Hello friends,

I am having list of questions regarding static frequency converter,

1. What is the reason for choosing Current source inverter for SFC even though Voltage source inverter having more efficiency?

2. While using Current source inverter, Why we are calculating the output power = DC link current * DC link voltage instead of (inverter output voltage * output current)?

3. In case of Current source inverter, What is the reason for having inverter output current only at the time of starting? After some time, y its getting reduced to almost zero?

4. How the overlapping time of thyristor is related to current..?

I hope,u guys will clear my doubts...

 

Chandru,

Doubts are not something we like to deal with here at control.com.

These are great questions for designers of electrical/electronic equipment. Many times the choice to use one type of equipment over another is based on designs currently being used or which have been proven to be more reliable than other designs.

Something you will learn as you spend more time around large plants and various kinds of equipments, "Engineering is a series of compromises." That doesn't sound like the best way to design something, but it's very, very true in just about every level of design and operation.

These questions have been asked very recently (in fact, your post looks like a copy-and-paste of the previous post!), and if someone had answers they probably would have posted them already. There is a great deal of knowledge and experience here at control.com, but there are questions and circumstances which fall outside the knowledge and experience of the people here.

For questions like these, I suggest you consult other sources on the World Wide Web and Internet for the benefits and disadvantages of these and other similar systems that apparently someone at your site is aware of. But always remember that many engineering design choices are simply the result of experience with a particular system or way of doing things, and also the cost of designing a completely new system versus using something that already exists. Economics figure greatly into many engineering choices, and also force many compromises.

Hope this helps!

 

Chandru..

If your application is related to a motor drive, then contact Danfoss

If your application is interfacing two power systems having differing frequencies, e.g., 60 to 25 Hz, then contact ABB!

Regards, Phil Corso

 

Phil Corso,

This application is for powering a synchronous machine as a motor to start a gas turbine. When the unit reaches approximately 95% of synchronous speed (approximately 2850 RPM), it is "switched" to a synchronous generator and the gas turbine drives the generator to produce electrical power.

 

> 1. What is the reason for choosing Current source inverter for SFC even though Voltage source inverter having
> more efficiency?

Thyristor commuting circuit is more easy and effective by means of controlling the current through the device.

> 2. While using Current source inverter, Why we are calculating the output power = DC link current * DC link voltage
> instead of (inverter output voltage * output current)?

Active Power is more easily calculated at the DC side variables. But is the same (but more difficult) if you calculate at the AC side.

> 3. In case of Current source inverter, What is the reason for having inverter output current only at the time of
> starting? After some time, y its getting reduced to almost zero?

The charging of the DC side inductor demands a high energy from the AC side. When charged the current slow down to balance the flow of power from input to output.

> 4. How the overlapping time of thyristor is related to current..?

The more current flowing through the thyristor the more time it takes to turn off. Under worst case conditions the device must have enogh time to "transfer2 the current from one device to the other.

> I hope,u guys will clear my doubts...

Hope it helps!

 

Hi John Correa,

Thank you so much for your reply..are you working in SFC field..?

While optimizing the inverter side, we minimised the angle between switching off thyristor current and its next zero crossing point, in order to get maximum power..we were able to see the current only at the time of starting and optimised based on current..that's y i asked about its role...at any case, this output current will not go to zero..right?..then whats a problem in calculating the o/p power from Vac, Iac...I am not able to get u at this point.

Regarding, forced commutation, u said it seems like dc variables (stator - Vac,Iac) from Inverter switching frequency point of view...what it means..can u please elaborate your explanation..

Thanks again.....

 

> Thank you so much for your reply..are you working in SFC field..?

You are welcome. I worked in a Power Electronics R&D department during my University years (15 years ago).

> While optimizing the inverter side, we minimised the angle between switching off thyristor current and its next zero
> crossing point, in order to get maximum power..we were able to see the current only at the time of starting and
> optimised based on current..that's y i asked about its role...at any case, this output current will not go to zero..right?

I need more specific information to help you.
May I need the circuit details, for better understanding your questions about it.

..then whats a problem in calculating the o/p power from Vac, Iac...I am not able to get u at this point.

