Posts Tagged ‘Power Electronics’

Energy Storage everywhere and a converter to connect it.

Wednesday, May 4th, 2022

Energy storage is everywhere or is being talked about everywhere. Adding an energy storage power electronic converter to the distribution or customer network allows

  • Load leveling across the day
  • Integration of local renewable sources such as solar and wind
  • Deferment of transmission investment
  • Reduction in CO2 emissions because peaking plants do not need to be run
  • Lowering energy cost by price arbitrage on daily demand variation

Historically the electrical grid has had energy flow from the large remote generator station out through the transmission and distribution network to customer loads. And the voltage profile was set up with transformer turns-ratios for this.

The transmission and distribution grid has historically transmitted power from central generation to the load. Energy storage power electronic converters can aid the integration of renewable generation
The transmission and distribution grid has historically transmitted power from central generation to the load

Energy storage is DC so there needs to be a reversible DC to AC power converter to interface between the battery and the grid. This is where power electronics provides the glue between the

Here at ELMG Digital Power, we’ve been working on grid-connected power converters since 1997 in applications including

  • Harmonic shunt active filters
  • Static VAr compensators from 2kVAr to 500MVAr
  • Solar Inverters
  • Micro Inverters
  • Grid Interactive UPS Inverters
  • Battery Energy Storage Inverters
tropical rain forests can be conserved by minimizing energy use with energy storage power electronic converters
Saving energy with grid energy storage reduces carbon dioxide emissions

Grid connection challenges

Grid connection challenges with an energy storage power electronic converter include grid synchronization usually with a PLL or a DSOGI, Low voltage fault ride through, designing the LCL filter, and connecting converters in parallel.

Grid synchronization for energy storage power electronic converter

The grid frequency and phase can vary, especially if there are faults or disturbances in the network.

PLL or DSOGI

The Phase-Locked Loop (PLL) keeps the inverter voltage synchronized to the grid. This allows the inverter digital control system to have the real and reactive power flow as requested. It also makes sure that if the grid frequency isn’t exactly the expected 50Hz, 60Hz or 400Hz then the system will still operate correctly.

The Dual second-order generalized integrator is used to implement an alternate type of frequency lock. It is often considered that DSOGI does a better job in the transient ride through. However, a well-tuned PLL is as effective as a DSOGI.

Sinusoidal grid voltages to synchronize energy storage power electronic converters
Grid voltages are sinusoidal for the most part

Low voltage fault ride through.

It is inevitable that there will be a fault in the AC system. Either two phases will connect together or one phase will connect to the ground or alternatively there can be some combination of phase to phase fault and ground fault. This causes a voltage disturbance that the inverter must “ride through”. To ride through the PLL must stay synchronized and the inverter must control the current as required by the grid code. (We’ll cover that later). So the PLL/DSOGI must be designed to ride through the voltage disturbance.

LCL Filter design

LCL filters are used to minimize the cost of the grid coupling filter. The higher filter has some poorly damped dynamics so synthetic damping is provided by control or via the addition of a small damping resistor. (Some grid codes now require this damping resistor).

LCL filters used in energy storage power electronic converters
LCL filters provide low loss low cost grid connection

Ensuring that the LCL is correctly designed so that it can be controlled with a suitably, low-cost controller means that LCL design is critical to a successful battery energy storage converter. The AC grid impedance range specified for the converter is really important.

Contact us for energy storage converters

Paralleling converters

It is a product manager’s dream to be able to put energy storage power electronic converters in parallel in any combination. If this is to be a possibility then all of the

  • LCL filter design
  • Controller choice,
  • PLL design
  • Parallel connection

need to be managed for parallel connection from the very beginning of the development. It is best to limit the required parallel connection combinations to a minimum as each requires verification and validation testing.

Battery energy storage interfacing to the battery

The key issues in interfacing an inverter to battery energy storage are

  • The allowed ripple current and voltage for the battery and what this means for the grid side LCL filter and voltage imbalance.
  • The charge and discharge rates allowed for the batteries
  • How to manage and coordinate with the battery management system (BMS)

Battery Types

There are two principal types of batteries used in energy storage

  • Lithium Ion
  • Lead Acid

Contact us for energy storage converters

Lithium Ion

Lithium-Ion (Li-Ion) batteries work unsurprisingly, by the movement of lithium ions. The energy density is high. The key safety issue with lithium-ion cells is the risk of fire from excess temperature. Li-Ion cell safety needs to be actively managed.

