Microgrids offer an economical way to ensure continuity of power supply and protection against grid faults and emergency situations, write Celine Mahieux and Alexandre Oudalov
|Microgrids are increasingly being deployed in grid-connected areas Credit: ABB|
Recent years have seen a significant growth in interest in microgrids as a way of providing access to electricity in off-grid locations like remote villages, mines and islands. Now, microgrids are increasingly being deployed as a way to improve local power resilience, reduce reliance on fossil fuels and defer large-scale grid investments in areas that have a connection to the main electricity grid.
This ‘grid-connected’ version of microgrids is growing in popularity as a way to meet rising power demands, take advantage of the falling cost of renewable sources, and improve supply resilience and autonomy (especially for critical applications). They provide an economical way of ensuring continuity of supply and protection against grid faults and emergency situations.
While many microgrids still rely on diesel generators as their energy source, the falling costs of wind and solar power, the availability of efficient energy storage technologies and the availability of affordable wide-area communication infrastructure are making microgrids based on multiple generation sources a highly attractive proposition. Modern microgrids combine distributed energy resources and loads in a controlled, co-ordinated way. Grid-connected microgrids can also deliver additional value by supporting the grid restoration process after a major failure (black-start capability) and bolstering the grid during periods of heavy demand.
At the same time, energy suppliers and industrial and commercial users are increasingly interested in moving away from reliance on fossil fuels and drawing from more sustainable and eco-friendly sources such as solar and wind. In areas where the grid is weak, microgrids can provide a reliable electricity supply while dramatically reducing fuel consumption and carbon footprint. They offer the flexibility and scalability to grow in line with demand, and can be deployed in significantly less time than that needed to complete a grid expansion project.
The ability to isolate such microgrids from the main grid seamlessly when needed is an important feature. Fast-reacting energy sources play a vital role in providing the resilience to ensure continuity of supply for critical loads.
The modern microgrid
In many ways, microgrids are scaled-down versions of traditional power grids. A key distinguishing feature is their closer proximity between generation sources and user loads. The system can be designed and controlled to increase power supply reliability. Microgrids typically integrate renewable energy sources such as solar, wind power, small hydro, geothermal, waste-to-energy and combined heat and power (CHP) systems. Microgrids are increasingly being equipped with energy storage systems, as batteries become more cost-competitive.
The system is controlled through a microgrid control system that can incorporate demand-response so that demand can be matched to available supply in the safest and most optimised way. A flywheel- or battery-based grid stabilising system may be included to offer real and reactive power support.
The microgrid control system performs dynamic control over energy sources, enabling autonomous and automatic self-healing operation. During normal usage the grid-connected microgrid will remain physically connected to the main grid. Microgrids interoperate with existing power systems and information systems and have the ability to feed power back to the grid to support its stable operation. At periods of peak load a microgrid may limit the power it takes from the grid, or even reduce it to zero. Only in the case of main grid failure or planned maintenance will it implement a physical isolation of its local generation and loads without affecting the utility grid’s integrity.
Resilience and independence
Even in developed markets with established grids, there are rising concerns over the resilience and quality of the power supply among certain end-users. In critical applications, grid-connected microgrids are able to disconnect seamlessly (becoming ‘islanded’) and continue to generate power reliably in the event of a fault, natural disaster or even outside attack. In areas where the grid is weak, such grid-connected microgrids satisfy the need to ensure continuity of supply. In recent years microgrids have been suggested as a potential solution after natural disasters in the US highlighted the vulnerability of distribution power grids based on overhead power lines.
While absolute power reliability is important in some sectors, many industries are also looking to reduce energy costs and reliance on fossil fuels for peak shaving or backup power, whatever the condition or availability of the main grid. Here, multi-generation microgrids provide the flexibility to take advantage of a number of options for self-consumption.
Utilities can choose to deploy grid-connected microgrids as a way of deferring investment in expansion or upgrading of the main grid. Such deferrals can produce financial value to utilities by reducing capital expenditure in the short to medium term. Smart control of the microgrid’s distributed energy resources and integration into markets enables the provision of ancillary services for the grid operator and creates new value propositions.
In grid-connected microgrids, the connection is made through a Point of Connection (POC) or Point of Common Coupling (PCC), which enables it to import or export electricity as commercial or technical conditions dictate.
|ABB’s South African factory is to host a solar-diesel microgrid Credit: ABB|
Modern microgrid solutions incorporate a number of key components.
The first is the microgrid control system, which uses distributed agents to control individual loads, network switches, generators or storage devices to provide intelligent power management and efficient microgrid operation. The system calculates the most economical power configuration, ensuring a proper balance of supply and demand to maximise renewable energy integration. It also optimises the network’s generator operations so the entire system performs at peak potential, and ensures a compliant grid-connected microgrid solution.
Power stabilisation and energy storage system
Second is energy storage that plays an important role both in microgrid stabilisation and in renewable energy time-shifts to bridge peaks and troughs in power generation and consumption. However, the two functions require very different technologies for energy storage.
Flywheel grid stabilisation technology enables a high instantaneous penetration of renewable generation sources by providing synthetic inertia and grid-forming capabilities. This stabilises power systems against fluctuations in frequency and voltage caused by variable renewable sources or microgrid loads. It stabilises the electricity network and reduces downtime by rapidly absorbing power surges or by injecting power to make up for short-term troughs, in order to maintain high-quality voltage and frequency.
For microgrid stabilisation the energy storage system must provide a very fast response while possibly being called several times per minute. This demands high power output but small stored energy.
For renewable energy time-shifts, battery-based energy storage systems should be capable of storing energy for a few hours to bridge the peaks of energy production and consumption.
