Over the last two years ELMG Digital Power CTO, Dr. Hamish Laird, has helped supervise (the now Dr.) Rabia Nazir in the pursuit of her Doctoral studies.
Hamish Laird says
“The research that Rabia has completed in the area of fractional delays in recursive filters for current control in grid tied inverters gives great control tools in the implementation of control for GTIs in grids where the AC system frequency is varying. It is always great to help with PhD research as I learn so much so thanks to Rabia for letting me help.”
Congratulations to Dr. Rabia Nazir on her successful oral defense of here work. Dr Laird again
“It was fantastic to attend Rabia’s defense. I am so proud of and pleased with the work she did in analysing, simulating and building power converter hardware to show her findings. It was a great learning experience for me.”
Recently (now Dr.) Rabia Nazir presented a paper at a conference in Sicily on the use of Taylor Series expansion based fractional delay filters for recursive control of grid inverter currents.
PCIM is the show where the large power electronics industry comes together to celebrate and show off. Three big trends at PCIM in May 2015 are off the shelf IGBT stacks, mature inductors and intelligent IGBT power stacks. No off the shelf controllers for these stacks were to be seen at the show.
By large converter I mean above 3kW. PCIM Europe is in Nuremberg, Germany. Other PCIM shows are in China and South America. When I attended this years PCIM show it was clear that three large trends are occurring in the power electronics market. With the off the shelf converters and inductors now available it is clear that intelligent stacks need digital power controllers.
Three trends at PCIM
The PCIM show always has significant numbers of new products launched. This year three trends were very clear.
Firstly there are more and more ready made stacks available. These are IGBT modules mounted on heatsinks with integrated gate drivers and DC bus capacitors. In some products there are fans included as does Semkron’s SEMIKUBE SlimLine, while other vendors, like AgileSwitch and Infineon, have left the cooling to the user. An interesting turn up is the Danfoss semiconductors stack that reportedly is a drop in replacement for the SkiiP product from Semikron. This might be that this is the result of Danfoss buying Vacon.
Infineon’s MiPaqPro is a new Intellingent IGBT Stack
Semikron also showed a watercolled megawatt converter in a cabinet for large wind and solar applications. They also had a really effective new parallel power converter system. Both are very impressive and useful for large grid connected converters.
When the Semikron, Danfoss, AgileSwitch (AgileStack) and Infineon (MiPaqPro) products are stacked up (sorry for the pun) together is it clear that off the shelf power converter stacks and systems are here to stay as all these vendors are truly committed. It also is clear that buying stacks off the shelf is going to be a valid solution instead of developing your own. I am not sure what that means for those who are still doing converter development. Time will tell. Probably purchasing volume and management commitment to building converters will be the deciding factors internally in the product companies. One colleague at the show thought that “If you stop developing your own converters you may as well just call it quits”.
Inductors are mature
I recall seven years ago struggling to find an inductor manufacturer that could design reasonable quality and that had a balance sheet that would pass any kind of financial stress test. And now there are at least three inductor companies who do a good job technically and who do not present too large a financial risk. Hoganas from Sweden have perhaps the strongest balance sheet of all the vendors at the show. And their inductors are good.
Monitoring of the IGBT performance while the stack is running looks like a great feature. Amantys had gate drives that did this. That company went by the wayside when they folded at the beginning of the year. . Two other vendors Infineon and AgileStack have created intelligent stacks and AgileSwitch are doing replacement gate drives for the Amantys part. Contact Rob at AgileSwitch for their intelligent gate drives.
AgileSwitch Power Stack – Intelligence and robust implementation.
These intelligent stacks have microprocessors or FPGAs that can monitor the stack performance and provide data for controller to do condition monitoring and system performance analysis. The Infineon MiPaqPro has a great water cooled heatsink plate that may become the new high performance low cost standard for heatsinks. The monitoring for the IGBTs is in the stack and there was a fiber optic interface also. Fiber optics are a good idea for monitoring as they provide great safety in the event of a fault or failure. Fiber optic systems are also very robust in highly noisy EM environments.
The AgileSwitch stack is a great little converter. It has integrated gate drives and has the ability to self monitor and provide digital output for status and current via a digital link. This link is electrical. AgileStack do not yet have a fiber optic link for their stack. Their system is however very flexible and all the external signals are differential which gives higher noise immunity.
The AgileStack system can also come with inbuilt low inductance DC bus capacitors. It looks to be a great solution.
All you need is a controller – Intelligent Stacks need Digital Power Controllers
The stacks at PCIM all need inductors and capacitors for the grid coupling filters. There were lots of capacitor and inductor vendors at the show. The stacks also need controllers to make them switch. How this controller is implemented is not the concern of the stack manufacturer as all they need is the gate dive signals. Ensuring that the gate drive signals make the converter do as required is the missing piece in the the three trends at PCIM.
It seems that power electronics companies can now purchase off the shelf power stacks. By adding inductors and capacitors from other vendors the converter is almost complete.
Then the controller is required. The stack manufacturers and the inductor manufacturers have solved two thirds of the power converter. The controller is the missing part of the puzzle. Get your digital power controller from ELMG Digital Power.
Get the ELMG Digital Power Controller Technology by clicking here
With inverter grid synchronisation the key problem is that the grid frequency can vary. In fact the frequencies of AC systems around the world are different and they are also constantly changing. This makes it a challenge to connect renewable generation to the network.
AC System Frequency
Aircraft AC systems run at 400Hz three phase. They do this to make the motors lighter with less iron due to the volt second integral being lower.
Some railway locomotive AC systems are 16.7Hz single phase. These frequencies are chosen to minimise the current needed to charge the capacitance of the overhead line and so minimises the number of substations needed. The 16.7Hz is chosen to so as to not be a multiple of 50Hz. The was made by rotary frequency changers (this is truly what they were called as power engineers often lack imagination) but is now also made by static power electronic frequency changers. With modern static frequency changers with inverter grid synchronisation 16.7Hz is achieved.
In Switzerland the railway has their own separate distribution grid.
There have been railway frequencies as low as 8Hz and some train locomotives even operate from very low frequency AC with a frequency of zero. DC is the AC you have when you are not having AC.
Early on GE decided that 40Hz would be good for AC distribution but it did not catch on.
There are aluminium smelters in Australia where they have or had 60Hz, 50Hz, 25Hz and 16 2/3 Hz AC systems and reportedly all at the same time.