With the turbulent world situation and increase in electric demand, energy has become an important topic around the world. In addition to actively increasing electricity generation capacity, the use of Energy Storage Systems (ESS) to conserve electricity and regulate electric demand is in present an important technology development direction. This article will introduce you to technological developments in ESS and the functionalities of Arrow Electronics’ ESS solutions.
ESSs can strengthen energy supply framework
Today, countries around the world often report tight power supply problems. In addition to calling for energy conservation amongst the population, the current energy framework needs to be adjusted. New power systems to be built to allow for integrated in-depth and coordinated interactions in terms of power generation, power grid, grid load, and energy storage, hence developing new models for flexible energy power systems. This requirement will bring about a wide gamut of energy storage application scenarios and installation requirements, with energy storage playing an irreplaceable and more important role in power system applications.
Energy storage technology refers to technologies which store energy for use when needed. During periods of low power grid demand, energy storage technologies can store excess power to be returned to the grid during peak demand. There are three common types of energy storage technologies. Firstly, mechanical power conversion. In the case of hydropower systems, additionally available power is used to pump water downstream of dams upstream through pipelines to preserve the potential energy of water upstream so that hydropower can later be used to drive the mechanical turbines to generate power. Secondly, thermal energy conversion. Equipment such as solar water heaters trap light energy (radiation) in hot water (heat). Thirdly, chemical storage of electricity. Electrical energy in batteries is stored in the form of chemical energy which is converted to electrical energy when the battery is used as a power source.
Application of silicon carbide technology to improve efficiency of battery energy storage
The rapid development of the new energy vehicle (NEV) industry in recent years has also led to the maturity of the lithium battery industry chain. The price of lithium batteries has fallen rapidly, with the cost of lithium batteries decreasing at 20%-30% per year. This trend promotes expanding application scenarios and business models for lithium batteries in energy storage and has also leads to increased demand for power converters.
Battery ESS operates by storing solar panel-generated power in batteries during the day for the stored power to be extracted for domestic or factory use at night or in emergencies.
During the power conversion process in an ESS, batteries must firstly be charged. The batteries are usually charged by DC which is converted from AC from the electrical grid via an inverter. During discharge, the inverter converts DC from the battery into AC which powers the electrical grid.
As silicon carbide (SiC) technology matures, current mainstream converts electrical power from AC to DC through SiC MOSFET. Due to MOSFET’s bidirectional switching characteristics, SiC MOSFETS also allow converting DC into AC. This switching process is controlled by the MCU.
SiC MOSFETs can operate at higher switching frequencies, are 50% smaller compared to traditional power converters and are more efficient in delivering bidirectional outputs achieved through simpler digital control and inversion output voltage through pure sinusoidal waves.
Modular bidirectional energy storage inverter reference design
For ESS applications, Arrow Electronics launched a bidirectional energy storage inverter reference design which allows AC/DC bidirectional power conversion, charging power of up to 6.6kW, AC input voltage from 180Vac to 265Vac at 50Hz, DC output voltage from 60Vdc to 90Vdc, with a maximum inverter power of 6.6kW. With the inverter rated input at 80Vdc, the inverter rated output is 220Vac at 50Hz, with a conversion efficiency > 93%.
The modular reference design includes Totem-Pole PFC bidirectional power converter board, CLLC bidirectional power converter board, AC-12V isolated auxiliary power board, 12V-12V isolated auxiliary power board, and isolated gate driver board. Customers can easily adapt the reference design based on product needs.
Core components used in this reference design include a monolithic control using ST STM32G474VBT6, CLLLC + PFC SiC MOSFET using ST SCTWA60N120G2-4, ST STGAP2SiCS isolated gate driver, ST A6986I and VIPER329HDTR isolated auxiliary power, ST L9616 CAN transceiver, ESD-protection ST HDMIULC6-4SC6Y/ST ESDCAN03-2BWY, ST TSZ181IYLT high-precision operational amplifiers, and Allegro current sensors ACS772LCB-050B-PFF-T, ACS772LCB-100B-PFF-T, ACS37002LMABTR-050B5-M, ACS72981LLRATR-100B5, ACS37610LLUA-020B5, TE T9V1K15-12S relay and muRata GCM43D7U3A472JX01L resonant capacitive tank, and electrolytic capacitors including Lelon VZH-471M1ETR-1010, 400BXW220MEFR18X50, KEMET F861DP155K310ZLH0J, R463W510050M1K, EDH477M025A9PAA, C4AEOBU4500A11J, etc.
