Power Electronics-based Energy Storage Devices

Article By : Pedro Esteban, Merus Power

Power electronics-based energy storage devices are among the fastest growing technologies for power quality improvement, the provision of ancillary services, clean energy development, and affordable energy access.

What’s happening in the power management space amid the never-ending drive to lower power consumption in more and more complex technologies and applications? What about in applications dealing with higher and higher voltages? This month’s In Focus highlights the various design developments and manufacturing strategies happening in the power management segment.


Most installations nowadays are exposed to multiple power quality problems and challenges to comply with grid code and sustainable energy requirements as they are not designed to support nonlinear, non-balanced and variable generators and loads that make up a large percentage of modern electric power systems.

These problems and challenges originate from equipment like renewable generation sources, fossil fuel generators, electric vehicle (EV) charging stations, data center servers, variable speed drives (VSD), welding machines, furnaces, lighting, underground high voltage cable (UHVC) networks, sensitive electronic equipment, highly dynamic loads, single-phase loads and railway electrification systems, to name a few. Events like capacitor switching (from existing capacitor banks or passive harmonic filters), auto-reclose operations of transmission and distribution lines or the starting of large motors also contribute to these problems and challenges.

Power electronics-based energy storage devices are among the fastest growing technologies for solving power quality problems, providing ancillary services, and supporting the development and access to affordable clean energy for a wide range of segments and applications. They are a high performance, flexible, compact, modular and cost-effective type of power electronics solutions with the ability to store energy that provide an instantaneous response in low or high voltage electric power systems. They protect critical equipment and facilities, facilitate the integration of distributed and intermittent generation sources, enable longer equipment lifetime, improved power system capacity and stability, and reduced energy losses, complying with most demanding power quality standards and grid codes, energy efficiency and sustainable energy requirements.

Power electronics-based energy storage devices

Energy storage-based devices have been around since the beginning of the 19th century. For example, electrochemical batteries have been used since the early 1800s and pumped hydro energy storage has been used since the early 1900s. Their use together with different types of power electronic switches is more recent, finding description of this topology and operating principle as far back as the 1960s.

Since the market introduction of energy storage devices with power electronic switches, they have been gaining considerable attention as replacement technology for conventional solutions like mechanically switched energy storage media and standby generators that have been traditionally used to deal with power quality problems and grid code and sustainable energy requirements. The drawbacks of these conventional solutions like their slow response time, bulky size and limited power quality and energy efficiency improvement capabilities together with the price reduction of power electronic switches resulted in the increased use of these devices in low and high voltage applications.

Modern power electronics-based energy storage devices can be controlled to act as current or voltage generators having an energy storage media able to provide active power for a certain amount of time when needed. Depending on the application, they can be connected in parallel or in series with the electric power system.

Types

Latest power electronics-based energy storage devices are typically built on neutral-point clamped (NPC) inverter technology. They can be installed to any point of the electric power system (low or high voltage) in parallel or in series with the equipment that causes problems, needs to be protected, or needs to comply with certain requirements. The devices connected in parallel are usually energy storage systems (ESS) and the devices connected in series are usually power protection systems (PPS).

Power electronics-based energy storage devices that can be found currently in the market include power protection systems like power and voltage conditioners (PVC), static UPS systems (SUPS), rotary UPS systems (RUPS) and continuous power supply systems (CPS), and energy storage systems like electrical energy storage systems (EESS), electrochemical energy storage systems (ECESS), mechanical energy storage systems (MESS) and hybrid energy storage systems (HESS).

Figure 1: Power electronics-based energy storage devices.

Over the years, the design of power electronics-based energy storage devices has been tailored to deliver specific functionalities. They are able to provide protection against power supply interruptions, compensate current-based distortions such as current harmonics and current unbalances, and compensate voltage-based distortions such as voltage harmonics, voltage fluctuations, voltage variations and voltage unbalances. They are able to provide ancillary services like frequency containment reserve, ramp rate control, voltage control, reactive power control, fault ride through capability and black start capability. They can also support the development and access to affordable clean energy by power factor correction, capacity firming, backup power supply, peak shaving, load levelling and load shifting.

Figure 2: Main capabilities of power electronics-based energy storage devices.

Energy storage technologies (EST)

Since the discovery of electricity, many different technologies to store energy have been developed, each with their strengths and weaknesses. Energy storage technologies (EST for short) have diverse classifications either based on their storage media or based on the functions they are capable to provide. According to their storage media, they can be classified into electrical, chemical, mechanical, electrochemical and thermal technologies. According to the functions they are capable to provide, they can be classified based on their energy density, their power density, their discharge time or their response time.

The design, functions, connection and applications of power electronics-based energy storage devices are mainly dependent on the type of energy storage technology the device will be using. The key energy storage technologies used by PPS and ESS can be classified into electrical, electrochemical and mechanical.

Figure 3: Key energy storage technologies for PPS and ESS.

