, the load is provided with positive voltage and vice versa. We could control the on-off states of the switches to adjust the AC magnitude and phase. We could also control the switches to eliminate certain harmonics. This includes controlling the switches to create notches, or 0-state regions, in the output waveform or adding the outputs of two or more converters in parallel that are phase shifted in respect to one another.

Another method that can be used is PWM. Unlike the basic frequency-variable bridge converter, in the PWM controlling strategy, only two switches S3, S4 can operate at the frequency of the AC side or at any low frequency. The other two would switch much faster (typically 100&nbsp;kHz) to create square voltages of the same magnitude but for different time duration, which behaves like a voltage with changing magnitude in a larger time-scale.

These two strategies create different harmonics. For the first one, through Fourier Analysis, the magnitude of harmonics would be 4/(pi*k) (k is the order of harmonics). So the majority of the harmonics energy is concentrated in the lower order harmonics. Meanwhile, for the PWM strategy, the energy of the harmonics lie in higher-frequencies because of the fast switching. Their different characteristics of harmonics leads to different THD and harmonics elimination requirements. Similar to "THD", the conception "waveform quality" represents the level of distortion caused by harmonics. The waveform quality of AC produced directly by H-bridge mentioned above would be not as good as we want.

The issue of waveform quality can be addressed in many ways. [[Capacitor]]s and [[inductor]]s can be used to [[electronic filter|filter]] the waveform. If the design includes a [[transformer]], filtering can be applied to the primary or the secondary side of the transformer or to both sides. [[Low-pass filter]]s are applied to allow the fundamental component of the waveform to pass to the output while limiting the passage of the harmonic components. If the inverter is designed to provide power at a fixed frequency, a [[resonance|resonant]] filter can be used. For an adjustable frequency inverter, the filter must be tuned to a frequency that is above the maximum fundamental frequency.

Since most loads contain inductance, feedback [[rectifier]]s or [[antiparallel (electronics)|antiparallel]] [[diode]]s are often connected across each [[semiconductor]] switch to provide a path for the peak inductive load current when the switch is turned off. The antiparallel diodes are somewhat similar to the ''[[flyback diode|freewheeling diodes]]'' used in AC/DC converter circuits.

{{Clear}}
{| class="wikitable" style="float:right; width:100px;"
|-
! Waveform
! Signal<br>transitions<br>per period
! Harmonics<br>eliminated
! Harmonics<br>amplified
! System<br>description
! [[Total harmonic distortion|THD]]
|-
| [[File:Square wave.PNG|160px]]
|  2 ||	 	            ||            || 2-level<br>square wave        || ~45%<ref name=Hahn/>
|-
| [[File:Sqarish wave, 3 level.PNG|160px]]
|  4 || 3, 9, 27, ...	||            || 3-level<br>modified sine wave || >23.8%<ref name=Hahn/> 
|-
| [[File:Sqarish wave, 5 level.png|160px]]
|  8 ||                 ||            || 5-level<br>modified sine wave || >6.5%<ref name=Hahn/>
|-
| [[File:Pwm 3rd and 5th harmonic removed, 2 level.PNG|160px]]
| 10 || 3, 5, 9, 27	    || 7, 11, ... || 2-level<br>very slow PWM      ||
|-
| [[File:Pwm 3rd and 5th harmonic removed, 3 level.PNG|160px]]
| 12 || 3, 5, 9, 27	    || 7, 11, ... || 3-level<br>very slow PWM      ||
|}

Fourier analysis reveals that a waveform, like a square wave, that is anti-symmetrical about the 180 degree point contains only odd harmonics, the 3rd, 5th, 7th, etc. Waveforms that have steps of certain widths and heights can attenuate certain lower harmonics at the expense of amplifying higher harmonics. For example, by inserting a zero-voltage step between the positive and negative sections of the square-wave, all of the harmonics that are divisible by three (3rd, 9th, etc.) can be eliminated. That leaves only the 5th, 7th, 11th, 13th, etc. The required width of the steps is one third of the period for each of the positive and negative steps and one sixth of the period for each of the zero-voltage steps.<ref>{{cite web |title=MIT open-courseware, Power Electronics, Spring 2007 |website=mit.edu |url=https://ocw.mit.edu/courses/6-334-power-electronics-spring-2007/resources/ch9/}}</ref>

Changing the square wave as described above is an example of pulse-width modulation. Modulating, or regulating the width of a square-wave pulse is often used as a method of regulating or adjusting an inverter's output voltage. When voltage control is not required, a fixed pulse width can be selected to reduce or eliminate selected harmonics. Harmonic elimination techniques are generally applied to the lowest harmonics because filtering is much more practical at high frequencies, where the filter components can be much smaller and less expensive. ''Multiple pulse-width'' or ''carrier based'' PWM control schemes produce waveforms that are composed of many narrow pulses. The frequency represented by the number of narrow pulses per second is called the ''switching frequency'' or ''carrier frequency''. These control schemes are often used in variable-frequency motor control inverters because they allow a wide range of output voltage and frequency adjustment while also improving the quality of the waveform.

Multilevel inverters provide another approach to harmonic cancellation. Multilevel inverters provide an output waveform that exhibits multiple steps at several voltage levels. For example, it is possible to produce a more sinusoidal wave by having split-rail [[direct current]] inputs at two voltages, or positive and negative inputs with a central [[ground (electricity)|ground]]. By connecting the inverter output terminals in sequence between the positive rail and ground, the positive rail and the negative rail, the ground rail and the negative rail, then both to the ground rail, a stepped waveform is generated at the inverter output. This is an example of a three-level inverter: the two voltages and ground.<ref>{{cite journal |last=Rodriguez |first=Jose |title=Multilevel Inverters: A Survey of Topologies, Controls, and Applications |journal=IEEE Transactions on Industrial Electronics |volume=49 |issue=4 |pages=724–738 |date=August 2002 |display-authors=etal |hdl=10533/173647 |hdl-access=free |doi=10.1109/TIE.2002.801052|url=http://americanae.aecid.es/americanae/es/registros/registro.do?tipoRegistro=MTD&idBib=3239971 }}</ref>