Bidirectional AC/DC Totem-Pole PFC
Arrow Electronics has launched a bidirectional AC/DC totem-pole PFC for energy storage applications. It is a bidirectional totem-pole PFC and CLLLC topology application solution based on ST high-performance MCU STM32G474 and ST SiC. It helps customers understand and learn about the characteristics, performance, and control of two-stages topology, and also visually assess the performance of main devices, thus accelerating customers’ product development Arrow Electronics’ ESS solution’s two-stages topology is divided into two boards including totem-pole PFC and CLLLC, with both boards working independently or in cascade to suit different customer needs. The following will highlight the key features of totem-pole PFC.
In the design and development process, the totem-pole PFC uses the lowest number of components in currently-known PFC topology and also has the advantages of minimum conduction loss and maximum efficiency. The totem-pole PFC has attracted increasing attention and is currently a preferred option for bidirectional AC/DC conversion.
The AC-DC rectifier mode (charging mode) for the totem-pole PFC solution is used for software implementation. In the PWM sequence, the totem-pole PFC power flow is divided into 4 stages, including two high-speed transistors with a switching frequency of 133kHz and two low-speed transistors with a switching frequency of mains frequency. Normal MOS transistors can be used as low-speed transistors.
The peak at the time of mains zero is processed by software for soft start/end of PWM. Due to the complementary output of PWM from the two transistors at the totem-pole High Frequency bridge, the two switching functions will interchange between negative and negative halves at the time of mains zero crossing, and the duty ratio will jump from 0% to 100%, or 100% to 0%, resulting in the current peak at the time of zero crossing. To reduce the current peak at the time of zero crossing, the software will soft start/end PWM processing at the time of zero crossing.
The totem-pole PFC controller plays two roles: one is to stabilize the DC output voltage of the PFC and the other is to control the input mains current. To reduce harmonic interference from the equipment to the grid and reduce the reactive power of the equipment and improve efficiency, a power factor closes to 1 is ideal. This requires controlling the input current waveform and phase to synchronize the input current with the input voltage phase, which will be realized by a software phase-locked loop, controlling the input current waveform, and stabilizing the DC output voltage which will be realized by a software double-closed loop.
In DC-AC inverter mode (discharge mode), the totem-pole PFC operates in reverse as an H-bridge inverter topology which can be applied to grid-connected inverters and off-grid inverters. Its pulse-width modulation method is commonly used in SPWM, which produces three types of output: unipolar SPWM, bipolar SPWM, and unipolar octave SPWM. Unipolar SPWM is used in this solution as it has a better THD than bipolar SPWM, but its disadvantage lies in a zero crossing peak problem like PFC, and the peak can be reduced by soft start/end at the time of zero crossing.
The PWM sequence in the discharge mode is divided into 4 stages in the H-bridge inverter power flow. There are also two high-speed transistors with a switching frequency of 133kHz and two low-speed transistors with a switching frequency of mains frequency. Normal MOS transistors can be used as low-speed transistors.
Inverter controllers, depending on application scenarios, can be divided into off-grid inverters and grid-connected inverters, both of which evidently differ in software control and focus. Grid-connected inverter controls are more like the reverse control of PFC rectifiers. As with rectifiers, the phase ring must be locked to synchronize voltage and current. The off-grid inverter control is more focused on the adaptability to load capacity at different loads. The inverters also control different parameters: Grid-connected inverters control grid connection current whereas off-grid inverters control output voltage.
The design elements of this totem-pole PFC solution include peak treatment at mains zero crossing, phase compensation, THD improvement, controller parameter adjustment, sampling signal processing, light load control, CBC current limiting and off-grid inverter load capacity, all of which can solve various design challenges faced by customers.
Conclusion
ESS has been one of the important applications for solving energy problems. In the future, more ESS applications of different grades and specifications in different fields such as home, industry, government, etc., will comprehensively improve grid operation efficiency and reduce energy waste. In accumulating operational experience on the STM32G474 in digital switching power supply and bidirectional totem-pole PFC, Arrow Electronics can provide hardware and software support for totem-pole PFC system solutions, address technical difficulties, share PCB design and debugging expertise, thereby speeding up customers’ product development. Should you have any related requirements, please feel free to conduct Arrow Technologies.
Courtesy: Arrow Electronics
