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AQ: Snubber circuit for IGBT Inverter in high frequency applications

Q:
First i had carried out experiments with a single IGBT (IRGPS40B120UP) (TO-247 package) 40A rating without snubber and connected a resistive load, load current was 25A , 400V DC and kept it on continuously for 20min . Then i switched the same IGBT with 10KHz without snubber and the IGBT failed within 1min. Then i connected an RC snubber across the IGBT (same model )and switched at 10KHZ. The load current was gradually increased and kept at 10A. This time the IGBT didn’t fail . So snubber circuits are essential when we go for higher switching frequency.

What are the general guide lines for snubber circuit design in You are not looking close enough at the whole system. My first observation is that you are using the slowest speed silicon available from IR. Even though 10KHz is not fast, have you calculated/measured your switching losses. The second and bigger observation I have, is that you think your circuit is resistive. If your circuit was only resistive, any snubber would have no effect. The whole purpose of a snubber is to deal with the energy stored in the parasitic inductive elements of your circuit. Without understanding how much inductcance your circuit has, you can’t begin designing a cost and size effective snubber.

To echo one of the thoughts of Felipe, you need to know the exact purpose of the snubber. Is it to slow down the dV/dt on turn off or is it to limit the peak voltage? Depending on which you are trying to minimize and your final switching frequency, will dictate which snubber topology will work best for you. The reason that so many snubber configurations exist, is that different applications will require different solutions. I have used snubbers in various configurations up to 100KHz.

AQ: Why BLDC motors are noiseless compare to Induction motor?

If referring to the acoustic noise generated at or around the PWM frequency of the PWM frequency. There are more laminations in an induction machine. This may account for some of the difference.

I also don’t know what the relative power difference is between the BLDC and the induction machine. If it’s about a 5Hp BLDC and a 100Hp induction machine, then you can bet that the PWM frequency of the BLDC is likely above the audible range and the PWM frequency of the induction inverter is well within the audible range.

These are just a few reasons that you may find subtle differences between the two. There are many factors and more information is needed to really help understand your specific situation.
I also believe there are simply sophomoric and unprofessional answers. My statement is based on the general rule that there is greater surface area between laminations of squirrel cage induction machine then there are in BLDC machines. Of course, if you want to state that you have a thin lamination on a long stack length design for a BLDC then there may be an argument that such a motor design when compared to a typical induction machine of the same power has a similar surface-to-surface lamination area. It is these laminations moving due to eddy currents at the PWM frequency that causes the audible noise.

BLDC can come with very small inductance which requires a higher PWM frequency, if you compare both them with controller that may cause different.
If you build 2 motor using exact mechanical shapes and electrical parameter they should be very close. You can build 2 induction machines from 2 different vendors to same electrical spec and they will not sound the same.

AQ: System configuration of grounding

There are different types of system configuration for grounding like TT,IT,TN-C etc. How do we decide which configuration is suitable for the particular inverter (string or central). What are the factors that help us to decide the configurations?

One of the main concerns in a system is to avoid large low impedance ground loops.
These are created by the return signal path connected to the chassis (metal work) at multiple points. The large current loop allows noise currents to radiate H fields and hence couple into other electronics. The antenna effect will be proportional to loop area.

Single point grounding of the return path to chassis prevents this. However single point grounding conflicts with good RF practice where you want to ground to chassis at the sending and receiving ends of a signal path. There is therefore no universal best practice.

In my field, spacecraft, the standard practice has all primary power electronics galvanically isolated from the spacecraft chassis. Individual modules must maintain the isolation with transformer coupled DC/DC converters. The centre tap of each PSU secondary output is then single point grounded to the module metalwork. We talk about primary side and secondary side electronics where only secondary side is grounded to the metalwork.

Anything powered directly from the primary bus must be isolated with a maximum capacitance to chassis of 50nF to avoid excess HF loop currents forming.

In general it depends on country specific law and standards required by Power Supply Operators. From design point of view it all depends on which point of grid you are going to connect and what type of inverter is used.

AQ: Lighting control panel to distribution board

There are a couple of construction differences which may be present, depending on the style of “lighting control panel”.

First, a distribution board typically has poly-phase branch breakers with the intention of feeding either other sub-panels or large loads — such as a motor with a motor controller.