Each energy storage technology offers several commercially available energy storage media designed in a wide range of ratings. While power and energy rating, and discharge duration of a certain energy storage media are important and often governs whether it should be considered for a particular application, there are numerous other physical and chemical features that contribute to the final selection decision, such as energy density, power density, storage duration, cycling, self-discharge and efficiency. While these features may determine which energy storage media may be preferred for a certain application, a very important factor that determines the feasibility of implementation is whether the benefits offered by a certain media exceed its cost.

Figure 4: Key energy storage media used by PPS and ESS

Power protection systems (PPS)

Modern electric power systems demand continuous high quality uninterrupted power supply free from any power quality problems. Momentary voltage variations or interruptions could damage electrical equipment, cause stoppages of manufacturing processes or even of the generation and supply of energy. These problems could be created either from the electrical supply side or from the load side of installations. Power protection systems are a cost-effective solution for protection against power supply interruptions and other power quality problems that can also help to comply with grid code and energy efficiency requirements.

PPS are connected in series with the equipment or facility that needs to be protected, directly or through a matching transformer or choke. The main energy storage technologies used by PPS are electrical (supercapacitors and capacitors), electrochemical (batteries and fuel cells) and mechanical (flywheels). Depending on their design and the functions provided, these devices can be divided into four groups:

  • Power and voltage conditioners (PVC).
  • Static UPS systems (SUPS).
  • Rotary UPS systems (RUPS).
  • Continuous power supply systems (CPS).

Figure 5: Types of power protection systems

Energy storage systems (ESS)

Modern electric power systems require that equipment and facilities comply with a wide range of power quality and energy efficiency standards and grid codes while generating and consuming low-priced high quality uninterrupted power free from disturbances. These equipment and facilities include electricity generating plants (renewable and non-renewable), consumers (loads) and electrical grids (transmission grids, distribution grids, minigrids and microgrids).

ESS provide a broad range of functions beneficial for generators, consumers and grid operators facilitating that equipment and facilities comply with power quality and energy efficiency standards and grid codes. They help reducing energy costs and environmental impact of operations while making the electric power system more resilient improving its overall efficiency, reliability, stability and availability. Together with the falling costs of energy storage media, the major drivers behind the rapid growth of ESS are the importance given to utilize more renewable energy sources, diminishing the use of fossil fuels, and the development of the future decentralised smart grid where end users are becoming electricity producers as well as consumers (or prosumers).

Thanks to their flexibility and their availability in a wide range of power and energy ratings, ESS can be used in many different applications. Some of the typical applications for ESS include power quality improvement, peak shaving, frequency support, capacity firming and ramp rate control for renewable energy sources, and integration of EV charging stations. They can also be used also in hybrid power systems (HPS) where multiple energy sources such as wind, solar, diesel or gas operate in parallel and where the integrator’s high level system controller coordinates the overall operation.

ESS are connected in parallel with the equipment or facility generating the power quality problems or that has issues to comply with grid code and energy efficiency requirements, directly or through an step-up transformer. The main energy storage technologies used by ESS are electrical (supercapacitors and superconducting magnetic energy storage), electrochemical (batteries and fuel cells) and mechanical (flywheel energy storage, pumped hydro energy storage, compressed air energy storage and liquid air energy storage). Depending on their design, these devices can be divided into four groups:

  • Electrical energy storage systems (EESS).
  • Electrochemical energy storage systems (ECESS).
  • Mechanical energy storage systems (MESS).
  • Hybrid energy storage systems (HESS).

Figure 6: Types of energy storage systems

Design

Power electronics-based energy storage devices are very customized solutions with a design based on end user’s technical and economic requirements.

Main components

The design of power protection systems and energy storage systems share some similarities. Both types of devices are equipped with an energy storage media connected to the electric power system through some kind of power electronics inverter. A control and protection system enables the devices to safely inject the required current/power/energy into the electric power system as demanded by the application.

Depending on design, PPS and ESS use different types of switchgear, transformers and chokes to connect to the electric power system including circuit breakers, bypass breakers, contactors, step-up transformers, delta transformers, injection/boost transformers, current and voltage transformers and coupling chokes.

Figure 7: Main components of power protection systems

Figure 8: Main components of energy storage systems

Voltage range

When connected to an AC electric power system, power electronics-based energy storage devices are offered in a range of voltages. Most common range is 200 V up to 690 V as the energy storage inverters are usually built using low voltage IGBT switches. Many manufacturers offer devices that can be connected directly to the electric power system within this range. It is possible to connect the devices to high voltage (over 1 kV) systems using a suitable step-up transformer.

Transformers should be studied carefully when designing a system with power electronics-based energy storage devices. Step-up or step-down transformers could reduce compensation performance due to increased impedance in between the device and the electric power system.

Figure 9: Connection possibilities of power electronics-based energy storage devices in an AC electric power system.

Internet-enabled technologies

Power electronics-based energy storage devices using industrial internet of things (IIoT) technologies can accurately and consistently capture and communicate data in real time. The adoption of the IIoT by power electronics-based energy storage devices is being enabled by the improved availability and affordability of sensors and processors. The IIoT allow the incorporation of machine learning and big data technology into the devices.

Figure 10: IIoT computing requirements power electronics-based energy storage devices.