Meeting both requirements typically requires a hybrid system with a combination of underlying storage technologies, each with different performance characteristics (cycle life and response time). A hybrid energy storage system will combine the benefits of each storage medium and offer lower total cost compared with individual units.
A protection system is needed to respond to utility-grid and microgrid faults. With a utility-grid fault, protection should immediately isolate the microgrid in order to protect the microgrid loads. For faults inside the microgrid, protection should isolate the smallest possible section of the feeder.
Optimal energy management system
Thermal loads usually represent a considerable part of total energy used by end consumers. There is significant potential for cost savings, particularly through the use of CHP systems, which allow consumers to realise greater efficiencies by capturing waste heat from power generators. Therefore, cost-effective microgrid energy management requires good co-ordination between thermal energy storage and other thermal sources, and between thermal and electrical systems.
System planning and design tools
System modeling is important during all phases of microgrid development – from the conceptual design and feasibility study, through construction, to final acceptance testing. For example, when an existing diesel-based backup power supply is extended with a large amount of fluctuating renewable energy resources, stable operation of the microgrid cannot be guaranteed. In order to optimally dimension a grid-stabilising device and to tune its control parameters, the dynamic behaviour of legacy diesel gensets has to be known.
Grid storage in Australia
Australian operator SP AusNet has deployed a containerised microgrid solution encompassing battery, transformer and diesel generator for a Grid Energy Storage System (GESS) in Melbourne, Victoria, Australia. This provides active and reactive power support during periods of high demand, and enables smooth transition into islanded/off-grid operation on command or in emergencies. It has also enabled investments in expanded power line capacity to be deferred.
AusNet Services, Victoria’s largest energy delivery service company, began investigating GESS in 2013. It chose to trial the technology to explore its ability to manage peak demand, with the potential to defer investment in network upgrades.
The GESS consists of a 1 MWh 1C lithium battery system operating in combination with a diesel generator, transformer and an SF6 gas circuit breaker-based ring main unit with associated power protection systems.
Located at an end-of-line distribution feeder in the northern suburbs of Melbourne, the system was commissioned in December 2014, and is currently undergoing a two-year trial. The GESS is the first system of this type and size in Australia, and the trial aims to explore the benefits to peak demand management, power system quality and network investment deferral.
AusNet Services is investigating the capabilities of grid-connected microgrids to provide peak demand support. With a generation source embedded close to the load, the utility aims to study the effect on postponing network investment in feeder line upgrades to support increased loads. The belief is that such an embedded generation source can also be used to provide peak load support by reducing the upstream feeder requirements during peak consumption periods by supplying the loads locally. AusNet is also investigating the effect on local system quality and stability that the GESS will provide, including power factor correction, voltage support, harmonics, flicker and negative sequence voltage suppression.
In addition, AusNet is investigating the capabilities of the GESS to operate as an islanded system, and how these improve the reliability of supply and system stability in the case of larger network faults. In the event of a fault, the GESS islands the downstream feeder, creating an islanded microgrid which the GESS supplies until its energy reserves are depleted or the fault is cleared. When the fault is cleared, the GESS reconnects to the grid and transfers the supply back to network and begins recharging the batteries on a scheduled, preset programmed time of day.
Heritage building goes carbon-neutral
A microgrid solution helped Legion House, an office building in Sydney’s central business district, become Australia’s first carbon-neutral and autonomous heritage-listed building. It generates its own power on-site from renewable sources, and can operate independently of the mains electricity grid.
The building’s owner Grocon, Australia’s largest privately-owned development, construction and investment management company, wanted to create its own renewable electricity on site through biomass gasification, fuelled by wood chips and waste paper collected from the 50-storey office block. Legion House can run in ‘islanded mode’, operating fully from on-site power generation.
The building’s location meant it was not able to rely on solar or wind for renewable power generation. Instead it uses two synchronised gas-fired generators connected to the stabilisation and storage system, which serve as a common power bus to provide a base electrical load, while the battery-based energy storage system dampens the effects of instantaneous load steps. The system exports spare electrical power to the adjacent tower building. The battery power system is also used to serve the overnight electrical load as well as minimise the generator operating hours.
The microgrid’s stabilisation and battery-based energy storage systems ensure the tenants have continuous access to a reliable electricity supply. They stabilise the internal (islanded) power network against fluctuations in frequency and voltage that can be caused by essential building services such as elevators and air conditioning systems. The solution uses advanced control algorithms to manage real and reactive power that is rapidly injected or absorbed to control the power balance, voltage, frequency and general grid stability.
The energy monitoring control system and battery monitoring system monitor and control the batteries to provide 100 kVA/80 kW power for up to four hours of electricity supply. The system monitors and controls various battery parameters, including battery temperature, to maximise service life, and it can also be remotely accessed.
Backup power for ABB in South Africa
ABB is itself installing an integrated solar-diesel microgrid at its Longmeadow premises in Johannesburg, South Africa. This will integrate multiple energy sources and battery-based stabilisation technology to ensure continuity of supply.
ABB’s 96,000 m3 facility houses the company’s country headquarters, as well as medium-voltage switchgear manufacturing and protection panel assembly facilities.
The microgrid solution includes a 750 kW rooftop solar photovoltaic (PV) array and 1 MVA/380 kWh battery-based grid stabiliser, which will help to maximise the use of clean solar energy and ensure uninterrupted power supply to keep the lights on and the factories running even in the event of a power outage on the main grid supply.
Celine Mahieux is Research Area Manager: Innovative Applications and Electrification at ABB. Alexandre Oudalov is Senior Principal Scientist with ABB Corporate Research. www.abb.com
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