A lighting control panel will have mostly single-pole breakers with phase-to-neutral branch circuits feeding lighting circuitry. There is the added possibility of having either ‘smart’ breakers or integral contactors included on the branch circuits to allow for a control means for area lighting beyond local control of an individual fixture/small group of fixtures, such as an office or conference room.

In general
1. The final branch circuits to be identified and rating load to be estimated.
2. Adequate utilization/diversity Factor to be applied if applicable (depends on the application).
3. To ensure the load balance over the 3 Phase as possible.
4. For Fluorescent light fixtures arrangement of the said fixtures with respect (RYB phases) is necessary to mitigate rendering/glaring and frequency affect.
5. Then size of cable from DB to LCP can be determined/sized, rating of the protective devices can be selected and type of CB(s) subject to type of lighting fixtures.
6. Verification of Voltage drop within the prescribed limit, otherwise select the next standard cable size.

A distribution board typically has poly-phase branch breakers with the intention of feeding either other sub-panels or large loads and lighting control panel (is also one type of distribution panel) will have mostly single-pole breakers with phase-to-neutral branch circuits feeding lighting circuitry.

AQ: The basis of rating a NGR in electrical system

NGR stands for Neutral Grounding Resistor. When an earth fault current occurs on a plant, assuming that there is no external device presented to limit the earth fault current, the magnitude of the earth fault current is limited only by the earth impedance presented between the point of fault (to earth) and the return path (typically a star point of a transformer). If the earth impedance is low (type of soil being one of the reason amongst others), the fault current magnitude can be significantly high, and if left unchecked could damage the primary equipment. It is therefore mandatory that the earth fault current be limited to a suitable value, which is typically the rated value of the plant as a thumb rule. Why use the rated value? Because the plant has been designed to carry the rated current continuously.

Let’s take an example: say you have a transformer 60MVA, 132/33kV Star-Delta transformer. It is required to calculate the value of NGR to be connected to the zig-zag transformer on the 33kV Delta. the value of the resistor required to limit the earth fault current to the transformer’s LV rated value is (33 x 33) / 60 = 18.15 Ohms.

(Earth Fault current limited to rated value = (60 x 1000) / (1.732 x 33) = 1050A) When you go to a supplier you might find he supplies only 20 ohms resistor (as you might not get the exact value that you have calculated theoretically). No problem, use the 20 ohms and calculate what your new value of earth fault current would be (33 x 1000 / (1.732 x 20) = 952.6A, which is less than the transformer’s rated LV current. So you’re safe. This is how I would go about. In fact I would go a step further and introduce a safety factor of 20% i.e. I’ll bump up the value of the resistor from 20 ohms by an extra 20% and buy a resistor/ NGR of 1.2 x 20 = 24 ohms. So I am 100% sure that the earth fault current is way below the rated value and my transformer will be safe, even if the fault current goes undetected for any unforeseen reason say my earth fault protection has failed to pick up.

Make sure however that the earth fault setting that you choose is sensitive enough to pick up for the earth fault current calculated. I would generally put two relays a 64 or REF designed to pick up and operate instantly backed by a 51N with a sensitive setting but with a delay of a couple of seconds to pick up in case the 64 has failed to pick up.

So that’s it. I have described how I would go about calculating the earth fault current, selection of NGR value and how I would protect it.
Protection and related devices aiding protection don’t come cheap. Also I assume by your comment “this method is the most expensive option available since the cost of the transformer shall be astronomical”, you are referring to the Zig-Zag transformer and not the actual 132/33kV Star-Delta power transformer, under question.

I have taken a very generic example and tried to focus on how to arrive at a suitable value of an NGR, assuming an Star HV and Delta LV. My aim being to calculate how I could limit the fault current on the Delta LV. Being a Delta winding, I have to use a Zig-Zag transformer, for providing a low zero sequence path for the flow of earth fault current. It is really the Zig-Zag trafo. that bumps up the cost.

Note: If the above transformer is one of a kind, i.e. this is the only transformer in an isolated network, then I simply disregard the Zig-Zag transformer + NGR method and use the 3 PT broken delta method for 3Vo detection to drive a 59N. My cost here would be very low.