State of charge and state of health systems for Li-Ion systems are widely available.

Lead-Acid

Lead-acid batteries have lead and sulphuric acid. The energy density of lead-acid batteries is lower than Li-Ion.

State of charge and state of health systems for lead-acid batteries are less effective than those for Li-ion.

Grid codes

The AC grid is one of the most reliable machines in the world. Blackouts in most countries occur irregularly. This reliability is the result of long experience and conservatism by system operators.

Battery energy storage converters connected near the load end of the network are an unmanaged and uncontrollable (in grid operator terms non-dispatchable) energy source. To ensure the AC grid machine stays reliable there are rules for connecting power and energy generation to the AC grid network. Energy storage is a generation source so it is covered by the rules for generators. These rules are called the Grid Code. Each country’s grid code is different as each country has a different AC network.

Contact us for energy storage converters

Power Ramp Rates

The most common and most useful rule for generating is that the real and reactive power ramp rates must be limited. The power cannot go up or down too fast. What does too fast mean? Typically the ramp rate limit is zero to maximum power in tens of minutes.

Reactive Power

Adding power to the grid causes the voltage to rise. And the grid was designed to supply power to the load rather than get power from the load. So the battery energy storage real power output may well cause the voltage to rise. This can be counteracted with reactive power draw to keep the voltage down. This reactive power requirement means that the inverter may well need a higher current rating and so will cost more.

Communication

Some Grid Codes have the requirement that each inverter be connected to a system operator communication network where the network operator can control the real and reactive power dispatch or how the real and reactive power are controlled together. These communication network standards are well established in mature solar markets, such as Germany.

Safety Disconnection

The principal safety requirement for the grid code is to disconnect the energy storage system if the AC grid fails. This is to ensure that anyone working on the AC lines is safe from electric shock. This anti-islanding is a requirement for all grid codes.

saving oil with energy storage power electronic converter
Grid energy storage allows the integration of renewable energy and the reduction in the use of fossil fuels

Energy storage power electronic converter FAQs

How do you implement anti-islanding?

The simplest way is with a frequency or phase angle perturbation. The best way is with a grid impedance change detection system.

Why do they keep changing grid codes?

The key reason is that adding more embedded generation and storage is threatening the reliability of the grid so they are changing the rules.

What is the best way to design an LCL filter for a grid-connected battery energy storage converter?

Well, this is a massive question. The best answer is to start with the low voltage fault ride-through (LVFRT) and the PLL and then work backward from there. A current control bandwidth target of 1kHz is useful.

Three-level or two-level inverter for an energy storage power electronic converter?

Get some help before you start if you are designing a three-level inverter. Common mode is easier to deal with for three-level. Modulation is less easy.

Can ELMG Digital Power Design us a Grid Connected Battery Energy Storage Converter for 1MW?

Yes. Any power from 1kw, 10kw, 20kw, 100kw 1MW, 10MW to 100MW.

Can you do MV or HV battery energy storage power electronic converter?

Yes, this is possible and we have worked on 11kV and 66kV MV UPS converters and their static transfer switches. Battery banks are best below 1500Vdc and so a multi-level multiple converter solution might be a good idea. Or alternatively, transformer coupling for the step up is also a useful and economically competitive approach.

Is thyristor, IGBT SiC Mosfet, or Si Mosfet the best switch choice?

Thyristors and IGBT are the most rugged so are more robust in grid-connected battery energy storage converters. SiC os Si Mosfets are also suitable choices.

Contact us for energy storage converters

Three Day Digital Control Course August 22-24 California

Friday, July 22nd, 2016

The ELMG Digital Power Electronics Control Course

Three days of focused unique training in digital control of power electronics!

Our Digital Power Electronics Control Course overs the essential knowledge and know-how for engineers to implement digital power electronic control!

Come to the Three Day Digital Control Course in Camarillo, California August 22-24, 2016.  Register here.

How did the course came about?

Essentially the course came about because we were asked by one of our customer’s to provide one. The story is we were in the middle of a “fix up” job where the power supply had shown some control instability at its final release testing. The testing that showed the problem was passing a short circuit test of parallel connected power supplies. When the short circuit was removed the supplies came out of current limit, however they did not come out of the limit at exactly the same time. This created an oscillation where individual power supplies came out of current limit and then returned to current limit.  It was possible for the oscillation to continue indefinitely.  This was an unacceptable and embarrassing problem.