There is an increasing market demand for smart devices and wireless connectivity technology for industrial equipment. Many power electronics-based energy storage devices in the market offer the possibility of remote asset connectivity, big data processing and analytics by using IIoT software platforms. This can improve the operational efficiency of installations (e.g., improved uptime and asset utilization) through predictive maintenance and remote management.

Functions

Power electronics-based energy storage devices provide valuable capabilities and flexibility to the electric power system. Depending on their design, they can combine different control functions in a single device. The vast array of functions that they can provide can be classified into four groups:

  • Power quality improvement.
  • Ancillary services.
  • Clean energy development.
  • Affordable energy access.

Some examples of the functions these devices can provide are explained hereunder.

Elimination of harmonics and interharmonics

Most electric power systems are not designed to support the nonlinear loads and renewable generators that nowadays make up a large percentage of the installed equipment. One of the main power quality problems that these devices bring are harmonic and interharmonic currents and voltages.

Figure 11: Harmonics waveform.

Power electronics-based energy storage devices can eliminate current or voltage harmonics (odd and even) and interharmonics by injecting the harmonic and interharmonic current signal measured into the electric power system. The injected current signal is of same magnitude but opposite in phase of the measured signal.

Protection against power supply interruptions

Continuous power supply within a certain voltage range is a critical issue for equipment, processes and facilities nowadays. Interruptions (complete loss of electrical supply for a duration of 0.5 cycles to one minute) or sustained interruptions (duration over one minute) can cause stoppage and damage of equipment and installations, disruption of processes, loss of production and data, plus the cost associated with downtime and the restart of the system.

The causes of interruptions can vary but are usually the result of equipment failure in the electric power system (insulation failure, insulator flashover, etc.), destructive weather and objects (trees, cars, etc.) striking T&D lines, fires, human errors, or bad coordination or failure of protection devices.

Power electronics-based energy storage devices can provide protection against power supply interruptions via active power injection. When the electrical supply is lost, power electronics-based energy storage devices immediately begin supplying active power from their energy storage through the energy storage inverter, guaranteeing maximum system availability and reliability.

Figure 12: Interruption waveform.

Power factor correction (PFC)

Low power factor is typically caused when inductive or capacitive loads like motors, transformers, cables or furnaces are present in the electric power system. Other contributors to low power factor are harmonic currents produced by nonlinear loads or renewable generators, or the change of load in the electric power system.

Figure 13: Low power factor waveform.

Power electronics-based energy storage devices can detect the phase angle difference caused by inductive or capacitive equipment and they can generate and inject in real time leading or lagging current into the electric power system, making the phase angle of the current the same as that of the voltage, which brings fundamental power factor to unity.

Peak shaving

Peak shaving refers to levelling out peaks in electricity use (mainly by industrial and commercial consumers with highly variable loads), reducing peak demand. With peak shaving, consumers reduce power consumption quickly for a short period of time avoiding consumption spikes. Peak shaving can be done by using power electronics-based energy storage devices, temporarily scaling down production, or activating an on-site generation system. The main benefits of peak shaving are:

  • Consumers save on electricity bills by reducing peak demand charges and/or reducing contracted power.
  • Electric utilities can reduce the operational costs of generating power during peak periods and avoid the investment on supplementary power plants.

Power electronics-based energy storage devices are ideally suited to serve as peak shaving assets as they can supply and absorb active power in real time when needed, supplying the peaks of a varying load and removing the need of bringing online a new generating source to cover the short-term demand.

These devices can be commanded to charge up during off-peak hours (when energy costs are low) and discharge during peak hours (when costs are high). In such cases the benefit of peak shaving is double; by reducing both the power fee and the cost of energy.

Peak shaving capabilities can be particularly beneficial in microgrids, where the expensive start-up of fossil fuel generators can be avoided by providing the extra power from an power electronics-based energy storage device during peak demand.

Figure 14: Peak shaving example.

Conclusions

Rise of nonlinear, non-balanced, variable and other challenging loads and generators in electric power systems present unique problems and challenges. Power electronics-based energy storage devices are the ultimate answer to power quality problems, providing ancillary services, and supporting the development and access to clean affordable energy for a wide range of segments and applications.

Power electronics-based energy storage devices are a high performance, flexible, compact, modular and cost-effective type of power electronics solutions with the ability to store energy that provide an instantaneous response in low or high voltage electric power systems. They protect critical equipment and facilities, facilitate the integration of distributed and intermittent generation sources, enable longer equipment lifetime, improved power system capacity and stability, and reduced energy losses, complying with most demanding power quality and energy efficiency standards, grid codes and sustainable energy requirements.

 

About the Author

Pedro Esteban is the Director of Asia Pacific for Merus Power Plc. Since 2002, he has extensive global experience in sustainable energy innovation and transformation, including renewable energy, power electronic solutions, energy storage, microgrid and its smart grid integration. He has been a leading expert in several marketing, strategic planning, business development, and communications positions at Areva T&D, Alstom Grid and General Electric. He has been based in Singapore since 2012.

To connect with Pedro, you may reach him at pedro.esteban@meruspower.com or linkedin.com/in/pedrojavieresteban.

 

 

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