1. Introduction
While the number of photovoltaic (PV) installations continues to grow, the imbalance between supply and demand in the solar power grid has become a major constraint. During the day, there is ample solar energy available, but demand is low. This means users will pay higher prices per watt during peak usage times in the morning and evening.
Energy storage systems (ESS) for solar installations in residential, commercial, and utility buildings use inverters to store electricity or power from the grid during the daytime when demand is lowest, and release the generated energy when demand is high. Adding an ESS to a solar grid-connected system allows users to save on costs associated with technologies known as "peak shaving."
2. Bidirectional power conversion
Traditional photovoltaic (PV) systems consist of unidirectional DC/AC and DC/DC power stages, but the unidirectional conversion method is a major obstacle to incorporating ESS (Emergency Safe Module). More components, modules, and subsystems are required, all of which significantly increase the cost of adding ESS to existing solar installations.
To add batteries to existing photovoltaic systems, the two paths of battery charging and discharging must be merged into a single path consisting of power factor correction (PFC) and the inverter power stage. ... But how can a bidirectional power converter be built to replace two unidirectional power converters?
Figure 1. Block diagram of bidirectional PFC and inverter stage
Hybrid inverters can effectively improve the efficiency of the conversion stage, but this efficiency improvement is more important for microgrids equipped with ESS that perform multiple power conversions. The power converter system manages DC/DC conversion to charge and discharge the battery. It also manages DC/AC and AC/DC conversion, converting the DC power stored in the battery into AC power for flow into and out of the grid.
3. High-voltage battery
In microgrid systems with batteries, the primary function of the batteries is to store photovoltaic energy and supply power to the grid on demand. Lithium-ion battery packs have a significantly higher storage capacity per unit than lead-acid batteries.
As 400V battery packs become increasingly popular in the electric vehicle (EV) sector, solar grid systems are also pushing battery pack voltages up from 48V. But how do you manage the power conversion of 400V battery packs?
In addition to the microcomputer with system control and communication functions integrating ESS into larger systems, the low-loss and high-efficiency power switches also improve the safety and reliability of energy storage systems. Compact power switches and real-time microcomputers based on silicon carbide (SiC) and gallium nitride (GaN) materials allow for modification of bidirectional converters to accommodate various DC energy storage units. See Figure 2.
Figure 2. Smooth bidirectional operation for battery charging and discharging applications.
4. Dual Active Bridge DC/DC Converter Design
Wide-bandgap semiconductors such as SiC and GaN play a crucial role in solving power conversion systems that can handle the ever-increasing range of battery voltages as converters increase power density and reduce switching losses. Power conversion systems also allow battery packs to better manage power fluctuations in distributed generation systems, enabling smarter and more resilient grid operation at higher and wider voltage ranges.
Ultimately, solar power systems may mimic the battery packs used in electric vehicles. The idea of recycling current battery packs used in electric vehicles as grid-connected energy storage systems (ESS) is becoming increasingly common.
5. Wide-bandgap materials required for efficiency and natural convection
To build an intelligent wall-mounted storage system, it is necessary to design an inverter that optimizes heat dissipation using minimal natural convection cooling. A distributed power architecture allows heat to be concentrated throughout the system. This architecture ensures that the required energy storage inverter can handle high current levels at varying voltages and reliably respond to rapidly changing load transients.
Such systems require high-speed switching at switching frequencies from 100kHz to 400kHz and gate drivers with protection. If the switching speed is not fast enough, you will find that the efficiency of the power conversion stage is significantly low.
This is where wide-bandgap materials with fast switching and high power density (such as SiC and GaN) come into play. These semiconductor devices facilitate system designs that do not require fan cooling. The LMG3425R030 GaN device, with its built-in driver and protection, offers a compact form factor, high power density, and fast switching capabilities.
The gate driver converts the controller's digital PWM signal into the current required by the SiC or GaN field-effect transistor (FET). PWM-based controllers allow for precise sampling of voltage and current across multiple power conversion stages.
Figure 3. A digitally controlled GaN CCM totem pole reference design using a C2000 real-time microcontroller and a fast-switching GaN device with built-in gate drivers and protection.
6. Current and voltage detection
High-frequency switching power supply designs face the challenge of accurate current and voltage sensing. Current measurement with a shunt not only improves accuracy but also speeds up response time, allowing you to react quickly to any changes in the power grid, thus enabling you to shut down the system in the event of a short circuit or disconnection.
Current measurement is crucial for inverter-centric designs because control algorithms require current sensing for operation. Some design solutions utilize isolated current measurement using amplifiers/modulators and power supplies that are isolated from external shunts.
The power converter needs to measure the current in the power grid to see if the current is in phase with the voltage. By measuring the current and voltage, in addition to controlling the battery charging current, the inverter's operation and overload protection functions are also controlled.
7. Summary
Hybrid inverters, which enable bidirectional power conversion between AC/DC and DC/DC converters, are expected to replace traditional solar inverters in the coming years. Solar inverter designers will be able to achieve power conversion with a wide range of output power and voltage using hybrid inverters.
Increasing battery voltage and expanding voltage range are important issues for solar inverters compatible with energy storage. With basic components such as microcomputer control with built-in gate drivers and protection, and wide-bandgap semiconductors, these higher and wider battery voltages can be supported, in addition to the need for high efficiency and natural convection.
