The ELMG Blog

Energy Storage everywhere and a converter to connect it.

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

The life of ELMG Digital Power Engineers

This is the life of ELMG Digital Power Engineers working for our global customers. So five years ago I was biking around Berlin after having spent the week with my ELMG Digital Power colleague and our customers and their customers, testing 12MW grid-connected power converters where the control did not yet quite work properly.

This is the Reichstag in the background. It was a beautiful, cool, Berlin day.

Eventually, the power converters worked really well and we eventually did the first commissioning for them when they were installed on the Qatar Metro Rail.

Berlin biking after power converter testing.

To address the issues we changed the software we had written and the FPGA code. This was to ensure that the gate pulses were always absolutely consistently in the right place at the right time. This is the constant challenge of power electronics. At 12MW it is very noticeable when gate pulses aren’t always in the right place.

Berlin biking route. Checkpoint Charlie, Brandenberg Gate and the Denkmal für die ermordeten Juden (Holocaust Memorial) on the route.
Berlin’s Holocaust monument.

ELMG Digital Power worked on a large-scale converter project, biked around Berlin on a day off, tested further in Japan, Turin, and Prague, and then commissioned the converters in Doha. As a result, our customer is now the leader in their market segment.

Working for our global customers’ success – this is the life of ELMG Digital Power Engineers.

This is one of many successful ELMG Digital Power Electronics engineering developments.

Contact ELMG for your power converter development.

Checkpoint Charlie in Berlin

FAQ

Do you do small converters?

Yes, all sizes from 100W to 500MW – we work on converters of all sizes. Grid storage, telco, motor drives, aerospace, traction. We do power converters for any application.

Where are your customers? Can you help us?

Arizona, Belgium, California, China, Denmark, England, France, Germany, India, Massachusettes, New Zealand, North Carolina, New South Wales, Scotland, South Australia, Switzerland, Sweden, Queensland, United Kingdom, United States of America.

Yes, we can help you.

Do you need to wear a bike helmet in Berlin?

No. There is no requirement by law.

What sort of control do you use?

Mostly DSP, Microcontrollers and FPGA. Our favorite microcontrollers/DSPs are Ti ti.com. Our favorite FPGAs are Xilinx xilinx.com.

Do you have any case studies?

Contact us and we’ll provide some.

Contact ELMG for your power converter development.

Do you have any references?

Yes.

ABB

Kraft PowerCon

TNEI

Comsys

How long have you been doing Digital Power Electronics Control?

As ELMG Digital Power Electronics since 2001.

ELMG Digital Power People have been working on digital power electronics control since 1992.

About ELMG Digital Power

ELMG Digital Power provides technology, know-how, and products to control, manipulate and measure electrical power. This means we design and build the best digital controlled power electronic systems and by doing that we change the world.
For the past twenty-five years, we have been working on digitally controlled power converters in

  • motor drives
  • industrial switch mode power supplies
  • reactive power compensation
  • medium voltage system
  • power quality systems
  • motor starters
  • appliances
  • telecom switch-mode power supplies
  • UPS
  • railway converters
  • grid connected energy storage converters

ELMG Digital Power helps our customers become leaders in digital power electronics.

Contact ELMG for your power converter development.

Happy Birthday Claude Shannon

Happy Birthday to Claude Shannon.

Shannon is known as the “father of information theory” because of the landmark paper he published in 1948. He is also known for founding both digital computer and digital circuit design theory in 1937. Shannon received the IEEE Medal of Honor in 1966 “For his development of a mathematical theory of communication which unified and significantly advanced the state of the art.” Claude Shannon would have been 106 on the 30th of April 2022.

Shannon had lots to do with the sampling theorem.

Happy birthday, Claude Shannon!

ELMG Digital Power Continues AMD Xilinx Partner Program

We are pleased to announce that ELMG Digital Power continues to be part of the AMD Xilinx Partner Program.

As ELMG Digital Power CTO Dr. Hamish Laird explains

“Being part of the Xilinx Partner Program has always been something that we at ELMG Digital Power are very proud of, grateful for, and happy with as it lets us do amazing things for our customers.”

He continues…

“It is fantastic to have the connection that gives us access to all the great AMD/Xilinx tools such as Vitis, HLS, IP Integrator, and Vivado with all its code templates. The combination of the toolchain and the devices from Spartan, Artix, Virtix, Zynq and UltraScale through to SoMs such as the Kria K26 means that ELMG Digital Power can provide great cost-effective, powerful solutions for our customers.”

