My Final Semester project: Over-current and Shoot-through protection of power electronic switches of a three-phase SPWM Voltage Source Inverter.

Over-current and Shoot-through protection of power electronic switches of a three-phase SPWM Voltage Source Inverter.

[My Final semester Bachelor project]

Hello! Today in this blog I am going to present my final year project in B.Tech to all of you.

We are a group of five people and our final year project is

“Over-current and Shoot-through protection of power electronic switches of a three-phase Sinusoidal Pulse Width Modulated Voltage Source Inverter”.

Voltage Source Inverter based AC motor drives are quite popular in many industries as well as traction based applications like high-speed railways. AC motor drives based on Inverters (also known as VFD/VVVFD/Soft-starter) helps in giving high starting torque to the drive with low starting current, constant as well as variable speed operation and overall efficient operation.

Thus inverters play a very important role in such places where motor drives are based on them. Any fault within the inverter must be detected, prevented and the system should auto-recovery post the fault clearance.

Voltage Source Inverter:

Above shown is the picture of an Inverter based 3 phase Induction Motor Drive. Switches shown by S_1, S_2, S_3, S_4, S_5, S_6 are known as Power Electronic Switches which can be an IGBT or MOSFET, generally used in Inverter. The aim of our project was to protect these switches against two major faults commonly found in industrial inverters

Over-current Fault
Shoot-Through Fault

Both of these faults cause rupture of costly Power Electronic switches and also causes an overall halt to the inverter based application. Thus, robust and reliable protection along with system auto-recovery after the fault clearance is what expected. Our project proposes and implements one such method.

1. Over-current protection

As the name suggests, over-current protection of IGBT in the inverter (We have used IGBT in the making of our inverter) is all about protecting it from carrying a huge amount of current i.e, more than the thermal rating. The over-current fault may appear due to a phase-phase fault at the inverter output, overloading or short-circuit. Thus active protection against the same is required.

How to protect?

Here is the schematic diagram of the design we are using

At the extreme right, the IGBT connection has been shown where G is Gate, C is collector and E is the emitter.

IGBT has an intrinsic property. Whenever the collector to emitter passing current increases or tends to increase beyond the normal working range of IGBT, the collector-emitter voltage begins to rise. This is the feature we are exploiting here for over-current fault protection.

From the above figure, you can see that a reverse biased diode is connected with the collector which finally goes to the non-inverting input of a comparator. This diode is known as a feedback diode. During the over-current situation, the collector-emitter voltage drop begins to increase. This is sensed by the feedback diode which ultimately feds it back to non-inverting pin of the comparator while the inverting input is connected to a set-point (Level Adjustment). As the voltage drop begins to increase beyond the tolerance limit, the non-inverting input to the comparator increases than the inverting one and hence an analog HIGH value appears at the comparator output. The output of the comparator is connected to the gate of a Bipolar Junction Transistor and as the comparator output is high, the transistor begins conduction. As a result, +15V DC which is the gate voltage for the IGBT disappears as the gate-current flows through the BJT where it gets less impedance. In this way, the gate-pulse to the IGBT is stopped thus stopping the IGBT from conducting under the over-current situation.

A push-pull or totem-pole configuration of NPN-PNP transistor has been used before the connection to the IGBT’s gate. Under normal situation, +15V turns on the NPN transistor and hence the gate pulse/gate drive appears at the gate of the IGBT. When there is a fault then the BJT connected to the comparator output turns ON and as a result, -9V appears at the gate of NPN and PNP transistor of the totem pole configuration. Since this is a negative voltage, the NPN transistor doesn't conduct while PNP transistor turns ON (PNP responds to negative gate pulse, opposite to NPN). This PNP transistor thus helps in Quick Discharge of IGBT’s gate capacitance. That’s why a negative reference voltage has been used instead of 0V as a reference. This ensures reliable over-current protection.

2. Shoot-through Protection

In the first picture, you can see the simple configuration of an inverter. There are three limbs with six switches, two in each limb. Each of these limbs is connected in parallel to the DC bus or DC supply. According to the working principle, it is ensured that no two IGBT in the same limb turns ON at the same time. An ideal power electronic switch during ON state behaves like a conductor and thus if two switches in the same limb turn ON simultaneously, they will cause the DC bus to short-circuit, This is called Shoot-through fault.

Although ensured through microcontroller used for gate-driving, there are various causes which may lead to a shoot-through fault to appear thus rupturing the costly switches. Few reasons are Electromagnetic Interference due to HF (high-frequency) operation, faulty snubber circuit design etc.

How to protect?

Here is the schematic

The picture shows the connection diagram of two IGBTs of a limb within the inverter. For protection against Shoot-through, we are using Opto-isolator. Simply speaking, an optoisolator is a kind of transistor manifested with an IR LED. Whenever the LED glows, it emits photons received by the gate of the transistor thus turning it ON.

In the above picture, you can see the transistor terminals of the optoisolator of both the IGBTs indicated by A_1, A_2 and B_1, B_2. Now the A_1 terminal is connected to the input line of the totem-pole configuration (NPN-PNP transistor input junction) of the lower IGBT (indicated by A_1). Similarly, the A_2 terminal of the upper portion is connected to the point indicated by A_2 in the lower half. The same thing has been done for B_1 and B_2 in the lower portion connected to B_1 and B_2 points of the upper portion.

Let’s say the upper IGBT gate pulse appears and it is conducting. This turns ON the LED of the optoisolator connected to the gate line of the upper IGBT. This causes the transistor of the optoisolator/optocoupler to turn ON and begin conduction. As the terminals of the upper opto-transistors (A_1 and A_2) are connected to the Gate and reference line of the lower IGBT, any stray gate pulse if tries to appear in the lower IGBT during conducting state of the upper IGBT will be short-circuited due to the upper transistor and hence the stray pulse is forbidden to appear at the gate of the lower IGBT under any condition.

Here’s what the Oscilloscope showed when we intentionally tried to create a shoot-through fault.

You can see how the lower IGBT gate pulse tries to appear but is quenched down thus ensuring protection against shoot-through.

So this is what our final project is all about. It was tiresome and we failed a lot of time before reaching here but it was the best thing at the end. Here are some physical image of the work and practical tests carried on the project.