A Smart Power System Made Possible by the Coordination Between Bidirectional DC-DC Converters and Energy Storage Systems

Charge and discharge (bidirectional) operations can be performed with a single unit and insulated as well as highly efficient power conversions can also be made

Demonstration experiments of smart grids, which are aimed at the optimized balance of supply and demand of power using ICT, are conducted in various locations serving as next-generation power infrastructures. Efforts are also being directed at ZEBs (Net Zero Energy Buildings) such as office buildings, which eliminate yearly energy budgets by using recyclable energy such as solar and wind powers. Systems like these will require the use of rechargeable batteries that store surplus power and technology for the efficient self-consumption of stored power and the cutting of power peaks.

This is why power supply systems require more saving of power. Commercial alternating currents undergo power conversions to become direct currents for electronic devices are driven by direct currents. However, the number of power conversions should be decreased in order to save more power because conversions are accompanied by heat generation loss in objects like semiconductor devices. Therefore, the introduction of DC power supply systems, where HVDC (high voltage direct current) buses for supplying DC power instead of conventional commercial alternating currents are constructed, electric/electronic devices are operated through HVDC buses, and rechargeable batteries are charged, is being considered for smart grids and ZEBs.

TDK's insulation type bidirectional DC-DC converters in the EZA series (products under the TDK-Lambda brand) are power supply units that serve as power conversion platforms in DC power supply systems and exchange DC power bidirectionally between HVDC bus and rechargeable batteries in a single unit. The incorporating of low-loss magnetic materials and the latest switching devices along with advanced digital control power supply technologies has led to extremely high bidirectional conversion efficiency. In addition to the conventional EZA2500 model with a maximum output power of 2.5kW, the lineup now also includes the EZA11K with a maximum output of 11kW with the same 1U full-rack size. Interconnections with energy storage systems in places such as office buildings, commercial facilities and public facilities, which use renewable energy, realize flexible and highly scalable power systems with various functions including those for DC power supply, leveling power consumption, and having backups during power failures.

Seamless switching between charging and recharging between HVDC bus and energy storage systems

In a DC power supply system, an HVDC bus of between 300 and 400V is constructed and power is supplied to electric/electronic equipment with DC inputs. The output power from solar power generation, wind power generation, fuel cells, and so on is DC and has the benefit of allowing systems to be expanded easily in collaboration with the HVDC bus. The generated power undergoes a voltage conversion with a DC-DC converter and supplied to the HVDC bus, and the surplus power is stored in an energy storage system that uses secondary batteries such as lithium ion batteries. The bidirectional DC-DC converter is a power supply unit that exchanges power between the HVDC bus and energy storage system.
Conventionally, when such a power supply was needed, there was no choice but to either develop a new one or to switch between two DC-DC converters, one for charging and the other for discharging. TDK's bidirectional DC-DC converters in the EZA series are state-of-the-art power supplies that realize both charge and discharge power conversions in just one DC-DC converter.

Stability of the HVDC bus voltage of DC power supply systems is a critical issue because power generated by sunlight and wind fluctuates according to the degree of solar radiation and wind speed. The charging and discharging of rechargeable batteries must be controlled precisely with no interruptions in order to stabilize voltage. TDK's bidirectional DC-DC converters in the EZA series offer high efficiency and seamless power conversions without interruptions even during frequent switching while showing excellent performance in stabilizing HVDC bus voltages.

An application model of TDK's bidirectional DC-DC converters in the EZA series for a smaller energy storage system mainly for industrial or commercial use is shown in Fig. 1.

Fig. 1 Application model of a bidirectional DC-DC converter for a smaller energy storage system (for industrial or commercial use)

Digital control power supply technologies have achieved multifunctionalization and the ability to design systems flexibly

Bidirectional DC-DC converters in the EZA series employ our accumulated digital control power supply technologies.
Switching power supplies, which are today's mainstream power supplies, have achieved stable output voltage by feeding back information on the monitored output voltage and controlling (PWM: pulse width modulation) the ON/OFF durations (duty cycles) of switching elements. In a conventional analog controlling power supply, the signals for controlling these ON/OFF durations are processed by an analog circuit. On the other hand, a digital control power supply performs this signal processing on a digital signal processing circuit realized by the A-D converter, DSP (digital signal processor) and software (Fig. 2).

