Compact mobile substation technology aids grid recovery

Fundamental changes to the energy sector are presenting transmission and distribution asset owners and operators with considerable challenges. Global trends like increasing urbanisation and widely distributed variable renewable sources of generation are forcing operators to manage scenarios never conceived when the now aging grid was largely designed and built. Furthermore, nowadays reinforcing the grid is an exercise typically beset by difficulties in execution. Urban landscapes are increasingly congested making the acquisition of land, securing permits and access for new power lines or substation infrastructure projects a significant business risk. Even where major infrastructure is completed successfully, by the time it has been commissioned such assets may already be superseded by new demands and new requirements. A major PV new plant could be developed and installed in a matter of months, for example. With high capital costs and a potentially troubled development timeframe that may span decades, conventional asset development pathways are often failing to keep pace with the demand for a more robust and flexible grid.

One approach that is gathering traction is the use of mobile substation technology to bolster key transmission grid nodes and build in resiliency.

Making transmission mobile

Several breakthroughs in solid state technology and high-voltage equipment – such as switch gear and power transformers – have emerged allowing engineers to make power systems far more compact. But to be mobile, they also need to withstand dusty, bumpy roads and challenging environmental conditions such as temperatures of over 55° C, and yet at the same time maintain structural and operational integrity. To minimise mechanical stresses on the pressure systems during transport, Siemens developed a single structural system design. In addition, elements like the high voltage bushings for gas insulated switchgear (GIS) can be rotated and are folded down during transport. These developments and others have resulted in mobile containerised modular units that can be rapidly deployed as required.

For instance, major US utility Dominion Energy is set to become one of the first companies to deploy Siemens’ mobile static synchronous compensator (STATCOM) technology. Some 50% smaller than conventional SVC (Static Var Compensator) designs, the technology is now compact enough to be mobile, an approach which gives an unprecedented level of flexibility. Instead of several years of planning and execution for a permanently installed substation, the mobile technology can be installed and commissioned within days.

During critical periods, such as maintenance outages or following catastrophic events like hurricanes or floods, Dominion Energy will deploy the mobile STATCOM. Once the emergency period has elapsed – a maintenance outage has been completed or when alternative infrastructure is in place – the asset can be relocated or returned to its depot for any potential future deployment.

Grid resilience

This kind of ‘plug and play’ mobile platform offers benefits in terms of grid resiliency. This is a growing issue in the USA, significant enough now for secretary of Energy Rick Perry to establish a new Office of Cybersecurity, Energy Security, and Emergency Response (CESER) at the Department of Energy this year. 

STATCOM keeps the grid stable during disturbances and faults by providing fast and controlled reactive power. Based on Modular Multilevel Converter (MMC) architecture, it uses Voltage-Sourced Converters (VSCs) to provide a nearly perfect leading or lagging sinusoidal waveform by virtue of the multilevel technology and generates significant capacitive or inductive output current independent of the AC system voltage. This makes the low-frequency harmonic filters often used in earlier designs superfluous and substantially reduces the space requirements for the overall unit.

Emerging as the state-of-the-art dynamic shunt compensator for reactive power control in transmission and distribution systems, the new Siemens SVC PLUS® mobile (a STATCOM on wheels) uses relatively few robust and proven components, such as typical AC power transformers, reactors, capacitors and industrial Insulated Gate Bipolar Transistors (IGBTs).

Because of its topology, it can control the current almost independently of the changing grid conditions, making it adaptable and providing longevity. This also avoids the need for repeated interventions and further investment to maintain stability.

By using these devices, Dominion Energy is meeting its ongoing need to maintain reliability but is also preparing the transmission system for future challenges. For example, the flexible nature of the STATCOM technology will help Dominion Energy manage the growing volume of renewable and distributed energy resources on its power grid.

With a temporary STATCOM installation in place, grid stability can be assured while the utility is able to allow sufficient time to plan for renewables on its system. The first mobile STATCOM is due for delivery as MPS goes to press, but Dominion Energy has already benefitted from mobile power transmission technology elsewhere on its network.

Mobile substation

The utility company previously deployed a mobile substation to support its operations during the Cartersville high-voltage transmission line rebuild project in Cumberland County, Virginia.

Typically, rebuild projects require a temporary transmission line to provide electric service to nearby communities during construction. For the Cartersville project the rough terrain and associated challenges prompted the use of a mobile solution and avoided the need to build a temporary transmission line while still maintaining a reliable connection. In this case mobile substation technology saved $4 million in project costs, according to the company.

The first 230/115 kV mobile installation in North America, the substation was originally designed as part of Dominion Energy’s resiliency strategy. The Cartersville project provided an opportunity to test the system in a planned environment, ahead of any emergency deployment.

Resilience transformers

Besides the mobile substation technology the PRETACT® resilience concept with its ‘Plug & Play’ resilience transformers developed by Siemens are a proven solution for bypassing transformers, either in emergency cases or for maintenance purposes.

Two projects have been executed in the US applying the modular architecture to achieve the customer’s special requirements to maximise grid resiliency. Features included are plug-in bushings and cable connections – and the possibility to use the transformers under different voltage levels and in different configurations. 