Six months of expertise in a three day course

During the six month project to rework the control code we spent lots of time teaching the team about the underlying issues that had been missed when the controller had been designed, coded and tested.  And part way through the “fix-up” the R and D manager suggested we could put a course together covering all that the team needed to know.

And so the digital control course was born

The first course covered exactly what we had discovered during the fix up job.  This included lots of digital expertise targeted for power electronics.  The areas we covered were diverse from;

  • Numeric precision loss in filters
  • Improvement of modulation spectral performance
  • Stability
  • The effect of numeric precision on stability
  • Best filter forms
  • Direct digital control design
  • Linearising control loops

What is covered in our course?

The course was created at the request of a Power Electronics Research  and Development manager.  He asked that we make it specific his team’s needs.  And this is why the course has the unique structure that it has.  We have been through the pain and heartbreak of having digital control development go wrong and have seen clearly where the repeated problems lie; our course addresses those areas.

Digital PWM and VPO modulators

One of the big differences between digital power electronics control and conventional analog control is the timer precision in digital modulators. This difference can be corrected or made negligible and in some cases can be made an advantage.  Spectral control in digital modulators is a focus area in the course as it is so effective.

Digital Precision in control blocks

It is possible to use a digital system and adjust the coefficients of the filters so that small inputs result in no output from the filter. Such scaling issues often lead to a loss of precision in the digital control system. The resulting slip-strike behavior can create limit cycle oscillations in the power converter output.

Direct Digital design of controllers

The “design then translation” approach of taking analog controllers to digital form can be avoided by using the direct digital design approach. This simple but powerful method of digital control loop design is covered in the course.

Converter non-linearity correction

Certain converter topologies are non-linear either in the control input to the output or the conversion ration.  Dealing with the converter non-linearity to achieve high bandwidth is key to stable parallel connected converters.

Stability

The course covers the fundamentals of stability from a physical basis with a focus on measurements of power converter transfers.  This along with a simple framework for managing margins and robustness is an integral part of the course.

Why we offer the course?

Understanding and implementing digital control of power electronics offers great advantages for configuration and flexibility. However, this is not without road blocks and issues that need to be designed around. This course provides the know how to get digital control working robustly and reliably.

How do I get on the course?

The course is next being run in Camarillo, California USA August 22-24.  To register for the course, click and visit the information page here. Press the ‘Register’ button on the page and this will take you to the shopping cart for the course. Complete the purchase to register for the course.

Next course

The next course is being held August 22-24 in Camarillo, California, USA.

Hotels

HOTELS

There are several hotels a short distance from the Ridley Engineering Design Center. The prices below reflect their current prices for August 2016. The last hotel listed is a nice beachfront resort if you do not mind the 25-minute commute to the office. Regardless of your selection, we recommend arriving on Sunday evening and departing Wednesday evening or Thursday.

 

Best Western Inn

295 E Daily Drive, Camarillo

0.3 mi.

805-987-4991

$100/night

Book.western.com

Residence Inn by Marriott

2912 Petit Street, Camarillo

2.8 mi.

805-388-7997

$185/night

Marriott.com

Courtyard by Marriott

4994 Verdugo Way, Camarillo

4.3 mi.

805-388-1020

$180/night

Marriott.com

Hampton Inn & Suites

50 W Daily Drive, Camarillo

1.1   mi.

805-389-9898

$175/night

Hilton.com

Hilton Garden Inn

200 Solar Dr., Oxnard

5.6 mi.

805-983-8600

$155/night

Hilton.com

Embassy Suites Mandalay Beach Resort

2101 Mandalay Beach Rd., Oxnard

15.4 mi.

805-984-2500

$200/night

Hilton.com

Travelling to the course

Transportation

Airports: There are three options for airports. Bob Hope Airport in Burbank will be the least congested and is serviced by American, United, Delta, Southwest and JetBlue:

Bob Hope Airport (BUR)

Los Angeles International Airport (LAX)

Santa Barbara Airport (SBA)

Shuttle: The Roadrunner Shuttle is a Camarillo-based service that provides door-to-door service from the airport. www.rrshuttle.com

Driving:

Bob Hope Burbank Airport (BUR) via US 101

https://goo.gl/maps/caMGB9QSEqP2

Los Angeles International Airport (LAX) via US 101

https://goo.gl/maps/kRBDQifyVfM2

Los Angeles International Airport (LAX) via Pacific Coast Highway (PCH)

https://goo.gl/maps/XPbBhNQYTzj

Santa Barbara Airport (SBA) via US 101

https://goo.gl/maps/2fGP3K7FMZx

About the presenter

3 Day Digital Control Course

Dr. Hamish Laird

Dr. Hamish Laird is a well regarded digital power electronics control engineer, researcher, lecturer and teacher.  Hamish is Chief Technology Officer at ELMG Digital Pwoer and holds a visiting academic position at the University of Canterbury in Christchurch, New Zealand.