Active power (P) is the same either you calculate at rectifier AC side, or the DC link side, or inverter AC side. Power flow can't be different (except if you consider the power losses) from input to output. Mathematically you need only 2 variables (I and V) to calculate active power in the DC link. Better yet you have constant current, so power is proportional to DC link voltage.
In the AC sides, you have 3 pairs of variables, each one not constant, so you have to calculate the mean of the product v*i over the time period to get mean power (P). Another way to calculate the same is doing by fourier series analysis.
You decide wich way to take to reach the same value.

> Regarding, forced commutation, u said it seems like dc variables (stator - Vac,Iac) from Inverter switching
> frequency point of view...what it means..can u please elaborate your explanation..

If the inverter is doing "soft start" of the motor the output frequency must start very low (let say 1 to 5 Hz) for a large period of time (depending on the power of your motor). The switching frequency of the thyristor must be high compared to this motor frequencies. From the point of the inverter dynamics the motor currents appear like "slow varying" DC currents. Also the motor reflected voltage are low amplitude because the slow speed. Then the only way to turn off the devices is by means of forced conmmuation (there is not enough load voltage to induce current to change, circuit must rely on its auxiliary stored energy devices, normally capacitors)

But when the motor is operating near the nominal speed, the inverter switching frequency and the motor voltage frequency are comparable and the load voltage is high enough to produce the current transfer from thyr to thyr , only by means of turning on the adecuate device

 

Hi John correa,

Thanks again.

As u mentioned, for forced commutation we are not relying on additional component like capacitor.We are making the DC link current to zero by switching the line side converter as inverter.When it happened, dc link current will become zero and there will not be any holding current for thyristors.But in the above process also i have some doubts,

1. while making the DC Link current zero, how the line side thyristors are conducting, bcoz they are unidirectional device..only allows the current to flow from anode to cathode...in this case i don't understand..?

For forced commutation, when i start to release firing pulse for machine converter, rotor will start to rotate slowly.Let say, i am giving a motor current to flow through a-b phase.Next before going to b-c phase, i need to commutate the previous set of thyristors.so suddenly i will make line rectifier to act as an inverter, dc link current will become zero.then within short period of time, again i need to switch from inverter to rectifier to get dc link current.release pulses for b-c phase.am i right...???...if its right,should i need to repeat this process until i get the sufficient back-emf...?

For load commutation, i will make the machine converter to supply motor current to stator windings.At the same time, there will be some back-emf in them because of rotor current.I read in one document, that synchronous motors are constructed in such a way to provide leading power factor which means the back emf will be greater that applied voltage from inverter.when i give a firing pulse to set of thyristor in machine converter, suddenly it will make back-emf to reverse bias and that reverse biased back-emf is helpful in commutating the thyristors.My question is...how the motor current from inverter output will make backemf to reverse bias..? and what exactly happening...is it making the voltage equal on both sides of thyristors equal(anode and cathode)..bcoz only at this case, there will not be any current flow..???

ok...now the thyristors are commutated...by load commutation..

 

> 1. while making the DC Link current zero, how the line side thyristors are conducting, bcoz they are unidirectional
> device..only allows the current to flow from anode to cathode...in this case i don't understand..?

The DC link current decay from Idc to 0 gradually.
AC side thyristors always conduct positive current until it reaches zero (and all stored energy from inductor is returned to the line). The power flow reversal occurs changing the voltage from the rectifier (acting like inverter)

> For forced commutation, when i start to release firing pulse for machine converter, rotor will start to rotate
> slowly. Let say, i am giving a motor current to flow through a-b phase. Next before going to b-c phase, i need to
> commutate the previous set of thyristors. so suddenly i will make line rectifier to act as an inverter, dc link
> current will become zero. then within short period of time, again i need to switch from inverter to rectifier to get
> dc link current.release pulses for b-c phase. am i right...???...if its right, should i need to repeat this
> process until i get the sufficient back-emf...?

Yes. This method of nulling (notching) the DC current is only possible when the motor frequency is slow (at starting time). At higher frequency the AC side dynamic is not enough to cope with the motor current dynamics. But when this happen you have the back-emf to do load conmutation.

