Voltage Source Inverter

Voltage source Inverter is a device that converts power from a DC constant voltage source into AC form by switching behavior. The output voltage has a switching waveform and the output current has AC form. It is classified into two inverters: square wave inverter and PWM inverter.

Fig 2. Voltage Source Inverter 2

Fig 1. Voltage Source Inverter 1

It consists of 3 pairs of 2 switching components. Depending on the switching condition, input and output voltage are determined. In order to prevent short circuit accident, switching component works in a commensurate way.

1. 3 Phase 2-Level Inverter

Fig 3. General Circuit of 3 phase 2-level inverter

Fig 3 is a common circuit of an 3 phase 2-level inverter. Line voltage has 3 level form which is +Vdc, 0, and -Vdc, The detail about this inverter is explained in one of my past posts titled as "Inverter: Single Phase Inverter and 3 Phase Inverter"

2. 3 Phase 3-Level Inverter

When the inverter transmits a wave, it contains harmonic components which interfere with actual power. For preventing this, we use 3 phase 3-level inverter. It contains fewer harmonics than a 2-level inverter.
It works as 3 levels: P, O, N. During P switching, 1, 2 switches are on, so the output voltage is Vdc/2. During O switching, 2, 3 switches are on, so the output voltage is 0. During P switching, 3, 4 switches are on, so the output voltage is -Vdc/2.
There are 2 types of 3 level inverter: T-type and NPC type

2.1 T-type

Fig 4. T-type

Fig 6. Negative direction

Fig 5. Positive direction

2.2. NPC type

Fig. 7. NPC type

Fig 9. Negative direction

Fig 8. Positive direction

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NPC Inverter and Psim Simulation

1. 3-Level NPC Type Inverter Fig 1. 3-Level NPC Type Inverter Fig 2, Switching Fig 1 is 3-Level NPC Type Inverter and Fig 2 indicates how it works. Fig 2 indicates how each switch is activated. There are 3 switching states which are called P, O, N. In each state, the output voltage becomes +V_dc/2, 0, -V_dc/2. Fig 3. Switching Mode Left in Fig 3 indicates circuits when positive voltage and the right one is for negative voltage. Let's check how it works! Considering P switching state in positive voltage, S1 and S2 are on, and S3 and S4 are off. So we can ignore the circuit below. As a result, the output voltage is +V_dc/2. In the same way, you can see how the rest works. 2. Control Fig 4, SPWM In Fig 4, we can see how SPWM works. The offset voltage is applied to control the circuit. The common value for offset voltage is 0. It makes the reference polarity voltage and the reference phase voltage same. 3. Circuit Fig 5. Circuit for Psim ...

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Harmonics

What is harmonics? When we think about harmonics, we can easily come up with harmony in music, which means the composition of individual sounds. What about electrical power? In an electrical power system, a harmonic is a voltage or current at multiple of the fundamental frequency of the system. Then, What is the fundamental frequency? Before we discuss harmonics, we should know what the fundamental frequency is. Maybe you know the relationship between voltage, current, and resistor, for example, V=IR. Assume that there is a system where a DC voltage supply and a resistor are connected in serial. Then, if the voltage is AC, the current is AC. And It's totally because of the resistor in this system. That is, the resistor is linear. So in this system, we have one voltage sine wave and one current sine wave. And they can be called a fundamental frequency wave. Fig.1 What if the resistor is replaced with other components which is non-linear? The current will ...

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What is Energy Storage System (ESS) and its role in Grid?

Energy Storage System (ESS) Methods of generating electricity by renewable energy such as wind power and solar power, rather than the methods by conventional power plants are becoming important both politically and technically not only in Korea but also around the world. In the case of Korea, the importance of renewable energy is emphasized as the country pushes for the "2050 carbon neutrality" policy. However, there are certainly drawbacks to renewable energy. Wind power and solar power are the representative sources of renewable energy. Wind power generators are affected by wind speed, and photovoltaic generators by solar radiation. Because of this inherent nature of renewable energy, the amount of power output from renewable energy is highly volatile. Many studies to reduce this volatility are going on , and in this post, we will first learn about ESS and how it is typically utilized to decrease bad natures of renewable energy and eventually to help stabilize ...

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Grid Tech

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