During his career Dr Laird has worked on the control for;

  • High Voltage Direct Current Transmission
  • Reactive Power Compensators
  • AC and DC Motor Drives
  • DC to DC converters including LLC and phase shifted bridges
  • Medium and low voltage AC motor starters

Dr. Laird has worked for;

  • Alstom Grid (GEC Alsthom)
  • Eurotherm Drives
  • University of Canterbury
  • Aucom

Through ELMG Digital Power Dr. Laird  has provided advice, services and products to;

  • ABB
  • Enphase
  • Comsys
  • Evashred
  • TNEI
  • Eaton

Dr Laird says

“In designing and presenting the course we aim to have engineers able to use digital control in power electronics to achieve robust and reliable results.  See you in Camarillo”.

 

How to Register

Click here to register.  

P.S. Please note that the ELMG Digital Power course is being hosted at the Ridley Engineering Centre in Camarillo, California.  Ridley Engineering are processing all course registrations viatheir webstore.  Click here to register.  

APEC Presentation Slide Correction for b1 Coefficient

Wednesday, July 20th, 2016

At APEC this year we presented High Performance Digital Control of Power Electronics.  We then made the slides available and repeated the presentation as two webinars.

Click here to download the slides if you do not have them already  https://info.elmgdigitalpower.com/high-performance-digital-control-presentation

Thanks

Thanks to some good attention by a person who worked through the controller example,  we have noticed an error in one of the coefficients on the slide as shown below.

The correct value for the b1 coefficient is 0.00625.

Thanks to the helpful person who found this.

APEC Presentation slide correction for b1 coefficient

APEC Presentation slide correction for b1 coefficient

 

 

 

 

 

Congratulations to Dr. Rabia Nazir on completing her PhD in fractional delays in repetitive control

Friday, December 18th, 2015

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.

Contact us for a copy of the paper.

 

 

Digital Power Electronics Control Group on LinkedIn

Wednesday, February 25th, 2015

We have started a group where we can share questions and discuss Digital Power Electronics Control on LinkedIn.

The group’s name is Digital Power Electronics Control.

The link to the group is https://www.linkedin.com/groups?home=&gid=6677852.

Look out for the group identified with the z-transform image like that shown above.

Recent Group Discussions

Recent discussions have included one on the effects of precision limits for PWM timers

Another thread covers how to get started in Digital Control of Power electronics.

There are some well known and expert members in the group who share experience and insight.

The group is private to prevent the spam that has been increasing on Linkedin so please ask to join.

Where to begin with digital control

The discussion on where to begin with digital control is an ongoing and popular thread.  The thread has 49 replies from group members offering advice on what is important in digital control of power electronics

Slip Strike

Another discussion covering a very specific issue is on handling slip-strike problems.  Slip strike is what happens where the system output gets “stuck” and then suddenly moves.  This typically happens in mechanical or hydraulic systems where the friction coefficient falls as movement begins.  A relatively simple solution is to use a high derivative gain to reduce the effect.  Other solutions include linearising the system to allow higher controller gains and higher bandwidth.

Choosing the control processor

The discussion on choosing the processor for digital control is also still popular.  This covers which is the best micro controller or processor for digital control of power electronics.  This is an ongoing and recurring theme as more people  use digital control for their power electronics.  Other subjects in the thread include

  • analogue to digital converter choice
  • aperture time
  • settling time and
  • how to design a digital loop compensator

Numeric precision and the effect of limited PWM timer bit limits are also mentioned.

Join the community

We look forward to seeing you there.

Come and join the community at the Digital Power Electronics Control group on LinkedIn.

https://www.linkedin.com/groups?home=&gid=6677852.

Download the report ‘Your Digital Power Future – Roadblocks to Avoid’ to learn about the three key issues to watch out for in the Digital Control of Power electronics.


Download report now

Is the Little Box Challenge something you are up for?