Constant professional development and training for ELMG Digital Power people allow us to use the best solution for our digital power electronics control platforms.

Dr. Laird again ” We are very grateful and humbled to be part of the Xilinx Partner program. We deliver the best power electronics controllers using this technology”

About AMD Xilinx

Xilinx is now part of AMD. AMD now has the industry’s broadest product portfolio and a highly complementary set of technologies, reaching customers in a diverse set of markets. Together, AMD and Xilinx leverage the right engine for the right workload to address the compute needs of our customers.

www.xilinx.com

About ELMG Digital Power

ELMG Digital Power provides technology, know-how, and products to control, manipulate and measure electrical power. We design and build the best digital controlled power electronic systems and by doing that we change the world.


For the past twenty-five years, we have been working on digitally controlled power converters in motor drives, industrial switch mode power supplies, reactive power compensation, medium voltage system, power quality systems, motor starters, appliances, energy storage converters, UPS, and telecom switch-mode power supplies.

ELMG Digital Power helps our customers become leaders in digital power electronics.

Inverter grid synchronisation – Six AC system frequency facts

With inverter grid synchronisation the key problem is that the grid frequency can vary.  In fact the frequency of AC systems around the world is different and is constantly changing.

  1. 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.
  2. 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.
  3. In Switzerland the railway has their own separate distribution grid.
  4. 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.
  5. Early on GE decided that 40Hz would be good for AC distribution but it did not catch on.
  6. 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.

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Other useful information for synchronisation.

  • NATO ships are 60 Hz,
  • The Swiss, German, and Austrian rail network reportedly changed to 16.7 Hz from 16 2/3 Hz  in 1995.  This to prevent over heating in rotary converters.
  • Japan has both 50 Hz and 60 Hz with the 50Hz built by the British and the 60Hz by the US after the second world war.  The two systems are linked by HVDC links.
  • 60 Hz is probably a better choice than 50Hz but it is too late now for the 50Hz countries.
  • Reportedly Tesla (not the car company) chose 50Hz first and then 60Hz.
  • There is a power station on the Argentina/Brazil border where half the generators make 60Hz for Brazil and half make 50 Hz for Argentina.
  • Sometimes, in some countries, on some days, 50Hz AC systems will run at 38Hz.  This presents an interesting inverter grid synchronisation problem.

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Grid frequency varies constantly as the mismatch between generator and load power changes.  In most countries the variation is not as large as the change to 38Hz.

Grid connected power converters must track or lock to the grid the grid.  The best way to do this is with a phase locked loop or a frequency locked loop.

Inverter Grid Synchronisation

Phase locked loops to track grid frequency are an essential part of grid connected converters. Click here to ask us about using an ELMG phase locked loop for inverter grid synchronisation.

Xilinx Partner Technical Recertification

ELMG Digital Power CTO Dr. Hamish Laird has completed Xilinx Partner technical recertification for ELMG Digital Power’s ongoing Xilinx partner program.

This ongoing commitment to capability, learning, and keeping up to date with Xilinx technology is an investment that pays dividends when we are implementing high-performance digital power controllers.

This year’s recertification covered the new Versal ACAP. This extended SoC device is powerful to the point of being daunting. As Dr. Laird says “I was suspicious of the suitability of the ACAP as a device for power control but I can now see how this device is suitable for almost every application we could ever consider.”

Xilinx Partner technical recertification – the certificate

Xilinx Partner Technical Recertification
Xilinx Partner Technical Recertification Certificate

Dr Laird continues “We are really thankful to Xilinx for our ongoing partnership program membership. The program is great and allows us access to world-leading devices, toolchains, and training. Completing this Xilinx Partner Technical recertification is always a great day in the year. The training material is really good and well worth the time and effort required.”

About Xilinx

We Are Building the
Adaptable, Intelligent World

Xilinx is the inventor of the FPGA, programmable SoCs, and now, the ACAP. Our highly flexible programmable silicon, enabled by a suite of advanced software and tools, drives rapid innovation across a wide span of industries and technologies – from consumers to cars to the cloud. Xilinx delivers the most dynamic processing technology in the industry, enabling rapid innovation with its adaptable, intelligent computing.

About ELMG Digital Power

We Provide Technology, Know-How and Products to Control, Manipulate and Measure Electrical Power.

ELMG Digital Power aims to design and build the best digital controlled power electronic systems.
By doing that we change the world for the better.

ELMG Digital Power is part of a select group of companies invited to be part of the Xilinx Partner Program. Each year ELMG people complete training in Xilinx tools and devices.