Posts Tagged ‘Digital 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.


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 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.


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

APEC Professional Education Seminar Well Received

Tuesday, April 19th, 2016

APEC 2016 at Long Beach, California

In March Dr Hamish Laird presented the High Performance Digital Control professional education seminar at this year’s Applied Power Electronics Conference in Long Beach California. Attendance at the seminar was high, with more than four hundred people coming to hear Dr. Laird speak. The large number of attendees asking questions at the end of the presentation showed how much demand there is for this type of knowledge and training.


The response to the presentation was very positive with a number of people asking if Dr. Laird could write a book containing information from the seminar.

Hamish Laird’s Reaction

Dr. Hamish Laird says, “As the power electronics design industry moves to digital control there is a great need to ensure that the quality of the digital control is high enough to ensure safe and reliable power supply operation. We at ELMG Digital Power hope to contribute to the constant improvement in the quality of the control”.

The ELMG Digital Power Chief Technology Officer also announced that “…as a result of the really large interest we are making the slides from the presentation available to download via our website. And we are working up a plan to present the three hour seminar content as a multipart webinar”.


Digital Modulators Explained

The first part of the APEC seminar covered how digital modulators such as Variable Period Oscillators (VPO) and Pulse Width Modulators PWM) differ from analogue modulators while the second part covered the other issues and know how to do with compensators, analogue to digital converters and anti-aliasing filters.

Dr. Laird again – “From the number of questions we received after the presentation it looks like digital control is well on the way to becoming established in the smaller switchmode power space. We look forward to helping achieve high quality digital control.”

Download the slides from the APEC presentation slides here.


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.



Latest news from the LinkedIn Digital Power Electronics Control Group

Monday, August 31st, 2015

What is the latest news from the Linkedin Digital Power Electronics Control group?

The group now has over 600 3440 members (as of January 25 2022). To join the group please click here.

In the group we have lively discussions with people contributing their knowledge and experience. To all who have contributed and made the group so much fun – a really big thank you.

In the group we have had discussions that cover

  • Digital PWM modulators
  • ADC (analogue to digital converters)
  • Choosing the number of bits in a digitally controller,
  • Implementing digital filters,
  • Processor choices and coding.

How many bits are required in digital control of power electronics

Recently the how many bits are required was brought up and addressed in this way by John S.

There are two aspects to selecting the number of bits: measurement resolution and control variable resolution. For measurement resolution, the ADC resolution is the deciding factor, and this can be increased by oversampling.

In a current control application I was recently working on, I required 14 bits of resolution to meet a target requirement of +-1% of ADC quantitization error at 10% current. This resolution was relatively simple to achieve by oversampling the 12 bit ADC in the uC.

The resolution of the control variable, e.g. uC timer resolution, is just as critical in fast systems. If the resolution is too coarse, jitter in the control variable will appear at the output. With slower system dynamics, e.g. in voltage controlled systems, jitter in the control variable causes less jitter in the output variable because of the smoothing effect of the output capacitor. Ideally the resolution of the control variable should match that of the measured variable.

Thanks John.

Thanks also to all the other contributors.

Other discussions cover measuring stability of power converters, what is the effect of PWM timer quantisation and what is the best converter for isolated bi-directional DC to DC.

 Join the group by clicking here.

The best integrator for digital control.

Look for this symbol to identify the group.

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

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.

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


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

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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