The new FACTS for grid stability

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Peter Jones, engineering manager for grid systems for ABB in the UK, explains how a new generation of FACTS (Flexible AC Transmission Systems) solutions combines fast-acting grid interface electronics with battery energy storage technology to help improve stability and power quality for grids with a significant penetration of renewable generation

 



Many transitions and distribution grids are already operating close to their limits. So as the penetration of intermittent renewable energy sources increases there is a greater risk of instability, especially at times of peak demand. Maintaining the grid frequency within acceptable limits is a primary concern should there be a sudden, unexpected loss of power generation or transmission. Normally, this is addressed by providing ancillary services in the form of generators that are on-line, spinning and loaded at less than full output.

 

Now there is an alternative in the form of a new generation of FACTS technology that combines fast-acting grid electronics with a battery energy storage system to enable dynamic control of active as well as reactive power in a power system, independently of each other. Through the control of the reactive power, grid voltage is controlled with high dynamic response while the active power element not only provides primary frequency control, but also enables a number of new services to be added including black start capability and peak load support.

A commercially available example of this dynamic energy storage solution for grid applications is ABB’s DynaPeaQ system that combines SVC Ligh technology with advanced Li-ion battery modules. DynaPeaQ provides the optimum mix of active and reactive power to support grids under high stress conditions. It features a modular, scaleable design for the creation of systems rated up to 50 MW for up to 60 minutes.

Combining an SVC Light with energy storage

A STATCOM, which is an element in the group of grid technologies known as FACTS, is a power electronic converter used to provide continuously variable, fast-acting control of the reactive power either absorbed from the grid or injected into it. A STATCOM utilises a VSC (voltage source converter) connected in shunt to the grid at both distribution and sub-transmission level. ABB’s STATCOM concept is known as SVC Light.
By combining an SVC Light with energy storage, DynaPeaQ can control both reactive power ‘Q’ – operating as an ordinary SVC Light – as well as active power ‘P’.

The grid voltage and the VSC current set the apparent power of the VSC, while the energy storage requirements determine the battery size. Consequently, the peak active power of the battery may be smaller than the apparent power of the VSC – for example a 10 MW battery for an SVC Light of +30 MVAr.
Since a grid contingency typically lasts for only a fraction of a second, the required backup power need only be made available for a short time. Similarly, an ancillary service like area frequency control will generally be needed for only a few minutes at a time. The energy storage system (ESS) can then provide the necessary burst of active power to maintain stability and is later recharged from the grid during normal operating conditions.

Li-ion battery system
The DynaPeaQ is designed for high-power applications, with series-connected IGBTs used to adapt the voltage level, so the pole-to-pole voltage is as high as 10 kV. Therefore, a number of battery modules are connected in series to develop the required voltage in a battery string. To obtain higher power and energy, several parallel battery strings may be added.

The Saft Li-ion battery technology incorporated in the DynaPeaQ offers a number of important features:
• 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

The calendar lifetime of the Li-ion cells is 20 years, with 3,000 cycles at a depth of discharge of 80 % or 1 million cycles at a depth of discharge of 3 %.
The battery system comprises rack-mounted Li-ion modules of 230 V each. An array of series and parallel connected battery modules provides the necessary rated DC voltage and storage capacity for each installation.

Power-frequency characteristics

Figure 1 illustrates the principle of the frequency profile (left) and the power-frequency characteristic of the primary frequency controller (right). When the frequency is within the deadband, no power is flowing to or from the battery. When the frequency moves outside the deadband, the battery will charge or discharge as necessary. So if the frequency exceeds the upper limit of the deadband, the battery charges (absorbing excess power from the grid). If the frequency falls below the lower limit of the deadband, then the battery discharges (injecting active power into the grid).
Clearly, it is important for the battery storage to be maintained in a state that allows both discharge (negative frequency deviation) as well as charge (positive frequency deviation), at any time. For this reason the average state of charge level of the battery storage is set at 50 %.

Main system components
• A complete DynaPeaQ comprises:
• Power transformer
• SVC light
• Battery system
• AC and DC high voltage equipment
• Control and protection system
• Auxiliary power equipment
The modular design of the DynaPeaQ makes it simple to scale, in power rating as well as energy. Its batteries and VSC are integrated, with detailed supervision and status check of both within the same system.

Dynamic energy storage applications
Dynamic energy storage is finding uses in many areas. Not only can it support the black start of grids, it can also provide bridging power until emergency generation is online and provide grid support with an optimum mix of active and reactive power.

This type of storage is an alternative to transmission and distribution reinforcements for peak load support, and enables optimum pricing. It becomes possible to 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 schemes, which are inherently intermittent.

UK Power Networks pilot installation

In May 2011, a pilot system was commissioned on an 11 kV radial distribution network operated by UK Power Networks (UKPN) at a site north of Hemsby in Norfolk. The battery capacity is sufficient to provide 200 kW of power continuously for one hour and a 600 kW peak output is possible for short durations. In addition to the real power capability a reactive power source/sink rated at 600 kVAr is also always available.

The DynaPeaQ has been placed at a normally open point near the remote ends of two 11 kV feeders from different substations. Only one feeder is connected to the system at any single moment, but it is easy to switch between feeders. Physical network information such as line and transformer data was provided by UKPN as well as half-hourly operational data comprising feeder current and DG (distributed generation) output.

A mixture of residential areas, rural areas and seasonally occupied accommodation are supplied by the feeders in this region. The typical load on the feeders is 1.15 MW and 1.30 MW with peaks of 2.3 MW and 4.3 MW respectively. A wind farm with 2.25 MW installed capacity is attached midway along the first of these feeders. This installation has fixed speed induction generators, so there is significant reactive power demand while generating.

Daily load profiles show the two feeders have quite different characteristics. On the first, the most significant demand occurs during the night, due to a high number of homes heated by night storage heaters. Summer loading is lower than that during winter. The second feeder has much less storage heating, and in this case summer loading is higher than during winter. These dissimilar characteristics mean events requiring grid support are likely to occur at different times, maximising the utilisation of this pilot system.

The installation is now undergoing operational testing and its effectiveness is being carefully monitored in collaboration with the University of Durham. Potentially, this solution could be replicated across parts of the UK where wind farms connect to the grid.