If the transformer is a Star-Star type with HV start solidly grounded, and LV star impedance (NGR) grounded, then I don’t need a Zig-Zag trafo. on the LV side. My cost is purely for the NGR alone.(Of course this transformer will have a Delta tertiary which may need it’s own protection depending on the whether one plans to load the tertiary or not. We could di

AQ: Difference between PLC and DDC system

PLC is defined as Programmable Logic Controller. It is a hardware, Includes processor, I/P & O/P Modules, Counters, Function Blocks, Timers,,, etc. The I/Os are either Analogue or Digitals or both. PLC can be configured to suit the application and to programmed in a logic manner by using one of the programing language such as Statement List, Ladder Diagram,, etc Interaction in real time between inputs and the resultant of the outputs through the program logic – PID – gives the entire Control System. While the Digital Control System I believe it is Software/ System that uses only Digital Signals for control and PLC/PC/Server/Central Unit may constitutes an Integral part of this system.

AQ: Power Transformer power losses

Power losses of ferromagnetic core depend from voltage and frequency. In case where is no-load secondary winding, power transformer has a power losses in primary winding (active and reactive power losses) which are very small, due to low current of primary winding (less than 1% of rated current) and power losses of ferromagnetic core (active and reactive power losses) which are the highest in case of rated voltage between ends of primary winding…

Of course, we can give voltage between the ends of primary winding of power transformer (voltage who is higher from rated voltage), but we need include some limits before that:

1. if we increase voltage in the primary winding of power transformer (voltage who is higher from rated voltage), we need to set down frequency, otherwise ferromagnetic core of power transformer will come in area of saturation, where are losses to high, which has a consequence warming of ferromagnetic core of power transformer and finally, has a consequence own damage,

2. if we increase voltage in the primary winding of power transformer (voltage who is higher from rated voltage), also intensity of magnetic field and magnetic induction will rise until “knee point voltage”: after that point, we can’t anymore increase magnetic induction, because ferromagnetic core is in area of saturation…

In that case, current of primary winding of power transformer is just limited by impedance of primary winding… By other side, in aspect of magnetising current, active component of this current is limited by resistance of ferromagnetic core, while is reactive component of this current limited by reactance of ferromagnetic core.

There is a finite amount of energy or power that can be handled by various ferromagnetic materials used for core material. Current increases greatly with relatively small voltage increases when you are over the knee of the magnetization curve characterized by the hysteresis loop. Nickel/steel mix materials saturate at lower flux densities than silicon steel materials. 50ni/50fe materials saturate at about 12kG; 80Ni/20Fe as low as 6kG. Vanadium Permendur material saturates at levels as high as 22kGauss- Nano-crystallines- 12.5kG (type), Ferrites -typically over 4kG at room, decreasing as temperature rises. What causes saturation?: Exceeding material limits.

AQ: What is ANSYS software?

This is a finite element analysis tool for various applications.
In power we get the voltage (stress) distribution in equipment like cables, bends in cables etc including stator winding of generators.

Once you go deep into it the applications become more apparent.  In mechanical engineering using FEM you can identify the stresses in each member of the structure and so on.

I believe ANSYS, Abacus, Nashtran etcare extensively used for detailed analysis of stresses including electrical stresses. Some of the above offer introductory courses on line.
One needs extensive and considerable insight into partial differential equations and advanced mathematics.

AQ: Transmission line low voltages and overload situations

Q: I want to know just what the surge impedance loading (SIL) is but its relevance towards the improvement of stability and reliability of a power network especially an already existing one with various degrees of low voltages and overload situations?

A: The surge impedance loading will provide you with an easy way of determining if your transmission line is operated as a net reactor (above SIL, so external sources of (2) line-voltage-drop limitation
(3) steady-state-stability limitation

In contrast with the line voltage drop limitation, the steady state stability limitation has been discussed quite extensively in the technical literature.

However, one important point is rarely made or given proper emphasis; that is, the stability limitation should take the complete system into account, not just the line alone. This has been a common oversight which, for the lower voltage lines generally considered in the past, has not led to significant misinterpretations concerning line loadability

At higher voltage classes such as 765 kV and above, the typical levels of equivalent system reactance at the sending and receiving end of a line become a significant factor which cannot be ignored in determining line loadability as limited by stability considerations, so surge impedance loading plays a fundamental role in reliability and stability.

AQ: Transformer Saturation