Fig. 2 Circuit blocks and features of digitally controlled bidirectional DC-DC converters

The applying of this digital control technology to bidirectional DC-DC converters in the EZA series has led to the types of operation modes, the first of which is the "external-control operation mode", where charge and discharge operations are switched under directions from host controllers such as PCs and PLCs (programmable logic controllers). The second is the "autonomous operation mode", where charge and discharge operations are independently decided in accordance with voltage fluctuations of the HVDC bus. This maintains stability of the DC bus voltage.
In addition, they are equipped with the RS485 serial communication interface as a standard, which allows changes in voltage and current setting values as well as in operation modes in real time during operations and the monitoring of input-output voltage and current values.

The greatest feature of bidirectional DC-DC converters in the EZA series is that they make coordination between inverters, BMUs (battery management units), and EDLCs (electric double-layer capacitors) easy in addition to allowing the controlling of the charging and discharging between energy storage systems and HVDC bus to meet your intended applications so that you can configure your system flexibly. A simple rewriting of control parameters is all that is required to realize complicated control by using digital controlling technology without the need to develop a new exclusive power supply.

Bidirectional DC-DC converters taking an active role in the use of regenerative energy of industrial equipment

Digital control power supplies are used as power supplies of xEVs (electric vehicles) because better efficiency can be obtained as the power supply of the output power increases. In addition, it is predicted that energy storage units of smart houses and in-vehicle batteries of xEVs will share power or be interconnected with grid power to contribute to peak cuts or peak shifts of power consumption in the coming era of smart grids. This is another area where bidirectional DC-DC converters play an important role.

Moreover, they can be used for various types of industrial equipment using motors such as cranes, elevators, AGVs, and forklifts. Such industrial equipment is started and stopped very frequently. Therefore, energy can be reused without being wasted by storing the regenerative energy generated during deceleration of the motors in rechargeable batteries and supplying the stored power at the startup of the motor for assistance when large amounts of torque is required, thereby promoting power conservation. Furthermore, power for evacuation functions can be supplied from batteries to devices during emergencies such as power failures (Fig. 3).

Fig. 3 An application mode of energy regeneration for industrial motors with bidirectional DC-DC converters in the EZA series

The role that power electronics take on is becoming more and more crucial as energy conservation and power saving are top priority issues in the industrial society. TDK's bidirectional DC-DC converters in the EZA series have been developed by incorporating core technologies that we have accumulated including materials technologies such as those for ferrites, transformer technologies, circuit technologies, and evaluation technologies. They are intelligent and high-performance power supply units for the age of IoT.

Main specifications / electrical characteristics

Model name	EZA2500-32048	EZA11K-320240
HV side voltage range	320VDC(300～380VDC)	320VDC(240～400VDC)
Efficiency	92%	95%
Secondary battery side voltage range	48VDC(36～60VDC)	240VDC(150～300VDC)
Max. output	2.5kW	11kW
Communication protocol	RS485
Operation mode	External control / Autonomous
Size (W×H×D)	422.8×43.6(1U)×400mm	422.8×43.6(1U)×530mm
Other	Input output isolation, expandable by parallel operation

Example of use

① This is an example application by Yutaka Electronics Industry.
》 Introduction of example application: Yutaka Electronics Industry Co., Ltd. "MPS (Multi Power System) using EZA2500 bidirectional DC-DC converters"

② This is an example application by eneman.
》 Introduction of example application: eneman Co., Ltd. "'eneman' independent power supply system using EZA11K bidirectional DC-DC converters"

③ This is an example application by Seiko Electric Corp.
》 Introduction of example application: Seiko Electric Corp. “DC microgrid system for renewable energy using the EZA series of bidirectional DC-DC converters”

④ This is an example application by Kanazawa Institute of Technology.
》 Introduction of example application: Kanazawa Institute of Technology. “Electrothermal coordinating DC micro-grid system that uses renewable energy using the EZA series of bidirectional DC-DC converters”