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The increasing penetration of distributed PV and other forms of distributed energy resources (DER) within low-voltage distribution networks presents significant challenges in maintaining grid stability and security.

Specifically, the inherently variable nature of DER can cause overloading, transient voltage variations and power-quality issues, such as harmonic distortion.

It is only in the past few years that energy-storage technologies have been developed into viable solutions for distribution-grid applications. Prior to these developments, only large-scale pumped-hydro storage and compressed-air storage systems were used to store large amounts of energy, usually in high-voltage transmission networks.

Recent advances in energy-storage systems now offer the potential to create new solutions that are both transportable and flexible, enabling their deployment on distribution networks to alleviate the problems introduced by DER. In particular, the capability of energy storage to deliver power to support peak loads or absorb energy when demand is low and production is high can be used for quality enhancement of the electricity grid and especially the regulation of power distribution.

In general, studies regarding the use of energy-storage systems to match generation and load profiles in distribution networks had been largely on a theoretical and conceptual basis. The completion of the GROW-DERS project, however, provides real-life practical test results. The project was funded by the European Commission, with project partners representing the entire electricity value chain, including KEMA, Liander, Iberdrola, MVV, EAC, Saft, Exendis, CEA-INES and IPE.

 

Storage technology

Currently, lithium-ion (Li-ion) batteries are considered one of the most suitable storage technologies for deployment in power grids. Li-ion technology offers advantages in performance and service life compared with conventional storage batteries. Li-ion batteries can offer 95% efficiency, as well as a long calendar life and cycle life - 20 years at 60% depth of discharge/day. Li-ion batteries are also sealed for life and maintenance free.

A new generation of industrial Li-ion battery systems was designed specifically for renewable energy applications. These were used in the GROW-DERS project to create a Li-ion battery unit consisting of two cabinets that contain the batteries, circuit breaker and battery management module. The 400 V battery system was designed for a power rating of 50 kW and an energy output of 40 kWh.

The separate energy storage inverter (ESI) monitors and manages the charging and discharging of the battery via a CANBus communication system utilizing information gathered on the cell voltages and temperature to optimize battery performance and life.

The project also investigated the use of flywheel energy-storage systems for short discharge applications where high power is required - e.g., to ride through voltage dips. Compared to battery systems, the energy output of these systems is usually considerably lower; therefore, batteries are mainly considered for applications requiring high power. In the GROW-DERS project, a 1 kW flywheel was used for demonstration purposes.

A portable ESI was designed and developed for the project. The inverter is a 60 kW bi-directional galvanic isolated power converter that supports an input of 450 V DC and an output of 3 x 400 V AC. Additional functions specifically developed for GROW-DERS include a UPS (islanding) mode, reactive power compensation, dip and flicker compensation, and active harmonic damping.

The ESI was developed to be remotely controlled by higher-level management systems using the Modbus protocol. It was used to manage both the battery systems and the flywheel system.

Management and coordination of the storage system components are performed by a remotely located energy management system (EMS). The EMS takes selected measurements from the energy-storage system and uses this information to calculate optimal charge and discharge set points in accordance with pre-defined objectives and constraints. Communication of the measurements and set points between the EMS and ESI takes place via general packet radio service.

The EMS also provides real-time control of the systems, which focuses on optimizing component life and equipment safety. Developments are currently under way to expand the functionality of the EMS to include the management of multiple storage systems on the same distribution grid.

 

Load management

The GROW-DERS project focused on energy-storage approaches for two main load management modes. In Mode 1, the focus is on planning to facilitate arbitrage - the purchase and sale of electricity at the optimum time. This would enable, for example, the owner of the battery system to store electricity when the forecast trading price is low and then release it during periods of high demand, when the price is high.

This mode also covers grid support and the improvement of power quality, such as compensation of under-and over-voltages through real-time management. The battery is charged when there is an over-voltage and discharged when there is an under-voltage. Power-quality support functions also include compensation of dip voltages and harmonics.

In Mode 2, the focus is on peak shaving to enable the generation profile to more closely match the load profile. The energy-storage system is charged fully at night and then discharged during the day, when the network is approaching the power or current threshold determined by local loads and/or the capacity of power cables.

This can be an effective way of deferring major investments in network infrastructure that would otherwise be required to handle these periods of peak demand. This mode also covers grid support (voltage control) and power-quality support.

