The thorn in the side of the supercapacitor is its limited lifetime. Supercapacitors can do many things in the field of energy storage and have be touted as being the future batteries. However any supercapacitor if not lifetime-engineered properly can fail prematurely. This leaves your company with potentially large warranty costs.
What is lifetime?
Supercapacitor lifetime stems from chemical reactions in the capacitor causing the capacitance to decrease. These reactions happen more often when it is hotter and thus lifetime can vary significantally with temperature. These reactions are governed by the Arhennius equation and appear on the datasheet as a lifetime. A typical lifetime for a supercapacitor is 1000 hours at 70C. This means that if your supercapacitor is at 70C it burns through a lifetime in 42 days! At only 83C some electrolytes can even boil causing your supercapacitor to rupture! However, if the temperature is decreased a supercapacitor’s lifetime can be increased greatly. It is all governed by this equation:
where t is lifetime, Tn is the nominal temperature, tn is the lifetime at the nominal temperature, Ea is the activation energy, T is temperature and kB is Boltzmann’s constant. In most cases this can be approximated by “halving the temperature doubles the lifetime”. If you are not using this equation in designing your supercapacitor systems then you better start using it fast! Note that this equation is for any electrolytic capacitor not just supercapacitors.
From the prior equation it can be seen that managing the temperature of supercapacitors can do wonders in increasing their lifetime. It is a lesser known fact that running them at voltages below the rated voltage also increases their lifetime.
Once you have your product in the field, re-engineering can become costly, so verification of your design and predicting failure rates becomes very important. This is done by measurement of the capacitors and then statistically analysing the results.
Measurement is important
Measurement of supercapacitors relies on the use of a simple capacitor model with just capacitance and ESR. This allows measurement by using the fact that the voltage response to a constant current source is a straight line. From this straight line you can thus measure the capacitance. When a constant current is applied there is also a small offset due to the ESR, which can be used to measure the ESR.
Once you made the measurements of a random group of supercapacitors you can begin to analyse the results. First of all you calculate the lifetimes that the supercapacitors have experienced up to the measurements based on their capacitance. Then these lifetime results typically fit a Weibull distribution. Using this fact, you can curve-fit your sampled data to a distribution with certain parameters and calculate the confidence of your fit. You now have a range of distributions that fit your data. From this range you simply find the worst case and use it to predict worst-case future failure rates. This information can be passed on to the accountant in the form of warranty liability.
How can we help?
ELMG Ltd have designed a system which can use this measurement technique to measure supercapacitor’s capacitance during both charging and discharging. This is done using a controlled constant current source. This system is highly scalable meaning that it can be used on capacitors ranging from a few farads to hundreds of farads. Once the measurements are made ELMG can then perform the statistical analysis and come up the results to tell your finance people what you warranty liability is.
ELMG Digital Power is an AMD-Xilinx Partner. As part of this we keep up to date with Xilinx parts, tools and technology with training. Consequently ELMG can provide the best digital control solutions for digital power electronics.
We started digital control of power electronics with Xilinx in 1997 so as a result ELMG Digital Power’s long history with Xilinx means that we have exceptional experience and expertise.
ELMG Digital Power for Digital Power Electronics Control expertise.
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.
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
Grid Interactive UPS Inverters
Battery Energy Storage Inverters
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 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.
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).
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.
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
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)
There are two principal types of batteries used in energy storage
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
industrial switch mode power supplies
reactive power compensation
medium voltage system
power quality systems
telecom switch-mode power supplies
grid connected energy storage converters
ELMG Digital Power helps our customers become leaders in digital power electronics.
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.