Thursday, August 7th, 2014

littlebox

Google are always shaking things up.  And they are shaking the digital power electronics and low power loss converter space with this competition.

For those of you who don’t know, the Little Box Challenge is a competition where you can win $1 Million USD for creating a 2kVA inverter with a power density of 50W per cubic inch.  For those not familiar with this unit I calculate this to be 3.4 kW per liter.  Other specifications are around efficiency, AC voltage and current THD  and noise,  and the DC side current ripple.  The inverter is a single phase device.

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This is an interesting challenge. It is also put together and presented in an interesting way. The marketing website has pictures of hybrid cars, laptops and washing machines.  The inverters in these are three phase inverters. The fan in your laptop is driven by a three phase inverter as is the motor in your hard drive.   Plugin hybrid car AC to DC converters for battery charging are single phase but they are probably not inverters and in a strange juxtaposition, there is a picture of a hairdryer.   Then I realised I had misunderstood.   The inverter is to power the appliances and charge the electric vehicle.

When you read on it becomes clear that the the little box challenge technical problem is single phase and more to do with solar inverters or probably standby inverters for your household single phase loads.  Sorry to those at Google for misunderstanding the picture.

The minimum efficiency is do-able now with silicon and/or SiC.  I have an in-production 2.88 kW isolated digitally controlled AC to DC converter that I had downstairs  on my desk now.  It is more efficient by some distance than the minimum efficiency required in the contest.  It has SiC diodes in the PFC stage.  Aside from that it is all silicon.  And it is from six years ago.   Just measuring it now and it is 10 inches by 1 and 5/8 inches by 4 and 3/4 inches for a total volume of 77.1875 cubic inches.  This gives 37 W /cubic inch so it is in the ballpark.  All of you who do this stuff know that to get to 50W per cubic inch will take some effort. Having said that looking in my power supply there is some air space.

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The Littlebox requires a a DC side ripple current that is quite low. I haven’t worked out the 120Hz filtering volume needed to do this with capacitors but am assuming the volume will be significant.  By adding the DC side ripple requirement the challenge becomes really quite different and is more about energy storage replacement or the equivalent of valley filling. The solution to that might be some sort of integrated reasonable switching frequency active filter on the DC side that only operates at certain conditions so the 20% current ripple can be met. Perhaps the active filter could gyrate the storage capacitor. Or maybe some sort of pulse number increase like the old HVDC ripple current re-injection schemes.  I might dig a few of those papers out to see what use they’d be.

The AC side specification doesn’t seem too harsh.  A good inverter feedforward lineariser for PWM precision, dead-time and minimum pulse width with a good repetitive or resonant notch filter based current controller will meet this THD and noise requirement.  It’d be good to know the measuring instruments noise measurement approach.  The magnetising current of the transformer is supplied by the inverter so this will mean that getting both the current THD and the voltage THD below a certain limit may not be actually possible.  This will depend on the transformer magnetising characterisitcs.

It is possible that you will be able to meet the power density with the minimum efficiency as the cooling might be OK. That said the description of the testing doesn’t cover exhaust air temperatures as would a UL standard. There may be room for that kind of really high temperature exhaust air that UL are always so troubled by.  It might be that the contest rules need to be tightened on this.

For those of you considering this from outside the US make sure you get some US fuses as the UL and IEC definition of the current is significantly different. Maybe a part number specification for the fuse for the testing would be useful. US fuses will be hard to buy outside the US in any case.

It’d also be good to get some idea of what the transformer in the test is so any EMC filter design can be optimised for that transformer.

The solution to the DC ripple current was solved by Tesla (Nikola – not the car company) back when he was doing his thing. It is three phase AC systems where balanced three phase currents give constant power flow and so constant DC side current.  If only we had three phase appliances.

Unfortunately most of the people who legitimately have a good shot at meeting this challenge will not be allowed to as they are already doing this type of stuff for a job and so will not be able to meet the disclosure requirements in the terms and conditions.

Other issues might be getting your gear into the US.

For power density trends, analysis and a good commentary on the current state of power converter density take a look at http://www.hpe.ee.ethz.ch/uploads/tx_ethpublications/IEEJ_PowerDensity_Paper_FinalRevised_03.pdf.

The paper presents data that shows that the switching frequencies that maximise power density are surprisingly low.

Maybe Professor Kolar and his team at ETH will have a go at the little box challenge.

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