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
- Solar Inverters
- Micro Inverters
- 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 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.
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
- 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)
There are two principal types of batteries used in energy storage
- Lithium Ion
- Lead Acid
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.
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.
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.