> For load commutation, i will make the machine converter to supply motor current to stator windings. At the same
> time, there will be some back-emf in them because of rotor current. I read in one document, that synchronous motors
> are constructed in such a way to provide leading power factor which means the back emf will be greater that applied
> voltage from inverter. when i give a firing pulse to set of thyristor in machine converter, suddenly it will make
> back-emf to reverse bias and that reverse biased back-emf is helpful in commutating the thyristors. My question
> is...how the motor current from inverter output will make backemf to reverse bias..? and what exactly happening...is
> it making the voltage equal on both sides of thyristors equal (anode and cathode)..bcoz only at this case, there
> will not be any current flow..???

The thyristor conducts while Vak> 0 and current is positive. Don`t forget the stator inductance, the voltage from this inductance in series with the back-emf and the DC side voltage forms the circuit of the inverter thyristor. Check the circuit state conditions previous to the firing.

 

Hi John correa,

Thanks a lot for your reply....now i got an idea about its operation..

 

HI!

Compared to the questions and answers I have been reading on this site so far, my question is going to sound some what, stupid For lack of a better word.

I am currently an operator employed to commission a brand new power station in my country.

At my current Pumped storage station, AVR's (AUTOMATIC VOLTAGE REGULATORS) with the aid of Liquid starters are made use of to excite the Generator, and provide starting torque for SCO and pump modes. However, at the power station I will be commissioning, they are using SFC's for this purpose. Please can any one explain to me, the operation of an SFC? What it is, what it does and how it works?

Please, I am looking to gather as much knowledge as I can before I start working there.

Thanks so much

 

Peaking.Duck,

Hello, and good on you for being proactive.

An SFC (Static Frequency Converter) is a device that converts AC (in this case, three-phase AC) to a variable frequency output. This output is then applied to the generator stator, and is used to accelerate the machine during starting.

In the past, an electric motor or a diesel engine, usually coupled through a hydraulic torque converter, were employed to accelerate the gas turbine during starting (to "break the shaft away" from zero speed, and to spin the axial compressor to purge the turbine and exhaust of combustible gases prior to admitting fuel and energizing the spark plugs). One can't just dump fuel in a gas turbine at zero speed and energize the spark plugs and accelerate the shaft to rated speed. Therefore some method of spinning the shaft to the point where fuel can be added to the turbine and flame established is necessary when starting the turbine.

After flame is established, the gas turbine is not usually "self-sustaining"--meaning that if one removed the torque assist from the starting means the unit could continue to accelerate without any assist. So, the starting means is used to help accelerate the turbine until it IS self-sustaining.

With the advent of better electronics and high-power conversion methods, it was decided to use the generator as a motor, temporarily, to spin and accelerate the turbine during starting. It eliminates some mechanical devices, and is a rather elegant solution to the problem of accelerating the turbine during starting.

When using an SFC, it is necessary to use the AVR (exciter) to apply current to the generator field to produce one of the two magnetic fields required for a motor to work.

Once the turbine reaches a certain speed, the static frequency converter is shut down, and some switching of contactors is done and the generator then becomes a true synchronous generator which is then synchronized to the grid to produce power for many loads.

This proves there is very little difference between a motor and a generator--only the direction of current flow and the application, or production of, torque. In a synchronous generator, torque is applied to the rotor and current flows "out" of the stator. In a synchronous motor, current is applied to the stator and torque "flows" out of the shaft (so to speak).

Now, I'm not familiar with many methods of pumped storage, but when the hydro "turbine" is to be employed as a pump to return the water to the upper reservoir I would imagine there would need to be some similar method required for reversing the hydro turbine from zero speed, and the "liquid starter" and AVR might be used to do that.

And, there is probably some high-power static frequency converter which provides variable frequency AC to the generator to use it as a motor to drive the hydro turbine when it's being used as a pump. And, the AVR would likely be providing current to the generator rotor during that time, also.

Hope this helps!

I'm not familiar with the term SCO, which is probably just another name for a variable frequency "source" that is used to drive the generator as a motor when using the hydro turbine as a pump. (I actually find most people use acronyms all the time without ever understanding exactly what they mean or what the equipment they are describing does. Amazing, actually.)