Combining storage and reactive grid support in a single facility

One of the challenges of a ‘smart grid’ its ability to cope with intermittent and variable power sources. In fact this is a must, since power sources such as wind and solar are becoming increasingly important. One way of meeting this challenge is through energy storage solutions. The newest member of the ABB FACTS family is one such solution, combining SVC Light and the latest battery energy storage technology. This marriage of technologies enables the balancing of power to accommodate large amounts of renewable energy. At the same time it can help improve stability and power quality in grids with a greater reliance on renewable generation.

The system being offered combines ABB’s SVC Light Static Var compensator with a substantial lithium-ion battery storage unit made by Saft. Together they can provide dynamic control of active as well as reactive power in an electricity supply system, independently of each other. By control of the reactive power, grid voltage and stability are safeguarded with high dynamic response. By control of active power, new services based on dynamic energy storage are added.

SVC Light is based on voltage source converters (VSC) which may be connected in shunt configuration to the grid at transmission level as well as sub-transmission and distribution level. The more or less obligatory insulated gate bipolar transistors (IGBTs) are utilised as switching devices. ABB is aiming at industry, distribution and transmission level energy storage applications, the focus being on applications where the combined use of continuous reactive power control and short term active power support is needed. It will become available commercially during 2010.

The system’s main functions would be:

• Black start support of grids

• Bridging power while emergency generation comes on line

• Taking advantage of price differences

• Balancing power for renewables generation

• Storing energy for peak load support

• Supplying power quality control in conjunction with railway electrification

• Smoothing peak power demand to gain on tariffs.

As a contingency typically lasts for only fractions of a second, the required backup power must be made available for only a short time. Similarly, an ancillary service like area frequency control will generally be needed for only a few minutes at a time. A battery based energy storage system can provide the necessary surplus of active power and later be recharged from the grid during normal conditions.

Dynamic energy storage is finding uses in many of areas. As well as supporting grid black start and providing bridging power it can provide grid support with an optimum mix of active and reactive power. This type of storage is an alternative to transmission and distribution reinforcement for peak load support, and enables optimum pricing. It becomes possible to store energy, and selling it at optimum prices, or reduce peak power to avoid high tariffs. Dynamic energy storage can also provide power quality control in conjunction with railway electrification, and help balance power in wind and solar generation, which exhibit an intermittent supply profile.

Smart grid applicability

There is now general acceptance that supply grids must be strongly modified to meet the demands of the future, and that these modifications present severe challenges. The steep rise in the installation of intermittent generation, mainly but not solely from renewables such as wind power, requires increasing capacity for dynamic voltage control as well as the quantity of balancing power. Bolt on systems such as the SVC with its energy storage device described here are therefore seen as enablers of the ‘smart grid’. The conventional power grid is based on centralised generation plants that supply end-users via long-established, unidirectional transmission and distribution systems. But times are changing. The increasing demand for power, and the need to simultaneously reduce CO2 emissions, require the integration of more renewable power and more distributed energy supplies into the system without compromising the reliability of the supply. An electricity system that can handle those challenges in a sustainable, reliable and economic way is required and essential. In other words, we need a smarter grid that can receive power of all qualities from all sources – both centralised and distributed – and deliver reliable supplies, on demand, to consumers of all kinds. The evolved system will be based on advanced infrastructure and tuned to facilitate the integration of all involved.

Basic mechanisms

SVC Light with Energy Storage is connected to the grid through a phase reactor and a power transformer. Having both capacitors and batteries, it can control reactive power (Q), as an ordinary SVC as well as active power (P) thanks to the batteries. The grid voltage and the VSC current set the apparent power of the VSC, while the energy storage requirements decide the battery size. As a consequence, the peak active power of the battery may be smaller than the apparent power of the VSC, for instance, 10 MW battery power for an SVC Light of 30 MVA.

To support the grid during contingencies, as well as for ancillary services, it is usually enough to have the necessary amount of power available during a relatively short time.

Typically a unit would be rated at 5-50 MW of active power, good for a period of 5-60 minutes, combined with a reactive power rating of ± 70 MVar.

System benefits from the point of view of applicability are said to be:

• Modular scalable design

• Integrated batteries and VSC

• Focus on safety and possibilities to handle the consequences of possible faults occurring.

• Low losses.

• Very high cycle efficiency.

The modularised design of the new energy storage technology makes it simple to scale, in power rating as well as energy. Its batteries and VSC are integrated, with detailed supervision and status checks of both within the same system. It focuses on safety and ensures the ability to respond to the consequences of possible faults. In addition, the solution boasts low losses and very high cycle efficiency.

Main system components

A complete SVC Light with Energy Storage comprises, together with its SVC and battery storage system:

• A power transformer

• AC and DC high voltage equipment

• A control and protection system

• Auxiliary power equipment

The VSC is built up of power IGBT and diode semiconductors. To handle the required valve voltage, the semiconductors are connected in series. Water cooling is utilised for the VSC, giving a compact converter design and high current handling capability.

Each IGBT and diode component is built up in a modular housing comprising a number of sub-modules, each containing a number of ABB StakPak semiconductor chips.

Battery system

The battery system supplied by Saft is made up of rack-mounted Li-ion modules. An array of battery modules provides the necessary rated DC voltage as well as storage capacity for each given case. The Li-ion battery technology is critical, because rechargeable cell chemistry results in a range of different characteristics for discharge curves, recharge rates and required recharge patterns, recharge capacity, repeatable cycling behaviour, and other factors. For this application the selected technology benefits from a number of features, according to Saft and ABB, namely:

• High energy density

• Very short response time

• High power capability both in charge and discharge

• Excellent cycling capability

• Strongly evolving technology

• High round trip efficiency

• High charge retention

• Maintenance free design

By excellent cycling capability the manufacturer means 3000 cycles at 80 % depth of discharge or 1 million cycles at 3 % depth of discharge. The system also offers high energy density and millisecond-level response time as well as long calendar life and an intelligent, self-diagnostic design.

The equipment itself provides precise information on the battery’s state of charge, which is a vital function in a dynamically operating energy storage system.

Other applications

The energy storage solution can be used for load support as well as ancillary grid services, or example regulating power frequency. Another promising use is as part of the infrastructure for PHEVs (plug-in hybrid electric vehicles). And its highly scalable ability to store energy is remarkable. At present, rated power and storage capacity are typically in the range of 20 MW; however, up to 50 MW for 60 minutes and beyond is said to be possible with this new FACTS technology. And as the price of batteries continues to drop, applications requiring larger battery storage will become viable, enabling for example multi-hour storing of renewable power during low demand, for release into the grid during higher demand.