by Mike Rycroft, Now Media
A microgrid is a miniaturised version of the larger grid, a configuration of energy resources, distribution wires and buildings, all within a distinct geographic footprint. There is no size limit, but microgrids tend to be scaled to discrete operations, such as a small village, neighbourhood, community, business park, education campus, mine, or industrial facility.
Microgrids developed in response to the growth in distributed generation, which brings generation closer to the point of consumption, and the microgrid concept allows local control over localised generation. Microgrids also allow the efficient use of renewable energy resources in communities isolated from the national grid. The size and development of microgrids allows incorporation of smart grid features which optimise power generation, storage and use.
A microgrid consists of a network which combines many components, including power generation, energy storage, grid connection and network control as shown in Figure 1.
This is generally taken to mean a network which combines distributed generation in the form of a combination of renewable and non-renewable energy sources. While the national grid is designed for top down power delivery, i.e. power is generated remotely and despatched via the grid to users across the country, microgrids, are configured for bidirectional power flow.
The heart of a microgrid system is the microgrid controller. All ancillary services are provided internally, including balancing the active and reactive power; stabilising the grid’s frequency and voltage,while providing quality, reliability and controlling power flow.An essential feature of the microgrid is that all elements are interconnected via the microgrid controller. The controller continuously monitors generation and consumption and balances energy generated with energy consumed and energy stored.
A microgrid may be used to serve a single site, such as an industrial or commercial complex, or several smaller individual sites,in a utility or communal microgrid application.
In function, the microgrid resembles a hybrid system by combining different generation and storage technologies. The main difference is that hybrid systems serve a single user or customer at a single location, whereas the microgrid is designed to serve multiple users at separate locations.
Off-grid microgrids are based on renewable energy as the primary energy source, combined with storage. Solar PV is the technology of choice for smaller systems, although small wind turbines are becoming available too.A solar PV based microgrid system is easier to design and manage, as the solar generation pattern is predictable, whereas wind can vary on a daily to seasonal basis.
Grid-connected microgrids use the grid as a primary source of energy, with renewable energy as an energy savings source, as well as a secondary source of energy, in case of grid failure. Storage and standby plant can be used to supplement the renewable energy resource during grid failure.
Grid connection transformer and controller
Currently, standard industrial transformers are used, but the development of solid-state smart (SST) transformers has the potential to reduce costs and add network power quality management features which currently require additional equipment. A problem with grid-connected microgrids is resynchronisation with the grid frequency when reconnecting. SSTs provide the means of smooth islanding and reconnection in the case of grid disconnection. Smart SSTs also allow for bi-directional current flow and can be used to control the amount of energy drawn from the grid or supplied to the grid.
Microgrid installations in South Africa
Essentially a stand-alone hybrid system serving the community on Robben Island, the microgrid is the conversion of an existing diesel generator-based system to include solar and storage, as well as upgraded control systems.
The system,provided by ABB, consists of a 1960 x 340 W PV array, a 500 kVA, 837 kWh Li-ion storage battery, and a diesel generator.The microgrid can operate from solar power through the day, while the battery bank provides power for approximately seven hours at night.Additional power is available on demand from the diesel generator. Control is by a local microgrid controller, backed up by a system which enables the microgrid to be monitored and operated from Cape Town.The generators on the island previously used about 600 000 ℓ of diesel per year, but the upgrade has resulted in 75% of that being saved.
Farm community in the Free State
Eskom has installed a pilot solar-powered microgrid at Wilhelmina Farm near Ficksburg in the Free State.The microgrid demonstration plant, which was completed in November 2017, provides electricity to the small Wilhelmina community. The microgrid consists of 32 kW of solar PV and a 90 kWh Li-ion battery, backed up by two 22kW diesel generators. The microgrid is linked to Eskom’s research, test and development centre in Gauteng.
ABB’s manufacturing complex in Longmeadow
This grid-connected microgrid, installed as a test and demonstration unit, powers the manufacturing and administrative loads at the ABB site. It consists of a 1 MVA/380 kWh PowerStore battery system,750 kWp rooftop PV array and Microgrid Plus, ABB’s dedicated control system for microgrids. This has been coupled to an existing pair of 600 kW diesel generators as a backup system. The microgrid technology deployed at Longmeadow is fully containerised and pre-designed for this type of application.
This microgrid, installed at Eaton’s manufacturing facility in Wadeville, Germiston, consists of two banks of rooftop PV panels rated at 230 kW, inverters, a 200 kWh storage battery, a 400 kW diesel generator as well as an electricity supply from the local utility. The microgrid serves to reduce demand from the local grid as well as to provide electricity in the case of grid failure. It can operate in islanded mode. This microgrid is unique in that the batteries used are second-hand, repurposed electric vehicle batteries which are still good enough for stationary applications.
Siemens’ head office in Midrand
A grid-connected microgrid, serving the Siemens headquarters, this consists of 1 MW PV-solar plant on the roof of campus’ buildings and on the carports in the parking area. It is integrated with a diesel generator and a 140 kWh SieStorage energy storage system via a Siemens microgrid controller. This microgrid has enabled a reduction of 40% in energy drawn from the grid.
Shopping mall in Clanwilliam
Initially, the developers of the Cedarmills mall applied for a 500 kVA connection to power the facility, but Eskom was only able to meet half of that due to constraints on the local grid. The problem was solved by using the mall’s extensive roof area to install a microgrid consisting of a 851kWp solar PV system and a 700 kWh Li-ion battery. The microgrid, designed and installed by SOLA Future Energy, made up for the power shortfall, thereby allowing the mall’sdevelopment to continue. The microgrid is grid-connected but can operate in an islanded mode in the case of grid failure.
As can be seen from the examples above, a microgrid can be used to provide power from multiple energy sources according the requirements of the client or application. Microgrids can be used in grid-connected or island mode depending on specific requirements. This means that a microgrid can offer significant financial savings to consumers, help to reduce air pollution by reducing demand on Eskom’s coal-fired plant without replacing it with local diesel-powered generators, and make additional power available to meet new demand where the local section of the national grid cannot.
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