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In case you missed it – The reversible substation

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Actively looking for new ways to improve its environmental footprint, Transport for London has recently performed a successful trial of Alstom’s reversible substation solution, also known as Hesop.

This new converter system can supply the train, providing voltage stability and regenerating the energy produced by braking trains and sending it back to the TFL electrical network to be used by other consumers or, potentially, sold back to the energy distributors. The results of this trial have highlighted the benefits of this new technology.

A challenging commitment

Public transport operators are expected to tackle, simultaneously, a number of challenges: improve energy savings, limit capital expenses, reduce life cycle cost, reduce carbon footprint, reduce heat emissions and improve the passenger experience.

While the Tube provides energy-efficient form of transport, London Underground’s electricity consumption is not negligible. The ‘LU Carbon Footprint Report’, published in 2008, states that London Underground’s electricity consumption represents 2.8 per cent of London’s total usage, making LU the largest consumer of electricity in the capital with an annual energy spend of over £100 million. So it is critical to increase efficiency, both from an environmental and a financial point of view.

Several applications have been developed to avoid energy losses and to reduce overall energy consumption. As a result, these systems can strongly impact operational costs linked to energy prices and substantially reduce emissions of CO2 and other harmful pollutants resulting from the generation of electricity in power plants. But finding the best-adapted technology and opting for the right implementation is not straightforward. It requires an analytical approach that takes many parameters into consideration.

For instance, on-board energy storage solutions, such as super capacitors or batteries, are great to store the excess energy temporarily and release it when needed. But, on the other hand, they are expensive to purchase and maintain, and they also make trains heavier, which increases the energy needed to move them and therefore the heat generated under braking.

So here’s the million pound question: how can an operator provide an environmentally friendly service without increasing costs?

Harnessing braking energy

The best solution is actually quite simple: use the energy elsewhere. This is currently made possible thanks to regenerative braking equipment on trains which allows the traction motor to work as a generator when the train is braking. The kinetic energy of the train can therefore be converted back into electricity.

A small portion of this energy can be reused to power the vehicle’s auxiliary systems (heating, cooling and lighting), while the remaining energy is returned to the network via the fourth rail system, (or via the overhead catenary or the third rail where applicable on other rail networks), to be used by other trains nearby that have a need for energy at the same time. Most modern rolling stock is now equipped with regenerative braking.


But, if there aren’t any other trains in the vicinity, this excess energy is generally wasted and has to be ‘disposed of’ in the braking resistors where it is simply dissipated as heat, thereby contributing to heating the Tube. And that’s the issue. There is not always a train nearby that needs energy (by accelerating) when the first train is giving-up energy (by braking).

Several further solutions have been developed to solve this problem, which can be classified in two families. On the one hand are the energy storage solutions. These can be either located on- board the train, where the energy can be used to power the vehicle and its auxiliaries, or they can be trackside, recovering the energy from any braking vehicle and powering any accelerating vehicle within the area of influence of the system.

On the other hand there are the reversible substation solutions, or ‘back to the grid’ solutions. The main difference with the previous applications is that ‘back to the grid’ applications do not store the recovered energy. Instead, they make it available to be used immediately by other consumers or potentially sold back to the energy distributors.

This is exactly what Hesop does. While most conventional substations only allow for unidirectional energy flow, Alstom’s reversible substation solution uses a purpose-designed converter, allowing the energy to flow in both directions.

The Hesop converter is a fixed piece of equipment that is installed within the power substation and allows for excess braking energy to flow back from the line to the distribution network. There, it can be used elsewhere in the substation, or in nearby stations for a variety of other purposes, such as lighting, cooling, lifts and escalators.

The benefits of Hesop

The Hesop product is the fruit of 10 years of development by a team drawn from various disciplines, from power electronics to traction components. It was developed jointly between two of Alstom’s Centres of Excellence, one in Paris, France, which covers railway infrastructure and turnkey engineering and the second one in Charleroi, Belgium, specialising, amongst other things, in traction and converter technology.

So what can this new technology help operators achieve? The simple answer is – energy savings. Hesop is able to recover 99 per cent of the available regenerated energy. The system senses the traction voltage and current to identify a braking profile and only then operates the inverter, prioritising the most efficient use of the regenerated energy.

To put a figure on the amount of energy that can be regenerated during a braking event, it generally represents around 40 per cent of the energy consumed by a train. Considering that, on average, 25 per cent of this energy can be used by other nearby trains (depending on parameters such as traffic density, distances between stations and slopes), this leaves us with an additional 15 per cent of energy that needs to be redistributed.

The more energy recovered, the less needs to be dissipated in the braking resistors, helping to reduce heat production and improve the passenger experience. Taken to its extreme, if the entire line is equipped with Hesop, the feedback control system effectiveness is such that the brake resistors could be entirely removed from the trains meaning zero heat emissions.

Saving energy in this way also means a net reduction in overall CO2 emissions.

The good news is that it saves energy with limited capital expense and reduced lifecycle costs. The regenerated energy is used elsewhere on the system, meaning that less energy is consumed overall, resulting in substantial reduction in operating expense (OPEX) and maintenance costs.

In the other hand, Hesop provides energy to the train with a constant voltage and therefore fewer power substations are required to run the system, resulting in substantial reduction in capital expense (CAPEX) costs.

Successful trial

The London Underground trial was a successful collaboration between three major UK-based organisations: London Underground, UK Power Network Services, and the Alstom Engineering team which is based in Victoria, London. Each organisation brought its own field of expertise and was fully committed to the trial’s success.

However, as can be expected for a research and development trial such as this one, extensive testing was required to ensure compatibility with the London Underground system.

Following initial factory testing and commissioning of the installation, it was gradually connected to the network in stages until it could finally be trialled using test trains during night shifts, to avoid disrupting the passenger service. The aim was to prove that the Hesop unit would have no effect on any of the existing subsystems, such as rolling stock or signalling.

The trial itself lasted five weeks and collected a large quantity of data on the resulting energy savings. Each braking event, on its own, is rather modest but, added up over a day, they represent an average of approximately 800kWh, enough to power 60 UK households or equivalent to the fuel needed for two round trips from London to Paris. And this was with a single unit.

London Underground confirmed that the energy saved over a week could power Holborn station for more than two days and save five per cent of its energy bill. This was without taking into account potential savings on capital investment (cooling system, braking resistors, and life cycle costs).

The results of this trial are a clear demonstration of the savings which the Hesop product and technology can deliver. Alstom’s Hesop solution has won awards for London Underground at the Railway Industry Association and London Transport Awards. Infrastructure owners in Milan, Sydney, Riyadh, and Panama City have all chosen Hesop for their latest urban transit infrastructure projects.

Written by Stewart Marshall, electrical design leader, and Xavier Billiard, electrification project manager at Alstom Transport

This article was first published in June 2016


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