Photovoltaic (PV) and wind turbine schemes play an ever increasing role in the global drive to reduce carbon dioxide (CO2) emissions and improve energy efficiency.
However, the increasing penetration of these distributed energy resources (DERs) within low-voltage distribution networks presents significant challenges in maintaining grid stability and security. Specifically the inherently variable nature of DERs can cause problems, such as overloading, transient voltage variations and issues concerning power quality, for example, 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 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 transportable and flexible, enabling their deployment on distribution networks to alleviate the problems introduced by DERs. In particular, the ability 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 have largely been theoretical and conceptual. But today real-life results from the GROW-DERS project are available. The European Commission’s Sixth Framework Programme funded this project, whose partners represent the entire value chain. They included KEMA, Liander, Iberdrola, MVV, EAC, Saft, Exendis, CEA-INES and IPE.
Lithium-ion (li-ion) batteries are considered one of the most suitable storage technologies for deployment in power grids. Their advantages when it comes to performance and service-life are significant when compared with conventional storage batteries. These include a very high efficiency of around 95 per cent, a long calendar life, a cycle life of 20 years at 60 per cent depth of discharge per day, lifetime sealing and freedom from maintenance.
|Li-ion battery system
A new generation of industrial li-ion battery systems are available specifically for renewable energy applications. GROW-DERS used these to create units of two cabinets that contain the batteries, a circuit breaker and a battery management module (BMM). The 400 V battery system had a power rating of 50 kW and a high energy output of 40 kWh. A separate energy storage inverter (ESI) monitors and manages the charging and discharging of the battery via a CANBus communication system. The ESI uses information gathered on the cell voltages and temperatures to optimise battery performance and life.
The project also investigated the use of flywheel energy storage systems for short discharge applications where high power is required, for example, to ride through voltage dips. But as flywheels have an energy output that is usually considerably lower than that of battery systems, the latter tends to be considered for applications requiring high power. GROW-DERS used a 1 kW flywheel for demonstration purposes.
The ESI was portable and acts as a 60 kW bi-directional galvanic isolated power converter that supports an input of 329-450 V DC and an output of three by 400 V AC. Additional functions specifically developed for the GROW-DERS project include an 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.
A remotely located energy management system (EMS) co-ordinated the storage system components. It takes selected measurements from the energy storage system and uses these data 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 GPRS, the data service for mobile personal electronic devices.
The EMS also provides real-time control of the systems that focuses on optimising component life and equipment safety. Developments are underway to expand the functionality of the EMS to include the management of multiple storage systems on the same distribution grid.
GROW-DERS focused on energy storage approaches for two main load management modes.
In the first of these the focus is on planning to facilitate arbitrage. This is 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 at a maximum.
|Peak shaving using energy storage (in Mode 2 operation) enables generation profiles to more closely match the load profile|
This mode also covers grid support and the improvement of power quality. This includes compensation of under and over-voltages through real-time management. So 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 for dip voltages and harmonics.
In the second mode, the focus is on peak shaving to enable the generation profile to more closely match the load profile. The energy storage system charges fully at night and then discharges during the day when the network is approaching the power or current threshold determined by local loads or the capacity of power cables, or both. This can be an effective way to defer major investments in network infrastructure that would otherwise be required to handle these periods of peak demand. This mode also covers voltage control grid support and power quality support.
Extensive laboratory testing has enabled the simulation of a wide variety of network conditions with varying degrees of intensity to validate the behaviour of the energy storage systems. Tests included active and reactive power scenarios, voltage dips, over 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 phenomena, such as voltage dips and flicker.
The storage systems were deployed at four field test sites. Each was selected with a specific area of focus to validate the transportability, installation and operation of the storage systems.Battery system one went to Zamudio in Spain; system two went to Chambery in France; the flywheel system went to Zutphen in the Netherlands; and a combination of systems went to Mannheim in Germany.
The Chambery site examined the development and evaluation of the EMS. The battery storage system was implemented in a live microgrid with connections to PV systems, transformers and loads.
At Zamudio, the focus was on the long-term operation of a storage system connected to a local grid serving a privately-owned technology centre, which comprised several buildings with some renewable energy sources connected to the grid.
The flywheel system was tested in Zutphen in an area with a large number of holiday homes. In this case its main use was to support power quality.
On completion of the initial field tests, all three systems were redeployed to the Mannheim site, close to a transformer station. This was an interesting grid location as it featured PV panels with a total maximum capacity of 80 kW and a local electricity demand. Two li-ion batteries and a flywheel were connected to the grid but at different nodes. During the field test there were no naturally occurring grid problems, so the project created some by artificially lowering the charge threshold of the cables. The field tests showed the energy systems reacted appropriately to alleviate the simulated grid problems.
The GROW-DERS project also focused on the development of an assessment tool called PLATOS that would determine the optimal location, size and type of storage system for any possible distribution grid configuration. The tool can also evaluate the technical and economical business case for storage systems, in both current and in future situations.
A key outcome of the project has been to show 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.
Of particular importance is that li-ion technology has shown that as well as handling a variety of functions 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 over-ridden by the voltage support function, reacting in real-time to actual grid circumstances. This requires dynamic, fast reacting charge-discharge capability and precise control of battery parameters, especially state of charge. Li-ion batteries meet these criteria, with the added benefit of high energy efficiency and long cycle life. Following the success of this project all the grid operators involved are positive about the opportunities for energy storage in their grids.
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. The vision for the future is to create an optimised energy system that integrates seamlessly the very best of distributed production, grid infrastructure, load management and storage. This will enable the most efficient use of local energy resources in a stable and reliable energy supply system.
Several projects are already underway to help turn this vision into reality. One is the Nice Grid project for the Carros district distribution network in France’s Var valley. Batteries of several hundreds of kilowatts will be installed along the distribution network to ensure the optimal integration of solar energy. These will ensure better management of the energy flow and voltage control. Nice Grid will also investigate islanding by enabling a branch of the network to operate autonomously through the use of batteries and PV panels.
In total 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 have an energy storage solution housed in a 20-foot (6-metre) container that delivers 1 MW of power. Furthermore, to control energy demands, batteries of around 10 kWh will be installed in homes to alleviate the load imposed on the network during peak periods.
Nice Grid is expected to deliver further evidence regarding the essential role that batteries can play in the management of distribution networks, especially in ensuring that supply and demand can be balanced flexibly and efficiently at the local level.
For more information on the GROW-DERS project, visit www.growders.eu.
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