Designed as single-phase units and to be as compact and lightweight as possible by applying high temperature insulation systems like NOMEX, they can be transported as completely assembled units and filled with environmentally-friendly MIDEL ester based fluid as well.

The compact design and the modular concept results in a low transport weight and enables very fast transportation and deployment.

Grid resiliency

Mobile substation technology also features in the resiliency plans of Consolidated Edison, the utility that serves New York City, among other locations.

In the wake of Hurricane Sandy, which brought floods and power disruption to the city, ConEd is currently executing a US$1 billion grid resiliency plan.

The plug and play nature of the mobile substation saw a transformer positioned and installed in just 30 hours. One of six units commissioned, Siemens mobile resilience transformers were developed in collaboration with ConEd as a part of the Siemens PRETACT® concept for maximum grid resiliency. Again designed as single-phase units and to be as compact and lightweight as possible, they are also transported complete with their MIDEL ester transformer fluid. Total transport weight is 216 000 lb, or less than 100 tonnes.

With the mobile substation, installation and commissioning can be cut to a few days if the configuration is known in advance and the mobile substation can be prepared and pre-configured.

Beyond the USA, the Middle East and Europe are also adopting mobile grid technology. For instance, National Grid SA, operator of the grid in the Saudi Arabian capital of Riyadh, is deploying portable power technology this year. Two extremely high-capacity mobile substations from Siemens with a capacity of up to 502 MVA and rated voltage of up to 420 kV are among equipment set to be deployed – allowing the company to bypass any of the 380 kV substations that feature in the national transmission system and connect the 380 kV grid with any of the three other grid voltage levels – 132,115 and 110 kV, respectively. The country already owns a fleetofseveraltensofunitsratedatupto 132 kV.

In the event of a failure or major scheduled maintenance work, each customised unit will be delivered to site aboard six trailers, including GIS, transformers and all necessary auxiliaries, as well as control and protection systems.

Portable power solutions

Mobile STATCOM is just one of the several designs available for mobile substations in a broader trend that is reducing risk for grid owners and operators. Increased resiliency and reliability are key to the central service provision, but mobile technology offers further advantages in reducing commercial risk.

For example, Portable Power Solutions include ‘plug and play’ high and medium-voltage substations built as a mobile unit on a trailer, skid-mounted or within a shelter as a so called “E-House”.

These E-houses are meant for semi-permanent installation and can be a solution for adding new assets to the grid quickly, saving time during expansion and renovation.

One function of these portable power solutions is to transfer a large amount of site work to the factory environment. This way the prefabricated substations can bring time savings of up to 30% in substation construction and, with full assembly and testing prior to shipment, the approach delivers a significant reduction in construction risk.

Making systems more modular, more capable and assembled in a controlled environment rather than on-site, eases the construction, installation and testing process, reducing on-site work and the need for skilled labour.

These installations can be used for mining applications or similar industries that need to operate for a few years and want to be moved later on somewhere else, for example. They are also increasingly being adopted in locations where the grid is expanding rapidly or its characteristics are changing even in very demanding locations – say in a very crowded urban area or in a residential area where disturbances must be minimised or for remote areas where it is hard to mobilise resources and materials or the costs may be prohibitive.

For example, Siemens has delivered more than 40 mobile substations to Algeria over the last decade at a number of voltage levels as the country rapidly expands its grid. The majority of these substations will remain several years in operation at the same location as the grid matures.

Similarly, some regions may be faced with environmental challenges such as extreme seasonal weather or wildlife constraints that require timely substation installation over a relatively short duration – say the short summer season in parts of Canada or Russia.

In all these cases an agile approach to minimise human risk, technical risk and construction risk appears in stark contrast to conducting all the works on-site in the conventional arrangement. Knocking out a chunk of the construction risk also makes such projects more easily financed at less cost.

The future is mobile

Portable power solutions enable faster and more reliable grid connections for critical infrastructure in electrical generation or process industries. Crucially, they allow utilities to build resiliency into their grid and future-proof their network in the face of a fundamental transformation that is underway across the energy sector.

Mobility enables grid operators to restore power after a substation failure within a short time frame and can also help avoid disruptions during grid maintenance and upgrade works. Mobility also facilitates a reduction in construction risk by minimising on-site works, a particular advantage in hostile environments or for projects that simply must be delivered on schedule. Moreover, mobile substation technology offers an ideal route to rapid grid reinforcement when networks are expanding quickly or when handling seasonal peak loads.

Siemens provides a range of prefabricated substations that are equally suited for either temporary or permanent use in challenging grid expansion and maintenance programmes up to 420 kV. Portable power solutions like high-voltage E-houses, skid- mounted systems and mobile substations are increasingly proven in numerous utility and industry applications that demand extra flexibility and speed. They are also attractive for both emerging and industrialised countries providing the greatest possible flexibility up to extremely high voltage levels to ensure robust and reliable grid operation. A risk reduction approach to the T&D sector is gathering momentum and the industry is moving to a point where the standard solution is probably going to be mobile or prefabricated – it’s just better business. Today it’s all about agility, but you can only be agile if you are mobile. 

*Alexander James is a freelance journalist