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Battery energy storage systems (BESS) are expected to play an important role in the decarbonization of consumer and industrial buildings and appliances. Demand for BESS is growing fast, driven by regulations and incentives put in place by governments, as well as by a desire to maximize the return on investments in renewable energy systems such as solar panel arrays. The rapidly falling price of lithium-ion cells has also boosted the market for BESS.
This Design Note provides a guide to the most common topologies, systems and components in BESS, and to advanced products from onsemi that will enable designers to develop competitive BESS designs.
Architecture of a BESS
A BESS is made up of four elements, shown in Figure 1. Providing for energy storage, the battery packs consist of cells which are combined in high-voltage modules: these modules are in racks or banks to give the required energy capacity. The charge and discharge voltages can range from 50 V to 1,100 V, depending on the battery voltage and the circuit topology.
The battery management system (BMS) is an electronic system for managing the packs’ rechargeable batteries by ensuring that they are working in the safe operating area. The BMS also monitors the system’s operating state, and calculates and reports on real-time data from sensors in the BESS to enable the BMS to optimize operation for long life.
The power conversion system (PCS) is a sub-system for bidirectional conversion of electrical energy between the battery pack and the grid and/or load. The PCS is responsible more than any other element for determining the cost, size and performance of the BESS.
The energy management system (EMS) is a software-based system of computer-aided tools used by the operator of the electric grid to monitor, control, and optimize the performance of the generation or transmission system.
Fig. 1: Typical system architecture of a BESS
Ac- or dc-coupled system
There are two types of BESS: ac-coupled or dc-coupled. An ac-coupled BESS is a discrete system which can be added to an existing solar or energy generation system or grid, as shown in Figure 2: this enables battery storage to be added as an easy upgrade. An ac-coupled system, however, requires additional power conversion stages to perform full charging and discharging, resulting in higher losses.
Fig. 2: Block diagram of an ac-coupled BESS
On the other hand, a dc-coupled system, commonly employed in residential hybrid solar inverters, offers extra energy storage capacity when connected to the dc bus. Its operation entails only a single dc-dc conversion step, but such a system can only be implemented during product design, as the dc bus voltage is often high, and so would give rise to safety risks when being supplied as a retrofit option.
Fig. 3: Block diagram of a dc-coupled BESS
Bidirectional operation
The PCS requires bidirectional operation. Normally, three-phase inverters can be bidirectional and behave as an ac-dc converter when operating in reverse mode, as in an electric vehicle motor’s regenerative braking mode.
In general however, power converters and the topologies are optimized for one use case and one direction in which current flows. This is reflected in the selection and relative sizing of switches and diodes. Therefore three-phase inverters used as ac-dc converters in PFC mode will not be as efficient as an optimized ac-dc PFC converter.
Even dc-ac topologies that are intended to be bidirectional will provide better performance in one direction than the other. So, it is important to keep in mind the most common use case. Also, bidirectionality is not achievable with all topologies, so selecting the right one is an important factor in BESS design success.
Superior performance of SiC devices in PCS
In terms of performance, a silicon carbide (SiC) power switch is preferable to a silicon IGBT in high-voltage and high-current applications, not least because it enables high-frequency switching. Although the IGBT remains the preferred choice in BESS designs today, incorporating SiC devices in certain sections of the circuit can yield superior performance.
For instance, in the bidirectional inverter using an A-type neutral point clamped (A-NPC) topology, SiC devices may be selected in the inner legs to reduce switching losses: this topology’s switching strategy requires high-frequency operation in the inner switches, while the other switches can use IGBTs with a low saturation voltage to keep bill-of-materials cost to a minimum.
Suitable SiC devices can be found in a new onsemi family of 1,200 V M3S planar SiC MOSFETs. These MOSFETs are optimized for high-temperature operation. They also offer improved parasitic capacitance for high-frequency switching.
Product options for BESS designs
Three-phase I-type NPC is a common bidirectional topology in the PCS stage to match the increasing bus voltage. Compared to two-level topologies such as three-phase half-bridge, I-NPC requires more components and driving signals, and a complicated switching scheme also makes the design more difficult to implement. But the advantages are that it reduces switching losses, lowers current ripple, and reduces EMI.
For this topology, the NXH600N65L4Q2F2 power integrated module (PIM) is suitable: it is a high-performance 650 V IGBT module which contains an I-NPC inverter. The module withstands high current in both directions, making it the best fit for commercial PCS rated at higher than 100 kW.
Desaturation is one of the important protection functions used in high-power conversion circuits. This can prevent IGBTs or MOSFETs from suffering damage caused by a short-circuit by shutting down the switches very fast. The onsemi NCD57000 gate driver integrates a desaturation detection function: when the saturation voltage reaches its target value, an internal soft turn-off MOSFET is activated to discharge the gate capacitor. This reduces the over-voltage stress and losses caused by a high transient voltage spike.
This single-channel gate driver has a high source/sink current of 4 A/6 A, provides 5 kVrms of galvanic isolation, and offers additional protection functions including under-voltage lockout and an active Miller clamp.
Auxiliary power supplies in BESS
Auxiliary power supplies are usually based on a flyback topology using a primary-side regulated, quasi-resonant flyback controller. The NCP1362 from onsemi is a primary-side PWM controller for low-power offline switch-mode power supplies. The advantages of using this controller is that it requires no optocoupler feedback, thus improving the reliability of the power supply.
Additionally, the NCP1362 turns off the switch at a low drain-source voltage to improve efficiency and reduce the generation of waste heat.
Ethernet interface for remote monitoring
A distributed energy storage system is often comprised of hundreds of PCS and control units. A modern control center must adopt sophisticated connectivity technology to meet the growing demand for remote monitoring capability.
To meet this need, BESS manufacturers can use the NCN26010 Ethernet controller from onsemi, one of the first IEEE 802.3cg-compliant controllers available on the market. The benefits include:
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