Extensive laboratory testing enabled the simulation of a wide variety of network conditions with varying degrees of intensity to validate the behavior of the energy-storage systems. Tests included active and reactive power scenarios, voltage dips, over-voltage and under-voltage conditions, harmonics and islanding.

The hardware successfully passed all the tests, and the results were used to enhance the ESI software to provide improved compensation for power-quality issues, such as voltage dips and flicker.

 

Field tests

The storage systems were deployed at four field test sites. Each was selected with a specific focus area to validate the transportability, installation and operation of the storage systems:

In Chambery, the focus was on the development and evaluation of the energy-management system. The battery-storage system was implemented in a live microgrid with connection to PV systems, transformers and loads.

The Zamudio field test focused on the long-term operation of a battery-storage system connected to the local grid serving a privately owned technology center. This business park comprises several buildings - mainly offices and some research facilities - with some renewable energy sources connected to the grid.

The flywheel system was tested in Bronsbergen in an area with a large number of vacation homes. In this case, it was mainly used for power-quality support.

Upon completion of the initial field tests, all three of these systems were redeployed to the Mannheim test site, which is located close to a transformer station. This was identified as an interesting grid location, as it featured both decentralized generation from PV with a maximum capacity of 80 kW, as well as local electricity demand.

Two Li-ion batteries and a flywheel were connected to the grid at different nodes. During the field test, no naturally occurring grid problems were experienced. Therefore, problems were intentionally created by artificially lowering the charge threshold of the different cables.

The field tests showed that the energy systems reacted appropriately to alleviate the simulated grid problems.

The GROW-DERS project also focused on the development of an assessment tool, known as PLATOS, to evaluate the optimal location, size and type of storage system for any possible distribution-grid configuration. This tool can also be used to evaluate the technical and economical business case for storage systems in current and future situations.

 

Feasibility

A key finding of the GROW-DERS project was that it is feasible to design and implement transportable and flexible energy-storage systems that can play a significant role in improving the management of distribution grids, especially when they are required to integrate increasing levels of DER.

Specifically, Li-ion technology showed that in addition to handling a variety of functions - including energy trading, voltage support and peak shaving - it can serve two or more of these functions simultaneously through suitable priority settings. For example, the planned charge/discharge profile for energy trading, set according to forecast prices, might be overridden by the voltage-support function, reacting in real time to the actual grid circumstances.

This flexibility requires dynamic, fast-reacting charge/discharge capability and precise control of battery-operation parameters, especially state of charge. Li-ion batteries meet these criteria. Following the success of this project, all the grid operators involved are positive about the opportunities for energy storage in their grids.

Overall, the GROW-DERS project has shown important potential for assisting the smooth integration of DER within distribution networks. It is also particularly relevant to the creation of smart grids, in which energy storage is expected to play a vital role.

One vision for the future is to create an optimized energy system that seamlessly integrates the very best in terms of distributed production, grid infrastructure, load management and storage. This setup will enable the most efficient use of local energy resources in a stable and reliable energy-supply system.

Several projects are already under way to help turn this vision into reality. One of the most important is the Nice Grid project for the Carros district distribution network in France’s Var Valley. In this project, in order to ensure the optimal integration of a large amount of solar energy, batteries of several hundred kilowatts will be installed on various branches of the distribution network.

These batteries are expected to help ensure better management of the energy flow and voltage control. The project will also investigate islanding, enabling a branch of the network to operate autonomously through the use of batteries and photovoltaic panels.

A total of 2.7 MWh of Li-ion batteries will be deployed across the network at various levels, from the transformer that links the transmission and distribution networks to the end-user.

The substation will be provided with an energy-storage solution housed in a 20-foot container that delivers 1 MW of power. Furthermore, to control energy demands, batteries of approximately 10 kWh will be installed in homes to alleviate the load imposed on the network during peak periods. S

 

Michael Lippert is marketing manager for the energy-storage division at Saft Batteries. He can be contacted at 33 49931784 or michael.lippert@saftbatteries.com.

Product: Energy Storage & Charge Controllers

Energy-Storage Applications For Renewable Energy On Distribution Networks

By Michael Lippert

Recent research has focused on using energy storage to support peak loads or absorb energy on grids with high levels of distributed PV.

 

 

 

 

 

 

 

 

 

 

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